Obtaining the NMR Spectrum

Three parameters affect an NMR spectrum the chemical shift, couphng, and nuclear relaxation. These must be accounted for when obtaining the NMR spectrum from the spectrometer s output. Obtaining the NMR spectral plot from the output (the free induction decay, FID) of a modern NMR spectrometer involves the analysis of the mathematical relationship between the time (i) and frequency (co) domains, known as the Fourier relationship  

The Fourier transform (FT) relates the function of time to one of frequency— that is, the time and frequency domains. The output of the NMR spectrometer is a sinusoidal wave that decays with time, varies as a function of time, and is therefore in the time domain. Its initial intensity is proportional to and therefore to the number of nuclei giving the signal. Its frequency is a measure of the chemical shift, and its rate of decay is related to T2. Fourier transformation of the FID gives a function whose intensity varies as a function of frequency and is therefore in the frequency domain. 

Each FID signal is accompanied by noise however, the noise is incoherent —sometimes positive, sometimes negative—so that it increases more slowly than the desired nuclear signal. A series of N FIDs has a signal-to-noise ratio Jn times better than a single FID, allowing spectroscopists to obtain useful chemical information from otherwise unreceptive nuclei or from dilute solution samples having few of the nuclei of interest. 

The output of the NMR spectrometer must be transformed from an analog electrical signal into digital information that can be stored in the computer s dedicated computer. The minicomputers used in NMR spectroscopy have memory used for data accumulation, programs for manipulating the data, and storage devices to store large collections of data for future or additional manipulation into useful spectral results. 

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The signal (FID, free induction decay) resulting from an NMR experiment contains the original data which are stored in the computer, and after the Fourier transformation (FT) we obtain the NMR spectrum itself. 

When attempts were made to prepare 102, a red brown 2-acetyloxepin (158) was obtained. The NMR spectrum showed very little change with temperature and was consistent only with oxepin structure 158. ... 

In order to obtain the NMR spectrum in an angular frequency, (Figure 4.30), it is necessary... 

Information on electron delocalization in the bicyclo[3.1.0]hexenyl cations is available from their reported NMR spectra Data obtained with a variety of systems point to a completely different charge delocalization pattern to that found with the homotropenylium ions. For example, Olah and colleagues have obtained the NMR spectrum of the parent ion"", 61, and compared this with those of 42 and 11. As can be seen from the data summarized in Scheme 18, the chemical shifts of the five-membered ring carbons of 61 resemble those of the cyclopentenyl cation. There is a considerable difference in chemical shifts, and hence charge distribution, at C(2), C(4) and C(3) of 61. There is no evidence for the fairly even charge distribution as is found for the homotropenylium and homocyclopropenium ions (see previous Sections III. A and III. B). It was also noted by Olah that the chemical shift of C(6) is consistent with large delocalization to this position, i.e. to conjugation of the allyl system of 61 with the external cyclopropyl bonds. 

Dimethyl(pentafluorophenyl)arsine is a colorless, malodorous liquid which has a nmr spectrum in dichloromethane consisting of a triplet centered at t8.51 with /PH = 1.1 Hz. A methyl iodide derivative of the arsine can be prepared by refluxing 1.7 g of the arsine with 15 ml of methyl iodide for 2 hr. After evaporation of the solvent and recrystallization of the product from a dichloromethane-benzene mixture, a colorless compound which melts at 167-168° is obtained. The nmr spectrum of the methyl iodide derivative is a triplet at t7.16 (Jfh = 1.4 Hz) in dichloromethane, and the infrared spectrum (Nujol mull) contains strong peaks at 1080 and 980 cm characteristic of the CeFs group. Anal, (done on the methyl iodide derivative). Calcd. for C9H9-FsAsI C, 26.09 H, 2.17 1, 30.68. Found C, 25.5 H, 2.21 I, 30.1. 

Yannoni et al. obtained MAS cross-polarization NMR spectra of the enriched 2-norbornyl cation in an Sbf s solid matrix at -196°C. The solid-state chemical shifts correlated well with the solution data except for the lack of resolution in the methylene region. Subsequently, they have been successful even in obtaining the NMR spectrum in the solid state at -268°C (5K) ... 

The experimental pulse response M it) (cf. eqn (2.2.19)) acquired in the absence of a field gradient is Fourier transformed to obtain the NMR spectrum,... 

Chemists generally obtain the NMR spectrum of a substance by following a typical routine. The substance must be dissolved in a solvent, and the solvent that is selected should have certain desirable properties. It should be cheap, it should dissolve a wide range of substances, and it should... 

Obtain the NMR spectrum of a straight-chain alkane, such as n-octane. Identify the methyl and methylene peaks. Note the chemical shift and the spin-spin splitting. Measure the total area of the methyl and methylene peaks and correlate this with the number of methyl and methylene protons in the molecule. 

Aris et al. (9) have obtained the NMR spectrum of dipropene silver tetrafluoroborate. They considered their data in terms of two bonding extremes, pure donor bonding olefin<2+>] and pure acceptor bonding... 

Obtain the NMR spectrum of a mixture of toluene and hexane. Integrate the peak areas of the different parts of the spectrum. Erom the ratio of the aromatic protons to the total protons, calculate the percentage of toluene in the mixture. 

Repeat Experiment 3.9 using margarine as the sample. First, dissolve a known amount of margarine in CCI4 and obtain the NMR spectrum. Compare the degrees of unsaturation of different brands of margarine. 

The reaction of 2,2-dimethoxypropane with acid to form methanol and acetone can be followed by both proton and C NMR. Into a dry NMR tube, add 0.6 mL of 2,2-dimethoxypropane. Obtain the NMR spectrum and integration. Then add 0.1 mL of 1 M HCl and mix well for about 5 min. Obtain the NMR spectrum again, (a) Write out the reaction, using structures. Predict how many proton and/or carbon peaks you should see in the first spectrum. Predict how many peaks should be seen in the second spectrum, (b) Look up the chanical shifts of the protons (and C, if obtained) in the tables in the chapter and predict where each proton or carbon will be observed relative to TMS. (c) Confirm the identity of the peaks in your spectra. 

If the epoxide is an oily semi-solid, it will not be possible to collect the material on a Hirsch or Buchner funnel. Weigh the material and calculate the percentage yield. Dissolve the sample in CDClg, and obtain the NMR spectrum as described in the next section. 

Assume you obtained the NMR spectrum of a mixture of 1- and 2-bromohexane and found that the integrations for the multiplets centered at 8 3.4 and 4.1 were 20 units and 40 units, respectively. What is the relative ratio of these two compounds in the sample ... 

OPTIONAL. Obtain the NMR spectrum of the product in CDCI3. Identify the resonances for the two protons attached to the halogenated carbon atoms, and ascertain that the product is a single diastereomer. This information will be useful in Experiment [F3],... 

OPTIONAL. Obtain the NMR spectrum of the aziridine. Establish from this spectrum and the NMR data obtained in Experiment [F2] if any unreacted dibro-mochalcone still contaminates the aziridine sample that has been purifiedpr use in the preparation of the photochromic target molecule. Determine the diastereomeric purity of your product, and determine that the aziridine product is indeed trans substituted. 

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