MM361_lec22-24

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Vibration Modes in Molecules

The vibrations of nuclei in a molecule can be characterized by normal mode

vibration. Each type of molecule has a defined number of vibration modes

and each mode has its characteristic frequency.

The simplest model of molecular vibrations is the diatomic model, which has

only one stretching mode.

However, a polyatomic linear molecule has a total of 04 vibration modes.

These vibration modes include two stretching modes and two bending

modes.


Stretching and bending

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For N atomic nuclei in a molecule, there are a total of 3N-5 number of total

vibration modes in a linear molecule. For example, CO2 has 3x3-5=4 vib

modes.

Among the total number of normal vibration modes in a molecule, only some

can be detected by IR spectroscopy. Such vibration modes are referred to

as infrared active.

To be infrared active, a vibration mode must cause alternation of dipole

moment in a molecule. A dipole is created if there is a separation of

negative and positive charge centres in a molecule.

IR Activity

The IR activity requires changes in the magnitude of normal vibration during

the vibration. The magnitude is commonly represented with a parameter q

(which is equivalent to x).

IR activity requires that the derivative of dipole moment with respective to

the vibration at the equilibrium position is not zero.

Where is the dipole moment.

It does not matter whether the molecule has permanent dipole moment,

because the dipole moment can be induced by the electric field of an

electromagnetic wave.

0

0

qq

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IR Activity

CO2 for example has no net dipole moment in the unperturbed state. One

if its normal mode of vibration is the symmetric. This does not induce the

dipole moment and is therefore IR inactive.

There is another vibration mode i.e. asymmetric stretch of CO2 resulting in

a net dipole change of the molecule upon vibration. This mode is IR active.

IR Spectroscopy

It has been established that certain functional groups in polymers (methyl,

ester, carbonyl etc.) absorb IR radiation at certain characteristic frequencies

which helps to indentify polymers with unknown compositions.

IR peak intensities are related to the concentration of the corresponding

functional groups which is associated with the absorption coefficient.

Intensity Io of IR radiation is attenuated to I as per the thickness bof the

sample.

Where a and c are abs. coeff. which

depend on the nature of the chemical

group and their concentration.

oI

IT %

)(I

IInA o

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NMR Spectroscopy:

Basics

In the radio frequency range, or microwaves there are transitions associated with the rotational energy levels in small molecules but not polymers. But if we apply magnetic field, certain nuclei such as Protons, Deuterons, 13C, 15N and 19F show absorptions because they have magnetic dipole moments due to the spin.

This spin give rise to a localised magnetic field so that the nucleus can be thought of as a small magnet with a magnetic moment . In the quantum mechanics the nuclear spin is characterized by the spin number, I (integral or half-integral values).

This spin number is related to the mass and atomic number.

The nucleus of most common isotopes of carbon and oxygen 12C and 16O are nonmagnetic because I=0

NMR Basics

In terms of characterizing polymers the most important isotopes are 1H, 13C

and 19F. Because these have a spin number I of 1/2.

If a magnetic nucleus is introduced into a uniform external magnetic field Ho it assumes a set of 2I+1 quantized orientations.

Therefore, these isotopes can only assume one of two possible orientations,

corresponding to energy levels of Ho

The magnetic moment of the lower energy +1/2 state (is aligned with the

external field, but that of the higher energy -1/2 spin state is opposed to

the external field.

*http://www2.chemistry.msu.edu/faculty/reusch/VirtTxtJml/Spectrpy/nmr/nmr1.htm

oHhE 2

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NMR Basics

The difference in energy between the two spin states is dependent on the

external magnetic field strength, and is always very small.

The two spin states have the same energy when the external field is zero,

but diverge as the field increases. At a field equal to Bx , a formula for the

energy difference is given (I = 1/2 and is the magnetic moment of the nucleus in the field).

Irradiation of a sample with radio frequency energy corresponding exactly

to the spin state separation of a specific set of nuclei will cause excitation

of those nuclei in the +1/2 state to the higher -1/2 spin state and

resonance occurs.

=2o

NMR Spectroscopy

Mass

number

Atomic no. Spin number Examples

Odd Even or odd , 3/2, 5/2, 1H or 13C

Even even 0 12C

Even odd 1,2,3, 2H

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NMR Instrumentation

NMR is non-destructive, and with modern instruments good data may be

obtained from samples weighing less than a milligram.

