Electronic Vocab 5 Flashcards
Relatively small scale (0 – 100 cm-1) energy transitions corresponding to wavelengths in the microwave region of the spectrum; only shown by molecules possessing a permanent electrical dipole moment; pure rotational spectra are observed for gas phase molecules where it is possible to distinguish transitions between rotational quantum energy levels.
rotational energy transitions
Medium scale (100 – 3000 cm-1) energy transitions corresponding to wavelengths in the infrared region of the spectrum; shown by molecules in which a change in the permanent dipole moment occurs during the vibrational motion; also seen in Raman spectra where a ∂Q
deformation of the overall polarizability of the molecule occurs during the vibrational motion
vibrational energy transitions
Large scale (10,000 – 50,000 cm-1) energy transitions corresponding to wavelengths in the UV/Vis spectral regions; seen in all molecules since changes in electron distribution are always accompanied by a dipole change.
electronic energy transitions
Since the energies of electronic transitions are so large, vibrational and rotational transitions are also excited by electronic energy transitions; therefore, for molecules in the gas phase, vibrational transitions appear as “coarse structure” and rotational transitions appear as “fine structure” on top of electronic spectra
rotational-vibrational fine structure
Defined as the region of the electromagnetic spectrum less than 200 nm; corresponds to electronic energy transitions greater than 50,000 cm-1.
far or vacuum ultra-violet spectral region
Defined as the region of the electromagnetic spectrum between 200 – 400 nm; corresponds to electronic energy transitions between 50,000 – 25,000 cm-1.
near ultra-violet spectral region
Defined as the region of the near ultra-violet between 315 – 400 nm.
UV-A spectral region
Defined as the region of the near ultra-violet between 280 – 315 nm.
UV-B spectral region
Defined as the region of the near ultra-violet between 200 – 280 nm
UV-C spectral region
Defined as the region of the electromagnetic spectrum between 400 – 780 nm; corresponds to electronic energy transitions between 25,000 – 12,821 cm-1.
visible spectral region
The molecular orbital that acts as an electron donor, since it is the outermost, i.e., highest energy, frontier orbital containing an electron.
HOMO
The molecular orbital that acts as an electron acceptor, since it is the innermost, i.e., lowest energy, frontier orbital that has room to accept electrons.
LUMO
These electrons form single covalent bonds between atoms, e.g. C – C, C – H, O – H, etc.; the strongest type of covalent bonds due to the direct overlap of orbitals; electrons are the most firmly bound to nuclei and require the most energy to undergo electronic transitions.
σ - Electrons
These electrons form multiple covalent chemical bonds, e.g. C = C, C = N, etc.; result from overlap of atomic orbitals that are in contact through two areas of overlap; are more diffuse than sigma bonds
π - electrons
These electrons are non-bonding electrons that can populate non-bonding molecular orbitals; occur in atoms to the right of C in the periodic table, e.g. N, O, and halogens.
n - electrons
group of atoms, with their associated electrons, in a molecule that produces an electronic absorption.
chromophore
Substituent groups attached to the basic chromophore structure that change the position and/or intensity of the chromophore’s absorption band; typical examples include methyl, hydroxyl, alkoxyl, halogen, and amino groups
auxochromophore
A shift in the absorption maximum to longer wavelength or lower energy, i.e. a red shift
bathochromic shift
A shift in the absorption maximum to shorter wavelength or higher energy, i.e. a blue shift
hypsochromic shift
An increase in band intensity
hyperchromic shift
A decrease in band intensity.
hypochromic shift
a transition in which a bonding s electron is excited to an anti bonding sigma orbital; not generally analytically useful since their energies fall outside the normal UV-Vis spectral range
σ - σ* transition
Insertion of a group containing n-electrons; common for O, N, S, and halogen containing chromophores
n - σ* transition
Promotion of an electron from a non-bonding molecular orbital to an anti-bonding orbital; common for heteroatom-containing molecules, e.g. O, N.
n - π* transition
Promotion of one electron from a p bonding molecular orbital to a π* antibonding orbital; common in unsaturated molecules, e.g. C = C
π - π* transition
a first approximation, the electronic, vibrational, and rotational energies of a molecule may be considered to be completely independent of one another
Born-Oppenheimer Approximation
An electronic transition in a molecule takes place much more rapidly than does the vibrational motion of the nuclei in a bond vibration. Therefore, the internuclear distance in the vibrating molecule can be regarded as fixed, i.e. it does not change, during an electronic transition. Alternate definition: An electronic transition is more probable if it begins in the middle of the v”=0 level and terminates toward either end of an excited v’ vibrational level.
