Heats of Formation of Krypton Fluorides and Stability Predictions for KrF₄and KrF₆from High Level Electronic Structure Calculations

Abstract

Atomization energies at 0 K and heats of formation at 0 and 298 K are predicted for KrF+, KrF-, KrF2, KrF3+, KrF4, KrF5+, and KrF6 from coupled-cluster theory (CCSD(T)) calculations with effective core potential correlation-consistent basis sets for krypton. To achieve near chemical accuracy (+/-1 kcal/mol), three corrections were added to the complete basis set binding energies based on frozen core coupled-cluster theory energies: a correction for core-valence effects, a correction for scalar relativistic effects, and a correction for first-order atomic spin-orbit effects. Vibrational zero point energies were computed at the coupled-cluster level of theory. The calculated value for the heat of formation of KrF2 is in excellent agreement with the experimental value. Contrary to the analogous xenon fluorides, KrF2, KrF4, and KrF6 are predicted to be thermodynamically unstable with respect to loss of F2. An analysis of the energetics of KrF4 and KrF6 with respect to fluorine atom loss together with calculations of the transition states for the intramolecular loss of F2 show that fluorine atom loss is the limiting factor determining the kinetic stabilities of these molecules. Whereas KrF4 possesses a marginal energy barrier of 10 kcal/mol toward fluorine atom loss and might be stable at moderately low temperatures, the corresponding barrier in KrF6 is only 0.9 kcal/mol, suggesting that it could exist only at very low temperatures. Although the simultaneous reactions of either two or four fluorine atoms with KrF2 to give KrF4 or KrF6, respectively, are exothermic, they do not represent feasible synthetic approaches because the attack of the fluorine ligands of KrF2 by the fluorine atoms, resulting in F2 abstraction, is thermodynamically favored over oxidative fluorination of the krypton central atom. Therefore, KrF6 could exist only at very low temperatures, and even the preparation of KrF4 will be extremely difficult.

High-Pressure Reactivity of Kr and F2—Stabilization of Krypton in the +4 Oxidation State

“…In contrast, the experimental enthalpy of formation at ambient pressure is slightly positive (0.21 eV/atom [16]). This value was well reproduced with rigorous coupled-cluster calculations of Dixon et al [55]. In our previous study, we found that the Perdew-Burke-Ernzerhof (PBE) functional used here tends to overestimate the thermodynamic stability of noble-gas compounds, and that more accurate values can be generally achieved with the application of the hybrid HSE06 functional [30].…”

“…In our previous study, we found that the Perdew-Burke-Ernzerhof (PBE) functional used here tends to overestimate the thermodynamic stability of noble-gas compounds, and that more accurate values can be generally achieved with the application of the hybrid HSE06 functional [30]. Indeed, calculation performed with the use of this functional gives ∆H f (KrF 2 ) = 0.06 eV/atom, much closer to the theoretical value of Dixon et al (0.21 eV/atom, [55]). However, the high computational cost of the HSE06 functional severely restricts the number of compounds that can be studied.…”

On the position of helium and neon in the Periodic Table of Elements

“…It was in the outburst of genius that Bartlett decided to react xenon with a small sample of PtF 6 which resulted in the isolation of the first noble gas compound (Bartlett 1962), what proved to be a mixture of several distinct compounds (Bartlett et al 1976), as it was finally confirmed only half a century from the initial discovery (Graham et al 2000;Craciun et al 2010;Seppelt 2015). The noble gas chemistry-particularly that of xenon and krypton, and including organoxenon chemistry (Naumann and Tyrra 1989;Frohn and Jakobs 1989)-has flourished since 1962 (Christe and Wilson 1982;Christe et al 1991;Dixon et al 2007), and even argon has proved to form a stable chemical compound, HArF, at sufficiently low temperatures (Khriachtchev et al 2000)-it remains the only isolated chemical compound of argon. Consequently, with over half a thousand of currently known compounds of noble gases, their chemistry has been extensively reviewed during the last three decades (Laszlo and Schrobilgen 1988;Lehmann et al 2002;Schrobilgen and Moran 2003;Grochala 2007;Grochala et al 2011;Brock et al 2013;Hope 2013;Haner and Schrobilgen 2015).…”

Selenium(IV) fluoride and oxofluoride anions

“…For the 19 F chemical shifts, the only exception for the calculated chemical shifts being larger than experiment is for SeF 6 2-where the trend is in the opposite direction and the calculated 19 F chemical shifts are more negative than experiment. This is no surprise because the free valence electron pair of Se is partially sterically active [8,49] requiring very high-level computations for reliable results [50][51][52][53]. The structures of the isoelectronic molecules KrF 6 and BrF 6 À are octahedral but the structures of XeF 6 and IF 6 À are C 3v with the octahedral structure slightly higher in energy.…”