A sample is placed in a uniform magnetic field whose field strength can be

varied and a transmitter applies a radio-frequency field by means of an

exciting coil.

By varying the frequency at fixed magnetic field strength, or by varying

the magnetic field strength at constant frequency, resonance conditions can

be found and detected.

oHhE 2

Superconducting

solenoid

Shielding Effect in NMR

It has been determined that a single proton would resonate at a lower field

strength than the nuclei of covalently bonded hydrogens. This is because of

the electron(s) surrounding the proton in covalent compounds and ions.

Electrons are charged particles, they generate a secondary field that

opposes the much stronger applied field. This secondary field shields the

nucleus from the applied field, so Bo must be increased in order to achieve

resonance. This is called shielding effect in NMR.

Similarly, the various hydrocarbon groups existing

in organic samples or polymers would resonate at

different frequencies due to the shielding effect.

For example, CH2 would resonate differently than

CH2 or CH.

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NMR Spectra

Two techniques are widely used 1H NMR and 13C NMR.

1H NMR is the most commonly used NMR which is based on the 1H nucleus or proton. It can give information about the structure

of any molecule containing hydrogen atoms.

The NMR spectra is a plot of intensity of NMR signals versus

the magnetic field (frequency).

1H NMR

A low resolution NMR spectra of ethanol shows three absorption peaks,

corresponding to the OH, CH2 and CH3 groups.

The areas of these peaks are in the ratio of 1:2:3. In NMR, the band

intensities give a direct measure of the number of nuclei they represent.

Now compare this spectrum to one taken at higher resolution. The peaks

due to the CH2, and CH3, protons appear as multiplets.

This splitting is due to the magnetic field of the protons on one group

influencing the spin arrangements of the protons on an adjacent group.

C2H5OH

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1H NMR:

Spin-Spin Coupling

The observed multiplicity of a given group of equivalent

protons depends on the number of protons on adjacent atoms,

n, and is equal to (n + 1).

Thus the two CH2, protons in the ethyl group of ethanol split the

CH3 resonance into a triplet, while the three protons on the CH3

group split the CH2 resonances into a quartet.

This is called Spin-Spin Coupling.

As per quantum mechanical selection rules, chemically

equivalent nuclei don't interact individually through spin-spin

coupling. Such as 2 protons in CH2 or 3 in CH3.

Rules for Spin-spin coupling

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Spin-spin coupling

Chemical Shift in NMR

The applied frequency to generate resonance also depend on

the magnetic field strength Ho. The large magnetic field

generated by the superconducting solenoid magnet may vary

in different NMR instruments resulting in a different resonance

frequencies for the same molecule.

In order to solve this problem, a parameter called chemical

shift was introduced which characterises the frequency with

respect to an internal standard i.e. Tetramethylsilane [Si(CH3)4]

TMS with zero chemical shift.

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Chemical Shift in NMR

Chemical shifts relative to TMS still depends on the Ho,

therefore, chemical shift is divided by the frequency of

spectrometer to normalise its value.

Where s stands for the resonant frequency of the sample. The units of chemical shift are ppm. Typically for 1H, a range of

10-12 ppm covers most organic molecules, for 13C the range

is 600 ppm.

erspectromet

sTMSppm

610)()(

Factors effecting the chemical

shift

Electronegative groups as they decrease the electron density

around the proton thus deshielding occurs and there is an

increase in the chemical shift

Hydrogen bonding

Protons that are involved in hydrogen bonding typically change

the chemical shift values. More the hydrogen bonding more

proton is deshielded and chemical shift goes higher.

Magnetic Anisotropy of -system, (Due to a nonuniform

magnetic field). Electrons in -systems (aromatics, alkenes,

alkynes, carbonyls etc.) interact with the applied field which

induced a magnetic field that leads to anisotropy and

shielding and deshielding of protons. For example, benzene.

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Signal Strength NMR

The magnitude or intensity of NMR resonance signals is

displayed along the vertical axis of a spectrum, and is

proportional to the molar concentration of the sample. Thus, a

small or dilute sample will give a weak signal, and doubling or

tripling the sample concentration increases the signal strength

proportionally.

If we take the NMR spectrum of equal molar amounts of

benzene and cyclohexane in CCl4 solution, the resonance signal

from cyclohexane will be twice as intense as that from benzene

because cyclohexane has twice as many hydrogens per

molecule.

