Determination of syn/anti Isomerism in DCNQI Derivatives by 2D Exchange Spectroscopy: Theoretical Underpinning

The dynamic syn/anti isomerism resulting from the inversion of the cyano group at the C=N double bond in a series of substituted DCNQIs has been investigated in solution by two-dimensional exchange spectroscopy (2D EXSY). The isomers formed were characterised by 1H NMR at 223 K to slow down the inversion process of the cyanoimine group as well as by H,H COSY spectra. Whereas compounds 6, 7, 9 and 10 show only one isomer, compounds 8 and 11 show two isomers with two inversion processes and compounds 12 and 13 show three isomers and four inversion processes. The energy barrier for the syn/anti isomerization of the NCN groups has been estimated from the rate constants determined from the 2D EXSY spectra, and very close values (13.16−13.93 kcal/mol for a single inversion) were found for all compounds. Isomerizations involving two NCN groups (compounds 12 and 13) exhibited higher activation free energy values (13.41−14.40 kcal/mol). Theoretical calculations of these free energy barriers are in excellent agreement with the experimental values, especially when solvent effects are taken into account. Theoretical calculations at B3LYP/6−31G* level predict planar geometries for the DCNQI derivatives studied (6 and 8), in particular when solvent effects are considered. This is in perfect agreement with the experimental data. Two-electron stabilizing interactions, as well as solvent effects, are related to the relative energies of the different stereoisomers. The relative equilibrium populations of the different isomers have been calculated using semiempirical energies and Boltzmann’s distribution. Although the most stable isomer as determined by AM1 method is in qualitative agreement with that deduced from NMR experiments in all DCNQI derivatives studied, better quantitative correlations are obtained at the B3LYP(L1A1)/6−31G* calculation level. Comparison of both semiempirical and ab initio calculations reveal that in these DCNQI systems AM1 is the method of choice for those larger systems whose size prevents energy calculations at higher levels.