Q13-31E - Organic Chemistry (Mcmurry)

Question

Question: When measured on a spectrometer operating at 200 MHz, chloroform $\left(CHC{l}_3\right)$shows a single sharp absorption at 7.3$\delta$ .

(a) How many parts per million downfield from TMS does chloroform absorb?

(b) How many hertz downfield from TMS would chloroform absorb if the measurement were carried out on a spectrometer operating at 360 MHz?

(c) What would be the position of the chloroform absorption in $\delta$ units when measured on a 360 MHz spectrometer?

Step-by-Step Solution

Answer

Answer

(a) The chloroform absorbs at 7.3 parts per million downfield from TMS.

(b) 2600 Hz downfield from TMS would chloroform absorb if the measurement were carried out on a spectrometer operating at 360 MHz

(c) The position of the chloroform absorption is still 7.3 $\delta$units, because the chemical shift measured in delta $\delta$ is independent of the operating frequency of the spectrometer.

Step 1: NMR spectroscopy

NMR stands for Nuclear Magnetic Resonance Spectroscopy. This is the technique used for determining the structure of the organic compounds. It is a non-destructive technique. The nucleus or the proton behaves as a tiny spinning bar magnet due to both electric charge and mechanical spinning.

It exploits the magnetic properties of certain atomic nuclei to determine physical and chemical properties of molecules in which they are contained. Any proton or nucleus with odd mass number spin on its axis.

 

Ste 2: Chemical Shift p

The shift in the position of PMR signals resulting through shielding and deshielding by circulation of electron in chemical bonds is called the Chemical shift. The relative position of the signal along the frequency axis in the delta scale is a  of the extent of electron density around the different type of protons.

Chemical shift is expressed in the following two scales:

The tau $\left(\tau \right)$ scale, $\tau =\left(10-\delta \right)$

The delta $\left(\delta \right)$scale
$\delta =\frac{\left(Chemical shift in hertz\right)}{\left(operating frequency of the instrument in MHz\right)}$

Step 3: Calculate (a), (b) and (c)

(a) Since the symbol $\delta$ indicates ppm downfield from TMS, therefore, the chloroform absorbs at 7.3 parts per million downfield from TMS.

 

(b)In order to determine the delta shift of the given peaks, we need to divide chemical shift in Hz by the spectrometer frequency in MHz

                                        $\delta =\frac{\left(Chemical shift in hertz\right)}{\left(operating frequency of the instrument in MHz\right)}$

As spectrometer is operating at frequency 360 MHz given, this gives

 

                                        $\delta \times 360MHz=Observed chemical shift in \left(Hz\right)$

We have 7.3$\delta$given

So, therefore

                                         $$\begin{gathered} \delta \times 300MHz=Observed chemical shift in \left(Hz\right) \\ 7.3ppm\times 360MHz=Observed chemical shift in \left(Hz\right) \\ 2600 Hz =Observed chemical shift in \left(Hz\right) \end{gathered}$$

So, 2600 Hz downfield from TMS would chloroform absorb if the measurement were carried out on a spectrometer operating at 360 MHz

 

(c)The position of the chloroform absorption is still 7.3 $\delta$units, because the chemical shift measured in delta $\delta$ is independent of the operating frequency of the spectrometer.