DISPERSED FLUORESCENCE (DF) SPECTROSCOPY OF
JET-COOLED METHYLCYCLOHEXOXY (MCHO) RADICALS
Jahangir Alam, Md Asmaul Reza, Amy Mason, Neil Reilly and Jinjun Liu
Department of Chemistry, University of Louisville.
1-MCHO, axial
1-MCHO, equatorial
2-MCHO
3-MCHO
4-MCHO
0
500
1000
1500
2000
2500
3000
relative frequency (cm-1)
International Symposium on Molecular Spectroscopy
University of Illinois Urbana-Champaign
06/26/15
1
Outline
 Introduction and Motivation.
 Conformational Studies of MCHOs.
 Quantum Chemical Calculations.
 Moderate Resolution LIF/DF Apparatus.
 LIF and DF Spectra of MCHOs.
 Franck-Condon Factor (FCF) Simulation.
 Summary and Future Work.
2
Introduction and Motivation
 Cyclohexoxy radicals and its substitutes like methylcyclohexoxy (MCHO) are
important reaction intermediates in combustion and atmospheric
chemistry.
 Analysis of the vibrational structures of nearly degenerate X̃ and à states of
MCHOs helps understand the coupling between close-lying potential energy
surfaces (PESs) including the pseudo-Jahn-Teller effect (pJTE).
 The mechanism of interaction between these two close-lying states is of
significant theoretical interests and can be used to benchmark quantum
chemical calculations.
 Experimentally obtained spectra are prerequisite of future kinetic studies.
 Investigation of the intramolecular interactions and the methyl substitution
effect on the vibrational structure will help understand chemical kinetics.
3
Conformational Studies of MCHO Radicals
4-, 3- and 2-MCHO
 Among possible chair and twisted-boat
conformers of MCHOs, only chair structures are
likely to be present under jet-cooled conditions
due to their lowest energy.
 Theoretical studies suggest that, for 4-, 3- and 2MCHOs, diequatorial conformers have the lowest
energy.
 For 1-MCHO, the chair-axial and chair-equatorial
conformers, defined by the orientation of the
oxygen atom, can co-exist in free jet expansion.
long boat
half chair
1-MCHO
boat-1
twisted
boat-a 1
chairequatorial
twisted
boat-a 2
chairaxial
minimum
-1
barrier
-1
minimum
-2
barrier
-2
chairequatorial
long
boat
twisted
boat-a 1
boat-1
twisted
boat-a 2
half
chair
chair-axial
1.60
10.22
5.65
6.91
5.88
10.43
0
J. Lin, Q. Wu, G. Liang, L. Zu; RSC Adv. 2, 583−589 (2012)
Q. Wu, G. Liang, L. Zu; J. Phys. Chem. A 116, 3156−3162 (2012)
minimum barrier
-3
-3
minimum
-4
4
Quantum Chemical Calculations on MCHOs
 The three lowest electronic states of 4-MCHO and 1-MCHO are X̃ 2A″, Ã2A′ and B̃ 2A′. 3-MCHO and
2-MCHO also have similar electronic configuration.
 The B̃ 2A′← X̃ 2A″ electronic transitions corresponds to the promotion of one electron from the σ
orbital of the C-O bond to the half-filled non-bonding orbital that is localized on the O atom.
 On the basis of symmetry of electronic states, both B̃ 2A′→Ã2A′ and B̃ 2A′→X̃ 2A″ electronic transitions
are allowed.
