Spectroscopy

Question 1

Spectroscopy

Answer 1

measures the energy difference between possible states of a system by looking at absorbed electromagnetic radiation

Question 2

Infrared Spectroscopy

Answer 2

measures molecular vibrations visualized as bond stretching, bending

infrared light is passed through a sample and the absorbance is measured

Question 3

Infrared Light Range

Answer 3

700nm - 1 mm

useful absorptions are from 2500-25000

Question 4

4 Types of Vibration

Answer 4

1) symmetric Bend
2) asymmetric bend
3) Symmetric Stretch
4) Asymmeric stretch

Question 5

Fingerprint Region

Answer 5

1500 - 400 cm^-1 range

specific absorbance pattern is characteristic of each individual molecule

Question 6

For an absorption to be recorded, the ____ must result in a change in the ________.

Therefore, molecules that are _____ or have atoms with _____ do not exhibit absorption/change in dipole moment

Answer 6

vibration

bond dipole moment

symmetrical

identical electronegativities

Question 7

Hydroxyl Group (OH)

Answer 7

broad wide peak at:

3300 for alcohls

3000 for carboxylic acids as the carbonyl pulls some electron density from OH bond shifting absorption to lower wavenumber

Question 8

Carbonyl Group (C=O)

Answer 8

1700 sharp peak

Question 9

Bond between any atom and hydrogen is relatively ____ and as we add more bonds between carbon atoms, the absorption frequency ____

Answer 9

high

increases

Question 10

Amines (N-H)

Answer 10

3100-3500

sharp peak instead of broad

Question 11

Ultraviolet Spectroscopy

Answer 11

obtained by passing ultraviolet light through a sample dissolved in inert, nonabsorbing solvent and absorbance is recorded and plotted against wavelength

absorbance is caused by electronic transitions between orbitals

most important info we get is wavelength of maximum absorbance which tells us extent of conjugation within conjugated systems.

Question 12

UV Q: The more _____ the compound, the ____ the energy of the transition and the _____ the wavelength of maximum absorbance.

Answer 12

conjugated

lower

higher

Question 13

UV spectroscopy works because molecules with ________ or ______ can be excited by ultaviolet light to ________ antibonding orbitals.

Answer 13

pi electrons

nonbonding electrons

higher energy

Question 14

HOMO

Answer 14

highest occupied molecular orbital

Question 15

LUMO

Answer 15

lowest unoccupied molecular orbital

Question 16

Molecules with a lower energy gap between ____ and _____ are more easily excited and can absorb longer wavelengths/_____ frequencies with lower energy.

Answer 16

HOMO

LUMO

lower

Question 17

Conjugated Molecules

Answer 17

unhybridized p orbitals

can be excited by ultraviolet light and shifts absorption spectrum to higher wavelengths and lower frequencies

larger conjugated molecules can lead to color by absorbing light to visible range

Question 18

Nuclear Magnetic Resonance (NMR)

Answer 18

based off the fact that certain atomic nuclei have magnetic moments oriented at random, in magnetic field the moments alight agaist or with the applied field

Question 19

Alpha State

Answer 19

nuceli with magnetic moments aligned with the field (said to be in lower energy state)

Question 20

Beta State

Answer 20

nuceli being irradiated with radiofrequency that match the energy gap between two states and excited lower energy nuclei into B state (higher energy)

Question 21

Magnetic Resonance Imaging (MRI)

Answer 21

noninvasive diagnostic tool that uses proton NMR

multiple cross sectional scans of body are taken and various chemical shiftts of absorbing protons are translated into shades of grey

dark area tends to water, light area is fatty tissue

comparison with normal MRI then allows to detect abnormalities in scanned region

Question 22

NMR spectrum

Answer 22

frequency vs absorption

chemical shift (delta) in ppm is the frequency

Question 23

The chemical shift in NMR is plotted on the _______ and it increases toward the left which is refered to as ______

Answer 23

x axis

downstream

Question 24

Tetramethylsilane (TMS)

Answer 24

a calibration standard to mark 0 ppm and does not count as a peak to see how downfield compounds are

Question 25

Proton NMR

Answer 25

hydrogen nuceli come into resonance at 0-10 ppm downfield from TMS

each distinct nuclei gives rise to a seperate peak

Question 26

If multiple protons are _____, having the same magnetic environment, they will lead to the same peak.

Answer 26

chemically equivalent

Question 27

Deshielding vs Shielding

Answer 27

more deshielding the more downfield you go as there are more electronegative atoms pulling electron density away from the proton

more shielding the more upfield you go as there are electron donating groups that shield the proton

Question 28

Integration

Answer 28

area under the peaks

ratio of protons that produced each peak (1:3 hence a could be 1 proton and b could be 3 protons)

Question 29

Spin-Spin Coupling/Splitting

Answer 29

two nuceli in nonmagnetically identical enviornments are adjacent to each other and impact the splitting

Question 30

DOublet

Answer 30

two peaks of identical intensity equally spaced around a chemical shift, both nuceli will have doublets where their shifts should be

Question 31

n + 1 rule

Answer 31

used to determine number of peaks present (doublet, triplet, etc.)

if a proton has n protons that are three bonds away, it will be split into n+ 1 peaks (no not include protons attached to oxygen or nitrogen)

Question 32

J

Answer 32

coupling constant which is the magnitude of splitting measured in hertz

Question 33

Multiplet

Answer 33

peaks that have more than four shifts

Question 34

H-NMR: Aldehyde

Answer 34

9-10 ppm

Question 35

H-NMR: Carboxylic Acids

Answer 35

10.5-12 ppm

Question 36

H-NMR: Aromatic Ring

Answer 36

6-8.5 ppm

Question 37

H-NMR: sp3 Carbons/alkyl groups

Answer 37

0-3 ppm (higher if electron withdrawing groups are around)

Question 38

H-NMR: sp Carbons/alkyne groups

Answer 38

2-3 ppm

Question 39

H-NMR: sp2 Carbons/alkenes

Answer 39

4.6-6.0 ppm