Spectroscopy and Structure

• Spectroscopy and structure involves characterizing organic compounds through element detection, molecular mass determination, and structural elucidation
• Spectroscopic methods are crucial tools for structural analysis
• The main spectroscopic methods are UV, IR, NMR, and mass spectrometry
• Mass spectrometry determines molecular mass and formula
• UV and IR spectroscopy identify functional groups
• NMR spectroscopy determines complete structure and stereochemistry
• Spectroscopic methods require small samples, minimal time, are reliable, and allow sample recovery (except mass spec)
• Spectroscopic techniques aid reaction mechanism and stereochemistry studies
• Spectroscopic methods are replacing classical methods
• Electromagnetic radiation has both particle and wave nature
• Light travels as electromagnetic waves with electric and magnetic fields
• Electromagnetic radiations travel at the speed of light (3 × 10^8 m/sec)

Important Characteristics of Electromagnetic Radiations

• Wavelength is the distance between two consecutive crests or troughs
• Frequency is the number of waves passing a point in one second
• Wave number is the number of waves per centimeter length (reciprocal of wavelength)
• Velocity is the linear distance traveled by a wave in one second
• Velocity is represented as C and generally expressed as cm/s or m/s
• The relationship between velocity (C), frequency (υ), and wavelength (λ) is C = υλ

Electromagnetic Spectrum

• Includes cosmic rays, X-rays, gamma rays, UV, visible light, IR, microwaves, and radio waves
• Arrangement of electromagnetic radiation by increasing wavelength or decreasing frequency
• Regions of maximum utility in organic chemistry are UV (100-400 nm), visible (400-800 nm), and IR (800 - 3 × 10^5 nm)
• Energy (E) of radiation is related to wavelength (λ) and frequency (ν) by E = hν = hc/λ

Types of Spectroscopy

• Emission spectroscopy and absorption spectroscopy
• Absorption spectroscopy is vital for organic compound structure analysis
• Absorption spectroscopy examines how substances absorb radiation to determine structure

Absorption Spectroscopy & Its Types

• Ultra-violet (UV) spectroscopy
• Infra-red (IR) spectroscopy
• Microwave spectroscopy
• Nuclear magnetic resonance (NMR) spectroscopy
• Electron-spin resonance (ESR) spectroscopy

Ultra-Violet (UV) Spectroscopy

• Measures light absorption in the UV and visible regions
• Often known as electronic spectroscopy
• The UV region ranges from 100-400 nm
• The UV region is divided into far UV (100-200 nm) and near UV (200-400 nm)
• Oxygen strongly absorbs below 200 nm
• Measurements are conducted with nitrogen flushing to avoid oxygen interference
• The technique is helpful for predicting conjugated double bonds, aromaticity, and α,β-unsaturated carbonyl groups

The Absorption Laws

• Light absorption is governed by Lambert's Law and Beer's Law
• Lambert's Law states that the rate of decrease in radiation intensity is proportional to the thickness of the absorbing medium
• Beer's Law states that the absorption of light is directly proportional to the concentration of the absorbing medium
• Beer-Lambert Law relates absorption of light to the number of molecules in the path of the light
• Mathematically expressed as log(Io/I) = εCl, where ε is molar absorptivity, C is concentration, and l is path length
• The expression log10(Io/I) is termed optical density or absorbance (A)

Principle of UV Spectroscopy

• Absorption of visible and ultraviolet light causes changes in the electronic states of molecules
• Electrons are excited from lower to higher energy levels, leading to electronic, vibrational, and rotational transitions
• Spectra consist of bands rather than peaks due to numerous possible transitions

Types of Electronic Transitions

• Radiant energy absorption is due to electron excitation
• Excitation occurs from bonding or non-bonding orbitals to antibonding molecular orbitals
• The three types of electrons present in the ground state of the molecules: σ-electrons, π-electrons, and n-electrons
• The electronic transitions which take place in the visible and UV regions of light are : σ-σ*, η-σ*, π-π*, η-π*

Relative Energies of Electronic Transitions

• The increasing order of energy is: η - π" < π - π* <η σ* <σσ
• σ-σ* transitions are high energy transitions
• η-σ* transitions occur in molecules that possess at least one heteroatom such as oxygen, nitrogen, sulphur,
• π-π* transitions occur in unsaturated compounds containing at least one multiple bond
• пл* transitions occur when the carbon atom is joined to a hetero atom

Probability of Electronic Transitions

• Allowed transitions are usually due to ππ* transitions.
• Forbidden transitions. The electronic transitions are ranging Emax, less than 1000, is forbidden transition

Role of Solvent in Electronic Transitions

• Solvent polarity affects electronic transitions in UV spectroscopy
• Solvent influences absorption band position
• Increased solvent polarity can cause shifts to shorter or longer wavelengths
• Solvents should not absorb in the region under investigation and should be less polar
• 95% ethanol is used because it has good dissolving power and is transparent above 210 nm

Chromophores and Related Terms

• Chromophore is a group that imparts color to a compound
• Chromophore a functional group absorbs electromagnetic radiations
• Bathochromic shift is a shift to longer wavelengths
• Hypsochromic shift is a shift to shorter wavelengths
• Hyperchromic shift brings about an increase in intensity
• Hypochromic shift causes decrease in intensity and disturbs the geometry of the molecule

Auxochromes

• Auxochromes do not function as chromophores
• When attached to chromophores cause bathochromic shift and increase intensity
• Common examples are -OH, -NH2, -OR, -NHR, -NR2, -SH, and -SR

Conjugated Chromophores

• When present, there is a large effect on the spectrum, it causes delocalization of the electron charge and decreases the energy of the excited state
• As a result, absorption takes place at a longer wavelength
• When double bond at alternating positions, this is a conjugated system

UV Absorption in Cyclic Dienes

• Cyclic dienes are of two types i.e. homoannular and heteroannular.
• Homo annular diene is the one in which the two double bonds are present in the same ring
• Hetero annular diene is the one in which the two double bonds are present in the adjacent rings

Nature of Double Bonds in the Cyclic Dienes

• A double bond is said to be endocyclic if it is present within a particular ring
• A double bond is known as exocyclic if it lies outside a particular ring
• In general, a homoannular diene absorbs at a greater wavelength than the heteroannular diene
• The presence of the exocyclic double bond generally increases the wavelength of absorption

Woodward Rules

• Woodward gave certain empirical rules for calculating the Amax in conjugated dienes, trienes and polyenes
• According to these rules, each diene has a certain fixed basic value and its actual Amax depends upon system that:
• The number of alkyl substituents or ring residues present on the diene system
• The number of double bonds extending conjugation beyond the diene system

Woodward and Fieser rules for calculating absorption maxima

• For (π - π*) band of a, ß unsaturated carbonyl compounds