FTIR spectroscopy of SO2 Essay


In this lab, the IR spectrum of SO2 gas was taken. The spectrum was so used to find which extremums corresponded to the vibrational manners of SO2­ . Once the manners had been determined, the experimental wavenumbers of the v1 and v3 manners and the overtones were used to find the anharmonicity of the two different manners. While there was a difference between the two manners ‘ anharmonicity, overall there was non a important difference. The manners and their corresponding wavenumbers were besides used to cipher the force invariables of SO2. The experimental informations produced a force changeless k1 with 3.112 % mistake while the K & A ; delta ; /l2 invariable had a 2.963 % mistake.


IR spectrometry is the sensing of a transmission or soaking up strength of alteration as a map of frequency1. In recent old ages, Fourier transform spectrometers replaced the traditional diffusing spectrometer because they are faster and more sensitive. They have made it possible to analyse many countries which were non possible with the diffusing spectrophotometer. The difference is the coincident scrutiny of all frequences. The three basic spectrometer constituents in a FT system are the radiation beginning, interferometer and sensor. The radiation beginning in preciseness FTIR instruments is frequently water-cooled in give it more power and stability2.

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Figure 1. shows a diagram of the interferometer and the schematics of the spectrophotometer as a whole. The interferometer has the undermentioned three constituents: a traveling mirror, fixed mirror, and a beamsplitter.. The beamsplitter is a semireflecting geranium thin movie of little atoms deposited on level KBr substrate. Radiation from the broadband IR beginning is focused into the interferometer, and hits the beamsplitter. Once the beam hits the beam splitter, half of it is transmitted to the fixed mirror while the other half is transmitted to the traveling mirror. The altering place of the traveling mirror relation to the fixed one generates an intervention form and causes the two beams to hover in and out of stage. When the beams are in stage, there is a constructive intervention ensuing in the maximal sensor response. However, when the beam is out of stage, there is a deconstructive intervention between the two beams. Once they have been reflected from both mirrors, they recombine at the beam splitter. The recombined beam base on ballss through the sample and so focuses on the detector2.

The strength of the radiation hitting the sensor will change in a sinusoidal mode while the mirror is traveling at changeless speed. The record of the intervention signal is the interferogram and is a clip sphere spectrum. The sensor ‘s response alterations versus clip within the mirror scan are recorded. When a sample absorbs at a certain frequence, the amplitude of the sinusoidal moving ridge reduces proportionately to the sum of sample in the beam. In an IR spectrophotometer, this procedure happens in three constituent frequences, which creates a more complex interferogram2.

To change over these interferogram recordings to the IR spectrum, a Fourier transmutation is used. Small, precise intervals are used during the mirror scan. The rate of the trying behaviour is controlled by a monochromatic beam produced by a He Ne optical maser focused on a separate detector2.

For this analysis, the mid IR spectrophotometer utilized a KBr beamsplitter and a quicksilver Cd telluride ( MCT ) sensor. MCT sensors are photon sensor with a dependance on the quantum nature of radiation. They besides exhibit really fast responses. They must be at a changeless temperature of 77^ ( O ) K, the temperature of liquid N. It is faster and more sensitive than the alternate sensor, the deuterated triglycine sulphate ( DTGS ) 2, which was used for the far IR analysis. The cell used to keep the SO2 gas can be seen in Figure 2.

A molecule ‘s energy can be split into three constituents: the negatrons gesture, the component atom ‘s quivers and the whole rotary motion of the molecule. While electronic passages happen on a short timescale, rotational passages happen on a longer clip graduated table. When a molecule is placed in an electromagnetic field, such as visible radiation, energy from the visible radiation is transferred from the field to the molecule. This happens upon the satisfaction of Bohr ‘s frequence status:

& A ; Delta ; E = hv

When a molecule is excited from one province to another, the energy difference between the two provinces is absorbed by the molecule. When the molecule reverts back to the old province, the alteration in energy which was absorbed upon excitement is so emitted1* . A molecule will be excited by photons which possess the appropriate energy3.

Vibrational passages are observed in the infrared ( IR ) spectra which are about the 103 ~ 104 cm-1 part. These passages are caused by the quiver of the karyon representing the molecule. The rotational passages occur at 1-103 cm-1 part, the microwave part, while the electronic passages occur at 104-106 cm-1 part, the UV-visible part. As the vibrational quantum figure V additions, the rotational intervals tend to diminish. The vibrational all right construction of electronic passages can give penetration to the structural and bonding information about molecules which are electronically excited1* .

