Nonadiabatic decomposition of gas-phase RDX through conical intersections: an ONIOM-CASSCF study.

Topographical exploration of nonadiabatically coupled ground- and excited-electronic-state potential energy surfaces (PESs) of the isolated RDX molecule was performed using the ONIOM methodology: Computational results were compared and contrasted with the previous experimental results for the decomposition of this nitramine energetic material following electronic excitation. One of the N-NO(2) moieties of the RDX molecule was considered to be an active site. Electronic excitation of RDX was assumed to be localized in the active site, which was treated with the CASSCF algorithm. The influence of the remainder of the molecule on the chosen active site was calculated by either a UFF MM or RHF QM method. Nitro-nitrite isomerization was predicted to be a major excited-electronic-state decomposition channel for the RDX molecule. This prediction directly corroborates previous experimental results obtained through photofragmentation-fragment detection techniques. Nitro-nitrite isomerization of RDX was found to occur through a series of conical intersections (CIs) and was finally predicted to produce rotationally cold but vibrationally hot distributions of NO products, also in good agreement with the experimental observation of rovibrational distributions of the NO product. The ONIOM (CASSCF:UFF) methodology predicts that the final step in the RDX dissociation occurs on its S(0) ground-electronic-state potential energy surface (PES). Thus, the present work clearly indicates that the ONIOM method, coupled with a suitable CASSCF method for the active site of the molecule, at which electronic excitation is assumed to be localized, can predict hitherto unexplored excited-electronic-state PESs of large energetic molecules such as RDX, HMX, and CL-20. A comparison of the decomposition mechanism for excited-electronic-state dimethylnitramine (DMNA), a simple analogue molecule of nitramine energetic materials, with that for RDX, an energetic material, was also performed. CASSCF pure QM calculations showed that, following electronic excitation of DMNA to its S(2) surface, decomposition of this molecule occurs on its S(1) surface through a nitro-nitrite isomerization producing rotationally hot and vibrationally cold distributions of the NO product.

Experimental and theoretical studies of reactions of neutral vanadium and tantalum oxide clusters with NO and NH3.

Reactions of neutral vanadium and tantalum oxide clusters with NO, NH(3), and an NO/NH(3) mixture in a fast flow reactor are investigated by time of flight mass spectrometry and density functional theory (DFT) calculations. Single photon ionization through a 46.9 nm (26.5 eV) extreme ultraviolet (EUV) laser is employed to detect both neutral cluster distributions and reaction products. Association products VO(3)NO and V(2)O(5)NO are detected for V(m)O(n) clusters reacting with pure NO, and reaction products, TaO(3,4)(NO)(1,2), Ta(2)O(5)NO, Ta(2)O(6)(NO)(1-3), and Ta(3)O(8)(NO)(1,2) are generated for Ta(m)O(n) clusters reacting with NO. In both instances, oxygen-rich clusters are the active metal oxide species for the reaction M(m)O(n)+NO→M(m)O(n)(NO)(x). Both V(m)O(n) and Ta(m)O(n) cluster systems are very active with NH(3). The main products of the reactions with NH(3) result from the adsorption of one or two NH(3) molecules on the respective clusters. A gas mixture of NO:NH(3) (9:1) is also added into the fast flow reactor: the V(m)O(n) cluster system forms stable, observable clusters with only NH(3) and no V(m)O(n)(NO)(x)(NH(3))(y) species are detected; the Ta(m)O(n) cluster system forms stable, observable mixed clusters, Ta(m)O(n)(NO)(x)(NH(3))(y), as well as Ta(m)O(n)(NO)(x) and Ta(m)O(n)(NH(3))(y) individual clusters, under similar conditions. The mechanisms for the reactions of neutral V(m)O(n) and Ta(m)O(n) clusters with NO/NH(3) are explored via DFT calculations. Ta(m)O(n) clusters form stable complexes based on the coadsorption of NO and NH(3). V(m)O(n) clusters form weakly bound complexes following the reaction pathway toward end products N(2)+H(2)O without barrier. The calculations give an interpretation of the experimental data that is consistent with the condensed phase reactivity of V(m)O(n) catalyst and suggest the formation of intermediates in the catalytic chemistry.

Unimolecular decomposition of tetrazine-N-oxide based high nitrogen content energetic materials from excited electronic states.

