Ab initio G4 calculations have been performed to investigate the singlet-triplet gap in a series of 66 simple carbenes. Energies and geometries were analyzed. An additive model has been explored that include four interaction terms. An abnormal behavior of the cyano group has been found. The C-13 absolute shieldings of the carbenic carbon atom were calculated at the GIAO/B3LYP/6-311++G(d, p). (C) 2017 Elsevier B.V. All rights reserved.