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
nm x 10^-7
then 1/answer
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
bc hunds rule: need to have opposite spins
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)
electronic spec: selection rule: change L = +-1
only transitions that change the angular momentum quantum number by 1 are allowed. s->p p-> d or s
transitions within the same sublevel are forbidden.
no s -> s or p->p
or pi to pi* but not pi to pi
l = 0 =
s orbital
l = 1 =
p orbital
l = 2 =
d orbital
electronic spec: selection rules: centrosymmetric systems
in centrosymmetric systems,, transitions are only allowed between orbitals of different symmetry ( u -> g) (g–>u)
pi u —> pi g***
centrosymmetric orbitals =
gerade orbitals
s
d
non centrosymmetric orbitals
ungerade - p
centrosymmetric molecules have aaa
basically a point of inversion
theres an identical part of a molecule on the opposite side of the symmetry centre // point of inversion
why is a p orbital ungerade
bc its shape!!
inverting it will change its phase meaning its not symmetric
why are s and d gerade orbitals
bc when u invert them,, their phases dont chnage.
meaning its symmetric
can forbidden transitions still occur
yes,, yes thery can
due to loss of symmetry // reduction of symmetry
hybridisation of orbitals
and vibronic coupling of electronic states
electronic transitions can occur between what
they can take place between many different kinds of orbitals
which are dependent upon the molecular structure.
many are measurable during uvvis spec
transitions seen in alkanes
alkanes: only sigma bonds
so transitions seen:
sigma –> sigma***
suchhhhh a big difference in energy tho,, so not normally seen in uvvis spec
how do we know the energy difference between sigma to sigma*** is brazy
think of the energy diagram for MO
sigma*
pi*
non bonding
pi
sigma
alkenes and aromatics see what type of transition
sigma and pi bonds
sigma–> sigma*
n –> pi*
pi –> pi*
l –> pi*
n = non bonding e- aka lone pair FROM the molecule itself
l = lone pair but from a substituent!!!
what transitions do we see in uvvis
pi –> pi*
n –> pi
l –> pi*
in n –> pi** how many e- move
just one of the e- in the lone pair move!!!!!
transitions we expect to see in benzaldehyde
its unsaturated aka double bonds: pi –> pi**
it has a carbonyl with an O:
l –> pi**
difference between pi–> pi** and n –> pi**
pi–>pi* : higher abs coefficient (> 5,000M-1cm-1) theyre symmetry allowed transitions.
HIGHER ABS,, HIGHER ENERGY (futher apart in AO energy diagram)
n–>pi: lower abs coefficient (<100M-1cm-1) symmetry forbidden transitions: poor overlap of the p orbital (with the lone pair) and the pi orbital. LOWER ENERGY, LOWER WAVELENGTH, CLOSER IN THE AO ENERGY DIAGRAM.
absorbance wavelength tells us about the
energy of the transitions
abs tells us about the
value of the molar absorbance// extinction coefficient.
why do n –> pi transitions have a low E value
bc they have poor overlap between the p orbitals the lone pair is in,, to the pi** orbitals.
what transition is seen in conjugated systems
pi –> pi***
the more conjugated smt iss theee
larger the E coefff
the larger the wavelength
lower energy transition
bc a greater degree of conjugation reduces the energy gap between the HOMO and the LUMO
what does conjugation do
more conjugation = lower energy gap between HOMO and LUMO
means larger wavelength and larger E coeff value.
conjugation and wavelength
more conj === longer wavelength == shift to the right
bathochromic shifttt
a highly pronounced vibrionic structure will also be seen for polyaromatic species (lots of peaks)
why is a compound colourless (one region)
bc it absorbs energy in the UV region::: meaning it doesnt absorb visible light
pronounced vibronic struicture could be due to which transition
S0 –> S1
pi –> pi***
higher energy transition:
larger arrow on the S0 with vib –> S1 with vib == higher energy = shorter wavelength peakkkk
energy difference between abs peaks relates directly to the spacing of vibronic states. (s0 –> s1 diagram with vib levels)
uv region ==
300nm - 400nm
calculating the energy difference between peaks
convert peak positions // wavelength (nm) into wavenumbers (cm-1)
( 1/ wavelength) x (1x E^7 )
do this for both peaks then do larger - smaller!! in cm-1
this doesnt give us a 100% accurate bond vibrational frequency!! just in the ballpark range.
