Photoelectron spectroscopy of the cyclopentadienide anion: Analysis of the Jahn-

Teller effects in the cyclopentadienyl radical

Takatoshi Ichino, Adam J. Gianola, and W. Carl Lineberger

JILA and Department of Chemistry and BiochemistryUniversity of Colorado, Boulder, Colorado 80309

John F. Stanton

Department of Chemistry and Biochemistry and Institute for Theoretical ChemistryThe University of Texas at Austin, Austin, TX 78712

Supported by

High-temperature oxidation of benzene

+ OH

HH

H

H H

O

HH

H

H H

H

H

H H

H

+ OHH

HH

H

H H

HH

H

H H

+ O2O

HH

H

H H

OH

H

H H

H

• Cyclopentadienyl radical is formed in combustion of benzene.

• Cyclopentadienyl radical is a resonance stabilized free radical which may play a role in growth of polycyclic aromatic hydrocarbons.

cyclopentadienylradical(C5H5)

Thermochemical property ofthe cyclopentadienyl radical

C–H Bond Dissociation Enthalpy of Cyclopentadiene

HH

H H

H H H

H

H H

H + H

(1) gas-phase equilibrium measurements (i.e., dissociation and recombination reaction rate constants): D0(C–H) = 80.8 ± 1.0 kcal mol-1. Roy et al., Int. J. Chem. Kinet. 2001, 33, 821(2) photoacoustic calorimetric measurements in benzene solution: DH298(C–H) = 85.6 ± 1.7 kcal mol-1. Nunes et al., J. Phys. Chem. A 2006, 110, 5130

(3) negative ion thermochemical cycle:D0(C–H) = acidH0(C–H) + EA(C5H5) – IE(H)

photoelectron spectroscopy of the cyclopentadienide anion (C5H5‾)

hemisphericalenergy analyzer

VelocityFilter

microwave dischargeflowing afterglow

ion source(~ 0.5 Torr He)

ion optics ion optics

(10-6 Torr) (10-8 Torr)

MCP+

positiondetector

electronoptics

argon ion laser (351.1 nm)

e‾

AOM

Photoelectron spectroscopy of C5H5‾

O + CH4 HO + CH3 HO + HH

H H

H H H

H

H H

H + H2O

Photoelectron spectrum of C5H5‾

electron Binding Energy (eV)1.82.02.22.42.6

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EA(C5H5) = 1.812 ± 0.005 eV

photon energy: 3.531 eV, magic angle

Enthalpy of formation of the C5H5 radical

HH

H H

H H H

H

H H

H + H

D0(C–H) = acidH0(C–H) + EA(C5H5) + IE(H) = 81.0 ± 0.6 kcal mol-1

DH298(C–H) = 82.9 ± 0.6 kcal mol-1

fH298(C5H5) = DH298(C–H) + fH298(C5H6) − fH298(H) = 62.9 ± 0.8 kcal mol-1

fH0(C5H5) = 65.6 ± 0.8 kcal mol-1

cf. fH298(C5H5) = 62.5 ± 1.0 kcal mol-1,fH0(C5H5) = 65.4 ± 1.0 kcal mol-1, Roy et al., Int. J. Chem. Kinet. 2001, 33, 821

Nonadiabatic effects in X 2E1″ C5H5

electron Binding Energy (eV)1.82.02.22.42.6

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Jahn-Teller effects?

X 1A1′ C5H5‾

X 2E1″ C5H5

− e‾

a2″

e1″

e1″

a2″

Laser excited dispersed fluorescenceA 2A2″ ― X 2E1″ C5H5 electronic transition

Applegate et al., J. Chem. Phys. 2001, 114, 4855, 4869

• Emission from the vibraional ground state as well as from the vibrationally excited states of the Jahn-Teller active modes.

• Spectral simulations based on a model Hamiltonian in terms of JT eigenfunctions.

• Ab initio evaluation of JT coupling constants.

• Measurements of isotopomers.

Model Hamiltonian for a degenerate system with linear Jahn-Teller coupling

2

2

21

iiiN q

T

20 2

1i

iiqV

wherenuclear kinetic energy operator

harmonic potential energy of the reference state

linear and bilinear JT coupling

Model potential has an expansion form around the reference geometry in terms of the reduced normal coordinates of the reference state (qi).

