CH3S radical

FIGURE 4 19 Bonding in methyl radical (a) If the structure of the CH3 radical IS planar then carbon is sp hybridized with an unpaired electron in 2p orbital (b) If CH3 IS pyramidal then car bon IS sp hybridized with an electron in sp orbital Model (a) IS more consistent with experimental observa tions... 

We start with the reaction of abstraction of a hydrogen atom by a CH3 radical from molecules of different matrices (see, e.g., Le Roy et al. [1980], Pacey [1979]). These systems were the first to display the need to go beyond the one-dimensional consideration. The experimental data are presented in table 2 together with the barrier heights and widths calculated so as to fit the theoretical dependence (2.1) with a symmetric gaussian barrier. 

Chung and coworkers tried to observe similar species in y-irradiated DMSO-h6 at 77 K, however, repeated attempts were unsuccessful. Besides no free -CH3 radicals were detected in the y-irradiated DMSO-h6. They suggested that this remarkable difference of an all-or-nothing deuterium effect might be connected with the very much larger reactivity of the methyl radical in a subsequent reaction of hydrogen abstraction due to the greater reactivity of the C—H over the C—D bond. 

Self-Test I3.9B Calculate (a) the number of half-lives and (b) the time required for the concentration of C2HA to fall to one-sixteenth of its initial value as it dissociates into CH3 radicals at 973 K. Consult Table 13.1 for the rate constant. 

The reactions of CH3 radicals and CI2 alone with CujSi have also been investigated. On pure Cu3Si, the dominant silane product from CH3 adsorption is SiH(CH3>3 and the temperature at which the surface is sputtered prior to methyl adsorption has a dramatic effect on the reaction rate (see section 3.3). The CI2 reaction gives SiCU evolution, and the reaction temperature is close to that for methylchlorosilane formation. 

In a separate set of experiments designed to follow the gas phase reactions of CHj-radicals with NO, CHj- radicals were generated by the thermal decomposition of azomethane, CHjN NCHj, at 980 °C. The CH3- radicals were subsequently allowed to react with themselves and with NO in a Knudsen cell that has been described previously [12]. Analysis of intermediates and products was again done by mass spectrometry, using the VIEMS. Calibration of the mass spectrometer with respect to CH,- radicals was carried out by introducing the products of azomethane decomposition directly into the high vacuum region of the instrument. 

Figure 6. Formation of CH3NO during the gas phase reaction of CH3- radicals with NO in a Knudsen cell O, mass 15 A, mass 28 , mass 45. Results were obtained at P(CHj-) = 0.7 mTorr, P(NO) = 1.4 mTorr and an electron impact energy of 19 eV.

The above result was used as a ground-stone of the well known kinetic method of detection which was initially proposed by Myasnikov [75] more than 30 years ago. Above paper dealt with experimental comparison of the change of relative concentration of CH3 radicals in gaseous phase using the stationary values of electric conductivity and initial rate of its change. The experiment yielded perfect coincidence of the measured values. Using methyl radicals as example of adsorption it was established that the resolution of this method was better than 10 particles per cubic centimeter of the ambient volume [75, 76]. 

Excrements show that all the alkyl, hydroxyl, and amine radicals which we have studied considerably reduce the conductivity and increase the work function of oxide semiconductors like ZnO, Ti02, CdO, WO2, M0O3, etc. during chemisorbtion. It should be noted that the revealed effects are rather profound especially if we are dealing with the effect of chemisorbtion of active particles on conductivity of a thin (less than 1 pm) sintered polycrystal semiconductor films. Thus, conductivity of such films in the presence of free CH3-radicals with the concentration of even 10 cm and less may change from initial value by dozens or hundreds percent depending on experimental conditions. 

Fig.4.1. Dependence of the concentration of free CH3 radicals in a quartz cylindrical cell on the distance between the film sensor and the point where the radicals are produced, for different temperatures of the vessel walls 25 C (/), 140 C (2), 24rC O), 269 C (4), and 300 C (5).

As an example the experimental results on heterogeneous recombination of CH3 radicals on glass at different temperatures are plotted on Fig. 4.1. The experimental conditions in this case are chosen in such a way that inequality (4.3) is satisfied (A < 1 cm, y is about 1(H, r = 3 cm). Thus, formula (4.1) holds in this experiment. This conclusion is supported by the fact that for all experimental series the results obtained at different temperatures of the reaction vessel walls are satisfactorily approximated by the same straight line. This means that methyl radicals on glass substrate undergo recombination governed by the first-order kinetics, and the activation energy is close to zero. 

