Spectroscopy A & B: Mass Spectrometry and Infrared Spectroscopy

Spectroscopy in Organic Chemistry

Introduction to Spectroscopic Methods

Spectroscopy is essential in organic chemistry for determining the structure and properties of organic molecules. The three main spectroscopic methods are mass spectrometry (MS), infrared spectroscopy (IR), and nuclear magnetic resonance (NMR). Each technique provides unique information about molecular structure and composition.

• Mass spectrometry (MS): Measures molecular weight and provides information about molecular formula and fragmentation patterns.

• Infrared spectroscopy (IR): Identifies functional groups by measuring bond vibrations.

• Nuclear magnetic resonance (NMR): (Mentioned for context) Provides detailed information about the carbon and hydrogen framework of molecules.

Example: The spectra of ethanol (C2H5OH) can be analyzed using all three methods to confirm its structure.

Mass Spectrometry

Principles and Instrumentation

Mass spectrometry is a technique used to determine the molecular weight and molecular formula of organic compounds. It works by ionizing molecules and measuring the mass-to-charge ratio (m/z) of the resulting ions.

• Ionization: A sample is vaporized and bombarded with a beam of electrons, forming a positively charged radical cation (M•+), known as the molecular ion or parent ion.

• Fragmentation: The molecular ion is unstable and breaks into smaller fragments (radicals and cations). Only cations are detected by the mass spectrometer.

• Detection: Ions are accelerated in an electric field and deflected in a magnetic field according to their m/z ratio. The instrument records the intensity of each ion versus its m/z value, producing a mass spectrum.

Key Equations:

• Mass-to-charge ratio: $\frac{m}{z}$

• For most organic ions, $z = +1$, so $m/z$ is effectively the mass of the ion.

Interpreting Mass Spectra

• Molecular ion peak (M•+): The peak corresponding to the unfragmented ion, indicating the molecular weight.

• Base peak: The tallest peak in the spectrum, assigned 100% relative abundance. It may or may not be the molecular ion peak.

• Fragment peaks: Peaks at lower m/z values, resulting from fragmentation of the molecular ion.

• M + 1 peak: A small peak one mass unit higher than the molecular ion, due to the presence of heavier isotopes (e.g., $^{13}$C).

Example: Methane (CH4)

• Molecular ion at m/z = 16 (M peak)

• Base peak at m/z = 16 (for methane, the base peak is the molecular ion peak)

• Fragment peaks at m/z = 15, 14, 13, and 12 due to loss of hydrogen atoms

• M + 1 peak at m/z = 17, due to 1.1% natural abundance of $^{13}$C

Isotopic Patterns: Alkyl Halides

Elements like chlorine and bromine have significant natural abundances of more than one isotope, leading to characteristic patterns in mass spectra.

Element	Major Isotopes	Natural Abundance	Characteristic Pattern
Chlorine (Cl)	$^{35}$Cl, $^{37}$Cl	76%, 24%	Two peaks (M and M+2) in a 3:1 ratio
Bromine (Br)	$^{79}$Br, $^{81}$Br	51%, 49%	Two peaks (M and M+2) in a 1:1 ratio

Example: The mass spectrum of CH3CH2Br shows two molecular ion peaks at m/z = 108 and 110, with nearly equal intensity, due to the two isotopes of bromine.

Infrared (IR) Spectroscopy

Principles of IR Spectroscopy

Infrared spectroscopy measures the absorption of IR radiation by molecules, which causes changes in their vibrational motions. Different types of bonds absorb IR radiation at characteristic frequencies, allowing identification of functional groups.

• Electromagnetic radiation: IR is a form of electromagnetic radiation with wavelengths longer than visible light.

• Key equations:

  • Speed of light: $c = \lambda \nu$

  • Energy of a photon: $E = h \nu$

• Wavenumber ($\tilde{\nu}$): The number of waves per centimeter, commonly used in IR spectroscopy. $\tilde{\nu} = \frac{1}{\lambda}$ (in cm-1).

IR Spectrum Regions

• Functional group region: 4000–1500 cm-1. Most functional groups absorb in this region, producing strong, characteristic peaks.

• Fingerprint region: 1500–400 cm-1. Contains many peaks unique to each molecule, useful for identification.

Example: O–H, N–H, and C–H stretching vibrations appear above 3000 cm-1; C=O stretching appears near 1700 cm-1.

Bond Vibrations and Absorption Frequencies

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

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

Key absorptions:

Bond	Approximate 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

Applications of IR Spectroscopy

• Identification of functional groups in unknown compounds

• Distinguishing between isomers based on their IR spectra

• Monitoring chemical reactions by observing changes in characteristic absorptions

Example: The IR spectrum of a carbonyl compound shows a strong absorption near 1700 cm-1 due to the C=O stretch.

Summary Table: Comparison of Spectroscopic Methods

Method	Information Provided	Typical Use
Mass Spectrometry (MS)	Molecular weight, formula, fragmentation pattern	Determining molecular mass and structure
Infrared Spectroscopy (IR)	Functional groups, bond types	Identifying functional groups
Nuclear Magnetic Resonance (NMR)	Hydrogen/carbon framework	Detailed structure elucidation

Additional info: NMR is mentioned for context but not covered in detail in these notes. High-resolution mass spectrometry (HRMS) and gas chromatography-mass spectrometry (GC-MS) are advanced techniques that provide more precise mass measurements and separation of mixtures, respectively.