charge resonance interaction and positive charge …...charge resonance interaction and positive...
TRANSCRIPT
Charge Resonance Interaction and Positive Charge Distribution in Aromatic Cluster Ions
(芳香族分子クラスターイオンの
電荷共鳴相互作用とその正電荷分布)
(IMS) Y. INOKUCHI and N. NISHI
2
Molecular Cluster Ions
ion core structure, positive charge distribution and its dynamic behavior.
What we want to figure out is…
Is the positive charge localized on the original molecule?
→ The answer is NO in a certain case.
e–
? +
3
Near IR–Visible Spectrum of (C6H6)2+
A strong band is observed at 10870 cm–1(920 nm). （Benzene and its cation have no electronic band in the near IR region.）
• This band is assigned to the charge resonance band (CR). • Appearance of this band implies that the positive charge is delocalized in the dimer.
1.0
0.8
0.6
0.4
0.2
0.025000200001500010000
Wavenumber / cm–1
CR 10870
(C6H6)2+
4
Charge Resonance Interaction
• Two degenerate states, φ(A1+)•φ(A2) and φ(A1)•φ(A2
+), are coupled, and the positive charge is delocalized in the dimer.
• An electronic transition between these charge resonance states is called charge resonance band (CR).
• Appearance of the CR suggests the positive charge delocalization.
Ψ+ = (0.5)1/2•φ(A1+)•φ(A2) +
(0.5)1/2 •φ(A1)•φ(A2 +)
The positive charge distribution is A1 side 0.5 A2 side 0.5.
CR
Ψ+
Ψ–
φ(A1+) • φ(A2)
(AA)+
φ(A1) • φ(A2+)
A+ + A Molecules A1and A2 are identical to each other in the dimer.
5
Structure of (C6H6)2+
3.35 Å
0.86 Å
Miyoshi et al. Chem. Phys. Lett. 275, 404 (1997). ab initio MO calculation.
6
increase in the number of molecules, or
contamination with another kind of molecules ?
How is the charge distribution affected
by
7
In This Study, • Aromatic cluster ions.
1. Benzene trimer ion. 2. Benzene–toluene mixed trimer ions.
• Positive charge distribution and its dynamic behavior.
• Mass-selected photodissociation spectroscopy. We obtain photodissociation (≈ absorption) spectra.
Near IR region. Appearance of the CR → Charge delocalization.
IR region. Vibrational structures.
8
Positive Charge Distribution and Charge Hopping
in Benzene Trimer Ion
9
Photodissociation Spectroscopy
(A•B)+
(A•B)+ * A+
B
Yield of A+
Wav
enum
ber
hν
Dissociation
(Dissociation Threshold)
10
Near IR–Visible Spectrum of (C6H6)3+
The spectrum is quite similar to that of (C6H6)2+.
(C6H6)3+ has a dimer ion core.
240002000016000120008000
Wavenumber / cm-1
Frag
men
t Ion
Inte
nsity
(C6H6)2+
(C6H6)3+ 0.67 eV
0.27 eV
11
IR Spectrum of (C6H6)3+
On the basis of the dimer ion core structure of (C6H6)3+, we assign
red component → ion core (C6H6)2+ blue component → solvent C6H6.
3150310030503000
Wavenumber / cm-1
Ion core (C6H6)2
+
Solvent C6H6
(C6H6)3+
Frag
men
t Ion
Inte
nsity
CH stretching region.
The spectrum can be reproduced by a strong Lorentzian band and absorption bands of neutral benzene.
（The solvent benzene in (C6H6)3+ has an absorption similar to that of neutral benzene.）
12
IR Spectra of Trimer Ions Containing C6D6
B=C6H6 D=C6D6
B2D1+→BD+
B1D2+→BD+
B2D1+
→B2+
B1D2+→D2
+
(C6H6)3+
Wavenumber / cm–1
Frag
men
t Ion
Inte
nsity
All the spectra resemble that of (C6H6)3+. Spectral features are independent of fragment species monitored.
13
Spectrum Interpretation for [(C6H6)2•C6D6]+
(C6H6)2+•••C6D6� (C6H6•C6D6)+•••C6H6�
+ +
= =
Band features predicted. （CH stretching region）
The spectral features are independent of fragment dimer species monitored, and are similar to those of (C6H6)3+.
The trimer ions lose information on the ion core in the course of photodissociation.
(C6H6)3+
（If fragment dimer ions are the same as ion core species,）
Structures
14
Intracluster Charge Hopping
A B +
B C +
C A +
hν
Dissociation
A B C +
C A B +
B C A +
A B C +
Intracluster charge hopping occurs in
vibrational excited states.
Vibrational excited states
15
Charge Hopping
hν
Excitation of intermolecular vibrations
The charge hopping is initiated by the light irradiation and assisted by intermolecular vibrations.
