Conformational Switching in Pyruvic Acid Isolated in Ar and N2

Article pubs.acs.org/JPCA

Conformational Switching in Pyruvic Acid Isolated in Ar and N2 Matrixes: Spectroscopic Analysis, Anharmonic Simulation, and Tunneling Igor Reva,† Cláudio M. Nunes,† Malgorzata Biczysko,‡,§ and Rui Fausto*,† †

CQC, Department of Chemistry, University of Coimbra, 3004-535 Coimbra, Portugal Scuola Normale Superiore, Piazza dei Cavalieri 7, I-56126 Pisa, Italy § Consiglio Nazionale delle Ricerche, Istituto di Chimica dei Composti OrganoMetallici (ICCOM-CNR), UOS di Pisa, Area della Ricerca CNR, Via G. Moruzzi 1, I-56124 Pisa, Italy ‡

ABSTRACT: Monomers of pyruvic acid (PA) isolated in cryogenic argon and nitrogen matrixes were characterized by mid- and nearinfrared spectroscopy. Interpretation of the experiments was aided by fully anharmonic calculations of the fundamental modes, overtones, and combinations up to two quanta, including their infrared intensities. The initially dominating PA conformer (Tc) has a cis CCOH arrangement and is stabilized by a strong intramolecular H-bond. Selective near-infrared excitation of Tc at the first OH overtone (6630 cm−1 in Ar, 6643 cm−1 in N2) induced a large scale conformational conversion to the higher-energy conformer (Tt) with trans CCOH arrangement. Tt was then converted back to Tc by selective NIR irradiation at the first Tt OH overtone (6940 cm−1 in Ar, 6894 cm−1 in N2). In N2 matrix, the Tt form was stabilized due to interaction between the OH group and the matrix molecules. This stabilization manifested itself in the absence of Tt → Tc relaxation and in a considerable change of the vibrational Tt signature upon going from argon to nitrogen matrix. In argon, the Tt form spontaneously decayed back to Tc in the dark (characteristic lifetime +16 h). In the presence of broad-band near-infrared light, the Tt → Tc relaxation speed considerably increased. The decay mechanisms are discussed. metal ions in condensed media14 indicate the existence of different reaction mechanisms and exit channels, dependent on the environment. Some of these mechanisms involve proton tunnelling.13 The most recent series of works on the atmospherically relevant chemistry of pyruvic acid has been published by Vaida and co-workers.15−17 In particular, they have shown that photochemical reactions of pyruvic acid can be initiated in the atmosphere by the vibrational OH overtone pumping by red sunlight, at energies well below the electronic transitions.18 Therefore, the experimental characterization of the near-infrared spectra in the volatile organic acids is relevant for understanding of their atmospheric chemistry.18 The microwave spectra of gaseous pyruvic acid are wellknown.19−21 The understanding of the main features of the rotational spectrum of pyruvic acid is now fairly comprehensive. Spectroscopic constants for the lowest vibrational states have been determined with precision suitable for astrophysical applications.22 The vibrational spectrum of PA was reported by Hollenstein et al. in 1978.23 However, all these studies

1. INTRODUCTION Pyruvic acid (PA) is well-known to participate in several fundamental metabolic pathways in biological systems, being a key intersection intermediate in both aerobic and anaerobic energy production processes in the cells.1 PA is an important organic acid widely used in the chemical, drug, and agrochemical industries. Industrially, it is used mainly as a starting material in the biosynthesis of pharmaceuticals, such as Ltryptophan, L-tyrosine, and alanine. It is also employed in the production of crop protection agents, polymers, cosmetics, and food additives.2 Pyruvic acid, central to leaf carbon metabolism, is a precursor of many volatile organic compounds that impact air quality and climate.3 PA is one of the four (formic, acetic, pyruvic, and oxalic acids) most abundant organic acids in the gas and aerosol phases, which are believed to make an important contribution to the formation of cloud condensation nuclei in such sources as vegetation emissions and biomassburning.4 The atmospheric reactions and energy disposal in decomposition of PA is an important research topic.5,6 The decomposition of pyruvic acid was studied for thermal reactions in the ground state7,8 as well as its photolysis.9 The photolysis of PA studied in aqueous solutions,10,11 in the gas phase,12 adsorbed on a metal surface in vacuo,13 and in the presence of © XXXX American Chemical Society

Special Issue: Markku Räsänen Festschrift Received: September 22, 2014 Revised: October 17, 2014

A

dx.doi.org/10.1021/jp509578c | J. Phys. Chem. A XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry A

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torsional barrier for the conformational interconversion (usually an OH stretching overtone). Then, in the course of vibrational relaxation to the torsional coordinate, a new conformer may be formed. This approach was pioneered in 1997 by Pettersson, Lundell, Khriachtchev, and Räsänen (PLKR).35 By pumping the OH stretching overtone of matrix-isolated formic acid, they were able to generate another formic acid rotamer. During the past few years the PLKR approach has developed as a very powerful and elegant technique35−43 and was successfully used for generation of high-energy conformers, otherwise not accessible experimentally, for several simple carboxylic acids (e.g., formic,44,45 acetic,36 glycolic,46 oxalic,47 and propionic acids48) and amino acids (e.g., glycine,49 alanine,41,50 β-aminoisobutyric acid42) as well as the trans−trans conformer of hydroxyacetone.38 Using narrow-band NIR radiation, it is possible to excite in situ, in a very selective way, only molecules adopting a particular conformation. Should such irradiation lead to conversion into another structure, then it is possible to totally depopulate a certain conformer in favor of another one. The high selectivity of NIR vibrational excitation of matrix-isolated molecules makes this procedure a very powerful technique in the optical control of the relative populations in conformational mixtures. In the present study of PA, we report an alternative to previous studies procedure for in situ generation of conformer Tt from the matrix-isolated most stable conformer Tc. The conformer-selective near-infrared (NIR) irradiation of the matrix-isolated PA resulted in a very efficient Tc → Tt conversion, on a very large scale. Moreover, it is also shown that subsequent narrow-band selective in situ NIR excitation of the optically generated Tt conformer can also be used to successfully convert this form back to the most stable Tc conformer. This is an illustrative example of a carboxylic acid based molecular system allowing for a successful bidirectional NIR-induced rotamerization. The selective narrow-band NIRinduced transformation of Tt into Tc is also compared with the spontaneous Tt → Tc conversion for the compound kept in the dark or exposed to the broad-band mid-IR radiation of the spectrometer IR source. The observed large scale NIR-induced rotamerizations in PA, combined with fully anharmonic calculations, accounting for both mechanical and electrical anharmonic effects,51 permitted a detailed characterization of the vibrational spectrum of the minor Tt form, including the positions and IR intensities of fundamental, overtone and combination bands.

concern the most stable conformer of PA dominating in the thermal equilibrium at room temperature. The molecule of PA has two conformationally relevant internal degrees of freedom (the rotations about the intercarbonyl CC and CO bonds), which may yield three different conformers of PA in its ground electronic state.24 We shall follow here the nomenclature used for the pyruvic acid conformers by Räsänen et al.25 (Figure 1).

Figure 1. Conformers of pyruvic acid. The uppercase letter (T, trans, = 180° and C, cis, = 0°) refers to the OCCO dihedral angle and the lowercase letter (t, trans, = 180° and c, cis, = 0°) refers to the CCOH dihedral angle.

Conformer Tc, bearing a strongly stabilizing OH···O intramolecular H-bond, corresponds to the lowest-energy form and has been extensively studied in the past both experimentally and theoretically.19−21,23−31 The highest-energy conformer, Ct, with a theoretically estimated relative energy within the range 10−18 kJ mol−1,24,25,28−31 has never been observed experimentally. The existence of a nonidentified second, in terms of energy, conformer Tt (between 5 and 11 kJ mol−1),24,25,28−31 of pyruvic acid in the gas phase had already been suggested.23,26 Indeed, several strongest infrared bands of this second conformer were unequivocally characterized experimentally in 2001 by Reva, Stepanian, Adamowicz, and Fausto (hereafter abbreviated as RSAF),24 using matrix isolation infrared spectroscopy. In the RSAF study, the Tt conformer was populated thermally in the gas phase. Its signature was identified by comparing the infrared spectra obtained by trapping the vapor of the compound initially at different temperatures (296 and 480 K) into a cryogenic (15 K) solid argon matrix with the theoretically predicted infrared spectra for its different conformers. Though the amount of the Tt conformer trapped from the PA vapor at 480 K was about 4 times that resulting from deposition of the PA vapor at 296 K, the relative population of this conformer in the matrix deposited from the vapor at 480 K was still below 10%.24 A further increase of the temperature of the gas led to thermal decomposition of the compound. If conformers of interest are not accessible thermally, they may be generated in matrixes by using UV irradiation.32 However, besides conformational conversion, the UV irradiation may lead to partial decomposition of the studied system.33 Recently, Gerbig and Schreiner (abbreviated as GS) reported on the UV-induced rotamerizations in a series of α-ketocarboxylic acids, including pyruvic acid.34 In the quest of a new conformer, we also tried UV irradiation of PA in this work. Only a limited amount (ca. 15−20% of the initially dominating Tc form) could be converted to the minor Tt conformer by UV irradiations in the 370−260 nm range. We do not further elaborate on this topic here, because our results were similar to those obtained in the GS study.34 Application of tunable narrow-band NIR light sources to promote conformational conversions represents an alternative approach of studying higher-energy conformers, especially in the cases when they are not accessible thermally. The idea consists of pumping a vibrational transition lying above the

