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 Angstrom. 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) Angstrom (amide-to-water) and 1.931(4) Angstrom (water-to-carbonyl oxygen), and this structure is nearly identical with the lowest energy ab initio structure.