Monomeric nickel(I1) thiosemicarbazone complexes are a n and X-ray structural analysis of the first trimeric nickel(I1) attractive new class of homogeneous catalysts for the activa-thiosemicarbazone complex. Aggregation was shown to tion of silanes. However, their activity is limited by the forma-proceed via Ni-0-Ni and Ni-S-Ni bridging, giving rise to tion of inactive oligo-and polymers. The pathway by which both fourfold planar and pseudooctahedral coordination of aggregation takes place was elucidated by the preparation the nickel ions.Nickel" complexes derived from the thiosemicarbazones of aromatic ortho-hydroxy aldehydes, in particular salicylaldehyde, have recently attracted considerable attention as homogeneous catalysts. As shown by Crabtree et al.['l and U S [~.~I , they are able to activate the Si-H bond in silanes. As a result, alcoholysis affording silyl ethers [','] (eq. 1) or the reduction of imines to primary or secondary amines1'I (eq. 2) can be catalyzed by these nickel chelates. R3Si-H + H-OR R3Si-OR + H2 nickel(//)-fhiosemicarbazones R3Si-H + R+NR R&H-NHRA crucial feature of nickel thiosemicarbazone complexes of the general structure 1 is their ability to aggregate to di-and oligomers or even polymers. Aggregation is induced by the tendency of the Lewis basic heteroatoms of the ligand of one chelate molecule to bind to the Lewis acidic nickel ion of another molecule etc. The X-ray crystal structures of a variety of monomeric nickel thiosemicarbazone complexes carrying inert coligands, like e.g. ammonia, at the fourth coordination site of the nickel ion are k n o~n [~*~I . These materials are all planar, tetracoordinated nickel complexes and catalytically inactive. As an example from our own laboratory, the crystal structure of the s~lfonated[~l complex l a is shown in Figure I . Clearly, only those monomeric complexes carrying a fourth, kinetically labile coligand. such as DMSO, are catalytically active. As a consequence, reactions catalyzed by nickel thiosemicarbazones are usually carried out in DMSO. This solvent is able to (partly) monomerize the polymeric catalyst preparations and it is a kinetically labile ligand['-3Jl, allowing the silane molecule to reach the fourth coordination site at the nickel ion.
EEG microstate sequence analysis quantifies properties of ongoing brain electrical activity which is known to exhibit complex dynamics across many time scales. In this report we review recent developments in quantifying microstate sequence complexity, we classify these approaches with regard to different complexity concepts, and we evaluate excess entropy as a yet unexplored quantity in microstate research. We determined the quantities entropy rate, excess entropy, Lempel-Ziv complexity (LZC), and Hurst exponents on Potts model data, a discrete statistical mechanics model with a temperature-controlled phase transition. We then applied the same techniques to EEG microstate sequences from wakefulness and non-REM sleep stages and used first-order Markov surrogate data to determine which time scales contributed to the different complexity measures. We demonstrate that entropy rate and LZC measure the Kolmogorov complexity (randomness) of microstate sequences, whereas excess entropy and Hurst exponents describe statistical complexity which attains its maximum at intermediate levels of randomness. We confirmed the equivalence of entropy rate and LZC when the LZ-76 algorithm is used, a result previously reported for neural spike train analysis (Amigó et al. 2004). Surrogate data analyses prove that entropy-based quantities and LZC focus on short-range temporal correlations, whereas Hurst exponents include short and long time scales. Sleep data analysis reveals that deeper sleep stages are accompanied by a decrease in Kolmogorov complexity and an increase in statistical complexity. Regarding the practical use of these methods, we suggest that LZC can be used as an efficient entropy rate estimator that avoids the estimation of joint entropies, whereas entropy rate estimation via joint entropies has the advantage of providing excess entropy as the second parameter of the same linear fit. We conclude that metrics of statistical complexity are a useful addition to microstate analysis and address a complexity concept that is not yet covered by existing microstate algorithms while being actively explored in other areas of brain research.
