Large Parity‐Violation Effects in Heavy‐Metal‐Containing Chiral Compounds

Abstract

The weak neutral current (Z-boson exchange) [1][2][3] between electrons and nucleons (quarks) introduces a small energy difference (DE PV ) between enantiomers of a chiral molecule owing to parity violation (P-odd effects). [4,5] There have been several attempts to measure the parity-violation (PV) difference between enantiomers, [6][7] however, such effects are estimated to be of the order of a few millihertz [8][9][10] for currently known molecules, and several orders of magnitude too low for the present ability of high-resolution spectroscopy. Here we report relativistic calculations of PV effects for several chiral compounds that contain a heavy-metal center; shifts in DE PV of up to 4.8 10 À14 au (300 Hz) are recorded for such compounds as [(h 5 -C 5 H 5 )Re(CO)(NO)I], which can easily be prepared. The high DE PV value is an indication that future P-odd experiments for chiral molecules should be directed towards heavy-transition-metal complexes.Since the creation of the unified electroweak theory by Weinberg, Salam, and Glashow, the investigation of PV effects has become one of the most exciting areas in particle physics. [1][2][3] Precise measurements of PV in atoms are now accurate enough for the standard model to be tested successfully.[11] Electroweak theory also predicts a small energy difference (DE PV ) between pairs of enantiomers, and thus a breakdown of mirror-image symmetry. The search for such effects currently involves vibrational, [5,[12][13][14] NMR, [15,16] electronic, [17,18] and Mössbauer [6,19] spectroscopy, electric-field optical activity, [20] and preferred crystallization.[21] Most notably, Chardonnet and co-workers performed a saturation spectroscopy experiment for CHFClBr in the 9.3 mm spectral range using a tunable CO 2 laser that provided a spectral purity of 6 Hz. [12] They obtained Dn PV = n RÀ Àn S+ = 9.4 AE 5.1 AE 12.7 Hz between the two enantiomers, where the first value of uncertainty is derived from statistical error and the second is determined by systematic effects. More recently, the same group reduced these uncertainties and obtained Dn PV = À4.2 AE 0.6 AE 1.6 Hz.[22] Thus the search for the breakdown of mirror-image symmetry in chiral molecules remains one of the most challenging tasks in molecular chemistry and physics.It is a nontrivial issue to find chiral compounds suitable for high-resolution spectroscopic measurements of parity nonconservation effects.[23] P-odd effects scale approximately like Z n (n % 5; Z = nuclear charge of the atom), [24] which limits the choice to heavy-element-containing compounds. Furthermore, in accordance with the single-center theorem of Hegstroem, the substance should contain more than one heavy element (either as ligands or chiral centers).[25] A detailed analysis of P-odd effects, with respect to the environment around a center of chirality, has been given by Faglioni and Lazzeretti.[26] Very recently they also proposed the hypothetical molecule [BiHFX] (X = Br, I) as a suitable candidate with PV effects, with the Dn PV value of the i...

Interactions between Physics and Biodisciplines within the Framework of Molecular Sciences

“…Recently, the case of a large parity-violation effect was announced [33]. I would like to close these remarks by expressing astonishment at the high symmetry of various supra-supramolecular systems, e.g., viruses.…”

The Search for Parity Violation in Chiral Molecules

“…For example, calculations for PV frequency shifts in fluorooxirane by Berger et al show rather small values, that is, Dn PV =n ¼ 1:2 Â 10 À18 for the C-F stretching mode [176]. The first large PV energy shifts of up to 300 Hz were obtained for chiral organometallic compounds such as [(g 5 -C 5 H 5 )Re(CO)(NO)I] [119]. These were DC-HF calculations, and from the more recent results for CHFClBr, we expect that these HF results are probably not very reliable and could change significantly upon inclusion of electron correlation.…”

“…Inzhechik et al claimed to have seen circular polarization of (0.9 AE 0.1) Â 10 À3 for the 23.9 keV transition in 119 Sn and (0.58 AE 0.12) Â 10 À3 for the 14.4 keV transition in 57 Fe [100][101][102]. This, however, was disputed later by Shuskov and Telitsin who obtained five orders of magnitude smaller values from the theory [103].…”

“…One has to avoid line broadening from nuclear quadrupole coupling, restricting the nuclei to nuclear spin I ¼ 1/2. There are not too many promising heavy element isotopes available satisfying this condition, and in addition forming stable chiral compounds, and possible choices are 117 Sn, 119 Sn, 187 Os, and 183 W as chiral centers [112]. Another possibility is to attach a heavy spin 1/2 nucleus to a chiral center.…”

“…In principle, electronic spectroscopy would be the most promising way to observe PV effects in chiral molecules, as PV shifts in the total electronic energy are largest, and are predicted to be in the range of a few 100 Hz for heavy element containing chiral compounds [119]. However, narrow line widths are only obtained in symmetry-or spin-forbidden electronic transitions with lifetimes of the excited electronic state of 1 ms or more.…”