Computational studies of hydrogen bond network dymanics in water

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C O M P U T A T IO N A L S T U D IE S OF H Y D R O G E N B O N D N E T W O R K

D Y N A M I C S IN W A T E R

B Y

A N IR B A N M U D I

D E P A R T M E N T O F C H E M I S T R Y

. Submitted

in fulfillment o f the requirements o f the degree o f

Doctor of Philosophy

to th e

IN D IA N IN ST IT U T E OF T E C H N O L O G Y , DELHI IN D IA

O C T O B E R 2005

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Certificate

This is to certify that the thesis titled “ C o m p u t a t io n a l S tu d ie s o f H y d r o g e n B o n d N e t w o r k D y n a m ic s in W a t e r ” is being subm itted by M r . A n ir b a n M u d i to the Department o f Chemistry, Indian Institute o f Technology, Delhi, for the award o f the degree o f D o c t o r o f P h ilo s o p h y . This thesis is a record o f bona-fide research work carried out by him under my guidance and supervision. In my opinion, the thesis has reached the standards fulfilling the requirements o f the regulations relating to the degree.

The results contained in this thesis have not been subm itted to any other university or istitute for the award o f any degree or diploma.

D r . C . C h a k r a v a r ty Associate Professor Department o f Chemistry Indian Institute o f Technology New Delhi - 110 016

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To my parents

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Acknowledgements

This thesis would not have been possible without the kind support, the trenchant cri

tiques, the probing questions, and the remarkable perseverance o f my thesis advisor - Dr. C.

Chakravarty. Th e best advisor and teacher I could have wished for, she is actively involved in the work o f all her students, and clearly always has their best interest in mind.

I thank Prof. R. Ramaswamy for the valuable ideas which forms the core o f this thesis.

I will always cherish the fruitful discussions, we had over wide range o f issues during the group meetings and otherwise. Dr. V. Banerjee, for her kind support. I also thank Dean o f PG SR , Prof. M. N. Gupta, for his timely support.

I would like to thank the Council for Scientific and Industrial Research for financial support and the Com puter Services Centre o f IIT Delhi for access to their comutational resources. IIT Delhi library, for the journals and books which proved more than useful for my research work. IIT Delhi, for being home away from home during my stay spanning more than six years.

Finally, I would like to thank those closest to me, whose presence helped to make the com pletion o f my Ph.D . work possible. Anuj, Riputapan, Krishna, Yuvraj, Sambit, Mayank, Rajeshwar, Meera Aunty, for adding extra dimensions to my life. Prosenjifc, Praveen, Puneeta, Jaspreet, Purnendu, Vishnu, Arunava, for teaching me various values o f life and making me a better person. Somendra, my friend, my labm ate and my brother, without whose support many things, that redefined my life, would not have been possible.

Ruchi, for being a great sport and a great labmate. Manish, thanks for all the lightning solutions to the technical problems and eatables during the course o f my Ph.D. work and otherwise. Smita, Supriya, Lavanya, Durba, thanks for the love, patience and understand

ing. Arpita, my sister, thanks for being a great m otivator and a source o f infinite courage.

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M y parents have been an inspiration throughout my life. They always supported my dreams and aspirations, and if I d o say so myself, I think they did a fine jo b raising me. I would like to thank them for all they are, and all they have done for me.

N e w Delhi A nirban M udi

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Abstract

This thesis em ploys classical molecular dynamics simulations and power spectral analysis o f fluctuations in tagged particle quantities to understand hydrogen-bond network dynamics and the connection between various anomalies o f water. It also explores the usefulness of power spectral analysis to characterise hydrogen b on d dynamics in aqueous solutions.

Th e first chapter presents an overview o f the com plex m acroscopic properties o f water, as exemplified through the unusual phase diagram and therm odynam ic and transport prop

erties o f water. T o understand the connection between the intermolecular interactions and the m acroscopic properties, it is necessary to employ com puter simulations. Th e possible approaches currently available for com puting intermolecular interactions in water, as a nec

essary input for com puter simulation studies, are discussed. T h e literature on computer simulations o f water is reviewed. These simulations have yielded im portant insights into the relevant features o f the potential energy surface and the degree o f accuracy required in order to m odel different m acroscopic aspects o f water. In the final section o f this chapter, the m otivation for this work and the organisation o f the thesis are described.

Chapter 2 discusses the relevant m ethodological and com putational aspects o f this thesis.

Details o f the m icrocanonical ensemble (N VE) molecular dynamics simulation techniques for integrating N ew ton’s equations o f m otion are discussed. T h e Verlet, SH AK E and the quaternion algorithm used in this work are discussed. Canonical ensemble (N V T ) ensemble and isothermal-isobaric (N P T ) ensemble molecular dynamics simulation techniques also used is this work are described. Analysis o f molecular dynamics trajectory data, with particular emphasis on power spectral techniques, is described in som e detail. T h e equation

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o f state calculations for S P C /E and T IP 5P -E , as well as extensive set o f tests o f the effect o f the Berendsen thermostat on the dynamics o f S P C /E water, are presented here.

Chapter 3 describes how the multiple time-scale behaviour o f power spectra is used to establish a quantitative connection between the com plex dynamics o f the fluctuating hydrogen bon d network and a long-tim e averaged transport property, such as the diffusivity.

Molecular dynamics simulations o f S P C /E water are used to show that monitoring o f 1

/ f a

behaviour is a simple and direct m ethod for linking phenomena on three distinctive length and time scales: the local molecular environment, hydrogen bond network reorganisations and the diffusivity.

Chapter 4 explores the connection between the structural, kinetic and thermodynamic anomalies o f water by analysing dynamical correlations in the fluctuations o f local tetrahe

dral order and local configurational energy. The boundaries o f the diffusionaly anomalous region have been m apped out using the m axima and minima o f the multiple time-scale exponent o f the tagged potential energy fluctuations. Higher moments o f the order param

eter distribution are shown to be useful indicators o f the transition from the anomalous to normal density regime. Th e final section o f the chapter focuses on the compressibility anomaly and explores it’s connection with the order map o f water.

Chapter 5 examines the effect o f ionic solutes on the hydrogen bond network dynamics by power spectral analysis and by monitoring static distributions o f tagged potential en

ergies. The local environment o f solvent and solute ions has been analysed based on the power spectral data. This approach provides a simple route to characterise the dynamical differences between hydrophobic and hydrophilic solutes. In the case o f hydrophilic solutes, such as NaCl, strong correlations between the exponents o f the multiple time-scale region and the diffusivities o f the anions, cations and water molecules have been observed, similar

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to those seen in bulk water.

Chapter 6 gives a summary o f the essential results contained in Chapters 3 to 5. The scope for extending power spectral analysis to study networked liquids such silica, beryllium fluoride and zinc chloride, is examined. Implications o f this work for experimental work on water and aqueous solutions have been discussed.

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Computational studies of hydrogen bond network dymanics in water