Formation of a dual hydrogen bond in the N-H...C=O moiety in the indole- (N-methylacetamide) 1 cluster revealed by IR-dip spectroscopy with natural bond orbital analysis

Abstract

IR-dip spectra in the NH stretch regions have been measured for the S0 state of the indole/N-methylacetamide 1:1 clusters (Ind- NMA1). We identified two structural isomers of Ind- NMA1 that possess an N-H...O=C hydrogen bond. The redshifts of the NH stretch fundamental of the indole moieties in In d- NMA1 are larger than that for Ind- (H 2O) 1 [Carney, Hagemeister, and Zweir, J. Chem. Phys. 108, 3379 (1998)], indicating that the strength of the N-H...O=C hydrogen bond in Ind- NMA1 is stronger than that of the N-H...O-H hydrogen bond in Ind- (H2O) 1. On the basis of the natural bond orbital analysis we suggest that two lone pair orbitals of the O atoms in the NMA moiety form a dual hydrogen bond with the NH group designated by N-H:::O=C. Owing to the dual nature of the N-H:::O=C hydrogen bond its strength in Ind- NMA1 is larger than that of the N-H...O-H hydrogen bond in Ind- (H2O) 1. © 2009 American Institute of Physics.