Microstate sequences summarize the changing voltage patterns measured by electroencephalography (EEG), using a clustering approach to reduce the high dimensionality of the underlying data. A common approach is to restrict the pattern matching step to local maxima of the global field power (GFP) and to interpolate the microstate fit in between. In this study, we investigate how the anesthetic propofol affects microstate sequence periodicity and predictability, and how these metrics are changed by interpolation. We performed two frequency analyses on microstate sequences, one based on time-lagged mutual information, the other based on Fourier transform methodology, and quantified the effects of interpolation. Resting-state microstate sequences had a 20 Hz frequency peak related to dominant 10 Hz (alpha) rhythms, and the Fourier approach demonstrated that all five microstate classes followed this frequency. The 20 Hz periodicity was reversibly attenuated under moderate propofol sedation, as shown by mutual information and Fourier analysis. Characteristic microstate frequencies could only be observed in non-interpolated microstate sequences and were masked by smoothing effects of interpolation. Information-theoretic analysis revealed faster microstate dynamics and larger entropy rates under propofol, whereas Shannon entropy did not change significantly. In moderate sedation, active information storage decreased for non-interpolated sequences. Signatures of non-equilibrium dynamics were observed in non-interpolated sequences only and decreased in moderate sedation. All changes occurred while subjects were able to perform an auditory perception task. In summary, we show that low-dose propofol reversibly increases the randomness of microstate sequences and attenuates microstate oscillations without correlation to cognitive task performance. Microstate dynamics between GFP peaks reflect physiological processes that are not accessible in interpolated sequences.
The determination of the coupling constants between a and @ protons in the NMR spectrum of isotactic vinyl polymers is of interest in determining whether they retain in solution the local conformation characteristic of the helical structure in the crystalline state. Bovey et a1.l have thus analyzed isotactic polystyrene, in which the two @ protons are coincidentally equivalent, and they conclude that the spectrum may be recalculated with a single vicinal coupling constant of 7 f 0.2 He.We report the measurement and analysis of the 100-Mc. spectrum of isotactic polyZvinylpyridine (PVP) which is expected to have a conformation very close to that of polystyrene. The presence of the heteroatom in the ring allows a precise determination of the chemical shift of the ring protons and produces a large non-equivalence of the two @ protons.The polymer was prepared by polymerization with phenylmagnesium bromide. Spectra were obtained on degassed 5% solutions in CDCla, odichlorobenzene, and pyridine in a Varian HA 100 spectrometer. Aromatic ProtonsThe An interesting obsei-vation is the upfield shift of the aromatic protons of PVP compared to ethylpyridine (6, = 8.51, 6b = 7.06, 6o = 7.56,6d = 7.20 pprn). The shifts of +0.76 ppm for d, +0.30 ppm for a and b, and +0.44 ppm for c may be compared with the observed shifts of +0.30-0.40 ppm for the meta and para protons and +0.70 ppm for the ortho protons, in ethylbenzene and polystyrene. As this shift is mostly due to the shielding of neighboring rings, it is consistent with a very similar relative positions of the rings in the two polymers. 1293 JOURNAL OF POLYMER SCIENCE: PART A-2 VOL. 5 (1967) Chain ProtonsThe chain protons may be treated as a cyclic six-spin system AA'BB'CC' where A and B are the 8 and C the a protons. We have not done a complete numerical analysis but we have treated separately the a and /3 protons using approximate analytical expressions (ABXX' for the fl proton and AzX& for the a proton.') We have checked on published data on isotactic poly(isopropy1 acrylate)' and isotactic polypropylenes that this type of analysis gives good agreement for the position of the lines compared to an exact six-spin calculation. The /3 part of the spectrum of Figure 2 (1)The set of values (1) gives only slightly better agreement than set (2) when compared to the rather wide lines of the actual spectrum.a Protons. The spectrum of the a proton calculated with the set of values (1) is consistent with the observed quintet. It seems therefore that the local conformation of isotactic PVP, as well as of isotactic polystyrene, is very well described by a single coupling constant of 7 He. between the a and 6 protons, a result which reflects the conservation of the local conformation of the 31 helix.We thank Dr. C. Loucheux for his gift of isotactic PVP and Mr. J. P. Bey1
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