9. Reaction dynamics /

Effect of isotope masses on reaction rates Áhrif samsætumassa á hraða efnahvarfa path path

path path

path path Effective activation energy

path path Effective activation energy

Photochemistry can be dramatically different from “ordinary” chemistry See example below

photochemistry example: Ljósefnafræði ; Dæmi / photochemistry example: Cl Ar

photochemistry example: Ljósefnafræði ; Dæmi / photochemistry example: . Cl . Ar .

photochemistry example: Ljósefnafræði ; Dæmi / photochemistry example: Ar Cl

photochemistry example: Ljósefnafræði ; Dæmi / photochemistry example: + - Cl Ar Cl

photochemistry example: Ljósefnafræði ; Dæmi / photochemistry example: Ar Cl Cl

„Photographing“ reactions; Nobel price in Chemistry, 1999 * Sjá: http://www.hi.is/~agust/

Reaction rate and mechanism: A + B C + D Hr.efnahv.

„Photographing“ reactions; Nobel price in Chemistry, 1999 Reaction rate and mechanism: A + B C + D Rate = k[A] [B]; k = Aexp(-Ea/RT); k <= [A] vs t measurements Arrhenius: Nobel price 1903 Hr.efnahv. Hr.efnahv.

Theory concerning transition state : 1930-1940: Theory concerning transition state : Henry Eyring Michael Polanyi # A..B 1 2 A + B C + D # short lived transition state: Lifetime » time of a vibration, typically 10-13 – 10-12 sec. 1: 2: Kenning

Experiments by use of molecular beams : 1975-1990: Experiments by use of molecular beams : Molecular beam collision Reagents and products detected Moveable detector -Transition state properties determined from scattering measurements John C. Polanyi Nobel price, 1986: Dudley Herzbach Yuan Lee Bunur

“Photographing reactions” 1985 - “Photographing reactions” i.e. Absorption measurements for transition state species vs. time (t) for Dt £ 10-12 s: Absorption measurements by light pulses 10-15 s AB CD A . .B C ..D A + B 0,000000000000001 s C + D “Femtosecond spectroscopy” Nobel price in chemistry 1999: Prof. Ahmed Zewail, Caltech. Ljosm.

Verðl.

http://www.its.caltech.edu/~femto/ Femtol.

All stages of the reaction simultaneously, i.e. How / What? A + B C + D All stages of the reaction simultaneously, i.e. Too much! Hvernig?

Coordination i needed : Example: “½ reaction”, e.g.: ICN# I + CN, i.e.: I CN#

Coordination i needed : Example: “½ reaction”, e.g.: ICN# I + CN, i.e.: I CN ICN# ICN I + CN Formed by absorption of a femtosec. laser puls Absorption measurements ½ hvarf

Procedure: ICN-Tilr.

In detail: ICN# I + CN Absorption measurements: ICN Results: ICN-niðurst.

(a) (b) [NaI# Na..I# ] Na + I NaI (a) NaI# (b) Na NaI

Energy Stilbene(1)

Stilbene(2)

IH..OCO [I..H..O..CO] I +HO+CO Van der Jet Waals expansion/ Molecular “Molecules” Jet expansion/ cooling Molecular beam IH+OCO

C4H8 C4H8# C2H4 + C2H4 ??? Cyclob.

Dissociation of C2F4I2 : . ?(1) . . . ?(2) . . C2F4I2 C2F4 + 2I: I F I C C C C F . F F I F I . I F F I F I F F . F ?(2) C C C C C C . F . F F I F I F F I C2F4I2

mynataka

Energy

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A + M ??? M1 + M2 ??? A + B C + D Reaction cross section:

A + M ??? M1 + M2 ??? A + B C + D Reaction cross section: Cross section = s = <d>2p d But ions:

But ions: Reaction cross section: A+ + B C + D + F µ 1/r2 - + +

But Ions: Reaction cross section: A+ + B C + D + - + +

+ + But ions: Reaction cross section: A+ + B C + D Collision frequency much larger +

+ A + M ??? M1 + M2 ??? A + B C + D Reaction cross section: Ions: Collision cross section = s >> <d>2p d