19 장 Fundamentals of Spectrophotometry Fundamentals of Spectrophotometry –19-1Properties of Light –19-2Absorption of Light –19-3 The Spectrophotometer –19-4 Beer’s Law in Chemical Analysis –19-5 What happens when a molecule absorbs light? –19-6 Luminescence

Spectrophotometry Any procedure that uses light to measure the concentration of a chemical species Light is composed of perpendicular, oscillating electric and magnetic fields Electric field y z x (t) Magnetic field Fig. 19-1

Properties of Light Wavelength ( ): crest-to-crest (or trough- to-trough) distance between waves, generally measured in nm Light described as “waves” y x

Properties of Light Frequency ( ): number of complete oscillations the wave makes each second (1/s, s -1, Hz) = c c  speed of light (in vacuum) = x 10 8 m/s Units for above equation: (s -1 )(m) = (m/s)

Properties of Light Refractive index (n): measure of angle at which light is bent Speed of light in a medium other than a vacuum c/n (n = 1 in a vacuum)

Properties of Light Photons (h ): energetic particles of light E = h E = hc/ = hc h  Planck’s constant = x J  s  wavenumber (1/ ) Light described as “particles” ~ ~

Electromagnetic Spectrum Compare energy of red and blue light: E = hc/ red > blue so E red < E blue Cosmic  -rays X-raysUVIR  -wave Radio Visible (Hz) (m) (nm): Fig. 19-2

19-2. Absorption of light Molecules absorb photons with energy (E) E = h Molecules gain that energy (E) when they absorb photons M is promoted from the ground state to an excited state Ground state Excited states Energy  M h

Absorption of light Molecules gain energy when they absorb photons Molecules lose energy when they emit photons Ground state Excited states Energy  Absorption gain h Emission lose h Fig. 19-3

Types of electromagnetic radiation (light) X-ray light –promotes core e - s to higher energy orbitals –breaks chemical bonds and ionizes molecules Ultraviolet and visible light (UV-VIS) –promotes valence e - s to higher energy orbitals Infrared light (IR) –stimulates vibrations of molecules Microwaves –stimulates rotational motion of molecules ENERGYENERGY

Energy levels Quantized: discrete levels, not continuous Energy states are “quantized” rotational levels vibrational levels v2v2 v1v1 S0S0 S1S1  E = h electronic levels

Example Problem By how many kJ per mole is the energy of O 2 increased when it absorbs UV radiation with a of 147 nm? New energy: E 2 Original energy: E 1 E 2 – E 1 = energy of photon absorbed

Example Problem

Reminder Prefixes of SI units piconanomicromillicentikilomega pn  mckM Example:

Measuring Absorption of Light selects that analyte will absorb sample absorbs radiation b source may contain many s Light Source MonochromatorDetectorSample P0P0 P detector looks for amount of radiation not absorbed

Measuring Absorption of Light Detector measures P Amount of light transmitted through sample is what is measured Transmittance (T): fraction of original light that passes through sample –Absorbance is measured INDIRECTLY radiant power not absorbed by sample incident radiant power =

Sept. 19 – Ch. 19 Fundamentals of Spectrophotometry 19-1Properties of Light –19-2Absorption of Light –19-3 The Spectrophotometer –19-4 Beer’s Law in Chemical Analysis –19-5 What happens when a molecule absorbs light? –19-6 Luminescence

Ch Fundamentals of Spectrophotometry 19-1Properties of Light –19-2Absorption of Light –19-3 The Spectrophotometer –19-4 Beer’s Law in Chemical Analysis –19-5 What happens when a molecule absorbs light? –19-6 Luminescence

Measuring Absorption of Light Detector measures P Amount of light transmitted through sample is what is measured Transmittance (T): fraction of original light that passes through sample –Absorbance is measured INDIRECTLY radiant power not absorbed by sample incident radiant power =

Measuring Absorption of Light 0 < T < 1 0 < %T < 100 Absorbance is related to T:

Absorbance vs. Transmittance When P decreases, A increases –less radiant power (light) is reaching the detector because the sample is absorbing light T and A are dimensionless (although some- times “absorbance units” are mentioned) P/P 0 %T A

Absorbance vs. Reflection When a molecule absorbs different s of white (visible) light, our eyes see the reflected s (the non-absorbed s) of Max. Absorption Color Absorbed violet violet-blue blue blue-green green yellow-green yellow orange red purple Color Observed green-yellow yellow orange red purple violet violet-blue blue blue-green green Table 19-1

Blue Blocker Sunglasses The infomercial claims that “harmful blue light” is blocked from damaging your eyes. Why are they orange?

