IR spectrometry\ Flashcards
What happens in IR spectroscopy
IR radiation is shone through a sample and certain frequencies are absorbed as molecules move to higher vibrational energies
IR in diatomics
gain a IR photon
closely spaced absorption sets are known as
bands
what are the transitions called
fundermental, first and second overtone
why are there multiple peaks for the same vibration peak?
different transitions in the rotational energy
why is the fundamental overtone frequency is half the size of the first?
reflects the spacing of energy levels
as bond strength increases
the greater the vibrational frequency
what are overtones?
vigorous extremes of the same vibrations
how many modes can there be ?
assymetric - there are two but they are degenerate
symmetric
bending
how to calculate the number of modes
3n-6 for non linear
3n-5 for linear
n is the number of atoms in a molecule
why is assymetric stretching at a higher wave number than the other two modes?
while some of the bonds get shorter, some other bonds become longer resulting in a change in the dipole moment of the molecule. So, asymmetric stretching appears at a higher wavenumber and absorbs or needs more energy than symmetric stretching.
why is assymetric stretching at a higher wave number than the other two modes?
while some of the bonds get shorter, some other bonds become longer resulting in a change in the dipole moment of the molecule. So, asymmetric stretching appears at a higher wavenumber and absorbs or needs more energy than symmetric stretching.
what causes the IR absorption strength ?
the change in dipole moment during the vibration.
if the dipole moment does not change during the bond vibration
no IR activity will be detected
why in linear symmetrical molecules IR may not show
dipole changes may cancel out
why can IR show in symmetrical molecules during symmetrical stretch?
the dipole moment changes direction depending on which side it stretch.
A dipole is formed
carbonyl attach to ring
the smaller the ring the greater the frequency of the c=o bond
why does ring size effect the bond frequency?
it reflects how how it is to disort the ring. smaller rings have a greater force constant
lone pairs can
conjuate pi bond. this is the alternating between double and single bonds
resonance structures
As soon as we introduce some resonance structure, or conjugation, now those electrons can delocalise over 3-4 atoms or even more. This weakens the C=O
bond, moving the IR peak down to lower wavenumbers.
ketone vs aldehyde
the sigma conjuation weakens the ketone carbonyl as alpha Hydrogen are sigma conjuated with carbonyl.
aldehyde have higher frequency
acyl chloride
the 3p lone pair orbital on cl does not overlap well with pi system.
c-cl bonds strengthens the c=o bond . so it has a higher frequency.
