Molecular Vibrations and IR Spectroscopy

<p>Degenerate (A)</p> Signup and view all the answers

<p>Two of the vibrational modes of $CO_2$ are degenerate and IR active. (D)</p> Signup and view all the answers

<p>The frequency is directly proportional to the square root of the force constant. (B)</p> Signup and view all the answers

<p>CO (C)</p> Signup and view all the answers

<p>Vibrational Frequency (B)</p> Signup and view all the answers

<p>The molecule must undergo a net change in dipole moment during its vibrational motion. (C)</p> Signup and view all the answers

<p>Wavenumbers ranging from about 12,800 to 10 cm-1. (D)</p> Signup and view all the answers

<p>Near-, mid-, and far-infrared radiation. (A)</p> Signup and view all the answers

<p>Presence of electronic transitions. (D)</p> Signup and view all the answers

<p>They are quantized and relate to energy differences in the mid-infrared region. (D)</p> Signup and view all the answers

<p>Decreases the percentage transmittance value (B)</p> Signup and view all the answers

<p>Globar (A)</p> Signup and view all the answers

<p>Both produce infrared radiation (A)</p> Signup and view all the answers

<p>Nernst Glower (A)</p> Signup and view all the answers

<p>Nitrogen molecules (C)</p> Signup and view all the answers

<p>Destructive Interference (B)</p> Signup and view all the answers

<p>Higher light intensity due to no slits (D)</p> Signup and view all the answers

<p>Light travels different distances leading to interference (A)</p> Signup and view all the answers

<p>Blank spectra (D)</p> Signup and view all the answers

<p>It extends into the sample and interacts with its composition (A)</p> Signup and view all the answers

<p>Cannot use thermal detectors due to speed (C)</p> Signup and view all the answers

<p>It enhances the signal-to-noise ratio (A)</p> Signup and view all the answers

<p>It splits light into two beams (C)</p> Signup and view all the answers

<p>Resistance decreases, leading to increased current (C)</p> Signup and view all the answers

<p>Lithium tantalate (C)</p> Signup and view all the answers

<p>It prevents photodegradation from excessive energy (A)</p> Signup and view all the answers

<p>IR transparent materials such as KBr pellets or NaCl (B)</p> Signup and view all the answers

<p>It produces an interference pattern that contains all IR data (B)</p> Signup and view all the answers

<p>10,000 - 333 cm-1 (D)</p> Signup and view all the answers

<p>High energy may cause photodegradation of samples (B)</p> Signup and view all the answers

<p>A diffractive element (D)</p> Signup and view all the answers

<p>It increases the ratio. (B)</p> Signup and view all the answers

<p>A change in polarizability of the bond. (A)</p> Signup and view all the answers

<p>Covalent bonds being more polarizable than ionic bonds. (A)</p> Signup and view all the answers

<p>Polarizability decreases with higher bond strength. (D)</p> Signup and view all the answers

<p>It reflects the change in polarizability rather than the molecule's dipole moment. (C)</p> Signup and view all the answers

<p>They result in a stronger signal than anti-Stokes lines. (A)</p> Signup and view all the answers

<p>Orientation with respect to an electric field. (C)</p> Signup and view all the answers

<p>It contributes to the number of electrons and their response to nuclear charge. (D)</p> Signup and view all the answers

The range of electromagnetic radiation containing wavelengths from 0.78 to 1000 micrometers (µm), encompassing near, mid, and far-infrared radiation.

Vibrational transitions occur when a molecule absorbs infrared radiation and changes its energy state, transitioning from the ground vibrational state to an excited state.

A molecule must undergo a change in its dipole moment during vibrations to absorb infrared radiation. Essentially, the distribution of charges within the molecule must change during vibration.

Infrared spectroscopy is a technique that analyzes the interaction of infrared radiation with molecules, revealing their vibrational properties and providing information about their structure, functional groups, and chemical bonds.

The part of the infrared spectrum where vibrational modes of chemical bonds in molecules are observed, providing valuable information about functional groups.

A light source for infrared spectroscopy that uses a cylinder of rare earth metal oxides (e.g., Zirconium, Cerium, Thorium) heated electrically. Current is passed through it, creating resistance and generating heat (1200-2200°C), producing IR radiation. It has a range of ~670-10,000cm-1.

A type of light source used in IR spectroscopy consisting of a rod made from silicon carbide. It works similarly to a Nernst Glower, but uses a different material. It has a range similar to the Nernst Glower.

A light source used in IR spectroscopy. It consists of a tightly wound wire (Nichrome or Rodium) which is heated by electricity, similar to the Nernst Glower. It's lower-intensity than the Nernst Glower or Globar but has a longer lifetime.

A type of light source using a gas mixture of Helium, Carbon Dioxide and Nitrogen. Applying voltage excites the Nitrogen, leading to collisions that create a specific type of IR radiation. It has a narrow spectral output, meaning it only produces light at specific wavelengths.

