6-Structure Determination (1H NMR, IR) Flashcards
what is molecular spectroscopy
The study of the interaction between matter and the electromagnetic radiation (absorption, emission or transmission of radiative energy).
what are the two ways to determine the molecular structure of a molecule
Nuclear Magnetic Resonance (NMR): connectivity, structure. (main one for organic molecules)
InfraRed (IR, FTIR): functional groups
what are the two three types of ground and excited states?
vibrational, rotational, electronic states
discuss radio waves with respect to nuclear spin transitions, NMR spectroscopy, and MRI
- Nuclear spin transitions
- nucleus matters here
- specific to the isotope, not the element
- NMR spectroscopy: safe method of analysis
- low frequency is not harmful
- MRI: same technique used in hospitals
- instead of a spectra, you get an image
how is a spectrum obtained from a sample in 1H NMR?
- NMR tubes are glass
- put sample into NMR tubes, then you put your tube into the NMR instrument
- because protons are very abundant, the spectra can be created very fast
- analyze the protons that exist in your molecule → able to connect the structure
why is deuterium used during proton nuclear magnetic resonance
deuterium is a H isotope that you use because it does affect the analysis of your structure and makes it easier to read your results, and identify your unknown
what is a spin in a nuclei and what do they generate?
- All atomic nuclei possess a spin (quantum mechanical property).
- Nuclei with spin and charge generate a magnetic moment (µ)
what is resonance frequency
the frequency at which a spin-flip occurs at the β state
describe free induction decay and how this leads to the NMR signal
- having different spin states will create different energy states
- When EM radiation is of the same frequency of ∆E, some nuclei are excited from α to β state.
- have an excess at the bottom, apply the magnetic field, an electromagnetic pulse with a radio frequency will move the population from the bottom to the top when the deltaE threshold is passed
- push the excess from alpha to beta, but then these will relax and go back to alpha
- when they relax, the send a radio frequency of emission = that is the signal that gets picked up
- push the excess from alpha to beta, but then these will relax and go back to alpha
- Relaxation with reorientation occurs in a process called free induction decay.
- This decay is the origin of the NMR signal.
what are the 4 characteristics of 1H NMR
- Number of signals
- Chemical shifts (δ in ppm)
- Integration of signal peaks
- Multiplicity/Spin-spin splitting
define chemical equivalence: number of signals
- Number of signals: number of chemically distinct protons.
- Chemically equivalent protons: protons found in chemically identical environment - same chemical shift.
explain symmetry and its relation to chemical equivalence
- If protons are related by symmetry (e.g. axis of symmetry), they will be chemically equivalent.
- if there is symmetry in your molecule, they can appear under one signal
what are protons are shielded by
electrons that surround them
what factors account for the variation in chemical shifts
Slight differences in the chemical environment and electron density of the protons result in variation in the shielding degree - variation in chemical shifts (δ)
describe the relationship between electron density and shielding/deshielding
- Shielding causes the signal to shift right (lower δ)
- increasing electron density
- Deshielding causes the signal to shift left (higher δ)
- decreasing electron density
why is chemical shift in ppm and not Hz
- To have values independent of the spectrometer intensity.
- have to eliminate the Hz using ppm → creates a unitless value
- can compare two spectra even if you are not using the same magnets
what are 6 chemicals that affect the chemical shift δ
- Electronegativity
- Hybridization
- Resonance
- Substituents effect (in aromatic)
- Magnetic anisotropy
- Hydrogen bonding and exchange
when electronegativity increases, what happens to the electron density, shielding, and chemical shift
- As electronegativity increases, the electron density at the proton decreases (less shielded, resonance moves downfield): the chemical shift increases
- More electronegative substituents, the stronger the effect.
- chemical shift increases when there is less distance between the substituents and the proton
when electronegativity decreases, what happens to the electron density, shielding, and chemical shift
- As electronegativity decreases, the electron density at the proton increases (more shielded, resonance moves downfield): the chemical shift decreases
- Less electronegative substituents, the weaker the effect.
- chemical shift decreases when there is further distance between the substituents and the proton
describe the effect of hybridization on chemical shift
- As s-character increases [sp3 (25%), sp2 (33%), sp (50%)], the valence electrons will be closer to the nucleus.
- They are held more tightly - expect deshielding - higher chemical shift.
