DSpace Collection:http://localhost:8081/xmlui/handle/123456789/242024-01-05T16:51:04Z2024-01-05T16:51:04ZDESIGN AND SYNTHESIS OF N, O-CONTAINING HETEROCYCLESKhan, Danishhttp://localhost:8081/xmlui/handle/123456789/156042023-07-27T11:55:00Z2020-10-01T00:00:00ZTitle: DESIGN AND SYNTHESIS OF N, O-CONTAINING HETEROCYCLES
Authors: Khan, Danish
Abstract: The thesis entitled “Design and synthesis of N, O-containing heterocycles” is reported
in five chapters.
The present work is aimed to design and synthesis of N and O containing
heterocyclic compound via different novel methodology involving C―C, C―O, C―N
bonds formation using different reactions like knoevenagel condensation, Aza Michael
addition with sequential dehydrogenation, oxiadative C―C bond cleavage and Ullman
type reaction. The synthesized compounds were characterization through different
analytical techniques such as 1H NMR, 13C NMR, HRMS and x-ray diffraction and their
biological evaluations were reported as calcium calmodulin dependent protein kinase
applications. Heterocyclic compounds are widely distributed in nature. All living things are made up
of genetic material and this genetic material are made up of DNA, Which contains
heterocycles moities like purine and pyrimidine as base. Beside these naturally occurring
alkaloids such as morphine, vinblastine, papaverine, phenazine; anibiotics such as
penicillin, cephalosporin moieties contains heterocyclic rings. Vitamin B complex
contains corrin and benzimidazole heterocyclic rings. There are many of natural product
having heterocyclic rings such as ascorbic acid, chlorophyll, hemoglobin, ATP, soft
drinks, dyes, amino acid. The colour of flowers due to anthocyanin which is the large
class of flavonoid family. Consequently the design of structurally diverse heterocyclic
compound is one of the driving forces for the development of organic chemistry. The
ability to create new heterocyclic compounds in an involuntary and efficient manner is
fundamental in many fields; for instance, in the industry heterocyclic compounds are used
in making cosmetics, textiles, plastics, lubricants and paints. Moreover they are most
extensively used in pharmaceuticals, because 90% of the medicines like analgesic,
antitumor, anticancer, antihypertensive drugs are made up of heterocyclic compounds.
Therefore, the precise synthesis of heterocyclic compounds is one of the main aims for
an organic chemist.2020-10-01T00:00:00ZSTUDIES ON NEW TRANSITION METAL COMPLEXES AND THEIR REACTIVITIESMawai, Kiranhttp://localhost:8081/xmlui/handle/123456789/148882021-06-22T12:05:10Z2018-12-01T00:00:00ZTitle: STUDIES ON NEW TRANSITION METAL COMPLEXES AND THEIR REACTIVITIES
Authors: Mawai, Kiran
Abstract: The thesis entitled “Studies on new transition metal complexes and their reactivities” is
divided into seven chapters. In this report, we have presented rationally designed
compartmental ligands which were utilized for the synthesis of mononuclear, dinuclear and
tetranuclear metal complexes based on first-row transition elements viz. manganese, iron,
nickel, copper and zinc. The chemical drawings of ligands utilized in this thesis are shown
below in Figure 1.
Figure 1 Chemical drawings of ligands utilized in the present thesis report.
Chapter 1: Introduction
This chapter includes the brief introduction of basic aspects of coordination chemistry
including different metal ions and ligand employed in the thesis report. The introduction of
several metalloenzymes, catalytic reactions and biomimetic reactivity studies were discussed.
The various chemical methods and spectroscopic techniques used were comprehensively
summarized in this chapter. The role of metal ions in association with rationally designed
ligand frame in exhibiting different type of catalytic activities.
