Lecture 25, December 2, 2009 - KOCW

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EEWS-90.502-Goddard-L15 1

Nature of the Chemical Bond

with applications to catalysis, materials science, nanotechnology, surface science,

bioinorganic chemistry, and energy

William A. Goddard, III, [email protected] WCU Professor at EEWS-KAIST and

Charles and Mary Ferkel Professor of Chemistry, Materials Science, and Applied Physics,

California Institute of Technology

Course number: KAIST EEWS 80.502 Room E11-101 Hours: 0900-1030 Tuesday and Thursday

Senior Assistant: Dr. Hyungjun Kim: [email protected]

Manager of Center for Materials Simulation and Design (CMSD) Teaching Assistant: Ms. Ga In Lee: [email protected]

Special assistant: Tod Pascal:[email protected]

Lecture 25, December 2, 2009

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Schedule changes

Dec. 2, Wednesday, 3pm, L25, additional lecture, room 101 Dec. 3, Thursday, 9am, L26, as scheduled

Dec. 7-10 wag meeting Pasadena; no lectures,

Dec. 14, Monday, 2pm, L27, additional lecture, room 101

Dec. 15, Final exam 9am-noon, room 101

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Last time

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Summary: Oxidation

Propene + O

₂

Acrolein + H

₂

O DG

₆₇₃

= -74.0 kcal/mol

propene 1/2 O₂ or [O]

DG₆₇₃~ 37.0

~O Bi

O~

HO

IV

2.5 Bi^III

~O O~

O~ Bi

~O O~

O~

O

V

allyl propene

allyl, H₂O -35.9

~O Mo^V

O~

O O CH₂CHCH₂

DG₆₇₃= 11.4

2 acrolein

~O Mo^IV

O~

~O Mo^V

O~

O OH

O

~O Mo^VI

O~

O O 2 allyl

~O Mo^VI

O~

O O

~O Mo^VI

O~

O O

~O Mo^IV

O~

O O CHCHCH₂

~O Mo^V

O~

O OH

~O Mo^VI

O~

O O

~O Mo^V

O~

O OH 1/2 O₂ or [O]

~O Mo^VI

O~

O O

~O Mo^VI

O~

O O

~O Mo^V

O~

O O CH₂CHCH₂

~O Mo^VI

O~

O O

~O Mo^V

O~

O OH

~O Mo^IV

O~

O O CHCHCH₂

~O Mo^IV

O~

O

~O Mo^V

O~

O OH

~O Mo^VI

O~

O O

~O Mo^V

O~

O OH

DG₆₇₃= 9.2

H₂O

O₂ or 2[O]

DG₆₇₃= -12.8

DG₆₇₃~ -105.6 DG₆₇₃~ -50.5

All in agreement with

experiment, except for

the role of Bi

^V

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Calculation: Allyl adsorption

• Chemisorption on Mo=NH is easier than on Mo=O by 10 kcal/mol

• Consistent with the assumption “k

_NI

>> k

_OI

”

• Spectator effect: Mo=O > Mo=NH by 7 kcal/mol

• Consistent with “k

_OI

>>k´

_OI

”

Cl

Mo^VI Cl O O

Cl

Mo^V Cl

O O

CH₂=CHCH₂

Cl

Mo^VI Cl O NH

Cl

Mo^V Cl O NH CH₂=CHCH₂

Cl

Mo^VI Cl O NH

Cl

Mo^V Cl O NH

CH₂CH=CH₂

Cl

Mo^VI Cl HN NH

Cl

Mo^V Cl HN NH

CH₂CH=CH₂ DDG₆₇₃= 5.7

DDG₆₇₃=12.8

DDG₆₇₃= -4.0

DDG₆₇₃= 2.9

O insertion N insertion

Spectator Mo=O

Spectator

Mo=NH

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One-center or Multi-center?

O/NH Insertion

Cl Mo

Cl O OH DDG₆₇₃=18.6

Cl Mo

Cl O O CH₂=CHCH₂

DDG₆₇₃= 4.6

Cl Mo O H

CHCH=CH₂

DDG₆₇₃=14.5

Cl O

Cl Mo

Cl O O

Cl Mo

Cl O OH

Cl Mo

Cl O O

CHCH=CH₂ CHCH=CH₂

di-oxo

Cl Mo

Cl HN NH₂ DDG₆₇₃= -4.8

Cl Mo

Cl NH NH CH₂=CHCH₂

DDG₆₇₃= -6.5 Cl

Mo HN H

CHCH=CH₂

DDG₆₇₃= 4.3

Cl NH

Cl Mo

Cl HN NH

Cl Mo

Cl HN NH₂

Cl Mo

Cl HN NH

di-imido

oxo-imido

Cl Mo

Cl HN OH DDG₆₇₃~ 4.4

Cl Mo

Cl NH O CH₂=CHCH₂

DDG₆₇₃= -9.2 Cl

Mo HN H

CHCH=CH₂

DDG₆₇₃~ 14.5

Cl O

Cl Mo

Cl O NH

Cl Mo

Cl O NH₂

Cl Mo

Cl HN O

Multi-center for di-oxo Multi-center for oxo-imido

May be one or two center for di-imido

All in agreement with experiment

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Vanadium Coordination

M1 2 site has V=O Vanadyl groups pointing into the C7 2 channel

M3 site, V=O vanadyl groups align along the c-axis,

similar to bulk V 2 O 5

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Final Configuration from ReaxFF- RD of Mo 3 VOx with Propane

propane molecules Yellow: in channel blue-gray: exterior

Initial Configuration final Configuration

final Configuration Top view

Mo = purple V = green O = red

3 propane molecules all

go into the C7

₂

channel

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3 Propane moved into C7

₂

Heptagonal Channel

Average channel radius = 4.6 Å length ~ 18Å Channel C7

₁

is

smaller with average radius = 4.1Å and remains empty

Cross-section final configuration from

the propane/Mo3VOx ReaxFF-RD

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Speculations about M1 selective oxidation from ReaxFF RD Simulations

We believe that the migration of propane into the heptagonal channels found in the RD plays an important role in the selectivity.

