The role of electron transfer stabilization in several gas phase ion–molecule reaction processes

A simple theoretical method is presented for estimating the bond energies of ion–molecule complexes such as O+2–M and NO+–M, where M is a neutral molecule. The theory has one adjustable parameter, H12, which is the electronic coupling between the state O+2–M (or NO+–M) and the charge– transfer state O2–M+ (or NO–M+). H12 has a fixed value for each ion. Good agreement is obtained with experimental bond energies where available. The theoretical bond energies are then compared with vibrational quenching rate constants and with three-body association rate constants measured for O+2–M and NO+–M systems. In each case there is a strong correlation, in agreement with earlier predictions. A similar comparison is made using the incremental bond energy which can be attributed to the H12 term. The correlation is even better, suggesting that the anisotropy in the ion–molecule interaction plays an important role in stabilizing the collision complexes.