Doubly hydrogen bonded dimer of δ-valerolactam: infrared spectrum and intermode coupling.

We report here the vibrational analysis of the infrared spectrum of doubly hydrogen bonded dimer of δ-valerolactam measured in CCl4 solution at room temperature (22 °C). The compound shows an equilibrium of population distributed over the monomer and doubly hydrogen bonded dimer, which is manifested by the isosbestic point in the normalized spectra for solutions of different concentrations. Dimerization induced changes in transition frequencies and intensities have been measured and compared with the computed results. Our results suggest doubling of the intensity of the amide-I (predominantly νC═O) band by double hydrogen bonding at the amide (-C(O)-N(H)-) interface. The amide-A (νN-H) spectral region appears broad and is featured with quite a few numbers of substructures. These substructures are theoretically reproduced by incorporating electrical anharmonicity to the vibrational states. Computational results at the MP2/6-311++G(d,p) level of theory are seen to nicely agree with the measured spectral data.