Infrared Spectroscopy and Mass Spectrometry: Structure Determination in Organic Chemistry

Introduction to Spectroscopy

Overview of Spectroscopy

Spectroscopy is a fundamental analytical technique in organic chemistry, used to determine the structure of compounds by measuring their interaction with electromagnetic radiation. Most spectroscopic methods are nondestructive, preserving the sample for further analysis. Absorption spectroscopy, a key method, measures the amount of light absorbed by a sample as a function of wavelength.

• Spectroscopy: Technique for structural determination of compounds.

• Nondestructive: Most methods do not destroy the sample.

• Absorption Spectroscopy: Measures light absorption versus wavelength.

Types of Spectroscopy

Major Spectroscopic Techniques

Several spectroscopic techniques are used in organic chemistry, each providing unique structural information:

• Infrared (IR) Spectroscopy: Measures bond vibration frequencies; identifies functional groups.

• Mass Spectrometry (MS): Fragments molecules and measures mass; determines molecular weight and functional groups.

• Nuclear Magnetic Resonance (NMR) Spectroscopy: Analyzes hydrogen environments; reveals alkyl and other functional groups.

• Ultraviolet (UV) Spectroscopy: Uses electronic transitions to determine bonding patterns.

Wavelength and Frequency

Wave Properties

Understanding the relationship between wavelength and frequency is essential for interpreting spectroscopic data.

• Frequency ($\nu$): Number of wave cycles passing a fixed point per second.

• Wavelength ($\lambda$): Distance between two consecutive peaks or troughs.

Key Equations

• $c = \lambda \nu$

• $E = h \nu$

Where c is the speed of light ($3 \times 10^{10}$ cm/sec), h is Planck's constant ($6.62 \times 10^{-37}$ kJ·sec).

The Electromagnetic Spectrum

Regions and Effects

The electromagnetic spectrum covers a wide range of wavelengths and energies, each associated with different molecular effects:

• Gamma rays, X-rays, UV, visible, IR, microwave, radio

• Molecular effects: ionization, electronic transitions, molecular vibrations, rotational motions, nuclear spin transitions

The Infrared (IR) Region

Characteristics of IR Radiation

• IR region: Just below visible light, above microwave frequencies

• Wavelengths: $2.5 \times 10^{-4}$ to $25 \times 10^{-4}$ cm

• Units: Wavenumbers (cm-1), proportional to frequency and energy

Molecular Vibrations

Bond Vibrations

Molecular bonds behave like springs, vibrating when stretched or compressed:

• Restoring force returns atoms to equilibrium bond length

• Vibration occurs when bond is released after stretching/compression

Bond Stretching Frequencies

Frequency Dependence

• Frequency decreases with increasing atomic mass

• Frequency increases with increasing bond energy

Table: Bond Stretching Frequencies

Bond Type	Frequency (cm-1)
C—C	1200
C=C	1660
C≡C	<2200

Vibrational Modes

Fundamental Vibrations

• Nonlinear molecule with n atoms: $3n - 6$ vibrational modes

• Example: Water (H2O) has 3 vibrational modes (2 stretching, 1 bending)

Fingerprint Region of the Spectrum

Unique IR Spectra

• No two molecules (except enantiomers) have identical IR spectra

• Fingerprint region: 600–1400 cm-1 (complex vibrations)

• 1600–3500 cm-1: Common vibrations, useful for functional group identification

Effect of an Electric Field on a Polar Bond

Dipole Moment and Field Interaction

• Bonds with dipole moments (e.g., HF) are stretched or compressed by electric fields

• Force on positive charge aligns with field; force on negative charge opposes field

The Infrared Spectrometer

Instrument Design

• Uses reference and sample beams

• Monochromator and detector record transmitted light

FT-IR Spectrometer

Advantages of FT-IR

• Better sensitivity, less energy required

• Rapid scans (1–2 seconds), averages multiple scans

• Laser calibration for accuracy

The Interferogram

Data Processing

• Interferogram: Time domain data

• Fourier transform converts to frequency domain

Characteristic IR Absorptions

Carbon–Carbon Bond Stretching

• Stronger bonds absorb at higher frequencies

• Conjugation lowers absorption frequency

Bond Type	Frequency (cm-1)
Isolated C=C	1640–1680
Conjugated C=C	1620–1640
Aromatic C=C	~1600

Carbon–Hydrogen Stretching

• sp3 C—H: 2800–3000 cm-1

• sp2 C—H: 3000–3100 cm-1

• sp C—H: 3300 cm-1 (sharp)