It has V=O chain, just like V

₂

O

₅

and VOPO that can break CH bond (E

_act

~ 28 kcal/mol)

After the activation, a 2

^nd

H can be transferred to any oxo group or ether group to form propene

But this propene is in a protected site inside the channel where the V=O chain has already been de-activated so that it can undergo selective activation of

allylic CH bond followed by trapping on a M=O bond to form M-O-CH2-CH-CH2

and then it continues the same as for propene selective oxidation in BiMoOx etc

We think that the unselective oxidation to CO

₂

occurs at the surfaces and grain

boundaries, where there may be multiple V=O sites, leading to rapid oxidation

Thus to obtain increased selectivity want to poison the surface V=O sites but

not the channel V=O chains. This might be done with bulky groups

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Hemoglobin

Blood has 5 billion erythrocytes/ml

Each erythrocyte contains 280 million hemoglobin (Hb) molecules

Each Hb has MW=64500 Dalton (diameter ~ 60A)

Four subunits (a1, a2, b1, b2) each with one heme subunit

Each subunit resembles myoglobin (Mb) which has one heme

Hb

Mb

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The action is at the heme or Fe-Porphyrin molecule

Essentially all action occurs at the heme, which is basically an Fe-Porphyrin molecule

The rest of the Mb serves mainly to provide a

hydrophobic

envirornment at the

Fe and to protect

the heme

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The heme group

The net charge of the

Fe-heme is zero. The VB structure shown is one of several, all of which lead to two neutral N and two negative N.

Thus we consider that the Fe is Fe 2+ with a d 6 configuration

Each N has a doubly

occupied sp 2 s orbital

pointing at it.

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Axial ligands to heme group

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Energies of the 5 Fe 2+ d orbitals

x 2 -y 2

z 2 =2z 2 -x 2 -y 2

xy

xz

yz

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Exchange stabilizations

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Summary 4 coord and 5 coord states

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Free atom to 4 coord to 5 coord

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Bond O2 to Mb

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Bonding O 2 to Mb

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Role of exchange energy

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compare bonding of CO and O2 to Mb

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New

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Bonding in metallic solids

Mosty of the systems discussed so far in this course have been covalent, with the number of bonds related to the number of

valence electrons.

Thus we have discussed the bonding of molecules such as CH4, benzene, O2, and Ozone.. The solids such as diamond, silicon, GaAs, are generally insulators or semiconductors

We have also considered covalent bonds to metals such as FeH+, (PH 3 ) 2 Pt(CH 3 ) 2 , (bpym)Pt(Cl)(CH3), The Grubbs Ru catalysts

We have also discussed the bonding in ionic materials such as (NaCl)n, NaCl crystal, and BaTiO3, where the atoms are best modeled as ions with the bonding dominated by electrostatics

Next we consider the bonding in bulk metals, such as iron, Pt, Li,

etc. where there is little connection between the number of bonds

and the number of valence electrons.

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Elementary ideas about metals and insulators

The first attempts to develop quantum theory started with the Bohr model H atom with

electrons in orbits around the nucleus.

With Schrodinger QM came the idea that the electrons were in distinct orbitals (s, p, d..), leading to a universal Aufbau diagram which is filled with 2 electrons in each of the lowest orbitals

For example:

O (1s) 2 (2s) 2 (2p) 4

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Bringing atoms together to form the solid

As we bring atoms together to

form the solid, the levels broaden into energy bands, which may

overlap . Thus for Cu we obtain Energy

Density states Fermi energy

(HOMO and LUMO

Thus we can obtain

systems with

no band gap.

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Metals vs inulators

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conductivity

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The elements leading to metallic binding

There is not yet a conceptual description for metals of a quality comparable to that for non-metals. However

there are some trends, as will be described

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Body centered cubic (bcc), A2

A2

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Face-centered cubic (fcc), A1

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Alternative view of fcc

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Closest packing layer

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Stacking of 2 closest packed layers

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Hexagaonal

closest packed

(hcp) structure, A3

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Cubic closest packing

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Double hcp

The hexagonal lanthanides mostly exhibit a packing of closest packed layers in the

sequence

ABAC ABAC ABAC

This is called the double hcp structure

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b c c h cp fcc m is

Structures of elemental metals

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Binding in metals

Li has the bcc structure with 8 nearest neighbor atoms, but there is only one valence electron per atom.

Similarly fcc and hcp have 12 nearest neighbor atoms, but Al has only three valence electrons per atom.

Clearly the bonding is very different than covalent One model (Pauling) resonating valence bonds

Problem is energetics:

Li 2 bond energy = 24 kcal/mol➔ 12 kcal/mol per valence electron

Cohesive energy of Li (energy to atomize the crystal is 37.7

kcal/mol per valence electron. Too much to explain with resonance New paradigm: Interstitial electron model (IEM). Each valence

electron localizes in a tetrahedron between four Li nuclei.

Bonding like in Li 2 + , which is 33.7 kcal/mol per valence electron

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GVB orbitals of ring M 10 molecules

Get 10 valence electrons each localized in a bond midpoint

Calculations treated all 11 valence electrons of Cu, Ag, Au using effective core potential.

All electrons for H and Li

R=2 a

₀

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Geometries of Li clusters

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