Polarization studies of Lyman-α radiation emitted in atom-H2-molecule collisions

A polarization study of Lyman-α radiation emitted in collisions of H⁺, H, He⁺, and He projectiles with H₂ molecules has been performed at projectile energies ranging from 1–25 keV. In H⁺?H₂ and H?H₂ collisions, the measured linear polarization is negative at low incident velocities, indicating a preferred alignment of the excited electron charge cloud perpendicular to the incident projectile direction. This is taken as significance for a rotational coupling which in these two collision systems predominantly populates the H(2p _±1) substates. In He⁺?H₂ and He?H₂ collisions, the measured linear polarization is about zero and independent of the projectile energy. In these more asymmetric systems,a′?a′ and a′?a″ couplings are now of comparable magnitude.     

The singlet and triplet states of phenyl cation. A hybrid approach for locating minimum energy crossing points between non-interacting potential energy surfaces

The phenyl cation is known to have two low-energy minima, corresponding to ¹ A ₁ and ³ B ₁ states, the first of which is more stable by ca. 25 kcal/mol. The minimum energy crossing point between these two surfaces, located at various levels including a hybrid method first described here, lies just above the minimum of the triplet, 0.12 kcal/mol at the CCSD(T)/cc-pVDZ//B3LYP/SV level, and there is significant spin-orbit coupling between the surfaces at this point. On the basis of these results, the lifetime of the triplet is expected to be very short. Received: 24 October 1997 / Accepted: 25 November 1997     

Study of ion structures produced by the reaction of 2-propyl cations with water and methanol; covalently bound v. Hydrogen-bridged adducts

In contrast to an earlier report,¹ the collisonally induced dissociation of protonated 2-propanol and t-butyl alcohol yields spectra that are indistinguishable from those of the corresponding [C₃H₇/H₂O]⁺ and [C₄H₉/H₂O]⁺ ions generated by the (formal) gas phase addition reactions in a high pressure ion source of [s-C₃H₇]⁺ and [t-C₄H₉]⁺ ions with the n-donor H₂O. Similarly, [s-C₃H₇/CH₃OH]⁺ ions generated by both gas phase protonation of n- and s-propyl methyl ethers and addition reactions of [C₃H₇]⁺ to CH₃OH display mode-of-generation-independent collisionally induced dissociation characteristics. However, analysis of the unimolecular dissociation (loss of propene) of the [C₃H₇/CH₃OH]⁺ system, including a number of its deuterium, ¹³C- and ¹⁸O-labelled isotopomers, supports the idea that prior to unimolecular dissociation, covalently bound [C₃H₇- O(H)CH₃]⁺ ions intercovert with hydrogen-bridged adduct ions, analogous to the behaviour of the distonic ethene-, propene- and ketene-H₂O radical cations.     

Regioselective synthesis of ω-bromo-3-ketosulfones, ω-bromo-3-ketonitriles and 2-(ω-bromoalkyl)benzofurans based on a ‘ring-closing/ring-opening’ strategy

ω-Bromo-3-ketosulfones, ω-bromo-3-ketonitriles and various functionalized 2-(ω-bromoalkyl)benzofurans were chemo- and regioselectively prepared by application of a ‘ring-closing/ring-opening’ strategy. The cyclization of 3-ketosulfone and 3-ketonitrile dianions with 1-bromo-2-chloroethane or 1,4-dibromobut-2-ene afforded functionalized 2-alkylidenetetrahydrofurans which were subsequently cleaved by reaction with boron tribromide or boron trichloride to give the final products.     

Preparation of (S)-2-substituted succinates by stereospecific reductions of fumarate and derivatives with resting cells of Clostridium formicoaceticum

Fumarate derivatives have been reduced to (S)-2-methylsuccinate 2a, (S)-2-ethylsuccinate 3a and (S)-2-chlorosuccinate 4a in up to 1 M concentrations with Clostridium formicoaceticum. Formate was the electron donor and viologens or anthraquinone-2,6-disulphonate acted as artificial electron mediators. Reductions with freeze-dried cells in ²H₂O-buffers led to the (2R,3S)-[2,3-2,²H]-dideuterated succinate derivatives. The productivity numbers¹ ranged from 450 to 5000 and the enantiomeric excess of all (S)-2-substituted succinates was 99 %.     

