Carbon resonant state

On the other hand, resonance assignments for CP of threonine and serine, and C and Cy of hydroxy proline, were difficult to make, because of their proximity to carbohydrate carbon resonances. In most cases then, the resonances were assigned on the basis of the effects of pH on the chemical shifts of those resonances. It was shown that the chemical shifts for the carbohydrate carbon resonances were virtually unaffected (AS 0.4 p.p.m.) when going from the cationic state (pH 2) to the anionic state (pH 11) of the amino acid residues. The chemical shifts of C and CP of the amino acid residues, however, shifted considerably (up to 3.1 and 6.6 p.p.m. for C" and CP, respectively see Table VI). 

Experimental NRM. Solid-state 13C NMR spectra were collected on a homebuilt spectrometer which operates at a proton resonance frequency of 127.00 MHz, corresponding to a carbon resonance frequency of 31.94 MHz, and employs a... 

Fig. 9 Examples of simplifying solid state NMR spectra by the TOSS and delayed decoupling pulse sequences. Shown is a comparison of the 31P CP/MAS NMR spectrum of fosinopril sodium utilizing the standard pulse sequence (A) and the TOSS routine (B). Also shown is the full 13C CP/MAS NMR spectrum of fosinopril sodium (C) and the nonprotonated carbon spectrum (D) obtained from the delayed decoupling pulse sequence utilizing a 80 /us delay time. Signals due to the methyl carbon resonances (0-30 ppm) are not completely eliminated due to the rapid methyl group rotation, which reduces the carbon-proton dipolar couplings.

Hoyle successfully predicts existence of a 7.6 MeV resonance state of the carbon-12 nucleus on grounds that otherwise little carbon would survive further processing into oxygen during stellar nucleosynthesis by helium burning, whereas in fact the C/O ratio is about 0.5. Discovery of strange particles. 

CPMAS spectrum typical of the regular twofold helical conformation (Figure 2.45b).147 191 Samples of sPP which present X-ray powder diffraction patterns typical of the isochiral form II (Figure 2.45c) show a different solid-state 13C NMR spectrum with additional signals in the region of the methylene and methyl carbon resonances (Figure 2A5d). These additional resonances have... 

Since the intensities of the resonances in the CPMAS experiment depend upon strengths of the dipolar couplings and molecular motions in the solid, it is not straightforward to obtain quantitative spectra. However, one may adjust the time during which magnetization is transferred from the proton to the carbon resonances, the CP time, and determine the time for which the solid state and solution integrals are equal. One can thereby obtain measurements of the aromatic to ethynyl ratio during the course of the cure. 

The cyclopropylidene W-complex appears to be indefinitly stable in the solid state only below -40 °C. It displays a characteristic downfield l3C NMR (THF-z/g, -60 °C) resonance at 8 255.5 ( Jc = 56.1 Hz), lower than the ligand carbonyl carbon resonances (8 206.5 and 210.7). 

It has been proved by X-ray analysis that, in the solid state, 1 is the tautomer present, but the claim has been made that,68 in solution, L-ascorbic acid exists as 2. Structures 3,4, and 5 were readily eliminated by a study of the l3C-n.m.r. spectrum,88,87 and, on the basis of the chemical shifts of the carbon resonances for C-l, C-2, and C-3, and the known chemistry of L-ascorbic acid, 1 is favored over 2 in solution. Berger98 claimed that the proton-carbon-coupled spectrum of L-ascorbic acid is consistent only with structure 1. Ogawa and coworkers87 studied the conformation of L-ascorbic acid and L-ascorbic acid-5-d in deuterium oxide by 13C-n.m.r. spectroscopy, and concluded that, in... 

In the Tm case, resonant photoemission proves that there is no significant hybridisation between the rare-earth 5d levels and the carbon electronic states thus the Tm ions have an essentially ionic interaction with their fullerene host. In the Gd case, the magnitude of the resonant enhancement of emission from the 4f levels signals the presence of hybridisation between the Gd valence levels and those of the carbon cage. However, this does not alter the conclusion as regards the Gd s trivalency and the transfer of essentially three electrons to the fullerene MOs. 

While many stable carbenes tend to be unreactive towards oxygen in the absence of a catalyst, Denk and co-workers found that a stable carbene could be oxidised to the urea 40 in the presence of a catalyst or by reaction with NO (Scheme 21).49 The solid state structure exhibits an oxygen-NHC bond length of 1.237(3) A, and the C2 carbon resonates at 152.7 ppm in the 13C NMR spectrum, 60.2 ppm upheld from the uncoordinated carbene. The bond length is elongated compared to a typical C=0 double bond... 

