Post on 18-Jun-2020
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Chapter 4
Part-A: Gas Phase
Studies of the Ionic
Liquids Through
Mass Spectrometry
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4(A).1.Introduction and Literature Survey
Chemical analysis plays an increasingly important role in our complex natural
and post-industrial world by helping to determine the nature of materials of all types.
Among analytical methods, mass spectrometry (MS) has the distinctive capability of
providing high sensitivity and high selectivity in the detection and quantitation of a
wide variety of chemical and biological compounds. Sir J. J. Thomson was pioneered
the term Mass Spectrometry, in 1910, the first mass spectra was recorded by Thomson
[190]. Organic Chemists never gave any importance to the mass spectrometry upto
1950. After that the knowledge of gaseous ion chemistry, improved the role of mass
spectrometry [191]. Today, organic mass spectrometry has become an integral part of
organic chemistry. Along with infrared, ultraviolet and nuclear magnetic resonance
spectroscopy, it is an indispensable tool for molecular structure elucidation. There are
many other applications in the field of chemistry. The rapid growth of mass
spectrometry has been due to the ingenious and intuitive applications in the areas of
drugs and pharmaceuticals, physical sciences, chemical sciences, polymer sciences,
life and environmental sciences [192-196]. It finds much of its importance in
increasing, not replacing, the effectiveness of other techniques. In addition to that the
linking up ability of mass spectrometry with chromatography (i.e. hyphenated
technique), its parallel unit (i.e. tandem mass spectrometry) and the use of computers
to analyze mass spectral data have added new dimensions to mass spectrometry [197-
199].
Molten salts, fused salts or ILs with their widespread applications demand an
inherent requirement to determine the nature and purity of synthesized ILs, and to
characterize and quantify the variety of compounds dissolved within them. These pose
an interesting analytical problem, because the properties of ILs do not always readily
53
lend themselves to conventional analytical techniques. A research article by Jackson
and Duckworth [200] gives an excellent account of the various aspects of mass
spectrometry in the study of ILs. It is particularly noteworthy that this gave an
excellent coverage for the early attempts to obtain electron ionization (EI) mass
spectra and chemical ionization (CI) mass spectra of ILs, which were unsuccessful,
presumably because ILs have negligible vapor pressure and cannot be readily
transferred to the gas phase. Because of their inherently low vapour pressure, fast
atom bombardment (FAB) MS has been the most extensively used MS technique for
characterizing ILs. The ILs may be dissolved in a matrix such as glycerol, or analysed
without dissolution. FAB-MS spectra of ILs are typically characterized by a dominant
peak for the unbound cation followed by clusters of the form , where A is
the cation and B is the anion. One may also perform mass analyses of ILs by
dissolving them in a suitable solvent and performing electrospray mass spectrometry.
By referring this, we report and discuss here an ESI-MS/MS studies of 1-ethyl-3-
methylimidazolium bromide,1-methyl-3-propylimidazolium bromide,1-butyl-3-
methylimidazolium bromide, and N-butyl-pyridinium bromide,3-3‘-bis-(indolyl)-
phenylmethane, 3-3‘-bis-(indolyl)-4-chlorophenylmethane, 3-3‘-bis-(indolyl)-4-
methoxyphenylmethane, 3-3‘-bis-(indolyl)-3,4-(dimethoxy)-phenylmethane, 3-3‘-bis-
(indolyl)-cinnamyl-phenylmethane, and the template used TBAB. Herein, we include
pyridinium salt in the category of ILs although it has a melting point higher than
1000C. The particulars of instrumentations and the mechanism involved in ESI, triple
quadrupole system followed by the detailed discussion on the results obtained by
means of pattern of fragmentation is presented in following sections.
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4 (A).1.2. Electrospray ionization (ESI)
ESI ―is a soft ionization technique that accomplishes the transfer of ions from
solution to the gas phase. The technique is extremely useful for the analysis of large,
non-volatile, chargeable molecules such as proteins and nucleic acid polymers.‖ In
which the solution is composed of a volatile solvent and the ionic analyte is at very
low concentration, typically In addition, the transfer of ions from the
condensed phase into the state of an isolated gas phase ion starts at atmospheric
pressure and leads continuously into the high vacuum of the mass analyzer. This
results in a marked softness of ionization and makes electrospray the ―wings for
molecular elephants‖. Another reason for the extraordinary high-mass capability of
ESI is found in the characteristic formation of multiply charged ions in case of high-
mass analytes. Multiple charging folds up the m/z scale by the number of charges and
thus compress the ions into the m/z range of standard mass analyzers.
