Structure of Polyhedral Oligomeric Silsesquioxanes (POSS) Oligomers Using Ion Mobility Mass

Spectrometry and Molecular Mechanics

by

Stanley Anderson (Westmont College)

Erin Baker, Connie Mitchell, Dena Bodzin, Michael T. Bowers (UCSB)

Tim Haddad, Ashwani Vij, Joe Schwab(Edwards AFB)

Why Study Silicon-Based Nanomaterials??• A wide range of application from

polymer modifiers to lubricants

• Robust physical and thermal properties of polymer systems useful for space applications

• Addition of POSS substituents gives polymers with– extended temperature ranges– reduced flammability– lower thermal conductivity– reduced viscosity– resistance to atomic oxygen– low density

• Major interest and funding by AFOSR!

Si

Si

O

O

Si

Si

Si

Si

O

O

O

O

SiO

Si

O

OO

OO

R R

R

R

R

R

R X

Anatomy of a Polyhedral OligomericSilsesquioxane (POSS®) Molecule

May possess one or more reactive groups suitable forpolymerization or grafting.

Thermally and chemically robust hybrid (organic-inorganic) framework.

Nanoscopic in size with an Si-Si distance of 0.5 nmand a R-R distance of 1.5 nm.

Nonreactive organic (R) groups for solubilization and compatibilization.

Precise three-dimensional structure for molecular levelreinforcement of polymer segments and coils.

PAS-03-082

Condensed POSS Cage Structures

R4D4(OH)4

R6D6(OH)2 R7D7(OH)3

Partially Condensed POSS Cage Structures Provide Synthetic

Routes to Monomers

Goals of POSS Work

• Understand how structure and functionality of POSS monomers affects polymer structure and properties

• Interact with synthetic chemists to characterize reaction steps and products

• Study oligomer structures in detail

• Create polymers with tailored properties.

v = const.v = K E

K = ion mobility

Concept of Ion Mobility

Drift cell

E

p(He)

Drift cell

E

p(He)

IonFelFfriction IonFelFfriction

Experimental Method

K = f (T, p, q, µ, σ)

T = temperaturep = pressureq = ion chargeµ = reduced massK = ion mobilityσ = collision cross section

σ = f ( )He–ion interactionIon shape

1500 1600 1700 1800 1900MASS

M + Li+

M + Na+

hν

SOURCE

TOFDETECTOR

TOF DRIFT CELL

QUADRUPOLE

DETECTORREFLECTRON

Trigger

TOF Mass Spectrum

Time-of-Flight (TOF) Mass Spectrometry

hν

SOURCE

TOFDETECTOR

TOF DRIFT CELL

QUADRUPOLE

DETECTORREFLECTRON

Trigger

Time-of-Flight (TOF) Mass SpectrometryArrival Time Distribution

One structure

Multiple structures

ATD

time

(Annealing/Energy Minimization)

AMBER 7 parameterized for Si

•Heat guess structure for 30 ps at 600-1400K

•Cool structure exponentially to 50K for 10 ps

•Energy minimize the structure

•Use final structure as initial structure for next cycle

Theoretical Method

Molecular Mechanics/Dynamics

Structures

Theoretical Method

Structures Collision Cross-Sections (σ)

Relative Energy (kcal/mol)

-5 0 10 15 20 25220

240

260

280

Cro

ss-S

ecti

on

(Å

2 )

5

Theoretical Method

Compare

Experimental Method

ARRIVAL TIME DISTRIBUTIONS (ATDs)

time

MOBILITIES

KoCOLLISION

CROSS-SECTIONS

σ

MOLECULAR MECHANICS/DYNAMICS

-10 0 10 20 30 40 50 60 70

840

860

880

900

920

940

960

1

2

3

4

5

6

7

8

9

101112

13

14

15

16

17

18

19

20

21

22

23

2425

2627

28

2930

31

32

33

34

35

3637

38

39

40

4142

43

4445

46

4748

495051

5253

54

55

5657

58

59

60

61

6263

64

65

66

67

68

6970

71

7273

74

75

76

77 78

7980

8182

83

84

85

86 87

88

8990

91

92

93949596 9798

99100

Cro

ss S

ectio

n (A

^2)

Relative Energy (Kcal/mol)

STRUCTURESCOLLISION

CROSS-SECTIONS

σ

Putting It All Together….Å

2 )

poly-Cp7T8(PMA)

Si

Si

O

O

Si

Si

Si

Si

O

O

O

O

SiO

Si

O

OO

OO

R R

R

R

R

R

R

OO

(CH2)3

C

CH3CH2

HH

Head

TailO

x

POSS Propylmethracylate Oligomers

S. E. Anderson, E. S. Baker, C. Mitchell, T. S. Haddad, and M. T. Bowers, “Structure of Hybrid Polyhedral Oligomeric Silsesquioxane Polymethacrylate Oligomers Using Ion Mobility Mass Spectrometry and Molecular Mechanics,” Chem Mater., 17, 2537 (2005).

