multiphoton ionization and ab initio calculation studies of pyridine–water mixed clusters using...

Multiphoton ionization and ab initio calculation studies ofpyridine±water mixed clusters using time of ¯ight mass

spectrometer

Yue Li 1, Richang Lu, Yongjun Hu, Xiuyan Wang *

State Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics, Chinese Academy of Sciences,

Dalian 116023, People's Republic of China

Received 11 January 2000; in ®nal form 12 March 2000

Abstract

Multiphoton ionization of binary mixed clusters (C5H5N)x±(H2O)y at 532, 355 and 266 nm laser wavelengths has

been investigated using TOF mass spectrometer. The experiments showed that almost all the products were protonated

ions. At 532 and 355 nm, the products were mainly protonated pyridine clusters (C5H5N)n±H�, while at 266 nm, mixed

binary cluster ions (C5H5N)m± (H2O)n±H� appeared. It was found that the abundance of the [(C5H5N)3±H2O±H]� ions

was abnormally high. The calculation indicated that the ion [(C5H5N)3±H2O±H]� is of a kind of magic number

structures with C3v symmetry. A stepwise reaction mechanism is suggested that photoionization is followed by

dissociation. Ó 2001 Elsevier Science B.V. All rights reserved.

1. Introduction

Hydrogen bond and its e�ect on solvation inaqueous solutions are the fundamental problemsin chemistry and biology [1]. Many processes inmetabolism involve hydrogen bonds and protontransfer. Clusters are special states of matter be-tween gas phase and condensed phase that consistof several to hundreds of composition units com-bined with hydrogen bonds or van der Waalsforce. Study of hydrogen bonded clusters couldreveal the e�ect of the cluster size and solvation on

its physicochemical properties [2], leading to un-derstand the mechanism of proton transfer reac-tions through a relatively simple system.

Pyridine (C5H5N) is a heterocyclic compoundwith hexacyclic ring, which is similar to benzenebut its two atoms (C±H) are substituted by atomN, showing aromaticity. In our previous paper [3],we reported the study of multiphoton ionization ofpyrrole±water mixed clusters which is a penta-heterocyclic compound. These kinds of hetero-cyclic compounds are the construction units formany biochemical supramolecules, in which theatom N could form hydrogen bond with watermolecule. Because of this, it is an ideal system forinvestigation of hydrogen bonded clusters.

Millen and Mines [4] observed the IR spectra ofC5H5N±H2O in gas phase. Destexhe et al. [5] ob-tained FTIR spectra of the same cluster in argon

5 January 2001

Chemical Physics Letters 333 (2001) 153±161

www.elsevier.nl/locate/cplett

* Corresponding author. Fax: +86-411-4675584.

E-mail address: [email protected] (X. Wang).1 Present address: Chemistry department, University of North

Carolina, Chapel Hill, NC 27599-3290, USA.

0009-2614/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved.

PII: S 0 0 0 9 - 2 6 1 4 ( 0 0 ) 0 1 3 0 0 - 2

isolated matrices and its vibration frequencies withab initio calculations. Their results showed thatthe hydrogen bond in the complex is quite strong.Zeng et al. [6] studied the in¯uence of existence ofwater on 1�nÿ p�� electronic transition of pyridinemolecule by theoretical computation. Their con-clusion was that a strong hydrogen bond betweenpyridine and water molecules can be formed atground state, but no hydrogen bond structure ex-ists in the p� electronic state. Martoprawiro andBacskay [7] obtained the structure, vibration fre-quencies and rotational constants of this binarycluster also from ab initio calculations.

To our knowledge, no study of ionization ofthis cluster system and properties of the fragmentions were reported so far. In this report, we willpresent the results of our studies on the multi-photon ionization of this binary mixed clustersystem at laser wavelength 532, 355 and 266 nmwith TOF mass spectrometer, and combined withab initio calculations.

