ch 18

frequency

waves (cycles) per second, Hz

v, nu

wavelength

λ

units depend on the part of the spectrum of interes

Ultraviolet-visible (UV-Vis)

usually in nanometers (nm) sometimes in angstroms (Å, 1 nm = 10 Å)

in a vacuum

λ × ν = C

electromagnetic radiation variables in a vacuum

λ × ν = C

electromagnetic radiation variables in any medium

λ × ν = C/n where n is the refractive index of the medium

n > 1, so light slows down; λ is shorter but ν is constant in all media

Various wavelengths of light are used in analytical chem

The interaction of light with matter enables both qualitative and quantitative measurements

Spectroscopy can be classified based on types of energy and types of interactions

Energy of a photon

E = ℏν, where ℏ is Planck’s constant

Electronic spectroscopy

involves selection/detection of light

Basic parts of spectrometer

Analyze the light transmitted through a sample

Consider a differential slice of a media with the variable

beer’s law

Calibration curves

give unknown concentrations

Dispersive Instruments

Diffraction gratings

help separate wavelengths of light

Important smaller components of spectrometer

Single path instruments

require fewer parts, budget friendly

first run the solvent blank – then run the sample – the computer subtracts one from the other

photodiode array detector

photodiode array detector

Spectrum can show multiple features depending on sample

there are absorbed and observed colors

duh

Peak width related to how the compound reacts to light

as phase photon absorptions by single atoms cause transitions among quantized

electronic states with no vibrations and no rotations

∴ spectral lines are extremely sharp for atoms

Molecules are more complicated. Each electronic state has a multitude of vibrational energy levels and rotational energy levels among which transitions can also occur

∴ spectral lines are generally very broad for molecules

Molecular orbitals across multiple atoms

Molecular orbitals delocalize across multiple atoms

molecular orbit is not confined to one atom, it’s a single wavelength spread over multiple atoms at once

on the molecule, there are omicron bonds (c-h, c-o), pi bonds (c=o) and nonbonding (n) electrons on oxygen

these combine into the molecular orbitals shown on right

pi orbital covers both c and o atoms, the electrons in this orbital are shared (delocalized) over both atoms, not sitting in just one

the omicron orbitals extend along bonds (c-h) (c-o), electron density is spread between atoms, not localized

the n orbital (non-bonding, on oxygen) is mostly centered on the oxygen atom (this is localized, not delocalized much)

when orbitals delocalize over multiple atoms, electrons can spread out, stabilizes bonding orbitals

When a molecule absorbs light it may change

geometry

S0 (ground state) - lowest energy state of the molecule, electrons occupy the lowest possible molecular orbitals, this is the molecules normal, stable state

S1 (first excited state) - a higher energy state reached after absorbing light, one electron has been promoted to a higher energy orbital, still a singlet - electron spins remain paired

slightly different shape because electron distribution changed

Photon is absorbed when

its energy matches the gap

Single electrons in different orbitals can become unpaired

intersystem crossing

involves a change in the spin multiplicity

Summary of a typical absorption process

Some molecules have electron spin in the ground state

Most molecules have vibrations with varying symmetry

Counting potential vibrational modes

Vibrations in some simple molecules

Morse Curve for a diatomic molecule, A–B

Morse Curve for benzene

represent each electronic state by its own Morse curve

Franck-Codon Principle

Nuclei move much slower than electrons

Nuclear positions don’t change during electronic transition