A review article attempted by Eberlin [201] gives an account of the important
applications of ESI in organic transformations. This review also enlightens the
significance of the conjugation of ESI-MS with its tandem version. More recently, a
review article by Traldi et al [202] has provided a comprehensive account of
developments in the ESI process. Though Fenn et al [203] have documented spray
action for ESI, but whole array of orders involved during the process of ionization,
modern aids in electrospray and formation of droplet was systematically described by
Hoffmann and Stroobant. Hoffmann and Stroobant‘s book entitled ―Mass
Spectrometry: Principles and applications‖ [204], gives considerable details of the
mechanism of ESI. A highly complex mechanism due to series of physical and
chemical processes result ions of the analytes starting from their solutions. The
Schematic of the mechanism of ion formation in ESI is shown in figure 4(A).1.
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Mechanically an ESI is produced by applying a strong electric field, under
atmospheric pressure, to a liquid passing through a capillary tube with a weak flux
(normally 1–10 μlmin−1
). The electric field is obtained by applying a potential
difference of 3–6 kV between this capillary and the counter-electrode, separated by
0.3–2 cm, producing electric fields of the order of 106Vm−1
(Figure 4(A).1).This field
induces a charge accumulation at the liquid surface located at the end of the capillary,
which will break to form highly charged droplets. A gas injected coaxially at a low
flow rate allows the dispersion of the spray to be limited in space. These droplets then
pass either through a curtain of heated inert gas, most often nitrogen, or through a
heated capillary to remove the last solvent molecules. H-ESI is now very popular
Strategy for mass spectrometer, its superiority and attachment ability draw more and
more attention for various improvements.
Figure 4(A).1: Diagram of electrospray sources, using skimmers for ion focalization and a
curtain of heated nitrogen gas for desolvation [top], or with a heated capillary
for desolvation (bottom)
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With this great development in the range and scope of mass spectrometry, a
book by Christopher G. Herbert and Robert A. W. Johnstone [205] entitled ―Mass
Spectrometry Basics‖ is a particularly important source of information on the design
and construction of high resolution mass spectrometers and gives an excellent account
of the salient features of the mass spectrometers and technical advances made in the
field.
Figure 4(A).2: Pneumatically assisted ESI
The schematic diagram of the process of pneumatic electrospray for the
formation of ionic aerosols before their generalization as inlets in organic chemistry
applications is shown in figure 4(A).2, in which the sample solution flows or is
pumped along a capillary tube, the end of which is held at a high positive or negative
electrical potential. Because of the electrical charge, the surface of the solution at the
outlet of the capillary also becomes charged and is repelled by the existing electric
field of the same sign. If the capillary tube is narrow enough, the liquid inside is
forced out of the end of the capillary, and the surface of the liquid is rounded with a
high radius of curvature. This point of liquid leads to repelling a steady stream of
charged droplet into a desolvation chamber. If the charged capillary tube is also
surrounded by an uncharged concentric capillary, nitrogen or other gas can be blown
through the annular space between the capillaries, which can be used to aid droplet
formation, as with concentric capillary pneumatic nebulizers.
57
As we are familiar with the process of formation of the spray, this could be
initiated at an ‗onset voltage‘ that, for a given source, depends on the surface tension
of the solvent. If one examines with a microscope the nascent drop forming at the tip
of the capillary while increasing the voltage, as schematically displayed in Figure
4(A).3, at low voltages, the drop appears spherical, then elongates under the pressure
of the accumulated charges at the tip in the stronger electric field; when the surface
tension is broken, the shape of the drop changes to a ‗Taylor cone‘ and the spray
appears. Gomez and Tang [204] were able to obtain photographs of droplets formed
and dividing in an ESI source. A drawing of a decomposing droplet is displayed in
Figure 4(A).3.
Figure 4(A).3: Decomposition of droplet
After the interpretation of the results, Gomez and Tang concluded that
breakdown of the droplets can occur before the limit given by the Rayleigh equation
is reached because the droplets are mechanically deformed, thus reducing the
repulsion necessary to break down the droplets. Figure 4(A).4 shows a model of the
deformation of droplet in an electrospray source.
Figure 4(A).4: A decomposing droplet in an ESI source: q- charge, є0- permittivity of the
environment, γ- surface tension, and d- diameter of supposed spherical droplet.
