G. Barney Ellison

Reinvestigation of the electron affinities of O2 and NO

Abstract We have re-examined the photoelectron spectra of O 2 − and NO − using negative ion photoelectron spectroscopy. We observe detachment to three electronic states ( 3 Σ g − , 1 Δ g , 1 Σ g + ) of O 2 and to one electronic state ( 2 Π) of NO. We find the following electron affinities: EA (O 2 )=0.451±0.007 eV and EA(NO)=0.026±0.005 eV. Analyses of the photoelectron spectra yield a value for the fundamental frequency of O 2 − (1073±50 cm −1 ) while the bond lengths are r e (O 2 − =1.347±0.005 A and r e (NO − = 1.271±0.005 A.

The electronic states of Si2 and Si−2 as revealed by photoelectron spectroscopy

We have measured the photoelectron spectrum of Si−2 and find that the molecular electron affinity is EA(Si2)=2.199±0.012 eV. This spectrum apparently involves multiple electronic states of the Si−2 ion as well as several electronic states of the final neutral, Si2. In order to unravel our experimental findings, we have carried out ab initio MCSCF+1+2 CI calculations on both species. These calculations suggest that there are two nearly degenerate states for both Si2 and Si−2. We can fit our experimental data by assuming detachment from two states of Si−2 :X (2Π) [re=2.187 A] and A (2Σ+g) [T0=0.117±0.016 eV, re=2.127 A]. We observe two final states of Si2: X (3Σ−g) [re=2.246 A] and A (3Πu) [T0=0.053±0.015 eV, re=2.171 A]. These assignments are confirmed by an experimental study of the angular distributions of the photodetached electrons.

Radical Chemistry in the Thermal Decomposition of Anisole and Deuterated Anisoles: An Investigation of Aromatic Growth

The pyrolyses of anisole (C(6)H(5)OCH(3)), d(3)-anisole (C(6)H(5)OCD(3)), and d(8)-anisole (C(6)D(5)OCD(3)) have been studied using a hyperthermal tubular reactor and photoionization reflectron time-of-flight mass spectrometer. Gas exiting the reactor is subject to an immediate supersonic expansion after a residence time of approximately 65 mus. This allows the detection of highly reactive radical intermediates. Our results confirm that the first steps in the thermal decomposition of anisole are the loss of a methyl group to form phenoxy radical, followed by ejection of a CO to form cyclopentadienyl radical (c-C(5)H(5)); C(6)H(5)OCH(3) --> C(6)H(5)O + CH(3); C(6)H(5)O --> c-C(5)H(5) + CO. At high temperatures (T(wall) = 1200 degrees C - 1300 degrees C) the c-C(5)H(5) decomposes to propargyl radical (CH(2)CCH) and acetylene; c-C(5)H(5) --> CH(2)CCH + C(2)H(2). The formation of benzene and naphthalene is demonstrated with 1 + 1 resonance-enhanced multiphoton ionization. Propargyl radical recombination is a significant benzene formation channel. However, we show the majority of benzene is formed by a ring expansion reaction of methylcyclopentadiene (C(5)H(5)CH(3)) resulting from methyl radical addition to cyclopentadienyl radical; CH(3) + c-C(5)H(5) --> C(5)H(5)CH(3) --> C(6)H(6) + 2H. The naphthalene is generated from cyclopentadienyl radical recombination; 2c-C(5)H(5) --> C(5)H(5)-C(5)H(5) --> C(10)H(8) + 2H. The respective intermediate amu 79 and 129 species associated with these reactions are detected, confirming the stepwise nature of the decompositions. These reactions are verified by pyrolysis studies of cyclopentadiene (C(5)H(6)) and C(5)H(5)CH(3) obtained from rapid thermal dissociation of the respective dimer compounds, as well as pyrolysis studies of propargyl bromide (BrCH(2)CCH).

Photoelectron spectroscopy of radical anions

Abstract The photoelectron spectroscopy of a number of radical anions has been investigated. We find the following electron affinities: EA(C3) =1.981 ±0.020 eV, EA(C3H) = 1.858 ±0.023 eV, EA(C3H2) = 1.794 ± 0.025 eV, EA(C3O) = 1.34±0.15 eV, EA(C3O2) = 0.85±0.15 eV, EA(C4O)= 2.05±0.15 eV, and EA(CS2) = 0.895± 0.020 eV. The structure and bonding for each of these ions is discussed.

