Key spectroscopic features of pyrazine lowest-lying 1B1u and 1B2u states

Abstract

We report new spectral assignments from a high-resolution vacuum ultraviolet (VUV) photoabsorption spectrum of pyrazine (C₄H₄N₂) lowest-lying valence excitations $\left(1{}^1{B}_{1u}\leftarrow \stackrel{\sim }{X}{}^1{A}_g\right)$ and $\left(1{}^1{B}_{2u}\leftarrow \stackrel{\sim }{X}{}^1{A}_g\right)$ in the photon energy range 3.7–6.2 eV (330–200 nm). The electronic state spectroscopy of C₄H₄N₂ has been investigated together with quantum chemical calculations at the time-dependant density functional theory (TD-DFT) providing vertical excitation energies and oscillator strengths. The vibronic excitation in the ¹B_1u and ¹B_2u states is accompanied by fine structure assigned to C–H stretching, $v_1^{\prime }\left(a_g\right)$, C–C stretching, $v_2^{\prime }\left(a_g\right)$, C–H bending, $v_3^{\prime }\left(a_g\right)$, ring breathing, $v_4^{\prime }\left(a_g\right)$, and ring deformation $v_5^{\prime }\left(a_g\right)$ modes. Harmonic frequencies for pyrazine neutral electronic ground- and first excited-states have been obtained at the B3LYP/aug-cc-pVTZ level of theory. Absolute photoabsorption cross-section values are also reported from 3.7 up to 10.8 eV (310–113 nm) and compared with previous data in the literature.