Infrared and Raman Basics

IR absorption occurs only when a molecule's dipole moment changes during vibration.

The product of bond distance and charge difference between bonded atoms.

Bonds with atoms of different electronegativities (like C-O, N-O, N-H, C-H) absorb IR light.

Molecules like H2, N2, and O2 do not absorb IR light because they have zero dipole moment change during vibration.

The study of how molecules absorb and emit infrared light, related to atomic vibrations around bonds.

The process where a molecule absorbs infrared radiation, causing vibrations in bonds.

A technique used to study how molecules absorb and scatter light (especially near infrared or visible), providing a different view than IR Spectroscopy

The frequency of vibration a molecule that causes a particular bond to oscillate or vibrate.

Describes the distribution of populations among energy levels in a system.

Different possible energy states a system can occupy.

The number of systems occupying each energy level.

Constant used to calculate population probabilities at different temperatures.

Measure of the average kinetic energy of particles in a system.

Difference in energy between different energy levels.

The number of systems in each energy level.

Defines the probability of populating a particular energy level at a given temperature

Certain molecular vibrations don't show up in infrared (IR) spectra because the molecule's symmetry prevents a change in dipole moment during the vibration.

A type of molecular vibration where atoms move in and out symmetrically along a bond axis, resulting in no change in the molecule's dipole moment.

A type of molecular vibration where atoms move in and out unequally along a bond axis, resulting in a change in the molecule's dipole moment.

Electromagnetic radiation with wavelengths longer than visible light, but shorter than microwaves.

Analysis of the interaction of a material with infrared light, often used to determine molecular structure through vibrations.

Sections of the infrared spectrum most useful for biologists.

The process by which objects emit infrared radiation due to their temperature.

The unit used to measure the frequency of infrared radiation absorbed by molecules in IR spectroscopy. It represents the number of wavelengths per centimeter.

Unique patterns in IR spectra that allow for the identification of specific molecules.

The mid-infrared (mid-IR) region is most useful for identifying biological molecules like proteins, carbohydrates, lipids, and nucleic acids.

The region of the IR spectrum between 1500 cm⁻¹ and 500 cm⁻¹ that contains complex and unique absorption patterns for different molecules. It is like a fingerprint that identifies a specific molecule.

Proteins have complex IR spectra due to the numerous modes of vibration present. However, some vibrations are more intense than others.

The portion of the infrared spectrum typically used in IR spectroscopy, ranging from 4000 cm⁻¹ to 400 cm⁻¹.

The most important diagnostic band for proteins, originating primarily from the C=O stretching vibration in peptide bonds.

IR spectroscopy identifies different molecules based on the unique way they absorb infrared light. The specific vibrational modes of bonds within a molecule determine its IR spectrum.

The UV-Vis spectrum provides information about electronic transitions (electrons moving between energy levels), while the IR spectrum reveals vibrational transitions (bonds stretching and bending) within a molecule.

The frequency of the amide I band changes depending on the secondary structure of a protein.

Molecules have multiple ways to vibrate, leading to complex IR spectra. Even simple molecules have multiple vibrational modes, contributing to a multitude of absorption peaks.

The strength of the hydrogen bond between C=O and N-H groups in peptide bonds influences the amide I frequency, which is different for alpha-helices and beta-sheets.

IR spectra can distinguish between molecules with similar structures but different functional groups. The fingerprint region is especially helpful in identifying molecules with unique patterns.

Changes in the amide I band frequency can be used to study and monitor changes in protein secondary structure.

IR spectra of mixtures show the combined absorption patterns of all components. However, overlapping peaks can make it challenging to analyze individual components.

IR spectroscopy is used in various fields including diagnostics, forensic science, and cell and tissue classification due to its ability to identify unique molecular fingerprints.

The tendency of an electron cloud to distort when an electric field is applied, causing temporary dipoles.

A technique that measures how molecules scatter light to reveal information about their vibrational modes.

Bonds with high polarizability, meaning their electron cloud easily distorts during vibration, resulting in strong Raman signals.

IR spectroscopy measures how molecules absorb infrared light based on changing dipole moments, while Raman spectroscopy measures how molecules scatter light based on changing polarizability.

Water strongly absorbs IR light due to its large dipole moment, making it difficult to study other molecules in IR. However, water is a weak Raman scatterer, making it a good solvent for Raman spectroscopy.

IR and Raman spectroscopy are complementary because they focus on different properties of molecules, providing a more complete picture of their structure and dynamics.

Different ways a molecule can vibrate, each with a specific frequency and energy level.

IR and Raman spectra often look similar because vibrational modes influence both techniques, but the intensity and presence of certain peaks can differ due to the different underlying properties they measure.