Unimolecular excited electronic state decomposition of novel high nitrogen content energetic molecules, such as 3,3(')-azobis(6-amino-1,2,4,5-tetrazine)-mixed N-oxides (DAATO(3.5)), 3-amino-6-chloro-1,2,4,5-tetrazine-2,4-dioxide (ACTO), and 3,6-diamino-1,2,4,5-tetrazine-1,4-dioxde (DATO), is investigated. Although these molecules are based on N-oxides of a tetrazine aromatic heterocyclic ring, their decomposition behavior distinctly differs from that of bare tetrazine, in which N(2) and HCN are produced as decomposition products through a concerted dissociation mechanism. NO is observed to be an initial decomposition product from all tetrazine-N-oxide based molecules from their low lying excited electronic states. The NO product from DAATO(3.5) and ACTO is rotationally cold (20 K) and vibrationally hot (1200 K), while the NO product from DATO is rotationally hot (50 K) and vibrationally cold [only the (0-0) vibronic transition of NO is observed]. DAATO(3.5) and ACTO primarily differ from DATO with regard to molecular structure, by the relative position of oxygen atom attachment to the tetrazine ring. Therefore, the relative position of oxygen in tetrazine-N-oxides is proposed to play an important role in their energetic behavior. N(2)O is ruled out as an intermediate precursor of the NO product observed from all three molecules. Theoretical calculations at CASMP2/CASSCF level of theory predict a ring contraction mechanism for generation of the initial NO product from these molecules. The ring contraction occurs through an (S(1)/S(0))(CI) conical intersection.

Photodissociation dynamics of nitromethane at 226 and 271 nm at both nanosecond and femtosecond time scales.

Photodissociation of nitromethane has been investigated for decades both theoretically and experimentally; however, as a whole picture, the dissociation dynamics for nitromethane are still not clear, although many different mechanisms have been proposed. To make a complete interpretation of these different mechanisms, photolysis of nitromethane at 226 and 271 nm under both collisional and collisionless conditions is investigated at nanosecond and femtosecond time scales. These two laser wavelengths correspond to the pi* <-- pi and pi* <-- n excitations of nitromethane, respectively. In nanosecond 226 nm (pi* <-- pi) photolysis experiments, CH(3) and NO radicals are observed as major products employing resonance enhanced multiphoton ionization techniques and time-of-flight mass spectrometry. Additionally, OH and CH(3)O radicals are weakly observed as dissociation products employing laser induced fluorescence spectroscopy; the CH(3)O product is only observed under collisional conditions. In femtosecond 226 nm experiments, CH(3), NO(2), and NO products are observed. These results confirm that rupture of C-N bond should be the main primary process for the photolysis of nitromethane after the pi* <-- pi excitation at 226 nm, and the NO(2) molecule should be the precursor of the observed NO product. Formation of the CH(3)O radical after the recombination of CH(3) and NO(2) species under collisional conditions rules out a nitro-nitrite isomerization mechanism for the generation of CH(3)O and NO from pi pi* CH(3)NO(2). The OH radical formation for pi pi* CH(3)NO(2) should be a minor dissociation channel because of the weak OH signal in both nanosecond and femtosecond (nonobservable) experiments. Single color femtosecond pump-probe experiments at 226 nm are also employed to monitor the dynamics of the dissociation of nitromethane after the pi* <-- pi excitation. Because of the ultrafast dynamics of product formation at 226 nm, the pump-probe transients for the three dissociation products are measured as an autocorrelation of the laser pulse, indicating the dissociation of nitromethane in the pi pi* excited state is faster than the laser pulse duration (180 fs). In nanosecond 271 nm (pi* <-- n) photolysis experiments, pump-probe experiments are performed to detect potential dissociation products, such as CH(3), NO(2), CH(3)O, and OH; however, none of them is observed. In femtosecond 271 nm laser experiments, the nitromethane parent ion is observed with major intensity, together with CH(3), NO(2), and NO fragment ions with only minor intensities. Pump-probe transients for both nitromethane parent and fragment ions at 271 nm excitation and 406.5 nm ionization display a fast exponential decay with a constant time of 36 fs, which we suggest to be the lifetime of the excited n pi* state of nitromethane. Combined with the 271 nm nanosecond pump-probe experiments, in which none of the CH(3), NO(2), CH(3)O, or OH fragment is observed, we suggest that all the fragment ions generated in 271 nm femtosecond laser experiments are derived from the parent ion, and dissociation of nitromethane from the n pi* excited electronic state does not occur in a supersonic molecular beam under collisionless conditions.

Experimental and theoretical exploration of the initial steps in the decomposition of a model nitramine energetic material: dimethylnitramine.

