Conformation of Alkali Metal Ion-Benzo-12-Crown-4 Complexes Investigated by UV Photodissociation and UV-UV Hole-Burning Spectroscopy

We measure UV photodissociation (UVPD) spectra of benzo-12-crown-4 (B12C4) complexes with alkali metal ions, M+center dot B12C4 (M = Li, Na, K, Rb, and Cs), in the 36300-37600 cm(-1) region. Thanks to the cooling of ions to similar to 10 K, all the M+center dot B12C4 complexes show sharp vibronic bands in this region. For UV-UV hole-burning (HB) spectroscopy, we first check if our experimental system works well by observing UV-UV HB spectra of the K complex with benzo-18-crown-6 (B18C6), K+center dot B18C6. In the UV-UV HB spectra of the K+center dot B18C6 complex, gain signals are also observed; these are due to vibrationally hot K+center dot B18C6 complex produced by the UV excitation of cold K+center dot B18C6 complex. Then we apply UV-UV HB spectroscopy to the M+center dot B12C4 complexes, and only one conformer is found for each complex except for the Li+ complex, which has two conformers. The vibronic structure around the origin band of the UVPD spectra is quite similar for all the complexes, indicating close resemblance of the complex structure. The most stable structures calculated for the M+center dot B12C4 (M = Li, Na, K, Rb, and Cs) complexes also have a similar conformation among them, which coincides with the UVPD results. In these conformers the metal ions are too big to be included in the B12C4 cavity, even for the Li+ ion. In solution, it was reported that 12-crown-4 (12C4) shows the preference of Na+ ion among alkali metal ions. From the similarity of the structure for the M+center dot B12C4 complexes, it is suggested that the solvation of free metal ions, not of the M+center dot 12C4 complexes, may lead to the selectivity of Na+ ion for 12C4 in solution.