R = -CH3, HLMe
-OCH3, HLOMe
-C(CH3)3, HLtBu
LNN
Thesis Abstract
v
Chapter 2: Dinuclear μ-phenoxo and μ-hydroxo bridged compartmental copper
complexes exhibiting broad spectrum applications in oxidation chemistry of phenols
and isoelectronic compounds: Cytotoxicity and evidences for cellular apoptosis
In this chapter three compartmental pentadentate ligands HLMe, HLOMe, HLtBu have
been synthesized, characterized and utilized for the synthesis of three dinuclear hydroxo and
phenolato bridged copper complexes viz. [Cu2(LMe)(μ-OPh)(μ-OH](ClO4)2]CH3CN; (1),
[Cu2(LOMe)(μ-OPh)(μ-OH)(μ-OH2)](ClO4)2.CH3CN; (2) and [Cu2(LtBu)(μ-OPh)(μ-
OH)(OH2)](ClO4)2; (3) respectively. All the three complexes were characterized by various
spectroscopic techniques. Tyrosinase activity was studied by in situ reaction of Cu(I) and
ligands exploiting 2,4-di-tert-butyl phenol as substrate leading to radical mediated C-C
coupling reaction. The catalytic activities of complexes 1-3 on di-phenols and related
substrate molecules were investigated and all three complexes efficiently oxidize 3,5-di-tertbutylcatechol
to 3,5-di-tert-butylquinone. Treatment of copper complexes on o-aminophenol
and o-phenylene diamine resulted in the oxidative coupling of substrate molecules, leading to
the formation of phenoxazinone and diaminophenazine respectively. Furthermore, oxidative
properties of reported copper complexes were studied in relevance to their ability in inducing
apoptotic cell death. Remarkably, all three copper complexes (1, 2 and 3) exhibited excellent
cytotoxic activity towards MCF-7 cell lines (human breast cancer cell lines) with IC50 values
of 1.6 μM, 2.0 μM and 0.2 μM respectively
Self-activated DNA cleavage Cellular apoptosis
Thesis Abstract
vi
Chapter 3: Facile synthesis and crystal structure of phenoxyl radical complex of zinc:
Synthesis of benzoxazole derivatives by C-H bond activation via hydrogen atom
transfer (HAT) and oxidation of dihydroanthracene
In this chapter a tetranuclear zinc complex [Zn4(LOMe)2(μ2-OPh)2(μ3-OH)2(μ2-
OAc)2](ClO4)3.2CH3CN (4) and a dinuclear diphenolato bridged complex [Zn2(LOMe)2(μ2-
OPh)2](ClO4)2.2H2O (5) have been synthesized from compartmental ligand HLOMe (4-
methoxy-2,6-bis(-(2-phenyl-2-(pyridine-2yl)hydrazono)methyl)phenol) using zinc acetate
(Zn(OAc)2).2H2O) and zinc perchlorate (Zn(ClO4)2.6H2O) as precursor salts respectively.
Complexes 4 and 5 were characterized by spectroscopic studies and molecular structures
were determined by X-ray crystallography. Presence of phenoxyl radical in the solid state of
complex 4 was authenticated by single-crystal X-ray diffraction, EPR, resonance Raman, and
UV-Vis spectral studies. Theoretical calculations were performed to investigate structural
parameters, spin density and resonance Raman spectral data. Redox properties of the
complexes were examined. Stable zinc phenoxyl radical complex 4 derived from
compartmental ligand served as a functional mimic of galactose oxidase enzyme. The
hydrogen atom abstracting tendency of phenoxyl radical complex was supported by
hydrogen atom transfer (HAT) reaction of 9,10-dihydroanthracene via C-H activation and
synthesis of anthracene. This has been exploited for oxidative cyclization of Schiff base
derivatives leading to the formation of benzoxazole derivatives via C-H bond activation
under very mild conditions.