Decomposition of dimethylnitramine (DMNA, (CH(3))(2)NNO(2)) has been studied extensively over the past decades. Although several different mechanisms have been proposed for the initial decomposition of DMNA, the dominant decomposition channel is still far from fully understood. In this report, we collect all the results reported in the literature, along with our new experimental and theoretical results, into a single reference for a sensible comparison in order to reach a general conclusion on DMNA decomposition. In this effort, nanosecond laser, energy resolved spectroscopy and complete active space self-consistent field (CASSCF) calculations are employed. The parent DMNA molecule is electronically excited using two different UV excitation wavelengths, 226 and 193 nm, to initiate the decomposition process. The NO molecule is observed as a major decomposition product with relatively hot (120 K) rotational and cold vibrational distributions by both time-of-flight mass spectrometry and laser induced fluorescence spectroscopy. On the basis of the experimental observations, a nitro-nitrite isomerization mechanism is predicted to be the major channel of decomposition of DMNA in the excited electronic state with a minor contribution from the HONO elimination mechanism. The branching ratio between nitro-nitrite isomerization and HONO elimination channels is estimated to be approximately 1:0.04. CASSCF calculations show that surface crossing (conical intersection) between upper and lower electronic states along the nitro-nitrite isomerization reaction coordinate plays an important role in the overall decomposition of DMNA. Presence of such an (S(2)/S(1))(CI) conical intersection in the nitro-nitrite isomerization reaction coordinate provides a direct nonadiabatic decomposition pathway from the Franck-Condon point of the S(2) surface, which is experimentally accessed by 226 nm photoexcitation. This excited state isomerization takes place through a loose geometry for which the NO(2) moiety interacts with the (CH(3))(2)N moiety from a long distance (approximately 2.8 A); however, in the ground electronic state, a similar (S(1)/S(0))(CI) conical intersection in this nitro-nitrite isomerization reaction coordinate hinders the isomerization exit channel, rendering NO(2) elimination as the major thermal decomposition channel of DMNA.

Reaction of carbon monoxide and hydrogen on neutral Nb8 clusters in the gas phase.

Reactions of neutral V(n), Nb(n), and Ta(n) metal clusters (n< or =11) with CO+H(2) mixed gases and CH(3)OH in a flow tube reactor (1-50 Torr) are studied by time of flight mass spectroscopy and density functional theory calculations. Metal clusters are generated by laser ablation, and reactants and products are ionized by low fluence (approximately 200 microJ/cm(2)) 193 nm excimer laser light. Nb(n) clusters exhibit strong size dependent reactivity in reactions both with CO+H(2) and CH(3)OH compared with V(n) and Ta(n) clusters. A "magic number" (relatively intense) mass peak at Nb(8)COH(4) is observed in the reaction of Nb(n) clusters with CO+H(2), and CH(3)OH is suggested to be formed. This feature at Nb(8)COH(4) remains the most intense peak independent of the relative concentrations of CO and H(2) in the flow tube reactor. No other Nb(n), Ta(n), or V(n) feature behaves in this manner. In reactions of CH(3)OH with metal clusters M(n) (M=V, Nb, and Ta, n=3-11), nondehydrogenated products M(n)COH(4)/M(n)CH(3)OH are only observed on Nb(8) and Nb(10), whereas dehydrogenated products M(n)CO/CM(n)O are observed for all other clusters. These observations support the suggestion that CH(3)OH can be formed on Nb(8) in the reaction of Nb(n) with CO+H(2). A reaction mechanism is suggested based on the experimental results and theoretical calculations of this work and of those in the literature. Methanol formation from CO+H(2) on Nb(8) is overall barrierless and thermodynamically and kinetically favorable.

Excited electronic state decomposition of furazan based energetic materials: 3,3'-diamino-4,4'-azoxyfurazan and its model systems, diaminofurazan and furazan.