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The solvent contained in the droplets evaporates, which causes them to shrink
and their charge per unit volume to increase. Under the influence of the strong electric
field, deformation of the droplet occurs. The droplet elongates under the force
resulting from the accumulation of charge, similarly to what occurred at the probe tip,
and finally produces a new Taylor cone. From this Taylor cone, about 20 smaller
droplets are released. Typically a first-generation droplet from the capillary will have
a diameter of about 1.5 μm and will carry around 50,000 elementary charges. The
offspring droplets will have a diameter of 0.1 μm and will carry 300 to 400
elementary charges. The total volume of the offspring droplets is about 2% of the
precursor droplet but contain 15% of the charge. The charge per unit volume is thus
multiplied by a factor of seven. The precursor droplet will shrink further by solvent
evaporation and will produce other generations of offspring. These small, highly
charged droplets will continue to lose solvent, and when the electric field on their
surface becomes large enough, desorption of ions from the surface occurs. Charges in
excess accumulate at the surface of the droplet. In the bulk, analytes as well as
electrolytes whose positive and negative charges are equal in number are present at a
somewhat higher concentration than in the precursor droplet. The desorption of
charged molecules occurs from the surface. This means that sensitivity is higher for
compounds whose concentration at the surface is higher, thus more lipophilic ones.
When mixtures of compounds are analyzed, those present at the surface of droplets
can mask, even completely, the presence of compounds which are more soluble in the
bulk. When the droplet contains very large molecules, like proteins for example, the
molecules will not desorb, but are freed by evaporation of the solvent. This seems to
occur when the molecular weight of the compounds exceeds 5000 to 10,000 Da.
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4 (A). 1.3. Triple Quadrupole Mass Spectrometers
The triple-quadrupole MS (TQMS) is a tandem arrangement, as illustrated in
(Figure 4(A).5). J. Gross [206] in his comprehensive book entitled ―Mass
Spectrometry: A Textbook‖ a set of two editions reported excellently step by step
modifications in the field with the help of citing standard literature. A somewhat
different type of book made its appearance in 2007, which was by Watson and
Sparkman [207] describing instrumentation, applications and strategies for data
interpretation involved in mass spectrometry.
Triple quadrupole mass spectrometers, QqQ, have almost become a standard
analytical tool for LC-MS/MS applications, in particular when accurate quantization
is desired. Ever since their introduction, they have continuously been improved in
terms of mass range, resolution, and sensitivity.
Figure 4(A).5: Schematic of a triple quadrupole mass spectrometer
To operate triple quadrupole mass spectrometers in the MS/MS mode, Q1
serves as MS1, the intermediate RF-only device, q2, acts as ―field-free region‖ for
metastable dissociations or more often as collision cell for CID experiments, and Q3 is
used to analyze the fragment ions exiting from q2. Typically, the mass selected ions
emerging from Q1 are accelerated by an offset of some ten electron volts into q2
where the collision gas (N2, Ar) is provided at a pressure of 0.1–0.3 Pa. In triple
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quadrupole instruments Q1 and Q3 are operated independently as MS1 and MS2,
respectively, making MS/MS a straightforward matter. The experimental setups for
product ion, precursor ion, and neutral loss scanning are summarized in Table 4(A).1.
Table 4(A). 1: Scan modes of triple quadrupole instruments
Scan Modea
Operation at Q1 Operation at q2 Operation at Q3
product ion,
define m1
no scan, select m1 metastable or CID scan up to m1 to
collect its fragments
precursor ion,
define m2
scan from m2 upwards
to cover potential
precursors
metastable or CID no scan, select m2
constant neutral loss,
define ∆m
scan desired range metastable or CID scan range shifted by
∆m to low mass a Masses for reaction, m1
+ = m2
+ + n
4(A).2.Experimental
All compounds were synthesized in the laboratory. The details of the synthetic
methodology and spectral characterization (FT-IR and 1H-NMR) have been reported
in chapter 2 and 3 of the thesis. We carried out the mass analysis with the help of TSQ
Quantum Access triple stage quadrupole mass spectrometer system by Thermo
Scientific (USA). The system also consists of Finnigan Surveyor Auto-sampler Plus,
and Surveyor MS pump Plus. The system was tuned by infusing a low concentration
of tuning and calibrating solution that contains polytyrosine-1, 3, 6 directly into the
H-ESI source using a syringe pump at a flow rate of 2 µL/min. so as to achieve the
best ion beam intensity and stability, the singly charged, positive ions for poly-
tyrosine monomer, trimer and hexamer: m/z 182, 508 and 997 respectively were
observed. The system was then flushed with the mobile phase Methanol: Formic acid
(0.1%) of solute in the composition of 75:25. A Full Scan positive mode analysis of
the sample in the Q1MS mode was carried out to find out the Parent masses of the
various ILs. The parent masses of the ILs are summarized in Table 4(A).2-4(A).6.The
system was then operated in the MS +MS/MS mode for optimization by introducing
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sample in the manual loop injection. The instrumental MS/MS conditions were: Spray
voltage: - 4500 V, Vaporizer temperature: - 150V, Sheath gas pressure: - 40,
Auxiliary Gas Pressure: -15, Capillary Temperature: - 300V, and the collision
Pressure: - 1.5 m Torr. The sample was injected and data acquisition was carried out
for all the ILS. The masses of the compounds in the sample were identified in this
mode. Argon was used as a collision gas and nitrogen as a nebulizing gas.
