Ethylcarbene versus Direct Propene Formation in the Near-UV Photodissociation of Ethylketene.

The competing pathways in the photodissociation of gaseous ethylketene at excitation wavelengths of 320.0, 340.0, and 355.1 nm were studied using photofragment translational energy spectroscopy. The primary dissociation channel was C═C bond fission producing ethylcarbene (CH3CH2CH; also known as propylidene) and CO. Product translational energy distributions are consistent with theoretical predictions that ground state ethylcarbene lies ∼34 kJ/mol higher in energy than its isomer dimethylcarbene (CH3CCH3). A second dissociation channel involved direct formation of propene prior to or concurrent with CO elimination. The measured product branching ratios indicate that the effective potential energy barrier for the direct propene channel lies below the energetic threshold for ethylcarbene formation. A minor C-C bond fission channel was also observed, leading to CH3 + CH2CHCO products. Comparisons are made to the results of our recent studies of methylketene and dimethylketene photodissociation.

Site-Specific Carbon-Carbon Bond Fission in Photoexcited Propyl Radicals Leads to Isomer-Selective Carbene and Radical Products.

Although there have been many studies of C-H bond fission in the UV photochemistry of alkyl radicals, very little is known about the possible occurrence of C-C bond fission. Here, we report that upon excitation at 248 nm, gaseous 1-propyl radicals primarily undergo C-C bond fission, producing methylene (CH2) and ethyl radicals (C2H5), rather than the more energetically favored methyl (CH3) and ethylene (C2H4). In contrast, the exclusive C-C bond fission products from 2-propyl radicals were ethylidene (CHCH3) plus methyl radicals (CH3). The isomer-selective formation of high-energy carbene + radical products involves excited-state site-specific C-C bond fission at the radical carbon, with quantum yields comparable to those for C-H bond fission. Our observations suggest that a general feature of alkyl radical photochemistry is predissociation of the initially formed Rydberg states by high-lying valence states, yielding high-energy carbene plus alkyl radical products.

Dimethylcarbene versus Direct Propene Formation in Dimethylketene Photodissociation.

Highly reactive carbenes are usually produced by photolysis of ketenes, diazoalkanes, or diazirines. Sequential kinetic pathways for deactivation of nascent carbenes usually involve bimolecular reactions in competition with isomerization producing stable products such as alkenes. However, the direct photolytic production of stable products, effectively bypassing formation of free carbenes, has been postulated for over 50 years but remains very poorly understood. Often termed "rearrangement in the excited state" (RIES), examples include 1,2-hydrogen migration within photoexcited carbene precursors yielding alkenes and the Wolff rearrangement in photogenerated carbonyl-substituted carbenes producing ketenes. In this study, the two competing CO elimination channels from photoexcited gaseous dimethylketene, producing dimethylcarbene and propene, were studied as a function of electronic excitation energy, under collision-free conditions, by using photofragment translational energy spectroscopy with vacuum ultraviolet photoionization of the products. A significant fraction of the dimethylcarbene → propene isomerization exothermicity (∼300 kJ/mol) was released as propene + CO translational energy, indicating that propene is formed prior to or concurrent with CO elimination. An increase in the propene yield with increasing excitation energy suggests that the effective potential energy barrier for this channel lies ∼24 kJ/mol above the energetic threshold for dimethylcarbene formation via C═C bond fission. Possible mechanisms for direct propene elimination are discussed in light of the observed energy dependence for the competing pathways.

Direct Observation of Ethylidene (CH3CH), the Elusive High-Energy Isomer of Ethylene.

Despite experimental efforts spanning more than 80 years, there has been no direct observation of free ethylidene (CH3CH), the simplest alkyl-substituted carbene. Here, we report that ethylidene is indefinitely stable in the absence of collisions if produced in the triplet ground state at energies below the threshold for intersystem crossing. Near-UV photolysis of gaseous methylketene, or propenal (followed by isomerization to methylketene), leads to CO loss producing triplet ethylidene, which is detected by photoionization mass spectrometry. Electronically excited singlet ethylidene is also produced, rapidly undergoing isomerization by a 1,2-hydrogen atom shift, producing highly vibrationally excited ethylene. The measured product translational energy distributions verify the theoretically calculated enthalpy of formation of triplet ethylidene and are consistent with a singlet-triplet energy gap of approximately 12.5 kJ/mol.

Subpicosecond HI elimination in the 266 nm photodissociation of branched iodoalkanes.

