Reaction Propensities Research Articles

Oxidation of deuterium-, acetylene-, and ethylene-terminated Si(100) by atomic oxygen

The oxidation of deuterium-, acetylene-, and ethylene-terminated Si(100) by gas-phase atomic oxygen is evaluated using Auger electron spectroscopy (AES). The atomic oxygen is generated by the 157 nm laser photolysis of O2, which results in nominally a 50/50 mix of the ground (3P) and electronically excited (1D) state species. O atom adsorption probabilities for oxygen coverages in the 1–3 ML regime are estimated to be on the order of 0.1 for these adlayer systems. For both C2H2 and C2H4 adlayers no effective loss of surface carbon is observed, indicating that there first must be extensive oxidation of the near-surface region of silicon (greater than about 3 ML) prior to the removal of chemisorbed carbon on Si(100). The O/Si AES ratio for the O atom oxidation for oxygen coverages of about 1–3 ML in the near-surface regime is found to be linear with O atom exposure for all three adlayers, with the propensity for reaction being given by the relative slopes of 1.0:1.4:2.0 for D-, C2H2-, and C2H4-terminated Si(100), respectively. The relatively low rate for O atom oxidation of D-terminated Si(100) suggests that the presence of defects facilitates the oxidation. While the difference in the oxidation rates for the C2H2 and C2H4 adlayers is statistically significant, this is most likely not due to initial adlayer coverage effects; the specific cause(s) of this difference, however, is not established.

Comparison of the effects of ultrasound and mechanical agitation on a reacting solid-liquid system

Ultrasound has been shown to have desirable effects on both homogeneous and heterogeneous reactions, such as increasing the conversion, enhancing the selectivity, and improving the yield. Enhancements due to ultrasound may be attributed to its chemical or mechanical effects, or to both simultaneously. The chemical effects of ultrasound are due to the implosion of microbubbles, generating free-radicals with a great propensity for reaction. Mechanical effects are caused by shock waves formed during symmetric cavitation, or by microjets formed during asymmetric cavitation. Research emphasis in this area has largely been restricted to the chemical effects of ultrasound and physical descriptions of cavitation. The present study is among a very few to attempt a chemical engineering analysis of the problem. More specifically, it seeks to discern the mechanisms behind the mechanical effects by selecting a model solid-liquid noncatalytic reacting system in which the chemical effects of ultrasound are negligible. Using several investigative techniques, the expected effects of ultrasound are observed, such as the degradation of the solid reactant, leading to increased surface area. More importantly, some novel findings of the effects of ultrasound on mass transfer parameters are reported. Results clearly show that ultrasound enhances the intrinsic mass transfer coefficient as well as the effective diffusivity of the organic reactant through the ionic lattice of the product layer. Discerning the effects of ultrasound on mass transfer parameters, as done in this paper, is an important step towards understanding the effects of ultrasound and determining its applications in the chemical industry.

Sequence-selective guanine reactivity by duocarmycin A.

High selectivity for covalent reaction at adenine N-3 within duplex DNA is a distinguishing feature of the CC-1065 and duocarmycin classes of natural products. Studies of the base and sequence selectivity exhibited by duocarmycins and CC-1065-based alkylating agents have focused on characterization of the predominant covalent adenine adducts that are formed. While information about minor DNA reaction products could provide valuable insights to our understanding the DNA recognition and reactivity properties of these agents, little characterization of such adducts by these agents has appeared in the literature. To broaden our structure-reactivity understanding of these DNA alkylating compounds, comparative investigations of the covalent sequence selectivity exhibited by compounds containing altered cyclopropapyrroloindole (CPI) alkylating subunits such as duocarmycin A were undertaken using the DNA polymerase inhibition assay. We were surprised to identify with this assay a DNA sequence with an unusual propensity for covalent reaction with duocarmycin A at a guanine nucleotide. Using the heat strand breakage assay with a duplex oligonucleotide containing this interesting sequence, we confirmed the site of alkylation to be the indicated guanine in the sequence 5'-CGCGTTG*GGAG-3'. The trimethoxyindole-CPI analog of duocarmycin A does not alkylate this guanine, suggesting that there are interesting features to the duplex recognition/reactivity exhibited by duocarmycin A. Herein we describe our identification of the first DNA sequence which covalently reacts with duocarmycin A at a guanine nucleotide in the absence of additional minor groove binding agents.

The effect of uniaxial stress on dislocation reactions at coherent twin boundaries in f.c.c. metals

Dislocations which react at coherent twin boundaries in f.c.c. metals can form a variety of products depending on the Burgers vectors of the reactants and the interactions between the leading partials. When reactants have longrange attractive forces, the reaction can lead to complete annihilation, to the formation of a single extended mixed dislocation or to the formation of twinning dislocations lying in the twin boundary. Reactants which have long-range repulsive forces can form stable products in two cases if the applied stress forces the reactants sufficiently close for reaction to occur. These products can be Shockley partials lying on the slip plane of a reactant screw dislocation, Frank partials lying in a twin and matrix separated by the twin boundary, or three-dislocation barriers consisting of a stair-rod dislocation and two Shockley partials from the original reactants. Suitable orientation of an applied stress can favor some cases over others. The propensity for reaction and the nature of the products formed for various orientations of a uniaxial stress are examined. Results are presented in terms of stereographic projections which illustrate orientations where various results can be obtained.

Vibrational excitation of the allene–fluorine reaction in cryogenic matrices: Possible mode selectivity

The infrared induced reaction of allene (C3H4) and fluorine in N2, Ar, Kr, and Xe matrices at 12 K has been studied. The reaction is promoted by the selective vibrational excitation of C3H4 ⋅ F2 reactive pairs and the course of the reaction followed by infrared spectroscopy. Four products are detected, consistent with the concerted addition of F2 across the C–C double bond to form vibrationally excited cis- and gauche-CH2CFCH2F which may then eliminate HF to yield either CH≡CCH2F ⋅ HF or CH2=C=CHF ⋅ HF pairs. The relative abundance of each product depends markedly upon the matrix. Selective vibrational photoisomerization of CH2=CFCH2F was used to distinguish and assign spectral features due to the cis- and gauche-rotameters as well as to provide evidence about the torsional barrier in the matrix. In a particular matrix, the allene–fluorine reaction product distribution is independent of the exciting frequency, with measurements at 1679, 1953, 1999, and 3076 cm−1. However, the relative quantum yield at 3076 cm−1 is three orders of magnitude higher than observed at the lower frequencies. Furthermore, the quantum yields at 1680, 1953, and 1999 cm−1 show differences that implicate mode selectivity in the allene–fluorine reaction. The CH2 rocking overtone 2ν9 at 1999 cm−1 and perhaps as well the CH2 bending overtone 2ν10 at 1679 cm−1 show a higher reaction propensity than that of the asymmetric skeletal stretching vibration ν6 at 1953 cm−1.