2. EXPERIMENTAL METHODS A commercial sample of pyruvic acid (PA, Acros Organics, 98% purity) was used. The sample was placed in a glass tube and then connected to the chamber of the cryostat through a needle valve (NUPRO SS-4BMRG). The sample compartment (glass tube) was kept at 273 K (in melting water ice) to provide the compound with adequate vapor pressure. The valve nozzle was kept at room temperature (298 K) to have the Tc conformer strongly dominate in the thermal equilibrium. Prior to usage, the sample was purified by using the standard freeze−pump−thaw method. To deposit the matrix, the vapor of PA was introduced into the cryostat through the needle valve, together with a large excess of argon (N60) or nitrogen (N60) gas, both supplied by Air Liquide, coming from a separate line. The used solute-to-matrix concentration ratios were kept low enough to ensure the absence of aggregates of PA in the matrixes. A CsI window, kept at 15 K during B



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4. RESULTS AND DISCUSSION 4.1. Potential Energy Surface and Matrix Isolation of Pyruvic Acid. The ground-state potential energy landscape of pyruvic acid has been investigated previously at different levels of theory.24,25,28−30 As found for other molecules bearing a methyl group adjacent to a carbonyl,66−68 in all minimum energy structures of PA the methyl group assumes a conformation where one of the hydrogen atoms is synperiplanar with respect to the carbonyl oxygen. This reduces the conformationally relevant degrees of freedom of the molecule to only the internal rotations about the intercarbonyl C−C and C−O bonds. A contour map representing the potential energy surface (PES) of PA as a function of these two coordinates is presented in Figure 2.

deposition and during the IR measurements, was used as the optical substrate for the matrixes. Its temperature was measured by a silicon diode sensor connected to a digital controller (Scientific Instruments, model 9650-1), which provides the stabilization accuracy of 0.1 K. In all experiments, an APD Cryogenics closed cycle helium refrigeration system with a DE202A expander was used. The mid-IR spectra were collected, with a resolution of 0.5 cm−1, using a Thermo Nicolet 6700 Fourier transform infrared spectrometer, equipped with a deuterated triglycine sulfate (DTGS) detector and a KBr beam splitter. The NIR spectra were recorded, with a resolution of 1 cm−1, using the same spectrometer and a mercury cadmium telluride (MCT-B) detector with a CaF2 beam splitter. To avoid interference from atmospheric H2O and CO2, a continuous flux of dry air was used to purge the system. To protect matrixes from light with wavenumbers above ∼4200 cm−1 or above ∼2200 cm−1, standard Edmund Optics long-pass filters were used (transmission cutoff values of 2.40 or 4.50 μm, respectively). The matrixes were irradiated, through an outer quartz window of the cryostat, using tunable narrow-band light (fwhm 0.2 cm−1) provided by the idler beam of a Spectra Physics MOPO-SL optical parametric oscillator pumped by a pulsed (pulse energy 10 mJ, duration 10 ns, repetition rate 10 Hz) Quanta Ray Pro-Series Nd:YAG laser.

3. COMPUTATIONAL METHODS All calculations were performed with the Gaussian 09 Rev. D01 program package52 using the B3LYP53−55 functional and the 6-311++G(d,p) basis set. Geometry optimizations were performed using TIGHT optimization criteria and were followed by harmonic frequency calculations, at the same level of theory, which also permitted us to characterize the nature of the stationary points. Anharmonic IR spectra were subsequently computed by means of a fully automated approach,56−58 set within second-order vibrational perturbation model (VPT2), 59 thus allowing for the evaluation of anharmonic infrared intensities of not only fundamentals but also overtones and combination bands.51,56,60 To compute anharmonic frequencies and IR intensities, the required semidiagonal quartic potential energy and cubic electric dipole moment surfaces were derived through numerical differentiations of the analytical second-derivatives of the energy computed at geometries displaced from equilibrium along the normal modes (with a 0.01 Å step). Fully anharmonic spectra were evaluated at the GVPT2/DVPT2 level, allowing for an effective treatment of anharmonic resonances (see ref 51 and references therein), applying default criteria for Fermi57,58 and 1-156 resonances.51 The relative energetics were computed beyond the harmonic approximation by means of simple perturbation theory (SPT)61,62 combined with the hinderedrotor anharmonic oscillator (HRAO) model,62,63 using resonance-free expression for the anharmonic zero point vibrational energies (ZPVE)64 and vibrational wavenumbers.62 For a graphical comparison of theoretical spectra with experiment, the calculated anharmonic frequencies, together with the calculated infrared intensities, were used to convolute each peak with a Lorentzian function having a full width at halfmaximum (fwhm) of 2 cm−1, so that the integral band intensities correspond to the calculated infrared absolute intensity.65 Note that the peak intensities (in units of “Relative Intensity”) in such simulated spectra are reduced by a factor of 0.3183 compared to the calculated intensity (in km mol−1).

Figure 2. Relaxed potential energy surface map of pyruvic acid calculated at the B3LYP/6-311++G(d,p) theory level, as a function of the OCCO and CCOH dihedral angles. The two dihedral angles were incremented in steps of 15°, and all the remaining coordinates were optimized. Dots (•) indicate positions of Tc, Tt, and Ct conformers and the Cc structure (transition state). The color bar on the right designates the energy scale defined relatively to the electronic energy of the lowest-energy form Tc (without the zeropoint vibrational correction). The isoenergy lines are traced using steps of 2 and 4 kJ mol−1, below and above 20 kJ mol−1, respectively.

The contour map in Figure 2 shows the location of the three PA minima (Tc, Tt, and Ct) on the molecule PES and their relative energies. All conformers belong to the Cs symmetry point group. The lowest energy of the Tc conformer results from the presence in this form of a stabilizing intramolecular H-bond (CO···HO; d(O···O) ∼ 204 pm) inserted in a five-membered ring, which energetically compensates the less favorable cis arrangement of the CCOH fragment.36,69−71 In the second most stable form, Tt (ΔE ∼ 9 kJ mol−1), the loss of the intramolecular hydrogen stabilizing interaction is partially compensated by the more favorable trans arrangement of the CCOH fragment.36,69−71 As shown below, these structural characteristics of the Tc and Tt conformers, in particular the inherently most stable arrangement of the CCOH fragment in Tt and the presence of the intramolecular H-bond in Tc stabilizing the intrinsically less stable cis arrangement of the CCOH fragment in this form, are fundamental to allow for the bidirectional optical C



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Figure 3. Fragments of spectra of pyruvic acid: (a) spectra in an Ar matrix at 15 K, immediately after deposition; (b) simulated spectra. See section 3 for the details of simulation. Note change of the absorbance scale factor from the near-IR to mid-IR range. Asterisks designate residual bands due to monomeric H2O.

control of the rotamerization reactions between these two forms discussed in the next sections of this article. The higher-energy Ct form differs from the Tt form by a 180° rotation about the intercarbonyl C−C bond. The repulsive interactions between the two carbonyl oxygen atoms in the Ct conformer are expected to be larger than the repulsive interactions involving the carbonyl and hydroxyl oxygen atoms in the Tt conformer, thus justifying the higher energy of Ct when compared with Tt. The Cc structure corresponds to a first-order saddle point, with an estimated relative energy above 45 kJ mol−1. The energy barrier for conversion of the Tt form into the most stable Tc conformer is estimated to be ca. ∼47 kJ mol−1 (∼56 kJ mol−1 in the reverse direction). On the other hand, the energy barrier associated with the Ct → Tt conversion amounts to only ca. 2 kJ mol−1 (Figure 2). Such a low barrier is not enough to allow the survival of the Ct form during deposition of the compound in a cryogenic matrix at 15 K, due to the phenomenon known as conformational cooling.37,72−74 Moreover, due to a high internal energy (ca. 14 kJ mol−1) the Ct conformer has a negligible population at the room temperature and only the two most stable conformers Tc and Tt are then expected to be trapped in the matrixes. The spectrum of PA freshly deposited in an argon matrix obtained in this work is essentially the same as in the previous RSAF matrix-isolation study.24 The amounts of the Tc and Tt forms in the cryogenic matrix prepared from the room temperature PA vapor (Tc ∼ 95%; Tt ∼ 5%) are in a good agreement with their predicted populations (Table 1). 4.2. Narrow-Band Selective NIR-Induced Rotamerization in Pyruvic Acid. In the present study, we used the NIR pumping to populate the Tt conformer of PA, through selective vibrational excitation of the OH stretching overtone vibration of the most stable Tc conformer. Such excitation introduces in the molecule an energy well-above the predicted energy barrier for the Tc → Tt conversion. Once generated, in this manner, the higher-energy Tt conformer could be subsequently characterized in detail spectroscopically.

