Physicochemical Properties of Deuterated Compounds. 8th Communication. The kinetic isotope effect for the reaction of hydrogen atoms with methylcyclohexane and perdeuteromethylcyclohexane in the gas phase

Using H-atoms produced in the radiolysis of water vapour, we determined the kinetic isotope effect for the reaction where RH and R′D are methylcyclohexane and perdeuterated methylcyclohexane, respectively. The result obtained from the isotope competitive method can be expressed with the Arrhenius-type equation k_H/k_D = (0.51 ± 0.04) exp[(8.58 ± 0.25) kJ mol^?1/RT] over the temperature range from 363 to 483 K. The occurrence of H abstraction from primary, secondary, and tertiary C? H bonds in methylcyclohexane is taken into consideration in the interpretation of the Arrhenius-type expression obtained. Theoretical interpretation of the kinetic isotope effect has been achieved on the basis of the transition-state theory and a semiempirical London-Eyring-Polanyi-Sato potential-energy surface. The tunnel effect is found to play a role in the H-abstraction. Several methods for estimating the tunnel correction factors have been discussed.     

Isotope shifts and hyperfine structures investigation of doubly excited levels in SrI

We measured isotope shifts and hyperfine structure of visible transitions of stable strontium isotopes by means of Doppler-free saturated absorption spectroscopy. In particular, we investigated transitions between excited states where the upper level involves two excited electrons. We report hyperfine coupling constants for the levels 5p4d¹D₂, 5p4d³F₂, 5s4d³D₁, 5s6s³S₁, 5s5p³P₁ and, for some of the studied transitions, we separate the specific mass and volume contribution to the isotope shifts.     

Stereoelectronic effects on the retro-Diels-Alder fragmentation of ionized bicyclo[4.3.0]nona-3,7-dienes

The electron impact and collision-induced dissociation mass spectra of cis- and trans-annulated bicyclo[4.3.0]nona-3,7-dienes differ in their relative abundances of [C₅H₆]⁺˙ fragments formed by the retro-Diels-Alder decomposition. The formation of [C₅H₆]⁺˙ is not preceded by hydrogen migration in the short-lived and long-lived molecular ions. The appearance energy of [C₅H₆]⁺˙ from both annulation isomers is identical within experimental error: AE_cis([C₅H₆]⁺˙)=10.56±0.10 eV and AE_trans([C₅H₆]⁺˙)=10.54±0.15 eV. The barrier to the retro-Diels-Alder fragmentation lies 68–76 kJ mol^?1 above the thermo-chemical threshold corresponding to [C₅H₆]⁺˙ + C₄H₆. Investigation of the two-dimensional reaction coordinate by the Topological Molecular Orbital treatment shows that the lowest energy path for the retro-Diels-Alder reaction involves a two-step dissociation of the C(5)? C(6) and C(1)? C(2) bonds in the molecular ion, the latter step overcoming a barrier, calculated as 80 kJ mol^?1 above the thermochemical threshold. The stereochemical difference between the geometric isomers is due to stereoelectronic assistance of the π orbitals of the cis-annulated isomer in the cleavage of the C(5)? C(6) bond. Other mechanisms of the retro-Diels–Alder reaction are discussed.     

Strahlungschemie von Kohlenwasserstoffen. 16. Mitteilung. Benzol-Brombenzol

The decrease of the concentration of bromobenzene and the formation of biphenyl and bromobiphenyl have been studied in the system benzene-bromobenzene under the influence of γ-rays. Bromobenzene is in competition with its own products for the reactive state of benzene or it can transfer its energy to its products. The rate of this transfer to decomposition is 74 l/Mol. With the help of additives it has been shown that they can act in two ways: firstly they compete with bromobenzene for an activated state of benzene in a reaction that does not depend on temperature. The nature of this activated state has not been determined. Secondly they scavenge phenyl radicals that have been formed in about 70% of the decomposed bromobenzene molecules. This yield depends slightly on temperature. The relative reactivities compared to bromobenzene for the first case and to benzene in the second have been determined as follows: naphthalene (0.6; 23); anthracene (5.8; 800); benzophenone (0.5; 17); cyclohexene (-; 20); N₂O (0.018; -); NO (1.7; 250); O₂ (-; 3400). Naphthacene in benzene is strongly decomposed in the presence of bromobenzene, but it also enhances the decomposition of the latter.     

