W2 Infrared and Raman Basics Flashcards
isotopic shift definition
the change in frequency observed when mass of one atom is changed
relationship between reduced mass and frequency
smaller reduced mass > higher frequency
3 modes of vibration of water and their corresponding wavelengths
symmetric stretching > 3240
asymmetric stretching > 3490
bending > 1640
how to subtract water contribution in IR using transmission cell
liquid sample placed between the 2 windows > collect spectrum in sample of water and spectrum of just water > subtract water from sample
what is attenuated total reflection
light bounces internally at surface of high refractive index material > sample in contact with ATR cell absorb part of IR light > generates evanescent waves > sample absorbs specific frequencies of infrared light
does IR or Uv-Vis contain more information
IR
what is Raman scattering
when light tranverses a transparent material, some of the deflected light changes its wavelength
why do Raman photons have different energy
some of their energy is added to, or removed from, the sample > this small amount of energy is of the order required for bond jumping between two vibrational levels
how is Raman radiation measured
raman spectra obtained by measuring light emitted by the sample (different from measuring light transmitted in UV)
difference between Rayleigh, stokes and anti-stokes photons
stokes: when molecule goes back to higher level > emitted photon (b) has less energy than original photon (a) > b have larger wavelengths
Rayleigh: molecule goes back to same level > both photons same energy and wavelength
anti-stokes: molecules goes to lower level > b has more energy than a > b has smaller wavelengths
difference between vibrational IR and Raman spectroscopy
IR: energy used for excitation of bonds is very small
raman: sample is excited with high energy
in raman, intensity detected of emitted photons can be increased by exciting sample for a long time but for UV/IR, duration of illumination has no effect on quality of spectrum (bc transmittance depends on ration between incident vs transmitted photons but this ratio same whether illumination short or long)
in IR, absorbance depends on difference in electronegativity between atoms involved; in Raman, emission intensity is stronger when polarisability of bonds is higher
where is the band for amide I, II and A located on the spectrum for proteins
amide I (C=O stretching): between 1600 to 1700cm-1
amide II (N-H bending): between 1500 to 1600 cm-1
amide A (N-H stretching): around 3300 cm-1
what are the different types of wavelengths on the electromagnetic spectrum
UV: 10-400nm
Visible light: 400-700nm
IR: 700nm - 1mm
difference between UV-Vis and IR
UV-Vis: use UV/visible light to see how electrons behave > find concentration and colour related properties
IR: use IR light to detect vibrations of chemical bonds > identify structure
