Shared Flashcard Set

Details

Video 13-15
2D NMR COSY-nOe effect
27
Chemistry
Graduate
10/23/2018

Additional Chemistry Flashcards

 


 

Cards

Term
Selective population inversion
Definition
trick to improve sensitivity, C-H irradiate 13C satellites in the 1H
Term
INEPT:
Definition
General way to invert 1H populations and enhance intensities
Term
Polarization transfer in INEPT occurs because:
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
Term
DEPT experiment
Definition
labeling of carbons occurs because pulse angle matching CH, CH2, CH3 gives different intensitives where tau(d) is 1/2J
Term
What is so important for DEPT with tau(d) to be 1/2J
Definition
insensitive to variations in tau(d), sensitive to pulse with feta(y) but this is easy to calibrate
Term
Double-quantum-filtered COSY
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
Term
DQF COSY useful for:
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
Term
Is symmetrization recommended for DQF-COSY?
Definition
NO
Term
Why is symmetrization not recommended?
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doesn't show the true quality of the experiment (not recommended for publication).
Term
nOe:
Definition
through space interactions
Term
What is Nuclear Overhauser Effect?
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.
Term
Intensity of nOe falls off with distance as
Definition
1/r^6
Term
equation for ni(s)
Definition
(I-I0)/I0*100%
Term
nOe at nucleus i when nucles s is saturated is the difference between...
Definition
the intensities of the unperturbed signal Io and the pertrubed signal divided by Io
Term
What happens when you saturate only the S transitions >
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!
Term
What is happening to W0 when we are no longer at equilibrium?
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.
Term
happening to W2 when we are no longer at equilibrium?
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
Term
Small molecules, non viscous solvents (long T1):
Definition
W2 is important +nOe
Term
Large molecules, or viscous solvents (short T1)
Definition
W0 is important - nOe
Term
Molecular correlatioon time constant gross approxiamation for random motions in a single parameter includes:
Definition
MW, solution viscosity, temperature, H-bonding
Term
NOE interpretation precautions
compare only similar sized molecules
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
Term
NOE interpretation precautions
restricted to rigid molecules..
Definition
typically cyclic systems or folded proteins or DNA
Term
NOE interpretation precautions
assumption of extreme narrowing has been used
Definition
so small molecules are best justified
Term
NOE interpretation precautions
should be no difference in
Definition
intramolecular interactions if you want to quantitate cross- peaks, however this is usually impossible, so just look for presence of an nOe
Term
NOE interpretation precautions
interaction of nucleus of interest
Definition
with the solvent must be the same from one molecule to another, so one should not be buried and the other exposed
Term
NOE interpretation precautions
must exclude
Definition
O2 and other paramagnetic species
Term
nOeis a direct result of...
Definition
relaxation processes; it can be optimized by waiting a time-period equal to T1 for nOe to build-up before acquiring.
Supporting users have an ad free experience!