A cold and slow molecular beam

Employing a two-stage cryogenic buffer gas cell, we produce a cold, hydrodynamically extracted beam of calcium monohydride molecules with a near effusive velocity distribution. Beam dynamics, thermalization and slowing are studied using laser spectroscopy. The key to this hybrid, effusive-like beam source is a "slowing cell" placed immediately after a hydrodynamic, cryogenic source [Patterson et al., J. Chem. Phys., 2007, 126, 154307]. The resulting CaH beams are created in two regimes. In one regime, a modestly boosted beam has a forward velocity of v(f) = 65 m s(-1), a narrow velocity spread, and a flux of 10(9) molecules per pulse. In the other regime, our slowest beam has a forward velocity of v(f) = 40 m s(-1), a longitudinal temperature of 3.6 K, and a flux of 5 × 10(8) molecules per pulse.     

A potential source of free radicals in iodine-based chemical oscillators

The iodide-peroxide system in an acidic medium was investigated as a potential source of free radicals in iodine-based chemical oscillators. The radicals were detected by EPR spin-trapping using spin-trap 5-(tert-butoxycarbonyl)-5-methyl-1-pyrroline N-oxide (BMPO), which forms stable spin-adducts with oxygen-centered radicals. The iodide-peroxide system is introduced as an easily available laboratory source of free radicals.     

High temperature molecular beam source reactor

Small low residence time flow tube reactors made of alumina and used as molecular beam sources are described. In these reactors, gas mixtures are rapidly heated and brought to reaction. The composition of the gas leaving the reactor is analyzed by molecular beam mass spectroscopy. For quantitative simulation of the reacting gas flow, the theory of one-dimensional compressible flow with friction, heat transfer, and chemical reaction is brought into a form suitable for practical computation. The system has been applied to study the thermal decompositions of O₃ and N₂O. The experimental results on both reactions can be well modeled by homogeneous reaction mechanisms with accepted rate constants. Heterogeneous reaction steps are shown to be unimportant.     

A new kinetic method for quantification phenoxyl free radicals

In this work, a new kinetic method was proposed for quantification phenoxyl radicals generated in enzyme reaction. Instead of direct detecting the spectral signals of phenoxyl radicals, a molecular probe, the reduced form of nicotinamide adenine dinucleotide (NADH), was employed to indicate the formation of phenoxyl free radicals. It was found that the reactions of NADH and phenoxyl radicals are very fast, but can be followed by using stopped-flow fast scanning spectrophotometric technique. The initial rate of accelerated-oxidation of NADH represents the reactivity of phenoxyl free radical, which is proportional in a certain range to the initial concentration of the parent chlorophenols of the radicals. With this method, the phenoxyl radicals generated in oxidation reaction of chlorophenols (2-CP; 4-CP; 2,4-DCP; 2,4,6-TCP and 2,3,4,6-Tetra-CP) with hydrogen peroxide, catalyzed by horseradish peroxidase, were investigated. The method is highly sensitive. Phenoxyl radicals generated from as low as 1 × 10⁻⁸ M 2,4-DCP, for example, can be readily detected with the proposed method. The results show that the reactivity of various phenoxyl radicals are in the following order: 2,4-DCP > 4-CP > 2-CP > 2,4,6-TCP > 2,3,4,6-Tetra-CP. A mechanism is proposed to explain the possible pathway of the probe reaction. The feasibility of this method was assessed by the determination of enzymatic generation of phenoxyl radicals in lake water samples.     

Preparation of18F source for slow positron beam by proton bombardment of18O-water

Fluorine-18 produced by the¹⁸O(p,n)¹⁸F reaction on¹⁸O-water has proved to be highly useful as a source for a slow positron beam. About 70 GBq of¹⁸F is produced routinely by an ultra-compact cyclotron. The¹⁸F formed in H₂ ¹⁸O target is sent through a fine pipe to the site of positron slowing-down, fixed on a small spot by adsorption or drying, and then placed close to the moderator foil. An automatic apparatus has been set up for the entire process including the recovery of H₂ ¹⁸O.     

