Predicting arene rate coe ffi cients with respect to hydroxyl and other free radicals in the gas-phase : a simple and e ff ective method using a single
Predicting arene rate coefficients with respect to hydroxyl and other free radicals in the gas-phase: a simple and effective method using a single topological descriptor M. R. McGillen, C. J. Percival, G. Pieterse, L. A. Watson, and D. E. Shallcross School of Earth, Atmospheric and Environmental Sciences, The University of Manchester, The Sackville Building, Sackville Street, P.O. Box 88, Manchester M60 1QD, UK Department of Air Quality and Climate Change, Energy Research Centre for the Netherlands, P.O. Box 1, 1755 ZG Petten, The Netherlands Biogeochemistry Research Centre, School of Chemistry, The University of Bristol, Cantock’s Close BS8 1TS, UK Received: 13 November 2006 – Accepted: 2 February 2007 – Published: 26 February 2007 Correspondence to: C. J. Percival (c.percival@manchster.ac.uk)
The Structural Interpretation of the RandiIndex
Motivation. The Randiindex is one of the most successful molecular descriptors in structure-property and structure-activity relationships studies. In spite of some attempts to interpret the physical meaning of this index it is still not clear what exactly this index measures in a molecular structure. The finding of the structural interpretation of this topological index will open the doors to novel applications as well as to a better understanding of the models developed with its use. Results. The inverse squared-root of the vertex degree is identified here as a measure of the relative accessible perimeter of an atom from the outside. These perimeters, which have length units, are proposed to be measured in a new unit called the Randi¸ (R). One Randicorrespond to the perimeter of a carbon atom with degree one. On this basis, the bond contributions to the Randiindex are relative areas of bond accessibility from the environment. Consequently, the Randiindex is interpreted as the relative molecular area of external accessibility, that is the sum of relative accessibility bond areas in a molecule. Conclusions. The results obtained here provide the Randiindex with a clear structural interpretation, which is very close to the physical thinking commonly used in chemistry.
Variable Connectivity Index for Cycle-Containing Structures
In the early applications of the connectivity index the index was empirically modified for some properties of cyclic structures by subtracting 1/2 from the computed value based solely on valence of vertices in the molecular graph. In this article we looked into the origin of this heuristic adjustment of the connectivity indices for cyclic structures. We have examined the relative role of carbon atoms in cycle-containing structures by differentiating carbon atoms making up a ring and carbon atoms in exocyclic bonds. We found in the case of the boiling points of cycloalkanes and alkylcycloalkanes that contributions of "cyclic" and "acyclic" atoms to molecular additivities differ somewhat.