Nuclear magnetic resonance NMR data

The Si MAS NMR spectra of Fe-mordenite show peaks around S= -100 ppm due to silanol groups and around 5= -110 ppm due to silicon atoms (SiOSi) [91C2]. In ferrimordenite, the analysis of A1 NMR line confirmed the absence of significant amounts of A1 in the sample. 

The A1 mas and MQ MAS NMR spectroscopy methods were used to study the A1 coordination in the parent H-mordenite and dealuminated sanqile [05C1]. The parent H-mordenite displayed signals at 55 and 0 ppm corresponding to framework tetrahedral and octahedral coordinated A1 species, respectively. The sample calcinated at 923 K depicts additionally a shoitlder at 5 = 30 ppm, attributed to a combination of both distorted [4]ai (WaID) and Al. In the ammoniirm-treated sample calcinated at 923 K most of the peak at S= 0 ppm was removed, but the shoulder at 5= 30 ppm remained. There are present both and Al species. The sample calcinated at 923 K and treated in autoclave in a anunoniiun-water solution evidenced one peak at 55 ppm and a shoitlder at 30 ppm corresponding to Al and AID, respectively. When dealuminated zeohtes were treated at 353 K in basic solutions, a combined dissolution and realumination phenomenon could occur [89K2, 05C1]. The calcination at 673 K of the ammonium-water-treated sample renders a tine at S = 0 ppm and a shorrlder at 30 ppm In the MQ MAS spectrum, besides A1, AID, and A1 species, traces of A1 were detected -Fig. 35a,b,d. The ammonium-water thermal treatment transformed almost all penta- and octahedral A1 into 

Multinuclear H, A1, Si MAS NMR and (HP) H/ Si CP NMR spectroscopic techniques were used in order to evaluate the dealumination process during steaming and leaching of mordenite [04Z2], A direct evidence was found for the existence of non-framework 4-coordinated Al species Al(OH)3-H20, giving rise to a 

The acidic properties were further studied by NMR, in correlation with Si/Al ratio of synthetic mordenites [98D1, 02K1, 1 IHl], According to [1 IHl], the H MAS NMR spectra were attributed to hydrogen-bonded Si-OH, Bronsted acid site, tighdy adsorbed water, Al-OH, and isolated Si-OH - Table 11. The H chemical shifts of Bronsted acid sites without proton exchange are in the following order Si/Al = 7.5 Si/Al = 9.0 Si/Al = 11.5. 

The acid strength of Bronsted acid sites increased with Si/Al ratio. Tiimethylposphine oxide (TMPO) was adsorbed as a probe molecule from the gas phase to avoid solvent effects. The MAS NMR spectra of TMPO-adsorbed samples showed a line at 5 s 46 ppm, ascribed to physisorbed or weakly adsorbed TMPO and a group of signals between 80 and 50 ppm ascribed to TMPO adsorbed on Bronsted acid sites or chemisorbed species. The analysis of the spectra showed that the average P chemical shift of chemisorbed TMPO and consequently the acidic strength increased with the Si/Al ratio. The multiple components of the P MAS NMR spectra showed that there are several kinds of sites with different acid strength. The Si and Al MAS NMR spectra were also studied - Table 11. The A1 spectra showed the presence of extra-framework A1 as well as invisible Al. 

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The (5,5) (2N2)-fused heterocyclic system contains three ring carbon atoms, one fusion carbon atom, and one additional nonfusion carbon atom in each five-membered ring. Only scattered H and 13C nuclear magnetic resonance (NMR) data are available for these systems. 

Similarly to the above section, no additional specific nuclear magnetic resonance (NMR) data have been published since CHEC(1984) and CHEC-II(1996) <1996CHEC-II(8)249> NMR data for new substituted compounds are routinely reported. Several X-ray structures of bioactive molecules possessing this heterocyclic core have been reported C1999BML1979, 1999JME779>. 

Proton nuclear magnetic resonance (NMR) data for pyrimidines was reviewed in CHEC(1984) together with C and NMR data, and NMR data for quinazoline and perimidines was discussed in CHEC-11(1996). A tabulation of H, N, and N NMR data for simple pyrimidines is available in the book by Brown <1994HC(52)1>. N... 

A fair amount of H nuclear magnetic resonance (NMR) data for various 1,4-oxazines exist, but the observed chemical shifts depend heavily on the substitution pattern as well as the number of ring double bonds. Representative data for most of the known types of 1,4-oxazines and dihydro-l,4-oxazines are given in Table 4. 

