Protonenresonanz-spektroskopie ungesättigter ringsysteme—IX

On the influence of vibrational modes and intramolecular isomerization processes on the NMR parameters of bullvalene: A Feynman path integral – ab initio investigation

“…In the bottom lines of Table 4 we have quoted the isotropic shieldings at the bullvalene nuclei under the premise of a dynamical equivalence between all 13 C ( 1 H) centres C av (H av ). It is this behaviour that is observed in NMR under ''fast'' isomerization conditions [18,[20][21][22].…”

“…The T dependence of the Cope rearrangement in bullvalene has characteristic consequences for the shape of the 13 C and 1 H HMR spectra [18,21,23]. For T 6 243 K this process is too slow to be detected in the NMR time-scale.…”

“…Next we comment on the definition of two NMR standards I and II which are required to transform the calculated absolute shieldings Q into chemical shifts v. Only the 13 C and 1 H shifts relative to TMS have been presented in recent NMR studies [21][22][23]. Standard I is based on experimentally estimated absolute 13 C and 1 H shieldings of TMS.…”

“…Its geometry is described by the 3n-dimensional vector R SI . The T dependence of the valence tautomerization leads to characteristic fingerprints in the 13 C and 1 H NMR spectra of bullvalene [21][22][23]. Indeed, the analysis of these spectra has provided evidence for its valence fluctuating character.…”

“…To derive the ensemble averaged absolute isotropic shieldings and chemical shifts in the second post-PIMC step, we have adopted these configurations as input for the ab initio based GIAO calculations. Experimental data have been reported only for the 13 C and 1 H chemical shifts relative to tetramethylsilane (TMS) [21][22][23] while absolute shieldings have not been recorded up to now. To calculate the chemical shifts, it is necessary to define proper NMR standards.…”