The 266 nm photodissociation dynamics of 1-iodopropane and 2-iodopropane were studied using photofragment translational energy spectroscopy using vacuum ultraviolet (VUV) photoionization and electron impact ionization detection of products. The photochemistry of 1-iodopropane was found to be similar to that of iodomethane and iodoethane, with dominant production of I*(2P1/2), and no evidence (<0.21%) for HI + alkene formation. Significantly different behavior was observed for 2-iodopropane, with dominant production of ground state I(2P3/2), and a HI yield >10.5%. The anisotropy (ß) parameters for all channels approached the limiting value of 2.0, indicating that 1,2-HI elimination occurs on subpicosecond timescales, like direct C-I bond fission, following excitation to 3Q0. The HI translational energy and angular distributions were similar to those for I(2P3/2), suggesting that motion of the heavy I atom in HI is largely derived from the repulsive nature of the 1Q1 surface correlating to R + I with the light H atom picked up by ground state I late in the exit channel producing highly vibrationally excited HI.

High intensity vacuum ultraviolet and extreme ultraviolet production by noncollinear mixing in laser vaporized media.

A method is described for generating intense pulsed vacuum ultraviolet (VUV) and extreme ultraviolet (XUV) laser radiation by resonance enhanced four-wave mixing of commercial pulsed nanosecond lasers in laser vaporized mercury under windowless conditions. By employing noncollinear mixing of the input beams, the need of dispersive elements such as gratings for separating the VUV/XUV from the residual UV and visible beams is eliminated. A number of schemes are described, facilitating access to the 9.9-14.6 eV range. A simple and convenient scheme for generating wavelengths of 125 nm, 112 nm, and 104 nm (10 eV, 11 eV, and 12 eV) using two dye lasers without the need for dye changes is described.

Crossed molecular beams studies of phenyl radical reactions with propene and trans-2-butene.

The reactions of phenyl radicals with propene have been studied at collision energies of 84 and 108 kJ/mol using the crossed molecular beams technique. The branching ratios between methyl radical elimination forming C8H8 and H-atom elimination forming C9H10 were found to be 10 ± 1:1 at 84 kJ/mol and 3 ± 1:1 at 108 kJ/mol. By using "soft" 9.9 eV vacuum ultraviolet photoionization for product detection, we were able to observe both product channels with negligible fragmentation of C9H10 to C8H8(+). Our finding that CH3 elimination is dominant is consistent with conclusions from a recent study employing a pyrolysis molecular beam reactor using photoionization detection. However, our C8H8/C9H10 branching ratios are significantly larger than inferred from previous CMB experiments and RRKM calculations. For comparison, we have also studied the reactions of phenyl radicals with trans-2-butene at Ecoll = 97 kJ/mol. In this case, the symmetry of trans-2-butene makes both alkene addition sites chemically equivalent. The intermediate formed in the reaction with trans-2-butene is similar to the 2-carbon addition intermediate in the reaction with propene. We observed only methyl elimination in the reaction with trans-2-butene, with no evidence for H-atom elimination, consistent with conclusions that C-C bond fission is the most favorable channel in these systems. Analogies between phenyl radical reactions with propene and trans-2-butene are used to provide insight into the mechanisms in the propene reaction.

High-intensity coherent vacuum ultraviolet source using unfocussed commercial dye lasers.

Using two or three commercial pulsed nanosecond dye lasers pumped by a single 30 Hz Nd:YAG laser, generation of 0.10 mJ pulses at 125 nm (6 × 10(13) photons∕pulse) has been demonstrated by resonance enhanced four-wave mixing of collimated (unfocussed) laser beams in mercury (Hg) vapor. Phase matching at various vacuum ultraviolet (VUV) wavelengths is achieved by tuning one laser in the vicinity of the 6 (1)S0 → 6 (3)P1 resonance near 253.1 nm. A number of different mixing schemes are characterized. Our observations using broadband lasers (~0.15 cm(-1) bandwidths) are compared to previous calculations pertaining to four-wave mixing of low intensity narrowband laser beams. Prospects for further increases in pulse energies are discussed. We find that VUV tuning curves and intensities are in good agreement with theoretical predictions. The utility of the VUV light source is demonstrated by "soft universal" single-photon VUV ionization in crossed molecular beam studies and for generation of light at 130.2 nm for oxygen atom Rydberg time-of-flight experiments.

Studies of bimolecular reaction dynamics using pulsed high-intensity vacuum-ultraviolet lasers for photoionization detection.

This article describes recent progress on the development and application of pulsed high-intensity (~0.1 mJ per pulse) vacuum-ultraviolet (VUV) radiation produced by commercial tabletop lasers for studies of gas phase chemical reaction dynamics involving polyatomic free radicals. Our approach employs near-triply resonant four-wave mixing of unfocussed nanosecond dye lasers in an atomic gas as an alternative to the use of synchrotron light sources for sensitive universal soft photoionization detection of reaction products using a rotatable source crossed molecular beams apparatus with fixed detector. We illustrate this approach in studies of the reactions of phenyl radicals with molecular oxygen and with propene. Future prospects for the use of tabletop laser-based VUV sources for studies of chemical reaction dynamics are discussed.

