M. E. Umstead

Dynamics of CO formation in the photodissociation of HNCO and CH2CO at 193nm

Abstract The photodissociation of isocyanic acid (HNCO) and ketene (CH 2 CO) at 193 nm was investigated using an ArF laser to dissociate the carbonyl compound and a CO laser to probe the resulting vibrationally excited CO. The dissociation of HNCO at 193 nm produces CO with an average vibrational energy of 4.6 ± 0.3 kcal/mol. The dissociation Gf CH 2 CO at 193 nm produces CO with an average vibrational energy of 6.4 ± 0.8 kcal/mol. The observed CO vibrational energy distributions were found to be in close agreement with those predicted statistically assuming NH(a 1 Δ) + CO and CH 2 ( 1 A 1 ) + CO were the photodissociation products.

The dynamics of reactions of O(3P) atoms with allene and methylacetylene

Abstract The reactions of O( 3 P) atoms with allene and methylacetylene: O+CH 2 =C=CH 2 CO+C 2 H 4 ,Δ H 1 0 = −119.4 kcal/mole, O+CH 3 -C CH CO+C 2 H 4 ,Δ H 2 0 = −117.8 kcal/mole were studied at 293 K with a CO laser resonant absorption and a discharge-flow GC-sampling method. The CO formed in reaction (1) was found to have a vibrational temperature of 5100 ± 100 K, compared with 2400 ± 200 K in (2). The good agreement between the observed CO vibrational distributions and those predicted by simple statistical models indicates that the reaction energies were completely randomized.The present results also showed unambiguously that CH 3 CH, instead of C 2 H 4 , was produced initially in reaction (2).

Kinetics and mechanism of the thermal decomposition of dimethylnitramine at low temperatures

Abstract Dimethylnitramine (DMNA) was pyrolyzed between 466 and 524 K at about 475 Torr pure DMNA pressure in static cells. A radical mechanism was proposed and computer-modeled to account for the disappearance of DMNA and the production of (CH3)2NNO and CH3NO2. The rate constant for DMNA decomposition into (CH3)2N and NO2, based on these low-temperature results and other high-temperature shock tube data, covering 460–960 K, can be given by k1 = 1015.9±0.2 exp(−22,000±200/T) sec−1. This result leads to values for the N-N bond energy of 43.3±0.5 kcal/mole and the heat of formation of the (CH3)2N radical, 35±2 kcal/mole at 298 K. Kinetic modeling of the CH3NO2 and (CH3)2NNO production profiles has been carried out.

CO product energy distribution in the photodissociation of methylketene and acrolein at 193 nm

CO product vibrational energy distributions in the photodissociation of the two C3H4O isomers, methylketene (CH3CHCO) and acrolein (CH2CHCHO), at 193 nm have been measured by CO laser resonance absorption. The CO from methylketene was found to be vibrationally excited up to v=7, and from acrolein v=6, with average vibrational energies of 3.4±0.3 and 2.7±0.7 kcal/mol, respectively. The similarities observed in the appearance times and in the vibrational energy content of the CO formed in the two systems support our previous conclusion that in the case of acrolein isomerization to methylketene takes place prior to the dissociation process: CH2CHCHO+hν→CH3CHCO*†→CH3CH+CO†. The CO vibrational energy distributions observed in both systems agree closely with the statistical distribution predicted assuming that ethylidene rather than ethylene is formed in the photodissociation reaction.

The dynamics of CO production from the O(3P) + methylacetylene reaction

Abstract The reaction of O(3P) atoms with methylacetlyene was studied at three temperatures between 260 and 350 K by means of a CO laser resonant absorption method. The CO formed in the reaction had a Boltzmann vibrational temperature of about 2400 K and carried only 1.9 ± 0.2% of the total available reaction energy, independent of the initial temperature of the system. Good agreement was obtained between the observed CO population distribution and that predicted by a simple statistical model, with the assumption that CH2CH, and not C3H4, was formed initially in the reaction. The kinetics of the CO formation along with the results from the statistical indicate that the dominant primary step of the reaction is: O(3P) + CH3C2H → CH3CH + CO.

Laser Applications To Heterogeneous Catalysis: Reactant Excitation And Product Diagnostics

Irradiation of gaseous NO2 with the 488 nm line of an argon ion laser during its reaction with C2H4 over a Pt catalyst at 250 C resulted in up to a fourfold increase in the CO2 product yield. This enhancement is believed to result from the reaction of vibrationally excited NO2 with adsorbed C2H4 or a species derived from it. The observed effect disappeared after a period of time due to surface poisoning. Hydroxyl radicals have been detected leaving the surface of Pt and Rh-Pt catalysts during the reaction of H2 and 02 at 600-800 C. The OH radical was detected by its fluorescence at 340 nm induced by a dye laser, both in the gas phase and in an Ar matrix at 10 K. The activation energies for OH production from Pt and Rh-Pt surfaces have been determined.

The dynamics of CO production from the reaction of O(3P) with 1-and 2-butyne

Abstract The reactions of 1and 2butyne with O(3P) were studied at 298 K by means of a CO laser resonant absorption technique. The CO formed O(3P)+CH3CH2CCH→CC3H6H3CH2CH+CO O(3P)+CH3CCCH3→CH3CC3H6CH3+CO From the kinetic modeling of the observed rates of CO formation, the rates of these reactions were found to be 5.0 × 1011 and 1.6 × 1012 ml

Photonitration of hydrocarbons with lasers

The photonitration of isobutane (i-C 4 H 10 ) has been investigated in the 458-515 nm region with an argon-ion laser as the radiation source, t-nitrobutane (t-C 4 H 9 NO 2 ) was the major product. Its rate of production was linear with laser intensity and increased with increasing photon energy. Computer modeling of possible reaction steps indicated that the reaction is initiated by the direct abstraction of hydrogen from i-C 4 H 10 by electronically excited NO 2 .

Laser-induced reactions of NO2 in the visible region. I: Kinetic modeling of nitrobutane formation in the NO2-isobutane system

The reaction of NO2 with isobutane, induced by 488 nm laser radiation, to form 2-nitro-2-methylpropane has been investigated and the results computer-modeled according to two possible reaction mechanisms. The first scheme involves the direct abstraction of H from isobutane by vibronically excited NO2 (NO2*Δ), and the second, abstraction by an intermediate NO3 radial produced by NO2*Δ+NO2. The modeling results strongly support the NO2*Δ scheme as the dominant reaction mechanism.

Chemical Applications of Lasers

The laser has become one of the most useful and powerful tools in chemistry as well as in many other branches of science and technology, due to its intensity, monochromaticity and tunability. In the Chemistry Division at the Naval Research Laboratory, we have applied a variety of lasers to different areas of chemical research, from reaction dynamics to homogeneously catalyzed polymerization. In this presentation, we shall discuss the results of our recent studies in the following areas of applications.