Internal energy content of n-butylbenzene, bromobenzene, iodobenzene and aniline molecular ions generated by two-photon ionization at 266 nm. A photodissociation study

A technique to investigate photodissociation kinetics on a nanosecond time scale has been devised for molecular ions generated by multiphoton ionization (MPI) using mass-analyzed ion kinetic energy spectrometry. The branching ratio or rate constant has been determined for the photodissociation of the n-butylbenzene, bromobenzene, iodobenzene, and aniline molecular ions generated by MPI at 266 nm. The ion internal energies have been estimated by comparing the measured kinetic data with the previous energy dependence data. The analysis has shown that only those molecular ions generated by two-photon ionization contribute to the photodissociation signals. Around half of the available energy has been found to remain as molecular ion internal energy in the two-photon ionization process. Copyright 1999 John Wiley & Sons, Ltd.     

Selective multiplex advantage with an electro-optic Hadamard transform spectrometer for multielemental atomic emission

A liquid-crystal spatial light-modulator Hadamard transform spectrometer is adapted for multielemental atomic spectrochemical analysis. The flame emissions of alkali metals are studied as a preliminary example. The multiplex disadvantage normally plaguing application of Hadamard and Fourier transform methods to atomic analysis is circumvented. Permanent electro-optic "closure" of certain Hadamard mask slits (corresponding to intense major element emissions) improves the signal-to-noise ratio (SNR) of the remaining trace element emissions. This approach to SNR enhancement of weaker spectral features by blocking known intense features is called the selective multiplex advantage. A problem with the contrast ratio (relative transmissions of the transparent and opaque states) of the liquid-crystal Hadamard mask has been identified in terms of "optical leakage". This produces an offset in the Hadamard encodegram, and leads to concentration-dependent baseline-offset effects in the transformed spectrum. A mathematical correction procedure was devised and evaluated experimentally.     

Gas-phase kinetic and mechanistic studies of some interconverting alkylcyclopropene pairs: involvement of dialkylvinylidene intermediates and their quantitative behaviour

The pyrolyses of two isomeric pairs of alkylcyclopropenes, namely 1,3-dimethyl-(15) and 1-ethyl-cyclopropene (16), and 1,3,3-trimethyl-(5) and 1-isopropyl-cyclopropene (17), have been studied in the gas phase. Complete product analyses at various conversions up to 95% were obtained for the decomposition of each compound at five temperatures over a 40 degrees C range. The time-evolution data showed that the isomerisation reactions 15<==>16 and 5<==>17 were occurring. Kinetic modelling of each system allowed the determination of rate constants for these and all other decomposition processes. Tests confirmed that all reactions were unimolecular and homogeneous. Arrhenius parameters are reported for overall reactions and individual product pathways. Further kinetic analysis allowed us to extract the propensities (at 500 K) for 1,3-C-H insertion of the dialkylvinylidene intermediates involved in the rearrangements as follows: k(prim):k(sec): k(tert)= 1:16.5:46.4. Additional experiments with 13C-labelled cyclopropenes yielded alkyl group migration aptitudes for the dialkylvinylidenes (from the pattern of 13C in the alkyne products) as follows: Me:Et:iPr=1:3.1:1.5. Explanations for these trends are given. Another important finding is that of the dramatic rate enhancements for 1,3-diene product formation from the 1-alkylcyclopropenes; this can be explained by either hyperconjugative stabilisation of the vinylcarbene intermediates involved in this pathway, or their differing propensities to 1,2 H-shift. The observed large variations in product distribution amongst these four cyclopropenes is interpreted in terms of these specific effects on individual pathways.     

Enhanced near-neighbour aggregation of dichloro- and trichloromethane in liquid methane solution from molecular dynamics simulation of radial site–site distribution functions: a simple predictor of solute–solvent compatibility

Molecular dynamics simulations have been performed for a range of equi-site and site–site radial distribution functions for the five-atom halomethane species dichloro-, trichloro-, and tetrachloromethane dissolved in the low-molecular weight hydrocarbons liquefied methane and cyclopropane, with the general aim of using this approach to predict good or bad solvent characteristics. It was found that methane solutions of dichloro- and trichloromethane showed an enhancement of near-neighbour occupancy, the methane solvent seemingly exhibiting a phobic, structure-promoting solvation behaviour towards the two solutes by increasing the number of nearest neighbours above the values that would result from a pure dilution effect caused by the solvent. It was verified that there were no significant regions of solid-like conformations nor remnants of imperfect average homogeneity within the system at the necessarily low temperature (183?K). On the other hand, simulated site–site radial functions with solvents tetrachloromethane and cyclopropane indicate normal solution characteristics towards solutes dichloro- and trichloromethane. The cause of the phobic solvation behaviour of solvent liquid methane towards di- and trichloromethane is not obvious, except that it seemingly involves the presence of hydrogen atoms on the solute species because the site–site centre-of-mass radial distribution functions of tetrachloromethane in liquid methane implied normal solution behaviour.     

