CHAPTER-1

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INTRODUCTION

1.1 General Introduction

Study of molecular interactions in pure liquids and liquid mixtures is a

challenging task and the nature of complex formation in binary mixtures is still far from

clear. In recent years, the term ‘dielectric’ has been applied to a broad group of non-

polar materials with small or negligible conductivity, when considering their interaction

with electric, magnetic or electromagnetic fields. It is a common property of a dielectric

medium to get polarized in an electric field. Solids and liquids have been subjected to

dielectric measurements at different frequencies and temperatures since the properties

shown are of interest and lead to a knowledge of the composition and molecular

structure of the material.

The interaction of electrical energy with matter is determined by

electromagnetic properties of the material. On a macroscopic scale under steady state

conditions, these properties are conventionally described by the permittivity and

permeability of the material. The increasing use of microwaves in fields like

communication, radar, medicine, biology, agriculture and industry demands accurate

data on dielectric properties of materials. The characterization of dielectric materials

includes the measurement of complex permittivity as a function of frequency at a given

temperature or as a function of temperature at a given frequency. The measurement of

dielectric properties over a wide frequency range gives the information regarding the

conduction mechanism, interfacial polarization, molecular dynamics and relaxation

behavior phenomena [1].

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Hydrogen bonding is complex in the liquid state due to the uncertainty in

identifying the particular bonds and the number of molecules involved. Study of

hydrogen bonding between –OH and -CH groups has been selected as it has extensive

applications in the biological research. The chosen alcohols and alkyl benzoates are

propan-1-ol, propan-2-ol, methyl benzoate and ethyl benzoate. In the present study, four

hydrogen-bonded systems are investigated. To understand the molecular behaviour of

associating molecules, it is necessary to determine the various dielectric parameters

which are related with inter and intramolecular association and internal rotations with

temperature variation.

Determination of Kirkwood correlation factor g [2, 3] provides the information

about intermolecular association, while the anomalous behaviour of molar polarization

[4] shows the intramolecular association in the associating molecules. Dielectric

relaxation times [5, 6] give confirmation about intramolecular rotations and steric

hindrance offered by the intramolecular association and the environment to group

rotations. One of the physical parameters used for conformational analysis of any

resultant structure is the net dipole moment. Dielectric studies are carried out on the

binary mixtures to determine the dipole moment and relaxation time.

The aim of the work is to understand the behavior of hydrogen bond between the

binary mixtures of alcohols and benzoates affecting the physical properties of the

systems such as dipole moment and relaxation time compared to the individual systems

as hydrogen bond involves –OH and -CH as functional groups. The interaction between

the molecules can be studied by using the quantum mechanical calculations such as ab-

initio and semiempirical methods. This quantum mechanical calculation gives the

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information regarding the energy, nature of chemical bond, dipole moment, vibrational

frequencies, optimized geometric structure of the molecule and thermo dynamical

parameters and can be verified from the experimental data.

Dielectric studies are of great help in the assessment of molecular structures or

configurations, particularly those of organic compounds. Although IR and NMR studies

are powerful tools for studying intramolecular H-bonds, dielectric studies provide

useful information about molecular association and intramolecular rotations [5].

1.2 The Hydrogen bond

Hydrogen bonding is a donor-acceptor interaction specifically involving

hydrogen atoms. A hydrogen bond is a link of the form A-H…B as described in Figure

1.1, where A and B are electronegative atoms and B has a lone pair of electrons. The

most significant hydrogen bonding occurs where A and B are atoms of Nitrogen (N),

Oxygen (O) and Fluorine (F). Weak hydrogen bonding can also be formed between less

electronegative atoms.

The simplest picture of a hydrogen bond is an electrostatic interaction between

the proton of A-H and the lone pair of B.

Figure 1.1: Hydrogen bond

The atom A needs to be electronegative in order to polarize the A-H

bond and hence leaves the charge of the proton partially unshielded. The atom B needs

to be electronegative such that it has restrained a high electron density in the molecule

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of which it may be a part and hence can interact reasonably strongly with the proton’s

exposed charge. In molecular orbital terms, a hydrogen bond consists of four electrons

in the three molecular orbitals that can be built from the appropriately directed atomic

orbitals of A, H and B.

Hydrogen bond energies extend from about 15-40 kcal/mol for strong hydrogen

bonds, 4-15 kcal/mol for moderate bonds and 1-4 kcal/mol for weak hydrogen bonds.

Strong hydrogen bonds are formed by groups in which there is a deficiency of electron

density in the donor group like O+, N+ or an excess of electron density in the acceptor

group like F-, O- and N-. This is to be expected since a deficiency of electrons of the

donor group further deshields the proton thereby increasing the positive charge. On the

other hand, while an excess of electrons on the acceptor group increases its negative

charge and interact with the deshielded proton. These bonds are also referred as ionic

hydrogen bonds.

Moderate hydrogen bonds are formed generally by neutral donor and acceptor

groups like –OH, –NH, O=C, in which the donor atoms (A) are electronegative relative

to hydrogen and the acceptor atoms (B) have lone pair unshared electrons. These are the

most common hydrogen bonds both in chemistry and nature. These are regarded as

normal hydrogen bonds and are essential components of the structure and function of

biological molecules.