Table 1. Relative Electronic Energies (ΔE), Relative ZPVE Corrected Energies (ΔEZPVE), and Relative Gibbs Energies at 298 K (ΔG298K) Calculated at the B3LYP/6-311++G(d,p) Level within Harmonic and Anharmonic Models, and the Equilibrium Populations of PA Conformers Estimated from the Relative Gibbs Energies at 298 K (P298)a harmonic (HO+RR)c

anharmonic (HRAO)d

structure

ΔE

ΔEZPVE

ΔG298K

P298

ΔEZPVE

ΔG298K

P298

Tc Tt Ct Ccb

0.0 9.5 15.2 46.2

0.0 8.8 14.3

0.0 6.8 11.3

93.1 5.9 1.0

0.0 8.9

0.0 6.7

93.8 6.2

Relative energies are given in kJ mol−1, populations in %. The absolute calculated E is −342.514415 au for the most stable Tc form. The graphical representation of the PA structures is given in Figure 1. b The Cc form was characterized as a first-order saddle point. c Harmonic oscillator and rigid rotor model. dHindered rotor + anharmonic oscillator model. Contributions computed by means of the HDCPT2 model62 in conjunction with simple perturbation theory (SPT).61,62 The two lowest vibrations have been described by hindered-rotor contributions computed using an automatic procedure.63,75 a

The experimental near-IR spectrum of PA isolated in an argon matrix was first collected to locate the spectral position of the νOH overtone of the Tc conformer. As shown in Figure 3a, the results indicated that 2νOH of Tc appears at ∼6630 cm−1, in good agreement with the B3LYP/6-311++G(d,p) calculated anharmonic frequency for this vibration (6671 cm−1; Table 2 and Figure 3b). The matrix-isolated PA monomers were then irradiated several times at 6630 cm−1. The progress of changes was controlled by registration of infrared spectrum after each irradiation. When the IR bands due to the Tc conformer decreased considerably (by more than 70% of their initial intensity), this series of NIR irradiations was terminated.76 The set of bands that intensify in the course of this irradiation has been previously identified as belonging to the Tt form.24 In the previous study, the maximum amount of Tt form could be D



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Table 2. Observed (Ar and N2 Matrix, 15 K) and Theoretical Anharmonic B3LYP/6-311++G(d,p) Wavenumbers (ν, cm−1) and Infrared Intensities (I, km mol−1) of the Pyruvic Acid Conformer Tc observed mode 2ν1 ν1+ν4 ν1+ν7 ν1+ν8 ν3+ν5 ν1+ν9 ν1+ν10 2ν4+ν11 ν1+ν12 ν1+ν20 ν1+ν21 ν1+ν13 ν2+ν11 ν1+ν15 2ν4 ν1 ν2 ν17 ν3 ν5+ν11 ν8+ν9 ν9+ν12 ν6+ν15 ν4 ν18+ν22 ν9+ν13 ν9+ν14 ν5 ν6 ν18 ν7 ν8 ν9 ν10 ν19 ν11 ν12 ν20 ν21 ν13 ν14 ν22 ν15 ν16 ν23 ν24

assignmenta

N2b 6643 5235 4823 4794 4647 4635 4573 4549 4198 4113

ν(OH) ν(CH3) as ν(CH3) as ν(CH3) s

ν(C3O)

ν(C2O) δ(CH3) as δ(CH3) as ν(CC) as δ(CH3) s δ(COH) ν(CO) γ(CH3) γ(CH3) ν(CC) s γ(C3O) τ(OH) δ(C2O) δ(C3O) γ(C2O) δ(CCO) δ(CCC) τ(CH3) τ(CC)

3823 3578 3434.6 3020 2932 2522 1959 1809 1797 1794 1790 1737 1733 1424 1406 1388 1355 1216 1140 1017 969 767 662 605

calculated

Arb,c

gasd,e

ν

6630.0b 5228.3b 4819.7b 4795.4b 4643.7b 4631.8b 4565.9b 4540.7b 4194.4b 4158.8b 4119.3b 4037.5b 4000b 3818.5b 3584.2c 3432.0c 3032.3c 2982.0c 2936.0c 2725b 2515b 1957.2b 1804.7b 1799.5c 1797.6b 1795.7b 1730.2c 1727.9c 1423.7c 1408.3c 1384.5c 1354.6c 1214.4c 1136.8c 1017.8c 968.4c 762.2c 678.8b 664.2c 603.8c 534.9c

6696

6671 5241 4807 4770 4677 4620 4554

3.30 0.27 0.43 2.03 0.09 2.10 1.17

4183 4164 4112 4039 3968 3820 3594 3438 3005 2948 2928 2711 2513 1938 1800 1809 1809 1794 1714 1751 1425 1415 1367 1350 1193 1120 1014 964 747 718 659 601 527 393 384 255 120 95

0.26 0.41 0.56 0.17 0.17 0.86 4.40 107.35 5.29 0.92 0.57 0.65 1.03 0.65 8.54 245.39 0.54 0.22 17.27 94.51 15.40 7.24 5.66 313.90 107.32 59.05 2.13 21.52 7.38 0.01 117.11 16.08 3.27 18.16 8.33 24.78 0.04 9.38

4851 4668 4589 4200 4145

3463 3025 2941 2565 1966 1804

1737 1424 1391 1360 1211 1133 1030 970 761 668 604 394 389 258 134 90

I

a Approximate description: ν, stretching; δ, bending; γ, rocking; τ, torsion. bThis work. cFrom ref 24. dGas phase results below 1000 cm−1 from ref 23. eGas phase results above 1000 cm−1 from ref 16.

tional change. For example, the observed shifts in the νOH mode to a higher frequency (from 3432 to 3556 cm−1, Figure 4a), and in the τOH mode to a lower frequency (from the 664 to 588 cm−1, Figure 4d), confirm this conclusion. The comparison of the experimental difference spectrum (Figure 5a), with that simulated on the basis of the calculated anharmonic spectra for the two conformers (Tc and Tt; Figure 5b), undoubtedly confirms that the NIR-generated species corresponds to the Tt form. This permitted a detailed

increased only up to 10% of the total conformational mixture, and only the eight strongest IR modes could be assigned. In the present work, the maximum amount of the Tt form could be increased up to 75% of the total conformational mixture (Figure 4a−d), owing to the selectivity of the NIR-induced conformational transformation.76 The observed changes in the IR spectrum of PA indicate that the intramolecular hydrogen bond existing in the initially present Tc conformer is disrupted in Tt after the conformaE



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Figure 4. Spectral indication of a large-scale conformational change in pyruvic acid isolated in an Ar matrix upon irradiation at 6630 cm−1 for 90 min: blue trace, spectrum taken immediately after deposition; red trace, spectrum after NIR irradiation.

Figure 5. (a) Experimental difference IR spectrum, the spectrum obtained after irradiation at 6630 cm−1 minus the spectrum of the freshly deposited PA in argon matrix at 15 K. (b) Simulated difference anharmonic IR spectrum at the B3LYP/6-311++G(d,p) level considering the quantitative conversion of the Tc into the Tt form (ratio 1:1). See section 3 for the details of simulation.

computations based on the double-harmonic approximation.51 Furthermore, a low value of obtained MUE demonstrates that the argon matrix is inert enough and spectra of pyruvic acid isolated in an argon matrix are in good agreement with results of anharmonic calculations that were carried for monomeric molecules in vacuo. That is also confirmed by comparison with the available gas-phase results reported by Vaida and coworkers.16 We note that the larger discrepancy is only observed for νOH, which is known to be more sensitive to the matrix environment.43,46,49,50,78−80 A low value of MUE for fully anharmonic calculations at B3LYP level is also in line with extended benchmark studies,51 giving us confidence in usage of the present theoretical approach for interpretation of the

assignment of many additional bands in the experimental Tt spectrum (Table 3). The mean unsigned error (MUE) resulting from the comparison between experimentally observed and nonscaled anharmonic B3LYP/6-311++G(d,p) wavenumbers (in the range below 1900 cm−1, for two conformers) amounts to 8.6 cm−1. This outperforms the scaled harmonic MUE value (10.9 cm−1) obtained from the B3LYP/aug-cc-pVDZ calculations used in the previous study on pyruvic acid24 and MUEs obtained from scaled harmonic calculations for other functionals and methods.77 Moreover, the anharmonic IR spectra provide also intensities for nonfundamental transitions, vanishing at the harmonic level, allowing for the assignment of overtone and combination bands, nonaccessible from any F



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Table 3. Observed (Ar and N2 Matrix, 15 K) and Theoretical Anharmonic B3LYP/6-311++G(d,p) Wavenumbers (ν, cm−1) and Infrared Intensities (I, km mol−1) of the Pyruvic Acid Conformer Tt observed mode 2ν1 ν1+ν4 ν1+ν5 ν1+ν7 ν1+ν8 ν1+ν9 ν3+ν4 ν1+ν10 ν2+ν6 ν2+ν18 ν2+ν7 ν17+ν18 ν3+ν18 ν1+ν20 ν1+ν12 ν1+ν21 2ν4 2ν5 ν1 ν2 ν17 ν3 ν5+ν10 ν8+ν9 ν10+ν12 ν4 ν9+ν13 ν5 ν7+ν15 ν19+ν20 ν18 ν6 ν7 ν8 ν9 ν10 ν19 ν11 ν20 ν12 ν21 ν13 ν14 ν15 ν22 ν16 ν23 ν24 a

assignmenta

N2 b 6900/6892.9 5293.2

4721 4660

ν(OH) ν(CH3) as ν(CH3) as ν(CH3) s

ν(C3O) ν(C2O)

δ(CH3) as δ(CH3) as ν(CC) as δ(CH3) s δ(COH) ν(CO) γ(CH3) γ(CH3) γ(C3O) ν(CC) τ(OH) δ(C2O) δ(C3O) δ(CCO) γ(C2O) δ(CCC) τ(CH3) τ(CC)

