Two-dimensional 1H—15N NMR HSQC/HMBC experiments enable the unambiguous determination of the protonation (methylation) position and tautomeric structure of nitrogen-containing heterocycles. In investigated thiopyrimidines protonation (or methylation) occurs at the N(1) atom of the pyrimidine ring. The tautomeric structures of these compounds were established based on the analysis of 1H—15N NMR spectra. Ab initio calculations of chemical shifts (GIAO B3LYP/6-31G(d)//HF/6-31G) are in full agreement with experimental values. The stability of various protonated (methylated) and tautomeric species is explained in terms of a thermodynamic approach.
                                    二维 1H-15N NMR HSQC/HMBC 实验能够明确确定含氮杂环的质子化(甲基化)位置和同分异构体结构。在所研究的
噻吩嘧啶中,质子化(或甲基化)发生在
嘧啶环的 N(1)原子上。根据 1H-15N NMR 光谱分析,确定了这些化合物的同分异构体结构。
化学位移的 Ab initio 计算(GIAO B3LYP/6-31G(d)//HF/6-31G)与实验值完全一致。热力学方法解释了各种质子化(甲基化)和同分异构体的稳定性。