A system displaced from its equilibrium force will be restored due to a reconstructing force provided by the snap of the system. However, there is a belongings of inactiveness which causes the system to over correct for the supplanting. The dorsum and Forth actions of snap and inactiveness cause the system to hold oscillating motion4. When the possible energy is graphed versus the internuclear separation, a perfect harmonic oscillator forms a parabola. The energy spacing in a harmonic oscillator does non alter throughout the well of the parabola and is equal to

H & A ; omega ; where & A ; omega ; =km12

and the nothing point energy is

Eo= 12h & A ; omega ;

When a system is non a perfect harmonic oscillator, it is considered anharmonic. Anharmonicity forces the right side of the parabola to widen and asymptotically attack nothing. The infinites between the permitted provinces are non equally spaced as they were in the harmonic system5. The comparing of the two graphs can be seen if Figure 3.

One of the possible ways to cipher xe, a term which shows the anharmonicity of a system is to chart? G/ ? versus ( ? +1 ) . This yields a graph with an equation as follows

& A ; Delta ; Gv=v+ 1xeve+ ve

By spliting the xeve term by ve, the xe term is found. The larger this figure, the more anharmonic the system is and vise versa5.

Covalent bonds of molecules are non stiff as ball and stick theoretical accounts would propose, but instead they can be compared to stiff springs which are capable of stretching and flexing. More energy is required to stretch and compact a bond than it does to flex it. There is a direct relationship between the energy or frequence which characterizes the stretching quiver of a bond and the bond dissociation energy3.

The major factors which are influencial in the stretching frequence of a covalent bond can be seen in the undermentioned equation:

V = 12rck ( m1+m2 ) m1+m2

where V is the frequence, K is the force invariable, degree Celsius is the velocity of visible radiation, and M1 and M2 are the multitudes of the two atoms on each terminal of the bond. This equation corresponds to the rigidity of the oscillation. However, it should be noted that non all molecular quivers are capable of being observed in the infrared part. In order to be seen in an IR spectrum, a quiver must do a alteration in the dipole of a molecule. This alteration in charge distribution allows the molecule to absorb infrared visible radiation. There is a relative relationship between the alteration in charge distribution and the soaking up: the greater the alteration, the stronger the absorption3.

All vibrating physical objects have a set of normal modes6. A normal manner can be defined as a simple harmonic oscillation which occurs about an country which is local and low in energy. The normal manners are determined by the system ‘s construction R and it ‘s energy map V ( R ) . Any gesture can be expressed as a superposition of normal manners when a pure harmonic V ( R ) is being considered. However, the close minimal potency can still be approximated by a harmonic potency for an anharmonic V ( R ) . Besides, small-amplitude gestures can still be described by the amount of normal manners. This means that all systems behave harmonically at low temperatures7.

For SO2, it is necessary to hold nine Cartesian co-ordinates in order to find the places of all three karyons. Therefore, the molecule is considered to hold nine atomic grades of freedom. The first three are necessary to depict the place of the centre of mass of the molecule. If these three grades change, it represents the translational motion of the molecule in infinite. The following three grades of freedom refer to the orientation of the molecule. These three grades can be described as the angles of the molecule. If these three grades change, so the molecule has rotated. The three staying co-ordinates are those used to depict the comparative places of the three atoms. These are called vibrational coordinates8.

To depict the quivers of a dead set rhombohedral molecule, it makes sense to utilize the valency co-ordinates. The valency coordinates consist of the two bond lengths and the bond angle. However, they do possess a drawback. If energy is put into a bond so that it stretches, to detect how the molecule reacts is hard due to the energy put into the stretched bond rapidly fluxing into the quivers of the other bond in the molecule. Because of this, it is said that the stretching of a individual bond and other vibrational gestures are coupled8.

By changing the co-ordinates, which are the additive combinations of alterations in the bond lengths and bond angles, a good uncoupled estimate can be made. These co-ordinates are called the normal co-ordinates. Gestures which take topographic point in these co-ordinates are suitably called normal manners of quiver. The centre of mass does non travel in these coordinates8.

A non symmetric molecule with N figure of atoms will hold 3N-6 normal manners. This means SO2 will hold 3 ( 3 ) -6 = 3 normal manners. The normal manners for SO2 can be seen in Figure 3. The symmetric stretch is labeled as v1, the crook is labeled v2, and the asymmetric stretch is labeled v3. When a molecule is exhibiting one of the vibrational manners, it travels the way indicated by the pointer, Michigans, and so returns back to its get downing position8.

