Amide-I and amide-A bands of δ-valerolactam and its dimers: a combined matrix isolation – FTIR spectroscopy and quantum chemistry study

Abstract

Singly and doubly hydrogen-bonded dimers of δ-valerolactam, a biologically important cyclic amide, are studied by matrix isolation FTIR spectroscopy and quantum chemistry methods. The spectral features of the doubly hydrogen-bonded dimer (DHBD) are nearly similar to those displayed by the dimer produced in CCl₄ solution at room temperature. On the other hand, the matrix environment allows identification of singly hydrogen-bonded dimer (SHBD) which is a transient species in the solution and gas phases. Identification of the two species are made by noting the spectral shifts of amide-I and amide-A bands corresponding to the two dimers and observed changes are consistent with predictions of electronic structure theory calculations. A cooperative strengthening of hydrogen bond is manifested in the large increment of C=O and N─H bond lengths in DHBD, as compared to those in SHBD.