1. IMPACT FT-MW Spectroscopy of Organic Rings: Investigation of the
Conformational Landscape
Dennis Wachsmuth, Jens-Uwe Grabow
Institut für Physikalische Chemie und Elektrochemie
Gottfried Wilhelm Leibniz Universität Hannover, Germany
Alberto Lesarri
Departamento de Química Física y Química Inorgánica
Universidad de Valladolid, Spain

2. Challenges for MW-spectroscopy
Large amplitude motions might cause unpredictable wide splittings
Examination of (hyper-)fine structure requires very high resolution (e.g. quadrupole coupling)
Acquisiton of wide spectral ranges can be tedious
Unstable molecular beam sources do not allow for long measurements (laser ablation, discharge, etc.)
 Broadband FT-MW spectroscopy
D. Banser et al., Angew. Chem. Int. Ed. 44 (2005), 6311–6315.

3. FT-MW spectroscopy - techniques
Balle-Flygare FT-MW spectrometers (Fabry-Pérot resonators)
Fixed frequency pulses
Advantages
Highest spectral resolution (<3 kHz)
Extremely sensitive
Disadvantages
Narrow frequency range ( 1MHz)
Laborious acquisition of wide spectral ranges
Requires good theoretical predictions
„chirp“ broadband FT-MW spectrometers
Frequency ramp pulses
Advantages
Rapid acquisition of wide spectral ranges
High resolution
Rough precalculations sufficient
Disadvantages
Less sensitive
Requires higher excitation power
COBRA
IMPACT
T. J. Balle et al., J. Chem. Phys. 72 (1980), 922.
J.‐U. Grabow et al., Rev. Sci. Instrum. 67 (1996), 4072.
G. G. Brown et al., J. Mol. Spec. 238 (2006), 200.
M. K. Jahn et al., J. Mol. Spec. 280 (2012), 54.

4. IMPACT FT-MW spectroscopy
In-phase/quadrature phase modulation passage-acquired coherence technique (IMPACT) – FT-MW spectrometer
Operational range: GHz
1 GHz broadband FT-MW spectrometer
Up to 20 single measurements per second
Sub-Doppler linewidth,
FWHM < 15 kHz ( cm-1)
Signal frequency determination accuracy < 1 kHz ( cm-1)
1,0 m
M. K. Jahn, D. Dewald, D. Wachsmuth, J.-U. Grabow, S. C. Mehrotra, J. Mol. Spec. 280 (2012),
J.-U. Grabow in „Handbook of High Resolution Spectroscopy“, Ed. M. Quack, F. Merkt, Wiley 2011,

5. IMPACT - signal generation and acquisition
-500…500 MHz sweep Δ from AWG
Carrier frequency (2 to 26.5 GHz) from microwave generator ν
One-step modulation with I/Q-modulator
Molecular signal detection
One-step demodulation to DC (phase stable)
Phase invariant repetition of experiment allows for averaging in the time domain
FFT of time domain molecular signal

6. IMPACT FT-MW spectroscopy
Horn antenna
Off-axis parabolic mirrors
Highest molecular density and strongest MW field strength overlap
Solenoid pulse valve
Planar mirror with nozzle

7. 2-Decalone Commercial sample is a mixture of trans- and cis-isomers
Several conformations with similar energies
Lowest conformers predicted by coarse ab-initio calculations (MP2/6-311G(d,p) )
Conformer
predicted
exp.
trans
A / MHz
(57)
DE=0
B / MHz
763.90
(38)
C / MHz
610.99
(26)
cis 2
(35)
DE=561.2 cm-1
867.57
(48)
738.74
(40)
cis 1
(11)
DE=667.1 cm-1
852.51
(34)
700.17
(28)

8. 2-Decalone 102,9 ← 92,8 38,0 ← 37,0 trans cis 1 cis 2
Broadband spectrum of 2-decalone
2 bar Ne; 200 µs FID
 / MHz

9. 2-Decalone All three trans- and cis-isomers identified in spectrum
High spectral density would not allow for immidiate assignment of single lines
Rapidly assigned due to wide overview and recognition of intensity patterns
Conformer
theo.
exp.
trans
A / MHz
(57)
DE=0
B / MHz
763.90
(38)
C / MHz
610.99
(26)
cis 2
(35)
DE=561.2 cm-1
867.57
(48)
738.74
(40)
cis 1
(11)
DE=667.1 cm-1
852.51
(34)
700.17
(28)

