DOI: 10.1002/cphc.200900022 Mode-Selective Promotion and Isotope Effects of Concerted Double- Hydrogen Tunneling in Porphycene Embedded in Superfluid Helium Nanodroplets Alexander Vdovin, [a, b] Jacek Waluk, [b] Bernhard Dick, [a] and Alkwin Slenczka* [a] Chemical reactions with nuclear rearrangement via tunneling through a potential barrier are challenging for experimentalists and theoreticians alike. For the experimental approach, low temperatures are mandatory to exclude the often dominating thermally induced path across the barrier. From the theoretical point of view, tunneling is a purely quantum phenomenon for which a semi-classical approach is inappropriate—even if it ex- plains some aspects conveniently. In speaking about tunneling, one distinguishes between incoherent and coherent tunneling. The first describes the unidirectional passage from an initial to the final nuclear configuration through a barrier, a process usu- ally characterized by a rate constant. In contrast, in classical terms coherent tunneling encompasses a dynamic equilibrium among different nuclear configurations separated by barriers which are tunneled bidirectionally. The appropriate quantum mechanical approach reveals a splitting of the energy levels with an energy difference classically related to the tunneling frequency. Quantum mechanically, the eigenstates are repre- sented by wave functions which are delocalized with respect to the tunneling barrier and which alternate in parity. The parity dictates the selection rules for spectroscopic transitions and, thus, is responsible for the typical tunneling multiplets in the molecular spectra. The rate constants or the tunneling splittings are affected by additional excitation of internal de- grees of freedom of the system and by isotope exchange of the tunneling nucleus. Systems exhibiting multiple nuclei tun- neling are particularly challenging. It raises the additional ques- tion on the correlation of the tunneling nuclei, which is classi- cally addressed as a concerted or stepwise tunneling mecha- nism. Several systems have been considered as a model for coher- ent multiple-hydrogen tunneling comprising both inter- and intramolecular cases with either stepwise or concerted tunnel- ing mechanism. [1] Herein, we concentrate on porphycene (Pc), a model system for intramolecular double hydrogen tunneling. Up to now, experimental data did not provide conclusive evi- dence for the stepwise or concerted mechanisms in Pc. [2, 3] Recent developments of theoretical methods have left behind the classical idea of a time-space trajectory for the nuclei. [4] Still, the question on the tunneling mechanism as deduced from theoretical investigations could not be confirmed by the experimental data for Pc. [5, 6] Therein, the authors express the demand for more experimental data to improve and test theo- retical tunneling models. Our paper provides new experimental data which have been obtained by applying a new experimen- tal technique for the preparation of cold isolated Pc, essential conditions for studying tunneling. High-resolution optical spec- troscopy is performed on Pc doped into superfluid helium droplets. A superfluid helium droplet is the most gentle cryomatrix available. [7] The superb heat conductivity of superfluid helium in combination with energy release by evaporative cooling at the droplet surface warrants for a temperature of only 0.37 K of any molecular system doped into a helium droplet. [7–9] More- over, the negligible viscosity is an advantage for the investiga- tion of unimolecular as well as bimolecular chemical processes. The superfluid droplet exhibits negligible inhomogeneity of the dopant system, which is ideal for high-resolution spectro- scopy. The spectral signature of coherent tunneling is multiplet splitting of spectroscopic transitions, which is easily hidden below inhomogeneous line broadening. If resolved, the multip- let fine structure reveals details of the tunneling mechanism of the particular system. A temperature of only 0.37 K warrants energetic conditions below the tunneling barrier. Additionally our experiment benefits from an almost instantaneous ther- malization of all nuclear degrees of freedom. [8, 9] Instantaneous in this respect means at a timescale short compared to any other decay process. Thus, helium droplets comprise a favora- ble host system for molecular spectroscopy and in particular for the investigation of inter- or intramolecular chemical proc- esses. The tunneling model system Pc is a tetrapyrolic compound with four nitrogen atoms bound in a p-conjugated ring. [10] The nitrogen atoms, arranged in rectangular configuration, com- prise four sites for two hydrogen atoms, which leads in general to six pairwise mirror-symmetric configurations, two trans and four cis, thus a rather complex tunneling system. [5, 6, 11, 12] Scheme 1 depicts all possibilities for tautomerization among the six configurations via tunneling of the two hydrogen atoms, distinguishing between concerted (c) and stepwise (a) processes, depending on the particular tautomers in- volved. An additional challenge is to study the influence of ad- ditional vibrational excitation on the intramolecular double hy- [a] Dr. A. Vdovin, Prof. Dr. B. Dick, Dr. A. Slenczka Institut für Physikalische und Theoretische Chemie Universität Regensburg Universitätsstrasse 31, 93053 Regensburg (Germany) Fax: (+ 49) 941-943-4488 E-mail : alkwin.slenczka@chemie.uni-regensburg.de [b] Dr. A. Vdovin, Prof. Dr. J. Waluk Institute of Physical Chemistry Polish Academy of Sciences, Warsaw (Poland) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cphc.200900022. ChemPhysChem 2009, 10, 761 – 765  2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 761