Conformational Changes in 5‑Methoxyindole: Effects of Thermal,Vibrational, and Electronic ExcitationsPublished as part of The Journal of Physical Chemistry virtual special issue “Veronica Vaida Festschrift”.

A. J. Lopes Jesus,†,‡ R. Fausto,† and I. Reva*,†

†CQC, Department of Chemistry, University of Coimbra, 3004-535 Coimbra, Portugal‡CQC, Faculty of Pharmacy, University of Coimbra, 3004-295 Coimbra, Portugal

*S Supporting Information

ABSTRACT: The molecule of 5-methoxyindole (5MOI) may adopt twoconformational states, syn and anti, with respect to the relative orientation of theNH and OCH3 groups. The structure of monomeric 5MOI was characterizedspectroscopically, in mid- and near-infrared domains. The conformationalcomposition of 5MOI could be controlled in three different ways. Thermally,two conformers of 5MOI could be trapped in xenon matrixes at 16 K. Uponannealing the xenon matrix to temperatures about 30−40 K, the higher-energy syn form converted to the ground-state anticonformer. Vibrational excitations in the near-infrared domain, at the frequency of the first NH stretching overtone, 6853 cm−1,afforded the inverse conformational transformation, and a part of the anti conformer was upconverted to the syn form. Electronicexcitations in the UV domain, at 315−310 nm, resulted in a total consumption of the syn form again, in favor of anti. Uponfurther irradiations at 308 nm, a partial repopulation of the syn form, at the expense of anti, was observed. We propose amechanistic explanation of the observed transformations, which is based on computations of the vibrational spectra of the twoconformers and also on computations of the ground state S0 and the first excited state S1 potential energy surfaces along thecoordinate for conformational isomerization. The highlights of the present work are the first experimental observation of theminor syn conformer of 5MOI, evidence of the long-range vibrational energy transfer resulting in conformational isomerizationupon excitation of the NH stretching overtone, and the possibility of partial conformational control of 5MOI by using electronicexcitations.

1. INTRODUCTION

Solar radiation is the single largest energy source on both theearly and modern Earth.1 Energy provided by the Sun hasaccess to different chemistries. The majority of photochemicalreactions considered in atmospheric models are reactions onmolecular electronic excited states requiring ultraviolet (UV)light for excitation. UV photons have energies equal to mostcovalent bonds and therefore may cause dissociation. However,certain atmospheric molecules (such as molecular oxygen andozone) absorb UV strongly and thus filter out the short-wavelength solar light. Unlike UV photons, visible (vis) andnear-infrared (NIR) light photons are significantly moreabundant in the atmosphere.2 When sunlight-driven excitedelectronic state reactions are not effective, photochemicalprocesses occurring by vibrational overtone excitation areimportant in reactions of oxidized atmospheric compounds(acids, alcohols, and peroxides) prevalent in the Earth’satmosphere.3 The fundamental energetic, mechanistic, anddynamical aspects of photochemical reactions occurring viavibrational overtone absorption have been reviewed by Vaidaand co-workers.4,5

Overtone excitation of X−H (X = C, N, O) oscillators hasbeen used extensively to prepare molecules in initial states withenergy localized in the X−H bond. The typical X−H stretchesdo not mix appreciably with other vibrations already at fairly

low energies (corresponding to 2−3 quanta in the stretch)6 andcan be well described by local mode basis sets.7,8 The overtonescorresponding to 2 quanta in the stretch appear in the NIRregion of the spectrum, and upon appearance of frequency-tunable optical parametric oscillators excitation of suchovertones became easily accessible. Molecules have beenprepared in excited X−H overtone states to study theintramolecular vibrational energy distribution, which manifestsitself in conformational changes following NIR excitations.9

The experimental technique of matrix isolation is particularlysuitable for characterization of the conformational structure ofmolecules.10,11 The most frequently reported cases concernmolecules possessing OH groups, mainly carboxylic acids.12−16

Narrow-band NIR excitations at frequencies corresponding tothose of the OH stretching overtones have proved to be anefficient method to induce selective conformational changes.The most common observed outcome of these irradiations is achange in the orientation of the same group that is beingexcited13,17 or of a molecular fragment occupying a closeposition (vicinal or geminal) relative to the excitedgroup.14,16,18,19 In only a few cases reported so far were the

Received: February 21, 2017Revised: April 5, 2017Published: April 14, 2017

Article

pubs.acs.org/JPCA

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excited and rotating groups separated by several covalentbonds.20−22

There were several attempts to induce conformationalisomerizations in molecules containing an NH2 group, suchas cytosine,23 5-substituted cytosines,24 oxamic acid,25 andamino acids glycine19 and alanine.26 However, all of thesemolecules contain an OH group, besides the NH2 group, andwhether conformational isomerizations occurred, or not, uponexcitations of pure NH stretching overtones in these moleculesremains inconclusive. Clearly observed conformational isomer-izations were only registered upon excitations of the OHstretching overtones in those molecules.23−26 Our attempts toinduce conformational isomerization by excitation of an NHstretching overtone in tetrazole−acetic acid failed.27 Thesuccessful isomerization induced by excitation of an NHstretching overtone was reported so far only for two molecules:6-methoxyindole22 (6MOI) and 2-thiocytosine.28

