Site-specific C content by quantitative isotopic C Nuclear Magnetic Resonance spectrometry: A pilot inter-laboratory study

Isotopic ¹³C NMR spectrometry, which is able to measure intra-molecular ¹³C composition, is of emerging demand because of the new information provided by the ¹³C site-specific content of a given molecule. A systematic evaluation of instrumental behaviour is of importance to envisage isotopic ¹³C NMR as a routine tool. This paper describes the first collaborative study of intra-molecular ¹³C composition by NMR. The main goals of the ring test were to establish intra- and inter-variability of the spectrometer response. Eight instruments with different configuration were retained for the exercise on the basis of a qualification test. Reproducibility at the natural abundance of isotopic ¹³C NMR was then assessed on vanillin from three different origins associated with specific δ¹³C_i profiles. The standard deviation was, on average, between 0.9 and 1.2‰ for intra-variability. The highest standard deviation for inter-variability was 2.1‰. This is significantly higher than the internal precision but could be considered good in respect of a first ring test on a new analytical method. The standard deviation of δ¹³C_i in vanillin was not homogeneous over the eight carbons, with no trend either for the carbon position or for the configuration of the spectrometer. However, since the repeatability for each instrument was satisfactory, correction factors for each carbon in vanillin could be calculated to harmonize the results.     

Enhanced (13)C PFG NMR for the study of hydrodynamic dispersion in porous media

PFG NMR methods are frequently used as a means of probing both coherent and incoherent molecular motions of fluids contained within heterogeneous porous media. The time scale over which molecular displacements can be probed in a conventional PFG NMR experiment is limited by the relaxation characteristics of (1)H - the nucleus that is typically observed. In multiphase systems, due to its sensitivity to susceptibility gradients and interactions with surfaces,(1)H signal is frequently characterized by rapid T(1) and T(2) relaxation. In this work, a heteronuclear approach to PFG NMR is demonstrated which allows the study of molecular displacement over extended time scales (and, consequently, length scales) by exploiting the longer relaxation time of (13)C. The method presented employs the DEPT technique of polarization transfer in order to enhance both the sensitivity and efficiency of (13)C detection. This hybrid coherence transfer PFG technique has been used to acquire displacement propagators for flow through a bead pack with an observation time of up to 35 s.     

Sound power flux measurements in strongly exited ducts with flow

This contribution describes new robust procedures for the measurement of sound power flux at appropriate axial positions along a duct with flow, using pairs of flush wall mounted microphones, or pressure transducers. The technology includes the application of selective averaging, order tracking, and optimized sampling rate methods to identify the small fraction of the total fluctuating wave energy that is being propagated along the flow path in a reverberent, or highly reactive duct system. Such measurements can also be used to quantify the local acoustic characteristics that govern the generation, transfer, and propagation of wave energy in the system. Illustrative examples include the determination of the acoustic characteristics of individual silencing elements installed in IC engine intakes and exhausts both on the flow bench and during controlled acceleration or run down on a test bed, where the wave component spectral levels approached 170 dB.     

The effect of reverberation on the damping of bubbles

The measurement of an acoustic emission, or scatter, from a bubble is not difficult. However, an accurate interpretation of that signal in terms of the bubble dynamics may require careful consideration. The study presented here is at first sight relatively simple: comparison of the predicted and measured quality factors of injected bubbles. While the measurement is normally done by monitoring the decay of passive emissions from a bubble, this technique becomes difficult with smaller bubbles. Therefore an active technique is introduced, which removes all the frequency-dependent effects on the measurement (such as transducer response) bar one. That, critically, is the effect of the change in the bubble resonance (frequency and damping) which results from the loading on the bubble due to the reverberant field. The vast majority of theoretical treatments of bubble acoustics assume free field conditions, yet the environmental conditions rarely if ever match these. Therefore measurements of bubble damping are compared both with the established free field theory, and with a new theory relevant to the prevailing reverberant conditions (whether caused by tank surfaces, monochromatic neighboring bubbles, or both).     

Heterocumulene metathesis by iridium guanidinate and ureylene complexes: catalysis involving reversible insertion to form six-membered metallacycles

The four-membered metallacycles Cp*Ir((NAr)2C=Z) have been prepared for Z = NAr and Z = O. These complexes undergo facile exchange with free heterocumulenes. This exchange process can be employed to effect the rapid, catalytic metathesis of aryl carbodiimides at room temperature and the metathesis of aryl carbodiimides with aryl isocyanates at slightly elevated temperature. The exchange process appears to proceed via a novel associative mechanism involving ring expansion to form a six-membered metallacycle rather than cycloreversion to give an imido complex. Observation of key intermediates and the results of crossover experiments support this hypothesis.     

Ramsey fringes in a Bose-Einstein condensate between atoms and molecules

In a recent experiment, a Feshbach scattering resonance was exploited to observe Ramsey fringes in a 85Rb Bose-Einstein condensate. The oscillation frequency corresponded to the binding energy of the molecular state. We show that the observations are remarkably consistent with predictions of a resonance field theory in which the fringes arise from oscillations between atoms and molecules.