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CHEM008B > Ch- 9: Mass spectrometry, NMR Spectroscopy, Structural Analysis. > Flashcards

Ch- 9: Mass spectrometry, NMR Spectroscopy, Structural Analysis. Flashcards

1
Q

What is the main use of IR?

A

To be able to tell what functional groups are present in the molecule.

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2
Q

How are we able to determine the formula based on spectra?

A

Mass Spectometry.

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3
Q

What is the main basis of Mass Spectroscopy?

A

Determining the mas to charge (m:z) ratio.

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4
Q

What are the axes of IR?

A

Y-Axis is intensity % transmittance; x-axis is absorbance frequency (cm^-1)

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5
Q

What are the axes of MS?

A

X-Axis is m:z and Y axis is the relative abundance.

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6
Q

What is the Molecular Ion (M+)?

A

the ion with the highest mass (largest m:z ratio).

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7
Q

Briefly explain the technique used in MS?

A

You shoot the sample with a beam of electrons, one of the electrons are knocked off, you end up with a radical cation (unstable). From there, fragmentation occurs and the molecules hit the detector based on the m:z ratio.

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8
Q

What is the charge when dealing with mass spectometry?

A

1, because one of the electrons are knocked off.

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9
Q

With fragmentation, what gets detected through mass spectometry?

A

Only the cation, not the radical cation.

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10
Q

What are the main isotopes of focus when dealing with MS?

A

C-13, Cl-37, and Br-81.

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11
Q

What % of carbons are C-13?

A

1.1%

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12
Q

How do you determine the height of the M+1 peak?

A

M+1= 1.1% . #C

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13
Q

What is the molecular ion region?

A

Where M+ is located and the parts to the right of it.

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14
Q

How can you tell the number of carbons based on the % of the C-13 peak?

A

1.1- 1
2.2-2
3.3-3
4.4-4
5.5-5

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15
Q

What is the ratio of Cl-35 to its isotope?

A

3:1

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16
Q

What is the ratio of Br-79 to its isotope?

A

1:1

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17
Q

What clue tells you that there’s a chlorine or bromine present in the molecule? How do you know if its Br or Cl?

A

The presence of the M+2 peak. If the ratio of the M to M+2 is 3:1, then it’s chlorine. If the ratio is 1:1, then it’s bromine.

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18
Q

What is the Nitrogen Rule?

A

If M+ is an odd #, that means that there are an odd # of hydrogens in the molecule.

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19
Q

How can you tell if Oxygen is part of the formula?

A

If the remaining mass once figuring the # of carbons is extremely large, then there has to be an oxygen present.

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20
Q

What does the degree of unsaturation refer to?

A

the presence of a pi bond, ring, or a combination of both.

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21
Q

What is the formula for calculating the degree of unsaturation?

A

HDI: ((2C+2)- (H+X-N))/2

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22
Q

Nuclei have _____.

A

Spin

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23
Q

What are the NMR active atoms?

A

1 H, 13 C, 19 F, 31 P

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24
Q

What are the NMR inactive atoms?

A

12C and 16O

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25
Q

How can spin states be oriented once placed in a magnetic field?

A

They can be aligned with or against the magnetic field.

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26
Q

What can be said about the energy of molecules when they are aligned with the magnetic field?

A

Low E.

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27
Q

What can be said about the energy of molecules when they are aligned against the magnetic field.

A

High E

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28
Q

When referring to electron density in chemical shift, what atom are we talking about?

A

Hydrogen

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29
Q

What do we have when there is HIGH electron density around the hydrogens?

A

Low Shift-Upfield-Shielded

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30
Q

What do we have when there is LOW electron density around the hydrogens?

A

Downfield-Deshielded-High Shift

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31
Q

What are the units for chemical shift and what is the standard?

A

Ppm- TMS

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32
Q

What impacts the chemical shift?

A

The presence of electronegative atoms, the distance from the electronegative atoms, and the presence of a pi bond.

33
Q

In what region (ppm) can sp3 C-H be found? What does it tell us if it’s closer to 4.5? Further?

A

0-4.5 ppm; the closer it is to 4.5, the closer it is to the electron-withdrawing group. The further away it is, the further it is from the electron-withdrawing group.

34
Q

The amount of ______ density around a nucleus determines the _____ seen by the nucleus.

A

Electron density- magnetic field.

35
Q

How does NMR work?

A

You put the sample in a magnetic field, apply a radio frequency, the chemically different H absorb at different frequencies. The resonance frequency (frequency of absorption) is dependent on the magnetic field, B0. However, electrons in molecules create an opposing circulating field.

36
Q

Why do double bonds and aromatic compounds have higher ppms?

A

Because the pi bond creates an extra circulating electric field.

37
Q

What is the range for alkenes ppm wise? Aromatic Compounds? Why are they different?

A

4.5-7 for alkenes. 7-8 for aromatic compounds. Aromatic compounds have more pi bonds, making them have a higher shift.

38
Q

In what range of chemical shift can an aldehyde be found? Why?

A

10 ppm. Because it has a double bond and an electron withdrawing atom (O).

39
Q

What can be said about the chemical shift of chemically identical nuclei?

A

They are the same.

40
Q

What can be said about the hydrogens in methyl groups? Why?

A

They are chemically identical. Because they have 120 degrees rotational symmetry.

41
Q

What are the things to look out for when identifying chemically identical nuclei?

A

1) The hydrogens in a CH2 are different if a chiral center is present in the molecule. (Each produce their own unique chemical signal).
2) the hydrogens for CH, CH2, and ch3 are identical if they can be interchanged by rotation or reflection.
3) all the hydrogens in a CH3 are equivalent, even if a chiral center is present.

42
Q

What are the positions of hydrogens relative to other things?