The case of 6-methoxyindole represents one of the mostnotable examples of long-range vibrational energy transfer inmatrix-isolated molecules.22 Excitation of the NH stretchingovertone in 6MOI induces a change in the orientation of theO−CH3 moiety, which is separated from the NH group by fourcovalent bonds. One important objective of this work is toinvestigate if conformational isomerizations by excitation of theNH stretching overtone can be also induced in 5-methoxy-indole (5MOI), in order to obtain experimental evidencewhether long-range vibrational energy transfer is still opera-tional in the extreme case of separation between the N−H andOCH3 groups in indole-based systems (i.e., in 5MOI).The conformational structure of 5MOI was investigated

theoretically29,30 and experimentally.29 The theoretical calcu-lations predicted two conformers for the isolated molecule(which are called anti and syn; see Figure 1), but only the

lowest-energy anti form was experimentally observed under jet-cooled conditions by using rotationally resolved electronicspectroscopy.29 For both conformers, the energy gap betweenthe two lowest-energy excited electronic singlet states (ππ*states), commonly designated as 1Lb(S1) and 1La(S2),

31,32 wascalculated to be larger than 4000 cm−1, thus proving that thevibronic coupling between these states is very small.29 It will beinteresting to investigate if electronic excitation of the ground-state anti conformer populated under experimental conditionswould lead to photogeneration of the syn conformer. Therefore,another aim of the present work is to attempt population of theminor conformer of 5MOI not only by vibrational overtonepumping but also by electronic excitation.

2. EXPERIMENTAL SECTIONCommercial 5MOI, acquired from Aldrich with a 99% puritydegree, was used in the matrix isolation experiments. A smallquantity of the solid substance was placed into a glass tube,

which was connected through a needle valve to the vacuumsystem of a closed-cycle helium cryostat (APD Cryogenics, witha DE-202A expander). Vapors of the compound, sublimating atroom temperature, were co-deposited with a large excess ofxenon (xenon N45, supplied by Air Liquide) onto a CsIwindow, which was kept, in different experiments, at 16 or 30K. The temperature of the CsI window was measured directlyat the sample holder by means of a silicon diode sensorconnected to a digital controller (Scientific Instruments, Model9650-1, accuracy of 0.1 K). A Thermo Nicolet 6700 Fouriertransform infrared (FTIR) spectrometer was employed torecord spectra in the near-infrared and mid-infrared ranges. Forthis purpose, the following combinations of beamsplitter/detector/resolution were used: mercury cadmium telluride(MCT/B) detector (cooled by liquid N2) and CaF2 beamsplitter (near-IR, 1 cm−1 resolution); deuterated triglycinesulfate (DTGS) detector and KBr beam splitter (mid-IR, 0.5cm−1 resolution). After deposition, the matrixes were irradiatedthrough the outer quartz window of the cryostat by usingmonochromatic (0.2 cm−1 spectral width, pulse energy 2−10mJ) NIR or UV light provided, respectively, by the idler beamor the frequency-doubled signal beam of a Quanta-Ray MOPO-SL optical parametric oscillator (OPO). The OPO was pumpedwith a pulsed Nd:YAG laser. The pulse duration time andrepetition rate were 10 ns and 10 Hz, respectively.

3. COMPUTATIONAL SECTION

For identification of the minimum energy conformationsof 5MOI, a relaxed potential energy scan around theC11−O10−C5−C6 (α) torsional angle was first carried out atthe DFT(B3LYP)33−35/6-311++G(d,p) level of theory. Com-plete geometry optimizations were then undertaken on theidentified minima by combining either B3LYP or MP236

methods with the 6-311++G(d,p) and 6-311++G(3df,3pd)basis sets. Harmonic vibrational calculations carried out at theB3LYP/6-31++G(d,p) level were followed by computations ofanharmonic infrared spectra using a fully automated second-order vibrational perturbative approach of Barone and co-workers,37,38 allowing for the evaluation of anharmonicvibrational frequencies and anharmonic infrared intensities upto 2 quanta, including overtones and combination bands.38−40

All above calculations were executed with the Gaussian 09program package (revision D.01).41 The computed harmonicvibrational wavenumbers above and below 3200 cm−1 werescaled by multiplicative factors of 0.95042 and 0.980,43

respectively. For graphical comparison between the exper-imental and theoretical spectra in the mid-IR range, the IRspectra obtained in the harmonic vibrational calculations wereconvoluted with Lorentzian functions having a full width athalf-maximum (fwhm) of 1 cm−1 and by setting the intensity atthe band maximum equal to the calculated absolute intensity.The Chemcraft software (version 1.8)44 was used for thispurpose. The harmonic vibrational calculations also allowed usto obtain the zero-point corrected energies (E0) for the 5MOIconformers (E0 = Eel + ZPVE), where Eel is the electronicenergy and ZPVE is the zero-point vibrational energy. TheCartesian coordinates of the geometries optimized at all levelsof theory, as well as the calculated wavenumbers and infraredintensities resulting from harmonic and anharmonic vibrationalcalculations, are provided as Supporting Information (TablesS1−S4). A Natural Bond Orbital (NBO)45 analysis was furthercarried out [B3LYP/6-311++G(d,p)] for the identified con-

Figure 1. Structures of the 5MOI conformers including numbering ofheavy atoms.

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formers of 5MOI by using the NBO 5.0 program46 asimplemented in the GAMESS program.47

Vertical excitation energies and oscillator strengths of thelow-energy electronic excited states were calculated by usingthe time-dependent version of the density functional method(TD-DFT)48,49 combined with the 6-311++G(d,p) basis setimplemented in the Gaussian 09 program (Table S5). For thespecific case of the lowest excited singlet S1 (or 1Lb) state,which is expected to be the only populated state in the courseof the UV excitations of the matrix-isolated 5MOI, geometryoptimizations were also performed at the TD-DFT level (TableS6).

