Spectroscopy Flashcards
What is the order of energy levels for vibrational, electronic and rotational?
Electronic > Vibrational > Rotational
What types of light on the EM spectrum are electronic?
UV/visible
What types of light on the EM spectrum are vibrational?
Infrared
What types of light on the EM spectrum are rotational?
Radio
What can UV/visible spectroscopy be used to investigate?
Presence of double bonds
Transition metals
Concentration
What can infrared spectroscopy be used to investigate?
Functional groups
What can radio (NMR) spectroscopy be used to investigate?
Molecular structure
How can you determine energy of radiation, using Plancks constant (formula + units)?
Energy of radiation = Plancks constant (J) x Speed of light (m/s) x Lamnda wavelength (m)
What is the visible spectrum in order of decreasing wavelength?
Red Orange Yellow Green Blue Indigo Violet
ROY G BIV
What is the Beer-Lambert Law (formula)?
Absorption = molar extinction coefficient x concentration x path length
What can you deduce from plotting absorption against concentration on a graph?
If there is a linear relationship (you get a straight line), it adheres to the Beer-Lambert law within that range of concentrations
What are electrons in bonding orbitals found?
In bonding orbitals, it is probable that the electrons will be found in between the two nuclei
What does a high molar absorption coefficient tell you?
That the compound is very effective at absorbing light (of a certain wavelength)§
What is the molar absorption coefficient?
A measurement of how strongly a chemical species absorbs light at a given wavelength
What are electrons in anti-bonding orbitals found?
In anti-bonding orbitals, it is probable that electrons will be found on opposite sides of the nuclei
Which orbitals are higher energy out of bonding and anti-bonding?
Anti-bonding orbitals are higher energy
On a UV spectroscopy diagram, which electron transition will be the higher peak?
The transition from anti-bonding to bonding orbitals, as there is a greater energy gap
On a UV spectroscopy diagram, what does the smaller peak represent?
The non-bonding to anti-bonding electron transition; this requires less energy, compared to bonding to anti-bonding
What does HOMO stand for?
Highest Occupied Molecular Orbital
What does LUMO stand for?
Lowest Unoccupied Molecular Orbital
When do electrons enter anti-bonding orbitals?
When they are excited
What is a conjugated molecule?
A molecule made up of alternating double and single bonds
What is the relationship between conjugation, and HOMO & LUMO?
With increasing conjugation, HOMO and LUMO get closer together
What does increasing conjugation mean wrt wavelength?
More conjugation = lower energy gap = longer wavelength
At what wavelengths will the substance appear colourless/white?
Less than 400nm or greater than 750nm
What are the two types of vibration in molecules?
Bending and stretching vibrations
What is a bending vibration?
Where the bond length doesn’t change
What is a stretching vibration?
Where the bond length is changing
What is the Y-axis on IR spectroscopy graphs?
% of either absorption or transmittance
What is the X-axis on IR spectroscopy graphs?
Wavenumber
What is the unit of wavenumber?
cm^-1
How do you convert from um to cm^-1?
Convert to cm, then take the reciprical
Example:
1um = 0.01 cm
1 / 0.01 = 100 cm-1
What is wavenumber determined by?
- Bond strength, quantified by force constant (k)
- Mass of atoms, quantified by reduced mass (u)
How can you deduce reduced mass (u)?
Multiply the atomic masses, then divide by the sum of the masses:
Reduced mass (u) = m1m2 / m1 + m2
What can you use to determine a predicted wave number?
Hooke’s law (equation given in exam)
What determines whether a bond is IR active or inactive?
For a bond to be IR active, the bond must polar, causing a vibrational stretch
What is the relationship between polarity of bond and intensity of absorption?
The more polar the bond, the more intense the IR absorption
What is the broadening of a peak on an IR spectroscopy diagram indicative of?
Hydrogen bonding, so often an O-H bond
Where is the functional group region on an IR spectroscopy diagram?
4000cm-1 to 1500cm-1
Where is the fingerprint region on an IR spectroscopy diagram?
1500cm-1 to 625cm-1
What does a strong sharp peak at around 1700cm-1 indicate?
A double bond, commonly C=O or C=C
How does bond strength affect wavenumber?
The stronger the bond, the higher the wavenumber (and the higher IR stretching frequency)
How does mass of atoms affect wavenumber?
The greater the mass, the lower the wavenumber (and the lower the IR stretching frequency)
At what frequency would a triple bond appear?
Around 2100cm-1
What bonds are found above 2800cm-1?
X-H bonds, like C-H, N-H and O-H
What affect does conjugation have on wavenumber?
