Pulse sequence HETCOR

This is the simple heteronuclear -X COSY (or HETCOR) pulse sequence adapted to long-range correlations simply by adjustment of the Ai and A2 delays. Small coupling constants rule out the use of broad-band proton decoupling during evolution time t by BIRD, and thus the correlation cross-peaks display the structure of proton multiplets, which reduces further the sensitivity of the experiment. 

Any HETCOR pulse sequence can readily be extended by a Hahn-echo pulse train [20] which is particularly useful in the case of Sn/ H HEED-HETCOR of tin-nitrogen compounds. This enables one to measure y( Sn, N), /( N, H) (together with the relative signs) and A / N( Sn), all at the same time (see Figure 13). Numerous other modifications of 2D HETCOR experiments are feasible, many of which are based on schemes of selective excitation and/or... 

Figure 3.25. Na- P 2D HETCOR spectra of Na3P309 A. Obtained with the normal HETCOR pulse sequence and B. with MQ-MAS incorporated in the pulse sequence to obtain higher resolution from Wang, De Paul and Bull (1997) with permission of the copyright owner.

Fig. 10.16. (A) 2D H—HETCOR pulse sequence and (B) 2D HETCOR spectrum for PMA/PVPh = 1/1 cast from methyl ethyl ketone. The interpolymer cross-peak between the carbonyl carbon of PMA (b) and the hydroxyl protons of PVPh (a) is appreciable. (Reprinted with permission from Ref. [26]. 1994 John Wiley, New York.)...

Mirau and White applied the HETCOR pulse sequence to PMA/PVPh, PMMA/PVPh and PC/polycaprolactone (PCL) blends [26, 124]. Figure 10.16(b) shows the HETCOR spectrum for PMA/PVPh = 1/1 cast from methyl ethyl ketone. The combined use of WIM-24 and MAS allows CP for H and C spins in close proximity, therefore, most of the cross-peaks appear between directly bonded C— H pairs. Interestingly, a cross-peak was observed for the nonprotonated carbonyl carbon of PMA. The spectrum shows... 

The original HETCOR experiment has been modified to incorporate the spin-diffusion period after the evolution period to examine the miscibility of PC and an aromatic dianine [125]. Li et al. [126] used the modified HETCOR pulse sequence to study d5-PS/PPO, d3-PS/PPO and d3-PS/d5-PS blends. The cross-peak intensities increase with increasing the spin-diffusion period, and the distance between interacting components was determined. Instead of inserting the spin-diffusion period, Takegoshi and Hikichi [127] used the conventional CP sequence for a mixing sequence to study an interpolymer interaction of PMA/PVPh. They found that with a CP contact time of 0.2 ms, no appreciable interpolymer cross-peaks appear, but with a CP time of 1 ms, the interpolymer cross-peak between the aromatic protons of PVPh and the carbonyl carbon of PMA appears. 

Acetohexamide solution in DHSO-de was used to obtain the PMR spectrum on a Varlan XL 200 MHz FT NHR spectrometer using TMS as Internal reference. The spectrum Is shown In Figure 5. The number of protons Is established by both Integration of the area under the curve and the multiplicities of the peaks. Table 3 assigns the chemical shifts to their respective protons. Further evidence for proton assignment Is obtained from the HETCOR pulse sequence (Figure 9). 

Preparation Evolution Mixing Detection Figure 7 The C-H HETCOR pulse sequence. 

The basic pulse sequence employed in the heteronuclear 2D shift-correlation (or HETCOR) experiment is shown in Fig. 5.40. The first 90° H pulse bends the H magnetization to the y -axis. During the subsequent evolution period this magnetization processes in the x y -plane. It may be considered to be made up of two vectors corresponding to the lower (a) and higher (/3) spin states of carbon to which H is coupled. These two... 

Figure 1. Pulse sequence for 27Al-29Si HETCOR experiment with sensitivity enhancement by RAPT and CPMG data acquisition.

Fig. 19 Pulse sequences and coherence transfer pathways used in MQMAS- >-HETCOR experiments. In MQMAS-/-HETCOR, CP is replaced by /-based transfer...

We introduce the use of solid-state 2D FSLG 13C-3H HETCOR experiments for correlating intermolecular interactions in miscible, hydrogen bonded polypeptide blends in this section. The pulse sequence that is used... 

Advantages of HETCOR over HMQC type pulse sequences became evident in cases where indirect detection of the high-y nucleus allows... 

The pulse sequence for this experiment, commonly called HETCOR (HETeronuclear CORrelation), is recorded in Figure 5.11a. During the evolution time U), the large one-bond heteronuclear 7-coupling (7Ch) is used for polarization transfer, and thus only 13C s bonded... 

HMQC and HSQC are about equally sensitive, the sensitivity being proportional to (Yx)5 2, but as the HSQC pulse sequence uses more than twice as many pulses as HMQC, the latter is preferred (especially on older spectrometers). Both use two polarization transfer steps while HETCOR uses only one transfer. If the coupling constants vary within the sample, HETCOR might be a better choice for lower losses in polarization transfers. Since the sensitivity of HETCOR is proportional to (yx)3 2(YY), the choice of nucleus for detection is not trivial. 

Very recently, an interesting modification of HETCOR correlation was published by Chan and co-workers.69 In this experiment, a DQ filter is incorporated into the pulse sequence of HETCOR spectroscopy so that a DQ excitation profile can be obtained by measuring a series of 2D spectra (Figure 14). This method offers a simple experimental approach to extract the van Vleck second moment of a multiple-spin system under high-resolution condition. Hydroxyapatite (Ca10(PO4)6(OH)2) and brushite (CaHP04 2H20) were used as reference samples. 

FIGURE 10.10 (a) Pulse sequence for HETCOR, as described in text. (f>) H-13C HETCOR spectrum (300 MHz) of allylbutyl ether in CDC13. The one-dimensional H spectrum is shown at the top and the proton-decoupled 13C spectrum at the left.The 2D plot clearly indicates the H-C correlations. Spectrum courtesy of Herman J. C.Yeh (National Institutes of Health). 

---