Zn1
Zn2
Zn4
Zn3
O13
O16
H16
N1
N3
N4
N6
C18
O14
O15
N12
N10
N9
N7
O17
O20
H20
O18
O19
O21
O22
C51
Thesis Abstract
vii
Chapter 4: Combined experimental and theoretical studies on selective sensing of zinc
and pyrophosphate ions by rational designing of compartmental chemosensor probe:
Dual sensing behaviour via secondary recognition approach and cell imaging studies
In this chapter ligand HLMe has been used as selective chemosensor probe for the
selective recognition of zinc ions over other transition metal ions via fluorescence “ON”
strategy. The chemo-sensing behaviour of HLMe has been demonstrated through
fluorescence, absorption and NMR spectroscopic techniques. The molecular structure of the
zinc complex [Zn2(LMe)2(μ2-OPh)2](ClO4)2.2H2O (6) derived from HLMe was determined by
X-ray crystallography. A probable mechanism of this selective sensing behavior was
described on the basis of spectroscopic results and theoretical studies by density functional
theory (DFT). The biological applicability of the chemosensor (HLMe) was examined via cell
imaging on HeLa cells. This zinc complex served as secondary fluorescent probe responding
for the pyrophosphate anion specifically over other anions. The fluorescence enhancement of
HLMe in association with Zn2+ ions was quenched due to the presence of pyrophosphate
(PPi). Thus a dual response has been established based on “OFF-ON-OFF” strategy for
detection of both cation and anion. This phenomenon was utilized in the construction of
“INHIBIT” logic gate.
Zn2+
PPi
450 500 550 600 650 700
0
20
40
60
80
100
120
Fluorescence Intensity (a.u.)
Wavelength(nm)
HL+Zn
HL
HL+Zn+
PPi
(Ba2+) (Co2+) (Mn2+) (Fe3+) (Lig) (Zn2+) (Na+) (Cu2+) (Hg2+) (Mg2+) (Cd2+)
Thesis Abstract
viii
Chapter 5: Study of cytotoxic activity of mononuclear Fe(II) and Mn(II) complexes
based on bidentate ligands: Metal ions controlling catalytic performance towards SOD
activity and nuclease activity
In this chapter, mononuclear iron(II) and manganese(II) complexes
[Fe(LNN)3](ClO4)2, (7) and [Mn(LNN)3](ClO4)2, (8) derived from bidenate ligand LNN, have
been synthesised and characterized by various spectroscopic techniques. The X-ray crystal
structures of the complexes revealed the tris coordination of three ligand moieties around
metal centres providing distorted octahedral coordination environment. The SOD activity of
both complexes was evaluated by following Xanthine/Xanthine oxidase assay. Complex 8
manifested efficient SOD activity with an IC50 values of 1.72 μM in contrast to the low-spin
iron(II) complex which showed some anomalous behaviour. DNA cleavage activity through
gel electrophoretic studies using pUC18 DNA were also studied revealing self activated
DNA cleavage activity of 7. The cytotoxic nature of both complexes was determined on
MCF−7 cell line. Complex 7 proved to be highly efficient in inducing nuclear deformation,
promoting the apoptosis in the cells having IC50 value of 2.97 μM while complex 8 did not
show cytotoxic activity. In situ reactive oxygen species generation has been further supported
via DPPH (2,2-diphenyl-1-picrylhydrazine) radical quenching studies and DCFDA images.
1 2 3 4 5 6 7 lane
Cleavage of supercoiled pUC18 plasmid DNA at 37 C (100 ng) by complex 2 using 2%
DMF as a medium after incubation at 37 C for 30 min. DNA control (lane 1); DNA +
Mn(ClO4)2.4H2O (lane 2); DNA + 2 (2μM) (lane 3); DNA + 2 (10μM) (lane 4); DNA +
2 (20μM) (lane 5); DNA+ 2 (60μM) (lane 6); DNA+ 2 (100μM) (lane 7).
Form II
Form III
Form I
Form II
Form III
Form I
1 2 3 4 5 6 7 8 lane
Cleavage of supercoiled pUC18 plasmid DNA at 37 C (100 ng) by complex 1 using 2%
DMF as a medium after incubation at 37 C for 30 min. DNA control (lane 1); DNA +
Fe(ClO4)2.xH2O (lane 2); DNA + ligand (20μM) (lane 3); DNA + 1 (2μM) (lane 4); DNA
+ 1 (10μM) (lane 5); DNA + 1 (20μM) (lane 6); DNA+ 1 (40μM) (lane 7); DNA+ 1
(60μM) (lane 8).
M = Fe, Mn
2+
Self-activated DNA cleavage
Cellular apoptosis
SOD activity
1 2 3 4 5 6
0
20
40
60
80
100
120
Concentration (M)
% Inhibition
8
Thesis Abstract
ix
Chapter 6: Study of cytotoxic activity of mononuclear Fe(II) and Mn(II) complexes
based on bidentate ligands: Metal ions controlling catalytic performance towards SOD
activity and nuclease activity
This chapter contains the report of synthesis and characterization of two nickel
complexes [Ni2LMe(μ−Oph)(μ−OH2)(μ-NO3)(H2O)](NO3) (9) and [Ni4(LMe)2(μ−Oph)2 (μ−
OH)2(μ-OAc)2](NaPF6) (10). Complex 9 is a dinuclear phenolato bridged, aqua bridged
complex synthesized on employing nickel nitrate tetrahydrate as precursor metal salt while
complex 10 was synthesized using nickel acetate as metal starting material. The
characterizations of both complexes have been done by different spectroscopic methods like
1H NMR, IR spectra and UV-visible spectral studies along with X-ray crystallographic
studies. Both complexes 9 and 10 were identified as paramagnetic complexes. The
complexes were testified for the Suzuki-Miyaura reaction and both 9 and 10 were proved to
be efficient cross-coupling agents under mild conditions and low catalyst loading for aryl
iodide and phenyl boronic acid leading to carbon‒carbon bond formation which ultimately
resulted in the synthesis of biphenyl. It is notable to mention here that no extra reducing
agent was added to carry out the cross-coupling reaction.
⁺
Catalysts
100 C, THF, Base
Iodobenzene
Phenyl boronic
acid
Biaryl (biphenyl)
Thesis Abstract
x
Chapter 7: Synthesis, characterization and reactivity studies on dinuclear manganese
(II,II) and mononuclear iron(III) complexes
This chapter reports the rational designing and synthesis of dinuclear diphenolato
manganese complex [Mn2(LMe)2(μ−Oph)2](ClO4)2 (11) and mono nuclear bis-coordinated iron
complex [Fe(LMe)2(μ− Oph)2](ClO4).2CH3OH (12) based on compartmental ligand HLMe. The
characterization of both complexes was done by different spectroscopic methods such as IR
spectral studies, UV‒visible spectroscopy, and electrochemical studies. Molecular structures
of both complexes were determined by X‒ray crystallography. The oxidative capability of
both complexes was investigated utilizing a range of substrates including hydrocarbons,
alcohols and substituted thio ether. Both the complexes were found to be efficient as
oxidation catalysts in presence of hydrogen peroxide. The generation of intermediate species
were justified using UV-Vis spectroscopy and ESI-MS spectral analysis2018-12-01T00:00:00ZDIVERSE FUNCTIONALIZED PORPHYRINS FOR CATALYTIC, EET AND SENSOR APPLICATIONSDar, Tawseef Ahmadhttp://localhost:8081/xmlui/handle/123456789/148782020-09-10T06:11:21Z2018-01-01T00:00:00ZTitle: DIVERSE FUNCTIONALIZED PORPHYRINS FOR CATALYTIC, EET AND SENSOR APPLICATIONS
Authors: Dar, Tawseef Ahmad
Abstract: Porphyrins and their metal derivatives are of considerable importance owing to their applications in various fields such as catalysis, sensors, non-linear optics, light emitting diodes and dye sensitized solar cells (DSSCs). In living systems, porphyrins are very vital to many enzymes and biomolecules which perform significantly important physiological processes like photosynthesis, gas transport, redox reactions and so on. Porphyrins are chemically versatile molecules due to their conformational flexibility and rich thermal and chemical stabilities. They also have remarkable optical, photophysical and electrochemical redox properties which make them robust precursors for obtaining industrially, medicinally and scientifically suitable chemicals. On account of the above properties, porphyrins are the most studied molecules in chemistry and are made to undergo innumerable amount of reactions. Herein, we report the synthesis and characterization of meso- and/or β-functionalized porphyrins and their utilization in a variety of applications like catalysis, energy transfer and sensing of anions and cations. The proposed thesis will consist of the following chapters.
Chapter 1 gives a general introduction to the field of porphyrins, their synthesis and different functionalization procedures undertaken to mould them into desirable target molecules. It also gives a peek into the material applications of porphyrins including dye sensitized solar cells, photodynamic therapy, catalysis, molecular sensing and nonlinear optics.
Chapter 2 describes the synthesis of vanadyl-5,10,15,20-tetrakis(3,5-dimethoxyphenyl) porphyrin and its hexadecabromo derivative through facile synthetic procedures. The synthesized compounds were characterized by various spectroscopic techniques including UV-Vis., FTIR, EPR spectroscopy, MALDI-TOF mass spectrometry and single crystal X-ray diffraction analysis. The hexadecabromo derivative is highly nonplanar as observed from its crystal structures and electrochemical studies. Catalytic studies (alkene epoxidation) were successfully carried out in CH3CN/H2O solvent mixture and ambient temperature resulting in significantly enhanced TOF numbers even with low catalyst loading. Remarkably, the hexadecabromo compound biomimics Vanadium Bromoperoxidase (VBrPO) enzyme with very high TOF value
vi
for oxidative bromination of thymol. Both the catalysts were successfully recovered at the end of the reactions indicating their viability and industrial applicability.
Chapter 3 describes the synthesis, characterization and catalytic applications in the epoxidation of alkenes by tetrasubstituted porphyrin molecules viz. vanandyl tetrabromotetraphenylporphyrin and vanadyl tetracyanotetraphenylporphyrin. The synthesized porphyrins are functionalized with oppositely natured ligands and were analyzed for their catalytic application in the epoxidation of differently structured alkenes. The catalytic reactions were carried out in CH3CN/H2O mixture in 3:1 ratio. Both displayed good efficiency in terms of mild reaction conditions, lower temperature and little catalyst amounts. Both the catalysts exhibited excellent selectivity, high conversion efficiency and huge TOF numbers in significantly less reaction times of 0.5 h. This work highlights that nature of the substituent (electron withdrawing or electron releasing) affects the efficiency of the catalyst whereas the ligand strength (strong or weak) does not influence it.
Chapter 4 describes the synthesis of a new series of aryloxyporphyrins bearing benzyl and naphthyl substituents viz., Zn(II)tetrabenzylporphyrin (ZnTBPP), Zn(II)tetranaphthylporphyrin (ZnTNPP), and Zn(II)octanaphthylporphyrin (ZnONPP). The synthesized porphyrins have been utilized as donors to interact with fullerene C60 acceptor to form 1:1 complex in toluene at 298 K. The subsequent investigation into fluorescence quenching measurements with concomitant increase in fullerene concentration revealed effective quenching constants. The calculated association constants were in the order of 103 M‒1. However, ZnTNPP exhibited higher binding constant as compared to other analogues due to effective π-π interactions. ZnONPP exhibited 3−4 folds lower association constant as compared to ZnTNPP due to steric hindrance offered by meso-3,5-dinapthyloxyphenyl groups. The geometric and electronic structure of Zn(II) porphyrin-fullerene dyad was optimized by DFT calculations which suggested the possibility of charge transfer from meso-aryloxyporphyrin core to fullerene C60.
