Basic Principles of Pulse FT-NMR: 2009

Table of Contents – Front

VIII. NMR Basic Principles Test

1. Diagram and label the three components of the 1PULSE FT-NMR experiment. Give the parameter names used for the three components in the Unity+300 and VXR-S 400 NMR spectrometers.

2. What is the result when you apply the FT to an FID (time domain signal)?

3. What is the relationship between number of points, spectral width, acquisition time, and digital resolution? Which of these parameters would you change if you wanted better digital resolution, and why? What are the parameter names for number of points, spectral width and acquisition time in the Unity+300 and VXR-S 400NMR spectrometers?

4. What is the peak shape found in most solution NMR spectra?

5. What shim(s) should be adjusted if the peak shape is asymmetrically distorted? Label the shim probably responsible for the distortions below.

6. What is the single best factor to tell whether a sample is poorly shimmed?

7. Given that after 100 transients (8.5 minutes) the S/N for a sample is 25:1 on the Unity+300, and 35:1 on the VXR-S 400, how long will it take to achieve a S/N of 350:1 on each instrument?

8. What are the six factors that can affect the accuracy of a ¹H integration? Why? Are there any additional factors that affect the accuracy of a ¹³C{¹H} integration? Why?

9. When would you use a homonuclear decoupling experiment?

10. Is there a difference between the 1PULSE FT-NMR experiment used to acquire ¹³C{¹H}NMR spectra and that used to acquire ¹H NMR spectra? If yes, what is the difference?

Take Home Lesson - V

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Obtaining useful ¹³C{¹H} spectra requires knowledge of the same basic principles as needed for obtaining useful ¹H spectra. When your spectrum does not look right, you can save yourself needless frustration on the instrument by taking a quick spectrum of a ¹³C standard and checking the S/N, or seeing if the standard is decoupled properly.

VII. 13C-{1H} NMR Spectra - II

The ¹³C{¹H} spectrum obtained using a standard 1PULSE experiment is not quantitative, i.e., the integration of the peaks will not give a true indication of relative ratios because of the nuclear Overhauser enhancement (nOe) of the ¹³C nuclei due to their attached ¹H nuclei. For the case of ¹³C spectra acquired with proton decoupling, an enhancement of up to 1.98 (i.e., 198%), or an almost threefold improvement in signal-to-noise is expected for those carbon nuclei that are attached to protons.

The ¹H digital resolution given by the default parameters on both the Unity+300 and VXR-S 400 differ from the ¹³C digital resolution by a factor of five. This is mainly due to the need for a larger spectral width used to accomodate the wider chemical shift range for ¹³C and the need to use a fewer points to conserve space on the hard disk. Due to the large spectral width typical of ¹³C{¹H} spectra, it is important that the number of points not be too small, or distortions of the peaks can occur. Figure VII-4 shows a spectrum of 30% menthol in CDCl₃ collected with only 8192 points at 75.432 MHz. Compare it to Figure VII-2 and note the anamolous peak heights in Figure VII-4, as well as the phase distortion in all the peaks. These effects are due to an insufficient number of data points and is not an instrument problem. The distortion can be eliminated by increasing the number of data points as is the case in the standard default parameters for ¹³C NMR spectroscopy on the Unity+300 and VXR-S 400 NMR spectrometers.

VII. 13C-{1H} NMR Spectra - I

This section gives you some useful information about¹³C{¹H}NMR spectroscopy. Examples of spectra of 30% menthol in CDCl₃ taken on the Unity+300 and VXR-S 400 are given in Figures VII-1 and VII-2 respectively. Some of the important default ¹³C NMR acquisition parameter values are given below.

