Introduction to Spectroscopy

Page 1

LECTURE 1

Spectroscopy is the study of the interaction of electromagnetic radiation, in all its forms, with matter.

Methods to determine the structure and purity of unknown compounds:

Conventional:

Physical properties (e.g. melting point)

Characteristic reactions (e.g. Tollen's test) and derivative preparations

Degradation to known fragments

Elemental analysis (microanalysis)

Spectroscopy:

Infrared spectroscopy

UV-vis spectroscopy

Mass spectometry

1H and 13C Nuclear Magnetic Resonance (NMR)

Advantages of spectroscopic methods:

Fast

Small sample sizes

Non-destructive (except mass spectrometry)

Can analyse mixtures/follow reaction progress

Electromagnetic spectrum - as as wavelength increases, frequency decreases

Gamma rays: nuclear decay

X-rays: X-ray tubes

UV: sun, Hg vapour

Visible: sun

Infrared: sun,combustion

Microwaves: electron excitation

Radio waves: electron excitation

E = hc/l

Energy = Planck's constant (6.262 x 10e-34) x speed of light/wavelength

Units:

E: J

h: J s

c: m/s

l: m

Spectroscopy measures the difference between the energy levels at which matter exists.

UV/vis

Energy levels: electronic

Information obtained: double bonds, concentration, functional groups?

Infrared

Energy levels: molecular vibrations

Information obtained: functional groups

NMR (radio waves)

Energy levels: nuclear spin

Information obtained: molecular structure

Mass spectrometry

Energy levels: -

Information obtained: structural formula

Page 2

LECTURE 2

UV/VIS ABSORPTION SPECTROSCOPY

Electrons and absorption

When a covalent bond is formed, two molecular orbitals are created: a bonding orbital, and an antibonding orbital.

Bonding state is lowest energy state (combination of electrons is favourable)

Antibonding state is higher in energy (combination of electrons is not favourable, so energy needs to be added to the system for such a state to exist)

If a bond receives energy (e.g. from a laser or intense light source), an electron may become excited into the antibonding energy level.

The electron has to receive precisely ΔE to jump up an energy level.

ΔE=hv

If radiation of such a precise frequency is given to a bond, the radiation will be absorbed by the bond. We can detect this absorption as light coming through the sample that is absorbed will not be recorded and thus its absence will be noted.

UV/vis spectrometry tools and methodology

Tools

UV-vis spectrometer (+ computer)

Cuvettes (transparent containers)

Sample compartment (top = reference; bottom = sample)

Methodology

Sample is irradiated with a sweeping range of frequencies to obtain the spectrum.

Since different molecules have different values of ΔE, each molecule has a characteristic pattern on the spectrum.

Spectra are plotted as wavelength (x-axis) vs absorbance (y-axis).

Why is the band broad and featureless?

When electron distribution is changed, the nuclei are subjected to different forces after transition and the molecule responds by vibrating.

The vibrational structure in liquids and solids (can be resolved for gases) merges to give a broad band.

The vibrational transitions and further complicated by rotational transitions.

UV absorptions are generally broad because vibrational and rotational levels are "superimposed" over the electronic levels.

For this reason, the wavelength of maximum absorption (λmax) is usually reported.

Electronic levels > vibrational levels > rotational levels

Beer-Lambert law (see image)

Also goes by the names of molar extinction coefficient and molar absorptivity, depicted with the symbol epsilon (ε).

It is a measure of how strongly a species absorbs light of a certain wavelength; a compound with a high ε is very effective at light absorption (at that specific wavelength).

It is an intrinsic property of the species.

ε will be largest at λmax.

When applying the law, the value of ε at the same λ that the absorbance is measured at must be used.

Limitations of Beer-Lambert law:

Linear for dilute concentrations only

Causes: changes in refractive index of the solution, band width of filters or monochromators in the instrument, equilibrium of absorbing species disrupted (e.g. by pH of solution)

Since the plot is a calibration graph, it shouldn't be extrapolated as this can lead to errors.

Good check of graph - calibration line should go through 0 (no concentration = no absorbance)

Using UV/vis spectroscopy to identify structures

When atomic orbitals combine to form molecular orbitals, they form different kinds of bonds - pi & sigma bonds, plus non-bonding electrons.

C-C and C-H bonds possess good orbital overlap and ΔE for σ --> σ* is relatively large. Transitions are usually <200 nm.

Excitation of π electrons require less energy (π --> π*)

Nonbonding electrons can also be excited into higher states: n --> π * and n --> σ*

We can use an understanding of these transitions to predict λmax from structure – Fieser-Woodward Rules.

Fieser-Woodward Rules

Parent heteroannular or open chain diene (s-trans) = 214 nm.

Parent homoannular diene (s-cis) = 253 nm.

s-trans and s-cis start off in the UV region.

Increments:

Additional conjugated double bond = 30 nm.

Adding double bonds - extending the conjugation of the molecule - shifts the absorption maximum into the visible region.

Each exocyclic double bond = 5 nm.

Substituents on the conjugated chain can shift λmax bathochromically (towards the red end of the spectrum):

Each alkyl substituent or ring residue = 5 nm.

-OAcyl = 0 nm.

-OR = 0nm.

-NR2 = 60 nm.

-SR = 30 nm.

-Cl or Br = 5 nm.

Conjugated double bonds

Molecules with conjugated double bonds absorb in the UV-visible region.

Highest Occupied Molecular Orbital (HOMO) and Lowest Unoccupied Molecular Orbital (LUMO) get closer together with increased conjugation.

More conjugation --> lower energy gap --> longer wavelength.

