Chapter 13: Introduction to Spectroscopy, Infrared Spectroscopy (IR), and Mass Spectrometry (MS)

Introduction to Spectroscopy

Overview of Spectroscopy

Spectroscopy is the study of the interaction between matter and electromagnetic radiation. It is a fundamental tool in organic chemistry for determining molecular structure and identifying functional groups.

• Spectrum: A graph showing the amount of radiation absorbed or transmitted as a function of wavelength or frequency.

• Types of Spectroscopy:

  • IR (Infrared): Identifies functional groups present in a molecule.

  • NMR (Nuclear Magnetic Resonance): Provides information on the number, connectivity, and environment of carbons and hydrogens.

  • UV-Vis (Ultraviolet-Visible): Gives information about π-electron systems.

• These techniques are often used together to elucidate unknown molecular structures.

Electromagnetic Radiation

Light as a Wave

Electromagnetic (EM) radiation, including visible, X-ray, and ultraviolet light, propagates as a wave and is characterized by its wavelength ($\lambda$) or frequency ($\nu$).

• Relationship: $\nu = \frac{c}{\lambda}$, where $c$ is the speed of light.

Light as a Particle

Light also exhibits particle-like behavior, with energy quantized in photons.

• Photon energy: $E = h\nu = \frac{hc}{\lambda}$

• Planck's constant: $h = 6.626 \times 10^{-34}$ J·s

• Energy, frequency, and wavelength are interrelated.

The Electromagnetic Spectrum

The electromagnetic spectrum covers a wide range of wavelengths and frequencies, from gamma rays to radio waves. IR, UV, and visible regions are particularly important in organic spectroscopy.

Absorption Spectroscopy

Principles

Absorption spectroscopy measures how much EM radiation is absorbed by a sample as a function of wavelength, frequency, or energy. The most common instruments are spectrophotometers or spectrometers.

Infrared (IR) Spectroscopy

Basics of IR Spectroscopy

IR spectroscopy records the light absorbed by a substance as a function of wavelength. The horizontal axis is the wavenumber ($\tilde{\nu}$, in cm-1), and the vertical axis is percent transmittance ($T$), related to light intensity ($T = I/I_0$).

IR Spectral Data

• Typical data: $\tilde{\nu}$, cm-1: 2980–2850 (s); 1470 (m); 1380 (m); 720 (w)

• s = strong, m = moderate, w = weak

Bond Vibrations and IR Absorbance

• IR absorbances result from quantized bond vibrations.

• Absorption occurs only when the frequency of IR radiation matches the frequency of bond vibration.

• Bonds vibrate with characteristic frequencies, and only those with a changing dipole moment absorb IR light.

IR Absorption and Chemical Structure

Functional Group and Fingerprint Regions

• Each functional group absorbs in a characteristic region of the IR spectrum.

• The fingerprint region (below 1500 cm-1) is unique for each molecule and is used for identification.

Wavenumber range (cm-1)	Type of absorption	Name of region
4000–2500	O–H, N–H, C–H stretching	Functional group
2500–2000	C≡C, C≡N stretching	Functional group
2000–1500	C=O, C=C, C=N stretching	Functional group
1500–400	Various bending vibrations	Fingerprint

Factors Affecting IR Absorption

• Bond strength: Stronger bonds absorb at higher frequencies.

• Atomic masses: Lighter atoms vibrate at higher frequencies.

• Type of vibration: Stretching vibrations occur at higher frequencies than bending vibrations.

Factors Affecting IR Absorption Intensity

• Overall intensity is related to sample concentration.

• Relative intensity is additive and affected by the number of similar groups and the dipole moment.

Dipole Moments and IR Absorbance

• Only bonds with a changing dipole moment absorb IR light.

• Bonds with no dipole moment (e.g., symmetrical C=C) are IR-inactive.

• Groups with large dipoles (e.g., C=O, O–H) provide intense absorptions.

