Classical physical organic molecular descriptors, such as logP, Hammett, Taft, or Charton parameters, were developed to independently probe isolated hydrophobic, electronic or steric effects. Such parameters are often used for the elucidation of reaction mechanisms, yet, cannot always account for the complex molecular structures and interactions invoked in modern synthetic chemistry. We recently established a method for the prediction of selectivity trends by applying a fundamental physical organic chemistry approach substantiated by several experimental catalytic case studies. A molecule's intricate structural features are embodied in its unique vibrational modes. Thus, we applied vibrational spectroscopy to depict modular non-additive steric and electronic perturbations, yielding correlations akin to free energy relationships. As substituted aromatic rings represent a ubiquitous synthetic scaffold, in the current work, we describe a set of normalized benzoic acid infrared vibrations as a generalized parameter system for the mathematical modeling of concomitant steric and electronic substituent modulations. This computationally-derived set of molecular vibrations can be employed for the prediction of reaction outcomes, while illuminating mechanistic aspects thereof.Figure 1. a.  Linear correlation between benzoic acid C=O vibration frequencies and Hammett s values. b.  Illustration of the benzoic acid C=O stretch. c.  Two-body harmonic oscillator equation: ? frequency, � reduced mass, and k force constant. d.  Schematic illustration of the dependence of frequency and intensity on the mass and charge of the atoms involved in a molecular vibration.

Infrared molecular vibrations as parameters for free energy correlations

248th National Meeting (2014)

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