Pyridine dipole

Interesting structures can be formed by combinations of ring and side-chain substituents in special relative orientations. As indicated above, structures (28) contain the elements of azomethine or carbonyl ylides, which are 1,3-dipoles. Charge-separated species formed by attachment of an anionic group to an azonia-nitrogen also are 1,3-dipoles pyridine 1-oxide (32) is perhaps the simplest example of these the ylide (33) is another. More complex combinations lead to 1,4-dipoles , for instance the pyrimidine derivative (34), and the cross-conjugated ylide (35). Compounds of this type have been reviewed by Ramsden (80AHCl26)l). 

Wall M C, Stewart B A and Mullin A S 1998 State resolved oollisional relaxation of highly vibrationally exoited pyridine ( ii = 38,000 om ) and CO2 influenoe of a permanent dipole moment J. Chem. Rhys. 108 6185-96... 

A solubihty parameter of 24.5-24.7 MPa / [12.0-12.1 (cal/cm ) ] has been calculated for PVF using room temperature swelling data (69). The polymer lost solvent to evaporation more rapidly than free solvent alone when exposed to air. This was ascribed to reestabUshment of favorable dipole—dipole interactions within the polymer. Infrared spectral shifts for poly(methyl methacrylate) in PVF have been interpreted as evidence of favorable acid—base interactions involving the H from CHF units (70). This is consistent with the greater absorption of pyridine than methyl acetate despite a closer solubihty parameter match with methyl acetate. 

Pyridine, 2-ben2oyI-dipole moment, 2, 163 <73JCS(P2)1461> Pyridine, 3-benzoyl-dipole moment, 2, 162 <73JCS(P2)1461) Pyridine, 4-benzoyI-dipole moment, 2, 162 <73JCS(P2)I461) Pyridine, 2-benzyl-... 

The positions of substitution, orientation, and configuration of the stable form are determined by a balance between opposing steric and dipole ef-fects. There is less agreement regarding the factors influencing kinetically controlled reaction (see below). Essentially neutral conditions, such as provided by an acetate or pyridine buffer, are required to avoid isomerization. Frequently, however, bromination will not proceed under these conditions, and a compromise has been used in which a small amount of acid is added to start and maintain reaction, while the accumulation of hydrogen bromide is prevented by adding exactly one equivalent of acetate... 

Table 16.1 Modulus of the electric dipole moment vector of pyridine...

Basis set dependence is important. The results in Table 16.1 were obtained for HF-LCAO calculations on pyridine. In each case, the geometry was optimized As a general rule, ab initio HF-LCAO calculations with small basis sets tend to underestimate the dipole moment, whilst extended basis sets overestimate it A treatment of electron correlation usually brings better agreement with experiment. 

Dipole moment data have provided valuable information for the study of the tautomerism of compounds such as isonicotinic acid, pyrid-4-one, and ethyl acetoacetate, However, this method must be used with discretion since it can lead to inconclusive results. Thus, the fact that 4-aminopyridine has a higher dipole moment than the algebraic sum of the dipole moments of pyridine and aniline was originally interpreted as proof that structure 54 exists with a strong contribution from 36, and it was stated that 55 w ould have a very low moment. Later, Angyal and AngyaF pointed out that the... 

Finally, two sets of physical properties have been correlated by the Hammett equation. Sharpe and Walker have shown that changes in dipole moment are approximately linearly correlated with ct-values, and Snyder has recently correlated the free energies of adsorption of a series of substituted pyridines with u-values. All the reaction constants for the series discussed are summarized in Table V. 

The triplet nitrenium ion, however, lacks symmetry but, unlike the excited triplet states of other azaaromatic systems such as pyridine, is surprisingly planar.283 Theoretical dipole moments for cyclopent[c]- and cyclopent[rf]azepine, and the molecular geometry of cyclopent[rf]-azepine, have been calculated using semiempirical ab initio methods.3... 

The steric and dipole-dipole effects of the CF3 group on valence isomerization in the Dewar pyridine-azaprismane-pyridine system have been studied. These reveal themselves in the high stability, compared to the pyridine, of the valence isomer arising out of the large activation energy for rearomatization. The transformation of a 1-Dewar to 2-Dewar pyridine was observed (89T3115). 

Sharp and Walker (36) have reported good linear plots of Mx - Mh for 3- and 4-substituted pyridines, pyridine-N-oxides and nitrobenzenes against the appropriate substituent constants. Charton (37) has reported correlations of dipole moments for substituted ethylenes and related compounds with eq. (1) using the oj, and Op constants. Best results were generally obtained with Op. 

Pyridine (62), like benzene, has six n electrons (one being supplied by nitrogen) in delocalised n orbitals but, unlike benzene, the orbitals will be deformed by being attracted towards the nitrogen atom because of the latter s being more electronegative than carbon. This is reflected in the dipole of pyridine, which has the negative end on N and the positive end on the nucleus ... 

Pyrrole (68) also has 6n electrons in delocalised n orbitals, but here the nitrogen atom has to contribute two electrons to make up the six (thus becoming essentially non-basic in the process, cf. p. 73), and the dipole of pyrrole is found to be in the opposite direction to that of pyridine, i.e. with the positive end on nitrogen and the negative end on the nucleus ... 

Azomethine ylides such as 412 react with triafulvenes again by analogy with cyclopropenones. (3 + 2) Cycloaddition of the 1,3-dipole to the CVC2 bond and subsequent loss of C02 produces l,4-dihydro-4-methylene-N-alkyl pyridines 559, which as merocyanines show marked solvatochromic and thermochromic effects260. ... 

A cycloaddition process involving dipole 22, readily prepared from thiazolidine 21, was reported to produce adducts such as 23 in the presence of sufficiently reactive dipolarophiles <2000T10011>. These adducts furnished substituted pyrazolo[l,5- ]pyridines 24 in fair yields upon further heating and extrusion of sulfur. However, diphe-nylacetylene did not react with dipole 22 (Scheme 10). 

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