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M102 > W22 Infra Red, UV/Vis, Mass,NMR Spectrometry > Flashcards

W22 Infra Red, UV/Vis, Mass,NMR Spectrometry Flashcards

1
Q

What are the different spectroscopic and spectrometric techniques?

A

1.Infrared Spectroscopy (IR): Measures vibrations of bonds to identify functional groups
2. Ultraviolet Spectroscopy (UV): Measures electronic transitions to provide information on the electronic bonding in the sample
3. Mass Spectrometry (MS): Breaks the molecule into fragments (destructive) and measures the MW of fragments to give information to the structure and functional groups present
4. Nuclear Magnetic Resonance (NMR): Uses
electromagnetic fields to measure spinning of nuclei to identify chemical environments leading to identification of functional groups and structure of the sample

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2
Q

Electromagnetic Spectrum
How do Electromagnetic Radiations travel?

A

Travel at the speed of light (c) but at different frequencies and wavelengths

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3
Q

Electromagnetic Spectrum
What is Frequency (v) ?

A

Number of complete wave cycles that
pass a fixed point per second, Hz (cycles per second)

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4
Q

Electromagnetic Spectrum
What is Wavelength (λ)?

A

distance between any two peaks

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5
Q

Electromagnetic Spectrum
What are Photons?

A

Massless packs of energy E = hv
(h=Planck’s Constant)

Molecule struck by a photon: may absorb the photon’s energy and will result in the molecule’s energy increasing by an amount equal to the photon’s energy
E = hv

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5
Q

What can be detected in the Infrared Region?

A

Molecular vibrations can be detected in the infrared region.
The covalent bonds between atoms act like springs. If the bond is stretched / compressed and then released, the atoms vibrate.

Heavier atoms vibrate more slowly.
Frequency decreases with increasing atomic mass
Stronger bonds are more rigid, so they vibrate faster.
More force needed to stretch / compress (higher frequency)

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6
Q

What are the 3 Vibrational Modes?

A
  1. Symmetric stretching
  2. Antisymmetric stretching
  3. Bending (scissoring)

Non-linear molecules with n atoms will have 3n –6 vibrational modes (i.e. 6 atoms = 12 modes)
There can also be combinations and overtones (multiples) of fundamental vibrations = complex vibrations

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7
Q

IR-Active and IR-Inactive Vibrations

A

IR-Active Vibrations:
* The bond must have a dipole
moment.
* The vibration must change the dipole
moment of the molecule
IR-Inactive:
* Symmetrical / has no dipole moment

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8
Q

What are the O-H Stretching Frequencies:

A

Alcohol (e.g. CH3OH): 3300 cm-1 , Broad
Acid (e.g. CH3COOH): 3000 cm-1 , Broad

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9
Q

What are the C-O Stretching Frequencies?

A

Alcohols and ethers: 1000 –1200 cm-1

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10
Q

What are the N-H Stretching Frequencies?

A

Amine: 3300 cm-1 , Broad
1 Spike = N-H of secondary amine (R2NH)
2 Spikes = N-H of primary amine (RNH2)

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11
Q

Infrared Spectroscopy of Ketones & Aldehydes
What is the range for C=O peaks? (4)
What is the range for a C-H peak?

A

C=O
* Intense absorption at 1700 cm-1
* Small overtone peaks at 3400 cm-1
* Aldehyde: 1725 cm-1
* Ketone: 1710 cm-1

C-H
Aldehyde has two: 2700 and 2800 cm-1

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12
Q

Infrared spectroscopy of C-N bond:

A

C triple bond N = 2,200-2250

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13
Q

Strengths and Limitations of IR Spectroscopy

A
  1. Ability to indicate functional groups in the compound
  2. Highlights the absence of other functional groups that would give strong absorptions if they were present
  3. Can confirm the identity of a compound by comparison with a known sample
  4. It is non-destructive (sample can be retrieved)
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14
Q

What is UV/Vis spectrophotometry?
What is measured? (2)

A
  • An instrument for irradiating a sample with photons of UV or vis light of particular wavelength/s
  • Liquid sample dispensed into a cuvette (solvent not abs. above 200nm) UV – quartz; vis – plastic, polystyrene
  • Must be run against a sample blank, (same solution but no drug)
  1. absorption - light not passing through sample
  2. transmission – light passing through sample
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15
Q

What are the different electromagnetic waves?
What waves have the longest wavelength?
What waves have the highest frequency?

A

RMIVUXG
Radio, Micro, IR, Visible, UV, Xray, Gamma
1. Radiowaves (size of mountains)
2. Gamma (size of atomic nuclei)

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16
Q

What is a Molecular Orbital (MO)?

A

The region in which there is the highest probability of finding an electron of a molecule

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17
Q

What is the Highest Occupied Molecular Orbital (HOMO)?
What is the Lowest Unoccupied Molecular Orbital (LUMO)?

A
  • Highest energy MO with any electrons in it
  • The next highest energy orbital. It will be empty and so is the lowest energy orbital in which to place an electron or excite one into.
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18
Q

What happens in Ultraviolet Absorption Spectroscopy?

