The dihydrogen radical cation H2+:: its EPR spectrum

A new interpretation of the EPR spectrum of irradiated solid hydrogen at 4 K is proposed; we suggest that the radical centre is H-2(+) rather than H-2(-) as previously suggested. The g-value of 2.0033, precisely given by the very narrow 0,0 feature, is greater than the freespin g-value, whereas, for H-2(-) it should be less. On the other hand this value is reasonable for H-2(+) in solid hydrogen. We have performed electronic structure calculations for a range of cluster structures to simulate the local environment of both radicals in a hydrogen matrix. No stable structure is found for the anion clusters, whereas H-2(+) remains strongly bonding with extensive delocalization onto neighbouring H-2 molecules. The extent of delocalization (ca. 30%) agrees well with expectation.