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CHAPTER 2
FOURIER TRANSFORM INFRARED STUDIES
2.1 INTRODUCTION
The infrared spectroscopy (IR) is one of the powerful tools for
identification and characterization of organic, inorganic, polymeric,
crystalline and coordination compounds (Skoog et al 1998; Sathyanarayana
1996; Aruldhas 2001). The IR region of the electromagnetic spectrum is
considered to cover the range from 50 to 12,500cm–1 approximately. It is
generally subdivided into three regions: near IR (12,500 – 4000 cm–1), mid IR
(4000 – 400 cm–1) and the far IR (400 – 50 cm–1). The mid IR is the region,
most commonly employed for standard laboratory investigations as it covers
most of the vibrational transitions. The far IR region is also equally important
when we deal with inorganic compounds. With most forms of spectroscopy,
the spectrum is a plot of the absorbance or transmittance of the sample against
wavelength; in infrared the absorbance or transmittance is plotted against
wave number. An infrared spectrum has four main features, namely (i) the
number of bands present (ii) the wave number positions (iii) shape of the
bands and (iv) the intensities of the bands. Knowing the structure of the
molecule, it is possible to predict the number of bands expected to appear in
an infrared spectrum in the case of very simple or highly symmetric
molecules. Complete vibrational analyses can be performed and the
vibrational changes deduced from this type of analysis are termed as
fundamental vibrations. The formula 3N – 5 for a linear molecule and 3N – 6
for a non – linear molecule allow the maximum number of fundamental bands
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possible to be calculated, but additional bands caused by other effects can also
appear in an infrared spectrum.
Infrared radiation in the range from 10000 – 10cm–1 is absorbed
and converted by an organic molecule into energy of molecular vibration.
This absorption is quantized, but vibrational spectra appear as bands rather
than as lines, because a single vibrational energy change is accompanied by a
number of rotational energy changes. The frequency or the wavelength of
absorption depends on the relative masses of the atoms, force constants of the
bonds and the geometry of the atoms. Band positions of the IR are presented
as wave numbers (), whose unit is the reciprocal of centimetre (cm–1).
When a molecule absorbs infrared radiation, the usual vibrational
transition is from the ground state to the first excited state, but other
transitions can also occur although not common, giving rise to weaker bands
than the fundamentals and are called overtones and combinations. If two
fundamental vibrations are simultaneously excited, then these are called
combination bands. However, the actual vibrations involved in a combination
band can be very complex. The bands in an infrared spectrum can occur over
a wide range of frequencies. The actual position of a band in an infrared
spectrum depends on the force constant that binds the atoms. It also shows
that vibrations involving lighter atoms or strong force constants will occur at a
higher frequency position than those involving heavy atoms or weak force
constants. So far only stretching vibrations have been considered. But other
vibrations, which involve bond angles, also can occur. These bending
vibrations are called deformation modes. When the reduced mass of the atom
is the same, the force constants for deformation modes are much lower than
that for the stretching vibrations. Because of this, the energy required for a
deformation is much lower and therefore deformation bands are found at
lower wave number positions than those of the stretching vibrations. The
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intensity of the absorption of an infrared band is proportional to the square of
the change in dipole moment during vibrations. If there is no change in the
dipole moment, then infrared band has zero intensity and hence that band
does not exist. This is the selection rule for infrared spectroscopy, which
states that for vibration to be active in the infrared there must be change in the
dipole moment of the bond. Monoatomic substances and homonuclear
diatomic molecules do not exhibit infrared absorption and another condition
for infrared absorption is that the change in vibrational quantum number
= 1.
2.2 FOURIER TRANSFORM INFRARED SPECTROSCOPY
Fourier Transform Infrared Spectroscopy (FTIR) is a simple
mathematical technique to resolve a complex wave into its frequency
components. The conventional IR spectrometers are not of much use for the
far IR region, as the sources are weak and the detectors are insensitive. FTIR
has made this energy-limited region more accessible. It has also made the mid
infrared (4000 – 400cm–1) more useful. Conventional spectroscopy, called the
frequency domain spectroscopy, records the radiant power as a function of
frequency. In the time domain spectroscopy, the changes in radiant power are
recorded as a function of time. In the Fourier Transform Spectrometer, a time
domain plot is converted into a frequency domain spectrum. The actual
calculation of the Fourier transform of such systems is done by means of
high-speed computers.
2.3 DESCRIPTION OF FTIR SPECTROMETER
The spectrometer consists of an infrared source, a sample chamber
with a provision for holding solids, liquids and gases, monochromator, a
detector and a recorder, which are integrated with a computer. At present, all
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commercially available infrared spectrophotometers employ reflection
gratings rather than prisms as dispersing elements. Interferometric multiplex
instruments employing the Fourier transform are now finding more general
applications for both qualitative and quantitative infrared measurements.
The interference pattern is obtained from a two-beam
interferometer. The path difference between the two beams is altered and then
Fourier transformed, gives rise to the spectrum. The transformation of the
interferogram into spectrum is carried out mathematically with a dedicated
online computer. The spectrometer consists of globar and mercury vapour
lamp as sources, an interferometer chamber comprising of KBr and mylar
beam splitter is followed by a sample chamber and detector. The schematic
diagram of a FTIR spectrometer is shown in Figure 2.1. This instrument
covers the entire region of 10 – 10000cm–1. The spectrometer works under
vacuum condition. Solid samples are dispersed in KBr or polyethylene pellets
depending on the region of interest. Signal averaging, signal enhancement,
baseline correction and other spectra manipulations are possible with the PC
attached to the system. The vibration - isolated optical bench protects the
optics from disturbance by other equipment.
