ISSN:
0030-493X
Keywords:
Chemistry
;
Analytical Chemistry and Spectroscopy
Source:
Wiley InterScience Backfile Collection 1832-2000
Topics:
Chemistry and Pharmacology
Notes:
The cis- and trans-annulated isomers of 8-(N-pyrrolidyl)bicyclo[4.3.0]nona-3,7-diene show different propensities for the retro Diels-Alder fragmentation following electron impact ionization. Molecular ions of the cis-annulated isomer decompose predominantly via the retro Diels-Alder reaction to give [C9H13N] +· fragments of the appearance energy (AE)=8.45±0.05eV and critical energy Ec=133±8kJ mol-1. The trans-annulated isomer gives abundant [M-H]+ (AE=9.34±0.08eV) and [M-C6H6]+· fragments, in addition to [C9H13N]+· ions of AE=8.98±0.05eV and Ec=181±8kJ mol-1. The ionization energies (IE) were determined as IEcis=7.07±0.05 eV and IEtrans=7.10±0.06eV. The stereochemical information is much less pronounced in unimolecular decompositions of long-lived (metastable) molecular ions which show very similar fragmentation patterns for both geometrical isomers. Nevertheless, the isomers exhibit different kinetic energy release values in the retro Diels-Alder fragmentation; T0.5=3.8±0.3 and 4.8±0.2 kJ mol-1 for the cis and trans isomer respectively. Topological molecular orbital calculations indicate that the retro Diels-Alder reaction prefers a two-step path, with a subsequent cleavage of the C(5)—C(6) and C(1)—C(2) bonds. The open-ring distonic intermediate represents the absolute minimum on the reaction energy hypersurface. The cleavage of the C(1)—C(2) bond is the rate-determining step in the decomposition of the cis isomer, with the critical energy calculated as 137 kJ mol-1. The cleavage of the C(5)—C(6) bond becomes the rate-determining step in the trans-annulated isomer because of stereoelectronic control. The difference in the energy barriers to this cleavage in the isomers (ΔE=95k Jmol-1) provides a quantitative estimate of the magnitude of the stereoelectronic effect in cation radicals.
Additional Material:
6 Ill.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1002/oms.1210220306
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