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15 Cards in this Set

  • Front
  • Back
why is it pointless to perform carbon decoupled 1H NMR
b/c the C13 has little efect on signal of the protons due to low nautural abundance
List 3 advantages of porton decoupled 13 C NMR
increases sensitivity (NOE)
faster T2 decrease relaxation time
simplified spectra

increases resolution
incease S/n ratio
2 disadvatnages to proton decoupled 13 C nmr
no conectivity data

no quantative data
how does noe affect an nmr signal?
in spin decoupling
1)it incraeases # of spin states on low energy side by cross polarization & increases intensity
2) it decreases T1 time
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?
amplitude increaes
vib frequency & interatomic distance staty the same
based on this diatomic harmonic oscillator mode, whichof the 2 stretching vibrations (c-h or c-cl) would vibrate at a higher frequency? why?
C-h would have a higher frequ b/c cl is a heavier mcule and therfoe would not vibrate as much
how do vibrationally excited mucles usually return back to their lowest energy state (v=o)?
give off thermal energy as heat
in ir what are we probing?
molecular vibrations
what kind of vibrations in ir?
stretching & bending
symmetric twisting wag rocking
whats important about dipole w/ir w/distance
w/distance b/t charge seperation creates dipole & magnitude dependant on seperation
why is anharmonic model more realistic than harmonic?
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
why are observed vibrational freq of funct groups often observed to be diff from that predicted w/harmonic oscillator mode?
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
vibrational coupling adds to complexty but helps w/fingerprinting of materials by 3 ways
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
what is uniform periodic motion explotied in ir spectroscopy? in raman spectroscopy?
vibrational motion for both
what conditoin must be met to observe the interacion f the electromagnet radiation w/uniform eriodic moition in ir spect? in raman spect?
ir-change in dipole moment
raman-change in polarizabiltiy