We present an application of the recently introduced Localized Pair Model (LPM) [Z. A. Zielinksi and J. K. Pearson, Comput. Theor. Chem., 2013, 1003, 7990] to characterize and quantify properties of the chemical bond in a series of substituted benzoic acid molecules. By computing interelectronic distribution functions for doubly-occupied Edmiston-Ruedenberg localized molecular orbitals (LMOs), we show that chemically intuitive electron pairs may be uniquely classified and bond strength may be predicted with remarkable accuracy. Specifically, the HF/u6-311G(d,p) level (where u denotes a complete uncontraction of the basis set) is used to generate the relevant LMOs and their respective interelectronic distribution functions can be linearly correlated to the well-known Hammett σp or σm parameters with near-unity correlation coefficients.
The so-called "Lewis pair" is a ubiquitous phenomenon in chemistry and is often used as an intuitive construct to predict and rationalize chemical structure and behavior. Concepts from the very general Valence Shell Electron Pair Repulsion (VSEPR) model to the most esoteric reaction mechanism routinely rely on the notion that electrons tend to exist in pairs and that these pairs can be thought of as being localized to a particular region of space. It is precisely this localization that allows one to intuit how these pairs might behave, generally speaking, so that reasonable predictions may be made regarding molecular structure, intermolecular interactions, property trends, and reaction mechanisms, etc. Of course, it is rather unfortunate that the Lewis model is entirely qualitative and yields no information regarding how any specific electron pair is distributed. Here we demonstrate a novel electronic structure analysis technique that predicts and analyzes precise quantitative details about the relative and absolute distribution of individual electron pairs. This Single Electron Pair Distribution Analysis (SEPDA) reveals quantitative details about the distribution of the well-known Lewis pairs, such as how they are distributed in space and how their relative velocities change in various chemical contexts. We show that these distributions allow one to image the explicitly pairwise electronic behavior of bonds and lone pairs. We further demonstrate how this electronic behavior changes with several conditions to explore the nature of the covalent chemical bond, non-covalent interactions, bond formation, and exotic 3-center-2-electron species. It is shown that indications of the strength of bonded and non-bonded interactions may also be gleaned from such distributions and SEPDA can be used as a tool to differentiate between interaction types. We anticipate that SEPDA will be of broad utility in a wide variety of chemical contexts because it affords a very detailed, visual and intuitive analysis technique that is generally applicable.
We assess the performance of six density functionals, each paired with one of five basis sets (a total of 30 model chemistries) for the prediction of geometrical parameters in the coordination sphere of nine vanadium complexes (for a total of 270 structural analyses). We find that results are generally consistent over the range of functionals tested and that none fail drastically. For bond lengths, the model chemistry PBE0/QZVP performed the best overall (having a MAD of only 0.02 Å from experiment) yet PBE0/6-31G* provides nearly identical results. For bond angles, PBE0 also performed best overall and, when combined with the 6-31G* basis, produces one of the smallest error distributions of any model chemistry tested. We subsequently applied the PBE0/6-31G* model chemistry to understanding the mechanism of action of a [BIMPY]VCl3 catalyst in the polymerization of styrene (Sty) and vinyl acetate (VAc). Our results indicate that the [BIMPY]VCl3 catalyst operates through a unique, two-step reaction pathway: dehalogenation to form a reactive V(II) intermediate (a highly favorable process) followed by a potentially reversible OMRP to control the polymerization of vinyl acetate. Control over vinyl acetate is facilitated by both the higher reactivity of the radical species and the participation of the ester group in the trapping step. In both the Sty and VAc cases we predict relatively poor control of the polymerization with the vanadium catalyst, which is in good agreement with our experimental results.
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