Chemical shift hybridization

NMR spectra are basically characterized by the chemical shift and coupling constants of signals. The chemical shift for a particular atom is influenced by the 3D arrangement and bond types of the chemical environment of the atom and by its hybridization. The multiplicity of a signal depends on the coupling partners and on the bond types between atom and couphng partner. 

There is one semiempirical program, called HyperNMR, that computes NMR chemical shifts. This program goes one step further than other semiempiricals by defining different parameters for the various hybridizations, such as sp carbon vs. sp carbon. This method is called the typed neglect of differential overlap method (TNDO/1 and TNDO/2). As with any semiempirical method, the results are better for species with functional groups similar to those in the set of molecules used to parameterize the method. 

Acetylenic hydrogens are unusual in that they are more shielded than we would expect for protons bonded to sp hybridized carbon This is because the rr electrons circulate around the triple bond not along it (Figure 13 9a) Therefore the induced magnetic field is parallel to the long axis of the triple bond and shields the acetylenic proton (Figure 13 9b) Acetylenic protons typically have chemical shifts near 8 2 5... 

Table 13.3 shows the correlation of H chemical shift with electronic environment in more detail. In general, protons bonded to saturated, sp3-hybridized carbons absorb at higher fields, whereas protons bonded to s/ 2-hybridized carbons absorb at lower fields. Protons on carbons that are bonded to electronegative atoms, such as N, O, or halogen, also absorb at lower fields. 

The 13C NMR data for representative three-membered sulfones and sulfoxides are given in Table 5. The chemical shifts of the sp3-hybridized a-carbon in the parent thiirane70 and the five-membered ring86 sulfide, sulfoxide and sulfone are 18.1, 31.7, 54.3 and 51.1, respectively, whereas those of cyclopropenone, diphenylcyclopropenone and dimethylcyclopropenone are 169.087, 148.788 and 157.9, respectively. 

Another useful tool for analyzing the structure of heteroallenes containing silicon is Si NMR. Typically, sp"-hybridized silicon atoms have a chemical shift... 

Several trends have emerged in the extensive carbon-13 NMR spectroscopy data that have been accumulated for sulfones and sulfoxides. Based on many studies of cyclic systems—particularly five- and six-membered ring sulfur compounds—these trends were shown to generally apply equally to both the cyclic and acyclic systems . Thus (a) oxidation of a sulfide to a sulfone results in a 20-25 ppm downfield chemical shift for sp -hybridized a-carbon atoms and 4-9 ppm upfield shift for / -carbons , and (b) there is very little difference between the chemical shifts of a-carbon atoms of sulfones and sulfoxides despite the difference in the inductive effects of these two functional groups . A difference is observed, however, in the H chemical shift of related cyclic sulfoxides and sulfones . 

Bienati, M., Adamo, C., Barone, V., 1999, Performance of a New Hybrid Hartree-Fock/Kohn-Sham Model (B98) in Predicting Vibrational Frequencies, Polarisabilities and NMR Chemical Shifts , Chem. Phys. Lett., 311, 69. 

The chemical shifts of jp3-hybridized silicon atoms, such as a trimethyl-silyl group attached to the jp2-hybridized silicon in silenes, are normally found from -10 to -18 ppm, as compared to the position at -9.8 ppm for the trimethylsilyl groups of (Me3Si)4Si (the central silicon atom of this compound resonates at -135.5 ppm). On the other hand, a trimethylsiloxy group attached to carbon, either sp2- or sp3-hybridized, normally absorbs in the range from +5 to +20 ppm. 

Although no 73Ge NMR spectra of stable doubly bonded germanium compounds have been recorded, NMR chemical shift data for the heteroatom of the double bond are available these are listed in Table II. The chemical shift of the, sp2-hybridized carbon atom of dimesitylneopentylger-mene (entry 1, Table II), without any polar substituents, falls at 8 124.2.26 The chemical shifts of the boron-substituted sp2-hybridized carbon atoms of two stable germenes (entry 2, Table II) fall in the upfield region of the... 

It is not clear why the sp2-hybridized carbon atom of a fluorenylidene germene (entry 3, Table II) is so shielded (8 79.8), unless this chemical shift is from the spectrum of the THF adduct.28 The chemical shift of the silicon atom in the germasilene (entry 4, Table II) and the phosphorus atom in germaphosphenes (entries 6-10, Table II) is deshielded compared to the corresponding silicon analogs. For example, for Mes2M=SiMes2... 

The relationship between the structure of 1,2,4-thiadiazolidines and their H NMR spectral solvent effects has been studied by measurement of the NMR chemical shift differences (Av) of 39 derivatives in various solvents (CgDg, CCU) for methyl or methylene groups attached to an sp2-hybridized nitrogen, Av correlates linearly with Hammett a constants and for those attached to an sp3 4-hybridized nitrogen, with Taft a° constants <1982AHC285>. 

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