2. Experimental

The experiments were carried out on the homemade TOF mass spectrometer, which has beendescribed in detail elsewhere [3]. Brie¯y pyridineand water were carried by helium into the ionizerthrough a pulsed valve, forming molecular beam,in which pyridine and water were in saturation atroom temperature. A focused laser beam crossedperpendicularly with the molecular beam at thecenter of the ionizer. A YAG laser with maximumoutput of 34 (532 nm), 9 (355 nm) and 6 (266 nm)mJ/pulse, respectively, was used in the experi-ments. The pulse width of the laser was 10 ns, andthe repetition rate was 5 Hz. No di�erence in themass distribution of the products with increase oflaser ¯uence was observed in our experimentrange. The ions produced in the ionizer were at-tracted and accelerated by a two-stage electric®eld, the voltage across the ®eld was 1000 V, di-vided at 1:4. The path of the free ¯ight was about1.2 m, the ions were detected with an MCP. Thesignals were ampli®ed and recorded by a transientrecorder (500 MHz, Model 9846-500, EG&G).The background pressure of the chambers was

8� 10ÿ5 Pa, while increased up to 1� 10ÿ3 Pawhen the molecular beam was on.

The pyridine (99%), water (deionized) andhelium (99.999%) were used without further puri-®cation.

3. Computational method

The program GAUSSIANAUSSIAN94 [8] was used in thecomputations. For di�erent initial con®gurationsof the molecules and clusters involved, their equi-librium structures, geometric parameters and en-ergies were calculated at HF/STO-3G, HF/6-31G�

and B3LYP/6-31G� levels, respectively, and geo-metrically optimized. The frequency calculationprovided the criteria for stability of a con®gura-tion (no imaginary frequency) and the zero pointvibrational energy. The values of B3LYP/6-31G�+ZPE(B3LYP/6-31G�) were adopted as theenergies because the calculation accuracy atB3LYP level is higher and the calculated values ofspin contamination (S2) at B3LYP level are veryclose to the ideal value of 0.75.

4. Results and discussion

The mass spectra obtained at 532, 355, and 266nm laser wavelength are presented in Figs. 1, 2 and3, respectively. The glaring peaks of the spectra inFigs. 1 and 2 are those at mass number 80, 159 and238, which correspond to the protonated pyridinecluster ions [C5H5N±H]�, [(C5H5N)2±H]� and[(C5H5N)3±H]�, respectively, almost no unproto-nated ion could be seen except a very smallC5H5N� peak in Fig. 1. While at 266 nm laserwavelength, all the protonated products werepyridine±water mixed cluster ions as shown inFig. 3.

It is a common phenomenon in the ionizationof hydrogen bonded clusters to produce proto-nated ions [9]. As discussed in our previous studyon the ionization of pyrrole±water cluster system[3], the protonated ions may be produced fromionization and dissociation of larger clusters. Inthe case of pyridine±water system, the formationmechanism of the ions is assumed to be a stepwise

154 Y. Li et al. / Chemical Physics Letters 333 (2001) 153±161

Fig. 2. MPI mass spectra of pyridine-water mixed clusters at 532 nm. Laser ¯uence: 34 mJ/pulse.

Fig. 1. MPI mass spectrum of pyridine±water mixed clusters at 355 nm. Laser ¯uence: 9 mJ/pulse.

Y. Li et al. / Chemical Physics Letters 333 (2001) 153±161 155

process, the ionization at ®rst, followed by disso-ciation. This assumption is based on considerationof the Franck±Condon principle that the elec-tron(s) moves so fast that the nuclei is like in sta-tionary states. Except for a resonance electronictransition upon laser excitation to a repulsive up-per electronic state of the cluster, the ionizationcould occur ®rst before any vibration mode is ex-cited. The stepwise process can be described as:

�C5H5N�x±�H2O�y � nhm

! ��C5H5N�x±�H2O�y �� eÿ;

��C5H5N�x±�H2O�y ��

! ��C5H5N�m�H2O�nH�� C5H5N�xÿm

� �H2O�yÿnÿ1 �OH

and/or

��C5H5N�x±�H2O�y �� ihm

! ��C5H5N�m�H2O�nH�� C5H5N�xÿm

� �H2O�yÿnÿ1 �OH:

The formation of the protonated cluster ions iscombined with breaking of an O±H bond and re-

arrangement of the H atom in the cluster that isso-called proton transfer reaction. The nascentions should be in unstable excited states since, ingeneral, there is plenty of excess energy left afterionization while additional photons may be neededbecause there are energy barriers at the reactionpath(s) of dissociation or proton transfer reaction.

For molecules in clusters linked with weak bondlike hydrogen bond, the molecule componentcould be selectively ionized [13]. The IP for pyri-dine in gas phase is 9.3 eV [10±12], and 12.62 eVfor water. For ionization of pyridine, at least four,three or two photons are necessary at 532, 355 or266 nm wavelength, respectively. The measuredlaser intensity indices for the yield of product ionsat 532 and 355 nm are listed in Table 1.

