Energetics of the Lighter Chalcogen Analogues of Carboxylic Acid Esters † Carol A. Deakyne* and Alicia K. Ludden Department of Chemistry, UniVersity of Missouri, Columbia, Missouri 65211-7600, United States Maria Victoria Roux and Rafael Notario Instituto de Quimica Fisica “Rocasolano”, CSIC, Serrano 119, 28006 Madrid, Spain Alexei V. Demchenko and James S. Chickos Department of Chemistry and Biochemistry, UniVersity of Missouri-St. Louis, One UniVersity BouleVard, St. Louis, Missouri 63121-4499, United States Joel F. Liebman Department of Chemistry and Biochemistry, UniVersity of Maryland, Baltimore County, 1000 Hilltop Circle, Baltimore, Maryland 21250-1000, United States ReceiVed: July 31, 2010; ReVised Manuscript ReceiVed: September 30, 2010 In the current paper we present the results of our quantum chemical (G2, G2(MP2), and G3) study of the structure and energetics of carboxylic acids and their chalcogen analogues. In the particular, calculations and accompanying natural bond orbital (NBO) and atoms in molecules (AIM) analyses were performed on all species with the generic formula RC(dX)YR′ (X, Y ) O, S, Se and R ) R′ ) CH 3 ). Energies, enthalpies, and free energies of formation, resonance energies, interchalcogen methyl transfer energies and their energies of activation, and heavy atom bond lengths and angles are all discussed. A comparison of the calculated results with the sparse experimentally available data shows good agreement. Trends are also presented. Introduction Carboxylic acid esters [RC(dO)OR′] are nearly ubiquitous. They are found in foods as definitional components of fats (cf. triglycerides) and in flavorings (e.g., methyl anthranilate and grapes). They are found in plastics as monomeric components, (e.g., methyl methacrylate, vinyl acetate) and as plasticizers (e.g., dibutyl phthalate). They are found in drugs, both legal (e.g., lidocaine) and illegal (e.g., heroin). It is thus not surprising that they have also caught the attention of the thermochemical com- munity: a quick look at diverse data compendiasDomalski, 1 Stull et al., 2 Pedley, 3 and the WebBook (Afeefy, Liebman, Stein chapter) 4 sshows data for well over a hundred esters. Carboxylic acid esters containing sulfur are rather much rarer in any of their three incarnations, S-thiocarboxylate esters (thiolesters, [RC(dO)SR′]), O-thiocarboxylate esters (thionesters, [RC(dS)- OR′]), and dithioesters [RC(dS)SR′], respectively. Of these, S-thiocarboxylate esters (thiolesters), as perhaps befits their relevance to biochemical energetics and coenzyme A, 5 have been studied by calorimetrists, but these efforts have been dominated by thiolacetates. 6-9 Exclusive of carbamic acid derivatives, 10-15 O-thioesters (thionesters) have seemingly been ignored by this community. Nonetheless, some qualitative inferences may be drawn, such as O-thioesters are less stable than the correspond- ing isomeric S-thioester as evidenced by the rearrangement for the former into the latter. 16-19 This oversight is surprising, indeed disappointing, given the extensive chemical interest in sulfur- containing species. Selenium-containing esters (with one or two seleniums, and if but one, with the other element oxygen or sulfur) likewise remain unstudied by thermochemical research- ers. This omission is less surprising given the paucity of such studies with selenium compounds in general as compared to those containing its lighter chalcogen congeners, as in the brief review in the “Patai series” on organoselenium and organotel- lurium thermochemistry, 20 as opposed to that on general sulfur compounds, 21 and the earlier, but more specialized one, on sulfonic acids and derivatives. 22 As befits our earlier studies of the energetics of chalcogen- containing 16 valence electron triatomics and pentaatomics 23-25 and of a pair of thiocarbamate and dithiocarbamate heterocycles, 15,16 we now report our computational study of S- and O-thiocar- boxylate esters (thiol and thiocarboxylates), dithiocarboxylate esters, and their selenium counterparts. In particular, we have investigated the structures and energetics of the nine compounds, H 3 CC(dX)YCH 3 (X, Y ) O, S, Se), at the G2, G2(MP2), and G3 levels of theory. The focus of this work is on the following properties of these compounds: resonance energies, enthalpies of formation, barrier heights for intramolecular 1,3-methyl migration, and syn (Z) vs anti (E) relative stabilities. We are interested in how these properties differ down the group and between the C(dX)Y and the C(dY)X arrangements. Relatively few previous quantum mechanical studies of sulfur- containing esters have appeared in the literature. Moreover, although the relative energies of HC(dSe)OH and HC(dO)SeH and the barrier for OfSe hydrogen transfer have been examined computationally, 26 as have the gas-phase acidities of the series of selenocarboxylic acids RCSeOH and RCOSeH, R ) H, F, Cl, NH 2 , CH 3 , 27 we have found no computational studies performed on selenoesters. The theoretical work with the most † Part of the “Robert A. Alberty Festschrift”. * To whom correspondence should be addressed. E-mail: deakynec@ missouri.edu, phone: 573-882-1347, fax: 573-882-2754. J. Phys. Chem. B 2010, 114, 16253–16262 16253 10.1021/jp107208q  2010 American Chemical Society Published on Web 10/28/2010