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Figure 2.1 Schematic diagram of a FTIR spectrometer
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2.4 SAMPLE HANDLING TECHNIQUES
Recording of IR spectra of solid sample is more difficult because
the particles reflect and scatter the incident radiation and therefore
transmittance is always low. Three different techniques are employed
commonly in recording such spectra.
2.4.1 Mulling Technique
A small amount of the sample is ground in an agate or mullite
mortar. Then a drop of paraffin oil, usually nujol is added and the grinding
continued till the mixture attains the consistency of a thin paste. It is
transferred to an infrared window and a second window is lowered on to it. A
thick film free of air bubbles should be produced. The two plates with a mull
between are placed in a cell holder and the spectrum is recorded. There will
be strong bands at 2900, 1470 and 1370cm–1 and a weak band at 720cm–1 due
to nujol. If the nujol bands are stronger than the peaks from the sample, then
more samples and less nujol must be ground. When the region near 2900cm–1
is important other mulling materials such as Fluorolube or
Hexachlorobutadiene must be used.
2.4.2 Pressed Pellet Technique
For solid compounds that are insoluble in the usual solvents, a
convenient sampling method is the pressed pellet technique. A few milligrams
of the sample are ground together in an agate or mullite mortar with about 100
times the quantity of a material (the matrix), transparent to the infrared. The
usual material is KBr, although other compounds such as CsI, TlBr and
Polyethylene are used in special circumstances. The ground powder is finally
introduced into a mini press made from two and a half inch diameter stainless
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steel bolts and a stainless steel nut. The ends of the bolts must be polished,
flat and parallel. One bolt is inserted about half way into the nut and the KBr
plus sample mixture added. The second bolt is then screwed into the nut and
pressure applied by tightening the bolts together. When the bolts are carefully
withdrawn a pellet suitable for infrared transmission work remains. The pellet
is not removed from the nut, which acts as a holder in the spectrometer.
2.4.3 Liquids and Solutions
Probably the easiest method to obtain a qualitative infrared
spectrum of a liquid is to place one drop of the liquid onto a disc of NaCl,
KBr etc., cover the drop with a second disc and mount the pair in a holder.
Teflon spacers may be used to give various path lengths. Alternatively liquid
samples can be run either as the pure liquid, if a cell of suitable thickness is
available (0.02mm) or a solution in a longer cell, if a suitable solvent can be
found. The best solvents for infrared use are non - polar, non - hydrogen
liquids such as CS2 or CCl4.
2.5 EXPERIMENTAL
The FTIR spectra of the synthesized crystals were recorded using
PERKIN ELMER Spectrum RX I spectrophotometer in the region
4000 – 450cm–1 by employing KBr pellet method. A spectral width of 4 cm–1
was used and the spectra are measured with a scanning speed of 1.87cm–1 per
minute. The frequencies for all sharp bands were expected to be accurate to
1cm–1. The spectra for the Picric Acid(PA), its salts and complexes are
presented in Figures 2.2 - 2.14. Some of the observed frequencies with their
assignments are summarized in Tables 2.1- 2.5.
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2.6 VIBRATIONAL BAND ANALYSIS
Mulliken’s theory of charge-transfer interactions produced between
an electron donor and electron acceptor has been successfully applied to many
interesting studies. Charge transfer complexes (Moamen S Refat et al 2006)
have great attention for their non - linear optical properties and electrical
conductivities. Also, they are known to take part in many chemical reactions
like addition, substitution and condensation. Electron donor - acceptor
interaction is also important in the field of drug - receptor-binding
mechanism, in solar energy storage, in surface chemistry and in many
biological fields.
The existence of molecular complexes between nitro substituted
benzenes and aromatic hydrocarbons has long been recognized. Charge
transfer complexes of picric acid are extensively studied because of their
special type of interaction, which is accompanished, by the transfer of
protons. The picric acid can form salts or complexes with aliphatic and
aromatic compounds. The molecular complexes have been formed via either
phenolic oxygen or ortho nitro oxygen or both. The picrate ion is a
monodendate when it coordinates with cation via phenolic oxygen. In
bidendate ligand, picrate coordinates the cation either by phenolic and the
ortho nitro oxygens. When the picrate is a tridendate ligand, it coordinates the
cation either by phenolic and the two ortho nitro oxygen or phenolic ortho
nitro and para nitro oxygen (Olsher et al 1996).
The study of vibrational spectra also reveals which parts of the
compounds play an active role in complex formation. The infrared spectra of
40 molecular complexes of picric acid with aromatic and heterocyclic
compounds have been investigated by Kross and Fassel (1956). They have
pointed out that the nitro asymmetric stretching and out of plane CH
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vibrations of the picric component are sensitive to the complex formation. In
general, the spectra of all molecular complexes appear to be a summation of
the spectra of the two components with the vibrational bands of picric acid
appearing more strongly than those of the other components.
In this chapter, some important vibrational species of the salts and
complexes of picric acid, which were prepared for this work have been
identified with reference to picric acid spectrum and discussions are made
herewith.
2.6.1 Vibrations of Picrate anion
NO2 Vibrations
Several researchers have studied intensively about the molecular
vibrations of nitrophenols by Fourier transform infrared (FTIR) and Raman
spectroscopies. In parallel, semi empirical, ab initio and various density
functional theory (DFT) methods are used to determine the geometrical,
energetic and vibrational characteristics of nitrophenols (Sundaraganesan et al
2006; Vasile Chis 2004; Abkowicz et al 1999; Socrates 1980). They have
reported that the asymmetric and symmetric stretching vibrations of the NO2
group have strong absorptions in the region 1570 - 1485cm–1 and
1370 - 1320cm–1 respectively. As expected, the strong band observed at
1341 cm–1 in the FTIR spectrum of Picric acid (PA), which is shown in the
Figure 2.2, could be attributed to NO2 symmetric vibrations. The asymmetric
stretching vibrations of the NO2 group appear at 1530cm–1.
