Solute Solvent Interactions of Benzonitrile in Solutions Studied by Sub-Picosecond Infrared Pump-Probe Spectroscopy

The vibrational energy relaxation of the CN stretching mode of benzonitrile (BN) in solution has been investigated by infrared (IR) pump-probe spectroscopy. The peak wavenumber, line shape, and cross section of the ground state absorption band due to the CN stretching mode depend on the solvent and concentration. From the experimental results of lR spectroscopy and calculations with density functional theory, the structures of solute-solute or solute-solvent complexes are suggested. In hexane, BN exists as a monomer or a dimer in which the two CN groups are arranged antiparallel. In ethanol (EtOH), it is suggested that BN forms the antiparallel dimer and a BN-EtOH hydrogen-bonded complex. In dimethyl sulfoxide (DMSO), BN probably forms a solute-solvent complex. From the results of IR. pump-probe spectroscopy, the decay time constant of the pump-probe signal shows a probe wavenumber dependence in hexane and DMSO, which indicates inhomogeneity of the microscopic environment around BN in these solvents. In EtOH, the wavenumber dependence was not observed. This suggests that the VER processes for the BN dimer and the BN-EtOH complex are accelerated from that for the monomer, and the VER times are almost identical among the two structures.