Under the terms of the Creative Commons Attribution (CC BY) license to their work.-- et al. ; We report on the optomechanical properties of a breathing mechanical mode oscillating at 5.5 GHz in a 1D corrugated Si nanobeam. This mode has an experimental single-particle optomechanical coupling rate of |g o, OM | = 1.8 MHz (|g o, OM |/2π = 0.3 MHz) and shows strong dynamical back-action effects at room temperature. The geometrical flexibility of the unit-cell would lend itself to further engineering of the cavity region to localize the mode within the full phononic band-gap present at 4 GHz while keeping high g o, OM values. This would lead to longer lifetimes at cryogenic temperatures, due to the suppression of acoustic leakage. ; This work was supported by the EU through the FP7 project TAILPHOX (ICT-FP7-233883) and the ERC Advanced Grant SOULMAN (ERC-FP7-321122) and the Spanish projects TAPHOR (MAT2012-31392). D.N-U and J.G-B acknowledge support in the form of postdoctoral fellowships from the Catalan (Beatriu de Pinòs) and the Spanish (Juan de la Cierva) governments, respectively. ; Peer Reviewed