4158.8 3501.8 3486.5 3535.8/3532.8 3028.6 2992.0

gasd

ν

I

6944/6940b 5325.5b 5316.0b 4934.0b 4907.2b 4750.3b 4702.5b 4670.6b 4441.1b 4400.2b 4381.0b 4381.0b 4329.8b 4288.0b 4288.0b 4143.7b 3504.3b 3484b 3556.2/3554.1c 3030.4b 2982.4b

6975

6977 5349 5337 4934 4891 4737 4706 4682 4416 4409 4367 4365 4347 4295 4290 4177 3536 3514 3571 3007 2954 2929 2875 2496 1829 1780 1762 1767 1746 1741 1420 1422 1365 1354 1208 1110 1018 954 723 722 605 590 513 382 378 257 121 39

5.15 0.15 0.63 0.14 0.97 0.29 0.06 0.07 0.53 0.23 0.21 0.26 0.19 0.09 0.16 1.02 3.50 3.24 65.93 6.20 2.78 0.004 0.88 1.151 1.784 200.38 2.91 209.47 1.31 1.36 10.03 11.21 10.11 34.33 22.94 226.02 1.61 44.77 31.34 11.54 97.15 69.71 1.65 1.30 0.05 10.01 0.0002 7.10

2878b 2496b 1818.6b 1763.7c 1761.4c 1750.8c 1749.4c 1749.4c 1426.1b 1422.3b 1387.0b 1356.9b 1205.0b 1118.8c 1116.6c 961.9c 722.7c 716.4c 588.2c 592.1c

2514.7 1868.5 1765 1762 1751

1430 1414 1363 1338 1197 1127.7 1120 966 726 626.6/618.6 594.1

calculated

Arb,c

3579

Approximate description: ν, stretching; δ, bending; γ, rocking; τ, torsion. bThis work. cFrom ref 24. dGas phase results from ref 16.

experiment. The results of anharmonic vibrational analysis of PA are presented in Tables 2 and 3. The observed experimental NIR-induced rotamerization in PA suggests a quantitative transformation from the Tc to Tt conformer. In a recent study on matrix isolated glycolic acid46 it was shown that the conformational transformations may occur stepwise. Monochromatic near-IR excitation of the most stable SSC conformer of glycolic acid (GA) produces directly only

one of the possible minor forms (GAC). The generation of additional minor GA form (AAT) needs excitation with another near-IR photon, acting on the GAC, which is then transformed into AAT.46 Following the same logic, it is plausible to assume that a third conformer of pyruvic acid might be generated in a stepwise process similar to glycolic acid, by selective NIR-excitation of the minor Tt form in PA. G



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The matrix previously subjected to irradiation at 6630 cm−1 (at the 2νOH wavenumber of the Tc conformer), thus containing an increased population of the Tt form, was irradiated at the 2νOH wavenumber of the latter conformer. This band appears at 6940 cm−1 in the experiment. It is already discernible in the NIR spectrum of the nonirradiated matrix (Figure 3) and increases considerably when the Tt form is produced. Its spectral position agrees well with the B3LYP/6311++G(d,p) calculated anharmonic value (6977 cm−1). As a result of this irradiation, no bands ascribable to a new, third conformer of PA (a putative Ct form) could be identified in the spectrum. Instead, the Tt form was efficiently converted back to Tc. Therefore, after Tt has been generated in the matrix (upon irradiation at 6630 cm−1), the reverse Tt → Tc transformation could be successfully induced by NIR irradiation at 6940 cm−1 (Scheme 1). The possibility of efficient bidirectional optical

Because of the long lifetime of the NIR-generated Tt conformer of PA in an argon matrix (several hours, see below), and indefinitely stable Tt form in a nitrogen matrix, it was possible to produce the Tt species in a large amount and characterize this conformer spectroscopically in detail. For PA isolated in a nitrogen matrix, the 2νOH overtone of Tc appears at ∼6643 cm−1, and selective irradiation at this frequency efficiently promotes its transformation to the Tt conformer, similarly to argon. The spectral indications of the NIR-induced rotamerization in argon and in nitrogen matrix are shown in the form of difference spectra in Figure 6a−d. As can be seen from the figure, the spectral manifestations of the Tc conformer are quite similar in Ar and in N2 (note negative blue and red bands in Figure 6). However, for the NIR-generated Tt conformer, the spectral signatures exhibit much larger discrepancies upon going from argon to nitrogen. Especially striking are these discrepancies related with positions of vibrations involving the OH group. In the Tc conformer, the OH group is “hidden” inside the molecule, the intramolecular hydrogen bond interaction is essentially the same (judging from the observed vibrational frequencies) in both types of matrix (Figure 6). In the Tt conformer, the intramolecular H-bond is disrupted, the OH group is oriented to the outside of molecule and must interact with the environment. Apparently, in argon such an interaction is weak (note a good agreement between calculation and experiment, and low value of MUE’s), as discussed above. In the nitrogen matrix, the vibrational frequencies of the Tt vibrational modes involving the OH group are strongly shifted (comparing with the Tt bands in argon), and these shifts occur in the direction of the Tc form (note shifts by 22 and 38 cm−1 for the νOH and τOH modes). This indicates that the OH group of the Tt form establishes a stabilizing (hydrogen-bondlike) interaction with nitrogen matrix. Such an interaction is strong enough to stabilize the Tt conformer in N2 indefinitely long against relaxation back to Tc. In addition to the mid-infrared spectra, the spectral signatures of the matrix-isolated PA were also recorded in the near-infrared domain, before and after the NIR-induced rotamerizations. The performed anharmonic calculations aided the assignments. The experimental difference spectra showing changes in the overtone range (7200−6400 cm−1) and in the range of combination bands (5500−3800 cm−1) are compared in Figure 7 with the anharmonic simulated spectra. Noteworthy, these shifts in frequencies from argon to nitrogen matrix observed for the fundamental bands of Tt conformer are also reproduced in the near-infrared domain. For instance, a shift of νOH fundamental Tt mode by −22 cm−1 corresponds to the shift of 2νOH overtone Tt by −46 cm−1. The shift of the (νOH + νCO) combination Tt mode by −23 cm−1 (appearing around 5300 cm−1) is well explained by the sum of shifts of the corresponding two fundamentals, where the νCO essentially does not change the spectral position from argon to nitrogen. The shift of the (νOH + τOH) combination mode by +15 cm−1 is also in agreement with the shifts of the fundamental modes by −23 and +38 cm−1 in the opposite directions (Figure 7a,b). Note that the intensities of the overtone and combination bands are by 2 orders of magnitude lower than the corresponding fundamental bands (compare the ordinate scales of Figures 6 and 7). The observation of the combination bands was made possible by the large scale of the NIR-induced transformation, and their assignment was facilitated by comparison between the experiments in argon and nitrogen,

Scheme 1. Bidirectional NIR-Induced Rotamerization Observed for Matrix-Isolated PA

control of the Tc ↔ Tt interconversion process (i.e., of the relative populations of the two PA conformers) makes this molecule a system satisfying the criterion of molecular optical switch.38,81 Following again the analogy with the recently reported case of glycolic acid,46 we studied the influence of the matrix host material on the conformational behavior of pyruvic acid. For glycolic acid, the NIR-induced photochemistry was different in argon and in nitrogen matrixes. A successful generation of a new, fourth GA conformer (SST) was achieved in solid nitrogen, whereas the minor forms of GA accessible in the argon matrix were not populated in nitrogen (and vice versa). The detailed description of the experiments with PA isolated in a nitrogen matrix will be given later in this work. Briefly, we have isolated PA in solid N2 and characterized its vibrational spectra in the mid-infrared and near-infrared domains. The processes occurring for PA in solid nitrogen were very similar to those just described for PA in solid argon. The minor Tt form was produced from Tc, and bidirectional Tc ↔ Tt transformations were successfully induced by irradiations at the respective 2νOH overtones (at 6643 and 6894 cm−1). There was no indication of generation of a putative third PA conformer (Ct) in the solid nitrogen matrix either. 4.3. Experimental Vibrational Signatures of Pyruvic Acid Isolated in Argon and in Nitrogen Matrixes. Using nitrogen as a matrix material was found previously to produce a stabilizing effect on the minor conformers of several matrixisolated molecules, such as formic,82,83 acetic,82 tetrazole− acetic,84 squaric,43 and glycolic46 acids, glycine,49 alanine,41,50 and cysteine,85 among others. The minor conformers of these compounds are stable in nitrogen but hard or impossible to detect by steady-state spectroscopic techniques in argon. A similar stabilization effect was found to occur also in pyruvic acid. As will be shown in section 4.4, the optically generated Tt conformer decayed back to Tc in argon matrixes, whereas no such relaxation occurred for the Tt form isolated in nitrogen matrixes. H



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Figure 6. Difference spectra showing effects of near-IR irradiations on PA isolated in Ar (irradiation at 6630 cm−1, blue line) and N2 (irradiation at 6643 cm−1, red line) matrixes. Positive bands are due to the growing Tt form generated at the expense of Tc form (negative bands). Numbers show shifts of vibrations related to the OH group upon going from argon to nitrogen.

