We have measured electronic and vibrational spectra of microsolvated crown ether complexes with metal ions under cold gas-phase conditions. The microsolvated complexes (M(CE)(L)n) are produced by an electrospray ion source, where M = K+, Rb+, Cs+, Ca2+, Sr2+, Ba2+, CE = dibenzo-18-crown-6 (DB18C6), benzo-18-crown-6 (B18C6), benzo-15-crown-5 (B15C5), L = H2O and CH3OH, and n = 1–8. The complexes are cooled to ~10 K in a cold, 22-pole ion trap and irradiated by an output of a UV laser. Ultraviolet photodissociation (UVPD) spectra of the complexes are obtained by plotting the yield of photofragment ions as a function of the UV wavenumer. Conformer-specific IR spectra of the complexes are obtained by IR-UV double-resonance spectroscopy. We determine the number and the structure of conformers by analyzing the UVPD and IR-UV spectra with the aid of quantum chemical calculations. For K+(DB18C6)(H2O)n with n = 2 and 3, there are at least two conformers even under the cold conditions, whereas Rb+(DB18C6)(H2O)3 and Cs+(DB18C6)(H2O)3 each exhibit only one isomer. The difference can be explained by the optimum matching in size between the K+ ion and the crown cavity; since the K+ ion can be deeply encapsulated by DB18C6 and the interaction between the K+ ion and the H2O molecules becomes weak, different kinds of hydration geometries can occur for the K+(DB18C6) complex, giving multiple conformations in the experiment. A similar trend in the number of conformers is found also for the microsolvated divalent-ion complexes. 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.

Microsolvation effects on the encapsulation of metal ions by crown ethers

2019 Fall National Meeting

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