It is perchance to show the three normal manners as a potential-energy map written in footings of bond stretching and angle bending as shown in the undermentioned equation:

V = 12k1 ( R1-Re ) 2+ 12k1 ( R1-Re ) 2+ 12kb ( & A ; theta ; – & A ; theta ; vitamin E ) 2

where R1 and R2 are the first and 2nd bond length of S-O, Re is the equilibrium S-O bond length, ? is the bond angle of O-S-O, and? vitamin E is the equilibrium value. The invariables Kansas and kb­ are for the stretching and bending respectively9­ . Though the derivations are hard, it was found that the undermentioned equations are derived from combining weight. ( 1 ) and are used to cipher both invariables:

4r2v32 = 1+2momssin2 & A ; alpha ; k1mo

16r4v12v22 = 21+2momssin2k1mo & A ; alpha ; k & A ; delta ; l2

4r2v12+v22 = 1+2momscos2 & A ; alpha ; k1mo+2mo1+2momssin2 & A ; alpha ; k & A ; delta ; l2

where V # is the wavenumber of that peculiar manner, 4? 3 is expressed as 5. 8918E-5 in order to obtain units of Nm-1, minute is the mass of O, MS is the mass of S, ? is 59.75^ ( O ) , and k? /l2 is the same as the kilobit invariable used in equation ( 6 ) 10.

Diatomic molecules possess merely one vibrational co-ordinate which is quantized. This means that merely specific consequences will be obtained for the value of the quiver. The quantum mechanical harmonic oscillator upon first estimate gives the allowed degrees of a diatomic molecule. Polyatomic molecules are similar. Each normal manner has quantized energy, and can be approximated by the harmonic oscillator theoretical account when at low energy degrees. The frequences associated with flexing tend to be lower than the frequences associated with stretching10.

It is possible to see normal manners via IR spectroscopy if they have a alteration in dipole in the molecule when it stretches or bends10. All of the normal manners in SO2 are IR active and hence can all be seen in the IR spectrum at the cardinal frequence. It is possible to detect other weak sets in the spectrum which are a consequence of overtones. Overtones occur because anharmonicities. They normally happen at whole number multiples of 2 or 3 of the cardinal frequences and are caused by two manners being at the same time excited10. These sets are located at frequences which are about the amount or difference of the two manners which were excited and are weak10.


About 1.5g of drierite was weighed out and placed in the barrel of a syringe and the speculator was inserted about wholly into the barrel. A 3 cm piece of gum elastic tube was attached to the tip of the syringe. A 1.5 g of Na H sulfite was measured and placed in a vial cap that was little plenty to suit into the syringe barrel. The filled vial cap was so into the syringe utilizing a dead set spatula to forestall the Na H sulfite from sloping into the barrel. The speculator was pushed into the syringe every bit far as it would travel. To guarantee that none of the Na H sulfite was spilled, the syringe was placed tip down in a beaker.

The following measure was puting 15 milliliter of 6 M HCl into a little beaker. All of the acid was so drawn into the syringe incorporating the vial cap really carefully as to non allow any of the acerb mix with the Na H sulfite. The plastic palpebra was so screwed onto the syringe. Once the cap was secure on the tip, the syringe was shaken so that the acid and the Na H sulfite assorted. As SO2 gas was being produced, the speculator on the syringe was pulled out at the same time. The high force per unit area of the gas in the syringe caused the cap on the tip to leak so it was necessary to use force per unit area to the tip to forestall it from ptyalizing acid out.

Once the reaction had stopped bring forthing gas, the syringe was inverted so that the tip was indicating up and the liquid was at the underside of the barrel. The cap was removed and the tip was connected to the other terminal of the gum elastic tubing attached to the syringe incorporating drierite. At this point the syringe incorporating drierite was above the syringe incorporating the SO2 gas. As the speculator in the bottom syringe was being pushed in, the speculator in the top syringe was being pulled out ; doing certain no liquid was pushed through the tube and into the top syringe. The top syringe, now incorporating the SO2 gas, was capped and allowed to sit for five proceedingss in order for the drierite to dry the SO2 gas.

