Keith D. King

Gas/gas and gas/wall average energy transfer from very low-pressure pyrolysis

Abstract It is shown that data obtained using very low-pressure pyrolysis (VLPP) on the pressure and temperature dependence of unimolecular rate coefficients of reactants with several reaction channels yield average energies transferred in gas/gas and gas/wall collisions (the wall being seasoned quartz at 800–1200 K). The downward average energy transferred, «Δ E a, for chlorocyclobutane/ethylene collisions is found to be 1600 cm −1 at 970 K; «Δ E a for chlorocyclobutane/wall collisions varies from 5000 cm −1 (wall efficiency β w = 0.8) at 930 K to 3500 cm −1 (β w = 0.4) at 1150 K; similar values are found from published data on cycloheptatriene and cyclopropane- d 2 . This indicates that the assumption of unit wall efficiency usually used in fitting VLPP experiments to RRKM theory needs revision.

Phenylgermane as a suitable precursor for laser flash photolysis measurements of germylene kinetics

Abstract R. Becerra, S.E. Boganov, M.P. Egorov, O.M. Nefedov and R. Walsh [Chem. Phys. Lett. 260 (1996) 433] reported the first direct kinetic measurements of germylene, GeH2, using two precursors, 3,4-dimethylgermacyclopentene and phenylgermane. They report that phenylgermane produces anomalously low reaction rate constants. We have re-examined this issue and measure rate constants using phenylgermane that agree with the values reported by Becerra et al. using the precursor 3,4-dimethylgermacyclopentene. The first direct rate constant measurement for GeH2+phenylgermane is reported. All rate constants are found to be independent of the total pressure (measured to 50 Torr). Photolysis of phenylgermane at 248 and 193 nm yields the same values for the rate constants within the experimental uncertainty, suggesting that relaxation of vibrationally excited germylene is not altering the measured reaction rate constants.

Solution of the master equation for unimolecular reactions

A method is given for computing the rate coefficient of a unimolecular reaction as an eigenvalue solution of an integral master equation, based on Nesbets algorithm, which overcomes computational difficulties associated with this problem. An illustrative fit to pressure-dependent data on the pyrolysis of azoethane is presented.

Heterotrophic growth and nutritional aspects of the diatom Cyclotella cryptica (Bacillariophyceae): Effect of some environmental factors

To investigate the nutritional value of the diatom Cyclotella cryptica (Reimann, Lewin, and Guillard) as an alternative feed for use in the aquaculture industry, the heterotrophic growth characteristics and resultant fatty acid profile of the microalga were studied when cultivated under a variety of controlled salinity and temperature conditions. In addition, the effects of pH on the growth characteristics were also studied. The maximum specific growth rate was affected by initial pH and cultivation temperature, but not by salinity. The optimal pH and temperature ranges for growth were 7.2 to 8.1 and 22.5 to 25.0 degrees C, respectively. Lipid accumulation and the fatty acid composition were also affected by cultivation temperature and salinity. The optimal temperature range and salinity level for lipid accumulation were 18.0 to 25.0 degrees C and 11.2 psu, respectively. In all cases the fatty acid distribution was similar, with the most abundant fatty acids being palmitic acid (16:0), palmitoleic acid (16:1 n-7), stearidonic acid (18:4 n-3, SDA), eicosapentaenoic acid (20:5 n-3, EPA), and decosahexaenoic acid (22:6 n-3, DHA).

Gas/gas and gas/wall energy transfer functions in the multichannel thermal decomposition of chloroethane-2-d1

Abstract The two-channel decomposition of chloroethane-2- d 1 (elimination of HCl and of DCl) has been studied using very low-pressure pyrolysis over the temperature range 975–1200 K (gas/wall collisions only) and over the range 1049–1130 K using Kr, Ne and He as bath gases. Fitting the data by solution of the integrodifferential reaction-diffusion master equation gives CH 2 DCH 2 Cl gas/wall downward energy collision transfer values of 6000-3500 cm −1 (975–1200 K) corresponding to collision efficiencies of 0.9-0.5; the wall is seasoned quartz. These collision efficiencies when applied to data obtained under the same conditions for the one-channel decomposition of CH 3 CH 2 Cl give extrapolated high-pressure Arrhenius parameters in excellent agreement with those obtained from conventional kinetic studies. The pressure-dependent data give gas/gas average downward collision energy transfer values of =600 cm −1 (Kr and Ne) and =700 cm −1 (He) over the observed temperature range (the values increasing slightly with increasing temperatures). The gas/gas energy transfer probability function, rather than being exponential in energy difference, is found to vary approximately as the exponential of the cube of the energy difference between initial and final states; the data are sensitive to this functional form since there is only a small difference between critical energies of each channel. Extrapolated high pressure rate coefficients for CH 2 DCH 2 Cl decomposition are 10 13.2 exp (−237 kJ mol −1 / RT ) s −1 (HCl elimination) and 10 13.1 exp (−244 kJ mol −1 / RT ) s −1 (DCl elimination). Unusual behaviour observed in the pressure dependence is interpreted as being caused by highly quantized structure in the microscopic reaction rate coefficients caused by the lack of low frequency ( −1 ) modes in the activated complexes.

