Proton decoupled 13C NMR spectra

C NMR of linear cross-linked PS. The proton decoupled 13C NMR spectra of linear and 1% cross-linked PS at 75 MHz in chloroform are illustrated in Figure 3. These spectra are similar to those for linear and cross-linked chloromethylated PS previously reported at lower field (14), although we have been able to resolve more structure in tHe" aliphatic and aromatic regions here. The quarternary and methylene carbon resonances at about 146 ppm and between 40 and 50 ppm respectively, are the most strongly affected by stereochemistry (20). The ortho and meta resonances at 128.4 ppm show partially resolved structure in the linear PS, as does the para carbon at 126.1 ppm. The methine resonance at... 

Proton-decoupled 13C-NMR spectra were recorded on a Varian XL-300 operating at 75.4 MHz. Approximately 250 mg of the sample was dissolved in 3 ml of deuterated chloroform. 13C chemical shifts were referenced internally to CDCL (77 ppm). A delay of 200s was used to ensure relaxation of all the carbon nuclei and 1000 transients were collected to assure a good signal-to-noise ratio. 

Heteronuclei such as 13C (this magnetically active nucleus has 1.1% natural abundant) and 15N (0.3% natural abundance) are routinely measured with modem NMR spectrometers. Proton decoupled 13C NMR spectra in natural abundance exhibit singlets for each specific carbon atom, which are easier to count than overlapping multiplet lines in H NMR. ID 13C NMR can be used to investigate whether a peptide exhibits a single set of lines or a double (or more) set, which indicate conformational or configurational isomers (see Section 7.5.3). However, ID 13C NMR is rather insensitive and if there is not enough material or the solubility is low, more sensitive techniques have to be applied. 

For illustration, the proton decoupled 13C NMR spectra of quinoline, obtained from one CW scan and one pulse, respectively, are compared in Fig. 2.18. A further signal noise enhancement of up to 40% arises from application of the QD technique outlined in Section 2.5.4.1 (Fig. 2.18(c)). 

Measurement of Proton-Decoupled 13C NMR spectra with Suppressed NOE... 

The stacked series of selectively proton-decoupled 13C NMR spectra with varying decoupling frequency is conveniently recorded fully computer-controlled, as can be seen for nicotine in Fig. 2.25 (b). The series clearly shows that 13C signals, e.g. those of C-2, C-6, C-4, C-5 and C-4, are unequivocally assigned if the protons attached to these carbons do not overlap with others in the H NMR spectrum. For overlapping proton resonances (e.g. the pairs 2 -H, 5 -H and 3 -H, CH3) more than one carbon is affected by decoupling, of course, and other assignment aids such as two-dimensional CH correlation (Section 2.10) have to be taken into account (e.g. for the pairs C-2, C-5 and C-3, CH3 of nicotine in Fig. 2.25 (b)). 

Fig. 4.18. Proton-decoupled 13C NMR spectra (22.63 MHz 100-150 mg/ mL deuteriochloroform) of guttapercha (trans-) (a) and natural rubber (m-polyisoprene) (b) [73 i].

In general, broad-band proton-decoupled 13C-NMR spectra of unenriched samples consist of single resonances for each type of carbon... 

Figure 1. The fully proton-decoupled 13C NMR spectra of amber samples from Simojovel and Totolapa, Chiapas, Mexico, taken on the solid samples with magic angle spinning and cross polarization. The top sample was orange-red and the middle sample yellow.

Unlike sugar phenylosotriazoles which possess a flexible side chain,130 some cyclitol phenylosotriazoles possess symmetrical structures, as indicated by the simplicity of their proton-decoupled 13C NMR spectra,392,532 Thus the 1H NMR spectra of inositol phenylosotriazoles, 292a, and their esters revealed the presence of a simple, twofold axis of symmetry and the ring protons were symmetrical about a midpoint (see 294), making them examples of four-nucleus AA BB systems. 

Proton noise decoupled 13C-NMR spectra of equimolar mixtures of the cyclic hexamer and metal thiocyanates showed that the signals of the carbonyl carbon and two methine carbons gave downfield shifts upon the addition of metal thiocyanates, while those of the three methylene carbons of the tetrahydropyran ring gave upfield... 

The proton noise-decoupled 13c-nmr spectra were obtained on a Bruker WH-90 Fourier transform spectrometer operating at 22.63 MHz. The other spectrometer systems used were a Bruker Model HFX-90 and a Varian XL-100. Tetramethylsilane (TMS) was used as internal reference, and all chemical shifts are reported downfield from TMS. Field-frequency stabilization was maintained by deuterium lock on external or internal perdeuterated nitromethane. Quantitative spectral intensities were obtained by gated decoupling and a pulse delay of 10 seconds. Accumulation of 1000 pulses with phase alternating pulse sequence was generally used. For "relative" spectral intensities no pulse delay was used, and accumulation of 200 pulses was found to give adequate signal-to-noise ratios for quantitative data collection. 

Figure 9.19—Proton decoupled 13C NMR spectrum of ethylbenzene. Each of the carbon atoms gives a signal consisting of a singlet. These large band decoupled spectra are simpler but contain less information.

C— lH) into singlets. NMDR spectra are often symbolized by putting the nuclei to be decoupled between brackets besides the nuclei to be observed A X. Proton decoupled 13C NMR experiments are thus referred to as 13C 1H NMR spectra. 

