Conformational Changes in 5-Methoxyindole: Effects of Thermal

Article pubs.acs.org/JPCA

Conformational Changes in 5‑Methoxyindole: Effects of Thermal, Vibrational, and Electronic Excitations Published 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 two conformational states, syn and anti, with respect to the relative orientation of the NH and OCH3 groups. The structure of monomeric 5MOI was characterized spectroscopically, in mid- and near-infrared domains. The conformational composition of 5MOI could be controlled in three different ways. Thermally, two conformers of 5MOI could be trapped in xenon matrixes at 16 K. Upon annealing the xenon matrix to temperatures about 30−40 K, the higher-energy syn form converted to the ground-state anti conformer. 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. Electronic excitations in the UV domain, at 315−310 nm, resulted in a total consumption of the syn form again, in favor of anti. Upon further irradiations at 308 nm, a partial repopulation of the syn form, at the expense of anti, was observed. We propose a mechanistic explanation of the observed transformations, which is based on computations of the vibrational spectra of the two conformers and also on computations of the ground state S0 and the first excited state S1 potential energy surfaces along the coordinate for conformational isomerization. The highlights of the present work are the first experimental observation of the minor syn conformer of 5MOI, evidence of the long-range vibrational energy transfer resulting in conformational isomerization upon excitation of the NH stretching overtone, and the possibility of partial conformational control of 5MOI by using electronic excitations. low energies (corresponding to 2−3 quanta in the stretch)6 and can be well described by local mode basis sets.7,8 The overtones corresponding to 2 quanta in the stretch appear in the NIR region of the spectrum, and upon appearance of frequencytunable optical parametric oscillators excitation of such overtones became easily accessible. Molecules have been prepared in excited X−H overtone states to study the intramolecular vibrational energy distribution, which manifests itself in conformational changes following NIR excitations.9 The experimental technique of matrix isolation is particularly suitable for characterization of the conformational structure of molecules.10,11 The most frequently reported cases concern molecules possessing OH groups, mainly carboxylic acids.12−16 Narrow-band NIR excitations at frequencies corresponding to those of the OH stretching overtones have proved to be an efficient method to induce selective conformational changes. The most common observed outcome of these irradiations is a change in the orientation of the same group that is being excited13,17 or of a molecular fragment occupying a close position (vicinal or geminal) relative to the excited group.14,16,18,19 In only a few cases reported so far were the

1. INTRODUCTION Solar radiation is the single largest energy source on both the early and modern Earth.1 Energy provided by the Sun has access to different chemistries. The majority of photochemical reactions considered in atmospheric models are reactions on molecular electronic excited states requiring ultraviolet (UV) light for excitation. UV photons have energies equal to most covalent bonds and therefore may cause dissociation. However, certain atmospheric molecules (such as molecular oxygen and ozone) absorb UV strongly and thus filter out the shortwavelength solar light. Unlike UV photons, visible (vis) and near-infrared (NIR) light photons are significantly more abundant in the atmosphere.2 When sunlight-driven excited electronic state reactions are not effective, photochemical processes occurring by vibrational overtone excitation are important in reactions of oxidized atmospheric compounds (acids, alcohols, and peroxides) prevalent in the Earth’s atmosphere.3 The fundamental energetic, mechanistic, and dynamical aspects of photochemical reactions occurring via vibrational overtone absorption have been reviewed by Vaida and co-workers.4,5 Overtone excitation of X−H (X = C, N, O) oscillators has been used extensively to prepare molecules in initial states with energy localized in the X−H bond. The typical X−H stretches do not mix appreciably with other vibrations already at fairly © XXXX American Chemical Society

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

DOI: 10.1021/acs.jpca.7b01713 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry A

which was connected through a needle valve to the vacuum system of a closed-cycle helium cryostat (APD Cryogenics, with a DE-202A expander). Vapors of the compound, sublimating at room temperature, were co-deposited with a large excess of xenon (xenon N45, supplied by Air Liquide) onto a CsI window, which was kept, in different experiments, at 16 or 30 K. The temperature of the CsI window was measured directly at the sample holder by means of a silicon diode sensor connected to a digital controller (Scientific Instruments, Model 9650-1, accuracy of 0.1 K). A Thermo Nicolet 6700 Fourier transform infrared (FTIR) spectrometer was employed to record spectra in the near-infrared and mid-infrared ranges. For this purpose, the following combinations of beamsplitter/ detector/resolution were used: mercury cadmium telluride (MCT/B) detector (cooled by liquid N2) and CaF2 beam splitter (near-IR, 1 cm−1 resolution); deuterated triglycine sulfate (DTGS) detector and KBr beam splitter (mid-IR, 0.5 cm−1 resolution). After deposition, the matrixes were irradiated through the outer quartz window of the cryostat by using monochromatic (0.2 cm−1 spectral width, pulse energy 2−10 mJ) NIR or UV light provided, respectively, by the idler beam or the frequency-doubled signal beam of a Quanta-Ray MOPOSL optical parametric oscillator (OPO). The OPO was pumped with a pulsed Nd:YAG laser. The pulse duration time and repetition rate were 10 ns and 10 Hz, respectively.

