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 (M2+ = Ca2+ , Sr2+ , Ba2+ , and Mn2+), solvated with an H2O or a CH3OH molecule: M2+ center dot B15C5 center dot H2O, M2+ center dot B15C5 center dot CH3OH, M2+ center dot B18C6 center dot H2O, and M(2+) center dot B18C6 center dot CH3OH. All the species show a number of sharp vibronic bands in the 36600-37600 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 similar to 3610 and similar to 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 similar to 3630 cm(-1). The H2O and the CH3OH components are directly bonded to the M2+ ion through the M2+ 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 Ca2+ , Sr2+ , and Mn2+, 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 Ba2+ the number of conformers with B18C6 (1 or 2) is almost the same as that with B15C5. This is probably because the Ba2+ ion is too large to be located in the cavity center of either B15C5 and B18C6, which provides an open site at the Ba2+ 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.