the absorption spectrum of anisole and the anisole co 1:1 … · 2004-03-16 · the absorption...

Z. Phys. Chem.218 (2004) 123–153 by Oldenbourg Wissenschaftsverlag, München

The Absorption Spectrum of Anisole andthe Anisole/CO2 1:1-Cluster.The Influence of Intermolecular Interactionon Intramolecular Vibrations

By A. S. Gemechu1, L. J. H. Hoffmann1, S. Marquardt1, C. G. Eisenhardt1,H. Baumgärtel1,∗, R. Chelli2, G. Cardini2, and S. Califano2

1 Institute of Chemistry, Physical and Theoretical Chemistry, Free University Berlin,Takustr. 3, 14195 Berlin, Germany

2 Dipartimento di Chimica, Universityof Florence, via G. Capponi 9, 50121 Florence,Italy

Dedicated to Prof. Dr. Herbert Zimmermann on the occasionof his 75th birthday

(Received October 25, 2003; accepted October 25, 2003)

Anisole / Anisole/CO2 / Quantumchemical Calculations / REMPI Spectra /Geometry / Vibrations

The S1(1B2) ← S0(

1A1) electronic transition of anisole and the anisole/CO2 aggregatecooled in a supersonic free jet has been investigated in detail using REMPI spectroscopyand quantumchemical model calculations. The 42 intramolecular modes of anisole in the1S1 state are assigned. Some previous assignments of modes of anisole in the1S1 statehave been improved, some of the assignments are still tentative.

The origin of the corresponding electronic transition in the 1:1-aggregate is blueshiftedby 117 cm−1 versus the 0-0 transition of anisole, the origine in the 1:2-agreggate isredshifted by 216 cm−1. Probably a second conformer of the 1:1-aggregate is formed inthe molecular beam. 36 fundamental modes of the anisole/CO2 1:1-cluster out of possible46 intramolecular modes are assigned. Spectral shifts of the fundamental modes in the1S1 state of anisole/CO2 due to the aggregation have been observed. The intermolecularmodes and their binary combinations with intramolecular modes have been analyzed.

1. IntroductionThe intermolecular interaction has been of interest since the early days of phys-ical chemistry. One of the most popular examples is the equation of state of realgases.

* Corresponding author. E-mail: [email protected]

124 A. S. Gemechuet al.

Intermolecular interaction results from the simultaneous action of differ-ent intermolecular forces. The natureof these forces are well-known and havebeen described in details [1]. The intermolecular forces modify genuin proper-ties of molecules. In the frame of this work we will report on the influence ofweak intermolecular interaction on the vibrational structure of the participatingmolecules anisole and carbondioxide in the anisole/CO2 1:1-complex.

The combination of molecular beam technique with laser spectrocopyallows the detailed experimental study in which way aggregation of thesemolecules influences their intramolecular vibrations in the1S1 state. In add-ition quantumchemical model calculations of the systems reveal the influenceof intermolecular interaction on the intramolecular modes in the electronicground state.

Molecular aggregates of aromatic systems with small molecules are verysuitable subjects for this kind of work. Therefore, numerous different mo-lecular clusters have been investigated by laser spectroscopy. The results aredocumented in several informative reviews [2–7]. Considering small 1:1- and1:2-aggregates, many of the investigations focused on systems with hydrogenbonds. Typical examples are clusters of phenol with water, alcohols, aminesetc. [8–16]. Many of the experiments were focused on the determination ofintermolecular vibrational modes which image the intermolecular interaction.The interpretation of the spectroscopic results was supported by quantumchem-ical calculations. They give informations on the total energy, the geometry andvibrations in the1S0 state.

However, so far the influence of aggregation on the intramolecular vibra-tions of the aromatic chromophore has not been investigated systematically,although the influence of solvents in solutions or solid matrices on the energyof vibrational modes has been observed since the early days of IR spectroscopy.In these early experiments small solvent shifts could not be assigned to a mo-lecular aggregate with specific structure.

Mass selected REMPI spectra offer the possibility to study vibrationalmodes in the1S1 state and their shifts in size selected aggregates. In the elec-tronic ground state the assignment of fundamental modes can be supported byquantumchemical model calculations, whereas the assignment of these modesin the 1S1 state is much more difficult. Firstly, the accuracy of routine quantum-chemical calculations for this state is still insufficient and, secondly, it is wellknown that fundamental modes may be considerably shifted in the excited statein comparison to the ground state. Therefore the analysis of vibrational modesin the excited electronic state remains difficult and the tentative character ofassignments has to be taken into account.

In this paper we will present a comparison between the REMPI spectra ofanisole and anisole/CO2. Spectroscopically anisole is a large system, but a firstattempt will be made to assign the 42 intramolecular vibrations of anisole in the1S1 state, because this is of key importance for forthcoming experiments withclusters containing anisole.

The Absorption Spectrum of Anisole and the Anisole/CO2 1:1-Cluster. . . 125

Fig. 1. Scheme of the experimental setup.

The paper is organized in the following way: First we give a short reporton the experimental setup, followed by the results of model calculations onanisole/CO2 in the electronic ground state. Then the REMPI spectra of anisoleand anisole/CO2 (1:1) will be analyzed and discussed.

For briefness we compile the results in tables. The figures illustrate parts ofthe spectra which are of interest for the discussion.

2. Experimental setup

A schematic drawing of the experimental setup is given in Fig. 1. It has beendescribed in detail previously [17, 18]. It consists of a supersonic beam coupledto a time of flight spectrometer, a tunable dye laser and a data aquisitionsystem.

The time of flight tube can be used for the analysis of ions as well as ofelectrons. Details of the construction have been given earlier [18, 19].

The stagnation pressure for the expansion of the gas mixtures (He/anisoleand He/anisole/CO2) has been varied between 0.2 and 2.5 bar. We useda 50 mm nozzle and a 300 mm skimmer to admit the aggregates from the mo-lecular beam source to the ionization chamber. The clusters are ionized by thefrequency-doubled output of a Nd-YAG pumped dye laser (Lambda PhysicsScanmate 2EC-400 OG) calibrated with a neon OG lamp. Coumarin 153 andCoumarin 307 dissolved in methanol have been used as dyes. The resolutionin the REMPI spectra generally was 0.1 cm−1, some energy ranges (not shownhere) have been measured with 0.025 cm−1 using an etalon.


3. Ab initio calculationsThe calculations were carried out by using the density functional theory(DFT) [20] as inplemented in the GAUSSIAN 98 package [21]. In DFT goodresults are achieved by using smaller basis sets than required in other corre-lated methods. However, it is importantto choose an appropriate combinationof basis set and exchange-correlation functionals as shown by Rauhut and Pu-lay [22] and by Scott and Radom [23]. The combination of the 6-31G(d) basisset [24] with B3-LYP exchange-correlation functional represents a good com-promise between accuracy and computer time cost. Of course, the agreement ofthe calculations with experimental data can be improved by using a larger ba-sis set. Therefore we used the B3-LYP functional and the 6-31G++(d,p) basisset. The “very tight” convergence criteria have been adopted for the minima lo-calization. A few starting configurations have been choosen, they all convergedeither in the global energy minimumA or at higher energy in a second mini-mum B. A very fine grid has been used for all the calculations to increase theaccuracy of the second derivative.

