Evidence for the H transfer

The H transfer mechanism has been evidenced in the phenol-NH3 system by accident.  In the figure two mass spectra are compared: the lower one is obtained when the pump (set on a 1-3 vibrationic band) and the probe laser are delayed by a few nanosecond and the upper mass spectrum has been recorded with a long delay (200 ns) between pump and probe (in this case 355 nm photon)  laser.

The remarkable fact is that the signal observed on the NH₄⁺(NH₃)_n=2,3 does not decay whereas the signal observed on the phenol-(NH₃)_n decays in time. At very long delays only the signal on protonated ammonia cluster remains. When  the pump laser is set on a the 0-0 band of the -1-2cluster, similar results are obtained with a probe laser at 290 nm, but not with the probe at 355 nm.

This result was interpreted as a Hydrogen atom transfer mechanism following :

 PhOH*(S₁)-(NH₃)_n=2,3® PhO^·+ (NH₄)(NH₃)_n-1.

The interpretation was made possible from the knowledge obtained by the Fuke group on solvated ammonium clusters. Indeed, the 355 nm (3.5 eV) probe photon used here is energetic enough to ionize the NH₄(NH₃)₂ dissociation product (ionization potential of 3.31 eV from the  work of Fuke et al.).  At this wavelength NH₄(NH₃)⁺ is not detected in agreement with its higher ionization threshold (3.88 eV). But when the probe wavelength is changed to 290 nm (4.3 eV), NH₄(NH₃)⁺ is observed. Moreover, the work of Fuke et al. shows that NH₄(NH₃)_n have long lifetimes (3 ms and 7 ms for n=1 and 2, respectively) which is necessary to observe a delayed ionization, whereas NH₄ is very short lived (16 ps)

Since this first observation, this mechanism has been strongly substantiated by other experiments.

a) spectroscopic evidence for the H transfer

    The infrared depopulation spectrum performed by Ishiuchi : in this experiment the NH₄(NH₃)_n cluster is produced by excitation of phenol (NH₃)₃. a second infrared laser induces the vibrational dissociation of the complex, while a third laser ionizes the surviving cluster. Scanning the IR laser gives the equivalent of the infrared absorption spectrum.  This experiment led to the observation of a broad vibrational band at 2900 cm-1 which has been associated with the excitation of the NH stretching vibration of the ammonium radical, which lead to a very fast predissociation of NH₄(NH₃)₂. A similar depopulation experiment has been performed by the same group in the visible region and then the spectrum obtained is very similar to the one obtained by the Fuke group on NH₄NH₃ clusters obtained by photolysis of NH₃ clusters.

b) Time evolution

The variation of the decay of phenol-(NH₃)_n with the excess energy. For the 1-1 complex the lifetime is strongly dependent on the vibrational coordinate. Exciting the stretching intermolecular vibration which is closely related to the reaction coordinate decreases the lifetime much more than exciting an intramolecular phenol vibration. As an example, the excitation of the +486 cm^-1 intramolecular vibration leads to a lifetime of 400 ps whereas the excitation of the intermolecular stretching mode +186 cm^-1 has a lifetime of 300 ps.  Similarly for the 1-2 complex the 0-0 transition has a lifetime of 400 ps whereas one quanta in the intermolecular vibration shortens the lifetime to 50 ps.  Lifetime decreases down to 50 ps for the 0-0 band of the 1-3 complex.

In deuterated clusters, the lifetimes come back to the nanosecond scale.

It was thus concluded that the H transfer mechanism was occurring via tunneling through a barrier. 

c)  H transfer : a general mechanism

This H transfer has been evidenced in many molecules

d)  It is a consequence of the SDDJ model