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Chem110 > Block 2: Spectroscopy > Flashcards

Block 2: Spectroscopy Flashcards

1
Q

What does mass spectroscopy determine?

A

The mass of individual ions derived from compounds in the gas phase

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2
Q

What is the concept of mass spectrometry?

A

The molecule is bombarded with a high energy electron beam where the ionised molecule results from the expulsion of one electron (therefore s radical cation)

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3
Q

What is a mass spectrum?

A

The plot of Kok abundance versus m/z (mass of ion/charge of ion)

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4
Q

Which value on a mass spectrum is the mass of the molecule?

A

The highest value

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5
Q

What size has a larger amount of deflection?

A

Small is more deflected (and only bends with magnetic field if a positive charge)

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6
Q

What happens when the mass of the M+ is measure to the nearest unit value?

A

Sometimes not possible to distinguish between certain molecular formulae

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7
Q

Limitation of MS (mass spectrometry)?

A

Not possible to distinguish between molecules with same molecular formula

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8
Q

What is spectroscopy?

A

The study of the interaction of electromagnetic radiation with matter

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9
Q

What is characteristic of specific structural units in molecules?

A

Absorption of light of particular frequencies

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10
Q

What spectrum has the lowest energy?

A

NMR (radio waves)

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11
Q

Whet does a longer wavelength correlate with?

A

Lower frequency and lower energy

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12
Q

What happens when the energy of the electromagnetic radiation exactly corresponds to the energy difference between two molecular energy levels?

A

The molecule can absorb the energy

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13
Q

Equation for velocity of light?

A

Wavelengthxfrequency

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14
Q

Equation for energy of radiation?

A

E = planck’s constant (h) x frequency

E= planck’s constant x velocity of light (c) / wavelength

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15
Q

Does energy have to match the energy gap exactly?

A

Yes, for transition state to occur

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16
Q

What is ultraviolet-visible (UV-VIS) radiation?

A

It’s of an appropriate energy to raise electrons in some molecules (especially pi bonds) from low energy bonding (or non bonding) to high energy anti-bonding molecular orbitals

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17
Q

What compounds are transparent to UV-VIS?

A

Those that contain only sigma bonds

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18
Q

What systems have absorption observed at a longer wavelength under UV-VIS?

A

Those in conjugated systems (ie alternating single and double bonds)

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19
Q

What is the absorption of any molecule containing conjugated double bonds greater than?

A

200nm

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20
Q

What is absorbance proportional to?

A

Concentration and path length

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21
Q

The more effective a molecule is at absorbing light at that wavelength…

A

The greater the absorbance

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22
Q

What is Beer’s Law?

A

Absorbance (A) = molecular absorbitivity in L mol-1 cm-1 (E) x path length in cm (b) x concentration in mol L-1 (c)

23
Q

How do you rearrange Beer’s Law to find concentration?

A

C = A/(E x b)

Remember units (mol L-1)

24
Q

What is IR (infra red) spectroscopy?

A

When IR radiation absorption corresponds to vibrational excitation m; that increasing the amplitude of the vibration

25
Q

Two types of bond vibrations in IR?

A

Stretching

Bending

26
Q

Do symmetrical bonds that are symmetrically substituted show absorption?

A

No (as a vibrational mode has to be accompanied by a change in dipole moment)

27
Q

What determines the position of IR absorption?

A

Relative masses of atoms and bond strengths

28
Q

What is the IR range for c=O?

A

1850-1600

29
Q

What’s the IR absorption for N-H?

A

3500-3100

30
Q

What’s the IR absorption for C-H?

A

3100-2800

31
Q

What is the utility of IR?

A
  1. Rapid identification of functional groups
  2. As a fingerprint as no two compounds have identical spectra (although you cant distinguish between functional groups)
32
Q

Do MS, UV-VIS or IR give information in regards to the hydrocarbon skeleton?

A

No

33
Q

What spectroscopic technique had the greatest impact on molecule structure determination?

A

Nuclear Magnetic Resonance (NMR)

34
Q

What does NMR provide information about?

A

Carbon and hydrogen framework

35
Q

What do nuclei do when placed in a magnetic field?

A

They align them selves with (parallel) or against (anti parallel) the external magnetic field

36
Q

Which nuclei have lower energy?

A

Those aligned with the magnetic field

37
Q

In NMR, is the absorption frequency the same for 1H or 13C?

A

No, it’s different for all unique 1H or 13C because the nuclei are in different chemical environment

38
Q

In NMR, what are positions of absorption called and what is it measured in?

A

Chemical shifts measured in delta units (ppm) from a peak of reference compound tetramethylsilane (TMS)

39
Q

What does a bigger PPM mean?

A

An increasing frequency

40
Q

What is 13C NMR?

A

A proton decoupled spectra: proton decoupling means one does not observe splitting of signals which would otherwise be seen because of 13C-1H

41
Q

What’s the chemical shift range for 13C?

A

Delta 0-210

42
Q

What carbons have the higher delta values?

A

Those bonded to electronegative substitutents (O, N, halogen) as opposed to hydrogen or carbon

43
Q

What does the electronegative atom do in regards to the attached carbon?

A

It pulls itself away from it, as its nucleus is deshielded

44
Q

In 13C NMR, what’s the signal of sp3 carbons?

A

Between 0-90

45
Q

In 13C NMR, what’s the signal for sp2 carbons?

A

Between 100-210

46
Q

In 13C NMR, is the signal strength indicative of the number of carbons giving rise to a signal?

A

No

47
Q

Which carbons display a signal in 13C NMR?

A

Every one in a unique environment

48
Q

Which hydrogens give rise to an absorption signal in H NMR?

A

Only hydrogens in unique environments (same environment=same signal)

49
Q

What increases a chemical shift range for hydrogen in H NMR?

A

An increased electronegativity (eg, C-N-O)

50
Q

Which hydrogens have a higher value?

A

Those bonded to sp2 carbons (as substituents containing double bonds deshield)

51
Q

What are signal areas of H proportional to?

A

The number of hydrogens giving rise to the signals

52
Q

When do absorption signals of H split?

A

When, on the atom directly bonded to one bearing the hydrogen(s) giving rise to the absorption signal, hydrogens are attached (ie vicinal/neighbouring Hs)

53
Q

What is the rule for absorption splitting due to vicinal hydrogens?

A

N+1

Chem110 (6 decks)

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