Ch. 12
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Absorption Spectroscopy
the measurement of the amount of electromagnetic radiation by a compound as a function of the wavelength
What are the 3 types of absorption spectroscopy
Infrared (IR), Nuclear magnetic resonance (NMR), ultraviolet (UV)
How are frequency and wavelength related?
Inversely
As energy increases, wavelenth _______, and frequency _____
Wavelength shortens, frequency increases
How does IR spectroscopy work
light causes molecular vibrations in the molecule
Wavenumber (v)
the number of wavelengths that fit into one centimeter that is proportional to the frequency
When a bond is stretched a restoring force _______; when a bond is compressed, a restoring force _______.
Pulls the two atoms together Pushes the two atoms apart
Heavier atoms vibrate _______ than lighter atoms
slower
Stronger bonds stretch or compress better/worse than weaker bonds
worse
Stronger bonds usually vibrate ______ than weaker bonds
faster
Simple stretching vibration region(IR)
most common area for characteristic and predictable vibrations between 1600 and 3500
IR-Active
a vibration that changes the dipole moment of the molecule and thus can absorb infrared light
IR-Inactive
a vibration that does not change the dipole moment of the molecule and thus cannot absorb infrared light
A molecule that is stretched has an _____ dipole moment. A molecule that is compressed has an ______ dipole.
Decreased Compressed
Infrared Spectrometer
a device that measures a compound’s absorption of infrared light as a function of frequency or wavelength
How long does dispersive IR take? How long does FT-IR take?
Dispersive takes a long time, FT-IR takes seconds
Why doesn’t a tertiary amine show up on a spectrometer? (IR)
Because there is no N-H bonds
Does resonance raise or lower carbonyl frequencies
lowers
Mass spectrometry
method to determine the molecular mass and other valuable information about the molecular formula
Mass spectrometer
An instrument that ionizes molecules, sorts the ions according to their masses and records the abundance of ions of each mass
Electron impact ionization
A sample is bombarded by a beam of electrons; an electron may ionize a molecules by knbocking out an additional electron forming a radical cation
Fragmentation
the breaking apart of a molecular ion upon ionization
Only __________ fragments are detected by the mass spectrometer
positively charged
How does the separation of ions of different masses occur?
magnetic deflection