Conformation and Hydrogen Bonding in the Alaninamide−Water van der Waals Complex

Five conformational minima of the alaninamide−water complex were found from MP2/6-31+G** calculations. The structure of the lowest energy conformation has a network of intermolecular hydrogen bonds from the amide to water and from water to the carbonyl oxygen; this structure is also based on the lowest energy conformation of the alaninamide monomer. The amide-to-water and water-to-carbonyl hydrogen bond lengths in the fully optimized structure are 2.061 and 1.914 Å. Rotational spectra have been recorded for four isotopomers of alaninamide−water, and the structure of the complex was fit to the experimental moments of inertia. The intermolecular hydrogen bond lengths in the best-fit structure are 2.042(6) Å (amide-to-water) and 1.931(4) Å (water-to-carbonyl oxygen), and this structure is nearly identical with the lowest energy ab initio structure.