Parameters
4-MCHO
3-MCHO
2-MCHO
N/A
1-MCHO (chairaxial)
77
1-MCHO (chairequatorial)
132
ΔE(Ã-X̃ ) (cm-1)
168
N/A
1υC-O stretch(B̃ ) (cm-1)
694
680
676
468
411
1υC-O stretch (Ã) (cm-1)
1060
N/A
N/A
903
902
1υC-O stretch (X̃ ) (cm-1)
1053
1120
1123
909
910
rC-O (B̃ ) - rC-O(X̃ ) (mÅ)
212
213
216
249
239
rC-CH3 (B̃ ) - rC-CH3 (X̃ ) (mÅ)
4
7
9
35
35
Calculated by Gaussian 09 at B3LYP/6-31+G(d) and CIS Level of Theory
5
Moderate-resolution LIF/DF apparatus
LIF: laser-induced fluorescence
DF: dispersed fluorescence
Methylcyclohexyl nitrite/He
Photolysis
Laser
Nd:YAG
νx3
Vacuum
Chamber
Doubling
Crystal
Δν~0.1 cm-1
Pulsed Dye
Laser
νx2
Excitation
Laser
OGC
PMT
Nd:YAG
B̃
Δν~30 cm-1
DF
BoxCar
spectrograph
LIF
iCCD
Computer
Ã
X̃
PMT=photomultiplier tube; OGC=optogalvanic cell
iCCD=intensified charge coupled device
6
LIF/DF Spectra of 4-MCHO
J. Lin, Q. Wu, G. Liang, L. Zu; RSC Adv. 2, 583−589 (2012)
Pumped LIF band: origin band at 26906 cm-1
1
n=0
2
nυCO (Ã)
3
3197
2165 2y
x+z
∆E=76(5)
y
x
0
500
1121
1006
1000
2111
x+y 2055
1170 z
2208 y+z
2269 2z
2x1939
1500
2000
Redshift/cm-1
2500
3000
3500
7
DF Spectra of 4-MCHO
Pumped LIF bands (with respect to origin band at 26906 cm-1)
n=0
∆E=76(5)
nυCO (Ã)+υpump(Ã)
2
f
b'
F (+632 cm-1)
1
1υCO (Ã)
2υCO (Ã)
1
2
3υCO
(Ã)
e
E (+542 cm-1)
d
D (+441 cm-1)
c
-1
C (+363 cm )
b
B (+271 cm-1)
n=0
3
nυCO (Ã)
A (Origin)
0
500
1000
1500
2000
Redshift/cm-1
2500
3000
3500
8
Proof of “Shift effect” for 4-MCHO
b'
f
F (+626 cm-1)
e
E (+538 cm-1)
d
D (+437 cm-1)
c
C (+358 cm-1)
-1
b
B (+268 cm )
A (Origin)
0
500
1000
1500
2000
2500
3000
3500
Redshift/cm-1
9
Duschinsky Mixing
The excited state PES is (i) displaced,
(ii) distorted, (iii) but not rotated
relative to the ground state.
The excited state PES is (i)
displaced, (ii) distorted, and (iii)
rotated relative to the ground state.
Vibrational modes Q1’’ and Q2’’ of the ground electronic state are mixed in the excited state
(Q1’ and Q2’).
C. W. Muller, J. J. Newby, C. Liu, C. P. Rodrigo, T. S. Zwier; Phys. Chem. Chem. Phys. 12, 2331–2343 (2010)
10
LIF/DF Spectra of 3-MCHO
J. Lin, Q. Wu, G. Liang, L. Zu; RSC Adv. 2, 583−589 (2012)
Pumped LIF band: origin band at 26677 cm-1
n=0
1
2
3
nυCO(X̃ /Ã)
∆E=62(5)
0
500
1000
1500
2000
-1
Redshift/cm
2500
3000
3500
11
DF Spectra of 3-MCHO
Pumped LIF bands (with respect to origin band at 26677 cm-1)
∆E=62(5)
n=0
1
-1
c'
C* (+1052 cm )
B* (+924 cm-1)
*
nυCO(X̃ /Ã)
*
*
*
A* (+684 cm )
*
*
*
-1
nυCO (Ã)+υpump(X̃ /Ã)
2
*
*
b'
680 cm-1
nυCO (Ã)+υpump(X̃ /Ã)
c
C (+376 cm-1)
b
B (+245 cm-1)
n=0
1
2
1000
1500
2000
Redshift/cm-1
3
nυCO(X̃ /Ã)
A (Origin)
0
500
2500
3000
3500
12
3-MCHO: Vibrational Population Relaxation in
B̃
State
ʋCO+ ʋ*
B̃
680 cm-1
~
~
Energy
ʋ=0
ʋCO
ʋ*
ʋCO+ ʋ*
ʋCO
ʋ*
ʋ=0
X̃ /Ã
rCO
13
