The H3+ + H2 isotopic system : origin of deuterium astrochemistry Dense cold molecular clouds reckoned to be stellar nurseries are the scene of an extreme molecular deuteration. Despite the cosmic D H ratio of 10^{ 5} molecular species in prestellar cores are observed to contain nearly as much deuterium as hydrogen. This astonishing deuterium enrichment promoted by low temperatures is the work of H3+. It is the key species which unlocks the deuterium from its HD reservoir via reactions like H3+ + HD > H2D+ + H_2 and drags it further to other species in successive reactions. For this reason the H3+ + H2 isotopic system is outstandingly critical for the astrochemistry of cold environments. However its understanding is yet incomplete and insufficient. This thesis thus focuses on the H3+ + H2 isotopic system from a theoretical experimental and astronomical point of view giving a particular look into the role of nuclear spins ...