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PMP 102 > Spectroscopy > Flashcards

Spectroscopy Flashcards

1
Q

what is infared spectroscopy (IR)?

A

measures vibrations of bonds to identify functional groups

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2
Q

what is ultraviolet spectroscopy (UV)?

A

measures electronic transitions to provide information on the electronic bonding in the sample

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3
Q

what is mass spectrometry (MS)?

A

breaks the molecule into fragments and measures the MW of fragemnts to give information to the structure and functional groups present

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4
Q

what is nuclear magnetic resonance (NMR)?

A

uses elctromagnetic fields to measure spinning of nuclei to identify chemical environments leading to dentification of functional groups and structure of the sample

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5
Q

what are electromagentic radiations?

A

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

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6
Q

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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7
Q

what is wavelength (upside down y)?

A

distance between two peaks

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8
Q

what are photons?

A

massless packs of energy
E=hv
(h= Planktons constant)

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9
Q

what are the characteristics of 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.
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.

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10
Q

what are IR-Active vibrations?

A

the bond must have a dipole moment
the vibration must change the dipole moment of the molecule

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11
Q

what are IR-Inactive vibrations?

A

symmetrical/ has no dipole moment

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12
Q

what are the carbon-carbon bond stretching?

A

c-c 1200
c=c 1660
c=-c <2200

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13
Q

what are the carbon-hydrogen bond stretching?

A

c-c-h 2800-3000
c=c-h 3000-3100
c=-c-h 3300

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14
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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15
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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16
Q

what are the C-O stretching frequencies?

A

Alcohols and ethers: 1000 –1200 cm-1

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17
Q

what are the c=o stretching frequencies?

A
  • Intense absorption at 1700 cm-1
  • Small overtone peaks at 3400 cm-1
  • Aldehyde: 1725 cm-1
  • Ketone: 1710 cm-1
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18
Q

what is the stretching frequency of C-H of an aldehyde?

A

2700 and 2800

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19
Q

what are the C-N stretching frequencies?

A

c-n 1200
c=n 1660
c=-n >2200

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20
Q

what are the regions of the IR spectrum?

A

4000-2500= z-H single bond stretches
2500-2000= triple bond stretches
2000-1500=double bond stretches
1500-1000=single bond stretches
1000-400=bending vibrations

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21
Q

what is used for UV/vs spectrophotometry?

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)
  • Measure either:
    i. absorption - light not passing through sample
    ii. transmission – light passing through sample
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22
Q

what are molecular orbitals?

A

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

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23
Q

what are HOMOs and LUMOs?

A
  • Highest Occupied Molecular Orbital (HOMO): Highest
    energy MO with any electrons in it
  • Lowest Unoccupied Molecular Orbital (LUMO): 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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24
Q

what happens in the 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, ᵰ to ᵰ , ᵰ to
    σ
  • The molecule will now be in an “excited state”
  • Energy will dissipate rapidly and will return to the
    “ground state”
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25
Q

what is lambda max?

A

wavelength showing the greatest abosorbance

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26
Q

what is the formula for beers law?

A

A=Ecl
A=absorbance
E= molar absorption coefficient
c=molar conc
l=optical path length

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27
Q

what are chromophores?

A

structural features of a molecule that absorb specific wavelengths

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28
Q

what are auxochromes?

A

auxiliary groups attached to a chromophore, unable to absorb, but cause shifts

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29
Q

what are the characteristics 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
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30
Q

what are the steps to recieve the mass spectrometry?

A
  1. Negatively charged accelerator plate with 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
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31
Q

what is the theory of NMR spectroscopy?

A
  • Nuclei of Elemental Isotopes:
    • Have a characteristic spin (l)
    • Some are integral (1,2,3), some are
      fractional (1/2, 3/2, 5/2), some have no spin,
      l = 0 (12C, 16O)
    • The most useful in organic chemistry are
      1H, 13C, 19F and 31P, all l = ½
  • Spinning Nuclei:
    • Create a magnetic moment (μ)
    • Can spin in all directions until an external
      field is introduced
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32
Q

what is the resonance theory of NMR spectroscopy?

