a study of 4,4 ΄ -dimethylaminobenzonitrile by chirped-pulsed fourier transform microwave...
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A Study of 4,4΄-Dimethylaminobenzonitrile by Chirped-Pulsed Fourier Transform
Microwave Spectroscopy
Ryan G. Bird, Valerie J. Alstadt, and David W. Pratt
University of Pittsburgh
Justin L. Neill and Brooks H. Pate
University of Virginia
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Prelude to Charge Transfer• DMABN is widely known
model for excited state charge transfer
• Charge transfer facilitated by twisting amino nitrogen
• Determine electronic density around amino nitrogen using quadrupole coupling terms
• DMABN = 2 nitrogen nuclei– Simplified model; DMA = 1
nucleusE. Lippart, W. Lüder, and H. Boos, in the Proceedings of the 4th International Meeting on Advances in Molecular Spectroscopy (1959), A. Mangini, ed., Pergamon Press, Oxford, 1962, pgs. 443-457.
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CP FTMW Spectrometer
MW Synthesizer
Arbitrary Waveform Generator
Fourier Transform
Free Induction Decay
Chirped Pulse240 MHz
500 MHzDigitizer(10 Gs/s)
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Spectrum of N,N΄-Dimethylaniline
10000 avg~500 MHz CP
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Inversion of DMA
505←404
1000 avg10 MHz CP
0+
0-
0-
0+
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Rotational Constants DMA
This work Lister1 mp2/6-31+g(d)
A(0+) (MHz) 3500.315(103) 3500.39 3500.042
B(0+) (MHz) 1226.667(4) 1226.646 1224.151
C(0+) (MHz) 923.345(3) 923.328 923.877
A(0-) (MHz) 3500.779(119) 3499.88
B(0-) (MHz) 1226.646(5) 1226.590
C(0-) (MHz) 923.609(5) 923.645
ΔI (amu Å2) -9.1 -9.0 -10.2
Lines 95
1Cervellati, R.; Borgo, A. D.; Lister, D. G. Journal of Molecular Structure 1982, 78, 161-167.
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Barrier to Inversion
Aniline1 N-methylaniline1 N,N-dimethylaniline
Inversion angle (φ) 37 ~20 22
V2 barrier (cm-1) 526 167 24 + 3
1Cervellati, R.; Esposti, A. D.; Lister, D. G.; Palmieri, P. Journal of Molecular Structure: THEOCHEM 1985, 122, 173-177.
ΔE = 4.5 + 0.5 cm-1
I is the integrated peak area
g is the statistical weight
F = ћ2/2Ir
Ir is the reduced moment of inertia of the inversion motion
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N,N΄-Dimethylaminobenzonitrile2 – 8 GHz190,000 avg9 GHz CP
6 – 18 GHz10,000 avg~500 MHz CP
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DMABN Constants
This work Endo1 LIF2 M052x
A (MHz) 3469.2(9) 3469.99 3470.3 3469.37
B (MHz) 578.583(2) 578.581 578.7 580.99
C (MHz) 499.612(2) 499.613 499.7 500.97
ΔI (amu Å2) -7.61 -7.58 -7.56 -6.73
Lines 293
1Kajimoto, O.; Yokoyama, H.; Ooshima, Y.; Endo, Y. Chemical Physics Letters 1991, 179, 455-459.2Nikolaev, A. E.; Myszkiewicz, G.; Berden, G.; Meerts, W. L.; Pfanstiel, J. F.; Pratt, D. W. Journal of Chemical Physics 2005, 122, 1-10.
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Quadrupole Splitting in DMABN2 – 8 GHz 6 – 18 GHz
313←212 909←808
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Amine Quadrupole Coupling
Compound Aniline1 DMA DMABNAmine N
χaa (MHz) 2.34(6) 2.58(3) 2.54(33)
χbb (MHz) 1.86(6) 2.80(6) 2.80(13)
χcc (MHz) -4.20(6) -5.39(6) -5.33(13)
Inversion Angle (deg) 37 22 6
1Hatta, A.; Suzuki, M.; Kozima, K. Bull. Chem. Soc. Jap. 1973, 46, 2321-3.
Lone pair density
Inversionangle
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Hyperconjugation
H
H
HN
-
+
C C
+
-
+
-
π-π donation π-π donation
N-
+
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Nitrile Quadrupole Coupling
DMABN Benzonitrile1 Ethyl Cyanide2
Nitrile N
χaa (MHz) -4.11(30) -4.23738(36) -3.309(33)
χbb (MHz) 2.40(13) 2.2886(11) 1.265(13)
χcc (MHz) 1.71(13) 1.9488(11) 2.044(20)
1Wohlfart, K.; Schnell, M.; Grabow, J. U.; Küpper, J. Journal of Molecular Spectroscopy 2008, 247, 119-121.2Li, Y. S.; Harmony, M. D. The Journal of Chemical Physics 1969, 50, 3674-3677.
C N-
+ +
-
π-π donation
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Townes and Dailey Populations1
Aniline2 DMA DMABN
χzz (MHz) 2.38 2.72 2.55
χxx(MHz) 1.86 2.8 2.8
χyy(MHz) -4.24 -5.52 -5.34
Nz 1.25 1.29 1.4
Nx 1.28 1.28 1.3
Ny 1.76 1.98 2.0
X
Y
1Townes, C. H.; Dailey, B. P. The Journal of Chemical Physics 1949, 17, 782-796.2 Lister, D. G.; Tyler, J. K.; Høg, J. H.; Larsen, N. W. Journal of Molecular Structure 1974, 23, 253-264.
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Aniline → DMA → DMABN
• Decrease in inversion angle and increase in amine χcc coupling term
• Caused by increase in electronic density around amine nitrogen
• Electronic density stabilized through hyperconjugation
• Increased density facilitates charge transfer• Quadrupole terms of excited state??
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Acknowledgements
Pratt Group: Pate Group:
Dr. David Pratt Dr. Brooks PateJustin Young Justin NeillA.J. Fleisher Matt MucklePhil Morgan Daniel ZaleskiJessica ThomasCasey Clements
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Quadrupole CouplingCompound Aniline1 DMA DMABN Benzonitrile2 Ethyl Cyanide3
Amine N
χaa (MHz) 2.34(6) 2.58(3) 2.54(33)
χbb (MHz) 1.86(6) 2.80(6) 2.80(13)
χcc (MHz) -4.20(6) -5.39(6) -5.33(13)
Nitrile N
χaa (MHz) -4.11(30) -4.23738(36) -3.309(33)
χbb (MHz) 2.40(13) 2.2886(11) 1.265(13)
χcc (MHz) 1.71(13) 1.9488(11) 2.044(20)
1Hatta, A.; Suzuki, M.; Kozima, K. Bull. Chem. Soc. Jap. 1973, 46, 2321-3.
2Wohlfart, K.; Schnell, M.; Grabow, J. U.; Küpper, J. Journal of Molecular Spectroscopy 2008, 247, 119-121.
3Li, Y. S.; Harmony, M. D. The Journal of Chemical Physics 1969, 50, 3674-3677.
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119°6°
Structures
DMABN
DMA113° 22°
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