A
  • Wavelengths of UV light absorbed by a molecule are determined by the electronic energy differences between the HOMO and LUMO.
  • Absorption is only possible if the molecule contains two molecular levels separated by energy (Transition from bonding to anti-bonding orbitals, to ᵰ , ᵰ , ᵰ to σ. (ᵰ =pi)
  • The molecule will now be in an “excited state”
  • Energy will dissipate rapidly and will return to the
    “ground state”
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19
Q

Ultraviolet Absorption Spectroscopy
What is Lambda max?

A

Wavelength showing the greatest absorbance

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20
Q

What is Beer’s law?

A

A=ecl

Where:
* Absorbance is what we measure and has arbitrary units (A)
* The molar absorption coefficient is a standard (e)
* The concentration is in moldm-3 (c)
* The optical path length is the width of the cuvette (normally 1 cm) (l)

21
Q

Beer’s Law graph plot:
What are the axis?
Shape of graph?
Significance of gradient?

A
  • We can measure absorbance as we vary the concentration
  • A standard calibration plot will be produced
  • If Beer’s Law is obeyed, a straight line will be produced
  • The gradient = the molar absorption coefficient
22
Q

Interpreting UV-Vis Spectra

A
  1. Wavelength (s) of maximum absorbance
  2. Molar absorptivity e at each maximum

Aromatic compounds, highly conjugated, absorb strongly in UV

Characteristic absorbance (lamda max) at 254nm

23
Q

What are Chromophores?

A
24
Q

Features of Mass spectrometry?

A
  • Provides Molecular Weight
  • Provides valuable information about the molecular formula
  • Does not use light (different from spectrometry)
  • Sample is destroyed
  • High-energy electrons fragment the molecules
  • Mass of the fragments is measured
  • Information used to “reconstruct” the molecule
25
Q

What is the process of mass spectrometry? (3)

A
  1. Negatively charged accelerator plate with a narrow slit to allow ions to pass through
  2. Charged particle passes through magnetic field
    (bends path) – Curvature of the bend depends on the m/z ratio (mass/charge)
  3. Slit followed by detector – at any given magnetic
    field, only ions of one particular mass are bent exactly the right amount to pass through the detector. The detector signal is proportional to the number of ions striking it
26
Q

Features of Mass spectrometry:

A

Peaks are vertical line
Masses rounded to nearest whole number mass unit
Base peak does not necessarily correspond to the mass of the molecular ion (M+)
Often very small peaks close to expect MW= Isotope peaks

27
Q

What is the molecular ion peak?
What is the m/z value?
What to do if no M+ observed

A

-The particle with the HIGHEST MW in the spectrum
Usually even-numbered mass
-m/z value- MW of compound
-Use a gentle ionisation

28
Q

Mass Spec Calculations

A
  • Calculation used to determine the molecular
    formula based on mass
  • Starts with the assumption of only having carbon
    and hydrogen atoms (C=12, H=1)
  • Then heteroatoms are explored until an exact
    combination or all possible combinations are
    identified
29
Q

Rule of 13:
a. What is the molecular formula with M+=78
b. M+=92
c. M+=152

A

a) 78/13 =6 so n=6 and CnHn so
C6H6
b) 92/13=7.07 so 7x13=91 (1 extra H)
so C7H8
c) C11H20

30
Q

What is Shielding?

A
  • Magnetic forcefield exerted by electrons: electrons
    are circulating and generating a small induced magnetic field that opposes the external field (B0)
  • Decrease in the magnetic field felt by the nucleus: this results in a stronger field being needed for resonance to occur
  • Electronegative atoms such as oxygen pull electrons
    away from neighbouring atoms, deshielding them
  • Shielded atoms need higher field energy to cause
    resonance
31
Q

The Chemical Shift (in ppm) (on exam)
What is the equation to calculate it?

A

…of a given proton is the same regardless
of the operating field and frequency of the spectrometer
Chemical shift=Shift downfield from TMS (in Hz) / Spectrometer frequency

32
Q

Chemical Shift - Variations

A
  1. Electronegativity (deshielding effect)
  2. Distance from Electron-Withdrawing Group
  3. Addition of More Electron-Withdrawing Groups
  4. Other Groups with Deshielding Effects e.g. vinyl, aromatic protons, aldehydes, OH and NH, COOH
33
Q

Integration: Areas of the Peaks

A

The area under the peak is proportional to the number of protons in that environment
The integral trace cannot specify the number of
protons but instead shows the ratio
(use ruler and measure length of the lines, then work out ratio)

34
Q

Spin-Spin Splitting

A

Protons from different environments can influence the magnetic field of each other if they are
significantly close enough to one another. This can result in the splitting of their peaks as seen in the
NMR spectrum – Chemically equivalent protons do not split each other

35
Q

Spin-Spin Splitting – N+1 Rule

A

The number of peaks (multiplicity) of an NMR signal is determined by the N+1 Rule
If the signal is split by N neighbouring equivalent protons, it will be split into N+1 peaks

36
Q

Unusual Splitting Patterns

A

Some aryl systems give complex coupling / splitting patterns:
* 1 strongly EWG or EDG
* Symmetrical ortho distribution
* Unsymmetrical para distribution
(aromatic rings)

37
Q

What are Coupling Constants?

