Heteroatomic dipoles

An alternative method of studying the molecular motions of a polymeric chain is to measure the complex permitivity of the sample, mounted as dielectric of a capacitor and subjected to a sinusoidal voltage, which produces polarization of the sample macromolecules. The storage and loss factor of the complex permitivity are related to the dipolar orientations and the corresponding motional processes. The application of the dielectric thermal analysis (DETA) is obviously limited to macromolecules possessing heteroatomic dipoles but, on the other hand, it allows a range of frequency measurement much wider than DMTA and its theoretical foundations are better established. 

For 2-methyl-5-phenyl-s-triazolo[3,4-61-1,3,4-thiadiazole (350) and 4-methyl-s-triazolo[3,4-6]benzothiazole (182, R = Me), dipole moments were found to be 4.96 and 5.98, respectively.435 For compounds containing only nitrogen as a heteroatom, dipole moment measurements have been used to study annular tautomerism (Section IV,A,1) using Eq. (36).356... 

Some diols of heterocycles have been subjected to pinacol rearrangement. Mundy found that (37) gave only (38) when treated with cold concentrated sulfuric acid (equation 21). It was noted that (38) is not the product expected from initial cation stability arguments based on anticipated heteroatom dipole effects. The basis for this selectivity remains unclear. An attempt to detect alternative products at short reaction times failed to shed light on this question. 

D. The calculations confirm that the negative end of the dipole is towards phosphorus. - The calculated dipole moments of phosphole (1.9 D) and pyrrole (2.0 D) are similar, and, unlike furan, the positive ends of the dipoles are towards the heteroatoms. Dipole moments have been used, in combination with results from other methods of study, to estimate the preferred conformations of the dichloride (179), of the phosphites (180), and of triarylphosphine oxides. The use of dipole moments to aid stereochemical studies of compounds has been reviewed. Additive polarizability parameters should not be used in the calculations, and it has been recommended that data should be obtained from model compounds containing identical environments for the phosphorus atoms. The sensitivity of bond moments to structural changes has been studied perfluoroalkyl groups lower the phosphoryl bond moment, and the P—N bond moment is very sensitive to the valence state of the phosphorus atom. The conformational analyses of phospho-nates, phosphonamides, silyl phosphates, and a number of dioxaphosphori-nans (181) - have been reported. The P—Se bond moment has been estimated to be 1.24 D. The zwitterionic structure (182) was identified by its high dipole... 

Synthesis of Five-membered Rings with Two or More Heteroatoms Table 4 1,3-Dipoles Without Octet Stabilization... 

Use of mesoionic ring systems for the synthesis of five-membered heterocycles with two or more heteroatoms is relatively restricted because of the few readily accessible systems containing two heteroatoms in the 1,3-dipole. They are particularly suited for the unambiguous synthesis of pyrazoles as the azomethine imine is contained as a masked 1,3-dipole in the sydnone system. An attractive feature of their use is that the precursor to the mesoionic system may be used in the presence of the cyclodehydration agent and the dipolarophile, avoiding the necessity for isolating the mesoionic system. 

Heats of formation, molecular geometries, ionization potentials and dipole moments are calculated by the MNDO method for a large number of molecules. The MNDO results are compared with the corresponding MINDO/3 results on a statistical basis. For the properties investigated, the mean absolute errors in MNDO are uniformly smaller than those in MINDO/3 by a factor of about 2. Major improvements of MNDO over MINDO/3 are found for the heats of formation of unsaturated systems and molecules with NN bonds, for bond angles, for higher ionization potentials, and for dipole moments of compounds with heteroatoms. 

Alkylvinylacetylenes react with 1,3-dipoles exclusively across the terminal unsubstituted bond, whether it is a double or triple bond (80UK1801). 1-Heteroalk-1-en-3-ynes behave quite differently in these reactions. Orientation is largely determined by the nature of heteroatom. 

Ionophores constitute a large collection of structurally diverse substances that share the ability to complex cations and to assist in the translocation of cations through a lipophilic interface.1 Using numerous Lewis-basic heteroatoms, an ionophore organizes itself around a cationic species such as an inorganic metal ion. This arrangement maximizes favorable ion-dipole interactions, while simultaneously exposing a relatively hydrophobic (lipophilic) exterior. 

Diazaphosphorinanes exist as a mixture of three conformers in solution [Eq. (51)], although all heteroatoms in the ring have substituents. Conformational equilibrium is due to the low inversion barrier of nitrogen studied by H NMR and the dipole moment method (83MI1 84MI1) [Eq. (51)]. The results are presented in Table III. 

The dipole moment of phosphabenzene is reinforced by the methyl group shown in (147) and increases it from 1.46 to 1.77 D thus it resembles pyridine, which has the heteroatom at the negative end of the dipole.181 The magnitudes and trends of the dipole moments of the methylphosphines have been investigated by MO studies and the dipoles partitioned into bond moments, bond polarization, and lone-pair moments.182 The reciprocal effects of the double bond and the phosphorus atom in... 

There is no simple explanation for the much more pronounced instability to pressure of CO compared to N2. Since the only structural difference arises from the heteroatomic character of CO, one could expect that the molecular dipole moment increases with pressure leading to a higher compressibility of CO. But no evidence for this is obtained from either the ab initio calculation or experimentally. In fact the equation of state of nitrogen and carbon monoxide are practically coincident in the pressure range of interest. One other point of interest is the head-to-tail disorder present in carbon monoxide because it has been observed in several high pressure experiments that defects and disorder can play an important role. 

It is assumed that most of the electron spin density resides on the metal, but that a certain small part of it, given by the quantity p , is delocalized to the ligand heteroatom L. The first term is the point-dipole interaction term, the second corresponds to the dipolar interaction between the nuclear spin under consideration and the spin-density on the atom L and the last term describes the cross-correlation of the two dipolar interactions (we discuss the issue of cross-correlation phenomena in more general terms in Section II. D and III.B). The quantity is the effective distance from the nuclear spin... 

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