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.

IR-dip spectra in the NH stretch regions have been measured for the S(0) state of the indole/N-methylacetamide 1:1 clusters (Ind-NMA(1)). We identified two structural isomers of Ind-NMA(1) that possess an N-H...O=C hydrogen bond. The redshifts of the NH stretch fundamental of the indole moieties in Ind-NMA(1) are larger than that for Ind-(H(2)O)(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-NMA(1) is stronger than that of the N-H...O-H hydrogen bond in Ind-(H(2)O)(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-NMA(1) is larger than that of the N-H...O-H hydrogen bond in Ind-(H(2)O)(1).