The crystal and molecular structures of (1) 2-methylpyrazine (2MP) with 2,5-dichloro-3,6-dihydroxy-p-quinone (chloranilic acid, CLA), (2) 2-methylpyrazine (2MP) with 2,5-dibromo-3,6-dihydroxy-p-quinone (bromanilic acid, BRA), (3) 2,3,5-trimethylpyrazine (TrMP) with 2,5-dichloro-3,6-dihydroxy-p-quinone (chloranilic acid, CLA), and (4) 2,3,5-trimethylpyrazine (TrMP) with 2,5-dibromo-3,6-dihydroxy-p-quinone (bromanilic acid, BRA) were analyzed in terms of the number of independent methyl groups in their crystal structure. The inelastic neutron back-scattering spectra at low temperature (4–40 K) were discussed in terms of methyl group tunnelling. The INS spectra were compared with the temperature dependence of the ¹H-NMR spin–lattice relaxation time, particularly at low temperatures, where CH₃ tunnelling is postulated. The infrared and Raman spectra at room temperature were recorded for all complexes under investigation. Furthermore, the vibrational spectra were discussed in terms of the structure of molecules and their interactions. The structural phase transition of the TrMP·CLA complex at 171/175 K (cooling/heating) was characterized by DSC and single-crystal X-ray diffraction. Full-geometry optimization was carried out in the solid state in order to obtain the minimum structures and bonding properties. The results are in very good agreement with the experimental data. The infrared spectrum in the harmonic approximation was calculated and a comparative vibrational analysis was performed. CRYSTAL09 vibrational results appear to be in good agreement with the experimental results.

中文翻译：

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甲基取代吡嗪与苯甲酸的晶体结构分析：衍射，非弹性中子散射，¹ H-NMR研究和理论方法的组合†

（所述的晶体和分子结构1）2-甲基吡嗪（2MP）与2,5-二氯-3,6-二羟基- p -quinone（氯冉酸，CLA），（2）2-甲基吡嗪（2MP）与2-， 5-二溴3,6-二羟基对苯醌（溴酸，BRA），（3）2,3,5-三甲基吡嗪（TrMP）与2,5-二氯-3,6-二羟基对苯醌（氯苯甲酸（CLA）和（4）2,3,5-三甲基吡嗪（TrMP）与2,5-二溴-3,6-二羟基-p根据其晶体结构中独立甲基的数量分析了对苯二酚（溴酸，BRA）。根据甲基隧道效应，讨论了低温（4–40 K）下的非弹性中子背向散射谱。将INS光谱与¹ H-NMR自旋-晶格弛豫时间的温度依赖性进行了比较，特别是在低温下，其中CH ₃隧道是假定的。记录了所研究的所有配合物在室温下的红外光谱和拉曼光谱。此外，根据分子的结构及其相互作用讨论了振动光谱。通过DSC和单晶X射线衍射表征了TrMP·CLA配合物在171/175 K（冷却/加热）下的结构相变。为了获得最小的结构和粘合性能，在固态条件下进行了全几何优化。结果与实验数据非常吻合。计算谐波近似中的红外光谱，并进行比较振动分析。CRYSTAL09的振动结果似乎与实验结果吻合良好。