Electron spin resonance and theoretical studies of the N-14 center dot center dot center dot center dot N-14 and N-15 center dot center dot center dot center dot N-15 spin-pair radicals in neon matrices: The effects of mixing among the (1)Sigma(+)(g), (3)Sigma(+)(u), (5)Sigma(+)(g), and (7)Sigma(+)(u) electronic states

The first experimental and theoretical study of the N .... N spin-pair radi cal 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-pa ir. To fully interpret the N .... N radical results it was necessary to inc lude electronic state mixing effects among all possible spin states, namely the (1)Sigma(g)(+), (3)Sigma(u)(+), (5)Sigma(g)(+), and (7)Sigma(u)(+) sta tes. Several different trapping sites were observed which indicated the int eraction of N atoms at distinctly different separation distances in the neo n lattice. Calculated J values at the complete active space self-consistent field (CASSCF) level (TZP basis set) were compared with the experimental r esults for the various trapping site distances. The N-15 ....N-15 radical i n the dominant trapping site had magnetic parameters of g=2.0016(2), A(N-15 )=15.9(1) MHz, D=-178(1)MHz and J=468(2) MHz. Using the point dipole approx imation this corresponds to a N .... N separation distance of 6.41 Angstrom . A most unusual type of magnetic dipole transition was observed that invol ves a transition between electronic states of formally different S values w here S is the total spin quantum number for a given electronic state. (C) 1 999 American Institute of Physics. [S0021-9606(99)00130-0].