Supporting Information. Surface Chemistry of 1- and 3-Hexyne on Pt(111): Desorption, Decomposition and Dehydrocyclization

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1 Supporting Information Surface Chemistry of 1- and 3-Hexyne on Pt(111): Desorption, Decomposition and Dehydrocyclization M. D. Rötzer 1, M. Krause 1, A. S. Crampton 1,2, E. Mitterreiter 1, H. H. Heenen 3, F. F. Schweinberger 1, K. Reuter 3, and U. Heiz 1,* 1 Technical University of Munich, Chair of Physical Chemistry, Catalysis Research Center and Chemistry Department, Ernst-Otto-Fischer-Str. 1 and Lichtenbergstr. 4, Garching, Germany 2 Harvard University, Department of Chemistry and Chemical Biology and School of Engineering and Applied Sciences, 12 Oxford Street, Cambridge, United States of America 3 Technical University of Munich, Chair of Theoretical Chemistry, Chemistry Department, Lichtenbergstr. 4, Garching, Germany * Corresponding Author January 18, 2018

2 Theoretical calculation of molecular orbitals To qualitatively support the experimental assignments in the UPS and MIES spectra, quantum chemical calculations are performed for the isolated molecules using the program Molpro [1]. To estimate the involved ionization potentials, initial and final state energies are computed. The initial state energies directly correspond to the molecular orbital energies of a Hartree-Fock (HF) calculation after Koopmans theorem. The final state energies correspond to excited electronic state energies of the cation referenced to the ground state energy of the neutral molecule. The latter is hereby representative of the relaxed electronic structure after ionization of different valence orbitals. A true ionization potential can be expected to lie between the initial and final state. The gas phase minimum energy structures are optimized on Møller-Plesset pertubation theory (MP2) level and electronic structure calculations for ground state molecular orbitals are performed on HF level. Excited state energies are obtained via complete active space self-consistent field (CASSCF) with multireference pertubation theory (CASPT2). All levels of theory employ a cc-pvdz basis set. The 10 lowest excited states are computed for the cation using a state averaged SA(10)-CASSCF calculation. The active space is chosen to include all orbitals with a contribution larger than 2% for the excited states. For 3-hexyne follows a (17,11) and a (18,11) active space for the cation and neutral molecule and for 1-hexyne a (19,12) and a (20,12) active space for the cation and the neutral molecule, respectively. The large number of occupied orbitals in the active space results from the cationic excited states including ionization of low lying molecular orbitals (s) (c.f. table S1 and S2). As seen in table S1 and S2, the rather crude Koopman approximation and the high-level CASPT2 calculations deviate quantitatively. Although most of the core-level ionizations are also found in the CASPT2 calculations, in the case of 3-hexyne also mixed ionization-excitation configurations are seen for the two highest excited states. Note that the declaration of the s changes partially due to a different energetic ranking in the HF and SA(10)-CASSCF formalism. Nevertheless, both methods show clearly that the photoelectron peaks around 10 ev correspond to π C=C orbitals while peaks at higher energies correspond to σ orbitals. References [1] Werner, H. J., Knowles, P. J., Lindh, R., Manby, F. R., Schütz, M., Celani, P., Korona, T., Rauhut, G., Amos, R. D., Bernhardsson, A. et al. LPRO, version 2006, a package of ab initio programs. School of Chemistry, Cardiff University, Cardiff, UK [2] Murillo, L. E.; Khan, N. A.; Chen, J. G. The Effect of Hydrocarbon Structure and Chain Length on the Low-Temperature Hydrogenation Activity on Ni/Pt(111) Bimetallic Surfaces. Surf. Sci. 2005, 594, S1

3 Table S1: Theoretical photoelectron peak assignment for a gas phase spectrum of 3-hexyne, comparing molecular HF orbital energies ( ε), corresponding to an initial state, with ionization potentials (IP) on SA(10)-CASPT2 level corresponding to a final state after a photoelectron transition. Given is the respective peak energy, the declaration or predominant electronic configuration as well as the character. ε / ev IP / ev a1 (πc=c ) a1 1 (πc=c ) a1 (πc=c ) a1 1 (πc=c ) a1 (σc H ) a a1 (σc H ) a1 1 (σc H ) a a a a1 1 (σc H ) a1 (σc C ) a a1 (σc C ) a a1 (σc H ) a1 1 23a1 25 a a a1 1 23a1 24a1 1 1 S2

4 Table S2: Theoretical photoelectron peak assignment for a gas phase spectrum of 1-hexyne, comparing molecular HF orbital energies ( ε), corresponding to an initial state, with ionization potentials (IP) on SA(10)-CASPT2 level corresponding to a final state after a photoelectron transition. Given is the respective peak energy, the declaration or predominant electronic configuration as well as the character. ε / ev IP / ev a1 (πc=c ) a1 (πc=c ) a1 (πc=c ) a1 (πc=c ) a1 (σc H ) a a a1 (σc H ) a a a1 (σc H ) a a a a a1 (σc H ) a a a a1 S3

5 3-hexyne 1-hexyne dn/de /a.u Pt C kinetic energy /ev Figure S1: Auger spectra for 3- and 1-hexyne on Pt(111) after TPD to 800 K. The ratio between C (275 ev) and Pt (241 ev) is used to calculate the amount of carbon residues on the surface. The calculation is performed according to reference [2] and takes two effects into account:(1) screening of the Pt AES signal by surface carbon atoms and (2) contribution from Pt layers in the bulk. S4

6 3-hexyne m/z=67, uncorrected m/z=67, corrected ion current /a.u. m/z= T /K Figure S2: Background correction for the desorption of 3-hexyne: The cracking pattern of hexene (m/z = 69) was determined in situ and the obtained sensitivity factor is used to substract the contribution of m/z = 69 (hexene) from m/z = 67 (hexyne) resulting in the background corrected traces, which were used to determine peak areas. S5

7 transmittance /% 375 K 300 K 285 K 250 K 200 K 100 K 0.5% wavenumber /cm 1 Figure S3: Temperature dependent IRRA spectra of 1-hexyne on Pt(111): 7 L of 1-hexyne were dosed either at 200 or 250 K. After dosage at 250 K the sample temperature was stepwise increased to 285, 300 and 375 K. All spectra were recorded at the dosage temperature. Adsorption at temperatures higher than 100 K leads to disappearance of all signals indicating that dehydrogenation takes place at low temperatures in accordance with the mechanism proposed for dehydrocyclization. S6