4. RESULTS AND DISCUSSION4.1. Conformers of 5MOI and Their Relative Stability.

The geometries of the syn (OCH3 is syn-parallel to the NHbond) and anti (OCH3 is anti-parallel to the NH bond)conformers of 5MOI are represented in Figure 1, while thepotential energy scan for the internal torsion of the OCH3fragment is shown in Figure S1. In Table 1 are listed the relative

electronic energies (ΔEel) and zero-point corrected energies(ΔE0) calculated for the two conformers at different levels oftheory. From these results, it is found that all methods predictanti as the lowest-energy conformer. The B3LYP energydifference between the two forms is 3.5−4.1 kJ mol−1,increasing to 5.3−5.8 kJ mol−1 at the MP2 level. The MP2relative energy is very close to that calculated at the CC2/cc‑pVTZ level.29,50 The preference of the OCH3 substituent toadopt an anti orientation, instead of syn, when it is attached toposition 5 of the indole ring has also been found for5‑methoxytryptamine51 and melatonin.52 Similar conforma-tional behavior was also observed when this substituent wasreplaced by an OH group in 5-hydroxyindole.53

Because the two 5MOI conformers are structurally verysimilar and no specific interactions (e.g., intramolecularhydrogen bonds) are present, their stability difference is mostprobably related with subtle differences of electronic delocaliza-tion between the benzene ring and the exocyclic OCH3 group.Using NBO theory, the values of the second-order perturbationenergies [E(2)] corresponding to the orbital interactionsbetween filled and vacant NBOs of the two moieties were

calculated, and the most significant interactions are listed inTable 2. The stabilizing effect given by the sum of the E(2)

values [∑E(2)] is greater in the anti (larger negative values; seeTable 2) than that in the syn conformer, which helps to explaintheir order of stability. These results reveal also that the mostsignificant contribution for this difference of electronicstabilization comes from the Lp2(O10)→ π*(C4C5) conjugativeinteraction.Considering a Boltzmann distribution based on the values of

ΔE0 calculated at the B3LYP level, the gaseous compoundimmediately before the matrix deposition is predicted to beconstituted by ∼80% anti and ∼20% syn (Table 1). For thehigher-energy conformer, its efficient trapping in the freshlydeposited low-temperature xenon matrix and subsequentsuccessful experimental detection is dependent on variousfactors.54 These factors include the nature of the matrix gas, thematrix temperature during deposition, as well as the energybarrier separating the conformers.55,56 In 5MOI, this barriercorresponds to the syn → anti rotamerization. Indeed, if thisbarrier is very low, conformational cooling may occur duringthe matrix deposition,57 thereby preventing stabilization andconsequently spectral identification of the syn conformer in thedeposited frozen sample. In the present case, the value of thebarrier height corresponding to the syn → anti transformationwas calculated to be ∼6 kJ mol−1 [B3LYP/6-311++G(d,p); seeFigure S1], which is even lower than the value calculated for6MOI at the same theory level (∼9 kJ mol−1).21,30 Therefore,the deposition temperature for 5MOI was set in the presentwork at 16 K, lower than that used for 6MOI (20 K).21 Thischoice of temperature allowed for successful trapping of both5MOI conformers after isolating the compound in solid xenonat 16 K, as shown below.

4.2. Spectral Identification of 5MOI Conformers. Themid-infrared spectrum of 5MOI in the 1700−680 cm−1 region,recorded immediately after isolating monomers of thecompound in solid xenon at 16 K, is represented in Figure2a, while Table 3 lists the positions of the experimental bandsand their approximate description based on the animation ofvibrations calculated for the two forms.

Table 1. Relative Electronic Energies (ΔEel),a Relative Zero-

Point Corrected Energies (ΔE0),a and Equilibrium

Populations Calculated for 5MOI Conformersa at DifferentLevels

syn

level of theory ΔEel ΔE0b pop. (%)c

B3LYP6-311++G(d,p) 4.11 3.58 19.16-311++G(3df,3pd) 3.96 3.49 19.7MP26-311++G(d,p) 5.80 − −6-311++G(3df,3pd) 5.27 − −CC2/cc-pVTZ 5.55d − −

aEnergies (kJ mol−1) of the syn conformer relative to the most stableanti conformer, whose relative energy was assumed to be zero at alllevels. bE0 = Eel + ZPVE (zero-point vibrational energy). cEstimatedfrom the Boltzmann distribution at 298.15 K and the values of ΔE0calculated at the B3LYP level. The sum of syn and anti populationsmakes 100%. dElectronic energies taken from ref 29.

