Chemistry Y1: Spectroscocpy Flashcards
What are the advantages of spectroscopic methods
- Fast
- measure small sample sizes
- Non-destructive
- Can analyse mixtures
- Sample doesn’t need to be pure
What are the spectroscopy techniques
*Identify compounds by determining absorption/ emission spectra
*Quantify substances by measuring appropriate wavelength
*determine molecular structure
*Follow reactions
Energy of radiation equation?
E = hc/λ
h= planks constant (J/s)
C= speed of light (ms-1)
Λ= lambda wavelength (m)
How is EM radiation absorbed by atoms/molecules?
- Atoms and molecules exists in quantised energy levels
- Bonds may bend or rotate only
*e can only move between specific orbitals - these energy differences quantify absorbtion
Order Rotational levels, Electronic levels and vibrational levels in terms of size of energy gaps
Rotational > Vibrational > electronic
What is the Energy range of UV?
1nm-400nm
What wavelength have electronic energy levels?
UV/ Visible
What wavelength have molecular vibrations as energy levels?
IR
What wavelength have Nuclear spin energy levels?
Radio (nmr)
What is the Energy range of Visible light?
400nm-750nm
How can you determine the concentration of a sample?
by measuring how much light has been absorbed
What factors control Amax?
*Concentration of a species
*Path length of the cuvette (wavelength)
What is beer lamberts law, provide an equation
A linear relationship between A and c
(Value of ε and A must be at the same wavelength)
A=εcl
ε=Molar extinction coefficient (L mol-1 cm-1)
C= concentration
L= path length (cm)
What does a higher molar coefficient (ε) mean
The species is more effective at absorbing light at a given wavelength
When is molar coefficient (ε) highest?
At Amax
What are negative and positive deviation
Negative= conc lower than expected for beer lambert
Positive= conc higher than expected for beer lambert law
what 2 orbitals are formed from the overlap of 2 atomic orbitals
2 molecular orbitals:
1. Bonding molecular orbital
2. Anti-bonding molecular orbital
Which bond requires more energy: Lp -Pi or Pi-Pi
Pi-Pi = 190nm MORE ENERGY
lP-pi = 270nm LESS ENERGY
What causes the peak in alkenes?
The energy gap between Pi-Pi*
Pi= Highest occupied (molecular orbital)
Pi* = Lowest occupied (molecular orbital)
Do Larger or smaller alkenes have the bigger wavelength?
What does this mean?
Larger alkenes have the bigger wavelength, SO the energy gap (HOMO) is getting smaller
Do increased number of double bonds increase or decrease Lander max
more double bonds = Higher lander max
Conjugation of double bonds:
What happens when conjugation increases?
The Pi and Pi* orbitals get closer together = longer wavelength
What does conjugation do?
It allows free movement of e within a molecule
More conjugation, the smaller the HOMO-LUMO energy gap
What are the 2 vibrational modes?
- Bending vibrations
- Stretching vibrations
Convert 10um into wavenumbers (sm-1)
10um = 10 x 10-6
=10 x 10-4
=1.0 x 10-3
so, 10um = (1.0 x 10-3)/ (1.0 x 10-3) = 1000cm-1
1cm -cm-1
What phases can IR spectroscopy samples be in?
(g), (aq), (s)
What is the reduced mass equation/
μ=m1m2/ m1+m2
μ= Mass of atoms (amu)
*Convert to Kg
What is Hooke’s law?
⋁=1/2πc √k/μ
V= frequency of vibrations (cmS⁻¹)
K= bond strength (KgS⁻¹)
μ = mass of atoms (Kg)
c= Speed of light (cmS⁻¹)
What must there be for IR absorption to occur
A dipole change during vibration
Dipole changes: What does X-X mean
No dipole
No change on stretch
IR inactive
Dipole changes: WHat does X-Y mean?
Has dipole
Increases on stretch
IR active
What happens to IR absorption with decreased bond polarity?
iR absorption becomes less intense
Stronger bonds = Bends
Weaker bonds = Stretches
TRUE OR FALSE?
FALSE
Stronger bonds = Stretches
Weaker bonds = Bends
What is below 1500cm⁻¹ known as?
The fingerprint region
-Allows us to distinguish between 2 molecules with the same functional group
What is above 1500cm⁻¹ known as on spectroscopy?
The functional group region
What peak would we expect from C=O
A sharp/strong peak at 1700cm⁻¹
*The stretch frequency can be affected groups bonded to the carbonyl
*A sharp peak means neighbouring group is quite polar
What does conjugation with N (amide) do to Carbonyl?
*The carbonyl gets weakened
*The conjugating group (N), lowers the IR stretching frequency by ~30-40cm⁻¹ usually
* THe C(=O)N bond creates a dipole where N gets a slightly +ve charge
What happens when Carbonyl is bonded to an electronegative species?
IR stretching frequency increases
The bond is strengthened
What does a C-H peak tend to look like
*Can be quite a strong peak despite not being v electronegative
*Their peak depends on what other molecules are bonded to e.g. C-O
*The peaks are the cumulation of all the C-H bonds in the molecule
-Making it look quite strong
Why do we need NMR spectroscopy?
