Crossed molecular beam studies of bimolecular reactions of atomic oxygen with nitrogen-bearing organic molecules (nitriles and N-heterocyclic)

In this contribution, dedicated to the memory of Prof. Gian Gualberto Volpi, we provide a short review of recent work carried out in our laboratory on reactive scattering studies of the reaction dynamics of atomic oxygen with nitrogen-bearing organic molecules. Specifically, we focus on the polyatomic bimolecular reactions of atomic oxygen, both in the ground and first excited state, O(3P) and O(1D), with the simplest unsaturated nitriles, namely HCCCN (cyanoacetylene) and CH2CHCN (cyanoethylene, or acrylonitrile), and with the simplest six-member ring N-heterocyclic compound, pyridine (C5H5N). Using the crossed molecular beam (CMB) scattering technique with universal electron-impact ionization mass-spectrometric detection and time-of-flight analysis to measure product angular and velocity distributions, the primary product channels and their branching fractions were determined, thus assessing the central role played by intersystem-crossing (ISC) in this class of reactions. The experimental work was synergistically accompanied by theoretical calculations of the relevant triplet and singlet potential energy surfaces (PESs) to assist the interpretation of experimental results and elucidate the reaction mechanism, including extent of ISC. Cyanoacetylene and cyanoethylene are of considerable interest in astrochemistry being ubiquitous (and relatively abundant) in space including comets and the upper atmosphere of Titan. Being oxygen the third most abundant element in space, the title reactions are of considerable relevance in the chemistry of extraterrestrial environments. In addition, they are also important in combustion chemistry, because thermal decomposition of pyrrolic and pyridinic structures present in bound N-containing fuels generates N-bearing compounds including, in particular, the above two nitriles.