The B3-LYP functional is defined in terms of the Dirac–Slater (DS),Hartree–Fock (HF), Becke (B88) [25], Lee–Yang–Parr (LYP) [26] and Vosko–Wilk–Nussair/VWN) [27] functionals according to the expression:

FB3-LYP = 0.8Fx(DS)+0.2Fx(HF)+0.88Fx(B88)+0.81Fc(LYP)

+0.19Fc(VWN) .

A satisfactory fit of the experimental frequencies in the electronic ground stateis obtained by scaling the calculated frequencies in the range up to 2000 cm−1

by the factor 0.973 and those in the higher frequency range by the factor 0.963.

4. Results and discussion4.1 Geometry and symmetry of anisole and anisole/CO2

The geometry of anisole and of its clusters with small molecules is of con-siderable interest when this system is considered as a model for the study ofintermolecular interactions.

There are several experimental investigations [28] andab initio calcula-tions [29, 30] which reveal that the equilibrium conformation of anisole in theelectronic ground state is planar. This configuration is energetically favored be-cause of the conjugation of one of the lone pair electrons of the methoxy groupwith the aromaticπ-system. However steric effects may constrain the systemin a nonplanar conformation. The analysis of the very high resolution spectrumof the 1S1 ← 1S0 transition of anisole [28] confirmes the planar geometry ofanisole.

In the 1S1 state, however, there are changes of the geometry in compari-son to the electronic ground state. The C–O–CH3 angle increases by 2◦ with


Fig. 2. Structure of the anisole/CO2-aggregates. Isomer A (on top) is more stable than iso-mer B (below).

respect to the neutral molecule. In addition, the length of the two C–C bondsadjacent to the C–O bond increases, whereas the two next C–C bonds decreaseand the other two C–C-bonds increase. These changes indicate the tendencyto a more quinoidal structure. Quantumchemical calculations show that thistendency continues in the anisole ion [29].

The geometry of the anisole/CO2 1:1-cluster has been obtained from quan-tumchemical model calculations. In many cases the potential energy surfaceof molecular aggregates shows several different local minima. The geometryof the minimum energy anisole/CO2 1:1-aggregate (conformer A) in the1S0

state as obtained from our calculations is shown in Fig. 2. The results of themodel calculations and the results from ahigh resolution spectroscopy studyof the 0-0 transition of this complex [31] fit together very well. The complexA is planar and the geometry of anisole and CO2 are not changed in compar-ison to the isolated compounds. In the electronic ground state the center of


mass distance of anisole and carbondioxide comes to 44.3 pm and the angleαbetween the pseudo-C2axis (C(6)–C(3)–O(12)) of anisole and the axis of car-bondioxide is about 40◦. In the 1S1 state the center of mass distance betweenthe two molecules comes to 44.7 pm andα is increased by 15◦. The rota-tion of the methoxy group in the aggregate generates two rotational isomers.The rotational barrier in isolated anisole is 3.9 kJ/mol [30]. In the aggregatefree rotation is not possible because of steric hindrance by the carbondioxidemolecule.

The model calculations revealed a second minimum corresponding to ananisole/CO2-aggregate B with a slightly altered structure (Fig. 2). The totalenergy of aggregate B is 5.5 kJ/mol higher than that of the aggregate A. Thepopulation of the different minima in the molecular beam is a priori not known,but the conformer assigned to the global minimum may dominate among thedifferent species with a given cluster size. The inspection of the REMPI spec-trum of the 1:1-aggregate exhibits signals of low intensity which point toa small contribution of the structure Bbesides the more stable conformer A.

From the structure of anisole follows that its molecular symmetry shouldin principle be classified according to Longuet–Higgins theory as belonging tothe G4 group, since in anisole the O–CH3 group can undergo a large scale ro-tation about the C–O bond. The G4 group is isomorpheous with the C2v groupand has the same character table. According to previous papers we classify theskeleton modes toA1, A2, B1, B2 of the C2v group, this makes the comparisonwith skeleton modes of other benzene derivatives easier. Fundamental modesof the methoxy group are classified to the symmetry species a′ and a′′ of theCs group. For the vibrations of the aromatic system we apply the well-knownWilson notation [32].

The anisole/CO21:1-cluster belongs to the Cs symmetry group, however,the chromophor in the complex is anisole and therefore from the spectroscopi-cal point of view we prefer for the discussion the use of the C2v symmetry ofanisole. Finally it should be noticed that a very rigid application of symme-try selection rules for the analysis of the optical transitions in the cluster is notpossible.

4.2 Vibrational structure of anisole and anisole/CO2 in the 1S0 state

The vibrational structure of the electronic ground state of anisole is well char-acterized. The 42 normal vibrations have been assigned by IR- and Ramanstudies and by quantumchemical model calculations [29, 33].

Before entering the detailed discussion of the REMPI spectra the vibra-tional structure of the aggregates A and B (Fig. 2) according to the results ofthe model calculations of the1S0 state shall be described. In the 1:1-aggregatesone expects in addition to the 42 intramolecular modes of anisole five intermo-lecular modes and 4 intramolecular modes of CO2. The values resulting fromthe model calculations are compiled in Table 1.


Table 1. Calculated modes of intermolecular and intramolecular vibrations of anisole,anisole/CO2 (A) and anisole/CO2 (B).

Nr. assignment anisole anisole/CO2 (A) anisole/CO2 (B)

1 – 21 62 – 29 113 – 46 174 – 59 225 – 83 30

6 COC torsion,a′′ 90 95 917 10b, B1 203 206 2008 9b, B2 250 256 2489 O–CH3, torsiona′′ 266 268 263

10 16a,A2 412 409 409

11 COC bending,a′ 433 429 42912 16b,B1 502 500 50013 6a,A1 543 539 53914 6b,B2 610 604 60315 ν2(CO2) – 619 627

16 ν2(CO2) – 630 62717 4, B1 669 669 66518 11,B1 738 734 73219 C–OCH3 stretching,A1 777 767 76920 10a,A2 806 802 803

21 17b,B1 867 863 86022 17a,A2 941 934 93323 5, B1 956 951 94724 12, A1 981 971 97125 1, A1 1015 1005 1004

26 18a,a′ 1041 1028 103027 18b,B2 1075 1066 106428 CH3 rocking,a′ 1138 1129 112629 15,B2 1147 1138 113630 9a,A1 1165 1155 1154

31 CH3 rocking,a′′ 1173 1164 116232 7a,A1 1248 1228 123633 3, B2 1305 1293 129234 ν1(CO2) – 1313 131335 14,B2 1330 1317 1317

36 CH3 sym. deformation,a′ 1437 1424 142537 19b,B2 1449 1435 143538 CH3 asym. deformation,a′ 1456 1446 144239 CH3 asym. deformation,a′′ 1469 1454 145340 19a,A1 1492 1477 1477


Table 1. continued.