rememebr cm-1 = freq (v) but its proportional to E.
how do we determine the influence of a substituent on an aromatic
we can draw the ground state resonance structure: aka draw the movement of the lone pair if u have an edg
EDG and an aromatic: effect on conjugation
increase in conjugation!!!
bc the e- are pushed into the aromatic // THE PI MOLECULAR ORBITAL
and conjugation is increase
phenol/// NH2 edg or ewg and what transition will we expect
its an EDG:
lone pair is added into the pi molecular orbital and conjugation is increased
we expect a
- pi –> pi**
- l–> pi** transition bc the lone pair came from the substituent, OH.
nitro and coo- groups are EDG or EWG??
theyre EWG (= N(+) - r(-) -r )
double bond is resonated to the N and then the o to form o-. make sure the charges are still balanced.
we still increase the degree of conjugation bc we move the e- onto the substituent
EWG what transition is expected
n –> pi*
bc the lone pair comes from the aromatic itself
what does conjugation do
conjugation reduces the energy of the pi –> pi* transition
benzene with an amine on it
aniline
both edg and ewg
both increase conjugation and the substituent is now also involved in the pi system
both help extend the pi system .
this helps reduce the energy gap between the pi –> pi** transition.
what do ewg and edg do
provide opportunities for additional electronic structures
if the energy between pi –> pi* is reduced by the substituent,, what else does the substituent effect
it alters the absorption wavelength of the pi –> pi*
how does basic ph affect substituted aromatics
can make the substituent a better EDG by giving it a (-) charge due to deprotonation.
this causes a bathochromic shift (red shift - longer wavelength = lower energy = shifts to the right)
eg phenol –> o-
how does acidic ph affect substituted aromatics
acidic ph can protonate the substituent giving it a (+) charge which reduces its EDG effects.
causes a hypsochromic shift (blue shift,, shorter wavelength = larger energy = shifts to the left)
what shift does a more EDG give the peak
a bathochromic shift: red shift
moves to the right
longer wavelength,, lower energy!!
what shift does a less EDG than originally give
a more hypsochromic shift
blue shift
lower wavelength
higher energy
shifts to the left
whats a ICT
an intramolecular charge transfer
when do intramolecular charge transfer transitions occur
when an aromatic has an EDG and an EWG as a substituent!!
in an intramolecular charge transfer transition what occurs
electron desnity is redistributed along the molecule when light is absorbed // when there is sufficient energy.
from the EDG to the EWG.
intramolecular chargw transfer and pH
more basic pH = high number = less acidic
deprotonates OH’s etc.
forms better EDG: O-
this increases the intensity of the intramolecular charge transfer: higher absorbance but also increases the wavelength: bathochromic shift. lowering the energy.
what else can influence the wavelength of absss
the polarity of the solvent
what are the two types of solvatochromism?
(+) (-) solvatochromism
what normally shows solvatochromism behaviour
normally aromatics with both EDG and EWG: ones that undergo a intramolecular charge transfer,, ICT,, will show solvatochromic behaviour.
whats the most common form of solvatochromism
normally positive solvatochromism is seen most
this is where the solvent is very polar and causes a bathochromic shift!!
increase in solvent polarity = increase in absorption wavelength // bathochromic shift
= lower in energy
when we think of smt shifting to the right,, what should we think
we should think that there is a bathochromic shift
higher wavelength
lower energy
reduced energy gap between the ground and excited states.
positive solvatochromism!!
