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The surface-enhanced Raman scattering (SERS) spectroscopy and inelastic electron tunneling spectroscopy (IETS) are employed to study trans-1,2-bis (4-pyridyl) ethylene (BPE)/gold system. Both junction and complex forms are considered for the SERS simulations. It is predicted that the peak at 1581 cm - 1 is more intense in the junction forms than that in the complex forms. Time dependent density functional theory calculations show that the relative intensity is mainly controlled by the excitation energy derivative respect to the normal modes, and the total intensity is governed by the excitation energy of the excited states. The C H bending modes dominate the IET spectra when BPE adsorbed on the flat gold surfaces. While, the pyridyl ring deformation modes are more active when BPE adsorbed on the edge of the gold clusters. For BPE adsorbed on the tip of gold clusters, the pyridyl ring and C C stretching modes show significant contribution to the IET spectra. [ABSTRACT FROM AUTHOR]

• Rise in inelastic transmission on changing the molecule-electrode coupling strength. • Relative displacements of the encapsulated metal also plays very prominent role. • In the present study electrode-molecule separation has been varied from 1.6 to 2.2 Å. • Inelastic transmission in strong coupling case is 100 times of weak coupling case. • It is also notable that in stronger coupling regime phonon mode numbered 5 contributes to current in μA range and relevant excitation takes place at much lower energy which is obviously in complete contrast with weakly coupled system; where higher energy phonon modes numbered 50–56 have chance to contribute current in μA range. Hence, in respect of low energy inelastic transmission strongly coupled system is preferable. • Larger packing of in-phase tunneling electron within strongly coupled Hf@C 28. Herein, we report how tuning of molecule-electrode coupling strength can turn on the coupling of tunnelling electrons with vibrational modes of central molecule in the two-probe configuration of Hf@C28 self-assembled between two Au(1 1 1) electrodes and thereby activating inelastic transmission path. The in silico study compares quantum transport properties of optimized configuration having C-Au bond length of 2.01 Å (weakly-bound), with strongly-bound configuration having C-Au bond length of 1.6 Å. It is observed that in weak-coupling the inelastic transmission coefficient is insignificant, however with strong-coupling symmetric and asymmetric part of inelastic transmission together contributes to rise in inelastic current. This rise in inelastic current is attributed to the resonance of phononic vibration corresponding to low-energy phonon-mode and energy of tunnelling electron close to applied bias 0.15–0.25 V. Observed results have been substantiated through an analysis of high density in-phase transmission pathways within the molecular part mediated by resonating electrons and phonon modes. [ABSTRACT FROM AUTHOR]