Molecular Spectroscopy Flashcards
Molecular spectroscopy
interaction of EM radiation with matter(molecules)
wave number
Number of complete waves or cycles
contained in unit distance cm-1
Atomic spectra
electronic transitions from one atomic orbital to
another => line spectra
Molecular spectra
electronic transitions from the Highest occupied
molecular orbital(HOMO) to Lowest unoccupied
molecular orbital(LUMO)
=> Band spectra
due to vibrational and rotational transitions along with electronic
Absorption and Emission spectrum
Absorption spectrum:
absorbs E and moves from lower energy
level to higher energy level (black spectra lines against light)
Emission spectrum:
higher energy level to lower energy level
(light spectra lines against black background)
Spectroscopic units
energy expressed in wave numbers
1cm^-1 = 1.99*10^-23 J
Radio Frequency region
10-1cm
NMR, ESR
(Nuclear and electron spin reversal)
VHF, AND UHF
Microwave
1cm- 100 micro-meter
Rotational levels
Infra red
100 micro-meter - 1 micro-meter
Vibrational levels
Visible-UV
1 micro-meter - 10 nm
Electronic transitions
Change in electronic distribution of
valence electrons
X-ray
10nm-100pm
X-ray Spectroscopy
Ionization
Change in electronic distribution of
inner electrons
y-ray region
100pm-1pm
nuclear effects
γ- ray spectroscopy
Rearrangement of nuclear particles
Raman Spectroscopy
monochromatic light thru transparent medium
most scattered radN = incident radN (λ) => Rayleigh Scattering
some scattered radN, diff λ from incident radN
stokes lines
λ scattered rad > incident radiation => Stokes lines
E of Stokes is lesser, < λ
λ scattered rad <incident radiation => anti -Stokes lines
E∝ 1/λ
Homonuclear of diatomic molecules
are microwave and Infra-red inactive (rotational and vibrational )=> Raman active
Born-Ophenheimer Approximation
E = Etrans + Erot + Evib + Eelec
Translational energy is negligibly, thus its removed
all these energies are independent of each other
Erot «_space;Evib «_space;Eelec
How is molecular spectroscopy recorded and which show the spectroscopy
When the frequencies match, resonance occurs and the molecule absorbs energy and goes to the higher rotational
levels => spectrum
HCl and CO show rotational spectra
MUST possess permanent dipole moment
while H2, Cl2 and
CO2 do not
Rotations of a molecule having permanent dipole moment= changes in electric dipoles = interact with electrical component EM
Gross selection rule for rotational
molecule should possess permanent dipole moment
* Δ J = ± 1
J => rotational quantum number
Vibrational Spectroscopy
-> absorbs IR radiation
-> requires change in dipole moment
Vibrations of molecules => change electric dipole=> interact with electrical component of EM rad
When frequencies match, resonance occurs =>
molecule absorbs energy and goes to the higher vibrational levels => vibrational spectrum
Vibrational Spectroscopy molecules examples
HCl, CO and H2O will show vibrational, CO2 also
spectra while H2, Cl2 will not
CO2 molecule
CO2 => no permanent dipole moment is IR
active due to change in dipole
- CO2 symmetric stretch is IR inactive => no change in dipole
- CO2 asymmetric stretch is IR active
- CO2 bending mode is IR active
how is each Vibrational spectroscopy unique
each bond => unique natural vibrational frequency
Bonds are not static but vibrating in different ways
Gross selection for vibrational
When the molecule vibrates => change in
the dipole moment
* Δv = ± 1
difference between two consecutive energy levels = v dash cm^-1
one line is observed in the vibrational spectrum, without changing its rotational states
what is zero point ISA 2M
a molecule at ground state will be vibrating at natural frequency
