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14 Cards in this Set
- Front
- Back
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True or False: IR and NMR spectroscopy are emittance spectroscopy.
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False, they are absorption spectroscopy.
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What do moecules prefer: ground state or excited state?
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They tend to prefer their ground state rather than an excited state, but in order for them to return to their ground state, they must lose the energy they have gained.
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What occurs in absorption spectroscopy?
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- scientists induce the absorption of energy by a sample of molecules by exposing the same to various forms of light, thereby exciting molecules to a higher energy state.
- they then measure the energy released as teh molecules relax back to their ground state. - energy released can reveal structural features of the molecules in the sample. |
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What EM radiation wavelengths have the proper energy to cause bonds in organic molecules to become vibrationally excited during IR spectroscopy? What occurs?
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2.5 to 20mm;
Their covalent bonds will begin to vibrate at distinc energy levels (wavelengths, freq) within this region. |
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IR spectroscopy:
What is a wave number? What is it proportional to? |
wave # = 1 / lambda = v / c
(v is frequency here) It's directlhy proportional to both the frequency and the energy of the radiation. ==> The higher the wavenumber, the higher the frequency and the greater the energy. |
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How does hz relate to angular frequency?
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w=2 pi v
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Low transmittance correpsonds to what?
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absorbance
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IR spectroscopy: carbonyl group.
Centered where? Characteristics? What type is this? |
- 1700 cm^-1
- very strong and sharp / intense. - double bond stretch |
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IR spectroscopy: alkenes
Centered where? what type is this? |
- centered around 1650 cm^-1.
- double bond stretch |
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In any IR spectrum, what should you look for first?
Why? |
carbonyl double bond stretch;
- if its not present, you can eliminate a wide range of compounds that contain a carbonyl group inclduing aldehydes, ketones carboxylic acids, acid chlorides, esters, amides, and anhydrides. |
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If the carbonyl stretch is present in IR spectrum, what are the possibilities?
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contains a carbonyl group inclduing aldehydes, ketones carboxylic acids, acid chlorides, esters, amides, and anhydrides.
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IR spectroscopy: Triple bond stretch
- centered where? |
C tripled bonded to C
or C triple bonded to N - 2600-2100 cm^-1 |
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IR spectroscopy: O-H stretch
- characteristics? |
- OH and R-OH
- O-H stretch is strong and very broad. - broadness is reflected as a wide U shaped appearance on the absorption specturm, as opposed to a V or spiked shape. - broadness is due to H bonding. - centered 3600-3200 cm^-1. - amines also have stretches in this region. |
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IR Spectroscopy: C-H for sp^3, sp^2, sp carbon.
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3000 to 2850 cm^-1 for sp^3 carbon
3150 to 3000 cm^-1 for sp^2 carbon - aliphatic C-H bonds stretch at wavenumbers a little less than 3000 cm^-1 and aromatic C-H bonds stretch at wavenumber slightly greater than 3000 cm^-1. 3300 cm^-1 for sp carbon |