Table 2. Comparison of the Second-Order PerturbationEnergies [E(2)/kcal mol−1] Corresponding to the MostSignificant Donor−Acceptor NBO Interactions between theOCH3 Moiety and the Indole Ring Calculated for the TwoConformers of 5MOI at the B3LYP/6-311++G(d,p) Level

conformer

donor acceptor anti syn

Lp2(O10) π*(C4−C5) −28.33 −25.05Lp1(O10) π*(C4−C5) −7.61Lp1(O10) σ*(C5−C6) −7.72σ(C4−C9) σ*(C5−O10) −4.89 −3.63σ(C5−C6) σ*(O10−C11) −3.23σ(C6−C7) σ*(C5−O10) −3.11 −4.33σ(C4−C5) σ*(O10−C11) −3.18σ(O10−C11) σ*(C5−C6) −2.96Lp1(O10) Ry*(C5) −2.95Lp1(O10) Ry*(C11) −2.83σ(O10−C11) σ*(C4−C5) −2.82σ(C5−O10) σ*(C6−C7) −1.54 −1.23σ(C5−O10) σ*(C4−C9) −1.31 −1.63∑ −45.08 −41.60

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A theoretical infrared spectrum of the gas-phase conforma-tional mixture at 298.15 K is shown in Figure 2b. This spectrumwas simulated from the results of the B3LYP/6-311++G(d,p)harmonic vibrational calculations carried out for the twoconformers, with the infrared intensities calculated for each oneof them scaled by the respective gas-phase Boltzmannpopulation estimated at the same level of theory (80% anti;20% syn). It is noteworthy the very good agreement betweenthe experimental and theoretical spectra, both with respect tothe position and relative intensity of the bands, thus suggestingthat both forms are present in the matrix. In fact, a moredetailed spectral comparison by including the individualwavenumbers and absolute infrared intensities calculated forthe two conformers (Figure 2c) permits identification in theexperimental spectrum of individual bands belonging to eachconformer. For example, the experimental features located at∼1593, 1484, 1453, 1327, 1126, 940, and 828 cm−1, as well asthe multiplet centered at ∼794 cm−1 (marked with closedcircles in Figure 2a), should be ascribable to the most stableanti form, while those found at ∼1587, 1464, 1317, 855, 784,and 776 cm−1 (marked with open circles) are likely to beexplained by the less stable syn form. Note that all of the synbands have very weak experimental intensities (relatively to theanti bands), which is consistent with the expected minorcontribution of the syn form in the conformational mixture.In order to undoubtedly confirm these assignments and to

identify other nontrivial spectral signatures of both conformers,the deposited xenon matrix was annealed from 16 to 40 K, andin the course of this process, various infrared spectra wererecorded at increments of 2 K. Owing to the moderate value ofthe energy barrier corresponding to the internal torsion of theOCH3 group in the direction of the conformational relaxation,if both conformers are trapped in the deposited matrix, then,

annealing of the sample should promote a syn → antitransformation.21 In the experiment, this would be manifestedby an increase of bands due to the most stable form at theexpense of bands due to the less stable form, as shown in Figure3a. The comparison of the difference IR spectrum, shown inFigure 3b, obtained by subtracting the spectrum recordedbefore annealing (16 K) from that recorded after annealing ofthe matrix up to 40 K with a calculated difference spectrum(anti minus syn, Figure 3c) leaves no doubt that duringannealing the syn conformer relaxes to the anti one. This provesthat both forms are present in the freshly deposited sample andthat anti is actually the lowest-energy conformer, astheoretically predicted. The comparison of the spectra obtainedbefore and after annealing (see Figures 3 and S2 for morespectral regions) also permits a more reliable assignment ofexperimental bands to each conformer (see Table 2). It isworth noting that this is the first time that the less stableconformer of 5MOI has been experimentally identified. In fact,as referred to before, rotationally resolved electronic spectra ofthis compound obtained in jet-cooled conditions only led toidentification of the lowest-energy anti form.29

Having confirmed the presence of the two rotamers in thedeposited xenon matrix at 16 K, their relative abundance([anti]/[syn]) at this temperature can be estimated from theintegrated absorbance of selected pairs of experimental bands,with each component assigned to a different form (Aanti andAsyn), and from the corresponding calculated IR intensities (Iantiand Isyn), by applying the following expression: [anti]/[syn] =(Aanti/Asyn) × (Isyn/Ianti). Using the 1327/1317, 1193/1188,900/896, and 855/843 cm−1 pairs of absorptions, the average[anti]/[syn] ratio was estimated to be 5:1 (±0.2), which meansthat the deposited sample is composed of 83% anti and 17% syn(±3%). These values are close to the Boltzmann populations

Figure 2. (a) Experimental mid-IR spectrum of 5MOI recorded immediately after isolating the compound in a low-temperature xenon matrix at 16K. (b) Simulated infrared spectrum of the gas-phase conformational mixture at 298.15 K, weighted by the calculated Boltzmann populations at298.15 K for the conformers (80% anti + 20% syn). (c) Individual stick spectra calculated for the anti (closed red circles) and syn (open blue circles)conformers at the B3LYP/6-311++G(d,p) level. Details of the simulation of the calculated spectra are given in section 3.

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estimated for the gas-phase conformational mixture at 298.15 K(see Table 1). During annealing, the population of the lessstable conformer was found to decrease from 17% (16 K) to

∼9% (30 K) and ∼6% (40 K) (see Figure S3), and fortemperatures higher than 40 K and up to the limit for thethermal stability of the host material (near 70 K for xenon), we

Table 3. Experimental Bands of the Spectrum of 5MOI Isolated in a Xenon Matrix at 16 K, Together with the Wavenumbers (ν,cm−1) and Infrared Intensities (I, km mol−1) Calculated for the Two Conformers of 5MOI at the B3LYP/6-311++G(d,p) Level

calcd antib calcd synb

exptl Xe matrix (16 K)a ν I ν I symmetry approximate descriptionc

3498 (sh,m)/3496 (s) 3493 76.9 3494 75.4 A′ νNH1632/1627 (vw) 1627 42.7 1636 31.9 A′ νCC benz1593/1587 (vw) 1588 28.7 1583 51.8 A′ νCC benz1556 (vw) ?1542 (vw) ?1519/1515/1513 (vw) 1513 19.5 1519 21.3 A′ νC2C3

