Transcript meth-pyrim.ppt

High Resolution Electronic Spectra of
Methylpyrimidines in the Gas Phase.
Radiationless transitions in the intermediate molecule limit
coupled with methyl torsional motion.
2MP
5MP
Leonardo Alvarez-Valtierra, Xue-Qing Tan, and David W. Pratt
Department of Chemistry
University of Pittsburgh
Pittsburgh, PA 15260
Radiationless transitions
A radiationless transition is a change in the electronic state of a molecule that occurs
without the absorption or emission of radiation.
The behavior of a molecule following
excitation depends primarily on:
α
a) The energy level density  i
b) The decay widths  i
c) The s  i
ε
 couplings  si
The important parameter characterizing
the different molecule “limits” is x i
Jablonski diagram
xi  i  i
Decays in different molecule limits
A pure singlet state.
I Fl  I 0 e
0
t / rad
No interactions
Exponential decay lifetime
0
 rad
ex. naphthalene, phenanthrene
e
0
 t / n rad
xi  1
Small molecule limit.
Few n triplet levels are coupled with the singlet
Fluorescence is modulated by a high frequency
2 /  n
ex. NO2, SO2
xi  1
Intermediate molecule limit.
ex. pyrazine, pyrimidine
e
e  t / NR
0
 t / n rad
Statistical limit.  (E ) is huge
xi  1
The fluorescence decays faster, dominated
by  NR
ex. Large heterocycles
FD05
Intermediate case molecular limit: Pyrimidine
Selected transitions in the R-branch of the rotationally resolved LIF spectrum
of pyrimidine in a supersonic jet.
Extra-lines appear in the spectrum
Coupling with dark states
J. A. Konings, W. A. Majewski, Y. Matsumoto, D. W. Pratt, and W. L. Meerts, J. Chem. Phys. 89 (1988), 1813.
Role of the methyl group in the ISC mechanism
2MP
5MP
Vibrationally resolved electronic spectra
R. E. Bandy, J. Nash, and T. S. Zwier, J. Chem. Phys. 95, (1991), 2317.
R. E. Bandy, A. W. Garrett, H. D. Lee, and T. S. Zwier, J. Chem. Phys. 96, (1992), 1667.
Experimental apparatus
Rotationally resolved electronic spectrum of 2MP
2-methylpyrimidine
30456.8
E-subband
Frequency (cm-1)
A-subband
Band splitting: ~157 MHz
30461.4
2MP at full experimental resolution
2-methylpyrimidine
pure “c”-type transitions
“Missing
transitions”
A - subband
30456.8
Frequency (cm-1)
~ 0.3 cm-1
E 30461.4
- subband
2MP – Experimental inertial parameters
PARAMETER
E - subband
A - subband
Microwavea
A"/MHz
6106.4 (3)
6095.6 (1)
6096.9 (1)
B"/MHz
2770.2 (2)
2751.7 (1)
2751.7 (‹1)
C"/MHz
1867.5 (2)
1895.5 (1)
1895.6 (‹1)
ΔI"/amuÅ2
5.40b
0.04
0.06
A'/MHz
6397.8 (3)
6373.4 (1)
B'/MHz
2600.1 (2)
2590.2 (1)
C'/MHz
1823.7 (2)
1842.0 (1)
ΔI'/amuÅ2
3.76b
-0.04
OMC/MHz
15.7
4.5
a)
W. Caminati, G. Cazzoli, and D. Troiano, Chem. Phys. Lett. 43, (1976), 65.
b)
Inertial defect values are likely to be positive when a torsional level lies above the barrier;
L. Alvarez-Valtierra, J. T. Yi, and D. W. Pratt, J. Phys. Chem. B 114, (2006), 19914.
2MP – Potential energy surfaces
S1
10
1e
Potential
Energy
5
0a1
(cm-1)
V6'  8.28 cm1
0
S0
10
Potential
Energy
1e
5
(cm-1)
0a1
0
0
π
Methyl torsional coordinate
2π
V6"  1.56 cm1
Rotationally resolved electronic spectrum of 5MP
5-methylpyrimidine
E-subband
A-subband
30797.5
Frequency (cm-1)
Band splitting: ~7,800 MHz
30801.3
5MP at full experimental resolution
5-methylpyrimidine
pure “c”-type transitions
30797.5
Frequency (cm-1)
~0.13 cm-1
Some shifted
and missing
transitions are
also observed.
