CHEM 344

Spectroscopy of Organic Compounds

Lecture 14th and 5th September 2007

Modern Spectroscopic Methods

• Revolutionized the study of organic chemistry

• Can determine the exact structure of small to medium size molecules in a few minutes

• Nuclear Magnetic Resonance (NMR) and Infrared Spectroscopy (IR) are particularly powerful techniques which we will focus on and use in this course

Interaction of Light and MatterThe Physical Basis of Spectroscopy

• Spectroscopy: the study of molecular structure by the interaction of electromagnetic radiation with matter

• Electromagnetic spectrum is continuous and covers a very wide range of wavelengths

• Wavelengths () range from 103 to 10-15 meters

The Electromagnetic Spectrum

Relationship Between Wavelength, Frequency and Energy

• Speed of light (c) is constant for all wavelengths

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

c

• Energy of a photon is directly proportional to frequency

E = hc/h(where h = Plank’s constant)

Energy Levels in Molecules

• Energy levels within a molecule are discrete (quantized)

• Transitions between various energy levels occur only at discrete energies

• Transition caused by subjecting the molecule to radiation of an energy that exactly matches the difference in energy between the two levels

Eupper – Elower = ΔE = h

Wavelength/Spectroscopy Relationships

Spectral Region Photon Energy Molecular Energy Changes

UV-visible10-7-10-8

m

1016-1017 Hz

~ 100 kcal/mole

~ 420 kJ/mole

Electronic transitions

(e.g. HOMO-LUMO)

Infrared 10-3-10-5

m

1012-1015 Hz

~ 10 kcal/mole Bond vibrations

(e.g. C≡O stretching)

Radio (used for NMR)

103-101 m

106-107 Hz

< 0.1 kcal/mol Flipping a nuclear spin state in a magnetic field

Nuclear Spins

• Spin ½ atoms: mass number is odd 1H, 13C, 19F, 29Si, 31P

• Spin 1 atoms: mass number is even 2H, 14N

• Spin 0 atoms: mass number is even12C, 16O, 32SNO NMR SIGNAL

Magnetic Properties of the Proton Related to Spin

Energy States of Protons in a Magnetic Field

Spin states degenerate

Random orientations

Two allowed orientations (2I+1) = 2

Aligned with or against direction of Bo

No External Mag. Field External Mag. Field Bo

Nuclear Magnetic Resonance (NMR)

• Nuclear – spin ½ nuclei (e.g. protons) behave as tiny bar magnets

• Magnetic – a strong magnetic field causes a small energy difference between + ½and – ½ spin states

• Resonance – photons of radio waves can match 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

• The sample, dissolved in a suitable NMR solvent (e.g. CDCl3, CCl4, C6D6), is placed in the strong magnetic field of the NMR spectrometer

• The sample is bombarded with a series of radio frequency (Rf) pulses and absorption of the radio waves is monitored

• The data are collected and manipulated on a computer to obtain an NMR spectrum

An NMR Spectrometer

Our NMR Spectrometer

PNNL NMR Spectrometer

The NMR Spectrum

• The vertical axis shows the intensity of Rf absorption• The horizontal axis shows relative energy at which

the absorption occurs (parts per million, ppm)

• Tetramethylsilane (TMS, SiMe4) is included as a standard zero point reference (0.00 ppm)

• The area under any peak corresponds to the number of hydrogens represented by that peak

NMR Spectrum of p-Xylene

Chemical Shift ()

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

= shift from TMS (in Hz)

radio frequency (in MHz)

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

2.3 (6H, singlet) 7.0 (4H, singlet)

• Hydrogen atoms in identical chemical environments 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

Structural 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 (downfield).

Structural Effects on Shielding

The deshielding effect of an electronegative substituent can be seen in the 1H-NMR spectrum of 1-bromobutane:

Br – CH2-CH2-CH2-CH3

(ppm): 3.4 1.8 1.5 0.9

No. of H’s: 2 2 2 3

Some Specific Structural Effects on NMR Chemical Shift

Hydrogen Environment (ppm)

Alkyl (C – H) 0.8 – 1.7

Alkyl Halide (RCH2X) 3 - 4

Alkene (R2C=CH2) 4 - 6

Aromatic (e.g. benzene) 6 - 8

Carboxylic Acid (RCOOH)

10 - 12