Tunnelling of H atoms

The tunneling of H atoms in irradiated crystals of dimethylglyoxime has been studied in refs. 78 and 79. Radical pairs of the J and K types are formed in the crystal when y-irradiation is used (Fig. 30). After irradiation, radical pairs J are transformed into radical pairs K. No chemical transformation occurs here but the positions of the H atom in the initial and in the final states are different, and this allows the process to be studied with the help of EPR spectroscopy. The kinetics of the process have been measured [79] at temperatures from 50 to 150 K and at 4.2 K. At 50 T 150 K the activation energy of the process was found to decrease with decrease of temperature (see Fig. 30). The process rates at 50 and 4.2 K are practically the... 

Tunneling of H atoms occurs in some enzyme-catalyzed reactions see Quantum Tunnelling in Enzyme-Catalyzed Reactions, R. Allemann andN. Scrutton (eds.), RSC Publishing, 2009. 

Hydron atoms readily dissolve into bulk Pd, where they can reside in either the sixfold octahedral or fourfold tetrahedral interstitial sites. Determine the classical and zero-point corrected activation energies for H hopping between octahedral and tetrahedral sites in bulk Pd. In calculating the activation energy, you should allow all atoms in the supercell to relax but, to estimate vibrational frequencies, you can constrain all the metal atoms. Estimate the temperature below which tunneling contributions become important in the hopping of H atoms between these two interstitial sites. 

Electron transfer from the excited states of Fe(II) to the H30 f cation in aqueous solutions of H2S04 which results in the formation of Fe(III) and of H atoms has been studied by Korolev and Bazhin [36, 37]. The quantum yield of the formation of Fe(III) in 5.5 M H2S04 at 77 K has been found to be only two times smaller than at room temperature. Photo-oxidation of Fe(II) is also observed at 4.2 K. The actual very weak dependence of the efficiency of Fe(II) photo-oxidation on temperature points to the tunneling mechanism of this process [36, 37]. Bazhin and Korolev [38], have made a detailed theoretical analysis in terms of the theory of radiationless transitions of the mechanism of electron transfer from the excited ions Fe(II) to H30 1 in solutions. In this work a simple way is suggested for an a priori estimation of the maximum possible distance, RmSiX, of tunneling between a donor and an acceptor in solid matrices. This method is based on taking into account the dependence... 

In reactions with the participation of H atoms, both classical (overbarrier) and quantum (underbarrier, tunneling) modes of overcoming an activation barrier are possible. The value of the isotope effect suggests that the reaction occurs via a classical (overbarrier) path at room temperature. Assuming a normal value of the pre-exponential factor (10-11 cm3/molecule sec) for the activation energy of this reaction, we obtain E — 7.7 (8.7)kcal/mol (processes with the participation of H2(D2) molecules). 

Lundstrdm, 1981 Lundstrom and Soderberg, 1981 Lundstrdm et al., 1975 Shivaraman et al., 1976), (2) absorbed H reaction with Pd to form a hydride with lower work function (Steele et al., 1976), and (3) formation of a Pd-H system without chemical bonding of H atoms, with a lower work function (Lewis, 1967). The second one concerns the penetration of H atoms through the oxide layer and the formation at the SiO /Si interface of trapping states that determine tunneling of electrons between the metal and the semiconductor (Keramati and Zemel, 1982 Zemel et al., 1980 Kera-mati, 1980). 

An indication of the tunneling mechanism of transfer was also obtained in the reaction of H-atom abstraction from methyl isocyanide molecules by methyl radical, where a decrease in the apparent activation energy from 0.2 to 0.06 eV in the range 125-77 K and the absence of CH3 decay under complete deuteration of the matrix were observed [11]. 

These two properties of H-bonds are relatively well known and documented. This is not so of the third fundamental property of H-bonds, the importance of which became known only recently, but already suggests more important consequences under certain conditions that are not yet fully understood, transfers of protons or of H-atoms occur through H-bonds. It clearly points to H-bonds as being basic centres of reactivity, particularly in aqueous media at room temperature. Theoretical descriptions of these transfers in simple and isolated H-bonded systems show that such transfers are much more complex than simple tunnellings and that many other parameters, especially the presence of other molecules and also coordinates, other than the X-H distance, influence them. These transfers are particularly important in aqueous media where the numbers of H-bonds are especially great, and where these... 

Ch. 25 by Christmann is devoted to the interaction and reaction dynamics of hydrogen and simple molecules containing a hydroxy group such as water and methanol with transition metal surfaces. In particular, the possibility of H transfer via lateral diffusion or proton tunneling within the adsorbed layers is discussed. It is shown that lateral diffusion and transfer of H atoms does indeed occur via both... 

Current understanding of adsorption on Si(lOO) is summarized below. More detailed reviews are readily available. The structure of the clean surface has been determined experimentally by electron diffraction [9] and x-ray diffraction [10]. Liu and Hoffman [11] have given a concise review of the geometry and electronic structure of the bare surface. The classic review of Appelbaum and Hamann [12] describes the electronic structure of the bare surface and the bonding of H atoms. Accurate diffraction studies of the H-covered surfaces are not available. Infrared spectroscopy of H-covered Si surfaces has been reviewed by Chabal et al. [13], and the tunneling microscopy of these systems has been reviewed by Boland [14]. 

Calculations suggest that this species exchanges a pair of H atoms through a tunneling path involving a transient H2 complex, and more recent calculations show that an complex 19 is a transition state in the H-exchange process with a... 

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