Investigations of Mixed Ionic Clusters using Vibrational Predissociation Spectroscopy

• Main Authors: Chih-Che Wu, 吳志哲
• Other Authors: Y. T. Lee
• Format: Others
• Language: en_US
• Published: 2003
• Online Access: http://ndltd.ncl.edu.tw/handle/48694436399736110003

博士 === 國立臺灣大學 === 化學研究所 === 91 === This thesis depicts new experimental investigations of mass-selected mixed cluster ions, produced under supersonic expansion conditions of using a vibrational predissociation ion trap spectrometer equipped with a corona discharge ion source and coupled with an infrared laser system. Density functional theory calculations are performed at B3LYP/6-31+G* level to assist interpretation of the spectra. The combined investigation provides information on structure and proton transfer dynamics of ionic clusters in the gas phase. Subjects studied include hydrophobic interactions, competitive proton solvation by solvents, hydrolysis/hydration of the peptide analogue, and the first overtone spectroscopy of ionic clusters. The experimental layout of the vibrational predissociation ion trap (VPIT) spectrometer and involved parameters in the calculations using density functional theory (DFT) are described in Chapter One. A description of the notations used throughout this work is given therein. Chapter Two illustrates proton assisted hydrophobic interactions in the gas phase. Four modeling coumpounds, (CH3)2O dimer, (CH3)(C2H5)O dimer, CH3C(O)CH3 dimer, and CH3C(O)H dimer, with single protonated water molecule are investigated by infrared spectroscopy under various beam conditions. In Chapter Three, the behavior of the excess proton (H+) in binary solvent is closely examined. Proton competition between methanol and water sheds insight into anomalous high proton conductivity in solution. Chapter Four characterizes the hydration/hydrolysis of peptide-containing molecule (formamide) by both infrared and mass spectroscopy. First infrared spectroscopic evidence of O-protonation rather than N-protonation of protonated mixed formamide clusters is found, which is in line with the prediction from the amide resonance model. Acid-assisted hydration of CH3C(O)CH2C(O)CH3 , the peptide chain analogue, is inspected in Chapter Five. Water-bridged conformation, either H3O+-bridged or H5O2+-bridged structure, is produced under various supersonic beam conditions. In all the mixed water cluster ions, both the sample-centered and the H3O+-centered isomers are identified on the basis of both experimental observations and theoretical calculations. Proton pulling effects have been discussed in these modeling systems. The first overtone spectroscopy of protonated water and methanol complexes is demonstrated in Chapter Six. Although there have been a number of publications for overtone spectroscopy of single molecule and neutral complexes, there are only few investigations on overtone spectroscopy of ionic clusters. We present in this chapter the first spectroscopic identifications and derive harmonic frequencies and anharmonic coefficients of the OH stretches in H+(H2O)3-5, and H+(CH3OH)4 from their first overtone and combination bands in the near infrared region using a tunable optical parametric oscillator (OPO) laser system.