a study of 4,4 ΄ -dimethylaminobenzonitrile by chirped-pulsed fourier transform microwave...

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

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.

CP FTMW Spectrometer

MW Synthesizer

Arbitrary Waveform Generator

Fourier Transform

Free Induction Decay

Chirped Pulse240 MHz

500 MHzDigitizer(10 Gs/s)

Spectrum of N,N΄-Dimethylaniline

10000 avg~500 MHz CP

Inversion of DMA

505←404

1000 avg10 MHz CP

0+

0-

0-

0+

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.

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

N,N΄-Dimethylaminobenzonitrile2 – 8 GHz190,000 avg9 GHz CP

6 – 18 GHz10,000 avg~500 MHz CP

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.

Quadrupole Splitting in DMABN2 – 8 GHz 6 – 18 GHz

313←212 909←808

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

Hyperconjugation

H

H

HN

-

+

C C

+

-

+

-

π-π donation π-π donation

N-

+

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

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.

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??

Acknowledgements

Pratt Group: Pate Group:

Dr. David Pratt Dr. Brooks PateJustin Young Justin NeillA.J. Fleisher Matt MucklePhil Morgan Daniel ZaleskiJessica ThomasCasey Clements

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.

119°6°

Structures

DMABN

DMA113° 22°