how does a more polar solvent cause a bathochromic shift
bc a polar solvent can stabilise a more highly dipolar excited state.
in an aromatic with a EDG and EWG together,,, what state has a larger dipole moment // long u
the excited state has a larger dipole moment
the one where the charge transfer has taken place from the EDG to the EWG
the one u can draw the resonance for to get
how can u draw a dipole moment
an arrow where the tail has a + sign and is where the positive side of the dipole is
the arrow head is where the negative side of the dipole is.
what else can solvent poalrity change
can chnage the wavelength that absorbance occurs in and the intensity of the absss
can also effect the energy gap between ground and excited states.
how can the polarity of the solvent effect the energy gap between ground and excited states (think of the resonance molecule ground and excited state)
bc the dipolarity of the ground and excited states change when the ICT occurs.
a polar solvent will stabilise what
a polar solvent can stabilise a highly dipolar electronic state
it can lower its energy
idk tbh but think hard and hard
a non polar solvent can destabilise
can destabilise a highly dipolar electronic state
aka it can increase the energy of the molecule.
think soft and hard,, these dont rlly like eachother.
in terms of energy gaps,, what does a more polar solvent do
non polar: energy gap is higher // normal
polar solvent:: energy gap of excited and ground state gets smaller
the energy change is the difference between the 2 excited state energy levels!! bc one will be lower due to being in a polar solvent which stabilises it and thereofre lowers its energy. - bathochromic shift
positive solvatochromism
bathochromic shift
we like it bc it reduces the energy of a dipolar electronic state. which stabilises it.
increases its wavelength of abs,, lowers its energy gap between its excited and ground state.
negative solvatochromism
increase in the solvent polarity gives a hypsochromic shift
increase in energy
,, lower wavelength.
dipolar moment of ground state > dipolar moment of excited state.
it basically decreases the energy of the ground state,, making the energy gap between the gs and es larger. the energy gap is the diff between the gs for both solvents.
when we think of negative solvatochromism what should we think
(-) so think of the bottom
think of the ground state
think its negative so bad
so an increase in energy and a lower wavelength
think hypsochromic shift.
ground state has a larger dipolar moment
what should we think when we see positive solvatochromism
we should think positive so good,, and positive sso high up
good: lowers energy,, stabilises things,, gives a bathochromic shift.
also bc high up we know its stabilising the excited state which has a larger dipolar moment!!
dielectric constant is used to measure what
the polarity of the different solvents
most polar –> least polar solvent
water - 80
acetonitrile - 39
ethanol - 24
acetone - 20
chloroform - 5
hexane - 2
what is needed for a charge transfer transition
one molecule needs an EDG and an EWG in the same molecule
is ketone EWG or an EDG
its an EWG
is amine an EDG or an EWG
amine is an EDG due to its lone pair.
whats an unsubstituted azobenzene
benzene ring attached to N = N attached to a benzene ring
whats the azo group
N=N
whats cool about unsubstituted azobenzene
its an organic compounds that can exist in a cis and trans form
what transitions do cis and trans unsubstituted azobenzene compounds show
strong pi –> pi* transitions
weak n –> pi* transitions
why are the unsubstituted azobenzene compounds showing weak n –> pi* transitions + why can we write n.
bc these transitions are symmetry forbidden
this means they have a low E value which means the probability of a transition occuring is low!! (450nm)
lone pair comes from the N
why are the unsubstituted azobenzene compounds showing strong pi –> pi* transitions
bc theyre an allowed transitions
theres a larger E value measning the possibility of this transition occuring is higher!
is trans azobenzene or cis azobenzene more stable
trans azobenzene is more stable!!!
less steric hinderance. due to the H’s on the benzene being further apart.
describe trans azobenzene
benzenes in a Z shape with N=N in the middle.
describe cis azobenzene
C shape on opp sides of the N=N
describe the plot of E against wavelength with trans and cis azobenzene
trans has a large peak that kinda just looks like a hill
the cis has 2 mini peaks at the beginning with the first one being slightly larger.