According to Kross et al (1956), these two vibrations are more
sensitive to complex formation. Due to the formation of complex, the nitro
group vibrations split and some additional vibrations appear in the spectra of
the complexes. Also the frequencies of the nitro group have been shifted to
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either side from the original value. In the spectra of 4FAP, 4MAP, PIPP,
MORP, DCHAP, COAP, DBCOP, FMAP, HAP, the NO2 asymmetric
stretching vibrations appear in the region 1557 – 1570 cm–1. The two bands
that appear in the region 1330 - 1370cm–1 in the spectra of the above salts
could be attributed to NO2 symmetric stretching vibrations. The shift in the
frequencies of NO2 asymmetric and symmetric vibrations on the higher side
suggests the formation of picrate salt between the picric acid and
corresponding parent compound. This shift is due to the formation of
intermolecular and intramolecular hydrogen bonding of the picric acid and the
parent compound (Szafran et al 1997).
From the spectra of 2NAPA, 4NAPA and IPA complexes, no
significant variations were observed in the frequencies of NO2 asymmetric
and symmetric stretching vibrations. This suggests that only very weak
hydrogen bonds are formed in the complex.
The scissoring mode of NO2 vibrations often gives rise to only
weak infrared bands in the region 800 – 890 cm-1 whereas the wagging mode
shows a strong absorption in the region 700 – 760 cm-1 (Syam Sundar 1985;
Kuwae and Machida 1979; Stewart et al 1986; Venkataramana Rao 1998, and
Lambert et al 1976). As expected, in the spectrum of picric acid, the
scissoring mode of NO2 vibration appears at 825cm–1 with weak intensity.
The strong band at 731 cm-1 could be attributed to wagging vibration of NO2.
The rocking vibration of nitro group appears at 523cm–1 as sharp bands with
medium intensity.
On the formation of complexes and salts, the intensities and
positions of scissoring, wagging and rocking vibrations of nitro group are
modified. This could be due to the strong and weak bonds that have been
formed between nitro oxygen group of the picrate anion and the cation. In the
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spectra of picrates, the weak bands centered at around 545cm–1, 520cm–1 and
505cm–1 could be attributed to NO2 rocking vibrations. The nitro wagging
vibrations appears in the picrate salts at around 740cm–1. This vibration
appears at 732cm–1, 735cm–1, 737cm–1 and 726cm–1 in the spectra of the IPA,
2NAPA and 4NAPA complexes. The medium intensity bands centered at
around 745cm–1 could also be attributed to wagging vibrations of nitro group.
The formation of additional band in the spectra of the salts is due to the
formation of the salts. The scissoring vibrations appear in the spectra of the
picrate salts 4MAP, 4FAP, FMAP, DCHAP, MORP and DBCOP at 813cm–1,
828cm–1, 850cm–1, 840cm–1 and 837cm–1 respectively. This vibration appears
at 860cm–1, 842cm–1 and 862cm–1 in the spectra of 2NAPA, 4NAPA and IPA
respectively.
CH Vibrations
The aromatic ring CH vibrations of benzene derivatives appear
generally above 3000cm–1 (Varsanyi, 1969). In consonance with this, the
strong band observed in the spectrum of PA at 3103cm–1 is assigned to CH
asymmetric stretching vibrations. The corresponding symmetric stretching
vibration appears at 2984cm–1 with medium intensity. The strong and
medium intensity bands centered at around 3086cm–1 in the spectra of the
picrate salts are attributed to aromatic CH asymmetric stretching vibration.
This vibration appears in the spectra of IPA, 4NAPA and 2NAPA at 3102cm–
1, 3109cm–1 and 3090cm–1 respectively.
The inplane aromatic CH deformation of benzene ring occurs in the
region 1300 – 900cm–1. In the spectrum of PA, this vibration appears at
1174cm–1 and 1086cm–1 with strong intensity. In the spectra of picrates, the
aromatic CH inplane bending vibrations appear in the regions 1154 –
1182cm–1 and 1080 – 1087cm–1.
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The aromatic CH out of plane vibration of PA usually appears at
780cm–1. The medium intensity band centered at 783cm–1 in the spectrum of
PA could be attributed to out of plane CH vibrations. According to Kross et al
(1956), this band is also sensitive to complex formation. As expected, in the
spectra of picrate salts, this vibration is shifted to 795 cm-1 with weak to
medium intensities. The aromatic CH out of plane vibrations appears in the
spectra of complexes in the region 776 – 782cm–1.
CO Stretching Vibration
In the spectra of alcohols and phenols, characteristics bands are
observed due to OH stretching and CO stretching. These vibrations are
sensitive to hydrogen bonding. The C - O stretching and O - H bending modes
are not independent vibrational modes because they couple with the vibrations
of the adjacent groups (Silverstein et al 1991). The CO stretching frequency
of nitro phenols (Abkowicz et al 1999) appear at around 1260cm–1. The CO
stretching vibration appears in the spectrum of picric acid at 1263cm–1. When
the charge transfer complexes are formed, there is always a transfer of proton
from donor to acceptor moieties. During the formation of picrate salts also,
proton transfer takes place and this transfer affect the neighboring
environment. In picrates, due to the removal of the phenolic hydrogen atom,
the bond length of C - O is slightly decreased showing partial double bond
character (Tanaka et al 1994 and Muthamizhchelvan et al 2005b). The double
bond nature of C - O always increases its absorption frequency. As expected,
in all picrate salts, the carbon - oxygen vibrations are slightly increased to the
higher wave numbers and appear in the range 1267 - 1279cm–1. The crystal
structure analyses of the picrate salts confirm the partial double bond nature
of CO bond. In the case of complexes, this vibration appears at 1250cm–1,
1265cm–1 and 1258cm–1 in IPA, 4NAPA and 2NAPA respectively.
