Transcript pMPEA-water 06-20-09.ppt

Pure Rotational and Ultraviolet-Microwave
Double Resonance Spectroscopy of
Two Water Complexes of
para-methoxyphenylethylamine (pMPEA)
Justin L. Neill, Matt T. Muckle and Brooks H. Pate,
Department of Chemistry, University of Virginia
Ryan G. Bird, David W. Pratt,
Department of Chemistry, University of Pittsburgh
Spectroscopy of pMPEA and pMPEA-water
Unamuno et al.
pMPEA
Seven conformers reported:
Martinez et al, J. Mol. Spectrosc. 158 (1993) 82-92.
Complete (correct) structural assignment:
Robertson, Simons, and Mons, J. Phys. Chem. A
105 (2001) 9990.
Yi, Robertson, and Pratt, Phys. Chem. Chem. Phys.
4 (2002) 5244-5248. (rotationally resolved LIF)
Douglass et al., MF02, ISMS (2006)
(CP-FTMW and UV-MW)
Cortijo, Alonso, and López, Chem. Phys. Lett.
466 (2008) 214-218. (MW)
Unamuno et al.
pMPEA-water
Two clusters found, binding energies measured,
assigned to structures:
Unamuno et al, Chem. Phys. 271 (2001) 55-69.
Conformational Landscape of pMPEA
B
B
AA
C
C
F
F
G
G
DD
EE
Conformer
E (cm-1)
E-8
0
D-5
34
C-7
38
A-4
54
B-1
351
F-2
489
G-3
498
6
675
9
678
mp2/6-31g**
Yi, Robertson, and Pratt, Phys. Chem. Chem. Phys. 4 (2002) 5244-5248.
New CP-FTMW Measurements
Sample acquired from Aldrich (98%);
placed in reservoir within nozzle, heated
to approximately 100°C, seeded in He/Ne
supersonic expansion. (No water added)
CP-FTMW sample reduction techniques:
3 nozzles
10 FIDs per valve pulse
Collected 995,000 FIDs, using
298,500 valve pulses, equivalent
sensitivity to 8.955 million pulses with
a single nozzle!
Measurement time: 48 hours (consecutive)
G.G. Brown, B.C. Dian, K.O. Douglass, S.M. Geyer, S.T. Shipman, and B.H. Pate, Rev. Sci. Instrum.
79 (2008) 53103-1-13.
New CP-FTMW Measurements
3 nozzles, 995,000 FIDs (298,500 pulses) versus 1 nozzle, 80,000 FIDs (80,000 pulses)
Scaled to match signal heights on strongest transitions
New CP-FTMW Measurements
3 nozzles, 995,000 FIDs (298,500 pulses) versus 1 nozzle, 80,000 FIDs (80,000 pulses)
Scaled to match signal heights on strongest transitions
2008 spectrum much richer than 2006…(used same bottle)
New Transitions
Several unassigned Q-branches observed. For near-prolate top, b/c-type Q-branches are located
at approximately (A-(B+C)/2)*(2K-1), so the ratio between two Q-branches gives you their K assignments
and A-(B+C)/2.
Pattern of the Q-branches gives (B-C), then (A+B+C) can be varied until the strong b/c-type R-branches
are fit. Two new spectra were assigned this way.
New Transitions
pMPEA-water Fit Parameters
Conformer D-water
A/MHz
B/MHz
C/MHz
χaa/MHz
χbb-χcc/MHz
Nlines
rms error/kHz
µa/D
µb/D
µc/D
Experiment
1740.6781(7)
430.5044(4)
380.0915(4)
0.20(6)
-2.419(26)
224
24.2
b≈c
Theory
1769.85
426.01
376.65
0.41
-2.19
0.64
2.84
2.60
Conformer E-water
A/MHz
B/MHz
C/MHz
χaa/MHz
χbb-χcc/MHz
Nlines
rms error/kHz
µa/D
µb/D
µc/D
Experiment
1533.8873(10)
457.8598(5)
398.9191(9)
0.785(47)
-2.31(44)
137
12.7
c only
Theory
1552.23
452.47
383.61
0.97
-2.04
0.47
0.49
2.32
Ab initio: b3pw91/6-311+g(df,pd), using effective Q and recommended basis set of W.C. Bailey
(http://homepage.mac.com/wcbailey/nqcc/)
All fits performed using SPFIT (Pickett), with standard errors determined by PIFORM (Kisiel). (Quartic
distortion parameters not listed)
Observed structures are analogous to those of other similar structures: tryptamine (Felker, J. Phys. Chem. 96 (1992) 7844);
2-phenylethylamine (Melandri, et al, RC13)
Coherence-Converted Population Transfer UV-FTMW Spectroscopy
Continuum
MW
Synthesizer
2 GS/s AFG
Nd:YAG
ν0
10 Hz rep. rate
v0 + 30 MHz
200 mJ/p 532 nm
Single Sideband
Pulsed 1 watt amp
ν0
Dye laser
Lambda Physik
0.025
cm-1
5 mJ/p UV
All spectra are ~0.1 cm-1 blue-shifted
due to coaxial arrangement.
