Positron scattering from the cyclic ethers oxirane, 1,4-dioxane, and tetrahydropyran

In this paper we report original measurements of total cross sections (TCSs) for positron scattering from the cyclic ethers oxirane (C(2)H(4)O), 1,4-dioxane (C(4)H(8)O(2)), and tetrahydropyran (C(5)H(10)O). The present experiments focus on the low energy range from ～0.2 to 50 eV, with an energy resolution smaller than 300 meV. This study concludes our systematic investigation into TCSs for a class of organic compounds that can be thought of as sub-units or moieties to the nucleotides in living matter, and which as a consequence have become topical for scientists seeking to simulate particle tracks in matter. Note that as TCSs specify the mean free path between collisions in such simulations, they have enjoyed something of a recent renaissance in interest because of that application. For oxirane, we also report original Schwinger multichannel elastic integral cross section (ICS) calculations at the static and static plus polarisation levels, and with and without Born-closure that attempts to account for the permanent dipole moment of C(2)H(4)O. Those elastic ICSs are computed for the energy range 0.5-10 eV. To the best of our knowledge, there are no other experimental results or theoretical calculations against which we can compare the present positron TCSs. However, electron TCSs for oxirane (also known as ethylene oxide) and tetrahydropyran do currently exist in the literature and a comparison to them for each species will be presented.     

Rate constants for the gas‐phase reactions of Cl atoms with a series of ethers

The laser photolysis–resonance fluorescence technique has been used to determine the absolute rate coefficient for the Cl atom reaction with a series of ethers, at room temperature (298 ± 2) K and in the pressure range 15–60 Torr. The rate coefficients obtained (in units of cm³ molecule⁻¹ s⁻¹) are dimethyl ether (1.3 ± 0.2) × 10⁻¹⁰, diethyl ether (2.5 ± 0.3) × 10⁻¹⁰, di‐n‐propyl ether (3.6 ± 0.4) × 10⁻¹⁰, di‐n‐butyl ether (4.5 ± 0.5) × 10⁻¹⁰, di‐isopropyl ether (1.6 ± 0.2) × 10⁻¹⁰, methyl tert‐butyl ether (1.4 ± 0.2) × 10⁻¹⁰, and ethyl tert‐butyl ether (1.5 ± 0.2) × 10⁻¹⁰. The results are discussed in terms of structure–reactivity relationship. © 2000 John Wiley & Sons, Inc. Int J Chem Kinet 32: 105–110, 2000     

Photoinitiated cationic polymerization of oxetane formulated with oxirane

Photoinitiated cationic polymerization of oxetane, oxirane (epoxide), and a formulation of both was carried out and their reactivity compared. To investigate a formulated system of oxetane and oxirane in photoinitiated cationic polymerization, computational and experimental methods were used. In the computational study, we employed a semiempirical molecular orbital method (AM1). On the other hand, the reactivities of each system were evaluated and compared experimentally by a real-time FT-IR method. The computational study reveals that oxetane seems to polymerize in S_N2 mechanism, but two possibilities, of S_N1 mechanism through the α-cleavage and of S_N2 mechanism through β-cleavage, are implied for oxirane. Using the real-time FT-IR method, the formulation of oxetane and oxirane was proved to possess rather high reactivities of oxetane toward photoinitiated cationic polymerization. The formulated system exhibited slightly lower number-average molecular weight than oxetane but higher than oxirane. These experimental observations are well explained in terms of the calculated reaction paths. © 1995 John Wiley & Sons, Inc.     

A theoretical prediction of stability in hydrogen-bonded complexes formed between oxirane and oxetane rings with HX (X=F and Cl)

The optimised geometries of heterocyclic hydrogen-bonded complexes, C2H4O...HX and C3H6O...HX, where X=F or Cl, were determined at DFT/B3LYP/6-311++G(d,p) computational level. Structural, electronic and vibrational properties of these complexes are used in order to compare the strained ring, which confer the great reactivity of these heterocyclic rings with monoprotic acids, forming a primary hydrogen bond. A secondary hydrogen bond between the hydrogen atoms of the CH2 groups and the halide species also takes place, thus causing a nonlinearity (characterized by the theta angle), in the primary hydrogen bond.     

Insertion reactions of organochlorosilanes with cyclic ethers

Phenyl glycidyl ether and allyl glycidyl ether (3-phenoxy- and 3-allyloxy-1,2-epoxypropane) cause fission of the silicon---chlorine bonds in a number of alkylchlorosilanes to form chloro-substituted alkoxysilanes. Relative reactivities of cyclic ethers with chlorosilanes have been discussed. Structures of the adducts are suggested on the basis of their IR and proton magnetic resonance spectra.     

