Conformational Changes in Thiazole-2-carboxylic Acid Selectively

Article pubs.acs.org/JPCA

Conformational Changes in Thiazole-2-carboxylic Acid Selectively Induced by Excitation with Narrowband Near-IR and UV Light Anna Halasa,† Igor Reva,*,‡ Leszek Lapinski,† Maciej J. Nowak,† and Rui Fausto‡ †

Institute of Physics, Polish Academy of Sciences, Al. Lotnikow 32/46, 02-668 Warsaw, Poland CQC, Department of Chemistry, University of Coimbra, 3004-535 Coimbra, Portugal

‡

S Supporting Information *

ABSTRACT: Conformers and photoinduced conformational transformations were studied for monomers of thiazole-2-carboxylic acid (TCA). The matrix-isolation technique and excitations with narrowband near-IR and UV light, tuned in an optical parametric oscillator, were used for this purpose. Form I, with the carboxylic moiety in the trans orientation and with the hydrogen atom of the OH group directed toward the nitrogen atom of the ring, was the most abundant in low-temperature argon or nitrogen matrixes. Conformer II, differing from I by 180° rotation of the OH group around the C−O bond, was also trapped in the matrixes, but in much smaller amount. The abundance of form II was experimentally determined as ∼6% of the total amount of TCA molecules. Selective excitation of I with narrowband near-IR laser light resulted in I → II transformation. This near-IR-induced conformational change was photoreversible: form II converted back to I upon selective excitation of II with near-IR light of different wavelength. Conformational conversions of I into II, or vice versa, were also induced in TCA monomers by narrowband UV excitations at 300 nm (for I → II transformation) and at 305 nm (for II → I transformation). A spontaneous conversion of photogenerated II into the most stable form I was observed for the compound trapped in the matrix at 15 K and kept in the dark. This process was very slow; the estimated half-life time of conformer II was longer than 50 h. Finally, TCA was shown to thermally decompose at room temperature, yielding CO2 and thiazole.

1. INTRODUCTION In small carboxylic acids, such as formic and acetic acids,1−4 the conformers having the cis orientation of the OC−O−H moiety are more stable than the trans forms (see Chart 1). The reasons for the greater stability of the cis conformers have been addressed in detail elsewhere.5−9 In compounds where the carboxylic fragment is attached at the α-position to a heterocyclic ring (Chart 1), the interaction between the lone electron pair(s) of the ring heteroatom and the OH group can lower the energy of the conformers with trans orientation of the carboxylic fragment. A clear example of such stabilization was observed in the experimental studies on pyridine-2-carboxylic acid (2-picolinic acid).10 For this compound, the conformer with the carboxylic fragment in trans orientation dominates very strongly in the gas phase and in low-temperature matrixes.10−12 On the other hand, the experimental study on furan-2-carboxylic acid (2-furoic acid)12 demonstrated that the trans conformers are not necessarily the most stable in all compounds where the carboxylic group is attached at the α-position to a heterocyclic ring. In furan2-carboxylic acid, the distance between the ring heteroatom and the carboxylic OH group (enforced by a five-membered furan ring) is longer than in the case of pyridine-2-carboxylic acid (where the ring is six-membered). Moreover, the interaction of the OH group with O-heteroatom is weaker than the analogous interaction with N-heteroatom. As a consequence of these two factors, the trans conformer of furan-2-carboxylic acid is less stable than the conformers with cis orientation of the OC− © XXXX American Chemical Society

O−H fragment and the structures of the compound with the cis conformation of the carboxylic group were nearly exclusively populated in low-temperature matrixes trapped from the gas phase.12 So far, pyridine-2-carboxylic acid and furan-2-carboxylic acid are the only compounds with an OC−O−H group attached at the α-position to a heterocyclic ring, for which the conformational equilibria have been investigated. In the present study, the conformational space of monomeric thiazole-2-carboxylic acid (TCA) has been investigated using the matrix isolation method. The heterocyclic ring of thiazole-2-carboxylic acid is fivemembered, hence the interaction between the ring nitrogen atom and the OH group should be weaker than in pyridine-2carboxylic acid, where the ring is six-membered. In comparison with furan-2-carboxylic acid, the N···H−O interaction stabilizing the trans conformer of TCA should be stronger, because nitrogen atoms usually form stronger hydrogen-bond-like bridges with O−H groups than oxygen atoms. Taking these two factors into account, one could expect comparable stabilities of TCA forms with the carboxylic moiety in cis and trans conformations (Chart 1). The experimental verification of this expectation was one of the motives that prompted us to conduct the current study. Received: November 27, 2015 Revised: March 15, 2016

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DOI: 10.1021/acs.jpca.5b11615 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry A

together with a large excess of argon or nitrogen (purity 6.0 for both gases), onto a CsI window cooled to 15 K. An APD Cryogenics closed-cycle helium refrigeration system with a DE202A expander was used in the matrix-isolation experiments. The IR absorption spectra were recorded in the 4000−400 cm−1 range, with a resolution of 0.5 cm−1, using a Thermo Nicolet 6700 FTIR spectrometer equipped with a KBr beam splitter and a DTGS detector. Matrixes were irradiated with narrowband, frequency-tuned near-IR light of the idler beam of the pulsed optical parametric oscillator Quanta-Ray MOPO-SL (fwhm ∼0.2 cm−1, repetition rate 10 Hz, pulse energy ∼6 mJ), pumped with a pulsed (10 ns) Nd:YAG laser. The frequencydoubled signal beam (pulse energy ∼1.5 mJ, repetition rate 10 Hz) of the same optical parametric oscillator (OPO) was used to irradiate the matrixes with UV light. In some experiments, to avoid excitation of matrix-isolated molecules with light of wavenumbers higher than 2100 cm−1, the mid-IR spectra were recorded only in the 2100−400 cm−1 range, with a standard Edmund Optics long-pass filter placed between the spectrometer sources and the cryostat. Thiazole (98% purity, supplied by TCI Europe) was isolated in cryogenic matrixes in order to confirm the nature of the TCA decomposition product. The mixtures of thiazole and argon or nitrogen (1:1000 molar ratio or more diluted) were prepared by the standard manometric method. These mixtures were deposited onto the CsI window kept at 15 K and the obtained matrixes were spectroscopically characterized as described above. The relative energies of the isomers of TCA were calculated at the DFT(B3LYP),15−17 MP2,18 and QCISD19 levels of theory. The infrared spectra of these forms were calculated within the harmonic approximation and using an anharmonic approach20−23 (Tables S1−S10 in the Supporting Information). The details of these calculations are analogous to those described in ref 12.