Nr. assignment anisole anisole/CO2 (A) anisole/CO2 (B)

41 8b,B2 1542 1573 157242 8a,A1 1606 1589 157143 ν1(CO2) – 2322 232244 CH3 sym. stretching,a′ 2903 2908 290945 CH3 asym. stretching,a′′ 2964 2970 2971

46 CH3 asym. stretching,a′ 3034 3045 303347 7b, A1 3063 3063 306248 13,B2 3070 3071 306959 2, A1 3089 3087 308550 20b,B2 3093 3097 3092

51 20a,A1 3101 3105 3106

As expected, the intermolecular modes appear at very low energy. Theintramolecular modes of anisole in the cluster are shifted in comparison tothe isolated system. Due to the weak interaction between anisole and carbon-dioxide these spectral shifts are small. The maximum shifts are observed withmode 7a, which is downshifted in cluster A by 20 cm−1 and with mode 8b,which is upshifted in cluster A by 31 cm−1 in comparison to anisole. The dif-ferences between the frequencies of the intramolecular modes of anisole in theaggregates A and B are negligibly small.

Another remarkable result of the calculations concernes the vibrations ofCO2 in the anisole/CO2 aggregates. The calculated values of the CO2 modesare smaller in the anisole/CO2 aggregate than the experimental values of iso-lated CO2. In the cluster A the bending modeν2 is not degenerate. The differ-ence between the in plane and out of plane bending mode comes to 11 cm−1.However, in the cluster B theν2 mode of CO2 remains degenerate.

We have performed no calculations for the1S1 state of anisole and the ag-gregates with CO2 because we expect the accuracy of the results obtained withstandardab initio programs not to be sufficient for comparing the fundamentalmodes in anisole and the anisole/CO2 aggregates. Therefore the assignment ofthe REMPI signals to normal modes, as given below, is based on the compari-son with data of related benzene derivatives.

4.3 The REMPI spectrum of anisole

Anisole is spectroscopically a large system and consequently the REMPI spec-trum exhibits many structural featureseven at medium spectroscopic resolutionas used in this experiment.


Fig. 3a. REMPI spectrum of anisole. The figures a–e show parts of the spectrum in whichmost of the intramolecular modes appear.

The number of aromatic molecules for which excited state vibrations havebeen assigned remains small in comparison to the large number of aromaticmolecules for which ground state vibrations have been studied in detail. Con-sequently many of the excited state vibrational modes up to now remain unas-signed. The REMPI spectrum of anisole due to overtones and combinationsshows much more features than the signals of the 42 fundamental modes. Itcan be compared with the conventional gasphase spectrum [34]. This compari-son reveals corresponding signals for many fundamental modes in the1S1 state.However, the conventional spectrum measured at room temperature showsmany features close to the origine which are not observed in the REMPI spec-trum.

The REMPI spectrum of anisole is the key spectrum for further investi-gations of clusters containing anisole and small molecules. Therefore strongefforts are made to determine the frequencies of fundamental modes in the1S1

state.We measured the REMPI spectrum of anisole in the range 36200–

40200 cm−1. The lowestπ–π * transition ( 1S1(1B2) ← 1S0(

1A1)) is elec-tric dipol allowed for single photon absorption. We observed the origin at36394±2 cm−1. The spectrum is shown in Fig. 3a–e and may be comparedwith the conventional absorption spectrum measured at room temperature by


Fig. 3b. continued.

Balfour [34]. He finds the origin of theπ–π * transition at 36386.4 cm−1, thedifference of these values may be due to the calibration in both experiments.The REMPI spectrum shows more signals than the spectrum reported by Bal-four. In both spectra the main intensities are observed at 757 cm−1 and in therange 920–960 cm−1 above the origin. The symmetry forbidden transitionsto vibronic states withB2 modes, the overtones and combinations generatea considerable number of weak signals. There are some differences betweenthe REMPI spectrum and Balfours spectrum considering the wavenumbers ofcorresponding signals and some assignments. This will be adressed below indetail. Moreover there are differences between these two spectra due to thedifferent temperatures of the probes. Even at the lowest expansion pressurethe temperature in the molecular beam exceeds hardly 50 K. Therefore in the


Fig. 3c. continued.

Fig. 3d. continued.


Fig. 3e. continued.

REMPI spectrum we observe only a few hot bands with very low intensityclose to the origine. Balfour has assigned many signals to hot bands. In add-ition the spectral resolution in the REMPI spectrum is improved in comparisonto Balfours spectrum.

For the assignment of vibrational modes in the1S1 state relevant data fromthe literature are helpful.

The evaluation of vibrations in the1S1 state by quantumchemical calcula-tions has been reported in the literature [35–38]. The results of these calcu-lations are valuable and may be used as afirst approximation for vibrationalmode shifting in going from the1S0 to the 1S1 state. However, they are not pre-cise enough to give reliable support for the assignment of the signals in theREMPI spectrum. To make the assignment of fundamental modes in the1S1

state of anisole as confident as possible we compare our results with studieson p-dimethoxy-benzene [38],p- and m-difluorobenzene [39, 40] and othersubstituted benzenes.

Generally most of the ring modes have lower frequencies in the1S1 statethan in the electronic ground state due to the occupation of an antibondingπ *

orbital which leads to ring expansion andlower force constants. Consequentlyone expects downshifts of the vibrational modes which are connected with mo-tions of atoms involved in theπ system. This tendency can be documented byshifts of the normal modes of benzene. In this molecule the modes 1, 19, 6, 9, 8


are downshifted in1S1 in comparison to1S0 by 70–87 cm−1, mode 3 is onlyshifted by−23 cm−1, mode 15 is not altered (+2 cm−1).

A remarkable exception is the frequency upshift of the Kekule-typ mode 14(b2u) in the 1B2u electronically excited state of several aromatic hydrocarbonsand some of their derivatives. In benzene mode 14 is upshifted by 261 cm−1 inthe 1B2u state versus the1A1g electronic state. This unusual behaviour has beendiscussed by Y. Haaset al. [41]. They assume that theσ frame is essentially re-sponsible for the D6h structure of benzene, whereas theπ electrons prefer a D3h

distortion. The transfer of an electron in the antibondingπ * orbital increasestheσ character of the benzene ring mode. This causes an increase in the forceconstant along this coordinate resulting in an increase of the corresponding fre-quency. Similar results have been reported forp- andm-difluorobenzene [40].

