Computational Study of the NH-Tautomerism in Calix[4]phyrins Observed by NMR Spectroscopy
Jan Horníček1,2, Hana Dvořáková3 and Petr Bouř1
1Institute of Organic Chemistry and Biochemistry of the ASCR, v. v. i., Flemingovo nám. 2, 166 10 Praha 6
2Department of Natural Compounds
Chemistry, Institute of Chemical Technology, Technická 5, 166 28, Prague 6,
Czech Republic,
3Central Laboratories, Institute of
Chemical Technology, Technická 5, 166 28, Prague 6, Czech Republic
Calix[4]phyrins are novel compounds derived from porphyrins. They can be used as excellent agents in complexes and macromolecular chemistry[1]. For some derivatives the inner hydrogen movement can bee seen in NMR spectra, where the NH-tautomerism can be followed by measurement of the coalescent temperatures. The transition energy barrier for the phenomenon can be obtained from the NMR data. Because of the importance of the hydrogen transfer processes we studied the effect in detail, by simulating the transition path and two-dimensional potential energy surfaces. The PM3, HF/3-21G and DFT BPW91/6-31G** and B3LYP/6-31G** methods were used for the ab initio computations. The Qgrad program[2,3] using normal modes coordinates was adapted for suitable coordinate definition corresponding to the hydrogen movement. Calculated barriers and equilibrium geometries well correspond to the observed data.
[1] Dolenský, B.; Kroulík, V.; Král, V., Sessler, J. L.; Dvořáková, H.; Bouř, P.; Bernátková, M.; Bucher, C.; Lynch, V., J. Am. Chem. Soc. 2004, 126, 13714-13722.
[2] Bouř, P., Collect. Czech. Chem. Commun. 2005, 70, 1315-1340.
[3] Bouř, P.; Keiderling, T. A., J. Chem. Phys. 2002, 117, 4126-4132.