Browsing by Author "Jimeno, María Luisa"

The stability of all possible conformers of hexa(pyrazol-1-yl)benzene [(pz)6bz], hexakis(3,5-dimethylpyrazol-1-yl)benzene [(dmpz)6bz], 1,2,4,5-tetra(pyrazol-1-yl)-3,6-bis(3,5-dimethylpyrazol-1-yl)benzene [(pz)4(dmpz)2bz] and 1,4-di(pyrazol-1-yl)-2,3,5,6-tetrakis(3,5-dimethylpyrazol-1-yl)benzene [(pz)2- (dmpz)4bz] has been determined by semiempirical AM1 calculations. In all series the absolute minimum corresponds to the conformation where the nitrogen lone pair of consecutive pyrazole rings is alternatively oriented to opposite sides of the benzene plane. The case of (pz)2(dmpz)4bz has been studied by 1 H NMR studies in solution. Three isomers have been identified and all the signals assigned. The slow evolution to other isomers from those found in the solid state has been followed by 1 H NMR spectroscopy thus allowing determination of the barrier to rotation of a (dmpz) substituent (ÄG‡ = 21–22 kcal mol21 ).

Hybrid ab initio calculations (GIAO/B3LYP/6-31G*) together with new DNMR experiments (1H and 13C) have been used to clarify the problem of the conformation of 5,6,11,12-tetrahydrodibenzo[a,e]cyclooctene. The carbon-13 chemical shifts have been assigned unambiguously to the chair C and twist-boat TB conformations. Lineshape analysis of the 1H NMR spectra has been carried out at several temperatures using an estimated set of Karplus-type vicinal coupling constants.

The IH and I3C NMR spectra of the title compounds have been recorded. While the spectrum of 6,7,14,15-tetrahydrobisbenzimidazo[ 1,2-a: l',2'-e] [ i ,5]diazoeine 3 does not show any dependence with the temperature in the studied range (-65 to +75 "C), that of 6,7,13,14-tetrahydrobispyrido[1,2- a: l',2'-e] diazocinediium dibromide 4 shows a clear variation, narrow lines appearing only at + 85 "C. The differences in the dynamic behaviour of these compounds has been analyzed using different semi-empirical calculations (MNIX), AMI, PM3) of the ground and transition states. For comparison purposes, two other compounds belonging to the 6,8,6 series of ring derivatives have also been calculated, 5,6,11,12- tetrahydrodibenzo[a,e]cyclooetene 1 and 5,6,12.13 -tetrahydrobispyrazolo [ 1,2 -a: 1 ',2'-e] [ 1,2,5,6] tetrazoeinediium cation 2.