4(A).3. Results and Discussion
This section is divided in two parts out of which first having a discussion on
ESI-MS/MS studies of the products obtained from Menschutkin quaternization
reaction (synthesis of ILs) of 1-methylimidazole and Pyridine with alkyl halide(s) by
employing molten TBAB as a template. In the similar way the second section covers
the detail study of the compounds which were synthesized via condensation reaction
between indole and aromatic aldehydes.
4(A).3.1. ESI-MS/MS studies of ILs
To our knowledge, only a few published reports deal with mass spectral
analysis of ILs [196, 202, 204-210]. The details of mass spectrums of the template
used, and all the four synthesized ILs are presented in figures 4(A).6 - 4(A).10 and
discussed below. In addition to this, we present here a likely detailed pattern of
fragmentation, which is evidenced earlier in the literature by using different ionization
methods. This involves the migration of β-proton with subsequent removal of the
neutral species i.e. alkenes, ring expansion through tropylium ion, loss of alkenes, the
McLafferty rearrangement, and the removal of heterocycle i.e. aziridine and pyridine.
The aqueous chemistry of tetra-alkyl-ammonium salts is very important. Such salts
also form clathrate-hydrates [215]. The properties like heat capacity, partial molal
volumes and entropies have led to concepts like hydration, solute-solute hydrophobic
62
interaction, micellization water structure making effect etc. [216, 217]. Therefore it
was felt that the mass spectral study of this compound would be interesting. The mass
spectrum is presented and analyzed.
The ESI-MS/MS spectral data obtained for tetra-butyl-ammonium ion is
depicted in Table 4(A).2 (entry 1a-1j) and our obtained pattern is shown in figure
4(A).6. The thermal decomposition mass spectrum for the said compound (TBA
cation) is reported in the literature [206] and shows close resemblance with the
present profile. The literature data have been explained on the basis of decomposition
of tri-n-butyl ammonium system. We observed the peaks at m/z = 242.315, 186.234,
142.129, 130.179, 100.204, 72.239, 58.245 (Table 4(A). 2), respectively which are in
harmony with the reported pattern. However, we observe additional peak resolutions
at 446.348, 414.253, 349.848 m/z respectively. These are being attributed to cation-
cation-cation or cation-anion-cation association in gas phase.
63
Table 4(A).2: ESI-MS/MS data for tetra-butyl-ammonium bromide
Compound Entry Observed m/z$
Calculated m/z*
Formula
Tetra-butyl-ammonium
bromide
1a 446.348 446.541 [C28H68N3]+
1b 414.253 414.479 [C26H60N3]+
1c 349.848 351.237 [C17H40N2Br]+
1d 242.315 242.285 [C16H36N]+
1e 186.234 186.222 [C12H28N]+
1f 142.129 142.159 [C9H20N]+
1g 130.179 130.160 [C8H20N]+
1h 100.204 100.113 [C6H14N]+
1i 72.239 72.081 [C4H10N]+
1j 58.245 58.066 [C3H8N]+
$- Experimentally observed values
* - Values calculated using Isotopic mass of the element [206]
The mass spectrum of tetra-butyl-ammonium ion (m/z – 242.165) can be
accounted in terms of loss of neutral species (butene and propene), migration of a
proton, and the McLafferty rearrangements as shown in scheme 4(A).1 and referring
to Table 4(A).2. The imminium ion obtained at m/z = 58.245 involves two
consecutive classical McLafferty rearrangements, as shown in scheme 4(A).1. The
peak observed at m/z=72.081 is due to removal of ethylene from the C6H14N+
(m/z =
100.113) ion.
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N+
C4H9
C4H9
C4H9
H9C4CH2
CHCH2
CH3
N+
H
C4H9
C4H9
H9C4N+
H
C4H9
C4H9
H
CH2
CHCH2
CH32
1g [C8H20N]+
(m/z - 130.160) 1d [C16H36N]+ (m/z - 242.285) 1e [C12H28N]
+ (m/z - 186.222)
H2
N+
CH2
C4H9
H9C4
1f [C9H20N]+ (m/z - 142.159)
CH3
CCH2
H
N+
CH2
CH3
H9C4
CH3
CCH2
H
N+
CH2
CH3
CH3
CH2 CH2N
+
CH2
CH3
H5C2
1i [C4H10N]+ (m/z - 72.081) 1h [C6H14N]
+ (m/z - 100.113)
1j [C3H8N]+ (m/z - 58.066)
CH2
CHCH2
CH33
N+
H
C4H9
H
HH2
N+
C4H8
H
H
1i [C4H10N]+ (m/z - 72.081)
CH2 CH31. 2.