A

Alpha (adjacent), beta (2 away), and gamma (3 away).

43
Q

What are the different chemical shifts for standard protons?

A

Methyl (ch3) —-> 0.9 ppm
Methylene (CH2) —-> 1.2 ppm
Methine (CH) —-> 1.7 ppm

44
Q

What is the affect of a OXYGEN of an alcohol or ether on an alpha proton?

A

+2.5 ppm

45
Q

What is the effect of the oxygen of an ester on an alpha proton?

A

+3 ppm

46
Q

What is the effect of a carbonyl group on alpha protons?

A

+1 (carbonyl groups include ketones, esters, aldehydes, etc.)

47
Q

What happens to the effect of protons in ppm as the protons go from alpha to beta to gamma?

A

1/5 with every increasing position.

48
Q

What is the chemical shift of allylic protons?

A

Around 2 ppm

49
Q

What is the chemical shift of alkynyl protons?

A

Around 2.5

50
Q

What is the chemical shift of benzylic methyl protons?

A

Around 2.5.

51
Q

What is the chemical shift of an alkyl halide proton?

A

2-4

52
Q

What is the chemical shift of an alcohol proton?

A

2-5

53
Q

What is the chemical shift of a vinylic proton?

A

4.5-6.5

54
Q

What is the chemical shift of an aryl proton?

A

6.5-8

55
Q

What is the chemical shift of an aldehyde proton?

A

10

56
Q

What is the chemical shift of a carboxylic acid proton?

A

12

57
Q

What does integration allow us to decipher?

A

The number of protons giving rise to the signal.

58
Q

How do you determine the number of protons giving rise to the signals based on the area provided under each curve?

A

Divide each area by the smallest number to get the relative ratio. Then, based on the molecular formula, you can get the EXACT number of protons.

59
Q

How do we determine the overall chemical shift of something provided there are multiple functional groups around?

A

You include all of them, taking the effect and positioning of each into consideration.

60
Q

What does multiplicity refer to?

A

The number of peaks seen in the signal.

61
Q

How many peaks do singlets, doublets, triplets, etc have?

A

1,2,3

62
Q

How can you determine how much splitting occurs?

A

Based on the # of neighboring protons relative to another proton. n+1. If it is adjacent to 2 protons, for example, the resulting peak will be a triplet.

63
Q

What are the 2 main factors to look at when considering splitting?

A

1) chemically equivalent protons do not split each other.
2) protons that split one another are typically split by 2 or three sigma bonds.

64
Q

When determining if protons are chemically equivalent, what is something helpful to abide by?

A

Look at the “environment” they are in: neighboring atoms, bonds, etc.

65
Q

What are the common pattern recognitions for NMR? (J coupling)

A

Ethyl: Quartet with an integration of 2H and triplet with an integration of 3H. EQT

Applies to multiples of this as well (i.e quartet with an integration of 4H and triplet with an integration of 6H means two ethyl groups are present).

Isopropyl: Septet (7) with an integration of 1H and doublet with an integration of 6H. ISD

Tert-Butyl: Singlet with an integration of 9H.

66
Q

When do we see complex splitting occur? Example?

A

When non-equivalent neighboring protons have SIGNIFICANTLY DIFFERENT J values. An example can be seen in alkenes where the J values between cis and trans hydrogens are different. For cis, it’s 10 and for trans it’s 18.

67
Q

What is generally the coupling constant for acyclic alkane CHs?

A

7 Hz.

68
Q

What is the range of IR absorption for C double bonded to O? What effects lead to the minimum and maximum of this range? What is the frequency of absorption of a ketone that is standard?

A

1680-1735. The more resonance incorporated into the C double bond O, the weaker the bond. As a result, the frequency of absorption significantly decreases as well. 2600 cm^-1.

69
Q

Describe the effect of leaning.

A

If two different protons are coupled to each other and have a close chemical shift, then the inner peaks will be higher than the outer peaks.

69
Q

What is the range of absorption for sp3-CH bonds? (IR)

A

Less than 3000

70
Q

What is the range of absorption for sp C-H bonds?

A

3250-3350

70
Q

What is the range of absorption for sp2 C-H bonds?

A

3000-3150

71
Q

When would you get a doublet of doublets?

A

When you have coupling with 2 protons that have different coupling constant.

71
Q

What is the difference between carboxylic acid and hydroxyl absorption in IR?

A

Carboxylic acids are VERY BROAD and around 2600. On the other hand, OH’s are broad and found around 3300 cm^-1.

72
Q

Where does the difference in coupling constants stem from?

A

For our purposes, cis and trans hydrogens on an alkene will have different coupling constants. Cis will be lower and trans will be higher. Additionally, if we have a stereocenter in the molecule, geminal hydrogens on the CH2’s can couple to one another and the adjacent hydrogens.

73
Q

How are OH peaks seen in NMR?

A

They are seen as broad peaks in NMR that do not couple to anything else.

74
Q

When attempting to determine the chemical structure of an unknown molecule, what should you do?

A

Calculate the degrees of unsaturation, look at the IR stretches if provided, take a look at integration, coupling, and chemical shift in that order. Chemical shift should be the last thing that is looked at.

75
Q

What are things to note about homotopic and enantiotopic hydrogens?

A

These hydrogens have the exact same chemical shift and are chemically equivalent. Homotopic hydrogens have all symmetry elements, meaning they have mirror and reflection symmetry. On the other hand, enantiotopic hydrogens have one symmetry element.

76
Q

What are the important general ranges for Carbon 13 NMR?

A

0-80 for sp3 carbons, 110-150 for double bond carbons, and C=O carbons (esters, acids, and amides) from 160-180. Aldehyde from 190-200, and ketones from 205-220.

CHEM008B (2 decks)

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