Evaluation of the distance between the hydrogen atom and hydrogen molecule in the tunneling reaction HD + D → H + D2 in an argon matrix at 20 K

A hydrogen atom-hydrogen molecule pair entirely isolated in an Ar matrix has been investigated by high-resolution ESR spectroscopy from 4.2 to 23 K. A doublet ESR spectrum of the hydrogen atom was separated by 51.18 mT and further split isotropically into nine lines with superhyperfine coupling constants of ca. 0.06 mT and peak intensities of 1:1:2:1:2:1:2:1:1 in a solid Ar matrix containing HD (3.3 mol%) at 20 K. The superhyperfine structure can be attributed to HHD and HD2 pairs. The HD2 pair is a product of the tunneling reaction HD + D → HD2. The mean distance between the hydrogen atom and the hydrogen molecule in the tunneling reaction in the Ar matrix at 20 K is experimentally determined to be 1.73 Å.