Figure 7. (a, b) Experimental difference NIR spectra in the range of the OH str overtone (left) and combination bands (right) showing effects of NIR irradiations at (a) 6643 cm−1 and (b) 6630 cm−1 of PA isolated in (a) nitrogen and (b) argon matrixes at 15 K. Negative and positive bands are due to the Tc and Tt conformers, respectively; assignments are given in Tables 2 and 3. (c) Simulated difference anharmonic IR spectrum considering the quantitative conversion of the Tc into the Tt form (ratio 1:1). See section 3 for the details of the simulation. The region below 4000 cm−1 in (b) contains a few residual positive bands due to the not entirely compensated atmospheric water vapors. The gray bar with blue asterisk in (c) is discussed in section 4.4.

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and also by comparison with the fully anharmonic calculations in this range of spectrum, including the infrared intensities (Figure 7c). The assignment of the full mid-infrared spectra of Tt is presented in Table 3. It completes the previously proposed assignments for this conformer presented in the RSAF paper.24 The assignment of several overtone and combination Tt bands is also included. Table 2 provides the corresponding data for conformer Tc and suggests some reassignment of combination bands reported by Vaida et al.16 Some of the combination bands observed in the present work may play an important role in the conformational relaxation of PA, as discussed below. 4.4. Conformational Tt → Tc Relaxation of Pyruvic Acid in the Dark and in the Presence of Broad-Band Mid-Infrared and Near-Infrared Radiation. For many previously studied carboxylic acids and amino acids, the NIRgenerated higher-energy conformers quickly (in the course of a few minutes) convert back in an argon matrix to the starting conformer by quantum-mechanical tunneling. For the matrixisolated PA in argon, the Tt conformer also converted to the most stable form Tc, but this process took a much longer time (many hours) compared to the other systems.40,43,86−88 The explanation for this fact relies on the structural characteristics of the relevant conformers. For most of the previously studied systems, the lower-energy conformers had their carboxylic fragment in the intrinsically more stable trans CCOH (same as cis OCOH) orientation, whereas the NIR-generated forms had their carboxylic moiety in the less stable cis CCOH (same as trans OCOH) arrangement. Because of this, the CO cis ↔ trans energy barriers in these molecules are lower (by 10−20 kJ mol−1) than in case of PA, where the NIR-generated conformer has the carboxylic fragment in the intrinsically more stable trans arrangement. The increase of the barrier height in PA thus reduces its permeability to tunneling and increases the lifetime of the higher energy conformer. It has been reported previously that exposure of the matrix isolated molecules to the broad-band IR light produced by the IR spectrometer source may also lead to cis ↔ trans CO rotamerization, through excitation of the νOH fundamental, like in cytosine.40 The Tt → Tc spontaneous relaxation process in PA was also investigated for the sample exposed to such radiation. Once the Tt form was generated by irradiation at 6630 cm−1, several kinetic observations, in independent experiments, were performed. In one case, the sample was exposed to the unfiltered IR beam of the spectrometer. In other cases, a long-pass cutoff infrared filter was placed between the spectrometer source and the sample. Two different cutoff filters were applied: transmitting only light with wavenumbers (a) up to 4200 cm−1 (∼50 kJ mol−1) and (b) up to 2200 cm−1 (∼25 kJ mol−1). In all cases the Tt → Tc decay process was followed spectroscopically over the time. The decay rates in these experiments depended on the applied filter range (Figure 8). In the case of the filter transmitting only light up to 2200 cm−1 the decay was considerably slow: it took ∼17 h to convert a half of Tt form back to Tc. In the experiments without filter or with filter transmitting in the whole mid-IR range (transparent up to 4200 cm−1), the decay rates were much faster (∼5 and ∼6 h). In the statistical analysis of the observed decay kinetics, the experimental integrated intensities of the two strongest bands of conformer Tt (1764 and 1119 cm−1) were used as the measures of the evolution. The intensity of each of these bands at the beginning of measurement was normalized to unity, and the average value over two bands is plotted in Figure 8. The

Figure 8. Decay kinetics of Tt conformer in Ar matrix at 15 K. The spectra were recorded: without filter (red squares); with a cutoff filter transmitting only up to 4200 cm−1 (green triangles); with a cutoff filter transmitting only up to 2200 cm−1 (blue circles). The red, green, and black lines represent fits using the classic single exponential kinetics. The blue line represents the fit with equations of dispersive kinetics.

kinetic analyses of the amount of conformer Tt, designated as n(Tt), were carried out with the classical monoexponential model: n(Tt) = n(Tt)t = 0 exp( −kt )

(1)

The corresponding decay rates k obtained from the monoexponential fit were 2.34 × 10−3 min−1 (no filter), 1.97 × 10−3 min−1 (filter 4200 cm−1), and 6.94 × 10−4 min−1 (filter 2200 cm−1). These decay rates yield the corresponding optimized classical half-life times t1/2 of 297, 351, and 999 min, respectively.89 The R2 correlation coefficients obtained for the above fits are 0.99948, 0.99971, and 0.99514, respectively. From these R2 values, it is obvious that the classical fit (black line in Figure 8) does not reproduce the kinetics observed with filter 2200 cm−1 as well as the two other fits. Initially, the decay process with this filter is faster than the best fit to classical kinetics equation. At later stages, the decay clearly slows down. This suggests that the probability of the Tt → Tc relaxation is dependent on slight differences in the matrix microenvironments. Some matrix cavities allow for a faster conversion of the trapped molecules that takes place at the initial stages of the experiment. At later stages, the conversion becomes slower. Such behavior is typical of transformations of molecules embedded in inhomogeneous media.90 Usually, the time evolution of such processes follows the equations of the dispersive kinetics:91−93 n(Tt) = n(Tt)t = 0 exp( −Bt α) α−1

(2) α

where k(t) = Bt (B, α are constants; B ≡ α/τ disp). The dispersive kinetics fit for the decay observed in PA with filter 2200 cm−1 resulted in a much better correlation coefficient R2 = 0.99941, with the fitted parameters B = 0.00143, α = 0.892. The lower the numerical value of α, the greater the dispersivity of the process. The limit of classical kinetics corresponds to α = 1. The value of α ≈ 0.89, obtained for the Tt → Tc decay at 15 K, suggests that, although the Ar matrix environment is not very J



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PA molecules are exposed to the spectrometer source without filter, or with filter transmitting up to 4200 cm−1, the Tt conformer may absorb at these frequencies. Then the vibrationally excited PA molecules may isomerize via overthe-barrier mechanism, provided there are levels of corresponding energy on the side of Tc conformer. Indeed, there is a pool of densely spaced combination bands in the Tc form at 4400− 4300 cm−1 (gray rectangle in Figure 7c and in Figure 9), which on the common energy scale are isoenergetic with the νOH fundamental transition of Tt. These combination bands may ensure an efficient vibrational coupling between Tt and Tc. It must be noted that the energy diagram shown in Figure 9 is constructed on the basis of the experimentally observed wavenumbers, whose vertical transitions are built starting from the electronic energies calculated at the B3LYP/6-311+ +G(d,p) level (Tt−Tc difference of 9.5 kJ mol−1). Based on comparison with reported CCSD(T) results,24,34 it is plausible to assume that the calculated energy gap between the ground states of Tc and Tt is in reality slightly larger (by 1−2 kJ mol−1). Also in such cases, the qualitative picture should remain the same, as there are still many energy levels of the Tc form that can be responsible for the vibrational coupling, for example such as those appearing at 4194.4 and 4819.7 cm−1 (Figures 7 and 9, note the gray bar). The νOH fundamental transition of the Tc form appears at 3432 cm−1 and is accessible to the infrared irradiation without filter. However, this vibrational level of the Tc form does not have a counterpart level on the other side of the barrier (to satisfy the coupling condition, the Tt form would need to have an energy level of about 2682 cm−1). The excitation of Tc at 3432 cm−1 should not be effective in conformational isomerization (crossed arrow in Figure 9). First, it would imply a process below the barrier, and second, there are no matching vibrational levels on the Tt side of the molecule. Altogether, the energy level scheme of vibrational fundamental and combination transitions in pyruvic acid suggests that it should relax from Tt to Tc more efficiently, when the excitations above the barrier are allowed. As discussed above, it is confirmed by the accelerated decay kinetics (Figure 8) in the experiments without filter, or with filter transmitting up to 4200 cm−1. This direction of isomerization is also consistent with the absorption cross sections reported by Vaida et al. for the OH stretching fundamental transitions, the overtones, and the combinations bands in PA.16 It provides additional insight into why the relaxation occurs in the direction from Tt to Tc.

disordered, the inhomogeneous character of this medium cannot be neglected. Earlier, the inhomogeneous character of argon matrixes was also confirmed during the studies of tunneling in matrix-isolated cytosine.40 In that case, the dispersive kinetic fit resulted in α ≈ 0.8. The dispersive kinetics decay cannot be characterized by one numeric value related with the half-life of the reacting species, because the speed of conversion constantly changes with time. Instead, the dispersive kinetics can be characterized by the time constant ταdisp, which can be derived from the B ≡ α/ταdisp expression. For B = 0.00143 and α = 0.892, the resulting ταdisp = 1360 min. The experimental observations for different Tt → Tc relaxation times can be explained in terms of the rotamerization barrier, separating the Tt and Tc forms, and the existing vibrational transitions in two conformers. The calculated relaxed potential energy profile connecting the two PA conformers via the intramolecular torsion of the OH group is depicted in Figure 9. The value of barrier in the Tt → Tc

Figure 9. Relaxed potential energy scan for the intramolecular OH torsion in PA calculated at the B3LYP/6-311++G(d,p) level. The horizontal colored lines (Tc, blue; Tt, red) designate energy levels obtained from the experimental values of the τOH and νOH modes in two conformers, as observed for monomers of PA in Ar matrix. Selected energy levels for some observed combination bands are also shown. The gray rectangle designated with a blue asterisk is commented in the text. Dashed curved arrows (black) indicate possible couplings between the closely spaced energetic levels in Tc and Tt. The vertical blue and red arrows are plotted for transitions with energies lower than 4200 cm−1. Note that the ordinate scale is given in cm−1 (left) and in kJ mol−1 (right).

