Intermolecular vibrational modes and orientational dynamics of cooperative hydrogen-bonding dimer of 7-azaindole in solution

We observed the low-frequency Raman-active intermolecular vibrational modes of 7-azaindole in CCl4 by femtosecond Raman-induced Kerr effect spectroscopy. To understand the dynamical aspects and vibrational modes of 7-azaindole in the solution, the ultrafast dynamics of 1-benzofuran in CCl4 was also examined as a reference and ab initio quantum chemistry calculations were performed for 7-azaindole and 1-benzofuran. The cooperative hydrogen-bonding vibrational bands of 7-azaindole dimer in CCl4 appeared at 89 cm(-1) and 105 cm(-1) represent the overlap of stagger and wheeling modes and the intermolecular stretching mode, respectively. They are almost independent of the concentration in the solution. We further found from the low-frequency differential Kerr spectra of the solutions with neat CCl4 that the intermolecular motion in the low frequency region below 20 cm(-1) was less active in the case of 7-azaindole/CCl4 than in the case of 1-benzofuran/CCl4. The slow orientational relaxation time in 7-azaindole/CCl4 is similar to 3.5 times that in 1-benzofuran/CCl4 because of the nature of the dimerization of 7-azaindole. (C) 2011 American Institute of Physics. [doi:10.1063/1.3583642]