Reading Notes for FT-NMR Articles


Van Bramer, S.E. Chemical Shifts in NMR, General. In The Encyclopedia of Analytical Chemistry, Myers, R. Ed; Wiley: New York, 2000 (In Press).

This is a chapter on calculating proton and carbon chemical shifts. It has lots of tables and examples. Look through it carefully and then use it as a reference. This article shows you how to verify the chemical shifts for a compound.


Lambert, J.B.; Shurvell, H.F.; Lightner, D.A.; Cooks, R.G. Organic Structural Spectroscopy; Prentice-Hall: Upper Saddle River, NJ, 1998

  1. Chpater 2, Indroduction and Expermental Methods. This chapter desribes the basics of NMR, read it very carefully. Understanding this chapter is critical for understanding how to setup the spectrometer to obtain quality spectra.
  2. Chapter 3, The Chemical Shift. The chemical shift is one of the clues that a NMR spectrum provides for the identification of a moleucle. Section 1 discusses what effects proton shifts, read this carefully. Section 2 discusses the proton shifts of different functional groups, skim this section and use as a reference. Chapters 3 & 4 are on carbon chemical shifts, read section 3 carefully and skim section 4. Section 5 is a set of tables for calculating chemical shifts. Note this is greatly expanded upon in Van Bramer, S.E. Chemical Shifts in NMR, General. In The Encyclopedia of Analytical Chemistry, Myers, R. Ed; Wiley: New York, 2000.
  3. Chapter 4, The Coupling Constant. This is another clue that the NMR spectrum provides. The concept is initially very simple, however, it becomes much more complex for nuclei that are not magnetically equivilent. Stereochemistry is important in this chapter, use your models from organic chemistry to help understand this chapter. This chapter goes into more depth than is required for the samples in this course, so read carefully but don't get bogged down.
  4. Chapter 5, Further Topics in One-Dimensional NMR. This chapter includes a variety of other topics that are important in really understanding NMR spectroscopy at a level needed to obtain high quality spectra and to get the most information about a molecule. Read this chapter carefully.
  5. Chapter 6, Two Dimensional NMR. Read this chapter carefully to fully understand the information provided by 2D NMR spectra.


Experiments

This listing of basic NMR experiments should serve as a useful reference.
  1. Homo J Resolved Spectroscopy. These are techniques for studying the J values (Coupling constants). They are either homonuclear or heteronuclear (coupling between the same element or coupling between different elements).

  2. COSY H-H correlation spectroscopy. Shows connections (coupling) between protons to determine which protons are coupled to which.

  3. APT Attached Proton Test. This is an almost bizarre experiment, but it produces a C-13 spectrum where the peaks for carbons with 0 or 2 protons point up, but the peaks for carbons with 1 or 3 protons point down.

  4. DEPT Distortionless Enhancement by Polarization Transfer. This is a series of three experiments used to determine the number of protons attached to a carbon. Because this is a "Polarization Transfer" experiment the S/N is very good.
    1. DEPT 45 - All carbons with attached protons are observed with + peaks (quaternary carbons are not observed).
    2. DEPT 90 - Only CH carbons are observed.
    3. DEPT 135 - methyl (CH3) and methyne (CH) carbons are up. Methene (CH2) carbons are down. From this you can identify quaternary carbons (not observed in DEPT), CH carbons (DEPT 90), CH2 carbons (Down in DEPT 135), and CH3 carbons (remaining).

    4. HETCOR (Heteronuclear Correlation Spectroscopy). Shows connections (coupling) between nuclei of different elements, ie: C-H coupling to show which protons are coupled to which carbons.

    5. INADEQUATE (Carbon Carbon correlation). This technique shows which carbon is coupled to which. This is very difficult because the natural abundance of C-13 is very low, so the chance of two adjacent C-13 is EXTREMELY low.

    6. NOESY (Nuclear Overhauser Effect Correlation). This technique shows which nuclei are close to each other. Unlike COSY which only shows which H are coupled to which H, NOESY shows if they are nearby in three dimensional space.


