uv vis Flashcards
brief overview about an IR spectra,, whats on the axis,, whats its measuring and what we can find
transmission %
cm-1
vibrational frequency of bonds
common functional groups
brief overview on mass spec
ionisation techniques
% abundance vs m/z (mass charge ratio)
fragmentation patters and parent ions
UV- vis and absorption spec can also be called what
electronic spectroscopy
a shorter wavelength means
a higher energy
longest wavelength =
lowest energy
radio waves
shortest wavelength =
lowest energy
gamma rays
large wavenumber =
high energy
bc units are cm-1 and we know this is proportional to energy.
IR spectra brief explanation
transmittance (%) against wavenumber (cm-1)
large wavenumber and therefore energy is usually on the lhs
eg where the carboxylic acid broad peak is
in IR,, most organic carbonyl groups have a wavenumber of what
fromm 1680-1800cm-1
UV VIS is involved in what transition
its involved in electronic transitions
the movement of e- between orbitals
whats an electronic transition
movement of an e- between orbitals
electronic transitions are the result of what energy
uv vis energy
what transtion does IR do
vibrational transitions
can promote vibrational transitions.
whats higher in energy: uv vis // IR + what does this mean
uv vis is higher in energy than IR
it means that the energy gap between electronic levels is larger than the energy gap between vibrational levels.
each electronic transition has what
it has vibrational sublevels
due to vibrational transitions requiring less energy than an electronic transition.
eg the movement of an e- between orbitals
electronic energy levels have the symbol
S
so s0 -> s1 is a transition from the lowest energy to one higher
how do u convert nm to cm-1
cm-1 = 10^7 / nm
when we analyse smt using uvvis spec we know what
we normally know the concentration of the dilute solution
uv vis spec basissss
light source and reference beam is detected to find a reference
then light is shone - incident beam ,, through a curvette with a sample in it,, the transmitted beam is then detected by a detector and a spectrum is prepared.
absorbance =
E c l
E =
molar absorbance coefficient
c =
conc
l =
path length
cm
Abs units
unitless
E must have units that get rid of all the other units
Transmittance (%) =
trnamitted I / incident I
then x100 bc %
Abs in terms of transmittance
Abs = log(T)
abs = log(I/I0)
what is the beer lambert law
a linear relationship between abs, conc, molar abs coefficient and optical path length.
beer lambert: what is on what axis
abs against wavelength (nm)
high performance liquid chromatography is aaaa
purification technique used in pharmaceuticals
uv/vis is used as a detector mode in HPLC - high performance liquid chromatography.
applications of uvvis// electronic spec
pharma industry to identify pharma compounds
whats a chromophore
any light absorbing molecule // molecular fragment
whats an auxochrome
a group of atoms attached to a chromophore that modify its absorbance properties.
whats a bathochromic shift
also known as a red shift
shifts the a longer wavelength (700nm bc red = longer wavelength = lower energy)
LNGER WAVELENGTH AND LOWER ENERGY
whats a hypsochromic shift
blue shift (500nm)
so smaller wavelength = larger energy!!
hyperchromic effect
an increase in intensity
hypernium = new tp ride = hyper = up = high intensity bc its a tall ride // hyper so tall energy.
hypochromic effect
a decrease in intensity
‘oh’ bc its going down in intensity.
isosbestic point
wavelength where 2 or more different molecular species in a mixture exhibit the same extinction coefficient.
wavewlength where 2 different molecules in a micture have the same E value ,, extinction coefficient
rememeber that a bathochromic shift is a red shift which means the wavelength increases which means it shifts to the right!!
red shift goes right
bc lower energy
larger wavelength
rememeber hypsochromic shift is a blue shift which means an increase in energy and a shorter wavelenfgth which means it shifts to the
lefttttt
shorter wavelength = larger energy = shift to the left
what is an electronic transition
when the wavelength of absorbed light has sufficient energy and is used to promote//excite an e- from a FULL bonding// nonbonding orbital to an EMPTY antiboniding orbital
electronic transition when the tranition hasnt occured yet is called the…. also descrie this
its called the ground state
its when both e- in the pair are in the lower energy full bonding//nonbonding orbital.
remmeber its a full MO.
electronic transition when the trantion was occurred… describe this and what its called
its called the excited state.
its when one e- from the full nonbonding// bonding MO is excited to the empty antibonding MO. this is higher in energy
in the excited state,, what is important about how the e- are drawn
one in the no longer full ,, nonbonding///bonding MO pointing up
one in the once empty antibonding MO pointing down
E energy =
hv
excited state issss
generally more reactive and unstable
it will deactivate rapidly.
S is the
sum of the e- spins
eg a +1/2 and a -1/2 gives an S=0
spin multiplicity equation
2s + 1
where s is the sum of e- spin states
ground state can be given the symbol
So
excited state can be given the symbol
S1
singlet state
when all electron spins are paired
in the same energy levels: must have opposite spins but still paired and still and singlet state
ground state has what state
a singlet state
bc 2s + 1
s = 0 bc one is 1/2 and other is -1/2
excited state has what state
a singlet state
bc 2s + 1 = 1
where s = 0 bc one is -1/2 and one is 1/2
what is the molar absorbance coefficient
E
the probability of a transition occuring
the higher the excited state energy level
the more energy is needed for the transition to occur from the ground state to the excited state.
eg So –> S1 needs less energy than S0 –> S4
between the different S levels (electronic levels//states) there are
many vibrational sublevels.
in an absorbance spectrum curves are seen when abs is plotted against wavelength however is it rlly a curve
nope
its an average
theres rapid electronic transitions occurring due to molecules being in many different vibrational states at the specific time of the transition.
the anharmonic oscillator difference between ground state and excited state
the ground state one is lower in energy
the excited state is higher in energy and also has a slightly larger nuclear coordinate
when energy is plotted against nuclear coordinate
using the graph of absorbance against nuclear coordinate and the 2 unharmonic oscillators,, how can we represent an excitation and a relaxation
excitation: one of the vib levels from the GS unharmonic oscillator to the ES unharmonic oscillator vib levels
relaxation: one of the vib levels of the unharmonic oscillators of the ES,, to a lower vib sublevel of the GS unharmonic oscillator.
the excited state has moreeeee
antibonding character
why are absorbance peaks broad at room temp
bc the spectrum has many vibrational energy levels in the electronic ground state (think unharmonic oscillator)
the abs band is made up of various transitions which all involve the movement of an e- to an antibonding orbital
if the absorbance coefficient has a small value theres a lower level of what. small E in A=Ecl meansss
theres a lower level of absorbance.
selection rules:
- spin rule
- change in L rule
- transitions and orbitals of different symmetry
representing electronic transitions can be done using the
jablonski energy level diagram
each electronic state has associated vibrational sublevels
larger Sn means
higher energy electronic energy level
electronic spec: selection rule: spin rule exp
only 1e- is involved in a transition
transitions are allowed when there is no change in the spin multiplicity (2s+1) (s= sum of individual e- spins in a molecule) of the ground and excited states
changeS = 0
singlet to singlet is allowed but singlet to triplet isnt.
one spin up and one spin down. not both spin up of that will be 0 (-1/2+1/2)–> 1(+1/2+1/2)