Unity+300	VXR-S 400
Spectrometer Frequency (sf) = 75.432 MHz	Spectrometer Frequency (sf) = 100.580 MHz
Spectral Width (sw) = 18,859 Hz	Spectral Width (sw) = 25000 Hz
Acquisition Time (at) = 0.820 sec	Acquisition Time (at) = 0.819 sec
Number of Points (np) = 30912	Number of Points (np) = 40960
Digital Resolution = 1.22 Hz	Digital Resolution = 1.22 Hz
Line Broadening = 1.00 Hz	Line Broadening = 1.00 Hz

The symbol ¹³C-{¹H} implies a ¹³C NMR spectrum where the ¹H nuclei are decoupled from the ¹³C nuclei. This is a double resonance experiment, just as described in section VI for homonuclear decoupling except that now the observed nucleus (¹³C) and decoupled nucleus (¹H) are not the same. This experiment is called heteronuclear decoupling, and is a 1PULSE experiment, as described in section II, with the addition of the decoupling field (Figure VII-3). It should be noted that the proton decoupling field is left on during the entire experiment.

Take Home Lesson - IV

Homonuclear decoupling is an effective way to establish that two nuclei are spin coupled, and to simplify a complex coupling pattern for further analysis. It can be difficult to obtain definitive data if the two nuclei are closer than 0.5 ppm to each other.

VI. Homonuclear Decoupling

This section will explain what homonuclear decoupling does. Examples of homonuclear decoupled spectra taken on the Unity+300 and VXR-S 400 are given in Figure VI-1.

Homonuclear decoupling is a double-resonance experiment because it uses two RF fields to affect magnetically active nuclei. Homonuclear decoupling involves applying a second ¹H RF field to cause selective saturation of a nucleus A while observing all other nuclei in the molecule; B, C, D, etc. If nucleus A is spin-coupled to nucleus B and if the second RF field is strong enough, the result is that A is effectively prevented from spin-spin interacting with B. The observed B nucleus spectrum will appear as if it is not coupled to A. The A resonance commonly appears as a tall spike or glitch as a result of this experiment. In Figure VI-1, if the triplet is decoupled, the quartet collapses to a singlet. Similarly, if the quartet is decoupled, the triplet collapses to a singlet.

Take Home Lesson - III

“The moral of this section is that there are numerous contributions to the error in a quantitative measurement made by FTNMR, and while each of them may be reduced to 1% or so in apractical fashion, the combined error is still likely to be significant. I am always skeptical of measurements purporting tobe accurate to better than a few percent overall, unless they come with evidence that careful attention has been paid to the above details.

Taken from Derome (p.172)

V. Integration - II

Continuing from Integration - I

4. The spectrum should have a S/N of atleast 250:1 for the smallest peak to be integrated. The S/N measured for the quartet in Figure V-3, is 213, which is close to fulfilling this criterion. We can improve this number by choosing a line broadening that is approximately equal to the true (natural) linewidth of the peaks.

5. The baseline should be flat. Distortion due to phase problems should be corrected. Baseline distortion due to non-optimum parameter selection causing a baseline roll are not discussed here.

6. The peaks need to be sufficiently digitized, as discussed in Section II. If the linewidth at half-height is 1 Hz, you need a digital resolution of less than 0.5 Hz.

7. The same area should be included or excluded for all the peaks. For example, all peak integrals should be measured +/- 5 Hz around each peak, not +/- 20 Hz around one peak, +/- 10 Hz around a second peak, etc. Spinning sidebands are included in this category, and should consistently be either included or excluded.

The reason the integrals for the phenyl region is so inaccurate for the spectrum in Figures V-1 and V-2 is because the recycle time or pulse repetition time was too short. Obviously, if you did not know the phenyl region represented 5 instead of 4 protons, you could draw an erroneous conclusion about the structure of this compound from the integrals in Figures V-1 and V-2.

V. Integration- I

This section will show you how to obtain good integrals. Examples of spectra of 0.1% ethylbenzene in CDCl₃ from the Unity+300 and VXR-S 400 NMR spectrometers are given in Figures V-1 and V-2 respectively. Both spectra were taken using the default parameters for acquiring ¹H spectra. If we assign an integral of 3.00 to the CH₃ triplet, then the phenyl region integrates to 4.52 protons, while the CH₂ qurtet integrates to 2.08 protons. Thus the integral for the phenyl protons is 9.6% too small, while the integral for the CH₂ quartet is off by 4.0%. The 9.6% error for the phenyl protons is not due to spectrometer error; it is because we have chosen parameters for acquiring the spectrum which guarentee we will get inaccurate integrals.

