Quadrupole Coupling Constants and Mössbauer Isomeric Shifts in Antimony Compounds within Gaussian 98 O. Kh. Poleshchuk a and J. N. Latosińska Institute of Physics, Adam Mickiewicz University, Umultowska 85, 61-614 Poznan, Poland a Permanent address: Tomsk Pedagogical University, Komsomolskii 75, 634041 Tomks, Russia Reprint requests to Dr. J. N. L.; Fax: +48-61-8257758; E-mail: jolanala@amu.edu.pl Z. Naturforsch. 55 a, 276-280 (2000); received August 23, 1999 Presented at the XVth International Symposium on Nuclear Quadrupole Interactions, Leipzig, Germany, July 25 - 30, 1999. The electron density and quadrupole coupling constants of molecules containing Sb are analysed. The NQCC for antimony, calculated using the extended basis 6-311G** are much lower than the experimental data, while the use of the small 3-21G* basis led to NQCC closer to the experimental ones. Key words: DFT; QCC; Isomeric Mössbauer Shifts; Antimony Compounds. Introduction Compounds of non-transition elements contain- ing tin and antimony were studied by nuclear qua- drupole resonance spectroscopy and Mössbauer ef- fect [1 - 5]. The parametres like quadrupole splitting or quadrupole coupling constant (QCC) as well as Mössbauer isomeric shifts on nuclei 121 Sb and 119 Sn (6) were determined. The studies of the electronic density distribution in a molecule and environment of the central atom, i.e. non-transition element, were performed mainly on the basis of semiempirical cal- culations. It is well known that such methods cannot be applied for a detailed analysis of bonding param- eters or interpretation of the electronic structure of molecules. The correlations between the experimen- tal and calculated Mössbauer isomeric shifts or QCC could also be significantly improved by the applica- tion of non-empirical methods. It should be pointed out that these methods have been widely used for large molecular systems. The semiempirical meth- ods of quantum chemistry do not give comprehensive information concerning electronic structure of com- pounds of non-transition elements, for example those from group Kothekar [6] studied similar molecules by CNDO and obtained negative charges on central atoms (antimony and tin). The Townes-Dailey approximation, in which vari- ous integrals are neglected and configuration interac- tions are taken into account to a different degree [7], is widely used for the estimation of QCC and Mössbauer isomeric shifts. Köster [8] suggests that, if we con- sider only one configuration for a ground state, it is equivalent to the assumption that each electron in an atom is in a field of the averaged central potential. The Coulomb repulsion between electrons is responsible for the mixing of various configurations of electrons in an atom. The magnitude of configuration interac- tion is especially important for heavy atoms, where the Coulomb repulsion between electrons is relatively larger than attraction to nuclear charge. The main idea of the Townes-Dailey approxima- tion [9] is that the main contribution to the electric field gradient comes from valence electrons of the atom considered. Therefore, we expected that the best QCC values (i. e. the closest to the experimental re- sults) could be calculated using a nuclear core pseu- dopotential. Non-empirical calculation performed by us for tin, antimony and iodine molecules, with the use of the extended 6-311G** basis set [10, 11] provided much lower QCC values than the experimental ones, whereas the NQR frequencies from chlorine atoms were well correlated with experimental values. Computational Details The calculations were performed within the Gaus- sian 98 [ 12] package at the B3LYP level of the theory 0932-0784 /2000/ 0100-0276 $ 06.00 © Verlag der Zeitschrift für Naturforschung, Tübingen • www.znaturforsch.com Unauthenticated Download Date | 9/22/15 6:54 AM