Volume 128, number 4 CHEMICAL PHYSICS LETTERS 1 August 1986 AB INITIO COUPLED HARTREE-FOCK CALCULATION OF THE 29Si NMR SHIELDING CONSTANTS IN SiH,, Si,H,, Si,H, AND H,!$O J.A. TOSSELL Department of Chemistry, university of Maryland College Park, MD 20742, VSA and Paolo LAZZERETTI Dipartimento di Chimica, Vniversitir degli Studi di Modena, Via Camp. 183, 41100 Modena Itab Received 29 January 1986; in final form 16 May 1986 Ab initio coupled Hartree-Fock perturbation theory (CHFPT) using large, flexible bases of gaugeless Gaussian orbitals is used to calculate the %i chemical shielding tensors, o,,, of SiH4, Si2H6, SiZH4 and Hr.Si0.‘Si2H4 is deshielded with respect to SiH, by an average of 134 ppm and the range of principal axis values of the shielding tensor is 291 ppm. In contrast, Si2H6is shielded with respect to SiH, by 21 ppm on the average and its range of principal axis shielding values is only 23 ppm. The large deshielding and anisotropy of S&H4 arise primarily from excitations of electrons from the Si-Si sigma bonding (8) MO into the Si-Si ti orbital induced by the magnetic field component lying in the molecular plane and perpendicular to the Si-Si axis. 1. Introduction As silicon is an element similar to carbon in its chemical properties it is not surprising that com- pounds with double bonds involving silicon have long been sought. In recent years stable compounds con- taining Si=C and Si-Si bonds have been character- ized [ 1,2] and compounds with Si-0 bonds have been implicated as reaction intermediates [ 11. Ex- tensive theoretical studies have also been performed on relevant simple molecules containing the Si mul- tiple bonds, such as H2Si=CH2 (silaethene) [ 31, H2Si=SiH2 (disilene) [ 41 and H$i=O (silanone) [ 41. Calculated double bond distances in those mol- ecules are in good agreement with experimental val- ues for substituted doubly bonded systems containing bulky hydrocarbon groups [2,3]. In addition to X- ray structural information, UV [2] and 29Si NMR spectra [ 51 are available for several of these species. By analogy with carbon doubly bonded systems one expects silicon doubly bonded systems to be de- shielded with respect to singly bonded compounds 420 (i.e. to have a smaller average nuclear shielding con- stant, a,“) and to show a larger NMR shift aniso- tropy. This expectation has been confirmed for Si-Si compounds [ 2,5 1. We have recently applied ab initio coupled Har- tree-Fock perturbation theory (CHFPT) [ 61 to the calculation of 29SiNMR shifts in SiH4 [ 71, SiF, [ 81 and other molecules. We here present CHFPT cal- culations of 29Sichemical shielding tensors for SiH4, Si2H6, Si2H, and H2Si0. Experimental geometries were employed for SiH4 and Si2H6 [ 91 ( D3,, sym- metry was utilized for Si2H6 to shorten the calcula- tion) and optimized geometries from high level ab initio SCF calculations were employed for Si,H, [ lo] and H2Si0 [ 41. The geometries employed are shown in fig. 1. Our basis set for Si was the (13~9~) primi- tive basis of Gianolio et al. [ 111, contracted to [ 6s/ 5p] and augmented with three Si 3d functions with exponents of 1.86, 0.56 and 0.21. The H basis was the (8s) set of van Duijneveldt [ 121, contracted to [ 6s] and augmented by a H 2p function of exponent 0.66 and the 0 basis was the van Duijneveldt (9s/ 0 009-2614/86/$ 03.50 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)