excited and rotating groups separated by several covalent bonds.20−22 There were several attempts to induce conformational isomerizations in molecules containing an NH2 group, such as cytosine,23 5-substituted cytosines,24 oxamic acid,25 and amino acids glycine19 and alanine.26 However, all of these molecules contain an OH group, besides the NH2 group, and whether conformational isomerizations occurred, or not, upon excitations of pure NH stretching overtones in these molecules remains inconclusive. Clearly observed conformational isomerizations were only registered upon excitations of the OH stretching overtones in those molecules.23−26 Our attempts to induce conformational isomerization by excitation of an NH stretching overtone in tetrazole−acetic acid failed.27 The successful isomerization induced by excitation of an NH stretching 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 most notable examples of long-range vibrational energy transfer in matrix-isolated molecules.22 Excitation of the NH stretching overtone in 6MOI induces a change in the orientation of the O−CH3 moiety, which is separated from the NH group by four covalent bonds. One important objective of this work is to investigate if conformational isomerizations by excitation of the NH stretching overtone can be also induced in 5-methoxyindole (5MOI), in order to obtain experimental evidence whether long-range vibrational energy transfer is still operational in the extreme case of separation between the N−H and OCH3 groups in indole-based systems (i.e., in 5MOI). The conformational structure of 5MOI was investigated theoretically29,30 and experimentally.29 The theoretical calculations predicted two conformers for the isolated molecule (which are called anti and syn; see Figure 1), but only the

3. COMPUTATIONAL SECTION For identification of the minimum energy conformations of 5MOI, a relaxed potential energy scan around the C11−O10−C5−C6 (α) torsional angle was first carried out at the DFT(B3LYP)33−35/6-311++G(d,p) level of theory. Complete geometry optimizations were then undertaken on the identified 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 the B3LYP/6-31++G(d,p) level were followed by computations of anharmonic infrared spectra using a fully automated secondorder vibrational perturbative approach of Barone and coworkers,37,38 allowing for the evaluation of anharmonic vibrational frequencies and anharmonic infrared intensities up to 2 quanta, including overtones and combination bands.38−40 All above calculations were executed with the Gaussian 09 program package (revision D.01).41 The computed harmonic vibrational wavenumbers above and below 3200 cm−1 were scaled by multiplicative factors of 0.95042 and 0.980,43 respectively. For graphical comparison between the experimental and theoretical spectra in the mid-IR range, the IR spectra obtained in the harmonic vibrational calculations were convoluted with Lorentzian functions having a full width at half-maximum (fwhm) of 1 cm−1 and by setting the intensity at the band maximum equal to the calculated absolute intensity. The Chemcraft software (version 1.8)44 was used for this purpose. The harmonic vibrational calculations also allowed us to obtain the zero-point corrected energies (E0) for the 5MOI conformers (E0 = Eel + ZPVE), where Eel is the electronic energy and ZPVE is the zero-point vibrational energy. The Cartesian coordinates of the geometries optimized at all levels of theory, as well as the calculated wavenumbers and infrared intensities resulting from harmonic and anharmonic vibrational calculations, are provided as Supporting Information (Tables S1−S4). A Natural Bond Orbital (NBO)45 analysis was further carried out [B3LYP/6-311++G(d,p)] for the identified con-

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

lowest-energy anti form was experimentally observed under jetcooled conditions by using rotationally resolved electronic spectroscopy.29 For both conformers, the energy gap between the two lowest-energy excited electronic singlet states (ππ* states), commonly designated as 1Lb(S1) and 1La(S2),31,32 was calculated to be larger than 4000 cm−1, thus proving that the vibronic coupling between these states is very small.29 It will be interesting to investigate if electronic excitation of the groundstate anti conformer populated under experimental conditions would lead to photogeneration of the syn conformer. Therefore, another aim of the present work is to attempt population of the minor conformer of 5MOI not only by vibrational overtone pumping but also by electronic excitation.

