Conformational and NBO analysis on cis and trans isomers of methyl-1-(4-hydroxy-3-methoxyphenyl)-1,2,3,4-tetrahydro-9H-β-carboline-3-carboxylate

The cis and trans-methyl-1-(4-hydroxy-3-methoxyphenyl)-1,2,3,4-tetrahydro-9H-β-carboline-3-carboxylates were prepared and 1H and 13C NMR spectra were recorded for both isomers. Conformational and NBO analysis were carried out for the cis and trans isomers. Conformer structures were pre-optimized using the hybrid method B3LYP along with the 6-311+G(d) basis set. Frequency calculations were employed to confirm the structures as minimum points. Potential energy surfaces (PES) were built at the same level of theory. Geometries obtained from DFT calculations were used to perform NBO analysis by the NBO 3.1 module in GAUSSIAN 03. The results obtained through theoretical calculations revealed that the shielding observed at C1 for the trans isomer can be attributed to carbomethoxy γ-effect, together with the hyperconjugative effect, while only hyperconjugative effects were found to explain the shielding of C3. The higher chemical shift value of C3 of the cis isomer was attributed to the carbonyl substituent, which plays an important role by capturing part of the electronic density in C3.