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

Abstract

The kinetics of the hydrogen migration between the degenerate tautomers of meso-tetraphenyl-porphine ([¹H₂]TPP) and of the deuterated species ([²H₂]TPP) both dissolved in various media has been studied over a wide range of temperatures by n.m.r.-lineshape analysis and by measurements of the longitudinal relaxation times in the rotating frame, T_1ρ, of selective lines. As proven by the ¹H-n.m.r.-spectra of [¹H₂¹⁵N₄]TPP, the rate constants obtained are entirely due to a random walk of the inner hydrogen atoms between the four nitrogen atoms, and not to intermolecular proton transfer. In the slow exchange range a doublet splitting of the inner proton signal shows that each proton is coupled with one ¹⁵N-spin with a coupling constant of J15_N—H= 101 Hz. However, in the fast exchange range a pentet splitting is observed with a line distance of J15_N—H/4. Consequently, in the slow exchange range, within the n.m.r.-timescale, each proton is localized at a single nitrogen atom. In the fast exchange range, within the n.m.r.-timescale, the inner protons are localised with equal probability at each of the four nitrogen atoms due to rapid random walk. The rate constants do not depend on the type of the solvent used. Their dependence on the temperature is given by k^H= 5.0 ± 0.5 + exp (26.5 ± 1) exp (–43.4 ± 1.3) kJ mol^–1/RT), 160 ⩽T⩽ 323 K, k^D= exp (29.7 ± 0.7) exp (–57.3 ± 1.3 kJ mol^–1/RT), 213 ⩽T⩽ 305 K.

The results cannot be explained in terms of the transition state theory, but only in terms of a vibrational model of tunnelling in a symmetric double minimum potential with quantized vibrational levels along the reaction coordinate. The rate constants are related to the tunnel splittings of barrier-separated degenerate states. The hydrogen motion proceeds by tunnelling between the ground states and the first excited vibrational NH-stretching states, the deuteron motion between the second excited ND-stretching states. It is probable that hydrogen tunnelling in TPP is a synchronous process like hydrogen tunnelling during the inversion of ammonia.