Measuring Absorption of Light P (for absorption by sample) = P 0 – P (reflected) – P ( scattered) Reference blank: solution containing all components of a sample except analyte A (analyte) = A (measured) – A (reference blank) DetectorSample P0P0 P scattering reflections

Absorbance Spectrophotometry: Any procedure that uses light to measure the concentration of a chemical species Beer’s Law: A =  bc   Molar absorptivity (or extinction coefficient) b  pathlength light travels through cuvet c  concentration of analyte Absorbance is directly proportional to concentration

Beer’s Law Best applied when c  0.01 M –when c > 0.01 M, solute molecules influence one another because they are closer together –physical properties of molecules change when they are close together –physical property that will change relevant to our discussion of Beer’s Law: 

Beer’s Law Molar absorptivity (  ): characteristic of a substance that tells how much light is absorbed at a particular wavelength (   is -dependent AND analyte-dependent because different analytes absorb different amounts of light at different s Pathlength (b): width of cuvet; dependent on instrumental setup A =  bc = (M -1 cm -1 )(cm)(M) A is dimensionless

Example Problem A 3.15 x M solution of a colored complex exhibited an absorbance of at 635 nm in a cm cuvet. A blank solution had an absorbance of Find the molar absorptivity of the colored complex.

Double-beam Scanning Spectrophotometer Alternately measuring P 0 and P by diverting light beam through reference cuvet light source monochromatordetector amplifier computer/ recorder sample cuvet reference cuvet mirror mirror/ beam chopper mirror mirror/ beam chopper Fig 19-6 P0P0 P P0’P0’

Spectrophotometers Single-beam spectrophotometer –insert reference blank once at beginning of exp’t –only measure absorbance of EITHER sample OR reference blank at a time Double-beam spectrophotometer –continuously checks reference blank to account for changes in source intensity (P 0 ) detector response –if absorbance measurement from reference blank changes, spectrophotometer corrects for that change to find true absorbance of analyte

Using a spectrophotometer Picking the (the source of the light) –Choose the at which the analyte absorbs the most ( max ) measurement is most sensitive at max Keep samples clean and dust free Analyte solution should absorb in the range of 0.4 < A < 0.9 –dilute solution if it is too concentrated reduce c to reduce A –use cuvet with longer pathlength increase b to increase A

Why should 0.4 < A < 0.9? Consider when A < 0.4 –P is almost as large as P 0 –difficult to see a small difference between two large numbers Consider when A > 0.9 –P is very very small –difficult to see a small amount of light –stray light reaching the detector could compete with P Adjust experimental parameters to keep A in an intermediate region

What happens when a molecule absorbs light? Consider Molecular Orbital (MO) Theory –describes the distribution of electrons in the molecular orbitals of a molecule Electronic transition –e - from one MO moves to another MO Ex: Formaldehyde –4  bonds –1  bond –1 lone pair (the other lone pair is mixed with the  bond orbitals) –refer to Figure C H H O

What happens when a molecule (Formaldehyde) absorbs light? 11 22 33 44  n  singlet or 11 22 33 44  n  triplet E 11 22 33 44  n  Ground State Electronic States Molecule absorbs light ( ) n   * transition

What happens when a molecule (Formaldehyde) absorbs light? Difference between 2 excited states is spin of e - Singlet –spin still opposite from e - it was originally paired with Triplet –spin parallel with spin of e - it was originally paired with In general, T 1 < S 1 11 22 33 44  n  singlet (S 1 ) or 11 22 33 44  n  triplet (T 1 ) Electronic States

Processes that occur when a molecule absorbs light The above “state  state” transitions are only examples –S  T or T  S: different spins –S  S: same spins Absorbance S 0  S 1 Fluorescence S 1  S 0 Phosphorescence T 1  S 0 Radiational Transitions Internal Conversion S 1  S 0 Intersystem Crossing T 1  S 0 Relaxations (within a state) Radiationless Transitions

Jablonski diagram S1S1 S0S0 T1T1 A Absorbance F Fluorescence P Phosphorescence IC Internal Conversion ISC Intersystem Crossing R Relaxation Fig

Processes that occur when a molecule absorbs light Internal conversions (IC) and intersystem crossings (ISC) –no gain or loss in energy –IC: S  S –ISC: S  T or T  S Fluorescence (F) and phosphorescence (P) are rare processes –molecule emits photons (loss in energy) –F: S  S –P: T  S –examples of Luminescence (section 19-6)