关键词: 核磁共振, 化学位移, 乙酰水杨酸, 量化计算, 密度泛函

Abstract: Prediction of chemical shifts using quantum chemical calculation is helpful for assigning NMR spectra, and of great importance in structure determination. In this paper, we evaluated the performance of different quantum chemical methods for calculating 13C chemical shifts of acetylsalicylic acid and its derivatives. When Hartree-Fock (HF) and density functional theories (DFT) were used to compute phenyl 13C chemical shifts, both the CSGT method and the GIAO method yielded results that agreed reasonably well with the experimental values, while the former method often showed better performance than the latter. Calculated phenyl 13C chemical shifts showed excellent agreement with the experimental results when B3PW91//CSGT at 6-311G(d, p) level was used. When the B3LYP//GIAO method was used for calculation, the difference between the results obtained by applying 6-31G(d, p) and 6-311++G(3df, 3pd), respectively, was only δ 0.01~2.04. The MP2 method was often more time-consuming, but the results it gave show only marginal improvement in terms of calculation accuracy. It was found that the greater effect of electronic correlation, the more the electronic atmosphere of C atoms affects the accuracy of chemical shift calculation. For the reason, the HF method performed not so well because the calculation did not take electronic correlation into account. It was also found that the length of C chain also has effects on the calculated results.

Key words: NMR, chemical shifts, acetylsalicylic acid, quantum chemical calculation, density function theory (DFT)