A cryogenic beam of refractory, chemically reactive molecules with expansion cooling

Cryogenically cooled buffer gas beam sources of the molecule thorium monoxide (ThO) are optimized and characterized. Both helium and neon buffer gas sources are shown to produce ThO beams with high flux, low divergence, low forward velocity, and cold internal temperature for a variety of stagnation densities and nozzle diameters. The beam operates with a buffer gas stagnation density of ～10(15)-10(16) cm(-3) (Reynolds number ～1-100), resulting in expansion cooling of the internal temperature of the ThO to as low as 2 K. For the neon (helium) based source, this represents cooling by a factor of about 10 (2) from the initial nozzle temperature of about 20 K (4 K). These sources deliver ～10(11) ThO molecules in a single quantum state within a 1-3 ms long pulse at 10 Hz repetition rate. Under conditions optimized for a future precision spectroscopy application [A. C. Vutha et al., J. Phys. B: At., Mol. Opt. Phys., 2010, 43, 074007], the neon-based beam has the following characteristics: forward velocity of 170 m s(-1), internal temperature of 3.4 K, and brightness of 3 × 10(11) ground state molecules per steradian per pulse. Compared to typical supersonic sources, the relatively low stagnation density of this source and the fact that the cooling mechanism relies only on collisions with an inert buffer gas make it widely applicable to many atomic and molecular species, including those which are chemically reactive, such as ThO.     

A new tin-free source of amidyl radicals

[reaction: see text] The readily available N-(O-ethyl thiocarbonylsulfanyl)amides are powerful amidyl radical precursors that undergo 5-exo cyclization to give pyrrolidinone derivatives via a radical-chain reaction initiated by a small amount of lauroyl peroxide.     

A family of new boron-containing free radicals

The first spectroscopic evidence for the existence of the HBX and DBX (X = F, Cl, Br) free radicals has been obtained from laser-induced fluorescence and wavelength-resolved emission spectra. The vibrational frequencies measured in emission are in excellent agreement with our ab initio (CCSD(T)/aug-cc-pVTZ) predictions. The patterns of resolved rotational sub-band structure and deuterium isotope effects have been used to positively identify the radicals. The experimental data and ab initio predictions will be useful in searches for the matrix infrared and microwave spectra of these new radicals.     

A long-lived luminescence and EPR bimodal lanthanide-based probe for free radicals

We developed a novel spin-labeled terbium complex Tb(3+)/cs124-DTPA-TEMPO (1) by covalently labeling a nitroxide radical on the terbium complex for monitoring free radicals of various areas. This lanthanide complex probe shows a high EPR signal which resulted from the nitroxide radical moiety, and is weakly luminescent which resulted from the intramolecular quenching effect of the nitroxide radical on sensitised terbium luminescence. The intensity of both the EPR and luminescence can be modulated by eliminating the paramagnetism of the nitroxide radical through recognition of a carbon-centered radical analyte and thus gives a quantification of the analyte. We have preliminarily applied this probe in the luminescent detection of model carbon-centered radicals and hydroxyl radicals (·OH). This probe is water-soluble and contains lanthanide-luminescence properties, favorable for the time-resolved luminescence technique. The investigation of the intramolecular quenching process has showed that the labeled nitroxide radical quenches multiple excited states of the terbium complex, resulting in highly efficient quenching of terbium luminescence. This probe is the first example of intramolecular modulation of lanthanide luminescence by a nitroxide radical.     

Relation of the decay of free radicals in irradiated polyethylene to the molecular motion of the polymer and the configurations of the free radicals

Decay reactions of the free radicals produced in irradiated polyethylene (high-density and low-density materials) were examined in connection with the molecular motion of the matrix polymer. Three temperature regions, in which the free radicals decay very rapidly, at around 120, 200, and 250°K, were designated T_A, T_L, and T_B, respectively. The decay of the free radicals at these temperatures had activation energies in high-density polyethylene of 0.4 kcal/mole for T_A, 9.4 kcal/mole for T_L, and 18.4 kcal/mole for T_B. In low-density polyethylene these quantities were 0.7 kcal/mole for T_A, 23.1 kcal/mole for T_L, and 24.8 kcal/mole for T_B. Comparison of time constants for the decay reactions and for molecular motion of the matrix polymer indicate that the decay in T_A and T_B is closely related to molecular motion in the amorphous regions of the polymer. The decay of the free radicals at T_L in high-density polyethylene is due to molecular motion associated with local mode relaxation at lamellar surfaces, while that of low-density polyethylene is due to local mode relaxation in the completely amorphous region. Steric configurations of the free radicals which decay in the respective temperature regions were also investigated.     

Modeling a reaction path by molecular mechanics: Dimerization of carbon free radicals

A variable force field model for radical dimerization was developed. It uses MM2 force constants for most atoms and new parameters only for the core atoms involved in bonding changes. The change in hybridization from sp² to sp³ is modeled using distance-dependent switch functions. The validity of the model has been tested by calculating the minimum energy path of the dimerization of di-tert-butylmethyl radicals. The calculated and experimental values for the enthalpy of activation of both dimerization of the radicals and dissociation of the dimer are in excellent agreement. The model has also been successfully applied to the stereoselective dimerization of 1-phenylneopentyl radicals: The form of the potential energy surface yields an explanation for the observed stereoselectivity. Another common feature in radical dimerization seems to be the formation of adsorption complexes prior to dimerization that can lead to increased reactivity. The results suggest that it is important to analyze the whole reaction path and not only the transition state alone.     

