Microsolvation of formamide:: A rotational study

Microsolvated formamide clusters have been generated in a supersonic jet expansion and characterized using Fourier transform microwave spectroscopy. Three conformers of the monohydrated cluster and one of the dihydrated complex have been observed. Seven monosubstituted isotopic species have been measured for the most stable conformer of formamide center dot center dot center dot H2O, which adopts a closed planar ring structure stabilized by two intermolecular hydrogen bonds(N-H center dot center dot center dot O(H)-H center dot center dot center dot O=C). The two higher energy forms of formamide center dot center dot center dot H2O have been observed for the first time. The second most stable conformer is stabilized by a O-H center dot center dot center dot O=C and a weak C-H center dot center dot center dot O hydrogen bond, while, in the less stable form, water accepts a hydrogen bond from the anti hydrogen of the amino group. For formamide center dot center dot center dot(H2O)(2), the parent and nine monosubstituted isotopic species have been observed. In this cluster the two water molecules close a cycle with the amide group through three intermolecular hydrogen bonds (N=H center dot center dot center dot O(H)-H center dot center dot center dot O(H)-H center dot center dot center dot O=C), the nonbonded hydrogen atoms of water adopting an up-down configuration. Substitution (r(s)) and effective (r(0)) structures have been determined for formamide, the most stable form of formamide center dot center dot center dot H2O and formamide center dot center dot center dot(H2O)(2). The results on monohydrated formamide clusters can help to explain the observed preferences of bound water in proteins. Clear evidence of sigma-bond cooperativity effects emerges when comparing the structures of the mono- and dihydrated formamide clusters. No detectable structural changes due to pi-bond cooperativity are observed on formamide upon hydration.