Dipole moments analysis

N=P(Oe) (for a general review see 1969, 26). This implication is in contradiction with indirect conclusions drawn from estimation of Lewis basicity (see Section IV) and from dipole moment analysis/1970,11 A more profound analysis is certainly required in order to elucidate this problem. 

Tetrahydro-l,3-oxazines are normally assumed to adopt a chair conformation in which the NH bond has an axial orientation. This view is substantiated by NMR spectroscopy and also by dipole moment analysis <73JCS(P2)325). The ring system is not stable and, for example, when the parent molecule is allowed to stand, it slowly ring opens and then forms a trimer (Scheme 33) (78AF937). In acidic media ring fission is accelerated and the open-chair imines may then hydrolyze. This property has been utilized in the synthesis of aldehydes as previously noted (see Section 2.27.2.2.4(0). 

The remainder of the optimization output file displays the population analysis, molecular orbitals (if requested with Pop=Reg) and atomic charges and dipole moment for the optimized structure. 

Repeat your analysis for tautomeric equilibria between 4-hydroxypyridine and 4-pyridone, 2-hydroxypyrimidine and 2-pyrimidone and 4-hydroxypyrimidine and 4-pyrimidone. For each, identify the favored (lower-energy) tautomer, and then use equation (1) to calculate the ratio of tautomers present at equilibrium. Point out any major differences among the four systems and rationalize what you observe. (Hint Compare dipole moments and electrostatic potential maps of the two pyridones and the two pyrimidones. How are these related to molecular stability )... 

A more thorough analysis shows that one should not expect the electric dipole moment to remain constant, because real molecules have polarizability. The polarization of the dielectric in the electric field of the molecule itself gives rise to a reaction field, which tends to enhance the electrical asymmetry. 

The foregoing conclusions are further supported by a refined X-ray analysis of pyrid-2-one, which indicated that the mobile hydrogen atom is attached to the nitrogen atom in the solid state and that individual molecules are bound into helices by N—H- -0 hydrogen bonds. An oxo structure is also indicated by the molar refractivity of pyrid-2-one. The dipole moment of 4-methoxypyridine is ca. 3.0 debyes in dioxane, whereas the values for pyrid-4-one and its 1-methyl derivative are much higher, ca. 6.0 debyes indicating the... 

Since the elementary quantum-mechanical treatment does not seem to give a high enough barrier, various treatments of the problem have been proposed which use empirical data such as bond dipole moments and steric repulsive forces. These treatments do not introduce any new forces which would not be included in a proper quantum-mechanical analysis, but they attempt to short-circuit these difficult and uncertain calculations. 

SOLUTION The data in Fig. 2.12 show that electronegativity differences decrease from 1 HC1 to HI, and so the dipole moments decrease as well. Therefore, dipole-dipole forces decrease, too, a trend suggesting that the boiling points should decrease from HQ to HI. This prediction conflicts with the data so we examine the London forces. The number of electrons in a molecule increases from HQ to HI, and so the strength of the London interaction increases, too. Therefore, the boiling points should increase from HCl to HI, in accord with the data. This analysis suggests that London forces dominate dipole-dipole interactions for these molecules. 

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