Nonoptical excited state spectroscopy of CHF2Cl: Characterization of nondipole n→σ* valence transitions by angle-resolved electron energy loss spectroscopy

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Abstract
A nondipole low‐lying excitation feature at 8.0 eV in difluorochloromethane (CHF2Cl) has been observed in angle‐resolved electron energy loss spectra obtained at 2.5 keV impact energy. Based on the results of a single‐excitation configuration interaction calculation, we assign this feature predominantly as electronic transitions from Cl nonbonding 14a’ and 7a‘ orbitals [the highest occupied molecular orbital (HOMO) and second HOMO, respectively] to an antibonding σC–Cl* 15a’ orbital (the lowest unoccupied molecular orbital), the so‐called n→σ* type transitions. The generalized oscillator strength (GOS) profile of this transition has been determined and found to be similar in shape and magnitude to that of a recently reported nCl → σC–Cl* (7e→11a1) transition in CF3Cl [Ying et al., Chem. Phys. Lett. 212, 289 (1993)]. Both GOS profiles are found to have a shape characteristic of a quadrupole transition, with a maximum at momentum transfer of ∼1 a.u. The similarity in the nature of these nCl → σC–Cl* transitions in CHF2Cl and CF3Cl to that of a p‐to‐p transition in a chlorine atom can be demonstrated qualitatively by using contour maps of the density functions of the dominant initial‐state and final‐state orbitals generated from ab initio self‐consistent field wave functions. The homology of transition‐related properties (including the GOS profile) among the nCl → σC–Cl* transitions in ‘‘simple’’ polyatomic molecules that contain a single highly localized C–Cl bond is also discussed. Furthermore, the calculated potential energy diagram for CHF2Cl along the C–Cl bond direction suggests that electronic transitions from the 14a’ and 7a‘ orbitals to the 15a’ orbital may lead to dissociation of the C–Cl bond. Such predissociation may represent a possible common consequence of the nCl → σC–Cl* transitions in these monochloro‐substituted chlorofluorocarbons and related derivatives.