Conformational Isomerization of Formic Acid by Vibrational Excitation at Energies below the Torsional Barrier Mika Pettersson,* ,† Ermelinda M. S. Mac ¸ o ˆ as, †,‡ Leonid Khriachtchev, † Rui Fausto, ‡ and Markku Ra ¨ sa ¨ nen † Laboratory of Physical Chemistry, P.O. Box 55, FIN-00014, UniVersity of Helsinki, Finland, and Department of Chemistry-CQC, UniVersity of Coimbra, P-3004-535, Coimbra, Portugal Received November 27, 2002; E-mail: petters@csc.fi Understanding the structural and dynamical conformational properties of molecules is of fundamental and practical value in chemistry. One of the most fascinating issues in this respect is the folding of proteins, which represents a very important but simul- taneously extremely complicated and multidimensional problem. The most detailed information can be obtained by studying small molecules. In this respect, formic acid (FA), being the simplest organic acid, represents a good model system for studying the conformational properties of carboxylic acids and the carboxylic group in general. It is known that interconversion of different conformers can be induced in the solid state by vibrational excitation. 1 Coupling of the initially excited state with other modes of the isolated molecule and with lattice phonons leads to the flow of energy into the reaction coordinate, finally promoting the change of the geometry of the molecule from one conformation to another. 1,2 Usually, it is natural to assume that to induce the conformational conversion, the initially excited state should lie well above the reaction barrier. 2,3 In this communication, we show that in FA, due to tunneling, the conversion from the lower energy trans to the higher energy cis form can be induced with high efficiency at energies well below the reaction barrier. In the experiments, FA (KEBO lab, >99%) was mixed with argon (AGA 99.9999%) in a glass bulb with a ratio FA/Ar ) 1/2000. The mixture was deposited on a CsI window held at 15 K in a cryostat (APD DE 202 A). After deposition, the sample was cooled to 8 K. The IR pumping was carried out with pulsed (∼5 ns) narrowband IR radiation of an optical parametric oscillator (Sunlite with an infrared extension, Continuum, fwhm ≈ 0.1 cm -1 ). The absolute wavelength was determined with a wavemeter (Burleigh WA-4500), and the pulse energy was measured with a pulsed energy meter (Molectron). The pulse energies were 0.5-1 mJ, and the repetition rate was 10 Hz. The IR spectra were measured with a FTIR spectrometer (Nicolet 60 SX) with a resolution of 1 or 0.25 cm -1 . The experimental setup was arranged in such a way that IR spectra could be recorded during irradiation. Suitable interference filters were used to suppress the effect of the glowbar radiation on the IR induced isomerization. It was also verified that the room-temperature radiation from the surroundings had a negligible effect on the results. An important value for the present work is the barrier height for the trans f cis conversion. There are various estimates for the barrier in the literature, and the high level ab initio calculations give ∼50 kJ/mol (4500 cm -1 ). 4,5 We performed additional calcu- lations within the GAUSSIAN 98 program package. 6 Our value for the barrier height at the CCSD(T)/aug-cc-pVTZ//MP2/aug-cc- pVTZ level is 3921 cm -1 (corrected for the zero-point-energy (ZPE) using the calculated frequencies at the MP2 level). At the same level of theory, the energy difference between the cis and trans forms is 1396 cm -1 , the experimental value being 1365 ( 30 cm -1 . 7 An additional factor to consider is the effect of the environment on the energetics. We have estimated the effect of the medium on the barrier height within the PCM (polarized continuum model) solvation model in GAUSSIAN 98 using a dielectric constant of 1.63 for argon. 8 According to these results, the barrier height is reduced by 111 cm -1 from the gas phase to solid argon. Combining these computational results, we reliably estimated the barrier height for the solid-state trans f cis conversion in an Ar matrix to be 3810 ( 100 cm -1 . Cis FA was prepared by pumping various vibrational transitions of trans FA. 9 Cis FA decays back to trans FA in a time scale of minutes even at 8 K due to tunneling. 9,10 In effect, under continuous IR pumping, a photoequilibrium is established, and the equilibrium concentrations of the two forms [cis] eq and [trans] eq are given by the pumping efficiency and the tunneling rate for the backreaction. The pumping efficiency is determined by the radiation intensity, absorption cross section, and the quantum yield for the isomeriza- tion, the first two values being experimentally measurable. The backreaction rate can also be measured. 10 Thus, the quantum yield for the isomerization at different excitation wavelengths can be extracted. The results of these experiments are shown in Figure 1, presenting the quantum yield for pumping several different fundamental and combination vibrations in the energy interval 2900-5000 cm -1 . The position of the barrier for isomerization determined from the ab initio calculations is given in the figure as well. The isomerization efficiency remains surprisingly high, being about 20% down to ∼4100 cm -1 . There is very little difference in † University of Helsinki. ‡ University of Coimbra. Figure 1. The quantum yield for the trans f cis isomerization of formic acid via pumping various vibrational transitions. The line connecting the points is for guiding the eye. The position of the barrier for the isomerization from the ab initio calculations is indicated in the figure with a vertical line, and the estimated uncertainty is indicated by a shaded region. The points correspond to the following vibrational modes: 4651 cm -1 , νOH + COH- CO def.; 4184 cm -1 , νOH + τOH; 4174 cm -1 , νCH + CO-COH def.; 3552 cm -1 , νOH; 3516 cm -1 ,2νCO; 2955 cm -1 , νCH. Published on Web 03/14/2003 4058 9 J. AM. CHEM. SOC. 2003, 125, 4058-4059 10.1021/ja0295016 CCC: $25.00 © 2003 American Chemical Society Downloaded by PORTUGAL CONSORTIA MASTER on July 10, 2009 Published on March 14, 2003 on http://pubs.acs.org | doi: 10.1021/ja0295016