Chapter 4 - Experimental techniques

Question 1

Analyte

Answer 1

The component being analyzed in a system.

Question 2

Recrystallization

Answer 2

A purification step involving the dissolution and crystallization of a solid from a solvent or solvent mixture.

Question 3

Atomic absorption spectroscopy (AAS)

Answer 3

The quantitative determination of a metal carried out by observing the diagnostic absorption spectrum of gaseous atoms of the metal.

Question 4

Thermogravimetric analysis (TGA)

Answer 4

In TGA, the change in mass of a sample is monitored as the sample is heated.

Question 5

Solvate

Answer 5

A compound formed by the interaction of a solute with a solvent.

Question 6

Fast atom bombardment (FAB)

Answer 6

Ions are produced by bombarding the sample (neutral molecules or ionic salts) with high energy xenon or argon atoms. It is a soft technique.

Question 7

Matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF)

Answer 7

A UV laser is used to generate atoms, M, of the sample, and [M+H]+ or [M+Na]+ ions are typically detected. It is a soft technique.

Question 8

IR and Raman spectroscopies are concerned with

Answer 8

transitions between vibrational energy levels.

Question 9

For a vibrational mode to be IR active, it must

Answer 9

give rise to a change in dipole moment.

Question 10

For a vibrational mode to be Raman active, it must

Answer 10

give rise to a change in polarizability.

Question 11

Zero point energy

Answer 11

corresponds to the energy of a molecule’s lowest vibrational level (vibrational ground state).

Question 12

Simple harmonic oscillator equation

Answer 12

Ev = (v + 1/2)hv (the last v is nu, frequency)
first v = energy level

Question 13

The vibrational frequency of the bond XY depends on

Answer 13

the masses of X and Y, and the force constant, k.

Question 14

Wavenumber equation for IR spectra

Answer 14

wavenumber = 1/(2pi(c)) sqrt(k/u)
c = speed of light
k = force constant
u = reduced mass (mu)

Question 15

IR region

Answer 15

Far IR = 20 cm-1
Near IR = 14000 cm-1
Mid IR = 400-4000 cm-1

Question 16

Rayleigh scattering

Answer 16

When radiation of a frequency, v0, falls on a vibrating molecule, most of the radiation is scattered without a change in frequency.

Question 17

Electronic spectroscopy

Answer 17

is concerned with transitions of electrons between energy levels and covers both absorption and emission spectroscopies.

Question 18

Electronic spectra arise from

Answer 18

transitions of electrons between energy levels.

Question 19

Transitions from lower to higher energy levels produce

Answer 19

absorption spectra.

Question 20

Transitions from higher to lower energy levels produce

Answer 20

emission spectra.

Question 21

Franck-Condon approximation

Answer 21

states that electron transitions are very much faster than nuclear motion; thus, an electronic transition occurs without any significant change in the positions of the nuclei in a molecule giving rise to a so-called ‘vertical transition.’

Question 22

A UV-VIS spectrophotometer typically records absorption spectra in the ________ range.

Answer 22

200-800 nm

Question 23

Transmittance (T)

Answer 23

the ratio of the intensity of the transmitted radiation (I) to that of the incident radiation (I0).

Question 24

Absorbance equation

Answer 24

A = -logT = -log(I/I0)

Question 25

E (epsilon)

Answer 25

molar extinction (or absorption) coefficient

Question 26

Bathochromic effect (red shift)

Answer 26

a shift in an absorption towards the red end of the spectrum (longer wavelengths, lower energy).

Question 27

Hypsochromic effect (blue shift)

Answer 27

a shift in an absorption towards the blue end of the spectrum (shorter wavelength, higher energy).

Question 28

Luminescence

Answer 28

The spontaneous emission of radiation from an electronically excited species. Covers both fluorescence and phosphorescence.

Question 29

Nuclear magnetic resonance spectroscopy.

Answer 29

A resonance technique involving absorption of radiofrequency energy.