It is obvious that all the indices are smaller thanthe necessary photon number and not integer,which implies that the ionization was not a uniqueelementary reaction, but a stepwise process, andfurther photodissociation with less photons fol-lowing the ionization may occur. It was probablebecause the laser pulse lasted 10 ns. The questionsare why there was no pure pyridine cluster ionsproduced at all the three laser wavelengths and

Fig. 3. MPI mass spectra of pyridine±water mixed clusters at 266 nm. Laser ¯uence: 6 mJ/pulse.


Table 2

The calculated energy, spin contamination S2 and zero point energy (Hartree)

Species HF/6-31G� hS2i ZPE B3LYP/6-31G� hS2i ZPE

OH )75.3822753 0.755 0.009105 )75.7234548 0.752 0.008304

H2O )76.0107463 ± 0.022965 )76.4089533 ± 0.021160

H3O� )76.2893384 ± 0.036728 )76.6890842 ± 0.034307

C5H5N )246.6958198 ± 0.095440 )248.284973 ± 0.089042

C5H5N� )246.411676 1.259 0.092477 )247.9584996 0.757 0.086625

C5H5N±H� )247.070856 ± 0.110461 )248.6569776 ± 0.103261

C5H5N±H2O )322.7161774 ± 0.121543 )324.707168 ± 0.113742

C5H5N±H2O�(I) )322.4707098 0.763 0.121273 )324.401373 0.752 0.113432

C5H5N±H2O�(II) )322.4411677 1.257 0.117771 )324.3903649 0.756 0.110314

C5H5N±H2O±H� )323.1092695 ± 0.136198 )325.0981453 ± 0.126933

C5H5N±H2O�ver ± ± ± ±324.3610505 0.758 0.110314

Table 3

The calculated reaction energiesa

Reaction Energy The other results

(kcal/mol) (kcal/mol)

C5H5N! C5H5N� IP� 203.3 214.4 b

H2O�H� ! H3O� PA� 167.5 166.7 c

C5H5N�H� ! C5H5N±H� PA� 224.5 221 d

C5H5N±H2O! C5H5N�H2O D0 � 6:1 5.7 c, 6.2 e, 7.6 f

C5H5N±H2O! C5H5N±H2O� IP� 191.7

C5H5N±H2O! C5H5N±H2O�verg IPver � 215:0

C5H5N±H2O�H� ! C5H5N±H2O±H� PA� 237.1

C5H5N±H2O��I� ! C5H5N� �H2O D0 � 17.7

C5H5N±H2O��I� ! C5H5N±H� �OH D0 � 12.0

C5H5N±H2O��II� ! C5H5N� �H2O D0 � 12:7

C5H5N±H2O��II� ! C5H5N±H� �OH D0 � 7:0

a B3LYP/6-31G� + ZPE values.b Ref. [18].c Ref. [17].d Ref. [16].e Ref. [7].f Ref. [5].g The subscript ver indicates the vertical ionization.

Table 1

Laser ¯uence indices for production of various ion species

Wavelength H2O� C5H5N� C5H5N±H� (C5H5N)2±H�

(nm)

355 2.5 0.9 1.8 1.5

532 3.2 ) 2.6 2.7


why the products were protonated pyridine clusterions at 532 and 355 nm wavelengths and proto-nated pyridine±water mixed cluster ions only at266 nm laser wavelength? The ®rst question couldbe understood that the intense laser probablyfragmented the pyridine monomers, few numbersof pyridine ions could be survived. The evidence isthe small fragments observed in the mass spectra.The protonated ions were produced from dissoci-ation of larger clusters, the energy absorbed couldspread into more vibration modes and carried outby the scattering fragments. Then the smallerfragment cluster ions could be stabilized and sur-vived from being broken further. If it is true, the

survival cluster ions would be those with stablestructure or stronger bonds.

The second question cannot be simply inter-preted. It seems that at 532 and 355 nm, a pyridinemolecule in the mixed cluster is excited and ionized®rst, then the excess energy transfers to the hy-drogen bond that is formed with H atom of thewater molecule, and breaks the H±O bond, form-ing the protonated pyridine ions. While at 266 nm,the photon energy is higher, could excite watermolecule. Suppose a water molecule in a mixedcluster is excited and ionized ®rst, the excess en-ergy may ¯ow from the water molecule to pyridinein the cluster, and breaks the hydrogen bond at

Fig. 4. The equilibrium structures and parameters of the species of the pyridine±water mixed clusters.


