1H chemical shifts in NMR. Part 20 — Anisotropic and steric effects in halogen substituent chemical shifts (SCS), a modelling and ab initio investigation

Abstract

The 1H NMR spectra of 1-halonaphthalenes were recorded and assigned. These data together with the known 1H chemical shifts of the halobenzenes and of H-5 in 4-halophenanthrenes were used to investigate different models of halogen SCS using the CHARGE program. Good agreement was obtained with the observed shifts for the fluoro compounds, but a new model which included the anisotropy of the C-X bond (X = Cl, Br, I) and steric and electric field effects was required to give an accurate description of the observed chemical shifts for the other halogens. A previous observation of an anomalous 1H SCS on the meta protons in halobenzenes was further investigated using the 1-halonaphthalenes plus the observed 1H shifts of a number of 2-substituted bromobenzenes. The meta SCS of the bromo substituent in the bromobenzenes was only observed on H-5. When a substituent was attached to C-2, H-3 showed no appreciable effect. This remarkable effect was investigated by CHARGE and GIAO calculations and shown to be due to the pi-electron system. It was reproduced in CHARGE as a gamma effect from the C-X carbon atom. The observed 1H chemical shifts in the above compounds were compared with those calculated by CHARGE, the GIAO technique and by the ACD database program. The recommended ab initio basis set for the iodo compounds (Lan12DZ) gave very poor calculated shifts, which suggests that 1H chemical shifts of fourth-row atoms when calculated with the GIAO technique should be viewed with caution. In contrast, the recommended 6-31G** basis set with the B3LYP technique in the Gaussian98 program gave calculated values in reasonable agreement with the observed data, as did the ACD package. These different prediction methods were compared by pie charts, scatter plots and r.m.s. errors and the CHARGE program was shown to be more accurate for the compounds considered here than the other two methods. The different philosophies of these programs are discussed together with the results obtained.