Computational Studies on the Anharmonic Dynamics of Molecular Clusters Open Access

Mancini, John S. (2015)

Permanent URL: https://etd.library.emory.edu/concern/etds/gb19f643s?locale=en
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Abstract

Molecular nanoclusters present ideal systems to probe the physical forces and dynam- ics that drive the behavior of larger bulk systems. At the nanocluster limit the first instances of several phenomena can be observed including the breaking of hydrogen and molecular bonds. Advancements in experimental and theoretical techniques have made it possible to explore these phenomena in great detail. The most fruitful of these studies have involved the the use of both experimental and theoretical tech- niques to leverage to strengths of the two approaches.

This dissertation seeks to explore several important phenomena of molecular clus- ters using new and existing theoretical methodologies. Three specific systems are considered, hydrogen chloride clusters, mixed water and hydrogen chloride clusters and the first cluster where hydrogen chloride autoionization occurs. The focus of these studies remain as close as possible to experimentally observable phenomena with the intention of validating, simulating and expanding on experimental work. Specifically, the properties of interested are those related to the vibrational ground and excited state dynamics of these systems. Studies are performed using full and reduced dimensional potential energy surface alongside advanced quantum mechan- ical methods including diffusion Monte Carlo, vibrational configuration interaction theory and quasi-classical molecular dynamics.

The insight gained from these studies are great and varied. A new on-they-fly ab initio method for studying molecular clusters is validated for (HCl)1-6. A landmark study of the dissociation energy and predissociation mechanism of (HCl)3 is reported. The ground states of mixed (HCl)n(H2O)m are found to be highly delocalized across multiple stationary point configurations. Furthermore, it is identified that the con- sideration of this delocalization is required in vibrational excited state calculations to achieve agreement with experimental measurements. Finally, the theoretical infrared spectra for the first case of HCl ionization in (H2O)m is reported, H+(H2O)3Cl-. The calculation indicates that the ionized cluster's spectra is much more complex than any pervious harmonic predictions, with a large number of the system's infrared active peaks resulting from overtones of lower frequency molecular motions.

Table of Contents

1 Introduction ... 1

1.1 Molecular Insight ... 1

1.2 Enhanced Understanding with Theory ... 2

1.3 Structure of Dissertation ... 3

2 Computational and Theoretical Methods ... 4

2.1 Overview... 4

2.2 Potential Energy Surfaces ... 5

2.2.1 Introduction ... 5

2.2.2 Full-Dimensional Fitting ... 8

2.2.3 Many-BodyApproximation ... 11

2.2.4 Procedure for Generating New Potential Energy Surfaces ... 13

2.2.5 Reduced-Dimensional Fitting ... 15

2.3 VibrationalSpectroscopy ... 17

2.3.1 Introduction ... 17

2.3.2 DiscreteVariableRepresentation ... 18

2.3.3 Multimode ... 19

2.3.4 Diffusion Monte Carlo ... 22

2.4 Quasi-Classical Simulation ... 25

3 (HCl)n Clusters ... 29

3.1 Overview ... 29

3.2 On-the-fly Ab Initio Calculations of Anharmonic Vibrational Frequencies: Local-Monomer Theory and Application to HCl Clusters ... 30

3.2.1 Context ... 30

3.2.2 Methods for Preforming On-the-Fly Vibrational Calculations ... 35

3.2.3 On-the-Fly (HCl)1-6 Vibrational Frequencies ... 38

3.2.4 Summary and Conclusions ... 49

3.3 A New Many-Body Potential Energy Surface for HCl Clusters and Its Application to Anharmonic Spectroscopy and Vibration-Vibration Energy Transfer in the HCl Trimer ... 50

3.3.1 Context ... 50

3.3.2 (HCl)n Many-Body Potential Energy Surface ... 51

3.3.3 Calculations of the HCl Trimer ... 55

3.3.4 Calculations of Larger HCl Clusters ... 65

3.3.5 Summary and Conclusions ... 68

3.4 Experiment and Theory Elucidate the Multichannel Predissociation Dynamics of the HCl Trimer: Breaking Up Is Hard To Do ... 69

3.4.1 Context ... 69

3.4.2 Theoretical Methods for the Study of (HCl)3 Predissociation ... 70

3.4.3 Ground State Properties and Dissociation Energies of (HCl)2-3 ... 74

3.4.4 Rotational and Translation Distributions of (HCl)3 Fragments ... 76

3.4.5 Dissociation Mechanism of (HCl)3 ... 79

3.4.6 Summary and Conclusions ... 80

4 Mixed (HCl)n and (H2O)m Clusters ... 82

4.1 Overview ... 82

4.2 A New Ab Initio Potential Energy Surface for HCl-H2O, Diffusion Monte Carlo Calculations of D0 and a Delocalized Zero-point Wavefunction ... 83

4.2.1 Context ... 83

4.2.2 Theoretical Methods for the Study the (HCl)(H2O) ... 86

4.2.3 Ground State Properties of the (HCl)(H2O) ... 90

4.2.4 Summary and Conclusions ... 93

4.3 Effects of Zero-Point Delocalization on the Vibrational Frequencies of Mixed HCl and Water Clusters ... 93

4.3.1 Context ... 93

4.3.2 Many-Body Potential for (HCl)n(H2O)m Clusters ... 95

4.3.3 Theoretical Methods for the Study the (HCl)n(H2O)m ... 98

4.3.4 Vibrational Ground State Properties of (HCl)n(H2O)m ... 102

4.3.5 Vibrational Excited State Properties of (HCl)n(H2O)m ... 106

4.3.6 Summary and Conclusions ... 113

5 H3O+(H2O)3 Cl- Solvent Ion Pair Cluster ... 115

5.1 Overview ... 115

5.2 Isolating the Spectral Signature of H3O+ in the Smallest Droplet of Dissociated HCl Acid ... 116

5.2.1 Context ... 116

5.2.2 Theoretical Methods for the Study of H3O+(H2O)3Cl- ... 118

5.2.3 IR Spectrum of H3O+(H2O)3Cl- ... 124

5.2.4 Summary and Conclusions ... 131

References ... 141

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