Diffusion of Tracer in Altered Tonalite: Experiments and Simulations with Heterogeneous Distribution of Porosity

Numerical time-domain-diffusion simulations were used for studying the diffusion behavior of tracer molecules in rock matrix with homogeneous and heterogeneous porosity. As the heterogeneous sample in these simulations, a 3D tomographic image of altered tonalite was used, in which the mineral components and the pores resolved by X-ray microtomography were represented by their respective intragranular porosities determined previously by the ¹⁴C-PMMA method. The apparent diffusion coefficient of a tracer in altered tonalite was determined experimentally, and was then used in the simulations. In the altered tonalite analyzed, inclusion of heterogeneity in the porosity increased the diffusion coefficient by 16 %. Altered and pristine feldspar was the main mineral component in the sample (72 %), and it also provided the dominant contribution to tracer diffusion, explaining alone 52 % of the diffusion coefficient. The large pores resolved by microtomography (6 %) and altered and pristine mica (22 %) gave an equal contribution to the diffusion coefficient. The simulation method applied was also validated by comparing the results to both an analytical and a numerical solution to the diffusion equation in a homogenous medium. In addition, the method was compared to discrete-time random-walk simulations in the case of randomly placed overlapping spheres.     

Comparison of methods for detecting nondeterministic BOLD fluctuation in fMRI

Functional MR imaging (fMRI) has been used in detecting neuronal activation and intrinsic blood flow fluctuations in the brain cortex. This article is aimed for comparing the methods for analyzing the nondeterministic flow fluctuations. Fast Fourier Transformation (FFT), cross correlation (CC), spatial principal component analysis (sPCA), and independent component analysis (sICA) were compared. 15 subjects were imaged at 1.5 T. Three quantitative measures were compared: (1) The number of subjects with identifiable fluctuation, (2) the volume, and (3) mean correlation coefficient (MCC) of the detected voxels. The focusing on cortical structures and the overall usability were qualitatively assessed. sICA was spatially most accurate but time consuming, robust, and detected voxels with high temporal synchrony. The CC and FFT were fast suiting primary screening. The CC detected highest temporal synchrony but the subjective detection for reference vector produced excess variance of the detected volumes. The FFT and sPCA were not spatially accurate and did not detect adequate temporal synchrony of the voxels.     

Ab initio structural and vibrational investigation of sulfuric acid monohydrate

We employ ab initio methods to find stable geometries and to calculate potential energy surfaces and vibrational wavenumbers for sulfuric acid monohydrate. Geometry optimizations are carried out with the explicitly correlated coupled-cluster approach that includes single, double, and perturbative triple excitations (CCSD(T)-F12a) with a valence double-ζ basis set (VDZ-F12). Four different stable geometries are found, and the two lowest are within 0.41 kJ mol(-1) (or 34 cm(-1)) of each other. Vibrational harmonic wavenumbers are calculated at both the density-fitted local spin component scaled second-order M?ller-Plesset perturbation theory (DF-SCS-LMP2) with the aug-cc-pV(T+d)Z basis set and the CCSD-F12/VDZ-F12 level. Water O-H stretching vibrations and two highly anharmonic large-amplitude motions connecting the three lowest potential energy minima are considered by limiting the dimensionality of the corresponding potential energy surfaces to small two- or three-dimensional subspaces that contain only strongly coupled vibrational degrees of freedom. In these anharmonic domains, the vibrational problem is solved variationally using potential energy surfaces calculated at the CCSD(T)-F12a/VDZ-F12 level.     

Positivity of second order linear recurrent sequences

We give a decision method for the Positivity Problem for second order recurrent sequences: it is decidable whether or not a recurrent sequence defined by u_n=au_n-1+bu_n-2 has only nonnegative terms.     

A time-domain approach to estimating the plucking point of guitar tones obtained with an under-saddle pickup

A method for estimating the plucking point of guitar tones is proposed. The algorithm is based on investigating the time lag between two consecutive pulses arriving at the bridge of the guitar. The signal is detected with an under-saddle pickup attached to the bridge. The method determines the minimum of the autocorrelation function for one period of the signal. The time lag of the minimum can be converted into the distance from the bridge where the string was plucked. The results obtained with the method are good, the error remains smaller than one centimetre, except for a few outliers. The algorithm is easy to implement and can be used to analyse playing styles. The efficiency of the method gives the potential to also use it in real-time computer music applications.     

Electron impact induced fragmentation of dihydro-1,4-oxathiines. 1. 2,3-substituted 5,6-dihydro-1,4-oxathiines

In the electron impact induced fragmentation of 2,3-disubstituted 5,6-dihydro-1,4-oxathiines, the cleavage of the heterocyclic ring proceeds through the retro Diels-Alder type of reaction. The further fragmentation of the resulting radical cation gives rise to substituted carbonyl or thiocarbonyl cations. This fragmentation favours the formation of an ion which, in the electron-deficient carbon-oxygen or carbon-sulphur triple bond, is stabilized by an electron-releasing group. The ring fragmentation was observed to be dependent on the nature of the 3-substituent of the ring when a series of 3-substituted 5,6-dihydro-2-methyl-1,4-oxathiines was investigated.     

Quantum chemical modeling of chiral catalysis. Part 8. On the conformational freedom of the ketone of ketone-borane complexes of oxazaborolidines used as catalysts in the enantioselective reduction of ketones

Standard ab initio molecular orbital methods were employed to study conformational freedom of the ketone of ketone-borane complexes of chiral oxazaborolidines used as catalysts for the enantioselective reduction of ketones (CBS reduction). A formaldehyde-borane complex of 1,3,2-oxazaborolidine was used as a model system. A new conformation was found which was energetically more advantageous than the original one predicted by Corey et al. The new conformation was predicted to be destabilized by bulky substituents at the C-5 of the ring. A new class of potential oxazaborolidine catalysts for the enantioselective reduction of ketones was invented.     

Quantum mechanical investigation of effects in formation of carbonyl and thiocarbonyl cations. Part I. Fragmentation reactions of 1,2-hemithiodione radical cations

In order to study decomposition reactions of ionic oxygen and sulphur-containing compounds, such as hemithiodione radical cations, a quantum chemical investigation of the formation of formyl, thioformyl, acyl and thioacyl cations and radicals was performed. Calculations were carried out mainly at the 6–31G^* level involving complete geometry optimizations. In the ionization of aldehydes and thioaldehydes, no important energy differences were found between the oxygen and sulphur analogues studied. A stepwise generation of formyl and thioformyl cations from formaldehyde and thioformaldehyde, by hydrogen atom abstraction followed by expulsion of unpaired electrons from the resulting radicals, showed the radicalization of formaldehyde to be only 12.6 kJ mol^?1 more favoured than that of thioformaldehyde. The electron expulsion from formyl radical was 23.8 kJ mol^?1 more favoured than that from thioformyl radical. Substitution of hydrogens of formyl and thioformyl groups by methyls lowered the total formation energies of carbonyl and thiocarbonyl cations 119.2 and 96.2 kJ mol^?1. The formation energy difference between acyl and thioacyl cations was also very small.