Spectroscopy & Analytical Methods Flashcards
True or false:
The perceived color of a substance is complementary to the color of the wavelength that was actually absorbed by the substance.
True
A substance which appears green, absorbs what color and wavelength range?
Absorbs red light ranging from 650-740 nm.
The difference between the HOMO and the LUMO, is inversely proportional to (energy/wavelength).
Wavelength; the higher the difference between the states, the higher the energy of the photon.
If the photon has high energy, its wavelength is decreased (short).
How does conjugation affect wavelength properties of a photon?
Conjugation makes a molecule more stable, so there is a lower energy. Lower energy means you have a longer wavelength.
What causes a solution to change color?
The electronic structure of the molecule in the solution changes. Basically, the wavelength/color that the solution can absorb depends on the electronic structure (electrons in the solution).
What does IR spectroscopy specifically measure?
The frequencies of electromagnetic radiation absorbed by molecules, the different molecular vibrations
What are the different vibrations IR spec can detect?
Bond stretching, bond bending, twisting & folding
What can you detect with IR Spec?
The presence of specific functional groups and connectivity of bonds (backbone) of a molecule
Define the fingerprint region
The the area of the IR spectrum between 500 and 1450 cm-1. It is unique and complex for every molecule, and is used to identify a specific molecule by chemists.
FG with a peak ranging from 1810-1650 cm-1 is?
C=O, carbonyl group
Aldehydes have a very strong characteristic peak at this specific range:
1740-1720 cm-1
What are the IR peaks for the sp, sp2, and sp3 carbons?
OH group from an alcohol absorbs at what frequency vs. an OH from a carboxylic acid
- Alcohol - broad peak 3300 cm-1
- Carboxylic Acid - very broad peak 3000-2400 cm-1
A sharp peak at 3300-3400 cm-1 indicates
An N-H bond
A general rule in IR spec is that heavier atoms, record lower wavenumbers because they have slower frequencies.
Which bond will have a higher wavenumber:
C-H
C-N
C-H
UV-Spec measures the absorption of ______ causing electronic excitations that distinguish the different bonds.
UV-light
UV-Spec is specifically designed to analyze what kinds of molecules ?
Conjugated systems
-Double & triple bonds
Molecules (heteroatoms) with nonbonding pi electrons
Molecules containing transition metals
True or False:
The more conjugated the compound, the greater the wavelength of maximum absorbance for UV-Spec.
True
What is NMR spectroscopy?
Nuclear magnetic resonance spectroscopy which measures the nuclear spin of atoms, most frequently H protons.
How to read an NMR Spec:
-X-axis measures chemical shift in ppm
-Further to the left = more downfield
-Further to the right = more upfield
-TMS peak at 0 is used as a reference, so don’t count this peak
The more electron-withdrawing groups there are, the more (downfield/upfield) the proton is.
Downfield, since electron density is pulled away from the proton, the proton is deshielded and is downfield.
Resonance structures contribute to (shielding/deshielding) a proton.
Deshielding, because they take electron density away from the proton.
True or false:
Proton NMR is used to determine the relative # of protons and their chemical environments.
True
What does the height/area of a peak in proton NMR tell you?
It tells you the number of identical protons it contains. A higher peak = more # of protons which are identical in chemical shift
What are the chemical shifts for the following functional groups you should know:
Aromatic ring (benzene)
Aldehyde
Carboxylic acid
Ester
Alkyne (sp)
Alkene (sp2)
Alkane/aliphatic protons (sp3)
Aromatic ring (benzene): 6.5-8.5 ppm
Aldehyde: 9-10 ppm
Carboxylic acid: 10.5-12 ppm
Ester: 2-2.5 ppm
Alkyne: 2.0-3.0 ppm
Alkene: 4.0-6.0 ppm
Alkane: 1.0-2.0 ppm
What does splitting of a peak (into a doublet, triplet, etc) tell you?
The peak still represents only 1 proton of reference we are looking at, but it shows how many adjacent protons are around the proton of reference.
In proton NMR, how can one determine the amount of splitting of a single peak?
Look at your proton of reference, count the # of adjacent protons around it and use the (n+1) rule to determine the splitting.
n = # of adjacent protons
If you have a molecule with a plane a symmetry, how do you determine how many unique proton signals there are?
- Draw the plane of symmetry
- Count the H’s on each carbon ONLY on one side of the plane
- Example: in this compound, there are only 3 unique hydrogens due to plane of symmetry
When asked to find the integration of a specific peak, how do you go about this?
Assuming you are given a drawing of the molecule, count the # of protons corresponding that specific chemical environment.
In this example there are 4 H’s for environment A, which means the integration will be 4. If there are multiple chemical environments, there will be multiple peaks on the NMR.
The integration of a peak (the # of identical Hs the peak corresponds to) is equal to the _________ of the peak.
Height
What does multiplicity of a peak refer to?
The amount of spin-spin splitting that occurs for that peak due to adjacent hydrogens.
Alkyl groups contribute to (shielding/deshielding) the proton.
Shielding
Mass spectrometry is used for?
Measuring the mass of molecules and the molecular fragments that make up that molecule.
How do you figure out the total mass of a molecule by looking at its mass spec?
You find the M+ (molecular ion) peak, which is usually the peak furthest to the right, with the highest m/z value.
Mass spec is only able to detect (cations/anion) fragments.
cation fragments
What are the isotope signature mass specs of carbon, bromine, and chlorine?
The isotopic abundance rate corresponds to what aspect of a mass spec?
Corresponds directly to the height of the M+ peak on the mass spec. The cluster of peaks within the molecular ion peak, represent the abundance of each isotope, which the highest peak being the most abundant isotope.
How can you determine the most stable cation/molecular fragment on a mass spec?
Identify the base peak which is the tallest peak, has the highest abundance.
How do you figure out the dilution factor?
What happens is there are two dilutions in sequence?
Final volume/initial volume = factor
If several dilutions happen in sequence, multiply the factors together to get a final dilution
Spectrophotometry
A technique that uses light absorption (UV or visible light) to measure the concentration of an analyte in solution
Beer’s law
A = εbc
A is absorbance of solution
ε is a constant (molar absorptivity of the analyte)
b is the distance light travels through the solution (tube)
c is concentration of solution
True or false:
The higher the concentration in solution, the lower the intensity of light that is transmitted through the solution.
True, concentration and transmittance are inversely related.
How do you calculate transmittance? How do you calculate absorbance?
Transmittance = Intensity of light put out ÷ Intensity of light put in
Absorbance = -log(transmittance)
True or false:
The higher the concentration of solution, the more it absorbs light.
True, absorbance & concentration are directly related.
