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ochem2 > spectroscopy > Flashcards

spectroscopy Flashcards

1
Q

What is spectroscopy?

A

When molecules absorb different energies of electromagnetic radiation (light) results in different types of motions in molecules

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2
Q

What does spectroscopy allow for?

A

To measure the energy differences of state energies in an allowed system via frequencies by determining the amount of energy absorbed

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3
Q

What is the main characteristic of energy states? In what direction to molecules go once absorbed?

A

They are quantized or set, like stairs. Go from lower to higher energy states by absorbing energy.

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4
Q

What is the equation for the difference in energy states? What is the relationship between change in energy and frequency?

A

Planks constants(frequency). Difference in energy is proportional to frequency

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5
Q

What is resonance in terms of spectroscopy?

A

The energy match between radiant light and the difference of energy between the two different quantized energy states that brings about the transition between the states

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6
Q

Infrared light changes what state?

A

Vibrational

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7
Q

UV/visible light changes what state?

A

Electronic

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8
Q

Radio frequency light changes what state?

A

Spin states of an atomic molecule

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9
Q

What does NMR stand for?

A

Nuclear magnetic resonance

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10
Q

What energy absorption is measured in NMR?

A

Energy absorption corresponds to energy difference between two spin states of a nucleus in a magnetic field

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11
Q

What nuclei are most commonly observed in NMR?

A

Proton and C13

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12
Q

What direction of spin and magnetic moment results in alpha spin?

A

Counter clockwise spin and upwards magnetic moment

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13
Q

What is the relative energy level of alpha spin and its relation to external magnetic field (Bo)?

A

Lower energy (more stable)
Aligned with Bo

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14
Q

What direction of spin and magnetic moment result in beta spins?

A

Clockwise spin and downward magnetic moment

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15
Q

What is the relative energy and alignment of beta spins to Bo?

A

Higher energy (less stability)
Aligned against Bo

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16
Q

What is the direction of spins in free space?

A

They orient in any direction

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17
Q

What is the orientation of spins when influenced by an external magnetic field?

A

Spins will go towards or against the Bo, a majority will be alpha, meaning they go along the same plane as the Bo.

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18
Q

What is the relative energies of alpha and beta spins in free space compared to when Bo is present?

A

They have equal energy in free space but when external magnetic field is present (Bo) then alpha is lower energy

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19
Q

What is the difference in energy levels of alpha and beta spins dependent on?

A

Dependent on the external magnetic field (Bo) energy level

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20
Q

Can alpha spin nuclei be changed to beta? T/F

A

True, through the nrg absorption it can change spin state. Go from lower to higher energy level

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21
Q

The lower the strength of Bo the larger the difference in energy levels of alpha and beta states? T/F

A

False. Frequency and magnetic field are proportional. Increase in Bo= increase frequency = increase difference in energy level

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22
Q

What is the equation to determine frequency?

A

Frequency= (gamma- magnetogyric ratio nucleus constant)(B-magnetic field @nucleus)/2pi

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23
Q

How do we distinguish nuclei when using NMR?

A

They all exist in different chemical environments

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24
Q

What are the forms of electronic charge clouds? Where are they located? What is its function?

A

Electronic charge clouds are surrounding each nucleus and are found in the form of bonds to other atoms and atom spatial awareness. Functions to shield from magnetic field.

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25
Q

Does the magnetic field affect electronic motion? T/F

A

True

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26
Q

What is the result of electronic motion on magnetic field?

A

Electronic motion creates its own magnetic field locally that opposes the applied magnetic field

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27
Q

What is the equation to determine the magnetic field felt at the nucleus?

A

B (magnetic field felt at nucleus)= Bo-B’( induced e cloud)

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28
Q

What is the function of the electronic cloud?

A

To shield the nucleus from the external electronic magnetic field (Bo)

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29
Q

Is the magnetic field felt at all nuclei the same?

A

No, it is different due to the difference in their chemical environments and will have different resonances (types of light energy they can absorb)

30
Q

What is deshielded? What other term is it relative to?

A

Smaller electronic clouds meaning they feel more of the Bo and greater velocity and greater difference in energy levels. Relative to downfield

31
Q

What is shielded and what term is it relative to?

A

Greater electronic cloud so it feels less of the Bo and therefore less frequency and difference in energy. Relative to upfield

32
Q

What is chemical shift?

A

When resonance frequencies are compared relative to a standard (TMS)

33
Q

What is the standard used in chemical shift calculations? What is its frequncy?

A

TMS (tetramethylsilane) is the standard and has a frequency of 0ppm.

