ELECTRON-SPIN-RESONANCE AND GROUND-STATES OF THE BORON AND ALUMINUM TRIMERS

Electron-spin-resonance (ESR) spectra of B-11(3)(i = 3/2) in neon, argon, and krypton, B-10(3)(i = 3) in argon, and Al-27(3)(i = 5/2) in argon, krypton, and methane matrices at 3-30 K indicate that both trimers contain one unpaired electron (S = 1/2) and three equivalent nuclei (D3h symmetry). The derived hyperfine (hf) parameters for B-11(3) in argon are isotropic \A parallel-to\ = \A(l)\ = 130 MHz, yielding about 15% s character in the wave function of the unpaired spin. Rotation of B3 in the matrices, causing spin relaxation and consequent linewidth variation as a function of M(I), also causes the anisotropic hf contributions to average to zero. Some hf anisotropy was observed for Al-27(3) in solid argon \A parallel-to\ = 109, \A(l)\ = 100 MHz and varied with the matrix gas, indicating some hindering of free rotation. The derived s character for Al3 in its ground state is approximately 8%. It is concluded that both trimers have 2A1' ground states, in agreement with ab initio theory and, in the case of Al3, with the Stern-Gerlach experiment of Cox et al.