The mechanism of H-bond rupture: the vibrational pre-dissociation of C2H2–HCl and C2H2–DCl

Abstract

Pair correlated fragment rovibrational distributions are presented following vibrational predissociation of the C₂H₂–DCl van der Waals dimer initiated by excitation of the asymmetric (asym) C–H stretch. The only observed fragmentation pathways are DCl (v = 0; j = 6–9) + C₂H₂ (ν₂ = 1; j = 1–5). These and previously reported data on the related C₂H₂–HCl species are analysed using the angular momentum (AM) method. Calculations accurately reproduce fragment rovibrational distributions following dissociation of the C₂H₂–HCl dimer initiated either by excitation of the asym C–H stretch or via the HCl stretch, and those from C₂H₂–DCl initiated via asym C–H stretch excitation. The calculations demonstrate that the dimer is bent at the moment of dissociation. Several geometries are found that lead to H-bond breakage via a clearly identified set of fragment quantum states. The results suggest a hierarchy in the disposal of excess energy and angular momentum between fragment vibration, rotation and recoil. Deposition of the largest portion of energy into a C₂H₂ vibrational state sets an upper limit on HCl rotation, which then determines the energy and AM remaining for C₂H₂ rotation and fragment recoil. Acceptor C₂H₂ vibrational modes follow a previously noted propensity, implying that the dissociating impulse must be able to induce appropriate nuclear motions both in the acceptor vibration and in rotation of the C₂H₂ fragment.