Al-Rusaese S, Al-Kahtani A A, El-Azhary A A
Chemistry Department, Faculty of Science, King Saud University, P.O. Box 2455, Riyadh 11451, Kingdom of Saudi Arabia.
J Phys Chem A. 2006 Jul 20;110(28):8676-87. doi: 10.1021/jp062654l.
The vibrational, Raman, and IR, spectra of the five 12-crown-4 (12c4) complexes with Li+, Na+, K+, Rb+, and Cs+ alkali metal cations were measured. Except for a small shift of the position of some bands in the vibrational spectra of the Li+ complex, the vibrational spectra of the five complexes are so similar that it is concluded that the five complexes exist in the same conformation. B3LYP/6-31+G* force fields were calculated for six of the eight predicted conformations in a previous report (J. Phys. Chem. A 2005, 109, 8041) of the 12c4-Li+, Na+, and K+ complexes that are of symmetries higher than the C1 symmetry. These six conformations, in energy order, are of C4, Cs, Cs, C(2v), C(2v), and Cs symmetries. Comparison between the experimental and calculated vibrational frequencies assuming any of the above-mentioned six conformations shows that the five complexes exist in the C4 conformation. This agrees with the fact that the five alkali metal cations are larger than the 12c4 ring cavity. The B3LYP/6-31+G* force fields of the C4 conformation of the Li+, Na+ and K+ complexes were scaled using a set of eight scale factors and the scale factors were varied so as to minimize the difference between the calculated and experimental vibrational frequencies. The root-mean-square (rms) deviations of the calculated frequencies from the experimental frequencies were 7.7, 5.6, and 5.1 cm(-1) for the Li+, Na+, and K+ complexes, respectively. To account for the earlier results of the Li+ complex that the Cs conformation is more stable than the C4 conformation by 0.16 kcal/mol at the MP2/6-31+G* level, optimized geometries of the complex were calculated for the C4 and Cs conformations at the MP2/6-311++G** level. The C4 conformation was calculated to be more stable than the Cs conformation by 0.13 kcal/mol.
测量了五种含有Li⁺、Na⁺、K⁺、Rb⁺和Cs⁺碱金属阳离子的12-冠-4（12c4）配合物的振动光谱、拉曼光谱和红外光谱。除了Li⁺配合物振动光谱中某些谱带位置有小的位移外，这五种配合物的振动光谱非常相似，因此得出结论，这五种配合物以相同的构象存在。针对先前报告（《物理化学杂志A》2005年，第109卷，8041页）中预测的八种构象中的六种，计算了12c4-Li⁺、Na⁺和K⁺配合物的B3LYP/6-31+G力场，这些构象的对称性高于C1对称性。这六种构象按能量顺序分别为C4、Cs、Cs、C(2v)、C(2v)和Cs对称性。假设上述六种构象中的任何一种，对实验和计算的振动频率进行比较表明，这五种配合物以C4构象存在。这与五个碱金属阳离子大于12c4环腔这一事实相符。使用一组八个比例因子对Li⁺、Na⁺和K⁺配合物C4构象的B3LYP/6-31+G力场进行缩放，并改变比例因子，以使计算和实验振动频率之间的差异最小化。Li⁺、Na⁺和K⁺配合物计算频率与实验频率的均方根（rms）偏差分别为7.7、5.6和5.1 cm⁻¹。为了解释Li⁺配合物早期的结果，即在MP2/6-31+G*水平下Cs构象比C4构象稳定0.16 kcal/mol，在MP2/6-311++G**水平下计算了该配合物C4和Cs构象的优化几何结构。计算得出C4构象比Cs构象稳定0.13 kcal/mol。
