Analytical Test III (chap 13-17)

What is attenuated total reflection (ATR) useful for? How does it work?

Effective for samples (solids of limited solubility, films, threads, powders) When a beam of radiation passes from a more dense to a less dense medium, reflection occurs. If the incident angle is greater than the critical angle, total reflection appears. The reflected radiation will attenuate in intensity and include the spectral information of the less dense medium Absorbance of ATR is dependent on angle of incidence but independent of thickness of sample

What is the energy comparison for NIR, mid-IR, Far-IR?

Far-IR<mid-IR<Near-IR λFar IR>λmid-IR>Near-IR

How does rigidity affect fluorescence intensity?

Fluorescence intensity favored in molecules with rigid structures because of the decrease in internal conversion ex)biphenyl

Is the lifetime of the triple state or the single state bigger?

Lifetime of singlet state<<<lifetime of triplet state

Which of the two types of charge transfer electrons are easier to happen?

Ligand to metal charge transfers (LMCT)

What are the two types of charge transfer electrons (charge-transfer complexes)?

Ligand to metal charge transfers (LMCT) Metal to ligand charge transfers (MLCT)

Transducer and radiation source used for FT-IR

Transducer: Photoconducting and Pyroelectric Radiation: an inert solid that is heated electrically to a temperature between 1500K and 2000K

Transducer and radiation source used for dispersive instrument

Transducer: Thermal and Pyroelectric Radiation source: an inert solid that is heated electrically to a temperature between 1500K and 2000K

Transducer and radiation source used for nondispersive instrument

Transducer: Thermal and Pyroelectric Radiation source: an inert solid that is heated electrically to a temperature between 1500K and 2000K

What are the radiative processes from the Jablonski diagram?

Phosphorescence and Florescence

What is a singlet state? What is it related to Phosphorescence or Fluorescence?

a molecular electronic state such that all electron spins are paired. That is, the spin of the excited electron is still paired with the ground state electron ( spin multiplicity = 1); related to Fluorescence

What is quenching?

a non-radiative energy transfer from an excited species to other molecules (due collisions of excited molecules with solvent)

Which technique is more sensitive, spectrofluorometry or spectrophotometry? Why?

Spectrofluorometry is more sensitive. It measures the intensity of fluorescence, while spectrophotometry (absorption spectroscopy) measures the ratio of the intensities of two beams (incident and passed beams). The fluorescence signal is directly proportional to the intensity of the excitation source.

Mid IR involves the use of what three things?

absorption, emission, and reflection

What do infrared sources consist of?

an inert solid that is heated electrically to a temperature between 1500K and 2000K

What type of cuvette is used in UV-vis spectroscopy?

fused glass/silica

What is the difference between fingerprint region and group frequency?

group frequency - identification of functional group fingerprint region - small differences in structure and constitution of a molecule that result in significant changes in the region

what is a nondispersive instument

in these instruments, wavelength which passes through the sampling chamber is not pre-filtered. The infrared energy is allowed to pass through the atmospheric sampling chamber without deformation; used in the determination of a variety of organic species in the atmosphere

What is the main difference between phosphorescence and fluorescence?

lifetime Phosphorescence - the emission of radiation in a similar manner to fluorescence but on a longer time scale, so that emission continues after excitation ceases. (longer lifetime) Fluorescence - should disappear immediately upon termination of the external radiation

What are the four types of bending vibrations?

scissoring Rocking Wagging Twisting

How does concentration affect fluorescence intensity?

should be kept small in order to avoid red‐shift, distortion, and even quenching which often occur especially in the excitation excitation spectra at relatively higher concentrations

How does temperature affect fluorescence intensity?

lowering the temperature allows for the aversion of external conversions; low temperature is better

What are reflection losses with respect to limitations to Beer's Law

the redirection of a beam of light through boundaries called interfaces

What is a quenching agent?

the species that collides with the excited species that leads to the transfer of energy; causes the drop in fluorescence spectrum

Is the wavenumber of stretching or the wavenumber of bending greater?

wavenumber of stretching > wavenumber bending

How many vibrational modes does a linear molecule have?

3N-5

How many vibrational modes does a nonlinear molecule have?

3N-6

What is internal conversion?