For stretching and bending vibrations which involve the COC substructurea small upshift may be expected because the C–OCH3 bond becomes strongerin the 1S1 state due to the increased interaction between the aromatic ring andthe methoxy group.

The C–H stretching modes of the aromatic ring and the CH3 group arehardly influenced by electronic excitation. However, a slight increase of thecorresponding modes due to the increased force constant can not be ex-cluded. The frequencies of the C–H bending and torsional modes of theOCH3 group are unchanged or slightly decreasing as anisol is excited tothe 1S1 state.

For the readers convenience details of the spectrum are discussed in foursections. The assignments of fundamental modes of anisole in the1S1 state asproposed from the results of our measurement are compiled in Table 2.

4.3.1 Origin – 900 cm−1 range

In the electronic ground state one observes in this range the COC torsion mode,the COC bending mode, the O–CH3 torsion mode, the C–OCH3 stretchingmode and the modes 10b, 9b, 16b, 16a, 6a, 4, 6b, 11, 17b, 10a, 17a. In theexcited state one expects a comparable number of fundamental modes.

According to the IR spectrum of anisole in the1S0 state the COC torsionmode and mode 10b are expected to have very low energies. Balfour assignesto the COC torsion mode in the1S1 state the value 82.9 cm−1. However in ourmeasurements no signal has been observed which could be assigned unambi-geously to the COC torsion mode.

There are several weak signals around 200 cm−1. The weak signals at234 cm−1 and 269 cm−1 are assigned tentatively to mode 10b and 16b. The sig-nal observed at 258 cm−1 is in agreement with the value given by Balfour, whoassigned it to the 18b mode. However, according to the results of our calcula-tions it is more likely to assign this signal to the 9b mode.

The weak signal at 269 cm−1 may be due to mode 16b, but this assignmentis very tentative, in fact, there is another weak signal at 289 cm−1 which also

136 A. S. Gemechuet al.Ta

ble

2.S

hifts

∆of

the

fund

amen

tal

mod

esin

the1 S 0

and

the

1 S 1st

ate

caus

edby

the

form

atio

nof

the

anis

ole

/C

O2

(1:1

)-ag

greg

ate

(A).*

Sym

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sign

men

tan

isol

e∆

anis

ole/

CO

2an

isol

eIn

tens

.∆

anis

ole/

CO

21 S 0

1 S 01 S 1

1 S 1(c

m−1

)(c

m−1

)(c

m−1

)(c

m−1

)re

l.%(c

m−1

)(c

m−1

)ca

lc.

calc

.ex

p.ex

p.

A1

6a,

Xse

nsiti

ve54

3−4

539

500

15.6

−21

479

C–O

CH 3

stre

tchi

ng77

7−1

076

775

799

.5−3

754

1298

1−1

097

194

096

.4−8

932

1(r

ing

brea

thin

g)10

15−1

010

0593

588

.7+2

937

9a11

65−1

011

5599

433

.6−1

398

17a

1248

−20

1228

1273/

7030

.4+2

012

9319

a14

92−1

514

7715

1715

.5−2

314

948a

1606

−17

1589

1543

––

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3063

±030

6330

64–

––

230

89−2

3087

3081

––

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a31

01+4

3105

3106

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sens

itive

250

+625

625

810

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024

86b

610

−660

452

627

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523

18b

1075

−910

6694

685

.3−7

939

1511

47−9

1138

1098

6.8

−17

1081

313

05−1

212

9311

2714

.4−1

611

1119

b14

49−1

414

3512

8724

.9+9

1296

8b15

42+3

115

7315

28–

––

1413

30−1

313

1715

71–

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1330

70+1

3071

3074

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3097

3097

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perim

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−1


Tabl

e2.

cont

inue

d.

Sym

.as

sign

men

tan

isol

e∆

anis

ole/

CO

2an

isol

eIn

tens

.∆

anis

ole/

CO

21 S 0

1 S 01 S 1

1 S 1(c

m−1

)(c

m−1

)(c

m−1

)(c

m−1

)re

l.%(c

m−1

)(c

m−1

)ca

lc.

calc

.ex

p.ex

p.

A2

16a

412

−340

938

43.

1−1

536

910

a80

6−4

802

706

7.4

−14

692

17a

941

−793

474

914

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742

B1

10b

203

+320

623

41.

9−2

221

216

b50

2−2

500

269

2.5

−326

64,

Xse

nsiti

ve66

9±0

669

507

9.9

+551

211

738

−473

465

14.

35−8

643

17b

867

−486

368

35.

8−8

675

595

6−5

951

955

91.7

−694

9

a′C

OC

bend

ing

433

−442

942

57.

45−4

421

O–C

H3

stre

tch.

(18a

)10

41−1

310

2895

113

0+8

959

CH

3ro

ckin

g11

38−9

1129

1140

7.6

−17

1123

CH

3sy

m.

defo

rmat

ion

1437

−13

1424

1443

9.8

−21

1422

CH

3as

ym.

defo

rmat

.14

56−1

014

4614

5510

.9−1

114

44C

H3

sym

.st

retc

hing

2903

+529

0829

65–

––

CH

3as

ym.

stre

tchi

ng30

34+1

130

4529

82–

––

a′′C

OC

tors

ion

90+5

9595

???

+398

O–C

H3

tors

ion

266

+226

829

94.

7+6

305

CH

3ro

ckin

g11

73−9

1164

1179

18.3

−11

1168

CH

3as

ym.

defo

rmat

.14

69−1

514

5414

6811

.8−1

1467

CH

3as

ym.

stre

tchi

ng29

64+6

2970

2976

––

–


could be correlated with this mode. Balfour assignes to mode 16b a signal at447 cm−1, however, we find no corresponding signal.

The signal at 299 cm−1 is assigned to the O–CH3 torsional mode, whichis found in the ground state at 266 cm−1. The shift to higher values may beexplained by the increased C–O bond order in the excited state. Balfour as-signed to this energy the first overtone of mode 16a, but the intensity of thesignal seems to be too high for an overtone. A weak signal at 384 cm−1 couldbe assigned to the excitation of mode 16a. The corresponding transition issymmetry forbidden in C2v but the limitations by symmetry are not strictlyvalid.

At 425 cm−1 we observe a signal which fits the value calculated for theCOC bending mode (a′) in theS0 state (433 cm−1) reasonably well. This signalhas not been reported by Balfour [34].

The modes 6a and 6b are clearly recognized at 500 cm−1 (553 cm−1 in 1S0)and at 526 cm−1 (618 cm−1 in 1S0). This assignment has already been givenearlier. In benzene mode 6 is shifted from 608 cm−1 ( 1S0) to 521 cm−1 ( 1S1).

Between these two signals two weaker signals at 507 cm−1 and 515 cm−1

are observed in the REMPI spectrum of anisole. The signal at 507 cm−1 is as-signed to mode 4 which in the1S0 state is observed at 669 cm−1. The weaksignal at 515 cm−1 may be the first overtone of the 9b mode.

