Instanton theory of tunnelling in molecules with asymmetric isotopic substitutions

Abstract

We consider quantum tunnelling in asymmetric double-well systems for which the local minima in the two wells have the same energy, but the frequencies differ slightly. We derive a generalization of instanton theory for these asymmetric systems, leading to a semiclassical expression for the tunnelling matrix element and hence the energy level splitting. We benchmark the method using a set of one- and two-dimensional models, for which the results compare favourably with numerically exact quantum calculations. Using the ring-polymer instanton approach, we apply the method to compute the level splittings in various isotopic-substituted versions of malonaldehyde in full dimensionality and analyse the relative contributions from the zero-point energy difference and tunnelling effects.

arXiv comment13 pages, 4 figures

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• Tunnel splittings for one dimensional potential wells revisited
• Derivation of instanton rate theory from first principles
• Full- and reduced-dimensionality instanton calculations of the tunnelling splitting in the formic acid dimer
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Cited by in corpus (3)

• Perturbatively corrected ring-polymer instanton theory for accurate tunneling splittings
• Exact tunneling splittings from symmetrized path integrals
• Dynamical system analysis of quantum tunneling in an asymmetric double-well potential