5. CONCLUSIONS In this investigation, the Tt conformer of pyruvic acid has been successfully populated by narrow-band selective pumping of the first νOH overtone of most stable Tc conformer isolated in argon and nitrogen matrixes. The ground-state potential energy surface of the molecule has been investigated in detail using DFT calculations, and the located minima characterized structurally and vibrationally also by performing fully anharmonic vibrational calculations, which allow for simulation of band position and infrared intensities, for the fundamental modes, overtones, and combinations up to two quanta. Because, contrary to what happens most frequently, the conformational ground state of PA (Tc) has its carboxylic group in the intrinsically less stable cis CCOH configuration and the conformer produced after NIR-induced conversion of this form (Tt) has this group in the trans CCOH arrangement, the stability of the NIR-produced form has been found to be

direction was calculated to be ∼47 kJ mol−1. The usage of a filter with cutoff at 2200 cm−1 is equivalent to irradiations of the sample with energies no more than 25 kJ mol−1. Such energies are not sufficient to promote transitions over the barrier. Therefore, in a slow relaxation Tt → Tc process observed with this filter the quantum-mechanical tunneling mechanism must dominate. Exposing the matrix-isolated pyruvic acid to the infrared light up to 4200 cm−1, allows excitation of transitions due to Tt conformer appearing in an argon matrix at 3556.2 cm−1 (equivalent of 42.5 kJ mol−1) and some combination bands, such as at 4143.7 cm−1 (equivalent of 49.5 kJ mol−1). They can be ascribed to the νOH fundamental transition and the νOH + τOH combination, respectively (Table 3). These transitions of the Tt form appear near the top or above the torsional barrier of pyruvic acid (Figure 9, red arrows). When the matrix isolated K



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(2) Li, Y.; Chen, J.; Lun, S.-Y. Biotechnological Production of Pyruvic Acid. Appl. Microbiol. Biotechnol. 2001, 57, 451−459. (3) Jardine, K. J.; Sommer, E. D.; Saleska, S. R.; Huxman, T. E.; Harley, P. C.; Abrell, L. Gas Phase Measurements of Pyruvic Acid and Its Volatile Metabolites. Environ. Sci. Technol. 2010, 44, 2454−2460. (4) Yu, S. Role of Organic Acids (Formic, Acetic, Pyruvic and Oxalic) in the Formation of Cloud Condensation Nuclei (CCN): A Review. Atmos. Res. 2000, 53, 185−217. (5) Grosjean, D. Atmospheric Reactions of Pyruvic Acid. Atmos. Environ. 1983, 17, 2379−2382. (6) Rosenfeld, R. N.; Weiner, B. Energy Disposal in the Photofragmentation of Pyruvic Acid in the Gas Phase. J. Am. Chem. Soc. 1983, 105, 3485−3488. (7) Ding, W.; Fang, W.; Liu, R. Mechanisms of Unimolecular Reactions for Ground-State Pyruvic Acid. Acta Physicochim. Sin. 2004, 20, 911−916. (8) Taylor, R. The Mechanism of Thermal Eliminations Part XXIII: [1] The Thermal Decomposition of Pyruvic Acid. Int. J. Chem. Kinet. 1987, 19, 709−713. (9) Mellouki, A.; Mu, Y. On the Atmospheric Degradation of Pyruvic Acid in the Gas Phase. J. Photochem. Photobiol. A Chem. 2003, 157, 295−300. (10) Leermakers, P. A.; Vesley, G. F. The Photochemistry of α-Keto Acids and α-Keto Esters. I. Photolysis of Pyruvic Acid and Benzoylformic Acid. J. Am. Chem. Soc. 1963, 85, 3776−3779. (11) Epstein, S. A.; Nizkorodov, S. A. A Comparison of the Chemical Sinks of Atmospheric Organics in the Gas and Aqueous Phase. Atmos. Chem. Phys. 2012, 12, 8205−8222. (12) Vesley, G. F.; Leermakers, P. A. The Photochemistry of α-Keto Acids and α-Keto Esters. III. Photolysis of Pyruvic Acid in the Vapor Phase. J. Phys. Chem. 1964, 68, 2364−2366. (13) Jones, G.; Jenkins, S. J. Insight into the Reduction of Pyruvic Acid to Lactic Acid Over Cu{110}: The Crucial Role of Intramolecular Tunneling in Direct Hydrogenation. J. Am. Chem. Soc. 2008, 130, 14483−14492. (14) Gachko, G.; Kivach, L.; Maskevich, S. A.; Sokov, K. V.; Podtynchenko, S. G. Spectroscopic Investigations of Specific Interactions of Pyruvic Acid in Model Systems. J. Appl. Spectrosc. 1990, 52, 555−560. (15) Takahashi, K.; Plath, K. L.; Skodje, R. T.; Vaida, V. Dynamics of Vibrational Overtone Excited Pyruvic Acid in the Gas Phase: Line Broadening Through Hydrogen-Atom Chattering. J. Phys. Chem. A 2008, 112, 7321−7331. (16) Plath, K. L.; Takahashi, K.; Skodje, R. T.; Vaida, V. Fundamental and Overtone Vibrational Spectra of Gas-Phase Pyruvic Acid. J. Phys. Chem. A 2009, 113, 7294−7303. (17) Larsen, M. C.; Vaida, V. Near Infrared Photochemistry of Pyruvic Acid in Aqueous Solution. J. Phys. Chem. A 2012, 116, 5840− 5846. (18) Vaida, V. Spectroscopy of Photoreactive Systems: Implications for Atmospheric Chemistry. J. Phys. Chem. A 2009, 113, 5−18. (19) Marstokk, K.-M.; Møllendal, H. Microwave Spectrum, Conformation, Barrier to Internal Rotation and Dipole Moment of Pyruvic Acid. J. Mol. Struct. 1974, 20, 257−267. (20) Dyllick-Brenzinger, C. E.; Bauder, A.; Günthard, Hs. H. The Substitution Structure, Barrier to Internal Rotation, and Low Frequency Vibrations of Pyruvic Acid. Chem. Phys. 1977, 23, 195−206. (21) Meyer, R.; Bauder, A. Torsional Coupling in Pyruvic Acid. J. Mol. Spectrosc. 1982, 94, 136−149. (22) Kisiel, Z.; Pszczółkowski, L.; Białkowska-Jaworska, E.; Charnley, S. B. The Millimeter Wave Rotational Spectrum of Pyruvic Acid. J. Mol. Spectrosc. 2007, 241, 220−229. (23) Hollenstein, H.; Akermann, F.; Günthard, Hs. H. Vibrational Analysis of Pyruvic Acid and D-, 13C- and 18O-Labelled Species: Matrix Spectra, Assignments, Valence Force Field and Normal Coordinate Analysis. Spectrochim. Acta, Part A 1978, 34, 1041−1063. (24) Reva, I. D.; Stepanian, S. G.; Adamowicz, L.; Fausto, R. Combined FTIR Matrix Isolation and Ab Initio Studies of Pyruvic

considerably higher than in other carboxylic acids. Moreover, the used strategy allowed promoting a large scale conformational Tc → Tt conversion (not reachable either by thermal population of the Tt form in the gas phase or by in situ UV irradiation of the initially deposited in a cryogenic matrix Tc form). Owing to this fact, the experimental spectra of both conformers, especially the minor Tt form, could be thoroughly studied in every detail. After the higher-energy Tt form was generated in the matrixes, it could be converted back to Tc using a similar strategy, i.e., by selective NIR irradiation at the respective first OH overtone. In addition, it was also observed that the Tt form, once produced in argon matrixes, spontaneously decays back to the most stable Tc form in the dark, by tunneling, in a process obeying the dispersive-type kinetics, with a characteristic lifetime of more than 16 h. Noteworthy, in the presence of near-infrared broad-band light, the speed of relaxation of the Tt form back to Tc considerably increases, as a result of a contribution of the over-the-barrier mechanism. This conclusion is supported by a deep analysis of the vibrational manifold characteristics of the ground-state potential energy surface region interconnecting the two conformers. In an N2 matrix, the Tt form, which has its OH group not involved in any intramolecular H-bond (in Tc, this group establishes an intramolecular H-bond to the carbonyl oxygen), was found to be considerably stabilized at the cost of interaction between the OH group and the matrix N2 molecules. This stabilization manifested itself in the absence of Tt → Tc relaxation and in a considerable change of the vibrational Tt signature upon going from argon to nitrogen.

