group meeting on photochemistry...2015/02/11 · photochemistry lamps intensity hazards • for a...

Group Meeting on Photochemistry

Safety

Equipment

Reactivity – Part 1

How photochemists think

Quantum yields and rates

Reactivity – Part 2

Safety

Safety

• Radiation hazards

• Ozone

• Lamp explosion

• Electrical shock

• EMI

• Heat

photokeratitis (“sunburn

of the eye,” snow blindness,

arc welder’s eye)

cataracts

Radiation Hazards

• Three types of radiation hazards

– high intensity of the radiation

– UV radiation

– IR radiation

Radiation Hazards

• Three types of radiation hazards

– high intensity of the radiation

– UV radiation

– IR radiation

• Filters to shield the eyes from the high

intensity, from UV, and from IR are

absolutely necessary when working around

photochemistry lamps

Intensity Hazards

• For a high pressure mercury arc lamp, the intensity is

beyond the safe limit of the eye. It’s like looking at

the sun.

• At most wavelengths, a typical high pressure mercury

arc lamp is about 1/10 the intensity of the sun; at the

Hg lines the intensities are comparable.

UV Hazards

UV Radiation

RegionAlso known

asRange (nm)

Hazard

Potential

Damage

Mechanism

UV-A near UV 320-400 lowest cataracts

UV-B mid UV 290-320 mid to highskin and eye

burns, cancer

UV-C far UV 190-290 highestskin and eye

burns, cancer

Glass Filters? No!

cataractseye burns

skin cancer

Plastic Filters? No!

Recommendations

• Wear appropriate UV-blocking glasses or

googles

• Even with UV-blocking glasses or googles,

never look directly at the light source

• Even with UV-blocking glasses or googles,

never look directly at a light-source reflection

Recommendations


googles

• Never look directly at the light source

• Never look directly at a light-source reflection

• If possible, contain the light source and all

reflections and scattered light within a closed

box (properly vented and cooled, of course)

Recommendations


googles

• Never look directly at the light source

• Never look directly at a light-source reflection

• If possible, contain the light source and all

reflections and scattered light within a closed

box (properly vented and cooled, of course)

• OK to view arc through #12 welding glass or sun-viewing filters. Most

other dark transparent materials can transmit too much IR, allowing

thermal damage to the eye.

Ozone

• Ozone can inflame delicate tissues in

the lungs, leaving them open to asthma

and infections

• Ozone free quartz: blocks wavelengths

below 254 nm

Lamp Explosion

• High pressure mercury arc lamp

– 8 atm cold

– up to 75-100 atm when running

• Explosion of a running lamp releases high velocity

shards of red hot quartz (800°C / 1472°F) and metal

particles

• Do not touch the lamp with your bare skin. Skin oils

will causing local overheating and lead to

devitrification. The resulting strain leads to

catastrophic failure. Wipe lamp with alcohol if you

touch it. (quartz glass, touch, hot spot forms, devitrification of that spot by excess heat (it becomes

crystalline), quartz crystal is weaker, explosion results)

Electrical Shock

• A high (>20kV) transitory voltage is used to ignite the lamp. Lamp current is up

to 22 amperes. Additionally, line voltage is supplied to the housing to power the

fan and elapsed timer. This line voltage will still be present even when the power

supply is disconnected from the housing.

• Recommendation:– follow standard electrical safety precautions

Electromagnetic Interference (EMI)

Recommendations:

1 Start the arc lamp before booting up nearby computer systems.

2 Keep the computer at least 6 feet away from the ignitor/power supply.

3 Use a different outlet and line for the computer and ignitor/power supply

4. Mike Strain asks that we not use the lamps near the NMR spectrometers

EMI: Any electromagnetic disturbance that interrupts, obstructs, or

otherwise degrades or limits the effective performance of electronics and

electrical equipment. It can be induced intentionally, as in some forms of

electronic warfare, or unintentionally, as a result of spurious emissions

and responses, intermodulation products, and the like

Heat

• The discharge tube of high pressure mercury arc lamps consists of

quartz glass. Only quartz glass can withstand the high mechanical

loads caused by the high operating pressure as well as the high

thermal loading (surface temperatures of up to 1000 °C).

• The IR output of a lamp is high. This radiation can damage absorbent

materials without the need for a thermal pathway

• Arc lamps become very hot after only minutes of operation, and remain

very hot for up to 30 minutes after being shut off.

end safety section

Equipment

• Light Sources (Lamps)

• Filters

• Reaction Vessels

• Solvents

Light Sources

• The sun

• Mercury arc lamps

– low pressure

– medium pressure

– high pressure

• Lasers

The Sun

• Photochemistry was

developed in Italy by

Giacomo Ciamician at the

U. Bologna

• The sun was their lamp– they did experiments in the

summer

– they wrote papers in the winter

Giacomo Ciamician,

the “father of photochemistry”

Giacomo

Low Pressure Hg Arc Lamps

Medium Pressure Hg Arc Lamps

These types of lamps

are typically “immersion”

lamps.

