11.3 Nuclear Magnetic Resonance Spectroscopy

Question 1

what type of nuclei have magnetic moments

Answer 1

those with odd mass numbers, odd atomic numbers, or both

(aka odd protons or odd neutrons)

Question 2

nuclear magnetic resonance spectroscopy

Answer 2

based on the fact that certain atomic nuclei have magnetic moments that are oriented at random

when such nuclei are placed in a magnetic field, their magnetic moments tend to align with or against the direction of the applied field

Question 3

alpha state

Answer 3

the lower energy state of nuclei with magnetic moments that are aligned with the field

Question 4

beta state

Answer 4

the higher energy state of nuclei once irradiated with uses that match the energy gap between the 2 states

Question 5

magnetic resonance imaging

Answer 5

a noninvasive diagnostic tool that uses proton NMR

multiple cross sectional scans of the patients body are taken, and the various chemical shifts of absorbing protons are translated into specific shades of grey

produces a picture that shows the relative density of specific types of protons

Question 6

NMR spectra are generally plotted as…

Answer 6

frequency vs absorption of energy

Question 7

standardized NMR spectrum plot (as seen on MCAT)

Answer 7

chemical shift plotted on x-axis

Question 8

chemical shift units

Answer 8

parts per million (ppm) of spectrophotometer frequency

Question 9

H-NMR

Answer 9

NMR used to study H nuclei (protons)

the only type of NMR tested by the MCAT

Question 10

alpha spin state

Answer 10

when the magnetic field of the protons align against the external magnetic field applied

lower energy than beta spin state

Question 11

beta spin state

Answer 11

when the magnetic field of the protons align against the external magnetic field applied

Question 12

NMR spectroscopy

Answer 12

measures the alignment of nuclear spin with an applied magnetic field, which depends on the magnetic environment of the nucleus itself

used for determining the structure/connectivity of a compound, including functional groups

Question 13

how are NMR spectra calibrated?

Answer 13

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

TMS is used because it has one of the “most upfield” positions due to the electron-DONATING properties of the S atom; making it VERY shielded

Question 14

higher chemical shifts are located _____ (downfield/upfield)

Answer 14

downfield

Question 15

lower chemical shifts are located _____ (downfield/upfield)

Answer 15

upfield

Question 16

each unique group of protons…

Answer 16

has its own peak

Question 17

integration

Answer 17

the area under the curves/peaks

proportional to the number of protons contained under the peak

Question 18

deshielding

Answer 18

when electron-withdrawing groups pull electron density away from the nucleus, allowing it to be more easily affected by magnetic field

moves a peak further downfield

Question 19

n+1 rule: how many peaks will Ha have

Answer 19

Ha is connected to one other hydrogen that is 3 bonds away

so it will have 1+1 = 2 peaks

Question 20

n+1 rule: how many peaks will Ha have

Answer 20

Hb is connected to two other hydrogens that are 3 bonds away

so it will have 2+1 = 3 peaks

Question 21

n+1 rule

Answer 21

if a proton has n protons that are 3 bonds away, it will be split into n+1 peaks

(BUT do NOT include protons attached to N or O)

Question 22

spin-spin coupling (splitting)

Answer 22

occurs when 2 protons close to one another have an effect on the other’s magnetic environment

results in the splitting of peaks into doublets, triplets, and multiplets, depending on the environment

Question 23

general ppm of aldehydic hydrogens

Answer 23

between 9 and 10 ppm

Question 24

general ppm of carboxylic acid hydrogens

Answer 24

between 10.5 and 12 ppm

Question 25

general ppm of aromatic hydrogens

Answer 25

between 6.0 and 8.5 ppm

Question 26

spin-spin coupling (splitting)

Question 27

general ppm range for sp3 hybridized carbons (alkanes/ alkyl groups)

Answer 27

0.0 to 3.0 ppm

(higher if electron withdrawing groups are present)

Question 28

general ppm range for sp2 hybridized carbons (alkenes)

Answer 28

4.6 to 6.0

Question 29

general ppm range for sp hybridized carbons (alkynes)

Answer 29

2.0 to 3.0 ppm

Question 30

hack for test day: counting the number of peaks and unique hydrogens may get you the correct answer (???)

Question 31

the height of each peak is proportional to…

Answer 31

the number of protons (H atoms) it contains

Question 32

each peak or group of peaks (a multiplet) represents ______

Answer 32

a single group of equivalent protons

Question 33

the relative area under each peak reflects:

Answer 33

the ration of the protons producing each peak

Question 34

the position of the peak (upfield vs downfield) reflects ______

Answer 34

the chemical environment of the protons

downfield = surrounded by electronegative atoms

Question 35

the splitting of a peak represents:

Answer 35

the number of adjacent hydrogens

a peak is split into n+1 peaks, where n is the number of adjacent hydrogens