A 193nm laser photofragmentation time-of-flight mass spectrometric study of chloroiodomethane

Abstract

The photodissociation dynamics of chloroiodomethane (CH2ICl) at 193nm has been investigated by employing the photofragment time-of-flight (TOF) mass spectrometric method. Using tunable vacuum ultraviolet undulator synchrotron radiation for photoionization sampling of nascent photofragments, we have identified four primary dissociation product channels: CH2Cl+I(P1∕22)∕I(P3∕22), CH2I+Cl(P1∕22)∕Cl(P3∕22), CHI+HCl, and CH2+ICl. The state-selective detection of I(P3∕22) and I(P1∕22) has allowed the estimation of the branching ratio for I(P1∕22):I(P3∕22) to be 0.73: 0.27. Theoretical calculations based on the time-dependent density-functional theory have been also made to investigate excited electronic potential-energy surfaces, plausible intermediates, and transition structures involved in these photodissociation reactions. The translation energy distributions derived from the TOF measurements suggest that at least two dissociation mechanisms are operative for these photodissociation processes. One involves the direct dissociation from the 2A′1 state initially formed by 193nm excitation, leading to significant kinetic-energy releases. For the I-atom and Cl-atom elimination channels, the fragment kinetic-energy releases observed via this direct dissociation mechanism are consistent with those predicted by the impulsive dissociation models. Other mechanisms are likely predissociative or statistical in nature from the lower 1A′1 and 1A″1 states and/or the ground X̃A′1 state populated by internal conversion from the 2A′1 state. On the basis of the maximum kinetic-energy release for the formation of CH2Cl+I(P1∕22), we have obtained a value of 53±2kcal∕mol for the 0K bond dissociation energy of I–CH2Cl. The intermediates and transition structures for the CHI+HCl and CH2+ICl product channels have been also investigated by ab initio quantum calculations at the MP2(full)∕6-311G(d) and B3LYP(full)∕6-11G(d) levels of theory. The maximum kinetic-energy releases observed for the CHI+HCl and CH2+ICl channels are consistent with the interpretation that the formation of CHI and CH2 in their ground triplet states is not favored.