Thermochemistry of the benzyl and allyl radicals and ions

Abstract We have studied the thermochemistry of the resonantly stabilized radicals, C6H5CH2 and CH2CHCH2 and their corresponding cations and anions. A flowing afterglow/selected ion flow tube instrument has been used to measure the rates of reaction: C6H5CH3 + CH3O− ⇌C6H5CH2− + CH3OH C6H5CH3 + CD3O− ⇌C6H5CH2− + CD3OH C6D5CD3 + CH3O− ⇌C6D5CD2− + CH3OD C6D5CD3 + CD3O− ⇌C6D5CD2− + CD3OD CH2CHCH3 + HO− ⇌CH2CHCH2− + H2O The ratio of the rate constants gives a free energy change for each reaction and use of the established gas phase acidity of CH3OH or H2O provides values for the acidities. We calculate the entropy changes, ΔacidS300(C6H5CH3) and ΔacidS300(CH2CHCH3), to extract values for ΔacidH300(C6H5CH3) and ΔacidH300(CH2CHCH3). Earlier photoelectron experiments have provided ionization potentials and electron affinities for the benzyl and allyl radicals. Use of the IPs and EAs, together with the enthalpies of deprotonation, provides values for the C H bond enthalpies at 300 K and the C H bond energies at 0 K. These bond energies are used to compute the heats of formation of the radicals and ions as well as the cation hydride affinities, HA (all values in kcal mol−1): C6H5CH2−H CH2CHCH2−H ΔacidG300(R−H) 374.9 ± 0.2 383.8 ± 0.1 ΔacidH300(R−H) 382.3 ± 0.5 391.1 ± 0.3 DH300(R−H) 89.8 ± 0.6 88.8 ± 0.4 D0(R−H) 88.1 ± 0.6 87.4 ± 0.4 ΔfH0(R) 54.1 ± 0.6 44.4 ± 0.5 ΔfH300(R) 49.7 ± 0.6 41.4 ± 0.4 ΔfH0(R−) 33.1 ± 0.6 33.1 ± 0.5 ΔfH300(R−) 28.7 ± 0.5 30.1 ± 0.4 ΔfH0(R+) 221.3 ± 0.6 231.6 ± 0.7 ΔfH300(R+) 216.8 ± 0.6 228.9 ± 0.6 HA0(R+) 237.9 ± 0.7 254.7 ± 0.7 HA300(R+) 239.5 ± 0.6 258.9 ± 0.7 In addition, we find ΔacidG300(C6D5CD3) = 377.0 ± 0.3 kcal mol−1.

Photoelectron spectroscopy of HNO− and DNO−

We have obtained the photoelectron spectra of HNO− and DNO− using a recently constructed negative ion photoelectron spectrometer. We observe detachment to three different electronic states of HNO and DNO, namely the X 1A′, a 3A′′, and A 1A′′ states. The electron affinity of HNO is determined to be 0.338±0.015 eV while that of DNO is 0.330±0.015 eV. The a 3A′′ state, which has not been observed directly before, is found to lie 0.778±0.020 eV above the X 1A′ state in HNO and 0.785±0.020 eV in DNO. We see vibrational excitation up to ν′2=3 of the ν2 (N–O stretch) mode in the HNO and DNO X 1A′ state. In the a 3A′′ and A 1 A′′ states, we see excitation of both ν2 and ν3 (H–N–O bend) modes. The fundamental frequencies for the a 3A′′ state are ν2 =1468±140 cm−1 and ν3=992±150 cm−1 for HNO and ν2= 1452±140 cm−1 and ν3=750±140 cm−1 for DNO. Analysis of transitions from vibrationally excited ions gives a ν2 fundamental frequency of 1153±170 cm−1 in HNO− and 1113±170 cm−1 in DNO−. A study of the angular dis...

Intense, hyperthermal source of organic radicals for matrix-isolation spectroscopy

We have incorporated a pulsed, hyperthermal nozzle with a cryostat to study the matrix-isolated infrared spectroscopy of organic radicals. The radicals are produced by pyrolysis in a heated, narrow-bore (1-mm-diam) SiC tube and then expanded into the cryostat vacuum chamber. The combination of high nozzle temperature (up to 1800 K) and near-sonic flow velocities (on the order of 104 cm s−1) through the length of the 2 cm tube allows for high yield of radicals (approximately 1013 radicals pulse−1) and low residence time (on the order of 10 μs) in the nozzle. We have used this hyperthermal nozzle/matrix isolation experiment to observe the IR spectra of complex radicals such as allyl radical (CH2CHCH2), phenyl radical (C6H5), and methylperoxyl radical (CH3OO). IR spectra of samples produced with a hyperthermal nozzle are remarkably clean and relatively free of interfering radical chemistry. By monitoring the unimolecular thermal decomposition of allyl ethyl ether in the nozzle using matrix IR spectroscopy, w...