2. EXPERIMENTAL SECTION Commercial 5MOI, acquired from Aldrich with a 99% purity degree, was used in the matrix isolation experiments. A small quantity of the solid substance was placed into a glass tube, B


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The Journal of Physical Chemistry A formers of 5MOI by using the NBO 5.0 program46 as implemented in the GAMESS program.47 Vertical excitation energies and oscillator strengths of the low-energy electronic excited states were calculated by using the time-dependent version of the density functional method (TD-DFT)48,49 combined with the 6-311++G(d,p) basis set implemented in the Gaussian 09 program (Table S5). For the specific case of the lowest excited singlet S1 (or 1Lb) state, which is expected to be the only populated state in the course of the UV excitations of the matrix-isolated 5MOI, geometry optimizations were also performed at the TD-DFT level (Table S6).

calculated, and the most significant interactions are listed in Table 2. The stabilizing effect given by the sum of the E(2) Table 2. Comparison of the Second-Order Perturbation Energies [E(2)/kcal mol−1] Corresponding to the Most Significant Donor−Acceptor NBO Interactions between the OCH3 Moiety and the Indole Ring Calculated for the Two Conformers of 5MOI at the B3LYP/6-311++G(d,p) Level conformer

4. RESULTS AND DISCUSSION 4.1. Conformers of 5MOI and Their Relative Stability. The geometries of the syn (OCH3 is syn-parallel to the NH bond) and anti (OCH3 is anti-parallel to the NH bond) conformers of 5MOI are represented in Figure 1, while the potential energy scan for the internal torsion of the OCH3 fragment is shown in Figure S1. In Table 1 are listed the relative a

Table 1. Relative Electronic Energies (ΔEel), Relative ZeroPoint Corrected Energies (ΔE0),a and Equilibrium Populations Calculated for 5MOI Conformersa at Different Levels syn level of theory B3LYP 6-311++G(d,p) 6-311++G(3df,3pd) MP2 6-311++G(d,p) 6-311++G(3df,3pd) CC2/cc-pVTZ

ΔEel

ΔE0b

pop. (%)c

4.11 3.96

3.58 3.49

19.1 19.7

5.80 5.27 5.55d

− − −

− − −

donor

acceptor

anti

syn

Lp2(O10) Lp1(O10) Lp1(O10) σ(C4−C9) σ(C5−C6) σ(C6−C7) σ(C4−C5) σ(O10−C11) Lp1(O10) Lp1(O10) σ(O10−C11) σ(C5−O10) σ(C5−O10) ∑

π*(C4−C5) π*(C4−C5) σ*(C5−C6) σ*(C5−O10) σ*(O10−C11) σ*(C5−O10) σ*(O10−C11) σ*(C5−C6) Ry*(C5) Ry*(C11) σ*(C4−C5) σ*(C6−C7) σ*(C4−C9)

−28.33 −7.61

−25.05

−4.89 −3.23 −3.11

−7.72 −3.63 −4.33 −3.18

−2.96 −2.95

−1.54 −1.31 −45.08

−2.83 −2.82 −1.23 −1.63 −41.60

values [∑E(2)] is greater in the anti (larger negative values; see Table 2) than that in the syn conformer, which helps to explain their order of stability. These results reveal also that the most significant contribution for this difference of electronic stabilization comes from the Lp2(O10) → π*(C4C5) conjugative interaction. Considering a Boltzmann distribution based on the values of ΔE0 calculated at the B3LYP level, the gaseous compound immediately before the matrix deposition is predicted to be constituted by ∼80% anti and ∼20% syn (Table 1). For the higher-energy conformer, its efficient trapping in the freshly deposited low-temperature xenon matrix and subsequent successful experimental detection is dependent on various factors.54 These factors include the nature of the matrix gas, the matrix temperature during deposition, as well as the energy barrier separating the conformers.55,56 In 5MOI, this barrier corresponds to the syn → anti rotamerization. Indeed, if this barrier is very low, conformational cooling may occur during the matrix deposition,57 thereby preventing stabilization and consequently spectral identification of the syn conformer in the deposited frozen sample. In the present case, the value of the barrier height corresponding to the syn → anti transformation was calculated to be ∼6 kJ mol−1 [B3LYP/6-311++G(d,p); see Figure S1], which is even lower than the value calculated for 6MOI at the same theory level (∼9 kJ mol−1).21,30 Therefore, the deposition temperature for 5MOI was set in the present work at 16 K, lower than that used for 6MOI (20 K).21 This choice of temperature allowed for successful trapping of both 5MOI conformers after isolating the compound in solid xenon at 16 K, as shown below. 4.2. Spectral Identification of 5MOI Conformers. The mid-infrared spectrum of 5MOI in the 1700−680 cm−1 region, recorded immediately after isolating monomers of the compound in solid xenon at 16 K, is represented in Figure 2a, while Table 3 lists the positions of the experimental bands and their approximate description based on the animation of vibrations calculated for the two forms.