bandwidth
Rhodamine 6G dye,
doubled with BBO
SHG crystal
1 Gs/s Oscilloscope
(30 MHz Carrier)
Free Induction Decay
T.J. Balle and W.H. Flygare, Rev. Sci. Instrum. 52, 33 (1981)
M. Nakajima, Y. Sumiyoshi, and Y.Endo, Rev. Sci. Instrum. 73, 165 (2002)
R.D. Suenram, J.U. Grabow, A. Zuban, and I. Leonov, Rev. Sci. Instrum. 70, 2127 (1999)
Douglass, Johns, Nair, Brown, Rees, and Pate, J. Mol. Spectrosc. 239, 29 (2006)
pMPEA-water UV-FTMW
Flowed He/Ne gas over cooled (0°C)
water reservoir before entering
chamber; increased signals by
around a factor of 5 (as strong as
monomer)
With the water reservoir at room
temperature, signal started to
drop again (higher water clusters?)
Ab Initio Relative Energies (cm-1)
Conformer
A-4
B-1
C-7
D-5
E-8
F-2
G-3
6
9
Monomer
54
351
38
34
0
489
498
675
678
1023
1427
1005
44
0
918
946
1210
816
Water cluster
mp2/6-31g**
pMPEA(E)-water
pMPEA(C)-water
pMPEA(9)-water
pMPEA-water UV-FTMW
The assignments of Unamuno et al are correct—35670 cm-1 feature is due to
water with conformer 5; 35681 cm-1 feature is due to water with
conformer 8.
Their assignments were based on structural stability—conformer 8+water goes
to strongest peak, conformer 5+water to second-strongest—and
low-frequency vibrational mode calculations.
Residual Spectrum
Strongest pMPEA transition intensity 120 µV
No residual transitions with resolved quadrupole hyperfine splitting—not pMPEA, or simply a function
of cluster size? (large number of hyperfine-resolved transitions for assigned pMPEA-H2O clusters)
Possibilities:
Other conformers with water; water molecule on the methoxy group? (Unlikely due to energetics)
Two waters or more? (more likely—ab initio calculations needed)
Remeasure CP-FTMW spectrum with water added!
MW-MW double resonance spectroscopy needed
Acknowledgements
Funding:
NSF CRIF:ID (CHE-0618755)
Jefferson Scholars Foundation (J.Neill)
Tektronix
pMPEA-water Fit Parameters
Conformer D-water
A/MHz
B/MHz
C/MHz
DJ/kHz
DJK/kHz
DK/kHz
dJ/kHz
dK/kHz
χaa/MHz
χbb-χcc/MHz
Nlines
rms error/kHz
µa/D
µb/D
µc/D
Experiment
1740.6781(7)
430.5044(4)
380.0915(4)
0.0496(13)
-0.223(6)
2.586(13)
0.0114(5)
0.32(5)
0.20(6)
-2.419(26)
224
24.2
b≈c
Theory
1769.85
426.01
376.65
0.41
-2.19
0.64
2.84
2.60
Conformer E-water
A/MHz
B/MHz
C/MHz
DJ/kHz
DJK/kHz
DK/kHz
dJ/kHz
dK/kHz
χaa/MHz
χbb-χcc/MHz
Nlines
rms error/kHz
µa/D
µb/D
µc/D
Experiment
1533.8873(10)
457.8598(5)
398.9191(9)
0.0573(35)
-0.063(12)
1.468(34)
0.0113(20)
0.23(8)
0.785(47)
-2.31(44)
137
12.7
c only
Theory
1552.23
452.47
383.61
0.97
-2.04
0.47
0.49
2.32
Observed structures are analogous to those of other similar structures: tryptamine (Felker, J. Phys. Chem. 96 (1992) 7844);
2-phenylethylamine (Melandri, et al, RC13)