2000

Mass / charge

1400

Inte

nsi

ty

800 2600 3200

2-mer 3-mer

1-mer

[MALDI-TOF Spectrum of (PMA)Cp7T8]x.Na+

…adding ATDs andexperimental cross-sections

σexpt = 248 Å2

σexpt = 377, 402 Å2 σexpt = 539 Å2

244

246

248

250

252

254

256

258

0 0.5 1 1.5 2 2.5 3

Relative Energy (kcal/mol)

Cro

ss-S

ectio

n(PMA)Cp7T8

.Na+ Scatter Plot

(PMA)Cp7T8.Na+ 1-mer Structure

σexpt = 248 Å2

σtheory = 252 Å2

Na+

365

370

375

380

385

390

395

400

405

0 5 10 15 20 25 30

Relative Energy (kcal/mol)

Cro

ss S

ectio

n (Å

2 )[(PMA)Cp7T8]2

.Na+ 2-mer Scatter Plot

365

370

375

380

385

390

395

400

405

0 5 10 15 20 25 30

Relative Energy (kcal/mol)

Cro

ss S

ectio

n (Å

2 )[(PMA)Cp7T8]2

.Na+ 2-mer Scatter Plot

1-mer scatter plot range

365

370

375

380

385

390

395

400

405

0 5 10 15 20 25 30

Relative Energy (kcal/mol)

Cro

ss S

ectio

n (Å

2 )[(PMA)Cp7T8]2

.Na+ 2-mer Scatter Plot

trans

365

370

375

380

385

390

395

400

405

0 5 10 15 20 25 30

Relative Energy (kcal/mol)

Cro

ss S

ectio

n (Å

2 )[(PMA)Cp7T8]2

.Na+ 2-mer Scatter Plot

trans

cis

365

370

375

380

385

390

395

400

405

0 5 10 15 20 25 30

Relative Energy (kcal/mol)

Cro

ss S

ectio

n (Å

2 )[(PMA)Cp7T8]2

.Na+ 2-mer Scatter Plot

extended trans

cis

trans

[(PMA)Cp7T8]2.Na+ (2-mer) Structures

transσexpt = 378 Å2

σtheory = 377 Å2

cisσexpt = 402 Å2

σtheory = 393 Å2

extended transσexpt = 402 Å2

σtheory = 393Å2

370

380

390

400

410

420

430

0 500 1000 1500 2000

Time (ps)

Cro

ss-s

ectio

ns (Å

)

cis

300K 800K

"cis" "cis"

"trans" "trans“ + “ext. trans”

[(PMA)Cp7T8]2.Na+ Dynamics

[(PMA)Cp7T8]3.Na+ 3-mer Regioisomers

HH2C

H2C

H3CR

R CH3

CH3

CH3 R

HH2C

H2C

H3CR

H3C R

CH3

CH3 R

R = POSS(propylmethacrylate)

syndiotactic isotactic

R = POSS(propylmethacryl)

* * * *

* = chiral center

[(PMA)Cp7T8]3.Na+ 3-mer Scatter Plots

510

520

530

540

550

560

570

580

590

600

610

0 5 10 15 20 25 30 35 40Reltive Energy (kcal/mol)

Cro

ss-s

ectio

n (A

2 )

Relative Energy (kcal/mol)

510

520

530

540

550

560

570

580

590

600

610

0 5 10 15 20 25 30 35 40Reltive Energy (kcal/mol)

Cro

ss-s

ectio

n (A

2 )

Relative Energy (kcal/mol)

[(PMA)Cp7T8]3.Na+ 3-mer Scatter Plots

Syndiotactic isomer

isotactic isomer

[(PMA)Cp7T8]3.Na+ Syndiotactic Isomer

σexpt = 539 Å2

σsyn = 540 Å2

σiso = 565 Å2

| 8.3 – 8.9 Å |(cage centers)

8-mer POSS 8-mer

Non-POSS PMA 8-mer vs. [(PMA)Cp7T8.]8 8-mer

POSS-PMA Oligomer Summary

1. Low energy structures obtained by molecular modeling agree with experiment within ~2%.

2. Cis, trans, and extended trans structures of the 2-mer give rise to the two ATD features.

3. Structures seem to be determined primarily by non-bonded interactions of the cyclopentyl capping groups that cause the cages to pack in a variety of ways.

4. 3-mer structure is consistent with the syndiotacticregioisomer; it shows cage-cage non-bondedinteractions similar to 2-mer.

