Cyclohexane radical decay

The extinction coefficients for the T-T neutral aminyl, and cation radical absorptions of 97 were used to calculate the quantum efficiencies for N—H cleavage and photoionization. The results indicate that in cyclohexane, the efficiency of cleavage is ca. 90%. Thus, roughly 90% of those upper triplet states that do not relax to T undergo cleavage. In acetonitrile on the other hand, the efficiencies for neutral and cation radical production are 0.53 and 0.45, respectively. In other words, of the upper triplets that do not regenerate Tv half decay to neutral radical and the other half to cation radical. It should be noted that the actual proportion of direct cleavage events may be smaller than indicated from the efficiencies because one of the cation radical decay routes is deprotonation to form the neutral radical. 

Figure 6 shows the -resolved ESR spectra of the alkyl radicals in alkanes and polyethylene obtained by using the ESE technique as described in Sect. 2.5. For cyclohexane and polyethylene, the spectral shape is essentially due only to the six-line component of the inner radical. For n-alkanes, the spectral shape is the superposition of the six-line component and the seven-line component. The latter due to the penultimate radical decays much more rapidly during the... 

The radical decays to dimer according to second-order kinetics ( 2 = 1-67x10 sec M at 31°C in cyclohexane activation enthalpy= 5-9 kcal mole ) (Evans and Evans, 1965). 

The study of the detailed mechanism of free radical initiation (rate constant k ) and ozone decay (rate constant d) by the reaction with cyclohexane, cumene, and aldehydes gave the following results (7 = 298 K) ... 

Free radicals are formed in the hydrocarbons are the result of decay of excited molecules (see earlier). The value of G(R ) from cyclohexane (RH) is 5.7 [222]. Various alkyl radicals are... 

Class (3) reactions include proton-transfer reactions of solvent holes in cyclohexane and methylcyclohexane [71,74,75]. The corresponding rate constants are 10-30% of the fastest class (1) reactions. Class (4) reactions include proton-transfer reactions in trans-decalin and cis-trans decalin mixtures [77]. Proton transfer from the decalin hole to aliphatic alcohol results in the formation of a C-centered decalyl radical. The proton affinity of this radical is comparable to that of a single alcohol molecule. However, it is less than the proton affinity of an alcohol dimer. Consequently, a complex of the radical cation and alcohol monomer is relatively stable toward proton transfer when such a complex encounters a second alcohol molecule, the radical cation rapidly deprotonates. Metastable complexes with natural lifetimes between 24 nsec (2-propanol) and 90 nsec (tert-butanol) were observed in liquid cis- and tra 5-decalins at 25°C [77]. The rate of the complexation is one-half of that for class (1) reactions the overall decay rate is limited by slow proton transfer in the 1 1 complex. The rate constant of unimolecular decay is (5-10) x 10 sec for primary alcohols, bimolecular decay via proton transfer to the alcohol dimer prevails. Only for secondary and ternary alcohols is the equilibrium reached sufficiently slowly that it can be observed at 25 °C on a time scale of > 10 nsec. There is a striking similarity between the formation of alcohol complexes with the solvent holes (in decalins) and solvent anions (in sc CO2). 

As shown by Tagawa et al. [74], the alkane excited molecules have a broad absorption band in the visible region with maxima increasing from —430 to —680 nm between C5 and C20 for the -alkanes. This spectrum strongly overlaps with the absorption spectrum of the radical cations with low carbon atom number alkanes the two maxima practically coincide. With increasing carbon atom number, the red shift in the radical cation absorbance is stronger than in the Si molecule absorbance [47,49,84-86]. The decay of the excited molecule absorbance was composed of two components with 0.1- and 1.0-nsec decay times in cyclohexane, and 0.17 and 2.7 nsec in perdeuterocyclohexane [47]. The nature of the faster-decaying component is as yet unclear. 

Reaction of dimethoxyFL (23a), which is known to have a singlet ground state (AGst = 2 kcal/mol), in cyclohexane supports this idea. These LFP studies in cyclohexane and cyclohexane-i/ia indicate that a similar small kinetic isotope effect of 1.7 is observed for the decay of the transient absorption bands associated with 23a and the corresponding radical. ... 

The absorption band around 520 nm is very similar to that of polystyrene excimer (2,3,5). The decay follows first order kinetics with a lifetime of 20 ns. The decay rate agrees with that of the excimer fluorescence and excimer absorption. The longer life absorptions, attributed to the triplet states and free radicals (2,5), were observed at wave lengths <400 nm, although the anionic species of polystyrene with the absorption maximum at 410 nm as seen in solid films (cf. Figure 5) was not observed. Figure 9 shows the absorption spectrum observed in the pulse radiolysis of CMS solution in cyclohexane. 

