Fulvene, energy

The first study was made on the benzene molecule [79], The S ISi photochemistry of benzene involves a conical intersection, as the fluorescence vanishes if the molecule is excited with an excess of 3000 crn of energy over the excitation energy, indicating that a pathway is opened with efficient nonradiative decay to the ground state. After irradiation, most of the molecules return to benzene. A low yield of benzvalene, which can lead further to fulvene, is, however, also obtained. 

Figure 9.12. Potential energy profile along (adapted from reference 10) near the fulvene conical intersection. The branching space consists of stretching and skeletal deformation of the five-membered ring.

F ure 9.13. Potential energy profile (adapted from reference 10) for fulvene in the space spanned by Xj and the coordinate (torsion). 

Kinetic analysis shows that the formation of tropone through a hydroxyphenyl-carbene intermediate (which exhibits the lowest activation energy 69.3 kcal/mol) dominates o-QM decomposition process up to 1500 K, with fulvene + CO formation becoming competitive at higher temperatures. In fact, the latter decomposition mode although disfavored by its higher activation enthalpy (75.4 versus 69.3 kcal/mol) becomes competitive due to its more positive activation entropy. 

Dimethylfulvene 93 also reacts with sydnone 89, albeit sluggishly, to form the dihydrocyclopenta[c]pyrazole 94 after elimination of carbon dioxide and hydrogen (Equation 10). Molecular orbital energies and coefficients of 3-phenylsydnone 89 and fulvenes 91 and 93 have been calculated (PM3-MNDO), but when orbital symmetries... 

The limitations of this naive approach are immediately obvious, if one considers that the molecules [3]radialene 173, 3,4-dimethylidenecyclobutene 174 and fulvene 175 give rise to the same graph Q (see display 23) and thus to identical predictions for their three 7T ionization energies. Using the above parameters one finds /j calc = I 2 calc = 9.0 eV,... 

Although valence isomerization reactions of aromatic compounds have found little by the way of practical application, they are a fascinating area for mechanistic and theoretical study. The details are not completely dear, but it seems that, for benzene itself, benzvalene arises from the lowest excited singlet state, perhaps by way of a biradical intermediate (3.32) that could also be a precursor to fulvene bicyclohexadiene is probably produced from the second excited singlet state. For some other aromatic compounds the electronic nature of 5, and S2 may be reversed, or at least the states are much closer in energy, so that the preference for benzvalene or bicyclohexadiene formation under conditions of long-wavelength irradiation can be rationalized. 

The rearrangement of fulvene (64) to benzene has been studied by dieoretical methods.74 The favoured pathway involved rearrangement to iso fulvene (65) (barrier 41.6 kcal mol-1), ring opening to cyclohexadienecarbene (66) (barrier 74.3 kcal mol-1), and 1,2-Hf shift to form benzene (barrier 59 kcal mol-1). The discrepancy between die calculated activation energy and the value determined by very low-pressure pyrolysis is suggested to be due to surface effects. 

For relief and reassurance, Table 5.13 shows the relative energies of some isomers calculated at modest levels, namely HF/3-21G1 1, HF/6-31G, and MP2/6-31G. For a reality check, we also see values from G3(MP2) and experiment (experiment fulvene/benzene, [229/230] cyclopropane/propene, [231/231] dimethyl ether/ethanol, [232/233] methylcyclopentane/cyclohexane, [230/234]). The energy differences chosen for this illustration are enthalpy differences, because differences in heats of formation yield these, and heats of formation represent the most extensive compilations of experimental energy quantities relevant to our... 

The radical cations of fulvene systems are of interest, because steric and electronic factors might favor a perpendicular structure and because the energy difference between the respective cis and trans isomers are expected to be small. However, the chloranil photosensitized reaction resulted in CIDNP effects, indicating planar or slightly twisted structures. The Z- and E-2-tert-butyl-6-(dimethylamino)fulvene [20, R = — N(CH3)2] radical cations rearrange readily whereas di-/er/-butylfulvene [20, R = — C(CH3)3] showed no interconversion under comparable experimental conditions [160]. 

Fulvene has six electrons, so the HOMO is the third from the bottom. The scheme suggests that this orbital is the butadiene 2 MO. However, we need to ensure that 2 does not perturb n enough to move it above 2. Equation (3.9) shows that the change in the n orbital energy is... 

Applying Paddon-Row s method to the second reaction, and taking into account all four FOs, suggests that the 6 + 4 cycloaddition is the most favorable reaction (having an interaction energy of 0.546/ ) followed by the 4 + 2 wherein the fulvene acts as a diene (interaction energy 0.359/ ) and finally 4 + 2 where the fulvene provides the di-enophile component (interaction energy 0.284/ ). Nonetheless, experiments prove that this last compound is the main product. 

They react easily with electrophiles and add nucleophiles at C-6. In cycloaddition reactions they may react as 2jt, 4n, or 6 i compounds. According to frontier orbital considerations they readily react with electron-deficient dienophiles (e.g., silenes) in Diels-Alder reactions this is due to the strong interaction between the fulvene HOMO and dienophile LUMO [9]. Although the n and n orbitals of silenes are generally 1-2.5 eV higher in energy than is the case for the alkene congeners [10] a normal [4+2] cycloaddition behaviour for 3 is observed in earlier works [3-5]. 

CALCULATION OF ENERGIES FOR MOLECULES OF BUTADIENE, BENZENE AND FULVENE... 

The remaining integrals and the solution of the secular equation is obtained as in the case of benzene and we obtain for the energy of fulvene ... 

Knowledge of the energetics of (C-C3H2 )-CH 2 (15) gives information relevant to those of other cyclopropene derivatives. If one accepts the above experimentally determined heat of formation of C4H4, the theoretical energy difference of vinylacetylene and methyl-enecyclopropene, and an ill-defined experimentally derived heat of formation of vinylacetylene S the ionization potential of methylenecyclopropene is indirectly deduced to be 8.2 eV. This value is meaningfully compared to the 9.5 eV directly measured as the ionization potential of cyclopropenone. The derived 1.3 eV difference is comparable to the 1.1 eV difference for the antiaromatic cyclopentadienone (with a vertical value of 9.5 eV) and non-aromatic methylenecyclopentadiene (i.e. fulvene, with an IP of 8.4 eV). 

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