Dipole moment fulvene

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 -... 

Fulvenes, cyclic molecules with odd number of carbon atoms in the ring, represent one of the fundamental categories of cross-conjugated systems (see Chapters 1 and 6). According to the ring size of these molecules, the compounds are classified astriafulvenes 1, pentafulvenes 2, heptafulvenes 3, and so on (Scheme 7.1). Owing to their dipole moments, fulvenes possess unique reactivity, different from that of open-chain polyolefins and aromatic compounds. In 1989, Neuenschwander... 

Here the charges as indicated are no longer the same for each carbon atom. Actually these hydrocarbons have a dipole moment (azulene 1.0 D, fulvene 1.2 D). 

A primitive approach to treating the solvent effect was made with 6,6-diheterosub-stituted fulvenes by calculating the gas-phase barriers with the CNDO/2 method and the solvent stabilization of ground and transition states in the reaction field formalism with a spherical cavity. The result was found to be very sensitive to the cavity size and to the calculated ground-state and transition-state dipole moments, and no reliable numerical data could be obtained. The calculated gas-phase barriers were too high, although they fell in the correct order. 

MCP, the archetype of nonalternant hydrocarbon known as the fulvenes, is a molecule of considerable theoretical and experimental interest [82-90]. It was synthesized and characterized for the first time in 1984 Billups et al. [84] and Staley and Norden [85] independently. An ultraviolet (UV) spectrum was included in the latter study. The electric dipole moment and heavy-atom molecular structure were reported two years later [88]. The experimental value of the dipole moment, p = 1.90 0.02 D, remarkably large for such a small molecule, which has been related to the strong transfer of 7r-electron density out of the ring due to the two double bonds interacting in a T-type arrangement. The... 

This conclusion is supported also by LCAO-HMO calculations. It can be shown that the molecular orbitals of [ ] fulvenes and [n, w] fulvalenes belong either to the A2 representation of the point group C21, or to its representation. In the molecular orbitals of [/i] fulvenes which belong to A2, all the electron density is concentrated in the ring. The A -> transition in triafulvene is from a Bi molecular orbital to an A2 molecular orbital. Thus, in the lowest unoccupied molecular orbital, all the electron density is concentrated in the three-membered ring. Likewise, in 1,2-diphenyltriafulvene, upon A - Fi transition, the calculated dipole moment diminishes. 

Consider the case of triapentafulvalene (XIV) (calicene), a combination of tria-fulvene and pentafulvene. Hexaphenyltriapentafulvalene (XV) [37] was taken as a representative. This compound has the highest experimental dipole moment in the ground state ever recorded in a hydrocarbon - 6.3D (benzene, 30°C) [37]. A similar value was obtained in the case of the l,2-dimenthyl-3,4,5,6-tetraphenyl derivative [38]. 

A. Pullman In all fairness, it must be said that the quantum chemical computations, as a rule, and quite generally, are much less satisfactory, for excited states properties than for ground state properties. This is true particularly when quantitative evaluation of a property is requested. This inconvenience appears in all its fullness for non-benzenoid hydrocarbons because the dipole moment is a striking example which does not occur in the benzenoid series, but it is true for any quantity related to excitation. As concerns qualitative evolutions of various properties, theory has been extremely successful in the past in the very case of the fulvenes and has even been instrumental in the development of the field since it has often preceded experiment and served as an incentive for the chemist to synthetize the compounds. (See the numerous series of papers by ourselves and Dr Ernst Bergmann in the early 1950 s.)... 

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