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AUTHOR INFORMATION

Corresponding Author

*R. Fausto. E-mail: [email protected]. Telephone: +351-239854-483. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS These studies were partially funded by the Portuguese “Fundaçaõ para a Ciência e a Tecnologia” (FCT), FEDER, and project PTDC/QUI-QUI/118078/2010, FCOMP-010124-FEDER-021082, cofunded by QREN-COMPETE-UE. C.M.N. acknowledges the FCT for the Postdoctoral Grant No. SFRH/BPD/86021/2012. The Coimbra Chemistry Centre (CQC) is supported by the FCT through the project PEst-OE/ QUI/UI0313/2014. This work was also supported by Italian MIUR (under the project PON01-01078/8). M.B. gratefully thanks Prof. Vincenzo Barone and Prof. Cristina Puzzarini for helpful discussions, and the high-performance computer facilities of the DREAMS center (http://dreamshpc.sns.it) for providing computer resources. The support of the COST CMTS-Action CM1002 “COnvergent Distributed Environment for Computational Spectroscopy (CODECS)” is also acknowledged.

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REFERENCES

(1) Christofk, H. R.; van der Heiden, M. G.; Harris, M. H.; Ramanathan, A.; Gerszten, R. E.; Wei, R.; Fleming, M. D.; Schreiber, S. L.; Cantley, L. C. The M2 Splice Isoform of Pyruvate Kinase Is Important for Cancer Metabolism and Tumour Growth. Nature 2008, 452, 230−233. L



Article

Acid: Proof for Existence of the Second Conformer. J. Phys. Chem. A 2001, 105, 4773−4780. (25) Murto, J.; Raaska, T.; Kunttu, H.; Räsänen, M. Conformers and Vibrational Spectra of Pyruvic Acid: An Ab Initio Study. J. Mol. Struct.: THEOCHEM 1989, 200, 93−101. (26) Schellenberger, A.; Beer, W.; Oehme, G. Untersuchungen zur Theorie der α-Ketosäuren. XI. IR-Spektroskopische Untersuchungen an α-Ketosäuren im Gaszustand. Spectrochim. Acta 1965, 21, 1345− 1351. (27) Kaluza, C. E.; Bauder, A.; Günthard, Hs. H. The Microwave Spectrum of Pyruvic Acid. Chem. Phys. Lett. 1973, 22, 454−457. (28) Van Alsenoy, C.; Schäfer, L.; Siam, K.; Ewbank, J. D. Ab initio studies of Structural Features Not Easily Amenable to Experiment: Part 65. Ab Initio Molecular Structures of Glyoxylic, Pyruvic, and Propiolic Acid, and Comparison with Microwave Data. J. Mol. Struct.: THEOCHEM 1989, 187, 271−283. (29) Tarakeshwar, P.; Manogaran, S. An ab Initio Study of Pyruvic Acid. J. Mol. Struct.: THEOCHEM 1998, 430, 51−56. (30) Chen, C.; Shyu, S.-F. Theoretical Study of Glyoxylic and Pyruvic Acids: Rotamers and Intramolecular Hydrogen Bonding. J. Mol. Struct.: THEOCHEM 2000, 503, 201−211. (31) Zhou, Z.; Du, D.; Fu, A. Structures and Vibrational Frequencies of Pyruvic Acid: Density Functional Theory Study. Vibr. Spectrosc. 2000, 23, 181−186. (32) Barnes, A. J. Matrix Isolation Vibrational Spectroscopy as a Tool for Studying Conformational Isomerism. J. Mol. Struct. 1984, 113, 161−174. (33) Ivanov, A. Yu.; Plokhotnichenko, A. M.; Izvekov, V.; Sheina, G. G.; Blagoi, Yu. P. FTIR Investigation of the Effect of Matrices (Kr, Ar, Ne) on the UV-Induced Isomerization of the Monomeric Links of Biopolymers. J. Mol. Struct. 1997, 408−409, 459−462. (34) Gerbig, D.; Schreiner, P. R. Hydrogen-Tunneling in Biologically Relevant Small Molecules: The Rotamerizations of α-Ketocarboxylic Acids. J. Phys. Chem. B 2014, DOI: 10.1021/jp503633m. (35) Pettersson, M.; Lundell, J.; Khriachtchev, L.; Räsänen, M. IR Spectrum of the Other Rotamer of Formic Acid, cis-HCOOH. J. Am. Chem. Soc. 1997, 119, 11715−11716. (36) Maçôas, E. M. S.; Khriachtchev, L.; Pettersson, M.; Fausto, R.; Räsänen, M. Rotational Isomerism in Acetic Acid: The First Experimental Observation of the High-Energy Conformer. J. Am. Chem. Soc. 2003, 125, 16188−16189. (37) Fausto, R.; Khriachtchev, L.; Hamm, P. Conformational Changes in Cryogenic Matrices. In Physics and Chemistry at Low Temperatures; Khriachtchev, L., Ed.; World Scientific: Singapore, 2010; pp 51−84. (38) Sharma, A.; Reva, I.; Fausto, R. Conformational Switching Induced by Near-Infrared Laser Irradiation. J. Am. Chem. Soc. 2009, 131, 8752−8753. (39) Lapinski, L.; Nowak, M. J.; Reva, I.; Rostkowska, H.; Fausto, R. NIR-Laser-Induced Selective Rotamerization of Hydroxy Conformers of Cytosine. Phys. Chem. Chem. Phys. 2010, 12, 9615−9618. (40) Reva, I.; Nowak, M. J.; Lapinski, L.; Fausto, R. Spontaneous Tunneling and Near-Infrared-Induced Interconversion between the Amino-Hydroxy Conformers of Cytosine. J. Chem. Phys. 2012, 136, 064511. (41) Nunes, C. M.; Lapinski, L.; Fausto, R.; Reva, I. Near-IR Laser Generation of a High-Energy Conformer of L-Alanine and the Mechanism of Its Decay in a Low-Temperature Nitrogen Matrix. J. Chem. Phys. 2013, 138, 125101. (42) Kuş, N.; Sharma, A.; Peña, I.; Bermúdez, M. C.; Cabezas, C.; Alonso, J. L.; Fausto, R. Conformers of β-Aminoisobutyric Acid Probed by Jet-Cooled Microwave and Matrix Isolation Infrared Spectroscopic Techniques. J. Chem. Phys. 2013, 138, 144305. (43) Lapinski, L.; Reva, I.; Rostkowska, H.; Halasa, A.; Fausto, R.; Nowak, M. J. Conformational Transformation in Squaric Acid Induced by Near-IR Laser Light. J. Phys. Chem. A 2013, 117, 5251−5259. (44) Pettersson, M.; Maçôas, E. M. S.; Khriachtchev, L.; Fausto, R.; Räsänen, M. Conformational Isomerization of Formic Acid by

Vibrational Excitation at Energies below the Torsional Barrier. J. Am. Chem. Soc. 2003, 125, 4058−4059. (45) Maçôas, E. M. S.; Khriachtchev, L.; Pettersson, M.; Juselius, J.; Fausto, R.; Räsänen, M. Reactive Vibrational Excitation Spectroscopy of Formic Acid in Solid Argon: Quantum Yield for Infrared Induced Trans→Cis Isomerization and Solid State Effects on the Vibrational Spectrum. J. Chem. Phys. 2003, 119, 11765−11772. (46) Halasa, A.; Lapinski, L.; Reva, I.; Rostkowska, H.; Fausto, R.; Nowak, M. J. Near-Infrared Laser-Induced Generation of Three Rare Conformers of Glycolic Acid. J. Phys. Chem. A 2014, 118, 5626−5635. (47) Maçôas, E.; Fausto, R. Infrared-Induced Rotamerization of Oxalic Acid Monomer in Argon Matrix. J. Phys. Chem. A 2000, 104, 6956−6961. (48) Maçôas, E. M. S.; Khriachtchev, L.; Pettersson, M.; Fausto, R.; Räsänen, M. Internal Rotation in Propionic Acid: Near-InfraredInduced Isomerization in Solid Argon. J. Phys. Chem. A 2005, 109, 3617−3625. (49) Bazsó, G.; Magyarfalvi, G.; Tarczay, G. Tunneling Lifetime of the ttc/VIp Conformer of Glycine in Low-Temperature Matrices. J. Phys. Chem. A 2012, 116, 10539−10547. (50) Bazsó, G.; Najbauer, E. E.; Magyarfalvi, G.; Tarczay, G. NearInfrared Laser Induced Conformational Change of Alanine in LowTemperature Matrixes and the Tunneling Lifetime of Its Conformer VI. J. Phys. Chem. A 2013, 117, 1952−1962. (51) Barone, V.; Biczysko, M.; Bloino, J. Fully Anharmonic IR and Raman Spectra of Medium-Size Molecular Systems: Accuracy and Interpretation. Phys. Chem. Chem. Phys. 2014, 16, 1759−1587. (52) 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. (53) 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. (54) Lee, C.; Yang, W.; Parr, R. G. Development of the Colle-Salvetti Correlation-Energy Formula into a Functional of the Electron Density. Phys. Rev. B 1988, 37, 785−789. (55) Becke, A. D. Density-Functional Thermochemistry. III. The Role of Exact Exchange. J. Chem. Phys. 1993, 98, 5648−5652. (56) Bloino, J.; Barone, V. A Second-Order Perturbation Theory Route to Vibrational Averages and Transition Properties of Molecules: General Formulation and Application to Infrared and Vibrational Circular Dichroism Spectroscopies. J. Chem. Phys. 2012, 136, 124108. (57) Barone, V. Anharmonic Vibrational Properties by a Fully Automated Second-Order Perturbative Approach. J. Chem. Phys. 2005, 122, 14108. (58) Barone, V. Vibrational Zero-Point Energies and Thermodynamic Functions Beyond the Harmonic Approximation. J. Chem. Phys. 2004, 120, 3059−3065. (59) Mills, I. M. Vibration-Rotation Structure in Asymmetric and Symmetric Top Molecules. In Molecular Spectroscopy: Modern Research; Rao, K. N., Matthews, C. W., Eds.; Academic Press: New York, 1972; pp 115−140. (60) Barone, V.; Bloino, J.; Guido, C. A.; Lipparini, F. A Fully Automated Implementation of VPT2 Infrared Intensities. Chem. Phys. Lett. 2010, 496, 157−161. (61) Truhlar, D. G.; Isaacson, A. D. Simple Perturbation Theory Estimates of Equilibrium Constants from Force Fields. J. Chem. Phys. 1991, 94, 357−359. (62) Bloino, J.; Biczysko, M.; Barone, V. General Perturbative Approach for Spectroscopy, Thermodynamics, and Kinetics: Methodological Background and Benchmark Studies. J. Chem. Theory Comput. 2012, 8, 1015−1036. (63) Ayala, P. Y.; Schlegel, H. B. Identification and Treatment of Internal Rotation in Normal Mode Vibrational Analysis. J. Chem. Phys. 1998, 108, 2314−2325. (64) Schuurman, M. S.; Allen, W. D.; Schleyer, P. v. R.; Schaefer, H. F., III. The Highly Anharmonic BH5 Potential Energy Surface Characterized in the Ab Initio Limit. J. Chem. Phys. 2005, 122, 104302. M



