Conformational Analysis, Experimental and GIAO-DFT 13C NMR Chemical Shift Calculation on 2’-Hydroxy-3,4,5-trimethoxy-chalcone

In this paper we investigated the ability of the GIAO-mPW1PW91/6-31G(d)//mPW1PW91/6-31G(d) level of theory to predict the 13C nuclear magnetic resonance (NMR) chemical shifts of the 2’-hydroxy-3,4,5-trimethoxy-chalcone molecule. Two different approaches were used. First: the absolute shieldings σ for all carbon atoms in each geometrically optimized conformers of the 2’-hydroxy-3,4,5-trimethoxy-chalcone molecule were calculated at the GIAO-mPW1PW91/6-31G(d)//mPW1PW91/6-31G(d) level of theory. This approach is further used to generate weighted average values for each atom considering the previously obtained conformational distribution. Second: only the σ for the lowest energetic conformer will be taken to account. The robustness of the method was evaluated for two other chalcones: (E)-1-(4-hydroxy-3-methoxyphenyl)-3-(3,5-di-tert-butyl-4-hydroxyphenyl)prop-2-en-1-one and (E)-1-(4-aminophenyl)-3-(3,4-dimethoxyphenyl)prop-2-en-1-one, corroborating the ability of the method in chemical shift prevision. Although, both approaches were able to reproduce the chemical shifts of the 2’-hydroxy-3,4,5-trimethoxy-chalcone, significant differences in the calculated values for C-4 and methoxy carbons were observed. The best results were obtained using the second approach (II).

本研究探讨了GIAO-mPW1PW91/6-31G(d)//mPW1PW91/6-31G(d)理论级别对2’-羟基-3,4,5-三甲氧基查尔酮分子的碳-13核磁共振（NMR）化学位移的预测能力。本研究采用两种不同方案：其一，在GIAO-mPW1PW91/6-31G(d)//mPW1PW91/6-31G(d)理论级别下，计算该2’-羟基-3,4,5-三甲氧基查尔酮分子所有几何优化构象中各碳原子的绝对屏蔽值σ，并结合此前获取的构象分布，为每个原子生成加权平均数值；其二，仅采用能量最低构象的绝对屏蔽值σ进行分析。为评估该方法的稳健性，本研究还针对另外两种查尔酮展开验证：(E)-1-(4-羟基-3-甲氧基苯基)-3-(3,5-二叔丁基-4-羟基苯基)丙-2-烯-1-酮，以及(E)-1-(4-氨基苯基)-3-(3,4-二甲氧基苯基)丙-2-烯-1-酮，结果佐证了该方法在化学位移预测中的能力。尽管两种方案均能复现2’-羟基-3,4,5-三甲氧基查尔酮的化学位移，但研究观察到C-4原子与甲氧基碳原子的计算值存在显著差异，其中第二种方案（方法II）的预测效果最佳。