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

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.

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

Yoshiya Inokuchi, Thomas Rizzo

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.

American Chemical Society2011

Laser Spectroscopic Study of Cold Host–Guest Complexes of Crown Ethers in the Gas Phase

Yoshiya Inokuchi, Thomas Rizzo

A laser spectroscopic study on the structure and dynamics of cold host–guest inclusion complexes of crown ethers (CEs) with various neutral and ionic species in the gas phase is presented. The complexes with neutral guest species are formed by using supersonic free jets, and those with ionic species are generated with electrospray ionization combined with a cold 22-pole ion trap. For CEs, various sizes of 3n-crown-n ethers (n = 4, 5, 6, and 8) and their benzene-substituted species are used. For the guest species, water, methanol, ammonia, acetylene, and phenol are employed as neutral guest species, and for charged guest species, alkali metal cations are chosen. The electronic and vibrational spectra of the complexes are measured by using various laser spectroscopic methods; electronic spectra for the neutral complexes are measured by laser-induced fluorescence. Discrimination of different species such as conformers is performed by ultraviolet–ultraviolet hole-burning spectroscopy. The vibrational spectra of selected species are observed by infrared–ultraviolet double-resonance (IR–UV DR) spectroscopy. For the ionic complexes, ultraviolet photodissociation and IR–UV DR spectroscopy are applied. The complex structures are determined by comparing the observed spectra with those of possible structures obtained by density functional theory calculations. How the host CEs change their confor- mation or which conformer prefers to form unique inclusion complexes are discussed. These results reveal the key interactions for forming special complexes leading to molecular recognition.

2013