Fig. 2.21. Series of stacked proton off-resonance decoupled 13C NMR spectra with varying frequency offset Af2.

This relation (3.24) can be verified for simple rigid molecules, e.g. for adamantane having 7](ch) = 17.0 s and T1(CH2) = 7.8 s [159 ]. It is a valuable aid in the assignment of the I3C NMR spectra of large molecules, particularly when signal crowding precludes clear distinction between singlets, doublets, and triplets in coupled or proton off-resonance decoupled 13C NMR spectra. 

Fig. 4.13. Proton broadband-decoupled 13C NMR spectra of 3-methyl-5-oxo-1-phenyl-4,5-di-hydropyrazole (saturated solutions, 30 "C, 100.576 MHz, 8 scans, 2 s as relaxation delay) (a) in deuteriochloroform (b) in hexadeuteriodimethylsulfoxide.

The assignments of the purine resonances [460, 749, 751, 758, 759, 761] of proton broadband-decoupled 13C NMR spectra of nucleosides and nucleotides are made using the following aids [749, 761] Correlation with the signals of the parent bases and analogous nucleosides or nucleotides comparison of the 13C NMR spectrum with that of a specifically analogue correlations of the chemical shift values with the 7r-electron densities [749, 761] and, in addition, proton off-resonance decoupling, which affords the identification of quaternary carbon atoms. 

C-5 of the ribofuranose residue can easily be assigned. Another rule is found for the anomeric carbons C-l of the ribofuranose residue which always absorb at lowest field [696]. C-4 can be assigned by noting the phosphorus-carbon long-range coupling in the proton broadband-decoupled 13C NMR spectra of nucleotides [750] as demonstrated in Fig. 5.9. 

A second difficulty of fully decoupled 13C NMR spectra is that die connectivity in the molecule is difficult to establish (except by chemical shift correlation) because coupling patterns are absent. This dilemma is partially resolved by die use of a technique called off-resonance decoupling. In off-resonance decoupled 13C spectra, the carbons are coupled only to diose protons directly attached to diem and die coupling is first order. Thus quaternary carbons are singlets, methine carbons are doublets, methylene carbons are triplets, and methyl carbons are quartets. It is possible to use diis information to establish proton-carbon connectivity,... 

The 1H NMR spectra of parbendazole was recorded with a JEOL-PS 100 NMR spectrometer operating at a frequency of 100 MHz and a magnetic field strength of 2.349 T. Spectra were determined over the region 10.8-0.0 parts per million (ppm), with a sweep time of 250 s. Chemical shifts were recorded as S (delta) ppm downfield from tetra-methylsilane (TMS). Proton noise and off-resonance decoupled 13C NMR spectra were measured on a JEOL FX 90Q Fourier Transform NMR spectrometer operating at 90 MHR and spectral width of 5000 Hz (220 ppm). All measurements were obtained with the compound being dissolved in deuterated dimethyl sulfoxide (DMSO-d6) for dT NMR and in deuterated trifluoroacetic acid (TFA-dx) for 13C NMR. 

Sketch the proton decoupled 13C NMR spectrum and DEPT spectra for each of the compounds in question 4.1. 

On the other hand, 13C NMR is extremely effective in characterizing these types of polymers. These spectra not only display well-separated lines for the various structural isomers that may be present, but also provide sequence distribution information and readily allow quantitative analysis. For example, the proton decoupled 13C NMR spectrum of polybutadiene shown in Figure 7-32 has bands that can be assigned to specific toad sequences. We have marked with arrows those that have a central cis unit, like the sequence shown at the top of the figure. 

Fig.l a-c. Simulated proton noise decoupled 13C NMR spectra of a polyketide-derived moiety a at natural abundance b enriched from [1,2 13C2] acetate c after cleavage or rearrangement of an originally intact acetate unit... 

Frequency Decoupling) 13C NMR spectra, recorded in the same conditions, allowed chemical shift identification of the protonated carbons, while the quaternary aromatic carbons were identified through NOE-Time Dependence measurements. These assignments were further supported by the 13C NMR data of the a-dihydrolycorine and the corresponding lactame, which were consistent with those reported in literature [145] for other structurally related naturally occurring compounds [144]. 

Figure 6. Plot of peak frequencies in the 1H off-resonance selectively decoupled 13C nmr spectra of NAD+ at pH 7-05 as a function of position of irradiation in the 1H spectrum, expressed in ppm downfield from internal dioxan. The positions of the peaks in the 1H noise decoupled 13C spectrum are shown by lines on the ordinate and the position of the proton peaks by lines on the abscissa. The arrows t indicate the point of collapse of the 1 3C doublet and the connection between a given 3C peak and the assigned proton peak. The errors in the position of measurements are indicated by the size of the points except near the cross-over points where they are larger ( 0-15 ppm). (Taken from Birdsall and Feeney, 1972.)...

Hufford and Baker [56] reported the assignments of the 13C NMR spectra of three derivatives of primaquine namely 4-methylprimaquine, 5-methoxy-4-methylprima-quine, and 5-methoxyprimaquine. These assignments were based on comparison with those of primaquine, proton-coupled data, and selective long-range proton decoupling. 

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