Structures and energies for C4

Abstract

Optimized geometries and relative energies for three states of the C₄ molecule have been obtained from single‐reference configuration interaction (SRCI) calculations. The ¹Σ⁺_g state, which is formed without activation from the dimerization of ground state C₂ molecules, is calculated to lie approximately 25 kcal above the ³Σ⁻_g state. At the SRCI level, a rhombic form is calculated to lie 1.2 kcal below the triplet form; consideration of the Davidson correction reduces this difference to 0.4 kcal, inclusion of a second set of diffuse d functions increased the difference only by about 0.2 kcal. Consideration of these effects, the difference in zero‐point energy and previous results for methylene leads to a final estimated separation of 4.9 kcal, favoring the rhombus. Electron density distribution analysis for the rhomb is consistent with the existence of a bond between inverted sp² carbon atoms. To aid the detection of this unusual molecule, preliminary estimates of the lowest optical transitions were obtained from SRCI calculations and vibrational frequencies were obtained from SCF calculations. Comparison of the calculated results with experimentally obtained spectra suggest the possibility that both the linear triplet and the rhombus may have already been observed.