Merged-beams for slow molecular collision experiments

Molecular collisions can be studied at very low relative kinetic energies, in the milliKelvin range, by merging codirectional beams with much higher translational energies, extending even to the kiloKelvin range, provided that the beam speeds can be closely matched. This technique provides far more intensity and wider chemical scope than methods that require slowing both collision partners. Previously, at far higher energies, merged beams have been widely used with ions and/or neutrals formed by charge transfer. Here, we assess for neutral, thermal molecular beams the range and resolution of collision energy that now appears attainable, determined chiefly by velocity spreads within the merged beams. Our treatment deals both with velocity distributions familiar for molecular beams formed by effusion or supersonic expansion, and an unorthodox variant produced by a rotating supersonic source capable of scanning the lab beam velocity over a wide range.     

Thermolyses of O-phenyl oxime ethers. A new source of iminyl radicals and a new source of aryloxyl radicals

Six O-phenyl ketoxime ethers, RR'C=NOPh 1-6, with RR' = diaryl, dialkyl, and arylalkyl, together with N-phenoxybenzimidic acid phenyl ether, PhO(Ph)C=NOPh, 7, have been shown to thermolyze at moderate temperatures with "clean" N-O bond homolyses to yield iminyl and phenoxyl radicals, RR'C=N(*) and PhO(*). Since 1-6 can be synthesized at room temperature, these compounds provide a new and potentially useful source of iminyls for syntheses. The iminyl from 7 undergoes a competition between beta-scission, to PhCN and PhO(*), and cyclization to an oxazole. Rate constants, 10(6) k/s(-1), at 90 degrees C for 1-6 range from 4.2 (RR' = 9-fluorenyl) to 180 (RR' = 9-bicyclononanyl), and that for 7 is 0.61. The estimated activation enthalpies for N-O bond scission are in satisfactory agreement with the results of DFT calculations of N-O bond dissociation enthalpies, BDEs, and represent the first thermochemical data for any reaction yielding iminyl radicals. The small range in k (N-O homolyses) is consistent with the known sigma structure of these radicals, and the variations in k and N-O BDEs with changes in RR' are rationalized in terms of iminyl radical stabilization by hyperconjugation: RR'C=N(*) <--> R(*)R'C[triple bond]N. Calculated N-H BDEs in the corresponding RR'C=NH are also presented.     

Capillary array as an effusive molecular beam source for high resolution recoil ion momentum spectrometry

The role of a capillary array as a molecular beam source for use in recoil ion momentum spectrometry in ion-atom collisions is investigated. Numerical simulations show that by using a capillary array it is possible to obtain a half-width of 1 meV at room temperature, while the sensitivity could be in the range as low as 100 µeV. Preliminary measurements with electron impact ionisation showed the effectiveness of this technique by providing an energy resolution of about 2 meV at room temperature, which is superior to the resolution reported using a gas cell cooled to 30° K.     

Free sodium-water clusters: photoionisation studies in a pulsed molecular beam source

Neutral Na·(H₂O) _n clusters are studied by near-uv one-photon ionisation and time-of-flight mass spectroscopy. The clusters are formed in a “pickup source” by injection of a beam of Na atoms into the expansion zone of a pulsed nozzle beam of water vapour seeded into an argon carrier gas. The performance of this novel technique for studying cold aggregates of potentially reactive species is discussed in detail. The photoion efficiency (PIE) spectrum of the monomer near its ionisation threshold (4.379(2) eV) shows a rich structure. Vibrational frequencies of the ion can be deduced and some indication of molecular Rydberg states is seen. Ionisation potentials for larger clusters and the binding energies of the neutral clusters up ton=5 are reported.     

Photosensitivity of methylene chloride,stimulated by free radicals

Conclusions The direct photolysis of CH₂C1₂ leads to the formation of HC1 and unsaturated compounds with conjugated C=C bonds. These same products are also formed under the influence of light that is not absorbed by CH₂C1₂ ( > 290 nm) in the presence of initiators of free radical reactions: benzoyl peroxide, benzophenone, anthraquinone, and Fe³⁺ chlorides.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 9, pp. 2094–2097, September, 1976.