CHEMICAL SHIFT

For the field strength of 1.4T, the protons have a Precessional frequency is 60 MHz. The Precessional frequency of all protons in the same of external applied field is not, however but the same and the precise values for any proton depending on number of factors. In the case of ethanol have three different values for the Precessional frequencies of the proton was detected corresponding to the three different environments, it's namely, methyl, methylene and hydroxyl groups. Because the shift in frequency depends on the chemical environment, this gave rise to the term is called " chemical shift" 

INTERNAL STANDARD IN NMR :-

                                

                  To measure the Precessional frequency of a group of all nuclei in absolute frequency units is very extremely difficult and rarely required. More commonly the difference in frequency are measured with the respect to some reference groups of nuclei. For protons, the universally accepted references is " tetramethylsilane", TMS

The reasons for choosing TMS as internal standards in nmr are :

1) it's giving an intense sharp signal even at low concentrations having 12 protons in magnetically equivalent positions. 

2) it's signals arising on the nmr spectrum well clear of most common organic protons. 

3) it's chemically inert 

4) it has a very low boiling point, so that it's is easily removed from the sample 

5) it's soluble in most of the organic solvents, but TMS is not soluble in water, hence for aqueous solutions sodium salt of 3-( trimethylsilyl) - propanesulphoric acids are used. 

UNITS OF CHEMICAL SHIFT :-

                  Usually Nmr spectrum is the plot of the intensity of absorption lines or peaks against the magnetic field strength. The peak at the high extreme right side of the spectrum is that of TMS. TMS is used as standard against which is the absorption positions of other compounds are measuring. That's the TMS absorption is placing at zero on the below chart, and the absorption positions of other compounds are measuring relative to the that of TMS.

                               The position of absorption of a proton relative to that of TMS is called " chemical shift". There are two types of ways of expression chemical shift. 

1) The first way is to measuring the number of cycles per second (hertz) downfield the line of interest is from TMS. The charts on which nmr spectra represented are calibration in Hertz, even though it's the applied field B₀ that is varied. This is permissible because it's field and frequency are proportional. 

2) The needs for a second convention for reporting chemical shift data is comes from the fact that chemical shift varies directly with the strength of the operating frequency υ (60MHz). Since the chemical shifting varies with the operating frequency, it's necessary either to specify the value of frequency is v when stating the chemical or to resort to a different types of convention for chemical shift that's independent of the operating frequency. The chemical shift δ is a quantity independent of the operating frequency :, it's defined by the ratio 

         Chemical shift in Hertz 

δ =  operating frequency in megahertz 

δ = v sample - TMS

               v

Where υ is operating frequency. 

                                    The δ unit is given in terms of " parts per million" (ppm). It a proportionality and therefore a dimensionless numbers. It's independent of field strength. Protons in common organic molecules it's give absorptions over a chemical shift range of about 0-15 ppm downfield from TMS, with most of the molecules absorbing in the 0-10 ppm range. 

UNITS :-

                 The δ units, Chemical shift values decreasing on the spectrum on going from left to right. In order to have the chemical shift values increasing from left to right, Tiers suggested that the T(tau) unit is 

                     T₀ = 10 − δ

Typical chemical shift values :

Solvents used in nmr :                                      it's advantages to use aprotic solvents. The following solvents are commonly used, many of which are normal organic solvents in which hydrogen have been replaced by deuterium. 

  CCl₄               carbon tetrachloride                 CS₂                   carbon disulphide                   CDCl₃          deuteriochloroform

C₆D₆             hexadeuteriobenzene

D₂O             deuterium oxide

(CD₃)₂SO      DMSO−d₆

(CD₃)₂CO      acetone−d₆