TiO2 and ZnO surface studies by electron spin resonance spectroscopy

The Molecular Structure of Allenes and Ketenes VII [1] 13CH Nuclear Spin‐Spin Couplings in Monosubstituted Allenes

“…The resulting long-range coupling constants of monosubstituted allenes are given in Table 2. In agreement with the generally adopted opinion [2] weassumeall the three-bond couplings in allenes to be positive. The determinations of the signs of the twobond coupling constants require further comment which will be given in 4.2.…”

“…The (relative) signs of the geminal spin-spin coupling constants were determined relating them to the absolute signs of some key values (of C -H couplings), which have been ascertained by appropiate NMR experiments, or by comparisons with substituent effects in C -H and H -H couplings. Another basis for the sign determinations was the assumption that electronegative substituents tend to increase the geminal carbon-proton coupling constants [2] and that in hydrocarbons two-bond carbon-proton coupling constants follow an overall pattern similar to that found for one-bond couplings From geminal carbon-proton coupling constants in acetylenes [2] it is known that phenyl and alkyl groups affect these coupling constants only to a negligible extent. If one assumes the general validity of a linear relationship between long-range carbon-proton and proton-proton coupling constants…”

“…In vinyl and methyl compounds 'J(13C-H) is increased substituting hydrogen for a phenyl group (C,H,) [2a, 22,231, whereas in allenes the one-bond coupling constants decrease as is observed for keteneimines 1251. Consequently, there is no general correlation between the one-bond coupling constants involving the substituted carbon atom of allenes and ethylenes [2,12,24] (Fig. 3).…”

“…The sign determinations for systems with substituents bonded via heteroatoms to the allenic skeleton (TI*) are based upon the observation that electronegative groups tend to increase the geminal carbon-proton coupling constants [2]. This requires a negative sign for trimethylsilylallene 3 and positive signs for all the other compounds.…”

“…' 3C-NMR-spectroscopy has been demonstrated to be an appropriate tool for the investigation of the electronic structure of carbon molecular skeletons [2]. '3C-chemical shifts of allenes have been reported recently [3,4] and discussed in part IV and part V of this series C3.51.…”