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Phenolic OH Vibration
In phenols, a broad absorption band appears in the region
2500 – 3500cm–1 which is primarily due to the OH stretching vibration
(Varsanyi 1969). The broad medium intensity band centered at 3417cm–1 in
the spectrum of PA could be attributed to phenolic OH stretching vibration.
The strong band observed at 1149cm–1 in the spectrum of PA could
be assigned to OH inplane bending vibration. The picric acid transfers the
hydrogen atom of the hydroxyl group to the acceptor atom forming the picrate
salts. As a result of this, OH vibrations disappear in the spectra of the picrates.
This is also further supported by the presence of N+H, NH3+ and NH2
+
vibrations in the spectra. In IPA, 4NAPA and 2NAPA, there is no complete
transfer of proton from PA to the counterpart and this is evidenced by the
presence of the characteristic band around 1149cm–1, the inplane OH bending
vibrations.
C - NO2 Vibration
In nitro benzenes, C - NO2 stretching vibration usually appear at
920cm–1 (Ansari and Verma 1979) with strong intensity. As expected, this
vibration appears in the region 906 – 921cm–1 with medium and weak
intensities.
Aromatic Ring C - C Vibrations
The ring aromatic C – C stretching frequencies are observed in the
region 1635 – 1430cm–1 (Socrates 1980). The frequencies of these modes are
fairly insensitive to substitution. As expected, three bands are observed at
1633cm–1, 1610cm–1 and 1432cm–1 in the spectrum of PA and these vibrations
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are due to aromatic ring C – C stretch. In the molecular complexes of picrates,
the ring C – C stretching vibrations appear at the same frequencies as that of
PA.
Some bands, which appear in the region below 700cm–1 are quite
sensitive to the changes in the nature and positions of substituents
(Varsanyi 1969). The medium and strong intensity bands observed in the
spectra of PA and its complexes at around 703cm–1 must be due to ring CCC
deformations. Similarly, the weak bands observed 490 - 460cm–1 could also
be due to ring CCC deformations. Some of the ring CC vibrations overlap
with methyl vibration.
2.6.2 4 – Fluoroanilinium and 4 - Methylanilinium Vibrations
Several theoretical studies have been carried out on aniline
derivatives through spectral measurements such as FTIR and Raman
Spectroscopies (Akalin et al 1999; Altun et al 2003 and Yurdakul et al
1999). From the spectral studies, it is reported that the NH2 (amine) group
gives rise to six characteristic vibrations – asymmetric and symmetric
stretchings, scissoring, rocking, wagging and twisting modes. The asymmetric
and symmetric stretching modes generally absorbs in the region 3550 –
3250cm–1, the former mode being at a higher magnitude than the latter. The
scissoring and twisting modes appear at 1619cm–1 and 1054cm–1 respectively.
Tanaka et al (1994) have reported that, due to the salt formation between
2 – iodoaniline and picric acid, phenolic hydrogen of picric acid is transferred
to the aniline forming NH3+ ion and hence the bond is formed between the
anion and cation. As a result of this, the vibrations of NH3+ ion are shifted to
lower wave numbers. The primary amine salts (Colthup et al 1975) are
characterized by strong absorption in the range 3200 – 2800cm–1, due to the
asymmetric and symmetric NH3+ stretch. From the Figure 2.3, the medium
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intensity band centered at 3149cm–1 in the spectrum of 4FAP could be due to
NH3+ asymmetric stretching vibration. The symmetric NH3
+ vibration appears
at 2860cm–1 in the spectrum of 4FAP. The same vibration appears at 2896cm–
1 in the spectrum of 4MAP which is presented in Figure 2.4.
The asymmetric NH3+ deformation absorbs at 1625–1560cm–1 and
the symmetric NH3+ deformation absorbs at 1550–1470cm–1
(Colthup et al 1975). As expected, the asymmetric NH3+ deformation bands
appear at around 1610cm–1. This vibration overlaps with the aromatic ring
C-C stretching vibration. The corresponding symmetric vibrations appear at
around 1483cm–1 and 1506cm–1. The strong absorption at 795cm–1 in the
spectra of 4FAP and 4MAP could be attributed to NH3+ rocking vibration.
The vibrational spectrum of 4 – Fluoroaniline has been investigated
by Lopez et al (2001). They have assigned the C – F stretching vibrations at
1225cm–1. As expected, this vibration appears at 1227cm–1 with medium
intensity.
The methyl (CH3) vibrations in aromatic complexes have its
asymmetric and symmetric vibrations in the region 2930–2860cm–1
(Colthup et al 1975). The asymmetric and symmetric deformations appear in
the region 1475 – 1370cm–1. The strong band centered at 2896cm–1 in the
spectrum of 4MAP could be attributed to methyl C-H stretching vibration.
The methyl deformation bands appear at 1437cm–1 and 1398cm–1 and this
vibration overlaps with benzene ring C-C vibration. The band that appears at
shoulder in the spectrum of 4MAP at 1047cm–1 could be assigned to CH3
rocking vibration. The presence of vibrations of NH3+ ion in the spectra of
4FAP and 4MAP, confirms the transfer of protons from PA to the counter
cation.
