Spectroscopy

Question 1

What does infrared spectroscopy measure? How does it work?

Answer 1

• molecular vibrations that can be seen as bond stretching, bending, or combinations of different vibrational modes
• infrared light is passed through a sample, and the absorbance is measured

Question 2

When light of wavelengths 4000 to 400 is absorbed, what are the 4 types of vibrations that can occur?

Answer 2

1. symmetric bend
2. asymmetric bend
3. symmetric stretch
4. asymmetric stretch

Question 3

What is one reason that a molecule won’t show up in IR spectra?

Answer 3

if it is symmetrical (because there is no net dipole movement)

Question 4

What is the IR peak for O-H alcohol and carboxylic acid bond, N-H bond, and C=O bond?

Answer 4

• O-H alcohol: broad 3300cm^-1
• O-H carboxylic acid: broad 3000 cm^-1
• N-H: sharp 3300 cm^-1
• C=O: sharp 1750 cm^-1

Question 5

How does conjugation effect UV spectroscopy?

Answer 5

the more conjugated the compound, the lower the energy of the transition state and the greater the wavelength of maximum absorbance

Question 6

What does UV light do to conjugated molecules?

Answer 6

• it can excite conjugated molecules causing a shift in the absorption spectrum resulting in higher maximum wavelengths

Question 7

What is nuclear magnetic resonance (NMR) based on?

Answer 7

based on the fact that certain atomic nuclei have magnetic moments that are oriented at random

Question 8

Nuclei with magnetic moments that are aligned with the field are said to be in the ___-___________ (lower energy). These nuclei can be irradiated with radiofrequency pulses that will excite some lower-energy nuclei into the ____-________ (higher energy)

Answer 8

α-state; β-state

Question 9

What is magnetic resonance imaging (MRI)? How does it work?

Answer 9

• a noninvasive diagnostic tool that uses proton NRM
• multiple cross-sectional scans of the patient are taken and various chemical shifts are translated into shades of grey
• dark areas = water and light areas = fattier tissue

Question 10

What is chemical shift? Which direction does it increase in on a graph?

Answer 10

• measured in ppm; the difference between the absorption frequency of proton in TMS and proton in the sample; x-axis
• increases towards the left (referred to as downfield)

Question 11

Where downfield are most hydrogen nuclei found?

Answer 11

1 to 100 ppm downfield; each distinct set of nuclei gives rise to a separate peak

Question 12

What does the height of a peak indicate? What is integration?

Answer 12

• the height of each peak is proportional to the number of protons it contains
• integration is the area under the peaks; corresponds to the ration of protons produced by each peak

Question 13

What do electron-withdrawing groups and electron-donating groups do to the reading for NRM?

Answer 13

• electron-withdrawing: deshield (pull electron density away from) and move further downfield (left)
• electron-donating: shield (pull electron density towards) and move further upfield (right)

Question 14

What is the n+1 rule?

Answer 14

• if a proton has n protons that are 3 bonds away, it will be split into n+1 peaks (do not include protons attached to O or N)

Question 15

When is a multiplet?

Answer 15

peaks that have more than 4 shift are referred to as a multiplet

Question 16

What is the ppm of alkyl groups, alkynes, alkenes, aromatic H, aldehydes, and carboxylic acids?

Answer 16

• alkyl groups: 0 to 3 ppm
• alkynes: 2 to 3 ppm
• alkenes: 4.6 to 6 ppm
• aromatic hydrogens: 6 to 8.5 ppm
• aldehydes: 9 to 10 ppm
• carboxylic acids: 10.5 to 12 ppm