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CHEM 110 > Spectroscopy > Flashcards

Spectroscopy Flashcards

1
Q

What are the 4 types of spectroscopy?

A

Mass spectrometry (MS)
Infrared spectroscopy (IR)
UV-Visible spectroscopy (UV-VIS)
Nuclear magnetic resonance spectroscopy

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2
Q

What is mass spectrometry?

A

Examination of ions (fragments) derived from molecules that allow the calculation of molecular mass

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3
Q

What is infrared spectroscopy?

A

Determines the presence or absence of particular functional groups in a molecule

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4
Q

What is UV-Vis spectroscopy?

A

Determines the presence of pi-bond electrons in a molecule as well as the presence of any conjugation in the structure of the compound

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5
Q

What is NMR?

A

Provides information about the carbon and hydrogen framework of a molecule

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6
Q

What is molecular spectroscopy?

A

IR, UV-VIS, NMR - depend on the interaction of molecules with radiation of specific energy

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7
Q

What is the relationship between energy and frequency? Provide an equation

A

E= h x v

where
h is plancks constant
v is frequency of radiation

Hence energy is directly proportional to frequency of radiation

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8
Q

What is the relationship between energy and wavelength? Provide an equation

A

c = v x lambda

where
c is speed of light
v is frequency of radiation
lambda is wavelength

therefore sub into E = hv

E = hc/lambda

Therefore energy is inversely proportional to wavelength of radiation

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9
Q

How does mass spectrometry work?

A

A molecule is bombarded with high energy electron beam -> this causes expulsion of an electron from the molecule creating a positively charged molecular ion (radical cation)

The radical cation formed is fragmented forming other positive ions and neutral fragments

A plot of relative ion abundance vs m/z forms the mass spectrum (mass of ion/charge of ion (+1))

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10
Q

How is the molecular mass of the molecule determined from the mass spectra?

A

The molecular ion (radical cation) must be the ion of the highest m/z hence the highest m/z value corresponds to the molecular mass of the parent ion

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11
Q

How does infrared spectroscopy work?

A

Absorption occurs when the infrared radiation frequency matches the frequency of bond vibration (specific to the type of bond)

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12
Q

How is the functional groups determined from IR spectra?

A

The relative masses of atoms involved and bond strengths determine the position of IR absorption displayed on spectrum.

Hence specific bonds will show absorption at specific ranges of the spectra

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13
Q

What are the strengths and weaknesses of MS

A

Only information about molecular mass - cannot determine between isomers

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14
Q

What are the strengths and weaknesses of IR spectroscopy?

A

Fast identification of functional groups

Compounds containing same functional groups will show very similar spectra
No indication on number of functional groups present

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15
Q

How does UV-Visible spectroscopy work?

A

UV-VIS radiation raises electrons in molecules (with pi bonds) from lower energy bonding and non-bonding molecular orbitals to higher energy anti-bonding molecular orbitals

The wavelength at which maximum absorption occurs is considered

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16
Q

What is a conjugated pi-system?

A

When the molecule contains a pi bond - sigma bond - pi bond

17
Q

What is the wavelength range of UV?

A

200-400nm

18
Q

What is the wavelength range of Visible light?

A

400-800nm

19
Q

What is an anti-bonding MO?

A

All bonding orbitals have an anti-bonding orbital

e.g. pi - bonding orbital
pi star - anti-bonding orbital

20
Q

What are the types of non-conjugated functional groups that show absorption on UV-VIS spectra?

A

C=C 170-180nm

C=O 170-180nm 280nm

21
Q

What absorptions are seen for C=C (non-conjugated system) and why?

A

The excitation of an electron from the pi bonding MO to the anti-bonding MO shows absorption at 170-180nm

22
Q

What absorptions are seen for C=O (non-conjugated system) and why?

A

The excitation of an electron from the lone pair on oxygen to anti-bonding pi orbital shows weak absorption at 280nm

The usual absorption at 170-180nm from excitation of pi orbital electron is also seen.

23
Q

Does UV-VIS provide info on the number of pi bonds?

A

No

24
Q

What is the relationship between absorption and wavelength for conjugated systems compared to non-conjugated systems?

A

Absorption occurs at longer wavelengths for conjugated systems as the energy gap between the molecular orbitals are small

Usually >200nm

25
Q

What is the relationship between number of double bonds in conjugation and wavelength?

A

The greater number of double bonds in conjugation = the smaller the gap in energy between molecular orbitals = larger the longest wavelength value

26
Q

What is Beer’s Law

A

A = Ebc

where A is absorbance
b is path length (cm)
c is concentration (mol L-1)
E is molar absorptivity (L mol-1 cm-1)

Hence absorbance is directly proportional to concentration and path length

27
Q

What is the relationship between amount of conjugation and molar absorptivity?

A

No relation

28
Q

How does NMR work?

A

Nuclear magnetic resonance provides information about the carbon and hydrogen frameworks of molecules.

A magnetic field is applied to a number of nuclei causing excitation from a lower to higher energy spin state

29
Q

What is the factor that affects the energy required for a flip in nuclei spin?

A

Shielding of protons due to electrons

30
Q

What does shielding do to the chemical shift value?

A

Shielding decreases chemical shift

31
Q

What is the chemical shift range for 13C NMR spectrum?

A

delta 0-220

32
Q

What is the chemical shift range for sp3 hybridized carbon atoms?

A

0-90 delta

33
Q

What is the chemical shift range for sp2 hybridised carbon atoms?

A

100-220 delta units

34
Q

How does shielding change?

A

Shielding decreases when electronegative groups of atoms are bonded to the carbon as it pulls electrons away from the molecule

35
Q

With proton NMR what are the 4 components of spectra?

A

Number of absorption signals
Position of absorption signals
Relative areas under absorption signals
Splitting pattern for absorption signals

36
Q

What information does the number of absorption signals provide?

A

Number of H nuclei in different environment

37
Q

What information does the position of absorption signals provide?

A

The amount of shielding of the proton nuclei

38
Q

What information does the relative area under absorption signals provide?

A

The number of protons in the same environment

39
Q

What does the splitting pattern for absorption signals provide?

A

The number of neighbouring hydrogens - each neighbouring proton will cause the signal to split once

CHEM 110 (16 decks)

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