Photodissociation dynamics of gaseous CpCo(CO)2 and ligand exchange reactions of CpCoH2 with C3H4, C3H6, and NH3.

The photodissociation dynamics of CpCo(CO)(2) was studied in a molecular beam using photofragment translational energy spectroscopy with 157 nm photoionization detection of the metallic products. At 532 and 355 nm excitation, the dominant one-photon channel involved loss of a single CO ligand producing CpCoCO. The product angular distributions were isotropic, and a large fraction of excess energy appeared as product vibrational excitation. Production of CpCO + 2CO resulted from two-photon absorption processes. The two-photon dissociation of mixtures containing CpCo(CO)(2) and H(2) at the orifice of a pulsed nozzle was used to produce a novel 16-electron unsaturated species, CpCoH(2). Transition metal ligand exchange reactions, CpCoH(2) + L → CpCoL + H(2) (L = propyne, propene, or ammonia), were studied under single-collision conditions for the first time. In all cases, ligand exchange occurred via 18-electron association complexes with lifetimes comparable to their rotational periods. Although ligand exchange reactions were not detected from CpCoH(2) collisions with methane or propane (L = CH(4) or C(3)H(8)), a molecular beam containing CpCoCH(4) was produced by photolysis of mixtures containing CpCo(CO)(2) and CH(4).

Reactions of neutral gas-phase yttrium atoms with two cyclohexadiene isomers.

The reactions of neutral ground-state yttrium (Y) atoms with 1,3- and 1,4-cyclohexadiene (CHD) were studied using crossed molecular beams. Formation of YC(6)H(6) + H(2) and YH(2) + C(6)H(6) was observed for both isomers at collision energies (E(coll)) of 31.3 and 13.0 kcal/mol. Measured product branching ratios at E(coll) = 31.3 kcal/mol indicated that YH(2) + C(6)H(6) was the dominant channel, accounting for >97% of the products. An additional minor product channel, YC(4)H(4) + C(2)H(4), was observed for 1,3-CHD at the higher E(coll). The reaction threshold for YC(4)H(4) formation was determined to be 29.5 ± 2.0 kcal/mol based on fits to the data.

Improved piezoelectric actuators for use in high-speed pulsed valves.

We describe the assembly of piezoelectric actuators suitable for use in Proch-Trickl pulsed gas valves employed in pulsed molecular beam experiments. In their simplest form, these actuators have performance equal to and resistance to chemical attack far superior to the recently discontinued commercial actuator around which the valve was originally designed. New actuators have been designed employing several different dual piezo configurations. The dual piezo design reliably produces shorter pulses than the original actuator, resulting in reduced gas consumption and lower average source chamber pressures for a given backing pressure and pulse rate. By electrically isolating the actuator assembly, active adjustment of the closed position may be achieved by simple addition of a continuous voltage bias.

Vibrational vs. translational energy in promoting a prototype metal-hydrocarbon insertion reaction.

The reaction Y + CH(4) --> HYCH(3) --> YCH(2) + H(2) is initiated by C-H insertion involving a 20 +/- 3 kcal/mol potential energy barrier. The reaction is studied in crossed molecular beams under two different conditions with nearly the same total energy. One experiment is carried out at a collision energy of 15.1 kcal/mol with one quantum of CH(4) antisymmetric (nu(3)) stretching vibrational excitation (8.63 kcal/mol), the other at a collision energy of 23.8 kcal/mol. The reaction cross-section for C-H stretch excited methane (sigma(s)) is found to be at least a factor of 2.2 times larger than for ground-state methane (sigma(g)) at the same total energy.

C-C versus C-H bond activation of alkynes by early second-row transition metal atoms.

The reactions of Y (a2D), Zr (a3F), Nb (a6D), Mo (a7S), and electronically excited-state Mo* (a5S) with propyne (methylacetylene) and 2-butyne (1,2-dimethylacetylene) were investigated using crossed molecular beams. For all of the metals studied, reactions with propyne led to H2 elimination, forming MC3H2. For Y + propyne, C-C bond cleavage forming YCCH + CH3 also was observed, with an energetic threshold in good agreement with an earlier determination of D0(Y-CCH). For Y + 2-butyne, three reactive channels were observed: YC4H4 + H2, YC3H3 + CH3, and YC3H2 + CH4. The C-C bond cleavage products accounted for 21 and 27% of the total products at Ecoll = 69 and 116 kJ/mol, respectively. For Zr and Nb reactions with 2-butyne, competition between H2 and CH4 elimination was observed, with C-C bond cleavage accounting for 12 and 4% of the total product signal at Ecoll = 71 kJ/mol, respectively. For reactions of Mo and Mo* with 2-butyne, only H2 elimination was observed. The similarity between reactions involving two isomeric species, propyne and allene, suggests that H atom migration is facile in these systems.

