Background to organic spectroscopy diatomic molecules such as Hydrogen (H 2), Nitrogen (N 2) and...

Lecture 3 31/10/2012

11/2/2012 Organic Spectroscopy

Introduction to IR

Vibrational modes in IR spectroscopy (Stretching and Bending)

Schematic representation of IR spectrometer

The IR spectrum (scale, band intensity, FGs region, Fingerprint

region)

Chemical bonds vs IR absorption

Interpretation of IR spectrum

http://www.chem.ucalgary.ca/courses/350/Carey5th/Ch13/ch13-ir-1.html

A chemical bond is compared with two weights connected by a spring as derived from “Hooke’s law”.

That the total amount of energy is proportion to the frequency of vibration, that is;

E oscillation a hνosc.


http://en.wikipedia.org/wiki/Hooke's_law

http://en.wikipedia.org/w/index.php?title=File:Spring-mass2.svg&page=1

Therefore frequency of absorption is

proportional to the square root of the force

constant for the bond divided by the reduced

mass

K is force constant in dynes/cm and

m is the reduced mass

m = m1m2/m1 + m2 masses of atom in grams

Or

m = M1M2/(M1 + M2)(6.02 ×1023)

masses of atoms in amu. 11/2/2012 Organic Spectroscopy

The smaller the atom the better the overlap and the higher the frequency of absorption For a stronger bond (larger K value), v increases,

for heavier atoms attached (larger m value), v decreases.


Molecules absorb infrared light when one of six

possible bonding vibrations occurs:

Recall

Bonds can have

◦ symmetrical and asymmetrical; stretching,

◦ scissoring, rocking, wagging and twisting.

The energies of the peaks must be matched to

known bond vibrations.


The C-H stretching and bending energies

are clearly visible but the C-C bands are not

seen, as they are either too weak or too low

in energy.

For a molecule to be IR active the vibration

must result in a change of dipole moment

during the vibration.


Homonuclear diatomic molecules such as

Hydrogen (H2), Nitrogen (N2) and Oxygen

(O2) have zero dipole moment.

No infrared absorption are observed.


For heteronuclear diatomic molecules

such as carbon monoxide (CO) and

hydrogen chloride (HCl) are IR active.

They possess a permanent dipole

moment, because bond stretching

leads to a change in dipole moment

( Dipole Moment = Charge x Distance


Usually symmetrical vibrations are weak or

not visible.

Larger changes in dipole moment of bond

lead to more intense IR absorption

It is not necessary for a compound to have

a permanent dipole moment to be infrared

active.


In the case of carbon dioxide (CO2) the

molecule is linear and centrosymmetric and

therefore does not have a permanent dipole

moment.

This means that the symmetric stretch will

not be infrared active.


However, in the case of the

asymmetric stretch a dipole moment

will be periodically produced and

destroyed resulting in a changing

dipole moment and therefore infrared

activity


C OO C OOC OO


http://chemwiki.ucdavis.edu/Physical_Chemistry/Spectroscopy/Vibrational_Spectroscopy/Vibrational_Modes


If there is an absence of major functional

group bands in the region 4000-1300 cm-1

(other than C–H stretches), the compound is

probably a strict hydrocarbon.


Aromatics, alkyl halides, carboxylic

acids, amines, and amides show

moderate or strong absorption bands

(bending vibrations) in 900-650 cm-1.


Absorption frequencies Typical bonds

3700-2500 cm-1

2300 –2000 cm-1

1900- 1500 cm-1

1400- 650 cm-1

Single bonds to hydrogen

Triple bonds

Double bonds

Single bonds (other than

hydrogen)




Look for a carbonyl in

the region

1760—1609 cm-1

Is an O-H band

also present?

A carbonylic acid

group

Is a C-O band

also present?

An ester

Is an aldehydic

C-H band also

present

An aldehyde

Is an N-H band

also present?

An amide

Are none of the

rest present?

A ketone


Other structural

features from 4000—

1300 cm-1

Are there C-O

stretches?

An ether

(or an ester if there is

a carbonyl band too)

Is there a C=C

stretching bands?

