Measures molecular vibrations including bond stretching, bending and rotation and can be symmetric or asymmetric A typical spectrum is obtained by passing IR light of frequencies 4000-400 cm-1 through a sample and recording absorbtion pattern. % transmittance (1/absorption) is plotted against frequency

Most of the useful functional group information is found between 1400 and 4000 cm-1

Certain nuclei have magnetic moments that are normally oriented at random When such nuclei are placed in a magnetic field the magnetic moments tend to align either with (α state, lowest energy) or against (β state, highest energy) the direction of the applied field If the nuclei are then irradiated with EM radiation, some will be excited into the α state. The absorption corresponding to this excitation occurs at different frequencies depending on an atom's environment - nearby atoms also possess magnetic moments

SNS - Organic Chemistry - Spectroscopy Flashcards by emily groves

Spectroscopy

Measures energy differences betwwen the possible states of a molecular system by determining the frequencies of elecromagnetic radioation (light) absorbed by the molecules

The possible ststes are quantized energy levels associated with different types of molecular motion, including molecular rotation, vibration of bonds and electron movement

Different types of spectroscopy measure different types of molecular motion

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Infrared

Measures molecular vibrations including bond stretching, bending and rotation and can be symmetric or asymmetric
A typical spectrum is obtained by passing IR light of frequencies 4000-400 cm-1 through a sample and recording absorbtion pattern.
% transmittance (1/absorption) is plotted against frequency

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Infrared

Vibrations and Regions

Useful absorptions of IR light occur in the 3000-30,000 nm region corresponding to wavenumbers 3,500-300 cm-1
Bond stretching involves the highest change in energy and is observed in the region 1500-4000 cm-1
Bending vibrations are observed at 400-1500cm-1
More complex combinations caudes by the motion of the whole molecule are seen at 1,500-400 cm-1 - the fingerprint region - used to identify compounds

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Infrared

Recordable Absorptions

For an absorbtion to be recorded, it must involve a change in bond dipole moment
Molecules comprised of atoms with the same electronegativity or symmetrical molecules therefore won’t experience a changing dipole moment and don’t exhibit absorption
For example, O2 and Br2 don’t absorb, HCl and CO do
Symmetrical bond stretches don’t show up in spectra as they involve no change in bond dipole moment

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Infrared

Application

Most of the useful functional group information is found between 1400 and 4000 cm-1

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Infrared

Absorption Peaks

Alkanes

C-H = 2800-3000
C-C = 1200

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Infrared

Absorption Peaks

Alkenes

=C-H = 3080 - 3140
C=C = 1645

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Infrared

Absorption Peaks

Alkynes

≡C-H = 3080-3140
C≡C = 1645

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Infrared

Absorption Peaks

Aromatic

C-H = 2900-3100
C-C = 1475-1625

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Infrared

Absorption Peaks

Alcohols

O-H = 3100-3500

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Infrared

Absorption Peaks

Ethers

C-O = 1050-1150

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Infrared

Absorption Peaks

Aldehydes

(O)C-H - 2700-2900
C=O - 1725-1750

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Infrared

Absorption Peaks

Ketones

C=O - 1700-1750

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Infrared

Absorption Peaks

Carboxylic Acids

C=O - 1700-1750
O-H = 2900-3300

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Infrared

Absorption Peaks

Amines

N-H - 3100-3500

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NMR

Certain nuclei have magnetic moments that are normally oriented at random
When such nuclei are placed in a magnetic field the magnetic moments tend to align either with (α state, lowest energy) or against (β state, highest energy) the direction of the applied field
If the nuclei are then irradiated with EM radiation, some will be excited into the α state. The absorption corresponding to this excitation occurs at different frequencies depending on an atom’s environment - nearby atoms also possess magnetic moments

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NMR

¹H NMR

Most 1H nuclei come into resonance between 0 and 10δ downstream of TMS (reference peak, designated t=0)
Protons attached to different groups have different magnetic environments and thus resonate at different frequencies.
Protons attached to the same group are magnetically equivalent due to rotation and resonate at the same frequency
Each distinct set of nuclei gives rise to a separate peak

NMR

1H NMR

Position of Peaks

The position of a peak due to shielding or deshielding effects reflects chemical environment
Electron clouds usually screen the nuclei slightly from the magnetic field, electron withdrawing groups decrease this effect (deshield) and thus the nucleus resonates in a lower field than would otherwise
Electron donating groups enhance the effect (shield), causing it to resonate at a higher field

NMR

1H NMR

Size of Peaks

The relative area of peaks represent the ratio of types of protons

NMR

1H NMR

Splitting

Represents the numbe of adjacent hydrogens
Peaks are split into n+1 where n = the number of adjacent hydrogens

1H NMR

Chemical Shifts

Order:

RCH3
-NH2
-CHOH-CH2OH
RCH2
R3CH
RCHCO-
-CHCOOH / -CHCOOR
-C≡CH
-CHX
-CHOH / -CHOR
ArOH
-CH≡CH
Ar-H
RCHO
-COOH

1H NMR

Chemical Shifts

RCH3

0.9

1H NMR

Chemical Shifts

-C≡CH

2-3

1H NMR

CHemical Shifts

RCH2

1.25

1H NMR CHemical Shifts R3CH

1.5

1H NMR CHemical Shifts -CH=CH

4.6-6

1H NMR CHemical Shifts Ar-H

6-8.5

1H NMR CHemical Shifts -CHX

2-4.5

1H NMR CHemical Shifts -CHOH / -CHOR

3.4-4

1H NMR CHemical Shifts RCHO

9-10

1H NMR CHemical Shifts RCHCO-

2-2.5

1H NMR CHemical Shifts -CHCOOH / -CHCOOR

2-2.6

1H NMR CHemical Shifts -CHPH-CH2OH

1-5.5

1H NMR CHemical Shifts ArOH

4-12

1H NMR CHemical Shifts -COOH

10.5-12

1H NMR CHemical Shifts -NH2

1-5

NMR ¹³C NMR

* Very similar to 1H NMR except occur 0-200δ downstream of TMS and only 1.1% C is ¹³C. Means that a much larger sample is needed (~50x) and coupling (splitting) between carbon atoms not observed * Coupling is still observed between carbon atoms and the protons directly attached to them, for example if a C is attached to two H, signal is split into a triplet * Can also record a spectrum without coupling of adjacent protons - spin decoupling

Spectroscopy UV

* Obtained by passing UV light through a chemical sample (usually dissolved in an inert nonabsorbing solvent) * Absorbance plotted against wavelength * Wavelength of maximum absorbance provides info on the extent of the conjugated system and other structural or compositional info

Spectroscopy Mass

* Not true spectroscopy - no absprbtion of EM radiation involved and is destructive * Most commonly, high speed beam of electrons used to ionize the sample. Then placed into a particle accelerator with magnetic field to deflect the accelerated cationic fragments (only cationic gragments show up on the spectrum). A detector records the number of particles of each mass exiting the deflector area

Spectroscopy Mass Spectrum

* Spectrum plots mass/charge ratio against relative abundance * The tallest peak belongs to the most common ion - base peak - and is assigned the relative abundance value of 100% * The peak with the highest m/e ratio is generally the molecular ion peak, M+ from which the molecular weight can be obtained * Fragmentation patterns provide indications of structure