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Organic Chemistry > Ch 12 IR and Mass Spectrometry > Flashcards

Ch 12 IR and Mass Spectrometry Flashcards

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1
Q

Infrared spectroscopy

A

Measures bond vibration freq.s in molecule

Used to determine functional group

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2
Q

Mass spectrometry

A

Fragments molecules and measures masses

Does not involve direct approach of light

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3
Q

Nuclear magnetic resonance NMR

A

Detects signals from hydrogen (and c and other atoms)

Can be used to distinguish isomers
Most informatics techniques

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4
Q

Ultraviolet spectroscopy

A

Uses electron transitions to determine bonding patterns

Invisible light region of electromagnetic spectrum

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5
Q

Spectroscopy

A

Analytical technique to help determine structure

Use of absorption, emission, or scattering of emagnetic radiation by matter to study physical processes

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6
Q

Wavelength

A

Physical distance in direction of propagation a photon makes one complete oscillation

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7
Q

Relationship of frequency and wavelength

A

Inversely proportiona

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8
Q

Photon

A

Discrete packet of energy w magnetic field and electric field components

These fields are perpendicular/orthogonal to each other and to direction of propagation of photon

Flip direction as photon travels

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9
Q

Frequency

A

Number of flips/oscillations that occur in one second

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10
Q

E=hv

A

Energy is plancks constant x frequency

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11
Q

v=c/λ

A

Frequency is spd of light divided by wavelength

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12
Q

E=hc/λ

A

Energy is plancks x spd of light / wavelength

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13
Q

Infrared region

A

Vibrational IR : 2.5 micrometers to 25 micrometers

Absorption of IR radiation in this region causes bonds to change from lower vibrational energy level to higher (causes chemical bonds)

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14
Q

Wave numbers

A

Frequency of IR radiation expressed in wave numbers

Number of waves per centimeter (Can be directly translated to v)

Vibrational IR from 4000 cm^-1 to 400cm^-1

Higher wave number, higher energy

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15
Q

Covalent bonds

A

Two vibrating masses connected by chemical bonds

As vibrates, energy changes btwn KE and PE and vv
Total energy EsubK + EsubP is proportional to v of vibration

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16
Q

Energies associated with bonding vibrations

A

Are quantized –> in a molecule, only certain vibrational energy levels are allowed

Usually btwn 2-10kcal/mol
(Within IR rgn of 1-11)

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17
Q

IR absorption positions

A

Affected by

  • strength of bond (stronger bonds at higher freqs)
  • masses of atoms in bond (lighter masses at higher freqs)
  • type of vibration observed (stretching at higher freqs than bending)
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18
Q

IR absorption intensity

A

-overall peak intensity directly related to [ ] of sample
-relative peak intensity is additive
large number of similar grps will increase intensity of given peak
-relative peak intensity influ. by dipole moment

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19
Q

Stretching frequencies

A

Heavier mass, decrease stretching frequency

Frequency increases with increasing bond energies

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20
Q

How many vibrations allowed for nonlinear molecule?

A

3n-6

21
Q

Simplest vibrational motions

A

Stretching and bending

22
Q

Fingerprint region

A

600-1400cm^-1

Complex vibrations

23
Q

How many molecules will give same IR spectrum?

A

None

Except enantiomers

24
Q

Stretching freq.s are _________ than corresponding bending freq.s

A

Higher

It is easier to bend a bond then stretch/compress it

25
Q

Bonds to hydrogen have _________ stretching freq.s than those to heavier atoms

A

Higher

26
Q

Triple bonds have ________ stretching freq.s that corresponding double bonds

A

Higher

27
Q

Double bonds have ________ frequencies than single bonds

A

Higher

Except for bonds to hydrogen

28
Q

O-H

A

3200-3650cm^-1

Strong and broad
Strong bc polar–> polar bonds, strong IR absorption

29
Q

N-H

A

3100-3500cm^-1

Medium

30
Q

C-H

A

2850-3300

Medium to strong

31
Q

C=O

A

1630-1810

Strong
Very important

32
Q

C=C

A

1600-1680

Weak

Usu hard to see

33
Q

C-O

A

1050-1250

Strong

34
Q

For a molecule to absorb IR radiation, bond undergoing vibration…

A
  • must be polar
  • it’s vibrations must cause periodic change in bond movement
       (Dipole moment leads to active IR absorption)
35
Q

IR inactive

A

Covalent bonds that don’t meet criteria (polar and vib.s that cause periodic change in movement)

C-C double and triple bonds of symmet. substituted alkenes and alkynes don’t absorb rad. bc they aren’t polar bonds
Compression/stretch doesn’t lead to dipole moment

36
Q

FT-IR spectrometer

A 
Faster
Uses interferometer
Better sensitivity
Less energy req. from source
Scan takes ~1-2 sec
Laser beam keeps it calibrated
37
Q

IR spectroscopy of hydrocarbons

A

Stronger bonds absorb at higher freq.s
C-C, C=C, C|||C
Conjug lowers freq
Isolated C=C,conjugated C=C, aromatic C=C

38
Q

IR absorption
C-C
C=C
C|||C

A

1200cm^-1
1660
2200
(Weak or absent if internal, bc C atoms not diff from each other)

39
Q

IR absorption
Isol. C=C
Conj. C=C
Aromatic C=C

A

1640-1680 cm^-1
1620-1640
~1600

40
Q

C-H bond stretching

A

Bonds w higher s character absorb at higher frequency

41
Q

Sp^3 CH absorption freq

A

Just below 3000cm^-1

42
Q

Sp^2 CH absorption freq

A

Just above 3000cm-1 ( to left)

43
Q

Sp CH absorption freq

A

At 3300cm^-1

44
Q

OH absorption freq and pattern

A

Around 3300cm^-1

Broad with rounded tip (don’t confuse with amines)
Looks different bc O is more eneg than N, so looks diff

45
Q

Secondary amine freq and absorption pattern

A

Around 3300cm^-1

R2NH- broad with one sharp spike
Only one sharp spike bc only one N-H bond

46
Q

Primary amine freq and absorption pattern

A

Around 3300cm^-1

RNH2- broad with two sharp spikes (one per N-H bond)

47
Q

C=O bond absorption freq

A

Around 1710cm^-1
1715+/- 5cm^-1

Usually strongest IR signal

48
Q

Carboxylic acids IR freq

A

Will have C=O and O-H absorption freqs (broad btwn 2500-3500cm^-1)

O-H absorbs broadly bc of strong H bonding- gigantic broad stretch confirmed by C=O

49
Q

Aldehydes absorption freq

A

C=O and two C-H signals around 2700-2800cm^-1

Organic Chemistry (1 decks)

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