The two signals at 651 cm−1 and 683 cm−1 are assigned to mode 11 andmode 17b, which are found in the1S0 state at 752 cm−1 and 880 cm−1 respec-tively.

At 706 cm−1 a weak signal is recognized which probably indicates themode 10a, but this assignment is a very tentative one. In the1S0 state this modeis represented by a weak signal at 819 cm−1.

Another small signal appears at 749 cm−1 as a shoulder in front of the verystrong signal at 757 cm−1. The small signal may be assigned to mode 17a,a C–H wagging mode (956 cm−1 in 1S0).

One of the strongest signals of the spectrum is observed at 757 cm−1. Bal-four also finds this very strong signal at 759 cm−1 and assigns it to mode 12whereas we prefer to this signal the designation “C–OCH3 stretching mode”.This difference is based on the results of our model calculations which revealthat both modes contribute significantly to this signal. In this paper mode 12 isassigned to a strong signal at 940 cm−1.

Besides the signals discussed above one observes numerous signals withlower intensity which are caused by combinations and overtones of vibrationsin the 1S1 state. We have used the experimental values of the fundamentalmodes in the excited state as given in Table 2 to calculate expected values ofovertones and binary combinations. In Table 4 these expected values are com-pared with the signals observed in the REMPI spectrum. The calculated andthe experimental values agree remarkably well within the error limit. This sup-ports the values of fundamental modes given in Table 2, however, it supportsnot specific assignments.


4.3.2 900 cm−1 – 960 cm−1 range

In this narrow range a very characteristic accumulation of strong signals at935 cm−1, 940 cm−1, 946 cm−1, 951 cm−1, 955 cm−1 is observed. Their assign-ment is still under discussion because relevant data are hardly available fromthe literature. Balfour reports in this range the fundamental modes 1 and 18aat 937.4 cm−1 and 952.6 cm−1, respectively. Smalleyet al. [42] measured theREMPI spectrum of toluene and assigned signals at 933.4 cm−1 to mode 12 andat 964.9 cm−1 to 18a. In our previous work [29] we assigned two of the signalsto the modes 12 and 1. Only few data on the frequency shifts in going from the1S0 to the 1S1 state are available. In benzene the frequencies of modes 1, 12, 18aare lower in the1S1 state than in1S0 state (1:−70 cm−1, 12: −42 cm−1, 18a:−118 cm−1). Tzenget al. [38] postulate small downshifts of the modes 18a(−35 cm−1), 1 (−35 cm−1), 12 (−15 cm−1) in the 1S1 state of p-dimethoxy-benzene in comparison to the1S0 state.

Taking into account similar shifts in anisole the contribution of modes 1,12, 18a in this range is very reasonable. However, the number of strong signalsstill exceeds the number of modes discussed so far.

There are two possible explanations for the two remaining signals. Firstly,additional signals in this range may bedue to other modes. Secondly, the num-ber of signals may be increased due to Fermi resonances.

The wagging vibration 5 (975 cm−1 in 1S0) may be hardly influenced by thechange from1S0 to 1S1, therefore this mode may correspond to the signal at955 cm−1. Another candidate is the O–CH3 stretching mode (1039 cm−1 in S0)

which could cause one of the strong signals in this range.Fermi resonance occurs only between a fundamental mode and a combi-

nation or between two combinations when the resonating transitions have thesame symmetry. This is fullfilled for the modes 1, 12, 18a and 6a, which belongto A1. However the first overtone of mode 6a is expected around 1000 cm−1

which seems to be too high for Fermi resonance with the other modes. On theother side Fermi resonance between the combination (17b+ 16b) and mode 5cannot be excluded.

In summary, we favour the contribution of additional fundamental modesto this very characteristic accumulation of signals, however, we can not ex-clude that Fermi resonance plays also a role. A definite signal to mode as-signment in this narrow range of the spectrum seems to be impossible atpresent.

4.3.3 960 cm−1 – 1350 cm−1 range

In this range of the spectrum several signals can be assigned to fundamen-tal modes of anisole. The signal at 994 cm−1clearly correlates with a signal at992.6 cm−1 reported by Balfour [34], it can be assigned unambigeously to themode 9a. There is a weak signal at 1127 cm−1 which seems to be too strong foran overtone or combination, but could represent mode 3.


The signal at 1140 cm−1correlates well with the CH3 rocking mode (a′),which is observed in the1S0 state at 1143 cm−1. The correspondinga′′ modeis observed at 1179 cm−1 (1180 cm−1 in 1S0). Between these two signals at1098 cm−1 a weak signal is recognized which, according to its shape, could bedue to mode 15.

At 1270 cm−1 and 1273 cm−1 two intensive signals appear which are fol-lowed by an intensive signal at 1287 cm−1. For the interpretation of thesesignals modes of the CH3 group can be excluded. We assign the signals at1270 cm−1 and 1273 cm−1 to mode 7a (1282 cm−1 in 1S0) which is downshifted and splitted by Fermi resonance with the combination of 6a and theC–OCH3 stretching mode. Inp-dimethoxybenzene a small upshift has beenreported [38] for this mode.

The source of the signal at 1287 cm−1 may be the mode 19b, which is ob-served in the ground state at 1455 cm−1. A downshift of 52 cm−1 has beenreported for this mode inp-dimethoxybenzene [38].

Between 1287 cm−1 and 1400 cm−1 a great number of weak overtones andcombinations is observed (Table 3).

4.3.4 1400 cm−1 – 1620 cm−1 range

In this part of the spectrum a considerable number of fundamental modes isexpected and, in fact, one observes another accumulation of signals around1450 cm−1, but with much lower intensity in comparison to the accumulationaround 950 cm−1.

Among the fundamental modes in this range there are the deformationmodes of the CH3 group. In the1S0 state the following modes have been ob-served [33]: sym. deformation mode (a′) 1442 cm−1, asym. deformation mode(a′) 1452 cm−1 and asym. deformation mode (a′′) 1469 cm−1. The frequency ofthese modes in the1S1 state should be nearly the same. In the REMPI spec-trum appear weak signals at 1443 cm−1,1455 cm−1, 1468 cm−1 which may beassigned to these deformation modes (Table 2). It should be mentioned thatin this energy range the number of signals increases considerably, so that therather weak signals of the CH3 modes are embedded in an accumulation of sig-nals with weak to medium intensity. Therefore, the error limit in the assignmentof these modes may be somewhat higher than for the other fundamental modes.

The remaining signals can be assigned unambigeously. The signal at1517 cm−1 is assigned to the 19a mode (1497 cm−1 in 1S0). It is one of thestrongest signals in the REMPI spectrum. The reason for the high intensityof this transition may be a Fermi resonance with the first overtone of theC–OCH3 stretching mode at 757 cm−1. For mode 19 in benzene one has ob-served a downshift of 79 cm−1 [43].

