1. Department of Chemistry, Soong Sil University, Sang Do 5 Dong 1-1, Dong Jak Gu, Seoul 156-743, Korea
2. Department of Chemistry, Chungbuk National University, Cheongju, Chungbuk 361-763, Korea
3. Department of Chemistry, Korea Advanced Institute of Science and Technology, 373-1 Kusung-dong Yusung-gu, Taejon 305-701, Korea
4. Baker Laboratory of Chemistry, Cornell University, Ithaca, New York 14853-1301