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Calculation of vibrational spectra of linear tetrapyrroles. 3. Hydrogen-bonded hexamethylpyrromethene dimers
Citation key Mroginski2005
Author Mroginski, M. A. and Nemeth, K. and Bauschlicher, T. and Klotzbucher, W. and Goddard, R. and Heinemann, O. and Hildebrandt, P. and Mark, F.
Pages 2139–2150
Year 2005
Journal Journal Of Physical Chemistry A
Volume 109
Number 10
Abstract The structure and vibrational spectra of hexamethylpyrromethene (HMPM) have been investigated by X-ray crystallography, IR and Raman spectroscopies, and density functional theory calculations. HMPM crystallizes in the form of dimers, which are held together by bifurcated N-H(center dot center dot center dot N)(2) hydrogen bonds, involving one intramolecular and one intermolecular N-H center dot center dot center dot N interaction. The monomers are essentially planar, and the mean planes of the monomers lie approximately perpendicular to one another, so that the four N atoms in the dimer form a distorted tetrahedron. The structure of the HMPM dimer is well-reproduced by B3LYP/6-31G* calculations. A comparison of the calculated geometry of the dimer with that of the monomer reveals only small changes in the N-H center dot center dot center dot N entity and the methine bridge angles upon dimerization. These are a result of weakening of the intramolecular N-H center dot center dot center dot N hydrogen bond and the formation of a more linear N-H center dot center dot center dot N intermolecular hydrogen bond. Using an empirical relation between the shift of the N-H stretching frequency of pyrrole and the enthalpy of adduct formation with bases [Nozari, M. S.; Drago, R. S. J. Am. Chem. Soc. 1970, 92, 7086-7090], estimates of the strength of the intra- and intermolecular hydrogen bonds are obtained. IR and Raman spectroscopies of HMPM and its isotopomers deuterated at the pyrrolic nitrogen atom and at the methine bridge reveal that the molecule is monomeric in nonpolar organic solvents but dimeric in a solid Ar matrix and in KBr pellets. The matrix IR spectra show a splitting of vibrational modes for the dimer, particularly those involving the N-H coordinates. Due to intrinsic deficiencies of the B3LYP/6-31G* approximation, a satisfactory reproduction of these modes of the monomeric and dimeric HMPM requires specific adjustments of the NH scaling factors for the calculated force constants and, in the case of the NH out-of-plane modes of HMPM dimers, also of intra- and intermolecular coupling constants. This parametrization does not significantly affect the other calculated modes, which in general reveal a very good agreement with the experimental data.
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