Triplet-singlet energy gaps in iodo-carbenes (I-C-X): Remarkable discrepancy between theory and experiment

The triplet-singlet energy gaps in iodo-carbenes, I-C-X with X=H, F, Cl, Br and I, were computed using the MP2, CCSD(T), CASSCF/CASPT2, MR-SDCI, MR-AC PF and B3LYP methods, with basis sets up to 6-311++G(3df,2p) and aug-cc-pvT Z and effective core potentials. Corrections for relativistic effects were also incorporated. Our results indicate that diiodo-carbene (CI2) is likely to possess a singlet ground state lying around 30 kJ mol(-1) below the low est triplet state, thus at variance with that of a recent negative ion phot oelectron spectroscopic study (R. L. Schwartz, G. E. Davico, T. M. Ramont a nd W. C. Lineberger, J. Phys. Chem. A, 1999, 103, 8213). In addition, the p resent study confirms the singlet character of other iodo-carbenes with sub stantial triplet-singlet gaps and also points out the remarkably large fluc tuations and discrepancies in the absolute values of the energy splittings, not only between experiment and theory, but also between ab initio quantum chemical methods.