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32 Cards in this Set
- Front
- Back
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Spectroscopy
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Measures the energy differences between the different energy states of a molecular system. By determining the frequencies of electromagnetic radiation, spectroscopy can accurately identify unknown compounds.
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IR Spectroscopy
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Measures molecular vibrations which are the combination if different vibrational modes which put in other terms is simply the BOND STRETCHING OR BENDING of a molecule
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Wavenumber
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The analogue of frequency that is used to represent the frequency of the light given off by a molecule
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What is the purpose of NMR?
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To deduce the structure of a molecule
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How does NMR work?
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Magnetic field is created. Radio waves are directed to the molecule. At a certain magnetic field, the nucleus of a molecule will absorb radio waves. Based on the absorption we can deduce which nuclei are present and which are not
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What do the peaks on a graph mean?
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They refer to the absorption at a given magnetic field.
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Fingerprint Region
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When a molecule absorbs radiation, its bonds will stretch and bend at different vibrations. The combination of these vibrations is characteristic of a given molecule which results in a characteristic absorption reading. The region that gives the information about a given molecule is known as the fingerprint region
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What is absorption based on?
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It is based on molecular vibrations which are themselves based on changing dipole moments.
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Can you do NMR on a single type of molecule?
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No, because there is no changing dipole on the molecule
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What happens to the frequency as you add bonds to the molecules?
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The frequency goes up
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What is absorption frequency of anything with an OH group?
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3,300 cm -1 with a broad peak. this is an approximation not an exact number
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What is the absorption of a carbonyl group?
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1, 700 cm -1 with a sharp peak. this is an approximation not an exact number
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TMS
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It is the reference peak used during NMR spectroscopy. It is an arbitrary point used to set a standard for varying absorptions. The TMS point is designated and the difference between that point and the point given by spectroscopic analysis correspond to a given nucleus.
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Chemical Shift
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It is the difference between the TMS point frequency of absorption point given by the graph. It is given in units of Parts Per Million which simply represents frequency.
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Down Field
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Moving towards the left on the graph. Represents levels of high frequency absorption.
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Up Field
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Moving to the right on the graph. Represents levels of low frequency absorption
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What do the different peaks on an H-NMR graph represent?
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The different peaks represent different resonances (absorption) of different H molecules. All the equivalent Hs throughout the molecule will resonate at the same frequency. The difference is the resonance patter of the different H atoms in a molecule is due to the different connectivity of the H molecules. The H atoms on a CH3 group will have a difference magnetic environment than that molecules on a NH2 group and as a result will resonate at different frequencies.
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Shielding vs Deshielding
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An H atom that is attached to a electronegative group will have its electrons pulled from it given the pull of the electronegative atom. The fact that the H atom has less of an electron could means that it will resonate at higher frequencies - DOWN FIELD. This is known as deshielding. In contrast, it an H atom is attached to an electron donating group, the electron cloud of the H atom will be enhanced thus shielding it from the radio waves. This results in a resonance peak at lower frequency- UP FIELD. This is known as shielding
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Coupling (Splitting)
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When there are two H atoms that are THREE BONDS apart (two atoms) then the magnetic field of one will affect the magnetic field of the other. What does this mean? This means that when an absorption spectrum is taken, one given proton will have a number of peaks depending on how many H atoms are within a three bond range. If there is one H atom, there will be 2 peaks. The presence of another H atom will move the absorption peak both up and down field with the true absorption at the center.
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n+1 Rule
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This rule will tell us how many absorption peaks there will be for a given H atom. n is the number of H atoms in a three bond radius.
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Magnitude of the splitting between the H atoms
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Coupling Constant, J
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Triplet, Doublet, and Multiplet
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These terms refer to the number of peaks that you find a given H atom relative to the number of H atoms that affect its magnetic environment. There can be 2, 3, and multiple peaks.
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Peak for Alkyl group
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1 to 3 ppm
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Peak for Alkene
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4 to 7 ppm
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Peak for Aldehyde
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9 to 10 ppm
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Can a sample that has undergone Mass Spectroscopy be reused?
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No. Mass spectroscopy destroys the sample
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How does Mass Spectroscopy Work?
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A beam of electrons are directed in a sample. This will, ironically, eject an electron creating an M+ ion (just a cation of the original molecule). This molecule will generally decompose into a number of different cationic and radical fragments. A detector records the mass and charge of fragments
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Mass/Charge Ratio
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A mass spectrographic analysis has a mass to charge ratio on the X axis and the abundance of those fragments on the Y axis. The Mass to Charge ratio is simple a number ratio of the mass of the fragments of the original molecule and charge of those fragments.
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Base Peak
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The peak that corresponds to the fragment with the highest abundance (greatest intensity).
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Molecular Ion Peak - Parent Ion Peak (M+)
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This is the peak that has the highest mass/charge (m/e) ratio which is the peak that is the furthest to right on the graph. The designation 'Parent Ion' makes sense because this is the fragment that has the highest mass and represents the greatest portion of the original molecule.
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What can the parent ion peak be read as?
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The parent ion peak can generally be read as the mass of the molecule itself.
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A Parent Ion with a peak of 72 m/e means what?
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It means 72 g/mol
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