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Thermochemical property of the cyclopentadienyl radical C–H Bond Dissociation Enthalpy of Cyclopentadiene (1) gas-phase equilibrium measurements (i.e., dissociation and recombination reaction rate constants): D 0 (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: DH 298 (C–H) = 85.6 ± 1.7 kcal mol -1. Nunes et al., J. Phys. Chem. A 2006, 110, 5130 (3) negative ion thermochemical cycle: D 0 (C–H) = acid H 0 (C–H) + EA(C 5 H 5 ) – IE(H) photoelectron spectroscopy of the cyclopentadienide anion (C 5 H 5 ‾)
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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.