UV Spectroscopy

12.5, 14.7, 14.8

What is spectroscopy

A collection of techniques that study how matter interacts with energy (light)

• Act as molecular/atomic fingerprint analysis tools that help identify composition, structure, and properties

• when a sample undergoes some kind of reversible change (no broken bonds) in which we call a “transition”, with the type of transition depending on the type of electromagnetic radiation

• A detector is able to tell how much of the radiation the sample absorbed at a given wavelength or energy allowing us to deduce information about the structure of the molecules in a sample

Why do we do spectroscopy?

To find or verify the structure of an organic compound

• for each different spectrum, we derive different type of structural information

How does infrared, ultraviolet, and nuclear magnetic resonance spectroscopies differ from mass spectrometry?

These methods are nondestructive, meaning the changes to the molecules are reversible and involve the interaction of molecules with electromagnetic energy rather than an ionizing, destructive force

Define the electromagnetic spectrum going from right to left

Radiowaves → Microwaves → Infrared Waves → Visible Light → Ultraviolet Light → X Rays → Gamma Rays

What two entities does light behave as?

A particle (photon), and as an energy wave

Since light can be characterized as a way, what are the characteristics of light?

1. Wavelength (λ) - The distance from the middle of one wave to the next

2. Frequency (ν) - The number of waves that pass by a fixed point per unit of time, usually given in reciprocal seconds (s^-1) / Hertz (hz)

3. Amplitude - the height of a wave, measured from the middle to its’ peak.

What is the speed of all forms of light?

c = λν

3 × 10^ m/s

What is the fundamental unit of light

Quanta

• Light is transmitted only in discrete amounts called quanta or a quantum

What is the equation to find the energy of 1 quantum of light

ε = hv = (hc)/λ, The plank equation

• h = plank’s constant

• High frequencies and short wavelengths correspond to high energy radiation

• Low frequencies and large wavelengths correspond to low energy radiation

What is the measurement for one mole of photons?

E = N_Ahc /λ

What is the absorption spectrum of a molcule

A characteristic pattern of wavelengths of electromagnetic radiation that a molecule absorbs

• When an organic compound is exposed to a beam of light, it absorbs energy of some wavelengths but passes, or transmits energy of other wavelength

What does an absorption spectrum chart

• The horizontal axis - Records wavelength

• The vertical axis records intensity

  • The baseline/top corresponds to 0% absorption (100%) transmittance so the more downward a spike/peak is means energy absorption has occured at that wavelength

When a molecule absorbs radiation what happens on the molecule

• Its energy is distributed over the molecule.

• The energy absorbed causes bonds to stretch and bend more vigorously

• Different radiation frequencies affect molecules in different ways, but each provides structural information when the results are interpreted

What type of information do you gain from ultraviolet (uv) spectroscopy?

Identifying conjugated pi electron systems

What is a conjugated compound?

A compound with alternating single and multiple bonds in a continous chain or ring

What is the range of UV light organic compounds care about (not a #)?

The one closets to the visible light spectrum

With IR radiation, the energy that is absorbed corresponds to what?

The amount of energy needed to increase molecular cibration

With UV radiation, the energy absorbed corresponds to what?

The amount needed to promote an electron from a lower-energy orbital to a higher - energy one in a conjugated molecule

What is the mechanism that occurs when UV strikes a conjugated compound

1. The compound absorbs energy

2. A pi electron is promoted from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO)

• Because the electron is promoted from a bonding pi molecular obital to an antiboding pi* molecular obital we call this p → pi star excitation

• An ultraviolet spectrum (graph) is recorded by irradiating a sample with UV light of continuously changing wavelegnth. The absorption is detected and displayed on a chart that plots wavelength versus absorbance (A)

What is the equation of absorbance on an ultraviolet spectrum

A = log (I₀/I)

• A is absorbance

• I₀ is the intesinty of the incident light

• I is the intensity of the light transmitted through the sample

• Baseline of 0 is at the bottom of the chart so that a peak indicates absorption

What is the amount of UV light usually expressed by and why?

A sample’s molar absorptivity (ε) because it is a physical constant, characteristic of the particular substance being observed and thus characteristic of the particular pi electron system in the molecule

• equation also allows you to measure concentration of a sample in a solution when A, ε, and I are known

What is the molar absorptivity (ε) equation?

ε = A /(c * l)

• A = Absorbance

• c = concentration in mol/L

• l = sample pathlength in cm

What are the factors affecting the wavelength of UV absorption by molecule?

• The extent of conjugation

• The energy gap between HUMO and LUMO

• As conjugation increases, the energy difference between HOMO and LUMO decreases

What level of transitions does UV/visible light undergo

• At the level of the electronic state

• Electron excitations from nonboding → antiboding orbitals (n → pi star) or from pi bonding to pi antibonding orbitals

If there is more than one single bond separting two double bonds, are they still conjugated double bonds?

No, they are nonconjugated or isolated

Do conjugated double bonds have to be C based?

No, can be other elements

When looking at an UV spectra, what are we interested in?

The location where maximum absorption comes from, our λ max

• The more conjugated bonds are in a molecule, the lower energy or longer wavelength the UV/visible absorption will be, which is why we’re interested in the location of maximum absorption not necessarily the value of absorption

How do you interpret an UV spectra

• Organic compounds w/ no double bonds or only isolated double bonds tend to absorb in the high - energy far ultraviolet ranges (λ max of 200 nm or less)

• As more conjugated double bonds are introduced into the structure, the absorption shifts more to higher wavelengths/lower energies

• With enough double bonds, the compound will absorb visible light