The results on pyrolysis of acetone displayed in Fig. 4.5 are consistent with formula (4.8). Thus, variation of the concentration of free radicals near the sensor surface and, consequently, variation of the value idv/dt)tMi = o as functions of the filament temperature are governed by relation (4.8). As the acetone pressure increases, this relation fails because of fast interaction of CH3 radicals with acetone molecules. 

Fig. 4.27. Variation of electric conductivity of ZnO film under the influence of adsorption of CH3 radicals at room temperature for various pressures of acetone vapours 1,2 - 200 Torr 5-1 Torr 1, 2 - before and after immersion of the film in liquid acetone 4 - the film covered with a liquid layer.

Fig. 12. Internal energy spectrum of the CH2 fragment from photolysis of the CH3 radical at 216.3 nm. The combs above the figure indicate the expected TOFs of H atoms formed, in association with triplet methylene CH2(X3Bi) or singlet CH2( i1Ai) respectively as a function of V2, the vibrational quantum number for their respective bending mode. (From Wilson et al,113)...

Fig. 25. Schematic C%v potential energy surfaces for the CH3S radical as a function of C—S bond length. (From Hsu et a/.,163 Cui et a/.,161 and Bise et a/.164)...

Tram-anular interactions, which would create an active radical site via hydrogen transfer through 98, cannot be invoked to explain the specific loss of a CH3 radical from the ether side chain. This conclusions is based upon the following experimental observations. The radical cation of the tetrafluoro substituted compound 101 eliminates CH3, but loss of CH3 from the para-isomer 102 is not observed. If a transanuiar process according to 97- 98 were operative, then such a reaction is not expected to be suppressed upon substitution of H by F as is known for many examples from the field of photochemistry of fluoro substituted compounds41 (23). 

FIGURE 9.1 EPR spectra of spin adducts recorded during the Fenton reaction in DMSO at different H202 concentrations ([FeClJ = 1 mM), (1), (2), and (3) are OH, OOH, and CH3 radicals, respectively. 

In reactions [5]-[8] pure electron addition occurs, but in reaction [9] addition and dissociative electron capture giving loss of MeO occur concurrently. Furthermore, CH3 radicals are also formed, together, presumably, with (Me0)2P02 this being an alternative dissociative route. Reaction [10] occurs in methanol, there being no clear sign of the parent anion, P(0Me)3 . This protonation step is also accompanied by dissociative electron capture to give P(0Me)2 radicals. 

In conventional experiments the gas and catalyst are maintained at the same temperature. In microwave experiments the power is deposited within the catalyst, which is cooled by the gas flow and thermal conduction to the surroundings. If the catalyst bed is not thick, the gas is always at a lower temperature than the solid catalyst. The increased loss factor of the catalyst favors the formation of CH3 radicals because they are produced at active 02 sites and these specific sites are preferentially excited by the microwave field. Hence the observed enhancement of C2 selectivity is,... 

G. A. Ozin Responding to your first comment. The 2D Cu atom is produced indirectly by 370-400 nm photofragmentation of Cu2 entrapped in solid CHi. In this photolysis we observe rapid bleaching of the Cu2 absorptions, complete quenching of the 2D emissions of Cu atoms, efficient photoproduction of 2S Cu atoms and the observation of some CH3 radicals with trace amounts of H atoms. 

In response to your second enquiry, we do not have any evidence for a reaction of the photoproduced CH3 radical with its hydrogen atom partner (or CHi itself) after photolysis. However,... 

To select between these two alternative structures it was necessary to synthesize a labeled analog. Three hydrogen atoms of the methyl moiety of the ester group were substituted for deuterium. One of the principal pathways of fragmentation of [M N2]+ ions involves the loss of CH3 radical. Since all R substitutes in diazo ketones 4-1 were also methyls it was important to detect what group exactly is eliminated from the [M N2]+ ion. The spectrum of deuterated sample has confirmed that the methyl radical of the ester moiety leaves the parent ion. As a result the cyclic structure 4-2 was selected as the most probable. The ketene structure 4-3 is hardly able to trigger this process, while for heterocyclic ion 4-2 it is highly favorable (Scheme 5.22). 

It is probable that the CH3 radical is the most abundant chain carrier and therefore, neglecting steps (vii) and (viii), the equation may be written as... 

---