Gas Phase Diagnostics of CVD Diamond Growth From Chloromethane and Chloroform

Low energy electron impact ionization (LEEII) time of flight mass spectrometry (TOFMS) has been used to interrogate gases sampled through an orifice from a diamond growth surface. Methane, chloromethane and chloroform precursor gases were examined to produce electron energy plots of the fragment ions that occur using LEEII. Filament temperature studies were completed covering temperatures from room temperature to over 2000 °C, and the results were compared to the LEEII plots. The monochloromethyl radical has been detected in the experiments using chloroform and the effect of a pretreatment with this radical present has been shown to enhance diamond nucleation. Scanning electron microscopy (SEM) and Raman spectroscopy have been used to confirm diamond growth

Diamond Growth Reactor Chemistry and Film Nucleation Enhancement Using Chlorinated Hydrocarbons

The chemistry of diamond film growth from chlorinated hydrocarbons has been investigated using a hot filament reactor coupled to an orifice sampling mass spectrometer. The relative concentrations of the species present near the growth surface have been determined as a function of filament temperature for dilute mixtures of CH4, CH3Cl, CH2Cl2 and CHCl3 in H2. Mass spectral analysis indicated that chlorinated hydrocarbons are sequentially dechlorinated in the presence of hydrogen at moderate reactor temperatures. A dark film was deposited on all surfaces of the reactor during studies of this dechlorination of CHCl3. Raman analysis indicated that these deposits are small particle polycrystalline graphite. Pretreatment of a Si 〈111〉 substrate under conditions that create the graphite deposit is seen to produce a significant nucleation enhancement of diamond when followed by growth at a higher temperature. Chemical mechanisms for some of these processes are proposed

Mechanistic evidence for a front-side, SNi-type reaction in a retaining glycosyltransferase

A previously determined crystal structure of the ternary complex of trehalose-6-phosphate synthase identified a putative transition state–like arrangement based on validoxylamine A 6?-O-phosphate and uridine diphosphate in the active site. Here linear free energy relationships confirm that these inhibitors are synergistic transition state mimics, supporting front-face nucleophilic attack involving hydrogen bonding between leaving group and nucleophile. Kinetic isotope effects indicate a highly dissociative oxocarbenium ion–like transition state. Leaving group 18O effects identified isotopically sensitive bond cleavages and support the existence of a hydrogen bond between the nucleophile and departing group. Brønsted analysis of nucleophiles and Taft analysis highlight participation of the nucleophile in the transition state, also consistent with a front-face mechanism. Together, these comprehensive, quantitative data substantiate this unusual enzymatic reaction mechanism. Its discovery should prompt useful reassessment of many biocatalysts and their substrates and inhibitor