Article

(65) Irikura, K. K. Program SYNSPEC; National Institute of Standards & Technology: Gaithersburg, MD; http://www.ccl.net/ cca/software/MS-DOS/synthetic-spectrum/SYNSPEC.TXT.shtml (accessed October 16, 2014). (66) Liao, D. W.; Mebel, A. M.; Hayashi, M.; Shiu, Y. J.; Chen, Y. T.; Lin, S. H. Ab Initio Study of the n-π* Electronic Transition in Acetone: Symmetry-Forbidden Vibronic Spectra. J. Chem. Phys. 1999, 111, 205−215. (67) Fausto, R.; Batista de Carvalho, L. A. E.; Teixeira-Dias, J. J. C. Molecular Structure and Properties of Thioacetic Acid. J. Mol. Struct.: THEOCHEM 1990, 207, 67−83. (68) No̅sberger, P.; Bauder, A.; Günthard, Hs. H. A Versatile Method for Molecular Structure Determinations from Ground State Rotational Constants. Chem. Phys. 1973, 1, 418−425. (69) Teixeira-Dias, J. J. C.; Fausto, R. A Molecular Mechanics Force Field for Conformational Analysis of Simple Acyl Chlorides, Carboxylic Acids and Esters. J. Mol. Struct. 1986, 144, 199−213. (70) Teixeira-Dias, J. J. C.; Fausto, R. Molecular Structure of Methyl Acrylate: The High Energy s-trans-(CO) Conformer. J. Mol. Struct.: THEOCHEM 1993, 282, 123−129. (71) Fausto, R.; Maçôas, E. M. Photochemical Reactivity of MatrixIsolated Monomeric Carboxylic Acids. J. Mol. Struct. 2001, 563, 27− 40. (72) Rosado, M. T. S.; Lopes Jesus, A. J.; Reva, I. D.; Fausto, R.; Redinha, J. S. Conformational Cooling Dynamics in Matrix-Isolated 1,3-Butanediol. J. Phys. Chem. A 2009, 113, 7499−7507. (73) Reva, I. D.; Lopes Jesus, A. J.; Rosado, M. T. S.; Fausto, R.; Eusébio, E. M.; Redinha, J. S. Stepwise Conformational Cooling Towards a Single Isomeric State in the Four Internal Rotors System 1,2-Butanediol. Phys. Chem. Chem. Phys. 2006, 8, 5339−5349. (74) Borba, A.; Gómez-Zavaglia, A.; Fausto, R. Conformational Cooling and Conformation Selective Aggregation in Dimethyl Sulfite Isolated in Solid Rare Gases. J. Mol. Struct. 2006, 794, 196−203. (75) Stein, S. E. Accurate Evaluation of Internal Energy Level Sums and Densities Including Anharmonic Oscillators and Hindered Rotors. J. Chem. Phys. 1973, 58, 2438−2445. (76) Typically, the total time of NIR irradiations was between 1 and 2 h. We did not attempt to irradiate longer and to obtain a maximum possible amount of transformation of Tc into Tt. As described in section 4.4, the near-IR generated Tt form decays spontaneously back to Tc in the dark. Then, during longer irradiations, generation of Tt competes with its decay. Further, the decay process accelerates considerably if the sample is exposed to the broad-band infrared light of the spectrometer source. Such decay must inevitably occur during registration of the mid-infrared and near-infrared experimental spectra presented in sections 4.2 and 4.3. (77) Duarte, L.; Reva, I.; Cristiano, M. L. S.; Fausto, R. Photoisomerization of Saccharin. J. Org. Chem. 2013, 78, 3271−3275. (78) Reva, I. D.; Stepanian, S. G.; Adamowicz, L.; Fausto, R. Conformational Behavior of Cyanoacetic Acid: A Combined Matrix Isolation Fourier Transform Infrared Spectroscopy and Theoretical Study. J. Phys. Chem. A 2003, 107, 6351−6359. (79) Reva, I. D.; Jarmelo, S.; Lapinski, L.; Fausto, R. IR-induced Photoisomerization of Glycolic Acid Isolated in Low Temperature Inert Matrices. J. Phys. Chem. A 2004, 108, 6982−6989. (80) Reva, I. D.; Jarmelo, S.; Lapinski, L.; Fausto, R. First Experimental Evidence of the Third Conformer of Glycolic Acid: Combined Matrix Isolation, FTIR and Theoretical Study. Chem. Phys. Lett. 2004, 389, 68−74. (81) Feringa, L. B. Molecular Switches; Wiley-VCH: New York, 2001. (82) Lopes, S.; Domanskaya, A. V.; Fausto, R.; Räsänen, M.; Khriachtchev, L. Formic and Acetic Acids in a Nitrogen Matrix: Enhanced Stability of the Higher-Energy Conformer. J. Chem. Phys. 2010, 133, 144507. (83) Marushkevich, K.; Räsänen, M.; Khriachtchev, L. Interaction of Formic Acid With Nitrogen: Stabilization of the Higher-Energy Conformer. J. Phys. Chem. A 2010, 114, 10584−10589.

(84) Araujo-Andrade, C.; Reva, I.; Fausto, R. Tetrazole Acetic Acid: Tautomers, Conformers, and Isomerization. J. Chem. Phys. 2014, 140, 064306. (85) Najbauer, E. E.; Bazsó, G.; Góbi, S.; Magyarfalvi, G.; Tarczay, G. Exploring the Conformational Space of Cysteine by Matrix Isolation Spectroscopy Combined with Near-Infrared Laser Induced Conformational Change. J. Phys. Chem. B 2014, 118, 2093−2103. (86) Pettersson, M.; Maçôas, E. M. S.; Khriachtchev, L.; Lundell, J.; Fausto, R.; Räsänen, M. Cis→Trans Conversion of Formic Acid by Dissipative Tunneling in Solid Rare Gases: Influence of Environment on the Tunneling Rate. J. Chem. Phys. 2002, 117, 9095−9098. (87) Maçôas, E. M. S.; Khriachtchev, L.; Pettersson, M.; Fausto, R.; Räsänen, M. Rotational Isomerism of Acetic Acid Isolated in Rare-Gas Matrices: Effect of Medium and Isotopic Substitution on IR-Induced Isomerization Quantum Yield and Cis→Trans Tunneling Rate. J. Chem. Phys. 2004, 121, 1331−1338. (88) Maçôas, E. M. S.; Khriachtchev, L.; Pettersson, M.; Fausto, R.; Räsänen, M. Rotational Isomerization of Small Carboxylic Acids Isolated in Argon Matrices: Tunnelling and Quantum Yields for the Photoinduced Processes. Phys. Chem. Chem. Phys. 2005, 7, 743−749. (89) While this manuscript was in preparation, tunneling in matrixisolated pyruvic acid was reported by Gerbig and Schreiner.34 They analyzed the decay kinetics using the classical monoexponential model and reported the measured half-life times for PA isolated in argon: t1/2 = 12.7 h at 8 K and t1/2 = 11.8 h at 20 K. These numbers lie between the t1/2 half-life times obtained in the present study with 2200 cm−1 filter (16.6 h) and with 4200 cm−1 filter (5.8 h) for PA in argon at 15 K. (90) Siebrand, W.; Wildman, T. A. A Structural Approach to Nonexponential Processes in Disordered Media. Acc. Chem. Res. 1986, 19, 238−243. (91) Plonka, A. Dispersive Kinetics; Kluwer Academic: Dordrecht, The Netherlands, 2001. (92) Skrdla, P. J. Comparison of Two Types of Dispersive Kinetic Approaches in Relation to Time-Dependent Marcus Theory. J. Phys. Chem. A 2007, 111, 11809−11813. (93) Gębicki, J.; Plonka, A.; Krantz, A. Photochemical Generation and Detection of an Elusive Rotamer of Matrix-Isolated Mesityl Oxide. Dispersive Kinetics of the Thermal Isomerization: Twisted s-trans→ s-cis Forms. J. Chem. Soc., Perkin Trans. 2 1990, 2051−2054.

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Conformational Switching in Pyruvic Acid Isolated in Ar and N2

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