In IR spectroscopy, when a molecule absorbs IR radiation, its structural bonds vibrate. These vibrations can reveal the molecule's structure. IR radiation can be used to identify functional groups in organic molecules.

The number of specific vibrational modes a molecule can exhibit is determined by its structure. Non-linear molecules have 3N-6 vibrational modes, while linear molecules have 3N-5, where N is the number of atoms.

Vibrational modes are caused by the movement of atoms within a molecule, resulting in changes in bond lengths and angles. Each mode corresponds to a unique frequency of vibration.

The intensity of an IR absorption peak is related to the change in dipole moment during the vibration. Larger changes in dipole moment result in stronger absorption.

In order to be observed in an IR spectrum, a vibrational mode must cause a change in the molecule's dipole moment. If the vibration does not alter the charge distribution, it will be IR inactive.

Degenerate modes are vibrational modes that have the same energy. In IR spectroscopy, they appear as a single peak because they are indistinguishable.

The natural frequency of a vibrating system is determined by the mass of the atoms involved and the strength of the bond (force constant). The higher the mass or weaker the bond, the lower the frequency.

The equation for vibrational frequency can be modified to account for two masses connected by a spring, representing a bond between two atoms. This allows us to predict vibrational frequencies for diatomic molecules.

IR spectroscopy measures the absorption of IR radiation by molecules. This enables scientists to analyze the vibrational modes of molecules and identify functional groups within a molecule.

A combination of two waves that results in an increase in the amplitude of the resulting wave.

A combination of two waves that results in a decrease in the amplitude of the resulting wave.

The Michelson Interferometer separates a beam of light into two paths, recombines them, and then analyzes the interference pattern. The difference in the path lengths traveled by the two beams leads to constructive or destructive interference, which is observed as a change in the signal.

The Michelson Interferometer is used to measure very small distances, such as the wavelength of light.

In FTIR, the spectra of all wavenumbers are collected simultaneously, while in normal dispersive IR spectroscopy, wavenumbers are measured sequentially.

FTIR spectroscopy is faster, uses signal averaging, and has higher light intensity due to the absence of slits, providing a greater signal-to-noise ratio.

FTIR requires a blank measurement and cannot work with slow thermal detectors.

ATR is a technique used in FTIR that measures the absorption of light by a sample through the evanescent wave generated when light is totally reflected at the interface between a crystal and the sample.

The ability of a semiconductor material to become electrically conducting when exposed to infrared light is due to the absorption of this light, which excites electrons from a non-conducting state to a conducting state.

When a material's resistance decreases, the flow of electric current through it increases.

The interaction between a material and infrared light causing changes in its electrical properties.

A pyroelectric detector is a device that measures the change in polarization of a pyroelectric material due to temperature variations. It exploits the temperature-dependent polarization of a pyroelectric material (often ceramic or lithium tantalate), meaning the material's separation of positive and negative charges changes with its temperature.

Pyroelectric detectors are known for their quick response time, making them suitable for applications like FTIR spectroscopy, where the rapid change of infrared light needs to be captured efficiently.

Materials like potassium bromide (KBr) pellets or sodium chloride (NaCl) are commonly used in infrared spectroscopy because they are transparent to infrared radiation.

A monochromator (often using a reflective grating) is crucial in IR spectroscopy because it separates infrared light into its different wavelengths or frequencies. This ensures that the analysis of a sample focuses on specific wavelengths, providing a detailed spectrum.

Placing the monochromator after the sample cell in IR spectroscopy helps minimize sample degradation due to excessive energy. IR light is less energetic than UV/Vis, which requires a different approach to prevent photodegradation.

The ratio of anti-Stokes to Stokes Raman scattering intensities increases with temperature because a larger fraction of molecules occupy the first vibrational excited state at higher temperatures.

A vibration is Raman-active if it leads to a change in the molecule's polarizability, which describes the ease with which the electron cloud can be distorted.

The polarizability of a molecule is affected by various factors including atomic number, bond length, size, molecular orientation, bond strength, and electronegativity difference.

The polarizability of a molecule is directly proportional to its size and bond length, and inversely proportional to the bond strength and electronegativity difference between atoms.

Covalent bonds are more polarizable than ionic bonds because electrons in covalent bonds are more loosely held and can be more easily distorted by an electric field.

The Raman depolarization ratio is a measure of the anisotropy (directionality) of the polarizability tensor of a molecule, and it provides information about the symmetry of the vibrational mode.

Raman spectroscopy is a vibrational spectroscopy technique that uses inelastic scattering of light to probe molecular vibrations. It's useful for identifying molecules and determining their structure, symmetry, and bonding.

Raman spectroscopy is a versatile technique that can be used on samples in various states (gas, liquid, solid) without requiring special preparation, making it convenient for diverse applications.