Conjugated molecules have a lower wavenumber (typically lower than all others)
Why can a peak look strong despite the bond not being strong?
The accumulation of many bonds (such as many C-H) in a molecule can make the peak look strong, even though individually they are weak
What are the two types of nuclei?
With spin
Without spin
What does it mean for a nucleus to be with spin?
It has an odd mass number and/or atomic number (i.e is an isotope)
What are some examples of nuclei with spin?
1-H (proton) and Carbon-13
What are the two possible spins of isotopes?
+1/2 and -1/2
What are nuclear magnetic moments generated by?
They are generated by the charged spinning nucleus
What is the orientation and energy of nuclear magnetic moments in the presence of no magnetic field?
The magnetic moments are randomly oriented, and are equal in energy
What is the orientation of nuclear magnetic moments in the presence of an externally applied magnetic field?
They will either spin parallel/aligned with the magnetic field, or antiparallel/aligned against the magnetic field
What are the respective energies of magnetic moments in the presence of a magnetic field?
Magnetic moments that are antiparallel/aligned against the magnetic field will be of higher energy, compared to those aligned with the field
What is delta E wrt nuclear magnetic moments?
The energy required to flip the nuclei from spin aligned to spin opposed
What two main factors determine delta E?
- Identity of the element
- The size of the externally applied field
What three other factors can affect delta E?
- Inductive effects by electronegative groups
- Magnetic anisotrophy
- Hydrogen bonding
What is the shielding magnetic field,
It is generated by electrons surrounding the nucleus when an external magnetic field is applied, and opposes it
What is the relationship between electron density and delta E?
The greater the electron density around the nucleus of the H, the greater the shielding, and so the lesser the delta value
What happens if hydrogen is bonded to a very electronegative element (e.g. O-H)?
There will be a lesser electron density around the nucleus of the H, so less shielding, meaning a greater overall field and delta value
What is magnetic anisotrophy?
Compounds with pi bonds can either be shielding (decrease delta) or deshielding (increase delta)
What is magnetic anisotrophy dependent on?
The positioning of the proton relative to the pi system
How does hydrogen bonding affect delta value in NMR?
Compounds containing NH and OH and undergo hydrogen bonding are difficult to predict where the peak will be on the delta scale
What is the unit of delta value in NMR?
ppm
What is typically the delta value of 1-H in aromatic compounds (Ar-H)?
6-7ppm
What does a low delta value for 1-H indicate?
Alkyl groups
What does a large range of delta values indicate?
Hydrogen bonding (compounds containing N-H/O-H)
Spin spin splitting
Do practice questions
If a NMR peak disappears after being shaken with D2O, what is likely to be present in the compound?
An NH or OH group
What is the natural abundance of carbon-13, and what are the implications of this for coupling?
1.1%, so it is unlikely that you will find 13-C next to a 13-C, so very low chance of seeing this coupling
Why may there be splitting on 13-C NMR?
Because 1-H can commonly be found next to a 13-C; proton noise decoupling can be used to remove these peaks
What 3 features will there be in DEPT spectrums?
- CH and CH3 groups will be pointing up
- CH2 peaks will be pointing down
- Quaternary C will have no peak
Summarise briefly how mass spectrometers work:
- Sample vaporised
- Electron beam ionises sample
- Particles accelerated into magnetic field
- Magnetic field separates particles based on mass/charge ratio
- Particles detected
What is fragmentation is mass spec?
The parent/molecular ion can fragment into daughter ions; the most intense peak on mass spec indicates the most stable fragment
What are the two main isotopes of Cl, and what are their relative abundances?
35-Cl and 37-Cl, with a natural abundance of 75:25 respectively
What are the two main isotopes of Br, and what are their relative abundances?
79-Br and 81-Br, with a 50:50 natural abundance
What does a m+1 small peak indicate?
Most commonly 13-C
What does a m+2 small peak indicate?
18-O
How do you work out the number of double bond equivilance?
(2n + 2) - no. hydrogens / 2
This will give you the number of DBE present
What is the effect of nitrogen on DBE?
For each nitrogen, add 1 H. So the equation is:
(2n + 2 + no. N) - no. hydrogens /2
What is the effect of oxygen on DBE?
Oxygen has no effect on DBE equation, so use normal (2n + 2) - no. hydrogens /2
What is the effect of halogens on DBE?
Each halogen will replace 1 H. So the equation is:
(2n + 2 - no. halogens) - no. hydrogens / 2
What does one DBE indicate?
One double bond (e.g. C=O, C=C, C=N) or a saturated ring structure
What does two DBE indicate?
C triple bond N
or
Saturated ring + double bond
or
Two double bonds
What does four DBE indicate?
Benzene ring
