Fundamentals of Spectroscopy & Applications of Spectroscopy (L02; chptrs 18 & 19)

CHEM 310: Foundations of Analytical Chemistry

Spectroscopy

measuring the extent to which radiant energy (light) is absorbed by a chemical system as a function of wavelength of radiation (i.e. spectral scanning) as well as absorption measurements at a fixed wavelength

electromagnetic spectrum

various optical phenomenon are best interpreted in terms of the idea that light is propagated in the form of transverse waves

spectroscopy vs. spectrometry

spectroscopy = experiments with electromagnetic radiation

spectrometry = the application of spectroscopy so that there are quantifiable results

UV Spectroscopy: Terminology for absorptive shifts - Bathochromic shift

shift of absorption to a longer wavelength

UV Spectroscopy: Terminology for absorptive shifts - hypsochromic

shift of absorption to a shorter wavelength

UV Spectroscopy: Terminology for absorptive shifts - hyperchromic effect

an increase in absorption intensity

UV Spectroscopy: Terminology for absorptive shifts - hypochromic effect

a decrease in absorption intensity

UV Spectroscopy: Conjugation Effects - the greater the conjugation…

the lower the energy required to induce electronic transitions (the longer the wavelength)

UV Spectroscopy: Conjugation Effects - lengthening conjugation will…

increase band intensity (and make the molar absorptivity greater)

UV Spectroscopy: Conjugation Effects - adding substituents may…

increase band intensity (and make the molar absorptivity greater) by a much smaller degree than lengthening conjugation would

UV Spectroscopy: Fieser-Kuhn Rules for Dienes

• used to reliably predict absorption wavelength in dienes, enone, and (to a lesser extent) aromatic systems

tip: watch some videos on YT about this because I don’t really know what this slide is trying to say

UV Spectroscopy: Fieser-Kuhn Rules for Polyenes

max wavelength = 114 + 5M + n(48-1.7n) - 16.5(# Rendo) - 10(# Rexo)

• M = # alkyl substituents

• n = # conjugated double bonds

• Rendo = # rings with endocyclic double bonds

• Rexo = # rings with exocyclic double bonds

Photon Energies

each packet has an associated energy: E_photon = hv = (hc) / 𝜆 = ℎ𝑐 ̅𝜈, where h = 6.626 × 10^-34 J

Interaction of light with molecules: electronic energy, E_elec

absorption of UV and visible light increases electronic energy that causes electrons to be excited to higher energy levels

Interaction of light with molecules: vibrational energy, E_vib

changed in vibrational energy are seen in the infrared region

Interaction of light with molecules: rotational energy, E_rot

rotational transition requires relatively little energy and are induced by radiation of very low frequency (long wavelength) typically in the far-IR or microwave region

Light Absorption Process

In general, when light causes electronic transitions, vibrational and rotational transitions occur as well. Electronic absorption bands are usually broad because of many different vibrational and rotational transitions.

internal conversion

radiation-less transition between levels with the same spin states (i.e. S_n → S₁ excited vibrational level)

intersystem crossing

radiation-less transition between levels with different spin states (i.e. S₁ → T₁)

fluorescence

emission of a photon during a transition between levels with the same spin states (i.e. S₁ → S₀)

phosphorescence

emission of a photon during a transition between levels with different spin states (i.e. S₁ → S₀)

What are the basic parts of a (single beam) spectrometer?

Light source: Tungsten/Halogen lamp

Filter (Absorption filter/Interference filter)

Sample Holder

Detector = silicon photodiode (photoemmisive or photovoltaic cell)

Galvanometer

Application of single-beam spectrometry?

to obtain an absorption measurement in the visible region

What are the basic parts of a double-beam spectrometer?

1. Light source

2. Diffraction grafting

3. Slit

4. Beam splitter

5. Sample cell → mirror & mirror → reference cell

6. Beam splitter

7. Detector and computer

8. Chart recorder

Application of double-beam spectrometry?

UV and Visible region (280-800 nm); obtains a photometric measurement, spectral scan, and kinetic measurement of two cells simultaneously

Single-beam Spectrometer vs. Double-beam Spectrometer

Light source is non-split light beam with high throughout of light vs. a split-into-two-fractions beam with low throughput of light

Used for simple structures and requires fewer components vs. complex structures and requires more components

Data quality: low reproducibility in the absorption measurement, so a less accurate result is obtained than what you’d get with a double-beam spectrophotometer

& the error due to the intensity radiation in double-beam is eliminated, which is a nice advantage

Transmittance (T)

the fraction, ranging from 0-1 (and usually referred to in %), of the original light that passes through the sample

Absorbance (A)

is directly proportional to the concentration of the light absorbing molecular species.. in other words, it is directly proportional to the concentration of the sample

Why do we use blanks in a spectrometer?

they’re used to compensate for any absorbance originating from reagents or contamination

spectrophotometry, simply explained:

transmittance (the amount of light you see come out divided by the amount of light you put in) will decrease if you increase concentration to make your solution more opaque AND/OR have a wider (more volume) cuvette that the light has to pass through because light has to “bump into” more molecules of solvent as it passes through the cuvette

Beer-Lambert Law, simply explained

absorbance increases as your concentration increases, and transmittance increases as absorption decreases (T and A are inversely related)

Absorbance and Beer’s Law

𝐴 = 𝜀 b c
𝜀 = molar absorption coefficient (M^-1cm^-1) [aka extinction coef., molar absorptivity coef.] b = cell path length (usually 1 cm)*
𝑐 = molar concentration (mol/L)

𝐴 = − log 𝑇
𝑇 = 10^logT = 10^-A

Limitations of Beer’s Law in Absorbance

1. Applies to monochromatic radiation only

2. Works for dilute solutions, usually ≤ 0.01 𝑀𝑀

3. Absorbing species (i.e. the chromophore) does not participate in a concentration-dependent equilibrium

chromophore

the absorbing species

observed color of light is called the …

compliment of the color absorbed by the chromophore

When Beer’s Law Fails:

• positive deviations: due to chemical effects

• negative deviations: due to chemical effects, polychromatic light, or stray light

• when your sample has a high concentration and the solute molecules have interacted with one another (because of the concentration-dependent chemical equilibrium, absorptivity changes with concentration)

Beer’s Law only applies to monochromatic light! It assumes that all light reaching the detector has passed through the sample. Scattered light may happen when you’re having solubility issues / free particles in your solutions / aggregation.

Isosbestic Point

when one absorbing species is converted into the other absorbing species