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Diradicals, antiaromaticity, and the pseudo-Jahn-Teller effect: Electronic and rovibronic structures of the cyclopentadienyl cation
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10.1063/1.2748049
/content/aip/journal/jcp/127/3/10.1063/1.2748049
http://aip.metastore.ingenta.com/content/aip/journal/jcp/127/3/10.1063/1.2748049

Figures

Image of FIG. 1.
FIG. 1.

One- and two-photon excitation schemes used to record photoionization and PFI-ZEKE photoelectron spectra of the cyclopentadienyl radical. The positions and symmetries of the low-lying electronic states of the cyclopentadienyl radical and cation are represented on the left-hand side of the figure by the horizontal lines and capital letters, respectively. The electronic configuration of the molecular orbitals from which the electronic states derive are represented schematically in the central column. The right-hand side represents schematic cuts through the potential energy surface of the electronic states along nuclear displacements of symmetry.

Image of FIG. 2.
FIG. 2.

Frost-Musulin diagram of the lowest-lying electronic configuration of the cyclopentadienyl cation (left-hand side) and energetic ordering of the corresponding electronic states in symmetry (right-hand side). represents the exchange integral (see text).

Image of FIG. 3.
FIG. 3.

(a) Schematic representation of the minimum energy pseudorotation path in the lowest singlet state. A distortion along the dimension preserves symmetry and takes the molecule to either the allylic or dienylic structure. If vibronic coupling terms of third and higher orders are neglected, the displayed minimum energy path is isoenergetic. (b) Electronic configurations of the state at symmetry (middle) and of the distorted dienylic and allylic structures (left and right, respectively). The ordering of the and molecular orbitals originating from the degenerate orbital is opposite in the dienylic and allylic structures.

Image of FIG. 4.
FIG. 4.

Eigenvalues of the PJT Hamiltonian displayed as a function of the coupling parameter assuming . The values are given with respect to the lowest level which is the doubly degenerate level for and the nondegenerate level for .

Image of FIG. 5.
FIG. 5.

(a) Structures obtained from geometry optimization at the CASSCF(4,5)/cc-pVTZ level of theory. The dienylic structure (left) corresponds to a first-order saddle point and the allylic structure (right) to a minimum at this level of theory. These structures result from a PJT distortion along an mode. (b) Minimum energy structure with a ground electronic state optimized at the same level of theory. This structure results from a JT distortion along an mode. (c) One-dimensional cuts through the potential energy surfaces of the states deriving from the , , and states of at geometry. The cuts are displayed along effective modes of symmetry (left) and (right) connecting the points to the relevant minimum energy structures. The distortion modes preserve symmetry which is used to label the electronic symmetries.

Image of FIG. 6.
FIG. 6.

One-photon VUV photoionization (dotted lines) and PFI-ZEKE photoelectron spectra (full lines) in the region of the adiabatic ionization threshold of (a) and (b) and simulations using a rotational temperature of for and for (insets). The spectra are assigned to the origin of the transition.

Image of FIG. 7.
FIG. 7.

Two-photon resonance-enhanced ionization spectrum of the transition of with assignment of the vibrational structure.

Image of FIG. 8.
FIG. 8.

Two-photon resonant photoionization and PFI-ZEKE photoelectron spectra (dotted and full lines, respectively) recorded via the vibrationless level of the state of (panel a) and (panel b). Vertical arrows mark the positions of the adiabatic ionization thresholds corresponding to the formation of the state and the lower component of the state. The horizontal lines with vertical assignment bars label autoionizing Rydberg series. The horizontal axis corresponds to the sum of the wave numbers of both lasers.

Image of FIG. 9.
FIG. 9.

PFI-ZEKE photoelectron spectra of recorded following two-photon resonant excitation via selected vibrational levels of the state. The selected intermediate levels are indicated on the right-hand side above the spectra. The horizontal axis corresponds to the sum of the wave numbers of both lasers. The lowest trace is the PFI-ZEKE spectrum of cyclopentadiene recorded following single-photon VUV excitation from the neutral ground state.

Image of FIG. 10.
FIG. 10.

PFI-ZEKE photoelectron spectra of recorded following two-photon resonant excitation via selected vibrational levels of the state. The intermediate levels are indicated on the right-hand side. The horizontal axis corresponds to the sum of the wave numbers of both lasers.

Image of FIG. 11.
FIG. 11.

Rotationally resolved resonance-enhanced two-photon ionization spectrum of the vibrationless transition in . The inset shows a magnification of the central part of the spectrum, the rotational assignments in the form , and a simulation using the parameters reported in Ref. 22.

Image of FIG. 12.
FIG. 12.

Rotationally resolved PFI-ZEKE photoelectron spectra of recorded following two-photon resonant excitation via selected rotational levels of the vibrationless level of the state. The values of the total angular momentum quantum number (excluding spins) of the intermediate level and of the ionic level are indicated on the left-hand side and top, respectively. The horizontal axis corresponds to the total wave number above the (, ) level of the ground state.

Image of FIG. 13.
FIG. 13.

PFI-ZEKE photoelectron spectra of (panel a) and (panel b) recorded following two-photon resonant excitation to the lower component of the state via selected vibrational levels of the state. The intermediate level is indicated on the right-hand side above the spectra. The spectra are compared to simulations (vertical stick spectra) of the vibronic structure including the modes , , and in and and in . The bands marked with an asterisk coincide with lines of the precursor (see Fig. 9).

Tables

Generic image for table
Table I.

Jahn-Teller and pseudo-Jahn-Teller activity of the doubly degenerate modes of the cyclopentadienyl cation up to fifth order.

Generic image for table
Table II.

Vibrational frequencies in the state of and calculated ab initio at the CASSCF(4,5)/cc-pVTZ level of theory.

Generic image for table
Table III.

Intervals between the lowest electronic states of . The first interval is given at the optimized geometry of the triplet state at the CASSCF(4,5)/cc-pVTZ level of theory. The energies of the singlet states in the last two intervals are evaluated after optimizing their geometry at the CASSCF(4,5)/cc-pVTZ level of theory.

Generic image for table
Table IV.

Harmonic frequencies , stabilization energies , and reduced PJT coupling constants for the four vibrational normal modes of symmetry calculated at the CASSCF(4,5)/cc-pVTZ level of theory for and (see also text). The normal modes were obtained from a calculation of the state at the CASSCF(4,5)/cc-pVTZ level. The last two columns show the values obtained after adjustment to the experimental spectra.

Generic image for table
Table V.

Adiabatic ionization thresholds of the and photoionizing transitions of the cyclopentadienyl radical and first singlet-triplet interval of .

Generic image for table
Table VI.

Positions and assignments of vibronic bands in the PFI-ZEKE photoelectron spectra of . The total wave number with respect to the neutral ground state of the radical is indicated. The table also lists the wave numbers relative to the origins of the and ( component) states and deviations between observed and calculated vibronic intervals.

Generic image for table
Table VII.

Positions and assignments of vibronic bands in the PFI-ZEKE photoelectron spectra of . The total wave number with respect to the neutral ground state of the radical is indicated. The table also lists the wave numbers relative to the origins of the and ( component) states and deviations between observed and calculated vibronic intervals.

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2007-07-17
2014-04-16
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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Diradicals, antiaromaticity, and the pseudo-Jahn-Teller effect: Electronic and rovibronic structures of the cyclopentadienyl cation
http://aip.metastore.ingenta.com/content/aip/journal/jcp/127/3/10.1063/1.2748049
10.1063/1.2748049
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