Experimental and theoretical studies of the reactions Y (a2D) + H2CO and Y (a2D) + CH3CHO.

The reactions of ground state Y (a(2)D) with H(2)CO and CH(3)CHO were studied at a range of collision energies in crossed molecular beams. For reaction with H(2)CO, three product channels were observed: formation of YH(2) + CO, YCO + H(2), and YHCO + H. Reaction with CH(3)CHO led to three analogous product channels involving formation of HYCH(3) + CO, YCH(2)CO + H(2), and YCH(3)CO + H. The calculated CCSD(T) energetics and DFT geometries for key intermediates in both reactions, together with RRKM theory, are used to calculate a priori the branching ratios between various product channels. These calculated values are compared to those obtained experimentally.

Reactive quenching of OH(A 2Sigma+) by D2 studied using crossed molecular beams.

Reactive quenching of OH(A 2Sigma+,v=0) by D2 forming HOD+D was studied in crossed molecular beams. The D atom products are primarily forward scattered relative to the incident D2. The dominant mechanism involves a direct reaction from relatively large impact parameters with approximately 88% of the available energy appearing in HOD internal excitation.

Reaction dynamics of CN + O2 --> NCO + O(3P2).

We have used oxygen Rydberg time-of-flight spectroscopy to carry out a crossed molecular beam study of the CN + O2 reaction at collision energies of 3.1 and 4.1 kcal/mol. The O(3P2) products were tagged by excitation to high-n Rydberg levels and subsequently field ionized at a detector. The translational energy distributions were broad, indicating that the NCO is formed with a wide range of internal excitation, and the angular distribution was forward-backward symmetric, indicating the participation of NCOO intermediates with lifetimes comparable to or longer than their rotational periods. Rice-Ramsperger-Kassel-Marcus modeling of the dissociation of NCOO to NCO + O suggests that Do(NC-OO) > or = 38 kcal/mol, which is consistent with several theoretical calculations. Implications for the competing CO + NO channel are discussed.

Photodissociation dynamics of N2O at 130 nm: the N2(A 3Sigmau +,B 3Pig)+O(3PJ = 2,1,0) channels.

Oxygen Rydberg time-of-flight spectroscopy was used to study the vacuum ultraviolet photodissociation dynamics of N(2)O near 130 nm. The O((3)P(J)) products were tagged by excitation to high-n Rydberg levels and subsequently field ionized at a detector. In agreement with previous work, we find that O((3)P(J)) formation following excitation to the repulsive N(2)O D((1)Sigma(+)) state produces the first two electronically excited states of the N(2) counterfragment, N(2)(A (3)Sigma(u) (+)) and N(2)(B (3)Pi(g)). The O((3)P(J)) translational energy distribution reveals that the overall branching ratio between N(2)(A (3)Sigma(u) (+)) and N(2)(B (3)Pi(g)) formation is approximately 1.0:1.0 for J = 1 and 2, with slightly less N(2)(B (3)Pi(g)) produced in coincidence with O((3)P(0)). The angular distributions were found to be independent of J and highly anisotropic, with beta = 1.5+/-0.2.

Crossed beams study of the reaction 1CH2+C2H2-->C3H3+H.

The reaction of electronically excited singlet methylene (1CH2) with acetylene (C2H2) was studied using the method of crossed molecular beams at a mean collision energy of 3.0 kcal/mol. The angular and velocity distributions of the propargyl radical (C3H3) products were measured using single photon ionization (9.6 eV) at the advanced light source. The measured distributions indicate that the mechanism involves formation of a long-lived C3H4 complex followed by simple C-H bond fission producing C3H3+H. This work, which is the first crossed beams study of a reaction involving an electronically excited polyatomic molecule, demonstrates the feasibility of crossed molecular beam studies of reactions involving 1CH2.

Competition between C-C and C-H insertion in prototype transition metal-hydrocarbon reactions.

The competition between C-C and C-H insertion in model transition-metal reactions with cyclopropane and propene (C3H6) was studied as a function of total energy. Insertion of neutral transition metal atoms M (= Y, Zr, Nb, and Mo*) into the C-C bonds of cyclopropane led to formation of MCH2 + C2H4, whereas C-H insertion produced MC3H4 + H2. The measured product branching ratios verify the relative potential energy barrier heights for C-C and C-H insertion predicted by ab initio calculations.