Effect of vinylene and 1,4-phenylene spacers on efficiency of the ground-state intramolecular charge-transfer in enlarged 4-dimethylamino-1-methylpyridinium cations

Abstract

15 N NMR chemical shifts of the exo-and endocyclic nitrogen atoms show how efficient is the ground-state intramolecular charge transfer between these sites in 4-dimethylamino-1-methylpyridinium cation (increased contribution of the quinoid resonance form results in a shielding and deshielding effect of their NMR signals, respectively). As it was anticipated, insertion of vinylene and/or 1,4-phenylene spacers to the cation considerably hinders the ground-state charge transfer. This hypothesis is further supported by an analysis of the C-NMe 2 bond lengths (X-ray data show that spacers elongate this bond). The selected valence angles in the compounds studied are also linearly dependent on d( 15 N endo ) and d( 15 N exo ) values. Although the correlation coefficient for the d( 15 N endo ) versus d( 15 N exo ) dependence is equal to 0.983, decrease of the net charge on one nitrogen atom is not compensated entirely by its increase on another nitrogen atom. This shows that exocyclic nitrogen atom is not the only acceptor of the positive charge in the molecule.The natural population analysis shows that the positive charge is transferred not only to the exocyclic N but also to, e.g., 1-and N-methyl C as well as to C3 and C5 atoms in pyridine ring. Ground-state charge transfer through the p-phenylene moiety was found to be less effective than through the trans-vinylene bridge.

Structure-based evaluation of the resonance interactions and effectiveness of the charge transfer in nitroamines

“…basicities, dipole moments, and spectral parameters, are significantly affected by the intramolecular charge transfer [1]. A series of such compounds has been recently studied by us [2]. Since nitro and amino groups are among the most common acceptors and donors known, nitramines R 2 N-NO 2 are subjected to extremely effective electron transfer.…”

“…p-Nitroanilines also contain these two strongly interacting groups where they are separated by the p-phenylene moiety. There is a question about the effect of conjugated spacers on the charge transfer in compounds of general formula R 2 N-G-NO 2 . This problem has been studied only occasionally.…”