1501 (vw) ?1484 (m) 1482 74.0 1481 6.5 A′ δCH3 as1464 (vw) 1472 103.9 A′ δCH3 as1453 (w) 1456 72.4 1455 20.4 A′ δC4H + δC7H + δNH1436 (m) 1441 38.4 1438 47.8 A′ δCH3 s1412 (vw) 1415 7.0 1417 6.2 A′ δNH + νC2N + δCH1353 (vw)/1350 (sh, vw) 1343 12.0 1346 21.2 A′ νCC + δCH1327 (w) 1324 12.6 A′ δCH + δNH1317 (vw) 1316 45.8 A′ δCH + δNH1286 (m)/1284 (sh, w) 1281 60.9 1274 62.6 A′ νC5O + δC4H1244 (w) 1239 10.1 1239 25.3 A′ δCH + δNH1226 (vs) 1219 136.0 1214 113.8 A′ δC4H + νC8N + νC5O1193 (w) 1188 16.3 A′ ρCH3

1188 (vw) 1184 10.5 A′ ρCH3

1158/1154 (m) 1153 97.3 1151 126.4 A′ δCH + ρCH3

1136 (vw) 1134 17.5 A′ δC6H + δC7H1126 (w) 1120 27.7 A′ δC6H + δC7H1082 (vw) 1085 4.4 1088 26.9 A′ δNH + δC2H1066 (w) 1066 11.0 1066 5.1 A′ δC2H + δC3H1042 (m) 1036 40.0 1038 40.7 A′ νC10O + δC4H940 (vw) 935 8.5 A′ δ benz900 (w)/896 (vw) 893 12.1 889 10.8 A′ δ py855 (vw) 857 25.8 A″ γC2H + γC3H + γC4H843 (w) 842 15.8 A″ γC2H + γC3H + γC4H828 (w) 826 17.5 A″ γC2H + γC3H + γC4H806 (vw)/797 (sh, vw) / 792 21.7 A′ δ ind796 (w)/794 (w)/791(vw) 790 25.2 A″ γC6H + γC7H784 (vw) 781 7.5 A′ δ ind776 (vw) 772 19.1 A″ γC6H + γC7H751 (w) 745 10.4 745 26.1 A″ γ ind739 (vw) 739 5.1 A′ δ ind732 (vw) 733 10.6 A′ δ ind716 (w)/714 (s) 704 67.5 707 60.9 A″ γC2H + γC3H602 (vw) 597 1.5 A′ δ ind593 (vw) 591 6.9 A″ γ ind589 (vw) 585 6.5 A″ γ ind540 (vw) 536 4.2 A′ δCOC520 (vw) 516 2.1 A′ δCOC452 (vw) 451 3.3 A′ δ benz425 (vw) 420 7.3 423 5.9 A″ γ benz

aExperimental intensities are expressed in a qualitative way: vs = very strong; s = strong; m = medium; w = weak; vw = very weak. Because of theirminor relevance for the present study, absorptions falling in the 3200−2800 cm−1 region corresponding to the CH and CH3 stretching vibrations arenot shown. Bold wavenumbers refer to bands assigned to the anti conformer, while underlined wavenumbers refer to bands assigned to the synconformer. This assignment was based on the comparison between the calculated and experimental spectra and also took into account the individualspectra of the two conformers extracted from spectral changes induced by annealing and by NIR excitations. bCalculated harmonic wavenumbers arescaled by 0.950 (above 3200 cm−1) and 0.980 (below 3200 cm−1). cBased on ChemCraft and Gaussview animation of the vibrations of theconformers. Abbreviations: ν, stretching; δ, in-plane deformation; γ, out-of-plane deformation; ρ, rocking; s, symmetric; as, antisymmetric; ind,indole ring; py, pyrrole fragment; benz, benzene fragment; sh, shoulder.

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observed a complete depopulation of the less stable form.Interestingly, this thermal behavior of 5MOI in a xenon matrixis similar to that observed earlier for conformational relaxationin trimethyl phosphate, another compound where conformersdiffer only by orientations of the OCH3 group separated bysimilar barriers.58

4.3. NIR-Induced Rotamerization around the C−OBond. A NIR spectrum of 5MOI isolated in solid xenon at 16K is shown in Figure 4a. For wavenumbers above 6500 cm−1,only one band is dominating, near 6853 cm−1. Thiswavenumber corresponds to approximately twice the positionof the fundamental νNH stretching absorption (3496 cm−1; seeFigure 4b), and the two bands exhibit similar profiles. This factstrongly suggests that the band appearing in the NIR regionshould be ascribed to the first overtone transition of the NHstretching vibration (2νNH). Usually, for matrix-isolatedmolecules, the observed and calculated anharmonic frequenciesshould be in good agreement.59 And rightly so, our anharmoniccalculations carried out for the two 5MOI conformers furthersupport this assignment. The frequency of the 2νNH overtoneis predicted to occur at wavenumbers close to the position ofthe observed absorption: 6887.4 cm−1 for anti and 6890.1 cm−1

for syn; see Figure 4b and Table S4. The above valuescorrespond to experimental anharmonicity of the NHstretching vibration in 5MOI equal to −70 cm−1 and computedanharmonicity equal to −71.2 cm−1, for both 5MOI con-formers. This finding agrees with the general trend thatanharmonicity of the XH stretching vibrations (X = O; N; C;S) decreases from OH to NH to CH to SH.60 Indeed, for OHstretching vibrations, Havey and Vaida reported largeranharmonicity, near −85 cm−1.61 Significant differences existalso in the intensity of the X−H stretching vibrationaltransitions of different heavy atoms X. Intensities of typicalO−H, C−H, and S−H oscillators decrease from OH to CH to

