Microhydration of Dibenzo-18-Crown-6 Complexes with K+, Rb+, and Cs+ Investigated by Cold UV and IR Spectroscopy in the Gas Phase

In this Article, we examine the hydration structure of dibenzo-18-crown-6 (DB18C6) complexes with K+, Rb+, and Cs+ ion in the gas phase. We measure well-resolved UV photodissociation (UVPD) spectra of K+·DB18C6·(H2O)n, Rb+·DB18C6·(H2O)n, and Cs+·DB18C6·(H2O)n (n = 1–8) complexes in a cold, 22-pole ion trap. We also measure IR-UV double-resonance spectra of the Rb+·DB18C6·(H2O)1–5 and the Cs+·DB18C6·(H2O)3 complexes. The structure of the hydrated complexes is determined or tentatively proposed on the basis of the UV and IR spectra with the aid of quantum chemical calculations. Bare complexes (K+·DB18C6, Rb+·DB18C6, and Cs+·DB18C6) have a similar boat-type conformation, but the distance between the metal ions and the DB18C6 cavity increases with increasing ion size from K+ to Cs+. Although the structural difference of the bare complexes is small, it highly affects the manner in which each is hydrated. For the hydrated K+·DB18C6 complexes, water molecules bind on both sides (top and bottom) of the boat-type K+·DB18C6 conformer, while hydration occurs only on top of the Rb+·DB18C6 and Cs+·DB18C6 complexes. On the basis of our analysis of the hydration manner of the gas-phase complexes, we propose that, for Rb+·DB18C6 and Cs+·DB18C6 complexes in aqueous solution, water molecules will preferentially bind on top of the boat conformers because of the displaced position of the metal ions relative to DB18C6. In contrast, the K+·DB18C6 complex can accept H2O molecules on both sides of the boat conformation. We also propose that the characteristic solvation manner of the K+·DB18C6 complex will contribute entropically to its high stability and thus to preferential capture of K+ ion by DB18C6 in solution.

Microhydration Effects on the Encapsulation of Potassium Ion by Dibenzo-18-Crown-6

Yoshiya Inokuchi, Thomas Rizzo

We have measured electronic and conformer-specific vibrational spectra of hydrated dibenzo-18-crown-6 (DB18C6) complexes with potassium ion, K+center dot DB18C6 center dot(H2O)(n) (n = 1-5), in a cold, 22-pole ion trap. We also present for comparison spectra of Rb+center dot DB18C6 center dot(H2O)(3) and Cs+center dot DB18C6 center dot(H2O)(3) complexes. We determine the number and the structure of conformers by analyzing the spectra with the aid of quantum chemical calculations. The K+center dot DB18C6 center dot(H2O)(1) complex has only one conformer under the conditions of our experiment. For K+center dot DB18C6 center dot(H2O)(n) with n = 2 and 3, there are at least two conformers even under the cold conditions, whereas Rb+center dot DB18C6 center dot(H2O)(3) and Cs+center dot DB18C6 center dot(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; because 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+center dot DB18C6 complex, giving multiple conformations in the experiment. For K+center dot DB18C6 center dot(H2O)(n) (n = 4 and 5) complexes, only a single isomer is found. This is attributed to a cooperative effect of the H2O molecules on the hydration of K+center dot DB18C6; the H2O molecules form a ring, which is bound on top of the K+center dot DB18C6 complex. According to the stable structure determined in this study, the K+ ion in the K+center dot DB18C6 center dot(H2O)(n) complexes tends to be pulled largely out from the crown cavity by the H2O molecules with increasing n. Multiple conformations observed for the K+ complexes will have an advantage for the effective capture of the K+ ion over the other alkali metal ions by DB18C6 because of entropic effects on the formation of hydrated complexes.

American Chemical Society2014

Ion Selectivity of Crown Ethers Investigated by UV and IR Spectroscopy in a Cold Ion Trap

Yoshiya Inokuchi, Thomas Rizzo

Electronic and vibrational spectra of benzo-15-crown-5 (B15C5) and benzo-18-crown-6 (B18C6) complexes with alkali metal ions, M+•B15C5 and M+•B18C6 (M = Li, Na, K, Rb and Cs), are measured using UV photodissociation (UVPD) and IR-UV double resonance spectroscopy in a cold, 22-pole ion trap. We determine the structure of conformers with the aid of density functional theory calculations. In the Na+•B15C5 and K+•B18C6 complexes, the crown ethers open the most and hold the metal ions at the center of the ether ring, demonstrating the optimum matching in size between the cavity of the crown ethers and the metal ions. For smaller ions, the crown ethers deform the ether ring to decrease the distance and increase the interaction between the metal ions and oxygen atoms; the metal ions are completely surrounded by the ether ring. In the case of larger ions, the metal ions are too large to enter the crown cavity and are positioned on it, leaving one of its sides open for further solvation. Thermochemistry data calculated on the basis of the stable conformers of the complexes suggest that the ion selectivity of crown ethers is controlled primarily by the enthalpy change for the complex formation in solution, which depends strongly on the complex structure.

American Chemical Society2012

UV and IR Spectroscopy of Cold H2O+-Benzo-Crown Ether Complexes

Yoshiya Inokuchi, Thomas Rizzo

The H2O+ radical ion, produced in an electrospray ion source via charge transfer from Eu3+, is encapsulated in benzo-15-crown-5 (B15C5) or benzo-18-crown-6 (B18C6). We measure UV photodissociation (UVPD) spectra of the (H2O·B15C5)+ and (H2O·B18C6)+ complexes in a cold, 22-pole ion trap. These complexes show sharp vibronic bands in the 35 700–37 600 cm–1 region, similar to the case of neutral B15C5 or B18C6. These results indicate that the positive charge in the complexes is localized on H2O, giving the forms H2O+·B15C5 and H2O+·B18C6, in spite of the fact that the ionization energy of B15C5 and B18C6 is lower than that of H2O. The formation of the H2O+ complexes and the suppression of the H3O+ production through the reaction of H2O+ and H2O can be attributed to the encapsulation of hydrated Eu3+ clusters by B15C5 and B18C6. On the contrary, the main fragment ions subsequent to the UV excitation of these complexes are B15C5+ and B18C6+ radical ions; the charge transfer occurs from H2O+ to B15C5 and B18C6 after the UV excitation. The position of the band origin for the H2O+·B18C6 complex (36323 cm–1) is almost the same as that for Rb+·B18C6 (36315 cm–1); the strength of the intermolecular interaction of H2O+ with B18C6 is similar to that of Rb+. The spectral features of the H2O+·B15C5 complex also resemble those of the Rb+·B15C5 ion. We measure IR–UV spectra of these complexes in the CH and OH stretching region. Four conformers are found for the H2O+·B15C5 complex, but there is one dominant form for the H2O+·B18C6 ion. This study demonstrates the production of radical ions by charge transfer from multivalent metal ions, their encapsulation by host molecules, and separate detection of their conformers by cold UV spectroscopy in the gas phase.

American Chemical Society2015

Microhydration Effects on the Encapsulation of Potassium Ion by Dibenzo-18-Crown-6

Yoshiya Inokuchi, Thomas Rizzo

American Chemical Society2014