The Photorearrangements of a Naphthobarrelene-like System. Dependence on excited-state spin multiplicity and electronic configuration,and evidence for biradical intermediates. Preliminary communication

8-Benzoyl-9-deuterio-naphtho [de-2.3.4]bicyclo [3.2.2]nona-2,6,8-triene ( 1 ) rearranged quantitatively in a photochemical di-π-methane-type process to 2-, 6-, and 9-deuteriated 1-benzoyl-naphtho [de-2.3.4]tricyclo [4.3.0.0^2,9]nona-2, 6-diene ( 8a–c ). The phenylhydroxymethyl analogue 2 underwent a similar regioselective rearrangement to 9a–c . The rearrangement 1 → 8a–c is proposed to proceed along three reaction paths evolving from two primary photochemical processes of naphthylvinyl and vinyl-vinyl bonding ( 1 → 3 + 6 ). Evidence for a competition between several paths and involvement of biradical intermediates derives from changes in the isotopomeric composition with temperature, and from laser flash detection (λ_exc 353 nm) of a transient. The dependence of the quantum yield for product formation from 1 on excitation wavelength and sensitizer triplet energy leads to the conclusion that reaction to the primary biradicals occurs directly from the S₁ (n, π*) and T₂ (n, π* ) states, and that reaction from T₁ (π, π*) and from S₂ (π, π*) proceed either directly or via T₂.     

Subalgebras that are cyclic as submodules

Let R be an associative, commutative, unital ring. By a R-algebra we mean a unital R-module A together with a R-module homomorphism : _R ⁿ AA (n2). We raise the question whether such an algebra possesses either an idempotent or a nilpotent element. In section 1 an affirmative answer is obtained in case R=k is an algebraically closed field and dim_kA<, as well as in case R=, dimS<, and n0(2). Section 2 deals with the case of reduced rings R and R-algebras which are finitely generated and projective as R-modules. In section 3 we show that the generic algebra over an integral domain D fails to have nilpotent elements in any integral domain extending its base ring D_n,m, and thus acquires an idempotent element in some integral domain extending D_n,m.Partially supported by National Science Foundation Grant GP-38229.     

On purely radiative space-times

The existence of space-times representing pure gravitational radiation which comes in from infinity and interacts with itself is discussed. They are characterized as solutions of Einstein's vacuum field equations possessing a smooth structure at past null infinity which forms the future null cone at past timelike infinity with complete generators. The pure radiation problem is analysed where free initial data for Einstein's field equations are prescribed on the null cone at past time-like infinity. It is demonstrated how the pure radiation problem can be formulated as a local initial value problem for the symmetric hyperbolic system of reduced conformal vacuum field equations. Its solutions are uniquely determined by the free data.Work supported by a Heisenberg-fellowship of the Deutsche Forschungsgemeinschaft     

Decomposition of neutral, singly and doubly protonated benzoquinone in the gas phase

The unimolecular fragmentations of singly and doubly protonated ortho-, meta-, and para-benzoquinones (BQH(+) and BQH(2)(2+), respectively) are studied by tandem mass spectrometry. The dominant fragmentation pathways lead to the elimination of a neutral CO molecule from BQH(+) and, by charge separation, to the expulsion of protonated CO from BQH(2)(2+). Reaction mechanisms are elucidated based on labeling experiments and UB3 LYP calculations. These results reveal that the respective reactions proceed in an analogous fashion to the decarbonylation of neutral benzoquinones, which decompose into carbon monoxide and cyclopentadienone. Single protonation facilitates all steps on the reaction pathway with neutral CO and O-protonated cyclopentadienone as final products. In contrast, double protonation leads to an increase of the barriers for the decomposition yielding CO.H(+) and O-protonated cyclopentadienone. This major process of BQH(2)(2+) is accompanied by two minor channels, which lead to the elimination of neutral carbon monoxide and water, respectively. The proton affinity of the para-BQH(+) monocation is estimated as 3.6+/-0.3 eV.