Question 30

3 main criteria for suitable nuclei for NMR

Answer 30

1) The nucleus possesses a value of the nuclear spin quantum number I greater than or equal to 1/2. 2) The nucleus should occur in significant abundance (advantageous, but not essential) 3) The nucleus possess a relatively short spin-relaxation time (T1).

Question 31

More positive (less negative) chemical shifts

Answer 31

= higher frequency = downfield (older term)

Question 32

Less positive (more negative) chemical shifts

Answer 32

= lower frequency = upfield (older term)

Question 33

Standard reference for 1H and 13 C NMR spectroscopies.

Answer 33

tetramethylsilane = SiMe4 (TMS)

Question 34

Samples for solution NMR spectroscopy are generally prepared using

Answer 34

deuterated solvents.

Question 35

The number of spins of attached nuclei determine the

Answer 35

multiplicity (number of lines) and patter of the NMR spectroscopic signal of the observed nucleus.

Question 36

Multiplicity equation

Answer 36

Multiplicity (number of lines) = 2nI + 1 the nucleus being observed is coupled to n equivalent nuclei with quantum number I.

Question 37

Redistribution reaction

Answer 37

one in which substituents exchange between species but the types and numbers of each type of bond remain the same.

Question 38

Electron paramagnetic resonance (EPR) spectroscopy

Answer 38

is a resonance technique involving microwave-induced transitions between magnetic energy levels of electrons which possess a net spin and orbital angular momentum.

Question 39

Zeeman electronic effect

Answer 39

Under an applied magnetic field, B0, the interaction between an unpaired electron and the magnetic field results in a splitting of the energy levels.

Question 40

X-band frequency

Answer 40

9-10 GHz

Question 41

L-band frequency

Answer 41

1-2 GHz

Question 42

S-band frequency

Answer 42

2-4 GHz

Question 43

Q-band frequency

Answer 43

35 GHz

Question 44

W-band frequency

Answer 44

95 GHz

Question 45

Gamma energy range for Mossbauer spectroscopy

Answer 45

10-100 keV

Question 46

The Mossbauer effect is the

Answer 46

emission and resonant absorption of nuclear gamma rays studied under conditions such that the nuclei have negligible recoil velocities when gamma ray are emitted or absorbed.

Question 47

The Mossbauer effect is only achieved by working with

Answer 47

solid samples in which the nuclei are held rigidly in a crystal lattice.

Question 48

Most abundant isotope used in Mossbauer spectroscopy

Answer 48

Iron - 57

Question 49

Isomer shift

Answer 49

The velocity of movement required to bring about maximum absorption relative to alpha-iron (defined as arbitrary zero for Iron - 57).

Question 50

In X-ray diffraction (XRD),

Answer 50

X-rays are diffracted by electrons surrounding the nuclei in atoms in a crystalline or polycrystalline solid.

Question 51

Monochromatic radiation

Answer 51

X-rays of a single wavelength

Question 52

Scattering power depends on

Answer 52

the number of electrons in an atom.

Question 53

Bragg's equation

Answer 53

2dsin(theta) = n(lambda)

Question 54

Analysis by single crystal X-ray diffraction leads to

Answer 54

the full determination of the structure of a compound.

Question 55

Ideal crystal dimensions for single crystal X-ray diffraction

Answer 55

0.1 to 0.3 mm

Question 56

Unit cell

Answer 56

The smallest repeating unit in a crystal lattice.

Question 57

1 Angstrom = _____ m

Answer 57

10^-10

Question 58

Polymorph

Answer 58

Different phases of the same chemical compound with different crystal structures.

Question 59

Powder X-ray diffraction data are routinely used for

Answer 59

identifying a bulk sample of a material, and for screening different phases of a compound.

Question 60

In neutron diffraction,

Answer 60

Bragg scattering of neutrons occurs when neutrons interact with the nuclei of atoms in a single crystal.

Question 61

Gas phase electron diffraction

Answer 61

Electrons are scattered by the electric fields of atomic nuclei in gas phase molecules; intramolecular bond parameters are determined.

Question 62

Photoelectron spectroscopy (PES)

Answer 62

is a technique used to study the energies of occupied atomic or molecular orbitals.