Weak hydrogen bonds are formed when the hydrogen atom is covalently bonded

to a slightly more electro neutral atom relative to hydrogen, as in C–H, or when the

acceptor group has no lone pairs but has π electrons such as an aromatic ring. These

interactions have similar energies and geometries to those of van der Waals complexes,

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and are distinguished from them by the evidence of a directional involvement of A-H

bond.

The concept of hydrogen bond, naturally, evolves from consideration of

Pauling’s atomic electronegativities. As a consequence of the greater electronegativity

of A relative to H in an A-H bond, the hydrogen proton is stripped of some of its

electron density and is descreened. This results in a dipole at the terminus of the A-H

bond, which interacts with the monopole or dipole of the lone pairs on the acceptor

atom and to a lesser degree with more distant bond dipoles. Consequently hydrogen

bond donor strengths are qualitatively proportional to these differences in

electronegativities. F-H > O-H > N-H > C-H, and the hydrogen bond has a directional

property, being the strongest when A-H…B = 180°. The angles are easily bent from

linearity for moderate and weak bonds. Bent bonds are entropy favored. The peak in the

hydrogen bond angle distribution curve is ~155°. This is known as the conic factor or

correction. The other components of the hydrogen bond energy are identified as

delocalization, repulsion and dispersion. Coulson [7] partitioned the hydrogen bond

system into contributions from the five valence bond structures shown below.

ψ HB = a ψ a+ b ψ b + c ψ c + d ψ d+ e ψ e

ψ a = A–H …..B covalent A—H bond

ψ b = A-__H+ …..B ionic A—H bond

ψ c = A-–H …..B+ charge transfer, A…B bond

ψ d = A+__H- …..B ionic A—H bond

ψ e = A–H- …..B+ charge transfer, H…B bond

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His conclusion is that for O–H ….O bonds at O….O = 2.8 A°, the sum of the

electrostatic components, ψ b + ψ d has contributed about 65% of the hydrogen bond

energy. As the O….O distance becomes closer the quantum mechanical charge transfer

contributions became more important, while for longer weak bonds, the interaction

became more electrostatic. Later ab-initio molecular orbital calculations supported this

general chemical concept. As a criterion, it is reasonable to assume that if the vibration

frequency and the bond length of the covalent A—H bond are not significantly altered

on the hydrogen bond formation, the major component of the bonding is electrostatic.

Computers make it possible for ab-initio molecular orbital methods to explore

the potential energy for hydrogen bonded dimers with a variety of likely configurations

and conformations. The lowest energy minima are sought with the structures and

energies associated with them. Decomposing these energies into components, known as

the Morokuma [8] decomposition method, subdivide the total bonding energy into

electrostatic (es), polarization (pl), exchange repulsion (ex), charge transfer (ct), and

coupling (mix). The electrostatic component includes monopole-monopole (r-1),

monopole-dipole (r-2), dipole-dipole (r-3) terms and higher combinations of classical

interactions between undisturbed monomer charge distributions. The electron

distributions of the molecules are disturbed by the close approaches due to the hydrogen

bonding. This gives rise to polarization and quantum mechanical interactions, exchange

repulsion, charge transfer, and dispersion. The polarization is the effect of the distortion

of the electron distributions of A—H by B and B by A—H. This is a stabilizing

interaction. The exchange repulsion is the short-range repulsion of the electron

distributions of the donor and acceptor groups. It accounts for the overlap of charges in

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occupied orbitals of both donor and acceptor. Charge transfer is the result of the transfer

of electrons between occupied orbitals on the donor to vacant orbitals on the acceptor

and vice versa. The coupling term allows for the fact that these four interactions are not

strictly independent of each other. It is small, except for large bonding energies. The

electrostatic, polarization, and charge transfer are attractive at equilibrium distances

while the exchange repulsion is balancing them.

Hydrogen bonds which involve two acceptors are termed as three centered

hydrogen bonds, since the hydrogen is bonded to three atoms: one by a covalent bond

and two by hydrogen bonds. Three center bonds are also described as bifurcated

hydrogen bonds. Sometimes, a bond occurs between two donor hydrogen’s and one

acceptor. Such bonds are known as Chelated hydrogen bonds. Chelated hydrogen bonds

and three centered hydrogen bonds are shown in Figure 1.2. The hydrogen bond plays a

significant role in most biological systems. In these systems, the donor and acceptor

atoms are either Nitrogen (N) or Oxygen (O).

Figure 1.2: Bifurcated Hydrogen bonds

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The discovery of hydrogen bonding between the bases adenine and uracil and

between guanine and cytosine by Watson and Crick together with the crystal structure

analyses of the proteins, hemoglobin and myoglobin by Perutz and Kendrew and their

co-workers, initiated a new science known as molecular biology [9]. Nearly all

biological molecules contain hydrogen bonds, which play a critical role for the function

of biological macromolecules and the maintenance of living organisms. The function of

biological macromolecules is closely associated with their structures. These structures

are stabilized by covalent bonds which link individual units such as amino acids,

nucleotides or sugars.

