CHEM 210: LT3 IR Spectroscopy Notes

LT3: IR Spectroscopy - Comprehensive Study Notes

1. Introduction to IR Spectroscopy

Definition: Infrared (IR) spectroscopy is a technique that involves shining infrared light on a sample. The light that is transmitted (passes through the sample) is then detected.
Principle: Certain frequencies of IR light are absorbed by the sample, and these absorptions indicate the presence of specific types of bonds within the molecule.

2. Learning Objectives

By the end of this study, you should be able to:

Explain how IR spectroscopy works.
Identify functional groups from an IR spectrum.
Know the characteristics of a spectrum (signal wavenumber, strength, and shape).
Determine bond strength from IR spectra.
Use Mass Spectrometry (MS) and IR to determine molecular formulas.

3. General Chemistry Foundations: Spectroscopy and Light

Spectroscopy: This field involves the interaction between matter and light (electromagnetic radiation).
Nature of Light: Light can be understood as either waves of energy or discrete packets (particles) of energy called photons.
Properties of Light Waves: Key properties include wavelength ( $\lambda$ ) and frequency ( $\nu$ ).
- Frequency and Energy Relationship: Energy ( $E$ ) is directly proportional to frequency ( $\nu$ ). Mathematically this is expressed as $E = h u$ , where $h$ is Planck's constant. Thus, as frequency increases, energy increases.
- Wavelength and Energy Relationship: Energy ( $E$ ) is inversely proportional to wavelength ( $\lambda$ ). This is expressed as $E = hc/ u$ , where $c$ is the speed of light. Thus, as wavelength increases, energy decreases.
Quantized Energy: Energy is quantized, meaning only specific, discrete amounts of energy can be absorbed by molecules. This absorbed energy can cause electrons to become excited or bonds to vibrate or rotate.
Electromagnetic Spectrum & Molecular Changes:

Type of Spectroscopy	Electromagnetic Radiation	Molecular Change Induced	Information Obtained
UV-VIS	Visible and Ultraviolet	Excitation of electrons in conjugated $\pi$ systems	Presence of any conjugated $\pi$ system in the compound
IR	Infrared	Vibration and rotation of bonds	The functional groups present in the compound
NMR	Radio waves	Nuclear spin transitions	The arrangement of all carbon and hydrogen atoms

4. How IR Spectroscopy Works: Bond Movements and Dipole Moments

Molecular Vibrations: Bonds within molecules are not static; they naturally move (vibrate) at specific natural frequencies or wavenumbers.
Types of Movements (Vibrations): These movements include:
- Stretches: Rhythmic elongations and compressions along the bond axis.
- Bends: Changes in the bond angle.
IR Absorption Mechanism: When a molecule absorbs IR radiation, these natural movements are amplified.
Dipole Moment Requirement: For a bond vibration to be IR active (i.e., produce a signal), there must be a change in the bond's dipole moment during the vibration.
- This change in dipole is what interacts with the electromagnetic field of the IR light and is subsequently recorded as an IR signal.
Signal Strength and Polarity: The strength (intensity) of an IR signal is directly related to the bond's polarity. More polar bonds (larger change in dipole moment during vibration) will typically result in stronger signals.
Non-IR Active Bonds: Bonds that possess no dipole moment (e.g., symmetrical diatomic molecules like $O2$ or $N2$ ) or those where the vibration does not cause a change in dipole moment will not give rise to an IR signal.
- Examples of Bonds with Non-Zero Dipole Moments: $O-H$ , $C=O$ , $N-H$ , $C-Cl$ . These bonds will show IR signals. The atom with higher electronegativity will have a partial negative charge ( $ext{ }\delta^{-}$ ), and the other atom a partial positive charge ( $ext{ }\delta^{+}$ ).
- Examples of Bonds with Zero Dipole Moments (or negligible for IR): $C-C$ (in alkanes), $H-H$ . These typically do not produce IR signals, or very weak ones if their environment induces a temporary dipole.