Chart 1. Structures of Cis and Trans Conformers of Simple Acids and Compounds with Carboxylic Group Attached in α-Position to a Heterocyclic Ringa

a

The name cis or trans of the more stable form is bold and underlined.

3. RESULTS AND DISCUSSION 3.1. Relative Energies of Thiazole-2-carboxylic Acid Conformers. The TCA conformers (shown in Table 1) differ from each other by the rotation of the whole carboxylic group with respect to the thiazole ring and by the rotation of the OH group around the C−O bond within the acid moiety. Structures of conformers I−IV were optimized using a variety of methods and basis sets (Table 1 and Table S11 in the Supporting Information). At all the applied levels of theory, the structure of form I (trans) is planar with the NC−CO and OC−O−H dihedral angles both equal to 180°. According to the calculations, form I should be the most stable conformer of TCA. The structure of conformer II (cis) differs from I by 180° rotation of the hydrogen atom around the C−O bond in the COOH group. The computed relative energies of conformers I − IV of TCA are collected in Table 1 and Table S11 in the Supporting Information. The calculations carried out using the MP2 and QCISD methods and the large 6-311++G(3df,3pd) basis set resulted in prediction of the ZPE corrected energy difference between conformers II and I equal to 7.9 and 7.5 kJ mol−1, respectively (Table 1). As it will be demonstrated in section 3.3.1, these predictions are in a good agreement with the experimental estimations. When the smaller 6-311++G(d,p) basis set was used with the MP2 and QCISD methods, the predicted energy gap between forms II and I (6.3 and 5.8 kJ mol−1, respectively) reproduced less accurately the experimental assessment. The number of polarization and diffuse functions in the basis set was

Other objectives of the present work are related with possibility of light-induced conformational transformations in matrix-isolated TCA. For this purpose, TCA monomers trapped in low-temperature argon or nitrogen matrixes were selectively excited with narrowband near-infrared light. Using this method, the structure of TCA conformers was manipulated in a reversible way. Effects of irradiation of the matrixes with narrowband tunable UV light were also examined. Finally, the spontaneous conversion of a higher-energy conformer of TCA into its most stable form has been monitored for 25 h. This spontaneous conformational transformation (at cryogenic temperature 15 K) must occur by hydrogen-atom tunneling.

2. EXPERIMENTAL AND THEORETICAL METHODS The sample of thiazole-2-carboxylic acid (98%) used in the present study was a commercial product supplied by Sigma. At room temperature, the compound forms colorless crystals whose structure was solved by Rossin et al.13 The main geometrical feature of the crystal is the hydrogen-bonding between the OH moiety of the carboxylic group and the nitrogen atom of the adjacent thiazole ring in the lattice, so that the COOH group adopts the cis orientation in all TCA molecules of the crystal.14 In order to prepare low-temperature argon or nitrogen matrixes, a sample of solid TCA was placed in a miniature glass oven located in the vacuum chamber of a helium-cooled cryostat. During deposition of the matrixes, the oven was either kept at room temperature (295 K) or was slightly heated (up to 305 K). Vapors of TCA coming out of the oven were deposited, B


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mol−1, should be much lower and constitute, at room temperature, approximately 5% of the total TCA population. The population of form III is expected to be such low (near 1%) that its experimental detection should be difficult, and the population of IV should be negligible. The barrier height for the II → I conversion was predicted by the MP2/6-311++G(3df,3pd) calculations to be approximately 46 kJ mol−1. Such a high barrier should prevent quick transformation of II into I under matrix conditions. Therefore, after trapping TCA molecules from the gas phase into lowtemperature matrixes, both I and II forms are expected to be observed with stationary spectroscopic methods.28 3.2. Conformers of Thiazole-2-carboxylic Acid Trapped in Low-Temperature Matrixes. Monomers of TCA were deposited, together with a large (>1000-fold) excess of inert gas (argon or nitrogen), onto a cryogenic (15 K) CsI substrate. The mid-IR spectra of TCA isolated in argon and nitrogen matrixes are presented in Figure 1a,b, Figure 2a,b and Figure S1 in the

Table 1. Calculated Relative Energies of Thiazole-2carboxylic Acid Conformers I−IVa,b,c

a

Electronic energy Eel, zero-point corrected energy Etotal (Etotal = Eel + ZPE), and Gibbs free energy ΔG0 in kJ mol−1. bAn extended version of this table, including the absolute calculated energies of conformer I, as well as results of B3LYP calculations with different basis sets, is provided in the Supporting Information. cKey (c−e): ΔZPE taken from the MP2/6-311++G(d,p) calculations. dFor conformer IV, a single-point QCISD/6-311++G(3df,3pd) energy calculation was carried out for the geometry optimized at the MP2/6-311+ +G(3df,3pd) level. For other conformers, QCISD electronic energies were calculated at the fully optimized QCISD geometries. eΔZPE taken from the MP2/6-311++G(3df,3pd) calculations. The zero-point energies and Gibbs free energies (for the room temperature) were calculated using unscaled vibrational frequencies.