10. Seven-membered rings: oxepane
Common motif in biological systems
Several conformations possible, twist-chair structures are the most stable
Previously studied with various quantum chemical techniques
Initial structures taken from Freeman et al. (B3LYP/ G(d,p) for structure, CCSD(T)/ G(d,p) for energies)
B3LYP/CCSD(T): J. Dillen, Struct. Chem. 24 (2013), 751.
B3LYP, CCD, CCSD, QCISD: Freeman et al., Intl. J. Quantum Chem. 108 (2007), 339.
Semiemperical potential: D. F. Bocian, H. L. Strauss, J. Am. Chem. Soc. 99 (1977), 2876.
P. Khalili, J. Chem. Phys. 138 (2013),

11. Oxepane – broadband spectrum
1 GHz sections out of the GHz range allow for rapid assignment
Linewidth: 13 kHz (FWHM)
Repetition rate: 12 Hz
n / GHz
30,320,2
200µs
Doppler splitting from coaxial arrangement of molecular beam and MW propagation direction

12. Oxepane Experimental spectrum
GED
XRD MW
𝒅 𝟏𝟐 / Å
1,419
1,430
1,397(17)
𝒅 𝟐𝟑 / Å
1,531
1,527
1,551(21)
𝒅 𝟑𝟒 / Å
1,525
1,523(4)
𝒅 𝟒𝟓 / Å
1,538(5)
𝒅 𝟓𝟔 / Å
1,530
1,528(4)
𝒅 𝟔𝟕 / Å
1,533
1,579(36)
𝒅 𝟕𝟏 / Å
1,428
1,374(37)
𝜶 𝟏𝟐𝟑 / °
109,0
110,0
110,6(6)
𝜶 𝟐𝟑𝟒 / °
112,2
114,1
114,6(6)
𝜶 𝟑𝟒𝟓 / °
112,6
115,1
115,3(5)
𝜶 𝟒𝟓𝟔 / °
111,9
113,3
112,9(4)
𝜶 𝟓𝟔𝟕 / °
112,7
114,7
114,6(14)
𝜶 𝟔𝟕𝟏 / °
113,8
114,6
114,6(3)
𝜶 𝟕𝟏𝟐 / °
112,1
114,6(18)
𝝉 𝟏𝟐𝟑𝟒 / °
77,0
73,0
70,6(12)
𝝉 𝟐𝟑𝟒𝟓 / °
-52,9
-51,0
-49,1(16)
𝝉 𝟑𝟒𝟓𝟔 / °
71,9
68,4
68,0(11)
𝝉 𝟒𝟓𝟔𝟕 / °
-88,8
-83,9
-81,7(17)
𝝉 𝟓𝟔𝟕𝟏 / °
36,1
34,1
30,5(39)
𝝉 𝟔𝟕𝟏𝟐 / °
49,4
49,5
54,3(37)
𝝉 𝟕𝟏𝟐𝟑 / °
-102,7
-98,6
-100,2(17)
Experimental spectrum
High discrepancy between simulated and measured rotational spectrum (shifted by approx. 100 MHz)
Easily assigned due to wide overview
Isotopologues could be measured for rs structure
and fitted to Kraitchman‘s equations
404 ← 313
414 ← 303
13C isotopologues
~100 MHz shift
×10
Theoretical prediction (MP2/ G(d,p) )
GED: J. Dillen, H. J. Geise, J. Mol. Struct. 64 (1980), 239.
XRD: P. Luger et al., Acta Cryst. C47 (1991), 102.
J. Kraitchman, J. Am. J. Phys. 21 (1953), 17.

13. Summary The IMPACT FT-MW spectrometer provides
High spectral resolution ( 13 kHz)
High signal frequency determination accuracy
Wide spectral range from 2 to 26.5 GHz
Broad acquisition range of 1 GHz
Advantages of the IMPACT scheme
Phase invariance allows for averaging in the time domain
Cheaper than chirp generation in arbitrary waveform generator and direct detection
Easily adoptable to higher frequency ranges (20-40 GHz, GHz)
Applications
Fast acquisition and interpretation of the rotational spectra of multi-conformational and/or flexible molecules
Rapid identification of wide splittings, e.g. from internal rotation
Determination of (hyper-)fine splitting constants
Search for „unpredictable“ transition frequencies (e.g. avoided crossing in heavy atom diatomics)

14. Acknowledgements The Grabow group Montse Vallejo Jan Borter
Special thanks to the mechanical and electronical workshop