Chemical Spectra Flashcards
Describe how an infrared spectrometer works
A beam of infrared (IR) radiation is passed through a sample. The spectrometer will then analyze the amount of radiation transmitted (transmittance %) through the sample as the incident radiation is varied (wavenumber in cm^-1).
What are intramolecular rotations?
The rotations of molecules about their centre of gravity. The difference in rotational energy levels is inversely proportional to the moment of inertia of a molecule. Rotational energy is quantized and gives rise to absorption spectra in the microwave region of the electromagnetic spectrum.
What are intramolecular vibrations?
How does this relate to IR spectra?
The bending and stretching motions of bonds within a molecule. The relative spacing between vibrational energy levels increases with the increasing strength of an intramolecular bond. Vibrational energy is quantized and gives rise to absorption spectra in the infrared region of the electromagnetic spectrum.
Thus there are two types of bond vibration: stretching and bending. That is, after exposure to the IR radiation, the bonds stretch and bend (or contract) to a greater degree once energy is absorbed. In general, bending vibrations will occur at lower frequencies (higher wavelengths) than stretching vibrations of teh same group. So each group will have two characteristic peaks, one due to stretching and one due to bending.
Give the frequency range (cm^-1) that would appear on IR spectra for the following groups
Alkyl (C-H): Alcohol (O-H): Carbonyl (C=O): Aldehyde: Ketone: Carboxylic acid: Amine (N-H):
Alcohol (O-H): 3200 - 3650 Amine (N-H): 3300 - 3500 Alkyl (C-H): 2850 - 2960 Ketone: 1735 - 1750 Carboxylic acid: 1710 - 1780 Carbonyl (C=O): 1630 - 1780 Aldehyde: 1680 - 1750
What is proton nuclear magnetic resonance (proton NMR) spectroscopy?
Examines the environment of hydrogen atoms in a molecule.
Can determine number and types of hydrogens with NMR spectroscopy.
NMR spectrometers measure the absorption of energy by the hydrogen nuclei in an organic compound (which is partially dependant on 1/2 or -1/2 spin of hydrogen nucleus/proton)
Using proton NMR, how can you determine how many hydrogens are at a peak?
How can you determine how many neighbouring hydrogens there are?
The relative height of a peak indicates the number of hydrogens represented by that peak.
The splitting of a peak indicates the number of neighbouring ‘non-equivalent’ hydrogens (peaks: n +1, where n is the number of non-equivalent neighbouring protons)
A chemist obtains a proton NMR spectrum of a product sample. If the sample were contaminated with acetone, how many extra signals corresponding to the acetone would be present in the spectrum?
The six hydrogen atoms in acetone are magnetically equivalent and their proton NMR signal appears as a singlet near δ = 2 ppm. Thus only ONE extra signal will be observed.
You have four compounds, each has a carbonyl (C=O) group and either an -OH or -NH group. The IR spectrum of each compound should contain a strong, sharp band at approximately ____ cm^-1.
1700 cm^-1
A C=O group displays a sharp, intense IR band between 1650 and 1800 (solution).
THe -OH and -NH groups generally display intermolcular hydrogen bonding, which causes their IR bands to be broad, extending over a wide spectral region (eg. 3600-2700).
The IR spectra of alcohols shows what type of bands?
A broad band centered near 3500 cm^-1
Define UV spectroscopy
Ultraviolet–visible spectroscopy or ultraviolet-visible spectrophotometry (UV-Vis or UV/Vis) refers to absorption spectroscopy or reflectance spectroscopy in the ultraviolet-visible spectral region. This means it uses light in the visible and adjacent (near-UV and near-infrared [NIR]) ranges. The absorption or reflectance in the visible range directly affects the perceived color of the chemicals involved. In this region of the electromagnetic spectrum, molecules undergo electronic transitions. This technique is complementary to fluorescence spectroscopy, in that fluorescence deals with transitions from the excited state to the ground state, while absorption measures transitions from the ground state to the excited state.
In what units are the chemical shifts of hydrogens in a nmr spectrum measured?
ppm (parts per million)
The chemical shift of a peak in a NMR spectrum is caused by what?
The electronic environment (shielding and deshielding)
The carbons furthest from a EN atom are those that are most upfield (closer to TMS/right hand side).
What causes a peak in NMR spectrum?
The absorption of electromagnetic radiation.
Why is the range of absorption 2100-2260 for a triple bond when it is 1620-1680 for a double bond in IR spectra?
Triple bonds are stiffer, and the stiffer the bond the higher the frequency of vibration when IR radiation is absorbed.
What is the purpose of the TMS signal in proton NMR?
To reference the spectra at 0 ppm
TMS stands for tetramethylsilane. The molecule is tetrahedral and all hydrogens are equal.