Kinetics of the reactions of fluorine and chlorine atoms with ethylene oxide (oxirane)

The kinetics of the reactions of F and C1 atoms with ethylene oxide have been studied using relative rate techniques in 10–700 Torr of either nitrogen or air diluent at 295 ± 2 K; k(F + C₂H₄O) = (9.4 ± 1.6) × 10^?11 and k(C1 + C₂H₄O) = (5.0 ± 0.9) × 10^?12 cm³ molecule^?1 s^?1. The result for k(F + C₂H₄O) is in good agreement with the literature data. The result for k(C1 + C₂H₄O) is a factor of 5.6 lower than that reported previously. It seems likely that in the previous study most of the loss of C₂H₄O attributed to reaction with C1 atoms was actually caused by unwanted secondary reactions leading to an overestimate of k(C1 + C₂H₄O). © 2001 John Wiley & Sons, Inc. Int J Chem Kinet 34: 122–125, 2002     

Next-generation quantum theory of atoms in molecules for the photochemical ring-opening reactions of oxirane

The conical intersections corresponding to the C─O and C─C ring opening were optimized and the reaction paths traversing these intersections were obtained. Investigation of the C─O ring opening revealed that when traversing the lowest energy conical intersection, the reaction path returns to the closed ring geometry. The C─O path traversing the intersection featuring torsion of terminal CH₂ group however, led to a ring-opened geometry, an H-shift and the formation of acetaldehyde that can undergo further dissociation. The observation of different reaction paths was explained by the 3-D paths from quantum theory of atoms in molecules (QTAIM) that defined the most preferred direction of electronic motion that precisely tracked the mechanisms of bond breaking and formation throughout the photo-reactions. The size, orientation, and location of these most preferred 3-D paths indicated the extent and direction of motion of atoms, bonds, and the degree of torsion or planarity of a bond indicating a predictive ability.     

Photoaddition reactions of fluoroolefins with diols and cyclic ethers

Fluoroalkylation of aliphatic diols, tetrahydrofuran and 1,3-dioxolanes with perfluoroolefins under photochemical initiation was studied. The aim of the study was the preparation of fluoroalkylated diols as intermediates for special fluorinated monomers. Fluoroalkylation was only partly successful in the case of 1,4-butanediol and quite successful with tetrahydrofuran and 2,2-dimethyl-1,3-dioxolane. The fluoroalkyl diols obtained were converted to monomethacrylates that are amphiphilic monomers.     

The dynamics of reaction of Cl atoms with tetramethylsilane

Rotational state distributions and state-selected CM-frame angular distributions were measured for HCl (v' = 0, j') products from the reaction of Cl-atoms with tetramethylsilane (TMS) under single collision conditions at a collision energy, E(coll), of 8.2 +/- 2.0 kcal mol(-1). The internal excitation of these products was very low with only 2% of the total energy available partitioned into HCl rotation. A transition state with a quasi-linear C-H-Cl moiety structure was computed and used to explain this finding. A backward peaking differential cross section was also reported together with a product translational energy (T') distribution with a maximum at T' approximately E(coll). This scattering behaviour is accounted for by reactions proceeding through a tight transition state on a highly skewed potential energy surface, which favours collisions at low impact parameters with a strong kinematic constraint on the internal excitation of the products. The large Arrhenius pre-exponential factor previously reported for this reaction is reconciled with the tight differential scattering observed in our study by considering the large size of the TMS molecule.     

Ring-opening reactions of cyclic ethers with diiodo- and dibromodimethylsilane equivalents

Ring-opening halosilation of cyclic ethers with reagents of (Me₂N)₂SiMe₂/4MeI (1a) and (Me₂N)₂SiMe₂/4allylBr (1b) was studied. Tetrahydrofuran and cyclohexene oxide reacted with 1a and 1b to give ring-opened di(haloalkoxy)dimethylsilanes in good yield. With less strained tetrahydropyran, however, only reagent 1a gave the ring-opened product. Reactions of reagents 1a and 1b with propylene oxide also proceeded smoothly, although the regioselectivity was rather low. When similar reactions were carried out with (Me₂N)₂SiMe₂/2MeI (2a) and (Me₂N)₂SiMe₂/2allylBr (2b) in a ratio of cyclic ethers/2a or 2b = 1/1, the corresponding 1:1 adducts were obtained.     