The extra HCl in the reaction syringe was expelled into a waste beaker. 15 milliliter of NaOH was placed in a beaker and so pull up into the syringe in order to destruct any staying SO2. The NaOH was so besides expelled into the waste beaker. After the syringe incorporating the gas had sat for five proceedingss, the IR gas cell was placed in the goon. The syringe incorporating the SO2 was so attached connected to the gas cell utilizing another piece of gum elastic tube. Both turncocks on the gas cell were opened and the gas was pushed into the cell. Both turncocks were so instantly closed to forestall any of the SO2 from leaking out. A spectrum in the scope of 700-2500 cm-1 was obtained utilizing an FTIR spectrophotometer. In order to acquire a good spectrum from the mid IR scope, the cell was undiluted. However, to obtain a good spectrum in the far IR scope, it was necessary to thin the gas cell.

Once the spectrum had been obtained, the gas cell was placed inside a fume goon. Both turncocks were opened up and a syringe was used to blush air through the gas cell. The gas cell was so placed in a vacuity sealed dessicator with the turncocks open in order to dry out any wet that may hold entered the cell during the experiment.


The IR spectra of SO2 can be seen in Figure 5. By looking at what wavenumbers the extremums appeared at, it could be concluded which extremum corresponded to each vibrational manner of SO2. The bending of a molecule happens at lower wavenumbers, so it was concluded that graph in the top right corner corresponds to the? 2 quiver. It was known from literature that the stretches occur someplace between 1000 and 1500 cm-1 so the graph in the bottom right must match to the overtones of SO2 ‘s? 3 and? 1 manners.

It is known that asymmetric stretches ever correspond to higher wavenumbers. So it was concluded that the following two extremums on the spectrum were? 1 and? 3 severally. The existent experimental wavelengths of each manner can be seen it Table 1. There are two overtones present, one from the? 1 manner and another from the? 3 manner. The lower frequence overtone corresponds to the lower-frequency manner. Thus the lowest overtone is that of? 1 while the 2nd seen overtone comes from the? 2 manner.

Using the experimental wavenumbers for each manner, both invariables could be found utilizing combining weight. ( 7 ) foremost to work out for k1. This values was calculated to be 1000.858 Nm-1. The litereature value is 1033 Nm-1 and the per centum mistake in the experimental value was 3.112 % The deliberate value of k1 was so used in combining weight. ( 8 ) to happen the K? /l3 invariable. The 2nd invariable was calculated to be 78.60 Nm-1. Literature value for this invariable is 81 Nm-1 and the per centum mistake in the experimental computation was 2.963 % . To measure the effectivity of this method for happening the invariables, both sides of combining weight. ( 9 ) were solved for. The left side equaled 93.77 Nm-1 while the right side equaled 95.54 Nm-1. The per centum difference between these two values is 1.85 % .

In order to find the harmonicity of each of the manners of quiver, the ve and vexe values were calculated. This was done by charting? G/v versus ( 5 + 1 ) in Microsoft Excel. The? G corresponds to the wavenumber of the overtone seen on the IR spectrum. ? G was so divided by v. The overtones corresponded to v=2 while the normal manner sets corresponded to? =1. Graphs for both the? 1 manner and? 2 manner can be seen in Figure 6.

Excel was so used to suit a tendency line and bring forth a Y = maxwell + B equation for the information. The incline of the equation was vexe and the intercept was ve. To find the anharmonicity of the two manners, it was necessary to work out for Xe. This was done utilizing combining weight ( 4 ) . The deliberate values for Xe in the? 1 manner was 1.0612 and for the? 3 manner was 0.07891. This means that the? 1 manner is more anharmonic than the? 3 manner.


For this lab, SO2­ was prepared and so studied via FTIR spectrometry. The three manners of SO2 were identified on the IR spectra obtained. It was determined that the lowest energy of flexing correlated to the lowest frequence extremum. The 2nd highest frequence extremum was determined to be? 1 since the symmetric stretch is lower in energy than the asymmetric stretch ( ? 3 ) which is the 3rd highest frequence extremum. The wavelengths determined from the IR spectra were used to cipher the invariables k1 and k? /l3. It was determined from the Numberss crunched from combining weight. ( 6 ) that the used method of finding the invariables was an accurate method. Besides, the anharmonicity of the manners? 1 and? 2 were calculated and compared. The graph of? G/vversus ( 5 + 1 ) produce an equation of Y = maxwell + B which provided the values of xeve and v­e. These values were so used to happen xe, which described the anharmonicity of each manner. The? 1 manner was found to be more anharmonic due to its greater xe value while the? 3 was found to be more harmonic.


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