Solvent Effects on the Encapsulation of Divalent Ions by Benzo-18-Crown-6 and Benzo-15-Crown-5

We measure UV photodissociation (UVPD) spectra of cold benzo-15-crown-5 (B15C5) and benzo-18-crown-6 (B18C6) complexes with divalent ions (M(2+) = Ca(2+), Sr(2+), Ba(2+), and Mn(2+)), solvated with an H2O or a CH3OH molecule: M(2+)·B15C5·H2O, M(2+)·B15C5·CH3OH, M(2+)·B18C6·H2O, and M(2+)·B18C6·CH3OH. All the species show a number of sharp vibronic bands in the 36,600-37,600 cm(-1) region, which can be attributed to electronic transitions of the B18C6 or B15C5 component. Conformer-specific IR spectra of these complexes are also obtained by IR-UV double-resonance spectroscopy in the OH stretching region. All the IR-UV spectra of the H2O complexes show IR bands at ∼3610 and ∼3690 cm(-1); these bands can be assigned to the symmetric and asymmetric OH stretching vibrations of the H2O component. The CH3OH complexes also show the stretching vibration of the OH group at ∼3630 cm(-1). The H2O and the CH3OH components are directly bonded to the M(2+) ion through the M(2+)···O bond in all the complexes, but a small difference in the conformation results in a noticeable difference in the OH stretching frequency, which enables us to determine the number of conformers. For Ca(2+), Sr(2+), and Mn(2+), the number of conformers for the B18C6 complexes is in the range of 2-5, which is clearly larger than complexes with B15C5 (1 or 2). However, for Ba(2+) the number of conformers with B18C6 (1 or 2) is almost the same as that with B15C5. This is probably because the Ba(2+) ion is too large to be located in the cavity center of either B15C5 and B18C6, which provides an open site at the Ba(2+) ion suitable for solvation with H2O or CH3OH. The more conformations a complex can take, the more entropically favored it is at nonzero temperatures. Hence, the larger number of conformations suggests higher stability of the complexes under solvated conditions, leading to a higher degree of ion encapsulation in solution.