- the s orbital is spherical - the nucleus has a strong hold on its electrons (better retained)
- more s character = electrons more retained by the nucleus
- if you increase electron density, you remove that shield = higher values of chemical shift
what is the difference between the actual and expected order with hybridization in chemical shifts
- expected order: sp > sp2 > sp3
- actual order: sp > sp3 > sp2
- to explain this order we will have to refer to magnetic anisotropy later on
describe the effect of resonance /conjugation on chemical shift
- negative charge = increased electron density = more shielded = lower value of chemical shift
- positive charge = decreased electron density = less shielded = higher value of chemical shift
what happens to the chemical shift when a EWG is added to your molecule + explain the example with NO2
An electron-withdrawing substituent has a deshielding effect (higher δ)
- NO2 is an electron withdrawing group
- draw a resonance form to generate a positive charge in the ortho position and para position
- ortho and para are the most deshielded compared to meta which is the most shielded
- ortho and para experience the most decrease in electron density because the take on a positive charge
- higher chemical shift → electron density is increased
what happens to the chemical shift when a EDG is added to your molecule + explain the example with OMe
An electron-donating substituent (by resonance form or by inductive effect) has a shielding effect (lower δ).
- OMe is an electron donating group
- draw a resonance form to generate a negative charge in the ortho position and para position
- ortho and para have the most electron density compared to meta
- ortho and para are the most shielded compared to meta which is the most deshielded
- lower chemical shift → electron density decreased
- draw a resonance form to generate a negative charge in the ortho position and para position
when do we talk about magnetic anisotropy
when we have pi bonds we talk about magnetic anisotropy
where is magnetic field found and where is ring current found
linear: electrons in pi bonds create two induced magnetic fields
aromatic rings: Electrons in an aromatic ring system are induced to circulate around the ring: RING CURRENT
explain the difference in magnetic fields on top of aromatic rings vs outside of aromatic rings
- On top of the benzene (interior), the induced magnetic field opposes the applied field - area of shielding
- Outside of the aromatic ring, the induced field aligns with the applied field - area of deshielding
describe the effects of the magnetic field when the proton is aligned with the field vs when the proton is outside of the field
- if the proton is the field that aligns with the applied field
- makes it feel even a stronger magnetic field - if the proton is in the field that opposes the applied area
- makes it feel a weaker magnetic field
describe the pattern of shielding for alkynes and alkenes
- π bonds of alkenes and carbonyls: The hydrogens at the periphery are deshielded.
- outside of cone = desheilded = higher value chemical shift
- inside of cone = shielded = lower value chemical shift
- whenever we have an alkyne, the H of the alkyne exists in the cone = lower chemical shift
- whenever we have an alkene, the H of the alkenes exist outside the cone = higher chemical shift
- benzene follows the alkene rules
describe the effect of H-bonding and exchange on the chemical shift in OHs and amines
- with an OH or NHR, we should see the protons attached to these atoms
- have the possibility of doing H-bonding in an intermolecular way
- if you have a low [], you are preventing H bonding = more shielded
- if you have a high [], you encourage H bonding = more deshielded
- instead of appearing as sharp signals, they will appear as broad signals
what is an issue with chemical shifts? how can you distinguish protons on OH/NH from the protons on C?
- Hydrogen atoms capable of hydrogen bonding may also exchange with other hydrogens.
- Signals from exchangeable hydrogens can be identified by adding a drop of D2O
- protons that are attached to OH or NHR will quickly changed to OD or NDR, the signal will disappear
define what integration is and what do you need to find in order to determine the number of H’s under each peak
- Integral: the area under the NMR peak that is indicative of the relative number of protons at that signal.
- ratio
- NMR spectrometers integrate the signals to display the relative intensities.
how do you determine the number of protons for each signal after measuring
- Add the integral heights.
- With the molecular formula given, divide the total integral heights by # of hydrogens in the molecule.
C5H10O
- # H’s per signal = integral height ÷ height per H
what is multiplicity and how does spin-spin coupling relate to it
- The multiplicity (number of spikes) is significant of the number of adjacent hydrogens (three bonds away from each other).
- Origin of multiplicity: spin-spin coupling Weak magnetic fields by nearby nuclei
what rule is used to explain spin-spin coupling
- The spin-spin splitting is explained empirically by (n+1) Rule where n = # of H’s neighbors.
- n+1 rule represents the multiplicity youre seeing
- n being the number of neighbouring chemically non-equivalent protons
list the multiplet, abbreviation, # of adjacent H (n), and intensity ratio
- singlet, s, 0, 1
- doublet, d, 1, 1:1
- triple, t, 2, 1:2:1
- quartet, q, 3, 1:3:3:1
- anything 5 > , multiplet
what is a diastereptopic hydrogen
hydrogens that are not equivalent and will appear at different chemical shifts. because they have different chemical shifts, they can undergo spin-spin coupling to each other