An alkene

Are there aromatic

stretching band?

An aromatic

Are there –NO2 band?

A nitro compound

Is there a CC triple

bond band?

An alkyne




It has few strong absorption bands whose

spectrum has the various CH stretch bands

that all hydrocarbons show near 3000 cm-1.

There is a weak alkene CH stretch above

3000 cm-1.

This comes from the C & emdash; H bonds on carbons 1 and 2,

the two carbons that are held together by the double bond.


http://webbook.nist.gov/cgi/cbook.cgi?ID=C592416&Type=IR-SPEC&Index=1#IR-SPEC


The strong CH stretch bands below 3000

cm-1 the CH2 and CH3 groups.

There is an out-of-plane CH bend for

the alkene in the range 1000-650 cm-1.

There is also an alkene C=C double

bond stretch at about 1650 cm-1.





http://www.chem.ucalgary.ca/courses/350/Carey5th/Ch13/ch13-ir-1.html




Frequency cm-1 Bond Functional group

3640–3610 (s,

sh)

O-H strength, frequency, free

hydroxyl

Alcohols, phenols

3500–3200 (s,b) O-H strength, frequency, H-bonded Alcohols, phenols

3400–3250 (m) N-H stretch 1o, 2o amine, amide

3300–2500 (m) O-H stretch Carboxylic acids

3330–3270 (n,s) -CC-H: C-H stretch Alkynes (terminal)

3100– 3000 (s) C-H stretch aromatic

3000– 2850 (m) =C-H stretch alkenes

2830– 2695 (m) H-C=O: C-H stretch aldehyde

2260– 2210 (w) CN stretch nitriles

2260– 2100(w) -CΞC stretch alkynes

1760– 1665 (s) C=O stretch Carbonyl (general)

1760–1690 (s) C=O stretch Carboxylic acids

1750–1735 (s) C=O stretch esters, saturate aliphatic

1740–1720 (s) C=O stretch aldehydes, saturate aliphatic

1730–1715 (s) C=O stretch a,b-unsaturated esters

1715 (s) C=O stretch ketones, saturated aliphatic

1710–1665 (s) C=O stretch a,b-unsaturated aldehydes,

ketones

1680–1640 (m) -C=C– stretch alkenes

Typical absorption frequencies of some FGs

a) Give the IUPAC names of the following compound 1 to 5.(start slide show and click on numbers, then

go to # 3. Group Frequencies)

b) Which one of these compounds best match the given IR spectrum Q?

c) Give reason to why it does match.

d) Why each of the rest do not comply with the spectrum? Suggest their absorption patterns and frequencies

OH

O

O OH O

(1) (2) (3) (4) (5)


http://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/Spectrpy/InfraRed/infrared.htm



IR spectrum Q


a) Find the number of vibrational modes for the following molecules:

NH3, C6H6, C10H8, CH4, C2H2(linear).

b) State which of the following vibrations are IR active: N2, CO, CO2

(stretching), HCl

c) Calculate the vibrational frequency of CO given the following data:

mass of C = 12.01 amu, mass of O = 16 amu, the force constant k

= 1.86X103kgs-2.

d) Calculate the vibrational energy in Joules per mole of a normal

mode in question 3, in its ground state of n=0.

e) Assuming the force constant to be the same for H20 and D2O. A

normal mode for H20 is at 3650 cm-1. Do you expect the

corresponding D2O wave number to be higher or lower?

11/2/2012


Organic Spectroscopy

Different chemical bonds give rise to characteristic IR absorption

frequencies and intensities.

The energy required for bond vibration depends on the size (mass)

of the atoms and bond strength.

Strong bonds absorb at higher frequencies

Frequency of absorption is proportional to the square root of the

force constant (K) for the bond divided by the reduced mass (m)

Homonuclear diatomic molecules have zero dipole moment hence

they are infrared inactive.

It is not necessary for a compound to have a permanent dipole

moment to be infrared active


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Background to organic spectroscopy diatomic molecules such as Hydrogen (H 2), Nitrogen (N 2) and...

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