The extreme quantum nature of muons and muonium in condensed matter is the result of having a mass that is nine times lighter than that of a hydrogen atom. While this quantum nature may often be masked by thermal excitations, at low temperatures its dramatic effects can become more apparent. This was clearly demonstrated in solid state benzene ALC spectra below 120 K, where we have observed both intramolecular and intermolecular multisite quantum tunnelling of radical states. In this proposal we aim to use molecular engineering to control the sites available to form quantum tunneling states and we will use low temperature solid state ALC spectroscopy to confirm the effect of the different modifications we have made to our reference molecule. The modifications will take the form of selective substitutions of the benzene molecule and four different examples will be studied.