On apparent quantized transition-state thresholds in the photofragmentation of acetaldehyde

Recent photofragmentation experiments have observed stepwise increases in the dissociation rate for CH3CHO (T-1)--> CH3 ((X) over tilde (2)A(2)) + HCO ((X) over tilde (2)A') as a function of excitation energy. In accord with the Rice-Ramsperger-Kassel-Marcus (RRKM) form of transition-state theory, these steps were interpreted as corresponding to vibrational levels of the fragmentation transition state on the triplet surface. We have investigated this acetaldehyde dissociation using coupled cluster (CC) and density functional (DFT) methods with [C,O/H] atomic-orbital basis sets ranging in quality from [4s2p1d/2s1p] to [6s5p4d3f2g1h/5s4p3d2 f1g]. A high-level focal point analysis, along with harmonic force field computations, results in predictions of the dissociation energy, D-0 = 1583 cm(-1), and the association barrier height, V-0* = 3149 cm(-1). With a basis set of triple-zeta plus double-polarization plus f(TZ2Pf ) quality, the DFT method UB3LYP and the CC method RCCSD predict barrier frequencies of 355i cm(-1) and 516i cm(-1), respectively, while the empirical value inferred from RRKM models is only 60i cm(-1). The RRKM-derived frequencies for the degrees of freedom orthogonal to the reaction path are more reasonable but still not in convincing agreement with electronic structure theory. Thus, while the experimental steps in the dissociation rate of acetaldehyde (as well as ketene) have yet to be satisfactorily explained, proven ab initio methods provide strong evidence that simple RRKM fits to the k(E) profile provide misleading vibrational frequencies of the transition state on the corresponding triplet potential energy surface. (C) 2000 American Institute of Physics. [S0021-9606(00)30811-X].