Coherence order

Structured (correlation). If the coherently ordered surface areas (islands, domains) are smaller than the transfer width of the LEED system and at the same vertical height, the width of these areas. Aw, is directly related to the width of the LEED spots in fe-space, Afen ... 

Transverse magnetization represents a particular type of coherence involving a change in quantum number p of 1. Each coherence a is equal to the difference in magnetic quantum numbers of the nuclei r and 5, i.e., the coherence order is M,—M and pulses cause transitions to occur... 

Band-pass filter A filter used for selecting the desired coherence order, p. Carrier frequency The transmitter frequency that consists of high-frequency pulses. 

Coherence order, p The difference in the magnetic quantum number, of the two energy levels connected by the same coherence. 

High-pass filter A filter used to select a coherence order > p. INADEQUATE Correlation spectroscopy in which the directly bonded C nuclei are identified. 

Low-pass filter A filter used for selecting coherence orders >/>. 

It is convenient to express these frequencies in term of p (coherence order) and q (satellite order), following the convention described in Sect. 2.3.1 ... 

This term is independent of the coherence order p. Furthermore, it vanishes for all symmetric transitions (q = 0), and thus can be disregarded when describing the SQ CT coherence or the symmetric MQ coherence. Note, however, that this is not the case for the STs (q 0), which are therefore strongly affected by the first-order quadrupolar interaction, except when the sample is spun at the magic angle. 

The structural chemistry of these species has been the subject of a remarkable number of misconceptions and only now is beginning to emerge as a coherent, ordered discipline. Thus, prevailing attitudes on the nature of even the simplest... 

COHERENCE ORDER USING GRADIENTS TO SELECT A COHERENCE PATHWAY... 

In general, the sensitivity to twisting of a particular spin state can be classified by something called its coherence order. Thus an ordinary magnetization vector in the x -y plane has coherence order of 1 (single-quantum coherence, p = 1) and a magnetization vector along the z axis has coherence order zero (p = 0). Only the coherence order of 1... 

The pathway S, S is unaffected by the first gradient (Fig. 8.26) because z magnetization does not precess, so the 13C SQC (Sx) is only twisted by the second gradient and arrives at the FID in a coherence helix that adds to zero over the whole sample. There is no need to subtract it out—it never reaches the receiver. We can add up the twists imparted by the two gradients using the fact that coherence order (p) equals zero for z magnetization ... 

Because the sum is not equal to zero, we end up with twisted coherence and no signal in the receiver. We call this a gradient-selected experiment because the gradients are being used to specifically refocus coherence in the desired coherence transfer pathway (XH SQC -> 13C SQC) and to reject all others. In Chapter 10, we will develop the idea of coherence order in a more precise manner, and we will see that coherence order can be either positive or negative. 

Furthermore, the coherence order (sensitivity to twisting of a coherence by a gradient) is ambiguous, at least with respect to the sign... 

The most important thing about the raising and lowering (or spherical ) operators is the way they react to gradients, which is to say their coherence order. The coherence order is no longer ambiguous. For the heteronuclear system... 

Note that the phase of the pulse does have an effect It introduces a phase factor (in this case 1 or -1) in front of the operator, but it does not change the coherence order. Most of the time, we will be ignoring these phase factors. 

The effect of delays is even simpler the coherence order does not change during a delay. For example, I- remains I- after a delay r ... 

We can also have /-coupling evolution from 1+ to I+Sz or from I SZ to I-, but the coherence order (1 and -1, respectively) does not change. Because ZQC and DQC do not undergo/-coupling evolution, I+S+ will stay as I+S+ andl+S- will stay as I+S- during a delay (times a phase factor for DQ or ZQ chemical-shift evolution) and the coherence order (5 and 3, respectively) will not change. 

---