Mode Selective Stereomutation and Parity Violation in Disulfane Isotopomers H2S2, D2S2, T2S2

Abstract

Dedicated to Edgar Heilbronner on the occasion of his 80th birthdayWe report quantitative calculations of stereomutation tunneling in the disulfane isotopomers H 2 S 2 , D 2 S 2 , and T 2 S 2 , which are chiral in their equilibrium geometry. The quasi-adiabatic channel, quasi-harmonic reaction path Hamiltonian approach used here treats stereomutation including all internal degrees of freedom. The torsional motion is handled as an anharmonic reaction coordinate in detail, whereas all the remaining degrees of freedom are taken into account approximately. We predict how stereomutation is catalyzed or inhibited by excitation of the various vibrational modes. The agreement of our theoretical results with spectroscopic data from the literature on H 2 S 2 and D 2 S 2 is excellent. We furthermore predict the influence of parity violation on stereomutation as characterized approximately by the ratio (DE pv /DE AE ) of the (local or vibrationally averaged) parity violating potential DE pv and the tunneling splittings DE AE in the symmetrical case. This ratio is exceedingly small for the reference molecules H 2 O 2 and D 2 O 2 , and still very small (2´10 À6 cm À1 ) for H 2 S 2 , which, thus, all exhibit essentially parity conservation in the dynamics. However, for D 2 S 2 it is ca. 0.002, and for T 2 S 2 it is ca. 1, which seems to be the first case where such intermediate mixing through parity violation is quantitatively predicted for spectroscopically accessible molecules. The consequences for the spectroscopic detection of molecular parity violation are discussed briefly also in relation to other molecules.

Two-Step Enantio-Selective Optical Switch

“…The calculated tunneling splitting of the lowest torsional states gives enantiomeric lifetimes of several msec, in accordance with previous reports on mode selective stereomutation [14]. Thus, although D 2 S 2 is not a chiral molecule in the conventional sense, molecular configurations described by superpositions of the lowest torsional states, localized in one minimum of the double well potential, stay chiral for sufficiently long times to be detected.…”

Stereomutation dynamics in hydrogen peroxide

“…We may mention in this context as examples on the one hand hydrogen bond dynamics in isotopomers of hydrogen fluoride dimers [46][47][48][49][50] and inversion in Ammonia isotopomers [51][52][53][54][55] and on the other hand stereomutation dynamics in hydrogen peroxide and related systems [56][57][58][59][60][61][62][63][64]. Hydrogen peroxide in its equilibrium geometry exists in two enantiomeric forms separated by a low torsional barrier (of about 360 cm À1 ) conventionally labelled the P and M enantiomers (see Fig.…”

“…Indeed, hydrogen peroxide has been a prototype molecule for theoretical calculations of molecular parity violating potentials (see [67][68][69][70][71] and the reviews [72,73] for further references). We have discussed the importance of the interplay of parity violation with tunneling dynamics in this context [58][59][60][61][62][63]72,73].…”

The Search for Parity Violation in Chiral Molecules

“…Interestingly, a fit to a f ðZÞ ¼ aZ n function gives basically the same Z scaling law of n ¼ 6.17-6.18 for all levels of theory applied, and is therefore above the Z 5 scaling as suggested by Zeldovich [82]. A number of research groups in the past studied PV on the test molecules H 2 O 2 or H 2 S 2 or similar systems such as H 2 SO [151,156,[161][162][163][164][165][166].…”