earlier shown to be very important for precise prediction of bond lengths and angles in molecules where hypervalent sulfur atoms are involved.24−27 In the present work, it was found that also for molecules with divalent sulfur (such as TCA and thiazole) a considerable number of polarization and diffuse functions in the basis set is important for a better prediction of relative energies and vibrational spectra. In the structure of conformer III, the carboxylic group adopts the cis configuration and is rotated as a whole with respect to the thiazole ring so that the OH group and the S atom are located at the same side. Theoretical calculations, carried out with MP2 and QCISD methods and the large 6-311++G(3df,3pd) basis set, predict the ZPE corrected energy of form III to be higher by 11 kJ mol−1 than the energy of the most stable form I. For conformer IV, with a trans carboxylic moiety and the OH group pointing to the ring sulfur atom (see structure in Table 1), the computations predict a very high relative energy. The very large energy difference (ca. 40 kJ mol−1) between forms IV and I shows how much weaker is the interaction of the OH group with the S atom of the five-membered ring in comparison to the analogous interaction with the N atom. According to the theoretically calculated relative Gibbs energies ΔG0, a very high relative population of form I (90% or more) should be expected for the gaseous TCA. The population of II, with predicted relative ΔG0 equal to ca. 7 kJ

Figure 1. Infrared spectra of thiazole-2-carboxylic acid isolated in (a) Ar matrix and (b) N2 matrix, compared with (c) the theoretical spectra of conformer I (blue) and conformer II (magenta), calculated within the harmonic approximation at the B3LYP/6-311++G(3df,3pd) level. The calculated wavenumbers were scaled by a single factor of 0.95. The calculated infrared intensities were scaled by factors of 0.9 and 0.1 for forms I and II, respectively.

Supporting Information. Comparison of the mid-IR spectra with the spectra calculated for I and II (Figure 1c and Figure 2c) shows that form I dominates in the matrixes. Indeed, nearly all of the observed experimental mid-IR absorption bands are well reproduced by the spectrum predicted for I. In addition, the spectral indication of the presence of a small amount of II in the matrixes is also clearly visible, in accord with the theoretical predictions of the relative energy of the two conformers. The observation of the low-intensity bands due to II, alongside the intense bands due to I, is also in agreement with the remark made on this subject in the earlier work of Maier et al.29 C


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Figure 2. Infrared spectra of thiazole-2-carboxylic acid isolated (a) in an Ar matrix and (b) in an N2 matrix, compared with (c) the theoretical spectra of conformer I (blue) and conformer II (magenta), calculated within the harmonic approximation at the B3LYP/6-311++G(3df,3pd) level. The calculated wavenumbers were scaled by a single factor of 0.98, and the calculated infrared intensities were scaled by factors of 0.9 and 0.1 for forms I and II, respectively. Asterisks indicate the bands due to water impurity; letters “T” indicate the bands due to thiazole (for details see text and Figure 10).

In the spectrum of I trapped in solid argon, a structured band due to the νOH vibration appears at 3449 cm−1 (Figure 1a, Table S9 in the Supporting Information). For a νOH band, this is quite a low wavenumber. The position of the νOH band in the spectrum of I is determined by the strength of the interaction between the carboxylic OH group and the N atom of the five-membered thiazole ring. In the spectrum of the trans isomer of furan-2carboxylic acid,12 where the interaction of the carboxylic OH group with the O atom of the five-membered furan ring is weak, the νOH band was observed at a much higher wavenumber 3572 cm−1 (Ar). On the other hand, in the spectrum of the trans isomer of pyridine-2-carboxylic acid, where the interaction of the OH group with the N atom of the six-membered pyridine ring is quite strong,10,12 a broad νOH band was observed at a wavenumber as low as 3340 cm−1 (Ar). This comparison shows that in the currently considered form I of TCA, the hydrogen-bond-like O−H···N interaction is of intermediate strength. It is stronger than the O−H···O interaction in furan-2carboxylic acid, but it is significantly weaker than the O−H···N interaction in pyridine-2-carboxylic acid (see the structures of the trans isomers of these compounds shown in Chart 1). These observations confirm our expectations discussed in the Introduction. Alongside the νOH band of TCA conformer I (at 3449 cm−1; Ar), a less intense νOH band was observed at 3559 cm−1 (Ar), see Figure 1a and Table S10 in the Supporting Information. The latter band is the spectral signature of the second most stable conformer II trapped in a low-temperature matrix. The wavenumber 3559 cm−1 (Ar) is typical of the νOH bands due to OH groups in carboxylic OC−O−H moieties in the cis configuration. In the spectra of the cis isomers of oxamic,30 pyruvic,31 cyanoacetic,32 glycolic,33,34 and acetic3 acids isolated in Ar matrixes, the νOH bands were found at very close wavenumbers, namely at 3555, 3556, 3560, 3561, and 3563 cm−1, respectively. Analogous pattern of absorption bands was observed in this range of the spectra of TCA isolated in N2

matrixes (see Figure 1b and Tables S9 and S10 in the Supporting Information). Another clear indication of a small population of form II in the matrix samples was observed in the 1850−1700 cm−1 spectral range, where the bands due to the stretching vibration of the C O group (νCO) are expected. In this spectral region (Figure 2a,b), a very strong band appears at 1786 cm−1 (Ar), whereas a significantly less intense band is observed at 1747 cm−1 (Ar). Following the theoretical predictions of the mid-IR spectra of the TCA conformers (Figure 2c), the experimental bands at 1786 and 1747 cm−1 should be attributed to the νCO vibrations of forms I and II, respectively. The presence of conformer II in the freshly deposited matrixes was further confirmed by the near-IR-induced transformation of conformer I into II described in section 3.3. Upon this conformational transformation, intensities of the bands due to conformer I decreased. Concomitantly, intensities of a set of small bands (including those at 3559 and 1747 cm−1, Ar), present already in the initial spectrum, grew several times. These growing bands match well the spectrum predicted for conformer II and provide an unquestionable proof of the existence of II in the samples prior to any irradiation. No clear evidence of the presence of conformer III was found in the spectra recorded directly after deposition of Ar or N2 matrixes. According to the theoretical predictions, no population of the very-high-energy conformer IV should be expected. No mid-IR bands that would suggest the presence of any small amount of IV in the matrixes were observed. 3.3. Light-Induced Selective Conformational Changes in Matrix-Isolated Monomers of Thiazole-2-carboxylic Acid. 3.3.1. Near-IR-Induced Conformational Transformations. For TCA isolated in Ar and N2 matrixes, the near-IR and mid-IR spectra were recorded prior to any irradiation. In the near-IR range (Figures 3a and 4a), low-intensity bands due to 2νOH overtone vibrations were observed. In the spectrum of the compound isolated in solid Ar, a single, relatively weak absorption was found at 6670 cm−1 (Figure 3a). The spectral D


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Figure 3. Fragments of the near-IR spectra of thiazole-2-carboxylic acid isolated in an Ar matrix: (a) recorded before any irradiation and (b) recorded after 30 min of irradiation at 6670 cm−1, compared with (c) the theoretical wavenumbers and infrared intensities of the 2νOH modes of conformer I (black) and conformer II (red), obtained in an anharmonic calculation at the B3LYP/6-311++G(3df,3pd) level (not scaled). Fine structure of the band due to the photogenerated product is shown in the circle.

