Lecture 36: IR spectroscopy

what does spectroscopy measure?

the interaction of light with matter

what do UV/VIS do?

UV(100-400nm) + VIS(400-700nm) is high energy light that excited e- causing electronic transitions

what does IR light do?

its very long(700-1000nm) so its lower in energy that excites bonds causing molecular vibrations

i.e stretching and bending bonds

why does IR have extensive use in the B.P?

it is low cost, small and portable

what are the features of IR?

• it is a qualitative method(if present or not)

• it uses wavenumbers instead of wavelengths

• its measure in cm^-1 and the range is 4000-400cm^-1

what are the 2 types of stretching ?

symmetric and asymmetric

what are the 4 types of bending?

in plane:

• rocking

• scissor

out of plane:

• wagging

• twisting

there are 6 vibration modes just with CH2 group

• it can get very complicated

• it is therefore unique to a compound

what are the uses of IR?

• fingerprint region(1500-500 cm^-1) is unique to a molecule and this region is used in the B.P to confirm the identity of a compound

• x-y covalent bond- can be considered spring therefore can be described by simple harmonic motion: hookes law- calculating bond strength in P4

can be used to identify some functional groups(different stretching and bending)

• OH + NH have very broad peaks as H is jumping on and off O and N as they are H bond donors

• CH are sharp

• C=O has a very characteristic peak(1800-1650 cm^-1)