UV Spectroscopic Studies of Cold Alkali Metal Ion–Crown Ether Complexes in the Gas Phase

We report UV photodissociation (UVPD) spectra of dibenzo-18-crown-6 (DB18C6) complexes with alkali metal ions (Li+, Na+, K+, Rb+, and Cs+) in a cold, 22-pole ion trap. All the complexes show a number of vibronically resolved bands, but the vibronic structure changes drastically in the series between Na+ and K+. The Li+ and Na+ complexes show extensive and intense activity of low-frequency vibrations, indicating that the β carbon atoms are largely displaced from the plane of the phenyl rings. For the K+, Rb+, and Cs+ complexes, such a long progression is not observed, suggesting that the β carbon atoms are almost in-plane of the phenyl rings. To hold Li+ and Na+ ions, DB18C6 shrinks the ether ring to fit the cavity size to the diameter of Li+ and Na+. In the K+, Rb+, and Cs+ complexes, the DB18C6 component opens the cavity the most by locating the β carbon atoms in the plane of the benzene rings, giving a boat-type open conformation. The K+ ion is effectively captured in the hole of the open conformer thanks to the optimum matching of the cavity size and ion diameter. The Rb+ and Cs+ ions are thought to be on the ether ring because the ion is too large to enter the cavity of the open conformer.

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

UV and IR spectroscopy of cold 1,2-dimethoxybenzene complexes with alkali metal ions

Yoshiya Inokuchi, Thomas Rizzo

We report UV photodissociation (UVPD) and IR-UV double-resonance spectra of 1,2-dimethoxybenzene (DMB) complexes with alkali metal ions, M+•DMB (M = Li, Na, K, Rb, and Cs), in a cold 22-pole ion trap. The UVPD spectrum of the Li+ complex shows a strong origin band. For the K+•DMB, Rb+•DMB, and Cs+•DMB complexes, the origin band is very weak and low-frequency progressions are much more extensive than that of the Li+ ion. In the case of the Na+•DMB complex, the spectral features are similar to those of the K+, Rb+, and Cs+ complexes, but the vibronic bands are not resolved. Geometry optimization with density functional theory indicates that the metal ions are bonded to the oxygen atoms in all the M+•DMB complexes calculated. In the Li+ and Na+ complexes in the S0 state, the metal ions are located almost in the same plane as the benzene ring, while the K+, Rb+, and Cs+ ions are substantially off from the plane. In the S1 state, the Li+ complex has a structure similar to that in the S0 state. In contrast, the other complexes show a large structural change upon S1–S0 excitation to the out-of-plane direction of the benzene ring, which results in the extensive low-frequency progressions in the UVPD spectra. For the Na+•DMB complex, fast charge transfer occurs from Na+ to DMB after the UV excitation, making the bandwidth of the UVPD spectrum much broader than that of the other complexes and producing photofragment DMB+ ion.

2012

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

Yoshiya Inokuchi, Thomas Rizzo

American Chemical Society2014