Figure 4. Fragments of the near-IR and mid-IR spectra of thiazole-2carboxylic acid isolated in an N2 matrix. Lower panel: fragments of the near-IR spectra, (a, black) recorded before any irradiation, (b, black) recorded after all of the irradiations: no. 1 at 6723 cm−1, no. 2 at 6697 cm−1 and no. 3 at 6739 cm−1. Upper panel: (c, black) fragment of the mid-IR spectrum recorded before any irradiation; (blue) difference spectrum obtained by subtraction of the spectrum recorded before any irradiation from that recorded after irradiation no. 1 at 6723 cm−1; (green) difference spectrum obtained by subtraction of the spectrum recorded after irradiation at 6723 cm−1 from that recorded after subsequent irradiation no. 2 at 6697 cm−1; (red) difference spectrum obtained by subtraction of the spectrum recorded after irradiation at 6697 cm−1 from that recorded after subsequent irradiation no. 3 at 6739 cm−1. Blue, green and red arrows graphically present the wavenumbers at which irradiations no. 1, no. 2, and no. 3, respectively, were carried out. Note difference in the ordinate scales.

position of this band is in a close agreement with the theoretical frequency 6717 cm−1 (Figure 3c, Table S5 in the Supporting Information), predicted for the 2νOH transition in conformer I, by the anharmonic calculation carried out at the B3LYP/6-311+ +G(3df,3pd) level. The matrixes were irradiated with monochromatic light tuned to the wavenumbers at which the 2νOH near-IR absorption(s) due to the most abundant conformer I appear. Upon irradiation of an Ar matrix at 6670 cm−1, the intensities of the IR bands due to the initially most populated form I decreased, whereas the intensities of a set of the initially weak bands (including those at 3559 cm−1 and at 1747 cm−1 ) increased substantially. Comparison of the spectra decreasing and increasing upon irradiation at 6670 cm−1 with the spectra theoretically calculated for conformers I and II is shown in Figure 5a,c. The good agreement between the experimental and theoretical spectra (in the full mid-IR range) allows a reliable interpretation of the observed phototransformation in terms of a selective I → II conformational conversion (Scheme 1). For TCA isolated in solid nitrogen, the 2νOH absorption of conformer I appears, in the spectrum recorded directly after deposition of the matrix, as a triplet with maxima at 6697, 6723 and at 6739 cm−1 (Figure 4a). The observation of three components of this band suggests that TCA is trapped in three spectrally distinguishable sites of the nitrogen matrix. Also the band due to the fundamental νOH vibration of conformer I is split in three components, appearing at 3473, 3467, and 3456 cm−1 (Figure 4c). A sequence of near-IR irradiations (at 6723, 6697 and at 6739 cm−1) demonstrated that, for the compound in solid nitrogen environment, the conformational conversion occurs mostly in a site-selective way (Figure 4). Site-selective conformational transformations were observed earlier for alanine35 and squaric acid36 isolated in nitrogen matrixes. Of the three applied irradiations of TCA in solid nitrogen, that at 6723 cm−1 induced most pronounced conformational conversion. Similarly to the effect of near-IR irradiation on TCA isolated in solid argon, also for the compound isolated in solid nitrogen, the mid-IR

experimental spectrum growing upon irradiation at 6723 cm−1 and the spectrum decreasing in intensity are well reproduced by the spectra theoretically predicted for conformers II and I, respectively (Figure 5b,c). This provides a further confirmation that the observed phototransformation is indeed a conversion of form I into conformer II. The agreement between the theoretical and the experimental spectra of I and II allowed a reliable assignment of the observed IR bands to the theoretically predicted normal modes. This assignment is presented in Tables S9 and S10 in the Supporting Information. The bands of II, growing upon selective near-IR excitation of I, were observed also in the near-IR range of the spectrum. The relatively strong absorptions appeared as split bands at 6954/ 6951 cm−1 (Ar, Figure 3b) and at 6922/6914 cm−1 (N2, Figure 4b) in the spectra of the photogenerated conformer. The spectral positions of these bands are in a close agreement with the anharmonic frequency 6963 cm−1 predicted [at the B3LYP/6311++G(3df,3pd) level] for the 2νOH transition in conformer II (Figure 3c; Table S5 in the Supporting Information). Subsequent irradiation of Ar matrixes, containing significant populations of photogenerated II, with monochromatic light at 6954 or 6951 cm−1 (Figure 6) led to the II → I transformation (Scheme 1). Analogous II → I transformation occurred, for TCA isolated in N2 matrixes, upon irradiations at 6922 cm−1 or at 6914 E


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Figure 6. Near-IR-induced transformations of I into II (left panel) and of II into I (right panel): (a) spectrum recorded after deposition of thiazole-2-carboxylic acid isolated in an Ar matrix; (b) spectrum recorded after irradiation at 6670 cm−1; (c) spectrum recorded after subsequent irradiations at 6951 and 6954 cm−1; (d) difference spectrum, trace b minus trace a; (e) difference spectrum, trace c minus trace b.

