  We develop a series of propensity rules for interpreting Inelastic Electron
Tunneling (IET) spectra of single-molecule transport junctions. IETS has no
selection rules, such as those seen in optical, infrared and Raman spectra,
because IETS features arise not from the field-dipole interaction
characterizing these other spectroscopies, but from vibronic modification of
the electronic levels. Expansion of the Landauer-Imry formula in Taylor series
in molecular normal coordinates gives a convenient, accurate perturbation-type
formula for calculating both frequency and intensity of the IETS spectrum.
Expansion in a Dyson-like form permits derivation of propensity rules, both
symmetry-based and pathway-deduced, allowing correlation of structure and
coupling geometry with the IETS spectrum. These propensity rules work very well
for the calculated spectrum of four typical molecular bridges.