IR Spectra of Functional Groups

Alkanes

• Show C—H and C—C stretching/bending

• C—H stretch: 2800–3000 cm-1

• Absence of other bands indicates no other functional groups

Alkenes

• C=C stretch: ~1642 cm-1

• Unsaturated C—H stretch: ~3080 cm-1

Alkynes

• C≡C stretch: ~2200 cm-1

• sp C—H stretch: ~3300 cm-1

Alcohols

• O—H stretch: Broad, intense, ~3300 cm-1

• Broadness due to hydrogen bonding

Amines

• N—H stretch: Broad, ~3300 cm-1

• Secondary amine: One sharp spike

• Primary amine: Two sharp spikes

• Tertiary amine: No N—H signal

Ketones, Aldehydes, and Acids

• C=O stretch: ~1710 cm-1

• Carboxylic acids: Broad O—H stretch (2500–3500 cm-1)

• Aldehydes: Two C—H stretches (2700, 2800 cm-1)

Amides

• C=O stretch: 1640–1680 cm-1

• N—H stretch: ~3300 cm-1 (if N—H present)

Esters and Strained Cyclic Ketones

• Esters: ~1735 cm-1

• Strained cyclic ketones: Higher frequency due to angle strain

Nitriles

• C≡N stretch: 2200–2300 cm-1 (intense, sharp)

• More polar than C≡C, stronger absorption

Summary Table: IR Stretching Frequencies

Frequency (cm-1)	Functional Group	Comments
3300	Alcohol O—H, Amine N—H, Alkyne ≡C—H	Always broad (O—H), may be sharp or broad (N—H), sharp (≡C—H)
3000	Alkane C—H	Just below 3000 cm-1
2200	Alkyne C≡C, Nitrile C≡N	Just above 2200 cm-1
1710	Carbonyl C=O	Esters, acids, ketones, aldehydes
1660	C=C, C=N	Conjugation lowers frequency

Problem Solving with IR Spectra

Functional Group Identification

• Identify peaks outside the fingerprint region

• Compare observed frequencies to known functional group absorptions

• Absence of a signal confirms absence of a functional group

Strengths and Limitations of IR Spectroscopy

• Cannot determine complete structure alone

• Some signals may be ambiguous

• Functional group usually indicated

• Absence of signal is definitive for absence of group

• Comparison with known spectra confirms identity

Mass Spectrometry (MS)

Principles and Applications

• Determines molecular weight and formula from small samples

• Destructive technique: sample is not recoverable

• High-energy electrons fragment molecules; masses and abundances of fragments reveal structure

Radical Cation Formation

• Loss of one electron forms a radical cation (positive charge, unpaired electron)

Electron Impact Ionization

• Fragments formed by breaking C—C or C—H bonds

• Only positive fragments detected

Mass Spectrometer Operation

• Ions separated by magnetic deflection

• Path curvature depends on mass-to-charge ratio ($m/z$)

• Most ions have $z = 1$; path depends on mass

Mass Spectrum Interpretation

• Base peak: Tallest peak, assigned 100% abundance

• Molecular ion (parent peak, M+): Corresponds to molecular weight

Gas Chromatography–Mass Spectrometry (GC-MS)

• GC separates mixture components

• MS scans mass spectra of components as they elute

High-Resolution MS

• Measures masses to high accuracy (1 part in 20,000)

• Distinguishes compounds with similar nominal masses

Isotopic Abundance in MS

• Isotopes present in natural abundance

• Carbon: $^{13}$C (1.1%) gives M+1 peak

• Bromine: $^{79}$Br (50.5%), $^{81}$Br (49.5%) gives equal M and M+2 peaks

• Chlorine: $^{35}$Cl (75.5%), $^{37}$Cl (24.5%) gives M and M+2 peaks (3:1 ratio)

• Sulfur: $^{32}$S (95%), $^{34}$S (4.2%) gives larger M+2 peak

Fragmentation Patterns

• Alkanes: Loss of one or more carbon fragments

• Branched alkanes: Most stable carbocation fragments predominate

• Alkenes: Allylic cleavage forms resonance-stabilized cations

• Aromatic rings: Benzylic cleavage forms resonance-stabilized cations

• Alcohols: Characteristic fragmentation patterns

Conclusion

Infrared spectroscopy and mass spectrometry are essential tools for identifying functional groups and determining molecular structure in organic chemistry. Mastery of these techniques enables chemists to analyze unknown compounds and confirm molecular identities with precision.