6-Structure Determination (1H NMR, IR)

Question 1

what is molecular spectroscopy

Answer 1

The study of the interaction between matter and the electromagnetic radiation (absorption, emission or transmission of radiative energy).

Question 2

what are the two ways to determine the molecular structure of a molecule

Answer 2

Nuclear Magnetic Resonance (NMR): connectivity, structure. (main one for organic molecules)
InfraRed (IR, FTIR): functional groups

Question 3

what are the two three types of ground and excited states?

Answer 3

vibrational, rotational, electronic states

Question 4

discuss radio waves with respect to nuclear spin transitions, NMR spectroscopy, and MRI

Answer 4

• 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

Question 5

how is a spectrum obtained from a sample in 1H NMR?

Answer 5

• 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

Question 6

why is deuterium used during proton nuclear magnetic resonance

Answer 6

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

Question 7

what is a spin in a nuclei and what do they generate?

Answer 7

• All atomic nuclei possess a spin (quantum mechanical property).
• Nuclei with spin and charge generate a magnetic moment (µ)

Question 8

what is resonance frequency

Answer 8

the frequency at which a spin-flip occurs at the β state

Question 9

describe free induction decay and how this leads to the NMR signal

Answer 9

• 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
• Relaxation with reorientation occurs in a process called free induction decay.
• This decay is the origin of the NMR signal.

Question 10

what are the 4 characteristics of 1H NMR

Answer 10

• Number of signals
• Chemical shifts (δ in ppm)
• Integration of signal peaks
• Multiplicity/Spin-spin splitting

Question 11

define chemical equivalence: number of signals

Answer 11

• Number of signals: number of chemically distinct protons.

- Chemically equivalent protons: protons found in chemically identical environment - same chemical shift.

Question 12

explain symmetry and its relation to chemical equivalence

Answer 12

• 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

Question 13

what are protons are shielded by

Answer 13

electrons that surround them

Question 14

what factors account for the variation in chemical shifts

Answer 14

Slight differences in the chemical environment and electron density of the protons result in variation in the shielding degree - variation in chemical shifts (δ)

Question 15

describe the relationship between electron density and shielding/deshielding

Answer 15

• Shielding causes the signal to shift right (lower δ)
  
  • increasing electron density
• Deshielding causes the signal to shift left (higher δ)
  
  • decreasing electron density

Question 16

why is chemical shift in ppm and not Hz

Answer 16

• 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

Question 17

what are 6 chemicals that affect the chemical shift δ

Answer 17

• Electronegativity
• Hybridization
• Resonance
• Substituents effect (in aromatic)
• Magnetic anisotropy
• Hydrogen bonding and exchange

Question 18

when electronegativity increases, what happens to the electron density, shielding, and chemical shift

Answer 18

• 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

Question 19

when electronegativity decreases, what happens to the electron density, shielding, and chemical shift

Answer 19

• 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

Question 20

describe the effect of hybridization on chemical shift

Answer 20

• 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

Question 21

what is the difference between the actual and expected order with hybridization in chemical shifts

Answer 21

• expected order: sp > sp2 > sp3
• actual order: sp > sp3 > sp2
  
  • to explain this order we will have to refer to magnetic anisotropy later on

Question 22

describe the effect of resonance /conjugation on chemical shift

Answer 22

• negative charge = increased electron density = more shielded = lower value of chemical shift
• positive charge = decreased electron density = less shielded = higher value of chemical shift

Question 23

what happens to the chemical shift when a EWG is added to your molecule + explain the example with NO2

Answer 23

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

Question 24

what happens to the chemical shift when a EDG is added to your molecule + explain the example with OMe

Answer 24

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

Question 25

when do we talk about magnetic anisotropy

Answer 25

when we have pi bonds we talk about magnetic anisotropy

Question 26

where is magnetic field found and where is ring current found

Answer 26

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

Question 27

explain the difference in magnetic fields on top of aromatic rings vs outside of aromatic rings

Answer 27

• 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

Question 28

describe the effects of the magnetic field when the proton is aligned with the field vs when the proton is outside of the field

Answer 28

• 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

Question 29

describe the pattern of shielding for alkynes and alkenes

Answer 29

• 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

Question 30

describe the effect of H-bonding and exchange on the chemical shift in OHs and amines

Answer 30

• 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

Question 31

what is an issue with chemical shifts? how can you distinguish protons on OH/NH from the protons on C?

Answer 31

• 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

Question 32

define what integration is and what do you need to find in order to determine the number of H’s under each peak

Answer 32

• 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.

Question 33

how do you determine the number of protons for each signal after measuring

Answer 33

• 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

Question 34

what is multiplicity and how does spin-spin coupling relate to it

Answer 34

• 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

Question 35

what rule is used to explain spin-spin coupling

Answer 35

• 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

Question 36

list the multiplet, abbreviation, # of adjacent H (n), and intensity ratio

Answer 36

• 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

Question 37

what is a diastereptopic hydrogen

Answer 37

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