Spectroscopy

Question 1

integration of peaks in HNMR indicate

Answer 1

how many hydrogen atoms are present of each different type of hydrogen

Question 2

number of peaks are associated with:

Answer 2

number of different types of hydrogen or carbon

Question 3

NMR only works for:

Answer 3

atoms with either an odd number of protons or neutrons and thus have a property called “spin”

Question 4

an external magnetic field causes:

Answer 4

nuclear magnetic moments to align parallel and antiparallel to the applied field (more are aligned parallel)

Question 5

without an external magnetic field:

Answer 5

the distribution of nuclear spins is random and thus the net magnitude of nuclear magnetic moments is zero

Question 6

what happens to the energy gap between parallel and antiparallel spin as applied energy field increases?

Answer 6

the energy gap increases

Question 7

in order for the photon to cause a “spin flip”, the energy of the photon must:

Answer 7

match the energy gap

Question 8

each type of hydrogen in the molecule absorbs at a different frequency due to:

Answer 8

its molecular environment, thus provides a different peak

Question 9

general process of how NMR works

Answer 9

application of a radio wave to flip nucleus from lower to higher energy state (spin flip), upon returning to the lower energy state, the nucleus emits the energy as a tiny pulse of radio frequency electromagnetic radiation which is what is detected

Question 10

changes in the distribution of electrons around a nucleus affect:

Answer 10

local magnetic field that the nucleus experiences, frequency at which the nucleus resonates, the chemistry of the molecule at that atom

Question 11

chemical shift

Answer 11

variation in frequency, provides information about the chemical environment experienced by the nucleus

Question 12

direction of induced magnetic field is:

Answer 12

opposite to that of the applied field (this causes shielding so that a stronger external field is needed for energy different between spin states to match r.f. radiation)

Question 13

chemical shift is a measure of:

Answer 13

the degree to which a molecule is shielded

Question 14

associate downfield with:

Answer 14

decreased shielding, deshielding, higher ppm, less electrons around

Question 15

associate upfield with:

Answer 15

increased shielding, more electrons around atom

Question 16

what are two principle factors that affect the chemical shift of H:

Answer 16

shielding (due to electronegativity factors) and magnetic anisotropy (due to pi systems)

Question 17

examples of how shielding influences chemical shift:

Answer 17

inductive effects (polarization of a sigma bond due to differences in electronegativity)
orbital hybridization (if H is bonded to sp or sp^2, greater s character means electrons are held closer to other nucleus so more deshielded)
hydrogen bonding (the H involved is acidic, easily exchanged, totally deshielded, the more acidic the more deshielded)

Question 18

magnetic anisotropy is caused by:

Answer 18

electrons in pi systems interacting with the applied field to create their own field (i.e. a non-uniform magnetic field results)

Question 19

number of sets of peaks (HNMR)

Answer 19

how many types of hydrogen are present

Question 20

integration of peaks (HNMR)

Answer 20

how many are hydrogen atoms are there of each type

Question 21

chemical shift (HNMR)

Answer 21

what types of hydrogen are present

Question 22

coupling patterns (HNMR)

Answer 22

how are the pieces connected together

Question 23

coupling patterns are caused by:

Answer 23

influence of magnetic field of a proton from the magnetic field of neighbouring protons (use the n+1 rule)

Question 24

coupling constant (J)

Answer 24

the distance or separation between lines/peaks caused by the strength of interaction of the proton (for a pair, the J value must be the same)

Question 25

what protons do not have coupling patterns?

Answer 25

an example is -OH because it is an exchangeable proton, very fast transmission that is not detected by NMR

Question 26

equivalent protons

Answer 26

protons in very similar chemical environments or are the same type and interact with each other that have no coupling observed and appears as a multiplet (can also be caused by accidental equivalence)

Question 27

complex coupling

Answer 27

occurs when a proton has neighbours of different types and there are different coupling constants

Question 28

what is more sensitive? CNMR or HNMR?

Answer 28

HNMR

Question 29

why is C coupling not observed?

Answer 29

because of the low natural abundance of carbon-13

Question 30

a CNMR does not have:

Answer 30

integration and coupling patterns