SH.5 Fewer studies have been done on N−H transitions.Kjaergaard and coauthors showed that changes of dipolemoment functions in a set of seven NH-containing moleculeshave a considerable range of variation, and hence, intensities of

Figure 3. Experimental evidence of occurrence of conformational relaxation in 5MOI monomers isolated in a xenon matrix during annealing. (a)Fragments of the experimental infrared spectra of (blue) the sample immediately after deposition at 16 K and (red) the same sample after annealingto 40 K. Arrows indicate the direction of changes from 16 to 40 K. (b) Experimental difference spectrum obtained by subtracting the spectrumrecorded at 16 K from that recorded at 40 K. Positive bands correspond to those growing upon annealing. Gray rectangles designate regions shownin panel (a). (c) Simulated B3LYP/6-311++G(d,p) difference spectrum obtained as the spectrum of the anti form minus the spectrum of the synform (positive bands are due to anti).

Figure 4. Fragments of (a) near-IR and (b) mid-IR experimentalspectra of 5MOI isolated in a xenon matrix at 16 K, showing theabsorption bands assigned to the (a) 2νNH overtone transition and(b) νNH fundamental transition (all data shown in black; ordinates onthe left). The color sticks represent wavenumbers and intensitiescalculated for the (a) 2νNH overtone and (b) νNH fundamentaltransitions of the anti (closed red circles) and syn (open blue circles)conformers obtained in anharmonic vibrational calculations at theB3LYP/6-31++G(d,p) level (ordinates on the right).

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νNH and 2νNH transitions vary a lot.62 For gaseous pyrrole,the intensity drop of about 8 from νNH to 2νNH60 is similar tothat reported for a relative change in intensities of νOH and2νOH bands, which drop by a factor of about 10 in the gasphase.5 In this respect, it appears interesting to us to report thatfor matrix-isolated 5MOI the experimental integrated inten-sities of νNH and 2νNH differ by a factor of 34 (see Figure 4).This is similar to the intensity drop from νNH to 2νNH in6MOI (near 35),21,22 as well to the intensity drop from νOH to2νOH in furoic acid63 (near 35) and in thiazole-carboxylicacid64 (near 38), all these compounds studied in the samematrix isolation experimental setup.65

On the basis of the experimental NIR spectrum of matrix-isolated 5MOI, the OPO was tuned at 6853 cm−1 (maximum ofthe 2νNH absorption band), and monomers of the compoundisolated in solid xenon at 16 K were exposed to thismonochromatic NIR light for about 30 min (three successiveirradiations, 10 min each). In this experiment, the matrix wasfirst deposited at 30 K and then cooled down to 16 K. Thismeans that before the NIR irradiations, the conformationalcomposition of the matrix-isolated compound was largelyshifted toward the most stable form (∼93% anti and ∼7% syn;see Figure S3).The spectral changes observed in the mid-IR region resulting

from this series of NIR irradiations are illustrated in twofragments of the difference IR spectrum (“after irradiation”minus “before irradiation”) shown in Figure 5a (a difference

spectrum covering a wider mid-IR region is given in Figure S4,and the changes in the 2νNH region are shown in Figure S5).The comparison of the experimental difference spectrum in thetwo selected fragments with the spectra calculated for the antiand syn conformers (Figure 5b) reveals in a very clear way thatexcitation of the matrix-isolated compound at the wavenumbercorresponding to the maximum of the 2νNH overtone band

induces rotamerization around the C−O bond in 5MOI. Infact, as illustrated in Figure 5, upon NIR irradiation, the bandsascribed to the syn conformer grow up (1350, 1317, 1284, 896,and 855 cm−1), while those assigned to the anti conformerdecrease in intensity (1353, 1327, 1286, 900, 843, and 828cm−1). This experimental finding provides another remarkableexample of conformational isomerization in matrix-isolatedmolecules where the vibrationally excited group (N−H) andthe group that undergoes isomerization (O−CH3) are situatedat remote ends of the molecule, which are even more distant in5MOI than in 6MOI.22 This proves the occurrence of long-range intramolecular vibrational energy transfer, wherein theenergy absorbed by the excited vibrational coordinate (in thiscase an N−H stretching vibration) is transferred to a remotefragment (OCH3) that undergoes internal torsion. It should benoted that so far the conformational isomerizations resultingfrom remote intramolecular vibrational energy redistributionhave been reported only for a reduced number of matrix-isolated molecules: 5MOI (this work), 6MOI,22 2-thiocyto-sine,28 and kojic acid.20 In 2-thiocytosine and kojic acid, themolecular fragments changing their orientation are the light Hatoms of the OH or SH groups, while in the twomethoxyindoles (5MOI and 6MOI), the group undergoinginternal rotation is a heavy CH3 fragment of the methoxygroup. The fact that the remote intramolecular vibrationalenergy transfer takes place in both methoxyindole isomersproves that this effect previously observed for 6MOI was not afortuitous event and that it is independent of the relativeposition of the methoxy fragment relative to the vibrationallyexcited NH group.After exposing the sample to narrow-band NIR light at 6853

