High resolution spectroscopy of methyltrioxorhenium: towards the observation of parity violation in chiral molecules

Originating from the weak interaction, parity violation in chiral molecules has been considered as a possible origin of the biohomochirality. It was predicted in 1974 but has never been observed so far. Parity violation should lead to a very tiny frequency difference in the rovibrational spectra of the enantiomers of a chiral molecule. We have proposed to observe this predicted frequency difference using the two photon Ramsey fringes technique on a supersonic beam. Promising candidates for this experiment are chiral oxorhenium complexes, which present a large effect, can be synthesized in large quantity and enantiopure form, and can be seeded in a molecular beam. As a first step towards our objective, a detailed spectroscopic study of methyltrioxorhenium (MTO) has been undertaken. It is an ideal test molecule as the achiral parent molecule of chiral candidates for the parity violation experiment. For the 187Re MTO isotopologue, a combined analysis of Fourier transform microwave and infrared spectra as well as ultra-high resolution CO2 laser absorption spectra enabled the assignment of 28 rotational lines and 71 rovibrational lines, some of them with a resolved hyperfine structure. A set of spectroscopic parameters in the ground and first excited state, including hyperfine structure constants, was obtained for the antisymmetric Re=O stretching mode of this molecule. This result validates the experimental approach to be followed once a chiral derivative of MTO will be synthesized, and shows the benefit of the combination of several spectroscopic techniques in different spectral regions, with different set-ups and resolutions. First high resolution spectra of jet-cooled MTO, obtained on the set-up being developed for the observation of molecular parity violation, are shown, which constitutes a major step towards the targeted objective.Comment: 20 pages, 6 figure

THE MOLECULAR STRUCTURE OF TETRACARBONYLDIHYDRORUTHENIUM - MICROWAVE MEASUREMENTS AND DFT CALCULATIONS

Author Institution: Department of Chemistry, The University of ArizonaThe microwave rotational spectra for seven isotopomers of tetracarbonyldihydroruthenium were measured in the 4-12 GHz range using a Flygare-Balle type microwave spectrometer. The measured transition frequencies could be fit to within a few kHz using a rigid rotor Hamiltonian with centrifugal distortion. The rotational constants for the most abundant isotopomer are $A=1234.2762(4), B=932.7016(6)$ and $C=811.6849(6) MHz$. The measured 21 rotational constants were used to deterine the following structural parameters: $r(Ru-H) = 1.710(23){\AA}, r(Ru-Cl) = 1.952(21) {\AA}, r(Ru-C3) = 1.974(28) {\AA}, /(H-Ru-H) = 87.4(2.4)^{\circ}, /(Cl-Ru-C2) = 160.6(4.3)^{\circ}, \angle(C3-Ru-C4) = 101.4(1.5)^{\circ}$, and $\angle(Ru-Cl-O1) = 172.6(7.6)^{\circ}$. These structural parameters are in excellent agreement with the substitution coordinates determined from the Kraitchman equations, and with the structural parameters calculated using density functional theory. There was no previous structural data on this complex. The results of the microwave data and with theoretical calculations both indicate $C_{2u}$ molecular symmetry, and show that the H atoms are separated by about $2.36 {\AA}$. These results indicate that this complex is clearly a `classical dihydride' rather than an $\eta^{2}$ - bonded. `dihydrogen' complex. Fairly large deuterium isotope effects were observed for the Ru-H bond length and H-Ru-H angle. The $r_{0}$, Ru-D bond lengths were observed to be $0.033 {\AA}$ shorter than the $r_{0}$, Ru-H bond length. The D-Ru-D angle is $1.1^{\square}$; less than the H-Ru-H angle indicating that the anharmonicity effects are larger for the bond lengths than for the interbond angle. The new results on this complex are compared with previous results on the similar dihydride complexes, $H_{2}Fe(CO)_{4}$, and $H_{2}Os(CO)_{4}$

MEASUREMENT OF THE MICROWAVE SPECTRUM, STRUCTURAL PARAMETERS, AND QUADRUPOLE COUPLING FOR METHYL RHENIUM TRIOXIDE

Author Institution: Department of Chemistry, University of ArizonaThe J = 1 $\leftarrow$ 0, and 2 $\leftarrow$ 1 transitions for six isotopomers of methyl rhenium trioxide were measured using Flygare-Balle type, pulsed-beam, Fourier-transform spectrometer with a heated nozzle system. The observed hyperfine structure splittings due to the $^{185}$Re and $^{187}$Re quadrupole coupling interactions are significantly smaller in this complex than for $HRe(CO)_{4}$. The Re-C and Re-O bond lenghths are compared with the corresponding solidstate values obtained from neutron diffraction work