At 1528 cm−1 and at 1543 cm−1 we observe two signals, which can beassigned unambigeously to modes 8b and 8a. They are shifted to lower en-ergies in comparison to the1S0 state by 60 cm−1 and 56 cm−1, respectively.


Table 3. Overtones and binary combinations of anisole and anisole/CO2 (A) in the 1S1

state* .

anisole 1S1 anisole/ 1S1

CO2

expected observed expected observed assignment

A1 1000 1005 958 955 6a2

1257 1259 1233 1236 6a+ C–OCH3 stretch.1440 1442 1411 1410 6a+ 121435 1435 1416 1416 6a+ 11494 1494 1460 1460 6a+ 9a

1773/70 1771 1772 – 6a+ 7a2017 2021 1973 – 6a+ 19a2043 2044 – – 6a+ 8a3564 3571 ? – – 6a+ 7b3581 3583 – – 6a+ 23606 3608 – – 6a+ 20a1514 1516 1508 – (C–OCH3)2

1697 1701 1686 – C–OCH3 + 121692 1696 1691 – C–OCH3 stretch.+ 11751 1753 1735 – C–OCH3 stretch.+ 9a

2030/27 2025 2047 – C–OCH3 stretch.+ 7a2274 2278 2248 – C–OCH3 stretch.+ 19a2300 2304 – – C–OCH3 stretch.+ 8a3821 3829 ? – – C–OCH3 stretch.+ 7b1880 1880 1864 – 122

1875 1875 1869 – 12+ 11934 1933 1913 – 12+ 9a

2213/10 2214 2225 – 12+ 7a2457 2459 2426 – 12+ 19a2483 2483 – – 12+ 8a1870 1870 1874 – 12

1929 1928 1918 – 1+ 9a2208/05 2207 2230 – 1+ 7a

2452 2457 2431 – 1+ 19a2478 2481 – – 1+ 8a1988 1982 ? 1962 – 9a2

2267/64 2268/66 2274 – 9a+ 7a2511 ??? 2475 – 9a+ 19a2537 2540 – – 9a+ 8a

2546/40 2538 2586 – 7a2

2790/87 2783 2787 – 7a+ 19a2816/13 2815/09 – – 7a+ 8a

3026 3024 2988 – 19a2

3060 3058 – – 19a+ 8a3086 3086 – – 8a2

B2 516 515 496 496 9b2

784 786 771 772 9b+ 6b1204 1203 1187 1187 9b+ 18b


Table 3. continued.


CO2


1356 1361 ? 1329 1330 9b+ 151385 1387 1359 1359 9b+ 31545 1537 ? 1544 – 9b+ 19b1786 1786 – – 9b+ 8b1829 1831 – – 9b+ 143332 3334 – – 9b+ 133355 3355 – – 9b+ 20b1052 1050 1046 1046 6b2

1472 1471 1462 1464 6b+ 18b1624 1625 1604 – 6b+ 151653 1652 1634 – 6b+ 31813 1809 1819 – 6b+ 19b2054 2055 – – 6b+ 8b2097 2094 – – 6b+ 143600 3599 – – 6b+ 133623 3626 – – 6b+ 20b1892 1895 1878 – 18b2

2044 2039 ? 2020 – 18b+ 152073 2072 2050 – 18b+ 32233 2228 2235 – 18b+ 19b2474 2472 – – 18b+ 8b2517 2519 – – 18b+ 142196 2195 2162 – 152

2225 2219 ? 2192 – 15+ 32385 2377 ? 2377 – 15+ 19b2626 2621 – – 15+ 8b2669 2667 – – 15+ 142254 2255 2222 – 32

2414 2416 2407 – 3+ 19b2667 2667 – – 3+ 8b2698 2696 – – 3+ 142574 2568 ? 2592 – 19b2

2815 2809 ? – – 19b+ 8b2858 2851 ? – – 19b+ 143056 3052 – – 8b2

3099 3099 – – 8b+ 143142 3135 ? – – 142

A2 768 768 738 735 16a2

1090 1086 1061 1062 16a+ 10a1133 1135 1111 1113 16a+ 17a1412 1410 1384 1385 10a2

1455 1457 1434 1435 10a+ 17a1498 1498 1484 1483 17a2


Table 3. continued.


CO2


B1 468 470 424 423 10b2

503 505 478 480 10b+ 16b741 742 724 723 10b+ 4885 886 855 854 10b+ 11917 916 887 889 10b+ 17b

1189 1191 1161 1161 10b+ 5538 537 532 533 16b2

776 777 778 777 16b+ 4920 921 sh 909 910 16b+ 11952 949 941 943 16b+ 17b

1224 1225 1215 1215 16b+ 51014 1014 1024 1023 42

1158 1155 1155 1154 4+ 111190 1186 1187 1187 4+ 17b1462 1462 1461 1460 4+ 51302 1307 ? 1286 1287 112

1334 1330 1318 1320 11+ 17b1606 1607 1592 – 11+ 51366 1365 1350 1349 17b2

1638 1638 1624 17b+ 51910 1906 1898 52

a′ 850 851 842 839 (COC bend.)2

1376 1379 1380 1379 COC bend.+ O–CH3 stretch.1565 1563 1544 – COC bend.+ CH3 rock.1868 1868 1843 – COC bend.+ CH3 sym. def.1880 1880 1865 – COC bend.+ CH3 asym. def.3390 3391 – – COC bend.+ CH3 sym. stretch3407 3406 – – COC bend.+ CH3 asym. stretch1902 1902 1918 – (O–CH3 stretch.)2

2091 2094 2082 – O–CH3 stretch.+ CH3 rock.2394 2396 2381 – O–CH3 stretch.+ CH3 sym. def.2406 2407 2403 – O–CH3 stretch.+ CH3 asym. def.2280 2278 2246 – (CH3 rock.)2

2583 ??? 2545 – CH3 rock.+ CH3 sym. def.2595 2594 2567 – CH3 rock.+ CH3 asym. def.2886 2889 2844 – (CH3 sym. def.)2

2898 2897 2866 – CH3 sym. def.+ CH3 asym. def.2910 2908 2888 – (CH3 asym. def.)2

a′′ 190 195 ? 196 197 (COC tors.)2

394 400 ? 403 407 COC tors.+ OCH3 tors.1274 ??? 1266 1270 COC tors.+ CH3 rock.1563 1563 1565 – COC tors.+ CH3 asym. def.


Table 3. continued.


