AA XX spin system

Figures 282-MHz F-NMR spectrum of 1,1-difluoroethene, an AA XX spin system (Reproduced with permission of Japan HalonA anan )...

The symmetry of this molecule makes the fluorines chemically equivalent, but not magnetically equivalent. Examination of the three staggered conformations of AA XX spin system (Fig. 2.8) helps one understand this situation. 

As further proof of the course of the reaction between 85 and lithium alkyls, the primary product 86, resulting from the opening of the ring in compound 85 according to Eq. (13), was reacted anew with MeCl (44). At —40°C, formation of the symmetrical n-tetraphosphane 92 takes place, as shown in Eq. (17). This is an equally stable derivative, whose P H -NMR spectrum shows the characteristic resonances of an AA XX spin system. 

Figure 10.12. Calculated and experimental P H NMR spectra of 14 with the XX multiplets of two AA XX spin systems. Left meso (centered at -201.8 ppm) right d,l (centered at -211.8 ppm).

Bertrand and coworkers were also the first to report white phosphorus (P4) activation using carbenes [227]. Reacting 2 equivalents of CAAC B with P4 in hexanes instantly gave a deep-blue-colored solution. P NMR spectroscopy revealed a mixture of two AA XX spin systems in a 9 1 ratio, which upon analysis appeared to arise fi-om the E and Z isomers of 2,3,4,5-tetraphosphatriene. X-ray crystallography confirmed the assignment and revealed the isomer to feature a P4 chain capped by two CAACs, as shown in Figure 15.53. 

The P H NMR spectra of 32-35 show two triplets with additional signals of low intensity indicating an AA XX spin system with V(p coupling constants close to zero (Table 4.1). These results are in agreement with previous work by Pidcock and co-workers reporting slow rotation (on the NMR timescale) of the... 

Double resonance in the A A XX spin system with J(AA ) = 0 is mentioned briefly in the previous report. (1) Subsequently the use (435) of A- X experiments to measure J(XX ) by locating weak lines in the X spectrum has found many applications, coupling in metal... 

Detailed analysis of spectra of this type leads in principle to the evaluation of. /axI. 1- ax I I- aa I. /xx 1- General equations for line positions and intensities of the X part of an X AA X spin system have been presented by Harris for situations where the remote XX coupling is zero. The appearance of the X part of such a spectrum consists of a doublet of separation. /ax+- ax centred at vX, (the chemical shift of the X nucleus), which has relative intensity 2 " , ... 

On the other hand, the P H NMR spectrum of 37 consists of a set of three signals corresponding to an AA BB XX spin system (Table 4.1), which indicates that the dianionic P4 chain with the expected dW-trans conformation is retained in solution, as was observed in its molecular structure determined by X-ray diffraction. In the solid state, the nickel atom and the four phosphorus atoms of the chain form a puckered five-membered ring (Table 4.2). Interestingly, the nickel atom has pseudo-octahedral coordination, with the diphosphene ligand (P5 and P6) and the terminal phosphorus atoms of the (P4Ph4) ligand (PI and P4) in the equatorial positions, and the potassium atoms (K1 and K2) in the axial positions with short K--Ni distances of 318.5(2) pm for K1 and 310.7(2) pm for K2, which are the shortest distances reported for an Ni -K interaction in a complex (Fig. 4.18). 

In the case of (43) (R = Me or Et cis- and trans-isomcis are formed in comparable quantities, and these can be separated by fractional crystallisation. These compounds all have relatively large and positive exo-P—N—P spin-coupling (typically 350-400 Hz), and as shown in Scheme 7, this can be attributed to the preponderance of conformers a. The individual spin-couplings for the AA XX -type spin system exhibited by the P- H n.m.r. spectra for the m-isomer where R = Me are shown in Fig. 4, (i). The crystal structure of the same compound shows that the molecule also adopts the same conformation in the solid state. When R = Pr or Bu , only cw-isomers of (43) are formed a surprising result, which... 

Figure 1.38 HP NMR spectrum of 114. d P 138. The observed second-order spin system, AA XX , is consistent with the complete conversion of 113 into 114.

For a system of four nuclear spins, the analysis becomes extremely complicated. One important case is A2X2, which is two pairs of chemically equivalent nuclei with ax ax Actually, there are two possible A2X2 cases, according to whether we have one or two A-X coupling constants. For 1,1-difluoroallene (Fig. 8.8a), there is a single value of JAX, whereas for 1,1-difluoroethylene there are two different A-X coupling constants JAX and Jthis latter case is best designated AA XX. ... 

The pairs of fluorines in all of these molecules, except those in 1,1-difluoroethene, would also be magnetically equivalent. In order to be magnetically equivalent, nuclei that are chemically equivalent must have identical coupling constants to any other particular nucleus in the molecule, and it can be seen that the two protons in 1,1-difluoroethene do not have the same spatial relationship with respect to a given fluorine substituent. For example, the Fa substituent has a cis relationship to Ha, but a trans relationship to Hb (Scheme 2.30). A spin system such as this one is represented as an AA XX system, which contrasts with the A2X2, A2X, and A2XY systems in Scheme 2.29 wherein both fluorines in each of these systems have identical 2/HF coupling constants. 

If two protons in the same set (i.e., chemical-shift-equivalent protons in the same multiplet) couple equally to every other proton in the spin system, they are also magnetically equivalent, and the usual Pople notations apply A2, B2, X2 etc. However, if two protons in a set are not magnetically equivalent, the following notations apply AA, BB, XX, etc. To rephrase Two chemical-shift-equivalent protons are magnetically equivalent if they are symmetrically disposed with respect to each proton in the spin system. Obviously magnetic equivalence presupposes chemical-shift equivalence. In other words, do not test for magnetic equivalence unless the two protons in question are chemical-shift equivalent. 

Multiplicity b = broad, d = doublet, dd = doublet of doublets, ddq = doublet of doublet of quartets, dq = doublet of quartets, dqu = doublet of quintets, dspd = doublet of septet of doublets, dt = doublet of triplets, m = multiples q = quartet, s = singlet, sp = septet, t = triplet, AA = AA part of an AAXX spin system, XX = XX part of an AAXX spin system. If no multiplicity is given, the resonance appears... 

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