Entropy effects in conformational distribution and conformationally dependent UV-induced photolysis of serine monomer isolated in solid argon S. Jarmelo * , R. Fausto Department of Chemistry, University of Coimbra, 3004-535 Coimbra, Portugal Received 6 September 2005; received in revised form 27 September 2005; accepted 27 September 2005 Available online 7 November 2005 Abstract Monomeric serine can be trapped in low temperature argon matrices in different conformers, which can be classified in three groups (A, B, C) accordingly to the main intramolecular interaction they exhibit: A (OH A /N hydrogen bond), B (OH C /N) and C (OH A /Oa) (subscripts A and C stand for alcohol and carboxylic group, respectively). The OH C /N intramolecular interaction found in B-type conformers is considerably stronger than both the OH A /N and OH A /Oa hydrogen bonds, and leads to reduce the abundance of B-type form relatively to A and C forms at high temperatures due to entropy effects. When submitted to UV irradiation (lO200 nm), the main observed photoprocess is decarboxylation, leading to production of CO 2 and ethanolamine. A less important photochemical process is also observed, where the compound undergoes decarbonylation, with formation of CO, H 2 O and acetamide. The two observed photoprocesses were found to be dependent on the conformation assumed by the reactant molecule, with A- and C-type conformers of serine undergoing decarboxylation and B-type conformers decarbonylation. q 2005 Elsevier B.V. All rights reserved. Keywords: Serine; Matrix-isolation infrared spectroscopy; Entropy effect on conformational equilibrium; Conformationally dependent photodecarboxylation 1. Introduction The study of the neutral (non-zwitterionic) form of aminoacids by matrix isolation infrared spectroscopy has been receiving considerable interest in recent years [1–9]. Indeed, this experimental approach has been particularly well succeeded in elucidating many important details of the molecular physics and chemistry of this type of compounds. Some notable examples are (i) the first experimental observation of the third conformer of glycine [1], which until then had successively escaped to detection by other techniques [10], (ii) the first direct observation of conformationally selective aggregation in aminoacids, in the case, N,N- dimethylglycine [7], and (iii) the identification of the entropy associated with the intramolecular hydrogen-bonds present in the possible conformers of aminoacids as an important variable to determine their relative gas-phase populations [8,9]. Recently, matrix isolation connected with infrared spec- troscopy was also successfully used to investigate the molecular structure, preferred conformations and vibrational spectra of monomeric neutral form of serine [11]. In the gaseous phase, the compound was found to exist in several different conformers, which can be classified in three groups (A, B, C) accordingly to the main intramolecular interaction they exhibit: A (OH A /N hydrogen bond), B (OH C /N) and C (OH A /Oa) (throughout this article, in references to the OH groups, the subscripts A and C stand for alcohol and carboxylic group, respectively). Representatives of each of these groups could be trapped in low temperature argon matrices and characterized structurally and spectroscopically [11]. In this study, the conformational equilibrium of serine in the gaseous phase (TZ470 K) was re-investigated by taking into account entropy effects. In addition, the matrix-isolated serine was submitted to UV irradiation (lO200 nm) and the influence of the conformation of the reactant molecule in determining the preference for two observed reaction channels (decarboxyla- tion and decarbonylation) was investigated. 2. Experimental and computational details Both DL-serine and its D,LK3,3-dideuterated (O98% D) isotopologue used in this study were commercial products, obtained from Aldrich and Icon Stable Isotopes, respectively. Matrices were prepared as described elsewhere [11]. The temperatures of the vapor of the compound prior to deposition and of the cold tip of the cryostat were 470 and 10 K, Journal of Molecular Structure 786 (2006) 175–181 www.elsevier.com/locate/molstruc 0022-2860/$ - see front matter q 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2005.09.021 * Corresponding author. Tel.: C35 1239852080; fax: C35 1239827703. E-mail address: sjarmelo@qui.uc.pt (S. Jarmelo).