Crystal Structure of a PolyethyleneCrystal Structure of a PolyethyleneOxide-Urea Inclusion Compound: Oxide-Urea Inclusion Compound:

the End of a 45-year-old Storythe End of a 45-year-old Story

Laura AdduciProfessor Bruce Foxman

Brandeis UniversityApril 23, 2009

Inclusion CompoundsInclusion Compounds

O

C

H2N NH2

Urea

HO

OH

n

Polyethylene oxide(PEO)

• Host/guest structure

• Host forms “channels” that are held together by hydrogen bonding

• Polyethylene oxide is a better guest molecule than, for example, alkanes, because it can hydrogen bond to the host urea molecules

Background: 1964Background: 1964

Reported that a Polyethylene-oxide inclusion compound had been synthesized

No crystal structure

Reported hexagonal crystals

Suggested 9:4 ratio of PEO to urea molecules

Background: the 1991 paperBackground: the 1991 paper

Background: the Li Di thesisBackground: the Li Di thesis

Acquisition of New Reflection DataAcquisition of New Reflection Data• Saturated solutions of Polyethylene Oxide (average molecular weight 3400 g/mol) and Urea in ethanol were mixed in equal amounts

• Stirred for 2 hours

• Allowed to evaporate at room temperature over the course of 3 days

• Suitable crystal was selected and cut to an appropriate size

• X-ray diffraction data was gathered at 120 K on a Bruker-Nonius Kappa Apex II diffractometer equipped with Mo radiation

• Collected ______ reflections, as opposed to the ___ reflections from the 1991 paper and ____ reflections for the Li Di thesis

Current Work 1Current Work 1

Started with Li Di’s thesis (structure A) and the 1991 Macromolecules paper (structure B).

Transformed the coordinates of the Li Di structure so that they matched those in the 1991 structure.

Refined both structures separately. Decided to refine the non-hydrogen urea atoms anisotropically and all other atoms isotropically.

Current Work 2Current Work 2

Imported my new reflections into each solution. Refined.

Problem: CRYSTALS automatically detects atoms that are close to special positions and puts them on the special positions.

Solution: Manually specify which should be on special positions.

Attempt to restrain bond lengths and angles – unsuccessful.

Combine.

Current Work 3Current Work 3

More problems.

Return to modified Structure A.

Try to figure out connectivity of the polymer chain: easy for Structure B because end atoms are on two-fold rotation axes.

For Structure A: must examine bond lengths and angles to choose the most logical atom.

Current Work 4Current Work 4

Started with Li Di’s thesis and the 1991 Macromolecules paper.

Refined both structures separately.

x, y, z -y, x-y, z+1/3 -x+y, -x, z+2/3

Chains in Structure AChains in Structure A

Symmetryoperations:

x, y, z -y, x-y, z+1/3 -x+y, -x, z+2/3

Chains in Structure BChains in Structure B

Symmetryoperations:

Structure BStructures B and A

6 PEO Chains (Rotation as insert)6 PEO Chains (Rotation as insert)

6 PEO Chains (full-size rotation)6 PEO Chains (full-size rotation)

Structure BStructures B and AStructures B and AStructures B and AStructures B and AStructures B and AStructures B and A

Hydrogen Bonding in Urea ChannelHydrogen Bonding in Urea Channel

Hydrogen Bonding Within ChannelHydrogen Bonding Within Channel

View along the C-axisView along the C-axis

OccupanciesOccupancies

Occupancy of C6-C2: 0.16666

C101O102

C103

C104

O105C106

C107O108

C109

C110

O111

C112

O 1

C2

C3

O 4

C5

C6

O 7

Occupancy of each atom: 0.08333

Occupancy of O1 and O7:

0.08333

OccupanciesOccupanciesStructure A Structure B

Num. Occ. Total Num. Occ. Total

Carbon 8 x 0.08333 = 0.6666 4 x 0.1666 = 0.6666

Oxygen 4 x 0.08333 = 0.3333 1 x 0.1666 = 0.1666

2 x 0.5(0.1666) = 0.1666

} 1.333 carbons

} 0.6666 oxygens

31 symmetry: multiply all atoms by 3

1.333 x 3 = 4 carbon atoms 0.6666 x 3 = 2 oxygen atoms

Torsion Angles ???Torsion Angles ???

Future WorkFuture Work

• PEO-Thiourea inclusion compound (already powder cell, could not make single crystal

• PEO with different molecular weights

AcknowledgementsAcknowledgements

•Professor Bruce Foxman

•Josh Chen