Structure, IR Spectra, and NH-Tautomerism of Chlorin: Nonlocal Force Field Changes upon Rearrangement of the Central Protons
E. Yu. Stepanovich1, A. M. Likhter1, K. V. Berezin2, V.V. Nechaev3
1 V.N. Tatishchev Astrakhan State University, Astrakhan, Russia
2 N.G. Chernyshevsky Saratov State National Research University, Saratov, Russia
3 Saratov State Technical University named after Yuri Gagarin, Saratov, Russia.
Abstract
The structure and IR spectra of chlorin and its NH-tautomer, with the central protons localized on the pyrrole rings lying on the symmetry axis,
have been calculated using the hybrid density functional B3LYP/6-31G(d) approximation. A reduction in the symmetry of the chlorin macrocycle
from C2v to C2 is observed upon synchronous proton rearrangement, as well as upon their substitution by Mg or Zn.
A complete correlation of all normal vibration frequencies of porphine, chlorin, and its NH-tautomer has been performed.
It is shown that the band of the antisymmetric NH stretching vibration at 3318 cm⁻¹ is convenient for the experimental detection of the less
stable chlorin tautomer by IR spectroscopy. Comparison of the quantum-mechanical force fields in dependent natural coordinates for the two forms
of chlorin revealed the nonlocal nature of the force field changes and the synchronicity (symbaticity) between the signs of bond length changes
and the corresponding diagonal force constants.
Speaker
Ekaterina Yu. Stepanovich
V.N. Tatishchev Astrakhan State University, Astrakhan, Russia
Russia
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