The Dipole Moments of Some Purine and Pyrimidine Derivatives

Bases, Nucleosides, and Nucleotides

“…known, then the direction of /x of the parent molecule can be ascertained by vector addition. Weiler-Feilchenfeld and Bergmann [54] showed concern about the substantial difference of/x values observed between 1,3-dimethyluracil (3.9 D) and 5,6 dimethyluracil (4.7 D) and the large /x value of 5bromouracil (4.6 D) observed. This study, however, suggests that the H> (Caromatic-Cl) is 1.3 D for pyrimidine instead of 1.6 D for /x (C aromatic -Cl) for chlorobenzene.…”

“…These values were determined by the mass spectroscopy method in comparison with the known standard of the rare gases [51,53]. From these values Bergmann et al [51,54] have made the following conclusions: (a) the range of the ionization potentials of all these compounds is sufficiently small (9-10 eV), and yet the structural variation within this group is sufficiently large that one can conclude that this representative ionization potential is due to the loss of one of the 77-electrons in the first ionization step; (b) the sequence of order of the ionization potentials of these compounds is the same as that of the energies of the highest occupied molec ular orbital (HOMO), and as that of the calculated values of / based on MO procedure, though the calculated values are different from the observed values by 0.5-0.9 eV (some values are too high and others too low; see Pullman and Pullman [43,44]). Purines and pyrimidines may form chargetransfer complexes with various organic compounds and these complexes are formed by a partial transfer of an electron from the highest occupied MO of the "donor" to the lowest empty MO of the "acceptor."…”

“…In Table IV, the experimental values of dipole moment of various bases, derivatives, and analogs are cited [45,[54][55][56][57]. The observed values of /x for adenine (3.0 ± 0.2 D), purine (4.3 + 0.2 D), and 1,3-dimethyluracil (3.9 ± 0.1 D) are in reasonable agreement with those values obtained from MO calculations based on vector addition of appropriate molecule and bond moment values [1,[43][44][45].…”

“…From Pullman et al [57]. This method was employed successfully by Weiler-Feilchen feld and Bergmann [54] in the assignment of the direction of fx for pyrimidine (from C 5 -> C 2 ) by comparing the /x values of pyrimidine (2.3 D), 2-chloropyrimidine (3.6 D), 4,6-dichloropyrimidine (1.1 D), and 2,4,6-trichloropyrimidine (2.3 D same as pyrimidine). This method was employed successfully by Weiler-Feilchen feld and Bergmann [54] in the assignment of the direction of fx for pyrimidine (from C 5 -> C 2 ) by comparing the /x values of pyrimidine (2.3 D), 2-chloropyrimidine (3.6 D), 4,6-dichloropyrimidine (1.1 D), and 2,4,6-trichloropyrimidine (2.3 D same as pyrimidine).…”

Molecular Complexes, 14. Isomeric Arene Complexes of Caffeine and 1,3,7,9-Tetramethyluric Acid Detected with Toluene. Polar Interactions Inverted with Hexafluorobenzene

“…4: B) from 4 and T 2 . The other complex type throughout Table 2 is T-shaped (dipole moment of 4 [23]: 3.21 debye) either standing with 3-Me and 9-Me on the same D (see below) or, with a small D (B, T), consisting of two complexes since either 3-Me or 9-Me will stand on D. Both 4T of this edge-on type must be stabilized by dipole-dipole interactions ( Fig. 4: C, D) else edge-on 4T 2 should also arise.…”