Invited Speaker - Boris Galabov (Bulgaria)

Spectroscopic and Theoretical Parameters in Predicting Hydrogen Bond Strength, Conformational Stability and Chemical Reactivity
 
B. Galabov
Department of Chemistry, University of Sofia, Sofia 1164, Bulgaria
 
Abstract

     Molecular parameters derived from theoretical computations as well as characteristic vibrational group frequencies were employed to quantify the reactivity of molecules for the process of hydrogen bonding and for reactions of organic compounds. The theoretical quantities include: atomic charges derived using different approaches, the Parr electrophilicity index, and the atomic electrostatic potential. The predictive power of these theoretical parameters is compared with the performance of experimental indices and quantities, such as the Hammet constants and shifts in vibrational group frequencies. A number of relationships are derived between the energies of hydrogen bond formation as well as the activation energies of chemical reactions and some of the quantities considered. It is shown for the first time that the atomic electrostatic potential can be employed as a reliable local reactivity index. A number of interactions are studied: hydrogen bonding of aliphatic carbonyl and nitrile derivatives, solvolitic reactions of amides and esters, SN2 reaction in aliphatic systems, hydrogen transfer reaction in phenols. It is shown that the variations of the carbonyl stretching frequency in amides and esters are linearly linked to theoretically estimated activation energies for reactions of these systems. Such relationships are in some cases absent if higher order effects (such as Fermi resonance) are present in the spectra. It is found that vibrational frequency shifts upon complexation correlate excellently with theoretically estimated energies of hydrogen bonding in series of structurally related molecules.
     The conformational stabilities of aromatic amides and thioamides are rationalized in terms of fluctuations of atomic charges as well as variations in characteristic group frequencies. A perfect linear dependence is found between the magnitudes of rotational barriers in series of acetanilides and the ν(C=O) frequency shifts. The origin of the higher barriers of rotation in thioamides than in amides is definitively established. 
 
[1] B. Galabov, P. Bobadova-Parvanova, J. Phys. Chem. A 103 (1999) 6793.
[2] B. Galabov, S. Ilieva, B. Hadjieva, E. Dincheva, J. Phys. Chem. A 107 (2003) 5854.
[3] B. Galabov, D. Cheshmedzhieva, S. Ilieva, B. Hadjieva, J. Phys. Chem. A 108 (2004) 11457.
[4] B. Galabov, S. Ilieva, H. F. Schaefer III, J. Org. Chem. 71 (2006) 6382.