Temperature dependence of germylene reactions with acetylene, trimethylsilane, and phenylgermane

Abstract Gas-phase reaction rate constants have been determined over the temperature range 295–436 K for the reactions of germylene, GeH2, with acetylene (GeH2 addition across a triple bond), trimethylsilane (GeH2 insertion into a Si–H bond), and phenylgermane. The room-temperature rate constant for germylene reacting with benzene has been measured and is found to be a factor of ∼300 smaller than that for phenylgermane, indicating that the latter reacts by GeH2 insertion into the Ge–H bonds. A negative temperature dependence is observed in all cases. The activation energies, obtained from weighted linear fits to the data over the experimental temperature range, are −3.5±0.3, −11.0±0.4, and −3.6±0.3 kJ mol−1 for acetylene, trimethylsilane, and phenylgermane, respectively, while the respective frequency factors, log(A/cm3 molecule−1 s−1), are −10.5±0.1, −11.8±0.1, −10.1±0.1.

Vibrational energy transfer in shock‐heated norbornene

Recently, Kiefer et al. [J. H. Kiefer, S. S. Kumaran, and S. Sundaram, J. Chem. Phys. 99, 3531 (1993)] studied shock‐heated norbornene (NB) in krypton bath gas using the laser‐schlieren technique and observed vibrational relaxation, unimolecular dissociation (to 1,3‐cyclopentadiene and ethylene), and dissociation incubation times. Other workers have obtained an extensive body of high‐pressure limit unimolecular reaction rate data at lower temperatures using conventional static and flow reactors. In the present work, we have developed a vibrational energy transfer‐unimolecular reaction model based on steady‐state RRKM calculations and time‐dependent master equation calculations to satisfactorily describe all of the NB data (incubation times, vibrational relaxation times, and unimolecular rate coefficients). The results cover the temperature range from ∼300 to 1500 K and the excitation energy range from ∼1 000 to 18 000 cm−1. Three different models (based on the exponential step‐size distribution) for the a...

Intermolecular energy transfer in two-channel unimolecular reactions: the pyrolysis of 1-iodopropane

Abstract Pressure-dependent unimolecular reaction rate coefficients have been obtained for the two channels of decomposition of 1-iodopropane (dilute in Ne), using very low-pressure pyrolysis (VLPP). The interpretation, taking finite diffusion rates into account, gives convincing evidence for “weak” gas/gas collisions and “strong” gas/wall collisions.

Eigenanalysis of infrared mulitphoton decomposition kinetics

An eigenvalue/eigenvector analysis of the master equation for infrared multiphoton decomposition is used to derive a computational efficient means of analysing experimental data (such as fractional decomposition and, for multichannel reactions, branching ratios) to obtain laser absorption cross sections and information on gas/gas energy transfer. Reaction both during and after the pulse is incorporated, post-pulse collisional relaxation being found to be of both qualitative and quantitative importance in the interpretation of data. It is shown that fractional decomposition and branching ratios are essentially independent of the finer details of pulse shape, provided that the fluence remains fixed; it is am demonstrated that the analysis of the problem in terms of a single eigenvalue is inadequate.

Direct measurement of methylene removal rates by species containing the OH functional group

The technique of laser flash photolysis/laser absorption has been used to obtain absolute removal rate constants for singlet methylene (1CH2) with N2, ketene (CH2CO), H2O, D2O, methanol (CH3OH), ethanol (C2H5OH), and n-propanol (n-C3H7OH) at 296 ± 2 K. The rate constants were found to be (0.130 ± 0.012), (2.38 ± 0.18), (1.60 ± 0.17), (1.05 ± 0.13), (2.20 ± 0.19), (2.61 ± 0.23) and (4.15 ± 0.44)× 10–10 cm3 molecule–1 s–1, respectively. The removal rate constants for the alcohols are consistently greater than those for the corresponding alkanes.