IR Spectroscopy and Molecular Vibration Concepts

The greater the mass of the atoms, the lower the wavenumber. This is because heavier atoms vibrate at lower frequencies.

Vibrational transitions and rotational transitions.

Stretching these bonds does not change the dipole moment, and bending cannot occur with only two atoms.

<p>The frequency of a vibration is directly proportional to the strength of the bond.</p> Signup and view all the answers

<p>Molecules exhibit stretching, bending, and twisting vibrations.</p> Signup and view all the answers

<p>The absorption of IR radiation.</p> Signup and view all the answers

<p>The number of ways a molecule can vibrate is related to the number of atoms and bonds it contains.</p> Signup and view all the answers

<p>More complex molecules have more complex IR spectra.</p> Signup and view all the answers

<p>IR spectroscopy has a wider range of vibrational frequencies that can be detected, leading to a more detailed and specific fingerprint of the molecule.</p> Signup and view all the answers

<p>The narrow, sharp peaks in IR spectra can lead to deviations from Beer's Law, making it difficult to obtain precise absorbance measurements.</p> Signup and view all the answers

<p>Solid samples can be mixed with powdered KCl or KBr, compressed into a disc, and then placed in the IR instrument.</p> Signup and view all the answers

<p>Nujol is used to create a thick paste with the solid sample so that it can be easily pressed between NaCl plates or a disc.</p> Signup and view all the answers

<p>Water strongly absorbs IR radiation in the same region as many organic molecules, making it difficult to obtain a clear spectrum.</p> Signup and view all the answers

<p>A reference cell containing the pure solvent is placed in the reference beam to eliminate the solvent's IR bands from the spectrum of the analyte.</p> Signup and view all the answers

<p>An IR spectrum plots % transmittance on the y-axis and wavenumber (cm-1) on the x-axis.</p> Signup and view all the answers

<p>By analyzing the positions and relative intensities of the absorption bands, IR spectra can provide information about the functional groups present in a molecule and its molecular structure.</p> Signup and view all the answers

<p>Most bands in an IR spectrum are the result of combinations of different stretching and bending modes of the molecule's bonds.</p> Signup and view all the answers

<p>The majority of bands result from complex combinations of vibrational modes making it difficult to definitively assign a specific functional group to each one.</p> Signup and view all the answers

<p>The characteristic range of wavenumbers for carbonyl groups (C=O) in infrared spectroscopy is typically between 1680-1750 cm⁻¹.</p> Signup and view all the answers

<p>Both alcohols and carboxylic acids exhibit O-H stretching vibrations in the IR spectrum. However, the O-H stretch in a carboxylic acid is broader and appears at a lower wavenumber (2500-3500 cm⁻¹) due to strong hydrogen bonding. In contrast, the O-H stretch in an alcohol is typically sharper and appears at a higher wavenumber (3580-3650 cm⁻¹).</p> Signup and view all the answers

<p>A wide peak in this region suggests hydrogen bonding between molecules. Hydrogen bonding weakens the O-H bond, leading to a lower vibrational frequency and a broader, weaker peak.</p> Signup and view all the answers

<p>Infrared spectroscopy can be used to identify and quantify pollutants such as carbon monoxide (CO), sulfur dioxide (SO₂), and carbon dioxide (CO₂). The presence and concentration of these gases can be determined based on the characteristic absorption bands in their IR spectra.</p> Signup and view all the answers

<p>Sulfur dioxide (SO₂) is a bent molecule, while carbon dioxide (CO₂) is a linear molecule. The bent structure of SO₂ allows for three different modes of vibration, corresponding to three peaks in the IR spectrum. In contrast, the linear structure of CO₂ only allows for two distinct modes of vibration, leading to two peaks in its IR spectrum.</p> Signup and view all the answers

<p>The two main regions are the fingerprint region and the functional group region. The fingerprint region is unique to each molecule, while the functional group region is similar for molecules with the same functional groups.</p> Signup and view all the answers

<p>Infrared spectroscopy can provide information about the types of bonds present in a molecule, such as C-H, C=O, and O-H bonds.</p> Signup and view all the answers

<p>Stronger bonds tend to vibrate at higher frequencies, which correspond to higher wavenumbers in an IR spectrum.</p> Signup and view all the answers

<p>Infrared spectroscopy is used to identify and quantify various atmospheric pollutants, such as carbon disulfide, pyridine, and carbon dioxide.</p> Signup and view all the answers

<p>Infrared spectroscopy cannot determine the molecular weight of a substance.</p> Signup and view all the answers

<p>The C-H stretch in an alkane typically falls within the range of 2950-2850 $cm^{-1}$</p> Signup and view all the answers

<p>Infrared technology is widely used in thermal imaging cameras and remote controls.</p> Signup and view all the answers

<p>More complex molecules tend to have more complex IR spectra with a larger number of absorption bands.</p> Signup and view all the answers

<p>The absorption peak in this range is typically associated with the O-H bond in a carboxylic acid.</p> Signup and view all the answers

<p>While IR spectroscopy can identify functional groups, it does not provide information about the relative positions of these groups within a molecule.</p> Signup and view all the answers

Measurement of IR radiation absorption by compounds.

Vibrational motion associated with molecules.

Vibrations must change the dipole moment to absorb IR.

Quantifies energy of vibrational frequencies.

Molecules can transition in vibration and rotation within ground state.

Specific groups of atoms in organic compounds identifiable by IR spectra.

Certain molecules do not absorb IR, like symmetrical diatomics.

Use of IR spectroscopy to detect air pollutants.

Graphical display of IR radiation absorption frequencies and light transmittance.

Determining the presence of specific substances in a sample.

Measuring the amount of substances in a sample.

Relationship stating that absorbance is proportional to concentration.

Mixing solid samples with KCl or KBr to form a disc for IR analysis.

Done by smearing between NaCl plates or using a liquid cell.

Contains pure solvent to eliminate solvent’s IR bands in spectrum.

Reading the peaks in an IR spectrum plotted as % transmittance vs wavenumber.

Molecular vibrations involving bending, contributing to IR bands.

Typical wavenumbers for aldehydes and ketones range from 1680-1750 cm-1.

The O-H group in alcohols absorbs at 3580-3650 cm-1 in the IR spectrum.

A wide peak in the 2800-3500 cm-1 region indicates hydrogen bonding in alcohols or carboxylic acids.

Infrared spectroscopy can monitor air pollutants like carbon monoxide and sulfur dioxide, and measure carbon dioxide concentration.

Infrared spectroscopy can distinguish between functional groups in organic molecules by analyzing characteristic absorption peaks.

The unique spectral region specific to a molecule.

Spectral region with similarities among molecules with same groups.

Absorption peak range for alkyl groups is 2950-2850 cm-1.

Absorption peak range for alkenes is 1680-1620 and 3100-3010 cm-1.

Absorption peak range for alcohols is 3550-3200 cm-1.

Technology used to study heat patterns in bodies.

Absorption peak range for nitriles is 2280-2220 cm-1.

Absorption peak for amines is 3500-3300 cm-1.