Chapter 11- Spectroscopy

Question 1

spectroscopy

Answer 1

measures the energy differences between the possible states of a molecular system by determining the frequencies of electromagnetic radiation absorbed by the molecules.

Question 2

what do MRI scanners measure

Answer 2

they measure 1H-NMR spectra of water molecules in different environments in the body

Question 3

Infrared (IR) spectroscopy

Answer 3

measures absorption of IR light by specific bonds, which vibrate. these vibrations cause changes in the dipole moment of the molecule that can be measured. Once the bonds are determined, one can infer the presence of a number of functional groups to determine the identity of the molecule.

Question 4

IR light range

Answer 4

wavelength = 700nm to 1mm (useful absorptions for spectroscopy occur at wavelengths 2500 to 25000nm or 4000 to 400cm-1)

Question 5

fingerprint region

Answer 5

more complex vibration patterns, caused by the motion of the molecule as a whole and can be seen in the 1500 to 400cm-1 range.

Question 6

4 types of vibrations that can occur when molecules are excited

Answer 6

1. symmetric bend
2. asymmetric bend
3. symmetric stretch- do not show up in IR spectra because they involve no net change in dipole movement
4. asymmetric stretch

Question 7

broad (wide) peak at 3300cm-1

Answer 7

hydroxyl group (OH)

Question 8

sharp (deep) peak at 1700cm-1

Answer 8

carbonyl

Question 9

sharp (deep) peak at 3300cm-1

Answer 9

N-H bonds

Question 10

name the three peaks you need to memorize on the IR spectrum and where they are

Answer 10

hydroxyl (O - H) - wide at 3300
carbonyl (C – O) - narrow at 1700
amine (N - H) - narrow at 3300

Question 11

trend with bonds and absorption frequency

Answer 11

• any atom will have a higher absorption frequency if H is bonded to it
• as we add more bonds between carbon atoms the absorption frequency will increase

Question 12

IR spectrum

Answer 12

percent transmittance is plotted vs. frequency. the maximum absorptions appear at the bottom of the valleys on the spectrum

Question 13

what is UV spectroscopy used for

Answer 13

most useful for studying compounds containing double bonds and/or heteroatoms with lone pairs that create conjugated systems.

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

Question 14

conjugated molecules orbitals

Answer 14

unhybridized p-orbitals

Question 15

NMR spectroscopy

Answer 15

based on the fact that certain atomic nuclei have magnetic moments that are oriented at random. when a nuclei is placed in a magnetic field it will allign either with or against the applied field.

aligned with field: a-state (lower energy), but can be excited to B-state (higher energy)

-only need to know proton NMR

Question 16

Magnetic resonance imaging (MRI)

Answer 16

noninvasive diagnostic tool that uses proton NMR. different areas pop up based on density, less dense- lighter, more dense- darker

Question 17

axes on typical NMR spectrum

Answer 17

frequency vs. absorption of energy

frequency (x-axis): chemical shift (units- parts per million)

Question 18

tetramethylsilane (TMS)

Answer 18

-reference peak. the signal for its 1H atoms is assigned 0

this allows us to know how far to the left (downfield) we are on the spectrum. skip TMS peak when counting peaks.

Question 19

how proton NMR (1H-NMR) works

Answer 19

• each distinct set of nuclei give rise to a separate peak. so 2 protons that are chemically equivalent (same magnetic environment) will lead to the same peak
• relative area of each peak reflects the ratio of the protons producing each peak
• position of the peak (upfield or downfield) is due to shielding (smaller ppm, more to the right) or deshielding (larger ppm, more to the left), which reflects the chemical environment of the protons : as we add electronegative atoms this will deshield the protons and cause them to be further to the left!

Question 20

spin-spin coupling

Answer 20

when you have two protons that are in such close proximity to each other but are not magnetically identical

-know n+1 rule for proton of interest to determine number of peaks

Question 21

proton NMR (1H-NMR) is good for…?

Answer 21

• determining relative number of protons and their relative chemical environments
• showing how many adjacent protons there are by splitting patterns
• inferring certain functional groups

Question 22

chemical shift ranges for different types of protons (1H-NMR)

Answer 22

0-3: alkyl groups
2-3: alkynes 
4.6-6: alkenes 
6-8.5: aromatic ring hydrogens 
9-10: aldehyde
10.5-12: carboxylic acid

Question 23

what cannot be used for NMR

Answer 23

atoms/molecules with no magnetic movement.

-nuclei with even mass numbers

Question 24

before absorbing a UV photon, electrons can be found

Answer 24

HOMO (highest occupied molecular orbital)