Dipole moments pyrrole molecule

The dipole moment varies according to the solvent it is ca 5.14 x 10 ° Cm (ca 1.55 D) when pure and ca 6.0 x 10 ° Cm (ca 1.8 D) in a nonpolar solvent, such as benzene or cyclohexane (14,15). In solvents to which it can hydrogen bond, the dipole moment may be much higher. The dipole is directed toward the ring from a positive nitrogen atom, whereas the saturated nonaromatic analogue pyrroHdine [123-75-1] has a dipole moment of 5.24 X 10 ° C-m (1.57 D) and is oppositely directed. Pyrrole and its alkyl derivatives are TT-electron rich and form colored charge-transfer complexes with acceptor molecules, eg, iodine and tetracyanoethylene (16). 

The electric dipole moments in units 1 X 10 18 e. s. u. of these molecules and their derivatives by hydrogenation measured19 in benzene solution are the following furan, 0.670 2,5-di-hydrofuran, 1.53 tetrahydrofuran, 1.68 pyrrole, 1.80 pyrroline, 1.42 pyrrolidine, 1.57 thiophene, 0.54 and tetrahydrothiophene, 1.87. We now give a very rough interpretation of these quantities based on the bond moments given... 

Pyrrole forms weaker NH-0 hydrogen-bonded complexes with ethers.106,1 6,157,163 The dipole moment of the pyrrole-1,4-dioxane complex shows it to be predominantly a binary system in which the pyrrole molecule occupies an axial position to the dioxane ring163 (see Section III, A, 2). Particularly strongly hydrogen-bonded NH-0=C complexes are formed between pyrrole and ketones.147,149,156 IR studies of the effect of the complex formation upon both the NH- and the C=0 stretching frequencies have been made. 

The MM3 force field has been extended by Allinger and co-workers to cover aromatic heterocycles of the pyridine and pyrrole types <93JA11906>. Structures (32 compounds), dipole moments (35 compounds), heats of formation (35 compounds), and vibrational spectra (11 compounds) were examined. The results are good for structure and fair for the other items resonance energies were reported for the series benzene (17.79 kcal mol ), pyridine (17.02 kcal mol ), pyridazine (14.35 kcal mol ), pyrazine (17.01 kcal mol ), pyrimidine (15.60 kcal mol ), 1,3,5-triazine (13.51 kcal mol ), and 1,2,4,5-tetrazine (17.72 kcal mol ). Finally, ab initio studies of the dipole polarizabilities of conjugated molecules have been reported in which monocyclic azines (pyridine, pyridazine, pyrimidine, pyrazine, 5-triazine, and 5-tetrazine) are compared <94JST(304)109>. 

Furan has a dipole moment of 0.70 D, while thiophene has a dipole moment of 0.51 D. The dipole moments of furan and thiophene are in the opposite direction of pyrrole due largely to the relatively strong inductive effect caused by the oxygen and sulfur in relation to weaker resonance effects. In the case of pyrrole, as described in Chapter 2, the resonance hybrids of the molecule result in the inversion of the dipole. 

We shall review the electronic and the pure vibrational contributions to the hyperpolarizabilities of pyrrole [50]. The original article involved also dipole moment and polarizabilities. The molecule is placed on the yz plane (Fig. 5.2). The computations have been performed at the Hartree-Fock level, employing the Pol basis set [45]. 

Pyridine and pyrrole have contrasting resonance descriptions (Fig. 13.51), and this difference results in some very different physical properties. For example, the direction of the dipole moment is different in the two aromatic molecules. 

It is understandable that pyrrole should possess a finite dipole moment (p. 51). Further, the resonance description requires the molecule to be planar. The classical structure (15) could accommodate a pyramidal nitrogen atom, but although dipole-moment studies allow the extra-annular nitrogen valency an out-of-plane latitude of 7°, and Raman and infra-red spectroscopic studies favour the planar form but do not rigorously exclude a non-planar formas, the microwave rotational spectrum confirms the truly planar structure (18). 

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