cm−1 for about 30 min, the population of the syn conformer wasfound to increase from ∼9 to ∼17% and stabilize. Noteworthy,no clear-cut new absorption due to the syn form could besimultaneously observed to increase in the NIR region; only thepreviously existing band centered near 6853 cm−1 slightlychanged its shape. In an attempt to verify if the conformationalpopulation ratio may be also shifted in the opposite direction,that is, from anti to syn, additional NIR irradiations were carriedout by selecting other wavenumbers within the 2νNH bandprofile. Hence, the OPO was tuned at 6856 and 6847 cm−1, atpositions of the higher- and lower-frequency shoulders thatslightly increased near the 2νNH band maximum (Figure S5).However, the irradiations performed at both wavenumbers didnot result in any significant spectral modifications in the mid-IRregion that could be related to the shift of conformationalpopulations either in the anti → syn or in the oppositedirection. Hence, when the conformational composition of thesample was dominated by one structure (anti form populatedby means of the preceding annealing), the NIR-inducedconformational transformation of matrix-isolated 5MOI couldbe indeed observed. On the other hand, a prolonged NIRexcitation within the 2νNH overtone band profile (near 6853cm−1) most likely results in simultaneous vibrational excitationof both conformers, which is in agreement with the almostcoincident wavenumbers of the 2νNH overtones predicted forthe two conformers (Figure 4). The fact that the conforma-tional mixture in the matrix before the series of NIR irradiationsat 6853 cm−1 is largely dominated by the anti conformer helpsto understand why the net result of the NIR-inducedconformational changes of 5MOI is a decrease of thepopulation of the anti form and an increase of the populationof the syn form.

Figure 5. Two representative mid-IR spectral fragments showing theconformational transformations in matrix-isolated 5MOI (xenon, 16K) after three successive NIR irradiations at 6853 cm−1 (10 min each).(a) Difference spectrum obtained as the spectrum recorded after theNIR irradiations minus that recorded before any irradiation (positivebands correspond to those growing up during the irradiations); (b)Spectra of syn (blue) and anti (red) conformers of 5MOI simulated atthe B3LYP/6-311++G(d,p) level. Calculated infrared intensities of theanti conformer were multiplied by −1.

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4.4. UV-Induced Conformational Isomerization. TheUV irradiations of the matrix-isolated 5MOI were carried outon a sample containing the two 5MOI conformers (the minorsyn conformer was populated by preceding NIR irradiations).The origin of electronic transition measured for the anticonformer of 5MOI under jet-cooled conditions was found tobe 302 nm.29,66 For the syn conformer, which has not beenobserved previously, such a transition was estimated to occur at∼308 nm.29 These values are in excellent agreement withtheoretical adiabatic excitation energies of 398.1 kJ mol−1

(300.5 nm) for anti and 387.6 kJ mol−1 (308.6 nm) for syn,calculated in this work after full geometry optimizations in theground S0 and in the lowest-energy excited (S1) singlet state (atthe [(TD-DFT/B3LYP/6-311++G(d,p)] level for S1), includ-ing the ZPVE corrections in both states.Therefore, initially, the OPO was tuned at λ = 320 nm, and

further irradiations followed by gradual application of shorterwavelengths. After each irradiation (duration of 1−2 min), amid-IR spectrum was collected to monitor the UV-inducedstructural transformations in the isolated molecules. The onsetof spectral changes occurred upon exciting the sample at λ =315 nm. Changes of the same type, but more pronounced,continued after another excitation at λ = 310 nm. Thesechanges are illustrated as the difference spectrum in Figure 6a.

Comparing this difference spectrum with the calculateddifference spectrum shown in Figure 6b (anti minus syn), itcan be inferred that the spectral modifications induced by theUV irradiations at 315 and 310 nm correspond to a syn → anticonversion. A change of population ratio in the oppositedirection was observed upon 3 min of subsequent UV

irradiation at λ = 308 nm (see Figure 6c; note the arrowindicating the direction of changes), which means that the UV-induced rotamerization of the OCH3 group is photoreversible.It is also to be noted that conformational isomerization was thedominating phototransformation taking place in 5MOI uponshort irradiations at 315 > λ > 308 nm as no significant newbands would appear in the infrared spectra recorded after theseUV irradiations. The summary of UV-induced conformationalchanges in 5MOI is presented in Figure 7.

In an attempt to provide a mechanistic picture of the UV-induced conformational isomerization in 5MOI, we havecomputed a relaxed potential energy profile around theC11−O10−C5−C6 dihedral in the S1 excited state, which isexpected to be the only accessible singlet excited state upon UVirradiation within the 315−308 nm wavelength range (seeTable S4). The results of these calculations are represented inFigure 8, together with the relaxed potential energy scan

calculated for the electronic ground state (S0). Note that unlikeS0, syn is the most stable conformer in S1, and the energydifference of anti relative to syn amounts to 7 kJ mol−1. The factthat the syn → anti conversion occurred in our experiments atlonger wavelengths than the anti → syn population shift (forthe sample enriched with the anti form) is in agreement withthe order of the computed adiabatic excitation energiesdetermined for the two conformers (398.1 and 387.6 kJmol−1). The computed difference of ∼10.5 kJ mol−1 is in good

Figure 6. Effect of narrow-band UV irradiation on the conformationalcomposition of 5MOI isolated in a xenon matrix at 16 K. (a)Difference spectrum obtained as the spectrum recorded after UVirradiation (1.5 min at λ = 315 nm followed by 1 min at λ = 310 nm)minus that recorded before any UV irradiation. (c) Differencespectrum obtained as the spectrum recorded after UV irradiation atλ = 310 nm minus that after 3 min of UV irradiation at λ = 308 nm. Inframes (a) and (c), the vertical arrows indicate the directions of thebands growing upon irradiation. Note that they are opposite. (b)Simulated B3LYP/6-311++G(d,p) difference spectrum obtained as thespectrum of the anti form minus the spectrum of the syn form (positivebands are due to anti).

