Infrared Spectroscopy of H2 in Fullerite and MOF-5
Stephen FitzGerald Hugh Churchill, Christie Simmons,
Phil Korngut, and Yorgos Strangas
Oberlin College
• Diffuse Reflectance Spectroscopy
• Cold Spectra on H2 in solid C60
• H2 in Metal-Organic Frameworks
Low-temperature infrared spectroscopy of H2 in crystalline C60
Phys. Rev. B 73, 155409 (2006)
Why use Infrared Spectroscopy to Study Adsorbed H2
• Problem: H2 not infrared active: no dipole moment
• Host induces dipole moment on H2
• H2 becomes “visible” when adsorbed
• Spectrum is very sensitive to the intermolecular potential
• Problem: spectra are very weak
Diffuse Reflectance Spectroscopy
• Light bounces around
within powder sample
• Very long path length
enhances absorption signal
• Problem: requires large
collecting optics
Cold Finger Cryostat Assembly
Hydrogen Trapped in C60 Fullerite
• H2 trapped at center of
octahedral interstitial site
• Only one H2 per site
• Sites separated by 10 Å
• A quantum molecule in a box
Quantum Dynamics of Adsorbed H2
• Vibration
Ev  v  1 / 2  v0
0
= 4161 cm-1 for free H2
• Rotation
E J  J  J  1 B0
B0 = 59 cm-1 for free H2
• Translation
En  n  1 / 2  n0
On the order of 100 cm-1
Room Temperature Spectrum
• R. M. Herman and J. C. Lewis
Phys. Rev. B 73, 155408 (2006)
Theory of the fundamental
vibration-rotation-translation
spectrum of H2 in C60 lattice
• Spectrum contains more than
100 modes: quite broad and
overlapping
Selection Rules Cold
• Para H2 in J = 0 rotational
state
Ortho H2 in J = 1 rotational
state
• Transitions dominated by
translational sidebands
• Expect to see 3 main peaks
cold
J=3
E
J=2
J=0
J=1
u=1
S(1)
Q(1)
S(0)
Q(0)
J=4
J=3
J=2
u=0
J=0
J=1
Low Temperature Spectra
Frequency Relative to Gas Phase
• Frequency red shift increases from
-57 to -62 cm-1
• Theory predicts shifts of -57, 60 and 63 cm-1 for Q, S(0), and S(1)
Absorbance
• Change in H2 polarization from
ground to excited state causes change
in interaction energy
1, J f   0, J i
 
V i
 0, J i
S(1)
S(0)
Q(0 and 1)
-75
-65
-55
-45
-1
Frequency Shift (cm )
Translational Sidebands
• Translational center-of-mass
frequencies all quite similar
S(1)
Absorbance
• Fine structure arises from crystal
field effects and rotationaltranslational coupling.
Predicted by Yidirim and Harris
Phys. Rev. B 66, 214301 (2002)
S(0)
Q(0 and 1)
-75
0
25
50
75
-1
Frequency shift (cm )
100
Translational Line-Shapes
0.4
S(1)
• Lorentzians yield significantly better
fits than Gaussian
• All much broader than for pure
vibrational-rotational (zero-phonon)
modes
0.3
Absorbance
• Individual peaks widths vary from
2 to 9 cm-1
S(0)
0.1
Q(0 and 1)
0.0
90
100
110 120 130
-1
Frequency (cm )
140
150
Metal-organic frameworks (MOFs)
•
Metal ion clusters + organic linkers
•
Initial result (2003):
4.5 wt % @ 78 K and 20 bar
•
Follow-up (2005):
1.7 wt % @ 77 K and 67 bar
microporous
structure
http://www.public.asu.edu/~rosebudx/MOF-5N.jpg
25.8 Å
MOF-5:
Zn4O(BDC)3
BDC = 1,4-benzenedicarboxylate
Room Temperature Spectra for H2 in MOF-5
• Vibrational red-shift of 25 cm-1
less than ½ of that of C60
• Evidence for multiple sites
• Zero-phonon bands dominate
Absorbance
Q(1)
S(1)
S(0)
S(2)
• Sites without inversion
symmetry
S(3)
4000
4400
4800
5200
-1
Frequency (cm )
5600
H2 in MOF-5 at 30 K
-3
Q Peak
20
Absorbance
• Only Q peak present
• S peaks absent
30x10
10
0
4000
4200
4400
4600
4800
-1
Frequency (cm )
5000
H2 in MOF-5 at 30 K
• Only Q peak present
• S peaks absent
30x10
-3
• In C60 S peaks arise from H2
quadrupole inducing dipole in
neighboring atoms
• Requires high polarizability host
Absorbance
20
10
0
4000
4200
4400
4600
4800
-1
Frequency (cm )
5000
Metal-organic frameworks (MOFs)
•
Metal ion clusters + organic linkers
•
Initial result (2003):
4.5 wt % @ 78 K and 20 bar
•
Follow-up (2005):
1.7 wt % @ 77 K and 67 bar
microporous
structure
http://www.public.asu.edu/~rosebudx/MOF-5N.jpg
25.8 Å
MOF-5:
Zn4O(BDC)3
BDC = 1,4-benzenedicarboxylate
Conclusion
• Diffuse Reflectance Infrared Spectroscopy ideal for
probing adsorbed H2
• Low-temperature spectra contain detailed fine structure
revealing intermolecular potential
• MOF-5 low-temperature spectra yields information about
the binding site