Carbon chemical shifts dilution

PDHS Structures in Solution. The determination of the chain conformation of polysilylenes in solution, particularly the conformations at temperatures just above or below the low-temperature thermochromic transition, is of great interest. NMR spectroscopy is one of the most useful techniques for probing chain conformation in solution (2i), and NMR is especially effective because of the large sensitivity of the carbon chemical shift to bond conformation (22). Silicon nuclei are also very sensitive to chain conformation, but a good correlation between silicon chemical shift and bond conformation has not been established yet. Unfortunately, both of these nuclei suffer from low sensitivity, primarily because of their low natural abundance. In contrast, protons have an essentially 100% natural abundance, but compared with the carbon or silicon chemical shift, the proton chemical shift is not very sensitive to bond conformation. Efforts to use NMR to probe the low-temperature dilute-solution conformation of the polysilylenes have been unsuccessful thus far. The diflSculty is that PDBS and PDHS precipitate from solution in 20-30 min after cooling through the thermochromic tran-... 

The electronic effects of many substituents have been examined by studies of PMR118,119 sulfinyl and sulfonyl groups have been included in some of these. For example, Socrates120 measured the hydroxyl chemical shifts for 55 substituted phenols in carbon tetrachloride and in dimethyl sulfoxide at infinite dilution, and endeavored to... 

There is a continual tendency for the values of cr/ and aR (and other substituent chemical shifts for a series of mono-substituted benzenes in very dilute solution in cyclohexane, carbon tetrachloride or deuteriochloroform were the basis for a redefinition of the aR scale and some amendment of a R values. However, the value for the nitro group was confirmed as 0.15. 

The 119Sn chemical shift of dimethyltin dichloride in carbon tetrachloride and other non-polar solvents remains practically invariant to large changes in concentration. It has a value of ca. +140 ppm. This indicates the ease with which the molecules are able to dissociate into discrete tetrahedral species in solution as a result of the very weak inter-molecular Sn... Cl bonds which exist in crystalline dimethyltin dichloride. (55) On the other hand, a chemical shift-concentration study of trimethyltin formate in deuterochloroform solution (56) has revealed a dramatic change in chemical shift from +2-5 ppm for a 3 M solution to + 152 ppm on dilution to 0-05 m in the same solvent. This has been attributed to self-association of monomeric tetrahedral trimethyltin formate molecules, [3]. As the concentration is increased, five-coordinate oligomeric or polymeric species, [4], could be formed. These are known to exist in the solid state. (57)... 

Cogly, Butler and Grunwald were the first who determined solvation equilibrium constants from chemical shift measurements. The chemical shifts of water in propylene carbonate containing various salts were extrapolated to zero water concentration. The dependence of the chemical shift of water at infinite dilution in PC... 

Recent studies33 of solvent and concentration effects on the NMR of indoles have established that, in addition to the 2-proton, the 7-proton resonance in certain substituted indoles also undergoes downfield shifts in more polar and more dilute solutions. The chemical shifts of protons of indole and several methylindoles in two solvents, carbon tetrachloride and tetrahydrofuran, are listed in Ref. 33. Proton chemical shifts of the... 

Fig. 1. Linear correlation between NMR chemical shifts at infinite dilution of CF3I in various electron pair donor solvents and donicity (DN), referred to CCI3F as external reference. NB, nitrobenzene BN, benzonitrile AN, acetonitrile PDC, propanediol-1, 2-carbonate THF, tetrahydrofuran DMF, dimethylformamide EC, ethylene carbonate TMP, trimethyl phosphate DMA, N,N-dimethylacetamide DMSO, dimethylsulfoxide HMPA, hexamethyl-phosphoric amide.

The data in all of the tables are for the compound in dilute carbon tetrachloride or deuteriochloroform relative to internal TMS where such data were available. It is important to remember that solvent effects, especially in the case of aromatics, can cause significant variation in the observed chemical shifts. All chemical shifts listed are in ppm or 6. 

Figure 6 shows the chemical shift of the a-carbon of hexanol as a function of composition in three different kinds of solutions. The first represents hexanol in a 14.7 wt% SLS aqueous solution as a function of hexanol concentration. The concentration range covered runs from very dilute in hexanol up to systems in which a fairly viscous phase results. Next are the microemulsions containing a 25, 50, and 67% excess of MMA over that which saturates the corresponding L1 phase system. Lastly are water-saturated MMA solutions of hexanol in which the hexanol/MMA volume ratios bracket those found in the microemulsion. The a-carbon shifts of hexanol measured in the micellar solutions are about 0.6 ppm downfield from those measured in MMA solution. There is only a slight variation in shift for either the micellar or the MMA solutions with various hexanol concentrations. The hexanol in the microemulsion systems appears approximately halfway between that in the other two solutions. 

Solvent. For all practical purposes, NMR spectra are recorded in solution, although pure ( neat ) liquids and even gases can, in principle, also be examined. The solvents must meet certain requirements (Sec. 12.1) and a compromise must often be employed between using concentrated solutions (for high sensitivity) and dilute solutions (for measuring chemical shifts uninfluenced by solute-solute interactions). Besides the commonly used carbon tetrachloride, deuterochloroform, and D2O, a range of deuterated solvents (dimethyl sulfoxide, benzene, pyridine, acetone, dioxane) is commercially available. It must be emphasized that direct comparison of chemical shifts obtained in different solvents is invalid, as solvent-induced changes of up to... 

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