For gas-phase experiments, the IR spectrum is obtained using the same FTIR. Instead of externally coupling the IR beam to the matrix-isolation chamber, the IR beam remains internal, making a single pass through a gas cell that is located in the main sample compartment of the FTIR, before being focused into a deuterated triglycine sulfate (DTGS) detector. The threaded gas cell with J. Young® Valve (Thorlabs) is equipped with two MgF2 windows, limiting the lowfrequency spectral range. The spectra are recorded with a resolution of 0.125 cm -1 -which corresponds to a data spacing of 0.015 cm -1 -and background subtracted. The cis-and trans-ν3 modes of the gas-phase CH3ONO spectra are fit using the Multipeak Fitting package in IgorPro, defined by a Gaussian line profile (see Supplementary Material (SM), Section 2). The area of these fits is used to determine the integrated absorbance of each transition in the gas phase.