Frank-Condon Principle
An excited state de-excitation process that results in the emission of radiation at a longer wavelength than that of the initial absorption; the initial excitation is caused by photons.
Photoluminescence
An excited state de-excitation process that results in the emission of radiation at a longer wavelength than that of the initial excitation; the initial excitation is caused by high-energy particles.
radioluminescence
An excited state de-excitation process that results in the emission of radiation at a longer wavelength than that of the initial excitation; the initial excitation is caused by a chemical process.
chemiluminescence
The quantification of the amount of unpaired electron spin; equivalent to the total spin angular momentum
multiplicity
The case for uncharged organic molecules in which all electrons are paired; in this case S, the total spin angular momentum quantum number is 0
singlet state
The case for organic molecules in which an excited state electron has reversed its spin; in this case S, the total spin angular momentum quantum number is 1
triplet state
singlet multiplicity defined as
2S + 1 = 2 x 0 + 1 = 1
triplet state multiplicity defined as
2S + 1 = 2 x 1 + 1 = 3
An energy diagram that illustrates the electronic states of a molecule and the transitions between them; the states are arranged vertically by increasing energy and grouped horizontally by spin multiplicity.
Jablonski Diagram
A non-radiative means of dissipating excess vibrational energy in an excited electronic state; the excess energy is usually released as heat following collision with solvent molecules
vibrational relaxation
A non-radiative process by which an excited state molecule passes from its original excited state to a lower energy excited state of the same multiplicity
internal conversion
The radiative emission of energy from the first excited singlet state to the ground singlet state; i.e. the process of photon emission for de-excitation from S1 to S0 ; due to Boltzmann factors, the emission primarily occurs from the v’=0 vibrational level of S1; the time scale is 10-9 s.
fluorescence
The electronic transition between the lowest vibrational levels of the ground and excited electronic states that has the same energy in both absorption and fluorescence; i.e. it is the transition between the v”=0 level in S0 and the v’=0 level in S1.
“0 - 0” transition
A non-radiative de-excitation process that involves energy transfer between the excited state molecule and the solvent or other solute molecule.
external conversion
A non radiative process in which the molecule in an excited electronic state changes its spin multiplicity from a singlet state to a triplet state that is lower in energy than the initial singlet state.
intersystem crossing
The radiative emission of energy from the first excited triplet state to the ground singlet state; i.e. the process of photon emission for de-excitation from T1 ® S0 ; due to Boltzmann factors, the emission primarily occurs from the v’=0 vibrational level of T1; the time scale is much longer than that of fluorescence due to the long-lived, “spin-forbidden” T1 ® S0 transition, it can be on the order of 10-3 – 10 s
phosphorescence
Any non-radiative de-excitation process that depopulates the S1 or T1 excited states and competes with fluorescence or phosphorescence;
quenching
The difference in wavelength or energy between the position of the band maximum of the absorption band and the maximum of the fluorescence emission spectra; it is a measure of the change in geometry of the equilibrium configurations of the ground and excited states.
stokes loss or stokes shift
The fraction of the total number of photons absorbed that result in fluorescence emission; can be considered a conversion factor between the rate of photon absorption and the rate of fluorescence emission.
fluorescence quantum yield
The fluorescence spectrum that is observed as a function of scanning the excitation wavelength, with the emission wavelength held constant.
excitation spectrum
The fluorescence spectrum that is observed as a function of scanning the emission wavelength, with the excitation wavelength held constant.
emission spectrum
Fluorescence spectrum is observed as a three-dimensional representation or contour plot; shows the fluorescence signal as a function of both excitation and emission; also known as a total luminescence spectrum.
excitation-emission matrix
The fluorescence spectrum that is observed by simultaneously scanning both the excitation and emission monochromators with a small wavelength difference between them.
synchronous spectrum
i.e., excited state dimers; formed by the interaction of an excited singlet state with a ground state of an identical molecule; concentration increases as the monomer concentration increases; emission is red-shifted relative to monomer emission.
excimer
i.e. excited state complexes; formed by the interaction of an excited singlet state with the ground state of a non-identical molecule
exciplex
Quenching of fluorescence due to dynamic solution diffusion of quencher and collision with excited state fluorophore, thereby decreasing its excited state lifetime; examples include O2, H2O2, I2, NO, BrO4-, etc.
dynamic or collisional quenching
Quenching of fluorescence due to formation of a ground state non-fluorescent complex between fluorophore and quencher; when this complex absorbs light, it returns to ground state without emission of fluorescence; examples include stacking interactions in purine and pyrimidine nucleotides
static quenching
Quantitative linear relationship describing dynamic collisional quenching
Stern-Volmer equation
