The vacuum-ultraviolet photoelectron spectra of CH₂F₂ and CH₂Cl₂ revisited

AbstractThe threshold photoelectron spectrum (TPES) of difluoromethane and dichloromethane has been recorded at the Swiss Light Source with a resolution of 2meV or 16cm−1. Electronic and vibronic transitions are simulated and assigned with the help of Franck–Condon (FC) calculations based on coupled cluster electronic structure calculations for the equilibrium geometries and harmonic vibrational frequencies of the neutrals, and of the ground and excited electronic states of the cations. Notwithstanding a high-resolution pulsed-field ionisation study on CH2F2 (Forysinski et al., 2010) in which a number of transitions to the X∼+ state have been recorded with unprecedented accuracy, we report the first complete vibrationally resolved overview of the low-lying electronic states of CH2X2+, X=F or Cl. Hydrogen atom loss from CH2F2+ occurs at low energy, making the ground state rather anharmonic and interpretation of the X∼+ band challenging in the harmonic approximation. By Franck–Condon fits, the adiabatic ionisation energies to the A∼+ 2B2, C∼+ 2A2 and D∼+ 2B2 states have been determined as 14.3±0.1, 15.57±0.01 and 18.0±0.1eV, respectively. The first band in the CH2Cl2 TPES is complex for a different reason, as it is the result of two overlapping ionic states, X∼+ 2B2 and A∼+ 2B1, with derived ionisation energies of 11.0±0.2 and 11.317±0.006eV, and dominated by an extended progression in the CCl2 bend (in X∼+) and a short progression in the CCl2 symmetric stretch (in A∼+), respectively. Furthermore, even though Koopmans’ approximation holds for the vertical ionisations, the X∼+ state of CH2Cl2+ is stabilized by geometry relaxation and corresponds to ionisation from the (HOMO−1) orbital. That is, the first two vertical ionisation energies are in the same order as the negative of the orbital energies of the highest occupied orbitals, but the adiabatic ionisation energy corresponding to electron removal from the (HOMO−1) is lower than the adiabatic ionisation energy corresponding to electron removal from the HOMO. The second band in the spectrum could be analysed to identify the vibrational progressions and determine adiabatic ionisation energies of 12.15 and 12.25eV for the B∼+ 2A1 and C∼+ 2A2 states. A comparison of the assignment of electronic states with the literature is made difficult by the fact that the B1 and B2 irreducible representations in C2v symmetry depend on the principal plane, i.e. whether the CX2 moiety is in the xz or the yz plane, which is often undefined in older papers

A Halomethane thermochemical network from iPEPICO experiments and quantum chemical calculations

Internal energy selected halomethane cations CH3Cl+, CH2Cl2+, CHCl3+, CH3F+, CH2F2+, CHClF2+ and CBrClF2+ were prepared by vacuum ultraviolet photoionization, and their lowest energy dissociation channel studied using imaging photoelectron photoion coincidence spectroscopy (iPEPICO). This channel involves hydrogen atom loss for CH3F+, CH2F2+ and CH3Cl+, chlorine atom loss for CH2Cl2+, CHCl3+ and CHClF2+, and bromine atom loss for CBrClF2+. Accurate 0 K appearance energies, in conjunction with ab initio isodesmic and halogen exchange reaction energies, establish a thermochemical network, which is optimized to update and confirm the enthalpies of formation of the sample molecules and their dissociative photoionization products. The ground electronic states of CHCl3+, CHClF2+ and CBrClF2+ do not confirm to the deep well assumption, and the experimental breakdown curve deviates from the deep well model at low energies. Breakdown curve analysis of such shallow well systems supplies a satisfactorily succinct route to the adiabatic ionization energy of the parent molecule, particularly if the threshold photoelectron spectrum is not resolved and a purely computational route is unfeasible. The ionization energies have been found to be 11.47 ± 0.01 eV, 12.30 ± 0.02 eV and 11.23 ± 0.03 eV for CHCl3, CHClF2 and CBrClF2, respectively. The updated 0 K enthalpies of formation, ∆fHo0K(g) for the ions CH2F+, CHF2+, CHCl2+, CCl3+, CCl2F+ and CClF2+ have been derived to be 844.4 ± 2.1, 601.6 ± 2.7, 890.3 ± 2.2, 849.8 ± 3.2, 701.2 ± 3.3 and 552.2 ± 3.4 kJ mol–1, respectively. The ∆fHo0K(g) values for the neutrals CCl4, CBrClF2, CClF3, CCl2F2 and CCl3F and have been determined to be –94.0 ± 3.2, –446.6 ± 2.7, –702.1 ± 3.5, –487.8 ± 3.4 and –285.2 ± 3.2 kJ mol–1, respectively

Gaze, February 1984

Brennan Newton '19 uploaded this document to DLynx Fall 2017. It was digitized by the DPS Student Team Summer 2017