Energies (kJ mol−1) of the syn conformer relative to the most stable anti conformer, whose relative energy was assumed to be zero at all levels. bE0 = Eel + ZPVE (zero-point vibrational energy). cEstimated from the Boltzmann distribution at 298.15 K and the values of ΔE0 calculated at the B3LYP level. The sum of syn and anti populations makes 100%. dElectronic energies taken from ref 29. a

electronic energies (ΔEel) and zero-point corrected energies (ΔE0) calculated for the two conformers at different levels of theory. From these results, it is found that all methods predict anti as the lowest-energy conformer. The B3LYP energy difference 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 MP2 relative energy is very close to that calculated at the CC2/ cc‑pVTZ level.29,50 The preference of the OCH3 substituent to adopt an anti orientation, instead of syn, when it is attached to position 5 of the indole ring has also been found for 5‑methoxytryptamine51 and melatonin.52 Similar conformational behavior was also observed when this substituent was replaced by an OH group in 5-hydroxyindole.53 Because the two 5MOI conformers are structurally very similar and no specific interactions (e.g., intramolecular hydrogen bonds) are present, their stability difference is most probably related with subtle differences of electronic delocalization between the benzene ring and the exocyclic OCH3 group. Using NBO theory, the values of the second-order perturbation energies [E(2)] corresponding to the orbital interactions between filled and vacant NBOs of the two moieties were C


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Figure 2. (a) Experimental mid-IR spectrum of 5MOI recorded immediately after isolating the compound in a low-temperature xenon matrix at 16 K. (b) Simulated infrared spectrum of the gas-phase conformational mixture at 298.15 K, weighted by the calculated Boltzmann populations at 298.15 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.

annealing of the sample should promote a syn → anti transformation.21 In the experiment, this would be manifested by an increase of bands due to the most stable form at the expense of bands due to the less stable form, as shown in Figure 3a. The comparison of the difference IR spectrum, shown in Figure 3b, obtained by subtracting the spectrum recorded before annealing (16 K) from that recorded after annealing of the matrix up to 40 K with a calculated difference spectrum (anti minus syn, Figure 3c) leaves no doubt that during annealing the syn conformer relaxes to the anti one. This proves that both forms are present in the freshly deposited sample and that anti is actually the lowest-energy conformer, as theoretically predicted. The comparison of the spectra obtained before and after annealing (see Figures 3 and S2 for more spectral regions) also permits a more reliable assignment of experimental bands to each conformer (see Table 2). It is worth noting that this is the first time that the less stable conformer of 5MOI has been experimentally identified. In fact, as referred to before, rotationally resolved electronic spectra of this compound obtained in jet-cooled conditions only led to identification of the lowest-energy anti form.29 Having confirmed the presence of the two rotamers in the deposited xenon matrix at 16 K, their relative abundance ([anti]/[syn]) at this temperature can be estimated from the integrated absorbance of selected pairs of experimental bands, with each component assigned to a different form (Aanti and Asyn), and from the corresponding calculated IR intensities (Ianti and 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 means that the deposited sample is composed of 83% anti and 17% syn (±3%). These values are close to the Boltzmann populations

A theoretical infrared spectrum of the gas-phase conformational mixture at 298.15 K is shown in Figure 2b. This spectrum was simulated from the results of the B3LYP/6-311++G(d,p) harmonic vibrational calculations carried out for the two conformers, with the infrared intensities calculated for each one of them scaled by the respective gas-phase Boltzmann population estimated at the same level of theory (80% anti; 20% syn). It is noteworthy the very good agreement between the experimental and theoretical spectra, both with respect to the position and relative intensity of the bands, thus suggesting that both forms are present in the matrix. In fact, a more detailed spectral comparison by including the individual wavenumbers and absolute infrared intensities calculated for the two conformers (Figure 2c) permits identification in the experimental spectrum of individual bands belonging to each conformer. For example, the experimental features located at ∼1593, 1484, 1453, 1327, 1126, 940, and 828 cm−1, as well as the multiplet centered at ∼794 cm−1 (marked with closed circles in Figure 2a), should be ascribable to the most stable anti form, while those found at ∼1587, 1464, 1317, 855, 784, and 776 cm−1 (marked with open circles) are likely to be explained by the less stable syn form. Note that all of the syn bands have very weak experimental intensities (relatively to the anti bands), which is consistent with the expected minor contribution 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, and in the course of this process, various infrared spectra were recorded at increments of 2 K. Owing to the moderate value of the energy barrier corresponding to the internal torsion of the OCH3 group in the direction of the conformational relaxation, if both conformers are trapped in the deposited matrix, then, D