CO2


3071 3076 ? – – COC tors.+ CH3 asym. stretch.598 599 610 608 (OCH3 tors.)2

1478 1478 1473 1474 OCH3 tors.+ CH3 rock.1767 1771 1772 – OCH3 tors.+ CH3 asym. def.3275 3277 – – OCH3 tors.+ CH3 asym. stretch.2358 2358 2336 – (CH3 rock.)2

2647 2647 2635 – CH3 rock.+ CH3 asym. def.2936 2932 2934 – (CH3 asym. def.)2

* The maximal error limit is±4 cm−1; ? The difference between expected and ex-perimental value exceeds the error limit; ??? No corresponding signal in the spectrum;– No measurement in this range of energy

This correlates with the shift of mode 8 in benzene which is shifted down by85 cm−1 in the 1S1 state [37, 45] in comparison to the1S0 state. Fusonet al. [44]have assigned the more intensive signal at higher frequency in the spectrum oftoluene to vibration 8a. A strong downshift of mode 8b by more than 500 cm−1

as proposed forp-dimethoxybenzene [38] can not be confirmed by ourexperiments.

The prominent signal at 1571 cm−1 obviously is caused by the Kekulemode 14, for which strong upshifts have been observed in benzene and otheraromatic molecules [41]. Balfour [34] has observed a signal at 1567 cm−1

which he assigned to mode 8a.

4.3.5 The 2900 cm−1 – 3100 cm−1 range

In this range one expects the aromaticand aliphatic C–H-stretching modes.We observe the sym. stretching mode (a′) of the CH3 group at 2965 cm−1

and the asym. stretching mode (a′′) at 2976 cm−1. The positions of these modesare upshifted by a few wavenumbers in comparison to the values obtained forthe 1S0 state, whereas the asym. stretching mode (a′) at 2982 cm−1 is down-shifted by 23 cm−1.

The aromatic C–H-stretching modes show similar behaviour. Their wave-numbers in the1S1 state are slightly enhanced in comparison to the1S0 state. Inthe REMPI spectrum they are clearly recognized at 3064 cm−1 (7b), 3074 cm−1

(13), 3081 cm−1 (2), 3097 cm−1 (20b) and 3106 cm−1 (20a). Wavenumbers ofnumerous signals of overtones and combinations in this range of the spectrumare compiled in Table 4. They confirm the assignment of spectral features atlower wavenumbers to fundamental modes.


4.4 REMPI spectrum of anisole/CO2

Many REMPI spectra of complexes of aromatic molecules with rare gas atomsor with small molecules have been published [2–8]. However, generally theenergy range which has been considered was rather small because the maininterest was focused on the appearence of the intermolecular modes near theorigin.

We have measured the REMPI spectrum of anisole/CO2 (1:1) up to1520 cm−1 above the origin because we are interested on the influence of theintermolecular interaction on the intramolecular modes of anisole in the1S1

state and on the coupling of these modes with the intermolecular modes.For an approximative assessment of the mode shifting in the1S0 state

caused by weak intermolecular interaction we used the results of the quan-tumchemical calculations for the1S0 state of anisole [29] and the anisole/CO2

1:1-cluster (Tables 1 and 2). For the cluster the calculation reveals the intermo-lecular and intramolecular vibrational modes.

The mode shifting in the1S1 state due to intermolecular interactionmay be evaluated by comparing the REMPI spectra of pure anisole and ofanisole/CO2, provided the assignment of the intramolecular modes in the1S1

state of the cluster is possible. The REMPI spectrum of the anisole/CO2 1:1-aggregate exhibits more signals than the spectrum of anisole due to the coup-ling of intermolecular modes with intramolecular modes. The intensity of thesignals obviously is influenced by dissociation of the ionized aggregate intothe anisole cation and CO2.Therefore we registered simultaneously the REMPIspectrum in the mass channelm/e = 92. This REMPI spectrum is not identicalwith the REMPI spectrum of pure anisole because both the ions of pure anisoleand the fragment ions of the 1:1-cluster contribute to the spectrum. There is nodisturbing contribution of the anisole/CO2 1:2-cluster.

The REMPI spectrum of the anisole/CO2 aggregate will be analyzed in twosteps: The range close to the origin (Fig. 4) where only intermolecular modes(Fig. 5) are expected and the range where intramolecular modes appear. Twocharacteristic parts of the REMPI spectrum are shown in Fig. 6a and 6b incomparison to the spectrum of pure anisole.

4.4.1 The intermolecular modes of anisole/CO2

The origin of the 1S1 ← 1S0 transition of the anisole/CO2 complex A isblueshifted by 117 cm−1 in comparison to anisole and observed at 36511 cm−1.As already mentioned small amounts of the anisole/(CO2)2 complex have beenobserved. The 0-0 transition of this complex appears at 36178 cm−1 i.e. red-shifted by 216 cm−1 versus the corresponding transition of anisole. Due to thelow intensity of the signals of anisole/(CO2)2 and their redshift there is nosignificant interference between the REMPI spectra of these two complexes.

Close to the origin the spectrum is dominated by the signals of intermolecu-lar modes and their first overtones and combinations (Fig. 4). One recognizes


Fig. 4. REMPI spectrum of anisole/CO2 (1:1) close to the origin. The signal at 36583 cm−1

is assigned to the origin of the isomer B.

the signals of the five intermolecular modes, their first overtones and combina-tions. The assignment of these signals is compiled in Table 4.

A remarkable signal appears at 36583 cm−1. The position and intensity ofthis signal fit not into the frame of the intermolecular modes. Therefore weassume that this signal may be due to the origin of the1S1 ← 1S0 band of con-former B of the 1:1-aggregate. The observation that many of the strong signalshave satellites blueshifted by 72 cm−1 supports this assumption. The results ofthe model calculations have shown that the frequencies of the intramolecularmodes of anisole in both conformers are nearly identical, however, the intermo-lecular modes of conformer A and B are different (Table 1). The contributionsof conformer B to the spectrum are not discussed due to the low intensity of thecorresponding signals.

Another signal which fits not in the pattern of intermolecular modes ap-pears at 98 cm−1. We assigned it to the COC torsion mode (a′′) of anisole.It is observed in the1S0 state of anisole at 81 cm−1, however, it can not belocalized unambigeously for the1S1 state in the REMPI spectrum of pureanisole.

In beams generated with low stagnation pressure (0.2 bar) one observesseveral “hot bands”. They reveal the wavenumbers of intermolecular modes inthe 1S0 state, which can be compared directly with calculated values. The ex-


Fig. 5. Intermolecular modes of isomer A as obtained from the model calculations.


Fig. 6. a,b: Comparison of the REMPI spectra of anisole/CO2 and anisole in the range280–380 cm−1 (6a) and 900–980 cm−1 (6b) above the origine. The number of transitionsin the spectrum of the aggregate exceeds by far that in the anisole spectrum.


