Question

i have some questions about IR. my first one being I am anaylyzing an IR graph what are reasons for the frequency not lining up with literature values? secondly, why might an alkyl CH group not show up at all on an IR graph? and lastly, what do the different peak lengths mean? and why can you look at the same compound but on different graphs and have different peak lengths?

Answer 1

IR values of the literature are standard with respect to specific functional groups ,but our values are not exactly in the same environment they may experience more or less electronic pull.due to that force constant varying.

E.g. Carbony group have frequencies about 1700 cm-1 but it varying from 1680 - 1800cm-1 based on substituents attached to the carbonyl group.

More the electro negative atom is substituted more electron pull will observe and hence force constant will increase due to that absorption frequencies also increase.

Most of the organic compound have frequencies simmiler to that of C-H group ,hence it will difficult to distinguish between them , hence these frequencies are not shown on graph.

Different peak length of ir spectra are due to the difference in the Dipole moment of the molecule.

More is the dilpole moment more will be the peak length or peak is sharp.

Same compound give different peak length if the concentration of the sample is different.

Less concentrate molecules give sharp spectra but more transmittance. High concentration molecules give broad peak and less transmittance.