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Table 3. Experimental Bands of the Spectrum of 5MOI Isolated in a Xenon Matrix at 16 K, Together with the Wavenumbers (ν̃, 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 exptl Xe matrix (16 K)

a

3498 (sh,m)/3496 (s) 1632/1627 (vw) 1593/1587 (vw) 1556 (vw) 1542 (vw) 1519/1515/1513 (vw) 1501 (vw) 1484 (m) 1464 (vw) 1453 (w) 1436 (m) 1412 (vw) 1353 (vw)/1350 (sh, vw) 1327 (w) 1317 (vw) 1286 (m)/1284 (sh, w) 1244 (w) 1226 (vs) 1193 (w) 1188 (vw) 1158/1154 (m) 1136 (vw) 1126 (w) 1082 (vw) 1066 (w) 1042 (m) 940 (vw) 900 (w)/896 (vw) 855 (vw) 843 (w) 828 (w) 806 (vw)/797 (sh, vw) / 796 (w)/794 (w)/791(vw) 784 (vw) 776 (vw) 751 (w) 739 (vw) 732 (vw) 716 (w)/714 (s) 602 (vw) 593 (vw) 589 (vw) 540 (vw) 520 (vw) 452 (vw) 425 (vw)

calcd synb

ν̃

I

ν̃

I

symmetry

approximate descriptionc

3493 1627 1588

76.9 42.7 28.7

3494 1636 1583

75.4 31.9 51.8

A′ A′ A′

1513

19.5

1519

21.3

A′

1482

74.0

1456 1441 1415 1343 1324

72.4 38.4 7.0 12.0 12.6

1481 1472 1455 1438 1417 1346

6.5 103.9 20.4 47.8 6.2 21.2

1281 1239 1219 1188

60.9 10.1 136.0 16.3

1316 1274 1239 1214

45.8 62.6 25.3 113.8

1153

97.3

1184 1151 1134

10.5 126.4 17.5

1120 1085 1066 1036 935 893

27.7 4.4 11.0 40.0 8.5 12.1

1088 1066 1038

26.9 5.1 40.7

889 857

10.8 25.8

842 826 792 790

15.8 17.5 21.7 25.2 781 772 745

7.5 19.1 26.1

733 707

10.6 60.9

585 536

6.5 4.2

423

5.9

A′ A′ A′ A′ A′ A′ A′ A′ A′ A′ A′ A′ A′ A′ A′ A′ A′ A′ A′ A′ A′ A″ A″ A″ A′ A″ A′ A″ A″ A′ A′ A″ A′ A″ A″ A′ A′ A′ A″

νNH νCC benz νCC benz ? ? νC2C3 ? δCH3 as δCH3 as δC4H + δC7H + δNH δCH3 s δNH + νC2N + δCH νCC + δCH δCH + δNH δCH + δNH νC5O + δC4H δCH + δNH δC4H + νC8N + νC5O ρCH3 ρCH3 δCH + ρCH3 δC6H + δC7H δC6H + δC7H δNH + δC2H δC2H + δC3H νC10O + δC4H δ benz δ py γC2H + γC3H + γC4H γC2H + γC3H + γC4H γC2H + γC3H + γC4H δ ind γC6H + γC7H δ ind γC6H + γC7H γ ind δ ind δ ind γC2H + γC3H δ ind γ ind γ ind δCOC δCOC δ benz γ benz

745 739

10.4 5.1

704 597 591

67.5 1.5 6.9

516 451 420

2.1 3.3 7.3

a

Experimental intensities are expressed in a qualitative way: vs = very strong; s = strong; m = medium; w = weak; vw = very weak. Because of their minor relevance for the present study, absorptions falling in the 3200−2800 cm−1 region corresponding to the CH and CH3 stretching vibrations are not shown. Bold wavenumbers refer to bands assigned to the anti conformer, while underlined wavenumbers refer to bands assigned to the syn conformer. This assignment was based on the comparison between the calculated and experimental spectra and also took into account the individual spectra of the two conformers extracted from spectral changes induced by annealing and by NIR excitations. bCalculated harmonic wavenumbers are scaled by 0.950 (above 3200 cm−1) and 0.980 (below 3200 cm−1). cBased on ChemCraft and Gaussview animation of the vibrations of the conformers. 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.