The pyrolysis of anisole (C6H5OCH3) using a hyperthermal nozzle

Abstract We have investigated the pyrolysis of anisole (C6H5OCH3), a model compound for methoxy functional groups in lignin. An understanding of the pyrolysis of this simple compound can provide valuable insight into the mechanisms for the thermal decomposition of biomass. Our emphasis in this study is the formation of polynuclear aromatic hydrocarbons (PAHs) and in particular we investigate the formation of naphthalene. The route to the formation of naphthalene from anisole follows the simple unimolecular decomposition of anisole, which leads to the phenoxy radical and then cyclopentadienyl radical. This chemical pathway has been demonstrated before, but the subsequent reaction of two cyclopentadienyl radicals to give naphthalene has only been the subject of theoretical investigations. We have used matrix isolation FTIR spectroscopy together with photoionization time-of-flight (TOF) mass spectrometry to identify intermediates in this reaction mechanism. Using this technique, we have trapped phenoxy and cyclopentadienyl radicals and measured their IR spectra. The formation of these species is confirmed in our TOF mass spectrometer. We have also identified the formation of 9,10-dihydrofulvalene, the adduct from the reaction of two cyclopentadienyl radicals. Finally, we have used molecular beam mass spectrometry (MBMS) and factor analysis to demonstrate the formation of naphthalene from the pyrolysis of anisole.

The properties of a micro-reactor for the study of the unimolecular decomposition of large molecules

A micro-reactor system (approximately 0.5–1 mm inner diameter by 2–3 cm in length) coupled with photoionization mass spectrometry and matrix isolation/infrared spectroscopy diagnostics is described. Short residence time flow reactors (roughly ≤ 100 μs) combined with suitable diagnostic tools have the potential to allow observation of unimolecular decomposition processes with minimum interference from secondary reactions. However, achieving the short residence times desired requires very small micro-reactors that are difficult to characterise experimentally because of their size. In this article the benefits of using these micro-reactors are presented along with some details of the systems employed. This is followed by some general flow considerations and then some simple analyses to illustrate particular features of the flow. Finally, computational fluid dynamics simulations are used to explore the flow and chemical behaviour of the reactors in detail. Some findings include: (1) The reactor operates in the laminar domain. (2) Heating and large pressure differences across the reactor result in a compressible flow that chokes (meaning the velocity reaches the sonic condition) at the reactor exit. (3) When helium is the carrier gas, under some circumstances there is slip at the boundaries near the downstream end of the reactor that reduces the pressure drop and heat transfer rate; this effect must be accounted for in the simulations. (4) Because the initial reactant concentration is held to less than 0.1%, secondary reactions are minimised. (5) Although the fluid dynamical residence time from entrance to exit ranges from 25 to 150 μs, in practice the period over which reactions take place is much shorter. In essence, there is a ‘sweet spot’ within the reactor where most reactions take place. In summary, the micro-reactor, which has been used for many years to generate radicals or study unimolecular decomposition chemical mechanisms, can be used to extract kinetic information by comparing simulations and measurements of reactant and product concentrations at the reactor exit.

Unimolecular thermal fragmentation of ortho-benzyne

The ortho-benzyne diradical, o-C(6)H(4) has been produced with a supersonic nozzle and its subsequent thermal decomposition has been studied. As the temperature of the nozzle is increased, the benzyne molecule fragments: o-C(6)H(4)+Delta--> products. The thermal dissociation products were identified by three experimental methods: (i) time-of-flight photoionization mass spectrometry, (ii) matrix-isolation Fourier transform infrared absorption spectroscopy, and (iii) chemical ionization mass spectrometry. At the threshold dissociation temperature, o-benzyne cleanly decomposes into acetylene and diacetylene via an apparent retro-Diels-Alder process: o-C(6)H(4)+Delta-->HC triple bond CH+HC triple bond C-C triple bond CH. The experimental Delta(rxn)H(298)(o-C(6)H(4)-->HC triple bond CH+HC triple bond C-C triple bond CH) is found to be 57+/-3 kcal mol(-1). Further experiments with the substituted benzyne, 3,6-(CH(3))(2)-o-C(6)H(2), are consistent with a retro-Diels-Alder fragmentation. But at higher nozzle temperatures, the cracking pattern becomes more complicated. To interpret these experiments, the retro-Diels-Alder fragmentation of o-benzyne has been investigated by rigorous ab initio electronic structure computations. These calculations used basis sets as large as [C(7s6p5d4f3g2h1i)H(6s5p4d3f2g1h)] (cc-pV6Z) and electron correlation treatments as extensive as full coupled cluster through triple excitations (CCSDT), in cases with a perturbative term for connected quadruples [CCSDT(Q)]. Focal point extrapolations of the computational data yield a 0 K barrier for the concerted, C(2v)-symmetric decomposition of o-benzyne, E(b)(o-C(6)H(4)-->HC triple bond CH+HC triple bond C-C triple bond CH)=88.0+/-0.5 kcal mol(-1). A barrier of this magnitude is consistent with the experimental results. A careful assessment of the thermochemistry for the high temperature fragmentation of benzene is presented: C(6)H(6)-->H+[C(6)H(5)]-->H+[o-C(6)H(4)]-->HC triple bond CH+HC triple bond C-C triple bond CH. Benzyne may be an important intermediate in the thermal decomposition of many alkylbenzenes (arenes). High engine temperatures above 1500 K may crack these alkylbenzenes to a mixture of alkyl radicals and phenyl radicals. The phenyl radicals will then dissociate first to benzyne and then to acetylene and diacetylene.