Important for understanding structure of unknown molecules
What are the 2 classes of atomic nuclei?
With spin= ODD mass no. and or ODD atomic no.
without spin= everything else
What 2 things are needed for a magnetic moment?
- Object is spinning
- Object has a charge
What are the 2 possible spins of H1 and C12?
+1/2 and -1/2
Magnetic moments with NO field are?
*Randomly oriented
*Equal in energy
Magnetic moments WITH field are?
*Aligned with externally applied magnetic field
EITHER - Spin parallel/aligned with +1/2
OR - Spin anti-parallel/ aligned against -1/2
What is magnitude of ΔE determined by?
*Identity of the element
*Size of the externally applied field
How are nuclei ‘flipped’ from ‘Spin aligned’ to ‘Spin opposed’
Using radiowave EM radiation equal to ΔE
What do you use to represent a nuclear magnetic moment
A vector arrow
What is the chemical shift equation?
δ=Vsample-Vstandard/(Operating Frequency ) x10⁶
V’s relate to wavelength of RF emr absorbed
What is the standard compound of chemical shift?
Tetramethylsilane (CH3)4Si
What are the 4 factors affecting Proton chemical shifts?
1.Inductive effects by electronegative groups
2. Magnetic anisotropy
3.H-bonding
4. Magnetic fields of neighbouring protons
What causes nucleus ‘shielding’?
Circulating electrons generate a local magnetic field (Be), this shields the nucleus from the applied magnetic field (Bo)
*i.e the magnetic field felt by the nucleus is reduced (Beff)
What is the calculation for Beff
Beff = Bo-Be
Order: CHBr₃, CH₃Br, CH₄ and CH₂Br₂ in terms of chemical shift
CH₄< CH₃Br < CH₂Br₂ < CHBr₃
- Because number of e-ve groups increases
=decreased Be
=increased Beff
= increased δ
What fields are protons in Aromatics, Alkenes, Alkynes … etc with II bonds under?
-They interact with Bo (applied field)
1.Bo
2.Be (shielding field of valence e)
3.Field due to II system
-They all have either a ‘shielding’ or ‘deshielding’ effect
Why does H bonding affect H NMR
*The δ range is large + hard to predict
*Solvation
*Acidity
*Concentration
*Temperature
What are equivalent protons?
They’re chemically/ electronically/ Symmetrically similar.
They have the same δ on NMR spectrum
e.g. CH3CHClCH3
The H on each end methyl group are all equivalent
In Hnmr What determines the amount of peaks
The number of different H groups
How do we work out how many H are in each envorinment
Integration
What is the 3 bond rule?
Only non-identical protons on adjacent carbons will induce coupling
What is the n+1 rule?
*Do the hydrogen(s) of interest have an H’s on adjacent groups
NO- Singlet peak
YES - How many = n
Why to we get splitting into single, doublet, triplet… etc.
Spin-Spin splittng/coupling
What are the exceptions to coupling rules
NH and OH protons DON’T couple to adjacent Hs = ‘Exchangeable’ protons
*This can be exploited by adding D2O to sample - OH and NH disapear
Why is proton noise decoupling needed?
chance of finding C13-C13 is low, but C13-H1 is high
This stops H1 coupling
So, each C is a singlet
How do you interpret 13C nmr?
No. peaks= no. different C environments
δ is dependent of group C is bonded to
scale usually =1-230ppm
0ppm = δ of (CH3)4Si
What doest symmetry do to the number of peaks in 13C nmr
Reduced the number of peaks
What is distortionless enhancement by polarisation transfer? Why is it useful?
It’s a method for determining the presence of 1⁰, 2⁰, 3⁰ and 4⁰ C atoms
It differentiates between CH, CH2, CH3, CH4
Shows CH and CH3 pointing up (or down)
Shows CH2 pointing down (or up)
Shows no peak for 4⁰
What is the Half equation of electron impact?
M ——> M⁺ + e⁻
M⁺= moleculare ion/ parent ion
What does fragmentation of the Molecular ion/Parent ion create
A⁺ + B⁺ daughter ions
For Cl2 what 2 peaks would you expect to find- ISOTOPIC EFFECTS OF HYDROGEN
1 peak 35Cl : 1 peak 37Cl
3 : 1
In Experimental analysis (Empiricle/molecular formula) if the elements don’t add up to 100% what do we assume?
The rest is made up by Oxygen
How to do elemental analysis?
- Find %s
- Divide %s by the Mr of that element
- Divide them all by the smallest ratio
What is Empirical formula?
The simplest ratio of the elements using integers
What is molecular formula?
The actual ratio of the element using the total Mr and empirical formula
What is the equation for double bond equivalence?
DBE = (2c +2 -H) / 2
*Measuring the degree of unsaturation in a molecule
e.g. DBE of 2 would suggest 1 double bond or saturated ring
DBE of 3: triple bond/ double bond/ saturated ring anf double
Oxygen has no affect
Where would you expect a C=O
1745cm⁻¹
Where would you expect a C-(triple bond)-N
2250cm⁻¹
Where would you expect a O-H
3340cm⁻¹
