nathan r. pillsbury, timothy s. zwier, department of chemistry, purdue university, west lafayette,...
TRANSCRIPT
Nathan R. Pillsbury, Timothy S. Zwier, Department of Chemistry, Purdue University, West Lafayette, IN 47907; David F. Plusquellic, NIST, Gaithersburg, MD 20899; W. Leo Meerts, Radboud University Nijmegen, The Netherlands.
CONFORMATION-SPECIFIC ELECTRONIC SPECTROSCOPY OF JET-COOLED BIS(2-HYDROXYPHENYL)METHANE
O OH H
Why study Bis(2-hydroxyphenyl)methane (2HDPM)?
O OH H
1. It is similar to diphenylmethane which was recently studied in our lab which was found to have very interesting spectroscopy
2. No synthesis needed!
0.00 kcal/mole 0.69 kcal/mole
DFT Calculations
π H-bonded conformerOH…OH H-bonded conformer
2 low-energy minima
Flu
ore
sc
en
ce
Sig
na
l
36800366003640036200360003580035600Wavenumbers (cm
-1)
LIF Spectrum of 2HDPM
O OH H
Records the UV spectrum of a single conformationfree from interference from others present in the expansion
Laser Timing
50-500nsec
UVHole-burn
UVprobe
UV-UV Hole-burning Spectroscopy
UV Hole-burn laser fixed: Provides selectivity UV probe laser tuned
Boltzmann distributionof conformers in the pre-expansion
Collisional cooling to zero-point vibrational level
B*
B*
B*CA
B*
C
CB
A
A
A
CA
ABC C
AAB BBB B
B B
UV
UV
C
2HDPM* (S1)
2HDPM (S0)
Hol
e-bu
rn
Pro
be
Conformer AConformer B
Hol
e-bu
rn
Pro
beTFTF
Flu
ore
sc
en
ce
Sig
na
l
36800366003640036200360003580035600Wavenumbers (cm
-1)
LIF
A
B
LIF and UVHB Spectra of 2HDPM
Flu
ore
sce
nc
e S
ign
al
36000359003580035700Wavenumbers (cm
-1)
A
B
Origin Regions of Conformers A and B
A
B
26
41
22
Fluorescence-Dip Infrared Spectroscopy (FDIRS)
Records IR spectrum of single species free from interference from others present in the expansion
2HDPM *(S1)
2HDPM (A) (S0)
2HDPM *(S1)
2HDPM (A) (S0)
S0 FDIRS S1 FDIRS
Total Fluorescence
Total Fluorescence
IR
UV
IR
UV
Dep
leti
on
(a.
u.)
3650360035503500Wavenumbers (cm
-1)
S0 A
S0 B
S0 FDIRS
OH…OH H-bonded conformer
π H-bonded conformer
OH…OH(f)OH(d)/torsioncombination
band
OH(d)…O
OH(d)…
leak-thru from A
leak-thru from A
IR In
ten
sity
(a.
u.)
3650360035503500Wavenumbers (cm
-1)
Experiment
Calculated
S0 A
Conformer A Experimental OH-stretch vs. Calculated
IR In
ten
sity
(a.
u.)
37003650360035503500Wavenumbers (cm
-1)
Experiment
Calculated
S0 B
Conformer B Experimental OH-stretch vs. Calculated
De
ple
tio
n (
a.
u.)
370036503600355035003450340033503300Wavenumbers (cm
-1)
S1 A
S1 B
18 30
S1 FDIRS
S0
S0
OH/torsioncombination bands
Ring 1(?) Ring 2(?)
Two rings now inequivalent!Strong OH stretch/torsion coupling
leak-thru from A
leak-thru from A
VOH=1
En
erg
y
OH vibration and ring torsion are very strongly coupled in the excited state
Why do we see seven OH stretches instead of one in the excited state of B?
= dihedral angle between the two rings
ring torsion levels
S1 zero-point levelVOH=0
Flu
ore
scen
ce
In
ten
sit
y
20x103
100-10-20MHz
-800 -400 0MHz
High Resolution UV Spectrum of 2HDPM A Origin
Experimental Spectrum
Simulated Fit
D. Plusquellic (NIST)
What information can we get from the high resolution spectra?
2HDPM Conformer A Origin:
A = 1159.6B = 412.5 C = 348.1
ΔA = -39.5ΔB = 9.5ΔC = 1.2
%A = 67%B = 24%C = 9
Ground State Rotational Constants
Change in Constants Upon Excitation
Transition Moment Direction
Predicted Ground State Constants
A = 1168.7 B = 404.6 C = 336.9
DFT MP2
A = 1129.6 B = 418.7 C = 346.6
a
b
c
Flu
ore
scen
e In
ten
sity
80x103
6040200-20-40MHz
High Resolution UV Spectrum of 2HDPM B Origin Region
vibronic bands of A
B origin
Experimental Spectrum
Simulated Fit
Simulated Fit
High Res. Spectrum of 2HDPM B +22 cm-1 Vibronic Band
Flu
ore
sc
en
e I
nte
ns
ity
30x103
20100-10-20MHz
-12.5x103
-12.0 -11.5 -11.0MHz
Experimental Spectrum
Simulated Fit
2HDPM Conformer B +22 cm-1 Band:
A = 1326.7B = 402.4 C = 368.8
ΔA = 27.7ΔB = -1.2ΔC = -5.7
%A = 85%B = 15%C = 0
Ground State Rotational Constants
Change in Constants Upon Excitation
Transition Moment Direction
Predicted Ground State Constants
A = 1358.0 B = 389.7 C = 360.7
DFT MP2
A = 1302.6 B = 412.1 C = 376.9
Conformer B Vibronic Band Fit Results
a
b
c
Dispersed Fluorescence Spectra of AF
luo
resc
ene
In
ten
sity
3002001000Wavenumbers
D = 2.3
612.9852.21108.31344.3
26 cm-1
FC progressionA Origin
+ 31 cm-1
+ 62 cm-1
Flu
ore
sc
en
e I
nte
ns
ity
3002001000Wavenumbers
D = 2.3 612.9
852.2
1108.31344.3
26 cm-1
FC progression
A Origin
+ 41 cm-1
+ 83 cm-1
Dispersed Fluorescence Spectra of A Cont.
Flu
ore
scen
e In
ten
sity
3000200010000Wavenumbers
B origin
+ 22 cm-1
+ 44 cm-1
Dispersed Fluorescence Spectra of B
* * * * *
*
*
What could be causing these strange intensities?
1. There could be some vibronic coupling to the nearby S2 state
2. Duschinsky mixing
3. It is possible that we may be seeing fluorescence to another minimum in the ground state
Future Studies of 2HDPM
SEP-Population Transfer Study:
What are the barriers to isomerization between conformers A and B?
Predicted Transition States (DFT):
A
B1885 cm-1
1399 cm-1
TS
TS
Conclusions
1. The two conformations of 2HDPM have been assigned using FDIRS and high resolution UV spectroscopy
2. We observed strong coupling between the OH- stretch and inter-ring torsion in the S1 state of conformer B
3. Dispersed fluorescence spectra show interesting features which we are currently attempting to explain
Acknowledgements
Professor Timothy Zwier
The Zwier GroupJasper Clarkson*Talitha SelbyEsteban Baquero*V. Alvin ShubertTracy LeGreveJosh NewbyBill JamesChirantha RodrigoDr. Ching-Ping Liu
Collaborators
Dr. David F. Plusquellic (NIST)
Professor W. Leo Meerts (Radboud University)
FundingFrederick N. Andrews Fellowship
U.S. Department of Energy