1.3 Types of Hydrogen bonds

Hydrogen bonds are classified into two types as,

(i) Intermolecular Hydrogen bonds

(ii) Intramolecular Hydrogen bonds

(i) Intermolecular Hydrogen bonds

In such a type of linkages, two or more than two molecules of the same

compound combine together to give a polymeric aggregate.

Example: HF molecule

When a number of HF molecules are brought together, the positive end of one

dipole Hδ+—Fδ- attracts the –ve end of the other similar dipole, Hδ+

—Fδ- and these

molecules associated together to form a cluster, (HF)n shown as,

[ --- H δ+ – F δ- --- H δ+ – F δ- --- H δ+ – F δ- --- H δ+ – Fδ- --- ]n.

In the case of (NH3)n and (H2O)n molecules are associated together to form

clusters.

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(ii) Intramolecular Hydrogen bonds

In this type, hydrogen bonding occurs within two atoms of the same molecule.

Generally, this type of bonding is known as chelation. This is possible in ortho

substituted compounds.

An important example of a molecule having intra molecular H — bonding is

furnished by o – nitrophenol. The o – nitrophenol boils at 214oC, while o — and o —

isomer boil at 290oC and 270oC respectively. Thus we see that o — form has the

minimum boiling point which is accounted for the assumption that o — form contains

an intramolecular (internal) H – bonding which can be represented as shown below.

Figure 1.3: Intramolecular Hydrogen bonding in o – nitrophenol

This type of intra – molecular H – bonding is not possible in m- and p- isomers

because of the size of the ring.

Thus in m- and p- forms inter – molecular H – bonding takes place and this

results in some degree of association among a number of m- and p- forms. It is this

association which accounts for the higher boiling points of m- and p- isomers.

1.4 Different Techniques to study Hydrogen bonds

The different techniques to study hydrogen bonded systems are broadly

classified into three categories, as follows:

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(i) Physical methods: Physical methods include the study of molecular weight,

melting/boiling points, viscosity, dielectric properties and affect of pressure and

temperature.

(ii) Spectroscopic methods: Spectroscopic methods normally used to study the

hydrogen bonded systems are Vibrational Infrared Spectroscopy (IR and Raman), NMR

spectroscopy, Electron absorption studies and Gas-Phase Microwave rotational

spectroscopy.

(iii) Diffraction Methods: Neutron and X-ray diffraction are used in the analysis of

hydrogen bonds.

(iv) Computational Chemistry: Molecular mechanics methods, semiempirical methods,

ab-initio methods and Monte Carlo simulations are some of the computational

techniques adopted to study hydrogen bonds.

1.4.1 Physical methods to study Hydrogen bonds

Hydrogen bonding can affect the physical properties of gases, liquids and solids.

In the absence of hydrogen bonding, the physical properties of gases, liquids and solids

are said to be normal. That means, they are predictable from laws that relate physical

properties solely to the chemical composition of the molecules involved. In gases,

deviations from Raoult’s law for ideal gas properties,

( )VP = RTMw

,

is an evidence but not proof of hydrogen bonding. In liquids, deviations from Trouton’s

Rule, Vap

bp

∆H

T≈ 22 cal deg-1mol-1 ,

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is also an evidence. In crystals, hydrogen bonding in hydrates increases molar volume

whereas in organic compounds, it tends to decrease molar volume, because

intermolecular hydrogen bonding generally brings the molecules into closer contact

than van der Waals interactions.

(i) Molecular weight measurements

All inter molecularly hydrogen bonded systems form associated species. These

species increase the apparent molecular weight of the system under study. The apparent

molecular weight is measured from vapour density measurements and the ideal gas

equation is used for this purpose. Since PV = nRT where P and V are the pressure and

volume of gas, n = w/M is the number of moles of gas, R is the gas constant, T is the

temperature, w is the weight and M is the molecular weight of the sample.

(ii) Melting and boiling point measurements

Melting and boiling points of inter-molecular hydrogen bonded species are

significantly higher than those compared to similar molecular weight non-hydrogen

bonded species. This is because of the extra energy needed to break the hydrogen bonds

present.

(iii) Viscosity measurements

Viscosity of hydrogen bonded systems increases due to the formation of

associated species. The excess values associated with molecular weights, melting and

boiling points and viscosity values are true only for inter-molecular hydrogen bonded

species only since intramolecular hydrogen bonding does not lead to association of the

molecules. Nikiforov et al. [10] have suggested an expression for calculation of the

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viscosity of binary liquid systems with different types of interaction between the two

constituent species.

(iv) Dielectric permittivity measurements

Although there is no direct theory to predict the dielectric permittivity spectra of

hydrogen bonded systems, a few observations are generally true. Almost all hydrogen

bonded systems show multiple relaxation process very often with distinct relaxation

mechanisms. The relatively high dielectric permittivity of hydrogen bonded liquids is at

least partially due to a high ratio of dipole/quadrupole moment of molecules that can

form hydrogen bonds. Also direct interactions (next neighbour interactions) are seen to

contribute less to the dipole angular correlation function than pair correlations due to

indirect interactions.