The accuracy of the integrals obtained for most routine spectra is usually about 10-20%. This accuracy is sometimes sufficient especially if you already know what the compound is. However, this accuracy is usually not adequate to determine the exact number of protons contributing to a given peak, nor is it sufficient for quantitative applications such as kinetic experiments or assays of product mixtures where one demands an accuracy of 1-2%. For example, 20% accuracy is not sufficient to decide whether two peaks have a relative ratio of 1:3 or 1:4. Obtaining 1-2% accuracy can be achieved but you need to be aware of the factors that affect integrations. These factors are discussed below.

1. There should be no nuclear Overhauser effect contributions or any other effects that selectively enhance certain peaks. This is a problem only with X nuclei such as ¹³C.

2. No peaks should be close to the ends of the spectrum. The spectral width should be large enough such that no peak is within 10% of the ends of the spectrum. This is because the spectrometer uses filters to filter out frequencies that are outside the spectral width. Unfortunately, the filters also tend to decrease the intensities of peaks near the ends of the spectrum. For example, at 299.96 MHz, if two peaks are separated by 7 ppm, a spectral width of atleast 2100 Hz is sufficient to get both peaks in the spectrum and prevent foldovers. However, to avoid distortion of the integral intensities because of filter effects, the spectral width should be set 10% larger on each side, 210 Hz, giving a total spectral width of about 2520 Hz (8.4 ppm). Although the standard setup parameters on the Unity+300 and VXR-S 400 should easily satisfy this criterion, you should be prepared to make the spectral width larger if necessary.

3. The pulse repetition (recycle) time should be atleast five T₁s. Data should be collected under conditions which ensure that all the nuclei can fully relax before the next FID is taken, i.e., if 90 degree pulse width are used, relaxation delays of 5xT₁ of the longest T₁ of interest are necessary. In the case of ethylbenzene, the longest T₁ of interest is 9.8 sec for the phenyl protons, so the relaxation delay when using 90⁰ pulse width should be at least 49 sec (5x9.8). In Figure V-3, is shown the spectrum of 0.1% ethylbenzene in CDCl₃, using a 90 degree pulse width, realaxation delay of 60 sec, taking 32 transients, and using a line broadening of 0.1 Hz ( for comparison, the default parameters on the Unity+300 use a 45 degree flip angle and a recycle delay of 5.1 sec). If we assign an integral of 3.00 to the CH₃ triplet, then the phenyl region integrates to 5.17 protons, while the CH₂ quartet integrates to 1.98 protons. Thus, the integral for the CH₂ quartet is off by only 1.0%, while the integral for the phenyl prtons is now 3.2% too high. The errors for both the phenyl protons and the CH₂ protons are now comparable; they reflect a good choice for the recycle delay for this sample. However, the 3.2% error for the phenyl protons is still higher than the 1-2% error range one needs for accurate quantitaive measurements. In this particular example, the signal from the residual proton (CHCl₃, 7.29 ppm) contributes to the integral of the phenyl region making it higher than the true value. This problem can be solved by measuring the spectrum in a solvent such as CD₂Cl₂ whose residual proton signal will not overlap with any of the ethylbenzene protons.

Take Home Lesson - II

At some point, you may take a spectrum and wonder why the signals are so weak. Well over 75% of the time, the problem is not with the spectrometer, but with your sample. You can test this quickly by taking a spectrum of ¹H or ¹³C standards such as ETB (or menthol). In this way, you can save yourself needless frustration on the instruments by identifying problems that are due to a bad sample.

IV. Signal-to-Noise Measurement - II

Choice of a noise region must be consistently applied for standard samples, and for 0.1% ethylbenzene (ETB), we have chosen 5 to 3.5 ppm. It is also critical that the noise be amplified if necessary in order to make an accurate measurement using a ruler. There is not much point in

trying to compare the ratio of noise to a few millimeters high to a peak 20 cm high; plot the noise such that it is several centimeters high. If the instrument performs the signal-to-noise measurement and calculation, the relative intensities are not as critical. Both the Unity+300 and VXR-S 400 NMR spectrometers perform the S/N calculation by software commands.

