Laser spectroscopy of supersonic jet-cooled flexible molecules: Conformational preferences, spectral signatures, and method development

The coupling of supersonic expansions with laser spectroscopy has been fruitfully applied toward gaining insight the conformational preferences and spectral signatures of molecules important to a variety of disciplines. In this vein, a variety of multi resonance methods were employed and/or developed. Studies of O-(2-acetamidoethyl)-N-acetyltyramine (OANAT) and the single chain analogs found the presence of two categories of structures: those having interchain amide-amide hydrogen bonding over the chromophore and those with non-interacting, independent chains. This result was counter the theoretical predictions that the interchain structures should be the only ones according to relative energies. Resolution to this paradox came once entropy corrections were made to obtain the free energies at the sample temperature and showed the population of independent chain structures is enhanced relative to the interchain structures at higher temperatures. Crown ethers (benzo-15-crown-5 ether and 4\u27-aminobenzo-15-crown-5 ether) and their singly and doubly complexed water clusters were also investigated. Some monohydrated crown ether water clusters were found to have the asymmetric water OH stretch split into multiplets due to a 3:1 Fermi resonance with aromatic CO stretches. Electronic origin shifts when coupled with theoretical methods proved especially diagnostic for making conformational assignments as the S0-S1 transitions were very sensitive to local conformation at the chromophore. The infrared region, notably the alkyl CH stretch, proved useful as blue-shifted CHO H-bonds characterized the region above 2930 cm-1 for the crown monomer due to the preference of uncomplexed crowns to buckle in on themselves, allowing different parts of the crown cycle to interact. Conversely, the water complexes lacked the alkyl CH stretch features indicative of CHO interactions. It was concluded that the complexation of water dramatically alters the conformational preferences of the crown ether cycles, forcing it to open up, allowing water molecules better access to the crown oxygen binding sites. Complexation also alters the properties of water, reducing the potential coupling between OH stretching modes as observed in a reduced splitting between the OH stretch fundamental frequencies. The development of a new triple resonance method, IR-IR-UV hole-burning, is also reported