Term
| Selective population inversion |
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Definition
| trick to improve sensitivity, C-H irradiate 13C satellites in the 1H |
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Term
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Definition
| General way to invert 1H populations and enhance intensities |
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Term
| Polarization transfer in INEPT occurs because: |
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Definition
evolution time (tau)delay is related to couplings (J)
if intensities are off it is due to tau(d) being imperfect
experiment is very sensitivities to the delay time |
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Term
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Definition
| labeling of carbons occurs because pulse angle matching CH, CH2, CH3 gives different intensitives where tau(d) is 1/2J |
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Term
| What is so important for DEPT with tau(d) to be 1/2J |
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Definition
| insensitive to variations in tau(d), sensitive to pulse with feta(y) but this is easy to calibrate |
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Term
| Double-quantum-filtered COSY |
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Definition
some magnetization arises due to W2 pathways, can be filtered out with a third pi/2 pulse after a fixed delay
removes singlets from spectrum
avoids dispersive diagonal peaks that interfere with cross-peaks near diagonal
only 50% of intensity detected, so use only when strong singlets or peaks near diagonal present |
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Term
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Definition
strong methyl singlets that overlap peaks of interest
sugars: cross peaks mostly near diagonal due to low dispersion of frequencies
Geminal couplings near diagonal, but don't need that information |
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Term
| Is symmetrization recommended for DQF-COSY? |
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Definition
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Term
| Why is symmetrization not recommended? |
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Definition
cosmetic purposes only
scorned by NMR-jocks
takes two points on opposite sides of diagonal and substitutes the smaller of the two.
removes a lot of noise, but can create false crosspeaks at the intersection of two noise streaks
weak peaks can be lost entirely
doesn't show the true quality of the experiment (not recommended for publication). |
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Term
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Definition
| through space interactions |
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Term
| What is Nuclear Overhauser Effect? |
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Definition
Nuclei close to each other through space show cross-relaxation due to the interaction of the two dipoles
Nuclei may be coupled through bonds (scalar or J-coupled) but the nOe effect occurs because they are close in space. |
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Term
| Intensity of nOe falls off with distance as |
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Definition
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Term
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Definition
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Term
| nOe at nucleus i when nucles s is saturated is the difference between... |
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Definition
| the intensities of the unperturbed signal Io and the pertrubed signal divided by Io |
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Term
| What happens when you saturate only the S transitions > |
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Definition
| no longer at equilibrium, if only single quantum pathways are avaliable, saturation of s does not affect I, but W0 and W2 (zero and double quantum) pathways are available if nuclei are close in space! |
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Term
| What is happening to W0 when we are no longer at equilibrium? |
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Definition
| W0 transfers population from (b)(a) to (a)(b) to try to restore the difference to zero. But then this in creasing the population of the top level (a)(b) relative to (a)(a). So now an I transition is out of whack, so the net rate of relaxation will depend both on W0 and W1i and there is a negative nOe at I due to S irradaiton. |
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Term
| happening to W2 when we are no longer at equilibrium? |
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Definition
| W2 transfers from (b)(b) to (a)(a) to try to restore the difference of 4 which is in turn decreasing the population of the top of one of the I transitions and increasing the population of the bottom of the other one, thereby increasing the intensity of signal due to I |
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Term
| Small molecules, non viscous solvents (long T1): |
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Definition
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Term
| Large molecules, or viscous solvents (short T1) |
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Definition
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Term
| Molecular correlatioon time constant gross approxiamation for random motions in a single parameter includes: |
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Definition
| MW, solution viscosity, temperature, H-bonding |
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Term
NOE interpretation precautions
compare only similar sized molecules |
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Definition
| compare only similar sized molecules same conditons of solvent, temperature and concentration so that tau(c) is roughly the same, best to consider diasteromers |
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Term
NOE interpretation precautions
restricted to rigid molecules.. |
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Definition
| typically cyclic systems or folded proteins or DNA |
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Term
NOE interpretation precautions
assumption of extreme narrowing has been used |
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Definition
| so small molecules are best justified |
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Term
NOE interpretation precautions
should be no difference in |
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Definition
| intramolecular interactions if you want to quantitate cross- peaks, however this is usually impossible, so just look for presence of an nOe |
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Term
NOE interpretation precautions
interaction of nucleus of interest |
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Definition
| with the solvent must be the same from one molecule to another, so one should not be buried and the other exposed |
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Term
NOE interpretation precautions
must exclude |
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Definition
| O2 and other paramagnetic species |
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Term
| nOeis a direct result of... |
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Definition
| relaxation processes; it can be optimized by waiting a time-period equal to T1 for nOe to build-up before acquiring. |
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