Question 1

Interaction of light and matter

Accepted Answer

light is made of photons
when dealing with the interaction of light and matter we treat light as a wave
electromagnetic (EM) radiation has both a frequency and wavelength

Question 2

frequency (v)

Accepted Answer

number of waves produced by a source each second
measured in Hertz (Hz) or s-1

Question 3

wavelength (λ)

Accepted Answer

distance between a point on one wave and the same point on the next wave
measured in metres (m)
wavenumber (cm-1) = 1/λ (cm)

Question 4

equation

Accepted Answer

velocity (c) = frequency (v) x wavelength (λ)

Question 5

equation

Accepted Answer

energy (E) = frequency (v) x Planck's constant (h)

Question 6

infrared and raman spectroscopy

Accepted Answer

wavelength will be in the range from 400nm to 100,000nm
this covers the visible and infrared regions of the electromagnetic spectrum

Question 7

motion in molecules

Accepted Answer

3 diff ways molecules move:
- translation (movement in 3 dimensions)
- rotation (movement about an axis)
- vibration (movement on a bond)

Question 8

atoms in molecules

Accepted Answer

all molecules have 3N degrees of freedom, where N is the number of atoms
- translation uses 3
- rotation 2 or 3
- vibration uses the rest

Question 9

vibrational modes

Accepted Answer

linear molecules e.g. H2 and CO2 have 3N - 5 vibrational modes
Non-linear molecules e.g. H2O have 3N - 6 vibrational modes

Question 10

Motion requires energy

Accepted Answer

Translation requires the least (kinetic)
Rotation requires more (equivalent to microwave wavelength)
Vibration requires even more (equivalent to infrared wavelength)

Question 11

vibration in molecules

Accepted Answer

absorption of infrared radiation causes covalent bonds within molecules to vibrate
a molecule is only able to absorb radiation whose energy corresponds to the energy difference: E1 - E0

Question 12

vibration in molecules

Accepted Answer

most molecules have many covalent bonds hence infrared and raman spectra provide detailed info on molecular structure
bond vibrations that result in absorption of infrared radiation are either stretching or bending vibrations

Question 13

modes of vibration in molecules

Accepted Answer

- symmetric stretch
- asymmetric stretch
- twisting
- wagging
- scissoring
- rocking

Question 14

IR active or inactive?

Accepted Answer

not all vibrations in molecules can be seen by IR spectroscopy
only those termed as IR active will be visible

Question 15

selection rules for IR

Accepted Answer

gross selection rule - vibration must result in a change in the molecular dipole moment
specific selection rule - the change in vibrational energy level must be Δν = ±1

Question 16

examples of ir active or inactive

Accepted Answer

symmetric stretch - not infrared active
symmetric stretch - not infrared active
assymetric stretch - infrared active

Question 17

absorption

Accepted Answer

large change in dipole moment e.g. C=O results in an intense absorption
small change in dipole moment e.g. C=C results in a weak absorption

Question 18

interpreting IR spectra

Accepted Answer

the structure of an unknown molecule is not determined just by using IR
IR can be used for identifying certain functional groups but is complimentary to other techniques

Question 19

advantages of IR spectroscopy

Accepted Answer

- little sample prep required
- can handle any state of sample
- identifies functional groups

Question 20

disadvantages of IR spectroscopy

Accepted Answer

- not usually quantitative
- low sensitivity
- cannot detect IR inactive impurities

Question 21

raman spectroscopy

Accepted Answer

when radiation passes through a transparent medium, most of it is elastically scattered (Rayleigh scattering)
- this is where light is scattered at same frequency to the incident radiation

Question 22

raman spectroscopy

Accepted Answer

one in a million photons are inelastically scattered (Raman scattering)
- light is scattered at different frequency to the incident radiation
- use high-powered lasers

Question 23

stokes emission

Accepted Answer

- decrease in frequency = stokes emission
- increase in frequency = anti-stokes emission
typically we see stokes emission since anti requires the molecule to be in an excited state initially

Front	Back
Interaction of light and matter	light is made of photons when dealing with the interaction of light and matter we treat light as a wave electromagnetic (EM) radiation has both a frequency and wavelength
frequency (v)	number of waves produced by a source each second measured in Hertz (Hz) or s-1
wavelength (λ)	distance between a point on one wave and the same point on the next wave measured in metres (m) wavenumber (cm-1) = 1/λ (cm)
equation	velocity (c) = frequency (v) x wavelength (λ)
equation	energy (E) = frequency (v) x Planck's constant (h)
infrared and raman spectroscopy	wavelength will be in the range from 400nm to 100,000nm this covers the visible and infrared regions of the electromagnetic spectrum
motion in molecules	3 diff ways molecules move: - translation (movement in 3 dimensions) - rotation (movement about an axis) - vibration (movement on a bond)
atoms in molecules	all molecules have 3N degrees of freedom, where N is the number of atoms - translation uses 3 - rotation 2 or 3 - vibration uses the rest
vibrational modes	linear molecules e.g. H2 and CO2 have 3N - 5 vibrational modes Non-linear molecules e.g. H2O have 3N - 6 vibrational modes
Motion requires energy	Translation requires the least (kinetic) Rotation requires more (equivalent to microwave wavelength) Vibration requires even more (equivalent to infrared wavelength)
vibration in molecules	absorption of infrared radiation causes covalent bonds within molecules to vibrate a molecule is only able to absorb radiation whose energy corresponds to the energy difference: E1 - E0
vibration in molecules	most molecules have many covalent bonds hence infrared and raman spectra provide detailed info on molecular structure bond vibrations that result in absorption of infrared radiation are either stretching or bending vibrations
modes of vibration in molecules	- symmetric stretch - asymmetric stretch - twisting - wagging - scissoring - rocking
IR active or inactive?	not all vibrations in molecules can be seen by IR spectroscopy only those termed as IR active will be visible
selection rules for IR	gross selection rule - vibration must result in a change in the molecular dipole moment specific selection rule - the change in vibrational energy level must be Δν = ±1
examples of ir active or inactive	symmetric stretch - not infrared active symmetric stretch - not infrared active assymetric stretch - infrared active
absorption	large change in dipole moment e.g. C=O results in an intense absorption small change in dipole moment e.g. C=C results in a weak absorption
interpreting IR spectra	the structure of an unknown molecule is not determined just by using IR IR can be used for identifying certain functional groups but is complimentary to other techniques
advantages of IR spectroscopy	- little sample prep required - can handle any state of sample - identifies functional groups
disadvantages of IR spectroscopy	- not usually quantitative - low sensitivity - cannot detect IR inactive impurities
raman spectroscopy	when radiation passes through a transparent medium, most of it is elastically scattered (Rayleigh scattering) - this is where light is scattered at same frequency to the incident radiation
raman spectroscopy	one in a million photons are inelastically scattered (Raman scattering) - light is scattered at different frequency to the incident radiation - use high-powered lasers
stokes emission	- decrease in frequency = stokes emission - increase in frequency = anti-stokes emission typically we see stokes emission since anti requires the molecule to be in an excited state initially

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Spectroscopy