par Liévin, Jacques
Référence Journal of Molecular Spectroscopy, 156, 1, page (123-146)
Publication Publié, 1992-11
Article révisé par les pairs
Résumé : Ab initio calculations on the few lowest 1Ag states of trans-bent acetylene have been performed at different levels of theory (SCF, CASSCF, SD-CI, and SO-CI) and with basis sets of increasing flexibilities (3-21G + pol, TZ + 2P, TZ + 2P + diff). The results predict the existence of an equilibrium structure (RCC = 1.621 A ̊, RCH = 1.096 A ̊, and θHCC = 104.5°) on the second potential energy surface of 1Ag symmetry, corresponding to the valence configuration 1ag21bu22ag2 2bu23ag23bu 24ag2 arising from a double Π → Π* excitation applied to the ground state configuration. The calculated parameters for this state, labeled C ̃′ (A = 8.71, B = 0.912, C = 0.825 cm-1; ν1 = 3126, ν2 = 934, ν3 = 1386 cm-1; Te = 7.53 eV), strictly confirm the recent findings of UVODR experiments. The analysis of the wavefunctions allows an interpretation of some of the features of this state, namely, its valence nature, its geometry (strong bending and enlarged CC bond), and the occurrence of multiple avoided crossings, leading to a potential energy barrier, along the pathway linking the C ̃′ minimum to the linear ground state geometry. The reason why previous ab initio calculations did not report the existence of this state is also discussed. © 1992.