04/18/2023

All India Shri Shivaji Memorial Society’s College Of Pharmacy, Pune -411001

FACTORS AFFECTING MOLECULAR VIBRATIONS

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PREPARED BY AKSHAY PAHADE

F.Y.M-PHARM (QUALITY ASSURANCE)

GUIDED BY DR. S. V. GANDHI

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INTRODUCTION

Atoms in a molecule do not remain at fixed positions with respect to each other, but actually vibrate back and forth.

At room temp. most of the molecules are in the lowest vibrational state.

On absorption of infrared light of appropriate energy molecules get excited to second vibrational state.

For the excitation of a molecule from one vibrational level to another, the molecule has to absorb IR radiation of particular energy.

Molecular vibrations are also important in understanding infrared absorption and the mechanisms and kinetics of chemical reactions. 

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MODES OF VIBRATIONS

Stretching: Change in inter-atomic distance along bond axis

symmetric asymmetric

Bending: Change in angle between two bonds.

Rocking Scissoring Wagging Twisting

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Cont…

Wagging twisting rocking

scissoring

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METHYLENE GROUP BENDING VIBRATIONS

C

H

H

C

H

HC

H

H

C

H

H

C

HH

C

HH

Scissoring Wagging

Rocking Twisting

BendingVibrations

~1465 cm-1

~720 cm-1

~1250 cm-1

~1250 cm-1

in-plane out-of-plane

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Hooke’s law :

n =1

2pc Km

n : Frequency in cm-1

c : Velocity of light => 3 * 1010 cm/s

K : Force constant => dynes /cm

: m masses of atoms in grams

=mm1 m2

m1 + m2

=M1 M2

M1 + M2 (6.02 * 1023)

CALCULATING VIABRATIONAL FREQUENCIES

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Cont…

7

=1

2pcn Km

larger K,higher frequency

larger atom masses,lower frequency

constants

2150

1650

1200

C=C > C=C > C-C

C-H > C-C > C-O > C-Cl > C-Br3000 1200 1100 750 650

increasing K

increasing m

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Cont…

CH3 CH2 CH2 CH2 CH2 CH3

CHstretchingvibrations

includesCH3 sym and asym

CH2 sym and asym

CH bending vibrations

Hexane:

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FACTORS AFFECTING MOLECULAR VIBRATIONS9

COUPLED VIBRATIONS AND FERMI RESONANCE

ELECTRONIC EFFECT

HYDROGEN BONDING EFFECT

CONJUGATION EFFECT

BOND ANGLE EFFECT

RING SIZE EFFECT

SUBSTITUENT EFFECT

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Cont…

It is been found that the calculated value of wave number is never exactly equal to the experimental value.

The differences arise because the vibrations of each group get influence by the structure of the molecule in the immediate neighborhood of the bond.

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COUPLED VIBRATIONS AND FERMI RESONANCE

• It is expected that there is one stretching frequency for isolated C-H bond but in case of methylene(-CH-) group two absorptions occur which corresponds symmetric and asymmetric vibrations.• Asymmetric vibration occur at higher wave number compared to symmetric vibration.• These are known as coupled vibrations because these occur at different frequencies than that required for an isolated C-H stretching.

Symmetric stretching Asymmetric stretching

~2853 cm-1 ~2926 cm-1

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Cont…

It may happen that the energy of overtone level can coincide with the fundamental mode of different vibrations.

A type of resonance takes place as in case of coupled pendulums. This type of resonance is called as Fermi Resonance.

This can be explained by saying that a molecule transfers its energy from fundamental to overtone and back again.

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Cont…

Overtones – The term overtone is used in a general sense to apply to any multiple of a given fundamental frequency.

The transitions from v0 v2 and v0 v3 are the first and second overtones of the fundamental and require radiation of twice and thrice times its frequency.

For eg. The first overtone for the carbonyl fundamental at 1700 cm-1 will be 3400 cm-1.

Most overtones are found in the near infrared region beyond 4000 cm-1.

Such absorptions are much weaker. The intensity of overtone decreases as the order of the overtone

increases. Aromatic compounds exhibit overtone absorptions in 2000 – 1667

cm-1 region which are characteristic of the aromatic substitution.

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ELECTRONIC EFFECT

Inductive Effect

Mesomeric Effect

Field effect

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Cont…

Positive Inductive Effect:Introduction of alkyl group causes positive inductive effect which gives rise to the lengthening or weakening of the bond. Due to this absorbance takes place at lower wave number.

