Matrix Isolation Fourier Transform Infrared Study of Photodecomposition of Formimidic Acid Fabrice Duvernay, Aurelien Trivella, Fabien Borget, Stephane Coussan, Jean-Pierre Aycard, and Thierry Chiavassa* Physique des Interactions Ioniques et Mole ´ culaires, Unite ´ mixte de recherche 6633, UniVersite ´ de ProVence et Centre National de la Recherche Scientifique, Centre de St Je ´ ro ˆ me, case 252, 13397 Marseille Cedex 20, France ReceiVed: August 30, 2005; In Final Form: October 6, 2005 The UV isomerization of formamide (HCONH 2 ) trapped in xenon, nitrogen, argon, and neon cryogenic matrices has been monitored by Fourier transform infrared (FT-IR) spectroscopy. Formamide monomer is the only species present in the matrices after deposition; when UV-selective irradiation was carried out at 240 nm, the n f π* transition allowed us to observe the formation of several isomers of formimidic acid [H(OH)Cd NH]. On these latter species, we carried out selective IR irradiation of their OH stretching mode and compared the experimental and theoretical (B3LYP/6-311+G(2d,2p)) sets of bands. This study allowed us to characterize for the first time all the isomers of formimidic acid. We have then studied the vacuum UV photodecomposition (λ > 160 nm) of this molecule at 10 K in argon and xenon matrices. Several primary photoproducts such as HCN‚H 2 O, HNC‚H 2 O, and HNCO‚H 2 complexes, yielded by dehydration and dehydrogenation processes, were characterized. 1. Introduction Formamide/formimidic acid, of CH 3 NO molecular formula, represents the simplest form of amide/imidic acid tautomerism, which is known to play an important role in many areas of chemistry and biochemistry. 1 Selective UV irradiations at 193 nm of formamide isolated in argon matrix have been performed by Lundell et al., 2 leading to the characterization of the photoproducts: CO‚NH 3 and HNCO‚H 2 complexes. 2 On the other hand, UV irradiation at 248 nm in argon matrix with a KrF excimer laser was performed by Maier and Endres, 3 leading through a 1-3 hydrogen transfer to the formimidic acid (H(OH)CdNH), a tautomer of the formamide. Several cases of UV-induced intramolecular proton-transfer reaction from the NH group of amide or thioamide have been already observed in cryogenic matrix. 4-7 Maier and Endres 3 have shown by comparison between the experimental and calculated IR spectra that formimidic acid is present in argon matrix as two forms called (s-Z)-(E) and (s-Z)-(Z) as displayed in Scheme 1. Among the four possible conformers of formimidic acid, the most stable was found to be the (s-Z)-(E) form, predicted to be 50 kJ mol -1 less stable than formamide. 8 However, formimidic acid is found to be more stable than the other CH 3 NO isomers such as formaldoxime, nitrosomethane, nitrone, and oxaziridine. The barrier estimated by ab initio calculations in the ground state for the conversion between formamide and formimidic acid is relatively high (200 kJ mol -1 above formamide). 8 The energy needed to get over this barrier can be provided by the resonant excitation of the forbidden n f π* transition located at 219 nm (ca. 544 kJ mol -1 ). 9-11 In this paper we present selective UV irradiation (240 nm) of formamide in different environments (Xe, N 2 , Ar, and Ne) leading to the formimidic acid. Then we suggest the vibrational assignments of the different formimidic acid isomers obtained by selective IR irradiation in the ν(OH) region and supported by theoretical calculations. We report the results relative to the photolysis of the formimidic acid that have been performed with broad-band UV irradiation of a H 2 lamp irradiating at λ > 160 nm. We give evidence of dehydrogenation and dehydration processes that lead to the formation of complexes such as HNCO‚H 2 and HCN‚H 2 O. 2. Experimental Details Formamide was obtained from Aldrich (99% purity) and purified under vacuum and prolonged degassing at 50 °C. The vapor pressure of formamide is too low to be handled by the usual vacuum line method. Thus, it is placed into a small glass tube, which is connected to the cryostat and then entrained at room temperature with argon (Linde, 99.99% purity), neon (Air Liquide, 99,99% purity), xenon (Air Liquide, 99.99% purity), or nitrogen, N 2 (Air Liquide, 99.99% purity) on a Au-plated cube of copper cooled between 4 and 10 K. Under these conditions, we cannot determine accurately the formamide concentration in the host gases; we can only state that we worked with a large excess of host gases in the mixture (around 1/500). During the deposition, the cryostat is kept under a limit and constant pressure of 10 -7 mbar and the sample is deposited at 4, 20, 25, and 30 K in Ne, Ar, N 2 , and Xe matrices, respectively. * Corresponding author: Tel +33 491-288-580; fax +33 491-636-510; e-mail Thierry.chiavassa@up.univ-mrs.fr. SCHEME 1: Phototautomeric Reaction of Formamide 11155 J. Phys. Chem. A 2005, 109, 11155-11162 10.1021/jp054903w CCC: $30.25 © 2005 American Chemical Society Published on Web 11/19/2005