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).