k.l. sebastian ipc department, iisc kls chennai, september 14, 2005 molecular devices
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K.L. Sebastian
IPC Department, IISc
http://ipc.iisc.ernet.in/~kls
Chennai, September 14, 2005
Molecular Devices
• Motivation – examples from biology
• Molecular Rollers and Rocker
• Molecular Wheel
• Molecular Rattle
. Fluxionality for Rotational Motion
• Nature does it very well! (Biological Molecular Motors)
• Synthetic Molecular Motors
• Light driven molecular motor
Outline
Height ~ 8 nm Width ~ 10 n
Is that a flower?
It is a motor
Can one design molecules that would
prefer to roll on a surface?
Seems rather difficult, perhaps we can try to
use fluxionality!
What do you mean?
ROLLERMOLECULAR
To explain, let us start with Pentaprismane
Consider Hypostrophene-it is fluxional -perhaps we
can use this property!
!!
Pentaprismane (C10H10)
D5h
Hypostrophene
C2v
Symmetry is broken! It can be broken in FIVE different ways!
hypostrophene
pentaprismane
H
Five degenerate minima! It should be possible to jump from
one to the other
It does! Known as Degenerate Cope Rearrangement
Rate constant
Degenerate Cope Rearrangement for Hypostrophene
GS TS
Activation energy
Q and Q* are the partition functions of GS and TS
Rate constant ~ 1.8 X 10-5 sec-1
Activation energy
25.31
B3LYP/6-31G** (kcal/mol)
Think of Hypostrophene
adsorbed on Al(100)
Rolling-TS Eact ~ 18 kcal/mol
Translation-TS:Eact ~ 65.5 kcal/mol
Rolling Motion
Same thing can happen with syn-
TOD!
Molecular Roller
GS TS
Activation energy
24.25
B3LYP/6-31G**(kcal/mol)
Syn-TOD
C2v
Cubane
Td
Activation energy
13.6 kcal/mol
(B3LYP/6-31G** C,H and 3-21G for Al)
TS
We conclude that:
Hypostrophene and tricyclooctadiene when chemisorbed on Al(100) surface should behave as ‘Molecular roller’
Bidisa Das, K.L Sebastian, Chemical Physics Letters, 330, 433 (2000).
MOLECULAR ROLLERS
MOLECULAR ROCKER
GS GS
Cope Rearrangement of Semibullvalene
Activation energy
5.5
B3LYP/6-31G**(kcal/mol)
TS
Semibullvalene on Al(100)
Metal surface: cluster of 14 or 32 Al atoms in two layers Hydrogen atoms at the edges.
B3LYP/Al:3-21G, C,H:6-31G**Ea = 21.8 kcal/mol
MOLECULAR ROCKER
Fe(CO)3
Fe(C
O) 3
Fe(C
O)3
Fe(CO)3 moving around
hypostrophene
Fluxionality for Rotational Motion
Hypostrophene
33.6
kcal/mol
4.3 kcal/mol
203i cm-1
Molecular wheel
C5H5Ge(CH3)3 is known to be fluxional!
Eact ~ 16.0 kcal/mol
MOLECULAR WHEEL
The activation barriers and rate constants
3.7X1085.74Sn (C,H: 6-31G**
& Sn: 3-21G)
1.6X10312.21Ge (C,H: 6-31G** & Ge: 6-31G**)
3.2X10214.04Si (C,H: 6-31G** & Si: 6-31G**)
rate constant at 298.15 K
(sec-1)
Eact
(kcal/mol)M atom bonded to Cp and basis-sets used
Cyclopentadienyl adsorbed on Ge surface,
should move like a
wheel!
Eact ~ 11.9 kcal/mol
Adsorbed to the same site!
Hopping onto adjacent Ge
atoms
2.5X1085.97
1.9X10411.90
8.5X10213.45
Rate constant(sec-1)
298.15 KEact
(kcal/mol)
Sn (C,H: 6-31G** & Sn:
3-21G)
Ge (C,H: 6-31G** & Ge:
6-31G**)
Si (C,H: 6-31G** & Si:
6-31G**)
M atom bonded to Cp and basis-sets used
Molecular Wheel
Sn
Ea = 5.97 kcal/mol
B. Das and K.L. Sebastian: CPL 357, 25 ( 2002)
That is not bad!Why don’t you call it a molecular “seal”?