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

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


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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 annealing to 40 K. Arrows indicate the direction of changes from 16 to 40 K. (b) Experimental difference spectrum obtained by subtracting the spectrum recorded at 16 K from that recorded at 40 K. Positive bands correspond to those growing upon annealing. Gray rectangles designate regions shown in 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 syn form (positive bands are due to anti).

observed a complete depopulation of the less stable form. Interestingly, this thermal behavior of 5MOI in a xenon matrix is similar to that observed earlier for conformational relaxation in trimethyl phosphate, another compound where conformers differ only by orientations of the OCH3 group separated by similar barriers.58 4.3. NIR-Induced Rotamerization around the C−O Bond. A NIR spectrum of 5MOI isolated in solid xenon at 16 K is shown in Figure 4a. For wavenumbers above 6500 cm−1, only one band is dominating, near 6853 cm−1. This wavenumber corresponds to approximately twice the position of the fundamental νNH stretching absorption (3496 cm−1; see Figure 4b), and the two bands exhibit similar profiles. This fact strongly suggests that the band appearing in the NIR region should be ascribed to the first overtone transition of the NH stretching vibration (2νNH). Usually, for matrix-isolated molecules, the observed and calculated anharmonic frequencies should be in good agreement.59 And rightly so, our anharmonic calculations carried out for the two 5MOI conformers further support this assignment. The frequency of the 2νNH overtone is predicted to occur at wavenumbers close to the position of the observed absorption: 6887.4 cm−1 for anti and 6890.1 cm−1 for syn; see Figure 4b and Table S4. The above values correspond to experimental anharmonicity of the NH stretching vibration in 5MOI equal to −70 cm−1 and computed anharmonicity equal to −71.2 cm−1, for both 5MOI conformers. This finding agrees with the general trend that anharmonicity of the XH stretching vibrations (X = O; N; C; S) decreases from OH to NH to CH to SH.60 Indeed, for OH stretching vibrations, Havey and Vaida reported larger anharmonicity, near −85 cm−1.61 Significant differences exist also in the intensity of the X−H stretching vibrational transitions of different heavy atoms X. Intensities of typical O−H, C−H, and S−H oscillators decrease from OH to CH to

Figure 4. Fragments of (a) near-IR and (b) mid-IR experimental spectra of 5MOI isolated in a xenon matrix at 16 K, showing the absorption bands assigned to the (a) 2νNH overtone transition and (b) νNH fundamental transition (all data shown in black; ordinates on the left). The color sticks represent wavenumbers and intensities calculated for the (a) 2νNH overtone and (b) νNH fundamental transitions of the anti (closed red circles) and syn (open blue circles) conformers obtained in anharmonic vibrational calculations at the B3LYP/6-31++G(d,p) level (ordinates on the right).

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


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The Journal of Physical Chemistry A νNH and 2νNH transitions vary a lot.62 For gaseous pyrrole, the intensity drop of about 8 from νNH to 2νNH60 is similar to that reported for a relative change in intensities of νOH and 2νOH bands, which drop by a factor of about 10 in the gas phase.5 In this respect, it appears interesting to us to report that for matrix-isolated 5MOI the experimental integrated intensities of νNH and 2νNH differ by a factor of 34 (see Figure 4). This is similar to the intensity drop from νNH to 2νNH in 6MOI (near 35),21,22 as well to the intensity drop from νOH to 2νOH in furoic acid63 (near 35) and in thiazole-carboxylic acid64 (near 38), all these compounds studied in the same matrix isolation experimental setup.65 On the basis of the experimental NIR spectrum of matrixisolated 5MOI, the OPO was tuned at 6853 cm−1 (maximum of the 2νNH absorption band), and monomers of the compound isolated in solid xenon at 16 K were exposed to this monochromatic NIR light for about 30 min (three successive irradiations, 10 min each). In this experiment, the matrix was first deposited at 30 K and then cooled down to 16 K. This means that before the NIR irradiations, the conformational composition of the matrix-isolated compound was largely shifted 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 two fragments of the difference IR spectrum (“after irradiation” minus “before irradiation”) shown in Figure 5a (a difference

