Section B Flashcards
What is the equation for refractive index?
refractive index n = c/v
What is the speed of light to 4s.f.?
2.998 x 10^8m/s
What is the equation for c?
c = lambda . f (wavelength x frequency)
What is the difference in wavelength between red and blue light?
~300nm (red ~ 700nm, blue ~ 400nm)
What receptors in our eyes detect colour?
Cone receptors, photorhodopsin molceules
What colours are in a rainbow? (sing the song with me)
FUCKING SING IT YOU PRICK
What interacts in UV/Vis spectroscopy?
Electrons are raised energy levels
What does IR spectroscopy interact with?
Vibrations of whole molecules
What does NMR interact with?
spin flip transitions of atomic nuclei - magnetic part of light
What is another term for the ‘pilot wave’ released by a particle?
A photon
What is the units for Max Planck’s constant?
Js (Joule seconds)
What is Max Planck’s constant (4s.f.)?
6.626 x 10^-34Js
What is the equation for light energy E?
E = hc/lambda = hf (f can be funny v)
Electron transitions in the IR range correspond to tightly/weakly bound electrons?
Weakly bound electrons, IR has longer wavelength = less energy
What transitions is UV/vis spectroscopy used to measure in aromatic organic molecules?
pi-pi* transitions
What else is UV/vis used to measure (4 things)?
Carbon chain length, C=O/C=C, presence of chromophores, transition metals
What is a chromophore?
Light absorbing species
What gives rise to absorption bands in UV/vis?
Electrons jumping between molecular orbitals
How do you calculate wavenumber from wavelength and what are it’s units?
1/lambda, 1/cm
What region of the spectrum does the vibrational frequency of molecules correspond to?
Infrared region, fluctuating molecular electric field which resonates with electric field of light wave = absorption at vibrational frequency
In NMR, what does irradiation with light cause nuclei to do?
causes spins of nuclei to flip
What does the energy of the spin flip depend on (3)?
size of external field, type of nucleus, local environment
What happens when spin nuclei are place in a magnetic field?
The spins of the nuclei line up with the field direction
What region of the spectrum do spin flip energies occur in?
Radiowave (long wavelength), 1-500MHz
Between which wavelengths do electronic transitions occur in UV/vis?
200-700nm
What is Raman spectroscopy?
observes shifts in scattered light from laser due to molecular vibrations
How do you find absolute peak intensity?
Measuring absorbance values over several concentrations, use beer-lambert to find ε
What can you find using relative peak intensities in UV/vis?
find out what kind of electronic transitions are involved, find bonds
Which of carbon’s orbitals are used by other molecules in bonding?
2s, 2px, 2py, 2pz -> these are hybridised
Which hydrogen orbitals are used by other molecules in bonding?
1s
What is the symbol for a nonbonding orbital?
n
What arrangement are sp3 orbitals in alkanes?
Tetrahedral (s + px + py + pz)
What interactions are there between molecules in an sp2 hybridisation bond?
sp2 forms a double bond, there is one pi bond between the px’s and py’s of each atom (one pi bond = when both ends of p orbital overlap) and one sigma bond
Which bonds are stronger pi or sigma?
Sigma bonds are stronger, pi bonds form after sigma bonds
How many areas of electron shell overlap are there in a pi bond?
2 at top and bottom areas of pi orbitals, these equate to one bond
What is the respective s and p character of sp3 hybrid orbitals?
25% s character, 75% p character
What is the respective s and p character of sp hybrid orbitals?
50% s character, 50% p character
What are the relative lobe sizes of an sp3 hybrid orbital?
Contain a big lobe and a little lobe (little lobe usually ignored)
What type of overlap of orbitals occurs in a sigma bond?
Head on overlap, overlap indirection of ‘travel’
What is the length of a carbon-carbon bond?
1.54Å
What interactions are there between molecules in an sp hybridisation bond?
sp hybridisation forms a triple bond, there are 2 pi bonds and 1 sigma bond
What wavelengths do transitions between pi & pi* orbitals occur between?
200-300nm in simple organic molecules
What happens to the wavelength of the pi-pi* transition when carbon chain length of conjugated hydrocarbons increases?
wavelength becomes longer, by 30nm per extra C=C bond
What is a conjugated hydrocarbon?
A hydrocarbon containing alternating single and double carbon bonds e.g. -C=C-C=C- etc
What is an example of a common conjugated/polyunsaturated hydrocarbon?
Beta Carotene, 11 conjugated bonds, absorption in 480-510nm range (blue), causes beta carotene to appear orange
What important vitamin is beta carotene a precursor to?
Vitamin A, important for growth,infection resistance, night vision
What common molecule contains a polyunsaturated ring system?
Chlorophyll, absorbs in 400-450nm range, green-yellow light reflected
Using the beer-lambert law, what can be discerned from the molar extinction co.?
Helps work out electronic structure and bonding
If there is no UV/vis spectra, what type of molecule is it likely to be?