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2.6.3 Piperidinium and Morpholinium Vibrations
Piperidine is a cyclic amine with a six membered ring. Its
molecular formula is C5H11N. Morpholine is diethylene imide oxide with
molecular formula C4H9NO. Several researchers have investigated the
vibrational spectra and interactions with other compounds of piperidine,
morpholine and its derivatives (Vedal et al 1976; Hirokawa et al 1980;
Szafran et al 2002; Szafran et al 2003; Szafran et al 2004; Moamen S Refat
et al 2006 and Fernandes et al 2000). A free piperidine and morpholine have
NH group but during the formation of salt with PA, proton transfer from the
picric acid to the morpholine and piperidine. This is evident from the
presence of NH2+ ions in the spectra of the picrate salts of piperidine and
morpholine. The FTIR spectra for PIPP and MORP are shown in Figures 2.5
and 2.6. The medium and weak intensity bands that appear at 3274cm–1 and
3184cm–1 in the spectra of MORP and PIPP could be attributed to NH2+
asymmetric stretching vibrations. The corresponding symmetric vibration
appears at 2820cm–1 and 2806cm–1. The bands that appear in the region 1631
– 1560cm–1 are due to NH2+ deformation vibration and this overlap with the
NO2 vibrations. The out of plane deformation of NH2+ appear at 795cm–1 and
937cm–1.
The methylene (CH2) vibrations usually appear in the region 2950 –
2800cm–1. As expected, the strong band that appears at 2948cm–1 and
2920cm–1 in the spectra of PIPP and MORP are due to methylene C-H
asymmetric stretching vibration. The corresponding symmetric vibration
appears at 2851cm–1 and 2868cm–1 respectively. The deformations of
methylene groups give rise to absorptions at 1497cm−1 and 1483cm−1. The CN
stretching vibration of piperidine and morpholine ring appears at 1246cm−1
and 1535cm–1 and 1257cm−1 respectively. The CO stretching of morpholine
appears at 1190cm−1 and 1043cm−1. The presence of NH2+, CH2, CO and CN
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vibrations in the spectra of PIPP and MORP evidences the formation of the
picrate salt.
2.6.4 Dicyclohexylaminium, Cyclooctanaminium, and
1,4–Diazabicyclo[2.2.2]octanium Vibrations
The Dicyclohexylamine has NH group in the molecular structure.
On formation of picrate salt, this moiety gets protonated and hence in the
spectrum of DCHAP (shown in Figure 2.7), the NH2+ vibrations are present.
The weak intensity bands that appear at 3183cm–1 and 3142cm–1 could be
attributed to NH2+ asymmetric stretching vibrations. The corresponding
symmetric vibration appears at 2808cm–1. The bands that appear in the region
1610 – 1557cm–1 are due to NH2+ deformation vibration and this overlap with
the NO2 vibrations. The out of plane deformation of NH2+ appear at 795cm–1
and 935cm–1.
A free cyclooctanamine has NH2 group but during the formation of
salt with PA, it get protonated forming NH3+ ions. This is evident from the
presence of NH3+ vibrations in the spectra of the picrate salt. The weak
intensity band centered in the spectrum of COAP, which is shown Figure 2.8,
at 2867cm–1 is due to NH3+ stretching vibration. The asymmetric and
symmetric NH3+ deformation absorbs the energy in the expected region.
The 1,4–Diazabicyclo[2.2.2]octane contains no NH group and
hence in the spectrum of this compound does not have an NH stretch nor an
NH wag. However, during the salt formation between 1,4 –
Diazabicyclo[2.2.2]octane and PA, the hydrogen atom from the hydroxyl
group is shifted to 1,4–Diaza bicyclo[2.2.2]octane. As a result of this, the
absorption band of N+H vibration appears. As expected, this vibration appears
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in the FTIR spectrum of DBCOP, which is presented in Figure 2.9, at
3427cm–1.
The molecular structures of DCHAP, COAP, and DBCOP contain
methylene group and its presence confirmed by the corresponding vibrations
in the spectra of the picrate salts. The methylene (CH2) stretching vibrations
appear at around 2939cm–1. The deformations of methylene groups give rise
to absorption at 1483cm−1. The presence of NH+, NH3+ and NH2
+ vibrations
confirm the transfer of protons from the picric acid to the counter cation.
2.6.5 Furan-2-yl-Methanaminium and Hexanaminium Vibrations
The vibrational spectra of furan and its derivatives have been
investigated by Ferenc billes et al (2004). Furan derivatives have bands in the
region 1610 – 1560cm–1, 1520 – 1470cm–1 which are due to the C = C ring
stretching vibrations (Socrates 1980). As expected, several bands are
observed in this region and overlaps with benzene ring C – C vibrations. The
FTIR spectrum for FMAP is shown in Figure 2.10.
The out of plane deformation of the C – H group give bands in the
region 935 – 915cm–1, 885 – 880cm–1 and 835 – 780 cm–1. In consonance with
this, the weak bands observed at 885cm–1 and 795cm–1 could be assigned to
CH group out of plane vibrations.
All furans have a strong absorption near 595cm–1 that is probably
due to ring deformation vibrations. As expected, this vibration appears at
599 cm–1 with medium intensity. Furans have a strong absorption bond in the
region at 1100 – 1065cm–1 due to the C - O stretching vibration. The bands at
1069 cm–1 is due to C – O stretching vibrations of the ring.
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The transfer of protons from picric acid to furan is confirmed by the
presence of NH3+ vibrations. The strong bands appeared at 3244 cm–1 and
3133 cm–1 could be attributed to asymmetric and symmetric stretching
vibrations of NH3+. The inplane and out of plane vibrations of NH3
+ overlap
with the other vibrations of the furan and picric acid. The CH stretching
vibrations of methylene group appears at 3033 cm–1 with medium intensity.
The medium intensity bands centered at 2933 cm–1 and 2722 cm–1 could be
attributed to CH stretching vibration of the furan ring. The prominent
vibrations observed in the spectrum of FMAP confirm the presence of furan.
