Solvation and solvent effects on the short-time photodissociation dynamics of CH₂I₂from resonance Raman spectroscopy

Resonance Raman spectra of CH2I2have been obtained at excitation wavelengths of 369, 355, and 342 nm in cyclohexane solution and in methanol solution at excitation wavelengths of 355 and 342 nm. Resonance Raman spectra were also measured for CH2I2in the vapor phase with an excitation wavelength of 355 nm. The resonance Raman spectra of CH2I2exhibit most of their intensity in fundamentals, overtones, and combination bands of modes nominally assigned as the I-C-I symmetric stretch, the I-C-I bend, and the I-C-I antisymmetric stretch vibrations. The absorption spectra and resonance Raman intensities of the gas phase and methanol solution phase diiodomethane spectra were simulated using a simple model and time-dependent wave packet calculations. Normal mode coefficients from normal coordinate calculations were used to convert the motion of the wave packet on the excited electronic state surface from dimensionless normal coordinates into internal coordinates of the molecule. The short-time photodissociation dynamics of diiodomethane in the vapor phase shows that the two C-I bonds are lengthening by the same amount, the I-C-I angle becomes smaller, the H-C-I angles become larger, and the H-C-H angle becomes smaller. The two C-I bonds appear essentially equivalent in the Franck-Condon region of the gas phase photodissociation which implies that the molecule chooses which C-I bond is broken after the wave packet has left the Franck-Condon region of the potential energy surface. Comparison of the gas phase resonance Raman spectrum with solution phase spectra obtained in cyclohexane and methanol solvents reveals that the short-time photodissociation dynamics are noticeably changed by solvation with a large solvent-induced symmetry breaking observed. In the Franck-Condon region of the solution phase diiodomethane photodissociation in methanol solvent the two C-I bond become larger by differing amounts, the I-C-I angle becomes smaller, the H-C-H angle becomes smaller, and the H-C-I angles differ from the corresponding gas phase values. During the initial stages of the solution phase photodissociation (at least in methanol and cyclohexane solvents) the two C-I bonds are not the same and this suggests that the molecule chooses which C-I bond will be broken soon after photoexcitation.