5. Wavenumber and Bond Properties

Wavenumber Specificity: The specific wavenumber ( $\tilde{ u}$ ) at which an IR signal appears is characteristic of a particular type of bond (e.g., $C-H$ , $C=O$ ).
Stretching Frequency Formula (Hooke's Law Analogy):
- The stretching frequency of a specific bond can be calculated using the following formula, which is derived from Hooke's Law for a spring:
  $\tilde{\nu} = \frac{1}{2\pi c}\sqrt{\frac{k}{\mu}}$
  Where:
- $\tilde{\nu}$ is the wavenumber, typically expressed in reciprocal centimeters ( $cm^{-1}$ ).
- $c$ is the speed of light ( $3 \times 10^{10} \text{ cm/s}$ ).
- $k$ is the bond's force constant, which represents the bond strength (a stronger bond has a larger $k$ ).
- $\mu$ is the reduced mass of the two atoms involved in the bond.
Reduced Mass ( $\mu$ ):
- The reduced mass is calculated as: $\mu = \frac{m1 m2}{m1 + m2}$ , where $m1$ and $m2$ are the masses of the two atoms in the bond.
Key Relationships:
- Bond Strength ( $k$ ): Higher bond strength leads to a higher wavenumber (stronger bonds vibrate at higher frequencies).
  - Example: A $C=O$ bond has a higher wavenumber than a $C-O$ bond because the double bond is stronger ( $k$ is larger).
- Reduced Mass ( $\mu$ ): Larger reduced mass leads to a lower wavenumber (heavier atoms vibrate at lower frequencies).
  - Example: A $C-O$ bond has a higher wavenumber than a $C-Br$ bond because oxygen is lighter than bromine, resulting in a smaller reduced mass for $C-O$ . Thus, $C-O$ vibrates at a higher frequency than $C-Br$ .

6. Interpreting IR Spectra: Correlation Charts and Functional Groups

Correlation Charts: Since specific bonds and their characteristic stretches and bends occur at predictable wavenumber ranges, correlation charts are used as a primary tool to aid in the interpretation of IR spectra. (These charts are typically provided during assessments).
Key Functional Group Regions (General Ranges - precise values vary):
- O-H Stretch (Alcohols/Phenols): Typically a broad, strong peak in the range of $3200-3600 \text{ cm}^{-1}$ .
- O-H Stretch (Carboxylic Acids): Very broad, strong peak often overlapping with C-H, around $2500-3300 \text{ cm}^{-1}$ .
- N-H Stretch (Amines): Medium intensity peaks in the range of $3300-3500 \text{ cm}^{-1}$ .
  - Primary Amines ( $NH_2$ ): Two peaks (one symmetric, one asymmetric stretch).
  - Secondary Amines ( $NH$ ): One peak.
  - Tertiary Amines ( $N$ ): No N-H stretch.
- C-H Stretch (Alkanes): Sharp peaks just below $3000 \text{ cm}^{-1}$ (e.g., $2850-2960 \text{ cm}^{-1}$ ).
- C-H Stretch (Alkenes): Sharp peaks just above $3000 \text{ cm}^{-1}$ (e.g., $3020-3100 \text{ cm}^{-1}$ ).
- C-H Stretch (Aromatics): Sharp peaks just above $3000 \text{ cm}^{-1}$ (e.g., $3030 \text{ cm}^{-1}$ ).
- C-H Stretch (Alkynes): Sharp, moderately strong peak around $3300 \text{ cm}^{-1}$ .
- C=O Stretch (Carbonyls): A very strong, sharp peak typically in the range of $1660-1820 \text{ cm}^{-1}$ . The exact position depends on the type of carbonyl:
  - Aldehydes/Ketones: Around $1710-1725 \text{ cm}^{-1}$ .
  - Esters: Higher, often around $1735-1750 \text{ cm}^{-1}$ .
  - Carboxylic Acids: Around $1700-1725 \text{ cm}^{-1}$ .
  - Amides: Lower, around $1630-1690 \text{ cm}^{-1}$ .
- C=C Stretch (Alkenes): Typically around $1620-1680 \text{ cm}^{-1}$ , often weak to medium intensity.
- C=C Stretch (Aromatics): Several peaks in the $1450-1600 \text{ cm}^{-1}$ region (ring stretching).
- C $\equiv$ N Stretch (Nitriles): Medium, sharp peak around $2250 \text{ cm}^{-1}$ .
- C $\equiv$ C Stretch (Alkynes): Weak, sharp peak around $2100-2260 \text{ cm}^{-1}$ . Often absent if symmetrical.

