Rotational barriers relaxational analysis

The olefin bond rotation barriers have also been determined for the vinylaniline complexes 2 using several NMR techniques, namely selective inversion, total line shape analysis, and Tip measurements (i.e., spin-lattice relaxation rate in the rotating frame). 

Temperature-dependent lineshape changes were observed in an early study of the fluo-renyllithium(TMEDA) complex. A detailed study by lineshape analysis, which was also applied to the TMEDA complex of 2,3-benzofluorenyllithium(TMEDA) (Figure 29f, yielded barriers AG (298) of 44.4 and 41.9 kJmoD for the 180° ring flip in these systems, respectively . A second dynamic process, which was detected via the temperature dependence of, the spin-lattice relaxation time in the rotating frame, is characterized by barriers of 35.1 and 37.6 kJmoD, respectively, and may be ascribed to the ring inversion process. For the fluorenyl complex, a barrier AG (298) of 15.9 kJmoD for the methyl rotation in the TMEDA hgand was determined from temperature-dependent NMR spectra of the deuteriated system. 

It is noted that a similar approach carried out for polyethytene leads to an activation energy of 13.6 0.4 kJ/mol, for the relaxation of short (6-8 bonds) chain segments, regardless of the constraints imposed by chain connectivity [105]. This is in good agreement with the value 13.8kJ ol obtained by Brownian dynamics simulations of PE chains, and with NMR measurements of perfluoroalkane chains by Matsuo and Stodonayer, in which the mean orientational correlation times for C-F and C-H bends are found to have equal activation energies of about 12 kJ/mol. The barrier for internal rotation about typical CH2-CH2 bonds is slightly over 12 kJ/mol, from spectroscopic data and conformational analysis [3,106,107]. This value is exactly reproduced in the present theoretical treatment... 

---