CENTRAL BOND IN THE 3 CN-BULLET DIMERS NC-CN, CN-CN, AND CN-NC - ELECTRON PAIR BONDING AND PAULI REPULSION EFFECTS

The strength of the CN/CN' bond decreases in the series NC-CN (1), CN-CN (2), and CN-NC(3), while simultaneously it shortens. The explanation requires that. apart from the sigma-pair bond between the CN 5-sigma MO's, the following effects be taken into account: (a) Pauli repulsion ("steric hindrance") between the 4-sigma ("N lone pair") orbitals; (b) Pauli repulsion between the 4-sigma and 5-sigma orbitals; (c) donor/acceptor interaction between the 4-sigma and 5-sigma orbitals; (d) donor/acceptor interaction between the occupied 1-pi and unoccupied 2-pi*. Each of these contributions is numerically significant. The singly occupied 5-sigma plays, apart from the pair bonding, a dual role, causing Pauli repulsion (with thc N lone pair) as an occupied orbital and causing charge-transfer interaction in its capacity of unoccupied acceptor orbital. Detailed consideration of the balance between the repulsive and attractive energy components and their R dependence is required to explain why the central CN/CN' bond considerably weakens when going from 1 to 3, and why it still contracts. Density functionals that include gradient corrections for the exchange and electron gas parametrization for correlation between electrons of different spin (but no same-spin correlation) are shown to achieve an accuracy for the systems under consideration which is comparable to that of conventional high-level ab initio methods like CEPA and the CCD(+ST) coupled cluster approach.