Dipole moment of uracil

The first electrostatic moment from X-rays was obtained by Stewart (1970), who calculated the dipole moment of uracil from the least-squares valence-shell populations of each of the constituent atoms of the molecule. Stewart s value of... 

D higher than the dipole moment of uracil or thymine. All the quantum-mechanical calculations predicted a greater moment for cytosine (and guanine) than for uracil or thymine (and adenine). The agreement between the experimental dipole moment of cytosine and the values calculated by different methods is satisfactory (cf. Table XI)... 

It is interesting to look at the effect of substituents on the dipole moment of uracil (or thymine). An examination of the data collected in Table XX shows that among the four positions (N-l, N-3, C-5, C-6) available for substitution, the one at C-6 is the most sensitive. An alkyl substituent at C-6 of uracil (or thymine) increases the dipole moment by about 0.4 0.5 D, while the same substituent at N-l, N-3, or C-5 has only a small effect. Similarly, a halogen does not greatly alter the dipole moment of the molecule when at C-5 whereas 6-halogen decreases the value of the dipole moment considerably (cf. the data for 1,3- dimethyl-uracil and for its 5- and 6-chloro derivatives, Table XX). An attempt has been made by Kufakowska and Wierzchowski402 to interpret these... 

Dipole Moments of Uracils and Thymines0 Calculated by Different Theoretical Methods... 

As to the direction of the dipole moments in the ground state of the molecules, two kinds of experimental measurements are available. Stewart has calculated the X-ray dipole moment of uracil within the point-charge approximation using the atomic charges for a standard STO L-shell. The calculated dipole moment is 4.0 1.3 D and its direction of 71° + 12° from N-l-C-4 toward N-3 atom. These values obtained within a point-charge model have an estimated standard error of about 30%. Nevertheless, the X-ray dipole moment is of reasonable magnitude and in close agreement with the value of 4.1 D obtained by solution measurement of uracil in dioxan (Table XX). 

The evaluation of the dipole moment of uridine as a function of the rotation about the glycosidic bond has been carried out by the PCILO method by Weiler-Feilchenfeld et (for the C -endo gg and C -endo gg conformations). The same method has heen used by Berthod and Pullman to calculate the dipole moments of anti and syn conformers of uridine, Cg-endo gg (7.0 and 3.4 D, respectively) and those of deoxyuridine, Ca-endo gg (4.8 and 4.8 D, respectively). For the n--contributions for the dipole moments of uracils see refs. 174,177,407. 

For other CNDO/2 caloulations on dipole moment of uracils see refs. 150, 153, 215, 369. 

Pi-pi complex between a model for a polar component of a herbicide (uracil) and a model for a salt binding site (guanidinium bicarbonate) is shown in Figure 2. The geometry was computed from initio calculations (36). It should be noted that in the lowest energy geometry shown in Figure 2, the dipole moment of uracil is antiparallel to the dipole moment of the salt. 

Jasien and Fitzgerald62 demonstrated that the LDA dipole moments of such molecules as HF, H20, NH3, formamide, imidazole, pyridine, cytosine, match very closely the experimental ones (the relative errors between 1 and 7%). For uracil and thymine, and adenine, the differences between LDA and experimental dipole moments are slightly larger (relative errors up to 12%) and compared better to the ones derived from second-order Mpller-Plesset calculations. The authors underlined the noticeable effect the inclusion of the hydrogen 2p polarization... 

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). 

From a monopole population analysis of uracil, a dipole moment was determined.29 The result is shown in Figure 4. Note the error in the magnitude is 32 percent the estimated standard deviation in the angle is 12°. A similar monopole analysis for cyclotriborazane leads to a dipole moment of 3.0 D, which compares favorably with a different experimental value.30 The direction of the moment is along the threefold axis from the plane of the B atoms toward the plane of N atoms. The largest source of error in these dipole moment results is the error in the proton positions of the hydrogen atoms. 

The second experimental measure of the orientation of the dipole moment in the ground state of the bases was made by Seibold and Labhart405 for uracil and thymine. The direction of the ground state dipole moment with respect to the transition moment in the lowest excited singlet state has been determined from the influence of an... 

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