11 Spectroscopy

Question 1

infrared (IR) spectroscopy

Answer 1

measures molecular vibrations
bond stretching, bending, combinations of different vibrational modes

determine what bonds exist within molecule (infer functional groups)

Question 2

IR wavenumber

Answer 2

4000 to 400 cm^-1

Question 3

IR fingerprint region

Answer 3

1500 to 400 cm^-1

more complex vibrational patterns

specific absorbance patterns characteristic of each individual molecule

Question 4

IR vibration must result in change of bond dipole moment

molecules that don’t result in change in dipole?

Answer 4

atoms of same electronegativity

symmetrical molecules

Question 5

IR hydroxyl (O-H) absorption peak

Answer 5

3300 cm^-1 (alcohols)
3000 cm^-1 (carboxylic acid)

broad (wide) peak

Question 6

IR carbonyl (C=O) absorption peak

Answer 6

1700 cm^-1

sharp (deep) peak

Question 7

IR N-H absorption peak

Answer 7

3300 cm^-1

sharp (deep) peak

Question 8

ultraviolet (UV) spectroscopy

Answer 8

absorbance = electronic transitions between orbitals

wavelength of max absorbance extent of conjugation within conjugated systems

increased conjugation decreases energy of transition increases wavelength of maximum absorbance

Question 9

nuclear magnetic resonance (NMR) spectroscopy

MCAT = 1H (proton) NMR

Answer 9

certain atomic nuclei have magnetic moments that are oriented at random

when placed in magnetic field, moments align with/against field

Question 10

NMR alpha state

Answer 10

magnetic moments align with field (lower energy)

Question 11

NMR beta state

Answer 11

irradiation excited some lower-energy nuclei to higher state (higher energy)

Question 12

TMS

Answer 12

NMR calibration standard (reference peak)

Marks 0 ppm

Skip when counting peaks

Question 13

proton (1H) NMR graph

Answer 13

0 to 10 ppm downfield from TMS

each distinct set of nuclei give rise to a separate peak
(if multiple protons are chemically equivalent, they yield same peak)

height of each peak is proportional to # of protons it contains
(integration)

Question 14

NMR: deshielding

Answer 14

EWG’s pull from surrounding atoms (deshield downfield)

proton cannot shield itself from applied magnetic field (reading further downfield)

*everything is more deshielded than TMS

Question 15

NMR: shielding

Answer 15

EDG’s supply electron density

shield proton (reading further upfield)

Question 16

NMR: spin-spin coupling (splitting)

Answer 16

2 protons on adjacent atoms (close proximity) that are not magnetically identical
(within 3 bonds of each other)

Question 17

NMR: n+1 rule

Answer 17

if proton has “n” protons that are 3 bonds away, it will be split into “n+1” peaks; equally spaced; identical intensity

DO NOT INCLUDE protons attached to O or N

Question 18

NMR: coupling constant (J)

Answer 18

magnitude of splitting

Question 19

NMR: peaks that have more than 4 shifts

Answer 19

multiplet

Question 20

NMR: sp3-hybridized carbons (H) chemical shift range

Answer 20

0-3 ppm

Question 21

NMR: sp2-hybridized carbons (H) chemical shift range

Answer 21

4.6-6.0 ppm

Question 22

NMR: sp-hybridized carbons (H) chemical shift range

Answer 22

2.0-5.0 ppm

Question 23

NMR: aldehyde hydrogens chemical shift range

Answer 23

9-10 ppm

Question 24

NMR: carboxylic acid hydrogens chemical shift range

Answer 24

10.5-12 ppm

Question 25

NMR: aromatic hydrogens chemical shift range

Answer 25

6.0-8.5 ppm