Vibrational Dynamics of Hydrated Proton based on High-level Ab Initio Potential Energy Surface and Dipole Moment Surface Open Access

Yu, Qi (Summer 2019)

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The hydrated excess proton is a common species in both aqueous chemistry and gas-phase chemistry which complexes with water in a variety of structures. Previous theoretical tools are limited in investigating the structure, dynamics and vibrational spectra of hydrated proton with both accuracy and computational efficiency. In our work, an accurate ab initio potential energy surface (PES) and dipole moment surface (DMS) is developed. This PES/DMS, based on a many-body representation, accurately describe the electronic energy of the hydrated proton system including one excess proton and arbitrary number of water monomers. Each term of the many-body representation is fitted using linear least square fitting method with thousands of high-level ab initio electronic energies. Part 1 and Part 2 of the dissertation will review the PES/DMS fitting methods and introduce how the PES/DMS for hydrated proton is constructed.

The accuracy of constructed PES/DMS is verified in properties of small protonated water cluster, H3O+(H2O)n, n=0-5. We carry out detailed comparison with high-level benchmark electronic structure calculations for all current known low-lying isomers of those clusters in terms of structures, energetics and harmonic spectra. Taking advantage of the PES/DMS, we report fully quantum studies of the vibrational spectra of different protonated water clusters. This includes the infrared spectra of H7O3+, four isomers of H9O4+, Zundel and Eigen isomer of H+(H2O)6.

We also apply the PES/DMS to infrared spectra calculation of aqueous proton system. We obtain hundreds of protonated water clusters, H+(H2O)6, from reactive molecular dynamics trajectories. Anharmonic vibrational spectra are conducted for all selected clusters combining extended local monomer approach and quantum vibrational self-consistent and virtual state configuration interaction approach (VSCF/VCI)

Table of Contents

1 Introduction

I Theories and Methods

2 Potential Energy Surface

2.1 Born-Oppenheimer Approximation

2.2 Permutationally Invariant Potential Energy Surface

2.3 Permutational Symmetry

2.4 Many-body Expansion

3 Molecular Vibrations

3.1 Vibrational Self-Consistent Field and Virtual-state Configuration Interaction


3.2.1 Watson Hamiltonian

3.2.2 n-Mode Representation of the Potential

3.2.3 Infrared Intensity

3.3 Quantum Local Monomer Model

II Many-body Potential Model for Hydrated Proton

4 Many-body Potential Energy Surface and Dipole Moment Surface

4.1 Assignment of Monomers

4.2 H3O+ Potential Energy Surfaces

4.3 Hydronium Water 2-body Interaction

4.4 Hydronium Water 3-body Interaction

4.5 Hydronium Water 4-body Interaction

4.6 Water Potential

4.7 Dipole Moment Surface

4.8 Benchmark electronic structure calculations for H3O+(H2O)n, n=0-5 clusters

III Vibrational Dynamics of Gas-Phase Protonated Clusters

5 H3O+ PES Vibration and Vibrational Analysis

5.1 Fidelity of the Potential and Vibrational Calculations

5.2 Summary and Conclusions

6 Vibrational Dynamics of H7O3+ and H9O4+

6.1 First Trial of Vibrational Spectra of H7O3+ and H9O4+

6.1.1 Introduction 6.1.2 Results and Analysis 6.1.3 Conclusions

6.2 Vibrational Spectra of the Eigen, Zundel and Ring Isomers of H+(H2O)4-Find a Single Match to Experiment

6.2.1 Introduction 6.2.2 Computational Details and Results 6.2.3 Conclusion

6.3 Vibrational Spectra of the Protonated Water Trimer H7O3+-Combined Experimental and Theoretical Study

6.3.1 Introduction 6.3.2 Conclusion

6.4 Deconstructing Prominent Bands in the Terahertz Spectra of H7O3+ and H9O4+: Intermolecular Modes in Eigen Clusters

6.4.1 Introduction 6.4.2 Computational Details 6.4.3 Results and Discussion

6.5 Revisit of Vibrational Spectra of H7O3+ and H9O4+: Classical, Thermostatted Ring Polymer and Quantum VSCF/VCI Calculations

6.5.1 Introduction 6.5.2 Classical MD and TRPMD IR Spectra Calculation 6.5.3 VSCF/VCI and Quasi-classical MD IR Spectra Calculation

6.5.4 Results and Discussion 6.5.5 Conclusions

IV Vibrational Spectra of the Aqueous Proton

7 High-level VSCF/VCI Calculations Decode the Vibrational Spectrum of the Aqueous Proton

7.1 Introduction 7.2 Computational Details 7.3 Vibrational Spectra of Two Isomers of H+(H2O)6

7.4 Vibrational Spectra of 800 H+(H2O)6 clusters

7.4.1 Decomposition of Spectrum by Vibrational Character

7.4.2 Influence of Structural Parameters on Proton Stretch Frequency

7.4.3 Decomposition of Spectrum by

7.4.4 Challenges and Outlook

7.5 Summary and Conclusions

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