Figure 5. Identification of conformer II generated by near-IR excitation of TCA isolated in Ar and N2 matrixes at 15 K: (a) mid-IR difference spectrum obtained by subtraction of the spectrum of an Ar matrix recorded before any irradiation from that recorded after 30 min of nearIR irradiation at 6670 cm−1; (b) mid-IR difference spectrum obtained by subtraction of the spectrum of an N2 matrix recorded before any irradiation from that recorded after 1 h of irradiation at 6723 cm−1; (c) harmonic vibrational spectra calculated at the B3LYP/6-311++G(3df,3pd) level for conformers I (shown as negative bars) and II (shown as positive bars). The theoretical wavenumbers were scaled by a single factor of 0.98.

matrix was estimated as n(II)/n(I) = 0.06 ± 0.01. Should this ratio correspond to the equilibrium ratio of the two conformers at room temperature prior to matrix deposition, then the difference in their Gibbs free energies should be 7 ± 0.5 kJ mol−1. This experimental estimation is in a close correspondence with the predicted ΔG0 obtained in the calculations using the 6-311+ +G(3df,3pd) basis set, as it was pointed out in section 3.1. The very nice agreement, though the obtained high accuracy may be somewhat fortuitous, shows that the number of polarization and diffuse functions in the basis set is very important for prediction of relative energies of conformers. The calculations carried out with the smaller 6-311++G(d,p) basis yielded underestimated Gibbs free energy differences between conformers II and I (see Table 1 and Table S11 in the Supporting Information). 3.3.2. Effects of UV Irradiation on Matrix-Isolated Thiazole2-carboxylic Acid. In dedicated experiments, TCA monomers isolated in Ar or N2 matrixes were subjected to a series of consecutive irradiations with monochromatic UV light (from the 420−300 nm range). The first irradiation was performed at 420 nm, and in all subsequent irradiations the wavelength of the UV light was systematically reduced. After each irradiation the matrix was monitored by recording its mid-IR spectrum. No changes in matrix-isolated TCA molecules were induced by irradiations at λ ≥ 310 nm. For TCA isolated both in Ar and in N2 matrixes, irradiation at λ = 300 nm resulted in clearly observable changes in populations of I and II. Upon irradiation at this wavelength, the population of the initially most abundant conformer I decreased, whereas the population of II concomitantly grew. The spectral changes induced by UV excitation at 300 nm were just the same as those observed after near-IR excitations of the matrixes at 6670 cm−1 (Ar) or 6723 cm−1 (N2) (Figure 7). Hence, the photoisomerization induced by UV (λ = 300 nm) excitation should be interpreted in terms of a conformational change involving I and II forms of TCA. Similar photoisomerization, occurring in TCA upon UV (λ = 254 nm) irradiation, was reported by Maier et al.29 Matrixes with the population of form II substantially enriched, as a result of the previous UV (λ = 300 nm) irradiation (Figure 8,

Scheme 1. Near-IR Induced Conformational Transformations in Matrix-Isolated Thiazole-2-carboxylic Acida

The wavenumbers 6670 and 6954 cm−1 refer to excitations of the compound isolated in an Ar matrix.

a

cm−1. The photoreversibility of the I ↔ II conformational transformation allowed us to manipulate the relative populations of the two conformers by specific near-IR excitation of either form I or II. The quantitative character of the near-IR-induced I → II conversion, with the same amount of I consumed and II produced, was the basis for estimation of the ratio of the conformers in matrixes prior to any irradiation: (AI0 − AIirr ) AII0 n(II) = n(I) (AIIirr − AII0 ) AI0

(1)

(where A0I and Airr I are integrated intensities of a chosen band due to form I recorded before and after near-IR irradiation; A0II and Airr II are integrated intensities of a chosen band due to form II recorded before and after near-IR irradiation). Using formula 1 the ratio of populations of forms II and I in a freshly deposited F


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Figure 7. Comparison of the effects of UV and near-IR irradiations on thiazole-2-carboxylic acid isolated in argon matrixes (15 K). (a) Mid-IR difference spectrum obtained by subtraction of the spectrum recorded before any irradiation from that recorded after 30 min of near-IR irradiation (ν̃ = 6670 cm−1). Fragment of the spectrum recorded (b) before any irradiation and (c) after near-IR irradiation at 6670 cm−1. (d) Mid-IR difference spectrum obtained by subtraction of the spectrum recorded before any irradiation from that recorded after 30 min of UV irradiation (λ = 300 nm). Fragment of the spectrum recorded (e) before any irradiation and (f) after UV irradiation at 300 nm.

excitation at longer wavelength 305 nm) are in fair agreement with the TDDFT predicted differences in the UV spectra of forms I and II. For TCA molecules isolated in Ar and N2 matrixes and excited with UV (λ = 300 nm or λ = 305 nm) light, conformational I ↔ II conversions were clearly the dominating photoprocess. A more detailed examination of the spectra recorded after irradiation at 300 nm revealed that some other minor UV-induced changes also occurred. Among the low-intensity bands that grew upon 300 nm excitation, and that do not belong to the spectra of either I or II, those observed at 3554, 1776, and 1125−1132 cm−1 (N2) (Figure S2 in the Supporting Information) may indicate generation of a small amount of conformer III. Though, a reliable identification of III as a product of the UV (λ = 300 nm) induced transformations is not possible. Other very weak bands, appearing upon UV irradiations of Ar matrix (Figure S3 in the Supporting Information) and placed at 3412, 1389, 1209, 751/ 748, 716, 666, and 614 cm−1, coincide with the positions of the bands identified by Maier et al. as a complex 2,3-dihydrothiazol2-ylidene···CO2, being the product of photolytic decarboxylation of thiazole-2-carboxylic acid in an Ar matrix.29 3.4. Spontaneous Conversion of the Higher-Energy Conformer II into the Most Stable Form I. An Ar matrix with significant amount of II, generated by near-IR excitation of I at 6670 cm−1, was kept for a period of 25 h in the dark and at 15 K. The progress of the spontaneous II → I conformational transformation was periodically monitored by recording the mid-IR spectra in the range 2100−400 cm−1, with a long-pass filter (transmitting only light with wavenumbers lower than 2100 cm−1) placed between the spectrometer sources and the cryostat. When the spectra were not recorded, the spectrometer beam was completely blocked by a metal plate. The spontaneous II → I transformation was very slow (see Figure 9a). The half-life time of II, which was consumed in this process, was longer than 50 h. By the II → I conformational change only the hydrogen atom of the OH group significantly changes its position in the molecule. Hence, in such a process, involving only a displace-