Scheme 4(A). 1: Proposed pattern of fragmentation for tetra-butyl-ammonium bromide
The peak observed at m/z 446.348 is assigned to association of tri-n-butyl-
ammonium ion and two ions of di-n-butyl-ammonium ion (i.e. cation-cation-cation
association). The m/z 414.253 peak can be viewed as a combination of tetra-butyl-
ammonium cation (m/z=242.285), N-butyl-N-methyl imine cation (m/z= 100.113)
and N-ethyl-N-methylimine cation or butylimine cation (m/z= 72.081). A
combination of di-n-butyl-ammonium ion and N, N-dibutyl-imminium cation with a
65
bromine (anion) results in a species having molecular weight m/z=349.221. All these
mean that there is a clusterization of ammonium ions in the form of triplet cation-
cation-cation (+ + +) or triplet cation-anion-cation (+ - +) associations in gas phase.
The studies of excess thermodynamic properties of mixed aqueous solutions of
electrolytes have assumed great interest in recent years, since these provide useful
information on the interaction between ions of the same or different charge either
triplet or high order interactions [218-220]. In light of this, we studied the gas phase
behavior of tetra-butyl-ammonium bromide closely and which indicates the existence
of the triplet ion (+ + +) or (+ - +) interactions in gas phase. i.e. cation-cation-cation
or cation-anion-cation triplet forming species even in gas phase. We believe that
interactions get attenuated in presence of water and lead to the concepts involving
aggregation, micelles, and hydrophobic interactions.
66
Figure 4(A). 6: ESI-MS/MS spectrum of tetra-butyl-ammonium bromide. The inset describes the resolution of the parent TBA cation of tetra-butyl-
ammonium bromide
67
Table 4(A).3: ESI-MS/MS data for 1-ethyl-3-methylimidazolium bromide
Compound Entry Observed
m/z $
Calculated
m/z*
Formula
1-ethyl-3-
methylimidazolium
bromide
2a 303.113 302.111 [C12H22N4Br] +
2b 111.215 111.092 [C6H11N2]+
2c 83.227 83.061 [C4H7N2]+
$- Experimentally observed values
* - Values calculated using isotopic mass of the element [206]
N+
NCH3
CH3
NN+
CH3
CH3Br
-
NN+
CH3
CH3
Br-
NN+
CH3
CH3
[C12H22N4Br]+ (m/z = 302.111) [C6H11N2]+ (m/z = 111.092)
NN+
CH3
H
CH2 CH2NN
+
H CH3
[C6H11N2]+ (m/z = 111.092) [C4H7N2]
+ (m/z = 83.061)
Scheme 4(A).2: Proposed pattern of fragmentation for 1-ethyl-3-methylimidazolium bromide
The mass spectral data of 1-ethyl-3-methylimidazolium bromide are tabulated
in Table 4(A).3 (entry 2a-2c) and described in figure 4(A).7. It involves peaks at m/z
303.113, 111.215 and 83.227. The 303.113 peak is assigned to formation of the class
C2A+. We propose a pattern of fragmentation as shown in scheme 4(A).2 for 1-ethyl-
3-methylimidazolium bromide which involves migration of β-hydrogen, forming an
imidazolium cation having mass 83.061 (entry 2c).
68
Figure 4(A). 7: ESI-MS/MS spectrum with proposed pattern of fragmentation of compound 1-ethyl-3-methylimidazolium bromide
N NCH2
CH3
H3CBr
69
Table 4(A). 4: ESI-MS/MS data for 1-methyl-3-propylimidazolium bromide
Compound Entry Observed
m/z$
Calculated
m/z*
Formula
1-methyl-3-
propylimidazolium
bromide
3a 329.121 329.134 [C14H26N4Br] +
3b 125.225 125.108 [C7H13N2]+
3c 83.265 83.061 [C4H7N2]+
$- Experimentally observed values
* - Values calculated using isotopic mass of the element [206]
N+
NC3H7CH3
NN+
H7C3 CH3
Br-
NN+
H7C3 CH3
Br-
NN+
H7C3 CH3
[C14H26N4Br]+ (m/z = 329.134) [C7H13N2]+ (m/z = 125.108)
NN+
H7C3 CH3
CH2 CH3NN
+
H CH3
[C7H13N2]+ (m/z = 125.108) [C4H7N2]
+ (m/z = 83.061)
Scheme 4(A). 3: Proposed pattern of fragmentation for 1-methyl-3-propylimidazolium
bromide
The mass spectra of 1-methyl-3-propylimidazolium bromide reveals the peak
at m/z 83.265 is the daughter ion of the parent having m/z 125.225 (figure 4(A).8). In
the Table 4(A).4 entry 3a is the associated triplet (+ + -) having m/z = 329.121. We
suggest a scheme on the basis of the peaks observed which are accounted in terms of a
pattern of fragmentation as shown in scheme 4(A).3. It shows loss of propene
molecule through the migration of β-hydrogen.
