Kimika Teorikoa Seminar: Number of Bonds in Diatomic Molecules from Excited State Potential Energy Curves

The question of quadruple bonding in C2 has emerged as a controversial issue, with opinions sharply divided between the practitioners of Valence Bond (VB) and Molecular Orbital (MO) theory. We have systematically studied the Potential Energy Curves (PECs) of low-lying high spin sigma states of C2, N2, and HC ≡CH using several MO
based techniques such as CASSCF, RASSCF and MRCI. The analyses of the PECs for the 2S+1∑g/u (with 2S + 1 =1, 3, 5, 7, 9) states of C2 and comparisons with those of relevant dimers and the respective wavefunctions were
conducted. Our investigations reveal that the number of bonds in the ground state can be determined for 2nd row
dimers by figuring out at what value of spin symmetry a purely dissociative PEC is obtained. For N2 and HC≡CH
the purely dissociative PEC appears for the 7Σ+ spin symmetry as compared to that for the 9Σ+ in C2. This is
indicative of a higher number of bonds between the two 2nd row atoms in C2 as compared to those of N2 and HC ≡CH.