Ch 11 - Spectroscopy

Question 1

What does infrared (IR) spectroscopy measure?

Answer 1

absorption of infrared light, which causes molecular vibration (stretching, bending, twisting, and folding)

• these vibrations cause changes in the dipole moment of the molecule that can be measured
• once the bonds in a molecule are determined, one can infer the presence of a number of functional groups to determine the identity of the molecule
• most useful for distinguishing between different functional groups (double and triple bonds)

Question 2

How are IR spectra generally plotted? What is the normal range of a spectrum?

Answer 2

• as percent transmittance vs wavenumber (1/wavelength)

- 4000 to 400 cm-1

Question 3

What is the fingerprint region?

Answer 3

between 1500 and 400 cm-1

- contains a number of peaks that can be used by experts to identify a compound

Question 4

What is necessary of for a bond to appear on an IR spectrum?

Answer 4

vibration of a bond must change the bond dipole moment

Question 5

Which bonds have characteristic absorption frequencies that allow us to infer the presence of their functional group?

Answer 5

• O-H peak is a broad peak around 3300 cm-1 (alcohols, water, and carboxylic acids; though the carboxylic acid O-H peak will be shifted around 3000 cm-1)
• N-H peak is a sharp peak around 3300 cm-1 (some amines, imines, and amides)
• C=O peak is a sharp peak around 1750 cm-1 (aldehydes, ketones, carboxylic acids, amides, esters, and anhydrides)

Question 6

What does UV spectroscopy measure?

Answer 6

absorption of UV light, which causes movement of electrons between molecular orbitals
- most useful for studying compounds with double bonds or heteratoms with lone pairs that create conjugated systems

Question 7

How are UV spectra generally plotted?

Answer 7

percent transmittance or absorbance vs wavelength

Question 8

What is required of the molecule to appear on UV spectrum?

Answer 8

• a molecule must have a small enough energy difference between its highest occupied molecular orbital (HOMO) and its lowest unoccupied molecular orbital (LUMO) to permit an electron to move from one orbital to the other

Question 9

How does distance between HOMO and LUMO relate to wavelength?

Answer 9

• the smaller the distance between HOMO and LUMO, the longer the wavelengths a molecule can absorb

Question 10

When does conjugation occur and what does it do?

Answer 10

• occurs in molecules with unhybridized p orbitals

- shifts the absorption spectrum to higher maximum wavelengths (lower frequencies)

Question 11

What does NMR spectroscopy measure and why is it useful?

Answer 11

• measures alignment of nuclear spin with an applied magnetic field, which depends on the magnetic environment of the nucleus
• useful for determining the structure (connectivity) of a compound, including functional groups

Question 12

What is magnetic resonance imaging (MRI)?

Answer 12

medical application of NMR spectroscopy

Question 13

How does NMR spectroscopy work?

Answer 13

• nuclei may be in the lower energy alpha-state or higher energy beta-state
• radiofrequency pulses push the nucleus from the alpha-state to the beta-state and these frequencies can be measured

Question 14

How is NMR spectra generally plotted?

Answer 14

• frequency vs absorption of energy

- standardized by using chemical shift, measured in parts per million (ppm) of spectrophotometer frequency

Question 15

How are NMR spectra calibrated?

Answer 15

using tetramethylsilane (TMS) which has a chemical shift of 0 ppm

Question 16

Where are chemical shifts located based on their levels?

Answer 16

higher chemical shifts are to the left (downfield)

lower chemical shifts are to the right (upfield)

Question 17

What is the integration of proton NMR and what is it proportional to?

Answer 17

• area under the curve

- proportional to the number of protons contained under the peak

Question 18

When does deshielding of protons occur?

Answer 18

• when electron withdrawing groups pull electron density away from the nucleus, allowing it to be more easily affected by the magnetic field
• moves a peak further downfield

Question 19

What is spin-spin coupling?

Answer 19

• due to influence on the magnetic environment of one proton by protons on the adjacent atom
• a proton’s (or group of protons’) peak is split n+1 subpeaks, where n is the number of protons that are 3 bonds away from the proton of interest

Question 20

What range are sp3, sp2 , and sp hybridized carbons in protons NMR?

Answer 20

• sp3 usually in 0-3 ppm (but higher if electron withdrawing groups are present)
• sp2 usually 4.6-6
• sp usually 2-3

Question 21

What range are aldehydic, carboxylic acid, and aromatic hydrogens on proton NMR?

Answer 21

• aldehydic: 9-10 ppm
• carboxylic acid: 10.5-12 ppm
• aromatic: 6-8.5 ppm

Question 22

In NMR spectroscopy, what does each peak, relative area, and position of peak tell us?

Answer 22

• each peak or group of peaks that are part of a multiplet represents a single group of equivalent protons
• the relative area of each peak reflects the ration of the protons producing each peak
• the position of the peak (upfield or downfield) is due to shielding or deshielding effects and reflects the chemical environment of the protons

Question 23

What is proton NMR good for?

Answer 23

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

Question 24

What are the most common NMR shifts: alkyl, alkynes, alkenes, aromatics, aldehydes, carboxylic acids?

Answer 24

alkyl 0-3 ppm
alkyne 2-3
alkene 4.6-6
aromatic 6-8.5
aldehydes 9-10
carboxylic acids 10.5-12

Question 25

Which isotopes have nonzero magnetic moment which are not useful for NMR?

Answer 25

nuclei with odd mass numbers or those with even mass number but an odd atomic number

Question 26

What is the coupling constant, J?

Answer 26

a measure of the degree of splitting caused by other atoms in the molecule

Question 27

Where are electrons found before absorbing an ultraviolet photon?

Answer 27

• the HOMO only (highest occupied molecular orbital)

- only after absorbing UV light is an electron excited from the HOMO to the LUMO (lowest unoccupied molecular orbital)

Question 28

In an IR spectrum, how does extended conjugation of double bonds affect the absorbance band of carbonyl (C=O) stretches compared with normal absoprtion?

Answer 28

• the absorbance band will occur at a lower wavenumber because delocalization of pi electrons causes the C=O bond to lose double bond character, shifting the stretching frequency closer to C-O stretches
• high order bonds tend to have higher absorption frequencies, so loss of double bond character should decrease the absorption frequency of the group

Question 29

How do 2 enantiomers compare in the IR spectra?

Answer 29

• they have identical IR spectra because they have the same functional groups