Ab initio and semiempirical (AM1, PM3, and SAM1) molecular orbital studies on monomeric and dimeric acetic acid are reported. The highest-level ab initio calculations (MP2/6-311++G(d,p)) on the monomeric forms of acetic acid predicted the cis form to be preferred over the trans by 6.1 kcal/mol. Four different dimers were studied. The enthalpy of stabilization at 298 K is 11.8 kcal/mol at the MP2/6-31G(d) level after correction for BSSE and ZPVE for the cyclic dimer containing two O-H⋯-O hydrogen bonds, which is 2.4-3.1 kcal/mol more stable than twice the open dimer containing only one O-H⋯O interaction. We attribute this difference to cooperativity in the cyclic structure. Vibrational analyses at the MP2/6-31G(d) level confirm this assessment. According to the best dimer calculations (MP2/6-31G(d)), the hydrogen-bonding enthalpy of a C-H⋯O bond is about 0.5-1.0 kcal/mol, while that of a simple O-H⋯O is 4.7 kcal/mol. Vibrational analyses on the monomers and dimers are reported and compared to experimental results. We suggest two modifications of vibrational assignments. The semiempirical results are compared with the ab initio calculations.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry