Chapter 4 - Experimental techniques Flashcards
Analyte
The component being analyzed in a system.
Recrystallization
A purification step involving the dissolution and crystallization of a solid from a solvent or solvent mixture.
Atomic absorption spectroscopy (AAS)
The quantitative determination of a metal carried out by observing the diagnostic absorption spectrum of gaseous atoms of the metal.
Thermogravimetric analysis (TGA)
In TGA, the change in mass of a sample is monitored as the sample is heated.
Solvate
A compound formed by the interaction of a solute with a solvent.
Fast atom bombardment (FAB)
Ions are produced by bombarding the sample (neutral molecules or ionic salts) with high energy xenon or argon atoms. It is a soft technique.
Matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF)
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.
IR and Raman spectroscopies are concerned with
transitions between vibrational energy levels.
For a vibrational mode to be IR active, it must
give rise to a change in dipole moment.
For a vibrational mode to be Raman active, it must
give rise to a change in polarizability.
Zero point energy
corresponds to the energy of a molecule’s lowest vibrational level (vibrational ground state).
Simple harmonic oscillator equation
Ev = (v + 1/2)hv (the last v is nu, frequency)
first v = energy level
The vibrational frequency of the bond XY depends on
the masses of X and Y, and the force constant, k.
Wavenumber equation for IR spectra
wavenumber = 1/(2pi(c)) sqrt(k/u) c = speed of light k = force constant u = reduced mass (mu)
IR region
Far IR = 20 cm-1
Near IR = 14000 cm-1
Mid IR = 400-4000 cm-1
Rayleigh scattering
When radiation of a frequency, v0, falls on a vibrating molecule, most of the radiation is scattered without a change in frequency.
Electronic spectroscopy
is concerned with transitions of electrons between energy levels and covers both absorption and emission spectroscopies.
Electronic spectra arise from
transitions of electrons between energy levels.
Transitions from lower to higher energy levels produce
absorption spectra.
Transitions from higher to lower energy levels produce
emission spectra.
Franck-Condon approximation
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.’
A UV-VIS spectrophotometer typically records absorption spectra in the ________ range.
200-800 nm
Transmittance (T)
the ratio of the intensity of the transmitted radiation (I) to that of the incident radiation (I0).
Absorbance equation
A = -logT = -log(I/I0)
E (epsilon)
molar extinction (or absorption) coefficient
Bathochromic effect (red shift)
a shift in an absorption towards the red end of the spectrum (longer wavelengths, lower energy).
Hypsochromic effect (blue shift)
a shift in an absorption towards the blue end of the spectrum (shorter wavelength, higher energy).
Luminescence
The spontaneous emission of radiation from an electronically excited species. Covers both fluorescence and phosphorescence.
Nuclear magnetic resonance spectroscopy.
A resonance technique involving absorption of radiofrequency energy.
3 main criteria for suitable nuclei for NMR
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).
More positive (less negative) chemical shifts
= higher frequency = downfield (older term)
Less positive (more negative) chemical shifts
= lower frequency = upfield (older term)
Standard reference for 1H and 13 C NMR spectroscopies.
tetramethylsilane = SiMe4 (TMS)
Samples for solution NMR spectroscopy are generally prepared using
deuterated solvents.
The number of spins of attached nuclei determine the
multiplicity (number of lines) and patter of the NMR spectroscopic signal of the observed nucleus.
Multiplicity equation
Multiplicity (number of lines) = 2nI + 1
the nucleus being observed is coupled to n equivalent nuclei with quantum number I.
Redistribution reaction
one in which substituents exchange between species but the types and numbers of each type of bond remain the same.
Electron paramagnetic resonance (EPR) spectroscopy
is a resonance technique involving microwave-induced transitions between magnetic energy levels of electrons which possess a net spin and orbital angular momentum.
Zeeman electronic effect
Under an applied magnetic field, B0, the interaction between an unpaired electron and the magnetic field results in a splitting of the energy levels.
X-band frequency
9-10 GHz
L-band frequency
1-2 GHz
S-band frequency
2-4 GHz
Q-band frequency
35 GHz
W-band frequency
95 GHz
Gamma energy range for Mossbauer spectroscopy
10-100 keV
The Mossbauer effect is the
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.
The Mossbauer effect is only achieved by working with
solid samples in which the nuclei are held rigidly in a crystal lattice.
Most abundant isotope used in Mossbauer spectroscopy
Iron - 57
Isomer shift
The velocity of movement required to bring about maximum absorption relative to alpha-iron (defined as arbitrary zero for Iron - 57).
In X-ray diffraction (XRD),
X-rays are diffracted by electrons surrounding the nuclei in atoms in a crystalline or polycrystalline solid.
Monochromatic radiation
X-rays of a single wavelength
Scattering power depends on
the number of electrons in an atom.
Bragg’s equation
2dsin(theta) = n(lambda)
Analysis by single crystal X-ray diffraction leads to
the full determination of the structure of a compound.
Ideal crystal dimensions for single crystal X-ray diffraction
0.1 to 0.3 mm
Unit cell
The smallest repeating unit in a crystal lattice.
1 Angstrom = _____ m
10^-10
Polymorph
Different phases of the same chemical compound with different crystal structures.
Powder X-ray diffraction data are routinely used for
identifying a bulk sample of a material, and for screening different phases of a compound.
In neutron diffraction,
Bragg scattering of neutrons occurs when neutrons interact with the nuclei of atoms in a single crystal.
Gas phase electron diffraction
Electrons are scattered by the electric fields of atomic nuclei in gas phase molecules; intramolecular bond parameters are determined.
Photoelectron spectroscopy (PES)
is a technique used to study the energies of occupied atomic or molecular orbitals.