70
Figure 4(A).8: ESI-MS/MS spectrum with the proposed pattern of fragmentation of compound 1-methyl-3-propylimidazolium bromide
N NCH2
H2C
H3CCH3
Br
71
Table 4(A).5: ESI-MS/MS data for 1-butyl-3-methylimidazolium bromide
Compound Entry Observed
m/z$
Calculated
m/z*
Formula
1-butyl-3-
methylimidazolium
bromide
4a 359 358.170 [C16H30N4Br] +
4b 139.174 139.124 [C8H15N2]+
4c 83.209 83.061 [C4H7N2]+
4d 41.486 42.034 [C2H4N]+
$- Experimentally observed values
* - Values calculated using isotopic mass of the element [206]
N+
NC4H9CH3
Br-
NN+
H9C4 CH3
Br-
NN+
H9C4 CH3
[C16H30N4Br]+ (m/z = 358.170) [C8H15N2]+ (m/z = 139.124)
NN+
H9C4 CH3
CH2 C3H6NN
+
H CH3
[C8H15N2]+ (m/z = 139.124) [C4H7N2]
+ (m/z = 83.061)
NN+
H9C4 CH3
NN+
H CH3
N
H
CH N+
CH3
[C4H7N2]+ (m/z = 83.061) [C2H4N]+ (m/z = 42.034)
Scheme 4(A).4: Proposed pattern of fragmentation for 1-butyl-3-methylimidazolium bromide
The closer scrutiny of the mass spectrum of [bmim][br] [figure 4(A).9] shows
peaks at m/z 139.174 which is the daughter ion of the parent which have m/z = 359
(the associated triplet of + - + ions). Similarly the peak at m/z 139.174 gives the
daughter ion at m/z 83.209 (table 4(A).5). The occurrence of these peaks as shown in
figure 4(A).9 can be accounted by scheme 4(A).4. It includes pattern of
fragmentation, where we have utilized the rearrangement in which β-hydrogen
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migrates with loss of neutral species (i.e. Butene), resulting in an ion having m/z
83.061 (entry 4c). Further interesting pattern of fragmentation is observed in the case
of the peak at m/z 41.486, which involves removal of neutral species alkenes propene,
followed by loss of three member nitrogen heterocycle (i.e. Aziridine).
73
Figure 4(A).9: ESI-MS/MS spectrum with the proposed scheme of fragmentation of compound 1-butyl-3-methylimidazolium bromide
N NCH2
H2C
H3CCH2
CH3
Br
74
Table 4(A).6: ESI-MS/MS data for n-butyl-pyridinium bromide
Compound Entry Observed
m/z$
Calculated
m/z*
Formula
n-butyl-pyridinium
bromide
5a 351.3 351.143 [C18H28N2 Br] +
5b 136.127 136.113 [C9H14N]+
5c 80.16 80.05 [C5H6N]+
5d 57.421 57.07 [C4H9]+
$- Experimentally observed values
* - Values calculated using isotopic mass of the element [206]
N+
CH3
CH2CH3
N+
H
[C9H14N]+ (m/z - 136.113) [C5H6N]
+ (m/z - 80.05)
N+
CH3
N[C4H9]
+
[C9H14N]+ (m/z - 136.113) (m/z - 57.07)
Scheme 4(A).5: Proposed pattern of fragmentation for n-butyl-pyridinium bromide
The results depicted in Table 4(A).6 (entry 5a-5d) for n-butyl-pyridinium
bromide show the peaks at m/z =351.3 (an associated triplet of + - + ions), 136.127,
80.16 and 57.421 respectively. We suggest a pattern of fragmentation as described in
scheme 4(A).5. It seems that the rearrangement concept utilized earlier helps us to
identify the presence of ions having m/z value very close to 5b, 5c and 5d ions.
