Modern Analytical Techniques I Flashcards
What is the molecular ion peak in mass spectrometry?
This indicates the species formed from the molecule by the loss of one electron.
What is the M+1 peak and why does it form?
The M+1 peak is a small peak to the right of the molecular ion peak. It forms due to the compound containing some C-13, a naturally-occurring isotope of carbon.
What is fragmentation?
Fragmentation occurs when the molecular ion breaks into smaller pieces in a mass spectrometer, resulting in many different peaks at different m/z values.
What is the base peak in a mass spectrum?
The base peak is the tallest peak, representing the species with the greatest abundance.
What are stretching vibrations in infrared spectroscopy?
Stretching vibrations occur when a molecule absorbs infrared radiation and this causes certain bond lengths to increase and decrease.
What are bending vibrations in infrared spectroscopy?
Bending vibrations occur when a molecule absorbs infrared radiation and the bond angles increase and decrease.
What are the axes on an infrared spectrum?
The axes are transmittance (as a percentage) (y-axis) and wave number (cm-1) (x-axis).
What is the wavenumber?
The wavenumber of an infrared absorption represents the frequency of infrared radiation absorbed by a particular bond in a molecule.
What does the intensity represent?
The intensity describes the amount of infrared radiation absorbed. Weak intensities refer to high transmittance values and vice versa.
How do you interpret an infrared spectrum?
Look at the characteristic peaks of the spectrum and note the wavenumber values where they occur and the intensity of the peak. It also helps to see if the absorption is sharp (narrow wavenumber range) or broad (wide wavenumber range), as this can help distinguish between bonds. A table is provided in the exam with wavenumber ranges for different bonds, so you can identify the functional groups present.
What do the peaks often look like for the infrared spectra of alcohols, carboxylic acids and carbonyls?
- alcohols - wide peak at 3750-3200 cm-1
- carboxylic acid - very wide peak at 3300-2500 cm-1
- carbonyls (ketones/aldehydes) - sharp peak at 1740-1720 (aldehyde) or 1700-1680 (ketone)
What is the fingerprint region of an infrared spectrum?
The fingerprint region is the part of the spectrum in the wavenumber range 1500-500 cm-1. Most of these absorptions result from bending vibrations or from absorptions not given in the data booklet. You can ignore this region (though it will vary for different molecules).