Reaction of a stable carbene with sulfuric chloride results in abstraction of the chloride cation to give the adduct 48 (Scheme 27).57 The C2 carbon resonates at 133.05 ppm in the 13C NMR spectrum and the solid state structure exhibits a chlorine-NHC bond distance of 1.696(9) A. The fluor-ine-NHC analogue was prepared by Kuhn and co-workers by reaction of the stable carbene with S02F2.50 The solid state structure exhibits a fluorine-NHC bond distance of 1.291(14) A. 

Addition of the same NHC to Eu(thd)3 (thd — tetramethylheptanedioate) affords the europium(III) adduct Eu(thd)3(NHC). The europium-NHC bond distance of 2.663(4) A is shorter than that of the samarium(II) complex and is consistent with the higher oxidation state of the lanthanide centre. The yttrium(III) analogue was also prepared and characterised by NMR spectroscopy. The C2 carbon resonates at 199 ppm in the 13C NMR spectrum, with a yc coupling constant of 33 Hz. This indicates that the NHC remains bound to the metal centre in solution and does not dissociate on the NMR timescale. 

Some differences in the change of 13C NMR chemical shifts of nitrobenzothiazoles compared with nitrobenzimidazoles and nitrobenzoxazoles have been discussed. In Table 3.30 the 13C NMR chemical shifts of nitrobenzothiazoles are presented. The nitro group introduction into position 2 leads to a 10 ppm down field shift of the ipso-carbon resonance, whereas a similar effect of the ipso-substitution in the phenylene fragment of benzothiazoles is 20 ppm [778-781], The results of regression analysis of the 13C NMR chemical shifts of benzothiazoles in terms of the inductive and resonance constants of substituents (F and R, c, and cR, c, and aR°) provide evidence for the fact that the substituent effect transmission from positions 2-6 is approximately 30% weaker than in the opposite direction [779], As stated previously, an analogous picture is observed for benzimidazoles. 

In C NMR spectroscopy, the carbene carbon resonates at ca. 140-400ppm to low field of SiMe4. This is probably a result of the existence of low-energy electronic excited states for the complex, which leads to a large paramagnetic contribution to the shift. A proton substituent at the carbene carbon resonates from 4-10 to 4-20 ppm. 

Figure 15b shows a solid-state spectrum recorded under conditions such that only the mobile portions of the solid PDHS sample are observed. In this polymer (as previously indicated), the mobile portion of the sample consists of the locally disordered phase II and any amorphous material to the extent that it exists. The chemical-shift pattern for the carbons agrees very well with the solution spectrum (Figure 15a). Because carbon resonances are very sensitive to bond conformation (22), this result demonstrates that the phase II portion of the sample has the same average chain conformation as the polymer chains in solution. Although these NMR data permit a comparison of local bond conformations, they do not provide an indication of the more global chain dimensions. Figure 15b shows increased line widths for the carbons near the silicon backbone, with the C-1 resonance almost broadened into the baseline. This broadening reflects the severe restriction of motion near the backbone.

The parameters of the solid-state NMR experiment can be selected to permit observation of the carbon resonances in the entire polymer sample, including both rigid and mobile portions (29). The data from this experiment are shown in Figure 16a. The resonances of the well-ordered phase I carbons are underlined, whereas those of the mobile phase II carbons examined in Figure 15b are not underlined. The phase II resonance for each carbon is at a higher field or a more shielded position compared with the corresponding carbon of phase I. This difference refiects the presence of the more shielding... 

Isocyanates have three possible resonance states, as shown in Figure 2.18. The reaction occurs by addition to the carbon-nitrogen double bond. In case of compounds with active hydrogen, the hydrogen atom becomes attached to the nitrogen of the isocyanate and the remainder of the active hydrogen compound becomes attached to the carbonyl carbon ... 

Compounds with high internal mobility, such as adamantane, can have line widths as narrow as 0.01 ppm. This is why in solid-state NMR, adamantane is used not only as a chemical shift standard, but also for optimizing experimental variables and for shimming probes. A typical line width encountered in crystalline organic compounds is of the order of 0.5 ppm, but the exact value will be strongly dependent on the mobility of the compound. It is not unusual to resolve the majority of carbon resonances in compounds containing 40 or more carbons. 

It was found that 46 would form 1,3,6,8-tetramethylxanthene 49 on heating in dilute solution (Scheme 24), and would crosslink if heated by itself. C CP-MAS solid-state NMR showed that the crosslinked material had a new carbon resonance at 30 ppm, and this was shown to be due to a CH2 group. Neither of these reactions occurred when bis(2-hydroxyphenyl)methane was used. Solid-state NMR was used to show that the resole resin from 3,5-dimethylphenol also had a CH2 peak at 30 ppm when heated to 300 °C. 

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