Long Exam 2 Flashcards
What is IR spectroscopy? What is behind its concepts?
It is a technique based on the vibrations of the atoms of a molecule. It is based on the molecular vibrations because molecules behave like system of masses joined by bonds with different spring-like properties.
What is the consequence of Hooke’s Law?
stronger bonds absorb at higher frequencies
as the mass of atom increases, frequency decreases
What are the different types of molecular vibrations?
There are two categories and under the categories, there are two types
- In plane
-scissoring
-rocking - out of plane
-twisting
-wagging
How do we know the number of movement of molecules?
Linear Molecules: 3N-5 degrees of freedom
Non-linear Molecules: 3N-6 degrees of freedom
N=no. of atoms
[Type of vibration] Lower intensity vibration at an integral times the fundamental frequency
V? = n x Vfundamental
Overtone
The sum of two interacting vibrational frequencies but only certain combinations are allowed
V? = V1 + V2
combination bands
difference of two interacting vibrations
v? =V-V2
difference bands
When fundamental absorption couples overtone or combination band
This type of vibration is often observed in molecules having what functional group?
Fermi resonance; carbonyl
What are the factors that we consider when identifying if a signal will be intense or not
1.) symmetric vibrations are weaker than asymmetric vibrations. Note that dipole moment is a vector sum
2.) Absorption of IR radiation only occurs if there is a single change in the dipole moment.
3.) The greater the electronegativity differences between atoms in the molecule, the more polarized the molecule will be and thus there will be larger change in dipole and the the intense will be the absorption band
What are the different types of IR spectrometer? Differentiate
1.) Dispersive IR - measures one frequency of light at a time; scan speed is relatively slow.
has two types: single beam and double beam
single beam: 1940, prisms like NaCl
double beam: eliminates atmospheric absorption due to CO2 and H2O
2.) Fourier Transform
-measures all frequencies of light at a time
Draw the Dispersive IR spectrometer
.
what is the purpose of interferometer?
it allows two beams to recombine and interfere with each other, which will yield an interferogram, which is a function of optical path difference
what is the purpose of Nujol?
it suspends tiny particles that can interfere with the IR signals
what are possible errors in IR and their correction?
Instrument lineshape: apodisation (mathematical modification of the shape of a function)
unequal sample intervals: phase correction
resolution: limiting the path difference between two beams
