Matrix-formation dynamics dictate methyl nitrite conformer abundance

Emily K Hockey; Nathan McLane; Korina Vlahos; Laura M McCaslin; Leah G Dodson

doi:10.1063/5.0188433

Matrix-formation dynamics dictate methyl nitrite conformer abundance

J Chem Phys. 2024 Mar 7;160(9):094303. doi: 10.1063/5.0188433.

Authors

Emily K Hockey¹, Nathan McLane², Korina Vlahos¹, Laura M McCaslin³, Leah G Dodson¹

Affiliations

¹ Department of Chemistry and Biochemistry, University of Maryland, College Park, Maryland 20742, USA.
² Institute for Physical Science and Technology, University of Maryland, College Park, Maryland 20742, USA.
³ Sandia National Laboratories, Livermore, California 94550, USA.

PMID: 38436440
DOI: 10.1063/5.0188433

Abstract

Methyl nitrite has two stable conformational isomers resulting from rotation about the primary C-O-N-O dihedral angle: cis-CH3ONO and trans-CH3ONO, with cis being more stable by ∼5 kJ/mol. The barrier to rotational interconversion (∼45 kJ/mol) is too large for isomerization to occur under ambient conditions. This paper presents evidence of a change in conformer abundance when dilute CH3ONO is deposited onto a cold substrate; the relative population of the freshly deposited cis conformer is seen to increase compared to its gas-phase abundance, measured by in situ infrared spectroscopy. We observe abundance changes depending on the identity of the bath gas (N2, Ar, and Xe) and deposition angle. The observations indicate that the surface properties of the growing matrix influence conformer abundance-contrary to the widely held assumption that conformer abundance in matrices reflects gas-phase abundance. We posit that differences in the angle-dependent host-gas deposition dynamics affect the growing surfaces, causing changes in conformer abundances. Quantum chemistry calculations of the binding energies between CH3ONO and a single bath-gas component reveal that significant energetic stabilization is not observed in 1:1 complexes of N2:CH3ONO, Ar:CH3ONO, or Xe:CH3ONO. From our results, we conclude that the growing surface plays a significant role in trapping cis-CH3ONO more effectively than trans-CH3ONO, likely because cis-CH3ONO is more compact. Taken together, the observations highlight the necessity for careful characterization of conformers in matrix-isolated systems, emphasizing a need for further study into the deposition dynamics and surface structure of chemically inert matrices.