(v) Effect of pressure and temperature on Hydrogen bonded systems

Increase of pressure on hydrogen bonded system increases the number of

hydrogen bonds since more number of molecules come closer. Hydrogen bond

formation is exothermic whereas hydrogen bond breaking is endothermic. This implies

that the hydrogen bonded systems have lower energy than the sum of energies of the

non-bonded individual components. It is therefore possible to study the

thermodynamics of hydrogen bond formation either directly or by measuring the

equilibrium constants of any property that changes with temperature or concentration in

an inert solvent.

The major demerit associated with the physical methods is that none of them can

prove the existence of hydrogen bonds uniquely. They also cannot locate the exact

position of hydrogen bond in the system. These limitations can be overcome in

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spectroscopic techniques where the site of hydrogen bond and very often the number of

hydrogen bonds present in a system can be determined.

1.4.2 Spectroscopic studies of Hydrogen bonds

(i) Vibrational spectroscopy

A vibrational spectrum of hydrogen bonded system (A-H…B) in the IR region is

used as an important tool. The prominent features of the IR spectra of hydrogen bond

system are

a) Shift of fundamental A-H bond (ν (A-H)) to lower frequencies. This is because

hydrogen bond renders the A-H bond weaker in the pure state.

b) Shift of A-H in plane/out of plane deformation to higher frequencies,

c) Formation of a new A-H….B fundamental stretch/deformation band at lower

frequencies and

d) Change in the bond contour and integrated absorption intensities are also observed

but are very often difficult to analyze quantitatively

IR spectroscopy can identify the presence of monomers, dimers, trimers etc. in

species containing OH bonds. The broad OH band can be deconvoluted into the

individual monomer, dimer, trimer etc. bands since the full width at half maximum and

peak positions of these bands are all known. The overlap of water OH band with that of

the alcohol band makes it necessary to record the spectra under vacuum conditions so

that the sample does not absorb water.

(ii) NMR Spectroscopy

NMR spectroscopy is another extremely sensitive method for identifying

hydrogen bonding. NMR spectroscopists observe the chemical shift caused by the

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change in the electronic environment around the proton. It is less widely applied than

infrared spectroscopy because of the complexity of hydrogen bonding in solution. The

development of solid state NMR through the 13C - cross polarization magic angle

spinning methods (CP-MAS) and the more difficult 1H - combined rotational and

multiple pulse spectroscopy (CRAMPS) provides very effective tools for studying

hydrogen bonding in solids. The development of multidimensional methods has made

NMR spectroscopy a powerful tool for elucidating molecular structure in solution.

Nevertheless, it shows relatively little impact on the study of hydrogen bonds.

(iii) Electron absorption spectroscopy

This is useful technique if the hydrogen bond part of the molecule has

absorption in the visible/UV region (6000-2000 A0) of the electromagnetic spectrum.

The hydrogen bond acceptor (A-H) shows a blue shift and the hydrogen bond donors

show a red shift in the hydrogen bond species.

1.4.3 Diffraction studies of Hydrogen bonds

Diffraction methods depend on the three-dimensional periodicity of the atoms.

The ability of atoms to diffract or scatter x-rays is proportional to the atom’s electron

density. Hydrogen with one electron has a very small ability for diffraction. Therefore,

this method cannot be used to determine the position of hydrogen.

Single crystal structure analysis has a special role in the study of hydrogen

bonds as they provide direct information concerning the stereochemistry. Location of

hydrogen atoms is essential to understand the nature of the hydrogen bond. Strong,

moderate and weak bonds can be distinguished using crystal structure analysis and

infrared spectroscopy. Electron diffraction and X-ray diffraction of liquids do not

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provide information concerning the position of the hydrogen atoms and therefore they

are not used for the study of hydrogen bonds.

Neutron diffraction is a better tool to study hydrogen bonds since proton has a

larger cross section to scatter neutrons. The coherent scattering of the atoms is not a

function of atomic number like X-ray, but varies over a relatively narrow range.

Hydrogen has a scattering power for neutrons that is equivalent to half that of carbon

and oxygen while it is 1/50 compared to X-rays. This very large incoherent neutron

scattering factor for hydrogen helps us to derive information concerning the positions of

hydrogen atoms with accuracy up to a third decimal point in A0. But, this involves a

greater commitment in time and money. Corrections for thermal motion and

anharmonic stretching motion should be taken into consideration, if precise

comparisons are made between the hydrogen bond geometries at different temperatures,

or with theoretical calculations, which predict the geometries of molecules at rest.

It is important to realize that different methods provide atomic structure and

processes on different time scales. Spectroscopic methods provide information relating

to structure and processes on a picosecond’s time scale (10-10-10-15sec). Diffraction and

thermodynamic methods are at the other end of the timescale (10-103sec). Crystal

structure analysis gives an average structure with respect to both time and space. NMR

spectroscopy is midway at 10-10-4sec. Therefore, the spectroscopists and diffractionists

sometimes view hydrogen bonded structures from different perspectives and their

conceptual models do not always correspond to each other.

