Spectroscopy of the transition state (theory). 4. Absorption by HFH‡ in H + FH′ → HFH‡ → HF + H′

Absorption spectra of transition-state configurations have been computed for the reaction H + FH′ → HFH‡ → HF + H′. The density of HFH‡ in configuration space was obtained from a classical trajectory study of the collinear reaction on the Williams and Wright (WW) ab-initio 2Σ ground-state potential surface. An upper 2Π surface was calculated for the purposes of this study by using the same MRD-CI (multiple reference double-excitation; configuration interaction) method. Absorption spectra were constructed by mapping the densities in configuration space to the vertical energy difference between the upper and lower surfaces. Structured absorption is predicted in the range between 190 and 240 nm - a favorable spectral region for experimental study. The effect of increasing reagent translational and vibrational energy on the form of the transition-state spectrum is reported; the former shifts the spectral features to the red, and the latter to the blue. These shifts are associated with reaction through more compressed configurations in the first case and more extended in the second case.