Vibrational Spectroscopy of Gas Phase Functional Molecules and Their Complexes Cooled in Supersonic Beams

Abstract

Functional molecules and supramolecules are the assemble of molecules, which are bound by noncovalent interactions, such as dispersion force, coordinate-bonding, hydrogenbonding, etc. They exhibit special functions by forming regular high dimensional structures controlled by those weak interactions. The concept of the supramolecule was first proposed by Jean-Marie Lehn, who succeeded in synthesizing cryptand (Lehn, 1995). In the early stage, many studies have been carried out for host-guest complexes of crown ether (Gokel, 1991; Izatt et al., 1969; Pedersen, 1967; Pedersen & Frensdorff, 1972), calixarene (Atwood et al., 2002; Gutsche, 1998; Purse et al., 2005; Thallapally et al., 2005), and cyclodextrin (Brocos et al., 2010; Szejtli, 1988). These studies are extended to larger size systems built by several units, such as protein, LangmuirBlodgett (LB) film, self assembled monolayer (SAM), and liquid crystal. In addition, more complicated molecular assemblies, rotaxane, catenane, and molecular capsules, are synthesized. Also, many functional groups have been used to applications such as sensing and basic chemical research, by covalently linking fluorescent dyes, nanoparticles, proteins, DNA, and other compounds of interest. Biomolecules may also be categorized to the functional molecules. In the biomolecules, many units form high dimensional structure through noncovalent interactions and exhibit special functions which are not possible for each unit. Among many experimental studies on the functional molecules, vibrational spectroscopy is one of the most general methods. By examining the frequency shift and intensity change of the specific vibrations sensitive to the local interaction, we can study which part or site of the molecule bound for the complexation. In this sense, it is necessary to investigate vibrational spectra not only of the complexes but also of each unit under the isolated condition. By the comparison of the spectra at different conditions, we understand how the molecules change their initial structures or which conformer is preferred for the complexation. A problem for this study is that in most cases they have flexible structures so that the structures are affected by many factors, such as temperature, local environment, solvent molecules as well as the phases. These effects result in the homogenous and inhomogeneous broadening of the spectra, which leads to the difficulty to analyze the spectra. In this chapter, we describe the vibrational spectroscopic study of gas phase small size functional molecules cooled in the supersonic jet. The supersonic jet technique enables us to