Gas-phase reactions of lithium dimethylaminoborohydride and related species

[reaction: see text] Ab initio and density functional theory (DFT) calculations were used to examine the mechanisms of reduction and amination of chloromethane by lithium dimethylaminoborohydride (LAB) in the gas phase. For comparison, the amination of chloromethane by lithium dimethylamide and the reduction by borane, diborane, and borohydride ions were also examined. The reduction of chloromethane by LAB occurred most readily from a conformation that allowed coordination of the lithium atom to the chloride leaving group, and the most favorable amination pathway occurred by a backside attack of the nitrogen nucleophile on chloromethane.     

The subjective assessment of trombone quality

A number of subjective experiments designed not only to assess the tonal and playing qualities of trombones but also to aid the interpretation of objective data is described. Quantifying multi-dimensional quantities such as timbre or quality is usually attempted either by multi-dimensional scaling (MDS) or by semantic differential scaling (SDS) and the advantages and disadvantages of each for the present purpose are briefly reviewed. For both methods additional information is needed either (for SDS) to aid the selection of scales, or (for MDS) to interpret the final results of the computer analysis; in this study such information is available from the investigations of Edwards [1]. A preliminary experiment testing the ability of subjects to rate trombone timbre using SDS showed that the intersubject variance was very large but that the order of factors governing timbre is (i) instrument, (ii) player and (iii) mouthpiece. A second experiment shows that pitch and loudness have a much greater influence on listener's ratings than do player, mouthpiece of instrument. Players' attitudes tested in further experiments show that the task of discriminating instruments when extraneous cues are removed is very hard indeed for most players, but a very few can be of quite remarkable discriminatory ability using either SDS or MDS. Results from one such player indicate clearly that timbre is the predominant acoustic factor deciding discrimination.     

Low-intensity transitions in the decay of Rb

The γ-rays following the decay of ⁸⁸Rb have been studied with a 40 cm³ Ge(Li) detector with the aim of resolving discrepancies in some recent determinations of the decay scheme of this isotope. The existence of a 1799 keV γ-ray has been confirmed and three new γ-rays with energies of 1218, 1295 and 1368 keV have been found. All of these results are in good agreement with a decay scheme recently proposed by Ragaini and Knight.     

Dependence of photoluminescence of n-GaAs on surface treatment

The changes in the intensity, shape and location of characteristic photoluminescence lines caused by various surface treatments in Te doped GaAs samples at 80 K were studied. The depth of the damage caused by such treatment is estimated. An explanation of the shifts of the characteristic lines based on the varying contribution of far donor-acceptor pairs is proposed.     

A microwave-excited emissive detector for gas chromatography Further studies with sulphur compounds

Improvements in the design and operation of the microwave excited detector for gas chromatography have led to an increase in the sensitivity and a lowering of detection limits for sulphur compounds.     

Rotationally resolved electronic spectra of 2- and 3-methylanisole in the gas phase: a study of methyl group internal rotation

Rotationally resolved fluorescence excitation spectra of several torsional bands in the S1 <-- S0 electronic spectra of 2-methylanisole (2MA) and 3-methylanisole (3MA) have been recorded in the collision-free environment of a molecular beam. Some of the bands can be fit with rigid rotor Hamiltonians; others exhibit perturbations produced by the coupling between the internal rotation of the methyl group and the overall rotation of the entire molecule. Analyses of these data show that 2MA and 3MA both have planar heavy-atom structures; 2MA has trans-disposed methyl and methoxy groups, whereas 3MA has both cis- and trans-disposed substituents. The preferred orientations (staggered or eclipsed) in two of the conformers and the internal rotation barriers of the methyl groups in all three conformers change when they are excited by light. Additionally, the values of the barriers opposing their motion depend on the relative positions of the substituent groups, in both electronic states. In contrast, no torsional motions of the attached methoxy groups were detected. Possible reasons for these behaviors are discussed.     

Organocatalytic living ring-opening polymerization of cyclic carbosiloxanes

[reaction: see text] An organocatalytic route to narrowly dispersed poly(carbosiloxanes) of predictable molecular weight and end group fidelity is described. N-Heterocyclic carbenes (NHC) and 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD) catalyze the ring opening of cyclic carbosiloxanes. The pK(b) of the catalyst is important in preventing adverse transetherification reactions and obtaining well-defined polymers. Mechanistic studies indicate that hydrogen bonding to TBD or the NHC activates alcohols or silanols for ring-opening reactions.     

Analytical gradients of a state average MCSCF state and a state average diagnostic

An efficient method for calculating the Lagrange multipliers and the analytical gradients of one state included in a state average MCSCF wave function is presented. It is demonstrated that the state average energy of an ‘equal-weight’ scheme is invariant to rotations within the state average subspace and that the corresponding rotations should be eliminated from the Lagrangian equations. Finally, a diagnostic is presented, which gauges the energy difference between a state defined by a state average calculation and the corresponding fully variational multi-configurational SCF state.