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Springer Verlag, Structural Chemistry, 4(20), p. 655-662

DOI: 10.1007/s11224-009-9457-5

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

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Abstract

15N 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-NMe2 bond lengths (X-ray data show that spacers elongate this bond). The selected valence angles in the compounds studied are also linearly dependent on δ(15Nendo) and δ(15Nexo) values. Although the correlation coefficient for the δ(15Nendo) versus δ(15Nexo) 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. © Springer Science+Business Media, LLC 2009.