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1.4.4 Computational molecular modeling

Computational molecular modeling has matured with the development of

computers. The main requirement is that the model must correspond to what the

experiment says. Two approaches have been employed viz: molecular mechanics and

quantum mechanics.

(i) Molecular mechanics

Molecular mechanics has many facets in common with early molecular

spectroscopy, which applied methods of classical mechanics to an understanding of

vibrational and rotational spectra. Atoms are treated as hard spheres with fixed masses

connected by springs with assigned bending and stretching force constants. Electrostatic

attractions and repulsions are also included. The sum of the equations taking all these

factors into account is called the “force field” and molecular mechanics calculations are

sometimes called force field calculations. A number of force fields have been developed

in recent years, each with a slightly different set of basic assumptions and

approximations. A simple force field can be Etotal = Estretch + Ebend + Etorsion + Enonbonded.

When two atoms A and B approach each other, resulting in the formation of the

diatomic molecule A—B, the energy of the system follows the Morse Potential energy

plot. As A and B approach from infinity, the electrons around each nucleus become

polarized, leading to an attractive London or dispersion force. The energy continues to

decrease until the equilibrium bond length ro is reached. At shorter bond lengths,

repulsion between electron charge clouds becomes increasingly effective and the energy

quickly mounts due to van der Waals repulsion.

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Bond stretching forces may be represented by a Hooke’s law type of interaction,

Estr = kstr ( r- ro )2 / 2. Similarly the bending interactions may be approximated by Ebend =

kbend (θ - θo)2 / 2 and Etorsion = V F (sin ω), where ω is the angle of rotation. The final

term in the simplified force field is a nonbonding interaction reflecting the London

forces that are attractive at long distances and are counterbalanced by van der Waals

forces of repulsion. The bond is formed at a distance between the atoms as they

approach the sum of their van der Waals radii. This term has been represented in a

number of different forms by the developers of various force fields. Many of these start

with Leonard Jones potential

Enonbonded = e [ (ro/ r)12 — 2 (ro/ r)6 ]

where e is the energy value at the minimum in the curve coinciding with ro. Each force

field defines a mechanical model to be used in computing the molecular structure and

its accompanying energy.

Once an input structure has been entered into the program, a set of parameters

describing the molecular geometry is computed i.e., bond lengths, bond angles and

torsion angles. These values are then fed into the terms of the force field equation and a

steric energy is calculated. Generally, this energy is expressed in kcal/mol and is the

sum of all the potential energy terms contained in the force field. The steric energy is a

value specific for a given force field. Such numbers cannot be used to compare values

calculated by other programs. Furthermore, as a generalization, such steric energies

cannot be used to compare the relative stabilities of different molecules, though they

may be used to compare different conformations of the same molecule. To alter the

structure of the molecule in a systematic way to minimize the steric energy, a variety of

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mathematical approaches such as Newton-Raphson method are used. The minimum

energy structure obtained in this way is the nearest local minimum on the potential

energy surface of the molecule.

Molecular mechanics are used primarily to simulate the structures of larger

molecules and macromolecules. It can also be extended to arrays of hydrogen bonded

molecules. Molecular dynamics, which uses the same force fields, permits the

exploration of transitions between different conformations separated by torsional energy

barriers. It can also be extended to clusters of molecules and to simulate the effect of

solvation.

(ii) Quantum-mechanical calculations

Calculating the structure, energies and other physical properties of an assembly

of nuclei and electrons in molecules is an extraordinarily complex problem.

Nevertheless, an ab initio quantum mechanical calculation has become an important

method for understanding hydrogen bonding. After 1980, it has become possible for

theoreticians to aim for computations for simple dimers, which reproduced the binding

energies within the uncertainties of the experimental measurements.

a) Ab-initio calculation

Ab-initio molecular orbital theory is concerned with predicting the properties of

atomic and molecular systems. This method bases on the fundamental laws of quantum

mechanics and uses a variety of mathematical transformation and approximation

techniques to solve its fundamental equations.

The ab-initio methods seek minimum energy intra molecular and inter molecular

geometry by solving the wave equation using a Linear Combination of Atomic Orbitals

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known as the LCAO approximation. Coefficients are attached to each of the electron

atomic orbitals and varied to obtain the minimum energy. Polarization effects are

introduced by adding p-orbitals to the s-orbitals of hydrogen, d-orbitals to the s and p

orbitals of first row elements and so on. To facilitate the calculations, these atomic

orbitals have to be expressed in an analytical form using basic functions. This is known

as the basis set. When the basis set becomes more sophisticated, the computing time

needed increases considerably. The computing time is roughly proportional to the fourth

power of the basis functions. The greater the basis set, the closer the result comes to the

Hartree-Fock limit. However, sometimes simpler basis sets give better agreement with

experimental data due to cancellation of errors.

b) Semiempirical methods

In the late 1970s, interest in the development of semiempirical methods has

grown in the laboratories of M.J.S. Dewar at the University of Texas, as the ab-initio

methods are extremely time costly. No integrals are evaluated in these semiempirical

methods. When the integral values are required, experimental numbers are used in their

places. The parameterization is then tested against a limited set of molecules to ensure

its accuracy. Like the numeric iteration, this process is a continuing one, until

convergence in energy parameters is achieved. AM1 (Austin Model 1) is a recent model

parameterized to treat hydrogen bonds successfully.