Figure 7. Directions of UV-induced shifts of conformationalpopulations in 5MOI isolated in a low-temperature xenon matrix.

Figure 8. B3LYP/6-311++G(d,p) relaxed potential energy scans forinternal torsion in 5MOI as a function of the C11−O10−C5−C6 (α)dihedral angle calculated for the lowest excited singlet state (S1 or

1Lb,red) using the TD-DFT method and for the ground state (S0, blue).

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accord with the experimental difference of ∼8.6 kJ mol−1,obtained in the present work for the onsets of UV-inducedphotoreactions.From the results shown in Figure 8, it seems plausible to

admit that whatever conformer is being excited from S0 to S1, itis rather difficult to overcome the barrier measuring more than22 kJ mol−1 in the excited state. Therefore, it is likely that theconformational isomerization occurs preferentially after repo-pulation of the electronic ground state S0 via internalconversion or fluorescence, where the barrier to conformationalisomerization is significantly smaller (4 kJ mol−1) and therelaxing molecules have a large excess of vibrational energy.

5. CONCLUSIONS

In this study, besides the first experimental observation andspectroscopic characterization of the higher-energy syn con-former of 5MOI, occurrence of long-range vibrational energytransfer leading to conformational isomerization in thismolecule was demonstrated, and different ways to manipulatethe relative populations of its two conformers (anti and synforms) were described:

(i) Annealing of the xenon matrix deposited at 16 K totemperatures of about 30−40 K led to conversion of thehigher-energy syn form into the most stable anticonformer, a result that confirmed the relative order ofenergies of the two conformers theoretically predicted.

(ii) Upon NIR irradiation of the matrix-isolated compound atthe frequency of the NH stretching overtone band (6853cm−1), partial conversion of the most abundant anticonformer into the syn form, by internal rotation aboutthe remotely located C−O bond, was observed. The finalpopulation ratio of the two conformers upon 6853 cm−1

irradiation revealed that a photostationary state wasattained, which is in agreement with the almostcoincident frequencies of the 2νNH overtones predictedfor the two conformers. The NIR-induced observedconformational isomerization of 5MOI is one of the veryfew examples reported hitherto where the conforma-tional change results from intramolecular vibrationalenergy redistribution upon excitation of a vibration of aremotely located bond.

(iii) Conformational interconversion between the two con-formers was also achieved by using electronic excitation inthe UV domain. Excitation in the 315−310 nm range ledto total consumption of the syn form, in favor of anti,while irradiation at 308 nm was shown to promote theinverse process. A mechanistic explanation of theobserved transformations, based on computation of theground and first excited states’ (S0, S1) potential energysurfaces along the conformational isomerization coor-dinate, has been given. Specifically, taking into accountthe considerably large energy barrier (>22 kJ mol−1) forconformational isomerization in the excited state, it isproposed that the UV-induced conformational isomer-izations should occur preferentially after repopulation ofthe electronic ground state S0, where the barrier toconformational isomerization is only ∼4 kJ mol−1, lowenough to be surmounted by the relaxing moleculeshaving a large excess of vibrational energy.

■ ASSOCIATED CONTENT

*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acs.jpca.7b01713.

Tables S1 and S2, with Cartesian coordinates of the antiand syn conformers of 5MOI optimized at different levelsof theory; Tables S3 and S4, with results of harmonic andanharmonic vibrational calculations for the fundamentaland overtone transitions of the anti and syn conformersof 5MOI; Table S5, with vertical excitation energies andoscillator strengths calculated at the TD-DFT level for 12lowest-energy singlet states of the syn and anti con-formers of 5MOI; Table S6, with Cartesian coordinatesof the anti and syn conformers of 5MOI optimized forthe first excited singlet electronic state; Figure S1, with arelaxed potential energy scan computed for 5MOI as afunction of the internal torsion of the OCH3 group;Figure S2, showing IR spectra of matrix-isolated 5MOIbefore and after annealing; Figure S3, showing variationof conformational populations of matrix-isolated 5MOIduring annealing; and Figures S4 (extended version ofFigure 5) and S5, showing evidence of NIR-inducedconformational transformations in matrix-isolated 5MOI(PDF)

■ AUTHOR INFORMATION

Corresponding Author*E-mail: reva@qui.uc.pt.

ORCIDI. Reva: 0000-0001-5983-7743NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTS

This investigation was performed within the Project PTDC/QEQ-QFI/3284/2014-POCI-01-0145-FEDER-016617, fundedby the Portuguese “Fundacao para a Ciencia e a Tecnologia”(FCT) and FEDER/COMPETE 2020-UE. The CoimbraChemistry Centre (CQC) is supported by FCT, through theproject UI0313/QUI/2013, also cofunded by FEDER/COMPETE 2020-UE. I.R. acknowledges FCT for the“Investigador FCT” Grant. The authors also acknowledge theCoimbra LaserLab facility.

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