investigated experimentally (by H Rydberg atom photofragment translational spectroscopy) and/or theoretically (by electronic structure theory and 1- and 2-D tunneling ab initio calculations). Derived energetic and photophysical properties such as the O–H bond strengths, the S1–S0 excitation energies and the S1 predissociation probabilities (by tunneling through the barrier under the conical intersection between the and S1(11ππ*) S2(11πσ*) potential energy surfaces in the stretch coordinate) are considered within a RO–H like framework. The Y-dependent O–H bond strengths and S1–S0 term values are found to correlate well with a simple descriptor of the electronic perturbation caused by the aromatic substituent Y between (the Hammett constant, σp+). We also identify clear correlations σp+ and the probability of a photochemical process (predissociation). Such a finding is unsurprising, given that Y substitution will perturb the entire potential energy landscape, but appears not to have been demonstrated hitherto. The predictive capabilities of this approach are explored by reference to existing energetic data for larger 4-substituted phenols like tyramine, L-tyrosine and tyrosine containing di- and tri-peptides. Introduction The Hammett equation (1) has long been used by physical organic chemists to rationalize, and to predict, the effect of a given substituent on reaction rates and reactivities., (1) The equation is a linear free energy expression that relates the logarithmic ratio of the equilibrium constant, or the rate constant, of a reaction involving a substituted derivative K, k, to that for the unsubstituted molecule (K0 or in terms of just two parameters a k0) – substituent (or Hammett) constant, and a (reaction-type dependent) reaction constant, . The σ, and m = and p = substituent constants were derived from the meta para) original σm σp (i.e. ionization constants of benzoic acids in aqueous solution, but numerous subsequent
investigated experimentally (by H Rydberg atom photofragment translational spectroscopy) and/or theoretically (by ab initio electronic structure theory and 1- and 2-D tunneling calculations). Derived energetic and photophysical properties such as the O–H bond strengths, the S1–S0 excitation energies and the S1 predissociation probabilities (by tunneling through the barrier under the conical intersection between the S1(11ππ*) and S2(11πσ*) potential energy surfaces in the RO–H stretch coordinate) are considered within a framework. The Y-dependent O–H bond strengths and S1–S0 term values are found to correlate well with a simple descriptor of the electronic perturbation caused by the aromatic substituent Y (the Hammett constant, σp+). We also identify clear correlations between σp+ and the probability of a photochemical process (predissociation). Such a finding is unsurprising, given that Y substitution will perturb the entire potential energy landscape, but appears not to have been demonstrated hitherto. The predictive capabilities of this approach are explored by reference to existing energetic data for larger 4-substituted phenols like tyramine, L-tyrosine and tyrosine containing di- and tri-peptides. Introduction The Hammett equation (1) has long been used by physical organic chemists to rationalize, and to predict, the effect of a given substituent on reaction rates and reactivities., (1) The equation is a linear free energy expression that relates the logarithmic ratio of the equilibrium constant, K, or the rate constant, k, of a reaction involving a substituted derivative to that for the unsubstituted molecule (K0 or k0) in terms of just two parameters – a substituent (or Hammett) constant, σ, and a (reaction-type dependent) reaction constant, . The original σm and σp (i.e. m = meta and p = para) substituent constants were derived from the ionization constants of benzoic acids in aqueous solution, but numerous subsequent like
hammett_15_3_2013.docx