A
  • When a nucleus is subjected to the right combination
    of magnetic field and electromagnetic radiation the
    spin can flip back and forth
  • The nucleus is in resonance and this is the energy
    detected by NMR
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33
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
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34
Q

what are the different chemical shifts variations?

A

electronegativity (deshielding effect)
distance from electron-withdrawing group
addition of more electron withdrawing groups
other groups with deshelding effects

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35
Q

what is the chemical shift effect of electronegativity?

A

With increasing electronegativity, there is more deshielding of neighbouring
atoms as the electrons are drawn towards the more electronegative atom.

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36
Q

what is the chemical shift effect of distance from electron-withdrawing group?

A

The further away an atom is from the electronegative group, the less deshielding
effect is felt by the protons of that atom.

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37
Q

what is the chemical shift effect of addition of more electron withdrawing groups?

A

If more than one EWG is present, the deshielding
effects are nearly (but not quite) additive.

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38
Q

what is the chemical shift effect of other groups with deshielding effects?

A

Vinyl (δ5-6) and Aromatic Protons (δ6-9): Electron circulation causes deshielding so lower fields
observed.
Aldehydes (δ9-10): Carbonyl group has deshielding effect
OH and NH: These depend on the concentrations widely
- Concentrated solutions, deshielding caused by H-bonding, NH (δ3.5) and OH (δ4.5)
- Diluted solutions, no H-Bonding, NH and OH (δ2.0)
- Broad peaks are observed due to H-bonding and proton exchange
COOH (δ10+): C=O and O present so even greater deshielding

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39
Q

what does the number of NMR signals present represent?

A

the number of proton environments

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40
Q

what does chemically equivalent mean?

A

protons in identical chemical environments

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41
Q

what does the area under the peak represent?

A

the total number of protons in that environment (ratio)

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42
Q

what is 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

43
Q

what is the N+1 rule?

A

If the signal is split by N neighbouring equivalent protons, it will be split into N+1 peaks

44
Q

how many bonds need to be present for spin-spin splitting to happen?

A

no more than 3

45
Q

what are coupling constants?

A

the distance between the peaks of multipkets measured in Hz

46
Q

what is homotopic?

A

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

47
Q

what is enantiotopic?

A

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

48
Q

what is 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).

49
Q

how to work out the degree of unsaturation?

A

(2+2a-b+c-e)/2
for a compound formula of CaHbNcOdXe

50
Q

what is degree of unsaturation also known as?

A

double bond equivalents

51
Q

when is a ch3 group further down the proton nmr?

A

when its closer to an O

52
Q

what is contained within a fourier transform infared spectrometer?

A

detector, sample, moving mirror, beam splitter, laser callibration beam, IR sorse, He-Ne laser and fixed mirror

53
Q

wher do you measure a wavelength?

A

from crest to crest of a signal

54
Q

what are the components of plack’s constant?

A

e= energy of the packet
h= plancks constant (6.626x 10^-34 J.s)
v= frequency (cycles per second)

55
Q

what is the numerical value of planks constant?

A

6.626x10^-34 J per second

56
Q

what is the stretching frequency of C-H?

A

3000

57
Q

what is the stretching frequency of c-c?

A

1200

58
Q

what is the stretching frequency of c-c?

A

1200

59
Q

what is the stretching frequency of c=c?

A

1660

60
Q

what is the stretching frequency of c=-c?

A

2200

61
Q

what is the stretching frequency of c-n?

A

1200

62
Q

what is the stretching frequency of c=n?

A

1650

63
Q

what is the stretching frequency of c=-n?

A

2200

64
Q

what is the stretching frequency of c-o?

A

1100

65
Q

what is the stretching frequency of c=o?

A

1700

66
Q

what are the different vibrational moes in infared?

A

symmetric stretching
antisymetric stretching
bending (scissoring)

67
Q

what is symmetric stretching?