75
Figure 4(A).10: ESI-MS/MS spectrum with the proposed pattern of fragmentation of compound n-butylpyridinium bromide
N+
CH3Br
-
76
Lattice energy of ammonium salts (particularly imidazolium and pyridinium)
is responsible for their very low melting nature, such compounds consist of a large
cation and an anion that is very difficult to fit into the lattice. Mass spectrums
obtained by us (Figures 4(A). 6-10) also indicate presence of associations for tetra-
alkyl-ammonium ions, imidazolium ions and pyridinium ions in the form of triplet
interaction species [C2A]+. We believe that such association may be due to cation -
cation affinity assisted by π-π stacking interaction due to the ring electrons. In the
studies of physico-chemical processes involved in the existence of ionic liquids
(imidazolium and pyridinium salts), Dupont and Suarez [221] have indicated the
presence of π-π stacking via weak C-H—π interaction in methyl and imidazolium ring
π system in solid phase. Such cation-cation π-π stacking interactions have also been
indicated in theoretical studies in gas phase [222]. Association of several imidazolium
and pyridinium ions also has been postulated [C2A]+ by Gross [223]. It is difficult to
quantify the energetics of such interactions, however the spectral, simulation and ab
initio studies do indicate the vital structural effects [224, 225]. We suggest that such
molecular arrangements can generate channels in which the bromide anion is
accommodated as chains. The said structural pattern in the form C2A+ may be
visualized in a form as given in figure 4(A).11.
N+
NRCH3
NN+
R CH3
Where R-= b) -C2H5, c) -C3H7, d) -C4H9
N+
CH3
N+
CH3
Br- Br
-
Figure 4(A).11: Visualized associated triplets of imidazolium and pyridinium ions with
bromide anion
77
4(A).3.2. ESI-MS/MS analysis of BIMs
It is well known that, the exceptionally large number of alkaloids and
medicinally important compounds are derived from indole. Its reactions, and
particularly the synthesis of complex derivatives, occupy central stage in heterocyclic
chemistry. As comparable to other indole derivatives, family of bis-(indolyl)-
methanes (BIMs from here) are well known for their biological activity. BIMs are
potential chemo-preventive, and have been phyto-chemically derived from Brassica
vegetables [148].
In no other class of natural products has the utility of mass spectrometry been
recognized so rapidly as in the field of indole and related alkaloids. While the first of
several pioneering studies by Biemann and collaborator appeared in 1960, number of
articles from various laboratories had been published in the intervening years, and
there is no doubt that mass spectrometry has now become an indispensable
component of alkaloid chemist‘s armamentarium of physical methods. As will be
shown in this chapter of the thesis, the recent structure elucidation of BIMs alkaloids
rests on mass spectroscopic evidence obtained with minute amount of material. A
comprehensive book by Budzikiewicz, Djerassi and Williams [226] manifests the role
of mass spectrometry in the structure elucidation of natural products, this book
emphasizes the importance of mass spectrometry to probe the structures of alkaloids
of indole family.
There are two principal reasons why indole alkaloids lend themselves so
readily to mass spectroscopic investigation. First, they contain two centers the indole
nucleus (aromatic π electrons and /or indole nitrogen atom) and a basic nitrogen atom,
which have a great capability of for stabilizing a positive charge. Second in most
instances they possess a carbocyclic framework, where certain bonds are especially
78
prone to rupture, thus giving rise to a very few intense fragment ions. Even slight
changes in this carbocyclic framework can lead to a completely different mass
spectral fragmentation pattern, which can be used either for identification purposes or
at least for assignment of an unknown alkaloid to a specific sub-group.
The advancement in the field of mass spectrometry provides a broad spectrum
of applications in structure elucidation of variety of natural products. Herein, we
successfully applied an ESI-MS/MS to the characterization of the products obtained
from the condensation reaction between indole and aldehydes by employing molten
[bpy][Br] as a template, and the ESI-MS/MS spectral data obtained are tabulated in
Table 4(A).7. The details of mass spectrums (figure 4(A).12-16) of all the five BIMs
are discussed in the following pages. In addition to this, we report here a detailed
pattern of fragmentation, which involves formation of new six membered ring and
the ring expansion through tropylium ion, loss of phenyl ring, and the removal of
heterocycle i.e. indole ring.
