11 Spectroscopy Flashcards
What is Infrared (IR) spectroscopy used for?
Infrared (IR) radiation is absorbed by certain types of bond and causes them to stretch or bend. This can be used to gain information about the type of bonds within the molecule.
In order for a gas to absorb IR radiation, there must be
a change in the dipole moment of the molecule as the bonds undergo asymmetrical stretching and bending.
Explain why CO2 is IR active while N2 and O2 are IR inactive
CO2 undergoes asymmetrical stretching and bending –> change in the dipole moment –> IR active
The amount of IR radiation absorbed by a bond depends on
bond polarity - strongly polar bonds absorb more, weakly polar bonds absorb less, non-polar bonds such as C-C usually do not absorb IR radiation and do not show up on an IR spectrum.
The peaks on the IR spectrum corresponds to
the amount of IR radiation absorbed by the bonds in the compound
How do you calculate IHD (Index of Hydrogen Deficiency)?
(2C+2-H)/2
State the 4 stages in a mass spectrometer
- Vaporization
- Ionization
- Acceleration
- Deflection
- Detection
Victor Is A Daft Duck
Outline how mass spectrometry analyses positive ions generated by the electron bombardment of molecules
The atoms or molecules are ionized by bombarding with high energy electrons, forming a molecular ion (M+) peak, which corresponds to a molecule that has lost an electron (same formula as the original compound but with a positive charge)
The resulting molecular ion has such high energy that it splits up (fragments) inside the mass spectrometer into smaller ions.
By identifying the fragments on the mass spectrum, we can determine the structure of the molecule.
Describe the features of Cl mass spectra
- two M+ peaks due to isotopes Cl-35 and Cl-37
- 3:1 height ratio between Cl-35 and Cl-37
Describe the features of Br mass spectra
- two M+ peaks due to isotopes Br-79 and Br-81
- 1:1 height ratio between Br-79 and Br-81
What is Nuclear Magnetic Resonance (NMR) spectroscopy used for?
HNMR is a spectroscopic technique that can be used to identify the different chemical environments in a molecule that contain an odd number of H atoms (protons) in their nuclei. When placed in an external magnetic field, the molecules will spin. Some nuclei will line up WITH the magnetic field and some will line up AGAINST the magnetic field (not assessed but good to understand)
Which region of the electromagnetic spectrum is used for NMR spectroscopy?
radiowaves as the energy required to make a proton spin in the higher energy state is quite small
The number of peaks in a NMR spectrum gives
the number of different chemical environments in which H atoms/protons are located
The chemical shift in a NMR spectrum gives
the nature of the chemical environments in which H atoms/protons are located
The area under each peak in a NMR spectrum gives
the number of H atoms in a particular environment
