Chapter 13: IR Spectroscopy and Mass Spectrometry Flashcards
What is spectroscopy?
The interaction of matter and light to determine the structure of unknown molecules
What does IR spectroscopy reveal?
Provides information about the functional groups present in a molecule in a spectrum with % transmittance on the vertical axis (how much is not absorbed) and wavenumber (cm^-1) on the horizontal axis (the measure of light absorbed and tells how many waves are in a cm)
How does absorbance occur?
If the frequency of the IR wave equals the frequency of the bond vibration in the molecule, the bond energy is pushed into a higher energy regime which is shown as a drop in transmittance on the spectrum
What does it mean for energy to be quantized?
It’s the fundamental of quantum mechanics meaning that energy is not continuous at the atomic level. Bond vibrations go from one energy to another in a jump at a specific frequency as seen in IR spectroscopy
Why are there different wavelength regimes?
They are based on the mass of the bond and the strength of the atoms. A stronger bond has a higher frequency, and lighter atoms have a higher frequency
What are factors that affect IR absorbtion intensity?
Intensity is correlated to the concentration of the sample. The higher the concentration of a similar group added, the larger the peak.
Bond vibrations must affect the dipole moment of the molecule in order to be absorbed. For a molecule to be IR active, it must have a changing dipole moment to interact with the electric field of a light wave and absorb the IR light.
What are the trends in dipole moment?
As a bond stretches, the dipole increases as the electronegativity is increasing.
As a bond compresses, the dipole decreases as electronegativity decreases.
What is the trend of C-H bonds from alkanes to alkynes?
As the bond order increases, the strength of C-H bonds increases because stronger bonds absorb at higher wavenumbers.
Why are O-H stretches large and rounded when energy is quantized?
This is due to hydrogen bonding that have orbital interactions that strengthen and weaken eachother’s bonds. The broad stretch is multiple O-H bonds interacting with one another.
What is mass spectrometry?
It determines the molecular mass of an unknown molecule using a mass spectrometer. It destroys the sample.
It reveals fragmentation patterns as molecule is destroyed.
Electron Ionization Mass Spectrometry
The sample is vaporized in a vacuum and subjected to an electron beam that kicks out an electron and leaves behind a radical cation.
Only charged molecules are seen in mass spectrometry
Following the radical cation formation, fragment ions are separated out according to their mass-to-charge ratio (m/z) where z usually equals 1
What is a molecular ion?
The ion created after an electron ejection, no fragmentation has occurred yet
What is a base peak?
The molecule of highest abundance
What is relative abndance?
Relative abundance = #C atoms (natural abundance 13C/natural abundance of 12C)
What are the isotope abundances of Cl and Br?
Cl = 3:1
Br = 1:1
What is chemical ionization?
It is adding a proton to the parent molecule to produce an M+1 peak that reveals the molecular mass of the parent ion