Direct coulomb and exchange interaction in artificial atoms

We determine contributions from the direct Coulomb and exchange interactions to the total interaction in artificial semiconductor atoms. We tune the relative strengths of the two interactions and measure them as a function of the number of confined electrons. The electrons tend to have parallel spins when they occupy nearly degenerate single-particle states. We use a magnetic field to adjust the single-particle-state degeneracy, and find that the spin configurations in an arbitrary magnetic field are well explained in terms of two-electron singlet and triplet states.     

Vibrational spectra and molecular configuration of hexafluoroazomethane

Twenty-one of the 24 fundamental frequencies of CF₃N=NCF₃ have been identified from the infrared and Raman spectra of the vapor and condensed states. The spectra reveal a total lack of infrared-Raman coincidence, i.e., the rule of mutual exclusion is obeyed. This evidence strongly supports the existence of hexafluoroazomethane in the trans configuration and negates the results of a recent electron diffraction study which favor the cis form.     

Fourier-transform infrared photoacoustic spectroscopy evaluated for near-surface characterization of polymeric materials

Fourier-transform infrared photoacoustic spectroscopy was applied to the near-surface study of various polymeric materials including aluminum/polyethylene composite films, poly(vinyl chloride) composites, and various chemically-modified textile materials. Examples are given for surface identification of polymers, distribution of chemical additives in textile materials, and characterization of polymer surface degradation. This approach has various advantages over conventional infrared sampling techniques for the study of polymeric materials.     

A low frequency assignment for infrared and Raman spectra of (+) bornyl acetate using related compounds and deuterated derivatives

Twenty-one fundamentals of (+)-bornyl acetate and nine deuterium substituted modifications (2-d₁; 3,3-d₂; 2,3,3-d₃; acetate-d₃; 2-d₁ acetate-d₃; 3,3-d₂ acetate-d₃; 2,3,3-d₃ acetate-d₃; 10-d₁; 10,10,10-d₃) as well as (−)-isobornyl-1-10,10.10-d₃ acetate have been assigned between 200 and 850cm⁻¹. These results supplement the previous assignment of nineteen fundamentals of (−)-isobornyl acetate and seven deuterium substituted modifications (2-d₁,; 3,3-d₂; 2,3,3-d₃; acetate-d₃; 2-d₁ acetate-d₃; 3,3-d₂ acetate-d₃; 2,3,3-d₃ acetate-d₃) between 200 and 900cm⁻¹ [8]. These fundamentals are: skeletal vibrations of the quaternary carbons, ring breathing, bending, and twisting vibrations, and vibrations of the acetate group. Key model compounds used in this analysis are norbornane, neopentane, methyl acetate, cyclopentanol, and the (−)-isobornyl acetate system. A series of related compounds (norbornane, bornane, endo-norbomyl acetate, 1-methyl-endo-norbornyl acetate, apobornyl acetate, and (+)-bornyl acetate) is used to identify frequencies associated with the quaternary carbon and the acetate group. Raman spectra are more useful for the quaternary carbon frequencies and i.r. spectra are more useful for acetate group frequencies. Four exo stereoisomer alcohols (1-methyl-exo-norborneol, 1-methy d₃-exonorborneoI, apoisoborneol, (−)-isoborneol) and three endo stereoisomer alcohols (1-methyl-endo-norborneol, apoborneol, (+)-borneol) serve as model compounds for a modification of the earlier assignment [8] for the skeletal stretching of the quaternary carbons in the (−)-isobornyl acetate system and extension of this modified assignment to the (+)-bornyl acetate system. Quaternary carbon symmetric skeletal stretching is believed to be responsible for prominent Raman bands between 580 and 680cm⁻¹ in the 36 bicyclic ring compounds investigated to date. Fermi resonance is proposed as the explanation for a number of unexpected intensity patterns observed in the i.r. and Raman spectra.