Dipole moments of azulene

The dipole moment of azulene is small (0.80 D) [50] this suggests that the contribution of dipolar forms to the overall structure is small, but nonetheless real. The direction of the dipole is confirmed by dipole moment measurements on 2-halogeno- and 2-cyano-azulenes [51]. Calculations agree that the five- and seven--membered rings of azulene should carry, respectively, negative and positive charges. 

Azulene, dipole moment of. 54 electrostatic potential map of, 541 structure of, 533... 

Problein 10.32 The deep-blue compound azulene (C,yHg) has five- and seven-membered rings fused through two adjacent C s. It is aromatic and has a significant dipole moment of 1.0 D. Explain. M... 

Azulene can be written as fused cyclopentadiene and cycloheptatriene rings, neither of which alone is aromatic. However, some of its resonance structures have a fused cyclopentadienyl anion and cycloheptatrienyl cation, which accounts for its aromaticity and its dipole moment of 1.0 D. 

In sharp contrast to 40 and 42, azulene, a blue solid, is quite stable and many of its derivatives are known.116 Azulene readily undergoes aromatic substitution. Azulene may be regarded as a combination of 31 and 35 and, indeed, possesses a dipole moment of 0.8... 

Fig. ll. Comparison of the 0-0 bands of the first and second excited electronic states of azulene in naphthalene at 4.2°K. The second (emitting) excited state displays a sharper absorption line. (This work is taken from R. M. Hochstrasser and L. J. Noe, Dipole Moments of the Excited States of Azulene (72).)... 

The pKa values given for diphenylcyclopropenone, tropone and azulene-l-aldehyde were obtained by spectrophotometric titration with aqueous acids in the author s laboratory these figures are accurate to 0.3 pK units. The dipole moment of perinaphthenone in benzene was measured by R. J. Best and K. Mead of the Stamford laboratories of the American Cyanamid Co., Connecticut, U.S.A. 

We have seen that in molecules one can consider two families of wave functions the localizable a family and the non-localizable n family. The dipole moment of the molecule is the sum of the dipole moments of the various a bonds (bond contributions. But the values that would be obtained would not have the quasiinvariant character of the a bonds. For instance, in azulene the various CC bonds would have highly different moments. On the other hand, the partitioning into bond moments is of interest only when the moments obtained have a sufficient degree of transferability. We are interested here only in the a moment. 

One of the deficiencies of the MO methods, especially the simple ones, is that they tend to exaggerate uneven distribution of electrons in a molecule and thus make it more polar (with a higher dipole moment) than it actually is. The result is that dipole moments which are based on charge densities obtained from eigenfunctions of the MO approximations are usually considerably higher than the actual experimental dipole moments. Two old, well-known examples of theoretical dipole moments obtained by the HMO method are fiilvene (11) whose calculated dipole moment is 4.7 D [72-74] and the experimental value is 1.2 D [68], and azulene (15), with a calculated dipole moment of 6.9 D [72] and the experimental value of 1.0 D [68]. 

The dipole moment of 5,7-diazaazulene has been calculated by CNDOVSB, STO-3G and 4-3IG ab initio methods <89JCS(P2)103>. As would be expected, the presence of the electron-attracting nitrogen atoms means that it has a lower ground state dipole moment than either azulene or 1- and 2-azaanulenes. 

The dipolar nature of azulene, symbolized by the participation of mesomeric forms 232b and 232c, is expressed by an experimental dipole moment of about 1.0 (59MI1, p. 208 66MI1 85HOU(5/2c)127). 

Naphthalene is a colorless solid with a dipole moment of zero. Azulene is a solid with an intense blue color and a dipole moment of 1.0 D. Account for the difference in dipole moments of these constitutional isomers. 

One might think this a trivial result on the grounds that the carbon atoms in a neutral hydrocarbon obviously must be neutral—but this would be an incorrect intuition. Nonalternant hydrocarbons are polar and do have non vanishing dipole moments. Fulvene (5) and azulene (6) are good examples, each having dipole moments of about 1 D (Debye). In the case of an odd AH ion, the total Tc-electron density at atom i is 1 + ah for an anion and 1 -... 

Azulene does have an appreciable dipole moment (0.8 The essentially single-bond nature of the shared bond indicates, however, that the conjugation is principally around the periphery of the molecule. Several MO calculations have been applied to azulene. At the MNDO and STO-3G levels, structures with considerable bond alternation are found as the minimum-energy structures. Calculations which include electron correlation effects give a delocalized n system as the minimum-energy structure. ... 

Azulene, an isomer of naphthalene, lias a remarkably large dipole moment for a hydrocarbon (/i = 1.0 D). Explain, using resonance structures. 

The crucial structural feature which underlies the aromatic character of benzenoid compounds is of course the cyclic delocalised system of six n-electrons. Other carbocyclic systems similarly possessing this aromatic sextet of electrons include, for example, the ion C5Hf formed from cyclopentadiene under basic conditions. The cyclopentadienide anion is centrosymmetrical and strongly resonance stabilised, and is usually represented as in (7). The analogous cycloheptatrienylium (tropylium) cation (8), with an aromatic sextet delocalised over a symmetrical seven-membered ring, is also demonstrably aromatic in character. The stable, condensed, bicyclic hydrocarbon azulene (Ci0H8) possesses marked aromatic character it is usually represented by the covalent structure (9). The fact that the molecule has a finite dipole moment, however, suggests that the ionic form (10) [a combination of (7) and (8)] must contribute to the overall hybrid structure. 

As with azulene, redistribution of the n electrons of calicene produces a resonance form in which both rings are aromatic and which has a dipole moment. 

---