= Solution
When $|eV|$ reaches a vibrational quantum, a tunnelling <electron> can lose energy by exciting that vibration. Opening an inelastic channel changes $dI/dV$ at both voltage polarities. If the conductance increases in each direction, $d^2I/dV^2$ has a positive feature at positive threshold and a negative feature at the corresponding negative threshold. Approximate antisymmetry follows from approximate symmetry of the conductance; unequal tip and sample electronic structures can make amplitudes unequal.
The three energy scales are \b[8, 48 and 375 meV], corresponding to about \b[65, 387 and 3025 cm$^{-1}$]. A plausible assignment is a low-energy frustrated lateral translation or rotation, an <adsorbate>–metal stretch or hindered molecular mode at 48 meV, and a C–H stretching vibration at 375 meV. The terminal <carbon> of methyl isocyanide supplies a plausible <adsorption> contact, but exact mode assignment requires calculated <normal modes> or independent vibrational measurements. In particular the 48 meV band should not be labelled a C–N stretching vibration simply because the molecule contains that bond: its <frequency> is far too low for the usual internal stretching mode.
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