Spectroscopy & Analytical Methods

Question 1

True or false:
The perceived color of a substance is complementary to the color of the wavelength that was actually absorbed by the substance.

Answer 1

True

Question 2

A substance which appears green, absorbs what color and wavelength range?

Answer 2

Absorbs red light ranging from 650-740 nm.

Question 3

The difference between the HOMO and the LUMO, is inversely proportional to (energy/wavelength).

Answer 3

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).

Question 4

How does conjugation affect wavelength properties of a photon?

Answer 4

Conjugation makes a molecule more stable, so there is a lower energy. Lower energy means you have a longer wavelength.

Question 5

What causes a solution to change color?

Answer 5

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).

Question 6

What does IR spectroscopy specifically measure?

Answer 6

The frequencies of electromagnetic radiation absorbed by molecules, the different molecular vibrations

Question 7

What are the different vibrations IR spec can detect?

Answer 7

Bond stretching, bond bending, twisting & folding

Question 8

What can you detect with IR Spec?

Answer 8

The presence of specific functional groups and connectivity of bonds (backbone) of a molecule

Question 9

Define the fingerprint region

Answer 9

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.

Question 10

FG with a peak ranging from 1810-1650 cm-1 is?

Answer 10

C=O, carbonyl group

Question 11

Aldehydes have a very strong characteristic peak at this specific range:

Answer 11

1740-1720 cm-1

Question 12

What are the IR peaks for the sp, sp2, and sp3 carbons?

Answer 12

Question 13

OH group from an alcohol absorbs at what frequency vs. an OH from a carboxylic acid

Answer 13

1. Alcohol - broad peak 3300 cm-1
2. Carboxylic Acid - very broad peak 3000-2400 cm-1

Question 14

A sharp peak at 3300-3400 cm-1 indicates

Answer 14

An N-H bond

Question 15

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

Answer 15

C-H

Question 16

UV-Spec measures the absorption of ______ causing electronic excitations that distinguish the different bonds.

Answer 16

UV-light

Question 17

UV-Spec is specifically designed to analyze what kinds of molecules ?

Answer 17

Conjugated systems
-Double & triple bonds
Molecules (heteroatoms) with nonbonding pi electrons
Molecules containing transition metals

Question 18

True or False:
The more conjugated the compound, the greater the wavelength of maximum absorbance for UV-Spec.

Answer 18

True

Question 19

What is NMR spectroscopy?

Answer 19

Nuclear magnetic resonance spectroscopy which measures the nuclear spin of atoms, most frequently H protons.

Question 20

How to read an NMR Spec:

Answer 20

-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

Question 21

The more electron-withdrawing groups there are, the more (downfield/upfield) the proton is.

Answer 21

Downfield, since electron density is pulled away from the proton, the proton is deshielded and is downfield.

Question 22

Resonance structures contribute to (shielding/deshielding) a proton.

Answer 22

Deshielding, because they take electron density away from the proton.

Question 23

True or false:
Proton NMR is used to determine the relative # of protons and their chemical environments.

Answer 23

True

Question 24

What does the height/area of a peak in proton NMR tell you?

Answer 24

It tells you the number of identical protons it contains. A higher peak = more # of protons which are identical in chemical shift

Question 25

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)

Answer 25

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

Question 26

What does splitting of a peak (into a doublet, triplet, etc) tell you?

Answer 26

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.

Question 27

In proton NMR, how can one determine the amount of splitting of a single peak?

Answer 27

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

Question 28

If you have a molecule with a plane a symmetry, how do you determine how many unique proton signals there are?

Answer 28

1. Draw the plane of symmetry 2. Count the H's on each carbon ONLY on one side of the plane 3. Example: in this compound, there are only 3 unique hydrogens due to plane of symmetry

Question 29

When asked to find the integration of a specific peak, how do you go about this?

Answer 29

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.

Question 30

The integration of a peak (the # of identical Hs the peak corresponds to) is equal to the _________ of the peak.

Answer 30

Height

Question 31

What does multiplicity of a peak refer to?

Answer 31

The amount of spin-spin splitting that occurs for that peak due to adjacent hydrogens.

Question 32

Alkyl groups contribute to (shielding/deshielding) the proton.

Answer 32

Shielding

Question 33

Mass spectrometry is used for?

Answer 33

Measuring the mass of molecules and the molecular fragments that make up that molecule.

Question 34

How do you figure out the total mass of a molecule by looking at its mass spec?

Answer 34

You find the M+ (molecular ion) peak, which is usually the peak furthest to the right, with the highest m/z value.

Question 35

Mass spec is only able to detect (cations/anion) fragments.

Answer 35

cation fragments

Question 36

What are the isotope signature mass specs of carbon, bromine, and chlorine?

Answer 36

Question 37

The isotopic abundance rate corresponds to what aspect of a mass spec?

Answer 37

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.

Question 38

How can you determine the most stable cation/molecular fragment on a mass spec?

Answer 38

Identify the base peak which is the tallest peak, has the highest abundance.

Question 39

How do you figure out the dilution factor? What happens is there are two dilutions in sequence?

Answer 39

Final volume/initial volume = factor If several dilutions happen in sequence, multiply the factors together to get a final dilution

Question 40

Spectrophotometry

Answer 40

A technique that uses light absorption (UV or visible light) to measure the concentration of an analyte in solution

Question 41

Beer's law

Answer 41

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

Question 42

True or false: The higher the concentration in solution, the lower the intensity of light that is transmitted through the solution.

Answer 42

True, concentration and transmittance are inversely related.

Question 43

How do you calculate transmittance? How do you calculate absorbance?

Answer 43

Transmittance = Intensity of light put out ÷ Intensity of light put in Absorbance = -log(transmittance)

Question 44

True or false: The higher the concentration of solution, the more it absorbs light.

Answer 44

True, absorbance & concentration are directly related.