Infrared photodissociation of a water molecule from a flexible molecule-H₂O complex: Rates and conformational product yields following XH stretch excitation

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Infrared-ultraviolet hole-burning and hole-filling spectroscopies have been used to study IR-induced dissociation of the tryptamine·H₂O and tryptamine·D₂O complexes. Upon complexation of a single water molecule, the seven conformational isomers of tryptamine collapse to a single structure that retains the same ethylamine side chain conformation present in the most highly populated conformer of tryptamine monomer. Infrared excitation of the tryptamine·H₂O complex was carried out using a series of infrared absorptions spanning the range of 2470–3715  cm⁻¹. The authors have determined the conformational product yield over this range and the dissociation rate near threshold, where it is slow enough to be measured by our methods. The observed threshold for dissociation occurred at 2872  cm⁻¹ in tryptamine·H₂O and at 2869  cm⁻¹ in tryptamine·D₂O, with no dissociation occurring on the time scale of the experiment (~2  µs) at 2745  cm⁻¹. The dissociation time constants varied from ~200  ns for the 2869  cm⁻¹ band of tryptamine·D₂O to ~25  ns for the 2872  cm⁻¹ band of tryptamine·H₂O. This large isotope dependence is associated with a zero-point energy effect that increases the binding energy of the deuterated complex by ~190  cm⁻¹, thereby reducing the excess energy available at the same excitation energy. At all higher energies, the dissociation lifetime was shorter than the pulse duration of our lasers (8  ns). At all wavelengths, the observed products in the presence of collisions are dominated by conformers A and B of tryptamine monomer, with small contributions from the other minor conformers. In addition, right at threshold (2869  cm⁻¹), tryptamine·D₂O dissociates exclusively to conformer A in the absence of collisions with helium, while both A and B conformational products are observed in the presence of collisions with helium. Using resolution-of-identity approximation to second-order Møller-Plesset binding energies extrapolated to the complete basis set limit and harmonic vibrational frequencies and transition states calculated at the density functional limit B3LYP/6-31+G^* level of theory, Rice-Ramsperger-Kassel-Marcus (RRKM) predictions for the dissociation, isomerization, and water shuttling rates as a function of energy are made. At threshold, the experimental dissociation rate is almost 10³ faster than RRKM predictions. Reasons for this apparent non-RRKM behavior will be discussed. ©2007 American Institute of Physics