Charge Hopping
（Vibrational excitation and
IVR）
16
Positive Charge Distribution in
Benzene–Toluene Mixed Trimer Ions
17
Mixed Dimer Ion
The positive charge exists also on benzene, although benzene has an ionization potential (IP) higher than that of toluene.
Charge Distribution (%) 36 64
IP (eV) 9.24 8.83
CH3 +
Benzene Toluene
Ohashi et al. Chem. Phys. 239, 429 (1998).
18
Near IR Spectra
Both B1T2+ and B2T1
+ have a broad and strong band in the near IR region.
l The charge resonance interaction occurs in these clusters. l The positive charge is not localized in a single molecule.
Frag
men
t Ion
Inte
nsity
20000160001200080004000
B2+
B1T1+
T2+
B1T2+
B2T1+
B3+
Wavenumber / cm–1
B = C6H6 T = C6H5CH3
19
IR Spectra
• Band features of B2T1+ are fairly
different from those of other ions. ◆ No strong band is observed in
the alkyl CH region. ◆ The band in the aromatic CH
region is quite narrower than those of other ions.
3100300029002800Wavenumber / cm-1
30602905
30572910
30632919
3068
B1T1+
T2+
B1T2+
B2T1+
Frag
men
t Ion
Inte
nsity
Aromatic CH
Alkyl CH
B = C6H6 T = C6H5CH3
• CH stretching region
20
Positive Charge Distribution in B1T2+
• Possible structures are T2
+•••B B1T1
+•••T (B T T)+.
• It cannot be determined definitely from the near IR and IR spectra.
ü The maximum of the near IR band is located in the middle of those of B1T1
+ and T2+.
ü The IR spectrum is similar to those of both B1T1
+ and T2+.
• Ionization potential T2 8.34 eV B1T1 8.42 eV
• Prediction of the spectra of (B T T)+ is difficult.
20000160001200080004000
B2+
B1T1+
T2+
B1T2+
B2T1+
B3+
3100300029002800Wavenumber / cm-1
30602905
30572910
30632919
3068
B1T1+
T2+
B1T2+
B2T1+
B=C6H6 T=C6H5CH3
21
IR Spectrum of B1(C6D5CD3)2+ We use C6D5CD3 instead of C6H5CH3, and measure an IR spectrum.
3100300029002800Wavenumber / cm-1
B1T2+
B1(C6D5CD3)2+
neutral benzene
A. Fujii et al. J. Chem. Phys. 112, 6275 (2000)
The benzene molecule in B1T2+
is almost neutral.
The spectrum of B1(C6D5CD3)2+ displays three bands, which are characteristic of neutral benzene.
B=C6H6 T=C6H5CH3
Positive charge distribution of B1T2
+ is T2
+•••B.
22
Positive Charge Distribution in B2T1+
• Possible structures are B2
+•••T B1T1
+•••B (B B T)+.
• Ionization potential B2 8.65 eV B1T1 8.42 eV
20000160001200080004000
B2+
B1T1+
T2+
B1T2+
B2T1+
B3+
3100300029002800Wavenumber / cm-1
30602905
30572910
30632919
3068
B1T1+
T2+
B1T2+
B2T1+
• It cannot be determined definitely from the near IR and IR spectra.
ü The maximum of the near IR band is located near that of B1T1
+. ü The IR spectrum is similar to that of B2
+.
• Prediction of the spectra of (B B T)+ is difficult.
B=C6H6 T=C6H5CH3
23
IR Spectrum of (C6D6)2T1+
• No sharp band is observed at 3068 cm–1 in the spectrum of (C6D6)2T1
+. • The sharp band in the spectrum of
B2T1+ is quite similar to that of B2
+.
↓ The structure is like B2
+•••T1 ?
↓ The spectrum of (C6D6)2T1
+ is different from that of neutral toluene. The toluene molecule in B2T1
+ is NOT neutral.
3150310030503000295029002850Wavenumber / cm-1
B2T1+
(C6D6)2T1+
B2+
neutral toluene
We use C6D6 instead of C6H6, and measure an IR spectrum.
Positive charge distribution of B2T1
+ is (B B T)+.
24
Energy Diagram of B2T1+
• The energy difference between (B2++T)
and (B2+T+) is accidentally small (0.10 eV), inducing the charge resonance interaction between B2 and T.
• The wave function of the electronic ground state is
Ψ+ = (0.58)1/2•φ(B2
+)•φ(T) + (0.42)1/2•φ(B2)•φ(T+).
The positive charge distribution of B2T1
+ is B2 58% T 42%.
25
Summary
• Benzene trimer ion ◆ Dimer ion core structure (C6H6)2+•••C6H6 ◆ Charge hopping in vibrational excited states
• Benzene–toluene mixed trimer ions ◆ B1T2
+
T2+•••B; the ion core is T2
+. ◆ B2T1
+
The charge is delocalized in the trimer; the distribution is B2 (58%) and T (42%).