Chapter 5 describes the synthesis of pyrenyl porphyrin and its metal complexes. Pyrene is a polyaromatic hydrocarbon (PAH) and PAHs have made a lasting impression in the field of molecular electronics and organic semiconductors. They have an extended π-electron cloud which reduces their HOMO−LUMO gap and makes them more suited for such devices. They also have very high charge mobility of the order of 10 cm2V-1S-1 in certain cases. The synthesized porphyrins were studied for their relative electrochemistry with tetraphenyl
vii
porphyrins. DFT optimized geometries revealed that the porphyrin core and the pyrenyl substituents remain essentially orthogonal to each other in both free base and metallated porphyrins. Fӧrster energy transfer studies in toluene displayed efficient energy transfer of the order of 80 to 85 percent in case of free base pyrenylporphyrin and zinc pyrenylporphyrin. Furthermore, it was observed that energy transfer occurs mainly via through-bond (TB) interaction and very less via through-space (TS) interaction.
Chapter 6 describes the facile synthesis of nitrovanillin porphyrin. The hydroxyl group bearing porphyrin was synthesized in good yield and successfully utilized in colorimetric “naked eye” detection of toxic CN‒ and F‒ ions. The synthesized probe detects CN‒ and F‒ ions via anion induced deprotonation mechanism, which brings about drastic spectral and colorimetric changes. Other commonly known interfering anions such as OAc‒, H2PO4‒, Cl‒, Br‒, I‒, ClO4‒, HSO4‒ and PF6‒ failed to bring such changes. The toxic CN‒ and F‒ ions interact selectively with the –OH protons as was established via 1H NMR, UV-Visible and fluorescence spectroscopic methods. The sensor could detect F‒ ions up to ~1.1 ppm and CN‒ ions up to ~1.34 ppm which is very appreciable and promising.
Chapter 7 descirbes the synthesis of Ni(II) porphyrins having fused –NH groups. The synthesized porphyrins were used to detect species of opposite polarity. Fused –NH porphyrin was used to sense toxic anions viz. cyanide and fluoride ions, whereas fused –CHO porphyrin was used for the detection of some selective metal ions including toxic mercury(II) ions. Acidic –NH protons detect anions via hydrogen bonding interactions which leads to deprotonation. On the other hand, sensing of metal species takes place via weak charge transfer interactions between oxygen atom of formyl group and the metal ions. Water washing reverses the interactions and regenerates the original porphyrins which makes them reversible probes. The detection limit (LOD) was found to be 2.13 ppm for cyanide and 3.15 ppm for fluoride ions, respectively. Similarly, the detection limit was found to be 0.930 ppm, 2.231 ppm and 0.718 ppm for Cu(II), Fe(III) and Hg(II) ions respectively.
Chapter 8 serves as the concluding chapter summarizing the results of this work and also the future perspectives.2018-01-01T00:00:00ZMOLECULAR DYNAMICS STUDIES OF MIXED GAS HYDRATE MELTS AND AQUEOUS INTERFACESK S, Sujithhttp://localhost:8081/xmlui/handle/123456789/148772020-09-11T14:35:08Z2018-01-01T00:00:00ZTitle: MOLECULAR DYNAMICS STUDIES OF MIXED GAS HYDRATE MELTS AND AQUEOUS INTERFACES
Authors: K S, Sujith
Abstract: Increasing global energy demand has necessitated the search for energy resources which
can substitute the fast depleting conventional reserves. In the last few decades, gas hydrates have
gained significant attention as a promising future energy resource. The advantages of gas
hydrates as an energy resource includes their abundance in under ocean and permafrost regions
and the low level of CO2 emission during the combustion of natural gas compared to other
carbon based fuels. Several techniques for the extraction of natural gas from hydrate sediments
have been developed. All these techniques involve the dissociation of the crystalline hydrate
structure which is accompanied by the release of large amount of gas and water. The liquid phase
formed during hydrate dissociation contains hydrate forming gas molecules dissolved in it at
high concentrations. The evolution of dissolved gas molecules from this liquid is known to have
a significant effect on the kinetics of hydrate dissociation. Earlier studies on the evolution of
dissolved gas from the hydrate melt were limited to the case of only one type of gas molecule in
the melt. The effect of thermodynamic hydrate inhibitors on the evolution of dissolved gas is also
not well understood. The present thesis attempts to apply molecular dynamics simulations to
study the process of dissolved gas evolution from aqueous solutions containing one or more
types of the hydrate forming gases at conditions typical to natural gas extraction.