LIF/DF Spectra of 2-MCHO
J. Lin, Q. Wu, G. Liang, L. Zu; RSC Adv. 2, 583−589 (2012)
Pumped LIF band: origin band at 26275 cm-1
n=0
1
2
3
nυCO(X̃ /Ã)
∆E=110 (5)
0
500
1000
1500
2000
-1
Redshift/cm
2500
3000
3500
14
DF Spectra of 2-MCHO
Pumped LIF bands (with respect to origin band at 26275 cm-1)
∆E=110 (5)
n=0
1
2
nυCO (Ã)+υpump(X̃ /Ã)
f
-1
F (+537 cm )
e
-1
E (+494 cm )
d
D (+412 cm-1)
c
C (+400 cm-1)
b
-1
B (+331 cm )
n=0
1
2
1000
1500
2000
Redshift/ cm-1
3
nυCO(X̃ /Ã)
A (origin)
0
500
2500
3000
3500
15
FCF Simulation
Duschinsky mixing is taken into account in the simulation
2-MCHO
Experimental
Simulation
nυCO(X̃ /Ã)
3-MCHO
Experimental
Simulation
nυCO(X̃ /Ã)
4-MCHO
Experimental
Simulation
nυCO(X̃ /Ã)
0
500
1000
1500
2000
Red shift/cm-1
2500
3000
Mozhayskiy, V. A.; Krylov, A. I. ezSpectrum. http://iopenshell.usc.edu/downloads.
3500
16
LIF/DF Spectra of 1-MCHO
Origin band of Chair-axial
Origin band of Chair-equatorial
Pumped LIF band: chair-axial
origin band at 27278 cm-1
∆E=89 (5)
1
0
1
500
2
1000
35
1υC-O stretch (X̃ ) (cm-1)
910
1υC-O stretch (Ã) (cm-1)
1υC-CH3 stretch (X̃ ) (cm-1)
902
564
1υC-CH3 stretch (Ã) (cm-1)
559
nυC-O(Ã)
2
1
n=0
rC-CH3 (B̃ ) - rC-CH3 (X̃ ) (mÅ)
nυC-CH3 (Ã) +2υ C-O (Ã)
n=0
n=0
249
Q. Wu, G. Liang, L. Zu; J. Phys. Chem. A 116, 3156−3162 (2012)
1
n=0
rC-O (B̃ ) - rC-O(X̃ ) (mÅ)
3
3
2
2
3
4
4
4
5
1500
2000
2500
RedShift/cm-1
5
6
3000
5
7
3500
nυC-CH3 (Ã) +υ C-O (Ã)
nυC-CH3 (X̃ ) +υ C-O (X̃ )
nυC-CH3(Ã)
4000
17
Summary
 DF spectra of MCHOs provide vibronic structures of the nearly degenerate X̃ and à states, especially the
energy separation between the vibrational ground levels of these two states.
Isomers
4-MCHO
3-MCHO
2-MCHO
1-MCHO (chair-axial)
ΔE(Ã-X̃ ) (cm-1) (Expt.)
76
62
110
89
 The DF spectra are dominated byo C-O stretch progression when the origin and the CO-stretch bands of the B̃ ←X̃ LIF excitation spectra
were pumped.
o progressions of CO-stretch modes combined with the pumped vibrational mode when non-CO-stretch
vibrational levels of the B̃ state were pumped.
 The transition intensities can be explained on the basis of FCFs taking Duschinsky mixing into consideration.
 Excited state vibrational population relaxation has been observed in the DF spectra of 3-MCHO.
 Calculated FCFs well reproduce the DF spectrum of 4-MCHO but not as well for the other isomers.
Future Work
 DF spectra of 1-MCHO.
 High resolution LIF spectroscopy with rotational resolution.
 Ab initio calculations of X̃ /Ã state PESs.
 Theoretical investigation of pJTE in MCHOs and its effect on
vibronic structure.
19
Acknowledgements
Group Members:
Former Member:
Dr. Neil Reilly
UMass Boston
Funding:
J. Alam, Md. A. Reza, A. Mason, N. J. Reilly, and J. Liu, J. Phys. Chem. A (2015)
DOI: 10.1021/acs.jpca.5b03454