Functional-Group IR Absorptions

Alkanes

• C–H stretching: 2850–2960 cm-1

• C–H bending: fingerprint region

Alkyl Halides

• Low wavenumber end, often obscured by other peaks

• C–F stretch: 1000–1100 cm-1

• –CF3: 1300–1360 cm-1

• Mass spectrometry and NMR are more useful for identification

Alkenes

Functional group	Absorption
–CH=CH2 (terminal vinyl)	1640 cm-1 (m, sh)
–CH=CH– (terminal methylene)	1680–1675 cm-1 (m, sh)
=C–H stretching	3000–3100 cm-1 (m)

• Bending absorptions: 910–990 cm-1 (terminal vinyl), 900 cm-1 (trans alkene), 675–730 cm-1 (cis alkene), 800–840 cm-1 (disubstituted alkene)

Alkynes

• C≡C stretching: 2100–2200 cm-1

• 1-Alkynes: sharp absorption at 3300 cm-1 (≡C–H stretch)

• Symmetrical alkynes may show weak or absent peaks due to lack of dipole moment

Alcohols and Ethers

• O–H stretch (H-bonded): 3200–3400 cm-1

• O–H stretch (not H-bonded): 3600 cm-1

• C–O stretch: 1050–1200 cm-1 (ROH and ethers)

Obtaining an Infrared Spectrum

Instrumentation and Sample Preparation

• IR spectra are obtained using an infrared spectrometer.

• Modern instruments are typically Fourier-transform spectrometers (FT-IR).

• Liquid samples: Analyzed undiluted (neat) or in solution (e.g., CHCl3 or CH2Cl2).

• Solid samples: Analyzed as fused KBr pellets or as a finely ground dispersion in mineral oil (mull).

Attenuated Total Reflectance (ATR)

• A thin layer of sample is spread across or pressed onto a crystal support.

• The IR beam is directed through the crystal and at the sample, reducing the amount of IR that is reflected and allowing for rapid analysis.

Introduction to Mass Spectrometry (MS)

Principles of Mass Spectrometry

• Molecular mass is determined using a mass spectrometer.

• MS provides limited structural information and destroys the sample.

• Very small sample amounts (μg to pg) are sufficient.

The Mass Spectrometer

• Produces gas-phase ions by chemical ionization or electron ionization.

• Sorts ions by mass (magnetic sector).

• Detects the relative number of ions of each mass (ion collector).

Electron-Ionization (EI) Mass Spectrometry

• Sample is vaporized and subjected to a high-energy electron beam (~70 eV).

• Causes ejection of an electron, forming a radical cation (e.g., $\ce{CH4 + e^- \rightarrow CH4^{+\cdot} + 2e^-}$).

Fragmentation Reactions

• Radical cations fragment into smaller ions and radicals.

• Fragmentation can be shown with fishhook arrow notation.

• Only ions are detected in the mass spectrum; neutral fragments are not.

Separation and Detection

• Fragment ions are separated by their mass-to-charge ratio (m/z).

• Peak height indicates the relative abundance of each fragment ion.

Mass Spectrum Terminology

• Molecular ion (M): Ion formed by electron ejection only (no fragmentation).

• Base peak: Ion of greatest relative abundance.

• Molecular ion peak and base peak are often different.

Isotopic Peaks and Abundances

• Isotopic peaks (e.g., M+1) arise from naturally occurring isotopes (e.g., $^{13}$C).

• Relative abundance of M+1/M can be calculated: $\text{relative abundance} = (\text{number of C atoms}) \times 0.0111$

• Characteristic isotopic patterns can identify halides (e.g., $^{79}$Br/$^{81}$Br = 1:1, $^{35}$Cl/$^{37}$Cl = 3:1).

Element	Isotope	Exact mass	Abundance (%)
Hydrogen	1H	1.007825	99.985
Hydrogen	2H	2.0140	0.015
Carbon	12C	12.0000	98.90
Carbon	13C	13.00335	1.10
Nitrogen	14N	14.00307	99.63
Nitrogen	15N	15.00011	0.37
Oxygen	16O	15.99491	99.759
Oxygen	17O	16.99913	0.037
Oxygen	18O	17.99916	0.204
Fluorine	19F	18.99840	100

Fragmentation of the Molecular Ion

• Radical ions decompose into smaller fragments, producing either a radical and a cation or a neutral molecule and a radical cation.

• Only ions are detected by the spectrometer.

Summary Table: Key IR Absorption Ranges

Functional Group	Absorption Range (cm-1)
O–H (alcohol, H-bonded)	3200–3400
O–H (alcohol, free)	3600
C–H (alkane)	2850–2960
C=O (carbonyl)	1700–1750
C=C (alkene)	1640–1680
C≡C (alkyne)	2100–2200
C–O (alcohol/ether)	1050–1200

Additional info: For more advanced interpretation, consult full IR and MS tables and spectra in your textbook or laboratory manual.