    Supplemental Resources

    This series of articles appeared in the Journal of Chemical Education. They provide an excellent and through introduction to FT-NMR. Much of this information is also available in your textbook, but these articles have a more through discussion of instrumentation and data handling.
    1. The Fourier Transform in Chemistry. Part 1. Nuclear Magnetic Resonance: Introduction. (King, R.; Williams, K. J. Chem. Educ. 1989, 66, A213-A219). This article introduces the basics of why nuclei are magnetic, how they behave in a magnetic field, and how they are probed, and how the signal is detected.

      1. Fundamental Concepts of Magnetization. This section introduces the idea that nuclei have a magnetic moment and that if a nuclei is placed in a magnetic field it will precess (like a wobbling top). The frequency of this precession (the Larmor frequency) depends upon the nuclei and the strength of the applied field. For nuclei with spin 1/2 (H-1 and C-13 are most common) there are two different states, spin up and spin down. Equation 4 gives the energy difference between these two states. Equation 5 converts the energy to a frequency of radiation. This frequency corresponds to the frequency of the precession. The table in Figure 2 gives the magnetogyric ratio and the larmor frequency for different nuclei in a 7 Tesla magnet (This is the size we have at Widener). Figure 2 also shows how a group of nuclei all line up. Adding all these vectors together produces the "Net Magnetization" pointing up in the center. An important concept is that there are many nuclei precessing, but they are spread out around the circle (random phase)

      2. The RF Magnetic Field. This is how the NMR signal is produced. A second magnetic field is briefly applied (Using a RF coil, remember that electro-magnetic radiation can also act like a magnet). This causes nuclei to spin flip (Up goes down and down goes up). Because the nuclei that flip must be in phase with the applied RF field, the nuclei are no longer spread out around the circle. Now they are in phase. This has the effect of tipping the net magnetization vector. Now the signal may be detected.

      3. Nuclear Relaxation. This section describes how the system returns to equilibrium after the RF pulse is applied. The pulse is said to perturb the system. The signal is observed as the system returns to equilibrium. The relaxation is described as a combination of processes with different relaxation times (T's).

      4. The RF Pulse. This section describes how the RF pulse is produced and describes it's effect with a different model. This section is summarized in Figure 7. You should be able to follow and explain this figure.

      5. The Free Induction Decay. This section describes the motion of the system after the RF pulse. The net magnetization vector rotates in the XY plane. Slowly decaying as a result of the nuclear relaxation processes. This section uses the "rotating frame" to simplify what is observed. Pay special attention to Figure 9 and 10. Figure 9 shows the vector representation, figure 10 shows the FID (signal) and the transformed spectrum.

    2. The Fourier Transform in Chemistry. Part 2. Nuclear Magnetic Resonance: The Single Pulse Experiment. (King, R.; Williams, K. J. Chem. Educ. 1989, 66, A243-A248). This article describes the details of data acquisition and detection for an experiment with a single RF Pulse. Latter papers will describe what happens when a series of pulses is applied to a system.

      1. Data Acquisition. This section describes how the data is aquired.
        1. Signal Averaging. NMR signals are very small so they must be averaged. A unique problem with this is that it takes time for the system to return to equilibrium. As a result there is a compromise between pulse angle (a 90 degree angle gives the largest signal in the XY plane) and the relaxation rate (no pulse returns to equilibrium immediately). This is called the ernst angle and it depends on the relaxation time for the nuclei and the acquisition time.
        2. Digitization. It is important to remember that the data is aquired by a computer. So they consist of discrete points. This "sampling frequency" (the rate that data is collected) determines the "spectral window" (the frequency range that is properly measured). If there is a signal outside this spectral window it will fold in. This is called aliasing. It is like the wagon wheel effect in old western's.
        3. Quadrature Detection. This is a method for determining if the signal frequency is positive or negative (ie clockwise or counterclockwise rotation.)
        4. Dynamic Range. This is another effect of using a computer to acquire data. Computers measure signals by counting steps. The total number of steps determines how finely the signal is measured. This is called the dynamic range. If a computer has 8-bit resolution, that means that each time the signal is measured, there are 28 (256) steps. It is important that the largest signal will fill most of these steps, but not go past the top (clipping).