There is sufficient H nuclear magnetic resonance (NMR) data available on the various structural types of 1,3-thiazines to be able to predict the chemical shift of the ring protons <1996CHEC-II(6)383>. A compilation of the data for some new derivatives is displayed in Table 1. 

Proton nuclear magnetic resonance (NMR) data of parent A,B-diheteropentalenes have been reviewed <1984CHEC(4)1037, 1984CHEC(6)1027, 1996CHEC-II(7)1>. 

Selim and Selim152 reported nuclear magnetic resonance (NMR) data on various aminooxadiazole derivatives. Komatsu and co-workers120-122 recorded NMR data on oxadiazolidones. 

Comparison of Calorimetric and Nuclear Magnetic Resonance (NMR) Data... 

Essentially all publications published in the period since CHEC-II(1996) dealing with the synthesis, structural characterization, or properties of pyran-containing molecules are replete with nuclear magnetic resonance (NMR) data. While the empirical rules summarized in CHEC(1984) and CHEC-II(1996) remain of value for the... 

The design of these analogs was guided by preliminary nuclear magnetic resonance (NMR) data on the bioactive (tubulin-bound) conformation of epothilones, which indicated that the C16/C17 double bond and the aromatic C18-N bond were present in a transoid arrangement (corresponding to a 180° C16-C17-C18-N22 torsion angle). (These data have subsequently been consolidated and have recently appeared in the literature ). 

The structurally important nuclear magnetic resonance (NMR) data was thoroughly established in the previous chapter of CHECH <1996CHEC-II(1)333>. Since most of the structural evidence is based on "B NMR data, the values previously reported are given for comparison purposes. The "B chemical shifts for structures of type 1 are in the range from 70 to 73 ppm, for type 2 from -9 to -27 ppm, the type 3 compounds have values of 34-52 ppm, for 4 between 27-49 ppm, and for 5 between -10 and -20 ppm. Compound 6 was only determined theoretically. 

Most of the publications dealing with heterocycles contain nuclear magnetic resonance (NMR) data. While H and NMR data are the most important, NMR data are particularly relevant to boron-containing heterocyles. The H, B, and NMR data of unsaturated, anionic, and transition metal 7t-coordinated rings are useful probes of their electronic structures. 

Before the availability of Nuclear Magnetic Resonance (NMR) data revealing three a helices in a recombinant PrP (Riek et al., 1997), Muramoto et al. (1997) used a four-helix-bundle model of PrP (Huang et al, 1994) to construct proteins with each of the predicted helical regions deleted. The aim was to assess the role of each region of the putative secondary structure, as well as those adjacent and intervening... 

The dimethylarsinoyl derivative of sulfated ribitol (see Fig. 2, compound 15) was isolated from the red alga Chondria crassicaulis (38). It had been observed as a major arsenical in C. crassicaulis by high performance liquid chroma-tography-inductively coupled plasma mass spectrometry (HPLC-ICPMS), and was initially reported as an unknown because it did not match any available standard (39). Subsequently, the compound was isolated and a chemical structure was proposed chiefly on nuclear magnetic resonance (NMR) data chemical synthesis of authentic material confirmed the proposed structure (38). This com-... 

Some H and C NMR data for selected 1 -oxa-2,5-diazine systems are shown in Table 2. Additional examples may be found in the indicated references. No recent nuclear magnetic resonance (NMR) data are available for the corresponding thiadiazine derivatives. The reader is referred to Section 6.15.2.1.2 in CHEC-II(1996) for some earlier data. 

In summary, all semi-empirical MO methods discussed above have their strong and weak points, which unfortunately are not always known prior to the actual calculation. As with all evaluations of theoretical methodologies, making contact and comparing with experimental data is always a wise thing to do. It should be borne in mind that quantum chemical calculations are most often done on the isolated molecule, that is, in absolute vaeuum. More often than not, the results of these calculations are compared to the results obtained from X-ray (crystalline state) or nuclear magnetic resonance (NMR) data (liquid state), from which it follows that discrepancies between theoretieal and experimental results are not necessarily a reflection on the validity and aeeuracy of the former. 

According to the majority of the theoretical and experimental studies available in the literature, the simpler reasonable description of a proton in solution is the hydronium (HjO ) ion. The shape and size of the HjO" ion is fairly well established from nuclear magnetic resonance (NMR) data on solid acid hydrates (Bockris and Reddy [90]). The HjO ion exhibits a rather flat trigonal pyramidal structure with the hydrogen atoms at the corners of the pyramid and the oxygen at the center (Fig. 7.3). But no reliable experimental geometric information is available for the hydronium ion in solution. 

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