Double-bond equivalents, structure spectroscopy

Notice the positions labeled 1 and 2 are part of a cyclopropane ring in one structure, but position 2 is part of a double bond in the second structure. If this rearrangement is repeated so that every bond becomes involved, all 10 hydrogens in bullvalene will become equivalent. An examination of the temperature at which the multiplets coalesce into one broad singlet (-t-15°C) allows the energy of activation, and thus the rate constant, for the isomerization to be determined. This process would be virtually impossible to study by any other technique except NMR spectroscopy. 

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>. 

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