induces rotamerization around the C−O bond in 5MOI. In fact, as illustrated in Figure 5, upon NIR irradiation, the bands ascribed to the syn conformer grow up (1350, 1317, 1284, 896, and 855 cm−1), while those assigned to the anti conformer decrease in intensity (1353, 1327, 1286, 900, 843, and 828 cm−1). This experimental finding provides another remarkable example of conformational isomerization in matrix-isolated molecules where the vibrationally excited group (N−H) and the group that undergoes isomerization (O−CH3) are situated at remote ends of the molecule, which are even more distant in 5MOI than in 6MOI.22 This proves the occurrence of longrange intramolecular vibrational energy transfer, wherein the energy absorbed by the excited vibrational coordinate (in this case an N−H stretching vibration) is transferred to a remote fragment (OCH3) that undergoes internal torsion. It should be noted that so far the conformational isomerizations resulting from remote intramolecular vibrational energy redistribution have been reported only for a reduced number of matrixisolated molecules: 5MOI (this work), 6MOI,22 2-thiocytosine,28 and kojic acid.20 In 2-thiocytosine and kojic acid, the molecular fragments changing their orientation are the light H atoms of the OH or SH groups, while in the two methoxyindoles (5MOI and 6MOI), the group undergoing internal rotation is a heavy CH3 fragment of the methoxy group. The fact that the remote intramolecular vibrational energy transfer takes place in both methoxyindole isomers proves that this effect previously observed for 6MOI was not a fortuitous event and that it is independent of the relative position of the methoxy fragment relative to the vibrationally excited 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 was found to increase from ∼9 to ∼17% and stabilize. Noteworthy, no clear-cut new absorption due to the syn form could be simultaneously observed to increase in the NIR region; only the previously existing band centered near 6853 cm−1 slightly changed its shape. In an attempt to verify if the conformational population ratio may be also shifted in the opposite direction, that is, from anti to syn, additional NIR irradiations were carried out by selecting other wavenumbers within the 2νNH band profile. Hence, the OPO was tuned at 6856 and 6847 cm−1, at positions of the higher- and lower-frequency shoulders that slightly increased near the 2νNH band maximum (Figure S5). However, the irradiations performed at both wavenumbers did not result in any significant spectral modifications in the mid-IR region that could be related to the shift of conformational populations either in the anti → syn or in the opposite direction. Hence, when the conformational composition of the sample was dominated by one structure (anti form populated by means of the preceding annealing), the NIR-induced conformational transformation of matrix-isolated 5MOI could be indeed observed. On the other hand, a prolonged NIR excitation within the 2νNH overtone band profile (near 6853 cm−1) most likely results in simultaneous vibrational excitation of both conformers, which is in agreement with the almost coincident wavenumbers of the 2νNH overtones predicted for the two conformers (Figure 4). The fact that the conformational mixture in the matrix before the series of NIR irradiations at 6853 cm−1 is largely dominated by the anti conformer helps to understand why the net result of the NIR-induced conformational changes of 5MOI is a decrease of the population of the anti form and an increase of the population of the syn form.

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

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 the two selected fragments with the spectra calculated for the anti and syn conformers (Figure 5b) reveals in a very clear way that excitation of the matrix-isolated compound at the wavenumber corresponding to the maximum of the 2νNH overtone band G


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

4.4. UV-Induced Conformational Isomerization. The UV irradiations of the matrix-isolated 5MOI were carried out on a sample containing the two 5MOI conformers (the minor syn conformer was populated by preceding NIR irradiations). The origin of electronic transition measured for the anti conformer of 5MOI under jet-cooled conditions was found to be 302 nm.29,66 For the syn conformer, which has not been observed previously, such a transition was estimated to occur at ∼308 nm.29 These values are in excellent agreement with theoretical 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 the ground S0 and in the lowest-energy excited (S1) singlet state (at the [(TD-DFT/B3LYP/6-311++G(d,p)] level for S1), including the ZPVE corrections in both states. Therefore, initially, the OPO was tuned at λ = 320 nm, and further irradiations followed by gradual application of shorter wavelengths. After each irradiation (duration of 1−2 min), a mid-IR spectrum was collected to monitor the UV-induced structural transformations in the isolated molecules. The onset of 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. These changes are illustrated as the difference spectrum in Figure 6a.

