Conformational study and vibrational spectra of 18-crown-6 and its complexes with some metals

This study is conducted & Submitted in Partial Fulfillment of the Doctor of Philosophy Degree Department of Chemistry - College of Science King Saud University November 2006 G; Shawal 1427 H.The main goal of this thesis is to investigate the structure and the vibrational spectra of free 18-crown-6 (18c6) and its alkali metal cation complexes using accurate and more reliable experimental and theoretical methods than was used in previous studies. This study consists of two parts. The first part is the search of the possible conformations of free 18c6 and its alkali metal cation complexes and some specific conformations of alkaline earth metal cation complexes. The second part is the study of the vibrational spectra of 18c6 and its alkali metal cation complexes. The conformation in which 18c6 and its alkali metal cation complexes exists in is determined through the comparison between the experimental and calculated vibrational spectra. In the first part, an extensive conformational search was performed using the CONFLEX algorithm at the MM3 level, the number of 18c6 predicted conformations was 3136 conformations. The predicted conformations were geometry optimized at different levels of ab initio methods. Excluding the redundant conformations, optimized geometries and energies were calculated at the MP2 correlated level, the number of conformations was reduced to 47 unique energy conformations. The study concluded that the correlated MP2/6-31+G*//HF/6-31+G* level is the lowest reliable level for the accurate prediction of the conformational energy order. At all correlated levels the S6 conformations, was predicted to be more stable than the experimentally known Ci conformation of 18c6. The S6 conformation is calculated to be more stable by 1.84 kcal/mol than the Ci conformation at the MP2/6-31+G* level. It was rationalized that the S6 conformation is the lowest energy conformation of 18c6 since it has the largest number of CH…O interactions. The effect of hydrogen bonding on the relative energy order of certain conformations is not clear, especially in relation to the dihedral angles. For the 18c6 alkali metal cation complexes, optimized geometries were calculated for the 56 lowest energy conformations of the 18c6 at the HF/STO-3G level with the alkali metal cations positioned at the center of the 18c6 ring plane, in addition, at 3 Ǻ above the ring plane. Optimized geometries and energies of the lowest predicted conformations were determined at the correlated levels, MP2/6-31+G*//HF/6- 31+G*, MP2/6-31+G*//B3LYP/6-31+G*. It is concluded that the lowest energy conformation of the 18c6—Li+ complex is the D2 conformation, for the 18c6—Na+ and 18c6—K+ complexes it is the D3d conformation. For the larger metal cation 18c6—Rb+ and 18c6—Cs+ complexes, the C3v conformation is predicted to be the lowest energy conformation, rather than the D3d conformation. This is attributed to the large size of the Rb+ and Cs+ cations compared to the 18c6 ring cavity displacing both cations out of the ring plane producing a C3v conformation. For the 18c6—K+ complex, the metal cation fits well the 18c6 ring cavity and a D3d conformation is produced. For the 18c6—Na+ complex a folded D3d conformation is obtained since the Na+ cation is slightly smaller than the 18c6 ring cavity. For the 18c6—alkaline earth metal cation complexes assuming only the D3d and C3v conformations. At MP2/6-31+G*//B3LYP/6-31+G* level, the 18c6—Mg2+, 18c6— Ca2+ and 18c6—Sr2+ complexes are predicted to adopt the D3d conformation. The larger alkaline earth metal cation the 18c6—Ba2+ and 18c6—Ra2+ complexes are predicted to adopt the C3v conformation. The smaller cations of the alkali and alkaline earth metal cation complexes were found to bind 18c6 more strongly than the larger cations, and the divalent alkaline earth metal cations bind more strongly to 18c6 than alkali metal cations at all levels of calculations. This is attributed to the larger charge on the former than on the later. The second part of the thesis is the experimental and theoretical study of the vibrational spectra of 18c6 and its alkali metal cation complexes. FT-IR and FTRaman spectra were measured for the free 18c6 and its alkali metal cation complexes. Cartesian coordinate force fields were calculated at the corresponding optimized geometries at the B3LYP/6-31+G* for the Ci, S6, C3, D3d and C2 conformations of the free 18c6 and all of the predicted conformations of its alkali metal cation complexes of symmetries higher than the C1 symmetry. A set of scale factors was used to scale the force fields. The experimental vibrational frequencies were assigned to the calculated frequencies and the scale was varied to minimize the difference between the calculated and experimental vibrational frequencies. The assignment of the fundamental vibrational frequencies of 18c6 and its alkali metal cation complexes depend on highly accurately B3LYP force fields. In addition, for the first time to the best of our knowledge, the assignment of the fundamental vibrational frequencies of 18c6 is being performed using the CCl4 and CS2 solution phase. While that of its alkali metal cation complexes of mainly of the methanol solution phase. The experimental and theoretical study of the vibrational spectra indicated that the free 18c6 exists in the Ci conformation. For the 18c6—alkali metal cation complexes, The 18c6—Na+ and K+ complexes exist in the D3d conformation and the 18c6—Rb+ and Cs+ complexes exist in the C3v conformation. These findings are in agreement with the conformational search and with the previous X—ray results. The vibrational spectra of the 18c6—Li+ complex, is different from the other four 18c6—alkali metal cation complexes. It was not possible to predict in which conformation this complex exists in.1. King Abdul Aziz City for Science and Technology (KACST). 2. Research Center, King Saud University