Matrix-formation dynamics dictate methyl nitrite conformer abundance

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 similar to 5 kJ/mol. The barrier to rotational interconversion (similar to 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 (N-2, 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 N-2: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.