The last point concerning sensitivity is demonstrated during the ¹³C{¹H} part of the checkout. The checkout requires that you acquire three spectra, using 1, 4, and 16 transients. Signal-to-noise increases as the square root of the number of transients; to double the signal-to-noise you must take four times as many transients.

When using a concentrated sample such as 30% menthol for ¹³C, or when running routine ¹H spectra, the number of transients or scans is quite small, so the point discussed above may not seem important. However, suppose you are in the following situation: you have only a few mg of research sample, and after collecting a ¹³C{¹H} spectrum on the Unity+300 for 2 hours, you get peaks with S/N of only 5:1. Since the peaks are barely visible above the noise ( and you may have missed quarternary carbons), you want to re-collect the spectrum to get a S/N of 50:1, a value most typical for carbon NMR. Unfortunately, this will take 10x10x2 = 200 hours! In such cases, you should consider using a higher field strength spectrometer, which will be able to give you a spectrum with good S/N in much less time.

IV. Signal-to-Noise Measurement - I

The first spectrum described in the checkout involves determining a signal-to-noise measurement, or S/N. S/N is an important criterion for accurate integrations (see section V), and is also one of the best ways to determine how sensitive an NMR spectrometer is. In general, a higher S/N specification means that the instrument is more sensitive.

A typical result for the Unity+300 checkout is S/N = 87 and is shown in Figure IV-1, while a typical result for he VXR-S 400 checkout is S/N = 109 and is shown in Figure IV-2. If you measure S/N for the ethylbenzene standard that is significantly lower than these values, write your observations in the spectrometer logbook and inform S. Chandrasekaran .

S/N measurements for proton spectra are always determined using a sample of 0.1% ethylbenzene in CDCl₃ (ETB). It is important that the spectrum be acquired under standard conditions:

1. 90⁰ pulse width

2. Line Broadening = 1.0 Hz

3. Spectral Width = 15 to 5 ppm

4. A sufficient relaxation delay (at least 5xT₁)

5. A sufficient digital resolution (less than 0.5 Hz/point)

6. One transient acquisition

Optimum signal-to-noise ratio for any sample is achieved using a line broadening equal to the linewidth at one-half height. When this line broadening is applied, the linewidth at half-height doubles, i.e., it is the sum of the natural linewidth at one-half height plus the line broadening applied. The equation used for calculating S/N is:

S/N = (2.5 x A) / N_pp

A = height of the chosen peak N_pp = peak-to-peak noise

Peak-to-peak noise means exactly that - a measurement from the most positive to the most negative positions for the noise.

As shown below, the wildest differences occur with low probability, so there seems to be a large gap between them and what appears to be the main part of the noise; will-power must be exercised in order to truly measure this gap and not to dismiss it in our minds as “spikes” rather than the incoherent noise it is. True instrumental defects resulting in spikes can be dismissed, but only after several measurements have proven a spike is always present.

Take Home Lesson - I

Knowledge of correct lineshapes allows you to decide quickly whether your sample is correctly shimmed. You have to decide whether the return (a better lineshape) is worth the time spent achieving that lineshape.

III. Shimming, Linewidths and Lineshapes -III

Knowledge of correct lineshape can help in correcting problems such as those shown in Figure III-3. Although the peak in Figure III-3b may have a linewidth at half-height (LW_1/2) that is less than 0.50 Hz, the checkout requirement, it is obviously poorly shimmed. You should never accept a poorly shimmed lineshape as is shown in Figure III-3b, where a single line is expected.

On the pages that follow are some lineshape defects and the shims that should be adjusted to correct the problem. In general, odd-order shims (Z1, Z3) affect the lineshape symmetrically while eve-order shims (Z2, Z4) cause a non-symmetrical lineshape. The higher the order (Z4 is higher order than Z2), the lower (i.e., closer to the base of the peaks) the problem is observed.

III. Shimming, Linewidths and Lineshapes - II

It is important to have a basic understanding of lineshape to be able to judge when:

(1) The shimming is off, and

(2) More time should be spent in shimming the sample. The best way to avoid problems is to establish a procedure using the guidelines below.

1. Always load a shim file when you sit down at the instrument. You should never assume the previous user left the instrument with a standard shim file loaded. Without reloading standard shims, you will have to start where the last person stopped --- and that might include someone who shimmed for a short sample, a “bad” tube, a viscous sample, etc.