(iii) Monte Carlo simulations

The Monte Carlo method of searching uses probability theory. This method is

different from other molecular dynamic methods in solving the Newtonian equations of

motion without generating successive configurations. Instead, a starting configuration is

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subjected to a random series of changes in atomic positions. Each new configuration is

minimized, and the potential energy is compared to that of the preceding structure. If

the new energy is less, the structure is kept. If the new energy is greater, it is discarded

except in the case where the term exp (-∆V/kT) is less than a randomly chosen number

between 0 and 1. Monte Carlo methodology is important in studying large protein and

polymer molecules that are not readily studied by molecular mechanics. Monte Carlo

methods are often used to compute thermodynamic quantities by statistical

thermodynamics.

1.5 Chosen systems for the Present study

In the present work, the following compounds are chosen in order to study the

hydrogen bonding between two individual systems. These compounds of AR grade are

purchased from E. Merck, Germany and are purified by standard methods. The

formation of hydrogen bond between the individual systems is conformed from the

experimental FT-IR spectra in the range 400 cm-1 - 4000 cm-1 by using the Perkin Elmer

FT-IR spectrometer. The experimental dipole moment and FT-IR values are compared

with the theoretical quantum mechanical ab initio and semiempirical calculations and

are in reasonable agreement with one another within the error limits. Further, various

dielectric and thermo dynamic parameters are studied on these systems to analyze the

formation of hydrogen bonding.

The chosen pure systems (monomers) are:

Alcohols:

1) Propan-1-ol

2) Propan-2-ol

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Alkyl benzoates:

3) Methyl benzoate

4) Ethyl benzoate

Physical and chemical properties of the Pure Systems:

Alcohols:

Alcohols play an important role in many chemical reactions due to their ability

to undergo self-association with manifold internal structures. They are in wide use in

industry and science as reagents, solvents and fuels. Liquid alcohols represent a

favorable system for evaluating the importance of both hydrophilic and hydrophobic

interactions in determining the relevant properties of the liquid phase. A substantial

effort is being taken to understand the mechanisms of molecular interactions associated

with the hydrogen bonds. The knowledge and understanding of the molecular

mechanisms in systems containing alcohols constitute a valuable source of information

for theoretical as well as experimental analysis of the same mechanism in more general

systems.

1. Propan-1-ol:

Propan-1-ol is a primary alcohol with molecular formula C3H8O. It is also

known as 1-propanol, 1-propyl alcohol, n-propyl alcohol, n-propanol or simply

propanol with abbreviation 1PN. It is formed naturally in small amounts during several

fermentation processes. It is an isomer of propan-2-ol. It is used as a solvent in the

pharmaceutical industry, and for manufacturing resins and cellulose esters.

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2. Propan-2-ol:

Propan-2-ol is a secondary alcohol with molecular formula C3H8O. It is also

known as Isopropyl alcohol and 2-propanol with the abbreviation IPA. It is a colorless,

flammable chemical compound with a strong odor. In this compound, the alcohol

carbon is attached to two other carbons sometimes shown as (CH3)2CHOH. As a

biological specimen preservative, propan-2-ol provides a comparatively non-toxic

alternative to formaldehyde and other synthetic preservatives. Propan-2-ol solutions of

concentration 90-99% are optimal for preserving specimens, although concentrations as

low as 70% can be used in emergencies.

Alkyl Benzoates:

Alkyl Benzoate (Aromatic ester) solvents are a remarkable family of fluids with

technological applications in very different fields. This group of molecules is

characterized by the presence of a polarizable aromatic ring in the vicinity of the dipolar

ester group, the combination of both properties, together with their aprotic and

hydrophobic character; confer them with a highly selective solvent ability. Aromatic

ester packings are disrupted upon mixing although diverse effects arise depending on

the nature of the second compound of the mixture.

3. Methyl benzoate:

Methyl benzoate is an Aromatic ester with chemical formula C6H5COOCH3 and

is abbreviated as MB (It is formed by the condensation of methanol and benzoic acid, in

the presence of a strong acid such as hydrochloric acid). It is a colorless liquid that is

poorly soluble in water but miscible with organic solvents. Methyl benzoate has a

pleasant smell, strongly reminiscent of the fruit of the feijoa tree, and it is used in

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perfumery. It also finds use as a solvent and as a pesticide used to attract insects. It is

one of many compounds which is attractive to males of various species of orchid bees,

which apparently gather the chemical to synthesize pheromones. It is commonly used as

bait to attract and collect these bees for study.

4. Ethyl benzoate:

Ethyl benzoate is also an Aromatic ester with chemical formula C6H5COOC2H6

and is abbreviated as EB (It is formed by the condensation of benzoic acid and ethanol).

