Cyclopropenes dipole moments

It should be pointed out that there are some methylene cyclopropene derivatives, whose stability is ascribed mainly to inductive effects brought about by strongly electron-withdrawing substituents. Thus, l,2-bis(p-tolyl)-4,4-(bis-trifluoromethyl)-triafulvene (63) synthesized recently by Agranat66 is a perfectly stable molecule with a dipole moment (7.42 D) comparable to that of l,2-diphenyl-4,4-dicyano-triafulvene (64) of the resonance-stabilized type (l)67 (7.9 D). 

The calculations showed that the C=C w-MO ionization in cyclopropene (9.86 eV) correlates most closely with the second ionization level in cyclopropenone and not with the first band. It is stressed that the noticably high dipole moment of cyclopropenone represents a manifestation of the extreme MO delocalization. 

Fohlisch99 reported a remarkable dependence of the electron spectra of quino-cyclopropenes on their structure. As shown in Table 9, the merocyanine-like quino-cyclopropenes show positive solvatochromy when they contain an anthraquinonoid chromophore (198), but negative solvatochromy when they contain a benzoquino-noid system (199). This can be interpreted in terms of a markedly increased participation of dipolar resonance forms in the ground state of the benzoquinonoid 199 compared to the anthraquinonoid 198. From the dipole moment of 198 (9.4 D99 ) the dipolar contribution was estimated to be in the range of 23%. 

Hartree-Fock, local density, density functional and MP2 models provide a credible account of dipole moments in hydrocarbons. Even STO-3G and 3-2IG (Hartree-Fock) models appear to be suitable. Not only is the mean absolute error very low (0.1 debye or less), but all models properly account for a variety of subtle trends in the experimental data, for example, the increase in dipole moment in cyclopropene in response to methyl substitution on the double bond. Finally, note that there is very little difference in the performance of any of the models with 6-3IG and 6-311+G basis sets. 

For this study we have used methylen-cyclopropene (MCP) and acrolein (ACRO) in two solvents, an apolar (dioxane) and a polar one (acetonitrile). The selected transitions can be seen as representative examples of different types of electronic transitions for which different solvent responses can be studied for MCP the first 77 - 77 transition for MCP, and the first n -> 77 and 77 -> 77 transitions for ACRO. We note that in MCP the resulting excited state is characterized by a dipole moment which has an opposite direction with respect to that of the ground state, whereas in ACRO, the n -> 77 and 77 -> 77 transitions are characterized by a decrease and an increase in the dipole moment passing from ground to excited state, respectively. 

A previous study of the charge distribution in cyclopropene using the 6-3IG basis set suggested that jr-electrons are withdrawn from the double bond into the methylene group. ) This led to a theoretical dipole moment with its negative end at the CH2 group and the following n-orbital populations... 

The dipole moment is a manifestation of the overall charge distribution, and the calculated moments are generally in good agreement with the experimental values. Among cyclopropene derivatives, cyclopropene is the most interesting case. Here, the sign of the dipole moment is reversed from that found in cyclobutene or other cycloalkenes. The... 

The vibrational spectrum of cyclopropene has been examined in detail Again, the dipole moment derivative for the olefinic C-H stretch was considerably smaller than that for the vinyl C-H stretch, corresponding to the magnitudes of the CH bond dipoles. 

There are two additional features of the hydrocarbon, cyclopropene, which have not been considered and which may contribute to its reduced kinetic and thermodynamic acidity. First of all, the reversed sign (polarity) of the dipole moment of cyclopropene compared to other alkenes, including cyclobutene, and its larger than usual magnitude (0.455 D) , is indicative of significant electron shift from the double bond to the methylene group . This has been shown to be a result of electron transfer from the n c-orbital to the <7 component orbital . Related to this polarity reversal is the observation... 

The dipole moment of cyclopropene, measured as 0.454 D, is well reproduced by theory However, the direction of the dipole obtained by calculation opposes that deduced from experiments. The fact that 1-methylcyclopropene has a larger and the 3,3-dimethyl derivative a smaller dipole moment than the parent molecule supports the calculated sign with the double bond at the positive end of the dipole. 

Structural studies" show that the cr ring bonds of cyclopropenone are shorter than their cyclopropene counterparts, whilst the carbonyl bond is longer than model compounds. These effects are consistent with extensive n delocalization" and the high dipole moments (4.7-5.1 D), low pK values" and low field ( 9.0 ppm) proton resonances support significant aromatic stabilization. 

All three levels correctly calculate the decreasing dipole moment trend in the NH3, CH3 NH2, (CH3 )2 NH and H2 O, CH3 OH, (CH3 )2 O series. Similarly, the increase in going from HCHO to CH3CHO is handled adequately by all three methods. The dipole moments of propane, propene, and propyne are of interest since these molecules are the simplest stable polar hydrocarbons. The extended basis sets handle the dipole moments in these molecules somewhat more successfully than does STO-3G. The dipole moment in cyclopropene is calculated to have its negative end on the methylene group. From studies of the g values of cyclopropene and its 1,2-dideuterio derivative, Benson and Flygare31 have found the opposite result. However, it should be noted that the experimental error... 

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