  Using a first principles approach combined with analysis of heuristic
tight-binding models, we examine the connectivity dependence of two forms of
quantum interference in single molecules. Based on general arguments, Fano
resonances are shown to be insensitive to connectivity, while Mach-Zehnder-type
interference features are shown to be connectivity dependent. This behaviour is
found to occur in molecular wires containing anthraquinone units, in which the
pendant carbonyl groups create Fano resonances, which coexist with
multiple-path quantum interference features.