Kinetics of the reactions of pinonaldehyde with OH radicals and with Cl atoms

The rate constant for the reaction of OH radicals with pinonaldehyde has been measured at 293 ± 6 K using the relative rate method in the laboratory in Wuppertal (Germany) using photolytic sources for the production of OH radicals and in the EUPHORE smog chamber facility in Valencia (Spain) using the in situ ozonolysis of 2,3‐dimethyl‐2‐butene as a dark source of OH radicals. In all the experiments pinonaldehyde and the reference compounds were monitored by FTIR spectroscopy, and in addition in the EUPHORE smog chamber the decay of pinonaldehyde was monitored by the HPLC/DNPH method and the reference compound by GC/FID. The results from all the different series of experiments were in good agreement and lead to an average value of k(pinonaldehyde + OH) = (4.0 ± 1.0) × 10⁻¹¹ cm³ molecule⁻¹ s⁻¹. This result lead to steady‐state estimates of atmospheric pinonaldehyde concentrations in the ppbV range (1 ppbV ≈ 2.5 × 10¹⁰ molecule cm⁻³ at 298 K and 1 atm) in regions with substantial α‐pinene emission. It also implies that atmospheric sinks of pinonaldehyde by reaction with OH radicals could be half as important as its photolysis. The rate constant of the reaction of pinonaldehyde with Cl atoms has been measured for the first time. Relative rate measurements lead to a value of k(pinonaldehyde + Cl) = (2.4 ± 1.4) × 10⁻¹⁰ cm³ molecule⁻¹ s⁻¹. In contrast to previous studies which suggested enhanced kinetic reactivity for pinonaldehyde compared to other aldehydes, the results from both the OH‐ and Cl‐initiated oxidation of pinonaldehyde in the present work are in line with predictions using structure‐activity relationships. © 1999 John Wiley & Sons, Inc., Int J Chem Kinet 31: 291–301, 1999     

Imaging the dynamics of gas phase reactions

Ion imaging methods are making ever greater impact on studies of gas phase molecular reaction dynamics. This article traces the evolution of the technique, highlights some of the more important breakthroughs with regards to improving image resolution and in image processing and analysis methods, and then proceeds to illustrate some of the many applications to which the technique is now being applied--most notably in studies of molecular photodissociation and of bimolecular reaction dynamics.     

Theoretical approach to the mechanism of reactions between halogen atoms and unsaturated hydrocarbons: the Cl + propene reaction

The potential energy surface for the Cl + propene reaction was analyzed at the MP2 level using Pople's 6-31G(d,p) and 6-311+G(d,p), and Dunning's cc-pVDZ and aug-cc-pVDZ basis sets. Two different channels for the addition reaction leading to chloroalkyl radicals and five alternative channels for the abstraction reaction leading to C(3)H(5) (.) + HCl were explored. The corresponding energy profiles were computed at the QCISD(T)/aug-cc-pVDZ//MP2/aug-cc-pVDZ level of theory. Theoretical results suggest that the previously established mechanism consisting of (1) direct abstraction and (2) addition-elimination steps is instead made up of (1) addition through an intermediate and (2) two-step abstraction processes. No direct abstraction mechanism exists on the potential energy surface. The kinetic equations derived for the new mechanism are consistent with the pressure dependence experimentally observed for this reaction.     

Kinetics and mechanism of the reactions of diethylzinc with phenols in cyclic ethers

Reactions of diethylzinc and phenols (phenol, 2-ethylphenol, 2-chlorophenol, 3-ethylphenol, 3-chlorophenol, 4-ethylphenol and 4-chlorophenol) have been carried out in tetrahydrofuran and 1,4-dioxane as solvents. Monomeric ethylzinc phenoxide has been found to be a product of the diethylzinc and phenol (1:1) reaction in 1,4-dioxane solution. Kinetic studies on the ethylzinc phenoxides and phenols reaction in tetrahydrofuran solution established the rate constants and the S_Ei mechanism of the reaction.     

Rate constants for the reactions of Cl atoms with HCOOH and with HOCO radicals

Rate constants have been measured at room temperature for the reactions of Cl atoms with formic acid and with the HOCO radical: Cl + HCOOH → HCl + HOCO (R1) Cl + HOCO → HCl + CO₂ (R2) Cl atoms were generated by flash photolysis of Cl₂ and the progress of reaction was followed by time‐resolved infrared absorption measurements using tunable diode lasers on the CO₂ that was formed either in the pair of reactions ( R1 ) plus ( R2 ), or in reaction ( R1 ) followed by O₂ + HOCO → HO₂ + CO₂ (R3) In a separate series of experiments, conditions were chosen so that the kinetics of CO₂ formation were dominated either by the rate of reaction ( R1 ) or by that of reactions ( R1 ) and ( R2 ) combined. The results of our analysis of these experiments yielded: k₁ = (1.8₃ ± 0.12) × 10⁻¹³ cm³ molecule⁻¹ s⁻¹ k₂ = (4.8 ± 1.0) × 10⁻¹¹ cm³ molecule⁻¹ s⁻¹ © 2000 John Wiley & Sons, Inc. Int J Chem Kinet 32: 85–91, 2000     

35Cl NQR of cyclic α-chlorovinyl ethers and orbital interactions in O-C-Cl triad

Conclusions In a series of compounds with fragments OC_sp3-Cl or OC_sp2-Cl, O=CCl, the relationships observed in the changes in³⁵Cl NQR frequencies are explained by geometric and energy characteristics of the orbitals n(0) and*(CCl).Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 10, pp. 2204–2207, October, 1985.     

