Chapter 14- Infrared Spec and Mass Spec

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Wavelength

Inversely proportional to the energy

Frequency

directly proportional to the energy

Electromagnetic spectrum

The range of possible frequencies of light

Matter on the molecular scale

exhibits quantum behavior

Vibrational excitation

If a photo of light strikes the molecule with the exact amount of energy needed, the light is absorbed and it will occur

Frequencies absorbed by the sample tell us the

types of bonds (functional groups) that are present

IR absorption spectrum

Plots the % transmittance as a function of frequency

Peaks

absorption bands

Values range from

400-4000cm-1

IR spec three important characteristics

wavenumber, intensity, and shape

Wavenumber (frequency) depends on

1) bond strength

2) mass difference of atoms bonded together

Stronger bond=

higher stretching frequency

Larger mass difference=

higher stretching frequency

Region above 1500cm-1 is

the diagnostic region (simpler to understand)

Below 1500cm-1

fingerprint region (messy)

Higher the s character if the carbon,

stronger the C-H

Higher the stretching frequency

More delocalized the p electrons,

the weaker the p bond

lower the stretching frequency

Conjugated carbonyls have _____ stretching frequencies

lower

Bigger difference in distance

Stronger dipole

Greater bond polarity

Stronger IR signals

If the bond is completely symmetrical

No stretching frequency observed in IR spectrum

O-H stretching signals are

often broad

No H bonding ~3600

Free O-H

2 degree emines exhibit

one signal for N-H bonds

1 degree amines exhibit

two signals for the N-H bonds

1600-1850cm-1

double bonds

2100=2300cm-1

triple bonds

2700-4000cm-1

X-H bonds

400-1600cm-1

single bonds

Mass spectrometry

determines the molar mass and formula for a compound

Electron impact usually causes

an electron to be ejected from the molecule

only charged particles can go through

Mass of radial cation is

same as the parent comount

If radical cation remains intact, its known as

the molecular ion (M+) or parent ion

What percent of Carbon is C13

1.1%

The base peak is the

tallest peak and most abundant

For most compounds, the M+ peak

is not the base peak

Odd massed M+ peak means

odd number of N atoms in the molecule

Even massed M+ peak

indicates absence of nitrogen or an even amount of N atoms present

35Cl

76%

37Cl

24%

Amines undergo

alpha cleavage

Carbonyls generally undergo

McLafferty rearrangement

M-18

Loss of water from an alcohol

M-X

McLafferty rearrangement

Where X=even number

Ketone or aldehyde

Electrospray Ionization (ESI)

high voltage needed sprays a liquid solution of an analyte into a vacuum causing ionization

Saturated hydrocarbons

CnH2n+2

Degree of unsaturation

Pi bond or ring

For every degree, number of H is reduced by 2

Oxygen does not affect

HDI

Nitrogen ______ number of expected hydrogen atoms by 1

Increases

HDI=

(2C+2+N-H-X)/2