Molecular geometry: bridging qualitative modeling and accurate computations. A comparative study of simple nitrogen and phosphorus derivatives

The molecular geometries of a series of simple nitrogen and phosphorus deri vatives have been computed in order to probe the validity of simple qualita tive models of structural variations using a consistent set of data. The de rivatives included NX3, ONX3, PX3, and OPX3 (X = F, CH3, CF3, SiH3). The co mputed geometries are consistent with experimental structures, where they a re available, especially when parameter differences are considered. Further scrutiny of the bond-angle differences suggests that the MP2/6-31G* and MP 3/6-3 IGC levels of theory are preferable for the nitrogen and phosphorus d erivatives, respectively. Our observations include nitrogen bond lengthenin g and a slight closing of the XNX bond angles upon oxidation of nitrogen de rivatives and phosphorus bond shortening and considerable opening of the XP X angles upon oxidation of phosphorus derivatives. These trends as well as the variations observed in nitrogen versus phosphorus comparisons are in ag reement with the so-called ''octet rule'' and VSEPR predictions. As expecte d, the latter lose validity with increasing ligand versus central atom size and are replaced by the predominance of ligand-ligand non-bonded interacti ons. (C) 1998 Elsevier Science B.V. All rights reserved.