Calculation of nuclear quadrupole parameters in imidazole derivatives and extrapolation to coenzyme B-12. A theoretical study

The N-14 nuclear quadrupole coupling (NQC) constants (chi) and asymmetry pa rameters (eta) for a series of small nitrogen-containing imidazole derivati ves are investigated, by using density functional theory (DFT), in three cl early distinct environments: as free molecules, in the solid state, and in solution. The spectroscopic characterization is also extended to coenzyme B -12 and cob(II)alamin systems. The main findings can be summarized as follo ws: (i) Deviations in the calculated chi for the two nitrogen sites in the free imidazole molecule are small enough to allow quantitatively accurate p redictions for isolated molecules of substituted benzimidazole compounds. ( ii) Asymmetry parameters, however, are difficult to reproduce with accuracy ; only trends along a Series of compounds can be taken as informative. (iii ) Shifts of the NQC parameters on going from gas phase to solid state are r easonably well reproduced at a qualitative level by using a trimer model; t o reach similar levels of accuracy in the case of eta, both the continuum a nd the point-charge effects need to be included. (iv) Short-range effects p lay an important role in the N-14 NQC parameters of imidazole in solution, although the NQC constants are also sensitive to solvent molecules beyond t he first shell. (v) Long-range effects taken into account by an averaged co ntinuum model alone are not enough to reproduce the effect of hydrogen bond ing with-water on the N-14 NQC parameters of imidazole; whenever water mole cules are present, these solvent molecules have to be included explicitly. (vi) Metal-ligand interactions are relevant only for the NQC parameters of the proximal N, more specifically for the eta value of this atom; most of t he environmental effects in the real coenzymes are handled correctly by emp loying a solvated nonmetallic model. (viii) We estimate that the chi and et a for the proximal nitrogen atom in cob(II)alamin (which so far has escaped experimental observation) will be approximately 2.7 MHz and 0.8, respectiv ely, as indicated by our calculations.