Electron spin resonance and theoretical studies of the ¹⁴$N⋅⋅⋅⋅14N$ and ¹⁵$N⋅⋅⋅⋅15N$ spin-pair radicals in neon matrices: The effects of mixing among the ¹$Σg+,$ ³$Σu+,$ ⁵$Σg+,$ and ⁷$Σu+$ electronic states

The first experimental and theoretical study of the N⋅⋅⋅⋅N spin-pair radical is reported. Its high-resolution ESR (electron spin resonance) spectrum has been observed in neon matrices and interpreted on the basis of weakly interacting atoms using a model recently developed for the H⋅⋅⋅⋅H spin-pair. To fully interpret the N⋅⋅⋅⋅N radical results it was necessary to include electronic state mixing effects among all possible spin states, namely the ¹$Σg+,$ ³$Σu+,$ ⁵$Σg+,$ and ⁷$Σu+$ states. Several different trapping sites were observed which indicated the interaction of N atoms at distinctly different separation distances in the neon lattice. Calculated J values at the complete active space self-consistent field (CASSCF) level (TZP basis set) were compared with the experimental results for the various trapping site distances. The ¹⁵$N⋅⋅⋅⋅15N$ radical in the dominant trapping site had magnetic parameters of $g=2.0016(2),$ $A(15N)=15.9(1) MHz,$ $D=−178(1)MHz$ and $J=468(2) MHz.$ Using the point dipole approximation this corresponds to a N⋅⋅⋅⋅N separation distance of 6.41 Å. A most unusual type of magnetic dipole transition was observed that involves a transition between electronic states of formally different S values where S is the total spin quantum number for a given electronic state.