Eg. C=O i) Formaldehyde (HCHO) 1750cm-1

ii) Acetaldehyde (CH3CHO) 1745cm-1

iii) Acetone (CH3COCH3) 1715cm-1

Inductive Effect

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Cont…

Negative Inductive Effect:

Similarly introduction of electronegative atom or groups (Inductive effect) increases the wave number of absorbance.

i) Acetone (CH3COCH3) 1715cm-1

ii) Chloroacetone (ClCH2COCH3) 1725cm-1

iii) Dichloroacetone (Cl2CHCOCH3) 1740cm-1

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Cont…

Mesomeric Effect:

In many cases mesomeric effect works along with inductive effect and cant be neglected.

Conjugation lowers the wave number of absorbance. In some cases inductive effect dominates over mesomeric effect

whereas reverse holds for the other cases Eg. C=O str. In amides and esters-

As nitrogen atom is less electronegative than the oxygen atom, the electron pair on nitrogen atom in amide is more labile and participates more in conjugation. Due to this greater degree of conjugation, the C=O absorption of frequency is much less in amides as compared to the esters.

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Cont…

Eg. Benzamide Phenyl acetate

(C=O) (C=O) 1663cm-1 1730cm-1

NOTE: It is important note that only inductive effect is considered

when the compound is metasubstituted.

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Cont…

Field Effect:

Two groups often influence each other’s viabrational frequencies by a through-space interaction, which may be electrostatic and/or steric in nature.

Eg. The best example for field effect are interactions between carbonyl groups and halogen atoms;

in the α-chloroketone derivatives of steroids, C=O str. frequency is higher when Cl is equatorial than when it is axial. Presumably the nonbonding electrons of oxygen and chlorine undergo repulsion when they are close together in the molecule; this results in change in the hybridization state of oxygen, and therefore a shift in C=O str. Frequency.

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Cont…

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HYDROGEN BONDING

Hydrogen bonding gives rise to downward frequency shifts. Stronger the hydrogen bonding greater the absorption shift towards

lower wave number from the normal value. Intermolecular hydrogen bonds give rise to broad bands. These

bonds are concentration dependent. On dilution, the intensities of such bands decrease and finally disappear.

Intramolecular hydrogen bonds give sharp and well defined bands. These are independent of concentration.

Eg. 1. Amines show N-H str. At 3500cm-1 in dilute solution while in condensed phase spectra, absorption takes place at 3300cm-1.

2. Alcohol exhibiting intermolecular H-bonding absorb between 3400-3200cm-1 while O-H str. in free alcohol absorb near 3600cm-1

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STRONGER BONDS HAVE LARGER FORCE CONSTANTS AND ABSORB AT HIGHER

FREQUENCIES

3000

-C-H=C-H

31003300

=C-H=

2900 2850 2750

-CH=O(weak)

increasing CH Bond Strength

sp3-1ssp2-1ssp-1s

increasing frequency (cm-1)

aldehyde

increasing s character in bond

increasing force constant K(CH BASE VALUE =3000 cm-1 )

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CONJUGATION

Introduction of C=C adjacent to the carbonyl group result in delocalization pi –electron in the C=O and C=C bond.

Conjugation increases the single bond character of C=O and C=C bond in resonance hybrid and hence lowers their force constant, resulting in lowering of frequencies of carbonyl and double bond absorption.

Carbonyl with triple bond also shows this effect.

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RING SIZE EFFECT

Decreasing ring size increases frequency of C=O absorption.

All the functional groups listed below, which can form rings, give increased frequencies of absorption with increased angle strain.

Anhydride 1810 cm-1

Acid chloride 1800 cm-1

Ester 1735 cm-1

Aldehyde 1725 cm-1

Ketone 1715 cm-1

Carboxylic acid 1710 cm-1

Amide 1690 cm-1

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BOND ANGLES

In ketones, the correlation charge shows that highest C=O frequencies arise in the strained cyclo-butanone and we can explain this term of bond-angular strain: the C-CO-C bond angle is reduced below the normal 120°, leads to increased s-character in the C=O bond.

The C=O bond is shortened and therefore strengthened and so ʋ(C=O) increases.

If the bond angle is pushed outward above 120°, the opposite effect operates, and for this reason di-tert-butylketone has a very low ʋC=O (1697cm-1)._

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SUBSTITUENT EFFECT

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Cont…

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REFERENCES

1. Chatwal G. R., Anand S. K., ‘INSTRUMENTAL METHODS OF CHEMICAL ANALYSIS’, Himalaya Publishing House Pvt. Ltd., 1st ed., (reprint-2011), Mumbai.

2. Pavia D. L., Lampman G. M., Kriz G. S., Vyayan J. R., ‘SPECTRSCOPY’, Cengage Learning, 8th ed. (Indian reprint-2011), New Delhi.

3. Kemp W., ‘ORGANIC SPECTROSCOPY’, Published by Palgrave, 3rd ed., (reprint-2010), New York.

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THANK YOU!!

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