Figure 8. UV-induced partial transformations of conformer I into II (left panel) and of conformer II into I (right panel): (a) spectrum of TCA isolated in an Ar matrix recorded before any irradiation; (b) spectrum after irradiation at 300 nm; (c) spectrum recorded after subsequent irradiation at 305 nm; (d) difference spectrum, trace b minus trace a; (e) difference spectrum, trace c minus trace b.

left panel), were subsequently exposed to UV light with wavelengths from the 310−301 nm range. Excitation at 305 nm led to partial repopulation of form I at the cost of conformer II (Figure 8, right panel). Hence, not only a I → II shift of conformational populations can be induced by UV (λ = 300 nm) irradiation of matrix-isolated TCA, but also a population shift in the opposite II → I direction can be induced by UV excitation at 305 nm. UV spectra of forms I and II of TCA have been theoretically calculated at the TDDFT level. According to the results of these computations (Table S12 in the Supporting Information), conformer II should absorb at wavelength longer by 7 nm, in comparison to the predicted UV absorption of form I. Preferred directions of the observed phototransformations: I → II (induced by excitation at 300 nm) and II → I (induced by G


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result, upon exposure to unfiltered broadband mid-IR and nearIR light, the population ratio of forms I and II, was observed to shift in favor of conformer I.39 3.5. Thermal Decarboxylation of Thiazole-2-carboxylic Acid. Indications of thermal decomposition of TCA40−43 into thiazole and CO2 were observed in the mid-IR spectra of matrixes obtained by codeposition of Ar (or N2) and vapor of TCA sublimating at 295 or 305 K. The characteristic bands due to the antisymmetric stretching and bending modes of the CO2 molecule were detected at 2355−2330 and 665−660 cm−1 (Figure S1 in the Supporting Information). In addition, a set of much weaker mid-IR bands not originating from TCA was observed in the spectra of the matrixes (Figure 2 and Table 2). The most intense of them appear at 1240, 863/862, and 796 cm−1 (Ar matrixes) and at 1244/1240, 869/865, and 807/805 cm−1 (N2 matrixes). These bands were assigned to thiazole because their spectral positions are the same as those found in the spectra of matrixes obtained by deposition of vapors over a sample of authentic thiazole (Figure 10 a,b, Table 2). These spectral positions agree, within a few wavenumbers, with those of the mid-IR absorptions of thiazole in the gas phase.44−47 Relatively to the intensity of the IR bands of TCA, the bands due to thiazole were more intense in the spectra of matrixes prepared with the compound sublimating at 305 K than in the spectra of matrixes prepared with solid TCA evaporating at room temperature 295 K. The bands of thiazole did not change their intensities under irradiation with near-IR light nor with UV (λ > 300 nm) light, whereas the bands due to the TCA conformers were substantially changing their intensities upon such irradiations (Figure 10 b,c and Figure 11). Using the methods described above, thiazole was positively identified as a product of TCA decarboxylation, which was occurring (under high-vacuum conditions) already at room temperature. Fortunately, the major part of TCA molecules could be evaporated and trapped into low-temperature matrixes

Figure 9. Time evolution of II → I transformation observed for TCA isolated in an Ar matrix at 15 K. The progress of the conversion was monitored by following the decrease of the band at 1747 cm−1 due to form II photogenerated upon excitation of I at 6670 cm−1. Zero on the time scale indicates the moment when irradiation at 6670 cm−1 ceased: (a) decay of conformer II observed with a cutoff filter transmitting only light with wavenumbers lower than 2100 cm−1; (b) decay of conformer II observed without any filter.

ment of one light particle (the hydrogen atom), the tunneling mechanism must play a crucial role. The relaxed barrier for the I ↔ II interconversion was calculated at the MP2/6-311+ +G(3df,3pd) level. The height of the barrier for the II → I conversion was estimated as 47.2 kJ mol−1. Such a high barrier can be crossed at 15 K only by the tunneling mechanism.37,38 In an analogous experiment, but with the matrix exposed all the time to the unfiltered light from the spectrometer IR source, a much faster conformational conversion was observed (Figure 9b). Broadband mid-IR and near-IR light (from the spectrometer IR beam), exciting the thiazole-2-carboxylic acid molecules, should induce both I → II and II → I transformations (see section 3.3.1). In the present study, form II converted more efficiently into I, in comparison to the reverse I → II process. As a

Table 2. Spectral Positions of Infrared Bands Observed in the Spectrum of Thiazole Monomers Compared with Theoretical Harmonic Vibrational Frequencies (ν̃) and Absolute Infrared Intensities (Ath) Calculated at the B3LYP/6-311++G(3df,3pd) Levela mode no.

sym

ν̃ (cm−1)

Ath (km mol−1)

Ar matrixb

N2 matrixb

gasc

ν1 ν2 ν3 ν4 ν5 ν6 ν7 ν8 ν9 ν10 ν11 ν12 ν13 ν14 ν15 ν16 ν17 ν18

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

3131.2 3097.2 3089.3 1488.9 1392.2 1330.3 1239.9 1129.6 1046.8 871.7 850.8 738.5 605.0 899.0 795.1 718.8 609.6 469.0

0.9 0.01 3.3 26.1 20.2 4.1 11.3 5.7 7.0 5.5 48.2 0.06 1.0 0.7 46.4 21.2 16.3 0.01

3144 vw 3092 1484/1483 1386/1382 1325/1323 1240 1126/1124 1044/1043 878 863/862 vw vw vw 796 727/717 604 vw

3139 vw 3101 1484 1383/1382 1323/1321 1244/1240 1124/1123 1045/1042 881/880 869/865 vw vw

3140d

807/805 726/721 607 vw

3093d 1484d 1383.7 1325.8 1240.5 1125.1 1043.6 879.3 866.5 749.3 612e 888.7 797.4 717.6 603.9 467e