The molecular structure of hexylamine consists of NH2 group and
hence on protonation, it becomes NH3+ ion. The weak band observed in the
FTIR spectrum of HAP, which is presented in Figure 2.11, at 2873cm–1 could
be assigned to NH3+ stretching vibrations. The methylene and NH3
+
deformations appear in the expected region. This confirms the transfer of
protons from PA to hexylamine.
2.6.6 2-Nitroaniline and 4-Nitroaniline Vibrations
Some of the picrate salts with aniline derivatives such as
2 – Iodoaniline, 2 -Chloroaniline, 3 – Methylaniline etc., have been
investigated by several researchers (Tanaka et al 1994 and Muthamizhchelvan
et al 2005a) and it is reported that there is a transfer of protons from the PA to
the counter cation. In the present work, PA is complexed with 2 – Nitroaniline
and 4 – Nitroaniline and surprisingly, in these two complexes, no protonation
takes place. This is evidenced by the presence of OH and NH2 vibrations in
the spectra of the 2NAPA and 4NAPA at the same position as that of the
parent compounds. The FTIR spectra for 2NAPA and 4NAPA are presented
in Figures 2.12 and 2.13 respectively.
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The vibrational spectra of the nitrophenol and aniline derivatives
have been studied by several investigators (Vijaya Kumar et al 1992 and
Akalin and Akyuz, 1999). The phenolic OH vibration usually appears above
3400cm−1. The bands centered at 3489cm−1 and 3483cm−1 in the spectra of
2NAPA and 4NAPA are assigned to OH vibrations of the nitrophenols. The
inplane bending vibration of nitrophenols appears at 1149cm−1 and this
vibration appears at the same frequency in the spectra of the complexes.
The asymmetric and symmetric stretching vibrations of the amine
group are identified in the region 3400-3350cm−1. As expected, these
vibrations appear at 3376cm−1 in the spectrum of 2NAPA and 3389cm−1 and
3362cm−1 in the spectrum of 4NAPA respectively. The absorptions at around
1610cm−1 and 1000cm−1 are due to the deformations of NH2 group. The
benzene ring vibrations appear in the expected region. Hence, in 2NAPA and
4NAPA complexes, there is no complete transfer of protons from PA to the
counterpart and this is supported by the crystal structure determination.
2.6.7 Indole Vibrations
Indole is an aromatic heterocyclic organic compound. It has a
bicyclic structure consisting of a six membered benzene ring fused to a five membered nitrogen containing pyrrole ring. The vibrational spectra of the indole and its derivatives have been studied by several researchers (Suwaiyan et al 1986; Smith et al 1999; Talbi et al 1997; Klots et al 1995;
Suwaiyan et al 1997 and Hideo et al 1986). The FTIR spectrum for IPA is shown in Figure 2.14. The NH stretch in indoles causes its absorption in the
region 3400 – 3100cm–1 (Colthup et al 1975). The strong band centered at 3400cm–1 in the spectrum of IPA is the characteristic of NH bond. The phenolic OH vibration overlap with the NH vibration of indole and appear at
33
the same frequency as that of the nitrophenols confirming the non – protonation.
The weak intensity bands centered at 2938cm–1 and 2868cm–1 are assigned to CH asymmetric and symmetric stretching vibrations of the indole ring. The bands that appeared in the region 1500 – 1450cm–1 are due to CC,
CH and CN vibrations of the indole ring. The inplane bending vibration of NH of indole appears at 1275cm–1. As expected, this vibration appears at 1278cm–1 with strong intensity. The inplane and out of plane bending vibrations of indole ring are appeared below 1300cm–1. The indole vibrations
in the spectrum of the complex confirm its presence.
2.6.8 Hydrogen bonding in picrates
During the formation of salts and complexes with PA, intermolecular and intramolecular hydrogen bonds like long and short
hydrogen bonds N – H …O, C – H …O have been formed. Several researchers investigated the nature of hydrogen bonds with the help of crystal
structure analysis and FTIR spectra (Szafran et al 1997; Szafran et al 2002 and Marchewka et al 2003). It is reported that due to hydrogen bonding, several sub maxima have been formed in the region 2700 – 2000cm–1 in the spectra of picrates. As expected, the hydrogen bonding vibrations appear in
the above mentioned frequency region. The bands centered at around 3435cm–1 in the spectra of picrate salts could be attributed to O…H hydrogen bond stretching.
2.7 CONCLUSION
A satisfactory vibrational band assignment has been made on the
some of the specific important observed frequencies of the picrate salts and its
complexes. The changes in the frequency and intensity of NO2 vibration, ring
34
C-H vibration, C-O vibration, absence of phenolic O-H bands in the spectra of
salts, presence of NH3+ vibrations in the spectra of 4FAP, 4MAP, FMAP,
COAP and HAP, presence of NH2+ vibrations in the spectra of DCHAP, PIPP
and MORP, presence of N+H vibration in the spectrum of DBCOP confirms
the transfer of proton from the picrate anion to the cation. In the spectra of the
complexes (IPA, 2NAPA and 4NAPA), the phenolic OH vibrations and
amine vibrations appear at the same frequency as that of the parent
compound. This suggests that there is no protonation in these complexes,
which is also supported by the crystal structure analysis.
