Dipole moment esters

Ester group is polar and possesses a high dipole moment /jl [38]. 

These observations do not, however, mean that TBT carboxylates and TBTCl are ionic in nature. After detailed analysis of the physical evidence such as the low specific conductance and dipole moment of trialkyltin halides, Neumann has concluded that they have no "salt-like constitution" (6). Bonding in the trialkyltin carboxylates also is essentially similar to that in covalent alkyl esters, as evidenced by the low dipole moment of 2.2D for tributyltin acetate in benzene, as compared to 1.9D for alkyl acetates (7). 

One is the steric effect. As in the case of amides, the E conformation is disfavored by the steric effect when R is equal to or larger than a methyl group. The second factor is the dipole moment. The E conformation is much more polar than the Z conformation. Because of these factors, esters usually assume the Z conformation and no E conformation is observed. To observe the latter, a special probe is needed. From the foregoing discussion, it is apparent that if R is hydrogen and R a large group, the E conformation might be present. Thus the E form of tm-butyl formate was detected by H NMR spectroscopy at low temperature (84). The barrier was found to be ca. 11 kcal/mol (85). 

The l,3,4-oxadiazole-2-thione (156, R = Me, R = Ph) has a dipole moment of 9.10 D, which obviously supports its meso-ionic formulation. " The meso-ionic l,3,4-oxadiazole-2-thiones (156) form methiodides (157) which are easily hydrolyzed, yielding the acyclic esters (160a). The meso-ionic l,3,4-oxadiazole-2-thione (156, R = Me, R ... 

In transition state TS2, leading to the minor perepoxide intermediate PE2 in a limiting step, the oxygen is placed syn to the ester group, and the net dipole moment is expected to be larger than that in transition state TSi where the oxygen has an anti orientation with respect to the ester group. TS2 is therefore more polar than TSi, and expected to be stabilized better by polar solvents than TS]. Consequently, the ratio 79a/79b decreases with increase of solvent polarity. 

Figure 5 Calculated (AMI) heats of formation AHf), dipole moments (/r), and frontier orbital coefficient and energies (e) for methyl esters 31a and 8a.

The simplified two-rotational-state scheme previously used for PMA is not able to predict satisfactory values for its dipole moment. A more realistic scheme with four rotational states is introduced which allows for the distinguishing between different interactions for different orientations of the ester group lateral to the chain. Values of dimensions, dipole moments, stereochemical equilibria, and NMR coupling constants calculated using this scheme are in agreement with experimental results. However, this scheme falls to reproduce the experimental variation of dipole moment with temperature. 

Dipole moments of PMA are calculated with the RIS scheme with two states per bond. When the dipole moment of the ester group lateral to the chain is placed in the orientation deduced by semiempirical methods, calculated values of the dipole moment are in excellent agreement with experimental results without any kind of adjustement of the energy parameters obtained by Flory and co-workers (A 0751. 

---