Theoretically calculated harmonic frequencies were scaled by factors of 0.96 (above 2000 cm−1) and 0.98 (below 2000 cm−1). bHere, “vw” stands for the modes with intrinsically very weak infrared intensities. They could not be identified in the experimental spectra. cGas phase experimental values from ref 47, unless specified otherwise. dGas phase data from ref 44. eRaman data for liquid from ref 44.

a

H


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trapped molecules with narrowband near-IR or UV light tuned to a chosen wavelength. Approximately 94% of TCA monomers, trapped from the gas phase into low-temperature argon or nitrogen matrixes, adopted structure I where the carboxylic OH group is involved in an interaction with the nitrogen atom of the thiazole ring. Because of the O−H···N hydrogen-bond-like interaction, form I of TCA (with the trans orientation of the carboxylic OC−O−H group) is more stable than TCA conformer II. The energy gap between forms I and II must be not very large, as it is revealed by presence of ∼6% of TCA molecules in form II, trapped in the matrixes from the gas phase. It was demonstrated in the work of Rostkowska et al.48 that the strength of a hydrogen-bond-like interaction closing a fivemembered ring depends very much on intramolecular geometry restrictions determining the mutual positions of the OH group and the interacting heteroatom. The comparison of the molecules of thiazole-2-carboxylic acid and pyridine-2-carboxylic acid provides a good example of such a dependency. Because of the geometry of the five-membered thiazole ring, the (O)H···N distance in TCA is longer than the analogous distance in pyridine-2-carboxylic acid, where the carboxylic group is attached to a six-membered ring (see Figure S4 in the Supporting Information). Consequently, the O−H···N interaction in pyridine-2-carboxylic acid is much stronger and the conformer of this compound with the trans orientation of the carboxylic group is stabilized so much that it is nearly exclusively populated in the gas phase11 and in low-temperature matrixes.10,12 Comparison of the conformational equilibria observed for thiazole-2-carboxylic acid and for furan-2-carboxylic acid12 shows that the stabilization introduced by the O−H···N interaction in the former compound is much more pronounced than the stabilizing effect of the O−H···O interaction in the latter species. Although in both compounds the carboxylic OH group interacts with a heteroatom placed at the α-position of a five-membered heterocyclic ring, the conformer with trans orientation of the OC−O−H group is more stable for TCA, whereas the form with the OC−O−H group in cis orientation is the most stable isomer of furan-2-carboxylic acid. This shows how much the strength of the O−H···N or O−H···O interaction depends on the nature of heteroatom interacting with the OH group. The most stable conformer I of TCA isolated in argon or nitrogen matrixes was selectively excited with narrowband nearIR light. The vibrationally excited form I converted into conformer II with the cis orientation of the OC−O−H fragment. The photoreversibility of this process was demonstrated by observation of the II → I conversion occurring upon selective near-IR excitation of the molecules in form II. We have experimentally observed the II → I conversion occurring for matrix-isolated TCA exposed to broadband light from the spectrometer sources passed through a filter transmitting light with wavenumbers lower than 4100 cm−1. This conversion was significantly faster than in an analogous experiment but with filter transmitting only light with wavenumbers lower than 2100 cm−1 placed between the spectrometer sources and the matrix. It shows that mid-IR excitation of fundamental modes (such as νOH) also promotes the conformational transformations in the investigated compound. In the present work, we have also demonstrated that the I ↔ II conformational changes can be induced in matrix-isolated molecules of TCA by UV (λ = 300 nm or λ = 305 nm) excitations. Finally, a spontaneous conversion of the higher-energy form II into the most stable form I was observed to occur in the dark and

Figure 10. Identification of the products of thiazole-2-carboxylic acid decomposition: (a) experimental spectrum of the authentic sample of thiazole monomers isolated in an argon matrix at 15 K in a separate experiment; (b) mid-IR spectrum of TCA in an Ar matrix recorded after 30 min of near-IR irradiation at 6670 cm−1; (c) difference spectrum obtained by subtraction of the spectrum recorded before any irradiation (shown in Figure 2a) from spectrum shown in trace b. The bands which did not change their intensities upon irradiation at 6670 cm−1 were attributed to the decomposition products (thiazole and CO2) and are marked with red and magenta in trace b.

Figure 11. Comparison of the effects of UV and near-IR irradiations on Ar matrixes containing monomers of TCA and its decomposition product (thiazole). Lower panels: (black) spectrum recorded before any irradiation; (red) spectrum recorded after UV irradiation at 300 nm. Upper panels: (black) spectrum recorded before any irradiation; (red) spectrum recorded after near-IR irradiation at 6670 cm−1. We observe no change of intensities of the bands due to thiazole (marked with T) indicating no change of population of this compound during UV (300 < λ < 305 nm) and near-IR irradiations.

without undergoing decarboxylation. The amount of decarboxylation products (thiazole and CO2) trapped in the matrix was indeed small and did not disturb the studies on TCA in any significant way.

4. CONCLUDING DISCUSSION Conformers of thiazole-2-carboxylic acid were studied using the matrix-isolation method, combined with excitation of the I