Table 4. Intermolecular modes of the anisole/CO2 aggregate A in the1S1 state.

expect. (cm−1)** obs. (cm−1) assignment

– 25* ν1

– 39* ν2

– 45* ν3

– 57* ν4

– 80* ν5

50 48 ν21

64 64 ν1 +ν2

70 68 ν1 +ν3

82 83 ν1 +ν4

105 103 ν1 +ν5

78 75 ν22

84 85 ν2 +ν3

96 94 ν2 +ν4

119 120 ν2 +ν5

90 90 ν23

102 103 ν3 +ν4

125 124 ν3 +ν5

114 111 ν24

137 139 ν4 +ν5

160 158 ν25

* Error limit: ±2 cm−1; ** For comparison the values of theovertones and combinations calculated from the basic modesare compiled in this table.

perimental values obtained in this way come very close to the values calculatedfor conformer A and the1S0 state (Table 1). This confirms the assumption thatthe intermolecular vibrations in the1S0 and the1S1 state have similar frequen-cies.

4.4.2 Intramolecular modes

The fundamental intramolecular modes in the REMPI spectrum of the ag-gregate are recognized due to their relatively strong intensity. In compar-ison to anisole the positions of the intramolecuar modes in the aggregateare shifted. As a first approximation the corresponding shifts in the elec-tronic ground state as obtained from the model calculations (Table 1) canbe used for the assignment. In addition the signals of overtones and com-binations of fundamental modes are detected in the spectrum, their valuesconfirm the assignment of signals to fundamental modes. In Table 3 over-tones and the combinations belonging to the same symmetry are com-piled. Another characteristic feature is the coupling of intramolecular modeswith the five intermolecular modes which leads to five characteristic addi-


tional weak signals blueshifted from the signal assigned to the fundamentalmode.

The spectral shifts due to the interaction of anisole with carbondioxide aresmall. According to the quantumchemical model calculations in the1S0 statethe limits of the shifts are+31 cm−1 and−20 cm−1. In the 1S1 state these lim-its are+20 cm−1 and−23 cm−1. The amount of mode shifting is comparable inboth states because the structure of the complex in both states is similar. Theseshifts are much smaller than the mode shifting in going from the electronicground state to the excited state.

There remain weak signals in the spectrum which are not assigned here.They may be due to “symmetry forbidden”transitions and the coupling of over-tones and combinations with intermolecular modes.

The vibrational structure of carbondioxide is also affected by the inter-molecular interaction with anisole. This is clearly seen from the results ofthe model calculations. In isolatedcarbondioxide one observes three vibra-tional signals [46]: the doubly degenerate bending mode (ν2) at 672.95 cm−1,the symmetric (ν1) and asymmetric stretching mode (ν3) at 1354.67 cm−1 and2396.3 cm−1 respectively. The model calculations reveal a considerable de-crease of these values in the aggregate. The most interesting result of thecalculations is the lifting of the degeneration of the bending mode in the con-former A. This mode splits in two components, the bending mode of CO2 inthe plane of the complex at 619 cm−1 and the bending mode perpendicular tothe plane of the complex at 630 cm−1. In the conformer B the bending modeof CO2 remains degenerate (626.7 cm−1 and 626.6 cm−1). For both conformersA and B the calculated values of the CO2 stretching modes in the complex are1313 cm−1 (ν1) and 2322 cm−1 (ν3).

The vibrations of CO2 can not be excited directly in our experiment, but ifvibrational states of anisole in the excited electronic state are in resonance withthe CO2 modes indirect vibrational excitation of CO2 is possible. Modes 6b(523 cm−1), 11 (643 cm−1) and 17b (675 cm−1) in conformer A are the modesnext to the nondegenerate bendig modes of CO2. This is an off resonancesituation. However there exists the possibility that combinations of the bend-ing modes of CO2 with the intermolecular modes are in resonance with thesemodes. In fact within the error limits mode 11 of anisole fits to the combina-tion of the CO2 in plane bending mode with the intermolecular modeν1 andmode 17b fits to the combination of the in plane bending mode of CO2 with theintermolecular modeν4 as well as to the combination of the out of plane CO2

bending mode with the intermolecular modeν3. The combination of 6b withthe COC torsional mode fits only to the 618.9 cm−1 bending mode of CO2. Inthe spectrum of the anisole/CO2 complex one observes two signals at 623 cm−1

and 630 cm−1.In isolated CO2 the symmetric stretching modeν1 is influenced by Fermi

resonance. In anisole/CO2 this mode is expected at 1313 cm−1 in the electronicground state and Fermi resonance with the overtone of the bending mode can


be excluded. However, the first overtone of mode 11 in theS1 state of anisolefits nearly this value. In the REMPI spectrum of the complex one observesa pair of signals at 1301/1305 cm−1 which can not be explained by vibrationsof anisole. The resonance enhanced coupling of the modeν1 of CO2 with theovertone of mode 11 in theS1 state offers an explanation for this observation.

Finally a short remark on the intensity of the signals in the REMPI spec-trum of the aggregate shall be made.

The intensity of the signals in the REMPI spectrum of anisole/CO2 is in-fluenced by dissociation of the aggregate. The two-photon excitation of theaggregate leads to anisole/CO2 cations. The maximum internal energy of theseionic aggregates is at least 0.8 eV which exceeds considerably their intermo-lecular bond energy. The excess energy in the ionic aggregate is distributedamong the degrees of freedom of the system. The dissociation of the ionicaggregates has to be taken into account. It is indicated by signals which ap-pear at the same energy as well in the ion yield curve of the aggregate as inthem/e = 92 channel. In the anisole/CO2 system the lowest ones of such sig-nals are detected 77 cm−1 above the 0-0 transitiion of the conformer A. Due toa rough estimation at higher excitation energies about 90% of the ionig aggrre-gates dissociates into anisole cations and carbondioxide.

5. Summary

The REMPI spectrum of anisole has been measured in a wide range of en-ergy and a first attempt has been made to assign the 42 fundamental modesin the 1S1 state of anisole. Some assignments given in this paper are tenta-tive and more efforts will be made to make the analysis of the REMPI spectraat these points more reliable. The results can be summarized briefly as fol-lows: The fundamental modes which involve the motion of the carbon atoms inthe aromatic ring are strongly downshifted, except mode 14 which is stronglyupshifted. The aromatic C–H stretching modes show a moderate upshift, the vi-brations of the CH3 group are hardly influenced by the change of the electronicstate.

The REMPI spectrum of the anisole/CO2 aggregate exhibits five intermo-lecular modes, their overtones and combinations near the origin. The assign-ment of the intramolecular modes of anisole in the 1:1 complex is supportedby the knowledge of the corresponding vibrations in pure anisole because thesmall shifts caused by the intermolecular interaction with carbondioxide arenearly the same in the1S0 and the1S1 state. Up to now no systematic correla-tion has been found for these shifts.

The intermolecular modes readily couple with the fundamentals of the in-tramolecular modes. This and the manifold of overtones and combinations ofintramolecular modes lead to a very complex REMPI spectrum. About 90% ofthe ionic aggregates dissociate above an excitation energy of 36588 cm−1.


Acknowledgement

Financial support of these experimentfrom the Deutsche Forschungsgemein-schaft, the Fonds der Chemischen Industrie and the Free University Berlin isgratefully acknowledged.

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