Introduction to Spectroscopy

Overview

Spectroscopy is a fundamental analytical technique in organic chemistry that involves the interaction between matter and light (electromagnetic radiation). It is used to probe molecular structure and composition by analyzing how molecules absorb, emit, or scatter light.

• Light can be described as both waves of energy and as discrete packets (particles) of energy called photons.

• Key properties of light waves include wavelength and frequency.

• Wavelength is inversely proportional to energy:

• Frequency is directly proportional to energy:

Electromagnetic Spectrum

The electromagnetic spectrum encompasses all possible frequencies of light, ranging from gamma rays to radio waves. Different regions are used for different types of spectroscopy.

• Visible light: 400–700 nm

• Infrared (IR): 700 nm – 1 mm

• Ultraviolet (UV): 10–400 nm

• Radio waves: >1 mm

Classification of Spectroscopy

Various forms of spectroscopy utilize different regions of the electromagnetic spectrum to provide specific information about molecular structure.

Quantum Behavior of Matter

Matter exhibits both particle-like and wave-like properties. On the molecular scale, matter shows quantum behavior, meaning molecules can only rotate or vibrate at certain discrete energy levels.

• Macroscopic objects appear to behave continuously.

• Molecules can only absorb specific amounts of energy, leading to quantized transitions.

IR Spectroscopy

Vibrational Excitation

In IR spectroscopy, molecules absorb infrared light, causing vibrational excitation of covalent bonds. Energy levels for these vibrations are quantized.

• When a photon matches the energy gap between vibrational levels, it is absorbed.

• Different bonds absorb different IR energies, allowing identification of functional groups.

• Absorbed energy is eventually released as heat.

Bond Stretching and Bending

Molecular bonds can vibrate in several ways:

• Stretching vibration: Change in bond length.

• Bending vibration: Change in bond angle (scissoring, twisting).

This chapter focuses mainly on stretching frequencies.

IR Absorption Spectrum

An IR spectrum plots percent transmittance versus frequency (wavenumber). Peaks (signals) correspond to absorption bands, indicating specific bond vibrations.

• Wavenumber (): Frequency units in IR, measured in cm-1. Typical range: 400–4000 cm-1.

• Diagnostic region: Above 1500 cm-1, provides clear information about functional groups.

• Fingerprint region: Below 1500 cm-1, complex and unique to each molecule.

Signal Characteristics

Each IR signal (peak) has three important characteristics:

• Wavenumber: Indicates bond type and strength.

• Intensity: Related to bond polarity and number of bonds.

• Shape: Broad or narrow, affected by hydrogen bonding and molecular environment.

Wavenumber Trends

• Stronger bonds = higher stretching frequency.

• Greater mass difference between atoms = higher stretching frequency.

Examples:

• C–H: ~3000 cm-1

• C≡C: ~2200 cm-1

• C–O: ~1100 cm-1

Bond Type Regions

Resonance Effects

• Resonance delocalization weakens bonds, lowering stretching frequency.

• Example: C=O in ketone (~1720 cm-1) vs. conjugated ketone (~1680 cm-1).

Intensity

• Greater bond polarity = stronger IR signals.

• Symmetrical bonds (e.g., O=O) do not show IR absorption.

• Multiple identical bonds increase signal strength.

Shape

• Hydrogen bonding broadens O–H and N–H signals.

• Carboxylic acids show very broad O–H stretches due to strong H-bonding dimers.

• Primary amines: two N–H signals; secondary amines: one N–H signal.

Analyzing an IR Spectrum

Steps for Analysis

• Focus on the diagnostic region (above 1500 cm-1).

• Check for double bonds (1600–1850 cm-1), triple bonds (2100–2300 cm-1), and X–H bonds (2700–4000 cm-1).

• Analyze wavenumber, intensity, and shape for each signal.

IR spectroscopy is useful for confirming the presence or absence of functional groups after chemical reactions.

Mass Spectrometry

Overview

Mass spectrometry (MS) is used to determine the molecular mass and formula of a compound. It involves vaporizing, ionizing, and fragmenting a sample, then detecting the masses of the ions.

• Electron Impact (EI) is the most common ionization method.

• Ionization produces a radical cation (molecular ion, M+).

• Fragmentation yields smaller ions and radicals; only charged fragments are detected.

Key Features of Mass Spectra

• Base peak: Most abundant ion in the spectrum (set to 100% relative abundance).

• Molecular ion (M+): Peak corresponding to the intact molecule.

• Fragment peaks: m/z values less than M+, representing fragments.

Isotope Peaks

• (M+1)+ peak: Due to 13C; abundance increases with number of carbons.

• (M+2)+ peak: Indicates presence of Cl or Br; Cl gives a 3:1 ratio, Br gives a 1:1 ratio.

Fragmentation Patterns

• Loss of methyl, ethyl, propyl, butyl radicals.

• Alcohols: alpha cleavage and dehydration (loss of H2O).

• Amines: alpha cleavage.

• Carbonyls: McLafferty rearrangement.

High-Resolution Mass Spectrometry

High-resolution MS measures m/z to four decimal places, allowing distinction between compounds with similar molecular weights.

High-res MS is essential for distinguishing compounds with nearly identical molecular weights.

Gas Chromatography-Mass Spectrometry (GC-MS)

GC-MS combines separation of mixtures (GC) with identification (MS). It is highly effective for analyzing complex samples such as blood or urine.

Mass Spec of Large Molecules

Electrospray ionization (ESI) is used for large molecules, producing intact molecular ions with minimal fragmentation.

Degrees of Unsaturation

Concept and Calculation

The degree of unsaturation (hydrogen deficiency index, HDI) indicates the number of rings or multiple bonds in a molecule. Each degree reduces the number of hydrogens by two.

• Alkanes: saturated hydrocarbons, formula

• HDI formula:

• Halogens (X) are treated as hydrogens; oxygen does not affect HDI; nitrogen increases expected hydrogens by one.

If HDI = 0, the molecule contains no rings or multiple bonds.

Example

• C5H10: HDI = 1 (one ring or double bond)

• Chloroethane (C2H5Cl): Cl treated as H for HDI calculation

Additional info: These notes expand on the original slides and text, providing definitions, formulas, and context for key concepts in IR and mass spectrometry as used in organic chemistry.