Interaction of hexafluoroacetone and hexafluoroisopropylidenimine with some organosilicon- and organotin- amines,azides and phosphinimines

Metathetical and insertion reactions of perhalogenoketones and hexafluoroisopropylidenimine with organosilicon and organotin phosphinimines, dimethylaminotrimethyltin and trimethylsilyl azide are reported. Activation energies for nitrogen inversion in some of the organometallic ketimines produced have been determined.     

On the commutant algebras corresponding to the permutation representations of the full collineation groups of PG(k,s) and EG(k,s), s = pr,k ⩾ 2

In this paper, the dimension t and a linear basis of the commutant algebra corresponding to the representation of the full collineation group as matrices permuting the flags (incident point-line or point-hyperplane pairs) have been determined for each one of the four geometries PG(2, s), EG(2, s), PG(k, s), and EG(k, s), s = p^r, k ? 3. For the four geometries, t = 6, 7, 7, and 8, respectively, and the corresponding linear bases are (I, G, B, T, BT, TB), (I, G, B, T, BT, TB, BTB), (I, G, B, T, BT, TB, S), and (I, G, B, T, BT, TB, BTB, S). I, G, B, T are the relationship matrices of James (Ann. Math. Statist.28 (1957), 993–1082) and the matrix S was introduced by Sysoev and Shaikin (Avtomat. i Telemekh.5 (1976), 64–73).     

Interactions of speaking condition and auditory feedback on vowel production in postlingually deaf adults with cochlear implants

This study investigates the effects of speaking condition and auditory feedback on vowel production by postlingually deafened adults. Thirteen cochlear implant users produced repetitions of nine American English vowels prior to implantation, and at one month and one year after implantation. There were three speaking conditions (clear, normal, and fast), and two feedback conditions after implantation (implant processor turned on and off). Ten normal-hearing controls were also recorded once. Vowel contrasts in the formant space (expressed in mels) were larger in the clear than in the fast condition, both for controls and for implant users at all three time samples. Implant users also produced differences in duration between clear and fast conditions that were in the range of those obtained from the controls. In agreement with prior work, the implant users had contrast values lower than did the controls. The implant users' contrasts were larger with hearing on than off and improved from one month to one year postimplant. Because the controls and implant users responded similarly to a change in speaking condition, it is inferred that auditory feedback, although demonstrably important for maintaining normative values of vowel contrasts, is not needed to maintain the distinctiveness of those contrasts in different speaking conditions.     

Residue-specific 13C' CSA tensor principal components for ubiquitin: correlation between tensor components and hydrogen bonding

The residue-specific 13C' CSA tensor principal components, sigma(11), sigma(22), sigma(33), and the tensor orientation defined by the rotation angles beta and gamma have been determined by solution NMR for uniformly labeled ubiquitin partially aligned in four different media. Spurious chemical shift deviations due to solvent effects were corrected with an offset calculated by linear regression of the residual dipolar couplings and chemical shifts at increasing alignment strengths. Analysis of this effect revealed no obvious correlation to solvent exposure. Data obtained in solution from a protein offer a better sampling of 13C' CSA for different amino acid types in a complex heterogeneous environment, thereby allowing for the evaluation of structural variables that would be challenging to achieve by other methods. The 13C' CSA principal components cluster about the average values previously determined, and experimental correlations observed between sigma(11), sigma(22) tensorial components and C'O...H(N) hydrogen bonding are discussed. The inverse association of sigma(11) and sigma(22) exemplify the calculated and solid-state NMR observed effect on the tensor components by hydrogen bonding. We also show that 13C' CSA tensors are sensitive to hydrogen-bond length but not hydrogen-bond angle. This differentiation was previously unavailable. Similarly, hydrogen bonding to the conjugated NH of the same peptide plane has no detectable effect. Importantly, the observed weak correlations signify the presence of confounding influences such as nearest-neighbor effects, side-chain conformation, electrostatics, and other long-range factors to the 13C' CSA tensor. These analyses hold future potential for exploration provided that more accurate data from a larger number of proteins and alignments become available.     

Mechanism of nitrite reduction at T2Cu centers: electronic structure calculations of catalysis by copper nitrite reductase and by synthetic model compounds

The mechanism of nitrite reduction at the Cu(II) center of both copper nitrite reductase and a number of corresponding synthetic models has been investigated by using both QM/MM and cluster calculations employing density functional theory methods. The mechanism in both cases is found to be very similar. Initially nitrite is bound in a bidentate fashion to the Cu(II) center via the two oxygen atoms. Upon reduction of the copper center, the two possible coordination modes of the protonated nitrite, by either nitrogen or a single oxygen atom, are close in energy, with nitrogen coordination probably preferred. Further protonation of this species leads to N-O bond cleavage, and an electron transfer from the Cu(I) center to the N-O+ ligand, resulting in loss of NO and regeneration of the resting state of the enzyme having a bound water molecule.     

Investigation of ligand exchange reactions in aqueous uranyl carbonate complexes using computational approaches

Carbonate anion exchange reactions with water in the uranyl-carbonate and calcium-uranyl-carbonate aqueous systems have been investigated using computational methods. Classical molecular dynamics (MD) simulations with the umbrella sampling technique were employed to determine potentials of mean force for the exchange reactions of water and carbonate. The presence of calcium counter-ions is predicted to increase the stability of the uranyl-carbonate species in accordance with previous experimental observations. However, the free energy barrier to carbonate exchange with water is found to be comparable both in the presence and absence of calcium cations. Possible implications of these results for uranyl adsorption on mineral surfaces are discussed. Density functional theory (DFT) calculations were also used to confirm the trends observed in classical molecular dynamics simulations and to corroborate the validity of the potential parameters employed in the MD scheme.     

(E)-1,2-Difluoro-1-iodo-2-trialkylsilanes—A useful synthon for the addition of the [IFCCF] unit to activated electrophiles

Fluoride ion catalyzed reaction of (E)-IFCCFSiR₃ with activated aromatic aldehydes and ketones and activated perfluoroaromatics, such as pentafluoropyridine and perfluorotoluenes, transfers the [IFCCF] unit to the activated electrophiles to stereospecifically provide (E)-1,2-difluoro-1-iodosubstituted derivatives. Aluminum chloride catalyzed reaction of (E)-1,2-difluoro-1-iodo-2-trialkylsilanes with alkyl or aryl acyl halides gives the corresponding (E)-1,2-difluoro-1-iodoketones stereospecifically in excellent yield. The vinyl iodide product formed via this methodology could be coupled (with Pd(0)) catalysis to provide an entry to a polyfunctionalized derivative.     

Fractional differential equations and Lyapunov functionals

We consider a scalar fractional differential equation, write it as an integral equation, and construct several Lyapunov functionals yielding qualitative results about the solution. It turns out that the kernel is convex with a singularity and it is also completely monotone, as is the resolvent kernel. While the kernel is not integrable, the resolvent kernel is positive and integrable with an integral value of one. These kernels give rise to essentially different types of Lyapunov functionals. It is to be stressed that the Lyapunov functionals are explicitly given in terms of known functions and they are differentiated using Leibniz’s rule. The results are readily accessible to anyone with a background of elementary calculus.