A Large Isotope Effect on Formation of Tetramethylethylene Cations in .gamma.-Irradiated 2,3-Dimethylbutane at 77 K. Tunneling of H2 Molecule

Abstract

The significant isotope effect on hydrogen elimination reaction from 2,3-dimethylbutane (h-DMB) cations was studied by ESR at 77 K. When a h-DMB-SFa(0.6 mol %I) mixture is y-irradiated at 70 K, h-DMB+ ions in addition to h-DMB radicals are produced. When the irradiated mixture is stored at 77 K for 10 min, tetramethylethylene (TME) cations are produced by HZ elimination from h-DMB+ ions. The rate constant for the HZ elimination reaction was measured as 1.1 x s-l at 77 K. When a 2,3-dimethylbutane-2,3-d~ (d-DMB)-SF6 (0.6 mol%) mixture is y-irradiated at 70 K and then stored at 77 K, TME+ ions are scarcely produced upon the storage of the irradiated mixture at 77 K even for 5 days. The rate constant for the DZ elimination reaction from d-DMB+ ions was measured as 6.5 x s-l, which is much smaller than the rate constant for the HZ elimination reaction. The significant isotope effect on the reactions was explained in terms of a model of the tunneling elimination of a hydrogen molecule from a DMB+ ion.

Selective Alkane Transformations via Radicals and Radical Cations: Insights into the Activation Step from Experiment and Theory

“…Another reaction of branched hydrocarbon radical cations also is the loss of H 2 . These reactions were studied experimentally and computationally for the 2,3-dimethylbutane radical cation (eq 28), and significant tunneling was indicated …”

Reactions of Atomic Hydrogen with Formic Acid and Carbon Monoxide in Solid Parahydrogen II: Deuterated Reaction Studies

“…One of the first research groups to study pure tunneling reactions focused on creating H atoms in solid molecular hydrogen (SMH). − These are fascinating chemical systems due to the importance of nuclear quantum effects; the light mass of the H atom results in a large de Broglie wavelength, and the H 2 molecule undergoes both large amplitude translational zero-point motions and free rotation within SMH . Through a series of radiolysis and photolysis studies of solid H 2 –D 2 mixtures and solid HD, Miyazaki and his group (followed by Kumada) used electron spin resonance (ESR) spectroscopy to demonstrate a number of nonstandard chemical phenomena: (1) H atoms readily diffuse through SMH even at temperatures below 5 K by repeated H + H 2 → H 2 + H tunneling exchange reactions, (2) D atoms produced in solid H 2 rapidly react with the host thereby trapping the D atom in HD and creating mobile H atoms, and (3) H atoms in solid H 2 are remarkably stable on the time scale of hours due presumably to inefficient H + H → H 2 recombination in enriched parahydrogen crystals. − Proof of the importance of tunneling in the measured reaction dynamics is provided by comparison of the rate constant for the D + HD → H + D 2 reaction estimated experimentally to be 3.5 × 10 –27 cm molecule –1 s –1 at 4.2 K with the thermally activated rate constant calculated to be 10 –400 cm 3 molecule –1 s –1 . Further proof the reaction occurs exclusively through tunneling is provided by measuring the rate constants for different isotopic variants of this reaction.…”

Large Isotope Effects in Tunneling Chemical Reactions

“…T * + H 2 → HT + H (11) energy exceeds the activation energy for the reactions (10) and (11). The results in Fig.…”

“…The tunneling reaction of H 2 -molecule-transfer was discovered by Miyazaki et al When 2,3-dimethylbutane(h-DMB) containing SF 6 (0.6 mol%) is irradiated at 70 K by γ -rays and then stored at 77 K, tetramethylethylene cations (TME + ) are produced by H 2 detachment from h-DMB + ions. 10) Figure 3 shows the effect of the storage time on the h-DMB…”

“…In fact, the experimental rate constant (1.1×10 −3 s −1 ) is similar to those (1.1×10 −3 s −1 ; 1.4×10 −3 s −1 ) calculated for the tunneling detachment. 10,11) Therefore, a TME + ion is produced by the tunneling detachment of a H 2 molecule from an h-DMB + ion. Figure 3 shows that TME + ions, denoted by open squares, are scarcely produced upon the storage of the irradiated 2,3-dimethylbutane-2,3-d 2 (d-DMB)-SF 6 (0.6 mol%) mixture at 77 K even for 5 days.…”