A sum rule for Inelastic Electron Tunneling Spectroscopy: an ab initio study of a donor (TTF) and acceptors (TCNE, TCNQ and DCNQI) parallelly oriented on Cu(100).
We performed first-principles simulations of Inelastic Electron Tunneling Spectroscopy (IETS) for horizontally lying individual molecules that form popular donor-acceptor pairs (the TTF donor and its possible partner acceptors TCNE, TCNQ and DCNQI) on Cu(100). We find that the highest frequency C-H stretching modes are highly active for the (electron-rich) donor molecule but inactive for the (electron-poor) acceptors. We explain this contrasting response by the spatial extension of sp(3) rehybridization upon adsorption: the donor molecule entirely deforms into sp(3) while the acceptors rehybridize only at their outer ends leaving the central spacer unaffected. The sp(3)-induced buckling permits in-plane vibration modes to overlap with the π-type tunneling states and hence to be detected in IETS. In addition, the IET-spectra of a family of cyano-group acceptors, TCNE, TCNQ and DCNQI, show a recurring pattern of signals from vibrations involving their common CN outer ends plus a set of compound-dependent signals arising from the spacing moiety. The IET-response of individual chemical groups thus adds up for these flat-lying acceptor molecules, evidencing a sum rule that may facilitate their identification.