  Asymmetric line shapes can occur in the transmission function describing
electron transport in the vicinity of a minimum caused by quantum interference
effects. Such asymmetry can be used to increase the thermoelectric efficiency
of molecular junctions. So far, however, asymmetric line shapes have been only
empirically found for just a few rather complex organic molecules where the
origins of the line shapes relation to molecular structure were not resolved.
In the present work we introduce a method to analyze the structure dependence
of the asymmetry of interference dips from simple two site tight-binding
models, where one site corresponds to a molecular $\pi$ orbital of the wire and
the other to an atomic $p_z$ orbital of a side group, which allows us to
analytically characterize the peak shape in terms of just two parameters. We
assess our scheme with first-principles electron transport calculations for a
variety of {\it t-stub} molecules and also address their suitability for
thermoelectric applications.
