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

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.

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.

IMPACT FT-MW spectroscopy In-phase/quadrature phase modulation passage-acquired coherence technique (IMPACT) – FT-MW spectrometer Operational range: 2-26.5 GHz 1 GHz broadband FT-MW spectrometer Up to 20 single measurements per second Sub-Doppler linewidth, FWHM < 15 kHz (0.0000005 cm-1) Signal frequency determination accuracy < 1 kHz (0.00000003 cm-1) 1,0 m M. K. Jahn, D. Dewald, D. Wachsmuth, J.-U. Grabow, S. C. Mehrotra, J. Mol. Spec. 280 (2012), 54-60. J.-U. Grabow in „Handbook of High Resolution Spectroscopy“, Ed. M. Quack, F. Merkt, Wiley 2011, 723-800.

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

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

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 2219.00 2229.28279 (57) DE=0 B / MHz 763.90 757.765693 (38) C / MHz 610.99 604.276312 (26) cis 2 1975.61 1992.82809 (35) DE=561.2 cm-1 867.57 853.595668 (48) 738.74 726.007789 (40) cis 1 2004.69 1990.2279 (11) DE=667.1 cm-1 852.51 847.213462 (34) 700.17 696.887412 (28)

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

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 2219.00 2229.28279 (57) DE=0 B / MHz 763.90 757.765693 (38) C / MHz 610.99 604.276312 (26) cis 2 1975.61 1992.82809 (35) DE=561.2 cm-1 867.57 853.595668 (48) 738.74 726.007789 (40) cis 1 2004.69 1990.2279 (11) DE=667.1 cm-1 852.51 847.213462 (34) 700.17 696.887412 (28)

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/6-311++G(d,p) for structure, CCSD(T)/6-311++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), 184110.

Oxepane – broadband spectrum 1 GHz sections out of the 2-26.5 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

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 (MP2/6-311++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.

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, 40-60 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)

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