It is a colorless liquid that is almost insoluble in water, but miscible with most organic

solvents. Like many volatile esters, ethyl benzoate has a pleasant odor. It is a

component of some artificial fruit flavors. Ethyl benzoate is used in as a perfume scent

under the name essence de niobe.

The binary systems in the present study are:

1) Propan-2-ol + Methyl benzoate (System 1)

2) Propan-2-ol + Ethyl benzoate (System 2)

3) Propan-1-ol + Methyl benzoate (System 3)

4) Propan-1-ol + Ethyl benzoate (System 4)

24

Table 1.1: Physical and Chemical properties of the pure systems

Property Compound

Propan-1-ol Propan-2-ol Methyl Benzoate Ethyl Benzoate

Molecular

formula

C3H8O C3H8O C8H8O2 C9H10O2

Molar Mass 60.10 g mol-1 60.10 g mol-1 136.15 g mol-1 150.18 g mol-1

Appearance Colorless liquid Colorless liquid Colourless liquid Colourless liquid

Density 0.7955 g/cm3 0.785 g/ cm3 1.0873 g/ cm3 1.0425 g/ cm3

Melting Point 147 K 184 K 260.5 K 239 K

Boiling Point 370.1 K 355.5 K 472.6 K 484- 486 K

Dipole moment

(gas) [11]

1.66 D

1.66 D

1.86 D

1.95 D

Molecular

Structure

(Red: Oxygen, Black: Carbon and White: Hydrogen)

1.6 Literature survey

Most of the dielectric relaxation processes reported in the literature were studied

for dilute solutions of polar substance in non-polar liquids. The non-polar liquid does

not itself undergo relaxation but alters the relaxation time of the solute molecule by

reducing the internal field and changing the viscosity. Relatively little work has been

done on mixtures of polar components. The study of dielectric dispersion and

absorption of a binary liquid mixture provides a very sensitive tool for detecting

molecular interactions. The formation of complexes or presence of association leads to

25

relaxation times considerably higher than those for the uncomplexed (unassociated)

species. In many cases, the interaction between the constituents may not be sufficiently

strong to allow the formation of stable complex. Even in such cases, the relaxation time

has increased because of the increased resistance to rotation due to attractive influence

of neighboring molecules. It has been shown by Schallamach [12] and others [13] that a

binary liquid mixture in which both the components are either associated or non

associated shows a single relaxation time, whereas two distinct relaxation times are

observed in mixtures with one associated and other non associated components.

Crossley et al. [14], Garg and Grag [15] have studied the complex dielectric

permittivity of six isomeric octyl alcohols at different concentrations in a non polar

solution i.e., n-heptane at different microwave frequency ranges and showed the

existence of three relaxation process in alcohols, dominated by low frequency Debye

type process with a single relaxation time. The three dispersion regions of the pure

primary alcohols are described by a short relaxation time 3τ which is attributed to

hydroxyl group rotation, an intermediate relaxation time 2τ which is attributed to the

orientational motions of the small molecular species and the dominant long relaxation

time 1τ which is associated with the hydrogen bonded structure in the liquid.

Kadaba [16] observed that the concentration dependence of the Kirkwood

correlation factor showed a minimum at particular concentration while diluting alcohols

with non polar solvents. This means qualitatively that dilution leads to the formation of

cyclic multimers with anti parallel dipoles.

26

Winklemann [17] has developed comprehensive relations linking the complex

permittivity of binary mixtures with short range and long-range interactions as an

extension of Kirkwood theory.

The theoretical approach and computer simulation study were reported to

understand the relaxation behavior and the hydrogen bonding in alcohols by Minami et

al. [18] and Padro et al. [19].

The dielectric relaxation of mixture of water and primary alcohols using time

domain reflectometry technique was studied by Satoru Mashimo et al., [20].

Brilliantov et al. [21] has explained the continuum theory of the rotational

motion of a uniformly charged solute molecule in a non polar solvent which takes into

account dielectric friction, dielectric saturation and spatial dependence of the solvent

viscosity due to electrostriction.

The characteristic features of the supramolecular clusters which comprise the

QCE (Quantum Cluster Equilibrium) model are discussed in terms of binding energies

and geometries of alcohols by Huelsekopf and Ludwig [22].

Schwerdtfeger and Kohler [23] have studied the relaxation behavior of

monohydric alcohols with n-alkanes where as Hiejima and Yao [24] have studied

dielectric relaxation behavior of alcohols in fluid phase.

Conformational and dielectric analysis of hydrogen bonded polar binary

mixtures of methyl benzoate and N-methyl aniline have been reported by Chitra et al.,

[25].

27

Chaudhari et al. [26] have studied the parallel and anti parallel alignment of

dipoles with the variation of excess permittivity and rotation of dipoles with excess

inverse relaxation time.

Kaatze et al. [27] has explained the wait-and-switch model of dipole

reorientation in hydrogen bonded systems of alcohols in a polar and non-polar medium.

