Answer to Question #171868 in Physical Chemistry for Aarati

Question

I) Predict the NMR spectrum of CH3CH2OH.

II) “IR spectra is often characterised as molecular finger prints”. Comment on it.

III) C-C double bond and triple bonds of ethene (CH2=CH2) and acetylene do not absorb 

IR energy why?

IV) Methane doesn’t absorb IR energy. Why?

V) What is the shift of the resonance from TMS of a group of nuclei with δ = 3.50 and 

an operating frequency of 350 MHz?

Answer 1

1.

2. It usually contains a large number of peaks, making it difficult to identify individual peaks. However, the fingerprint region of a given compound is unique and, therefore, can be used to distinguish between compounds.

3.  To determine whether or not there will be a change in dipole moment, examine the bond that's stretching and look at the two fragments that it's connected to. If these two fragments are equivalent, there will be no net change in dipole moment for the symmetrical stretch and it will be IR-inactive.

4.  There is no IR absorption because these molecules have no intrinsic dipole or one caused by vibrations. Also as they have no permanent dipole there is obviously no dipole generated when molecules rotate.

5. As we know that chemical shift = resonance frequency of proton ÷ operating frequency of spectrometer ( in MHz) i.e resonance frequency of proton= chemical shift × operating frequency of spectrometer ( in MHz)

Rf= 3.5×350 =1225 Hz = 1.225 KHz