Mode Tracking of Preselected Vibrations of One-Dimensional Molecular Wires

The mode-tracking principle ( J. Chem. Phys. 2003, 118, 1634) for the direct quantum chemical calculation of preselected, characteristic molecular vibrations makes vibrational analyses of molecular wire junctions feasible. Characteristic vibrational parameters of molecular bridges such as vibrational frequencies and force constants can be of importance for subsequent treatments in physical model theories of electron transfer and conductance. We investigate how efficient the mode-tracking protocol can be applied to determine such vibrational parameters for a particular type of normal modes of dinuclear polyynediyl rhenium complexes. The frequencies of vibrations of the carbon chain in complexes of the type [Re]−(C≡C) n −[Re] with [Re] = (η 5 -C 5 Me 5 )Re(NO)(PPh 3 ) have been studied as a function of the chain length 2 n, leading to molecules with up to 144 atoms, for which harmonic wavenumbers are determined. The harmonic approximation for the potential-energy surface is compared to explicitly calculated electronic energy values along normal coordinates in order to get an estimate for the role of anharmonicity effects. A possible vibration-induced rupture of molecular bridges is discussed. Different bond-breaking positions for a rupture process in the carbon chain of the rhenium complexes are investigated.