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The Effect of an NH Tautomerism on the Structure and Vibrational States of Chlorin
K. V. Berezin and V. V. Nechaev
Chernyshevsky State University, Saratov, 410026 Russia
Received July 8, 2003
Abstract: The structure and IR spectrum of chlorin and of its NH tautomer whose central protons are localized on the pyrrole rings lying on the symmetry axis are calculated in the approximation of the hybrid density functional B3LYP/6-31G(d). It is found that the symmetry of the chlorin macrocycle lowers from C2v to C2 both due to a synchronous rearrangement of the central protons and upon their replacement with Mg or Zn. The assignments of the frequencies of all the normal vibrations of porphin, chlorin, and the above NH tautomer are compared. It is shown that the band at 3318 cm–1 of the antisymmetric stretching vibration of the NH bonds is the most convenient for experimental detection of the less stable tautomer of chlorin by IR spectroscopy. A nonlocal character of the changes in the force constants and a consistent behavior of the signs of the changes in the bond lengths and in the corresponding diagonal force constants are revealed from a comparative analysis of the quantum-mechanical force fields of these two forms of chlorin in dependent natural coordinates.
© 2004 MAIK "Nauka/Interperiodica".
Optics and Spectroscopy, Vol. 97, No. 5, 2004, pp. 707–713. Translated from Optika i Spektroskopiya, Vol. 97, No. 5, 2004, pp. 753–759. Original Russian Text Copyright © 2004 by Berezin, Nechaev.

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