Cyclopropanes, ionized structure

Cyclopropane (D3 , symmetry) has a degenerate pair of in-plane e orbitals (S, A). Vertical ionization leads to a doubly degenerate E state, and JT distortion results in two nondegenerate electronic states, A and 82 (Cav symmetry), corresponding to two different molecular structures. The Ai state (orbital S singly occupied) corresponds to a structure with one lengthened C—C bond it is lowest in energy for many cyclopropane radical cations (Fig. 6.10). 

For radical cations of norcaradiene and derivatives, the interaction of the cyclopropane in-plane e orbitals with the butadiene frontier MO favors the type B structure. The assignments are based on ab-initio calculations, CIDNP results, and the ET photochemistry. The norcaradiene radical cation (lla ) has a electronic ground state (Cj symmetry). The Cl—C6 bond is shortened on ionization (—3.4 pm) while the lateral bonds are lengthened (+2.8 pm). The delocalization of spin density to C7 (py = 0.246 py 5 = 0.359) and the hyperfine coupling constants of the cyclopropane moiety a e = 1.36 mT oysyn = —0.057 mT flvanti = —0.063 mT) support a type B structure. 

The two species of 202 amu are taken to be the initially formed Franck-Condon structure and the parent species giving coherent resonance motion loss of a bromine atom gives BrCH2CH2CH2, which is detected after ionization by the probe pulse as C3H5, at 41 amu. The decay of BrCH2CH2CH2 leads to cyclopropane, a product not ionized by the probe pulse, and hence not seen through mass spectrometry. 

The pump-probe-detect arrangements for the femtosecond experiments was similar to those described above. When cyclobutanone was pumped with two photons of a X = 307-nm femtosecond pulse, two consecutive C—CO bond cleavages led to the formation of the trimethylene diradical, detected as an easily ionized transient at 42 amu, with buildup and decay times of 120 20 fs. The decay presumably involves isomerizations to cyclopropane and to propylene— structures not ionized by the probe pulse and thus undetected during the experiment. 

A most careful study to probe experimentally the effect of internal energy on the structure and reactivity of ionized cyclopropane was recently published by Lias and Buckley They generated by charge transfer to cyclopropane from ... 

The formation of cyclic structures not only as transients but also as stable product ions is conceivable for quite a number of gas phase reactions. However, unambiguous assignments of the structures could in most cases be obtained only by combining various experimental techniques. Even the simplest process, i. e. the formation of CaH product ions from various precursors, has been debated controversially for many years, and only recently it was shown that ionized cyclopropane (1) is produced from the cation... 

In a series of papers Gross et provide experimental evidence that ionization of cyclopropane in the gas phase results in the formation of a trimethylene cation radical (2) isomerization of cold 2 to ionized propene (3) can be ruled out. The authors, moreover, stress that the structural changes of ionized cyclopropane in both the gas phase and in condensed phase are entirely analogous. 

The rather peculiar electronic structure of 49, as compared to cyclopropane and bicyclo[1.1.0]butane 1, is strongly reflected in the correlation diagram of Figure 27, in particular by the fact that the first ionization energy of 49 cannot be obtained by a linear extrapolation of the first ionization energies of cyclopropane and of 1. 

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