Dipole moments of pyrimidine

A number of authors have compared the validity of various semiempirical methods and optimized the parameters [103-106] It does not seem practical to give references to all calculations on this topic. Selected recent calculations of dipole moments of pyrimidine and purine bases and thdr derivatives and analogs will be mentioned here as practical examples [107-114]. Numerous references on semiempirical calculations of dipole moments of different types of heterocycles including pyrimidines and purines can be found in our publications [115-133]. 

In Debye units. In the case of the purines, the calculated values are for the 9 -tautomers wherever appropriate (as shown in the formulas). The numbers in the brackets are the references. In dioxane unless a different solvent is indicated. "Vector sum of the n- and o-components rt-component PPP method, o-component o-bond (and o-group) moments. References pyrimidines [122], purines [118,119]. For another set of values on CNDO/2 dipole moments of pyrimidines, see [122] (our calculations). SPARTAN, 6-3IG level. Our work, to be published [133]. 0is the angle between the positive direction of the x-axis and the calculated dipole moment (PPP + o) read counterclockwise, for the orientation of the structures as shown in the formulas... 

Electron-density calculations for quinazoline (which has no symmetry) vary markedly with the method used. The diagram (6) has the same bases as that given for pyrimidine above it will be observed that the 2- and 4-positions in quinazoline are comparable with the corresponding positions in pyrimidine and that the aromatic carbon atoms (C-5-C-8) in quinazoline are roughly comparable with C-5 in pyrimidine (67MI21300). The dipole moment of quinazoline does not appear to have been measured, but that of 2-methylquinazo-line is 2.2 D. 

The dipole moment is a fundamental property of a molecule (or any dipole unit) in which two opposite charges are separated by a distance . This entity is commonly measured in debye units (symbolized by D), equal to 3.33564 X 10 coulomb-meters, in SI units). Since the net dipole moment of a molecule is equal to the vectorial sum of the individual bond moments, the dipole moment provides valuable information on the structure and electrical properties of that molecule. The dipole moment can be determined by use of the Debye equation for total polarization. Examples of dipole moments (in the gas phase) are water (1.854 D), ammonia (1.471 D), nitromethane (3.46 D), imidazole (3.8 D), toluene (0.375 D), and pyrimidine (2.334 D). Even symmetrical molecules will have a small, but measurable dipole moment, due to centrifugal distortion effects. Methane " for example, has a value of about 5.4 X 10 D. 

As concerns the dipole moments of these pyrimidines, the CNDO/2 (without d-orbitals) method seems to give somewhat too high values for the sulfur compounds. In this case the simpler methods give more satisfactory results. For instance, the calculated dipole moments of thiouracils by Berthod and Pullman by means of the tt-HMO + cr-Del Re procedure are in very good agreement with the available experimental data (2-thiouracil pcaic- D, /a xp. 4.21 D 4-thio-uracil i oaio- 4.53 D, 4.47 D 2,4-dithiouracil /Xpaic- 4.59 D, pexp-4.67 D). 

New fused tetrazoloazines that have been reported include tetrazolo[5,l-a]isoquinolines,665 tetrazolo[5, l-6]benzothiazoles,666 and tetrazolopyridines.667 Reactions of fused tetrazoloazines, or the azido isomers, that have been reported include hydrolytic cleavage of the pyrimidine ring of tetrazolopyrimidines,66 pyrolysis of 3-azido-pyridazine 2-oxides,66 and photolysis of azido-l,3,5-triazine.670 Dipole moments of 3-azido-1,2,4-triazole derivatives have also been reported.671... 

The number of papers with ab initio calculations of dipole moments of heterocycles has been continually increasing throughout the last few years. Today, ab initio dipole moments are available for most major parent heterocyclic systems and numerous derivatives, especially the nitrogen-containing heterocycles in general and pyrimidine and purine bases in particular. A number of different basis sets and approaches were used. 

Experimental and calculated ground-state dipole moments of selected pyrimidines and purines... 

However, in many other series results have been obtained that are compatible with those from other methods and that gave the dipole-moment method an appearance of general reliability now known to be unjustified. Such compatible results include spiropiperidines (Section III,A,4), tropanes (Section III,B,4), 2-alkyltetrahydro-l,2-oxazines (Section III,C,2), perhydro-pyrido[l,2-c][l,3]oxazines (Section III,D,IX perhydropyrido[l,2-c][l,3]thi-azines (Section III,D,2), dialkylhexahydropyrimidines and perhydropyrido-[l,2-c]pyrimidines (Section III,D,3), 5-alkyldihydro-l,3,5-dithiazines (Section III,G,3), 3,5-dialkyltetrahydro-l,3,5-thiadiazines (Section III,G,4) and, in part, l,2,4,5-tetraalkylhexahydro-l,2,4,5-tetrazines (Section III,H,4) as well as piperidines, tetrahydro-l,3-oxazines, and tetrahydro-l,3-thiazines containing an N-H group. 

Molecular orbital computational analysis by PM3 CI UHF semiempirical methods have been used to support the contention that preferable HSOMO-LUMO interactions produce a favored biradical and explain the site selectivity in the sensitized photochemical [2 + 2] cycloadduct formation of 2-pyrones with electron-deficient ethylenes <92BCJ354>. The lowest ionization energies, dipole moment, and dominant electronic configurations of a 5-methylidenated version of 7-nitroso-oxazolo[4,5- ]cyclopenta[e]pyrimidine of unknown origin were calculated by the ADC(3) ab initio method <92CPHii>. An extensive semiempirical and ab initio investigation into the mechanism of oxidation of methanol by PQQ is cited in Section 7.22.12.4. 

In the crystal structures, homo base pairs with centrosymmetrical configuration are favored. This general statement is true for all the four different bases of pyrimidine and purine type. The individual bases exhibit considerable dipole moments, as shown in Thble 16.1, and we assume that the preference for base pairs with a center of symmetry is due to the antiparallel orientation of the dipole moments in this particular arrangement. This results in a favorable cancellation of the total electric field over the crystal volume. 

The pyrimidine moiety of purines is 7t-electron deficient, whereas the imidazole ring is a Jt-electron excessive system. The direction of the dipole moment is altered by the introduction of substituents, by protoiiation, tautomerization or base pairing. The 7t-excessive character of the imidazole moiety of various purines makes it suitable for anion formation upon treatment with sodium hydride, potassium hydroxide, potassium carbonate or other reagents which are used during electrophilic reactions, such as alkylation or glycosylation. The nucleophilic attack on carbons occurs in the order C8 > C6 > C2. A number of purine syntheses use the displacement of existing substituents. 

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