35
Figure 2.2 FTIR spectrum of Picric Acid
36
Figure 2.3 FTIR spectrum of 4FAP
37
Figure 2.4 FTIR spectrum of 4MAP
38
Figure 2.5 FTIR spectrum of PIPP
39
Figure 2.6 FTIR spectrum of MORP
40
Figure 2.7 FTIR spectrum of DCHAP
41
Figure 2.8 FTIR spectrum of COAP
42
Figure 2.9 FTIR spectrum of DBCOP
43
Figure 2.10 FTIR spectrum of FMAP
44
Figure 2.11 FTIR spectrum of HAP
45
Figure 2.12 FTIR spectrum of 2NAPA
46
Figure 2.13 FTIR spectrum of 4NAPA
47
Figure 2.14 FTIR spectrum of IPA
48
Table 2.1 Vibrational Band Assignments of the PA, 4FAP and 4MAP
Wave Number (cm–1) Vibrational band assignments PA 4FAP 4MAP
468 (w) 463 (w) Ring CCC deformation
523 (m) 503 (w) 521 (w) NO2 rocking
538 (w) 546 (w) NO2 rocking
703 (vs) 715 (m) 703 (m) Ring CCC deformation
731 (vs) 733 (w) 732 (w) NO2 wagging
745 (m) 745 (m) NO2 wagging
783 (m) 795 (w) 795 (w) Aromatic CH out of plane bending / NH3+ rocking
825 (w) 828 (m) 813 (m) NO2 scissoring
918 (vs) 908 (w) 907 (m) C - NO2 stretching
1047 (w) CH3 rocking
1086 (vs) 1085 (m) 1081 (m) Aromatic CH in plane bending
1149 (vs) OH in plane bending
1174 (s) 1166 (m) 1162 (s) Aromatic CH in plane bending
1227 (m) CF stretching
1263 (vs) 1278 (vs) 1267 (vs) C – O stretching
1341 (vs) 1334 (vs) 1330 (vs) NO2 symmetric stretching
1360 (vs) 1370 (s) NO2 symmetric stretching
1398 (w) CH3 deformation
1432 (vs) 1438 (m) 1437 (m) Aromatic ring CC stretching /CH3 deformation
1484 (s) 1483 (s) NH3+ symmetric deformation
1502 (vs) 1506 (s) NH3+ symmetric deformation
1530 (vs) 1558 (vs) 1559 (vs) NO2 asymmetric stretching
1567 (vs) 1570 (vs) NO2 asymmetric stretching
1610 (vs) 1608 (vs) 1612 (vs) Aromatic ring CC stretching / NH3+ asymmetric
deformation 1633 (vs) 1632 (vs) 1631 (vs) Aromatic ring CC stretching
2860 (s) 2896 (s) NH3+ symmetric stretching / CH3 symmetric stretching
2904 (w) Aromatic CH symmetric stretching
2984 (m) 2922 (s) 3035 (m) Aromatic CH symmetric stretching
3103 (vs) 3085 (m) 3085 (s) Aromatic CH asymmetric stretching
3417 (m) Phenolic OH vibration
3149 (m) NH3+ asymmetric stretching
3437 (w,bd)
3435 (w,bd) O...H hydrogen bonding stretching
49
Table 2.2 Vibrational Band Assignments of the PIPP and MORP
Wave Numbers (cm–1) Vibrational band assignments
PIPP MORP
521 (w) 521 (w) NO2 rocking 548 (w) 547 (w) NO2 rocking
703 (m) 703 (m) Ring CCC deformation
733 (w) 733 (w) NO2 wagging
746 (m) 745 (m) NO2 wagging
795 (w) 795 (w) Aromatic CH out of plane bending / NH2+ out of plane
deformation
840 (vw) NO2 scissoring
907 (m) 907 (m) C – NO2 stretching
937 (w) 937 (w) NH2+ rocking
1043 (m) CO stretching of morpholine
1081 (w) 1081 (m) Aromatic CH inplane bending
1162 (m) 1163 (m) Aromatic CH inplane bending 1190 (w) CO stretching of morpholine
1246 (m) 1257 (m) CN stretching of morpholine / CH2 twisting
1277 (vs) 1277 (vs) C – O stretching
1334 (vs) 1334 (vs) NO2 symmetric stretching
1369 (s) 1369 (s) NO2 symmetric stretching
1438 (s) 1437 (s) Aromatic ring CC stretching 1483 (s) 1483 (s) CH2 scissoring / NH3
+ deformation
1497 (s) 1497 (s) CH2 scissoring / NH3+ deformation
1535 (m) CN stretching of piperidine
1560 (vs) 1560 (vs) NO2 asymmetric stretching / NH2+ deformation
1612 (s) 1610 (vs) Aromatic ring CC stretching / NH2+ deformation
1631 (vs) 1631 (vs) Aromatic ring CC stretching / NH2+ deformation
2806 (s) 2820 (m) NH2+ symmetric stretching
2851 (m) 2868 (m) Methylene CH symmetric stretching
2948 (s) 2920 (m) Methylene CH asymmetric stretching
3071 (m) Aromatic CH symmetric stretching
3086 (m) 3086 (m) Aromatic CH asymmetric stretching
3184 (w) 3274 (m) NH2+ asymmetric stretching
3430 (w,bd) 3419 (s) O...H hydrogen bonding stretching
50
Table 2.3 Vibrational Band Assignments of the DCHAP, COAP and DBCOP
Wave Number (cm–1) Vibrational band assignments
DCHAP COAP DBCOP
477 (w) Ring CCC deformation
520 (w) NO2 rocking
547 (w) 546(w) NO2 rocking
710 (m) 703 (m) 709 (m) Ring CCC deformation
745 (m) 745 (m) NO2 wagging
795 (m) 787 (m) Aromatic CH out of plane bending / NH3+ rocking
850 (w) 837 (w) NO2 scissoring
907 (m) 908 (w) 906 (w) C - NO2 stretching
1080 (m) 1081 (w) 1071 (m) Aromatic CH in plane bending
1162 (m) 1163 (w) 1161 (m) Aromatic CH in plane bending
1269 (vs) 1279 (m) 1270 (vs) C – O stretching
1333 (vs) 1338 (m) 1330 (vs) NO2 symmetric stretching
