| Abstract
| - The gas phase structure of cis-formic acid dimers isinvestigated by high-level correlated ab initio molecularorbital methods using large basis sets augmented with both polarizationand diffuse functions (up to MP2/D95++(d,p) level). Seven stable dimer structures werelocated on the potential energy hypersurface of thedimer configurational space with two H-bonding interactions. Nostable dimer with only one H-bond wasfound. The heat of dimerization (10.7−11.3 kcal/mol) of the moststable, C2h cyclic dimer is inexcellentagreement with more recent experimental measurements. Beside thedimer with two equivalent O−H···OCH-bonds, there exist two local minima with significant stabilizationfrom C−H···O interactions. The threeweakest of the seven complexes contain exclusively C−H···OH-bonding interactions. The dimers caninterconvert to each other by rotation, disrupting one of theirH-bonds. The saddle points and the localminima are anticipated not to play important roles in the gas phase butcan have dominant influence on liquiddynamics. The interaction energies of the complexes allow us toassess the relative importance and approximateenergetic contributions of the individual H-bonds to the overallstability of the dimers. It is illustrated that,in addition to the inferred stabilization of two separateO−H···OC H-bonds, the most stableC2h complexis stabilized 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 contraction of the C−H bondlengths and positive frequency shift of theν(C−H) stretching modes relative to the noninteractingmonomer. We also show that the effect of BSSE onthe intermolecular potential surface of the dimers and, in particular,on the location of the true potentialminimum only negligibly influences the interaction energy, butsignificantly distorts the intermolecularequilibrium geometry.
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