2. Be aware of lock parameters, especially if you only shim on the lock display. Establish lock transmitter power and gain levels that work for most of your samples. If you encounter a sample that seems to require an unusually high power or gain setting, there is a problem with your sample and/or the instrument, and shimming on the lock level may be difficult or impossible.

3. Shimming problems are confirmed only if the “problem” is visible on every peak in your spectrum. If only one peak is doubled, the “problem” is sample related, and can’t be shimmed away. Remember, anomalies close to the base of intense single lines may not be visible on less intense peaks unless the vertical scale is increased.

4. Establish a shimming method. Shimming is an art form that requires patience and practice. You should always approach shimming with some method that works for you to give acceptable results. Example: load a shim file, adjust lock level to a maximum with Z1, then Z2, then Z1, then Z3, then Z1.

5. Spinning sidebands should always be below 2%. If spinning sidebands are above 2%, turn off the spinner air, optimize X and Y, then turn the spinner air back on and re-optimize Z1, Z2, and Z3. If this does not solve the problem, consider transferring your sample to another tube.

III. Shimming, linewidths, and lineshapes - I

It is important to have a knowledge of proper lineshape of an NMR signal and how improper shimming contributes to poor lineshape. Examples of reasonably well-shimmed NMR spectra from the Unity+300 and VXR-S NMR spectrometers are illustrated in Figure III-1 and III-2.

NMR peaks have a Lorenzian lineshape. The mathematical expression of a Lorentzian line and the three parameters associated with the line are shown below.

The minimum obtainable linewidth at half-height is directly related to the resolution of an instrument, i.e., how close two peaks can be and still be distinguishable. Resolution is usually measured using o-dichlorobenzene, which has very narrow lines its ¹H NMR spectrum. The manufacturer’s resolution specification is usually around 0.20 Hz, although linewidths of less than 0.10 Hz are obtainable with expert shimming.

Manufacturers of NMR instruments, however, have traditionally separated the resolution specification from the lineshape specification. Lineshapes for ¹H NMR spectra are usually specified using CHCl₃ and the specifications are stated in terms of linewidth at 0.55% and 0.11% height of the CHCl₃. These percentages are chosen because they are the height of the ¹³C satellites of the CHCl₃ line and one-fifth this height. However, these values are meaningful only when compared with the half-height width (LW_1/2). From the mathematical equation for a Lorentzian line, the linewidth at 0.55% height is calculated to be 13.5 times the LW_1/2, while the linewidth at 0.11% height is calculated to be 30 times the LW_1/2. So, if the linewidth at half-height is 0.30 Hz, the calculated values are 4.0 Hz at 0.55% and 9.0 Hz at 0.11%.

For comparison, the manufacturer’s specifications are 10-15 Hz and 20-30 Hz at 0.55% height and 0.11% height respectively. These values are larger than the theoretical values because the linewidths at 0.55% and 0.11% height are very sensitive to how well shimmed the magnet is.

Components: Relaxation Delay [d1]

While running repetitive transients (nt > 1, e.g.,) to obtain high signal-to-noise ratio in an NMR experiment, the time between each transient becomes important. This time, called the pulse repetition time (prt), also called recycle time should be long enough to allow complete relaxation of the nuclear spins between transients. Incomplete relaxation will lead to loss of signal and is one cause of inaccurate integrations. The pulse repetition time is given by the following equations.

prt = Pulse Width + Acquisition Time + Relaxation Delay or

prt = pw + at + d1

The relaxation time for a nucleus is called T₁ (longitudinal relaxation time) and if a 90 degree pulse is used to excite the spins (Figure II-2A), a pulse repetition time equal to or greater than 5xT₁ is required to have complete relaxation (see Figure II-5). If a pulse width less than 90 degree pulse width is used, the pulse repetition can be proportionally less. In the above equation, pw is in

microseconds whereas at and d1 are in seconds. Thus, pw can be ignored in the calculation of the pulse repetition time. In practice, on the Unity+300 and VXR-S NMR spectrometers, at will be a fixed parameter chosen for the resolution desired, and therefore, the user has the only option of setting the relaxation delay d1 to a proper value so that the pulse repetition time can be optimum.