We report the first experimental and theoretical study of gas phase excited electronic state decomposition of a furazan based, high nitrogen content energetic material, 3,3'-diamino-4,4'-azoxyfurazan (DAAF), and its model systems, diaminofurazan (DAF) and furazan (C2H2N2O). DAAF has received major attention as an insensitive high energy explosive; however, the mechanism and dynamics of the decomposition of this material are not clear yet. In order to understand the initial decomposition mechanism of DAAF and those of its model systems, nanosecond energy resolved and femtosecond time resolved spectroscopies and complete active space self-consistent field (CASSCF) calculations have been employed to investigate the excited electronic state decomposition of these materials. The NO molecule is observed as an initial decomposition product from DAAF and its model systems at three UV excitation wavelengths (226, 236, and 248 nm) with a pulse duration of 8 ns. Energies of the three excitation wavelengths coincide with the (0-0), (0-1), and (0-2) vibronic bands of the NO A 2Sigma+<--X 2Pi electronic transition, respectively. A unique excitation wavelength independent dissociation channel is observed for DAAF, which generates the NO product with a rotationally cold (20 K) and a vibrationally hot (1265 K) distribution. On the contrary, excitation wavelength dependent dissociation channels are observed for the model systems, which generate the NO product with both rotationally cold and hot distributions depending on the excitation wavelengths. Potential energy surface calculations at the CASSCF level of theory illustrates that two conical intersections between the excited and ground electronic states are involved in two different excitation wavelength dependent dissociation channels for the model systems. Femtosecond pump-probe experiments at 226 nm reveal that the NO molecule is still the main observed decomposition product from the materials of interest and that the formation dynamics of the NO product is faster than 180 fs. Two additional fragments are observed from furazan with mass of 40 amu (C2H2N) and 28 amu (CH2N) employing femtosecond laser ionization. This observation suggests a five-membered heterocyclic furazan ring opening mechanism with rupture of a CN and a NO bond, yielding NO as a major decomposition product. NH2 is not observed as a secondary decomposition product of DAAF and DAF.

On the excited electronic state dissociation of nitramine energetic materials and model systems.

In order to elucidate the difference between nitramine energetic materials, such as RDX (1,3,5-trinitro-1,3,5-triazacyclohexane), HMX (1,3,5,7-tetranitro-1,3,5,7-tetraazacyclooctane), and CL-20 (2,4,6,8,10,12-hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane), and their nonenergetic model systems, including 1,4-dinitropiperazine, nitropiperidine, nitropyrrolidine, and dimethylnitramine, both nanosecond mass resolved excitation spectroscopy and femtosecond pump-probe spectroscopy in the UV spectral region have been employed to investigate the mechanisms and dynamics of the excited electronic state photodissociation of these materials. The NO molecule is an initial decomposition product of all systems. The NO molecule from the decomposition of energetic materials displays cold rotational and hot vibrational spectral structures. Conversely, the NO molecule from the decomposition of model systems shows relatively hot rotational and cold vibrational spectra. In addition, the intensity of the NO ion signal from energetic materials is proportional to the number of nitramine functional groups in the molecule. Based upon experimental observations and theoretical calculations of the potential energy surface for these systems, we suggest that energetic materials dissociate from ground electronic states after internal conversion from their first excited states, and model systems dissociate from their first excited states. In both cases a nitro-nitrite isomerization is suggested to be part of the decomposition mechanism. Parent ions of dimethylnitramine and nitropyrrolidine are observed in femtosecond experiments. All the other molecules generate NO as a decomposition product even in the femtosecond time regime. The dynamics of the formation of the NO product is faster than 180 fs, which is equivalent to the time duration of our laser pulse.

Single photon ionization of hydrogen bonded clusters with a soft x-ray laser: (HCOOH)x and (HCOOH)y(H2O)z.

Pure, neutral formic acid (HCOOH)n+1 clusters and mixed (HCOOH)(H2O) clusters are investigated employing time of flight mass spectroscopy and single photon ionization at 26.5 eV using a very compact, capillary discharge, soft x-ray laser. During the ionization process, neutral clusters suffer little fragmentation because almost all excess energy above the vertical ionization energy is taken away by the photoelectron, leaving only a small part of the photon energy deposited into the (HCOOH)n+1+ cluster. The vertical ionization energy minus the adiabatic ionization energy is enough excess energy in the clusters to surmount the proton transfer energy barrier and induce the reaction (HCOOH)n+1+-->(HCOOH)nH+ +HCOO making the protonated (HCOOH)nH+ series dominant in all data obtained. The distribution of pure (HCOOH)nH+ clusters is dependent on experimental conditions. Under certain conditions, a magic number is found at n=5. Metastable dissociation rate constants of (HCOOH)nH+ are measured in the range (0.1-0.8)x10(4) s(-1) for cluster sizes 4

First Report of Sudden Death Syndrome of Soybean in Wisconsin.

