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.

硝基甲烷在 5.4–11.8 eV (43,500–95,000 cm ^-1) 介绍了使用同步辐射；首次报道了 > 9 eV 区域的 VUV 吸收光谱。观察到的光谱特征被分配给各种价态和里德堡跃迁，并得到使用 TDDFT 方法的量子化学计算的支持。6 eV 区域主要是一个宽的、强烈的吸收带，在 6.2 eV 处达到峰值，分配给 π-π* 价跃迁，然后是富里德堡级数收敛到硝基甲烷的前四个电离电位。轨道能量的理论计算以及里德堡级数分析表明，第三个 IP 位于 11.95 eV，而较早的估计值为 11.5 eV。在 60,^-1区域，暂时分配到涉及 NO ₂面内岩石和 CH ₃不对称变形模式的级数。在 DFT 计算的帮助下重新访问和分配液相红外光谱。观察到一些新频段并将其分配给泛音和组合模式。相对于 CN 和 NO 键长的前几个激发单重态和三重态的理论模拟势能曲线有助于解释硝基甲烷光解动力学的一些未完全理解的特征。发现在 Franck Condon 区域的直接解离是 CH ₃ NO + O 通道的可能机制，而单线态-三线态交叉在 CN 和 NO 键断裂中起重要作用。