Journal of Molecular Structure (Theochem), 121(1985) 109-114 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands zyxwvutsrqponmlkjihgfed AN MNDO STUDY OF THE &NH CARBENES MOZART N. RAMOS and B. B. NET0 Departamento de Qufmica Fundamental, Universidade Fedeml de Pernambuco, Cidade Universitdria, 50000, Recife, PE (Brasil) CELSO P. DE M EL0 Departamento de Fish, Universidade Fedeml de Pernambuco, Cidade Universitciria, 50000 Recife, PE (Brasil) (Received 5 June 1984) ABSTRACT The MNDO method has been used to calculate the geometries, dipole moments and force constants of the &NH carbenes. All the carbenes studied have been subjected to a complete geometry optimization for both singlet and triplet electronic states. The results indicate that these carbenes are nonlinear and singlet, and much less polar than they were reported to be. An ab initio calculation using a STOSG basis for C,NH confirms this result. The stretching and interaction bond-bond force constants have been determined, in order to best understand the nature of the chemical bonding in these molecules. INTRODUCTION The rate of discovery of molecular species in the interstellar medium has increased dramatically in the past decade [ 11. However, the search for new molecules is usually hindered by the fact that only those species whose spectral lines are accurately known can be sought [Z] . To reproduce in the laboratory, conditions prevailing in interstellar space is difficult: populations of the different states of molecules in the dust and gas clouds associated with young stars do not generally follow Boltzmann distribution and are not in thermodynamic equilibrium [ 31. As a consequence, there is a definite need for theoretical calculations of the structural and electronic properties of mol- ecules likely to be found in the interstellar medium. Since Douglas’s suggestion that the nature of the diffuse interstellar lines could result from absorption by longchain carbon molecules [4], a detailed knowledge of the related polar species cyanopolyacetylenes, H&N, and their carbene isomers, C&NH, was made desirable. Of the former, chains with n as large as 9 have been detected in the interstellar space [l, 51, and consider- able progress was attained in the synthesis and experimental characterization of several members of the family [6-81. As for the carbenes, although the infrared spectrum of hydrogen isocyanide, CNH, has been known for some time [9, lo], only recently was this molecule positively identified in inter- stellar space [ 111. 0166-1280/85/$03.30 0 1985 Elsevier Science Publishers B.V.