Dimer formation of NO ligands in the gas-phase halide ion clusters X −(NO) n enhanced by a frontier orbital interaction

Abstract

The gas-phase equilibria for the clustering reaction of halide ions(X −) with NO were measured with a pulsed electron-beam high-pressure mass spectrometer. Van't Hoff plots of equilibrium constants led to the determination of the thermochemical stabilities for X −(NO) n with n=1 and n=2. The bond energies of X −(NO) 2 (X −=Cl − and I −) are found to be about 1.5 times larger than those of X −(NO) 1. This anomaly is due to the electron-spin pairing effect, i.e., dimer pair formation of the NO ligands in the cluster ions X −(NO) 2. In the dimer, the lowest unoccupied molecular orbital of (NO) 2 has a low-energy level to accept the electronic charge from X − effectively.