Types inept

The INEPT experiment can be modified to allow the antiphase magnetization to be precessed for a further time period so that it comes into phase before data acquisition. The pulse sequence for the refocused INEPT experiment (Pegg et al., 1981b) is shown in Fig. 2.13. Another delay, A. is introduced and 180° pulses applied at the center of this delay simultaneously to both the H and the C nuclei. Decoupling during data acquisition allows the carbons to be recorded as singlets. The value of Z), is adjusted to enable the desired type of carbon atoms to be recorded. Thus, with D, set at V4J, the CH carbons are recorded at VsJ, the CH2 carbons are recorded and at VeJ, all protonated carbons are recorded. With D3 at %J, the CH and CH ( carbons appear out of phase from the CH2 carbons. 

The kinetics of hydrogenation transfer is covered by the use of an exchange superoperator assuming a pseudo first-order reaction. Thereby, competing hydrogenations of the substrate to more than one product can also be accommodated. In addition, the consequences of relaxation effects or NOEs can be included into the simulations if desired. Furthermore, it is possible to simulate the consequences of different types of pulse sequences, such as PH-INEPT or INEPT+, which have previously been developed for the transfer of polarization from the parahydrogen-derived protons to heteronuclei such as 13C or 15N. The... 

Poly(hydrosilane)s are stable compounds and can be manipulated in the air only for a short period since they are oxygen sensitive. In order to study the oxidation products, a xylene solution of poly(phenylhydrosilane)(Mw = 2340, Mw/Mn = 1.72) was refluxed (140 °C) for 12 h in a system exposed to the air [15]. Only minor changes were observed by GPC analysis whereas FTIR showed characteristic absorptions due to siloxane-type structures on the polymer backbone. A detailed NMR analysis, based on H NMR, Si INEPT and H- Si HMQC spectroscopies, indicated that the oxidized material contains the units 7-10 shown in Scheme 8.2. In particular, units 7,8 and 9+10 were present in relative percentages of 27%, 54% and 19%, respectively, which mean that more than 70% of the catenated silicons were altered. It has also been reported that silyl hydroperoxides and peroxides are not found as products in the autoxidation of poly(phenylhy-drosilane) [16]. 

In contrast to applications in structural biology where X/Y correlations are nowadays normally executed as H detected, three-dimensional experiments because of sensitivity reasons,14 many studies on inorganic or organometallic compounds are still performed as two-dimensional experiments with direct detection of one heteronucleus and under -decoupling. As compared to these two categories, one-dimensional polarisation transfer methods such as (semi) selective X/Y-INEPT or INDOR-type techniques, which had in the past been shown to be particularly useful for the characterisation of substrates with only one or two heteronuclei,11 have recently received less attention.15 NOE-based correlations, which are frequently employed for the structure elucidation of bio-molecules, remain rare, and apart from an earlier report of a 13C/6Li HOESY experiment,16 have not been further investigated. 

Values of /CH were also determined by two-dimensional /-resolved spectroscopy using an INEPT-type pulse-sequence,29 with a BIRD pulse for the suppression of "/c H and folding in the F, dimension. Rules for the calculation of the correct values of /c H from the reduced splittings observed were given. The values were also obtained from a set of /-modulated, one-dimensional 13C spectra via the... 

The width of the final1H pulse in the DEPT sequence ( ) affects the intensity and phase (positive or negative) of the 13C peaks in a way very similar to the length of the refocusing delay A in refocused INEPT, according to the relation = nJA. The intensities are shown below for the three types of carbon in DEPT ... 

Short mixing times (e.g., 30 ms) lead to INEPT-type spectra (or COSY-type 2D spectra), where transfer is mostly limited to a single jump over one J coupling. Unlike INEPT and COSY, however, the transfer results in an in-phase rather than antiphase signal. This is a significant advantage as the peaks have the same shape and pattern as they do in a ID spectrum. 

Coherent transfer experiments can roughly be divided into two classes pulse-interrupted free-precession experiments and Hartmann-Hahn-type experiments (Ernst et al., 1987). Examples of homo- and heteronuclear pulse-interrupted free-precession coherence transfer are COSY (correlation spectroscopy Aue et al., 1976), RELAY (relayed correlation spectroscopy Wagner, 1983), and INEPT (insensitive nucleus enhancement by polarization transfer) transfer steps (Morris and Freeman, 1979 Burum... 

In combinations of homonuclear and heteronuclear coherence-transfer steps, the homonuclear and/or heteronuclear transfer step may be realized by a Hartmann-Hahn mixing sequence. The first combination of a homonuclear H- H Hartmann-Hahn mbdng step with a heteronuclear INEPT-type transfer (Morris and Freeman, 1979 Sprensen and Ernst,... 

More modem one-dimensional (ID) sequences such as INEPT (Insensitive Nuclei Enhanced by Polarization Transfer) or DEPT (Distortionless Enhancement by Polarization Transfer) [6,13-15] have not been used extensively [16]. This type of experiment seems to be more powerful than the old INDOR technique. 

Fig. 3. Basic pulse sequences for 2D- X,"T H correlations. Tbe same notation as in Hg. 1 is used. Minimum phase cycles for selection of correlation signals are given, more elaborate schemes for quadrature detection in FI and phase-sensitive spectra may be applied following standard rules. (a) HETCOR (without 180° pulses)/INEPT (with 180° pulses), the refocusing delays A are optional in both experiments setting the mixing pulses 8 to 45°/135° instead of 90° allows to determine coupling signs in ABX-type spectra. (b), HSQC. (c), HMQC the refocusing delay A2 is optional.

Long-range correlations based on the HSQC sequence are generally less effective. Significant evolution of H- H couplings during the A period leads to unwanted COSY-type transfers among protons by the second 90° proton pulse of the INEPT sequence, a problem not foimd with HMBC. 

A second example of coherence selection using phase cycling is the suppression of the quaternary carbon signals in a polarization transfer spectrum. DEPT and INEPT type pulse sequences use a polarization transfer step to enhance the signal of an NMR insensitive nucleus such as which exhibits scalar coupling to a NMR sensitive nucleus such as IR. 

The APT sequence was the first experiment to decode the sign of the signal amplitude as a function of I S multiplicity. Because of the hardware restrictions in early NMR spectrometers, particularly applying simultaneous pulses on both the acquisition and second channel, emphasis was made on making the pulse sequence as simple as possible. The sequence only requires a 90° pulse and 180° pulse on the acquisition channel, which may be easily determined, and a simple decoupler switch-on/off on the second channel. Nevertheless the experiment is still included in modern pulse program libraries the experiment seems to be very robust and in contract to DEPT or INEPT type experiments quaternary carbon atoms can also be detected by the same experiment. 

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