meaning the trans isomer has a larger E value at a shorter wavelength …
the shorter wavelength one (first peaks) are the pi –> pi**
the peaks more on the RHS are the lower energy,, larger wavelength,, n –> pi** transitions.
does the cis or trans isomer has a larger energy for the pi –> pi* transition
the cis isomerrrrr
has a shorter wavelength for the pi–> pi* transition.
due to the change in conjugation due to the change in orientation due to the change in probability of the transition happening seen by the different E value.
what type of isomerisation is seen between the cis and trans azoenzene + what does it mean!!
photo isomerisation!!
using light to induce an isomeric change
what is the energy barrier for isomerisation between cis and trans azobenzene
100 kj mol-1
whats needed to go from trans to cis
uv light
whats needed to go from cis to trans
visible light // heat
how else can the cis isomer go to the trans isomer
the cis isomer can thermally relax back to the trans isomer in the dark
but can be accelerated using vis // heat
trip for rememebering what we need to go from cis to trans
starts with a C
at the sea u have visible light and heat bc its sunny and warm
also bc visible light has less energy than uv light,, and trans is more stable sp u would need less energy to go to the more stable one.
equation for energy using planks,, frequency of light and wavelength
E = hc / wavelength!!
energy per photon given!!
multiply this by avogadros constant to get energy for a mol!!
whats diff about cis azobenzene and what effects does this have
the H’s on benzene have steric hinderance due to being so close together!!
this reduces the molecules conjugation + leads to a higher energy pi –> pi* transition.
how can substituents on the azobenzene affect it
they can lower the energy of the LUMO (pi** orbital) and bathochromically shift the wavelength of the (to the right. - lower energy) the n –> pi* and pi –> pi* transitions.
uses of azo dye photoisomerism
- alters larger scale structures such as metal organic framwork (MOF’s)
- controls conformation of biological moelcules (peptides)
- photo release drug molecules from a host
describe methyl orange
methyl orange = disubstituted trans azobenzene with
NMe2 on one benzene
So3- on the other
describe the EWG and EDG on methyl orange and what methyl orange is used for
methyl orange is a PH indicator
SO3- = EWG
NMe2 = EDG
bc the methyl orange has both EWG and EDG,, it shows
intramolecular charge transfer transitions
theres a push-pull redistribution of e- density.
it shifts absorption properties to the visible region of the EMS; we can see the colour change.
how does methyl orange work as a PH indicator and what colours will it be at different pH’s.
- at low pH = high acidity ,, high conc of H+ ions,, the azo group,, N=N is protonated which enhances the charge transfer between the EWG and EDG!! (from NMe2 –> N=N+)
low pH = less than 3.1 = red
higher pH = more than 4.4 = yellow
methyl orange: plotting abs against wavelength : what are the shapes of the different colours seen at different pH’s + whats the isobestic point
red = down up down peak,, the up peak being large and at a higehr wavelength but also higher abs. = lower pH = more acidic!!
yellow: a small little up down peak at a lower wavelength and a lower abs. = BC N=N ISNT PROTONATED, bc its a larger PH,, lower acidity.
isobestic point: where they cross,, 350nm ,, where starting material and product have the same E value at a given wavelength.
lower pH = more acidic effect on methyl orange + describe methyl orange. + describe conjugation
methyl orange = trans,, disubstituted azobenzene with NMe2 on one benzene and SO3- on the other
it has an intramolecular chargw transfer due to having both EDG and EWG.
lower pH = more acidic = more H’s,, the N=N is protonated,, induces the charge transfer from the NMe2!!
N=N is protonated meaning one N is positive as its bonded 4 times,, N: is moved throughout the benzene via the double bonds and the N=N double bond has an arrow from it to the positive N. single bond between the NH and the NN, double bond between the N and the benzene!!
