Ionized cycloalkanes

Many other ion-molecule reactions involving highly unsaturated hydrocarbon ions and neutral olefins or the equivalent strained cycloalkanes have been studied by mass spectrometry98. For example, we may mention here the addition of ionized cyclopropane and cyclobutane to benzene radical cations giving the respective n-alkylbenzene ions but also isomeric cyclodiene ions such as ionized 8,9-dihydroindane and 9,10-dihydrotetralin, respectively. Extensive studies have been performed on the dimerization product of charged and neutral styrene4. 

Electron ionization (El) mass spectrometry has been successfully applied to establish effects caused by stereochemistry, degree of saturation, and substitution on the characteristic fragmentations of cycloalkane-fused 1,3-oxazines, 1,3-thiazines, and pyrimidines and some of their precursor amino alcohols. 

We turn to the chemical behavior of cycloalkane holes. Several classes of reactions were observed for these holes (1) fast irreversible electron-transfer reactions with solutes that have low adiabatic IPs (ionization potentials) and vertical IPs (such as polycyclic aromatic molecules) (2) slow reversible electron-transfer reactions with solutes that have low adiabatic and high vertical IPs (3) fast proton-transfer reactions (4) slow proton-transfer reactions that occur through the formation of metastable complexes and (5) very slow reactions with high-IP, low-PA (proton affinity) solutes. 

The orbital assignments of the first ionization potential, as well as of the higher bands which are broader and less intense, have been confirmed by ab initio MO calculations and by comparison with PE spectra of other small-ring cyclic ethers, amines, sulfides, silanes and cycloalkanes (77JA3226). 

The best estimates have been obtained to date by using the MINDO and PNDO methods. In Tables 6 to 8 we show the ionization potential values obtained by each of these methods for alkanes and cycloalkanes, alkenes, acetylenes and aromatic compounds. Dewar and Klopman (PNDO) and Dewar et al. (MINDO/2) also compared their calculated inner orbital energies with experimental ionization potentials obtained from photoionization spectra. The ionization potentials of methane and ethane have also been calculated by the PNDO method along the more sophisticated procedure of minimizing separately the energy of the ion and that of the molecule. In these cases, the experimental value of the first ionization potential was reproduced accurately 48>. 

Table 6. Comparisons of experimental ionization potentials with calculated orbital energies of alkanes and cycloalkanes...

The third class of organic donor molecules are a-donors, viz., alkanes and cycloalkanes. These substrates have inherently high ionization and oxidation potentials. Therefore, their radical cations are not readily available by photoinduced electron transfer, but typically require radiolysis and electron impact in the condensed phases or the gas phase, respectively. Thus, radical cations of simple alkanes (methane [206], ethane [207]) or unstrained cycloalkanes (cyclopentane, cyclohexane) [208] were identified and characterized following radiolysis in frozen matrices. In contrast, strained ring compounds have significantly lower oxidation potentials so that the radical cations of appropriate derivatives can be generated by photoinduced electron transfer. 

Taylor, R. P. (American Chemical Co.) Process for production of cyclic ketoximes and lactams from cycloalkanes by means of ionizing radiation. U.S.P. 3062812 (23.4.59/6.11.62). 

Many bicycloalkanes are as unreactive as alkanes or unstrained cycloalkanes. They have very high values and their low electron affinities make them poor electron acceptors. The ionization potentials of unstrained bicycloalkanes are somewhat lower than those of unstrained cycloalkanes—bicyclo[2.2.2]octane IP 9.47 eV), bicyclo[4.3.0]nonane [IP 9.46 eV), or bicyclo[4.4.0]decane [IP 9.32 eV) lie below the 9.75-9.90 eV range characteristic of cycloalkanes [5]. As a result, they are somewhat easier to oxidize in solution or in solid matrices. Strained bicycloalkanes have even lower ionization potentials (bicyclo] 1.1. Ojbutane, IP 8.7 eV) the ring strain... 

The first evidence for the protonation of alkanes under highly acidic (superacid) conditions was independently reported by Olah and Lukas, as well as by Hogeveen and coworkers. " Protolytic reactions of hydrocarbons in superacid media were interpreted by Olah as an indication of the general electrophilic reactivity of covalent C-H and C-C single bonds of alkanes and cycloalkanes. The reactivity is due to the donor ability of the a-bond electron pairs via a three-center, two-electron (3c-2e) bond formation and follows the trend tertiary C-H > C-C > secondary C-H primary C-H. The transition state for protolytic ionization of hydrocarbons was presumed to be linear. It was later suggested that such 3c-2e interactions in carbocations generally tend to be nonlinear (even in stericaUy crowded cases) in nature (see also Chapter 1) [Eqs. (6.3) and (6.4)]. That is, such interactions are similar to transition states proposed for frontside Se2 reactions. 

Solvent holes in neat cycloalkanes were generated by multiphoton ionization (3 x 4 eV or 2 x 5 eV) of the solvent at fluxes in excess of 0.01 J/cm [15]. In a typical experiment, the laser-induced dc conductivity was measured as a function of the delay time with resolution better than 3 ns. A similar setup was used to observe the dc conductivity in pulse radiolysis with fast 16 MeV electrons [14]. The decay kinetics of solvent holes in cyclohexane and decalins were consistent with the value of =1 for multiphoton laser ionization. For cyclohexane, a lower ratio of fh =0.5 was needed to account for the kinetics observed in pulse radiolysis. (Note that these ratios refer to the situation at ca. 10 ns after the ionization event the conductivity signal of the holes cannot be measured at earlier time). To be consistent with the observations, the simulations required a higher value for the mobility //jj of the... 

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