      2. Data Processing.
        1. Zero filling. This is adding a string of zero's to the data file. This has the effect of improving the frequency resolution of the spectrum (As though you aquired data for twice as long.)
        2. Apodization. This has a variety of effects on the spectrum, this usually improves the S/N or the resolution. There is often a trade off between these two factors.
        3. Phase Correction. Because there is a delay between the pulse and the acquisition of data (the ringdown time), the rotating vectors do not start out at 0. The faster they rotate the further they are from 0. Phase correction realigns everything to 0.

      3. The Frequency Spectrum.
        1. Spin Decoupling. In a normal spectrum nuclei can interact with each other (since they are small magnets). This interaction causes the lines in an NMR spectrum to split (depending on the spin + or - of the neighbor). Spin decoupling is a technique to remove these interactions. Since the lines are not split, the spectrum is simpler.
        2. The Nuclear Overhauser Effect. This effect is difficult to explain, but the basic idea is that it increases the signal.
        3. Quantitation. This section points out the difficulty in quantitating some NMR signals. The most significant problems are that NOE is not constant for all nuclei, so some are enhanced more than others. Also the relaxation rates are different for each nuclei. Since some do not return to equilibrium as quickly, they produce smaller signals.

    3. The Fourier Transform in Chemistry. Part 3. Multiple Pulse Experiments. (King, R.; Williams, K. J. Chem. Educ. 1990, 67, A93-A99). This paper describes several NMR experiments where a more complex series of pulses is used to probe the nuclear spin system.

      1. Relaxation Time Measurement and Spin Echoes.
        1. T1 by Inversion Recovery. (T1IR) This is a technique for measuring the T1 relaxation time. We will do this in lab, and I have several nice demonstrations of this. It is nicely summarized in Figure 1.
        2. The Spin Echo Effect and T2 (T2HSE). This is another experiment that we will preform in the lab, and which I have several nice demonstrations for. This one is nicely summarized in Figure 3.
        3. The Attached Proton Test. (APT) This is an almost bizarre experiment, but it produces a C-13 spectrum where the peaks for carbons with 0 or 2 protons point up, but the peaks for carbons with 1 or 3 protons point down.

      2. Polarization Transfer. This is basically a way to use a series of pulses to change the distribution of spin states. The effect is like changing the boltzman distribution to obtain much larger signals.
        1. Selective Polarization Transfer. This technique enhances the signal for a single multiplet in a spectrum.
        2. INEPT. Insensitive Nuclei Enhanced by Polarization Transfer. This pulse sequence is used as a part of other experiments.
        3. Spectral Editing. The idea here is to control different types of nuclei. This is usually done with a DEPT experiment. This is a modification to the INEPT sequence that can produce spectra that only contain certain types of Carbon (methyl, methylene, methyline, quaternary).

    4. The Fourier Transform in Chemistry. Part 4. Two-Dimensional Methods. (King, R.; Williams, K. J. Chem. Educ. 1990, 67, A125-A137). This paper describes the techniques used to obtain 2-D NMR spectra and discusses the information contained in these spectra.

      1. J Spectroscopy. These are techniques for studying the J values (Coupling constants). They are either homonuclear or heteronuclear (coupling between the same element or coupling between different elements).

      2. COSY H-H correlation spectroscopy. Shows connections (coupling) between protons to determine which protons are coupled to which.

      3. HETCOR (Heteronuclear Correlation Spectroscopy). Shows connections (coupling) between nuclei of different elements, ie: C-H coupling to show which protons are coupled to which carbons.

      4. INADEQUATE (Carbon Carbon correlation). This technique shows which carbon is coupled to which. This is very difficult because the natural abundance of C-13 is very low, so the chance of two adjacent C-13 is EXTREMELY low.

      5. NOESY (Nuclear Overhauser Effect Correlation). This technique shows which nuclei are close to each other. Unlike COSY which only shows which H are coupled to which H, NOESY shows if they are nearby in three dimensional space.

    5. The Fourier Transform in Chemistry. NMR A Glossary of NMR Terms. (King, R.; Williams, K. J. Chem. Educ. 1990, 67, A100-A105). This glossary contains definitions of many terms used to describe NMR experiments and brief explanations of many different NMR experiments.


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    Scott Van Bramer
    Department of Chemistry
    Widener University
    Chester, PA 19013

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