Accessing the terahertz (THz) spectral domain through surface-enhanced Raman spectroscopy (SERS) is challenging and opens up the study of low-frequency molecular and electronic excitations. Compared to direct THz probing of heterogenous ensembles, the extreme plasmonic confinement of visible light to deep sub-wavelength scales allows the study of hundreds or even single molecules. We show that self-assembled molecular monolayers of a set of simple aromatic thiols confined inside single-particle plasmonic nanocavities can be distinguished by their low-wavenumber spectral peaks below 200 cm−1, after removal of a bosonic inelastic contribution and an exponential background from the spectrum. Developing environment-dependent density-functional-theory simulations of the metal-molecule configuration enables the assignment and classification of their THz vibrations as well as the identification of intermolecular coupling effects and of the influence of the gold surface configuration. Furthermore, we show dramatically narrower THz SERS spectra from individual molecules at picocavities, which indicates the possibility to study intrinsic vibrational properties beyond inhomogeneous broadening, further supporting the key role of local environmentJ.J.B. acknowledges support from the European Research Council (ERC) under Horizon 2020 research and innovation programme PICOFORCE (883703) and POSEIDON (861950) and from the UK EPSRC (Cambridge NanoDTC EP/L015978/1, EP/L027151/1, EP/S022953/1, EP/P029426/1, and EP/R020965/1. R.A.B., R.E. and J.A. acknowledge support from PID2019-107432 GB-I00 and PID2022-139579NB-I00 funded by MCIN/AEI /10.13039/501100011033/ and by \u201CERDF A way of making Europe\u201D grant IT 1526-22 from the Basque Government for consolidated groups of the Basque University, and Elkartek project u4Smart of the Department of Economy Development of the Basque Country. BdN acknowledges support from the Leverhulme Trust and Isaac Newton Trust in the form of an ECF. F.A.G. acknowledges funding from PID2022-138470NB-I00 funded by MCIN/AEI, and computing time from Red Espa\u00F1ola de Supercomputacion through the project QHS-2021-2-0019 and Universidad Aut\u00F3noma de Madrid for funding a research stay with the \u201CRecualificaci\u00F3n\u201D programme (CA5/RSUE/2022-00234
