Chem. 17 - Chpt. 3 Reading Notes

Front 1

What are the three methods used to determine the structure of a molecule?

Back 1

Mass spectrometry: used to determine mass of molecule and atomic composition

Infrared spectroscopy: to determine functional groups in molecule

Nuclear Magnetic Resonance: Spetroscopy to determine the carbon skeleton of molecule

Front 2

What is mass spectrometry used to determine?

Back 2

Mass of molecule and atomic composition

Front 3

What is nuclear magnetic resonance spectroscopy used for?

Back 3

To determine carbon skeleton of molecule.

Front 4

What is infrared spectroscopy used for?

Back 4

To determine functional groups in molecule

Front 5

What are the two methods of mass spectroscopy?

Back 5

Electron impact and chemical ionization

Front 6

How does electron impact mass spectrometry work?

Back 6

Molecule is bombarded with highly energetic electrons that knock a weakly bound electron out of the molecule.

Front 7

What is left behind when the molecule is bombarded by the electron in electron impact mass spectrometry?

Back 7

A radical cation; (occurs if the electron beam supplies more than exactly the right amount of energy

Front 8

What is the base peak?

Back 8

Strongest peak assigned an 'abundance' of 100% ~ mass spectrometry by electron impact

Front 9

How does chemical ionization mass spectrometry work?

Back 9

Electron beam is used to ionize a simple molecule such as methane which in turn ionizes our molecule by collision and transfer of a proton.

(Methane loses a bonding electron to give CH4+ which reacts with an unionized methane molecule to give CH3 and CH5+, which is a very powerful, unstable acid that can protonate just about any other molecule)

Front 10

What is the different between the cations of direct electron impact versus chemical ionization mass spectroscopy?

Back 10

Direct electron impact: radical cation produced by removing an electron by bomardment

Chemical ionization: simple cation produced by adding a proton from something like CH5+

Front 11

What aren't exact masses whole integers?

Back 11

There is a slight mass different between a proton and a neutron

Front 12

How can different atomic compositions for the same molecule weight be distinguished?

Back 12

If we know the molecule weight, it can be determined, since individual isotopes have non-integral masses; mass measurements are called high resolution mass spectrometry

Front 13

Of C, O, and N, which is the only one that can form an odd number of bonds?

Back 13

N

Front 14

How does nuclear magnetic resonance work?

Back 14

1. Sample of unknown compound is dissolved in a suitable solvent and put in a very strong magnetic field. Any atomic nuclei with a nuclear spin now have different energy levels.

2. Sample is irradiated with a short pulse of radiofrequency energy, which disturbs the equilibrium balance between the two energy levels ~ some nuclei absorb the energy and are promoted to higher energy levels

3. Detect the energy given out when the nuclei fall back down to the lower energy levels using what is basically a sophisticated radio receiver

4. Results are displayed in the form of intensity against frequency

Front 15

What is the unfortunate thing about NMR?

Back 15

Energy difference between the nuclear spin being aligned with the magnetic field and against it is very small - so small that we need a very, very strong magnetic field to see any difference.

Front 16

Since the energy different between the two states in NMR is so small, what can be used to flip the nucleus?

Back 16

Radio waves

Front 17

What are the two axises of the NMR results?

Back 17

Frequency vs. intensity

Front 18

What are the two factor that affect the energy difference, (and, therefore, the frequency), in NMR?

Back 18

Magnetic field strength and what sort of nucleus is being studied

Front 19

When is a carbon more shielded - when it is attached to three H or two H and an OH?

Back 19

When it is attached to the three H, because the electrons are not pulled away from the outside

Front 20

Does a shielded or deshielded atom feel more force from the applied magnetic field? Which one resonates at a higher frequency?

Back 20

deshielded atom ~ resonates at a higher frequency

Front 21

In NMR, which is the precise value of the operating frequency?

Back 21

Frequency at which a standard reference sample resonates

Front 22

What is the reference sample usually used in NMR?

Back 22

Tetramethylsilane, (TMS)

Front 23

What is the formula for chemical shift?

Back 23

frequency - frequency TMS / frequency TMS

Front 24

How does the scale for chemical shift run?

Back 24

Runs to the left from zero, where TMS resonates

Front 25

Whenever you first look at a spectrum, what is the first thing you should do?

Back 25

Identify the peaks due to the solvent

Front 26

When a carbon atom is more shielded than those in TMS, what is the sign of the TMS value?

Back 26

Negative

Front 27

How are functional groups identified?

Back 27

Infrared spectra

Front 28

Which functional groups can be determined by NMR, rather than infrared spectra?

Back 28

C=O and C=C, because they contain carbon atoms; OH, because it influence chemical shifts of carbon atoms.

Front 29

How does the infrared spectra identify functional groups?

Back 29

Detects the stretching and bending of bonds rather than any property of the atoms themselves.

Front 30

What type of bonds is infrared spectra particularly good at detecting?

Back 30

Unsymmetrical bonds

Front 31

Why is infrared used in infrared spectrometry?

Back 31

Energy needed to stretch bonds falls in the infrared spectra

Front 32

How is the infrared spectra recorded?

Back 32

Recorded using a frequency measurement called wavenumber (cm^-1)

Front 33

How is the sample prepared for the infrared spectrometry?

Back 33

Either dissolved in a solvent, such as CHCl3, that has few IR absorptions, pressed into a transparent disc with powdered solid CBr, or ground into an oily slurry called a mull with a hydrocarbon oil called 'Nujol.'

Front 34

What does Hooke's Law say frequency of infrared spectrometry depends on?

Back 34

Mass and a force constant, (bond strength)

Front 35

If a bond stretches at a higher frequency, does that mean it's weaker or stronger?

Back 35

Stronger

Front 36

Do antisymmetrci vibrations require more or less energy? Do they vibrate at a higher or lower frequency?

Back 36

More energy; higher frequency

Front 37

If the absorption is lower, is the H bond stronger or weaker?

Back 37

Stronger

Front 38

How does dipole play into infrared spectrometry?

Back 38

For bonds between unlike atoms, the larger the difference in electronegativity, the greater the dipole moment, and the more it changes when stretched.

Front 39

For the absorptions in Ir spectra, what does the position of the band depend on?

Back 39

Reduced mass of atoms and bond strength

Front 40

For the absorptions in Ir spectra, what does the strength of the band depend on?

Back 40

Change in dipole moment

Front 41

For the absorptions in Ir spectra, what does the width of the band depend on?

Back 41

Hydrogen bonding

Front 42

What are strong peaks called, which are created by the bending of bonds, (particularly C-H and N-H)?

Back 42

Deformations

Front 43

What are double bonds of all kinds and rings called?

Back 43

Double Bond Equivalents, (DBEs)

Front 44

What are the 6 criteria for working out the DBEs of an unknown compound?

Back 44

1. Calculate the expected number of Hs in the saturated structure.
2. Subtract the actual number of Hs and divide by 2 to give the DBEs.
3. If there are other atoms, it is best to draw a trial structure
4. If there are few Hs, suspect a benzene ring
5. A benzene ring has four DBEs
6. A nitro group has one DBE only