A GIAO/DFT study of ¹H, ¹³C and ¹⁵N shieldings in amines and its relevance in conformational analysis

Abstract

The 1H, 13C and 15N absolute shieldings of 13 amines were calculated at the GIAO/B3LYP/6-311++G** level. For some compounds (ethylamine, piperidine and 1-methylpiperidine) two conformations were calculated. The 13C and 15N data could be correctly correlated with experimental chemical shifts, allowing the conformation of 1-methylpiperidine to be established. The 1H NMR absolute shieldings, although less well correlated with delta values, were used to account for the anisotropy effects of the N lone pair.

Structural determination of two new sesquiterpenes biotransformed from germacrone by Mucor alternata

“…[11,12] So the geometry of (2) was optimized by using B3LYP method and 6-31G * BSF. [13,14] The calculated harmonic frequency indicated that its molecule was located at the minimal and stable points. And then according to the fully optimized geometry, CD spectrum and NOESY experiment, the preferred conformation of (2) was as shown in Fig.…”

“…The purity was above 98% determined by HPLC. Its structure was characterized by 1 H NMR, 13 1 H NMR and 13 C NMR data see Table 1.…”

The prediction of 1H chemical shifts in amines: a semiempirical and ab initio investigation

“…One is the strong basicity of alkyl amines. In a recent investigation, Abraham et al 4 found that the 1 H shifts of the NH protons of aliphatic amines showed little consistency in either CDCl 3 or DMSO solvent and often the signals were too broad to be observed. They note that the NH 2 C proton signals of dimethylamine hydrochloride occur at 9.2 ppm in CDCl 3 compared to those of the free base at ca 1 ppm, 5 and therefore traces of acid will produce large shifts for these protons and also broaden the signal owing to exchange.…”

“…For example, the SCS of the amino group is dependent on the position of the lone pair. 3 In solution, the NH 2 group is usually not in a single orientation, but the precise populations of the different conformers are often unknown. For example, ethylamine has two distinct conformers with the lone pair anti or gauche to the C-C bond but the conformer energies in solution have not to our knowledge been measured accurately.…”

“…In previous parts of this series, the CHARGE program has been developed to provide a model capable of accurately predicting the 1 H chemical shifts of a variety of organic compounds in CDCl 3 6 -8 and DMSO 4 solutions. This program consists of a neural-network-data-based approach for one-, two-and three-bond substituent effects plus a classical calculation of the long-range effects of substituents.…”