A

atoms go away from the central atom

68
Q

what is antisymmetric stretching?

A

one atom goes towards the central atom and the other goes away

69
Q

what is bending (scissoring)?

A

outer atoms go towards each other with no regard for the central atom

70
Q

how many vibrational nodes do non-linear molecules have?

A

3n-6 vibrational modes
(ie, 6 atomes= 12 modes)

71
Q

what is the name when there are combinations of fundamental vibrations?

A

complex vibrations

72
Q

whuat are the rules for IR-active vibrations?

A

the bond must have a dipole moment
the vibration must change the dipole moment of the molecule

73
Q

what are the rules for an IR-inactive vibrations?

A

symmetrical
has no dipole

74
Q

what are the OH stretching frequencies?

A

alcohol= 3300 (broad)
acid= 3000 (broad)

75
Q

what is the c-o stretching frequencies?

A

alcohol and ethers= 1000-1200

76
Q

what are the N-H stretching frequencies?

A

amine= 3300 (broad)
1 spike= secondary amine
2 spikes = primary amine

77
Q

what can UV/Vis spectrophotometry meaure?

A

absorption (light not passing through sample)
transmission (light passing through the sample)

78
Q

what are wavelengths of molecule determined by?

A

electronic energy differences between HOMO and LUMO

79
Q

when is absorption only possible?

A

if the molecule contains two molecular levels seperated by energy
this is then turned into its excited state, then will return to its ground state rapidly

80
Q

what are chromophores?

A

structural features of a molecule that absorb specific wavelengthss

81
Q

what are auxochromes?

A

auxillary groups attached to a chromophore, unable to absorb, but cause shifts

82
Q

what are the components of the mass spectrometry machine?

A
  1. negatively charged accelerator plate with narrow slit to allow ions to pass through
  2. charged particles passes through magnetic field- curvature of the bend depends on the m/z ratio
  3. slit followed by detector- at any given magnetic field, only ions of one particular mass are bent exactly at the right amount to pass through the detector.
83
Q

what is the rule of 13?

A

calculation used to determine the molecular formula based on mass
starts with the assumption of only having carbon and hydrogen atoms
then heteroatoms are expored until exact combinations are identified

84
Q

what are the characteristics of nuclei of elemental isotopes?

A

have a characteristic spin
some are integral, some are fractional, some have no spin

85
Q

what are the characteristics of a spinning nuclei?

A

create a magnetic moment
can spin in all directions until an external field is introduced

86
Q

why is there a difference where absorption happens?

A

shielding,
less sheilded protons absorb at lower field

87
Q

what is the effect of electronegativity in chemical shifts?

A

with increasing electronegativity, there is more deshielding of neighbouring atoms as the electrons are drawn towards the more electronegative atom

88
Q

what is the effect of distance from electron withdrawing group on chemical shift?

A

the further away an atom is from the electronegative group, the less deshielding effect is felt by the protons of that atom

89
Q

what are the charecteristics of the carbon-13 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
90
Q

what is the 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
91
Q

what is the splitting for RCH3?

A

0-35

92
Q

what is the splitting for R2CH2?

A

15-40

93
Q

what is the splitting for R3CH?

A

25-50

94
Q

what is the splitting for R4C?

A

30-40

95
Q

what is the splitting for RC=-CR?

A

65-90

96
Q

what is the splitting for R2C=CR2?

A

100-150

97
Q

what is the splitting for benzene?

A

110-175

98
Q

what is the splitting for RCH2Br?

A

20-40

99
Q

what is the splitting for RCH2Cl?

A

25-50

100
Q

what is the splitting for RCH2NH2?

A

35-50

101
Q

what is the splitting for RCH2OH?

A

50-65

102
Q

what is the splitting for RCH2OR?

A

50-65

103
Q

what is the splitting for R(C=O)OR?

A

160-185

104
Q

what is the splitting for R(CO)R?

A

190-220

PMP 102 (29 decks)

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