Dipole moments mesomeric

The mesomeric dipole moment, /Lim, which is the difference between the measured values for the heteroaromatic and its fully saturated analogue (c/. Table 1), has been proposed as... 

The type of conjugation is also reflected in the frontier orbital profile, the charge distribution, and the permanent dipole moments. The results of semiempirical calculations on l-methylpyridinium-3-olate (16), Malloapeltine (17), Trigollenine (18), and Homarine (19) are presented in Scheme 7. Characteristically for the class of conjugated mesomeric betaines, the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) are distributed over the entire molecule as examplifled for l-methylpyridinium-3-olate. It was shown that 90% of the... 

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. 

Dipole Moments. The dipole moments of quinoxaline, several derivatives, some pyrazine analogs, and comparable homocyclic compounds have been measured in an attempt to rationalize the values in terms of mesomeric effects and/or conformational isomerism. ... 

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. 

Based on the fundamental dipole moment concepts of mesomeric moment and interaction moment, models to explain the enhanced optical nonlinearities of polarized conjugated molecules have been devised. The equivalent internal field (EIF) model of Oudar and Chemla relates the j8 of a molecule to an equivalent electric field ER due to substituent R which biases the hyperpolarizabilities (28). In the case of donor-acceptor systems anomalously large nonlinearities result as a consequence of contributions from intramolecular charge-transfer interaction (related to /xjnt) and expressions to quantify this contribution have been obtained (29). Related treatments dealing with this problem have appeared one due to Levine and Bethea bearing directly on the EIF model (30), another due to Levine using spectroscopically derived substituent perturbations rather than dipole moment based data (31.) and yet another more empirical treatment by Dulcic and Sauteret involving reinforcement of substituent effects (32). 

Some substituents induce remarkably different electronic behaviors on the same aromatic system (8). Let us consider, for example, the actions of substituents on an aromatic electron system. Some substituents have a tendency to enrich their electronic population (acceptors), while others will give away some of it (donors). Traditionaly, quantum chemists used to distinguish between long range (mesomeric) effects, mainly u in nature, and short range (inductive) effects, mainly a. The nonlinear behavior of a monosubstituted molecule can be accounted for in terms of the x electron dipole moment. Examples of donor and acceptor substituents can be seen on figure 1. 

He stressed the significance of measurements of molecular dipole moment, the principles of which had been established in 1912 by Debye.52 Ingold predicted that mesomeric systems would display measurable dipole effects. 53 He later said that he realized at this time that the theory of dipolar charge and intermediate form was... 

If pyran-4-one is a 677-electron aromatic system, it must adopt the betaine structure (212 X = 0), which would exhibit a large dipole moment. The observed moment for (18) is 3.7 D. A dipole moment of 1.9 D is obtained by vector summation of bond and group moments. Clearly, if mesomeric effects were absent, the difference between the calculated and observed moments would be very small. The magnitude of the observed dipole moment suggests there is considerable it- electron delocalization in pyran-4-one. However, the calculated value for the betaine structure (212 X = 0) is 21 D (73MI22203) and thus it appears that the extent of delocalization is, after all, quite small. 

Complexes of platinum(II) with /J-monothiodiketonates, derived by deprotonation of the parent acid (207), can be prepared from PtCl -. The dark red complex Pt(C3H(Ph)2SO 2 (R = R = Ph) shows IR bands at 1535 cm-1 [v(C=C)], 1410 cm-1 [v(O O)] and 1270 cm-1 [v(O S)].1824 Electronic spectra and dipole moment data for these complexes have been compared with the O.O -diketonate complexes.182s 1826 The structure of the phenyl derivative has been confirmed by X-ray crystallography.1827 Detailed dipole moment measurements using static polarization have been made with fluorinated jS-monothiodiketone complexes. Variations with substituent depend on the magnitude and vector directions of the Ph—X bond moments (aryl substituents), the inductive effect of the meta and para substituent on the phenyl ring, and the mesomeric effect of the substituent X.1828 A useful separation method for bis(monothiotrifluoroacetylacetonates) of platinum(II) is gas chromatography.1829... 

The differences in the value of the dipole moments of the complexes [M(RCS=CHCOCF3) ] (R = Ph or X-substituted phenyl) depend upon (a) the magnitude and vector direction of the Ph—X bond moments (b) the inductive effect arising from the difference in electron density of the C-l and C-5 atoms of the monothio-jS-diketone (this is affected by the nature and position of the substituent X on the phenyl ring) (c) the change in moment brought about by the mesomeric effect (56) of X.230-231... 

Dipole moments of nitrovinylamines 11 were larger than the sum of bond moments and the simple algebraic difference was taken as a measure of through-conjugation78. Conformation was assumed as planar. In this way, the exact direction of the mesomeric moment iam remained unknown evidently the structure of the compound would not allow one... 

The Cl2C=CH group moment was not further resolved and no mesomeric moment was calculated. Ethenethiol has sp conformation in the gas phase as determined by MW and IR spectroscopies at this opportunity the dipole moment was also measured87. 

FIGURE 1. Determination of the configuration (E or Z) and of die electron distribution (the mesomeric dipole moment /xm) of file amidine 23 on the basis of the experimental dipole moment [Pg.271]

It remains to mention derivatives with just one substituent on the NH2 group. Concerning the configuration, they should take an intermediate position in addition, one has to deal with the conformation on C—N. These compounds have been less studied. Compared to structurally very similar amidines, a remarkable difference was found101. While AW -dimethyl-d-nitrobenzamidine in the crystal is in the form E,sp (41), A -methyl-4-nitrobenzamidoxime is Z.ap (42). Dipole moments in solution are compatible with a conformational equilibrium, but analysis is made imprecise by the mesomeric moments which are not known exactly102. In a previous analysis129 of /z of derivatives similar to 42 (H and Cl in place of NO2) the form Z.ap was also preferred, although the presence of a minor conformer could not be completely excluded. 

The example of amides offers us an opportunity to present the problem of mesomeric dipole moments more generally. Let us suppose that the real structure of a molecule is represented as mesomeric between the two limiting structures the dipole moments of these structures can be anticipated as vectors A and B. The effective experimental dipole moment of the compound is then given by the vector equation 5, where p is a measure of relative importance of the two structures. Equation 5 is formally identical with equation 3 and is always valid for mesomeric structures, since these are assumed to equilibrate by... 

FIGURE 2. Analysis of the experimental dipole moment (E) of A. A -di methyl lomiamide and/or N,N-dimethylbenzamide in terms of the resonance structures 78a and 78b (A and B) and of the mesomeric dipole moment (pm) according to various approximations... 

There is some confusion concerning the conformation of substituted ureas196,197, when one difficulty is again in the small solubility. For a dipole moment study combined with IR spectroscopy, derivatives of 82 were selected196 which have only one axis of rotation. The mesomeric moment is of less importance and could be estimated in advance. Then conformation was solved in favor of the sp form 82. In hydroxyureas 83 there is one rotational axis more and the conformation found198 should be considered as approximate. 

---