The study of relationship between chemical geometry and electronic configuration, non-Walsh type B Part II A.H. Pakiari * , F. Nazari Department of Chemistry, College of Sciences, Shiraz University, Golestan Av., Shiraz 71454, Iran Received 1 September 2004; accepted 4 November 2004 Available online 26 January 2005 Abstract In this research, we will introduce non-Walsh type B and extra five driving forces in molecular geometry determination, and demonstrate them in this paper. The question first came to our mind when we considered that CH 3 radical has planar structure, but SiH 3 radical is in bent form, while both molecules have an electron on central atom from Lewis structure viewpoint, and they must have bent structure according to Walsh rule. We will show three contrasts in current theory; there are many molecules which do not follow Lewis valence theory and Walsh rule, and also they have spin multiplicity, but they do not show torsion in molecule. Therefore, we are looking for some types of existing interactions between different molecular orbitals in particular type of category of molecules: AX 3 , AH 2 X and AHX 2 in the form of radical, cation and anions, where A is C, Si, Ge, N or P atom, and X is F, Cl or Br atom. In spite of studying one category of compound in this article, a generalized method for investigation of these types of interactions will be presented, which can be used in other similar cases. We will demonstrate the reasons for such molecules having particular geometry, which are caused by some interactions such as lone pair of X-atom with radical electron on A-atom, lone pair of X and s-bond, hybridization of electrons on central atom, back bonding and spin multipilicity. B3lyp with 6-311G** basis set have been applied to our calculations, and atom in molecule (AIM), natural bond orbital (NBO), are also used for the results analysis. q 2004 Elsevier B.V. All rights reserved. Keywords: Walsh rule; Non-Walsh type B; Geometry of molecule; Lone pair–lone pair interaction; Methyl; Silyl; Germanyl; Nitrogen hydride and phosphine radical derivatives 1. Introduction We are going to investigate the relationship between electronic structure and geometry for one category of compounds, which cannot simply be found an explanation for their geometrical structure. These compounds are those, which have a similar appearance from Lewis structure point view, one has bent structure, but the other one is in non bent geometry. Lewis structure of these species have one lone pair or one electron (radical) on central atom, therefore, they must obey the Walsh rule, and must have the bent geometry, but some of them do not. It seems that Walsh rule is not enough to give us satisfactory explanation for this phenomenon. Hence, we are looking for the reasonable explanation for this behavior in this paper. Let us survey the literature about above matter. VSEPR model assumes that (i) the valence charge density is spatially localized into pairs of electrons and (ii) the geometrical arrangement of the ligands maximizes the inter-pair separation and minimizes the inter-pair repul- sion, including both bonded and lone pairs. The shape of trigonal pyramid, C 3v , is predicted for AX 3 with lone pair on central atom. The geometrical patterns due to lone pair activity are predicted and interpreted within the valence shell electron pair repulsion (VSEPR) model developed by Sidgwick and Powell [1], Lennard–Jones and Pople [2], Gillespie and Nyholm [3], Gillespie [4–6], Linnett [7–10] and others [11–14]. In 1947 Walsh [15–18] stated the following rule: if a group X attached to carbon is replaced by a more electronegative group Y than carbon, then the valence 0166-1280/$ - see front matter q 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.theochem.2004.11.023 Journal of Molecular Structure: THEOCHEM 717 (2005) 189–197 www.elsevier.com/locate/theochem * Corresponding author. Tel.: C98 71 1624 5849; fax: C98 71 1228 6008. E-mail address: pakiari@susc.ac.ir (A.H. Pakiari).