Basics of IR
What are the two types of bond vibrations?
Bending and stretching
What are the variables in Hooks Law that will remain unchanged?
(1/2pic) pi is a constant, and the length of light is a constant.
What is the difference between the fingerprint and diagnostic regions of the graph?
diagnostic: where most functional groups are found; 1500cm-1 and above
fingerprint: some functional groups here like c-o bonds, tell you about backbone of molecule; 1500cm-1 and below.
i have a sample that has a functional group but creates no signal on the IR. What can this be from?
a symmetrical molecule with no dipole moment between bonds. (an alkene)
Explain the stretching of a bond.
between 2 atoms. stretching can be asymmetric of symmetric, the stretch occurs along the length of the bond
Hooks law uses what two concepts to determine the stretching frequency of a molecule?
Force constant (bond strength)
Reduced mass (m1m2/m1+m2)
What are the three characteristics of signals?
Explain what each mean
Wavenumber, intensity, shape
What are some ways to identify a benzene on an IR spectrum?
What is bending? How many atoms must be present for it to occur?
Bending changes the bond angles between 3 or more atoms in a molecule
a stronger bond = ________ stretching frequency
smaller mass difference = ________ stretching frequency
higher, lower
What is the difference between an OH and COOH on an IR spectrum?
OH will have no C=O bond around 1600cm-1.
COOH will have both OH peak at 3300cm-1 and C=O at 1600cm-1
C=O is the same, so the nitrogen and oxygen have different effects on energy. nitrogen will donate electrons to the C=O where oxygen will withdraw electrons from the C=O. The donation of electrons will give the carbonyl more single bond characteristics, lowering the energy needed to vibrate the bond.
An IR graph can have two x axes. What variable is on each axis and where is energy the highest? How does that energy correlate to the variable on the x-axes?
one axis will have wavelength, one will have wavenumber.
Energy is highest toward the right side of the graph, wavenumber is the highest here and wavelength is the lowest.
What is the equation for hooks law? What can this equation tell you about the relationship between bond strength and mass difference?
v= (1/2pic)(f/mred)^1/2
since reduced mass is in the denominator, it is inversely proportional to wavenumber. As the difference in mass between 2 atoms decreases, the wavenumber increases. (ex C-H bonds having a high wavenumber)
as bond strength increases, wavenumber increases
around what region will you find C-H bonds? How will you know to differentiate between sp, sp2 and sp3?
about 2800-3300cm-1
using hooks law, a sp hybridized carbon will be the highest in energy while sp3 will be lowest.
Why do alcohols have an extra, less intense peak at 3800cm-1?
The OH is participating in hydrogen bonding with other OH molecules. The O-H bond is weakened and requires more energy to vibrate. Similar to inductive effects.
What does % transmittance tell us about a molecule?
How much IR light passed through the sample and was absorbed. The lower the % transmittance, the more energy a sample needed to vibrate.
What is the difference between a C=H, C=N and a C=O? Where would you expect these stretching vibrations to show up on a IR graph?
C=H have similar bond strengths, but a large mass difference (-3000cm-1)
C=N have similar masses, a double bond is higher in strength (1600cm-1)
C=O Have similar masses, a double bond is higher in strength (-1700cm-1)
What is the difference between a N-H bond, C-O bond, O-H bond and a C-H bond? Where can you find these located on a spectrum?
The relative polarity of the bonds differ. O-H has the highest polarity bond and is located around 3400cm-1. N-H has a less intense dipole moment as is located at 1600cm-1. C-H bond have little to no dipole moment and are located in the finger print region.
How does conjugation affect an IR spectrum? Why does it change it this way?
Conjugation will lower the wavenumber of a compound from its unconjugated counterpart. A conjugated molecule delocalizes electrons (the double bond character) around the molecule. this creates more single bond characteristics versus having no conjugation. using hooks law, single bond require less energy to vibrate than double bonds.