Metallocenes featuring two nearly parallel units that rotate in relation to one another around a metal center are often regarded as prototypes of molecular carousels.[1] Unsubstituted ferrocene is the simplest and the best-known archetype of molecular carousels among metallocenes. In ferrocene-based rotors, cyclopentadienyl rings can rotate around the Fe(II) metallic center into both staggered and eclipsed configurations, with an energy barrier that depends on the inserted substituents on the rings. For instance, the insertion of bulky groups, such as tert-butyl substituent, substantially increases the rotation barrier,[2] whereas the introduction of phenyl substituents leads to an unexpected concerted rotary motion.[3] Previous gas-phase investigations have shown that the intramolecular rotary motion in disubstituted ferrocene derivatives can be induced by protonation and deprotonation of carboxylate moieties, through which trans and cis conformations are selectively locked.[4] In recent attempts to understand the intramolecular dynamics in metallocene molecular carousels using rotational spectroscopy, we observed spectral signatures of quantum tunneling emerging in a ferrocene-based rotary system in locked staggered and eclipsed conformations. In this contribution, we will present the results of our broadband microwave experiments, and discuss our strategy for analyzing the rotational spectra of a disubstituted ferrocene derivative using complementary quantum chemistry calculations. References: [1] Browne, W. R.; Pijper, D.; Pollard, M. M.; Feringa, B. L.; Synthetic Molecular Machines, based on noninterlocked molecules; from concepts to applications. Solvay Conference. Wiley, 2008. [2] Luke, W.D.; Streitweiser, A.; J. Am. Chem. Soc., 1981, 103, 3241–3243. [3] Castellani, M.P.; Wright, J.M.; Geib, S.J.; Rheingold, A.L.; Trogler, W.C.; 1986, Organometallics, 1986, 5, 1116–1122. [4] Wang, X. - B.; Dai, B.; Woo, H. - K.; Wang, L. - S.; Angew. Chem. Int. Ed., 2005, 44, 6022–6024.

Methyl lactate, one of the simplest chiral esters, has been an iconic system for chiral recognition study in the past decades[1,2]. The methyl lactate monomer has the ability to form a strong intramolecular hydrogen bond between the hydroxy group and carbonyl group and its aggregates show a robust competition between intra- and intermolecular hydrogen bonding interactions[3,4]. Understanding its self-aggregation process is a fundamental step to draw a comprehensive picture of the conformational behavior of chiral α-hydroxy ketones, a ubiquitous structural unit found in natural products and pharmaceutical industries. In the current study, we applied chirped-pulse Fourier transform microwave spectroscopy (CP-FTMW) to detect homochiral and heterochiral binary aggregates of methyl lactate in the range of 2-6 GHz. Rotational spectra of several binary conformers of methyl lactate were assigned, aided by a conformational searching tool, CREST[5], and subsequent DFT calculations. The internal rotation of methyl rotors generated fine splitting structures of the rotational transition. This allows us to catch a glimpse of the symmetric geometry of the homochiral dimers. (see the figure) l0pt Figure References [1] T. B. Adler, N. Borho, M. Reiher, M. A. Suhm, Angew. Chem. Int. Ed. 2006, 45, 3440–3445. [2] J. Thomas, O. Sukhorukov, W. Jäger, Y. Xu, Angew. Chem. Int. Ed. 2014, 53, 1156–1159. [3] P. Ottaviani, B. Velino, W. Caminati, Chem. Phys. Lett. 2006, 428, 236–240. [4] N. Borho, Y. Xu, Phys. Chem. Chem. Phys. 2007, 9, 1324–1328. [5] S. Grimme, C. Bannwarth, P. Shushkov, J. Chem. Theory Comput. 2017, 13, 1989–2009.

r0pt Figure The rotational spectrum of methoxyflurane was recorded using molecular jet FTMW spectroscopy. Methyl internal rotation and nuclear quadrupole coupling (NQC) of two chlorine nuclei (Cl) complicated the analysis. Codes that treat internal rotation often allow for simultaneous but approximate treatment of NQC, with significant errors for the NQC of Cl. In contrast, the well-known SPFIT code provides an exact treatment of NQC, which can be combined with separate fits for each internal rotation species. However, SPFIT local fits also have limitations; they only allow fitting of NQC off-diagonal elements and internal rotation parameters when the internal rotation axis and the coupling nucleus are coplanar, which is not the case in methoxyflurane. Still, with a "not-too-small" internal rotation barrier of 350 cm⁻¹, a work-around is possible. The abstract-figure shows the J_Ka,Kc :3_2,1−2_2,0 transition - experiment (black), simulations of species A (blue), E (red) and both combined (green). (A and E each scaled to match peak marked with an asterisk).

