Week 3 Textbook Reading

Question 1

spin

Answer 1

Magnetic resonance in organic analyses
All atomic nuclei possess a quantum mechanical property called spin
The number of spin states that a nucleus has is determined by its atomic number and mass
Nuclei with even atomic numbers and masses have spins of 0

Question 2

magnetic moment

Answer 2

Quantum spin produces a magnetic moment
The magnetic moments of nuclei in an external magnetic field (B0) align either parallel to the external field (a, lower energy) or antiparallel to it (B, higher energy)

Question 3

The magnitude of delta E depends on the …

Answer 3

strength of the applied magnetic field, with a larger delta E resulting from a stronger magnetic field

Question 4

resonance frequency

Answer 4

When the nuclei in the a state are exposed to EM radiation of a frequency corresponding to delta E, some of the nuclei are excited from the a state to the higher energy B state, while others transition downward from B to a
The frequency at which this spin-flip occurs is called the resonance frequency

Question 5

rapid resonance frequency

Answer 5

A rapid resonance frequency EM pulse applied to a sample equalizes the populations of α and β states

Question 6

The equal number of hydrogens in the α and β states produces…

Answer 6

a detectable magnetic field that contains a frequency component for each hydrogen type

Question 7

after excitation…

Answer 7

After excitation, this magnetic field weakens as the system relaxes back to equilibrium, a process called free induction decay (FID)

Question 8

during relaxation…

Answer 8

During the relaxation stage, signal detection provides information about the magnetization experienced by each hydrogen type

Question 9

nuclear magnetic resonance

Answer 9

The different chemical environments in a molecule cause variations in the magnetic field around the nuclei in the molecule

These variations cause the nuclei to absorb energy at different frequencies, measured using nuclear magnetic resonance (NMR) spectroscopy

Question 10

signals/resonances

Answer 10

The first information to notice in an NMR spectrum is the number of different signals, or resonances
Each signal in a spectrum is produced by a hydrogen or group of hydrogens in a different chemical environment, which affects the energy gap between their a and B states
A signal may consist of a single peak or a collection of peaks grouped closely together in a symmetric pattern

Question 11

Chemically equivalent hydrogens …

Answer 11

are those in chemically identical environments and so have identical chemical shifts
Such H atoms are interchangeable by rotation about a single bond or by a plane of symmetry in a molecule

Question 12

intensity of each NMR signal

Answer 12

The intensity of each NMR signal corresponds to the area under the entire signal

Question 13

ratio of each NMR signal

Answer 13

The ratio of the areas under the signals indicates the relative amounts of each type of hydrogen atom producing the signal

NMR spectrometers integrate the signals to display the relative intensities

Question 14

how to determine a relative intensity from an integration line

Answer 14

measure the vertical distance from the point where the graph line starts to curve up to the point where it levels out again

Question 15

infrared spectroscopy

Answer 15

The determination of molecular structure is simplified when the types of functional groups in a molecule are known
The most direct way to determine functional groups is by using infrared (IR) spectroscopy

Question 16

Photons have an energy that depends on the …

Answer 16

frequency(v) or the wavelength of that oscillation (E=hv= hc/v)

This eqn shows that the energy of a photon is proportional to 1/wavelength

-This qty is expressed in the unit of ^-1 called a wavenumber

Question 17

what causes a change in bond vibrations

Answer 17

The bonds in a molecule absorb photons of light from the infrared region of the spectrum, which causes a change in the bond vibrations
Because the bonds in different functional groups vibrate at different characteristic energies, the wavenumber of the absorbed light reveals which functional groups are present in a molecule

Question 18

Measurement of bond vibrations by infrared spectroscopy

Answer 18

The bonds in organic molecules are constantly vibrating around a certain equilibrium distance (bond length) between them

Molecular vibrations described as stretching or bending motions

Bending modes of vibrations increase (and decrease) the angles between atoms relative to the equilibrium bond angles in the molecule

Question 19

Molecules that have a greater number of atoms undergo ____ stretching and bending vibrations than molecules with _____ atoms

Answer 19

more; fewer

Question 20

The number of these vibrations can be calculated using…

Answer 20

the formula 3n-6, where n is the number of atoms in the molecule

Question 21

Most vibrating bonds produce …

Answer 21

a change in bond dipoles

Question 22

When IR light passes through a sample of a compound, photons with frequencies that match those of the molecular vibrations are …

Answer 22

absorbed

Question 23

The photons that are not absorbed reach a detector and are converted to a …

Answer 23

spectrum

Question 24

Vibrations (whether stretching or bending) that do not change the molecular dipole ….

Answer 24

do not absorb IR light and do not appear in an IR spectrum

Question 25

what is the easiest way to identify functional groups in an organic molecule

Answer 25

The easiest way to identify functional groups in an organic molecule is by examining the absorbances(absorption peaks) in an IR spectrum Because each functional group has characteristic bonds, functional groups have characteristic bonds, functional groups have characteristic vibrations that can be distinguished in an IR spectrum Like a mass spectrum, an IR spectrum has many peaks

Question 26

4 key regions of the IR spectrum

Answer 26

1. hydrogen region (3600–2700 cm^–1) 2. triple bonds (2700–1900 cm^–1) 3. double bonds (1900–1500 cm^–1) 4. fingerprint region (1500–500 cm^–1)

Question 27

hydrogen region

Answer 27

The hydrogen region of an IR spectrum, 3600–2700 cm–1, gives information about the “types” of hydrogens, such as C–H bonds, O–H bonds, and N–H bonds Bonds involving hydrogen vibrate at the largest frequencies (highest wavenumbers) due to the relative lightness of hydrogen atoms -Because the location of some of these peaks overlap, both the location and the shape of the various peaks are important Note that primary amines and amides have two distinct vibrational modes (symmetric and antisymmetric stretching) for NH2 groups, appearing as double N–H peaks -In contrast, secondary amines and amides produce only one N–H peak

Question 28

triple bond region

Answer 28

The intensity of an IR signal depends on how much a particular dipole vibrates in response to the absorbed IR energy Specifically, a more intense absorption of IR radiation is associated with a greater change in the affected dipole -This is most noticeable in the absorption region of double and triple bonds The stretching of the terminal alkyne, such as what occurs in 1-hexyne, induces a greater change in the dipole, so terminal alkynes produce a stronger signal than internal alkynes such as 2-hexyne

Question 29

double bond region

Answer 29

Because C=O bonds are more polar than C=C bonds, they produce more intense C=O stretch peaks between 1850 and 1650 cm–1 -C=C bonds appear as weak signals between 1700 and 1600 cm–1 The C=O peaks are particularly important because carbonyl groups are present in several common functional groups and because the different types of carbonyl groups can often be differentiated based on their slightly different absorbance wavelengths -This range of frequencies correlates with the strength of the double bond and the relative contributions of resonance forms in each groups

Question 30

fingerprint region

Answer 30

This region contains a large number of peaks, and many of them provide info that can be easily obtained from other parts of the spectrum Three functional groups have key absorbances that are observed in the fingerprint region: ester, ether, and nitro groups Most often, the fingerprint region is too complex for direct identification of a compound but is very useful for comparing the IR spectrum of an unknown compound to a library of known IR spectra to positively identify its structure

Question 31

mass spectrometry

Answer 31

Mass spectrometry can provide a molecular mass, which can be used to calculate a molecular formula