GIAO NMR calculations for carbazole and its N‐methyl and N‐ethyl derivatives. Comparison of theoretical and experimental 13C chemical shifts

Abstract

High-level ab initio calculations were performed at the restricted Hartree-Fock (RHF) level of theory on carbazole and its N-methyl and N-ethyl derivatives. Single-point gauge-invariant atomic orbitals (SP GIAO) RHF NMR calculations on ab initio RHF optimized geometries were performed. The 6-31G* and 6-311 CC G** basis sets were used and some calculations were performed within a density functional theory using a recent B3PW91 hybrid functional. The theoretically predicted multinuclear magnetic resonance chemical shifts of carbazole and its N-methyl and N-ethyl derivatives in the gas phase are compared with experimental NMR data in CDCl 3 solutions. A revised assignment of 13 C NMR spectra of simple carbazoles is proposed.

Convergence of nuclear magnetic shieldings and one‐bond 1J(11B1H) indirect spin–spin coupling constants in small boron molecules

“…This indicates the importance of using very good basis set for reliable NMR calculations to avoid accidental error cancellation. In fact, a similar difference (about 9 ppm) was observed for nuclear isotropic shielding of carbon atoms in carbazole at B3LYP/6–311++G** level of theory and explained by overestimation of paramagnetic component of shielding by DFT . The above results show some serious problems for DFT‐B3LYP in case of untypical molecules such as BH or F 2.…”

Method and basis set dependence of the NICS indexes of aromaticity for benzene

“…It has been observed that due to overestimation of paramagnetic component of isotropic nuclear magnetic shielding of 13 C by DFT, this parameter calculated using RHF and B3LYP differed by about 10 ppm, and was also sensitive to the basis set quality . Thus, carbon isotropic shielding calculated with 6‐31G* and 6−311++G** basis sets, differs by about 20 ppm and this suggests that a comparison of theoretically predicted aromaticity indexes of two compounds, studied in various projects, could be in many cases unreliable.…”

Molecular modeling and experimental studies on structure and NMR parameters of 9-benzyl-3,6-diiodo-9H-carbazole

“…Nowadays, the assignment of experimental spectra is often supported by molecular modeling of the relevant NMR parameters [19][20][21], as it is possible to predict satisfactory chemical shifts for several different nuclei including 13 C, 17 O, 15 N, and 19 F [19,[22][23][24][25] using density functional theory (DFT) calculations in combination with gauge including atomic orbitals (GIAOs [26,27]) and employing the Becke three-parameter, Lee-Yang-Parr (B3LYP [27,28]) or the hybrid half-and-half (BHandHLYP) exchange correlation functionals [29]. Several NMR reports on the simplest carbazoles are available [30][31][32][33]. However, systematic studies of the correlation between the molecular structure of carbazoles and their NMR parameters are lacking in the literature.…”