Fig. compares the freshly deposited-as opposed to matrices that have undergone annealing or other processing-matrix-isolated spectra of the fundamental N=O stretch ν3 of 500:1 X:CH3ONO, (X = N2, Ar, Xe) from 1690-1590 cm -1 deposited at α = 20°. Full matrix-isolated spectra from 3500-700 cm -1 for each matrix at this angle can be found in SM Section 3. The most intense transition in this spectral region (whether it corresponds to cis-ν3 or trans-ν3 CH3ONO) was normalized to 1, and the absorbance of the remainder of the spectrum was scaled linearly. The cis-ν3 transition in N2 is centered at 1612 cm -1 with a secondary peak centered at 1615 cm -1 (more details on secondary peaks in the next paragraph), in Ar is centered at 1613 cm -1 , and in Xe is centered at 1606 cm -1 . The trans-ν3 transition in N2 is centered at 1664 cm -1 with a secondary peak centered at 1667 cm -1 , in Ar is centered at 1665 cm -1 , and in Xe is centered at 1662 cm -1 .