A simple flowcell for reaction monitoring by NMR

A simple, cheap and flexible flowcell based on a standard 5 mm NMR tube, designed for the monitoring of reactions but of wide applicability, is described. No modification of the NMR instrument is needed, allowing the system to be employed with any conventional NMR probe and magnet. The system is robust and economical in use of reagents, and can be used for studying both homogeneous and heterogeneous reactions. Copyright © 2010 John Wiley & Sons, Ltd.     

Ein Axiomensystem der Methode der kleinsten Quadrate

Ohne Zusammenfassung     

Über geometrische Wahrscheinlichkeit und über das Axiom der beschränkten Arithmetisierbarkeit der Beobachtungen

Ohne Zusammenfassung     

Pitch discrimination of diotic and dichotic tone complexes: harmonic resolvability or harmonic number?

Three experiments investigated the relationship between harmonic number, harmonic resolvability, and the perception of harmonic complexes. Complexes with successive equal-amplitude sine- or random-phase harmonic components of a 100- or 200-Hz fundamental frequency (f0) were presented dichotically, with even and odd components to opposite ears, or diotically, with all harmonics presented to both ears. Experiment 1 measured performance in discriminating a 3.5%-5% frequency difference between a component of a harmonic complex and a pure tone in isolation. Listeners achieved at least 75% correct for approximately the first 10 and 20 individual harmonics in the diotic and dichotic conditions, respectively, verifying that only processes before the binaural combination of information limit frequency selectivity. Experiment 2 measured fundamental frequency difference limens (f0 DLs) as a function of the average lowest harmonic number. Similar results at both f0's provide further evidence that harmonic number, not absolute frequency, underlies the order-of-magnitude increase observed in f0 DLs when only harmonics above about the 10th are presented. Similar results under diotic and dichotic conditions indicate that the auditory system, in performing f0 discrimination, is unable to utilize the additional peripherally resolved harmonics in the dichotic case. In experiment 3, dichotic complexes containing harmonics below the 12th, or only above the 15th, elicited pitches of the f0 and twice the f0, respectively. Together, experiments 2 and 3 suggest that harmonic number, regardless of peripheral resolvability, governs the transition between two different pitch percepts, one based on the frequencies of individual resolved harmonics and the other based on the periodicity of the temporal envelope.     

Structural characterization of G-quadruplexes in deoxyguanosine clusters using ion mobility mass spectrometry

The aggregation and conformation of deoxyguanosine (dG) in an ammonium acetate buffer solution were examined using mass spectrometry, ion mobility, and molecular mechanics/dynamics calculations. The nano-ESI mass spectrum indicated that 4 and 6 dGs cluster with 1 NH4+; 11 dGs with 2 NH4+; 14, 16, and 17 dGs with 3 NH4+; and 23 dGs with 4 NH4+. The collision cross sections with helium were measured and compared with calculated cross sections of theoretical structures generated by molecular mechanics/dynamics calculations. Three distinct arrival time distribution (ATD) peaks were observed for (4dG + NH4)+. One peak was assigned to the quadruplex structure of (4dG + NH4)+, while the other two peaks corresponded to the quadruplex structures of (8dG + 2NH4)2+ and (12dG + 3NH4)3+, all with the same m/z. Four ATD peaks were observed for (6dG + NH4)+ and assigned to the globular structure of (6dG + NH4)+, and the quadruplex structures of (12dG + 2NH4)2+, (18dG + 3NH4)3+, and (24dG + 4NH4)4+. Two ATD peaks were observed for (11dG + 2NH4)2+ and assigned to the quadruplex structures of (11dG + 2NH4)2+ and (22dG + 4NH4)4+. All of the other clusters in the mass spectrum (14, 16, and 17 dGs with 3 NH4+ and 23 dGs with 4 NH4+) only had one peak in their ATDs and in all cases the theoretical structures in a quadruplex arrangement agreed with the experimental cross sections. These results provide compelling evidence that quadruplexes are present in solution and retain their structure during the spray process, dehydration, and detection.     

Spectroscopic properties of rare earth borohydrides: Er(BH4)3·3THF in pure and mixed crystals

The optical spectra of Er(BH₄)₃·3THF neat crystals and La, Gd, Y(BH₄)₃·3THF mixed crystals are reported and analyzed. Lanthanum borohydride is found to have a different room temperature crystal structure (triclinic) from Er, Gd, Y(BH₄)₃·3THF (Pbcn). At low temperature the Pbcn crystals undergo a phase transition to a structure with two crystallographically inequivalent sites in a unit cell. The optical spectra of Er(BH₄)₃·3THF in Er, Y, Gd(BH₄)₃·3THF crystals clearly evidence these two sites. Large vibronic intensity is observed at 1.6 K and 77 K and nine “molecular” vibrations are assigned. These modes are quite similar to those found for U(BH₄)₄. Er (BH₄)₃·3THF spectra are very different: no vibronic transitions are observed but many (often upwards of fifty for a given manifold) weak sharp “satellite” lines are found associated with pure electronic transitions. These data are discussed in terms of structural differences and comments on bonding and covalent character in lanthanide borohydrides are made.     

Über die Darstellung der N-Chlor-N-Methylammoniumsalze (CH3)nNCl4–n+MF6− (n = 1–3; M = As,Sb) und (CH3)2NCIX+MF6− (X = F,Br)

About the Preparation of N-Chloro-N-Methylammonium Salts (CH₃)_nNCl_4–n⁺MF₆^? (n = 1–3; M = As, Sb) and (CH₃)₂NClX⁺MF₆^? (X = F, Br) Simple one-step methods for the preparation of the methylated chloroammonium salts (CH₃)_nNCl_4–n⁺MF₆^? (n = 1–3; M = As, Sb) and for (CH₃)₂NClX⁺MF₆^? (X = F, Br) are reported. Their vibrational and NMR-spectroscopical data are discussed in comparison.     

Variations in the strength of the infrared forbidden 2328.2 cm-1 fundamental of solid N2 in binary mixtures.

We present the 2335-2325 cm-1 infrared spectra and band positions, profiles and strengths (A values) of solid nitrogen and binary mixtures of N2 with other molecules at 12 K. The data demonstrate that the strength of the infrared forbidden N2 fundamental near 2328 cm-1 is moderately enhanced in the presence of NH3, strongly enhanced in the presence of H2O and very strongly enhanced (by over a factor of 1000) in the presence of CO2, but is not significantly affected by CO, CH4, or O2. The mechanisms for the enhancements in N2-NH3 and N2-H2O mixtures are fundamentally different from those proposed for N2-CO2 mixtures. In the first case, interactions involving hydrogen-bonding are likely the cause. In the latter, a resonant exchange between the N2 stretching fundamental and the 18O = 12C asymmetric stretch of 18O12C16O is indicated. The implications of these results for several astrophysical issues are briefly discussed.