In August of 2006, soybean (Glycine max (L.) Merr.) plants collected from Columbia, Dane, Green Lake, Walworth, Jefferson, and Waushara counties in southern Wisconsin exhibited symptoms typical of sudden death syndrome (SDS) caused by Fusarium virguliforme O'Donnell & Aoki [synonym F. solani (Mart.) Sacc. f. sp. glycines] (1). Foliar symptoms ranged from chlorotic spots to severe interveinal chlorosis and necrosis. Taproots of symptomatic plants were necrotic and stunted and stems exhibited a light tan discoloration, but never the dark brown discoloration typical for brown stem rot, a disease with similar foliar symptoms. Isolations from root and crown tissue of symptomatic plants were made using one-quarter-strength potato dextrose agar (PDA) amended with 100 ppm of streptomycin. Slow-growing, white-to-cream fungal colonies with blue and turquoise sporodochia were observed. Spores produced in sporodochia grown on PDA ranged in size from 32.5 to 70 µm long (average 53.1 µm) and 3 to 6 µm wide (average 4.4 µm) and with 3-5 septa (mode of 3). Isolates were characteristic of F. virguliforme based on colony morphology, spore morphology and size, and the absence of microconidia (3). The identity of F. virguliforme was confirmed by PCR amplification and DNA sequencing of the ITS, BT1, Act, and EF1B regions. All isolate sequences exhibited single nucleotide polymorphisms that matched the sequences of these regions of F. virguliforme. Koch's postulates were conducted to confirm that the causal agent of the observed symptoms was F. virguliforme. Inoculum of single-spore isolates was produced on sterilized sorghum seed. After 14 days of incubation at 20 to 22°C and a 12-h photoperiod, the sorghum seed was assayed to determine colonization incidence by transferring seeds to PDA. In all trials, sorghum seed was 100% infested. Infested sorghum seeds (35) were placed in potting soil at 2 cm beneath each seed of the susceptible soybean cv. Williams 82 (4). Noninfested sorghum seed was used for a noninoculated control. Three trials were performed, each using 15 replicates of several fungal isolates and 15 replicates of the noninoculated control. Plants were grown in water baths located in a greenhouse (trial 1) and in a growth chamber (trial 2) and both maintained at an average temperature of 25°C with a 14-h photoperiod (2). The third trial was conducted in the growth chamber without a water bath with the same temperature and light regimen. In all environments, inoculated plants developed chlorotic spots 14 days after planting. After 21 days, symptoms progressed to a range of chlorotic mottling to interveinal chlorosis and necrosis. Foliar and root symptoms that resembled those on the original plant samples infected with F. virguliforme appeared on 88% of inoculated plants. Isolates that resembled the original F. virguliforme were recovered from 75% of inoculated plants and from 88% of plants showing symptoms. No symptoms were observed and no isolates were recovered from noninoculated plants. There was a statistically significant difference between inoculated and control plants (P < 0.001) based on the presence of symptoms and isolation success using the Goodman χ2 analysis. The confirmation of the presence of SDS in five counties suggests that the disease is widespread in Wisconsin and could become a serious threat to soybean production in the future. References: (1) T. Akoi et al. Mycoscience 46:162, 2005. (2) R. Y. Hashmi et al. Online publication. doi:10.1094/PHP-2005-0906-01-RS. Plant Health Progress, 2005. (3) K. W. Roy et al. Plant Dis. 81:259, 1997. (4) J. C. Rupe et al. Can. J. Bot. 79:829, 2001.

IR plus vacuum ultraviolet spectroscopy of neutral and ionic organic acid molecules and clusters: acetic acid.

Infrared (IR) vibrational spectroscopy of acetic acid (A) neutral and ionic monomers and clusters, employing vacuum ultraviolet (VUV), 10.5 eV single photon ionization of supersonically expanded and cooled acetic acid samples, is presented and discussed. Molecular and cluster species are identified by time of flight mass spectroscopy: the major mass features observed are A(n)H(+) (n=1-9), ACOOH(+) (VUV ionization) without IR radiation present, and A(+) with both IR and VUV radiation present. The intense feature ACOOH(+) arises from the cleavage of (A)(2) at the beta-CC bond to generate ACOOH(+)+CH(3) following ionization. The vibrational spectrum of monomeric acetic acid (2500-7500 cm(-1)) is measured by nonresonant ionization detected infrared (NRID-IR) spectroscopy. The fundamentals and overtones of the CH and OH stretches and some combination bands are identified in the spectrum. Mass selected IR spectra of neutral and cationic acetic acid clusters are measured in the 2500-3800 cm(-1) range employing nonresonant ionization dip-IR and IR photodissociation (IRPD) spectroscopies, respectively. Characteristic bands observed at approximately 2500-2900 cm(-1) for the cyclic ring dimer are identified and tentatively assigned. For large neutral acetic acid clusters A(n)(n>2), spectra display only hydrogen bonded OH stretch features, while the CH modes (2500-2900 cm(-1)) do not change with cluster size n. The IRPD spectra of protonated (cationic) acetic acid clusters A(n)H(+) (n=1-7) exhibit a blueshift of the free OH stretch with increasing n. These bands finally disappear for n> or =6, and one broad and weak band due to hydrogen bonded OH stretch vibrations at approximately 3350 cm(-1) is detected. These results indicate that at least one OH group is not involved in the hydrogen bonding network for the smaller (n< or =5) A(n)H(+) species. The disappearance of the free OH stretch feature at n> or =6 suggests that closed cyclic structures form for A(n)H(+) for the larger clusters (n> or =6).

