Chemical shift calculations resonance

C NMR spectra are recorded for a low molecular weight atactic PP dissolved in a variety of solvents over a broad temperature range [293 - 393 K). Comparison of chemical shifts calculated via the y effect method with the observed resonances, whose relative chemical shifts are solvent independent, permits their assignment to most of the methyl heptad, methylene hexad, and methine pentad stereosequences. Agreement between observed and calculated chemical shifts requires y effects, he., upfield chemical shifts produced by a gauche arrangement of carbon atoms separated by three bonds, of ca. - 5 ppm for the methyl and methine carbons and ca. - 4 ppm for the methylene carbons. 

Fig. 14.4 ACD/NNMR chemical shift calculation for the simple alkaloid harmaline (2). The N6 resonance, as expected, has a chemical shift in a range typical for indoles while the N1 resonance exhibits a calculated chemical shift more typical of that of pyridine, which is well outside the Fi window provided for the survey conditions shown in Figure 3A.

Vibrational Frequencies and Nuclear Magnetic Resonance Chemical Shift Calculations... 

Bisquaric acid was also studied by 13C CPMAS NMR between 123 and 523 K, with powdered crystals.173 This material has also potential for nonlinear optical and dielectric applications. The low-temperature spectra resolve three peaks instead of four in SQA. This compound has no dipole moment, and no phase transition was detected in the studied temperature range, although the lineshape suggests the occurrence of a phase transition below 373 K. An explanation proposed by the authors is the lack of resolution due to the accidental overlapping of the two resonances of the C OH and C — O carbons participating to the HBs, an interpretation also supported by GIAO ab initio chemical shift calculations. 

Fig. 15.2-17 Simulation of NMR spectra of a two state DFG-in/DFG-out model. The grayscale represents the relative maximum peak intensities of the l5N-amide resonance of Phel69 as a function of the exchange rate and the population of the out state. The magnetic field strength is set according to a 1H resonance at 600 MHz. The chemical shift difference was set to 13.7 ppm, as predicted by chemical shift calculations applied to the published X-ray structures (1 P38.pdb and 1 KV1. pdb). The lowest...

Quantum chemical nuclear magnetic resonance (NMR) chemical shift calculations enjoy great popularity since they facilitate interpretation of the spectroscopic technique that is most widely used in chemistry [1-11], The reason that theory is so useful in this area is that there is no clear relationship between the experimentally measured NMR shifts and the structural parameters of interest. NMR chemical shift calculations can provide the missing connection and in this way have proved to be useful in many areas of chemistry. A large number of examples including the interpretation of NMR spectra of carbocations [12], boranes [10, 13], carboranes [10, 13-15], low-valent aluminum compounds [16-18], fullerenes [19-21] as well as the interpretation of solid-state NMR spectra [22-26] can be found in the literature. 

On the other hand, very recently Asakura et al. reported application of high-resolution H SS NMR performed under F-MAS and GIPAW chemical shift calculations for assignment of resonances and structure of alanine tripeptides [168]. The information on the exact H positions is important for fine refinement of peptides because their higher order structure is determined mainly by the intramolecular and intermolecular hydrogen bonds. The homonuclear DQMAS experiment allowed to precisely assign proton signals in the H spectra of (Ala)3 (Fig. 2.34). 

I. Ando, S. Kuroki, H. Kiarosu, T. Yamanobe, NMR chemical shift calculations and structural characterizations of polymers. Prog. Nucl. Magn. Reson. Spectrosc. 39 (2001) 79-133. 

When not assigned in the literature [10, 11], resonances were attributed on the basis of chemical shift calculations according to the y-gauche effect [12] and by comparative analysis with the 150 MHz C-NMR spectra of samples of isotactic... 

Except for ozonides obtained from vinylsiloxanes, the ozonides that result from these reactions are surprisingly stable. Figure 1 shows the C-NMR spectrum of an ozonide that was prepared from DC 7697 (n = 30). It contains resonances at 1, 18, 23, 29, 41, 94 and 104 ppm. Chemical shifts calculated for the carbon atoms in this compound (1.5(3,4), 15.7(1),24(2),26.3(6),30.4(7),97.3(5) and 99.3(6) ppm) correspond reasonably well with those observed. Polysiloxanes containing the ozonide functionality can, in fact, be used as macroinitiators for block copolymer synthesis (13). 

Comparison of the methyl resonances in P-VC and PP reveals a decreased sensitivity to stereosequence for the P-VC copolymer. The methyl carbon resonances in P-VC are sensitive to pentad stereosequences, whereas in PP heptad sensitivity is observed. In Table 2.6 the C chemical shifts calculated for the methyl carbons in several heptad stereosequences of P-VC and PP are compared. As observed, the methyl carbon chemical shifts calculated for P-VC are sensitive to pentads, but PP methyl carbons show significant heptad sensitivity. This difference in stereosequence sensitivity between the methyl carbons in P-VC and PP is directly attributable to differences in their conformational behavior as embodied in their RIS models. Local bond conformations reflect pentad sensitivity in P-VC and heptad dependence in PP. In addition, note that the overall spreads in methyl carbon chemical shifts observed in P-VC and PP are 2.7 and 2.0 ppm, respectively, with the P-VC methyl carbons resonating about 1 ppm upfield from those in PP. These observations are also reproduced by the calculated chemical shifts, which employ the same y-effect (ycHj.cH = Ppm)... 

These calculations indicate that the peaks at 48-52 ppm represent carbons bonded to sulfur (a to sulfur) in polysulfidic crosslinks and tertiary carbons in monosulfidic cyclic structures. The resonances at 34-40 ppm are due to methylene carbons to sulfur in monosulfidic and polysulfidic crosslinks and methylene carbons in the monosulfidic cyclics. The chemical-shift calculations suggest that the 48-52-ppm region contains all methine carbon resonances and that the resonances in the 34-40-ppm region are all methylene carbons. 

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