Matrix Effect on Hydrogen Atom Tunneling from Alkane to Free Deuterium Atoms in Cryogenic Solid

Abstract

ESR study on alkyl radicals generated in deuterated organic matrixes at 77 K by hydrogen atom tunneling from alkane molecules to free deuterium atoms has been carried out for elucidating control factors for the tunneling. For small alkane molecules, the tunneling rate is determined by the height of the potential energy barrier for the tunneling. For larger molecules in glassy matrixes, the rate decreases with increasing number and length of alkyl chains bonded to a carbon atom to be hydrogen abstracted. The tunneling rate from antepenultimate tertiary carbon is much slower than that from penultimate secondary carbon, even the potential energy barrier is lower. This abnormal effect is explained as being due to the steric hindrance by matrix molecules to the deformation of alkane molecules during the reaction. The alkyl chains surrounded by matrix molecules prevent the deformation of the chemical bonds of the carbon atom from the initial sp3 to the final sp2 configuration, which may cause the increase of the thickness of the potential energy barrier and thereby decrease of the tunneling rate. Free deuterium atoms in a crystalline adamantane matrix selectively abstract hydrogen atoms from the antepenultimate tertiary carbon, probably due to less steric hindrance to the deformation.

Matrix Effect on Hydrogen Atom Tunneling: Comparison between Hydrogen Addition and Abstraction

“…Hydrogen-atom abstraction from alkanes results in the change of the bond orbitals of the reacting carbon from the initial sp 3 to the final sp 2 configuration, which accompanies the change of the C−C−C bond angle. As shown in the previous paper, the optimized geometrical structure of the reaction system at the exit of the tunneling channel is not far from the product state, so that the suppression of the angular change by matrix molecules causes the increase of the potential energy and thereby a decrease of the tunneling rate. The suppression is larger for longer alkyl chains, so that hydrogen atoms are preferentially abstracted from carbon atoms with shorter alkyl chains, even if the rate of hydrogen-atom abstraction by thermal activation is the same or slower.…”

“…Although we do not know the reason for the discrepancy, it may be worthwhile to point out that the spectral separation of the resultant CH 3 C*HCH 2 CH 2 CH 3 and CH 3 CH 2 C*HCH 2 CH 3 radicals is very difficult, since both of them give octet spectra with the separation of 2.2 mT. 7 Our result is in agreement with that by Kelley et al 8 On the basis of product analyses, they concluded that the tunneling rate of hydrogen atoms to the penultimate vinyl carbon of CH 3 CHd CHCH 2 CH 3 was the same as that to the antepenultimate one.…”

“…We have shown in the previous paper 7 that the rate of hydrogen-atom tunneling from hexane to hydrogen atoms in cryogenic glassy matrices decreases with the increasing length of alkyl chains attached to reacting carbon atoms, because the increases of the length causes the increase of steric hindrance by matrix molecules to the deformation of hexane from the initial sp 3 to the final sp 2 configuration and thereby causes the increase of the area of the tunneling barrier. If this explanation were applicable to the hydrogen-atom addition, the addition to the penultimate vinyl carbon of CH 3 CHdCH(CH 2 ) 2 CH 3 would be faster than that to the antepenultimate one.…”

“…5,6 On the basis of the observation that the preferential position of hydrogen-atom tunneling from the branched alkane changed to the antepenultimate tertiary carbon by changing the matrix to crystalline adamantane, in which the branched alkane could move rather freely even at 77 K, we proposed that the peculiarity of the hydrogen abstraction in cryogenic solids arose from the resistance of matrix molecules to the deformation of alkane molecules from the initial sp 3 to the final sp 2 configuration. 7 The resistance increases with the increasing number and length of alkyl chains bonded to a carbon atom to be hydrogen-abstracted. Increase of the resistance causes the increase of the area of the potential energy barrier for the tunneling.…”

“…Although the activation energy and the bond dissociation energy is the lowest at the tertiary carbon of alkanes, Henderson and Willard found a very strange reaction that the rupture of C−H bonds by the attack of hydrogen atoms or by γ-ray and UV irradiation selectively took place in rigid cryogenic matrices at the antepenultimate secondary carbon of branched alkanes with tertiary carbon at the antepenultimate position . Ichikawa and Ohta then showed that the location of the selective rupture could be changed to the tertiary C−H bond by replacing the −CH 2 CH 3 group with −CD 2 CH 3 . , Using ESR and electron spin echo spectroscopy, we found that the selective C−H bond rupture at the penultimate secondary carbon was due to the conversion of primary alkyl radicals and free hydrogen atoms to penultimate secondary alkyl radicals by selective hydrogen-atom tunneling from the penultimate position of the carbon skeleton. , On the basis of the observation that the preferential position of hydrogen-atom tunneling from the branched alkane changed to the antepenultimate tertiary carbon by changing the matrix to crystalline adamantane, in which the branched alkane could move rather freely even at 77 K, we proposed that the peculiarity of the hydrogen abstraction in cryogenic solids arose from the resistance of matrix molecules to the deformation of alkane molecules from the initial sp 3 to the final sp 2 configuration . The resistance increases with the increasing number and length of alkyl chains bonded to a carbon atom to be hydrogen-abstracted.…”