Large Chemical Shift Differences Research Articles

Dependence of 19F chemical shielding on internal coordinates in CF4, SiF4, and BF3

The thermal average of chemical shielding for CF4-, BF3-, SF6-, HCN-, CO2-, and NH3-type molecules is explicitly expressed in terms of molecular constants such as fundamental frequencies and elements of the L matrix for the totally symmetric vibration, Coriolis constants, moments of inertia, and cubic force constants kss′s′. Expressions for the klss cubic force constants for a BF3-type molecule are derived and values of klss are calculated for CF4, SiF4, and BF3. The thermal average of chemical shielding calculated using these cubic force constants is compared with the experimental temperature dependence of the 19F NMR signal, yielding values of (∂σ/∂R) for CF4, SiF4, and BF3. Despite the large differences in chemical shifts the values of (∂σ/∂R) obtained are nearly identical: 1115, 1170, and 1115 ppm/Å, respectively.

A conformational study of adenylyl-(3',5')-adenosine and adenylyl-(2',5')-adenosine in aqueous solution by carbon-13 magnetic resonance spectroscopy.

The solution conformation of adenylyl-(3',5')-adenosine and adenylyl-(2',5')-adenosine in both the stacked and unstacked states was studied by carbon-13 magnetic resonance spectroscopy. Large chemical shift differences between the base carbons in the dimers and those in the corresponding monomers are attributed in part to the influence of base-base interaction. Carbon-phosphorus couplings across three bonds revealed the preferred populations for certain backbone rotamers, demonstrating that significant changes in conformation about the "c(3')-O and C(5')-O bonds do not occur in the temperature or salt-induced unstacking of adenylyl-(3',5')-adenosine. However, rotations about the C(2')-O and C(5')-O bonds occur in the temperature-mediated unstacking of adenylyl-(2',5')-adenosine.

Carbon-13 NMR of Polymers

Abstract Recent advances in 13C NMR spectroscopy instrumentation are causing an almost-explosive flurry of activity in organic molecular structure studies. This is the most powerful molecular structure tool to come upon the scene since proton NMR, and it will undoubtedly surpass the latter in importance. There are several reasons for this: (a) The l3C chemical shifts cover a range of about 300 ppm compared to only about 12 ppm for proton chemical shifts, i.e., there are more “boxes” in which to display the same information (b) 13C chemical shifts are much more sensitive to steric environment than proton chemical shifts. Thus two carbon nuclei whose local environments are comparable might have relatively large chemical shift differences. These shift differences depend primarily upon the local π-electron density, whereas proton shifts are more strongly influenced by neighboring bond anisotropy shielding effects [1,2]. (c) 13C spectra are simpler to interpret because all spin-spin interactions can be removed...

3-Substituted thietane derivatives : Stereochemical considerations

Stereochemical assignments could be made for a series of 3-substituted thietane 1-oxides from their NMR spectra. Both isomers appear to reside in conformations with the ring substituent equatorial. The cis isomer (equatorial SO bond) is characterized by a large chemical shift difference for the α-methylene hydrogens. The trans isomer is characterized by a significant deshielding of the β-hydrogen (relative to cis isomer, not relative to the parent sulfide). The dominant conformations of 3,3-dimethylthietane 1-oxide and the corresponding 1-tosylimine have the SX bond equatorial. Stereochemical assignments were made to the α-methylene protons and were confirmed by benzene-induced shift data. As expected for a hydrogen anti to the “lone pair” on sulfur the axial hydrogen of the sulfoxide appeared at higher field, the same was not true for the sulfilimine. Significant stereoselective benzene-induced shifts observed for the N-tosylsulfilimine and N-tosylsulfoximine of 3,3-dimetbylthietane indicate that they may constitute a method of configuration assignment for such compounds.

A heteronuclear double and triple magnetic resonance study of an AA′MM′X spin system

Heteronuclear 1H-{19F} double resonance experiments are used to obtain the magnitudes and some of the relative signs of the aromatic H-F and F-F coupling constants in 2,3,5,6-tetrafluoroaniline. It is shown that the precision attained is equal to that of the single resonance technique with direct observation of the 19F spectrum, even in the case of this complex spectrum. The large 19F chemical shift difference leads to certain degeneracies of transitions in the proton spectrum and hence to ambiguities in the signs of some of the coupling constants. These are resolved by novel triple resonance tickling experiments.

Nuclear magnetic resonance studies on the configurations and conformations of heterocyclic nitrosamines

The nuclear magnetic resonance spectra of various heterocyclic nitrosamines were studied in solution. A qualitative view of the long-range diamagnetic effects of a nitrosamino group is advanced and correlated with the chemical shifts of α-equatorial protons and the methyl group in those nitrosamines possessing a preferred conformation or a rigid system. The use of this correlation to decide the configuration and conformation of heterocyclic nitrosamines is discussed. A large chemical shift difference (ca. 3 p.p.m.) between a cis α-axial and a cis α-equatorial proton was noted.

Rotational isomerism in epifluorohydrin

The complete analysis of the 1H and 19F n.m.r. spectra of epifluorohydrin in four different solvents is reported. The parameters obtained are used to predict the predominant rotational isomers of the molecule. The long-range coupling constants to fluorine are opposite in sign, in accordance with recent results, and the long-range proton–proton coupling constants are small or negligible. A large chemical shift difference between the protons of the CH2F group is explained on the grounds of the proposed rotational isomers, and the anisotropic effect of the epoxide ring.

NMR Studies of Polyaldehydes

Abstract The large differences in chemical shifts that result from oxygen's being incorporated into the polymer backbone yield more accurate qualitative and quantitative analysis through NMR than those polymers that do not have oxygen in the backbone. Configuration effects are observed without interference from methylene groups that frequently interfere in the analysis of polymers. However, the results obtained on polyacetaldehyde with the new 200-MHz Varian Instrument at ambient temperature show a much more complete definition of the configuration than can be obtained with lower-frequency instruments. They show that there are more isotactic triads than syndiotactic triads for the methyl group in “atactic” polyacetaldehyde and that this polyacetaldehyde is nearly a random polymer. By means of the new 220-MHz Varian Instrument and heating of the samples to elevated temperatures for better-resolved lines complete configurational analysis of polyacetaldehyde should become possible.

A large chemical shift difference for geminal protons

A large chemical shift difference for geminal protons