17O NMR

Variable-temperature 17O NMR Water exchange rate (l/rM), q, Tle (through measurement of 170 T2n) rR (through measurement of 170 Tln)... 

D. W Boykin and A. L. Baumstark, 170 NMR spectroscopic data for carbonyl compounds I. Aldehydes and ketones II. Carboxylic acids and derivatives, in 17O NMR Spectroscopy in Organic Chemistry, D. W. Boykin, ed., CRC Press, Boca Raton, 1991. 

D. W. Boykin, 17O NMR Spectroscopy in Organic Chemistry. CRC Press, Boca Raton, FL, 1991. 

Figure 5. Four structural possibilities for the V-TPA complex. The number of17O NMR signals predicted for each structure and the experimentally observed 17O NMR spectrum are shown below the structures.

Analysis of the apparently competing factors which cause these differences between cyclic and acyclic systems is incomplete. The effect of substituents on the 17o NMR chemical shifts of simple cyclohexenones (31-33) has been investigated [67]. Both a and P alkyl groups cause shielding of the cyclohexenone carbonyl signal (cf. 32 and 33 with 31). 

Table 4 170 NMR Chemical Shifts for Thietane and Thiete S-Oxides and Related Compounds Compound Chemical shift [p.p.m.) Solvent/17O reference Ref.

A PC-based 1H-nmr database, which includes full spectrum search capability, is being constructed by the Toyohashi University of Technology (67). Speclnfo, owned by Chemical Concepts, offers a 150,000 spectra library and database system for mainframe computers, which includes H, 15n, 19f, 17o, 31P-nmr, and a large collection of 13C-nmr spectra compiled by Bremser at BASF (68,69). It also offers nB-nmr spectra compiled by Nu th at the University of Munich. 

Within last 15 years new application of isotope effects has been developed. Natural abundance precise NMR measurements of deuterium, 13C and sporadically 17O enable determination of isotope effects for all atoms within molecules. This is a new opportunity for organic and physical organic chemists probing the reaction mechanisms. Fast development of sensitivity of modern NMR spectrometers and continuously better access to them should encourage scientists to apply KIEs measured at natural abundance. 

Figure 4(B) shows the simulated 17O MAS spectra as a function of i]q from 0 to 1. These calculations were carried out using the same conditions of the 170 stationary NMR spectra. In the similar manner as in the case of the stationary NMR spectra, the MAS spectra exhibit characteristic line shapes from which the information on t]q as well as Cq can be extracted. The anisotropy of CS tensors is removed by MAS and only <5iSO will be obtained if it exists. When the sample spinning frequency is not high enough, the effect of spinning sidebands spaced at the spinning frequency around the central peak needs to be considered in the spectral simulation. The frequency contribution from the second-order quadrupolar interaction under slow/intermediate MAS conditions is given in the literature 46,47 A complicated line shape is expected to appear in the MAS NMR spectrum so that a computer simulation is not trivial (see Figure 15).

The ONIOM method can also yield 17O EFG tensors accurately (data not shown). At the present time, this approach is one of the best calculation methods for predicting 170 NMR tensors, and is also capable of CS tensor calculations for heavy-element systems, which will be applied to various research fields. 

The natural abundance of is only 17o> and thus spectra are difficult to observe in unenriched samples. A further disadvantage is that is a less good magnet than a proton. The overall loss of sensitivity compared to H is approximately 6000-fold. However, because of the central importance of carbon in organic chemistry, constant efforts to obtain NMR data have been made over the last 15 years. Recently (1970-1971) Fourier Transform NMR spectroscopy (see Sec. 

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