An important physical system which is of significance to gas hydrate formation as well as
atmospheric chemistry is the interface between liquid and gas. One of the most studied liquid-gas
interface is the one between water and methane which is known to act as preferred sites for
hydrate nucleation due to the high concentration of dissolved gas at the interface. The interaction
of methane with water at their interface also has an important role in atmospheric processes such
as adsorption of methane on aqueous aerosols. Despite this, the current understanding of
structure and dynamics of this interface is insufficient to explain important interfacial processes
such as methane dissolution. In the present thesis, molecular dynamics simulations are applied to
study the structure of methane-water interface at a molecular level. The role of interfacial
structure and the presence of an amphiphilic cosolvent on the adsorption and dissolution of
methane at the interface is also examined.
In Chapter 1 of the thesis, various methods for natural gas extraction through hydrate
dissociation and the factors affecting the rate of dissociation are discussed. The chapter also
reviews earlier studies on the evolution of dissolved gas molecules and its effect on hydrate
i
dissociation. Following this, findings from reported studies on the structure and dynamics of the
liquid-gas interface are briefly reviewed.
The second chapter briefly discusses the computational methods applied in the present
work. The functional forms of the various interaction potentials and integrator algorithms applied
in molecular dynamics simulations are discussed and a brief outline of the simulation procedure
is given.
In chapter 3, evolution of dissolved gas in the CH4-CO2-H2O ternary mixture is
investigated. The study of CH4-CO2-H2O mixture is important since it is formed during the
extraction of methane from hydrate sediments by its replacement in hydrate cages with CO2.
Classical molecular dynamics simulations of the ternary mixture of varying compositions were
performed which revealed that evolution of gas molecules from the mixture involves the
formation of nanobubbles. The study also revealed that an increase in the concentration of CO2
enhanced the formation of bubbles in the mixture. To understand the role of CO2 in promoting
bubble formation, the structure and composition of the nanobubbles formed were examined in
terms of the average distribution of gas molecules. The analysis revealed that bubbles formed in
the mixture are of mixed type with both gas molecules present inside them. The average
distribution of gas molecules in the bubble indicated that CO2 molecules accumulate at the
interface between the bubble and the surrounding liquid phase. The CO2 molecules at the
interface interact with CH4 through direct contact which is energetically favorable than gas-water
interactions. The value of surface tension at the bubble-water interface was calculated which
revealed that the presence of CO2 reduces the surface tension thereby enhancing the stability of
the interface. The greater stability of the interface decreases the critical size of the bubble nuclei
leading to rapid bubble formation. The results suggest that an increase in concentration of CO2
assists the evolution of dissolved gas from the CH4-CO2-H2O mixture thereby preventing the
accumulation of methane in the liquid phase. Thus, the presence of CO2 assists the
decomposition of methane hydrates in the initial stages of the replacement process.
