X-nucleus coil

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 adoption of the inverse approach also has implications for the design of the NMR instrument. Conventional probes have been constructed so as to optimise the sensitivity for observation of the low-y X-nucleus, which entails placing the X-nucleus coil closest to the sample and positioning the proton coil outside this. Inverse probes have this configuration switched such that the proton coil sits closest to the sample for optimum sensitivity, thus providing a greater filling factor. However, even with conventional probes, the proton detected experiments can still be performed, albeit with less than optimum sensitivity, and may still provide a faster approach than the former X-observe experiments. 

NMR probes are designed with the X-coil closest to the sample for improved sensitivity of rare nuclei. Inverse detection NMR probes have the proton coil inside the X-coil to afford better proton sensitivity, with the X-coil largely relegated to the task of broadband X-nucleus decoupling. These proton optimized probes are often used for heteronuclear shift correlation experiments. 

ID ll spectrum, and crosspeaks are arranged symmetrically around the diagonal. There is only one radio frequency channel in a homonuclear experiment, the H channel, so the center of the spectral window (set by the exact frequency of pulses and of the reference frequency in the receiver) is the same in If and F (Varian tof, Bruker ol). The spectral widths should be set to the same value in both dimensions, leading to a square data matrix. Heteronuclear experiments have no diagonal, and two separate radio frequency channels are used (transmitter for F2, decoupler for F ) with two independently set spectral windows (Varian tof and dof, sw, and swl, Bruker ol and o2, sw(If), and sw(I )). Heteronuclear experiments can be further subdivided into direct (HETCOR) and inverse (HSQC, HMQC, HMBC) experiments. Direct experiments detect the X nucleus (e.g., 13C) in the directly detected dimension (Ff) using a direct probe (13C coil on the inside, closest to the sample, H coil on the outside), and inverse experiments detect XH in the To dimension using an inverse probe (XH coil on the inside, 13C coil outside). 

As was discussed earlier in the chapter, probes typically include two coils H and X nucleus (e.g., or N). The inner (or observation) coil is more sensitive and requires more careful adjustment. Normal NMR spectra usually have such good signal-to-noise ratios that probe tuning is not critical for relatively concentrated samples. Tuning, however, is very important for both one- and two-dimensional X-nucleus-detected experiments and for many two-dimensional H-detected techniques. A surprising number of these experiments have failed simply because the X coil was not tuned to the correct nucleus In addition, if X-nucleus detection is to be conducted with proton broadband decoupling, as is usually the case (Section 1-5), then it is important that the H decoupling coil also be tuned optimally. 

Triple Inverse 5mm TBI probehead for H and C observation with one additional channel for X observation or decoupling (choice of one nucleus within the range P -" Ag), a Z-gradient coil and operable in the temperature range -50° to -t-80°C. 90° pulselenghts for H and "C were 6.9us (-6dB) and 13us ( dB) respectively. 

An architecture which offers more variability is based essentially on a multinuclear probe head whose H coil is triply tuned to deliver the additional " Y frequency and offers the possibility to perform triple-resonance experiments with a fixed nucleus Y, but a choice of "X. The selection of the fixed channel depends on the intended usage the most common options are and C which offer widespread applicability in organometallic and coordination chemistry. For a good performance it is mandatory that RF interferences between the different channels are eliminated by appropriate filtering. Even if this arrangement is still less flexible than a probe head in which both "X and ""Y are variable, it appears preferable because the presence of two tuneable broadband coils would probably lower sensitivity, and the handling would be rather difficult because of increased RF interference problems. 

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