| Abstract
| - The structure and vibrational spectra of hexamethylpyrromethene (HMPM) have been investigated by X-raycrystallography, IR and Raman spectroscopies, and density functional theory calculations. HMPM crystallizesin the form of dimers, which are held together by bifurcated N−H(···N)2 hydrogen bonds, involving oneintramolecular and one intermolecular N−H···N interaction. The monomers are essentially planar, and themean planes of the monomers lie approximately perpendicular to one another, so that the four N atoms in thedimer 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 onlysmall changes in the N−H···N entity and the methine bridge angles upon dimerization. These are a result ofweakening of the intramolecular N−H···N hydrogen bond and the formation of a more linear N−H···Nintermolecular hydrogen bond. Using an empirical relation between the shift of the N−H stretching frequencyof 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 atthe methine bridge reveal that the molecule is monomeric in nonpolar organic solvents but dimeric in a solidAr 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 requiresspecific adjustments of the NH scaling factors for the calculated force constants and, in the case of the NHout-of-plane modes of HMPM dimers, also of intra- and intermolecular coupling constants. This parametrizationdoes not significantly affect the other calculated modes, which in general reveal a very good agreement withthe experimental data.
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