Short-time photodissociation dynamics of iodoalkanes: refinement using resonance raman intensities of isotopic derivatives

Abstract

Resonance Raman spectra of 1,1,1,3,3,3-d6-2-iodopropane and d9-tert-butyl iodide in cyclohexane solution were obtained at excitation wavelengths of 266 and 217.8 nm. The 266 nm spectra, resonant with the directly dissociative A-state absorption, display long overtone progressions in the nominal C-I stretch (ca. 500 cm-1) and some intensity in fundamentals, overtones and combination bands of bending and C-C stretching modes. The 266 nm resonance Raman intensities and the A-band absorption spectrum are simultaneously modelled using wavepacket propagation on a multidimensional locally harmonic potential surface. The dimensionless normal coordinate wavepacket motions determined from the simulations are converted into internal coordinate motions by making use of previous results on the isotopically unsubsrituted compounds to remove some of the indeterminacy in the signs of the normal mode displacements. In the first 10 fs the C-I bond lengthens less in these higher alkanes than in iodoethane, suggesting that more of the available energy goes toward C-I bond cleavage in iodoethane. © 1997 John Wiley & Sons, Ltd.