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

In an attempt to provide a mechanistic picture of the UVinduced conformational isomerization in 5MOI, we have computed a relaxed potential energy profile around the C11−O10−C5−C6 dihedral in the S1 excited state, which is expected to be the only accessible singlet excited state upon UV irradiation within the 315−308 nm wavelength range (see Table S4). The results of these calculations are represented in Figure 8, together with the relaxed potential energy scan

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

Figure 8. B3LYP/6-311++G(d,p) relaxed potential energy scans for internal 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).

calculated for the electronic ground state (S0). Note that unlike S0, syn is the most stable conformer in S1, and the energy difference of anti relative to syn amounts to 7 kJ mol−1. The fact that the syn → anti conversion occurred in our experiments at longer wavelengths than the anti → syn population shift (for the sample enriched with the anti form) is in agreement with the order of the computed adiabatic excitation energies determined for the two conformers (398.1 and 387.6 kJ mol−1). The computed difference of ∼10.5 kJ mol−1 is in good

Comparing this difference spectrum with the calculated difference spectrum shown in Figure 6b (anti minus syn), it can be inferred that the spectral modifications induced by the UV irradiations at 315 and 310 nm correspond to a syn → anti conversion. A change of population ratio in the opposite direction was observed upon 3 min of subsequent UV H



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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-induced photoreactions. From the results shown in Figure 8, it seems plausible to admit that whatever conformer is being excited from S0 to S1, it is rather difficult to overcome the barrier measuring more than 22 kJ mol−1 in the excited state. Therefore, it is likely that the conformational isomerization occurs preferentially after repopulation of the electronic ground state S0 via internal conversion or fluorescence, where the barrier to conformational isomerization is significantly smaller (4 kJ mol−1) and the relaxing molecules have a large excess of vibrational energy.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.7b01713. Tables S1 and S2, with Cartesian coordinates of the anti and syn conformers of 5MOI optimized at different levels of theory; Tables S3 and S4, with results of harmonic and anharmonic vibrational calculations for the fundamental and overtone transitions of the anti and syn conformers of 5MOI; Table S5, with vertical excitation energies and oscillator strengths calculated at the TD-DFT level for 12 lowest-energy singlet states of the syn and anti conformers of 5MOI; Table S6, with Cartesian coordinates of the anti and syn conformers of 5MOI optimized for the first excited singlet electronic state; Figure S1, with a relaxed potential energy scan computed for 5MOI as a function of the internal torsion of the OCH3 group; Figure S2, showing IR spectra of matrix-isolated 5MOI before and after annealing; Figure S3, showing variation of conformational populations of matrix-isolated 5MOI during annealing; and Figures S4 (extended version of Figure 5) and S5, showing evidence of NIR-induced conformational transformations in matrix-isolated 5MOI (PDF)

5. CONCLUSIONS In this study, besides the first experimental observation and spectroscopic characterization of the higher-energy syn conformer of 5MOI, occurrence of long-range vibrational energy transfer leading to conformational isomerization in this molecule was demonstrated, and different ways to manipulate the relative populations of its two conformers (anti and syn forms) were described: (i) Annealing of the xenon matrix deposited at 16 K to temperatures of about 30−40 K led to conversion of the higher-energy syn form into the most stable anti conformer, a result that confirmed the relative order of energies of the two conformers theoretically predicted. (ii) Upon NIR irradiation of the matrix-isolated compound at the frequency of the NH stretching overtone band (6853 cm−1), partial conversion of the most abundant anti conformer into the syn form, by internal rotation about the remotely located C−O bond, was observed. The final population ratio of the two conformers upon 6853 cm−1 irradiation revealed that a photostationary state was attained, which is in agreement with the almost coincident frequencies of the 2νNH overtones predicted for the two conformers. The NIR-induced observed conformational isomerization of 5MOI is one of the very few examples reported hitherto where the conformational change results from intramolecular vibrational energy redistribution upon excitation of a vibration of a remotely located bond. (iii) Conformational interconversion between the two conformers was also achieved by using electronic excitation in the UV domain. Excitation in the 315−310 nm range led to total consumption of the syn form, in favor of anti, while irradiation at 308 nm was shown to promote the inverse process. A mechanistic explanation of the observed transformations, based on computation of the ground and first excited states’ (S0, S1) potential energy surfaces along the conformational isomerization coordinate, has been given. Specifically, taking into account the considerably large energy barrier (>22 kJ mol−1) for conformational isomerization in the excited state, it is proposed that the UV-induced conformational isomerizations should occur preferentially after repopulation of the electronic ground state S0, where the barrier to conformational isomerization is only ∼4 kJ mol−1, low enough to be surmounted by the relaxing molecules having a large excess of vibrational energy.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

I. Reva: 0000-0001-5983-7743 Notes

The authors declare no competing financial interest.

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

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REFERENCES

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