Ch 12 IR and Mass Spectrometry

Question 1

Infrared spectroscopy

Answer 1

Measures bond vibration freq.s in molecule

Used to determine functional group

Question 2

Mass spectrometry

Answer 2

Fragments molecules and measures masses

Does not involve direct approach of light

Question 3

Nuclear magnetic resonance NMR

Answer 3

Detects signals from hydrogen (and c and other atoms)

Can be used to distinguish isomers
Most informatics techniques

Question 4

Ultraviolet spectroscopy

Answer 4

Uses electron transitions to determine bonding patterns

Invisible light region of electromagnetic spectrum

Question 5

Spectroscopy

Answer 5

Analytical technique to help determine structure

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

Question 6

Wavelength

Answer 6

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

Question 7

Relationship of frequency and wavelength

Answer 7

Inversely proportiona

Question 8

Photon

Answer 8

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

Question 9

Frequency

Answer 9

Number of flips/oscillations that occur in one second

Question 10

E=hv

Answer 10

Energy is plancks constant x frequency

Question 11

v=c/λ

Answer 11

Frequency is spd of light divided by wavelength

Question 12

E=hc/λ

Answer 12

Energy is plancks x spd of light / wavelength

Question 13

Infrared region

Answer 13

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)

Question 14

Wave numbers

Answer 14

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

Question 15

Covalent bonds

Answer 15

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

Question 16

Energies associated with bonding vibrations

Answer 16

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

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

Question 17

IR absorption positions

Answer 17

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)

Question 18

IR absorption intensity

Answer 18

-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

Question 19

Stretching frequencies

Answer 19

Heavier mass, decrease stretching frequency

Frequency increases with increasing bond energies

Question 20

How many vibrations allowed for nonlinear molecule?

Answer 20

3n-6

Question 21

Simplest vibrational motions

Answer 21

Stretching and bending

Question 22

Fingerprint region

Answer 22

600-1400cm^-1

Complex vibrations

Question 23

How many molecules will give same IR spectrum?

Answer 23

None

Except enantiomers

Question 24

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

Answer 24

Higher

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

Question 25

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

Answer 25

Higher

Question 26

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

Answer 26

Higher

Question 27

Double bonds have ________ frequencies than single bonds

Answer 27

Higher Except for bonds to hydrogen

Question 28

O-H

Answer 28

3200-3650cm^-1 Strong and broad Strong bc polar--> polar bonds, strong IR absorption

Question 29

N-H

Answer 29

3100-3500cm^-1 Medium

Question 30

C-H

Answer 30

2850-3300 Medium to strong

Question 31

C=O

Answer 31

1630-1810 Strong Very important

Question 32

C=C

Answer 32

1600-1680 Weak Usu hard to see

Question 33

C-O

Answer 33

1050-1250 Strong

Question 34

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

Answer 34

- must be polar - it's vibrations must cause periodic change in bond movement (Dipole moment leads to active IR absorption)

Question 35

IR inactive

Answer 35

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

Question 36

FT-IR spectrometer

Answer 36

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

Question 37

IR spectroscopy of hydrocarbons

Answer 37

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

Question 38

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

Answer 38

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

Question 39

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

Answer 39

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

Question 40

C-H bond stretching

Answer 40

Bonds w higher s character absorb at higher frequency

Question 41

Sp^3 CH absorption freq

Answer 41

Just below 3000cm^-1

Question 42

Sp^2 CH absorption freq

Answer 42

Just above 3000cm-1 ( to left)

Question 43

Sp CH absorption freq

Answer 43

At 3300cm^-1

Question 44

OH absorption freq and pattern

Answer 44

Around 3300cm^-1 Broad with rounded tip (don't confuse with amines) Looks different bc O is more eneg than N, so looks diff

Question 45

Secondary amine freq and absorption pattern

Answer 45

Around 3300cm^-1 R2NH- broad with one sharp spike Only one sharp spike bc only one N-H bond

Question 46

Primary amine freq and absorption pattern

Answer 46

Around 3300cm^-1 RNH2- broad with two sharp spikes (one per N-H bond)

Question 47

C=O bond absorption freq

Answer 47

Around 1710cm^-1 1715+/- 5cm^-1 Usually strongest IR signal

Question 48

Carboxylic acids IR freq

Answer 48

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

Question 49

Aldehydes absorption freq

Answer 49

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