The lifetimes of gas phase CO2˙− and N2O˙− calculated from the transition probability of the autodetachment process A− → A + e−

A procedure to calculate the quantum mechanical transition probability of a unimolecular primary chemical process, A− A + e− is investigated for the circumstance where A− and A have different numbers of vibrational and rotational degrees of freedom (one is linear, the other not). A procedure is introduced to deal with the coupling between the vibrational and rotational motions. The proposed method was applied to calculating the lifetimes of CO2˙− and N2O˙− in the gas phase. The geometry optimizations and frequency calculations for CO2, CO2˙−, N2O, and N2O˙− are performed at HF, MP2, and QCISD(T) levels with 6-31G* or 6–31+G* basis sets, in order to obtain reliable geometric and spectroscopic information on these systems. Lifetimes are calculated for several of the lower vibrational–rotational states of the anions, as well as for the Boltzmann distribution of states at 298 K. The lifetime of the lowest vibrational–rotational state of CO2˙−, is 1.03 × 10−4 s, and of the lowest vibrational state with rotational levels weighted by Boltzmann distribution at 298 K, 1.50 × 10−4 s. These values are in good agreement with the experimental number, 9.0 ± 2.0 × 10−5 s, and support the experimental evidence that CO2˙− was formed in its ground vibrational level by the techniques used. The lifetime of CO2˙− calculated with Boltzmann distribution over its vibrational and rotational levels at 298 K, is 1.51 × 10−5 s. There are no direct measurements of the lifetime of N2O˙−, but it was estimated to be greater than 10−4 s from experimental evidence. The predicted lifetimes of N2O˙−, at its lowest vibrational–rotational state (0 K) and lowest vibrational state with rotational levels weighted by the Boltzmann distribution at 298 K, are 238 and 19.1 s, respectively. The lifetime of N2O˙− at thermal equilibrium at 298 K is 6.66 × 10−2 s, indicating that electron loss from the excited vibrational states of N2O˙− is significant. This study represents the first theoretical investigation of CO2˙− and N2O˙− lifetimes. © 1994 John Wiley & Sons, Inc.

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