Ab initio and direct dynamics study of the reaction of Cl atoms with HOCO

The reaction of Cl with HOCO has been examined using the coupled-cluster method to locate and optimize the critical points on the ground-state potential energy surface. The results show that the reaction produces the HCl and CO(2) products as experimentally observed. The reaction occurs via a HOC(O)Cl intermediate with an estimated heat of formation of -97.8+/-2.0 kcal/mol. A direct ab initio dynamics method has been used to provide insight into the reaction mechanisms and to determine the thermal rate coefficients in the temperature range of 200-600 K. At room temperature, the thermal rate coefficient is predicted to be 3.0x10(-11) cm(3) molecule(-1) s(-1) with an activation energy of -0.2 kcal/mol. Two kinds of reactive trajectories are found. One kind proceeds through short-lived HOC(O)Cl complexes with a lifetime of 310 fs while the other kind occurs via longer-lived intermediates with a lifetime of 1.9 ps.     

Classical trajectory study of the dynamics of the reaction of Cl atoms with ethane

We present an electronic-structure and dynamics study of the Cl + C2H6 --> HCl + C2H5 reaction. The stationary points of the ground-state potential energy surface have been characterized using various electronic-structure methods and basis sets. Our best calculations, CCSD(T) extrapolated to the complete basis limit, using geometries and harmonic frequencies obtained at the MP2/aug-cc-pVTZ level, are in agreement with the experimental reaction energy. Ab initio information has been used to reparameterize a semiempirical Hamiltonian so that the predictions of the improved Hamiltonian agree with the higher-level calculations in key regions of the potential energy surface. The improved semiempirical Hamiltonian is then used to propagate quasiclassical trajectories. Computed kinetic energy release and scattering angle distributions at a collision energy of approximately 5.5 kcal mol(-1) are in reasonable agreement with experiments, but no evidence was found for the low translational energy HCl products scattered in the backward hemisphere reported in recent experiments.     

State-to-state chemiluminescence in reactions of Mn atoms with S2Cl2

A combined experimental and time-dependent density functional theory (TDDFT) investigation of the title reaction is presented. Both 'hot' and 'cold' laser-ablated Mn atom beams have been employed to determine the translational excitation functions for production of MnCl*(c(5)Σ(+), d(5)Π, e(5)Δ, e(5)Σ(+), A(7)Π). Analysis in terms of the multiple line-of-centres approach shows that the 'hot' results are dominated by reactions of the second metastable state of Mn, z(8)P(J), all with very low thresholds; while the first metastable state, a(6)D(J), and the ground state, a(6)S, are the precursors in the 'cold' results, all with significant excess barriers. The post-threshold behaviour of most z(8)P(J) and a(6)D(J) reaction channels implies that the transition states shift forward with increasing collision energy. The TDDFT calculations suggest that, while Mn*(z(8)P(J), a(6)D(J)) insertion into the S-Cl bond is facile, the observed chemiluminescence channels mostly derive from abstraction in a preferred linear Mn-Cl-S configuration, and that the low z(8)P(J) thresholds originate from attractive but excited reagent potentials which either reach a seam of interactions in the product valley or (in the c(5)Σ(+) case) lead to an octet potential very close in energy to the product sextet. The excess barriers in the Mn*(a(6)D(J)) and Mn(a(6)S) reactions appear for the most part to derive from exit channel mixing with lower-lying product potentials. The observed transition state shifts are consistent with the system being forced to ride up the repulsive wall of the entrance valley as collision energy increases, the location of that wall being different for the z(8)P(J) and a(6)D(J) cases.     

Triblock copolymers based on cyclic ethers: Preparation and properties of tetrahydrofuran and 3,3-bis(azidomethyl) oxetane triblock copolymers

Novel energetic thermoplastic elastomers (TPEs) based on tetrahydrofuran (THF) and 3,3-bis (azidomethyl) oxetane (BAMO) were prepared in this present study by cationic living polymerization. A bifunctional catalyst, triflic anhydride [(CF₃SO₂)₂)O] was selected to be an initiator for the polymerization THF and BAMO. The resulting polymers were characterized by IR, NMR, and DSC, which demonstrated that triblock copolymers with A-B-A type were formed. The polymers were indicated from thermogravimetric analysis (TGA) to have decomposed at approximately 243°C. The decomposition enthalpies were determined by DSC. These enthalpies were varied with the poly-BAMO contents of the copolymers. The synthesized polymers exhibited relatively good mechanical properties and thermoplastic characteristics at room temperature. © 1994 John Wiley & Sons, Inc.