Table . Matrix-isolated conformer-specific abundances measured using the cis and trans-ν3 transition of freshly deposited X:CH3ONO (where X = N2, Ar, Xe) at α = 20° compared to the room temperature (RT) dilute gas-phase conformer abundance. A change in conformer abundance in freshly deposited matrix-isolated CH3ONO was unexpected, regardless of the matrix host, since this phenomenon is typically only observed in cases where the internal conversion barrier between conformers is relatively low. When these barriers are small, conformer interconversion can occur, allowing abundances to approach thermodynamic equilibrium at the lower temperature of the matrix; this is known as conformational cooling. However, the interconversion barrier between trans and cis-CH3ONO (~45 kJ/mol) is too large for isomerization to occur at any matrix temperature used in this study. We therefore must consider other physical mechanisms responsible for our observed changes in conformer abundance between the gas and matrix phases, as well as the observed lower-energy conformer enrichment following a cis abundance enhancement of N2 ≈ Ar < Xe.