AB-INITIO MOLECULAR-ORBITAL ANALYSIS OF DIMERS OF CIS-FORMIC ACID - IMPLICATIONS FOR CONDENSED PHASES

The gas phase structure of cis-formic acid dimers is investigated by h igh-level correlated ab initio molecular orbital methods using large b asis sets augmented with both polarization and diffuse functions (up t o MP2/D95++(d,p) level). Seven stable dimer structures were located on the potential energy hypersurface of the dimer configurational space with two H-bonding interactions. No stable dimer with only one H-bond was found. The heat of dimerization (10.7-11.3 kcal/mol) of the most s table, C-2h cyclic dimer is in excellent agreement with more recent ex perimental measurements. Beside the dimer with two equivalent O-H ... O=C H-bonds, there exist two local minima with significant stabilizati on from C-H ... O interactions. The three weakest of the seven complex es contain exclusively C-H ... O H-bonding interactions. The dimers ca n interconvert to each other by rotation, disrupting one of their H-bo nds. The saddle points and the local minima are anticipated not to pla y important roles in the gas phase but can have dominant influence on liquid dynamics. The interaction energies of the complexes allow us to assess the relative importance and approximate energetic contribution s of the individual H-bonds to the overall stability of the, dimers. I t is illustrated that, in addition to the inferred stabilization of tw o separate O-H ... O=C H-bonds, the most stable C-2h complex is stabil ized by about 0.4-0.6 kcal/mol internal cooperative effect, less than in the similar acetic acid dimer. The C-H H-bonding dimers display con traction of the C-H bond lengths and positive frequency shift of the v (C-H) stretching modes relative to the noninteracting monomer. We also show that the effect of BSSE on the intermolecular potential surface of the dimers and, in particular, on the location of the true potentia l minimum only negligibly influences the interaction energy, but signi ficantly distorts the intermolecular equilibrium geometry.