1,1,2,2-Tetracyanocyclopropane (TCCP) and its derivatives can serve as synthetically accessible and versatile synthons [1]. The four cyano groups create an electron-poor region in the center of the (NC)₂C-C(CN)₂ fragment, which can interact with an electron-rich partner, like the lone pair of tetrahydrofuran (THF), through tetrel bonds with the sp³ hybridized carbons [2]. Microwave spectroscopy, when combined with quantum chemistry calculations, can be a useful tool to study these interactions. However, the presence of four nitrogen atoms turns the analysis of the rotational spectrum into a much more difficult task. While the splitting pattern due to the quadrupolar electric field of a single nucleus is normally easy to resolve, each additional nuclei increases its complexity considerably. We will present and discuss our analysis of the broadband rotational spectra of 3,3-dimethyl-TCCP and 3,3-diethyl-TCCP and their complexes with THF, obtained from full and partial fits including nuclear quadrupole interactions. These results illustrate both the potential and limitations of broadband experiments in accurately determining information from hyperfine splitting. References: [1] Antonio Bauzá, Antonio Frontera and Tiddo J. Mooibroek, Phys. Chem. Chem. Phys., 2016, 18, 1693 [2] Victoria L. Heywood, Thomas P. J. Alford, Julius J. Roeleveld, Siebe J. Lekanne Deprez, Abraham Verhoofstad, Jarl Ivar van der Vlugt, Sérgio R. Domingos, Melanie Schnell, Anthony P. Davis and Tiddo J. Mooibroek, Chem. Sci., 2020, 11, 5289

In this presentation, I will discuss a new experimental platform designed to facilitate quantum logic control of polar molecular ions in a segmented ring ion trap, paving the way for precision measurements. This approach focuses on achieving near-unity state preparation and detection, as well as long spin-precession coherence. A distinctive aspect lies in separating state preparation and detection conducted in a static frame, from parity-selective spin-precession in a rotating frame. Moreover, this method is designed to support both temporally and spatially localized multiplexing measurements, enhancing the ability to probe and minimize potential systematic errors. While the primary focus of this talk is on detecting the electron’s Electric Dipole Moment (eEDM) using ²³²ThF⁺ ions, the proposed methodology holds promise for broader applications, particularly with ion species like ²²⁹ThF⁺ and ¹⁸¹TaO⁺ that exhibit enhanced sensitivity to the nuclear Magnetic Quadruple Moment (nMQM).

r0pt Figure Nuclear electric resonance (NER) spectroscopy is currently experiencing a revival as a potential tool for quantum computing based on nuclear spins. Access to nuclear spin states via electric fields is provided by the nuclear quadrupole moment, a common feature of many standard isotopes, caused by the non-spherical shape of their nuclei. Based on an in-depth analysis of the underlying coupling mechanism, we investigate the possibility of coherent spin control in atoms or molecules via nuclear quadrupole interaction from first principles. A general, time-dependent description is provided, which entails and reflects on commonly applied approximations often found in recent literature. This formalism is then used to propose a new method we refer to as `optical' nuclear electric resonance or `ONER'. Our protocol takes advantage of time-modulated optical excitations via UV/visible light, e.g. realized by a pulsed laser, to control the electric field gradient at the position of a specific nucleus by periodic changes of the surrounding electron density. The proposed method is theoretically investigated for the ¹S→ ¹P transition in ⁹Be as well as the ¹XΣ⁺ →  ¹AΣ⁺ transition in ⁷Li²³Na as first atomic and molecular benchmark systems, respectively.Johannes K. Krondorfer and Andreas W. Hauser. Nuclear electric resonance for spatially resolved spin control via pulsed optical excitation in the uv-visible spectrum. Phys. Rev. A, 108:053110, Nov 2023.Johannes K Krondorfer, Matthias Diez and Andreas W Hauser, Optical Nuclear Electric Resonance in LiNa: Selective Addressing of Nuclear Spins through Pulsed Lasers, Physica Scripta (submitted) Our findings suggest that it might be possible to shift complicated spin manipulation tasks in atomic, molecular or solid-state systems into the time domain by pulse-duration encoded laser signals.

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Johannes K. Krondorfer and Andreas W. Hauser. Nuclear electric resonance for spatially resolved spin control via pulsed optical excitation in the uv-visible spectrum. Phys. Rev. A, 108:053110, Nov 2023.

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Last year¹, we presented the rotational spectrum of cyclopropylchloromethyldifluorosilane (synthesised by the Guirgis group at the College of Charleston, SC) in the 5.0-19.0 GHz region of the electromagnetic spectrum. We noted that the quadrupole coupling tensor for the most populous conformation in the free-jet expansion was unusual, in that the value of χ_aa was very close to zero. Upon further analysis, and the subject of this talk, we have observed electric dipole forbidden, quadrupole allowed transitions (including the somewhat enigmatic x-type transition) in the rotational spectra of the ³⁷Cl isotopologue of the most populous conformation in the free-jet expansion, and the parent species of a higher-energy conformation also present in the free-jet expansion. These transitions are highly unusual in chlorine containing species owing to the relatively small quadrupolar moment of chlorine and the (generally) higher rotational constants in chlorine-containing species when compared to brominated or iodated species. We present and discuss pathways through which the zero-order rotational state wavefunctions can be viewed to mix together in order to give rise to these transitions.
¹ A. R. Davies, A. G. Hanna, A. Lutas, G. A. Guirgis and G. S. Grubbs II, TK05 - “Rotational spectroscopy and structure of cyclopropylchloromethyldifluorosilane”, International Symposium on Molecular Spectroscopy (ISMS), 2023.

