VIBRATIONAL LEVELS OF THE TRANSITION-STATE AND RATE OF DISSOCIATION OF TRIPLET ACETALDEHYDE

Fluorescence decay of the S-1 state of d(4)-acetaldehyde is measured. Below the dissociation threshold for formation of fragments of formyl and methyl radicals, single exponential decays of fluorescence are obs erved. Biexponential decay is observed when the excitation energies ar e near and above the dissociation threshold. Hence, in this region the mechanism for intramolecular relaxation of energy reaches the ''inter mediate case.'' Strong coupling between S-1 and T-1 states or reversib le reaction for S-1 <-> T-1 results in this biexponential behavior in the fluorescence decay of S-1. Rates of appearance of DCO from dissoci ation of acetaldehyde are measured. The stepwise increases in plots of both rate of appearance of formyl radicals and rate of decay of excit ed acetaldehyde versus excitation energy for both isotopic variants of acetaldehyde are observed. According to fits to Rice-Ramsperger-Kasse l-Marcus (RRKM) theory, these sharp increases correspond to vibrationa l levels of transition state in the triplet surface. Frequencies of to rsional and C-C-O bending modes are determined to be (73 +/- 10) and ( 200 +/- 10) cm(-1) for the transition state of CD3CDO and the torsiona l frequency (80 +/- 10) cm(-1) for CH3CHO. That these fitted frequenci es are less than values calculated with methods B3LYP and MP2 indicate s a looser transition state than calculated for dissociation. From the se fits the dissociation threshold for formation of radical fragments is determined to be 31 845 for CD3CDO and 31 650 cm(-1) for CH3CHO. (C ) 1998 American Institute of Physics. [S0021-9606(98)02545-8].