Factors affecting Vibration frequency


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1 A Presentation on factors affecting infrared spectroscopy Guided By:- Mr.Dharmendra Baria Lecturer Presented By:- Jinesh gandhi Roll No. 08 jineshsiper@gmail.com Department of QA DDPC

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2 Contents:- Vibrational coupling Requirment for vibration Fermiresonance Hydrogen bonding Electronic effect Resonance effect Inductive effect Bond angle Interaction b/w solute & solvent Concentration of analyte Nature of solvent Temperature effect

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3 Factors influencing vibrational frequency:- (1) Vibrational coupling:- Here, interaction of 2 vibrational level which vibrate at same frequency and near by in molecule  known as vibrational coupling Occur only when bonds are located closely to each other. Here,2 more peaks appear in spectra. (1)symmetric (2)Antisymetric

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4 Requirements:- 2 Stretching vibrational coupling occur when it is separated through common atom. 2 bending vibrational coupling occur when it is separated by common bond. Coupling can occur between stretching and banding if bond involved in stretching is involved in bending also. Both vibrator should vibrate with same frequency.

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5 If no.of bonds will increase or seperated by more than 2 atoms  vibrational coupling is negligible. E.g;-(1)-C-H  show streching vibration only. -C - H  appear 2 peaks H Antisymmetric:-at 3000cm-1 Symmetric :-at 2900cm-1

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6 If H - C - H  here,antisymmetric:-at 3100cm-1 H symmetric:-at 3000cm-1 (2) CO 2 it is linear molecule so no. of peaks should be 3n-5 = 3*3-5= 4 But here only 2 peaks appear in spectra that is anti-symmetric and bending. Antisymmetric peak appear at 2350cm- Here in symmetric peak doesn’t appear because of there is no change in dipole moment -Therefore it is IR inactive And instead of 2 bending peaks only one bending peak appear because energy of absorption is same.

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7 (3) Acetaldehyde (CH3-CH0) Here n=7 and molecule is non linear so no. of vibrational peaks should be 15 according to 3n-6 But here only 5 peaks appear in spectra (4) H 2 O Here 3 peaks appear according to 3n-6 Symmetric peak appear at 3650 cm-1 Antisymmetric peak at 3760 cm-1 Bending peak at 1595 cm-1

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8 (5) Anhydride:- R-C-O-C-R O O These give rise to 2-C=Ostr absorption,1 antisymmetric and 1 symmetric peak. Here,coupling between 2 carbonyl groups which are indirectly linked through –O-. (6) Amide R-CONH 2 Shows 2 absorption bands around 1600-1700cm-1 i.e. due to –C=O str and –N-H def.

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9 but due to coupling original character is changed. Here coupling between –C-N stretching and –N-H bending vibrational level takes place Here amide I peak  due to –C=O stretching and amide II peak  due to coupling.

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10 Fermiresonance:- interaction between fundamental vibrations and overtones known as Fermiresonance. E.g (1) aldehyde(R-CHO):- Here coupling between –C-H stretch and overtone of –C-H str occur

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11 (2)Inorganic structure :-  appearance of doublet of –C=O stretch of cyclopentanone ,Fermiresonance with an overtone or combination band of an alfa-methylene group shows 2 absorption in the carbonyl stretch region

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12 (2) Hydrogen bonding:- Possible only if any system contain 1 proton donor and 1 proton acceptor groups in system. Here,”s” orbital of proton donor overlaps ”p” or “  ”orbital of proton acceptor group. E.g of donors:--COOH -OH -NH2 -CONH2 E.g: of acceptor:- any system containing “=“ H bonding is denoted by “-----”

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13 2 types of H-bonding (1) inter molecular (2) intra molecular H-bond occur at how much extent depends on geometry, nature of proton donor/acceptor ,ring strain existing in molecule. At low concentration 2 peaks appear for alcohol. -the sharp band is of –O-H str in free alcohol -the broad band is due to H-bonded –O-H str

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14 E.g:- in which intermolecular H-bonding occur  alcohol, phenol,carboxylic acid,etc. E.g:- in which intramolecular H-bonding occur  o-chloro and o-alkoxy phenols,beta-hydroxy amino/nitro compounds

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15 but due to coupling original character is changed. Here coupling between –C-N stretching and –N-H bending vibrational level takes place Here amide I peak  due to –C=O stretching and amide II peak  due to coupling.

