Quantum interference distinguishes between constitutional isomers

Abstract

Matter waves, as introduced by de Broglie in 1923 [1], are a fundamental quantum phenomenon, describing the delocalized center of mass motion of massive bodies and we show here their sensitivity to the molecular structure of constitutional isomers.In quantum textbooks, matter wave phenomena are often associated with the equation λ dB = h/(mv), where the de Broglie wavelength λ dB is only determined by Plancks constant h and the particles momentum p = mv. It is a common conjecture that this relation still holds for bodies of arbitrary mass, size and complexity. But what is the role of the detailed internal molecular composition if λ dB does not include any such information?As already shown before [2], the centre of mass motion can be well described by quantum delocalization and interference of each entire molecule with itself -even if the internal atoms may populate a thermal mixture of vibrational and rotational modes at a temperature of about T = 500 K. The conceptual separation of the internal and external degrees of freedom allows us to discuss two different cases:If the interaction between the molecule and its environment makes it possible, even only in principle, to retrieve position information, the initially delocalized particle will be localized and quantum interference can no longer occur [3].The second case is of particular relevance for our present experiment: All individual atoms in a given molecular structure will add up to determine its global properties, and in particular also its electrical susceptibility. The susceptibility can couple to an external electric field and thus influence the center of mass motion of the entire particle. In this way, the internal structure becomes influential for the external motion, even though the molecule remains still widely delocalized and capable of showing de Broglie interference.First experiments along this line were recently able to apply this idea to near-field interferometry for measuring the static [4] and optical [5] polarizability α stat and α opt as well as the total electric susceptibility χ tot of molecules. The latter may also contain additional information about static or time varying electric dipole moments [6]. It has therefore been suggested that different molecular conformations might eventually also be distinguished in quantum interference experiments [7].For demonstration purposes we here compare two tailor-made model compounds 1 and 2. The design * http://www.quantumnano.at; markus.arndt@univie.ac.at † marcel.mayor@unibas.ch of both is based on our recent findings in molecular electronics which showed a considerable delocalization of the π-system in rod-like oligo phenylene ethynylene (OPE) [8,9] on the one hand and a partition of the π-system by conjugation interrupting subunits like e.g. platinum complexes [10], perpendicular torsion angles [11,12] or sp 3 -carbon atoms on the other hand. The constitutional isomers 1 and 2 both have the chemical formula C 49 H 16 F 52 and an identical molecular weight of 1592 g mol −1 , but different polari...

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