Abstract
We report the ground state tunneling splittings (ΔE± ) of a number of axially chiral molecules using the ring-polymer instanton (RPI) method (J. Chem. Phys., 2011, 134, 054109). The list includes isotopomers of hydrogen dichalcogenides H2 X2 (X = O, S, Se, Te, and Po), hydrogen thioperoxide HSOH and dichlorodisulfane S2 Cl2 . Ab initio electronic-structure calculations have been performed on the level of second-order Møller-Plesset perturbation (MP2) theory either with split-valance basis sets or augmented correlation-consistent basis sets on H, O, S, and Cl atoms. Energy-consistent pseudopotential and corresponding triple zeta basis sets of the Stuttgart group are used on Se, Te, and Po atoms. The results are further improved using single point calculations performed at the coupled cluster level with iterative singles and doubles and perturbative triples amplitudes. When available for comparison, our computed values of ΔE± are found to lie within the same order of magnitude as values reported in the literature, although RPI also provides predictions for H2 Po2 and S2 Cl2 , which have not previously been directly calculated. Since RPI is a single-shot method which does not require detailed prior knowledge of the optimal tunneling path, it offers an effective way for estimating the tunneling dynamics of more complex chiral molecules, and especially those with small tunneling splittings.
Highlights
In a symmetric double-well system, tunneling splittings are produced by the overlap of the tails of degenerate wavefunctions essentially localized in the wells and correspond to the energy difference between delocalized eigenstates that can be formed as superpositions of the localized states
H2X2 with X = O, S, Se, Te, Po and HSOH and S2Cl2 serve as prototype molecules for the stereorotation dynamics of an axially chiral molecule
ring-polymer instanton (RPI) neglects the weak dependence of ΔE± on the overall rotation of the system
Summary
Quantum tunneling in degenerate rearrangements of molecular clusters has been a topic of high interest in many fields of physics, chemistry, and biology.[1,2,3] In recent years, tunneling phenomena have attracted particular attention in molecular systems where the splittings give insights into the nature of rearrangement dynamics,[4,5] even at low temperature at which rearrangements are otherwise highly suppressed.[1,6] In a symmetric double-well system, tunneling splittings are produced by the overlap of the tails of degenerate wavefunctions essentially localized in the wells and correspond to the energy difference between delocalized eigenstates that can be formed as superpositions of the localized states. Schrödinger equation for multi-dimensional or reduced-dimensional rovibrational tunneling dynamics.[27,28,29,30,31,32] Some of the notable studies include the calculation of lowest rovibrational states and tunneling splittings in the intermolecular bound states of (H2O)[2,14,27,33] calculations on the tunneling splittings of the vibrational ground state and low-lying excited states of malonaldehyde,[30] the construction of a global analytical PES for the electronic ground state of NH3 from multireference configuration interaction and coupled cluster study[31] and calculation of the tunneling splittings of NH3+ , OH3+ , and so on.[32] Being computationally expensive due to the formal exponential scaling with the number of degrees of freedom, this is in full dimensionality only applicable to small gas-phase systems Methods such as diffusion Monte Carlo (DMC)[20,34,35] and path-integral techniques[36,37,38,39,40,41,42] are useful for larger systems.
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