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Silne wiązania wodorowe w kompleksach wybranych kwasów organicznych z tetrametylopirazyną

Bator G., Sobczyk L.

Wiadomości Chemiczne

2011

[Z] 65, 9-10

869-885

In the present review our interest is focused on the hydrogen bonded complexes of tetramethylpyrazine (TMP) with strong proton donors, in particular with chloranilic (CLA) or squaric (H2SQ) acid. The x-ray diffraction studies show that, depending on the proton donor, various assemblies with the acid are formed, e.g. the infinite O-H…N hydrogen bonded chains without proton transfer in the case of the complex with CLA. On the other hand with H2SQ the assemblies of [HSQ]2 2–-2TMPźH+ composition are created, in which the ionized HSQ–1 molecules are present in the form of dimers. These dimers are bound with the TMPźH+ cations on its both sides via the +N-H…O– hydrogen bonds. Picric acid forms with TMP the complex of the 2:1 composition with a double protonated TMP molecule. In the case of HI3 acid the interesting units of the (TMPźH+)2źTMP composition are formed, in which two TMPźH+ cations are coordinated with one TMP molecule through the +N-H…N bridges. In the infrared spectra of the TMP complexes, both with CLA and H2SQ, the similar absorption continua are observed. They can be interpreted in terms of an asymmetric potential for the proton motion, with either the double minimum or the single broad minimum potential for the CLA and H2SQ complexes, respectively. An analysis of the neutron scattering spectra concerns the phenomena of the tunneling splitting, quasielastic neutron scattering (QNS) and inelastic (INS) scattering. In the case of tunneling splitting neat TMP does not show any tunneling transitions in the ěeV energy region, because they are overlapped by the elastic scattering band. In the case of the TMPźCLA complex four tunneling transitions are seen corresponding to the four crystallographically nonequivalent CH3 groups in the TMP molecule. In the spectrum of the complex with squaric acid the observed two transitions are ascribed to the two different CH3 groups. The two remaining CH3 group tunneling transitions are overlapped by the elastic scattering. The measurements in various low temperature ranges yield information about the shape of the CH3 group rotational potential. The shape of the potential is also reflected in the spectra of quasielastic scattering. In particular the temperature dependence of the quasielastic band allows us to find the activation energy for the CH3 rotations. Finally the inelastic neutron scattering spectra are analyzed in the energy range of the CH3 torsional modes (below 200 cm–1 = 25 meV). The analysis shows that for the complexes the torsional vibration frequencies are markedly lower than those for neat TMP. In the case of the TMPźCLA complex frequencies found are particularly low. They are close to the frequencies calculated for the TMP+ cation. A general conclusion can be drawn that in the complexes the CH3 groups behave more loosely than in neat TMP.

Structure and Vibrational Spectra of the 1:1 Squaric Acid (H2SQ) – Tetramethylpyrazine (TMP) Adduct

Nowicka-Scheibe J., Grech E., Sawka-Dobrowolska W., Bator G., Sobczyk L.

Polish Journal of Chemistry

2007

Vol. 81, nr 5-6

643-652

The crystal structure of the title complex was determined at 100 K by X-ray diffraction. It consists of double ionized dimeric (HSQ)2 2 species and protoned TMPH+ cations. Strong charge assisted (OHO)– and (NHO)+ hydrogen bonds are the main factor of crystal packing. A wealth of unconventional C–HO and C–HN relatively short bridges should be emphasized. Particularly important are the latter ones binding two adjacent TMPH+ cations. The structure is well manifested in the IR and Raman spectra, particularly in the region of (OH) and (NH) as well as of (C=O) frequencies. The complexation of TMPleads to a marked change in the low and high frequency vibrations of the methyl groups. An anomaly on the DSC runs has been found at 163/175 K (cooling/ heating) which can be assigned to the freezing of the methyl group rotations. No change of the crystal symmetry is observed at this temperature.