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24 Cards in this Set
- Front
- Back
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What equation can be used to find the frequency or wavelength of an electromagnetic wave?
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speed of light= wavelength x frequency
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What equation can be used to determine the amount of energy in a mole of photons?
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Energy = Rydbergs constant x frequency
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What is the premise for molecular spectroscopy?
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Introduce a range of IR waves over a sample and measure which waves are absorbed and emitted. This absorption spectrum corresponds with vibrational energy transitions from lower to higher back to lower energy levels
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What is the vibrational IR region?
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2.5 micrometers to 25 micrometers
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The top of an IR spectrum indicates ____ transmittance?
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100%
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The bottom of the IR spectrum indicates ___ transmittance?
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0%
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Are ionic bonds seen on an IR spectrum?
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No
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What must occur for a molecule to absorb radiation?
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A substantial change to the bond dipole moment
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How many fundamental vibrations exist for a nonlinear molecule?
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3n-6
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How many fundamental vibrations exist for a linear molecule?
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3n-5
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What types of bonds are generally IR inactive?
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Ionic and non-polar covalent bonds in a molecule with symmetry such as CCl4
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What equation is used for the frequency of vibration between two atoms?
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wavenumbers = 4.12 * square root (K/u)
where K is force constant in dynes/cm (single bond=5E5, double=10E5, triple=15E5) and u is the reduced mass + (m1m2)/(m1+m2) |
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A stronger bond will move the IR band ____?
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Left
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What is one very unique IR band in aromatics?
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690-900
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How does hydrogen bonding shift the IR spectrum in alcohols?
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Shifts right
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N-H bonds in amines typically show up as?
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1-2 peaks around 3300-3500
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How are wavenumbers calculated?
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1 / wavelength in cm -or-
Frequency / Speed of light |
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What distinct peak appears in aldehydes?
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2720
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How does conjugation shift an IR spectrum?
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Shifts right
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What are the steps in trying to decipher an IR spectrum?
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1. Check around 3000 to determine alkane, alkene, alkyne
2. Check 3500 for OH 3. Check 1700 for C=O 4. 1-2 peaks around 3500 could mean amine |
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How do you determine if a molecule is an ether?
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1. Determine if there is a C-O bond
2. Rule out C=O which excludes carboxylic acids 3. Rule out O-H 4. This rules out all other possible molecules besides ethers |
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How was the equation for determining the number of fundamental vibrational frequencies of a non linear molecule established (3n-6)?
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Each atom (n) can move in the 3 planes of space thus giving 3n. Then 3 possibilities are removed to account for translational motion and an additional 3 possibilities are removed to account for rotation
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How are vibrational frequencies related to IR frequencies in spectroscopy?
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When the energies necessary to increase the vibrational frequency from one quantized state to the next is exactly equal to the energy provided by a particular IR frequency, the molecule will absorb a portion of the radiation and will be plotted on the spectra.
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What is the fundamental reason an O-H bond is to the left of an N-H bond or a C-H bond?
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The bond length is shorter and therefore stronger, thus requiring greater energy to change it's frequency state
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