Electronic Vocab 5

Question 1

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.

Answer 1

rotational energy transitions

Question 2

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

Answer 2

vibrational energy transitions

Question 3

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.

Answer 3

electronic energy transitions

Question 4

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

Answer 4

rotational-vibrational fine structure

Question 5

Defined as the region of the electromagnetic spectrum less than 200 nm; corresponds to electronic energy transitions greater than 50,000 cm-1.

Answer 5

far or vacuum ultra-violet spectral region

Question 6

Defined as the region of the electromagnetic spectrum between 200 – 400 nm; corresponds to electronic energy transitions between 50,000 – 25,000 cm-1.

Answer 6

near ultra-violet spectral region

Question 7

Defined as the region of the near ultra-violet between 315 – 400 nm.

Answer 7

UV-A spectral region

Question 8

Defined as the region of the near ultra-violet between 280 – 315 nm.

Answer 8

UV-B spectral region

Question 9

Defined as the region of the near ultra-violet between 200 – 280 nm

Answer 9

UV-C spectral region

Question 10

Defined as the region of the electromagnetic spectrum between 400 – 780 nm; corresponds to electronic energy transitions between 25,000 – 12,821 cm-1.

Answer 10

visible spectral region

Question 11

The molecular orbital that acts as an electron donor, since it is the outermost, i.e., highest energy, frontier orbital containing an electron.

Answer 11

HOMO

Question 12

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.

Answer 12

LUMO

Question 13

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.

Answer 13

σ - Electrons

Question 14

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

Answer 14

π - electrons

Question 15

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.

Answer 15

n - electrons

Question 16

group of atoms, with their associated electrons, in a molecule that produces an electronic absorption.

Answer 16

chromophore

Question 17

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

Answer 17

auxochromophore

Question 18

A shift in the absorption maximum to longer wavelength or lower energy, i.e. a red shift

Answer 18

bathochromic shift

Question 19

A shift in the absorption maximum to shorter wavelength or higher energy, i.e. a blue shift

Answer 19

hypsochromic shift

Question 20

An increase in band intensity

Answer 20

hyperchromic shift

Question 21

A decrease in band intensity.

Answer 21

hypochromic shift

Question 22

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

Answer 22

σ - σ* transition

Question 23

Insertion of a group containing n-electrons; common for O, N, S, and halogen containing chromophores

Answer 23

n - σ* transition

Question 24

Promotion of an electron from a non-bonding molecular orbital to an anti-bonding orbital; common for heteroatom-containing molecules, e.g. O, N.

Answer 24

n - π* transition

Question 25

Promotion of one electron from a p bonding molecular orbital to a π* antibonding orbital; common in unsaturated molecules, e.g. C = C

Answer 25

π - π* transition

Question 26

a first approximation, the electronic, vibrational, and rotational energies of a molecule may be considered to be completely independent of one another

Answer 26

Born-Oppenheimer Approximation

Question 27

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.

Answer 27

Frank-Condon Principle

Question 28

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.

Answer 28

Photoluminescence

Question 29

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.

Answer 29

radioluminescence

Question 30

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.

Answer 30

chemiluminescence

Question 31

The quantification of the amount of unpaired electron spin; equivalent to the total spin angular momentum

Answer 31

multiplicity

Question 32

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

Answer 32

singlet state

Question 33

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

Answer 33

triplet state

Question 34

singlet multiplicity defined as

Answer 34

2S + 1 = 2 x 0 + 1 = 1

Question 35

triplet state multiplicity defined as

Answer 35

2S + 1 = 2 x 1 + 1 = 3

Question 36

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.

Answer 36

Jablonski Diagram

Question 37

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

Answer 37

vibrational relaxation

Question 38

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

Answer 38

internal conversion

Question 39

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.

Answer 39

fluorescence

Question 40

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.

Answer 40

“0 - 0” transition

Question 41

A non-radiative de-excitation process that involves energy transfer between the excited state molecule and the solvent or other solute molecule.

Answer 41

external conversion

Question 42

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.

Answer 42

intersystem crossing

Question 43

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

Answer 43

phosphorescence

Question 44

Any non-radiative de-excitation process that depopulates the S1 or T1 excited states and competes with fluorescence or phosphorescence;

Answer 44

quenching

Question 45

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.

Answer 45

stokes loss or stokes shift

Question 46

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.

Answer 46

fluorescence quantum yield

Question 47

The fluorescence spectrum that is observed as a function of scanning the excitation wavelength, with the emission wavelength held constant.

Answer 47

excitation spectrum

Question 48

The fluorescence spectrum that is observed as a function of scanning the emission wavelength, with the excitation wavelength held constant.

Answer 48

emission spectrum

Question 49

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.

Answer 49

excitation-emission matrix

Question 50

The fluorescence spectrum that is observed by simultaneously scanning both the excitation and emission monochromators with a small wavelength difference between them.

Answer 50

synchronous spectrum

Question 51

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.

Answer 51

excimer

Question 52

i.e. excited state complexes; formed by the interaction of an excited singlet state with the ground state of a non-identical molecule

Answer 52

exciplex

Question 53

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.

Answer 53

dynamic or collisional quenching

Question 54

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

Answer 54

static quenching

Question 55

Quantitative linear relationship describing dynamic collisional quenching

Answer 55

Stern-Volmer equation