Gas-phase activation

Gas-phase activation above the deposition surface is essential for achieving appreciable diamond growth rates. The various CVD methods differ primarily in the way they produce gas-phase activation. The most abundant carbon-containing gaseous species present in most activated systems are methyl radicals and acetylene molecules which are also considered to be predominant growth precursors for diamond, almost independent of the deposition methods used. However, in systems that dissociate a significant fraction of H2, such as DC plasma arc-jet CVD, carbon atoms, aside from acetylene, are also abundant in the gas phase. 

One of the critical factors affecting growth rates is the gas-phase temperature which can be reached in a CVD method. A comparison of the various CVD methods to each other demonstratesl that typical linear growth rates correlate positively with estimated gas-phase temperatures (Fig. 1), approaching 1 mm h in atmospheric pressure plasma arc-jet CVD with temperatures around 6000 to 7000 K. The partial pressures of various gas species in typical CVD processes have been calculated as a function of 

High rate CVD methods obviously operate at high temperatures where neutral H and C atoms dominate the gas phase. In the complex C-H-0 systems, CO remains present at high temperatures, but the CO partial pressure hardly varies with temperature from 1000 to 6000 K and hence would not explain oily rate choligc. 

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Table 3. Gas phase activated carbon consumption. Reprinted from [11], copyright 1992...

Catalyst Activation Gas phase activation of supported DENs was examined using in-situ FTIR spectroscopy and FTIR spectroscopy of adsorbed CO. For in-situ dendrimer decomposition studies, the spectra were collected under a gas flow composed of 20% 02/He or 20% H2/He. The supported DEN sample was pressed into a self-supporting wafer, loaded into a controlled atmosphere IR cell, and collected as the sample background. The temperature was raised stepwise and spectra were collected at each temperature until little or no change was observed. After oxidation, the sample was reduced in 20% H2/He flow with various time/temperature combinations. The sample was then flushed with He for lhr at the reduction temperature. After cooling under He flow, a background spectrum was collected at room temperature. A 5% CO/He mixture was flowed over the sample for 15 minutes, followed by pure He. IR spectra of CO adsorbed on the catalyst surface were collected after the gas phase CO had been purged from the cell. 

Bourn (107) as 10.8 kcal mole-1 (AH4 temperature range 156—246 K solution of cyclohexane in carbon disulfide). By applying an interesting trapping technique Anet and coworkers (110) have recently measured a value of 5.3 kcal mole-1 for the barrier of the reverse twist/chair process (AH4 temperature range 72.5—74.0 K solid state). Our calculated (for the gas phase) activation enthalpies AH4 for the former process are at 73,... 

Ah initio calculations to map out the gas-phase activation free energy profiles of the reactions of trimethyl phosphate (TMP) (246) with three nucleophiles, HO, MeO and F have been carried out. The calculations revealed, inter alia, a novel activation free-energy pathway for HO attack on TMP in the gas phase in which initial addition at phosphorus is followed by pseudorotation and subsequent elimination with simultaneous intramolecular proton transfer. Ah initio calculations and continuum dielectric methods have been employed to map out the lowest activation free-energy profiles for the alkaline hydrolysis of a five-membered cyclic phosphate, methyl ethylene phosphate (247), its acyclic analogue, trimethyl phosphate (246), and its six-membered ring counterpart, methyl propylene phosphate (248). The rate-limiting step for the three reactions was found to be hydroxyl ion attack at the phosphorus atom of the triester. ... 

The term exp(— zkZb e2/4n o rrkT) modifies both the gas phase collision rate and the gas phase activation energy in a complex manner. Equation (7.3) cannot be written in a simple manner as... 

If the gas phase activity of the host is controlled by the presence of a pure condensed phase, solid or liquid, the equilibrium between host and guest in a stoichiometric clathrate can be described in terms of the gas phase pressure of the guest. This is, in effect, a vapor pressure for the guest. At higher pressures the guest will condense to form clathrate, and at lower pressures the clathrate will decompose. Temperature variation of this pressure will follow the Clapeyron equation which, with the usual assumptions (ideal gas behavior of the vapor and negligible volume of the condensed phase), reduces to the Clausius-Clapeyron equation ... 

A. Filippi, M. Speranza, Gas-phase activation and reaction dynamics of chiral ion-dipole complexes. Chem. Eur. J. 9, 5274—5282 (2003)... 