Proton chemical shift ranges

For samples in CDCl3 solution. The scale is relative to TMS at = 0

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1H NMR Table of standard chemical

shifts

Interpretation of 1H NMR Spectra

Number of signals -- indicates how many different kinds of

protons are present

Position of signals -- indicates something about magnetic

environment of protons

Relative intensity of signals proportional to the number of

protons present

Splitting of signals indicates the number of nearby

nuclei (spin-spin coupling)

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4 levels of information 1H NMR

Spectra

There are 4 levels of information to be gained from proton

NMR related to different aspects of the spectrum:

1. How many types of H ? ..........Look at how many basic groups

of signals there are.

2. How many H of each type ? .... Look at the integration

(relative area) of each group.

3. What is each type ? .................Look at the chemical shift of

each group and relate to tables of typical values.

4. What is the connectivity ? ........Look at the spin coupling

patterns. This tells you what is next to each group

Sampling for NMR

In order to take the NMR spectra of a solid, it is usually

necessary to dissolve it in a suitable solvent.

Deuterium labeled compounds, such as deuterium oxide (D2O),

chloroform-d (CDCl3), benzene-d6 (C6D6), acetone-d6

(CD3COCD3) and DMSO-d6 (CD3SOCD3) are now widely used

as NMR solvents.

Since the deuterium isotope of hydrogen has a different

magnetic moment and spin, it is invisible in a spectrometer

which is tuned to protons only.

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13C NMR

13C is only present to the extent of 1.1%, and the relative

sensitivity of a 13C NMR experiment is about 6000 times less

than that of a 1H experiment.

Modem instruments have solved this problem and high quality 13C NMR spectra can now be obtained with 13C-1H coupling

eliminated by a technique called proton decoupling.

13C NMR

The advantage of 13C relative to 1H NMR spectroscopy is the

higher resolution that can be obtained in the later. 13C

resonances of organic compounds are found over an enormous

chemical shift range of 600 ppm and one can frequently

identify resonances for individual carbon atoms in a molecule,

as also illustrated by the spectrum.

Note that the lines are not of equal

intensity, even though each is assigned

to an individual carbon atom.

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13C NMR

Unfortunately, in 13C NMR spectroscopy one cannot simply

relate the relative intensities to the number of equivalent

carbon atoms in a molecule; relaxation phenomena and

something called the nuclear overhauser effect have to be

taken into account.

Due to low abundance, we do not usually see 13C-13C coupling

as observed in 1H-NMR

Relative Advantages of 13C NMR and 1H NMR

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Applications of IR and NMR in

Polymers

Polymer Identification

Branching

Sequence isomerism

Structural isomerism

Tacticity

Molecular spectroscopy

Idenstification Chain others

IR and NMR in Polymer

Identification

IR spectroscopy is used for the qualitative identification of major

components through the use of group frequencies and distinctive

patterns in the "fingerprint" region of the spectrum.

From polystyrene spectra, we can determine that the sample

contains aliphatic and aromatic groups from the bands observed in

the 2800 to 3200 cm-1 region of the spectrum.

Secondly, we can initially eliminate such groups as hydroxyls, amines,

amides, nitriles, carbonyls etc., which all have distinctive group

frequencies.

Thirdly, the presence of a group of distinctive and relatively sharp

bands (e.g., the band at about 700 cm-1 and the two bands near

1500 cm-1) that are characteristic of monosubstituted aromatic rings

readily leads one to the conclusion that the spectrum resembles that

of a styrenic polymer.

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IR spectra of atactic polystyrene and polymethylmethacrylate

aliphatic and

aromatic groups

Monosubstituted

aromatic rings

Carbonyl

stretching

-C-O-

stretching


From the spectrum of a-PMMA, we find bands that are

associated with aliphatic CH2, and CH3, groups in the CH

stretching and fingerprint regions, but the dominant feature is

the presence of the carbonyl stretching vibration at about

1720 cm-1

The characteristic bands attributed to the -C-O- stretching

vibration near 1200 cm-1, and the precise pattern of the

bands in the fingerprint region, lead to the conclusion that this

polymer is poly(methyl methacrylate).

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Polymer Branching

Low density Polyethylene by IR

Polymer Branching

13C NMR allowed an analysis of branching at a far greater

level of detail because the 13C shifts of paraffinic

hydrocarbons depend strongly on their proximity to tertiary

carbons (i.e., the branch points).

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