E : energy of the degenerate states at the reference geometry

Köppel et al., Adv. Chem. Phys. 1984, 57, 59; Mayer et al., J. Chem. Phys. 1994, 100, 899

baEccbaE

IVTH N 0

i

iiqa linear intrastate coupling

ji

iyjijj

yjj qqqb,

ji

ixjijj

xjj qqqc,

Electronic structure calculations: the initial state (closed-shell anion)

cyclopentadienide anion (C5H5‾)

mode symmetry frequency mode symmetry frequency

1 a1′ 3221 8 e1″ 528

2 1157 9 e2′ 3169

3 a2′ 1267 10 1434

4 a2″ 618 11 1063

5 e1′ 3196 12 834

6 1488 13 e2″ 709

7 1021 14 604

CCSD/DZP calculation

H

H

H H

H

r(CC) = 1.4254 År(CH) = 1.0938 Å

X 1A1′ C5H5‾ is the reference state for the model potential.

in units of cm-1

Electronic structure calculations:the final state (neutral radical)

H

HH

H H

Equation-of-Motion Ionization Potential Coupled-Cluster method (EOMIP-CCSD)

H

HH

H H

cyclopentadineyl radical (C5H5)

2B12A2

C1–C2 1.4467 1.4084

C2–C3 1.3798 1.4794

C3–C4 1.4922 1.3696

C1–H 1.0898 1.0868

C2–H 1.0870 1.0895

C3–H 1.0886 1.0877

C5–C1–C2 108.88 107.27

C1–C2–C3 107.37 108.68

C2–C3–C4 108.19 107.68

H–C1–C2 125.56 126.36

C1–C2–H 125.71 126.18

C2–C3–H 126.80 125.21

2B1 (minimum) 2A2 (TS)

in units of angstroms and degrees

J. F. Stanton, J. Chem. Phys. 2001, 115, 10382

Ab initio parametrizationof the model potential

X 2E1″ C5H5 model potential parameter (eV)

linear intrastate coupling

2 0.0234

linear JT coupling

10 0.0626

11 0.1149

12 0.2125

bilinear JT coupling

2,10 −0.0001

2,11 0.0116

2,12 0.0281

linear coupling constants:

Geometry displacements from the initial (anion) to the final (radical) states are multiplied by the quadratic force constant matrix of the final state at its equilibrium geometry in terms of the anion reduced normal coordinates.

bilinear coupling constants:

The off-diagonal elements of the quadratic force constant matrix of the final state at its equilibrium geometry in terms of the anion reduced normal coordinates.

No energy barrier is assumed along the pseudorotation path in the model potential.

electron Binding Energy (eV)1.71.81.92.02.12.2

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Simulation based on the model Hamiltonian:linear intrastate coupling (a1′) only

No obeserved peak can be assigned to a1′ mode.

X 2E1″ C5H5

electron Binding Energy (eV)1.71.81.92.02.12.2

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Simulation based on the model Hamiltonian:linear intrastate (a1′) + linear JT (e2′) coupling

X 2E1″ C5H5

Observed peak positions are well reproduced by linear JT coupling.

electron Binding Energy (eV)1.71.81.92.02.12.2

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Simulation based on the model Hamiltonian:add bilinear coupling

X 2E1″ C5H5

Relative peak intensities are well reproduced by addition of bilinear coupling.cf. photoelectron spectrum of CH3O‾, Schmidt-Klügmann et al., Chem. Phys. Lett. 2003, 369, 21

The vibronic peaks for X 2E1″ C5H5

X 2E1″ C5H5

peak (J, n) position (eV) simulation (eV)

a 1/2, 0 0 0

b 3/2, 1 0.062 ± 0.002 0.0564

c 1/2, 1 0.108 ± 0.002 0.1061

d 1/2, 2 0.134 ± 0.003 0.1356

a1′, 1 ― 0.1414

e 1/2, 3 0.164 ± 0.005 0.1613

f 1/2, 4 0.188 ± 0.003 0.1845

1/2, 5 ― 0.2067

g 1/2, 6 0.213 ± 0.003 0.2136

1/2, 7 ― 0.2177

Jahn-Teller stabilization energy = 0.1961 eV

electron Binding Energy (eV)1.71.81.92.02.12.2

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a

bcd

e

fg

Conclusion• The 351.1 nm photoelectron spectrum of the cyclopentadienide anion has

been measured. The electron affinity of the cyclopentadienyl radical has been determined to be 1.812 ± 0.005 eV.

• The C–H bond dissociation enthalpy of cyclopentadiene has been derived as D0(C–H) = 81.0 ± 0.6 kcal mol-1 from a negative ion thermochemical cycle. The enthalpy of formation of the cyclopentadienyl radical has been derived to be fH298 = 62.9 ± 0.8 kcal mol-1.

• Model potentials of X 2E1″ C5H5 have been constructed around the equilibrium geometry of X 1A1′ C5H5‾ in terms of the anion reduced normal coordinate, based on the EOMIP-CCSD calculations. A simulation based on the model Hamiltonian reproduces the observed vibronic structure very well, revealing strong Jahn-Teller activity for e2′ modes in the spectrum. It is important to include the bilinear coupling between a1′ and e2′ modes in the model potential.

• The simulation is completely ab initio.