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

Inokuchi Yoshiya

Ebata Takayuki

Rizzo Thomas R.

Journal of Physical Chemistry A

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+•B15C5•H2O, M2+•B15C5•CH3OH, M2+•B18C6•H2O, and M2+•B18C6•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 ~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 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 non-zero 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.

alkaline earth

transition metal

18-crown-6

15-crown-5

encapsulation

water

crown ether

infrared

ion trap

electrospray

solvent effect

ultraviolet

conformation

Chemistry [ 430 ]

eng

American Chemical Society

Copyright (c) 2015 American Chemical Society

This document is the Accepted Manuscript version of a Published Work that appeared in final form in Journal of Physical Chemistry A, copyright © American Chemical Society after peer review and technical editing by the publisher. To access the final edited and published work see https://doi.org/10.1021/acs.jpca.5b04450.

[ISSN] 1089-5639

[ISSN] 1520-5215

[DOI] 10.1021/acs.jpca.5b04450

[DOI] https://doi.org/10.1021/acs.jpca.5b04450