The results of a combined vibrational and structural study of the acrylic acid monomer undertaken by matrixisolated low-temperature IR spectroscopy and ab initio SCF-HF and MP2 MO calculations are presented. in addition, both Raman and IR spectra of liquid acrylic acid and the Raman spectrum of the crystal are also reported and interpreted. It is shown that in both argon and krypton matrices acrylic acid monomer exists as a mixture of two conformers of similar energies, differing by the relative orientation of the C=C-C=O axis. Upon irradiation at 1 = 243 nm by a xenon lamp, the scis form (C-C-C-0 dihedral angle equal to 0"), corresponding to the conformational ground state, converts to the s-trans form (C=C-C=O dihedral angle equal to 180"). In the liquid phase, dimeric structures strongly predominate, but the existence in this phase of the two conformational states referred to above can also be inferred from the corresponding vibrational spectra. In turn, in the crystal only the thermodynamically most stable form (scis) exists. Results of ab initio SCF-HF and MP2 molecular orbital (MO) calculations, in particular optimised geometries, relative stabilities, dipole moments and harmonic force fields, for the relevant conformational states of acrylic acid are also presented and the conformational dependence of some relevant structural parameters is used to characterise the most important intramolecular interactions present in the studied conformers. Finally, the calculated vibrational spectra and both the results of a normal-mode analysis based on the theoretical harmonic force fields and of IR intensity studies based on the chargecharge flux-overlap (CCFO) model were used to help interpret the experimental vibrational data, enabling a detailed assignment of the acrylic acid spectra obtained in the different conditions considered.

Ab initio SCF-MO calculations have been carried out for formic, thioformic and dithioformic acids using the 6-31G* basis set. Fully optimized geometries, atomic charges, relative stabilities and harmonic force fields for s-cis and s-trans conformers of these molecules have been determined and the effects of oxygen-by-sulphur substitution analysed. A realistic description of the molecular charge distribution can be reached by introducing a quantum-mechanical correction to the Mulliken atomic charges, derived from the 'charge'-'charge flux'-'overlap' (CCFO) model. Unlike reported theoretical results, the present ab initio calculations yield relative stabilities of the thioformic acid conformers in agreement with experiment [s-cis (thiol) > s-trans(thio1) > s-cis(thione) > s-trans(thione)]. The success of these ab initio calculations should be partially ascribed to the inclusion of polarization functions on all non-hydrogen atoms. Dithiocompounds constitute an ideal res-'*lance Raman (RR) probe for monitoring catalytic events within an enzyme's active site.'-3 The catalylic hydrolysis of substrates containing the RC(=O)O fragment by cysteine proteases (e.g. papain) proceeds through the formation of a thiolacyl enzyme, RC(=O)S-Enz. Using a thion-substituted substrate, it is possible to generate an enzyme-substrate intermediate that contains the chromophore-C(=S)S-, thus differing from the 'natural' intermediate by a single atom substitution Interpretation of the RR spectra of dithioenzyme-substrate intermediates has relied heavily on joint Raman and crystallographic studies on suitable model c o m p~u n d s .~-~ While our understanding of some of these probes has reached a sophisticated level, it is apparent that the assessment of changes in molecular properties produced by oxygenby-sulphur substitutions in molecules containing the-C(=X)Y-, (X,Y = 0 or S) fragment has a major interest to further oiir understanding of the active site data. We have recently initiated a systematic approach to the study of thione-and thiolsubstituted carboxylic acids and esters using ab initio quantum-mechanical calcu'lations.7-' The results obtained for HCSSH using the 3-21G basis set8 were compared with previous data on HCOOH, HC(=O)SH and HC(=S)OH,'* and revealed a remarkable similarity between molecular properties of dithioformic and thiolformic acids. This trend was also observed along the series of the corresponding methyl esters, HCSSCH3 resembling more HC(=O)SCH, than the compounds possessing an oxygen ester at om. 7*9 The importance of rnesomerism in determining the properties of this kind of molecule (fig. 1) is well known, in particular, for their ground conformational state (the s-cis form). We have proposed' that the main effect resulting from C(=O)O-+ C(=O)S or C(=S)O-C (=S)$ substitutions is originated in the poorer conjugating properties of the sulphur 3p orbitals as compared with those of the oxygen 2p orbitals.

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In this theoretical work, the characterization of multiple proton donors in three isomers of BeH 2 ÁÁÁ2HCl trimolecular dihydrogen-bonded complex was developed through the analysis of structural parameters, electronic criteria, and analysis of vibrational harmonic spectrum, where all these data were obtained from the B3LYP/6-311++G(3d,3p) calculations. As dihydrogen bonds are formed by the interaction between two hydrogen atoms, proton donors and hydride hydrogen, the isomers of BeH 2 ÁÁÁ2HCl trimolecular dihydrogen-bonded complexes are stabilized by two, three, and four centers of proton donors. By analyzing the charge density integrations calculated by the theory of Atoms in Molecules, these multiple centers of proton donors were identified by quantification of the intermolecular electronic density, Laplacian operators, and charge transfers.