AA-BB systems

The exchange of the iV-proton between the 1-N and 3-N sites is fast on the H NMR time-scale at ambient temperature, as found by Vernin et al. (1977). In CDC13 two AA BB systems are observed only at low temperatures ( — 80 °C). 

In terms of chemical equivalence, (or more accurately, chemical shift equivalence) clearly, Ha is equivalent to Ha. But it is not magnetically equivalent to Ha because if it was, then the coupling between Ha and Hb would be the same as the coupling between Ha and Hb. Clearly, this cannot be the case since Ha is ortho to Hb but Ha is para to it. Such spin systems are referred to as AA BB systems (pronounced A-A dashed B-B dashed). The dashes are used to denote magnetic non-equivalence of the otherwise chemically equivalent protons. What this means in practise is that molecules of this type display a highly characteristic splitting pattern which would be described as a pair of doublets with a number of minor extra lines and some broadening at the base of the peaks (Spectrum 5.6). 

Another example of 19F coupling, this time in an aromatic system, (4-fluoro benzoic acid) is shown in Spectrum 6.11. Note how the 19F couplings to the aromatic protons give the AA BB system an... 

From the fact that the ABCD system of dibenzotetraazapentalene 255 did not become an AA BB system on heating, even at 225°C, it was concluded339 that the valence isomerization tetraazapentalene tetraazacyclooctatetraene (Scheme 14, Section IV,B,2) did not occur. Hall, Stephanie, and Nordstrom341 have effected an analysis of the ABCD system of 255 and revealed the presence of two superimposed ABCD systems that give the molecule an element of symmetry. Chemical shifts conformed to electron densities calculated by Chia and Simmons (Section V,B,3).388 The same authors also studied the ABCD-... 

The configurational stability of primary organolithiums, while of no synthetic consequence, has been determined by NMR methods. Analysis of the line shape of the AA BB system of 1713 or of the ABX system of 1814 gave half-lives for inversion of about 0.01 s for 17 or 18 in ether at 30 °C, increasing (for 17) to about 0.1 s at 0 °C and 1 s at -18 °C. The rate of inversion was decreased by a factor of 8 in pentane or 10 in toluene, but the addition of 1 equiv. TMEDA to pentane restored a rate of inversion more or less equal to that in pentane. 

Annellation of two benzene rings increases the barrier for the racemization process considerably and the -NMR-spectrum of (45) shows the expected splitting at room temperature120). Coalescence of the AA BB system for the CH2—CH2 moiety occurs at 128 °C (60 MHz) which corresponds to AG = 19.7 kcal/mole118,185>. By chromatography of (45) on a chiral phase (TAPA on silica gel) at —28 °C a partial separation of the enantiomers was achieved. 

The p.m.r. spectrum of l,6-dideoxy-2,5-0-methylene-D-mannitol (66 see p. 65) at 60 MHz has been classified as an AA BB M3M3 system, but the full complexity of this system was avoided by analysis of the spectrum of the related l,3,4,6-tetra-0-acetyl-2,5-0-methylene-D-mannitol-i,7,6,6 -d4 (76) as an AA BB system (compare myo-... 

Both H and C NMR spectra have been recorded for 8 at 193 K in acetone-d and at 213 K in a trichlorofluoromethane/l,2-dibromotetrafluoroethane/acetone-d6 (5 5 1) mix-ture. The fact that the compound has been handled under these conditions suggests that it will soon be the subject of chemical as well as physical study. The H NMR spectrum (acetone-dg) displays the AA BB system expected for the aromatic protons at 7.82 and 8.28 ppm while the C data show the quaternary carbons at 154.90 ppm (C=0) (cyclo-propenone 155.1 ppm) and 139.33 (Cda/s )). The non-equivalent methine signals resonate at 117.89 and 141.72 ppm and have been assigned to C0/4) amd C(2/s), respectively. These last assignments, made by comparison with cyclobutabenzene-l,2-dione and not verified, run counter to the expectation that of the cycloproparene are influenced... 

The two alternate explanations for the low-temperature nmr spectra of VI are potentially distinguishable. Even the spectra are not quite the same the twist-boats should give two ABCD systems (with suitable coincidences in chemical shifts), while the 1 1 mixture of the boat-chair and twist-chair-chair should give two different AA BB systems. Unfortunately, vicinal fluorine-fluorine coupUng constants are extremely small, so that two apparent AB systems are expected from both explanations. 

H2(3) appears as a low-field singlet and aromatic protons as an AA BB system. 

The NMR spectrum for phenazine shows an AA BB system for ring protons at <5 7.97 and 8.26 ppm in [ H Jdimethyl sulfoxide and coupling constants (Jj j = 9.0 Hz Jj 3 = 1.67 Hz Ji.i = 6.55 Hz) have been estimated. Similar coupling constants are also characteristic for substituted phenazines and phenazine A -oxides. H and CNMR data for phenazine and substituted phenazinesand CNMR spectra for phenazine di-A-oxide have been measured and assigned. NNMR data have also been reported for phenazine.Tire high resolution NMR spectra of phenazine have been measured in inert and proton donating solvents. 

Two-dimensional NMR spectroscopy ((double quantum fdtering (DQF), correlation spectroscopy (COSY), hetero-nuclear multiple quantum correlation (HMQC), heteronuclear multiple bond correlation (HMBC)) as well as liquid secondary ionization mass spectrometry (LSI MS) and UV-Vis spectroscopies were used to establish crown structures of TTFs 33 ( =l-3). In the case of the macrocycle 33 ( = 1), two protons of each methylene group of the SCH2CH2O fragments were not identical and gave an AA BB system. This observation was in accordance with the expected low conformational mobility of the polyether bridge in ( )-33 ( = 1) as compared with (Z)-33 ( = 1). The macrocycle ( )-33 ( = 2) behaved similarly to ( )-33 ( =1), whereas the protons under discussion were equivalent in ( )-33 ( = 3) <2001CFJ447>. 

For 3,3-dimethylbutylmagnesium chloride 2, the proton spectrum has been observed to be temperature-dependent [29,30]. At — 53°C, an AA BB system is obtained, whereas... 

Consideration of 9,10-ethano-9,10-dihydronapththalene, 23 122,123 a reasonable model for 20, further supports the delocalized nature of l,6-methano-[10]annulene. The AA BB system of olefinic protons in 23... 

A final sub-group within the bridged 10 7r-electron systems includes those compounds bridged by heteroatoms. The synthesis and n.m.r. investigation of l,6-oxido-[10]annulene 137-139,142) and many of its derivatives 17,125,138,140) have accordingly been executed. The n.m.r. spectrum 17 of l,6-oxido-[10]annulene itself (30, see Table 6) consists of an AA BB system centered at t 2.65 (in CCI4 also quoted at t 2.52... 

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