Excited cyclopropane

CD2CH2CD2 and CH2CD2CH2 diradicals decay at somewhat longer lifetimes, in 183 and 129 fs, respectively. Torsional motions of the terminal methylene groups are obviously critical to the reaction coordinate leading to vibrationally excited cyclopropane-dj products. 

The initial products may be considered to be excited cyclopropane derivatives formed by addition to the double bond and excited olefins formed by insertion into CH bonds. (The detailed reaction mechanisms are discussed in Sec. V.) The isomerization products are expected to be similar to those found in the thermal isomerizations of the corresponding cyclopropane derivative or olefins, the excitation energy being at least 80-85 kcal. in the former case and 85-90 kcal. in the latter (taking AHf° (CH2) 80-85 kcal.). The excitation energy is increased by any excess energy of methylene. 

The lifetime n of excited cyclopropane formed by reaction (2) with respect to reopening of the ring is given by... 

Methylene radicals may be caused to react with various molecules, and since the reations that occur are usually highly exothermic, molecules of very high energy are produced. Methylene radicals add, for example, to the double bond in ethylene to give excited cyclopropane, viz. 

If Vt is the rate of production of excited cyclopropane, assuming a stationary concentration of excited molecules leads to the conclusion that the rates of formation of normal cyclopropane and propene are... 

Fig. 5. Relative yield of cyclopropane as a function of pressure. The excited cyclopropane was produced from ethylene and ketene irradiated with light of wavelengths 3600 A (O) and 3100 A...

Von Bunau and Kuhnert studied the y-radiolysis of cyclopropane both in direct radiolysis and in Kr- and Xe-sensitized radiolysis. The sensitized radiolysis was studied earlier by Smith and coworkers who showed that the addition of a rare gas increases the percent of cyclopropane consumed for constant irradiation time. The sensitization increases with increasing pressure of the rare gas for constant pressure of cyclopropane. The degree of rare gas sensitization was found, as expected, to be in the order of its energy absorption characteristics (electron density), i.e. Xe > Kr > Ar. Von Bunau and Kuhnert studied the effect of electron scavenger (SFe) and radical scavenger (NO) on the yield of the various products in direct radiolysis and in the sensitized radiolysis. They explained the different effects of the additives in the three systems by assuming the formation of two reactive species, an excited cyclopropane molecule and an excited cyclopropane ion, whose yields are different in the three systems as can be seen in Table 3. 

They suggested that the excited cyclopropane can react in three different reactions (equation 10) ... 

The excited cyclopropane ion is either collisionally stabilized to CaH or decomposed to give CaH, CaH, CaHj and C2H3 However, the proportion of these different ions depends on the excitation energy of the (CP ) and consequently is different for the three methods of radiolysis. These ions react with another molecule of cyclopropane, according to equation (11) ... 

The decomposition of the excited cyclopropane molecule is suggested to occur either through (1) a one C-C bond rupture (equation 17) or (2) via simultaneous rupture of two C-C bonds (equations 18-22). The relative high yield of C2 products supports the suggestion of simultaneous rupture of two C-C bonds, i.e. that both ruptures occur before thermalization of the molecule. 

The application of both criteria to gas-phase reactions is complicated further by the formation of vibrationally excited products. Both the insertion and addition reactions of methylene are exothermic by approximately 93 kcal. mole (based on recent estimates of AH (CH2) = 94 kcal.mole" ). Vibrationally excited alkanes and alkenes may dissociate into free radicals, and excited cyclopropanes may undergo structural and geometrical isomerizations unless collisionally stabilized . The occurrence of hot molecule reactions excludes any reasonable estimation of singlet and triplet methylene fractions. The data presented in the following paragraphs have been taken from experiments at high-pressures", which are thought to ensure complete collisional deactivation of excited reaction products. 

There is little question, however, that considerable energy is left in the labeled alkane in some instances, whether the labeled molecule arose via a collision complex or via direct substitution. Lee et al. (1960b), used the known pressure-dependent isomerization of cyclopropane (eq. 12) as a model for studying this problem. The excited cyclopropane can be de-excited by collision with another molecule or it can imdergo isomerization before de-excitation if the rate of isomerization is sufficiently fast. The yield of the isomerization product approached that of the parent as the pressure decreased. Qualitative... 

Here C3H6 is an excited cyclopropane molecule. At low pressure, Step 1 is much slower than Step 2. Derive the rate law for this mechanism at low pressure. Explain. 

Figure IX-E-4 shows as a function of wavelength at two pressures the ratios of propene to cyclopropane products that are formed in alternative pathways for reaction of the vibrationally excited cyclopropane molecule. A large increase in this ratio is seen as the excitation energy is increased in photolysis at the shorter wavelengths. A cyclopropane molecule that is vibrationally richer results as the quantum energy is increased. A somewhat more efficient quenching of the vibrationally excited cyclopropane can be seen for the data from 11 Torr of cyclobutanone (dashed curve) compared to those for 5 Torr (solid curve). One expects that an atmosphere of air will result in more significant...

Figure iX-E-4. The ratio of propene/cyclopropane as a function of wavelength in the photolysis of cyclohutanone at two pressures. Propene and cyclopropane both can arise from process (II) cyclopropanone + /ju —> excited cyclopropane + CO (II). Collisional stabilization of the excited cyclopropane becomes less effective (and decomposition more effective) in photolysis at the shorter wavelengths figure from Calvert et al. (2008). 

Figure IX-E-7. Approximate j-values for cyclobutanone photodecomposition versus solar zenith angle for a cloudless day in the lower troposphere with an overhead ozone column of 350 DU. Process (I) cyclobutanone + hv CH2=CH2 + CH2=C=0 process (II) cyclobutanone + hv -> vibrationally excited cyclopropane + CO figure from Calvert et al. (2008).

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