The experimental gas-phase spectrum of CH3NO2 from ref. 84 is reproduced in Fig. 4 for comparison. As already noted, a low-intensity feature around 3000 cm–1, also appearing in the experimental spectrum, is introduced with the anharmonic treatment. The main peak of this feature, at 2955 cm–1, arises mainly from the 4171 combination band and a minor contribution from the 31 fundamental band. Apart from the 11 and 21 fundamental bands (3038 and 3005 cm–1, respectively), another low-intensity combination band, 4181 at 2933 cm–1, appears from the anharmonic treatment. Other low-intensity peaks, also present in the experimental spectrum, appear due to the 41111 combination band at 2471 cm–1 and the 72 overtone band at 2768 cm–1.