Joan A. Deiters

ENHANCED REACTIVITY OF PENTACOORDINATED SILICON SPECIES. AN AB INITIO APPROACH

Abstract Molecular orbital calculations using GAUSSIAN 86 on tetracoordinated silicon species SiH n F4-n are compared with calculations on the anionic species formed by the addition of fluoride ion [SiH n F5-n ]. Similar calculations on isoelectronic tetra- and pentacoordinated phosphorus species [PHnF4-n ]+ and PH n F5-n are also carried out. All bond lengths in the pentacoordinated species are longer than in the related tetracoordinated species, with the greater increase in bond lengths occurring in the axial positions from both the silicon and phosphorus series. Fluoride ion addition results in only slight changes in the positive charge on silicon and phosphorus and in an increase in negative charge on all substituents. These calculations are used to elucidate the enhanced reactivity of pentacoordinated silicon observed in nucleophilic reactions.

Foreword to Special Issue of Phosphorus, Sulfur, Silicon and the Related Elements

Taylor & Francis Group wishes to dedicate this issue of the journal in honor of Professor Robert R. Holmes who served as Editor-in-Chief of Phosphorus, Sulfur, and Silicon and the Related Elements from 1987 to 2008. His distinguished scientific career spans more than 50 years with more than 255 publications including two invited ACS monographs (Nos. 175 and 176) on pentacoordinated phosphorus. His reputation in research is highly regarded both nationally and internationally. He was the recipient of the 1997 Award for Research Excellence by the International Council on Main Group Chemistry. Professor Holmes gave the keynote lecture for the XII International Conference on Phosphorus Chemistry, Jerusalem, Israel, 1995 and the keynote lecture at the 7th International Conference on Heteroatom Chemistry, Shanghai, China, 2004. He also served as Chairperson of the IRIS international conference for Main Group Chemistry. The latest conference in the series, IRIS-14, was held in Regensburg, Germany in July 2015. Robert Holmes was born in Chicago, Illinois. He received his B.S. degree in 1950 at the Illinois Institute of Technology, Chicago, and his Ph.D. in 1953 with Nobel Laureate H. C. Brown at Purdue University. He taught at Carnegie Institute of Technology before joining the technical staff of Bell Laboratories, Murray Hill, New Jersey where he was given free reign in developing his research program in phosphorus chemistry. He was appointed as Professor of Chemistry with tenure at the University of Massachusetts, Amherst, in 1966 and continued there until his retirement from teaching in 1998 and his retirement from research in 2007. In graduate school, Holmes discovered his intense interest in both research and writing. These two interests shaped his career in establishing research direction, writing of grant proposals, and in the construction of publications and lectures. As a research director, he guided more than 40 graduate students at the master’s and doctoral level. One of these graduate students was Robert Holmes’ wife Joan, who later assisted in research with computations for X-ray structure determination, and who also greatly assisted in the editing of the journal, Phosphorus, Sulfur, Silicon and the Related Elements. Professor Holmes also guided the research of 14 postdoctoral students. I was fortunate to be among that number, and the postdoctoral year evolved into nearly 30 years of collaboration with Holmes and his research group. The topics of Holmes’ research started with the reactions of boron trihalides with Lewis bases while a graduate student with H. C. Brown, then moved into the formation of cage compounds with phosphorus trihalides and amines at Carnegie Tech. Soon he began a long-term interest in pentacoordinate molecules of main group elements. This interest extended to include the main group IV elements, silicon, germanium, and tin. He was

CYCLIC PENTAOXYPHOSPHORANES AS MODELS FOR cAMP ACTION. AN AB INITIO APPROACH

Abstract Ab inirio molecular orbital calculations on monocyclic pentaoxyphosphorane molecules as model states in cyclic AMP action are carried out. Five-membered-ring systems are included for comparison. Minimum energy geometries in trigonal bipyramidal configurations that position the rings in axial–equatorial (a–e) sites are compared with those for diequatorial (e–e) ring locations. The models are made in-creasingly more complex approaching proposed intermediates in enzymatic hydrolysis of cAMP. These contain ribose rings trans-fused to the saturated six-membered ring of the pentacoordinated phosphorus state. All calculations show the e–e ring location to be higher in energy than the a–e ring arrangement. The computed energies and P–O bond lengths are compared with VT NMR activation energics for ligand exchange and with X-ray structural studies, respectively, on pentaoxyphosphoranes. The computations strongly support proposals for in-line enzymatic hydrolysis of cAMP with the ring positioned at a–e site...