IR/NMR Spectroscopy Flashcards
(28 cards)
What should you do first with an IR spectrum?
Draw a line at 3000 and look for peaks immediately to the left (sp2 C-H’s) and the right (sp3 C-H’s). Then look for O_H peaks at around 3600-2800, and C=O double bonds around 1800.
Where does a carboxylic acid show up in NMR? An aldehyde proton?
Around 10-12ppm and 8-10ppm respectively.
What solvent peaks are present in NMR?
CDCl3 produces a singlet at 7.3 ppm in HNMR and a triplet at 7.7 in CNMR, and water produces a broad singlet at about 1.6ppm.
What are the units of IR or vibrational spectroscopy?
The wavenumber, or cm-1
What are the two fundamental IR vibrations?
Stretching and bending.
What are different qualitative aspects of peaks?
How large (intense) they are, aka strong or weak, and how much distance they cover (are they sharp or broad).
Where can you find a table of IR vibrational frequencies for functional groups?
page 313 on the pdf.
What is characteristic of carboxylic acids in IR spectra?
They have an unusually large O-H peak, from around 3200-2500 wavenumbers.
Where do sp3, sp2, and sp C-H’s occur in an IR spectrum?
Around 2800-3000, 3000-3200, and 3300 respectively.
Where are the triple bonds for CN and CC located in IR spectroscopy?
In the 2280-2100 region, strong to medium for the CN bond, and weak to medium for the alkyne. They are the only possible peaks in this region.
Where are C=O bonds found in IR spectroscopy?
Around 1850-1630 wavenumbers, with a strong and sharp peak.
Where are amide absorptions located on an IR spectrum?
Primary and secondary amides exhibit N—H stretching absorption bands in the 3500–3150 cm1 region, two bands for primary amides and one band for secondary amides.
Where is the C=C region in an IR spectrum?
Around 1440-1680 wavenumbers, with intensities varying from weak to medium.
Where is the C-O stretching bond located in an IR spectrum?
In the fingerprint region, so it will be very difficult to find.
Where can we find good information on diagnostic bonds in the textbook?
Page 323 in the pdf.
How are NMR spectrometers referred to by?
By their operating frequency, such as 60MHz.
Where does an additional noise peak for deuterated chloroform occur?
At 7.26ppm, because some of the deuterium is actually protium.
What happens in NMR spectroscopy as a carbon becomes more substituted?
The hydrogen attached to it becomes more deshielded, and is moved downfield (away from TMS).
What affects the intensity of a peak in IR spectroscopy, and how is this relevant to peaks that should exist but do not?
The dipole moment, and since only stretching vibrations that affect the dipole moment are seen on the IR spectrum, we do not see stretching vibrations that do not affect the dipole moment, such as molecules that have perfect symmetry.
Where can we find tables to predict NMR chemical shift of protons?
On page 351 in the pdf.
What is a vinyl proton?
A proton attached to a C=C double bond.
What is a geminal group?
A group attached to a =C double bonded carbon, which must also have a proton attached to the doubly bonded carbon.
How can H-bonding be prevented?
By excessive steric hindrance in a molecule. This will lead to a sharp O-H peak rather than a broad one.
Why does carboxylic acid have different IR spectra than other O-H bonds?
Because there are multiple sites for H-bonding, so the signal is much broader as opposed to other O-H bonds, and will often obscure the C-H bond stretch.
