NMR Lecture 5 Flashcards
What does a spin-echo pulse sequence do?
refocuses magnetizations of nuclei regardless of their chemical shift difference
Understand the general scheme of 2D NMR.
1) Preparation time: sample is excited by one or more pulses.
- resulting magnetization is allowed to evolve for the first time period, t1
2) Mixing time: more pulses allow magnetization to transfer
- the signal is recorded as a function of the second time variable, t2
3) Resulting fourier transformed twice to yield a spectrum as a function of two frequency variables
What is the advantage of 2D NMR over 1D NMR?
1) 2D NMR provides enhanced spectral resolution, allowing for the separation of overlapping peaks and facilitating the identification of individual signals
2) 2D NMR allows for the correlation of signals between different nuclei within a molecule. Cross-peaks in the spectrum represent correlations between specific pairs of resonances
3) COSY provides information about scalar coupling between nuclei
4) 2D NMR can reveal signals that may be hidden or obscured in 1D spectra. Cross-peaks in 2D spectra can highlight signals that are not easily discernible in 1D NMR.
5) NOESY in 2D NMR provides information about spatial proximities.
What respective NMR information does the diagonal peaks and cross peaks of 2D 1H COSY provide?
A diagonal peak represents the resonance of a specific proton at a certain chemical shift correlated with itself.
Cross peaks in COSY indicate a correlation between two protons that are directly coupled to each other. The position reveals the chemical shifts of both protons involved in the correlation and the relative position indicates which proton is in the x or y-axis. The intensity of the cross-peak is related to the strength of the coupling between the two protons.
What respective NMR information does the diagonal peaks and cross peaks of 2D 1H TOCSY provide?
Each diagonal peak corresponds to the resonance of a specific proton at a certain chemical shift.
TOCSY provides long-range coupling information, allowing the observation of correlations between protons that are not directly bonded but are coupled through multiple bonds. The intensity of a cross-peak is related to the strength of the coupling between the correlated protons.
In comparison with 2D 1H COSY, what additional information could 2D 1H TOCSY provide?
TOCSY provides additional information about spin systems and connectivity in a molecule.
1) Long-range coupling information, allowing for the observation of correlations between protons that are not directly bonded but are coupled through multiple bonds.
2) helps establish complete spin systems by connecting protons through all intervening bonds. It reveals correlations throughout the entire spin network, offering a more comprehensive view of connectivity.
3) displays cross-peaks for all protons that are coupled through j-couplings, providing a more inclusive representation of spin systems.
What does “spin system” mean?
It refers to a set of coupled nuclear spins that interact with each other through scalar (J) couplings. It’s a group of nuclear spins in a molecule that are interconnected by through-bond coupling interactions.
What respective NMR information does the diagonal peaks and cross peaks of 2D 1H NOESY provide?
Each diagonal peak corresponds to the resonance of a specific proton at a certain chemical shift.
The presence of a cross-peak suggests spatial proximity between the protons involved. The intensity of the cross-peak is related to the degree of spatial proximity and the strength of the NOE.
What information does the intensity of the NOESY cross peaks provide?
The intensity of NOESY cross-peaks is directly related to the magnitude of the NOE.
1) Strong intensity: protons will strong NOEs are likely to be in close contact
2) Weak intensity: weaker NOEs may be located at a greater distance from each other in the molecular structure
3) Zero intensity: absence of NOE implies unfavorable orientation for dipolar interactions or the protons are too far apart.
4) Structural insights: used to infer the relative positions of different protons in space, helping to determine molecular conformations and structural features
In a transient NOE experiment, what is the drawback if the mixing time τm is too short or too long?
1) Too short: not enough time for the nuclear spins to undergo significant relaxation and establish strong interactions.
- weak NOE effects, making it challenging to observe and quantify interactions
2) Too long: risk of over-saturation and depletion of nuclear spin polarization, leading to reduced NOE signals
- signal decay, excessive relaxation and loss of coherence among nuclear spins
- reduced sensitivity, depletion of nuclear spin populations
- distorted NOE effects: spin diffusion effects
What is spin diffusion?
involves the transfer of nuclear spin polarization between neighboring nuclear spins through dipolar interactions.