7. Characteristics of IR Signals: Wavenumber, Strength, and Shape

Wavenumber ( $\tilde{\nu}$ ): As discussed, indicates the type of bond and its vibrational mode. Expressed in $cm^{-1}$ .
Strength (Intensity): Refers to the extensiveness of the dipole moment change during vibration. Stronger signals indicate more polar bonds or a larger number of identical bonds vibrating:
- Strong (s): Very intense absorption.
- Medium (m): Moderate intensity absorption.
- Weak (w): Low intensity absorption.
- Absent: No detectable absorption.
Shape: Describes the appearance of the peak:
- Broad (br): Often characteristic of O-H groups due to hydrogen bonding, which causes a range of vibrational energies.
- Sharp (sh): Narrow, distinct peak, common for many C-H, C=O, or triple bond stretches.

Comparing and Contrasting Signals

C-H and O-H in Butanol (Alcohol):
- O-H: Typically appears as a broad, strong absorption in the $3200-3600 \text{ cm}^{-1}$ range due to hydrogen bonding.
- C-H: Appears as sharper absorptions below $3000 \text{ cm}^{-1}$ (alkane C-H).
N-H Stretches in Amines:
- 1-Hexanamine (Primary Amine, $RNH_2$ ): Will show two distinct N-H stretching peaks (asymmetric and symmetric stretches) in the $3300-3500 \text{ cm}^{-1}$ range.
- N-ethyl-1,1-dimethylethan-1-amine (Secondary Amine, $R_2NH$ ): Will show one N-H stretching peak in the same region.
C=O Stretches in Amides vs. Ketones:
- Butanamide (Amide C=O): The carbonyl stretch is typically observed at a lower wavenumber (e.g., $1630-1690 \text{ cm}^{-1}$ ) than a ketone due to resonance interaction with the nitrogen lone pair, which reduces the double bond character.
- Butan-2-one (Ketone C=O): The carbonyl stretch is typically observed at a higher wavenumber (e.g., $1710-1725 \text{ cm}^{-1}$ ).

8. Identifying Functional Groups & Determining Molecular Formulas (IR + MS)

IR spectroscopy is crucial for identifying functional groups within a molecule due to the unique vibrational frequencies of specific bonds.
Combined with Mass Spectrometry (MS):
- IR provides information about functional groups present.
- MS provides information about the molecular weight and fragmentation patterns.
- By integrating both datasets, along with other analytical techniques like NMR (Nuclear Magnetic Resonance) when available, one can often deduce the molecular formula and overall structure of an unknown compound.

9. Monitoring Reactions Using IR Spectroscopy

IR spectroscopy is an effective tool to monitor the progress of a chemical reaction by observing the disappearance of reactant functional group peaks and the appearance of product functional group peaks.

Example: Ketone reduction to a Secondary Alcohol
- Reactant (Ketone): Will exhibit a characteristic strong, sharp C=O stretch around $1715 \text{ cm}^{-1}$ .
- Product (Secondary Alcohol): Will exhibit a characteristic broad, strong O-H stretch around $3300 \text{ cm}^{-1}$ and the absence of the C=O stretch.
- Distinguishing Structural Features: The C=O bond in the reactant and the O-H bond in the product are key.
- Non-Distinguishing Features: Aliphatic C-H bonds or C-C single bonds typically remain largely unchanged unless the carbon skeleton is significantly altered.
- Monitoring Strategy:
  - Look for: The disappearance of the C=O absorbance ( $\approx 1715 \text{ cm}^{-1}$ ) and the appearance of a new O-H absorbance ( $\approx 3300 \text{ cm}^{-1}$ ).
  - Reaction Completion: The reaction is complete when the C=O peak fully disappears or the O-H peak intensity no longer increases (reaches a plateau).