For example, if T₁is 0.5 sec, the optimum prt should be 2.5 sec for a 90 degree pulse used for excitation. The standard parameters for the Unity+300 and VXR-S 400 contain at set to 4.096 sec and 4.099 sec. Since these values are greater than 5xT₁, d1 could be set to 0 to meet the condition that prt be equal to or greater than 5xT_1. The standard parameters on the Unity+300 and VXR-S 400 NMR spectrometers for ¹H and ¹³C include 45 degree pulse width for excitation and the proper d1 to give a prt of 5.1 sec which will be optimum for most routine NMR runs, but exceptions are possible!

Components: Number of Points [np] and Spectral Width [sw]

Performing a mathematical operation called the Fourier transformation (ft) on the FID (time-domain data) generates a spectrum which gives intensity of signals as a function of frequency (frequency-domain data), as shown in Figure II-3. This familiar NMR spectrum is characterized by

two important parameters, the spectrometer frequency (sf), and the spectral width (sw). As

shown in Figure II-4, at a spectrometer frequency of 299.957 Hz on the Unity+300, a spectral width (sw) of approximately 3000 Hz is required to observe 10 ppm (10 ppm x 300 Hz/ppm). At a spectrometer frequency of 399.964 MHz on the VXR-S 400, a spectral width (sw) of

approximately 4000 Hz is required to observe the same 10 ppm (10ppm x 400 Hz/ppm)

The spectral width sw in Hz, acquisition time at in seconds and the number of points np are related to each other by the following equation:

at = np/2sw equation 1

The digital resolution is related to acquisition time at as follows.

resolution = 1/at = 2sw/np equation 2

From equation 2, it follows that resolution or digital resolution is in the units of Hz/point. To obtain properly digitized NMR spectra, the digital resolution must be less than one-half the peak width at half-height of the narrowest peak in the NMR spectrum. Using this optimum digitization condition guarantees that each peak is represented by at least 3 points. For example, if the narrowest peak width is 0.7 Hz at half-height, the required digital resolution is less than 0.35 Hz. It follows that for a spectrometer frequency of 399.964 MHz, a spectral width of 4000Hz (10 ppm) and a digital resolution of 0.35 Hz/point, the number of points can be calculated from equation 2 as follows:

np = 2sw/resolution ; np = 2*4000/0.35 = 22857 points

Normally, the spectral width is fixed for a given NMR experiment and either the number of points or the acquisition time is modified to achieve the desired digital resolution. On both the Unity+300 and VXR-S 400 NMR spectrometers, it is best to modify acquisition time by setting at to the proper value to achieve the resolution (see equation 2). The spectrometer will automatically set the correct number of points np. The standard parameters for ¹H observation are as follows.

Unity+300 VXR-S 400

Spectral Width (sw) = 4000 Hz (13 ppm) Spectral Width (sw) = 5199.5 Hz (13 ppm)

Acquisition Time (at) = 4.096 sec Acquisition Time (at) = 4.099 sec

Number of points (np) = 32768 Number of Points = 42624

Digital Resolution = 0.24 Hz/point Digital Resolution = 0.24 Hz/point

Components: Acquisition Time [at]

After the radiofrequency pulse has been applied to the sample and turned off, the spins return to thermal equilibrium, and in the process induce a signal in the receiver coil. In Figure II-1, the decaying sine wave depicts this signal called the Free Induction Decay or FID. The FID is a plot of the intensity of the induced signal as a function of time, and is commonly referred to as the time-domain data. The time spent in acquiring the FID is called the acquisition time (at). The FID, which is an analog signal is digitized using an analog-to-digital converter which represents the FID by a series of points along the FID curve. The number of points thus used can be entered by setting np to the proper value on the Unity+300 and VXR-S 400 NMR spectrometers. As a general rule, more points result in higher resolution.

