Abstract
The significance of the heteroatom in influencing the acidity and binding affinity of the C(2)-H donor in five-membered heterocyclic rings is explored. The water clusters of benzoxazole (BOX) are studied in a supersonic jet by IR-UV double resonance spectroscopy and compared with those of benzimidazole (BIM) and its N-methyl derivative (MBIM). Two conformers are identified for the monohydrated cluster, both of which are O-H···N bound and differ in their orientation with respect to the intermolecular hydrogen bond. IR spectroscopy of the doubly hydrated cluster shows the presence of an intensity enhanced C(2)-H (carbon atom between the heteroatoms in the five-membered ring) stretching mode in addition to two red-shifted bound OH stretches, indicating that the water molecules form a hydrogen-bonded bridge encompassing the N acceptor and the weakly activated C(2)-H bond in oxazole. Comparison of the topological parameters of electron density, natural bond orbital analyses, and computed binding energies of the analogous hydrated structures of BOX, BIM, and MBIM indicates that the C(2)-H bond in the former is a more potent H-bond donor.
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