We have recorded and analysed the rotational spectrum of 3-iodo-1,1,1-trifluorobutane in the 6.0-18.0 GHz region of the electromagnetic spectrum using our chirped-pulse Fourier transform microwave (CP-FTMW) spectrometer. The rotational spectrum was incredibly dense (~1 transition every 4 MHz, on average) owing to the very low rotational constants and the quadrupolar iodine nucleus (I = 5/2) - the latter splits each rotational transition into multiple hyperfine components. However, the very large number of transitions allows accurate determination of all components of the quadrupole coupling tensor in the principal axis system. Additionally, this tensor is diagonalised into a space-fixed coordinate system to gain insight into the electric field (localised at the iodine atom) and is compared to related systems to understand the long-range effect, if any, of the three electron-withdrawing fluorine atoms on the iodine atom.

Through a long-standing collaboration with the Guirgis group (College of Charleston, SC), we have been trying to understand the physicochemical differences between silicon-containing species and carbon-containing species. Now, we present the rotational spectrum of 1,1-dichloro-1-silacyclohex-2-ene in the 5.50-18.75 GHz region of the electromagnetic spectrum and discuss the quadrupole coupling within this molecule and present a partial substitution (r_s) structure; the latter arises from the observation of various isotopologues in the free-jet expansion, in their natural abundances. The hyperfine structure is complicated and arises from the two non-equivalent chlorine nuclei (I = 3/2), although the quadrupole coupling constants for both atoms are very similar. This leads to challenging analysis as a large portion of the hyperfine splitting is unresolved - even with the resolution we achieve with our chirped-pulse Fourier transform microwave (CP-FTMW) spectrometer. We compare our experimental results to those predicted by high-quality ab initio calculations, as well as the related systems 1,1-difluoro-1-silacyclohex-2-ene and 1-silacyclohex-2-ene¹.
¹ N. T. Moon, A. J. Duerden, T. M. C. McFadden, N. A. Seifert, G. A. Guirgis and G. S. Grubbs II, J. Phys. Chem. A, 128, 10-19 (2024).

When considering a C_s molecule with a single spin source and single internal rotor, the group theoretically complete Hamiltonian up to 4th order has 168 allowed operators. To remove the redundancies and interdependencies between these operators, Watson's famous contact transformation procedure has been followed. However, only 43 operators can be removed thus resulting in a great many possibilities for the reduction. The second order Hamiltonian can be reduced to form either the Principal Axis Method or the Rho Axis Method. Reductions of the 4th order Hamiltonian will be carried out to resemble the Watson-A and the Watson-S as best as possible for the more complicated case of spin-torsion-rotation with lower symmetry. Alternative possibilities of the reduction as well as modifications to the westerfit package to accommodate these reductions will also be discussed.

The structure and phenyl group internal rotation vibration of the scaffold molecule flavone, its complex with water, and its derivative luteolin have been investigated by laser ablation chirped pulse Fourier transform microwave spectroscopy (LA-CP-FTMW). The investigation of the tunneling splittings detected in the spectrum has allowed the determination of the periodic potential function of the phenyl torsion vibration of flavone. The interaction with water may alter substantially this potential function depending on the site of flavone where interaction takes place. This behavior explains the flexibility and the variety of equilibrium structures found for this molecule in condensed phases. M. P. Waller, D. E. Hibbs, J. Overgaard, J. R. Hanrahan, T. W. Hambley Acta Crystallogr Sect E Struct Rep Online 2003, 59, 767–768^,M. Narwal, T. Haikarainen, A. Fallarero, P. M. Vuorela, L. Lehtiö, J. Med. Chem. , 2013, 56, 3507–3517^,S. Caddick, F. W. Muskett, R. G. Stoneman, D. N. Woolfson, J. Am. Chem. Soc.2006, 128, 4204–4205he shape of the phenyl torsion potential energy function also governs the behavior of luteolin, a flavone derivative with four OH group substituents.

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M. P. Waller, D. E. Hibbs, J. Overgaard, J. R. Hanrahan, T. W. Hambley Acta Crystallogr Sect E Struct Rep Online 2003, 59, 767–768\end M. Narwal, T. Haikarainen, A. Fallarero, P. M. Vuorela, L. Lehtiö, J. Med. Chem. , 2013, 56, 3507–3517 S. Caddick, F. W. Muskett, R. G. Stoneman, D. N. Woolfson, J. Am. Chem. Soc.2006, 128, 4204–4205T