Table 4(A).7: ESI-MS/MS data for the synthesized bis(indolyl)methane compounds
Entry Peak(s) R’ R Theoretical
Mass
Practical Mass Formulae
Obs. Mass Calc. Mass
1a
Ia
-H -H 322.157
321.09 [m-1] 321.149 C23H17N2+
ib 203.991 204.086 C15H10N+
ic 116.177 116.055 C8H6N+
id 77.132 77.039 C6H5+
2a
iia
-H -Cl 356.602
355.50 [m-1] 355.594 C23H16N2Cl+
iib 238.015 238.531 C15H9NCl+
iic 116.346 116.055 C8H6N+
2b
iiia
-H -OCH3 352.171
351.17 [m-1] 351.164 C24H19N2O+
iiib 233.996 234.102 C16H12NO+
iiic 77.204 77.039 C6H5+
2c
iva
-OCH3 -OCH3 382.187
381.2 0 [m-1] 381.179 C25H21N2O2+
ivb 244.929 245.117 C17H13N2+
ivc 116.954 116.055 C8H6N+
2d
va
-H -H 348.173
347.10 [m-1] 347.165 C25H19N2+
vb 230.026 230.102 C17H12N+
vd 103.186 103.056 C8H7+
79
NN
m-1
NNH H
H H
R
RR' R'
NN
H H
R
R'
m-1
BA
I)
NN
H H
m-1
NN
H H
m-1
NN
H H
AB
II)
Where:
Entry R R‘ m/z in Da.
1a -H -H 321.09
2a -H -Cl 355.50
2b -H -OCH3 351.17
2c -OCH3 -OCH3 381.20
2d -H -H 347.10
R
RR
R'R'
R'
Scheme 4(A).6: Proposed fragmentation pathway suggested for the formation of I] tropylium
cation, and II] aza-fulvenium cation through m-1 ion configuration
The close examination of mass spectra‘s of all the compounds, reveals the
peaks at 203.991, 238.015, 233.996, 244.929 and 230.026 Da. which are the daughter
ion peaks of their parent ions 1a, 2a, 2b, 2c and 2d respectively (Scheme 4(A).6).
Pattern of fragmentation suggested in scheme 4(A).7 is quite interesting and shows
formation of a 6-membered ring after subsequent loss of one of the indole ring in gas
phase.
80
NN
N N
H
N
H H
N
H H
H
R
R
RR
R
N
R
R'
R'
R'
R'
R'R'
- H2
Where:
Entry R R‘ m/z in
Da.
1a -H -H 203.991
2a -H -Cl 238.015
2b -H -OCH3 233.996
2d -H -H 230.026
I)
NH
NH
NH
NH N
II)R
R'
R'R
R'R
-H2
NH
NH
OCH3
OCH3
OCH3 OCH3
NH
NH
N NH
H
III)
Scheme 4(A).7: Proposed pattern of fragmentation, in which I and II] formation of 6-
membered cyclic structure after loss of one indole molecule for entry 1a, 2a,
2b and 2d, III] formation of an aza-fulvenium cation after loss of di-
substituted phenyl ring for entry 2c
81
The peaks at m/z 116.177 and 77.132 (1a), 116.346 (2a), 77.204 (2b), 116.954
(2c) are the characteristics of indole and benzene cation, while, the daughter ions
having m/z 103.186 is of 2d which is obtained after a loss of diindolylmethane species
from its parent species Va (Table 4(A).7).
The author is happy with these mass spectral studies. It has been shown
successfully, the occurrence of non-polar cation –non polar cation (low charge
density), -anion interactions even in gas phase for tetra-butyl-ammonium halides.
These leads to association in a gas phase in the form of dimer/trimer and higher
aggregation. The observations are important from the point of view of modeling,
computer simulation and clusterization studies.
Similarly, the analysis of mass spectrums of ILs yielded new information
about [C2A]+ ions in the form of π-π stacking and weak C-H-π interactions involving
imidazolium and pyridinium ionic species. The hypothesis advanced to account the
patterns for indole derivatives in the form of fragmentation pattern leads to a detection
of formation of tropylium ion with subsequent breakdown of phenyl and indole rings.
The experimental detection and analysis thus yielded satisfaction of doing a scientific
investigation.
82
Figure 4(A).12: ESI-MS/MS spectrum with the proposed pattern of fragmentation of compound 3-3‘-bis-(indolyl)-phenylmethane
NH
NH
83
Figure 4(A).13: ESI-MS/MS spectrum with the proposed pattern of fragmentation of compound 3-3‘-bis-(indolyl)-4-chlorophenylmethane
NH
NH
Cl
84
Figure 4(A).14: ESI-MS/MS spectrum with the proposed pattern of fragmentation of compound 3-3‘-bis-(indolyl)-4-methoxyphenylmethane
NH
NH
OCH3
85
Figure 4(A).15: ESI-MS/MS spectrum with the proposed pattern of fragmentation of compound 3-3‘-bis-(indolyl)-(3-4-dimethoxy)-phenylmethane
NH
NH
OCH3
OCH3
86
Figure 4(A).16: ESI-MS/MS spectrum with the proposed pattern of fragmentation of compound 3-3‘-bis-(indolyl)-cinnamyl-phenylmethane
NH
NH