Observed and Calculated Dipole Moments

The dipole moment of a molecule is a defined quantum mechanical property and may be obtained exactly, for a given wavefunction, by integration using the appropriate operator r. Comparison of calculated with observed dipole moments gives infoimation about the quality of a wavefunction. Since nearly every reported wavefunction includes the molecular dipole moment (if applicable), there is available an extensive literature of calculated dipole moments. These dipole moments may be calculated either at the observed molecular geometry or at wavefunction-optimized geometry. 

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The structure of the dimers from mero-substituted derivatives was initially determined by comparison of the observed and calculated dipole moments. For a head-to-tail dimer the dipole moments resulting from the 9 and 9 substituents should cancel each other and the resultant dipole moment should be essentially zero. For a head-to-head arrangement the dipoles would be in the same direction and the resultant should be considerably greater than zero. The dimers produced upon irradiation of 9-chloro and 9-bromoanthracene solutions were observed to be 0.36 and 0.60 D, respectively. Since these values are much less than expected for a head-to-head arrangement for these derivatives (3.8 D), it was concluded that both of these dimers were formed in a head-to-tail configuration/30 ... 

Observed and Calculated Dipole Moments for Some Typical Nonselect Solvents... 

The dipole moment of tributylpliosphine varies from 1.49 to 2.4 D according to the solvent used. Inductive effects in phosphines have been estimated by comparing the calculated and observed dipole moments, and the apparent dipole moment due to the lone electron pair on phosphorus has been estimated. A method of calculating the hybridization of the phosphorus atom in terms of bond angles is suggested which leads to a linear relationship between hybridization ratio and lone electron pair moment. The difference between experimental and calculated dipole moments for para-substitued arylphosphines, phosphine sulphides, and phosphinimines has been used to estimate mesomeric transfer of electrons to phosphorus. 

The cis addition observed with 6,6-diphenylfulvene and indene to give 6 and 8, respectively, may involve a concerted molecular addition115. The cis stereochemistry was inferred by comparison of measured and calculated dipole moments of the dinitrofulvene 6 and on the basis of a small difference in the chemical shifts of methylene hydrogens of dinitroindene 8, hence, it is possibly incorrect119. 

In case of 1,1-dimethylallene (2) (-0.549 vs. —0.613) and 1,3-difluo-roallene (10) (1.77 vs. 1.79) a good agreement between experimental and calculated dipole moments is observed. A larger deviation is found for 1,1 -difluo-roallene (9) (2.07 vs. 3.02). However, such discrepancies are always observed if two strongly polar groups are in geminal positions in the molecule (45). 

Observed integrated gas-phase infrared intensities of H2O and calculated dipole moment derivatives with respect to symmetry coordinates... 

The first empirical and qualitative approach to the electronic structure of thiazole appeared in 1931 in a paper entitled Aspects of the chemistry of the thiazole group (115). In this historical review. Hunter showed the technical importance of the group, especially of the benzothiazole derivatives, and correlated the observed reactivity with the mobility of the electronic system. In 1943, Jensen et al. (116) explained the low value observed for the dipole moment of thiazole (1.64D in benzene) by the small contribution of the polar-limiting structures and thus by an essentially dienic character of the v system of thiazole. The first theoretical calculation of the electronic structure of thiazole. benzothiazole, and their methyl derivatives was performed by Pullman and Metzger using the Huckel method (5, 6, 8). 

View molecular models of dimethyl ether and ethylene oxide on Learning By Modeling Which one has the greater dipole moment Do the calculated dipole moments bear any relation ship to the observed boiling points (ethylene oxide +10°C dimethyl ether —25°C) d... 

Repeat your analysis for tautomeric equilibria between 4-hydroxypyridine and 4-pyridone, 2-hydroxypyrimidine and 2-pyrimidone and 4-hydroxypyrimidine and 4-pyrimidone. For each, identify the favored (lower-energy) tautomer, and then use equation (1) to calculate the ratio of tautomers present at equilibrium. Point out any major differences among the four systems and rationalize what you observe. (Hint Compare dipole moments and electrostatic potential maps of the two pyridones and the two pyrimidones. How are these related to molecular stability )... 

In accord with experimental observations, SCFMO calculations indicate that an /V-acyl function should deactivate 5f/-dibenz[/>,/]azepine towards electrophilic substitution at the carbocyclic rings, and that substitution should occur preferentially at the CIO position.32 The calculated dipole moment (0.98 D) for 5f/-dibenz[A,/]azepine compares favorably with the measured value (0.96 D), and is expected for a nonplanar ring system.58... 

The problem of the structure of 1,2- and 1,4-quinone diazides was investigated by Le Fevre s group (1949, 1954) by measuring dipole moments. The observed moments in benzene are in the range 2.9 to 5.0 D, compared with calculated values of 1.6 to 4.0 D for the quinone diazide structure and 15.7 and 27.4 D respectively for the 1,2-and 1,4-zwitterionic forms. No attempts were made by Lowe-Ma et al. (1988) to calculate dipole moments for the mesomeric structure 4.4 that they proposed. 

The calculations so far have been concerned with the permanent charge distribution which the molecule would have when isolated, and which would be observed in its dipole moment. The factor that actually determines the orientation of the entering group, however, is the charge distribution at the instant of attack, which differs from the permanent charge distribution in consequence of polarization by the entering group. In most cases it is not necessary to take this latter effect into account explicitly, since it represents only a small correction. However, in certain cases it is apparently of decisive importance.7... 

The deformed-chair conformation of ring A in a 4,4-dimethyl-3-oxo-5or-steroid has been confirmed by an X-ray study of a 17 -benzoyloxyandrostane derivative. The results agree with those of an earlier study of the 17-iodoacetate, and with the geometry indicated by force-field calculations. Dipole moments calculated by the application of molecular mechanics to 5a-androstane-3,17-dione and its distorted 4,4-dimethyl derivative are larger than those observed the reasons for these deviations are not yet clear. 

The dipole moments of oxepin and benzene oxide have been calculated to be in the range 0.76-1.36 D and >1.5 D respectively using the ab initio SCF and MINDO/3 methods (80JA1255). The lower calculated dipole moment would be in accord with experimental observations where the equilibrium was found to favor oxepin (7) in less polar solvents. Coordination between the oxirane oxygen atom and polar solvent molecules would also strengthen the C—C bond of the epoxide and thus lead to a preference for the benzene oxide isomer <72AG(E)825). Thus the proportion of oxepin (7) was found by UV spectral analysis to be higher in isooctane solvent (70%) than in water-methanol (10%). 

De Kreuk [2] also calculated dipole moments using his observed values of e and n . His figures are given in Table 4. 

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