A crossover between two states of the same multiplicity (s→s or T→T). This is a consequence of overlap of the vibrational manifolds of the electronic states.

To absorb IR radiation, what condition does a vibrational mode have to meet?

A net change in dipole moment is required

What does each variable stand for in the Beer's Law equation? What are the units?

A=absorbance (no units) ε=molar absorptivity (M-1cm-1) b=path length (cm) c=concentration (M)

What is a ligand to metal charge transfer? What are some examples?

Absorption results when the ligand donates a nonbonding electron pair to the metal which has an empty orbital (metal - electron acceptor and ligand- electron donor) ex) thiocyanate and phenolic complexes of Fe(III) and the Iodide complexe of molecular I2

What is a metal to ligand charge transfer? What are some examples?

Absorption results when the metal donates a nonbonding electron pair to the ligand which has an empty orbital (metal - electron acceptor and ligand- electron donor) ex) O-phenanthroline of Fe(II) or Cu(I) much less common

What is an overtone peak?

An overtone band is the spectral band that occurs in a vibrational spectrum of a molecule when the molecule makes a transition from the ground state (v=0) to the second excited state (v=2) and rarely (v=3). Overtone bands are often only very weak and are only clearly possible to be identified for bands where the fundamental transition is fairly strong.

For the π→π* transitions, how does conjugation affect wavelengths?

Conjugation causes shift towards longer wavelengths (less energies)

Transitions of d electrons examples

Cu2+, Ni2+, Cr2O72-, Co2+

What is vibrational relaxation?

Dissipation of excess energy in a molecular excited state among the vibrational degrees of freedom. This is the means of the Stokes shift in molecular fluorescence (emitted light appears at longer wavelength than the excitation wavelength).

What is a fundamental peak?

Fundamental peak occur when the molecule makes a transition from the ground state (v=0) to the first excited state (v=1).

What kind of photoconductive cell detectors are commonly used for the mid-IR and far-IR regions?

Hg-Cd-Te

What is the difference between a Fluorescence excitation spectrum and a Fluorescence emission spectrum? Which resembles the absorption spectrum?

In a fluorescence emission spectrum, the excitation wavelength is held constant and the emission intensity is measured as a function of the emission wavelength. In an excitation spectrum, the emission is measured at one wavelength while the excitation wavelengths are scanned. The excitation spectrum closely resembles an absorption spectrum

What are the variables in the Stern Volmer equation?

Kq = rate constant for quenching F0 = signal in the absence of quencher F = signal in the presence of quencher

Transitions of f electrons examples

Lanthanide and Actinide ions, the absorption process results from electronic transitions of 4f and 5f electrons. Ho3+, Pr3+, Er3+, Sm3+

Applications of Near IR

NIR spectra less useful for determination of unknown Can be used to quantitatively analyse compounds containing functional groups made of C-H, N-H, O-H, Determine water amounts based on first overtone peak of O-H Helpful for determination of primary and secondary amines

Types of radiation sources and their IR range Nernst Glower Globar Source Incandescent wire source Mercury arc Tungsten Filament Lamp Carbon Dioxide Laser Source

Nernst Glower - Near and mid Globar Source- Near and mid Incandescent wire source - Near and mid Mercury arc- Far Tungsten Filament Lamp - Near Carbon Dioxide Laser Source - Mid

Three types of IT instruments used?

Non Dispersive, Dispersive, FTIR

What is Far-IR used for?

Particularly useful for inorganic studies because the vibrations generally occur at frequencies lower than 650cm-1 Provides information about lattice energies of crystals and transition energies of semiconducting metals Pure-rotational absorption by gases

What kind of photoconductive cell detectors are commonly used for the near-IR region

PbS and lnSb

What is the difference between Photoluminescence and Chemiluminescence?

Photoluminescence- molecules of the analyte are excited by absorption of photons (Xe lamp provides energy for excitation) Chemiluminescence- excited species of the analyte molecules are generated by a chemical reaction between analyte and a suitable reagent

What is intersystem crossing?

Processes in which the spin of an excited electron flips. For neutral molecules this is typically a singlet->triplet conversion.

Absorption in mid IR can analyze what materials?

Pure liquid Pure solid and gas solution

What is diffuse reflection?

