Nitrones dipole moments

Relative contribution of each of these structures differs significantly and is determined by internal structural characteristics of the nitrones and by the influence of external factors, such as changes in polarity of solvent, formation of a hydrogen bond, and complexation and protonation. Changes in the electronic stmcture of nitrones, effected by any of these factors, which are manifested in the changes of physicochemical properties and spectral characteristics, can be explained, qualitatively, by analyzing the relative contribution of A-G structures. On the basis of a vector analysis of dipole moments of two series of nitrones (355), a quantum-chemical computation of ab initio molecular orbitals of the model nitrone CH2=N(H)0 and its tautomers, and methyl derivatives (356), it has been established that the bond in nitrones between C and N atoms is almost... 

French scientists (270) suggested that the configurations of stereoisomeric acyclic alkyl nitronates can be determined from the relative dipole moments which for trans- isomers of nitronates containing EWG at the a-C atom are substantially larger than those of the cis isomers (Chart 3.5). 

The energetically favorable conformations of model alkyl nitronate (88) and related compounds (89 to 91) (283) were determined by the B3LYP/6-31G(d) method (see Chart 3.9). It appeared that the marked conformations with the minimized dipole moments are energetically most favorable for compounds (88-91) (the use of the dipole-dipole interaction model in conformational analysis was also described in the publication (284)). Calculations including n-n interactions (see Fig. 3.1) did not change this situation because the contribution of this effect is equal in magnitude for both limiting conformations shown in Chart 3.9. 

The H NMR spectra of monosubstituted, acyclic nitronates show two resonances in the range of 5.5 and 7.2 ppm (Table 2.17) (39,40). The peaks are nonequilibrating and are assigned to the two possible nitronate diastereomers. This result is in contrast to the silyl nitronates, in which the two diastereomers were observed to be in rapid equilibrium. Based on the measurement of their respective dipole moments (39), and comparison to known nitrones and oxime ethers (72,73), the downfield signal is assigned to the trans isomer. 

Nitrone derivatives of a unique structure were prepared recently130. The two stereoisomers 43a and 43b are in equilibrium in solution configuration was determined by means of NMR spectroscopy. Dipole moments were calculated by MNDO for both stereoisomers, but were not measured. In our opinion one suitable approach has thus been omitted. 

Intensely yellow leaflets from alcohol, solvated needles from chloroform, dec about 189. Practically inso] in water sol in alcohol, benzene, acetone, chloroform, ethyl acetate, dil acids slightly so] in ether. The ale soln undergoes partial decompn, indicated by a ted color, hence shonld be protected from light. Dipole moment in benzene 7,2. Solubility of some Nitron salts in 0,0lJV HCI Nitrate 0.0099%, perchlorate 0.008%, iodide 0.0] 7%, thiocyanate 0.04%, chromate 0.06%, chlorate 0.12%, nitrite 0.19%, bromide 0.61%. 

When molecules with a double bond are quite polar such as nitrone, the addition reaction rates of non-polar arythiyl radical are also reduced in polar solvents. The large dipole moment of nitrone in the reactant is reduced on the addition of ArS producing a less polar nitroxide radical, as shown Scheme 19 [55],... 

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