chapter 16 - infrared spectroscopy

what is a requirement for a molecule to be IR active?

it must undergo a net change in dipole moment upon sundering vibrational

stretching

needing two or more atoms, it is the continuous change in bond distant. the direction of the bond remains the same and dipole magnitude changes but it remains pointed in the same direction

bending

3 or more atoms are needed; this is when the angle between 2 bonds changes

hooke’s law

F = -ky

reduced mass equation

µ = (m1m2)/(m1 + m2)

harmonic oscillator

chemical bonds cause atoms to oscillate back and forth when it is displaced

attenuated total reflection

reflection occurs when radiation passes from a high RI phase to a lower RI phase; choose angle of input beam incidence where total reflection occurs; even at total reflection, there is a penetrating radiation; attenuation occurs at wavelengths where low RI material absorbs the evanescent radiation

exchange a proton with a deuterium in a probed bond; how does this change the wavenumber of the vibration band?

the larger mass of the atom causes the reduced mass to increase, which results in a decrease in the frequency of the vibration. thus, deuterium will have a lower wavenumber than a proton

think about the vibrational frequency equation

for certain compounds, why are there more vibrational bands observed than predicted?

• vibrational modes combine to create overtone bands

• a single photon can excite two different vibration modes simultaneously

for certain compounds why are there less vibrational bands than predicted?

• the symmetry of the molecule is such that the dipole does not change, so no IR absorption is observed

• frequencies of vibration overlap and they cannot be resolved

• vibrations may be outside of the region you are detecting

what is the penetration depth in FTIR-ATR?

roughy 2-10 microns

what kind of crystals are used in IR transducers?

single crystals of pyroelectric materials

how many vibrational modes will a molecule have?

3N - 6 for a nonlinear molecule

3N - 5 for a linear molecule

vibrational frequency equation

v = (1/2πc) * sqrt(k/µ)

does the wavenumber of a vibrational band increase or decrease as the force constant of the bond that is being probe increases?

it increases

what is the relationship between vibrations frequency and reduced mass?

they are inversely proportional

how would you find the force constant for a molecule?

find the mass of each atom in the molecules, and plug that into the reduced mass equation. then, use this value in the vibrational frequency equation and solve for k

what type of substances can ATR be used to measure the spectra of?

solids and liquids

true or false: ATR’s typically have crystals with 100% transmission of IR energy within the region of interest in mid-IR

false; the ATR crystal is interacting with the sample