We conclude that :
Bidisa Das, K.L Sebastian, Chemical Physics Letters, 357, 25 (2002).
The cyclopentadienyl co-adsorbed with hydrogen on Si/Ge/Sn (111) surfaces would form a system where the five membered ring can undergo spinning motion with low activation energies.
Molecular Rattle
A
B
B B
A
A
H
H
HHH
H
Does not happen! Ionization potential of
H too large!
Perhaps, in an excited state, this
might happen
H+
-
H+
-
Replace H with Li! Li+
-
Ring too small
Li+
-
These are the molecules that we studied but activation energies for the ‘umbrella inversion’
kind of motion was found to be high.
Eact~42.4 kcal/mol
Eact
a:315 kcal/mol
b:36.6kcal/mol
c:33.6kcal/mol
M. Oda, Pure & Appl. Chem. 58, 7 (1986), T.Z. Ktaz, P. A. Garratt, J. Am. Chem. Soc. 85, 2852 (1963).
C9H9-
Ea = 11.7 kcal/mol
B. Das and K.L. Sebastian: CPL, 365, 320 (2002)
Molecular Rattle
Proton going through benzene (C6H7
+)Mahapatra, Sathyamurthy, Current
Science, 1995
Cyclononatetraenyl-lithium
The activation barrier for the ‘umbrella
inversion’ in this case is ~11.5 kcal/mol
Normal mode analysis: 276 cm-1(GS), 274i cm-1(TS)
1D through ring motion calc.: 277 cm-1(GS), 267i cm-1 (TS)
Nature does it well!
Biological Molecular Motors
We know of several, efficient
molecular motors!
All of them occur in BIOLOGICAL
systems
Figure from: http://ccgb.umn.edu/~mwd/cell.html
Energy from photosynthesis
Figures and animation from: http://www.sp.uconn.edu/~terry/images/anim/ATPmito.html
ATP Synthase
Synthesizes ATP.Rotates while it
does this!
Most powerful known motor
ATP Synthase (Rotary)
Kinesin (Walker)
Proteins that WALK!
Works like a PORTER at the railway station
See animation at http://mc11.mcri.ac.uk/wrongtrousers.html
Myosin
Proteins that PUSH!
Myosin
For an animation, see the CD of the book: Molecular Biology of the Cell by
B. Alberts et. Al.
See also:
http://www.rpi.edu/dept/bcbp/molbiochem/MBWeb/mb2/part1/myosin.htm
Synthetic Molecular Motors
NO WAY near the natural
ones!
Rotaxane
shuttlestationstopper
station
-e+e
Electron Removal
+ H+
- H+
Proton Addition
Either electron removal or
proton addition
Catenanes
2-catenane 3-catenane
Switching by Oxidation-Reduction Reations
-e
+e
Catenanes – how to have light driven motor?
Leigh et. al. Nature, 424, 174 (2003)
Excitation of the station leads to unbinding!
station
shuttle
Light induced excitation of the shuttle is better!
K.L. Sebastian: Current Science, 87, 232 (2004)
Light induced conformational change driving translation!
Cis-trans isomerization as in Azobenzene could be
useful
Can one have……?
Brownian motion would drive the ratchet! But it can rotate only in
one direction! So thermal, random motion can be used to
drive the ratchet in one direction!
Molecular Ratchets, Second Law and Detailed Balance
[4]helicene
triptycene
Axle
It can undergo ratchet-like internal rotation!
A molecular ratchet was Synthesized and studied by Kelly et. al. Angewandte Chemie 109, 1969 (1997).
To probe rotational motion use NMR
That is clever!
Second Law verified!
But wait! Let us think about this!
There he goes! As mad as a
coot!
The fact that you can see rotation means
that your initial state is a non-equilibrium
one!So even if you had seen net rotation, that would not violate the second
law!
But you do not see any net rotation! What does
that mean?
Because of Detailed Balance!
K.L. Sebastian, Physical Review E61, 937 (2000)
CONCLUSIONS
Molecular Roller, Wheel, Rocker, Rattle!
Fluxional behavior can be used to get
interesting mechanical motion
Ratchets, Light driven motor……
Acknowledgements
Dr. Bidisa Das
Prof. Ashoka Samuelson
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