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44 Cards in this Set

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  • Back
What are the three methods used to determine the structure of a molecule?
Mass spectrometry: used to determine mass of molecule and atomic composition

Infrared spectroscopy: to determine functional groups in molecule

Nuclear Magnetic Resonance: Spetroscopy to determine the carbon skeleton of molecule
What is mass spectrometry used to determine?
Mass of molecule and atomic composition
What is nuclear magnetic resonance spectroscopy used for?
To determine carbon skeleton of molecule.
What is infrared spectroscopy used for?
To determine functional groups in molecule
What are the two methods of mass spectroscopy?
Electron impact and chemical ionization
How does electron impact mass spectrometry work?
Molecule is bombarded with highly energetic electrons that knock a weakly bound electron out of the molecule.
What is left behind when the molecule is bombarded by the electron in electron impact mass spectrometry?
A radical cation; (occurs if the electron beam supplies more than exactly the right amount of energy
What is the base peak?
Strongest peak assigned an 'abundance' of 100% ~ mass spectrometry by electron impact
How does chemical ionization mass spectrometry work?
Electron beam is used to ionize a simple molecule such as methane which in turn ionizes our molecule by collision and transfer of a proton.

(Methane loses a bonding electron to give CH4+ which reacts with an unionized methane molecule to give CH3 and CH5+, which is a very powerful, unstable acid that can protonate just about any other molecule)
What is the different between the cations of direct electron impact versus chemical ionization mass spectroscopy?
Direct electron impact: radical cation produced by removing an electron by bomardment

Chemical ionization: simple cation produced by adding a proton from something like CH5+
What aren't exact masses whole integers?
There is a slight mass different between a proton and a neutron
How can different atomic compositions for the same molecule weight be distinguished?
If we know the molecule weight, it can be determined, since individual isotopes have non-integral masses; mass measurements are called high resolution mass spectrometry
Of C, O, and N, which is the only one that can form an odd number of bonds?
How does nuclear magnetic resonance work?
1. Sample of unknown compound is dissolved in a suitable solvent and put in a very strong magnetic field. Any atomic nuclei with a nuclear spin now have different energy levels.

2. Sample is irradiated with a short pulse of radiofrequency energy, which disturbs the equilibrium balance between the two energy levels ~ some nuclei absorb the energy and are promoted to higher energy levels

3. Detect the energy given out when the nuclei fall back down to the lower energy levels using what is basically a sophisticated radio receiver

4. Results are displayed in the form of intensity against frequency
What is the unfortunate thing about NMR?
Energy difference between the nuclear spin being aligned with the magnetic field and against it is very small - so small that we need a very, very strong magnetic field to see any difference.
Since the energy different between the two states in NMR is so small, what can be used to flip the nucleus?
Radio waves
What are the two axises of the NMR results?
Frequency vs. intensity
What are the two factor that affect the energy difference, (and, therefore, the frequency), in NMR?
Magnetic field strength and what sort of nucleus is being studied
When is a carbon more shielded - when it is attached to three H or two H and an OH?
When it is attached to the three H, because the electrons are not pulled away from the outside
Does a shielded or deshielded atom feel more force from the applied magnetic field? Which one resonates at a higher frequency?
deshielded atom ~ resonates at a higher frequency
In NMR, which is the precise value of the operating frequency?
Frequency at which a standard reference sample resonates
What is the reference sample usually used in NMR?
Tetramethylsilane, (TMS)
What is the formula for chemical shift?
frequency - frequency TMS / frequency TMS
How does the scale for chemical shift run?
Runs to the left from zero, where TMS resonates
Whenever you first look at a spectrum, what is the first thing you should do?
Identify the peaks due to the solvent
When a carbon atom is more shielded than those in TMS, what is the sign of the TMS value?
How are functional groups identified?
Infrared spectra
Which functional groups can be determined by NMR, rather than infrared spectra?
C=O and C=C, because they contain carbon atoms; OH, because it influence chemical shifts of carbon atoms.
How does the infrared spectra identify functional groups?
Detects the stretching and bending of bonds rather than any property of the atoms themselves.
What type of bonds is infrared spectra particularly good at detecting?
Unsymmetrical bonds
Why is infrared used in infrared spectrometry?
Energy needed to stretch bonds falls in the infrared spectra
How is the infrared spectra recorded?
Recorded using a frequency measurement called wavenumber (cm^-1)
How is the sample prepared for the infrared spectrometry?
Either dissolved in a solvent, such as CHCl3, that has few IR absorptions, pressed into a transparent disc with powdered solid CBr, or ground into an oily slurry called a mull with a hydrocarbon oil called 'Nujol.'
What does Hooke's Law say frequency of infrared spectrometry depends on?
Mass and a force constant, (bond strength)
If a bond stretches at a higher frequency, does that mean it's weaker or stronger?
Do antisymmetrci vibrations require more or less energy? Do they vibrate at a higher or lower frequency?
More energy; higher frequency
If the absorption is lower, is the H bond stronger or weaker?
How does dipole play into infrared spectrometry?
For bonds between unlike atoms, the larger the difference in electronegativity, the greater the dipole moment, and the more it changes when stretched.
For the absorptions in Ir spectra, what does the position of the band depend on?
Reduced mass of atoms and bond strength
For the absorptions in Ir spectra, what does the strength of the band depend on?
Change in dipole moment
For the absorptions in Ir spectra, what does the width of the band depend on?
Hydrogen bonding
What are strong peaks called, which are created by the bending of bonds, (particularly C-H and N-H)?
What are double bonds of all kinds and rings called?
Double Bond Equivalents, (DBEs)
What are the 6 criteria for working out the DBEs of an unknown compound?
1. Calculate the expected number of Hs in the saturated structure.
2. Subtract the actual number of Hs and divide by 2 to give the DBEs.
3. If there are other atoms, it is best to draw a trial structure
4. If there are few Hs, suspect a benzene ring
5. A benzene ring has four DBEs
6. A nitro group has one DBE only