Predissociation of methyl cyanoformate: The HCN and HNC channels

TL;DR

This study combines experimental and theoretical methods to investigate the photodissociation of methyl cyanoformate at 193 nm, revealing new dissociation channels producing HCN and HNC, and estimating their quantum yields.

Contribution

It introduces spectral reconstruction analysis for enhanced detection of minor photoproducts and provides a detailed mechanistic understanding of the dissociation pathways.

Findings

Observed new HCN and HNC channels in photolysis.

Estimated HCN quantum yield of approximately 0.42%.

Validated dissociation mechanism with energy distribution data.

Abstract

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.