1370 (vs) 1369 (w) 1370 (s) NO2 symmetric stretching
1438 (m) 1439 (w) 1435 (s) Aromatic ring CC stretching
1483 (s) 1483 (w) 1484 (vs) CH2 scissoring / NH3+ deformation
1512 (m) 1522 (w) 1530 (vs) CH2 scissoring / NH3+ deformation
1559 (vs) 1561 (m) 1557 (vs) NO2 asymmetric stretching / NH2+ deformation
1610 (s) 1610 (vs) Aromatic ring CC stretching / NH2+ deformation
1633 (vs) 1631 (s) 1630 (vs) Aromatic ring CC stretching
2757 (s) 2792 (m) Methylene symmetric CH stretching
2808 (s) NH2+ symmetric stretching
2855 (s) 2820 (m) Methylene CH symmetric stretching
2867 (w) NH3+ stretching
2939 (vs) 2933 (w) Methylene CH asymmetric stretching
2962 (s) Aromatic CH symmetric stretching
3086 (m) 3089 (m) 3087(m) Aromatic CH asymmetric stretching
3142 (w) NH2+ asymmetric stretching
3183 (w) NH2+ asymmetric stretching
3427 (s) N+H stretching
3435 (w, bd) 3435 (s) O … H hydrogen bond stretching
51
Table 2.4 Vibrational Band Assignments of the FMAP and HAP
Wave Numbers (cm–1) Vibrational band assignments
FMAP HAP
503 (w) NO2 rocking
546 (w) NO2 rocking
599 (m) Furan ring deformation
712 (m) 704 (m) Ring CCC deformation
746 (s) NO2 wagging
795 (m) Aromatic CH out of plane bending / CH out of plane deformation of furan
828 (w) NO2 scissoring
885 (w) CH out of plane deformation of furan
908 (w) C – NO2 stretching
1069 (w) CO stretching of furan
1082 (m) 1081 (w) Aromatic CH inplane bending 1165 (m) 1163 (w) Aromatic CH inplane bending
1278 (vs) 1270 (m) C – O stretching
1339 (vs) 1334 (m) NO2 symmetric stretching
1369 (s) 1369 (w) NO2 symmetric stretching
1438 (s) Aromatic ring CC stretching
1483 (s) 1484 (w) CH2 scissoring / NH3+ deformation
1500 (s) 1520 (w) CH2 scissoring / NH3+ deformation
1561 (vs) 1567 (m) NO2 asymmetric stretching
1608 (vs) Aromatic ring CC stretching / Furan ring CC stretching
1632 (vs) 1632 (vs) Aromatic ring CC stretching / Furan ring CC stretching
2722 (m) CH stretching of furan
2873 (w) NH3+ symmetric stretching
2933 (m) 2939 (w) Methylene CH asymmetric stretching / Furan CH stretching
3033(m) Methylene CH asymmetric stretching
3086 (s) 3094 (m) Aromatic CH asymmetric stretching
3133 (s) NH3+ symmetric stretching
3244 (s) NH3+ asymmetric stretching
3435 (vs) 3445 (vs) O...H hydrogen bonding stretching
52
Table 2.5 Vibrational Band Assignments of the 2NAPA, 4NAPA and IPA
Wave Number (cm–1) Vibrational band assignments 2NAPA 4NAPA IPA
484 (w) 490 (w) Ring CCC deformation 505 (w) 521 (w) NO2 rocking 527 (w) 535 (w) 544 (w) NO2 rocking 581 (w) CC out of plane deformation of indole ring 632 (w) CH out of plane deformation of indole ring 714 (m) 698 (m) 708 (s) Ring CCC deformation 726 (m) NO2 wagging 737 (m) 735 (m) 732 (s) NO2 wagging 782 (w) 782 (w) 776 (m) Aromatic CH out of plane bending 795 (w) CH out of plane deformation of indole ring 842 (vw) 842 (m) NO2 scissoring 860 (w) 860 (vw) NO2 scissoring 920 (m) 918 (m) 917 (m) C - NO2 stretching 939 (w) CH deformation of indole ring 998 (w) NH2 deformation 1083 (m) 1087 (m) 1082 (s) Aromatic CH inplane bending 1149 (m) 1149 (m) 1154 (m) OH inplane bending 1169 (m) 1182 (m) Aromatic CH inplane bending 1258 (vs) 1265 (s) 1250 (m) C – O stretching 1278 (s) NH inplane bending of indole 1309 (vs) 1312 (s) NO2 symmetric stretching 1342 (vs) 1339 (vs) 1341 (vs) NO2 symmetric stretching 1431 (s) 1444 (s) 1437 (s) Aromatic ring CC stretching 1454 (m) CC stretching / CN stretching / CH deformation of indole 1484 (m) CC stretching / CN stretching / CH deformation of indole 1500 (m) CN stretching / CH deformation of indole 1546 (s) 1530 (s) 1530 (s) NO2 asymmetric stretching 1566 (s) 1564 (s) 1560 (vs) NO2 asymmetric stretching 1610 (s) 1604 (vs) 1606 (vs) Aromatic ring CC stretching / NH2 deformation 1626 (vs) 1632 (vs) 1632 (vs) Aromatic ring CC stretching 2868 (w) CH symmetric stretching of indole
53
Table 2.5 contd.,
2930 (w) 2912 (w) 2938 (w) Aromatic CH symmetric stretching / CH asymmetric stretching of indole
3080 (m) Aromatic CH asymmetric stretching
3090 (m) 3109 (m) 3102 (s) Aromatic CH asymmetric stretching
3219 (m) NH2 symmetric stretching
3362 (vs) NH2 symmetric stretching
3376 (vs) 3389 (s) NH2 asymmetric stretching
3400 (vs) NH stretching of indole / OH stretching of nitrophenols
3489 (vs) 3483 (s) OH stretching of nitrophenols