Ch. 11: Spectroscopy

Question 1

Infrared (IR) Spectroscopy

Answer 1

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

Question 2

Plotting IR spectra

Answer 2

Percent transmittance vs wavenumber (1/λ). The normal range is 4000 to 400 cm-1

Question 3

Fingerprint region of IR spectra plot

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

How does a bond appear on an IR spectrum

Answer 4

Vibration of a bond must change the bond dipole moment. Certain bonds have characteristic absorption frequencies, which allow us to infer the presence (or absence) of particular functional groups.

Question 5

O-H peak

Answer 5

Broad peak around 3300 cm-1. Molecules w O-H include alcohols, water, and carboxylic acids; the carboxylic acid O-H peak will be shifted around 3000 cm-1

Question 6

N-H peak

Answer 6

Sharp peak around 3300 cm-1. Molecules w N-H include some amines, imines, and amides

Question 7

C=O peak

Answer 7

Sharp peak around 1750 cm-1. Molecules w C=O include aldehydes, ketones, carboxylic acids, amides, esters, and anhydrides

Question 8

Ultraviolet Spectroscopy

Answer 8

Measures absorption of ultraviolet light, which causes movement of electrons between molecular orbitals

Question 9

UV spectra plotting

Answer 9

Percent transmission or absorbance vs wavelength

Question 10

How does a molecule appear on a UV spectrum?

Answer 10

To appear on a UV spectrum, a molecule must have a small enough energy diff between its highest occupied molecular orbital and its lowest unoccupied molecular orbital to permit an electron to move from one orbital to the other. Smaller the diff between the HOMO and LUMO the longer the wavelengths a molecule can absorb

Question 11

Conjugation

Answer 11

Occurs in molecules w unhybridized p orbitals. Conjugation shifts the absorption spectrum to higher max wavelengths (lower frequencies)

Question 12

Nuclear Magnetic Resonance Spectroscopy

Answer 12

Measure the alignment of nuclear spin w an applied magnetic field, which depends on the magnetic environment of the nucleus itself. Useful for determinging the structure (connectivity) of a compoud, including functional groups. 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 13

MRI

Answer 13

Magnetic resonance imaging Medical app of NMR spectroscopy

Question 14

NMR spectra plotting

Answer 14

Frequency vs absorption of energy. They are standardized by using chemical shift, measured in parts per million (ppm) of a spectrophotometer frequency

Question 15

NMR spectra calibration

Answer 15

Tetramethylsilane (TMS) is used to calibrate NMR spectra. It has a chemical shift of 0 ppm

Question 16

Chemical shifts

Answer 16

Higher chemical shifts are located to the left (downfield); lower chemical shifts are located to the right (upfield)

Question 17

Most common NMR

Answer 17

Proton (1H) NMR

Question 18

Each unique group of protons…

Answer 18

Has its own peak

Question 19

Integration

Answer 19

The integration of this peak (area under the curve) is proportional to the number of protons contained under the peak

Question 20

Deshielding

Answer 20

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

Question 21

Spin-Spin Coupling (splitting)

Answer 21

When hydrogens are on adjacent atoms, they interfere w each other’s magnetic environment, causing spin-spin coupling. A proton’s (or group of protons’) peak is split into n+1 subpeaks, where n is the number of protons that are 3 bonds away from the proton of interest

Question 22

Splitting patterns

Answer 22

Include doublets, triplets, and multiplets

Question 23

Where do protons on diff hybridized carbons fall?

Answer 23

Sp3: 0-3 ppm
Sp2: 4.6-6.0 ppm
Sp: 2.0-3.0 ppm

Question 24

Where do protons on diff compounds’ hydrogens fall?

Answer 24

Aldehydic hydrogens: 9-10 ppm
Carboxylic acid hydrogens: 10.5-12 ppm
Aromatic hydrogens: 6.0-8.5 ppm