Antiphase doublets

Figure Bl.13.8. Schematic illustration of (a) an antiphase doublet, (b) an in-phase doublet and (c) a differentially broadened doublet. The splitting between the two lines is in each case equal to J, the indirect spin-spin coupling constant.

A 90° Gaussian pulse is employed as an excitation pulse. In the case of a simple AX spin system, the delay t between the first, soft 90° excitation pulse and the final, hard 90° detection pulse is adjusted to correspond to the coupling constant JJ x (Fig- 7.2). If the excitation frequency corresponds to the chemical shift frequency of nucleus A, then the doublet of nucleus A will disappear and the total transfer of magnetization to nucleus X will produce an antiphase doublet (Fig. 7.3). The antiphase structure of the multiplets can be removed by employing a refocused ID COSY experiment (Hore, 1983). 

So far, we have not considered the so-called longitudinal two-spin order, represented by the product operator9 2J ff, a quantity related to the polarization of nuclei A and B. This spin state can be created in different ways. The easiest way is probably to let the system evolve under the sole Jab coupling so as to obtain an antiphase doublet, for instance the B antiphase doublet represented by 2//Vf (corresponding to the two proton-carbon-13 satellites in an antiphase configuration). 

Figure 3 Creation of the longitudinal order by cross-correlation as a function of the mixing time fm which follows the inversion of a carbon-13 doublet (due to a./-coupling with a bonded proton). The read-pulse transforms the longitudinal polarization into an in-phase doublet and the longitudinal order into an antiphase doublet. The superposition of these two doublets leads to the observation of an asymmetric doublet.

If the H-NMR spectrum is recorded with complete 31P decoupling, both hydride resonance at -9.4 ppm and -17.6 ppm collapse into a doublet of antiphase doublets. The remaining 13-Hz and 22-Hz couplings correspond to JHRh and JHrk,... 

Fig. 7. Spin states of a doublet (a) after a (Jt/2)j pulse (b) some time t later and its decomposition into an in-phase and an antiphase doublets.

The antiphase doublet (Fig. 6.14(c)) is dispersive because /-coupling evolution to the antiphase state moves the vectors by 90°, from the +x axis to the +/ and —/ axes. This dispersive antiphase doublet can be phase corrected by moving the reference axis from the +x axis to the +/ axis (90° zero-order phase correction). Now the C = a peak is positive absorptive and the C = ft peak is negative absorptive (Fig. 6.15) and the central 12CH3l peak is pure dispersive because the vector is on the -hx/ axis and the reference axis is now +y (90° phase error). 

There are two important consequences of this method. Both result from the fact that the magnetization that is being observed (13C antiphase doublet) arises from H magnetization that is rotated from its equilibrium state along the z axis by the first 90° proton pulse. The first consequence arises because the lH population difference at equilibrium (sometimes called polarization ) is four times the carbon population difference at equilibrium. This results from the larger energy separation between the a and states for protons ... 

Thus Fourier transformation in F will yield two peaks a positive peak at F = 2a — nJ and a negative peak at F = 2a + nJ. This is an antiphase doublet in F centered at frequency 2a and separated by 2nJ rad or / Hz. So we have a crosspeak that is an antiphase doublet in F2 (—2IyI observed in the FID) and an antiphase doublet in F (sin( 2aH)sin(7r/fi)), with both doublets showing a separation of /ab- This is the crosspeak fine structure shown in Figure 9.29. 

Acquisition simply turn on the analog-to-digital converter and record a 1H FID. Fourier transformation of this signal will give an antiphase doublet whose amplitude is modulated by a factor cos( 2c t ). 

The refocusing delay just before acquisition has been eliminated, so the peaks in F2 will be antiphase doublets separated by the long-range/cH (2-15 Hz). This splitting is in addition to any H splitting pattern already present in the ID proton spectrum (Fig. 11.11). The second 13 C 90° pulse is phase cycled as before to make sure that only H magnetization... 

Figure 3 N 90° decoupler pulse measurement via using Bruker pulse program DECP90F3, sample [Pt(C5H5 N)(PPli3)Cl2] in dg-THF. (a) decoupler off-resonance—antiphase doublet does not invert, gives a false result, (b) decoupler on-resonance, correct...

Figure 9 Determination of the separation of the signal maxima of an antiphase doublet for the absorptive (vj and dispersive W component. Based on these two parameters the coupling constant J can be calculated. Reproduced with permission of Wiley-VCH from Eberstadt et al (1995) Angewandte Chemie, International Edition in English 34 1671-1695.

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