Quantitative determination of finely ground solids Requires use of FT-IR Very effective for powder samples with minimum sample preparation Works when a beam of radiation strikes the surface of a finely divided powder leading to specular reflection at each plane in all directions.

What are the limitations to Beer's Law? Explain each

Reflection and scattering losses Too high concentrations - higher concentration means a closer distance between molecules and a stronger interaction; also ε might be altered at high concentration because of its dependence on RI Chemical reactions - reactions result in chemical deviations by reacting to produce a product that has different absorption spectrum from the analyte. (will result in nonlinear relationships) Random errors from instruments - can never be truly eliminated ex) Thermal noise, Flicker Noise or 1/F noise

Why do we use reflection in mid -IR?

Reflection is used because it can overcome difficulties involving sample preparation. FT-IR is used for reflection techniques

What is external conversion?

Relaxation of an excited electronic state without emission of a photon due collisions of excited molecules with solvent. This competes with fluorescence, reducing fluorescent quantum yield, and is often referred to as quenching.

What can emission analyze?

Remote analysis ex)components emitted from distant industrial plumes

What are the two types of NIR and Mid-IR (Vibrational transitions)?

Stretching vibrations Bending vibrations

What are the two types of stretching vibrations?

Symmetric - equal change in chemical bond length Asymmetric - an unequal change in the chemical bond length

What is quantum yield?

The ratio of the number of molecules that luminesce to the number of excited molecules. A quantification of the efficiency of luminescence.

When is standard addition method used?

Used when there is a severe matrix effect ex) soil, minerals

What are the variables for the symmetrical and asymmetrical stretching formulas?

V=frequency (cm-1) V w/ line = wavenumber (cm-1) k= force constant (N/m) μ= reduced mass (kg) c=cm/s

How does pH affect fluorescence intensity?

basic conditions increase the intensity and pH drops (towards acidic) cause a drop in intensity

What are bending vibrations?

change in the angle between two bonds

What are Stretching vibrations?

continuous change in chemical bond length

Are transitions of d electrons or f electrons influenced by factors such as ligands and solvents?

d electrons absorption bands are strongly influenced by factors such as ligands and solvents

What are the nonradiative processes from the Jablonski diagram?

deactivation processes include Vibrational relaxation internal conversion external conversion intersystem crossing

What is spin multiplicity?

defined as 2S+1, where S is the total spin angular momentum

Charge-Transfer Absorptions have a large ε which means what?

easy quantitative determination

Use of an FT-IR allows for what kind of analysis?

fast

What transitions are are located in the UV-vis specrtrum in π, σ, and n electrons transitions?

n→π star transitions n→σ star transitions π→π* transitions

Near IR is (qualitative or quantitative) in nature and involves what two things?

quantitative Absorption (quantitative) bands with low ε include overtones peaks of C-H, N-H, O-H, and carbonyl overtone peaks Diffuse reflection (quantitative) Widely used in determination of proteins, moisture, etc in agricultural products Quantitative determination of finely ground solids

What are scattering losses with respect to limitations to Beer's Law?

redirection of the 'beam' of light when a photon in solution is scattered in all directions from the surface of large particles. Scattering causes an apparent absorbance because less light reaches the detector

What are the four factors that influence fluorescence intensity?

rigidity, temp, solvent, pH and concentration

What is a triplet state? What is it related to Phosphorescence or Fluorescence?

spin of the excited electron is not paired with the ground state electron ( spin multiplicity = 3); related to Phosphorescence

What are the four parts of the Jablonski diagram?

vibrational relaxation internal conversion external conversion intersystem crossing

n→π* transitions ε values and examples

ε values are 10-100 Lcm-1mol-1 (weak absorption) Many inorganic anions exhibit UV-Vis absorption peaks due to this transitions ex) NO3-, N3-, CO32-

π→π* transitions ε

ε values are 1000-10000 Lcm-1mol-1 (strong absorption)

What is the ranking of wavelengths of Phosphorescence, excitation, and Fluorescence?

λ(excitation)<λ(Fluorescence ) < λ(Phosphorescence)

What are the three types of electronic transitions?

π, σ, and n electrons (any compound) d and f electrons (transition metal ions) Charge transfer electrons (charge-transfer complexes)

What transitions are not allowed in π, σ, and n electrons transitions?

π→σ* σ→π*