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15 Cards in this Set
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why is it pointless to perform carbon decoupled 1H NMR
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b/c the C13 has little efect on signal of the protons due to low nautural abundance
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List 3 advantages of porton decoupled 13 C NMR
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increases sensitivity (NOE)
faster T2 decrease relaxation time simplified spectra increases resolution incease S/n ratio |
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2 disadvatnages to proton decoupled 13 C nmr
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no conectivity data
no quantative data |
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how does noe affect an nmr signal?
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in spin decoupling
1)it incraeases # of spin states on low energy side by cross polarization & increases intensity 2) it decreases T1 time |
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assuming a simple harmonicoscillation what happens to the maximum vibrational amplitude, vibrational frequ, & average interatomic distnace of a simple diatomic mcule when it absorbs IR light?
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amplitude increaes
vib frequency & interatomic distance staty the same |
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based on this diatomic harmonic oscillator mode, whichof the 2 stretching vibrations (c-h or c-cl) would vibrate at a higher frequency? why?
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C-h would have a higher frequ b/c cl is a heavier mcule and therfoe would not vibrate as much
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how do vibrationally excited mucles usually return back to their lowest energy state (v=o)?
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give off thermal energy as heat
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in ir what are we probing?
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molecular vibrations
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what kind of vibrations in ir?
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stretching & bending
asy symmetric twisting wag rocking |
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whats important about dipole w/ir w/distance
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w/distance b/t charge seperation creates dipole & magnitude dependant on seperation
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why is anharmonic model more realistic than harmonic?
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the anharmonci takes into acount the distance of the bond & that the bond may break
also when energy levels get higher, the levels get closer together so you get overtones |
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why are observed vibrational freq of funct groups often observed to be diff from that predicted w/harmonic oscillator mode?
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the harmoci oscilator is ideal model & not all funct groups adhere to it. Some mcules may have high high repulsion for 1 antoher & want to break a bond. Also b/c of vibrational coupling
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vibrational coupling adds to complexty but helps w/fingerprinting of materials by 3 ways
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1.b/t stretching vib have to have an atome in common
2.b/t bending vib must have to have a comon bond to be coupled 3.b/t stretching & bending vib the stretch bond form 1 side of angle in bend bond |
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what is uniform periodic motion explotied in ir spectroscopy? in raman spectroscopy?
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vibrational motion for both
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what conditoin must be met to observe the interacion f the electromagnet radiation w/uniform eriodic moition in ir spect? in raman spect?
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ir-change in dipole moment
raman-change in polarizabiltiy |
