Halogen Migration in the Photofragmentation of Halothane

Abstract

The photofragmentation of halothane (CF₃CHBrCl) was studied with synchrotron radiation by photoionization efficiency (PIE) measurements and photoelectron-photoion coincidence (PEPICO) experiments, as well as by a theoretical exploration of potential energy surfaces. Among the other fragments, the formation of the CHClF⁺ and CHBrF⁺ ions, which involves the transfer of a F atom between the two moieties of the parent molecule, was observed. To understand the mechanisms leading to the halogen migration, a detailed theoretical study of the production of CHClF⁺, m/z 67⁺, based on DFT calculations and natural bond orbital (NBO) analysis was conducted. The results contribute to the understanding of the photochemistry of halothane, its polluting behavior in the high atmosphere, and the formation of highly reactive species.

Keywords: appearance energy; halogen migration; halothane; photofragmentation.

Scheme 1

Halothane (CF₃CHClBr) structure.

Figure 1

(a) The photoelectron spectrum of halothane at 21.5 eV. The bars are the calculated vertical ionization energies from ref. [5]; (b) the mass spectrum of halothane.

Figure 2

(a) The PEPICO yields of the fragments listed in the legend of the different panels; (b) the PIE curves of selected fragments used to calculate the AE. In the case of the parent ion, the fitting used to evaluate the AE_exp is shown.

Figure 3

Schematic of the first reaction pathway, with inverted position between Br and F in halothane, which leads to the release fragment of CHClF⁺ (m/z 67, TS1 AE_th = 13.91 eV) and two distinct final states (CF₂Br + CHClF⁺ 12.57 eV, CF₂ + Br + CHClF⁺ 13.98 eV). Total relative energies (in eV with zero-point energy, ZPE) at the CCSD(T)/6-311++G** level of theory with respect to the parent ion.

Figure 4

Schematic of the second reaction pathway, leading to the release of fragment CHClF⁺ (m/z 67, TS2 AE_th = 13.06 eV). Total relative energies (in eV with zero-point energy, ZPE) at the CCSD(T)/6-311++G** level of theory with respect to the parent ion.

Figure 5

Electron density distribution of orbital 42a.

Figure 6

Natural charges of halothane at the B3LYP/aug-cc-pVTZ: (A) neutral and (B) cation.

Figure 7

HOMO of halothane at the B3LYP/aug-cc-pVTZ level.

Figure 8

Natural charges of halothane at the B3LYP/aug-cc-pVTZ: (A) TS1 and (B) TS2 (see also Figure 3 and Figure 4).

Figure 9

Major NBO donor–acceptor interactions of TS2 for alpha spin.

Figure 10

Major NBO donor–acceptor interactions of TS2 for beta spin.

Figure 11

Scheme of the VMI-ToF spectrometer. In the two bottom panels, the photoelectron spectrum (PES), (right), and the mass spectrum, (left), of halothane measured on the VMI and TOF detectors, respectively, are shown (see Figure 1 for detailed plots of PES and MS spectra).