H±O of the water, the proton transfers from H2O�

to pyridine molecule, forming a stable structurelike [(C5H5N)n±H]�, or transfers to another watermolecule, which is linked with pyridine molecule,forming [(C5H5N)n±H±OH2]� ion.

Since the cluster system investigated is verycomplicated, we cannot clarify the mechanism justfrom the mass spectra. However, the ab initiocalculations of the structures and energy correla-tion of the related species could provide additionaluseful information. The results of our calculationsare presented in Tables 2 and 3, and the equilib-rium structures of the cluster ions and neutralclusters involved are shown in Fig. 4. The energy

levels of di�erent species are given in Fig. 5. Theseinformations are helpful for getting insight intosome aspects of the formation mechanism of thesecluster ions in the ionization.

The energy levels in Fig. 5 show that theenergy of [C5H5N±H]� + OH is lower than[C5H5N ]� + H2O, which implies that formation of[C5H5N±H]� is a favorable channel. This result isalso in agreement with the observed mass spectrain Figs. 1 and 2. The protonated pyridine clusterions [(C5H5N)n±H]� are the predominantproducts.

C5H5N is a hetero-hexacyclic compound withC2v symmetry. The calculated equilibrium structure

Fig. 4. (Continued).


of C5H5N and C5H5N±H2O in Fig. 4 are well inagreement with the reported [7,14] in literature. ForC5H5N±H� the most stable structure is that theattached H atom is linked to N atom of pyridine,the length of N1±H1 bond is 1.018 �A, and protona�nity is 224.5 kcal/mol, which is consistent withthe results obtained by Nguyen et al. [15].

The protonated binary ion [C5H5N±H2O±H]�

is probably produced from dissociation of[(C5H5N)3±H2O±H±OH]�. Breaking of the H±Obond of the second water molecule (underlined)results in releasing of the OH and the protontransferred to the water molecule that is linked tothe pyridine molecule. The existence of pyridine inthe cluster makes the proton a�nity of the watergreatly increased from 167.5 to 237.1 kcal/mol,even higher than the a�nity of pyridine, whichstabilizes the protonated ion [C5H5N±H2O±H]�.The bond lengths of N±H and H±O are 1.044 and1.698 �A, respectively. The calculated Mullikencharge distribution in the cluster shows that thecharge of H3O� decrease from +1 down to +0.54,transferred to C5H5N. It seems that the binarycluster ion [C5H5N±H2O±H]� looks like a complexof [C5H5N±H]� and H2O.

Similar to the case in protonated methanolclusters [16], the equilibrium structure of proto-nated cluster ions [(C5H5N)3±H2O±H]� with C3v

symmetry as shown in Fig. 4 should be the moststable as a magic number structure in the series of

[(C5H5N)n±H2O±H]�, which is in agreement withthe observed projecting peak in mass spectrum ofFig. 3. For the most species corresponding to thepeaks in the mass spectra, their equilibriumstructures were calculated, and shown in Fig. 4.

5. Summary

The multiphoton ionization of hydrogenbonded binary clusters (C5H5N)m(H2O)n has beenstudied with TOF mass spectrometer at 532, 355and 266 nm laser wavelength. The predominantproducts are protonated cluster ions at all thethree wavelengths. At 532 and 355 nm, the prod-ucts were protonated pyridine clusters, while at266 nm, mixed binary cluster ions appeared. It wasfound that the abundance of the [(C5H5N)3±H2O±H]� ions was abnormally high. The calculationindicated that the ion [(C5H5N)3±H2O±H]� is of akind of magic number structure with C3v symmetryand the H3O� located at the center.

A stepwise reaction mechanism is suggestedthat photoionization is followed by dissociation.The excess energy plays an important role in thedissociation. Ab initio calculations of the clusterstructures and energy levels were carried out. Theresults show that the products and their distribu-tions not only depend upon cluster size and theirstructure stability, but also upon the ionization

Fig. 5. The energy variance diagram for the ionization and dissociation of the C5H5N±H2O binary cluster.


laser wavelength, that is which molecule in theclusters is excited.

Acknowledgements

This work was supported by the NKBRSF.

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multiphoton ionization and ab initio calculation studies of pyridine–water mixed clusters using...

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