30801.3
5MP – Experimental inertial parameters
PARAMETER
E - subband
A - subband
Microwavea
A"/MHz
6100.6 (2)
6108.3 (1)
6108.4 (2)
B"/MHz
2640.5 (1)
2642.4 (1)
2642.2 (‹1)
C"/MHz
1847.6 (1)
1844.5 (1)
1844.2 (‹1)
ΔI"/amuÅ2
-3.14
0.00
0.03
A'/MHz
6357.8 (2)
6365.2 (1)
B'/MHz
2537.2 (1)
2538.9 (1)
C'/MHz
1818.9 (1)
1817.1 (1)
ΔI'/amuÅ2
-0.83
-0.32
OMC/MHz
9.8
3.0
a)
W. Caminati, G. Cazzoli, and A. M. Mirri, Chem. Phys. Lett. 31, (1975), 104.
5MP – Potential energy surfaces
S1
100
3a1
80
Potential
60
Energy
(cm-1)
2e
40
1e
0a1
V6'  58.88 cm1
20
0
Potential
Energy
S0
40
2e
20
1e
0a1
(cm-1)
0
0
π
Methyl torsional coordinate
2π
V6"  4.11cm1
Changes in electronic distribution induced by light
2MP
5MP
V6'  58.88 cm1
LUMO
V6'  8.28 cm1
0.686
0.682
HOMO-1
V6"  1.56 cm1
V6"  4.11cm1
Conclusions
* The high resolution electronic spectra of 2MP and 5MP exhibit two sub-torsional
bands on top of each other, which are assigned to the torsional motion of the methyl
group attached.
* Several rotational transitions were found to be perturbed in frequency and/or
intensity from the expected line positions by using a low-barrier torsional Hamiltonian.
* The E-subtorsional bands in 2MP and 5MP show to be more severely perturbed
than the corresponding A-subbands.
* Vibronic coupling with some nearby triplet state levels is the most likely responsible
of the perturbations observed in the spectra.
* The methyl torsional barrier heights were obtained from analyses of the data, which
are in good agreement with previous MW results.
* The values of the methyl torsional barriers in 2MP and 5MP are likely to be due to
the differences in electronic density (π-Bond Order) upon electronic excitation.
Ongoing and future work
* Analysis with further detail the ISC by deconvolution of the singlet-triplet molecular
eigenstates on these molecules.
“Lawrance-Knight deconvolution method”
W. D. Lawrance and A. E. W. Knight, J. Phys. Chem. 89, (1985), 917.
31607.3
Frequency (cm-1)
31611.4
Acknowledgements
Many thanks to:
• Dr. David Plusquellic (NIST) and Dr. David Borst (INTEL) for programming
support on the data analysis.
• The current Pratt group members at the University of Pittsburgh.
• The National Science Foundation (NSF) for its financial support (CHE-0615755).
• And…thanks to you, for your attention!
Hindered torsional model
It is often hindered by a torsional
barrier.
Tunneling is observed.
Energy level splitting
V(α)
S1
Splitting in the torsional
energy levels appears.
The properties of the potential
energy surface along the torsional
coordinate usually change upon
electronic excitation.
Tunneling
A
B
V3
0A1/1E
C
5E
4E
3A1
2E
S0
V3
Sometimes a change of ‘phase’
of the barrier accompanies
electronic excitation.
Selection Rules:
A↔A
E↔E
2E
0A1/1E
0°
60°
120°
180°
240°
Torsional angle (α)
Low-barrier torsional Hamiltonian
H  Fp 2  12 V6 1  cos 6   AF Pz2  BPx2  CPy2  2 AF pPz
where:
2
AF 
2 I z  I  
B
2
2Ix
C
p  i  
Rot. Const. where the methyl group is attached
Usual rigid rotor rotational constants
2
2I y
Angular momentum of the top
Interpretation of the barrier heights
* We believe that the barriers in 5MP are entirely steric in origin. The distance
between the methyl group hydrogens and the adjacent ring hydrogens is less than
the van der Waals radii, indicating significant nonbonding interactions.
* In 2MP this is no longer true; therefore, the barrier heights are extremely small.
Small torsional
barriers
Larger torsional
barriers