A

A coupling constant (J) is the distance between the peaks of multiplets measured in Hz
They do not vary with the field strength of the spectrometer
They are often used to distinguish between the isomers of a compound including stereoisomers (cis = 10Hz, trans = 15Hz)

38
Q

Are 2 Signals Equivalent or not?
Homotopic =

A

if any H are replaced by
deuterium (D) nothing changes as the
other two groups are equivalents
- In any solvent

39
Q

Are 2 Signals Equivalent or not?
Enantiotopic?

A

If any of the H are replaced
by D the molecule becomes chiral (so they
are called prochiral)

40
Q

Are 2 Signals Equivalent or not?
Diastereotopic?

A

If either of the prochiral H are replaced by D we create a second chiral centre and the two resulting molecules will be diastereomers (1x S,R, 1x R,R).

41
Q

Carbon-13 (13C) NMR

A
  • Determines the magnetic environments of the carbon atoms instead of protons
  • 12C (99%) has no magnetic spin
  • 13C (1%) does have a magnetic spin –but sensitivity is decreased by x100
  • The resonance frequency of 13C is only one fourth of that for 1H NMR
    (gyromagnetic ratio)
  • Hundred of spectra were taken and then averaged (old technique)
  • Now we have Fourier-Transform
42
Q

What us Fourier Transform NMR Spectroscopy?

A
  • Mathematical technique used to compute a spectrum from the FID
  • A pulse is used to introduce non-equilibrium nuclear spin magnetization
  • The relaxation of spins to their resonance is time-dependent and measured to create the spectrum
43
Q

Carbon Chemical Shifts

A
  • 13C signals are also deshielded by EWG (similarly to 1H NMR)
  • The carbon atom is one atom closer to the shielding or deshielding group than the attached hydrogen
    atom
  • Therefore, carbon chemical shifts are usually 15-20 times larger than proton shifts

EWG= Electron withdrawing group

44
Q

Comparison of 13C and 1H NMR

A
  • 1H Spectrum = 10ppm, 13C Spectrum = 200 ppm (20x larger scale)
  • Deuterated Chloroform (CDCl3) used as common solvent for 13C NMR
45
Q

Differences between 1H and 13C NMR:

A
  1. Operating frequency:
    * Gyromagnetic ration and resonance frequency of 13C is about ¼ of that for 1H NMR
  2. Peak Areas:
    * The areas of the peaks of 13C are not necessarily proportional to the number of carbons giving rise to those peaks –therefore can be used to identify the type of chemical environment but may not relate directly to the number of carbons present.
    * Carbon atoms with 2 or 3 protons attached usually give the strongest absorptions while carbon atoms with no protons attached usually give the weakest absorptions.
  3. Spin-Spin Splitting:
    * Since only 1% of Carbon atoms are the 13C isotope, carbon-carbon splitting is ignored
    * Carbon-Hydrogen splitting patterns can be complicated
46
Q

Proton Spin Decoupling:

A
  • H is continuously in resonance (rapidly flip spins) due to being irradiated by a broadband proton transmitter
  • The C nuclei see an average of the possible proton spin states
  • Each C signal appears as a single unsplit peak because any C-H splitting has been eliminated
47
Q

Off-Resonance Decoupling

A
  • Simplification of the spectrum that still allows some of the splitting information to be retained 13C nuclei are only split by the protons directly bonded to them
  • N+1 Rule Applies: C with 1H appears as a doublet, C with 2H gives a triplet etc.
  • TMS is a quartet at 0 ppm (split by 3 protons of each methyl group)

The spectrum is run twice:
1st: Broadband-Decoupled Spectrum (indicated number of non-equivalent environments of C
and their chemical shifts)
2nd: Off-Resonance-Decoupled Spectrum (multiplicities of signals indicate the number of H
bonded to each C)

48
Q

DEPT 13C NMR

A

DEPT: Distortionless Enhanced Polarization Transfer
* All peaks remain as decoupled singlets
* Has better sensitivity and avoids overlapping multiplets
* Each 13C nucleus is magnetically coupled to the protons bonded to it
* There is a transfer of polarization from the protons to the carbon nucleus
* How the polarization transfer occurs is determined by the number of protons bonded to the 13C nucleus

49
Q

What is polarisation transfer?

A

Transferring of spin that electrons have

50
Q

Interpreting 13C NMR Spectra

A
  1. The number of different signals implies how many different types of carbons are present
  2. The chemical shifts of those signals suggest what types of functional groups those carbon atoms belong to
  3. The splitting of signals in the Off-Resonance-Decoupled, DEPT-90 and DEPT-135 spectra indicate how many protons are bonded to each carbon atom

M102 (42 decks)

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