Ab initio g-tensor calculations of hydrogen bond effects on a nitroxide spin label

Hydrogen bonding effects on the electron paramagnetic resonance (EPR) g-tensor of a nitroxide spin label was investigated by quantum chemical calculations. The restricted open-shell Hartree–Fock (ROHF) linear response method with the atomic mean field approximation (AMFI) was used in the calculations. The results show that hydrogen bonding reduces the g-tensor component directed along the NO bond, gxx. This decrease is traced to higher excitation energy and lower spin–orbit coupling and angular momentum matrix elements for the n–π* excitation. The calculations show that the g-tensor is practically invariable when hydrogen bonding was modeled with methanol instead of water.