Electronic states of nitromethane: Experimental and theoretical studies

Highlights: • VUV absorption spectrum of nitromethane, 9–11.8 eV region reported for first time. • First report of detailed spectral analysis aided by quantum chemical calculations. • Identification of 3rd IP at 11.95 eV by theoretical calculations and Rydberg analysis. • IR spectrum revisited, a few new combination and overtone bands assigned. • New insights into photodissociation dynamics by simulation of excited state PECs. A comprehensive spectroscopic study of the UV–VUV photoabsorption spectrum of nitromethane in the energy region 5.4–11.8 eV (43,500–95,000 cm⁻¹) using synchrotron radiation is presented; the VUV absorption spectrum in the region > 9 eV being reported for the first time. The observed spectral features are assigned to various valence and Rydberg transitions, supported by quantum chemical calculations using the TDDFT method. The 6 eV region is dominated by a broad, intense absorption band peaking at ∼ 6.2 eV assigned to the π-π* valence transition, followed by rich Rydberg series converging to the first four ionization potentials of nitromethane. Theoretical calculations of orbital energies as well as Rydberg series analysis indicate that the third IP is located at 11.95 eV, in contrast to the earlier estimated value of 11.5 eV. Series of bands attributed to vibrational transitions accompanying the 3s (6a″), 3p(6a⁾, 3s(10a′) and 3s(5a″) Rydberg transitions are observed in the 60,000–73,000 cm⁻¹ region and are tentatively assigned to progressions involving the NO₂ in-plane rock and CH₃ asymmetric deform modes. The liquid phase IR spectrum is revisited and assigned with the help of DFT calculations. A few new bands are observed and assigned to overtone and combination modes. Theoretically simulated potential energy curves of the first few excited singlet and triplet states with respect to the CN and NO bond lengths are useful in explaining some of the incompletely understood features of the photodissociation dynamics of nitromethane. It is found that direct dissociation in the Franck Condon region is a possible mechanism for the CH₃NO + O channel, while singlet-triplet crossings play an important role in CN and NO bond scissions.