Deshielded in NMR

Figure 1.2. H NMR spectrum of ethyl dichloroacetate (CDCI3, 25 °C, 80 MHz). The proton of the CHCh group is less shielded (more strongly deshielded) in comparison with the protons of the CH2 and CHj residues...

Carbons next to amine nitrogens are slightly deshielded in the NMR spectrum and absorb about 20 ppm downfield from where they would absorb in an alkane of similar structure. In N-methylcyclohexylamine, for example, the... 

Deshielding (Section 13.2) An effect observed in NMR that causes a nucleus to absorb downfield (to the left) of tetramethylsilane (TMS) standard. Deshielding is caused by a withdrawal of electron density from the nucleus. 

The bonding situation in the stannanorbornyl cation 62 is reminiscent of the intriguing zwitterions of the types 63 and 64 in which triorganotin cations are intramolecularly stabilized by side-on coordination with an alkynyl group. The zwitterions 63 with a [3-l-1] coordination for the tin atom are characterized by a significantly deshielded Sn NMR resonance and sizeable j(SnC) couplings to the alkynyl carbon atoms (for = alkyl and R = Et 5" Sn = 160-215 J(SnC ) = 41 8Hz, J(SnC ) = 60-80 Coordination with the amino... 

Unlike the 13C-NMR method, H-NMR spectra are not applicable to 3-alkylaldehydes. For 3-arylaldehydes, the chemical shifts of 2-H, 3 -H, and 5-H appear deshielded in oxazolidines derived from chiral aldehydes with configuration A, where Rz is the aryl group. 

A proton NMR study by Mirek and Bogdanowicz-Szwed128 observed that the 5-proton was significantly deshielded in 3-amide, -thioamide, and -amidine derivatives, and this established the preferred conformation as 83. 

To study the possible stabilizing effect of [3-silyl cations, Olah and co-workers334 prepared the 2- [(1 -trimethylsilyl)vinyl]-2-adamantyl cation 132 [Eq. (3.43)] as well as the parent silicon-free carbocation. In contrast to the above observations, NMR data [the (Cl ), (C2), and (C2 ) carbons are more deshielded in 132 than in the parent ion] showed that cation 132 is destabilized compared with the silicon-free analog. Furthermore, at — 100°C the C(l) and C(3) carbons were found to be equivalent, whereas in the parent ion they were nonequivalent. This indicates a rapid rotation about the C(l)-C(3) bond in 132, which can be rationalized by assuming the intermediacy of the [3-silyl-stabilized cation 133. The difference between cation 132 and those having [3-silyl-stabilization discussed above may be the orthogonal arrangement of the [3-C-Si bond and the p-orbital of the carbocation center. 

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