Kinetic study of hydrogen tunnelling in meso-tetraphenylporphine by nuclear magnetic resonance lineshape analysis and selective T1ρ-relaxation time measurements

摘要: The kinetics of the hydrogen migration between degenerate tautomers meso-tetraphenyl-porphine ([1H2]TPP) and deuterated species ([2H2]TPP) both dissolved in various media has been studied over a wide range temperatures by n.m.r.-lineshape analysis measurements longitudinal relaxation times rotating frame, T1ρ, selective lines. As proven 1H-n.m.r.-spectra [1H215N4]TPP, rate constants obtained are entirely due to random walk inner atoms four nitrogen atoms, not intermolecular proton transfer. In slow exchange doublet splitting signal shows that each is coupled with one 15N-spin coupling constant J15N—H= 101 Hz. However, fast pentet observed line distance J15N—H/4. Consequently, range, within n.m.r.-timescale, localized at single atom. protons localised equal probability rapid walk. do depend on type solvent used. Their dependence temperature given kH= 5.0 ± 0.5 + exp (26.5 1) (–43.4 1.3) kJ mol–1/RT), 160 ⩽T⩽ 323 K, kD= (29.7 0.7) (–57.3 1.3 213 305 K.The results cannot be explained terms transition state theory, but only vibrational model tunnelling symmetric double minimum potential quantized levels along reaction coordinate. related tunnel splittings barrier-separated states. motion proceeds ground states first excited NH-stretching states, deuteron second ND-stretching It probable TPP synchronous process like during inversion ammonia.