Annotation Examples

IR Spectrum of 2-Butanol (an Alcohol):
- O-H Stretch: A very prominent, broad, and strong peak typically around $3300 \text{ cm}^{-1}$ .
- C-H Stretches: Peaks below $3000 \text{ cm}^{-1}$ representing aliphatic C-H bonds.
- Absence: No C=O group present, so no peak around $1715 \text{ cm}^{-1}$ .
IR Spectrum of 2-Butanone (a Ketone):
- C=O Stretch: A strong, sharp peak typically around $1715 \text{ cm}^{-1}$ .
- C-H Stretches: Peaks below $3000 \text{ cm}^{-1}$ representing aliphatic C-H bonds.
- Absence: No O-H group present, so no broad peak around $3300 \text{ cm}^{-1}$ .

10. Step-by-Step Approach for Structure Determination Using IR Spectroscopy

This systematic approach can help in interpreting an IR spectrum, especially when combined with other data like MS and NMR.

Check for C=O (Carbonyl):
- Look for a strong absorption in the region of $1820-1660 \text{ cm}^{-1}$ .
If C=O is Present, Further Differentiate Carbonyl Type:
- Acids (Carboxylic): Is an O-H absorption also present (very broad, strong; often overlapping with C-H, $2500-3300 \text{ cm}^{-1}$ )?
- Amides: Is an N-H absorption also present (medium; one or two peaks $3300-3500 \text{ cm}^{-1}$ )?
- Anhydrides: Are two C=O absorptions present (typically strong, e.g., $1820 \text{ cm}^{-1}$ and $1760 \text{ cm}^{-1}$ )?
- Aldehydes: Is aldehyde C-H present? Look for two weak absorptions near $2850-2750 \text{ cm}^{-1}$ (to the right side of aliphatic C-H absorptions).
- Esters: If the preceding four choices are eliminated, and the C=O absorption is typically above $1730 \text{ cm}^{-1}$ .
- Ketones: If the preceding four choices are eliminated, and the C=O absorption is typically below $1730 \text{ cm}^{-1}$ .
If C=O is Absent, Check for Other Heteroatom Functional Groups:
- Alcohols: Look for a broad, strong O-H absorption ( $3200-3600 \text{ cm}^{-1}$ ).
- Amines: Check for N-H absorptions (medium intensity; one or two peaks $3300-3500 \text{ cm}^{-1}$ ).
Check for Double Bonds and/or Aromatic Rings:
- Look for C=C absorptions (alkene or aromatic rings) usually in the $1600-1680 \text{ cm}^{-1}$ range.
- Also check for $=C-H$ stretches above $3000 \text{ cm}^{-1}$ for alkenes or aromatics.
Check for Triple Bonds:
- C $\equiv$ N (Nitrile): A medium, sharp absorption near $2250 \text{ cm}^{-1}$ .
- C $\equiv$ C (Alkyne): A weak, sharp absorption near $2150 \text{ cm}^{-1}$ . If the alkyne is internal and symmetrical, this peak may be absent.
Hydrocarbons: If none of the preceding functional groups are found, the compound may be a simple hydrocarbon. Major absorptions will be in the C-H region (typically below $3000 \text{ cm}^{-1}$ for saturated hydrocarbons).

Important Note: Other functional groups like ethers or alkyl halides often do not have strong, diagnostic IR signals. Mass Spectrometry (MS) and/or Nuclear Magnetic Resonance (NMR) data are more reliably used to identify these groups.