Carbon-13 N.M.R. chemical shifts and rotational barriers of para-substituted N,N-dimethylbenzamides

Abstract

The carbon-13 N.M.R. chemical shifts for a series of para-substituted N,N-dimethylbenzamides have been measured. The substituent induced 13C shifts have been examined by a dual substituent parameter (DSP) method using Hammett-type constants. The barriers to rotation have also been correlated with Hammett-type constants by the DSP method and related to 13C substituent induced shifts. Substituent effects of the bromomethyl, dibromomethyl and tribromomethyl groups have been examined by using the chemical shift and rotational barrier probes.

Solvent Effects on the Carbon – 13 NMR Chemical Shifts and Rotational Barriers of N,N-Dimethylbenzamide - Solvent Enhanced π Polarization

“…The meta carbon, C3, has always been assigned downfield from the ortho carbon atom. This assignment is in agreement with previous assignments [3,4] for a limited number of solvents.…”

“…We have recently established a linear relationship between the rotational barriers about the C-N bond, zlG + 298, in a series of ortho-and para-substituted N,N-dimethylbenzamides and the carbonyl 13 C substituent chemical shifts [3,4]. Hence it may be predicted that a similar relationship may be found in the parent compound, N,N-dimethylbenzamide, as ZlG+298 varies in different solvents.…”

“…The values of A G+298 were derived from signal shape analysis, or in the case of dichloromethane [19] an approximate signal shape analysis. These values can be considered accurate [3,16,17] to about 0.5 to 1.0 kJ mole- 1 .…”

“…Hence, as indicated by equation (1) rotational barriers increase proportionately as solvent polarity or hydrogen bond donor ability increase in magnitude. [17]; « from reference [18]; h from reference [19]; 1 from reference [3]; J from reference [16]. It is obvious from equations (1) and (2) Table I for numbering scheme.…”

“…All rotational barriers [3,16,17] were determined at a concentration of 0.5 mol dm -3 , or less, with the exception of the value for chlorobenzene [16] which was obtained from a 1.0 mol dm -3 solution. The values of A G+298 were derived from signal shape analysis, or in the case of dichloromethane [19] an approximate signal shape analysis.…”