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 gr oup at the C=N double bond in a series of substituted DCNQIs has been inves tigated in solution by two-dimensional exchange spectroscopy (2D EXSY). The isomers formed were characterised by H-1 NMR at 223 K to slow down the inv ersion process of the cyanoimine group as well as by H,H COSY spectra, Wher eas 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 kca l/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 thes e free energy barriers are in excellent agreement with the experimental val ues, especially when solvent effects are taken into account. Theoretical ca lculations at B3LYP/6-31G* level predict planar geometries for the DCNQI de rivatives studied (6 and 8), in particular when solvent effects are conside red. This is in perfect agreement with the experimental data. Two-electron stabilizing interactions, as well as solvent effects, are related to the re lative energies of the different stereoisomers. The relative equilibrium po pulations 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 corr elations are obtained at the B3LYP(L1A1)/6-31G* calculation level. Comparis on of both semiempirical and ab initio calculations reveal that in these DC NQI systems AM1 is the method of choice for those larger systems whose size prevents energy calculations at higher levels.