Premium
Speed of action of anesthetics and hydrogen‐bond proton‐acceptor properties. Estimation of the free energy of interaction of O ⃛H—O bonding in hindered phenols based on cndo/2 potentials
Author(s) -
Davies R. H.,
Mason R. C.,
Smith D. A.,
McNeillie D. J.,
James R.
Publication year - 2009
Publication title -
international journal of quantum chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.484
H-Index - 105
eISSN - 1097-461X
pISSN - 0020-7608
DOI - 10.1002/qua.560140723
Subject(s) - cndo/2 , hydrogen bond , chemistry , conformational isomerism , steric effects , acceptor , computational chemistry , interaction energy , bond energy , proton , molecule , stereochemistry , organic chemistry , physics , quantum mechanics , condensed matter physics
Estimates of the relative free energies of O ⃛H—O bonding in 2,6‐dialkyl hindered phenol‐water interactions have been made based on CNDO/2 potentials. To compute the free energies it was found necessary to examine a number of phenolic conformers for each interaction since conformers of small populations in the unbonded species became dominant in the bonded forms. An empiric potential was developed from the CNDO/2 results for a linear O ⃛H—O bond based on the Rydberg potential. The limitations of using electrostatic terms in the potential were examined. The steric effects of neighboring atoms were accurately incorporated into the potential by the use of repulsive exponential terms. Charge directionality from the proton acceptor was not observed. Reproducibility in hydrogen bonding was achieved to ±0.13 kcal over a 3–7 kcal range of hydrogen‐bond interaction. The statistics of the interaction were computed for 2,6‐dimethyl, 2,6‐diisopropyl, and 2,6‐di‐ t ‐butyl phenol conformer‐water interactions. The average energies of interaction were very similar, being 5.5, 5.4, and 5.4 kcal, respectively, compared to the minimum energies of interaction of 7.4, 7.4, and 6.9 kcal observed for each compound. While the differences between the free energies of interaction of the phenolic proton acceptors were entropy dominated, the estimate of the difference between the 2,6‐dimethyl and 2,6‐diisopropyl phenol‐water interactions was small (0.3 kcal). A similar difference comparison for the less entropy favored 2,6‐di‐ t ‐butyl phenol‐water interaction gave an estimate of 1.5 kcal.