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

Abstract

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-ACPF and B3LYP methods, with basis sets up to 6-311++G(3df,2p) and aug-cc-pvTZ 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 lowest triplet state, thus at variance with that of a recent negative ion photoelectron spectroscopic study (R. L. Schwartz, G. E. Davico, T. M. Ramont and W. C. Lineberger, J. Phys. Chem. A, 1999, 103, 8213). In addition, the present study confirms the singlet character of other iodo-carbenes with substantial triplet–singlet gaps and also points out the remarkably large fluctuations and discrepancies in the absolute values of the energy splittings, not only between experiment and theory, but also between ab initio quantum chemical methods.