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(5) Fausto, R.; Batista de Carvalho, L. A. E.; Teixeira-Dias, J. J. C.; Ramos, M. N. s-cis and s-trans Conformers of Formic, Thioformic and Dithioformic Acids. An Ab Initio Study. J. Chem. Soc., Faraday Trans. 2 1989, 85, 1945−1962. (6) Fausto, R. Bonding in Carbonyl and Thiocarbonyl Compounds: An Ab Initio Charge Density Study of H2C = X and HC(=X)YH (X,Y = O or S). J. Mol. Struct.: THEOCHEM 1994, 315, 123−136. (7) Hirao, H. Theoretical Study of Formic Acid: A New Look at the Origin of the Planar Z Conformation and C−O Rotational Barrier. Chem. Phys. 2008, 344, 213−220. (8) Ratajczyk, T.; Pecul, M.; Sadlej, J. The Nature of the Rotational Barriers in Simple Carbonyl Compounds. Tetrahedron 2004, 60, 179− 185. (9) Sadlej-Sosnowska, N. Energy Barriers to Internal Rotation: Hyperconjugation and Electrostatic Description. J. Phys. Chem. A 2003, 107, 8671−8676. (10) Miyagawa, M.; Akai, N.; Nakata, M. UV-Light Induced Conformational Changes of 2-Pyridinecarboxylic Acid Isolated in Low-Temperature Argon Matrices. J. Mol. Struct. 2015, 1086, 1−7. (11) Peña, I.; Varela, M.; Franco, V. G.; López, J. C.; Cabezas, C.; Alonso, J. L. Picolinic and Isonicotinic Acids: A Fourier Transform Microwave Spectroscopy Study. J. Phys. Chem. A 2014, 118, 11373− 11379. (12) Halasa, A.; Lapinski, L.; Reva, I.; Rostkowska, H.; Fausto, R.; Nowak, M. J. Three Conformers of 2-Furoic Acid: Structure Changes Induced with Near-IR Laser Light. J. Phys. Chem. A 2015, 119, 1037− 1047. (13) Rossin, A.; Di Credico, B.; Giambastiani, G.; Gonsalvi, L.; Peruzzini, M.; Reginato, G. Coordination Chemistry of Thiazole-Based Ligands: New Complexes Generating 3D Hydrogen-Bonded Architectures. Eur. J. Inorg. Chem. 2011, 2011, 539−548. (14) The experimentally observed band of the OH stretching vibration at 2727 cm−1 indicates a strong association of TCA in crystals. See: Pérez-Peña, J.; González-Dávila, M.; Arenas, J. F. Vibrational Spectra of Thiazole-2-Carboxylic Acid and Thiazole-2-Carboxylate Ion. Spectrosc. Lett. 1988, 21, 795−807. (15) Becke, A. D. Density-Functional Exchange-Energy Approximation with Correct Asymptotic Behavior. Phys. Rev. A: At., Mol., Opt. Phys. 1988, 38, 3098−3100. (16) Lee, C. T.; Yang, W. T.; Parr, R. G. Development of the ColleSalvetti Correlation-Energy Formula into a Functional of the ElectronDensity. Phys. Rev. B: Condens. Matter Mater. Phys. 1988, 37, 785−789. (17) Vosko, S. H.; Wilk, L.; Nusair, M. Accurate Spin-Dependent Electron Liquid Correlation Energies for Local Spin Density Calculations: a Critical Analysis. Can. J. Phys. 1980, 58, 1200−1211. (18) Møller, C.; Plesset, M. S. Note on an Approximation Treatment for Many-Electron Systems. Phys. Rev. 1934, 46, 618−622. (19) Pople, J. A.; Head-Gordon, M.; Raghavachari, K. Quadratic Configuration Interaction. A General Technique for Determining Electron Correlation Energies. J. Chem. Phys. 1987, 87, 5968−5975. (20) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; et al. Gaussian 09, Revision D.01; Gaussian, Inc.: Wallingford CT, 2009. (21) Keresztury, G.; Jalsovszky, G. An Alternative Calculation of the Vibrational Potential Energy Distribution. J. Mol. Struct. 1971, 10, 304− 305. (22) Rostkowska, H.; Lapinski, L.; Nowak, M. J. Analysis of the Normal Modes of Molecules with D3h Symmetry Infrared Spectra of Monomeric s-Triazine and Cyanuric Acid. Vib. Spectrosc. 2009, 49, 43−51. (23) Pulay, P.; Fogarasi, G.; Pang, F.; Boggs, J. E. Systematic Ab Initio Gradient Calculation of Molecular Geometries, Force Constants, and Dipole Moment Derivatives. J. Am. Chem. Soc. 1979, 101, 2550−2560. (24) Magnusson, E. The Role of d Functions in Correlated Wave Functions: Main Group Molecules. J. Am. Chem. Soc. 1993, 115, 1051− 1061. (25) Sordo, J. A. On the Important Role Played by Polarization Functions in Calculations Involving Hypervalent Molecules. Chem. Phys. Lett. 2000, 316, 167−170.

at low-temperature (15 K). This process was found to be very slow, with the half-life time of the consumed form II exceeding 50 h.

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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.5b11615. Figure S1, presenting the mid-IR spectra of TCA isolated in Ar and N2 matrixes; Figure S2, showing the IR bands of a photoproduct that appeared after UV (300 nm) irradiation of TCA isolated in an N2 matrix; Figure S3, illustrating the photodecomposition of TCA isolated an Ar matrix upon UV (300 nm) irradiation; Figure S4, showing cis and trans conformers of three acids with heterocyclic rings; Tables S1−S7, with harmonic and anharmonic infrared spectra of the four conformers calculated at different levels of theory; Table S8, collecting internal coordinates used in the normal-mode analysis; Tables S9 and S10, with assignments of the mid-IR bands observed in the spectra of conformers I and II, respectively, to the normal modes calculated for these structures; Table S11, with absolute calculated energies of conformer I and relative calculated energies of conformers II−IV; Table S12, with calculated vertical transition wavelengths and oscillator strengths for the lowest 12 excited singlet and triplet states of TCA conformers I and II; and Scheme S1, calculated bond lengths and partial Mulliken charges of the [2,3-dihydrothiazol-2-ylidene···CO2] complex. (PDF)

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AUTHOR INFORMATION

Corresponding Author

*(I.R.) E-mail: [email protected]. Telephone: +351 967215641. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work has been supported by the National Science Center (Poland) under the grant 2012/04/A/ST2/00100 and by the bilateral project No. 2505 for cooperation between Poland and Portugal. I.R. acknowledges the Portuguese “Fundaçaõ para a Ciência e a Tecnologia” (FCT), for the “Investigador FCT” grant. The research leading to these results has also received support from LASERLAB-EUROPE, Grant Agreement No. 284464, EC’s Seventh Framework Programme.

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REFERENCES

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Conformational Changes in Thiazole-2-carboxylic Acid Selectively

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