IR plus vacuum ultraviolet spectroscopy of neutral and ionic organic acid monomers and clusters: propanoic acid.

The vibrational spectrum of molecular propanoic acid, cooled in a supersonic expansion, in the region of 2500 to 7500 cm(-1) is obtained employing infrared plus vacuum ultraviolet nonresonant ionization detected spectroscopy. The fundamental and first overtone of the CH and OH stretch modes of cold propanoic acid molecules can be identified in the spectrum. Propanoic acid neutral and ionic clusters are also studied employing nonresonant ion dip and photodissociation spectroscopic techniques, respectively. For the neutral dimer, a sequence of features observed at ca. 2500-2700 cm(-1) can be assigned as combination bands of low frequency modes with the COH bending overtone; these features characterize the cyclic dimer ring structure. IR spectra of the larger neutral clusters n=3, 4, 5 indicate that they also have cyclic structures in which the OH groups are engaged in the cluster hydrogen bonding network. The CH groups are not involved in this hydrogen bonding structure. Free OH features are observed for the protonated ion clusters (C(2)H(5)COOH)(n)H(+), n=1,...,5, indicating that at least one OH group of these cluster ions is not involved in the cluster hydrogen bonding network. A comparison of the results for four hydrogen bonding neutral and ionic clusters (CH(3)OH, C(2)H(5)OH, CH(3)COOH, and C(2)H(5)COOH) is presented and discussed.

Reaction of niobium and tantalum neutral clusters with low pressure, unsaturated hydrocarbons in a pickup cell: from dehydrogenation to Met-Car formation.

Neutral niobium and tantalum clusters (Nbn and Tan) are generated by laser ablation and supersonic expansion into a vacuum and are reacted in a pickup cell with various low pressure (approximately 1 mTorr) unsaturated hydrocarbons (acetylene, ethylene, propylene, 1-butene, 1,3-butadiene, benzene, and toluene) under nearly single collision conditions. The bare metal clusters and their reaction products are ionized by a 193 nm laser and detected by a time of flight mass spectrometer. Partially and fully dehydrogenated products are observed for small (nor=m) neutral metal clusters, respectively, with m ranging from 2 to 5 depending on the particular hydrocarbon. In addition to primary, single collision products, sequential addition products that are usually fully dehydrogenated are also observed. With toluene used as the reactant gas, carbon loss products are observed, among which Nb8C12 and Ta8C12 are particularly abundant, indicating that the Met-Car molecule M8C12 can be formed from the neutral metal cluster upon two collisions with toluene molecules. The dehydrogenation results for low pressure reactions are compared with those available from previous studies employing flow tube (high pressure) reactors. Low pressure and high pressure cluster ion reactions are also compared with the present neutral metal cluster reactions. Reactions of unsaturated hydrocarbons and metal surfaces are discussed in terms of the present neutral cluster results.

Formation and distribution of neutral vanadium, niobium, and tantalum oxide clusters: single photon ionization at 26.5 eV.

Neutral vanadium, niobium, and tantalum oxide clusters are studied by single photon ionization employing a 26.5 eV/photon soft x-ray laser. During the ionization process the metal oxide clusters are almost free of fragmentation. The most stable neutral clusters of vanadium, niobium, and tantalum oxides are of the general form (MO2)0,1(M2O5)y. M2O5 is identified as a basic building unit for these three neutral metal oxide species. Each cluster family (Mm, m=1,...,9) displays at least one oxygen deficient and/or oxygen rich cluster stoichiometry in addition to the above most stable species. For tantalum and niobium families with even m, oxygen deficient clusters have the general formula (MO2)2(M2O5)y. For vanadium oxide clusters, oxygen deficient clusters are detected for all cluster families Vm (m=1,[ellipsis (horizontal)],9), with stable structures (VO2)x(V2O5)y. Oxygen rich metal oxide clusters with high ionization energies (IE>10.5 eV, 118 nm photon) are detected with general formulas expressed as (MO2)2 (M2O5)y O1,2,3. Oxygen rich clusters, in general, have up to three attached hydrogen atoms, such as VO3H1,2, V2O5H1,2, Nb2O5H1,2, etc.

