ir spectroscopy

further chem

IR radiation produces bond

total degrees of freedom is where is number of atoms in a molecule

bond energy is proportional to wavenumber

is the bond force constant (triple > double > single)

reduced mass: smaller reduced mass easier to vibrate

think of bond as spring, reduced mass acts as weight on end of spring

comparing wavenumber of carbonyl compounds:

CC(=O)Cl

highest (no resonance and inductive effect draws electron density away from carbon even more, strengthening double bond in carbonyl)

CC(=O)C

standard to compare to

CC(=O)C=C

slightly weaker due to resonance from C=C to carbonyl oxygen, giving carbonyl group single bond character

CC(=O)N

strongest resonance from nitrogen due to freely available lone pairs

ring strain and wavenumber

C1C(=O)CCCC1

highest wavenumber because properly sp2 satisfied

O=C1CCCC1

next, less satisfied with sp2

O=C1CCC1

next

O=C1CC1

lowest, least satisfied with sp2

as we increase ring strain, we increase s character in the carbonyl, increasing p availability for side on side overlap, thus shortening/strengthening carbonyl bond

degrees of freedom and IR (because IR-active requires change in dipole moment)

rotation has 2 or 3

• linear molecule has 2 because rotation along bond axis does “nothing”
• otherwise 3

translation has 3

• move in x, y, z

vibration has whatever is left

• linear
• non-linear

stretching a bond requires more energy than bending

• stretching bond needs to overcome strong covalent electrostatic attraction
• bending bond needs not this

in terms of energy/wavenumber
asymmetric stretch > symmetric stretch > in plane bending > out of plane bending

asymmetric stretch needs more energy than symmetric stretch because

• electron cloud distorts very severely (add on one side, remove on another, requires much to equilibrate)
• requires central atom to vibrate to stabilise centre of mass