Proton chemical shifts in NMR. Part 15?proton chemical shifts in nitriles and the electric field and ?-electron effects of the cyano group

Abstract

The proton resonance spectra of a number of nitriles of fixed geometry were recorded in dilute CDCl3 solution and assigned. These were trans- and cis-4-tert-butylcylohexanecarbonitrile, axial and equatorial cyclohexanecarbonitrile and ax–ax- and eq–eq-trans-1,4-dicyanocyclohexane, the latter compounds at −60 °C. The aromatic nitriles measured were benzonitrile, o-, m- and p-dicyanobenzene, 1- and 2-cyanonaphthalene and 9-cyanoanthracene. This plus previous literature data allowed the determination of the cyano substituent chemical shifts (SCS) in a variety of molecules. These SCS were analysed in terms of the CN electric field, magnetic anisotropy and steric effects for protons more than three bonds removed together with a model (CHARGE7) for the calculation of the two- and three-bond SCS. For the aromatic nitriles ring current and π-electron effects were included. The anisotropic and steric effects of the cyano group were negligible in all the compounds investigated and in the aliphatic nitriles the SCS were due only to the CN electric field plus for near protons electronic effects. For the aromatic nitriles the π-electron effects were calculated from Hückel theory with the values of the exchange and resonance integrals adjusted to give π-electron densities in agreement with those obtained by ab initio calculations. The ring current shifts of the cyano derivatives were assumed to be the same as those of the parent hydrocarbons. The model gives the first comprehensive calculation of the SCS of the cyano group. For the data set of 93 proton chemical shifts from 1 to 9δ, the r.m.s. error (observed vs calculated shifts) was 0.088 ppm. The breakdown of the CN SCS in the aromatic nitriles showed good agreement with the Swain and Lupton field and resonance (F and R) components of substituent effects. Copyright © 2000 John Wiley & Sons, Ltd.