Spectroscopy A & B: Mass Spectrometry and Infrared Spectroscopy

Spectroscopy A & B: Mass Spectrometry and Infrared Spectroscopy

Introduction to Spectroscopy in Organic Chemistry

Spectroscopy is a fundamental set of techniques in organic chemistry used to determine the structure, molecular weight, and functional groups of organic molecules. The three main methods of characterization are:

• Mass Spectrometry (MS): Determines molecular weight and formula.

• Infrared Spectroscopy (IR): Identifies functional groups via bond vibrations.

• Nuclear Magnetic Resonance (NMR): (Mentioned for context) Provides information about the carbon and hydrogen framework.

These methods are often used together to fully characterize organic compounds.

Mass Spectrometry

Principles and Instrumentation

Mass spectrometry is a technique for measuring the molecular weight and determining the molecular formula of an organic molecule. The process involves:

• Vaporizing a sample and bombarding it with electrons to form unstable radical cations (M+).

• Accelerating and deflecting these ions in a magnetic field according to their mass-to-charge ratio (m/z).

• Detecting and recording the abundance of ions at each m/z value to produce a mass spectrum.

Key components of a mass spectrometer:

• Electron beam (ionizes the sample)

• Accelerating plates

• Magnetic analyzer tube

• Detector (records ion abundance)

Mass Peaks and Fragmentation

• The radical cation M+ is called the molecular ion or parent ion.

• The molecular ion is often unstable and fragments into smaller ions and radicals.

• Mass spectrometers sort ions by their mass-to-charge ratio (m/z). Since z is usually +1, m/z ≈ mass of the ion.

• The base peak is the tallest peak in the spectrum and represents the most abundant ion.

Example: Methane (CH4)

• Molecular ion peak at m/z = 16 (12 for C + 4 for H).

• Smaller peaks at m/z = 17 (M+1, due to 13C isotope, ~1% abundance).

• Fragment peaks at lower m/z values due to breakdown of the molecular ion.

Isotopic Patterns and Alkyl Halides

• Elements like chlorine and bromine have significant natural abundances of multiple isotopes, leading to characteristic M+2 peaks.

• For example, chlorine (Cl) has two main isotopes: 35Cl and 37Cl, resulting in a 3:1 ratio of M to M+2 peaks.

• Bromine (Br) has a 1:1 ratio of 79Br to 81Br, so M and M+2 peaks are of equal intensity.

Fragmentation Patterns

• Hydrocarbons: Fragmentation often occurs at bonds to form carbocations of varying stability (tertiary > secondary > primary > methyl).

• Carbonyl Compounds: Undergo α-cleavage (breaking the bond adjacent to the carbonyl carbon), forming resonance-stabilized acylium ions.

• Alcohols and Amines: Undergo α-cleavage and may also lose water (dehydration) in the case of alcohols.

High Resolution Mass Spectrometry (HRMS)

HRMS measures m/z values to four or more decimal places, allowing for the distinction between compounds with similar nominal masses. For example, a molecular ion at m/z = 60 could correspond to different formulas, but HRMS can distinguish between them based on exact mass.

Gas Chromatography-Mass Spectrometry (GC-MS)

GC-MS combines separation (by gas chromatography) with mass analysis, allowing for the identification of components in complex mixtures.

Infrared (IR) Spectroscopy

Principles of IR Spectroscopy

IR spectroscopy involves the absorption of infrared radiation by molecules, causing changes in their vibrational motions. Only vibrations that result in a change in dipole moment are IR-active.

• The energy difference between vibrational states corresponds to specific IR frequencies.

• Absorption is measured as a function of wavenumber (cm-1), which is inversely proportional to wavelength.

Key equations:

• Wavenumber: $\tilde{\nu} = \frac{1}{\lambda}$ (in cm-1)

• Energy of a photon: $E = h\nu$

IR Spectral Regions and Functional Group Identification

• Bonds absorb in predictable regions of the IR spectrum, allowing identification of functional groups.

• Four main regions: (1) X-H stretching, (2) triple bond region, (3) double bond region, (4) fingerprint region.

• Functional group absorptions are the most distinct signals in an IR spectrum.

Common IR Absorption Bands

Bond	Wavenumber (cm-1)	Intensity
O-H (alcohol)	3200–3600	Strong, broad
N-H (amine)	3300–3500	Medium, sharp
C-H (alkane)	2850–2960	Medium
C=O (carbonyl)	1650–1750	Strong
C=C (alkene)	1600–1680	Variable
C≡C, C≡N	2100–2260	Variable

Factors Affecting IR Absorption

• The frequency of bond vibration depends on bond strength and the masses of the atoms (Hooke's law analogy).

• Stronger bonds and lighter atoms vibrate at higher frequencies (higher wavenumbers).

• Conjugation, hydrogen bonding, and ring strain can shift absorption frequencies.

Applications of IR Spectroscopy

• Identification of functional groups in unknown compounds.

• Distinguishing between isomers (e.g., alcohols vs. ethers, ketones vs. aldehydes).

• Monitoring chemical reactions by observing the appearance/disappearance of characteristic bands.

Summary Table: Comparison of Spectroscopic Methods

Method	Information Provided	Key Features
Mass Spectrometry (MS)	Molecular weight, formula, fragmentation pattern	m/z values, isotopic patterns, base peak
Infrared Spectroscopy (IR)	Functional groups, bond types	Characteristic absorption bands (cm-1)
NMR Spectroscopy	Hydrogen/carbon framework	Chemical shifts, splitting patterns

Additional info: NMR is mentioned for context but not detailed in these notes. Practice questions from the Smith textbook are recommended for further study.