13

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

14

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

15

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

16

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.

17

Figure 2.1 Schematic diagram of a FTIR spectrometer

18

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

19

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.

20

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

21

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

22

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

23

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.

24

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.

25

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

26

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

27

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

29

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

30

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.

31

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.

32

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