Dedicated to Richard N. Zare on the occasion of his 65th birthday Studies of isotope effects have a long tradition in providing fundamental insights into molecular spectroscopy and reaction dynamics, [1,2] usually dealt with theoretically on the basis of the electromagnetic interaction that is parity conserving, i.e. remains unchanged under space inversion at the origin. [3][4][5] Isotope effects are frequently caused by mass differences of the isotopes. There are also isotope effects due to the different nuclear spins of the isotopes, [6] and, in principle, isotope effects can arise independent of mass and spin because of symmetry restrictions on the molecular wavefunction leading to different symmetry selection rules for different isotopomers.[7] Here we report the first quantitative investigations of a new isotope effect, which leads to a ground-state energy difference D pv E % D pv H 0 0 /N A for the enantiomers of molecules that are isotopically chiral, i.e. chiral only by isotopic substitution (Figure 1). This parity-violating isotope effect arises from the electroweak interaction between electrons and nucleons, mediated by the Z-boson, and thus depends upon nucleonic composition. Our calculations are of interest in relation to efforts of measuring D pv E in enantiomers, [5,8] and they are also important for the fundamental understanding of isotope effects and molecular chirality. The present work opens a new avenue in this field by providing quantitative calculations on such chiral isotopomers in the framework of electroweak quantum chemistry [9] including the weak nuclear force. Since recent theoretical approaches predict absolute values of D pv E that can be orders of magnitude larger [9][10][11][12] than anticipated on the basis of earlier calculations, [14,15] there is new hope that accurate measurements and calculations, particularly for molecules with light atoms, will provide additional insights into the standard model of high-energy physics. [5,16] We refer here to recent articles with extensive further references. [4,5,10,12] In this context we address and answer the following questions: 1. How large is D pv E in isotopically chiral systems compared to "ordinary" enantiomers? 2. Is D pv E dominated here by the parity-violating potential at the equilibrium geometry or by vibrationally averaging the parity-violating potential? 3. How does vibrational excitation change D pv E (i.e. D pv E*) in such systems compared to "ordinary" enantiomers where this question was addressed previously? [17] The answers to these questions will help in planning future experiments possibly including isotopic enantiomers. We study the phosphane derivatives PHDX (X = F, 35 Cl, 37 Cl, 79 Br, 81 Br) and P 35 Cl 37 ClY (Y = F,H,D) with these goals in mind. While isotopic chirality has been considered for some time, [3,[18][19][20] as an isotope effect through variation of
