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1 H chemical shifts in NMR. Part 21—Prediction of the 1 H chemical shifts of molecules containing the ester group: a modelling and ab initio investigation
Author(s) -
Abraham Raymond J.,
Bardsley Ben,
Mobli Mehdi,
Smith Richard J.
Publication year - 2005
Publication title -
magnetic resonance in chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.483
H-Index - 72
eISSN - 1097-458X
pISSN - 0749-1581
DOI - 10.1002/mrc.1491
Subject(s) - chemistry , chemical shift , molecule , ab initio , steric effects , computational chemistry , anisotropy , carbon 13 nmr , ring (chemistry) , maleic anhydride , charge density , proton nmr , nmr spectra database , crystallography , spectral line , stereochemistry , organic chemistry , physics , quantum mechanics , copolymer , polymer , astronomy
The 1 H NMR spectra of 24 compounds containing the ester group are given and assigned. These data were used to investigate the effect of the ester group on the 1 H chemical shifts in these molecules. These effects were analysed using the CHARGE model, which incorporates the electric field, magnetic anisotropy and steric effects of the functional group for long‐range protons together with functions for the calculation of the two‐ and three‐bond effects. The effect of the ester electric field was given by considering the partial atomic charges on the three atoms of the ester group. The anisotropy of the carbonyl group was reproduced with an asymmetric magnetic anisotropy acting at the midpoint of the carbonyl bond with values of Δχ parl and Δχ perp of 10.1 × 10 −30 and −17.1 × 10 −30 cm 3 molecule −1 . An aromatic ring current (=0.3 times the benzene ring current) was found to be necessary for pyrone but none for maleic anhydride. This result was confirmed by GIAO calculations. The observed 1 H chemical shifts in the above compounds were compared with those calculated by CHARGE and the ab initio GIAO method (B3LYP/6–31G**). For the 24 compounds investigated with 150 1 H chemical shifts spanning a range of ca 10 ppm, the CHARGE model gave an excellent r.m.s. error (obs − calc) of <0.1 ppm. The GIAO calculations gave a very reasonable r.m.s. error of ca 0.2 ppm although larger deviations of ca 0.5 ppm were observed for protons near to the electronegative atoms. The accurate predictions of the 1 H chemical shifts given by the CHARGE model were used in the conformational analysis of the vinyl esters methyl acrylate and methyl crotonate. An illustration of the use of the CHARGE model in the prediction of the 1 H spectrum of a complex organic molecule (benzochromen‐6‐one) is also given. Copyright © 2004 John Wiley & Sons, Ltd.