Thermochemistry of Halomethanes CF_nBr_4–n (n = 0–3) Based on iPEPICO Experiments and Quantum Chemical Computations

Abstract

Internal energy selected bromofluoromethane cations were prepared and their internal energy dependent fragmentation pathways were recorded by imaging photoelectron photoion coincidence spectroscopy (iPEPICO). The first dissociation reaction is bromine atom loss, which is followed by fluorine atom loss in CF(3)Br and CF(2)Br(2) at higher energies. Accurate 0 K appearance energies have been obtained for these processes, which are complemented by ab initio isodesmic reaction energy calculations. A thermochemical network is set up to obtain updated heats of formation of the samples and their dissociative photoionization products. Several computational methods have been benchmarked against the well-known interhalogen heats of formation. As a corollary, we stumbled upon an assignment issue for the ClF heat of formation leading to a 5.7 kJ mol(-1) error, resolved some time ago, but still lacking closure because of outdated compilations. Our CF(3)(+) appearance energy from CF(3)Br confirms the measurements of Asher and Ruscic (J. Chem. Phys. 1997, 106, 210) and Garcia et al. (J. Phys. Chem. A 2001, 105, 8296) as opposed to the most recent result of Clay et al. (J. Phys. Chem. A 2005, 109, 1541). The ionization energy of CF(3) is determined to be 9.02-9.08 eV on the basis of a previous CF(3)-Br neutral bond energy and the CF(3) heat of formation, respectively. We also show that the breakdown diagram of CFBr(3)(+), a weakly bound parent ion, can be used to obtain the accurate adiabatic ionization energy of the neutral of 10.625 ± 0.010 eV. The updated 298 K enthalpies of formation Δ(f)H(o)(g) for CF(3)Br, CF(2)Br(2), CFBr(3), and CBr(4) are reported to be -647.0 ± 3.5, -361.0 ± 7.4, -111.6 ± 7.7, and 113.7 ± 4 kJ mol(-1), respectively.

Fast Dissociations of Halogenated Methanes: A Thermochemical Network

“…In such cases, the parent signal is always less than 100 %, and there is significant daughter ion signal, even at the ionization limit. This effect had been observed previously [17], and was first discussed in the TPEPICO study of CFBr 3 and CBr 4 [11].…”

“…Firstly, that CF 4 , CHF 3 , CFCl 3 and CCl 4 which are not included in this study, dissociatively photoionize forming no initial parent ion, and one can regard the ion potential well depths as so shallow they do not exist. In another study on CF n Br 4-n , Bodi et al [11]. found that CBr 4 + is formed and dissociated into CBr 3 + ?…”

“…However the ion potential well is shallow, like the ions CHCl 3 + , CHClF 2 + and CBrClF 2 + . Based on the cursory observation that the parent ion potential well is deepest for those molecules with more hydrogen atoms, onset energies from threshold coincidence experiments for a range of halogenated methanes can found in the literature, in addition to those included here [11,15,[26][27][28][29]. CH 3 Br + has the deepest potential well and CH 4 + is the next most stable ion.…”

“…The advantage of this approach over the purely ab initio route is that the network is pegged by the accurately measured onset energies and well-known enthalpies of formation such as those for CF 4 and CH 4 , thus reducing the plasticity of the ab initio network and eliminating systematic errors inherent in the original ab initio calculations. Such an approach has also been used to obtain updated enthalpies of formation for primary amines [21], as well as for bromofluoromethanes and their dissociative photoionization products [11].…”

“…Recent threshold photoelectron photoion coincidence (TPEPICO) experiments, in which both the photon and the photoelectron energies are known to within 1-2 meV (0.1-0.2 kJ mol -1 ), [1,[10][11][12] are capable of measuring dissociative photoionization onset energies in small to medium sized molecules with such levels of accuracy. To reiterate, in the absence of an overall reverse barrier, the onset energies, E 0 , correspond to the reaction energy at 0 K, and yield the enthalpies of formation for the parent ion, daughter ion or neutral fragment if two out of the three are known;…”