Isomeric C Hn radical ions fragment not very differently by the different mass spectro-metric methods. The metastable decays are nearly identical, but the collisionally activated spectra of 14 isomeric hexenes, measured by Nishishita and McLafferty240, exhibit some quantitative differences. Bensimon, Rapin and Gaumann251 compared the metastable decay and the photoinduced fragmentation by infrared photons of long-lived parent ions of six hexene isomers and cyclohexane. If the linear isomers are practically identical, some notable differences are observed for branched isomers. Cyclohexane behaves similar to n-hexenes. The metastable fragmentation of H/D-labeled 4-Me-2-pentene, 2-Me-2-pentene... 

High energy radiation generates ionic and free radical intermediates in styrene, and both ionic and radical polymerizations take place simultaneously. Therefore, the observation of neutral radicals related to the radical polymerization had been expected. The absorption of radicals with a maximum at 320-330 nm was actually observed in the pulsed styrene and a-methylstyrene it decayed over a time interval of several hundred microseconds without being affected by the presence of water [12 14]. This absorption was attributed for the main part to a radical with a benzyl-type structure. The similar absorptions were observed in the cyclohexane solutions [21], Swallow suggested that this intermediate would be formed by an ionic process, probably by the protonation of a... 

Figure 7. Transient absorption spectrum of 9-bromoanthracene cation radical obtained at 25 ps following the 532-nm CT excitation of the bromoanthracene-TiCU complex in cyclohexane. The inset shows the complete decay of the cation radical within 1 ns due to back electron transfer [116].

Most organometallic EDA complexes of arenes with titanium tetrachloride [116] in solution also follow the general reaction scheme in Eq. 15 in that no net chemical reaction is observed upon charge-transfer irradiation due to rapid back electron transfer (A et 10 ° s ). For example, the transient absorption spectrum of bro-moanthracene (BrAnt) cation radical generated by 532-nm laser excitation of the [BrAnt, TiCU] complex in cyclohexane (see Figure 7) decays completely to the spectral baseline within about 1 ns (see inset) due to back electron transfer [116], (Eq. 18) ... 

To ensure that photolysis is the only loss process for the aldehyde experiments can be carried out in the presence of an excess concentration of a radical scavenger such as cyclohexane. In cases where the high concentration of a scavenger is undesirable, e.g. because it causes saturation in the infrared absorption spectrum, a tracer compound, such as di-n butyl ether, can be used to correct for the measured decay of the aldehyde in order to obtain the j value. Typical starting concentrations used in photolysis experiments at EUPHORE are [aldehyde] = 0.5-1.5 ppmv, [scavenger] = 10-50 ppmv or [tracer] = 0.1-02. ppmv (Wenger et ai, 2004 and Magneron et al, 2002). 

No triphenylmethyl cation or radical was detected. Malachite green leuco-methoxide, 51, in cyclohexane behaved similarly. In contrast, 51 showed rapid growth of the triaryl cation from excited Si with a rise time of 1 ns in 9 1 acetonitrile/water, and an even faster rise time in methanol. No evidence for the triarylmethyl radical was obtained. Also, the dithioketal, 52, has been used to generate the corresponding relatively long-lived cations by photoheterolysis of the carbon-oxygen ether bond the half-life for cation decay in 1 1 ethanol/water is 0.17 s. Rate constants for the reactions of this cation with a number of amine and anionic nucleophiles were measured. 

Emission measurement from the excited states is also a powerful method to investigate the ion beam radiation chemistry because a very sensitive time resolved photon-counting technique can be applied. In 1970s, temporal behavior of the emission from benzene excited states in 40 mM benzene in cyclohexane irradiated with pulsed proton and He ion particles was measured and compared with UV pulse irradiation. It was found that immediately after the irradiation there is a short decay (< 10 ns) followed by a longer decay corresponding to the life-time of the benzene excited states (26-28 ns). The fraction of the shorter decay component increases with increasing LET of the particle. This was explained by a quenching mechanism that radical species formed in the track core attack and quench the benzene excited states, which would take place only shorter period less than 10 ns after irradiation [69]. 

Transient absorption spectra of some "satellite ions" closely resemble the spectra of olefin radical cations. In cyclohexane, a band centered at 270 nm (at 2 ns [22]) is observed from 250 ps [25] after the ionization event (this band overlaps with the strong 240 nm band of cyclohexyl radicals [22]). The scavenging behavior and the decay kinetics of the UV-absorbing species suggest that they are normally-diffusing radical cations [25]. In the first few nanoseconds after the ionization event, the VIS absorbance is dominated by solvent excited states [22,57]. When the thermalized electrons are rapidly scavenged using a suitable electron acceptor (halocarbons or N2O), this... 

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