A. Schmidt et al., Charge-separated modified nucleobases. On pi-interactions and hydrogen bonding of self-complementary cationic and betainic uracils, J ORG CHEM, 64(26), 1999, pp. 9499-9506
Depending on the conditions, reaction of 6-chloropyrimidine-2,4(1H,3H)-dion
e 1 with heteroaromatics such as 4-(dimethylamino)pyridine, 4-(pyrrolidin-1
-yl)pyridine, pyridine, and 1-methylimidazole, respectively, results in the
formation of uracil-6-ylhetarenium salts or cross-conjugated uracilylbetai
nes, the intramolecular interactions of which are examined. In addition, 1
forms uncharged 1:1 pi-stacks with DMAP and PPY, the electronic properties
of which are inverted in relation to natural nucleobase-heteroaromatic syst
ems. The geometry of the ct-complexes is governed by steric effects as well
as by the frontier orbitals of the uracil and the heteroaromatic, whereas
the dipole moments are almost completey uncoupled. 1H NMR experiments in DM
SO-dB at room temperature as well as ESI- and MALDIMS prove homo-intramolec
ular hydrogen bonding of the uracilylhetarenium salts and the uracilylbetai
nes, respectively. An X-ray single-crystal analysis shows the uracilylbetai
ne 7 associated with a second molecule via two strong hydrogen bonds, thus
forming a centrosymmetric dimer. In contrast to the Watson-Crick pairing mo
de of natural pyrimidine nucleobases, O(2) and N(3)-H of the uracilylbetain
es are involved in hydrogen bonding. This geometry of the dimer 7=7 gives r
ise to a coupling of the semiempirically calculated dipole moments of the m
onomeric cross-conjugated betaines.