Predissociation of methyl cyanoformate: The HCN and HNC channels
  We present a combined experimental and theoretical investigation of the 193
nm photolysis of the cyano-ester, methyl cyanoformate (MCF). Specifically,
nanosecond time-resolved infrared emission spectroscopy was used to monitor the
ro-vibrationally excited photoproducts generated in the photolysis reaction.
The signal-to-noise of all time-resolved spectra were enhanced using the
recently developed algorithm, spectral reconstruction analysis, which allowed
observation of previously obscured minor resonances, and revealed new
dissociation channels producing HCN and HNC. Spectral fit analysis of the
nascent HCN and electronically excited CN($A^2\Pi_1$) resonances yield a lower
bound estimate for the HCN quantum yield of ca. 0.42$\pm$0.24%.
Multi-configuration self-consistent field calculations were used to interrogate
the ground and four lowest energy singlet excited states of MCF. At 193 nm,
dissociation is predicted to occur predominantly on the repulsive S$_2$ state.
Nevertheless, minor pathways leading to the production of highly excited ground
state MCF are available via cascading internal conversion from nascent S$_2$
population. An automated transition-state search algorithm was employed to
identify the corresponding ground state dissociation channels, and
Rice-Ramsperger-Kassel-Marcus and Kinetic Monte Carlo kinetic simulations were
used to calculate the associated branching ratios. The proposed mechanism was
validated by direct comparison of the experimentally measured and
quasi-classical trajectory deduced nascent internal energy distribution of HCN,
which were found to be in near perfect agreement. The propensity for cyano
containing hydrocarbons to act as photolytic sources for ro-vibrationally
excited HCN and HNC, as well as their significance to astrophysical
environments, are discussed.