Infrared plus vacuum ultraviolet spectroscopy of neutral and ionic ethanol monomers and clusters.

A high sensitivity spectroscopy is employed to detect vibrational antiitions of ethanol neutrals and ions in a supersonic expansion. The infrared (IR) features located at 3682 and 3667 cm(-1) can be assigned to the OH stretch for the two neutral C(2)H(5)OH conformers, anti and gauche, respectively. Their overtone energies located at 7179 (anti) and 7141 (gauche) cm(-1) are also identified. The OH fundamental stretch for ethanol ions is redshifted around 210 cm(-1), while the CH stretch modes are unchanged for neutral and ionic C(2)H(5)OH at around 2900-3000 cm(-1). The charge on the ethanol ion is apparently localized on the oxygen atom. IR induced photodissociation spectroscopy is applied to the study of neutral and protonated ethanol clusters. Neutral and protonated ethanol cluster vibrations are observed. The CH modes are not perturbed by the clustering process. Neutral clusters display only hydrogen bonded OH features, while the protonated ionic clusters display both hydrogen bonded and non-hydrogen-bonded features. These spectroscopic results are analyzed to obtain qualitative structural information on neutral and ionic ethanol clusters.

Infrared plus vacuum ultraviolet spectroscopy of neutral and ionic methanol monomers and clusters: new experimental results.

We present new observations of the infrared (IR) spectrum of neutral methanol and neutral and protonated methanol clusters employing IR plus vacuum ultraviolet (vuv) spectroscopic techniques. The tunable IR light covers the energy ranges of 2500-4500 cm(-1) and 5000-7500 cm(-1). The CH and OH fundamental stretch modes, the OH overtone mode, and combination bands are identified in the vibrational spectrum of supersonic expansion cooled methanol (2500-7500 cm(-1)). Cluster size selected IR plus vuv nonresonant infrared ion-dip infrared spectra of neutral methanol clusters, (CH(3)OH)(n) (n=2,[ellipsis (horizontal)],8), demonstrate that the methanol dimer has free and bonded OH stretch features, while clusters larger than the dimer display only hydrogen bonded OH stretch features. CH stretch mode spectra do not change with cluster size. These results suggest that all clusters larger than the dimer have a cyclic structure with OH groups involved in hydrogen bonding. CH groups are apparently not part of this cyclic binding network. Studies of protonated methanol cluster ions (CH(3)OH)(n)H(+) n=1,[ellipsis (horizontal)],7 are performed by size selected vuv plus IR photodissociation spectroscopy in the OH and CH stretch regions. Energies of the free and hydrogen bonded OH stretches exhibit blueshifts with increasing n, and these two modes converge to approximately 3670 and 3400 cm(-1) at cluster size n=7, respectively.

Single photon ionization of van der Waals clusters with a soft x-ray laser: (CO2)n and (CO2)n(H2O)m.

Pure neutral (CO2)n clusters and mixed (CO2)n(H2O)m clusters are investigated employing time of flight mass spectroscopy and single photon ionization at 26.5 eV. The distribution of pure (CO2)n clusters decreases roughly exponentially with increasing cluster size. During the ionization process, neutral clusters suffer little fragmentation because almost all excess cluster energy above the vertical ionization energy is taken away by the photoelectron and only a small part of the photon energy is deposited into the (CO2)n cluster. Metastable dissociation rate constants of (CO2)n+ are measured in the range of (0.2-1.5) x 10(4) s(-1) for cluster sizes of 5< or =n< or =16. Mixed CO2-H2O clusters are studied under different generation conditions (5% and 20% CO2 partial pressures and high and low expansion pressures). At high CO2 concentration, predominant signals in the mass spectrum are the (CO2)n+ cluster ions. The unprotonated cluster ion series (CO2)nH2O+ and (CO2)n(H2O)2+ are also observed under these conditions. At low CO2 concentration, protonated cluster ions (H2O)nH+ are the dominant signals, and the protonated CO2(H2O)nH+ and unprotonated (H2O)n+ and (CO2)(H2O)n+ cluster ion series are also observed. The mechanisms and dynamics of the formation of these neutral and ionic clusters are discussed.