The mass spectrometric fragmentation of n-alkanes

The fragmentation of n-hexane, n-nonane and n-tetradecane under electron impact has been investigated, using ¹³C labelled compounds. The mechanism of the formation of the alkyl radical ions is quantitatively explained by using a method of calculation developed in an earlier publication for n-heptane. It is assumed that these ions are formed either by a direct C-C bond cleaveage or by a secondary olefin loss from an alkyl radical ion. In the latter case the probability for a particular carbon to be lost in the neutral fragment is assumed to be random. The probability for a direct cleavage to an alkyl ion is about 80% for an ion containing at least half of the number of carbon atoms of the molecular ion and 15% for the smaller ions. The [M? H]⁺ ion seems to be a special case not yet clearly understood. Former results about the loss of methyl from the molecular ion are confirmed.     

Mass Spectrometric Decomposition Processes in Labelled 1-Heptenes

The 70 eV mass spectra of a number of ¹³C- and D-labelled analogs of 1-heptene have been measured, as well as the metastable transitions in the non-labelled compound. Isotopic distributions in the major fragment ions have been calculated from the high and low resolution data. The results show that considerable skeletal rearrangement must take place before formation of most of the fragment ions. Loss of methyl and ethyl radicals occurs mainly from the two ends of the molecule. Ethylene fragments come primarily from the unsaturated end of the molecule, but show evidence of significant prior skeletal rearrangement. The predicted McLafferty rearrangement accounts for only 2/3 of the C₄H₈⁺ ions formed, less for the C₃H₆⁺ ions. At least 80% of C₄H₉⁺ ions appear to be formed by allylic cleavage, as expected, but this mechanism can only account for a maximum of 20% of the formation of the complementary ion C₃H₅⁺. Both, this latter ion and C₃H₆⁺, are probably generated by loss of hydrogen from C₃H₇⁺. Figures obtained for label retention in 1-[¹³C]- and 1-D-labelled analogs were nearly identical for most fragment ions, probably indicating that the hydrogen atoms in position 1 remain on C(1) even following skeletal rearrangement. A similar result was found for the 7-[¹³C]- and 7-D-labelled compounds. The main exceptions in the case of the products labelled in position 1 (C₄H₇⁺, C₃H₃⁺) seem to be due to initial loss of an hydrogen atom from this position followed by further fragmentation.     

Asymptotic linear expansion of regularized M-estimators

Annals of the Institute of Statistical Mathematics - Parametric high-dimensional regression requires regularization terms to get interpretable models. The respective estimators correspond to...     

Transcranial direct current stimulation of the prefrontal cortex modulates working memory performance: combined behavioural and electrophysiological evidence

Background  

Transcranial direct current stimulation (tDCS) is a technique that can systematically modify behaviour by inducing changes in the underlying brain function. In order to better understand the neuromodulatory effect of tDCS, the present study examined the impact of tDCS on performance in a working memory (WM) task and its underlying neural activity. In two experimental sessions, participants performed a letter two-back WM task after sham and either anodal or cathodal tDCS over the left dorsolateral prefrontal cortex (DLPFC).     

Influence of higher harmonics of the undulator in X‐ray polarimetry and crystal monochromator design

The spectrum of the undulator radiation of beamline P01 at Petra III has been measured after passing a multiple reflection channel‐cut polarimeter. Odd and even harmonics up to the 15th order, as well as Compton peaks which were produced by the high harmonics in the spectrum, could been measured. These additional contributions can have a tremendous influence on the performance of the polarimeter and have to be taken into account for further polarimeter designs.