Lecture 5: 2D NMR Spectroscopy Flashcards
What does a spin echo pulse sequence do?
-Refocuses magnetization of nuclei regardless of their chemical shift difference.
-As the net magnetic moment precesses, some spins slow down while others speed up. This makes the net magnetization spread.
~A 180 degree pulse is then applied so that the slower spins lead ahead of the main moment and fast spins trail behind.
~Progressively, the fast moments catch up with the main moment and the slow moment drift back toward the main moment.
Understand the general scheme of 2D NMR.
-In 2D NMR, the signal is recorded as a function of two time variables, t1 and t2, and the resulting data Fourier transformed twice to yield a spectrum which is a function of two frequency variables.
~In the first period, called the preparation time, the sample is excited by one or more pulses. The resulting magnetization is allowed to evolve for the first time period t1.
~In the second period, the mixing time, which consists of further pulses to allow the magnetization to transfer. After the mixing period, the signal is recorded as a function of the second time variable, t2.
-Many FIDs are recorded, each at a different t1 between pulses–each provide a 1D NMR spectrum.
-The time-variation of the maximum amplitude in the 1D NMR spectra is subjected to a Fourier Transform. The Fourier transform in the 2nd dimension.
What is the advantage of 2D NMR over 1D NMR?
-Although 1D NMR is sufficient to observe distinct peaks for the various functional groups of small molecules; but for larger molecules many overlapping resonance can make an interpretation of an NMR spectra difficult.
-2D NMR, however, allows one to circumvent this challenge by adding a second dimension; thus providing a better way to resolve signals.
~1D NMR has 2 pulses with time (t) in between each pulse and the Fourier Transform of the outgoing signal results in a 1D spectra as a function of chemical shift.
~A 2D experiment adds an additional dimension to the spectra by varying the length of time (t) the system is allowed to evolve after the first pulse. The result is an outgoing signal (t, t2), which when Fourier transformed, gives a 2D spectrum.
What respective NMR information does the diagonal peaks and cross peaks of 2D 1H COSY provide?
The cross peaks in the 2D 1H COSY spectrum provide connectivity and J coupling constant between two covalently-linked 1H nuclei. The J coupling constant needs to be large enough to be observed.
What respective NMR information does the diagonal peaks and cross peaks of 2D 1H TCOSY provide? In comparison with 2D 1H COSY, what additional information could 2D 1H TCOSY provide?
-Cross peaks and diagonal peaks are observed not only for nuclei which are directly coupled, but also between nuclei which are connected by a chain of couplings.
is-this makes it useful for identifying the larger interconnected network of spin couplings.
What does spin system mean?
- Spin system: is a group of nuclei connected by J couplings.
- For instance, for multinuclear NMR 1H NMR, 13C, and 15N of arginine.
What respective information does the diagonal peaks and cross peaks of 2D 1H NOESY provide?
-The cross peaks and diagonal peaks connect resonances from nuclei that are spatially close rather than those that are through-bond coupled to each other.
What information does the intensity of the NOEST cross peaks provide?
Strong NOESY peaks suggest protons are in close proximity (2A) apart. Weak peaks suggest protons are far apart (more than 5A). Because of 1/r^6 dependence, generally no NOESY peaks seen for protons further than 5-6 A apart.
In a transient NOE experiment, what is the drawback if the mixing time for tm is too short or too long?
-At longer mixing times, the NOE peaks are disappearing due to:
~Spin diffusion: describes a situation wherein the individual spins undergo continuous exchange of energy.
~The system relaxes back to equilibrium.
-Short mixing times will not allow the system to interact long enough for an accurate measurement.
What is spin diffusion?
Describes a situation wherein the individual nuclear spins undergo continuous exchange of energy.