Conclusion & limitations

UV/vis spectra give us a pattern which represents a unique behaviour of the sample.

However, it gives little information on the true structure of the molecule.

If the structure is known before hand, Fieser-Woodward Rules can be used to calculate the theoretical appearance of the sample's spectrum.

For the purpose of structure determination, this procedure has its limits.

Useful definitions

Chromophore: any group of atoms that absorbs light whether or not a colour is thereby produced.

Auxochrome: group which extends the conjugation of a chromophore through sharing of nonbonding electrons.

Bathochromic shift: a shift of absorption to a longer wavelength (red).

Hypsochromic shift: a shift of absorption to a shorter wavelength (blue).

Hyperchromic effect: an increase in absorption intensity.

Hypochromic effect: a decrease in absorption intensity.

Page 3

LECTURE 3

Infrared (vibrational) spectroscopy

Absorbing infrared radiation causes vibrational excitation in bonds within a molecule.

Bending vibrations = lower energy

Stretching vibrations = higher energy

When a molecule possesses excess vibrational energy, most of the bonds stretch and contract together – this isn’t useful for structure determination.

However, bonds containing functional groups generally are “IR active” as there is an associated change in dipole moment during vibration – these stretch independently of the rest of the molecule.

These independent stretches occur because the bond is weaker or stronger than others nearby, or the bond is between atoms that are much lighter or heavier than their neighbours.

If the vibration of bonds results in a change in the molecule's dipole moment, the molecule will absorb infrared energy at a frequency corresponding to the frequency of the bond's natural vibration.

This absorption of energy resulting in an increase of the amplitude of the vibrations is known as resonance.

no dipole --> no change on stretch --> IR inactive

has dipole --> increases on stretch --> IR active

Region is usually 100 - 10 000 cm-1 (wavenumber, 1/l), for organic compounds our range of interest is 400 - 4000 cm-1.

IR spectroscopy methodology

Solid samples

KBr discs containing analyte, requires press to form transparent disc.

Nujol mull samples; form very thick paste of solid in Nujol, spread onto KBr disc

Solution samples

Dissolved in solvent

Requires blank solvent reference

Gas sample cells

Analyse gaseous materials

Has transparent windows at each end

IR spectrum

Plotted upside down as transmittance is recorded, not absorbance.

Increased wavenumber = increased energy needed to vibrate bond

Larger energy means higher wavenumber (cm-1) or smaller wavelength (cm)

High number for wavenumber on left as it is really wavelength that is scanned.

Learning outcomes

State and apply the equation relating the wavenumber of an IR transition to the force constant and reduced mass.

v = 1/2πc x sqrt(k/μ) (Hooke's Law - movement of two masses attached to a spring)

v = frequency of vibration

k = force constant for strength of bond

μ = the reduced mass

μ = (m1 x m2)/(m1 + m2)

v increases with increasing bond strength

v decreases as mass of atoms involved in bond decreases

Discuss the idea of group frequencies, list the approximate wavenumber of the IR absorptions of the common functional groups and interpret and explain simple IR spectra.

IR spectrum is divided into functional group region (4000 - 1500 cm-1) and fingerprint region (1500 - 625 cm-1)

Functional group region divided into regions:

Bonds to hydrogen

Triple bonds

Double bonds

Single bonds

Primary (N-H) vs secondary amines (NH2) have different IR patterns.

N-H bonds have one strong peak at approximately 3300 cm-1, whether its an amine or an amide (RCONHR)

For NH2, the group is independent of the rest of the molecule, but but the two N-H bonds are identical and vibrate as one unit. This gives two equally strong bands - one for both N-H bonds vibrating in phase (symmetric) and one for the two bonds vibrating in opposition (antisymmetric).

Antisymmetric requires more energy --> higher frequency.

O-H bonds are stronger than others (C-H, N-H), so they occur at a higher frequency.

In the absence of hydrogen bonding, can appear as a sharp peak at approximately 3600 cm-1.

Hydrogen bonding causes the peak to broaden (covalent O-H bond is weakened and and frequency lowers)

Hydrogen bonds vary in length --> weaken covalent bonds by varying amounts

Gives a range of frequencies distributed about a mean value (broad band)

Double bond region

3 sharp bands

Carbonyl (C=O): intense band between 1900-1500 cm-1

Nitro (NO2): two intense bands, mid 1500s and mid 1300s cm-1

Alkene (C=C): one weak band at around 1650 cm-1.

Strength (intensity) of the band depends on the dipole moment when the bond is stretched: C=C is least active (may even be nonpolar in which case IR inactive), NO2 is highly polarised, C=O is very polar (usually the strongest peak in the IR spectrum)

The C=O region

Because frequency depends on bond strength, it is often possible to identify the type of carbonyl group in an IR spectrum.

Can observe more subtle effects, like conjugation, ring strain, adjacent atoms, etc.

Acid chloride: chlorine atom polarises C=O bond (poor lone pair donation, highly inductive electron withdrawing) – shift at 1815 cm-1

Anhydride: oxygen atom between 2 x C=O bond (poor lone pair donation due to “sharing”, inductive electron withdrawing) – shift at 1790 and 1810 cm-1 (symmetric and asymmetric C=O stretches)

Ester: oxygen atom (good lone pair donation, highly inductive electron withdrawing) affords a good balance– shift at 1745 cm

Amide: nitrogen atom causes conjugation of C=O bond (N atom strong electron donor) – shift at 1650 cm-1

The fingerprint region

The single bond region

Bond stretches and bends are present - requires less energy

Important, but not generally used for structure identification