The non-diamond carbon phase in polycrystalline diamond films (often referred to as graphite, although this conclusion is far from accurate [23]) is first and foremost the disordered carbon in the intercrystallite boundaries. Their exposure to the film surface can be visualized by using a high-resolution SEM techniques [24] the intercrystallite boundaries thickness comes to a few nanometers. In addition to the intercrystallite boundaries, various defects in the diamond crystal lattice contribute to the non-diamond carbon phase, not to mention a thin (a few nanometers in thickness) amorphous carbon layer on top of diamond. This layer would form during the latest, poorly controlled stage of the diamond deposition process, when the gas phase activation has ceased. The non-diamond layer affects the diamond surface conduc-... 

Table 5 contains the gas-phase activation parameters for syru- anti conformational exchange available for six primary alkyl nitrites. Two trends are apparent in the gas-phase activation parameters listed in this table. First, the barrier to sync anti... 

The endo/exo selectivity is also well predicted by the PCM and explicit water computations. The difference in the s-ds endo and exo gas-phase activation enthalpies is only 0.1 kcal mol (Table 7.5). This difference increases to 1.1 kcal mol with the PCM calculation or explicit water compntations. 

Fig. 5.7-2. Flow Chart of the Manufacture of Activated Carbon from Carbonized Feedstocks by Gas-Phase Activation...

One might still assume that the above analysis is inconclusive, considering the arguments of Ref. 87 that the EVB approach is not reliable, in view of what they considered as the gas phase results of Ref. 86. However, first the EVB is probably still the most reliable method in evaluating the catalytic effect (see discussion in Section 2) and is clearly more reliable than the QM/MM approach of Ref. 87, whose results changed drastically from their previous value.85 Second, what was considered as the gas phase results of Ref. 86 does not reflect the actual gas phase activation energy (since it is not evaluated at the gas phase minimum). This issue is further clarified in Ref. 88. Third, the analysis of Equation (20) does not depend on the EVB surface, but only on the use of reasonable solute charges, and mainly on the ability to perform reliable electrostatic calculations. 

Calculations at the B3LYP/6-31G level were used to show how hydrogen bond formation influences the chemical reactivity of ketones.70 The effect of the chloroform on the activation energies was modelled by means of discrete-continuum models. Explicit hydrogen bond formation to chloroform lowers the gas-phase activation barrier. A DFT analysis of the global electrophilicity of the reagents provided a sound explanation of the catalytic effects of chloroform (see Table 6 and Chart 3). The electrophilicity of acetone... 

Prior to the main study the experimental conditions for performing kinetic measurements were established. CH4 conversions of below 10% (reactants diluted with H O) could be obtained at temperatures below 923 K. The carbon limits, which correspond to the maximum partial pressure and minimum temperature under which formation of carbon could be observed as a weight increase by the microbalance, were also determined. In addition, test runs at the most severe conditions, i.e. 1073 K and 4.3 bar CH4, revealed that the support as well as the quartz and the alumina surfaces are sufficiently inert, and that gas phase activation of CH4 is not important. 

In the theory of active intermediates, decomposition of the intermediate does not occur instantaneously after internal activation of the molecule rather, there is a time lag. although infinite,simally small, during which the species remains activated, Zewail s work was the first definitive proof of a gas-phase active inieniiediate that exists for an infinitesimally shon time. Because a reactive intermediate reacts virtually as fast as U is formed, the net rate of formation of an active intermediate (e,g A ) is zero. i.e.. 

The computed activation energy of 19.6 kcal/mol in vacuum at 298 K is in excellent agreement with the experimental value of 19.7 kcal/mol [70]. In toluene, the experimental activation enthalpy is 15.8 1.4 kcal/mol, with an activation entropy of —38 4 cal/mol K [71]. Therefore the computed 19.1 kcal/mol gas phase activation enthalpy of acrolein and butadiene should be lowered by 3.3 kcal/mol in toluene. In a related Diels-Alder reaction of cyclopentadiene and methyl acrylate, the experimental activation enthalpy is 15.1 kcal/mol in toluene, which is ca. 0.7 kcal/mol lower than that reported for acrolein and butadiene [81],... 

An activation enthalpy decreasing with increasing solvent polarity is provided by the PCM method, as shown in Tables 4 and 5. The gas phase NC activation enthalpies decrease by 1.5 and 3.8 kcal/mol in benzene and water, respectively. In benzene (dielectric similar to toluene), the computed activation enthalpy is 17.6 or 1.5 kcal/mol lower than the gas phase activation enthalpy. 

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