Spectroscopy **

Question 1

IR spectroscopy

Answer 1

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

Question 2

IR spectra are generally plotted as

Answer 2

percent transmittance versus wavenumber . (1/lambda). Te normal range of spectrum is 4000 to 400 cm^-1. Then fingerprint region is between 1500 and 400cm^-1.

Question 3

To appear on an IR spectrum,

Answer 3

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 4

IR O-H peak is

Answer 4

a broad peak around 3300. Molecules with OH include alcohols, water, and carboxylic acids; the carboxylic acids OH peak will be sifted around 3000.

Question 5

N-H IR peak is

Answer 5

sharp peak around 3300. Molecules with NH includes amines, imines, and amides

Question 6

C=O peak is

Answer 6

sharp peak around 1750. This includes aldehydes, ketones, carboxylic acids, amides, esters, and anhydrides.

Question 7

UV spectroscopy measures

Answer 7

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

Question 8

UV spectra are

Answer 8

generally plotted as percent transmittance or absorbance vs wavelength.

Question 9

To appear in UV spectrum,

Answer 9

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 10

The smaller the difference between HOMO and LUMO,

Answer 10

the longer the wavelengths a molecule can absorb.

Question 11

Conjugation occurs in

Answer 11

occurs in molecules with unhybridized p-orbitals. Conjugation shifts the absorption spectrum to higher max wavelength (lower frequencies)

Question 12

Nuclear magnetic resonance (NMR) spectroscopy

Answer 12

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

Question 13

Nuclei may be in the

Answer 13

lower energy alpha state or higher energy beta state; radio frequency pulses push the nucleus from the alpha state to the beta state, and these frequencies can be measured.

Question 14

Magnetic resonance imaging is a

Answer 14

medical application of NMR spectroscopy.

Question 15

NMR spectra are calibrated using

Answer 15

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

Question 16

higher shifts are located

Answer 16

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

Question 17

Proton (H) NMR

Answer 17

most common. Each unque gropus of proton has its own peak.

Question 18

Desheilding

Answer 18

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

Question 19

integration

Answer 19

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

Question 20

Spin to spin coupling (splitting)

Answer 20

when hydrogens are on adjacent atoms, they interfere with each other’s magnetic environment. A proton/protons peak is split into n + 1 subpeaks, where n is the number of protons that are three bonds away from the proton of interest.

Question 21

Splitting patterns

Answer 21

doublets, triplets, and multiplets

Question 22

protons on sp3 hybridized carbons are usually in the

Answer 22

0 to 3 ppm range (but higher if electron if electron withdrawing groups are present). protons on sp2 hybridized carbons are usually in the 4.6 to 6.0 ppm range. Protons on sp-hybridized carbons are usually 2 to 3 ppm.

Question 23

Aldehydic hydrogens tend to

Answer 23

appear between 9 and 10 ppm.

Question 24

Carboxylic acids hydrogens tend to appear

Answer 24

between 10.5 and 12 ppm

Question 25

Aromatic hydrogens tend to appear

Answer 25

between 6 and 8.5 ppm.