Chapter 4 investigates the effect of thermodynamic hydrate inhibitors on natural gas
evolution from a hydrate melt. The effect of two common hydrate inhibitors, NaCl and CH3OH
on the formation of nanobubbles by dissolved gas molecules was studied. Both types of
inhibitors considered are found to assist the formation of methane nanobubbles in the CH4-H2O
system. NaCl promotes bubble formation by enhancing hydrophobic interaction bewteen
aqueous gas molecules. Whereas, enhanced bubble formation in the presence of CH3OH is
ii
attributed to its amphiphilic nature. These molecules are found to accumulate at the interface
between bubble and water with the methyl group oriented towards the gas phase. The presence of
CH3OH at the interface makes the nanobubble more stable by reducing the excess pressure inside
the bubble as well as surface tension at the interface. The evolution of dissolved gas from the
CH4-CO2-H2O mixture containing hydrate inhibitor was also examined. It is observed that, for a
given concentration of the inhibitor, nanobubble nucleation is more rapid in the CH4-CO2-H2O
ternary system compared to that in CH4-H2O system. The nanobubble formed in the ternary
system contains both CH4 and CO2 in it and the composition of the bubble is found to be
influenced by the type and concentration of the inhibitor molecules present. The stability and
properties of the nanobubble is related to its interaction with the surrounding liquid. A
quantitative analysis of the bubble-liquid interaction revealed that a frequent exchange of gas
molecules takes place between the bubble and the surrounding liquid. The frequency of this gas
exchange is found to decrease with an increase in the concentration of hydrate inhibitor and also
with an increase in the radius of the bubble. The observed trends in bubble-liquid interaction are
explained in terms of the excess pressure inside the bubble and the solubility of gas molecule in
the surrounding liquid phase.
Considering the significance of methane-water interface to gas hydrates and atmospheric
chemistry, the molecular level structure of this interface was investigated. The results of the
study are discussed in chapter 5. Earlier studies on the adsorption of methane molecules on the
water surface did not consider the effect of the inherent molecular level roughness of the surface
on gas adsorption. Therefore, adsorption of methane on water surface was examined by taking
into account the roughness of the surface. A quantitative analysis of roughness was performed in
which the extend of roughness was expressed in terms of the amplitude of humps and wells of
the surface as well as the frequency at which these are present at the surface. The simulation of
methane-water interface at different pressures indicated that an increase in the pressure makes
the water surface more rough in terms of amplitude of the humps and wells. The adsorption of
methane on the rough water surface was analyzed by identifying the methane molecules in direct
contact with the surface as well as those which are held slightly above the surface by attractive
non-covalent methane-water interactions. The analysis revealed that a greater fraction of
methane molecules in direct contact with the surface are present at the humps of the surface. In
contrast, the density of methane molecules above the surface is higher near the wells compared
to the humps. The results indicate a clear preference for methane to come in direct contact with
iii
the water surface at the humps rather than at the wells. This is caused by a lower density of water
at the humps of the surface layer which reduces the free energy penalty associated with the
formation of a cavity between water molecules which methane can occupy.
In chapter 6, dissolution of methane at its interface with methanol-water liquid mixture is
investigated. Methanol is a commonly used hydrate inhibitor and is also known to act as a
cosolvent for methane in water. The entry of methane into the bulk liquid region of the
methanol-water mixture is examined by determining the average density profile of methane
along the direction perpendicular to the interface. The results indicate that molecular level
surface roughness of the methanol-water liquid mixture has a role in methane dissolution with
humps of the surface acting as preferred channels for the entry of methane into the bulk liquid.
Analysis of surface roughness of the methanol-water mixture indicates that the surface becomes
more rough with an increase in the concentration of methanol. The humps and wells at the
surface of methanol rich mixtures are larger in terms of their amplitude compared to the case of
mixtures with lower concentrations of methanol. The larger humps on the surface of methanol
rich mixtures can assist the entry of methane into the liquid by acting as channels for methane
dissolution. The results suggest that the effect of methanol on roughness of liquid surface has a
role in the enhanced solubility of methane in aqueous methanol. The effect of methane on the
orientation of the methanol molecules at the surface of the methanol-water mixture was also
examined. The results indicate that the presence of methane significantly increases the tendency
of surface methanol molecules to have their methyl group oriented towards the gas phase of the
interface.
The summary and future scope of the present study is provided in chapter 7.2018-01-01T00:00:00Z