34
Q

What is the equation to determine chemical shift?

A

(Velocity of sample- Velocity of TMS / Velocity of instrument)(10^6ppm)

35
Q

What C isotope are we observing in C NMR?

36
Q

What is the spin quantum of 13C NMR?

37
Q

What is the range of 13C NMR? Is it simple? What is the signal like?

A

0-220ppm, much simpler compared to proton NMR, weaker signal

38
Q

What are the three differences of 13C to 1H NMR?

A
  1. Single, well defined peak per type of C
  2. Decoupled- no splitting is observed
  3. Intensity- not useful to determine quantity
39
Q

What are the three considerations of chemical shifts?

A
  1. EN of attached atoms
  2. Degree of substitution
  3. Hybridization
40
Q

How does electronegativity affect chemical shift of c?

A

More electronegative atoms pull electron density from the cloud around the C in questions and DESHIELDED it so more exposed to Bo so larger shift.

41
Q

How does degree of substitution affect chemical shift of C?

A

Increased substitution allows for more EN atoms to withdraw electron density from C resulting in DESHIELDING. More substitution = higher chemical shift and frequency

42
Q

What are two considerations when looking at 13C NMR?

A

Chemical equivalence and symmetry

43
Q

How does hybridization affect shielding and chemical shift?

A

Shielding: sp3>sp>sp2
Chemical shift: sp2>sp>sp3

44
Q

What is the main characteristic of peak intensity of 13C NMR?

A

Variable, quaternary C have low peak intensity

45
Q

What is the range of 1H NMR?

46
Q

What is integration in terms of 1H NMR?

A

The measure of relative peak areas giving the relative number of H’s contributing to each signal

47
Q

What is multiplicity when it comes to 1H NMR?

A

Signal splitting, tells the number of vicinal H’s due to the spin-spin coupling

48
Q

What are the 8 multiplicity values/names?

A

Singlet s- 1
Doublet d- 2
Triplet t- 3
Quartet q- 4
Pentent - 5
Sextet- 6
Septet-7
Multiple m- many, indecipherable

49
Q

What is the equation for multiplicity?

A

N+1 (n=of equivalent vicinal protons)

50
Q

What is the coupling constant?

A

The amount of space between downfield and upfield peaks

51
Q

What is the special condition of H in equivalent environments?

A

They do NOT split each others signal, even if they are vicinal

52
Q

How do you determine peak intensity?

A

By looking at the percent influence of varying spin states couplings

53
Q

How many peaks and peak intensity is in a singlet?

A

1 peak, 1 intensity

54
Q

How many peaks and peak intensity is in a doublet?

A

2 peaks, 1:1 ratio

55
Q

How many peaks and peak intensity is in a triplet?

A

3, 1:2:1 ratio

56
Q

How many peaks and peak intensity is in a quartet?

A

4 peaks, 1:3:3:1

57
Q

How many peaks and peak intensity is in a pentet?

A

5 peaks, 1:4:6:4:1

58
Q

How many peaks and peak intensity is in a sextet?

A

6 peaks, 1:5:10:10:5:1

59
Q

What are homotopic protons?

A

Protons that are equivalent to one another and have the same chemical shift when they are in the same equivalent environment

60
Q

How do you determine equivalency?

A

Look at symmetry and use replacement method

61
Q

What are diastereotopic protons?

A

Protons that are chemically non-equivalent and can and most often do have different chemical shifts

62
Q

What are steps to determining if protons are diastereotopic?

A

Determine a stereo center, draw one diasteromer, replacement method, determine if new stereocenter arises

63
Q

What is the number of peak and relative ratio of doublet of doublet splitting?

A

4 peaks, 1:1:1:1

64
Q

How many peaks and relative intensity is three is a doublet of triplet splitting?

A

2 sets of three peaks with 1:2:1 ratio

65
Q

What is the relative distance of coupling constant in geminal, cis and trans protons?

A

Geminal- small
Cis- medium
Trans- large

66
Q

What are enantiotropic protons?

A

Chemical shift of protons are equivalent and are in equivalent environments. Plane of symmetry

67
Q

If chemical shift is less than 100 what is the hybridization of the C?

68
Q

If chemical shift is greater than 100, what is the hybridization of the carbon?

69
Q

In what range would we see a chemical shift peak for C-O?

70
Q

In what range would we see a chemical shift peak for a C=O?

A

Around 200

71
Q

Where is solvent found in carbon NMR?

A

Around 75ppm

72
Q

Where is solvent peak found in proton NMR?

A

Around 7ppm

ochem2 (13 decks)

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