Inverse geometry

The last thing to mention about probes is that they can have one of two geometries. They can be normal geometry, in which case the nonproton nucleus coils would be closest to the sample or inverse geometry (the inverse of normal ). We mention this because it will have an impact on the sensitivity of the probe for acquiring proton data (inverse is more sensitive than normal). Most of the time this shouldn t matter unless you are really stuck for sample in which case it is a bigger deal... 

Inverse geometry Term used to describe the construction of a probe that has the 1H receiver coils as close to the sample as possible and the X nucleus coils outside these 1H coils. Such probes tend to give excellent sensitivity for 1H spectra at the expense of X nucleus sensitivity in 1-D techniques. They offer a lot of compensation in terms of sensitivity of indirectly detected experiments. 

The on-line SEC-NMR measurements were performed on a 750 MHz spectrometer with an inverse-geometry LC-NMR probe with pulsed-field gradient coils. The detection volume of the flow cell was approximately 60 pi A Shodex linear K-805L SEC column (8 mm x 300 mm) with a maximum porosity of 4 x 106 was used for the chromatography. 

An alternative and equally valid approach, used by Blostein el, al. [Blostein 2001], is to calculate C(t) by expressing E as a function of t and E0 and integrating over Eq. However, it is more convenient to use Eq. (6), when discussing an "inverse geometry" instrument such as VESUVIO, where the energy of the scattered neutron is analysed. It follows from (3) and (5) that... 

For an ideal inverse geometry instrument, in which Lp, Li, 0 are precisely known and only neutrons of a precisely defined energy Er are detected, i.e.,... 

D. Colognesi, C. Andreani E. Degiorgi (2003). J. Neutron Res., 11,123-143. Phonon density of states from a crystal analyzer inverse-geometry spectrometer a study on ordered solid hydrogen sulfide and hydrogen chloride. 

A concerted reaction must be stereospecific. The mechanism described by Fig. 5.2 requires inversion of configuration. An alternative direct displacement mechanism involving front-side attack would also exhibit second-order kinetics and respond similarly to structural and medium effects, but would require retention of configuration as the stereochemical course. As we shall see, the available data support fully the mechanism involving back-side displacement. Additionally, SCF-MO calculations of the hypothetical transition states for hydride displacement of hydride from methane indicate that the inversion geometry is 14.9 kcal/mol more favorable than the retention geometry ... 

A whole family of experiments have been developed that detect low-sensitivity nuclei such as or N indirectly by their spin coupling connectivity to protons in the molecule. This involves a series of pulses on both and the heteronucleus but allows detection at the much superior sensitivity of NMR. Special probes have been developed for such indirect detection experiments in which the coil is placed close to the sample, and the heteronucleus coil is placed outside it, the opposite or inverse geometry to a standard heteronuclear detection probe. 

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