ESR and ab initio theoretical studies of the cation radicals\u3csup\u3e14\u3c/sup\u3eN\u3csup\u3e+\u3c/sup\u3e\u3csub\u3e4\u3c/sub\u3e and \u3csup\u3e15\u3c/sup\u3eN\u3csup\u3e+\u3c/sup\u3e\u3csub\u3e4 \u3c/sub\u3eThe trapping of ion-neutral reaction products in neon matrices at 4 K

The 1 4N+ 4 and 1 5N+ 4 molecular cation radicals have been generated by the ion-neutral reaction N+ 2 +N2 and isolated in solid neon matrices at 4 K for detailed ESR(electron spin resonance) investigation. Both photonionization at 16.8 eV and electron bombardment (50 eV) were used in conjunction with the neon matrix trapping technique to produce the N+ 4 dimer cation. The ESR results clearly show that N+ 4 is linear and has a 2Σμground electronic state. The magnetic parameters in neon are: g -ˆ¥=2.0016(4) and g -Š¥=1.9998(2); A -ˆ¥(1 4N)=311(1) MHz and A -Š¥(1 4N)=264(1) MHz for the central atoms and -€-A -ˆ¥-€-=10.4(5) MHz and -€-A -Š¥-€-=20.4(1) MHz for the outer or terminal 1 4N atoms. Electronic structure information for N+ 4 was obtained from the ESR results and compared with a b i n i t i o CI calculations. The unpaired electron resides primarily on the inner nitrogen atoms with significant 2p σ and 2s character. Orbital characters obtained from the commonly applied free atom comparison method (FACM) were compared with the results of a Mulliken type spin population analysis conducted on the calculated wave function. The calculated nuclear hyperfine parameters (Atensors) showed reasonable agreement with experiment except for the very small A iso parameter for the outer nitrogen atoms. Benchmark calculations employing large basis sets were conducted for the free nitrogen atom; these efforts demonstrate the difficulty in computing the A iso parameter when inner shell effects are important