The effective fragment model for solvation: Internal rotation in formamide

The internal rotation of formamide with 0–5 water molecules oriented along the N–C bond has been studied by the full ab initio self-consistent field theory and using the effective fragment (EFP) method. For each case, the EFP geometries, harmonic vibrational frequencies, rotational barriers, and intrinsic reaction coordinates for the internal rotation are found to be in excellent agreement with their ab initio counterparts. The global energy minimum structures for four and five water complexes are predicted to be formamide bonded to two adjacent waters, with all water molecules in a ring. Probably due to the structural constraints, the complexes containing less than four waters have cyclic structures with the two ends of formamide connected by a sequence of water molecules. The internal rotation barrier of formamide–water complexes increases from 15.3 kcal/mol with no water to 19.0 kcal/mol with four waters and seems to saturate at four to five waters. When electron correlation corrections are added, the estimated internal rotation barrier is ∼20 kcal/mol, in very good agreement with experimental measurements.