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16 Hydrogen bonding is strongest when bonded structure is stabilized by resonance. E.g:- (1) salicylic acid (o-hydroxy benzoic acid/p-hydroxy benzoic acid):- here, both shows different peaks because OHBA shows intramolecular H-bond (2) phenol:- shows inter/intramolecular H-bonding

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17 (3) Enols/chelates:- here, H-bonding is so strong that even though diluting solution it can’t break -C- and –OH:-here, H-bond with O of –C=O so, decreased double bond characteristic. Depends on basicity of –C=O group. If more basic  stronger H bond. -COOH also shows H-bonding

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18 (4) Aromatic compounds posses pie-system:- -so. posses conjugation. -act as Lewis base. -so. decreased ‘=‘characteristic -so, deceased F (5) -NH 2 :- shows 2 peaks, at 3000cm-1 and 3600cm-1 -if free amino group  shows peak at 3600cm-1 -if H-bonded amino group  shows peak at 3000cm-1 - H-bond in –N-H is more weaker than that of in –OH.

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19 ( 3)Electronic effects:- depends on presence of substituent. (a)conjugation:- cause delocalization of electron, -so, decreases “=“c increases bond lenth decreases bond strength so decrease on vibrational frequency.

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20 0-here, delocalization of  -electron between –C=O and ring increases double bond character of bond joining them.

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21 (b)Resonance effect(mesomeric effect):- means single molecule can be represented in 2 or more than 2 forms -if electron releasing group present  increases in delocalization  decreases in “=“ characteristic  increases in bond length  decreases bond strength  so, decreases vibrational frequency.

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22 (c)Inductive effect:- -Inductive and resonance both type of effects are existing in molecule. -finally which type of effect is shown by molecule depends on which effect is predominant. E.g:- Amides:- (1)R-C-NH2:- NH2 is electronreleasing group  so, more resonance effect  so decreases F  so, decreases vibrational  frequency

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23 (2)R-CO-Cl:- -C=O is electron withdrawing O group  so,decreases in delocalization of electron  so, increases bond strength  so, increases in vibrational frequency (3)Esters:- R-C-0O-R1:-if alkyl esters  resonance effect is predominant If R=benzene  inductive effect is predominant.

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24 (4)Field effect:- If 2 functional group present and 1 affect the vibration of other  known as field effect. OCH3 C=O:- here, Cl- present in equatorial position 2 functional groups are near each other  so, repulsion occur When –C=O group is in axial position  no field effect

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25 (5)Bond angle:- Normal bond angle is of 120. -if decrease in angle strain occur  bond length is decreased and “=“characteristic is increased.  increased F  increadsed vibrational frequency. If ring expands  increased length of bond  so decreased F  so, decreased vibrational frequency

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26 6)Interaction between solute and solvent:- Due to this change in position and shape of IR peak. 7) Concentration of analyte :- Increased concentration of analyte  so, more interaction  so, band broadening

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27 (9)Temperature effect:- If increased temperature increased interaction so band broadening also, there may be change in position and shape of IR spectra

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28 References:- 1) Instrumental analysis By skoog;hooler;crouch India edition reprint 2008; page No.486-7 2) Handbook instru.techniques for analytical techniques By frank settle India reprint 2004;page No.251-2 3)Organic spectroscopy,william kemp,3 rd edition 4)Spectrometic identification of organic compound,Robert M.silverstein,francis X.webster,6 th edition 5)Introduction to instrumental analysis,Robert D.Broun

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29 Frequently asked question in exam:- Explain the principle of IR. Discuss factor affecting tbe carbonyl streching in IR spectroscopy.

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30 Thank you

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