Jet-Cooled Electronic and Vibrational Spectroscopy of Crown Ethers: Benzo-15-Crown-5 Ether and 4′-Amino-Benzo-15-Crown-5 Ether

Laser-induced fluorescence (LIF), ultraviolet hole-burning (UVHB), and resonant ion-dip infrared (RIDIR) spectroscopies were carried out on isolated benzo-15-crown-5 ether (B15C) and 4'-amino-benzo-15-crown-5 ether (ABC) cooled in a supersonic expansion. Three conformational isomers of B15C and four of ABC were observed and spectroscopically characterized. Full optimizations and harmonic frequency calculations were Undertaken for the full set of almost 1700 conformational minima identified in a molecular mechanics force field search. When compared with TDDFT predictions, the S-0-S-1 origin positions serve as a useful diagnostic of the conformation of the crown ether near the phenyl ring responsible for the UV absorption and to the position of the NH2 substituent. In-plane orientations for the beta carbons produce red-shifted S-0-S-1 origins, while out-of-plane "buckling" produces substantial blue shifts of 600 cm(-1) or more. Comparison between the alkyl CH stretch spectra of B15C and ABC divide the spectra into common subgroups shared by the two molecules. The high-frequency CH stretch transitions (above 2930 cm(-1)) reflect the number of CH center dot center dot center dot O interactions, which in turn track in a general way the degree of buckling of the crown. On this basis, assignments of each of the observed conformational isomers to a class of structure can be made. All the observed structures have some degree of buckling to them, indicating that in the absence of a strong-binding partner, the crown folds in on itself to gain additional stabilization from weak dispersive and CH center dot center dot center dot O interactions.