resonant transition = “spin flip” of electron allowed transitions: m s = + 1 = 9.6 ghz (x-band...

11
ESR Investigation of Matrix Isolated H 4 + Matt Corren ti Lon B. Knight and John Banisaukas

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Page 1: Resonant Transition = “spin flip” of electron Allowed transitions:  M s = + 1 = 9.6 GHz (X-band microwave) Ex) One unpaired electron: E  E = h M s =

ESR Investigation of Matrix Isolated H4

+

MattCorrenti

Lon B. Knight and John BanisaukasDepartment of Chemistry, Furman University, Greenville, S.C.

Page 2: Resonant Transition = “spin flip” of electron Allowed transitions:  M s = + 1 = 9.6 GHz (X-band microwave) Ex) One unpaired electron: E  E = h M s =

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

Page 3: Resonant Transition = “spin flip” of electron Allowed transitions:  M s = + 1 = 9.6 GHz (X-band microwave) Ex) One unpaired electron: E  E = h M s =

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

Page 4: Resonant Transition = “spin flip” of electron Allowed transitions:  M s = + 1 = 9.6 GHz (X-band microwave) Ex) One unpaired electron: E  E = h M s =

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

Page 5: Resonant Transition = “spin flip” of electron Allowed transitions:  M s = + 1 = 9.6 GHz (X-band microwave) Ex) One unpaired electron: E  E = h M s =

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

Page 6: Resonant Transition = “spin flip” of electron Allowed transitions:  M s = + 1 = 9.6 GHz (X-band microwave) Ex) One unpaired electron: E  E = h M s =

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

Page 7: Resonant Transition = “spin flip” of electron Allowed transitions:  M s = + 1 = 9.6 GHz (X-band microwave) Ex) One unpaired electron: E  E = h M s =

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

Page 8: Resonant Transition = “spin flip” of electron Allowed transitions:  M s = + 1 = 9.6 GHz (X-band microwave) Ex) One unpaired electron: E  E = h M s =

D4+

Magnetic Field

7 K 3300 33203280

21 3 4 5 6 7

Neon Matrix

Dep. Temp = 2.0 K

Ne : D2 = 200 : 1

Page 9: Resonant Transition = “spin flip” of electron Allowed transitions:  M s = + 1 = 9.6 GHz (X-band microwave) Ex) One unpaired electron: E  E = h M s =

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)

Page 10: Resonant Transition = “spin flip” of electron Allowed transitions:  M s = + 1 = 9.6 GHz (X-band microwave) Ex) One unpaired electron: E  E = h M s =

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

Page 11: Resonant Transition = “spin flip” of electron Allowed transitions:  M s = + 1 = 9.6 GHz (X-band microwave) Ex) One unpaired electron: E  E = h M s =

Acknowledgements

Furman University

Childers/Knight/Arrington Research Fund

Sponsored by Dr. Alfred G. Childers

Furman Advantage Program