Chemical shift at infinite dilution

Pinkerton and Thames have carried out a new, more extensive study of pd-rc-bonding in silicon substituted furans and have slightly modified the earlier, Soviet conclusions. The aromatic protons indicate by their chemical shifts at infinite dilution that there are simultaneous but opposing +1 and... 

The 31p chemical shifts of 0.05, 0.025 and 0.0125 M solutions in deuteriochloroform were recorded, except where solubilities were limited, and the chemical shifts at infinite dilution, 6P, obtained by extrapolation. The... 

Analysing these data, the most interesting result is the possibility of obtaining experimental values of 33S chemical shift at infinite dilution, i.e. a measure of nuclear shielding in the absence of intermolecular interactions. These values can be compared with the ones obtained in condensed phases, providing an estimate of the effect of intermolecular interactions on nuclear shielding. 

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 intramolecular process (a) amounts to hindered rotation about the C—0 bond of the phenol. It may be exclusively studied therefore by obtaining accurate chemical shifts at infinite dilution in an inert solvent at a series of temperatures. The equilibrium constant can be written... 

The chemical shifts of monosubstituted thiophenes relative to the a- and )8-hydrogens of thiophene at infinite dilution in cyclohexane are given in Table I and are discussed in the following. 

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... 

Water was also investigated as a proton donor for the hydrogen bond with DMSO and other Lewis bases at infinite dilution detected by means of 1H NMR54-69. A comparison of the hydrogen bonding ability of DMSO in various other aprotic solvents was presented by Delpuech70 who measured the H-NMR chemical shift of CHC13. 

Advanced Chemistry Development Inc. has built a sizeable proton chemical shift database derived from published spectra (most commonly in CDCI3 solution). Their H NMR predictor programme accesses this database and allows the prediction of chemical shifts. Whilst this software takes account of geometry in calculating scalar couplings, in predicting chemical shifts it essentially treats the structure as planar. It would therefore seem doomed to failure. However, if closely related compounds, run at infinite dilution and in the same solvent, are present in the database, the conformation is implied and the results can be quite accurate. Of course, the results will not be reliable if sub-structures are not well represented within the database and the wide dispersion of errors (dependent on whether a compound is represented or not) can cause serious problems in structure confirmation (later). ACD are currently revising their strict adherence to HOSE codes for sub-structure identification and this will hopefully remove infrequent odd sub-structure selections made currently. 

Fig. 7. 23Na-NMR spectra of (Na[2.2.2])+Na solutions in three solvents (chemical shifts are referenced to Na at infinite dilution 5 = 0 ppm) (reproduced with permission).

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... 

Table 6-7. H NMR chemical shifts of the alkyne H-atom of phenylacetylene in various solvents at infinite dilution [273],...

Magic angle spinning Si, Al and NMR spectra were recorded on a variety of spectrometers. The data reported here are from results obtained at 200 MHz proton. Thus, the Si, Al and frequencies are 39.7 MHz, 52.15 MHz and 81.0 MHz, respectively. Chemical shifts for aluminum are reported relative to A1(N03>3 in aqueous solution at infinite dilution and are not corrected for second-order quadrupole effects. The phosphorus and silicon chemical shifts are reported relative to 85 wt% H3P0 and Me Si, respectively. 

TABLE 1-34. PROTON CHEMICAL SHIFTS AND COUPLING CONSTANTS J OF THIAZOLE IN VARIOUS SOLVENTS AT INFINITE DILUTION (20 C) (235)... 

Huggins, Pimentel, and Shoolery measured NMR shifts of chloroform in acetone and in triethylamine (982). This study furnishes corroborative evidence that the chloroform-base interaction can be classified as a H bond. More important, however, it serves as a prototype of the use of NMR chemical shifts in the study of complex formation. Huggins et al, based their analysis on an expression analogous to equation (7). They show that two data—the experimental values of 5 in pure chloroform and at infinite dilution, combined with the equilibrium constant for association—permit csdculation of the entire concentration dependence of 5. This implies that the measurement of 5 over the range from pure liquid to infinite dilution gives an estimate of the equilibrium constant. Of course the temperature dependence of the constant gives the heat of association. The appropriate equations are given in reference 982 where they are used to obtain K and A// for the association of chloroform with the bases acetone and triethylamine. 

The measurement of solute chemical shifts in mixed solvents cannot be used in general to determine the exact solvation shell composition, in the situation of fast exchange, since for a solvation number of n there are (n — 1) compositions with unknown chemical shifts. However, the existence of preferential solvation can be shown from the analysis of the curvature of a plot of d at infinite dilution against solvent mol fraction. 

The solvent shift obs, taken as the change in chemical shift for a given solute resonance in a reference phase and at infinite dilution in a solvent, may arise from a number of mechanisms, such that... 

SR = calculated from measured spin rotation constants in molecular beam experiments. Aik.Hal. = from theoretical calculations of shieldings in alkali halide crystals. Aik.Hal. (P) = from pressure dependence of alkali halide shifts in combination with theoretical models for shielding values. T2 = calculated from experimental value of line width at infinite dilution. 6VS.T2 = calculated from concentration dependence of chemical shifts and line widths. H2O/D2O = calculated from solvent isotope effects on chemical shifts. NaVwa" = estimated from difference in chemical shift between Na and Na . Atomic beam = calculated from magnetic moment of free atom determined in atomic beam experiments. 

The difficulties, described above, in accounting for alkali and halide ion shielding in the simple situation of the ion at infinite dilution may look discouraging for the use of ion shielding data to probe into the interactions of the ions with other species in more complex systems. Indeed, quantitative interpretations of the changes in halide ion chemical shift with changes in solution composition are difficult and considerations in the literature have mainly been confined to... 

Fig. 6.1. Cl chemical shifts (in ppm) at 25°C for aqueous alkali chloride solutions as a function of salt molality. The shifts are given relative to the Cl" ion at infinite dilution in water with a positive shift denoting a shift to lower applied field. (From Ref. [2S0])...

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