Barrier Height for the Exchange Reaction F + HF → FH + F

Abstract

There exists a body of conflicting data as to the existence or nonexistence of FHF, ClHCI, BrHBr, and IHI as chemically bound molecular species. Ab initio quantum mechanical electronic structure calculations are presented which predict linear symmetric FHF to be unstable. The barrier height for the F + HF exchange reaction is suggested to be no less than 18 kcal/mol, much larger than expected either intuitively or on the basis of certain experiments on related systems. The expected reliability of the calculations is based upon comparable results for diatomic molecules and the F + H2 and H + F2 potential energy surfaces.The two classic systems for the study of reactive quantum mechanical tunneling are (1-3) H + H2 H2 + H and H + CH4 H2 + CH3The merits of these two systems with respect to tunneling are at least three: (a) both are relatively simple reactions, (b) both have relatively high barrier heights (about 10 kcal), and (c) both involve the transfer of the lightest atom, hydrogen. Thus H + H2 and H + CH4 are amenable to detailed calculation and at thermal energies, tunneling should contribute a significant fraction of the reactive cross sections.A related system which might be considered a candidate for reactive tunneling is the F + HF --FH + F reaction. Although the experimental activation energy is not known for the above reaction, in recent years it has been assumed (1) to be low, no more than 6 kcal. There are two reasons for this assumption. The first is Johnston's bond energy bond order (BEBO) method, which predicts the 6-kcal barrier. As ab initio theorists, we tend to be skeptical of empirical schemes such as BEBO. However, Truhlar (4) has recently shown that BEBO is remarkably accurate for the H + H2 and F + H2 reactions. Not only does BEBO accurately reproduce the activation energies, but it also predicts minimum energy paths within 0.03 A of those obtained from the most elaborate available ab initio calculations (5, 6). Thus the BEBO method for these two systems is far more accurate than could have been anticipated, and one should consider seriously the 6-kcal prediction for F + HF.The second indirect piece of evidence against the existence of a substantial barrier for F + HF is the report by Pimentel and coworkers (7)

New Landscape of Electron-Pair Bonding: Covalent, Ionic, and Charge-Shift Bonds

“…We would expect to see manifestations of CSB on reactivity for cases that involve cleavage of CS bonds. Our studies [81] showed that one of these manifestations is the computational [82][83][84][85][86] and experimental [87] results that halogen transfer reactions (and especially of fluorine), Eq. (13a), have much larger barriers (by >20 kcal/mol for X¼F) than the corresponding hydrogen transfer processes, Eq.…”

Identity Hydrogen Abstraction Reactions, X• + H−X‘ → X−H + X‘• (X = X‘ = CH3, SiH3, GeH3, SnH3, PbH3): A Valence Bond Modeling

“…The barrier calculated by the model for X ) X′ ) F is also in good agreement with the datum of Schaefer et al is 23.9 kcal/ mol. 35 In fact, the barriers in Tables 2 and 3 enable us to assess the model equation's ability to predict trends along a row of the priodic table. Thus, eq 22 predicts, in accord with the results of CCSD(T), that in each row the identity barrier is generally larger for the more electronegative groups, X and X′ (compare, e.g., the barriers for X ) F vs CH 3 , etc).…”

Reactions of fluorine atoms

“…-> F + FO +(101) F + F20 -F2 + FO(102) FO + FO -02 + 2F(88) F + F + M-*"F2 + M(97) An activation energy of 14.3 ± 1.5 kcal for reaction 102 was determined from the photochemical investigation,125 while the results of the thermal study126 gave k102 = 5.1 X 101°e xp[-(13.7 ± 1.0) X 103/fi7] cm3 mol-1 s-1.…”