5. Presence of the cation does not determine the oligomer backbone structure as in non-POSS oligomeric systems previously studied.

Si

Si

O

O

Si

Si

Si

Si

O

O

OSi

Si

O

OO

OO

Cy Cy

Cy

OCy

Cy

Cy

Cy

O

Cy

OSi

Si

O

O

Si

Si

Si

Si

O

O

O

O

SiO

Si

O

OO

OO

Cy

Cy

Cy

Cy

Cy

Cy

Si

Si

O

O

Si

Si

Si

Si

O

O

O

O

SiO

Si

O

OO

OO

Cy

Cy

Cy

Cy

Cy

Cy

Cy

Cy

O

OSi

O

Si

Si

SiO

O

O

OO

OO

Cp

Cp

Cp

Cp

O

Cp

Si

O

Si

Si

Si

O

Cp

Cp

Si

O

Si

Si

SiO

O

OSi

O

Si

Si

SiO

O

O

OO

OO

Cp

Cp

Cp

Cp

Cp

Cp

Cp

POSS Siloxanes

Cp = cyclopentyl

Cy = cyclohexyl

Cy = cyclohexyl

1000 1100 1200 1300 1400 1500 1600 1700 1800 1900

Mass

[Cp7T8]2O.Na+ Siloxane 2-mer Mass Spectrum

1000 1100 1200 1300 1400 1500 1600 1700 1800 1900

Mass

[Cp7T8]2O.Na+ Siloxane 2-mer Mass Spectrum

ATDσexpt = 355 Å2

350

355

360

365

370

375

0 2 4 6 8 10 12

Relative Energy (kcal/mole)

Cro

ss-s

ectio

n (A

2)[Cp7T8]2O.Na + Siloxane 2-mer Scatter Plot

σcalc = 357 Å2

σcalc = 365 Å2

σexpt = 355 Å2

(Å2 )

[Cp7T8]2O.Na+ Siloxane 2-mer Comparison

staggered eclipsed|

8.4

±0.

1 Å

|

σcalc = 357 Å2

σexpt = 355 Å2σcalc = 365 Å2

σexpt = 355 Å2

Cp = cyclopentyl

[Cy7T8]2O.Na+ Siloxane 2-mer Structures

|8.

4 ±

0.1

Å|

σcalc = 433 Å2

σexpt = 400 Å2

Cy = cyclohexyl

equatorial

No folds

[Cy7T8]2O.Na+ Siloxane 2-mer Structures

|8.

4 ±

0.1

Å|

σcalc = 395 Å2

σexpt = 400 Å2

Cy = cyclohexyl

axial

3 folds

[Cy7T8]2O Siloxane 2-mer X-Ray Structures

All Cy’s equatorial Cy’s disordered(chair to boat)

Cy = cyclohexyl

100 K 300 K

| |8.3 – 8.6 Å

Cage Centers

[Cp7T8]2O.Na+ Siloxane 2-mer Cage Separation

Covalent cage-cage (center) distance provides benchmarkfor non-bonded interactions!

[Cy7T8O-Cy8T6D2-OCy7T8].Na+ “3-Mer” Mass Spectrum

1500 2000 2500 3000 3500MASS

2.5 2.7 2.9 3.1 3.3

Arrival Time (ms)

σEXPT = 557 Å2 Cy 3-mer+ Na+

σcalc = 557 Å2

exo,exo

Siloxane Summary

• Low energy siloxane 2-mer and 3-mer structures obtained by molecular modeling agree with experiment within ~2%.

• Staggered or folded R group structures more compact.

• Structures seem to be determined primarily by non-bonded interactions of the cycloalkyl capping groups that cause the cages to closest pack.

• Distances between center of cages in siloxane 2-mer are in range from 8.3 – 8.6 Å. This provides a benchmark for cage-cage interactions.

Where are We Headed??1. Larger Oligomers – we have been limited to trimers 2. Detection issues – larger oligomers without electronegative

groups not detected! 3. Novel synthetic approaches to suitable oligomers:

• Prepare amino derivatives which can be protonated (Bryan Coughlin, U. Mass.)

• Fluoride derivatives – modify MALDI/TOF for routine negative ions or negative ion mode ESI

(CH3)4NF

(CH3)4N+

THF

F-

Ph8T8 Ph8T8F-

(CH3)4NF

(CH3)4N+

THF

F-

Ph8T8 Ph8T8F-

Acknowledgments

• National Research Council / National Academy of Sciences Senior Research Associateship, 2003-05, 2005-06.

• AFOSR $$$$

• ASEE Summer Faculty Fellowship, 2005.

• Bryan Coughlin, U. MassachusettsDavid Marten, Westmont College

The Bowers Group

Website: http://bowers.chem.ucsb.edu