  Based on density-functional theory calculations, we report a detailed study
of the single-molecule charge-transport properties for a series of recently
synthesized biphenyl-dithiol molecules [D. Vonlanthen et al., Angew. Chem.,
Int. Ed. 48, 8886 (2009); A. Mishchenko et al., Nano Lett. 10, 156 (2010)]. The
torsion angle {\phi} between the two phenyl rings, and hence the degree of
{\pi} conjugation, is controlled by alkyl chains and methyl side groups. We
consider three different coordination geometries, namely top-top,
bridge-bridge, and hollow-hollow with the terminal sulfur atoms bound to one,
two, and three gold surface atoms, respectively. Our calculations show that
different coordination geometries give rise to conductances which vary by one
order of magnitude for the same molecule. Irrespective of the coordination
geometries, the charge transport calculations predict a cos^{2}{\phi}
dependence of the conductance, which is confirmed by our experimental
measurements. We observe that the calculated transmission through biphenyl
dithiols is typically dominated by a single transmission eigenchannel formed
from {\pi} electrons. Only for a single molecule with a completely broken
conjugation we find a perfect channel degeneracy for the hollow-hollow-type
contact in our theory.