Single photon ionization of van der Waals clusters with a soft x-ray laser: (SO2)n and (SO2)n(H2O)m.

van der Waals cluster (SO2)n is investigated by using single photon ionization of a 26.5 eV soft x-ray laser. During the ionization process, neutral clusters suffer a small fragmentation because almost all energy is taken away by the photoelectron and a small part of the photon energy is deposited into the (SO2)n cluster. The distribution of (SO2)n clusters decreases roughly exponentially with increasing cluster size. The photoionization dissociation fraction of I[(SO2)(n-1)SO+] / I[(SO2)n+] decreases with increasing cluster size due to the formation of cluster. The metastable dissociation rate constants of (SO2)n+ are measured in the range of (0.6-1.5) x 10(4) s(-1) for cluster sizes 5< or =n< or =16. Mixed SO2-H2O clusters are studied at different experimental conditions. At the condition of high SO2 concentration (20% SO2 partial pressure), (SO2)n+ cluster ions dominate the mass spectrum, and the unprotonated mixed cluster ions (SO2)nH2O+ (1< or =n< or =5) are observed. At the condition of low SO2 concentration (5% SO2 partial pressure) (H2O)nH+ cluster ions are the dominant signals, and protonated cluster ions (SO2)(H2O)nH+ are observed. The mixed clusters, containing only one SO2 or H2O molecule, SO2(H2O)nH+ and (SO2)nH2O+ are observed, respectively.

Generation and detection of alkyl peroxy radicals in a supersonic jet expansion.

Alkyl peroxy radicals are synthesized in a supersonic jet expansion by the initial production of alkyl radicals and subsequent reaction with molecular oxygen. Parent ions CH3OO+/CD3OO+ are observed employing vacuum ultraviolet (VUV) single photon ionizationtime-of-flight mass spectroscopy (TOFMS). Employing infrared (IR) + VUV photofragmentation detected spectroscopy, rotationally resolved infrared spectra of jet-cooled CH3OO and CD3OO radicals are recorded for the A 2A' <-- X 2A" transition by scanning the IR laser frequency while monitoring the CH3 + and CD3 + ion signals generated by the VUV laser. The band origins of the A 2A'<--X 2A" transition for CH3OO and CD3OO are identified at 7381 and 7371 cm(-1), respectively. Rotational simulation for the CH3OO and CD3OO 0(0) 0 transitions of A<--X yields a rotational temperature for these radicals of approximately 30 K. With the aid of ab initio calculations, two and five vibrational modes for the A 2A' excited electronic state are assigned for CH3OO and CD3OO radicals, respectively. Both experimental and theoretical results suggest that the ground electronic state of the ions of ethyl and propyl peroxy radicals are not stable although their ionization energies (IE) are less than 10.5 eV. The C2H5OO+/C3H7OO+ cations can readily decompose to C2H5 +/C3H7 + and O2. This is partially responsible for the inability of IR+VUV photofragmentation spectroscopy to detect the near IR A<--X electronic transition for these radicals.

Dynamics and fragmentation of van der Waals clusters: (H2O)n, (CH3OH)n, and (NH3)n upon ionization by a 26.5 eV soft x-ray laser.

A tabletop soft x-ray laser is applied for the first time as a high energy photon source for chemical dynamics experiments in the study of water, methanol, and ammonia clusters through time of flight mass spectroscopy. The 26.5 eV/photon laser (pulse time duration of approximately 1 ns) is employed as a single photon ionization source for the detection of these clusters. Only a small fraction of the photon energy is deposited in the cluster for metastable dissociation of cluster ions, and most of it is removed by the ejected electron. Protonated water, methanol, and ammonia clusters dominate the cluster mass spectra. Unprotonated ammonia clusters are observed in the protonated cluster ion size range 2< or =n< or =22. The unimolecular dissociation rate constants for reactions involving loss of one neutral molecule are calculated to be (0.6-2.7)x10(4), (3.6-6.0)x10(3), and (0.8-2.0)x10(4) s(-1) for the protonated water (9< or =n< or =24), methanol (5< or =n< or =10), and ammonia (5< or =n< or =18) clusters, respectively. The temperatures of the neutral clusters are estimated to be between 40 and 200 K for water clusters (10< or =n< or =21), and 50-100 K for methanol clusters (6< or =n< or =10). Products with losses of up to five H atoms are observed in the mass spectrum of the neutral ammonia dimer. Large ammonia clusters (NH(3))(n) (n>3) do not lose more than three H atoms in the photoionization/photodissociation process. For all three cluster systems studied, single photon ionization with a 26.5 eV photon yields near threshold ionization. The temperature of these three cluster systems increases with increasing cluster size over the above-indicated ranges.