A fully saturated hydrocarbon (no double bonds)
Where in the electromagnetic spectrum do small unsaturated molecules have their absorption peaks?
UV region
Which groups provide the strongest absorption bands for pi-pi* transitions?
Aromatic groups, and unsaturated hydrocarbons
What transition occurs in molecules containing C, O, N, S and Cl?
n-pi* transitions, in the 200-400nm range
What is a vibrational mode?
The different well-defined patterns of vibration which occur around equilibrium positions in atoms in molecules
What is the equation for Hooke’s Law?
F = -kx (k = force constant/bond strength)
How do we calculate the reduced mass (mu)?
Use relative masses of both atoms: m1.m2/(m1+m2)
What is the point of calculating reduced mass?
Allows a 2 body to problem to be solved as 1 body problem
How do we calculate v0?
v0 = vibrational frequency = (1/[2pi]).sqrt(k/mu0)
What are the units of reduced mass?
kg
What are the units of k?
N/m or kg/s
How do we convert Hz (1/s) to wavenumber (1/cm)?
Hz/c (c= speed of light in cm/s 3 x 10^10)
What equation can be used to quickly calculate wavenumber?
v0 (1/cm) ~ 130.sqrt(k/mu)
What is changed by isotopes?
Changing of reduced mass due to extra neutron, leads to different vibrational frequency
What is the isotope substitution formula?
(v0 heavy)/(v0light) = sqrt[(mu light)/(mu heavy)]
What are the names of the 3 vibrational modes?
Symmetric stretching, bending, asymmetric stretching
Which occurs at a higher frequency symmetric or asymmetric stretching?
Asymmetric stretching
Do bending vibrations occur at a lower or higher frequency than stretching?
bending occurs at a ~50% LOWER frequency than stretching
Does H-bonding lower or increase OH stretching effects?
Lowers OH stretching frequency
What is the equation for absorbance (involving logs)?
Absorbance = log(I0/It), useful for quantitative studies of IR spectra
What type of molecule has no dipole moment?
Homonuclear molecules (molecules consisting of only one element)
What type of molecule does have a dipole moment?
Heteronuclear molecules e.g. HCl, so have IR absorption spectrum
What is the relationship between dipole moment change and IR absorption intensity?
If dipole moment change is large than IR absorption intensity is strong
Which vibrational modes are IR active?
Bending and asymmetric stretching are IR active, symmetric stretching is IR INactive
What provides the source of infrared spectra in IR spectroscopy?
Heated Globar, SiC (ceramic)
How did ‘old’ IR machines create spectra?
Scanned through all wavelengths, building up a spectra
How do current FTIR machines create spectra?
Use Fourier Transform mathematical techniques
Is it necessary to run a ‘blank’ to calculate background effects in FTIR?
No, FTIR spectroscopy calculates this first
What are the ways samples of different phase are run in an FTIR?
Solid phase: mixed with Nujol gel, pressed into KBr disc, Liquid phase: run between 2 KBr plates, Gas phase: placed in Gas cell with KBr windows
What is another name for absorption peaks in IR spectroscopy?
transmission minima
What groups would be present if there was a sharp peak 2940-2860cm^-1 region?
CH2 or CH3 groups, this area is characteristic of a C-H bond
What type of bond (including vibrational mode) do peaks between 2940-2860cm^-1 suggest?
Symmetric/Asymmetric C-H stretching in saturated hydrocarbons
What 6 vibrational modes occur between C and H atoms?
Symmetric stretching, Asymmetric stretching, Symmetric in plane bending, asymmetric bending, out of plane bending (wag), twisting/torsion
What wavenumber do unsaturated C-H bonds (alkenes, aromatics, alkynes) occur?
greater than 3000cm^-1, higher than C-H in saturated hydrocarbons, alkyne group even higher ~3310cm^-1
Other than alkyne bonds, what other bonds could be present ?
O-H or N-H bonds occupy a similar frequency
What is the difference in O-H bond environment if you see a sharp O-H bond peak or a broad one?
Sharp peak at 3650-3700 suggests O-H is non-bonded, Broad peak at 3600-2500 suggests O-H is bonded e.g COOH
What effect does intermolecular H-bonding have on OH stretch?
H-bonding causes OH stretch to occur at a lower frequency
What is the difference in peak between N-H and O-H bonding?
N-H bonds usually occur at a lower frequency and are usually a sharper peak
What would a peak in the range 1800-1600cm^-1 suggest?
A carbonyl group, C=O, position determines strength of C=O bond
Do IR peak positions change depending on protein conformation?
Yes, they change slightly if alpha helix or beta sheet
What happens to the C-O bonding frequency in pure ethanol?
Single bond C-O stretch occurs at a lower frequency ~1150cm^-1
In NMR, if electron spin aligned against an external magnetic field, is it in the unstable or stable configuration?
Unstable configuration, high energy state
What is the symbol for an external magnetic field in NMR?
B0
What is the Larmor frequency (vL)?
the frequency at which the nucleus precesses around the B0 direction (like a spinning top about to fall when it wobble)
What is the gyromagnetic ratio (gamma)?
Constant which determines Larmor frequency - different for each nucleus/isotope, varies with B0 also
Does the gyromagnetic ratio change with isotopes of the same element?
YES IT FUCKING DOES BARRY YOU INSOLENT PRICK
Other than isotope, what can cause the gyromagnetic ratio to vary?
The strength of the external magnetic field, B0
What is the typical strength of the external magnetic field used in NMR?
4-10Tesla
What range does the Larmor Frequency occur in?
Radiowave frequency (10-1000MHz), so NMR is otherwise known as Radiowave spectroscopy
What particles contribute to the spin of a nucleus?
Protons and Neutrons
What is the equation used to find the number of quantised spin states?
2 I + 1 (when I is total nuclear spin quantum number)
What is the equation used to find the difference in energy between spin states?
ΔE = (gamma.B0.h)/2pi (gamma=nuclear gyromagnetic ratio, h=planck’s constant)
What is the equation for ΔE using Larmor frequency?
ΔE = h.vL
What surrounds the superconducting magnet in an NMR machine?
Liquid nitrogen (77K) and liquid helium (4.4K)
What causes the spins of the nucleus to flip in NMR?
electromagnetic signal sent by a radiotransmitter provides enough energy
What frequency must we ‘tune’ the radio receiver to?
Larmor Frequency (vL)
What compound is added to calibrate the NMR machine?
Tetramethylsilane (Si(CH3)4))
What is done to ensure all the chemical environments are averaged in the magnetic field?
Sample is spun
What is the difference when NMR is undertaken on solids?
Speed of spinning is much increased (15-20kHz)
What is shielding?
An effect caused by electron clouds which create a small magnetic field due to their circulation, this magnetic field opposes the external magnetic field
What is the equation for the effective magnetic field (Beff)?
B eff = B0.(1-rho) where rho = shielding constant
Shielding in NMR causes a chemical shift in which direction?
Chemical shift values are usually smaller when shielded
If an adjacent species to a proton is electron donating what would happen to the NMR frequency?
shielding electron density around proton would increase, so rho (shielding constant) would increase, Beff decreases, NMR frequency decreases
What will be the electron density around the O-H protons in methanol be?
Electron density would be decreased compared to CH3 protons, as O is more electronegative than C, so draws more electron density to O nucleus and away from H nucleus, shielding smaller, higher NMR frequency
What does ‘magnetically equivalent’ mean in terms of NMR?
Same chemical shift values, e.g. all protons in CH3 are magnetically equivalent
Which is a better electron withdrawing group H or CH3?
solitary H is a better electron withdrawing group (more electronegative) than CH3, so a proton attached to a C next to a CH3 group will be better shielded and have a lower NMR frequency
What is the equation for delta (ppm)?
[(vsample - vreference)/vreference] x 10^6 = ppm, shift values relative to reference frequency
What is the chemical standard used in NMR?
TMS (tetramethylsilane) -> (CH3)4Si
Why is TMS as good standard?
unreactive, liquid (mixes w/ solvents), single NMR peak, peak at lower freq. (Si is v. electron donating)
Are most proton peaks at higher or lower delta than TMS?
Most are higher, as shielding effect is weaker than in TMS (due to v. electron donating Si)
Roughly what ppm do CH3/CH2 groups have in saturated hydrocarbons?
~1-4ppm, close to TMS peak, very highly shielded by electrons
Roughly what ppm do OH groups have?
~2-5ppm, less shielded due to electronegativity of O drawing electron density away from H
What 2 groups have a similar ppm range, but one shows a more broad peak?
O-H and N-H, N-H has a broader peak due to interaction of N14/N15 nuclear spins
Roughly what ppm do COOH/COH groups have?
~9-10ppm as they are highly deshielded, small shielding constant
What aspect of aromatic compounds causes high shift values (8-9ppm)?
Electrons travelling around ring create own magnetic flux (‘ring current’), which adds to B0 (reduces shielding) = greater shift values
Where in an aromatic compound are there likely to be protons with a negative shift?
On the inside of certain compounds, ring current opposes B0 e.g. annulene
What is changed in a high resolution NMR?
Stronger magnetic field used, allows splitting patterns to be analysed
How does a multiplet pattern arise?
protons on adjacent C, diff. environments, when one proton undergoes resonance & flips spin, other proton could be either in up or down position, changes environment
What is the magnitude of coupling constant (J)?
the strength of the interaction between two coupling protons
Why do triplet patterns form?
2 equivalent protons, so 3 possible combinations of spins: uu, 2ud, dd (middle peak = 2x as high because 2x as common)
What else can cause peak splitting?
Interactions with other NMR active nuclei
Why is MRI able to ‘see’ soft tissues?
MRI observes relaxation time of protons in H2O molecules, these times vary depending on the mobility of water -> shows density of tissue
Why is fMRI different to MRI?
fMRI observes spin state of Iron (Fe) in oxygenated haemoglobin, able to map brain activity in real time
