Chemistry 341 spectroscopy lecture

Interaction of Light and Matter The Physical Basis of Spectroscopy Quantum properties of light photons  Quantum properties of matter quantized energy states..  Photons of light act as

Chemistry 341

Spectroscopy of Organic

Compounds

Modern Spectroscopic Methods

medium size molecules in a few minutes

Infrared Spectroscopy (IR) are particularly

powerful techniques which we will focus on

Interaction of Light and Matter The Physical Basis of Spectroscopy

 Quantum properties of light (photons)

 Quantum properties of matter (quantized energy states).

 Photons of light act as our “quantum

probes” at the molecular level giving us

back precise information about the energy levels within molecules

The Electromagnetic Spectrum

 Continuous

 Covers a wide range of wavelengths of

“light” from radio waves to gamma rays.

 Wavelengths ( λ ) range from more than ten meters to less than 10-12 meter

The Electromagnetic Spectrum

Relationship Between Wavelength,

Frequency and Energy

 Speed of light (c) is the same for all wavelengths.

 Frequency ( ν ), the number of wavelengths per second, is inversely proportional to wavelength:

Wavelength/Spectroscopy

Relationships

Energy Changes

states in a magnetic field

Spin of Atomic Nuclei

Magnetic Properties of the Proton

Related to Spin

Energy States of Protons in a Magnetic

Field

Applied Magnetic Field

Hext

Nuclear Magnetic Resonance

(NMR)

tiny bar magnets

small energy difference between + ½

and – ½ spin states

the exact energy difference between the + ½

and – ½ spin states resulting in absorption of

photons as the protons change spin states

The NMR Experiment

magnetic field of the NMR

frequency (Rf) pulses and absorption of the radio waves is monitored

computer to obtain an NMR spectrum

The NMR Spectrometer

The NMR Spectrometer

The NMR Spectrum

absorption

which the absorption occurs (in units of parts per million = ppm)

standard zero point reference (0.00 ppm)

number of hydrogens represented by that peak

The NMR Spectrum

Chemical Shift ( δ )

 The chemical shift ( δ ) in units of ppm is defined as:

δ = distance from TMS (in hz)

radio frequency (in Mhz)

 A standard notation is used to summarize NMR spectral data For example p-xylene:

δ 2.3 (6H, singlet)

δ 7.0 (4H, singlet)

 Hydrogens in identical chemical environments

(equivalent hydrogens) have identical chemical shifts

Shielding – The Reason for Chemical Shift Differences

 Circulation of electrons within molecular

orbitals results in local magnetic fields that oppose the applied magnetic field.

 The greater this “shielding” effect, the

greater the applied field needed to achieve resonance, and the further to the right

(“upfield”) the NMR signal.

Structure Effects on Shielding

 Electron donating groups increase the

electron density around nearby hydrogen atoms resulting in increased shielding,

shifting peaks to the right.

 Electron withdrawing groups decrease the electron density around nearby hydrogen atoms resulting in decreased shielding,

(deshielding) shifting peaks to the left.

Structure Effects on Shielding

The Deshielding effect of an electronegative substituent can be seen in the NMR

spectrum of 1-Bromobutane

Br – CH2-CH2-CH2-CH3

δ (ppm): 3.4 1.8 1.5 0.9

Some Specific Structural Effects on

Spin-Spin Splitting

have different chemical shifts

carbon atoms spin-spin splitting will occur due to the hydrogens on one carbon feeling the

magnetic field from hydrogens on the adjacent carbon

hydrogens (measured in Hz) is the coupling

Spin-Spin Splitting Origin of the Doublet

Spin-Spin Splitting Origin of the Triplet

Spin-Spin Splitting Origin of the Quartet

The n + 1 Rule

If Ha is a set of equivalent hydrogens and Hx is an adjacent set of equivalent hydrogens which are not equivalent to Ha:

 The NMR signal of Ha will be split into n+1 peaks by Hx (where n = # of hydrogens in the Hx set.)

 The NMR signal of Hx will be split into n+1 peaks by Ha (where n = # of hydrogens in the Ha set.)

1H NMR Spectrum of Bromoethane

1H NMR Spectrum of

1-Nitropropane

Exceptions to the n+1 Rule

 The n+1 rule does not apply when a set of equivalent H’s is split by two or more other non-equivalent sets with different coupling constants.

 The n+1 rule does not apply to second

order spectra in which the chemical shift difference between two sets of H’s is not much larger than the coupling constant.

NMR: Some Specific Functional

Group Characteristics

no resolved splitting, and the chemical shift can vary greatly

adjacent carbon is small

NMR: Some Specific Functional

Group Characteristics

 Ortho splitting on aromatic rings is often

resolved, but meta and para splitting is

Infrared Spectroscopy

to differences in vibrational energy levels within molecules

types and bond strengths

bonds (functional groups) are present in the

molecule

IR Spectrum of Ethanol

IR Correlation Table

Key Functional Groups by Region

of the IR Spectrum

IR Spectrum of Benzaldehyde

IR Spectrum of Cyclohexanone

IR Spectrum of Propanoic Acid

Unknown A

7.1-7.5 (m, 5H)

Unknown B

4.1(quart., 1H), 7.2-7.4 (m, 5H)

Unknown C

2.90 (trip., 4H), 7.1-7.3 (m, 4H)