resonant transition = “spin flip” of electron allowed transitions: m s = + 1 = 9.6 ghz (x-band...
TRANSCRIPT
ESR Investigation of Matrix Isolated H4
+
MattCorrenti
Lon B. Knight and John BanisaukasDepartment of Chemistry, Furman University, Greenville, S.C.
The ESR SpectrometerResonant Transition = “spin flip” of electron
Allowed transitions: DMs = + 1
n = 9.6 GHz (X-band microwave)
Ex) One unpaired electron:
E DE = hn
Ms =12
Ms = - 12
Magnetic Field (H)
Hyperfine structure = due to interaction of electron spin (S) and nuclear spin (I)
Ex) Nuclear spin (I) = ½ : MI = ½, -½
One unpaired electron: MS = ½, -½
Allowed transitions: DMS = + 1 DMI = 0
# Hyperfine transitions = 2NI + 1
N = # equivalent magnetic nuclei
E
MI = -½
MI = ½
MI = ½
MI = -½ Ms =12
Ms =12
-
Magnetic Field (H)
DE = hn
The Matrix ApparatusGas Inlet
Liquid He Cooled
Cold Shield77 K
Copper Rod2 K
ESR Cavity
Magnets
+
+
+ +
-
-
--
- Neon Atom
} - Analyte Radicals
Under high vacuum10-8 – 10-12 Torr
Dihydrogen Cation: H2+
Neon Matrix
Temp = 2.0 K
Ne/H2 = 5,000 : 1
Expected Spectrum
two equivalent H
hyperfine = triplet
3020 3040 3060 3080
3670 3690 37103650
+ +-
Simplest Molecule!
A┴ = 851 MHzA = 940 MHz
g┴ = 2.0023g = 2.0010
Observed Spectrumouter lines of triplet (304.4 G)
center of triplet obscured at ge
Nuclear Spin (I)
H = ½
H3D+
Neon Matrix
Dep. Temp. = 2 K
Ne : (H2+HD) = 500 : 1H2 : HD = 3 : 1
3150 317031103090 3130Magnetic Field (gauss)
Observed Spectrum
Large triplet (33.1 G)
of small triplets (8.0 G)
H atom
H2D2+
H4+
1 2 3 4 5 6 7 8 9
H4+
2 K 31303110Magnetic Field (gauss)
Moyano, G.; Pearson, D.; Collins, M. J. Chem. Phys. 2004, 121, 12396.
Nuclear Spin Statistics• Fermi Principle•Only able to observe anti-symmetric lines
Resultant Pattern
1 1Doublet
Neon Matrix
Dep. Temp. = 2.0 K
Ne : H2 = 500 : 1
Expected Hyperfine Pattern
1 2 1 1 1 1
Symmetric Lines
Anti-Symmetric
11 1 1 12 2
H4+ Expected Hyperfine Pattern
1 3 3 1
Symmetric Lines
1 1 1 1
Anti-Symmetric
2 2
Nuclear Spin Statistics• Fermi Principle•Only able to observe anti-symmetric lines
Resultant Pattern
Doublet2 2
3110
Magnetic Field (gauss)
3130 7 K10 K
Neon Matrix
Dep. Temp = 2.0 K
Ne : H2 = 500 : 1
D4+
Magnetic Field
7 K 3300 33203280
21 3 4 5 6 7
Neon Matrix
Dep. Temp = 2.0 K
Ne : D2 = 200 : 1
8.3 G
5.2 G
78 G
155.8 G
512 G
464.5 G
466.7 G
7.8 G
H2
D2
HD
34003115 3680
D atomD atom
H atom H atom
D atom
Sample Gas
H4+
D4+
H3D+
ProposedCluster
Summary of ESR Spectra for H4+ Isotopomers
(in Neon at 2 K)
Dihydrogen cation (H2+): First observation in rare gas matrix (very reactive)
Requires extremely dilute matrix (Ne : H2 = 5,000 : 1) and low temperature (2.0 K)
H4+ isotopomers = structure dependent on hydrogen isotopes present and matrix temperature
H
H
HH
H
H
HH
H
HH HH
H
D
HH
H
D
HH
D
HH HH
H
H
DD
H
H
DD
H
DD DDD
D
D
H
DD
D
H
DD
D
D
DD
D
D
H4+ temperature dependence:
H4+ = only observed after 2.0 K deposition, but survives up to 9 K
H4+ appears to undergo structural changes as the thermal
energy in the matrix increases
By invoking nuclear spin statistics, the spectral patterns observed for H4
+ can be explained
Summary/Conclusions
Acknowledgements
Furman University
Childers/Knight/Arrington Research Fund
Sponsored by Dr. Alfred G. Childers
Furman Advantage Program