A car-Parrinello molecular dynamics simulation has been performed on fully

deuterated liquid methanol by Paglai et al. [28]. The results are compared with the latest

available experimental and theoretical data. It has been shown that the liquid aggregates

in chains of hydrogen bonded molecules. The structure of the aggregates is

characterized and it is found that the dynamics include fast and a slow regime. The

weak H- bond formed by the methyl group hydrogen and oxygen atoms of surrounding

molecules has been characterized.

Handgraal et al. [29] has explained the density-functional theory based

molecular dynamics of methanol. The structural, dynamical and electronic properties of

liquid methanol under ambient conditions were analyzed in the study.

Pawar et al. [30] have reported the temperature-dependent dielectric relaxation

study of chlorobenzene with n-methylformamide from 10 MHz to 20 GHz using time

domain reflectometry (TDR) in the temperature range 15 °C to 45 °C for 11 different

concentrations of the system.

Madhurima et al. [31] have studied the effect of steric hindrance of ketones in

the dielectric relaxation of binary mixtures of methanol and ketone and compared the

experimental and theoretical dipole moment values obtained from quantum mechanical

calculations such as ab initio and semiempirical methods.

28

Thenappan et al. [32] have studied the nature of intermolecular interactions

between associative and non-associative polar liquids and explained the formation of

cyclic and linear α-multimers in the binary mixtures.

Dharmalingam et al. [33] have studied the solute-solvent interaction which

explains the ordering of the molecules in the liquid phase with the help of Kirkwood

correlation factor, and also explained the creation of dimers and monomers with the

help of excess thermodynamical parameters.

Parthipan et al. [34] have studied the thermodynamical parameters of anisole

with 2-ethyl-1-hexanol and decyl alcohol to analyze the heteromolecular interactions.

Pang et al. [35] have measured the densities and viscosities of binary aqueous

solutions of 1-propanol and 2-propanol over the whole composition range at

temperatures between 293.15 K and 333.15 K. The activation free energies for viscous

flow for aqueous solutions of 1-propanol and 2-propanol were calculated and found to

be 17.94 and 22.16 kJ mol− 1, respectively.

Jimenez et al. [36] have studied the electromagnetic behaviour of polar and non-

polar dielectric mixtures of the type alcohol + n-alkane from primary alcohols 1-

pentanol, 1-hexanol, and 1-heptanol using TDR technique.

Vishwam et al. [37] have studied the formation of hydrogen bond between

propionaldehyde and isopropylamine from experimental and theoretical calculations

and explained the absence of ionic structure from excess dipole moment values.

Aparicio et al. [38] have determined the microwave dielectric spectroscopy of

methyl benzoate in different alkenes and alcohols and explained the different dielectric

parameters for the conformation of hydrogen bond.

29

Lone et al. [39] used the bilinear calibration method to obtain the dielectric

parameters viz., static dielectric constant ( 0ε ) and relaxation time. Using these

parameters the values of excess permittivity ( Eε ), excess inverse relaxation time,

Bruggeman factor ( Bf ) and thermodynamic parameters are determined.

Kinart et al. [40] have measured the densities and relative permittivities, at

T = (293.15, 298.15 and 303.15) K, in the binary mixtures of 2-ethoxyethanol with

ethylene glycol, diethylene glycol, tri ethylene glycol and tetra ethylene glycol as a

function of composition. From the experimental data the excess molar volumes and

deviations in the relative permittivity have been calculated. The results are discussed in

terms of intermolecular interactions and structural properties of studied binary mixtures.

Yomogida and Nozaki [41] have done the Complex permittivity measurements

on acetophenone and its derivatives o-hydroxy benzaldehyde, o-methyl acetophenone,

and o-hydroxy acetophenone are performed at frequencies between 1 MHz and 20 GHz

at temperatures from 273 to 323 K. The parameters obtained from the fitting of the

complex permittivity are analyzed in order to study the effects of the hydroxyl group

within a molecule on the dielectric relaxation phenomenon in these liquids. The analysis

indicates that dynamical properties are affected not only by the intermolecular hydrogen

bond but also by the slight change in molecular structure.

Lileev and Lyashchenko [42] have measured the high-frequency dielectric

permittivity, dielectric losses and low-frequency conductivity of aqueous solutions of

ammonium salts at 288, 298 and 308 K. These values characterize the mobility of water

molecules in solutions.

30

Prajapati et al. [43] have reported the dielectric relaxation and dispersion studies

of mixtures of 1-propanol and benzonitrile in pure liquid state at radio and microwave

frequencies and explained the molecular interaction between the molecular species of

the liquid mixtures with the dielectric parameters.

Yomogida et al. [44] have reported the complex permittivity measurements of a

series of normal alcohols (methanol, ethanol, 1-propanol, 1-butanol, and 1-pentanol)

which are performed by THz time-domain spectroscopy in the frequency range of 0.2–

2.5 THz at temperatures of 253–323 K.

Ghanadzadeh Gilani et al. [45] have made the dielectric measurements of the

binary polar mixtures of the butanediols with 2-ethyl-1-hexanol (2EH) for various

concentrations at 298.2 K to study the Kirkwood correlation factor, Bruggeman factor

and excess permittivity.