High Pressure Hg Arc Lamps(Short Arc Lamps)

High Pressure Hg Arc Lamps


bulb


mirror


focusing lens

Short Arc Lamps

The Lamp Output is a Continuum Across

the UV, Visible, and IR Regions

Lasers

Filters for Wavelength Selection

• Cut-off filters

– Long-pass

– Short-pass

• Bandpass filters

• Interference filters

• Filter solutions

• Monochromators

Cutoff Filters

Cutoff Filter Solutions

Bandpass Filters

Bandpass Filter Solutions

Water Filter

• A water filter is used to remove IR from

the output of the high-pressure mercury

arc lamp

Water Filter

• A water filter is used to remove IR from

the output of the high-pressure mercury

arc lamp

water or filter

solution can go

in here

Reaction Vessels

flow cell or batch

Turntable Reactors (Merry-Go-Round Reactors)

Freeze-Pump-Thaw Photochemical Glassware for

Mechanistic Work

Freeze-Pump-Thaw Photochemical Glassware for

Mechanistic Work

Note the “graded seals.”

All temperatures are

given in degrees C

Soft Glass

(Soda Lime)

Borosilicate

(Duran, Kimax,

Pyrex)

Quartz

(Fused Silica)

Softening Range 696 821 1580

Working Range 1000 1200 1800

Annealing Range 514 565 1050

Max Service Temp. 450 490 1100

Physical Properties of Common Glasses

By using a graded seal, one can join extremely short lengths of

glass that vary from each other by just a little bit and thereby achieve

a smooth transition from one type of glass to another, e.g.,

borosilicate to quartz.

Sometimes it becomes necessary to seal glasses of widely different compositions together

such as a soda-lime glass to a borosilicate glass or a borosilicate to fused silica. Due to

the great differences in coefficients of thermal expansions, these glasses cannot be

sealed directly together, but require intermediate glasses so that the coefficient of

expansion of each glass differs from that of its neighbor by a small increment. The

smaller one keeps the expansion mismatch between adjacent glasses, the less the strain

that will exist in the finished seal. A good graded seal should typically have six

intermediate glasses or steps.

http://www.ilpi.com/msds/ref/tempunits.html

Solvents

Solvents (con’t.)

• Greases luminesce

• Remove C=C

• Dirty cells are the greatest single source of error in

spectrophotometry. Common contaminants on cuvette

surfaces include:

• Synthetic detergent solutions or stopcock grease

• Filter paper which leaves phenol residues from the

original wood

Reactivity

• Overview

• The First Law of Photochemistry

• Bond cleavage reactions

• Coordination complexes

• M-CO complexes

• M-M bonds

Overview

fluorescence

phosphorescence

luminescence

emission

The First Law of Photochemistry

• The first law of photochemistry (the Grotthuss-

Draper law) states that light must be absorbed by a

compound in order for a photochemical reaction to

take place.

Reactivity: Bond Cleavage Reactions

Examples of Metal-Metal Bond Photolysis

A Precedent

+ CN–

+

C• + •CN

3

h

polar solvent

h

nonpolar solvent

CC C N

C C N

Calvert, J. Am. Chem. Soc. 1952, 74, 2101.

–

C

H

H

C

C

C C

C

C

H

+

A– B+

A–B A• + •B

A–B A– + B+

Another Example

Komatsu, J. Org. Chem. 1989, 54, 2038.

Caution: Homolysis followed by electron

transfer can be mistaken for heterolysis

PhCH2Cl PhCH2+ + Cl–

PhCH2• + •Cl

e.t.

PhCH2Cl PhCH2• + •Cl PhCH2–CH2Phh

C6H12

hzeolites

PhCH2

CH2Cl

Garcia et al. J. Chem. Soc., Chem. Commun. 1993, 1041.

How Photochemists Think

• Orbitals

– they use states if orbitals fail

• Analogies and Precedents

• Models and Calculations

– calculations on ground states

– calculations on excited states (not so much)

• Thermodynamics (not so much)

Homolysis or Heterolysis?

• Consider forming a bond from 2 radical fragments MLn

• There are four possible electronic configurations:

• Note the analogy to H2, i.e., the H-H bond

• * excitation should weaken or cleave the M-M

bond, but an orbital analysis cannot determine if the

cleavage is homolytic or heterolytic

• Four electronic configurations means there are 4

states, as shown next

singlet

singlet

singlet

triplet

The State Diagram for a Generic M-M Bond

Analogy: the Photochemistry of cis-trans

Isomerization in Alkenes

Analogy: the Photochemistry of cis-trans

Isomerization in Alkenes

singlet

triplet

singlet

singlet

Photochemical Isomerization of Alkenes

singlet

singlet

singlet

triplet

The State Diagram for a Generic M-M Bond

The Actual Mechanism of the “Heterolysis”

is Homolysis Followed by Electron Transfer

Mn(CO)5LMn(CO)5 + L•

"19e-"

Mn(CO)5L + Mn2(CO)10

19

Mn(CO)5L+ + Mn2(CO)10

–

Mn(CO)5 + Mn(CO)5–Mn2(CO)10

– •

Mn2(CO)10 2 Mn(CO)5h •

Overall: Mn2(CO)10 + L Mn(CO)5L+ + Mn(CO)5

–h

Mo(CO)6h Mo(CO)5 + CO

Co(CN)63– h

Co(CN)52– + CN–

NN+ h

H2O+ + N2

True Photochemical Heterolyses

hN=O + ONO2

CH3NCOh

CH3N + CO

True Photochemical Heterolyses

Photochemical Heterolysis

The relative energies of these

configurations depend on the

ionization potential of X and

the electron affinity of Y.

IP(X) > EA(Y)

Photochemical Heterolysis

To Kinetics

group meeting on photochemistry...2015/02/11 · photochemistry lamps intensity hazards • for a...

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