1. Department of Chemistry, Wake Forest University, Winston-Salem, North Carolina 27109-7486, RMN et Chimie du Solide, Université Louis Pasteur, 4, rue Blaise Pascal, 67070 Strasbourg, France, and Center of Interdisciplinary Magnetic Resonance, National High Magnetic Field Laboratory, 1800 East Paul Dirac Drive, Tallahassee, Florida 32310