The lowest singlet–triplet gap in CCl 2, CBr 2 and CI 2

Abstract

High-level ab initio calculations are performed to determine the lowest singlet–triplet (S–T) gap in CCl 2, CBr 2 and CI 2. For CCl 2 all-electron basis sets are used. For CBr 2 all-electron and effective core potential (ECP) basis sets are used, whilst for CI 2 only ECP basis sets are used. Methods up to CCSD(T) are employed, as well as the B3LYP method. The conclusions are that for CCl 2 the S–T gap is 20±1 kcal mol −1 and that the gaps for CBr 2 and CI 2 are 17±2 kcal mol −1 and 9±2 kcal mol −1, respectively. Some comments are made regarding the disagreement with recent photodetachment studies.