Progress in Physical Chemistry.Vol.1

IR Spectroscopy of Microsolvated Aromatic Cluster Ions: Ionization-Induced Switch in Aromatic Molecule Solvent Recognition (S. 39-40)

By Otto Dopfer
Institut für Physikalische Chemie, Universität Würzburg, Am Hubland, 97074 Würzburg, Germany

Ion Ligand Interaction / IR Spectroscopy / Cluster Ions / Ion Solvation / Aromatic Molecules

IR spectroscopy, mass spectrometry, and quantum chemical calculations are employed to characterize the intermolecular interaction of a variety of aromatic cations (A+) with several types of solvents. For this purpose, isolated ionic complexes of the type A+ Ln , in which A+ is microsolvated by a controlled number (n) of ligands (L), are prepared in a supersonic plasma expansion, and their spectra are obtained by IR photodissociation (IRPD) spectroscopy in a tandem mass spectrometer. Two prototypes of aromatic ion solvent recognition are considered: (i) microsolvation of acidic aromatic cations in a nonpolar hydrophobic solvent and (ii) microsolvation of bare aromatic hydrocarbon cations in a polar hydrophilic solvent.

The analysis of the IRPD spectra of A+ L dimers provides detailed information about the intermolecular interaction between the aromatic ion and the neutral solvent, such as ion ligand binding energies, the competition between different intermolecular binding motifs (H-bonds, p-bonds, charge dipole bonds), and its dependence on chemical properties of both the A+ cation and the solvent type L. IRPD spectra of larger A+ Ln clusters yield detailed insight into the cluster growth process, including the formation of structural isomers, the competition between ion solvent and solvent solvent interactions, and the degree of (non)cooperativity of the intermolecular interactions as a function of solvent type and degree of solvation. The systematic A+ Ln cluster studies are shown to reveal valuable new information about fundamental chemical properties of the bare A+ cation, such as proton affinity, acidity, and

1;Content;6
2;Preface;8
3;Attacking a Small Beast: Ar CO, a Prototype for Intermolecular Forces;10
3.1;1. Introduction;10
3.2;2. Quantum-mechanical description of Ar CO;15
3.2.1;2.1 The Ar CO Hamiltonian;15
3.2.2;2.2 Coriolis coupling;19
3.3;3. Theoretically predicted potential surfaces for Ar CO;20
3.4;4. Experimental results;22
3.4.1;4.1 High resolution vibrational-rotational spectroscopy;22
3.4.2;4.2 Ground state and propeller modes;24
3.4.3;4.3 Intermolecular modes: the intermolecular bending state;25
3.4.4;4.4 Coriolis decoupling;26
3.4.5;4.5 Intermolecular modes: the stretch;28
3.4.6;4.6 Intermolecular modes: combination bands;29
3.4.7;4.7 Coupling of intramolecular and intermolecular modes;30
3.5;5. Comparison between theoretical and experimental data;30
3.6;6. Semi-empirical fit of the potential surface;34
3.7;7. Perspectives;42
3.7.1;Acknowledgement;43
3.7.2;References;43
4;IR Spectroscopy of Microsolvated Aromatic Cluster Ions: Ionization- Induced Switch in Aromatic Molecule Solvent Recognition;46
4.1;1. Introduction;47
4.2;2. Experimental approach;49
4.3;3. Results and discussion;55
4.3.1;3.1 Complexes of acidic aromatic ions with nonpolar ligands;55
4.3.2;3.2 Complexes of bare aromatic hydrocarbon cations with polar ligands;70
4.4;4. Concluding remarks and outlook;82
4.4.1;Acknowledgement;84
4.4.2;References;84
5;Fluorescence Imaging of Reactive Processes;90
5.1;1. Introduction;90
5.2;2. Principles of LIF imaging;91
5.2.1;2.1 Fluorescence quenching;93
5.2.2;2.2 LIF in the gas phase;93
5.2.3;2.3 LIF in solutions;94
5.3;3. Experimental considerations;98
5.3.1;3.1 Set-up for gas phase diagnostics;98
5.3.2;3.2 Microscopic LIF imaging;102
5.4;4. Examples: gas phase chemistry;105
5.4.1;4.1 Reaction rate imaging;105
5.4.2;4.2 Turbulence/ chemistry interactions;107
5.5;5. Examples: biological applications;108
5.5.1;5.1 Protein mobility in live cells;109
5.5.2;5.2 Protein protein interactions;112
5.6;6.