Classical phys. org. mol. 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 mol. structures and interactions invoked in modern synthetic chem. We recently established a method for the prediction of selectivity trends by applying a fundamental phys. org. chem. approach substantiated by several exptl. catalytic case studies. A mol.'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 arom. rings represent a ubiquitous synthetic scaffold, in the current work, we describe a set of normalized benzoic acid IR vibrations as a generalized parameter system for the math. modeling of concomitant steric and electronic substituent modulations. This computationally-derived set of mol. vibrations can be employed for the prediction of reaction outcomes, while illuminating mechanistic aspects thereof.Figure 1. Linear correlation between benzoic acid C=O vibration frequencies and Hammett σ values. Illustration of the benzoic acid C=O stretch. Two-body harmonic oscillator equation: ν frequency, μ reduced mass, and k force const. Schematic illustration of the dependence of frequency and intensity on the mass and charge of the atoms involved in a mol. vibration. [on SciFinder(R)]
Milo, A., & Sigman, M. S. (2014). Infrared molecular vibrations as parameters for free energy correlations. Abstracts of Papers, 248th ACS National Meeting & Exposition, San Francisco, CA, United States, August 10-14, 2014, PHYS-582.