The mechanism of formation and infrared-induced decomposition of HXeI in solid Xe

Abstract

Ultraviolet ͑UV͒ irradiation of HI-doped xenon matrix dissociates the precursor and leads to the formation and trapping of neutral atoms. After UV photolysis, annealing of the matrix mobilizes the hydrogen atoms at about 38 K. The mobilized hydrogen atoms react with I/Xe centers forming HXeI molecules in a diffusion controlled reaction. The formed molecules can be photolyzed with infrared ͑IR͒ irradiation at 2950-3800 cm Ϫ1 and quantitatively regenerated thermally. The formation of HXeI from neutral atoms is proved by the quantitative correlation between neutral iodine atoms and HXeI molecules in selective IR photodissociation and thermal regeneration experiments. Kinetic measurements show that the formation of HXeI from atoms is prevented by a potential barrier, which is estimated to be 700 cm Ϫ1 in magnitude. The potential barrier is proposed to originate from the avoided crossing between neutral HϩXeϩI and ionic ͑HXe͒ ϩ ϩI Ϫ singlet surfaces. The dissociation energy D 0 of HXeI with respect to the top of the potential barrier is estimated to be 2950 cm Ϫ1 and D e about 4070 cm Ϫ1 in solid Xe. The weak IR photodissociation profile of HXeI around 3000 cm Ϫ1 is measured by irradiating the sample with tunable IR source and monitoring the changes in the fundamental region. The formation mechanism from neutral atoms is believed to be valid for other similar rare-gas compounds.

Neutralization of solvated protons and formation of noble-gas hydride molecules: Matrix-isolation indications of tunneling mechanisms?

“…It should be stressed that the delayed formation of the HNgY molecules has been usually promoted by annealing activating diffusion of hydrogen atoms in the matrix, with an important exclusion of IR-decomposed HXeI. 10,12 In the present work, it is found that HArF ͑preferably the unstable configuration͒ can be slowly but efficiently produced in a photolyzed HF / Ar matrix at 8 K. The formation of DArF at 8 K is much slower, by a factor of ϳ50, than the formation of HArF. The formation of HArF at 8 K follows different kinetics from the decay of the ͑ArHAr͒ + cations.…”

“…The tunneling mechanism was previously suggested to contribute to the local formation of HXeI after its IR decomposition. 12 The excitation of the second H-Xe stretching overtone produced short-rangeseparated H-Xe-I triads, which could restore HXeI via H atom tunneling. The short separation distance is needed for efficient H atom tunneling.…”

“…10 As an example, the first neutral ground-state chemical compound containing argon, HArF, was prepared using the vacuum-UV photolysis of HF in solid Ar at 7 K and by annealing at ϳ20 K. 11 The formation of HNgY molecules via the H + Ng+ Y reaction of the neutral fragments was directly demonstrated in the case of HXeI. 12 It was not an evident fact because of the charge-transfer nature of these molecules showing the strong ͑HNg͒ + Y − character and the presence of the ͑NgHNg͒ + ions in most of those experiments. Indeed, one could connect the annealing-induced decay of ͑NgHNg͒ + ions and the formation of HNgY molecules, but this does not seem to be the general case.…”

On theoretical predictions of noble-gas hydrides

“…5,6 The formation of HNgY molecules presumably occurs via the thermally activated H + Ng+ Y reaction of the neutral fragments. 7 Thus, it is crucial for this method that HNgY is somewhat lower in energy than the H + Ng+ Y reagents. This energy difference has been attributed as the first indication for a stable HNgY molecule and careful analysis of this energy diagram is of primary importance for the practical predictions of HNgY molecules.…”

Anomalous isotopic effect on vibrational properties of HXeOH

“…4,5 From the energetic point of view, these molecules are quite unusual due to the low dissociation energy ͑ϳ0.4 -1.5 eV͒, and we were able to photodecompose one of them ͑HXeI͒ directly by exciting the second H-Xe stretching overtone at ϳ3000 cm Ϫ1 with intense narrowband IR radiation. 6 The shallowness of the potential well suggests the importance of anharmonic effects in their ground-state dynamics. It follows that one should carefully account the anharmonicity of various normal modes and the anharmonic couplings between the modes when treating these molecules theoretically.…”