Components: Pulse Width [pw]

If the average equlibrium magnetization of all the spins in a sample placed in a magnetic field is represented as oriented along the Z axis in a cartesian coordinate system as shown in Figure II-2, then the length of time the radiofrequency pulse (rf energy) is applied along the X axis (for

example) will have a specific effect on the magnetization. The length of time the radiofrequency pulse is applied to the sample is called the pulse width (pw). The pulse width is normally defined in terms of a flip angle in degrees. As shown in Figure II-2, different pulse widths have different effects on the equilibrium magnetization. The 90 degree pulse width is the length of time the

pulse of rf energy is applied to a particular sample to flip (rotate) all the spins from the Z axis into the XY plane, as shown in Figure II-2A. The 90 degree pulse width, specified as pw90 on the Unity+300 and the VXR-S 400 NMR spectrometers depends upon the nucleus, solvent, and the instrument (probe, etc.,). On the Unity+300, pw90 is 13.3 microseconds for proton and 9.7 microseconds for carbon. On the VXR-S 400, pw90 is 35.0 microseconds for proton and 13.0 microseconds for carbon. Routine experiments on these spectrometers are performed using a pulse width much less than the 90 degree pulse width. The pulse width is entered in microseconds by setting pw to the proper value on both the NMR spectrometers.

Components: Spectrometer Frequency [sf]

The nuclei are excited by the radiofrequency pulse from a transmitter and upon returning back to thermal equilibrium during acquisition time induce a signal in the receiver coil which is detected as the NMR signal. The radiofrequency pulse has a characteristic frequency, named the spectrometer frequency (sf), which is dependent on the nucleus observed and the magnetic field strength of the NMR spectrometer used. For protons, sf on the Unity+300 is approximately 299.957 MHz, and on the VXR-S 400, it is 399.964 MHz. For carbons, sf on the Unity+300 is approximately 75.432 MHz, and on the VXR-S 400, it is 100.580 MHz. The spectrometer frequency specifies the center of the NMR spectrum acquired.

Components of the Pulse FT-NMR Experiment

This section will cover the most important parameters which affect any spectrum you may collect using an FT-NMR spectrometer. The standard names of the parameters and the specific names (in bold) by which these parameters are referred to on the Unity+300 and VXR-S 400 NMR spectrometers are listed below.

Spectrometer Frequency sf

Pulse Width pw

Acquisition Time at

Number of Points np

Spectral width sw

Relaxation Delay d1

The generic 1PULSE FTNMR experiment is illustrated in Figure II-1. The experiment essentially

Consists of three components: relaxation delay, pulse width and acquisition time, and is named a 1PULSE experiment because one radiofrequency (rf) pulse is applied per cycle or transient.

Scope

The purpose of this site is to give the the prospective FT-NMR spectrometer user (trainee) an understanding of the basic principles which govern the pulse FT-NMR experiment and the associated basic NMR instrument parameters. A good understanding of the basic NMR parameters is of utmost importance to avoid the most common errors in an FT-NMR experiment.The trainee is strongly urged to study the information presented here well enough to be able to answer all the questions contained in the NMR Basics Test which forms the last section of this document. The second step of the checkout consists of the trainee passing the NMR Basics Test by answering all the questions correctly from memory in 30 minutes under the supervision of A Professor who will administer the test. This test can be taken any number of times until the trainee passes it in the required time.

Modern NMR Techniques for Chemistry research

From A.E. Derome, Modern NMR Techniques for Chemistry research (1987):

“Modern pulse NMR is performed exclusively in the Fourier transform mode. Of course it is useful to appreciate the advantages of the transform, and particularly the spectacular results which can be achieved by applying it in more than one dimension, but it is also essential to understand the limitations imposed by digital signal analysis. The sampling of signals, and their manipulation by computer, often limit the accuracy of various measurements of frequency and amplitude, and may even prevent the detection of signals altogether in certain cases. These are not difficult matters to understand, but they often seem rather abstract to newcomers to FT-NMR. Even if you do not intend to operate a spectrometer, it is irresponsible not to acquire some familiarity with the interaction between parameters such acquisition time and resolution, or repetition rate, relaxation times and signal intensity. Many errors in the use of modern NMR arise because of a lack of understanding of its limitations.”

Basic Principles of Pulse Fourier Transform NMR

The NMR sample is prepared in a thin-walled gl... Table of Contents:

Basic Principles of Pulse FT-NMR