Hydrogen atom abstraction tunneling reactions

We now consider hydrogen transfer reactions between the excited impurity molecules and the neighboring host molecules in crystals. Prass et al. [1988, 1989] and Steidl et al. [1988] studied the abstraction of an hydrogen atom from fluorene by an impurity acridine molecule in its lowest triplet state. The fluorene molecule is oriented in a favorable position for the transfer (Figure 6.18). The radical pair thus formed is deactivated by the reverse transition. H atom abstraction by acridine molecules competes with the radiative deactivation (phosphorescence) of the 3T state, and the temperature dependence of transfer rate constant is inferred from the kinetic measurements in the range 33-143 K. Below 72 K, k(T) is described by Eq. (2.30) with n = 1, while at T>70K the Arrhenius law holds with the apparent activation energy of 0.33 kcal/mol (120 cm-1). The value of a corresponds to the thermal excitation of the symmetric vibration that is observed in the Raman spectrum of the host crystal. The shift in its frequency after deuteration shows that this is a libration i.e., the tunneling is enhanced by hindered molecular rotation in crystal. 

CFjCl, CFjCl CFaCl, and CFs CFaCT and CFCl, >-i have been made, and Arrhenius parameters for the rate-determining step F + RQ - R + CIF determined the heat of this reaction was estimated for each substrate, and the values were used to calculate a large number of C—Cl, C—F, and C—C bond strengths [e.g. Z)°(CC1 F—Cl) = 300 8 kJ mol ]. Kinetic isotope effects for H-atom abstraction reactions of fluorine atom with chloroform, methane, and hydrogen have been measured the results are those expected from the differences in zero-point energies for the relevant vibrations, so H-atom transfer by quantum mechanical tunnelling is apparently unimportant. ... 

ABSTRACT. The results of multireference singles and doubles Cl calculations of potential energy surfaces for hydrogen atom addition to O2, N2, and NO and recombination of OH -h O are discussed. The errors due to the use of externally contracted Cl and due to the neglect of correlation of O 2s and N 2s electrons are analyzed. Similarities and differences between the surfaces for the addition reactions are discussed. The calculated HN2 addition surface is used in a simple dynamical treatment (one-dimensional tunneling through an Eckart barrier) to estimate the lifetime of the HN2 species. The OH -h O recombination potential is found to exhibit complex features which require that electrostatic forces (dipole-quadrupole) and chemical forces be treated consistently. 

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