Question

what is the effect of substitute group on cnmr shift for benzene? in details,

Answer 1

Solvent The changes in 1 H resonance frequency affected by the solvent are usually smaller than 1 ppm. Complications caused by solvent effects can be avoided by using "inert" solvents (carbon tetrachloride, cyclohexane). On the other hand, strong solvent effects can be used in order to "simplify" the spectrum: if an aromatic solvent such as benzene is used, then this will solvate areas of low electron density and, due to ring current effects, this can change chemical shifts by as much as 1 ppm. NMR Spectroscopy 7 Temperature The chemical shift difference between the resonance signals in the 1 H NMR spectrum of a sample of liquid methanol is temperature dependent and is used for the purpose of accurate temperature calibration: as the temperature rises, the extent of H-bonding diminishes, and the resonance for the hydroxyl proton moves to low frequency, towards the resonance signal of the methyl protons. Unpaired electrons. Shift reagents The interaction between electron and nuclear spin manifests itself as the hyperfine splitting of the signal in ESR experiments. In NMR experiments this interaction leads to a shift of the signal to higher or lower frequency relative to the signal of the same proton in the diamagnetic compound (contact shift). Paramagnetic impurities significantly accelerate the spin-lattice relaxation of protons and cause severe line-broadening. The effects of paramagnetic lanthanide shift reagents can be used to simplify the 1H NMR spectra. A lanthanide shift reagent is an octahedral complex of a lanthanide element (Eu, Dy, Pr, Yb) with a ligand chosen to make the complex soluble in NMR solvents, and this is dissolved in a solution of the compound. The lanthanides are capable of assuming higher coordination numbers than 6 so that if the organic molecule possesses a suitable coordination site (generally, any atom with an available lone pair) it can interact with the complex. This produces a pseudocontact shift of the protons of the organic molecule. 13C chemical shifts Since the organic chemist is interested primarily in the molecular carbon skeleton, a 13C NMR spectrum yields structural information much more directly than a proton spectrum: quaternary carbons, as those of many functional groups (CN, C=O, C=NR), are detectable. In some cases, 13C NMR easily distinguishes between groups that might otherwise be confused, e.g. cyanide (110-120 ppm) and isocyanide (135-150 ppm).