Lecture 10 - IR Spectroscopy Flashcards

1
Q

spectroscopy:

A

field of study of the interaction of matter and light

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2
Q

what can spectroscopy be used to do?

A

spectroscopy can be used to determine unknown molecular structures

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3
Q

what is light?

A

electromagnetic radiation that comes in various waves

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4
Q

types of electromagnetic radiation:

A

visible, x-ray and ultraviolet light

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5
Q

what does electromagnetic radiation propagate as and what is it categorised by?

A

EM radiation propagates as a wave motion through space and is categorized by its wavelength (l) or frequency (n)

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6
Q

light particles are called ________

A

photons

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7
Q

how can light exist?

A

although light can be described as a wave it also exhibits particle-like behaviour

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8
Q

the most common type of spectroscopy used for structure determination:

A

absorption spectroscopy

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9
Q

the amount of electromagnetic radiation absorbed by a sample is determined by?

A

a spectrometer or spectrophotometer

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10
Q

what does infrared spectroscopy provide information about?

A

infrared provides information about what functional groups are present

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11
Q

what does nuclear magnetic resonance provide information about?

A

nuclear magnetic resonance provides information on the number, connectivity and functional group environment of C & H

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12
Q

what information does UV-visible spectroscopy provide information about?

A

UV-visible provides information about the types of p-electron systems present

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13
Q

how is the light absorbed by a substance is recorded?

A

as a function of wavelength

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14
Q

the horizontal axis is the __________ the ________ of the wavelength

A

wave number

inverse

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15
Q

what are the conventional units of the infrared spectrum?

A

reciprocal cm or inverse cm (cm^-1)

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16
Q

what is the y axis of infrared spectroscopy?

A

percent transmittance

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17
Q

what are absorptions in the IR spectrum the result of?

A

they absorbances are as a result of bond vibrations

18
Q

what must the radiation exactly match?

A

the frequency of radiation must exactly match the frequency of bond vibration

19
Q

what do peaks correspond to in an IR spectrometry graph?

A

peaks corresponds to absorption of energy by the vibration of a particular bond or group

not all peaks can be interpreted but many are diagnostic

20
Q

wavenumber for O-H, N-H & C-H:

21
Q

wavenumber for C///N, C///C:

22
Q

wavenumber for C=O, C=N, C=C:

23
Q

wavenumber for C—C, C—O, C—N:

24
Q

wavenumber for C—H:

25
fingerprint region:
absorptions in this region are not usually interpreted, but can serve as a valuable molecular fingerprint
26
Factors That Determine IR Absorption Position:
(1) strength of bond: stronger bond = higher frequencies (2) masses of atoms:lighter masses = higher frequencies (3) type of vibration: stretching vibrations = higher frequencies than bending ones
27
three types of normal vibrational nodes:
symmetrical stretch, unsymmetrical stretch, unsymmetrical in-plan bend (rock)
28
Factors That Determine IR Absorption Intensity:
• overall peak intensity is related to the concentration of the sample • relative peak intensity is additive: A large number of similar groups (e.g., alkyl) will increase the intensity of a given peak • relative peak intensity is also due to the dipole moment
29
dipole moment is related to:
the charge separation and distance between two atoms
30
as the bond stretches the dipole…
… dipole increases
31
as the bond compresses the dipole….
… decreases
32
The electric field of a light wave cannot interact with a bond that…
has no dipole bonds with no dipole will not absorb in IR
33
how do groups with large dipole interact with IR?
groups with large dipoles provide intense absorptions
34
Molecular vibrations that occur but do not give rise to IR absorption are said to be…
infrared-inactive
35
Any vibration that does give rise to an absorption is said to be…
infrared-active
36
IR Spectra of Alcohols and Ethers:
•O-H stretch (H-bonded): 3200-3400 cm-1 •O-H stretch (not H-bonded): 3600 cm-1 •C-O stretch: 1050-1200 cm-1 (ROH and ethers)
37
IR Spectroscopy of Aldehydes and Ketones:
Strong C=O stretch: 1700 cm-1
38
Conjugation with a p bond…
lowers the absorption frequency (typically by 20-30 cm-1)
39
The Infrared Spectrometer Machine:
•Most modern IR spectrometers are Fourier-transform spectrometers •Liquid samples can be analyzed undiluted (neat), as a mineral oil dispersion (mull), or as a solution (CHCl3 or CH2Cl2 as solvent) •Solid samples can be analyzed as a fused KBr pellet •Our instruments are attenuated total reflectance (ATR) instruments
40
spectroscopy deals with:
the interaction of matter and electromagnetic radiation
41
IR spectroscopy deals with:
molecular vibrations
42
there needs to be a change in ______ ______ on vibration
dipole moment