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 = 1 2 M s = Magnetic Field (H) Hyperfine structure = due to interaction of electron spin (S) and nuclear spin (I) Ex) Nuclear spin (I) = ½ : M I = ½, -½ One unpaired electron: M S = ½, -½ Allowed transitions:  M S = + 1  M I = 0 # Hyperfine transitions = 2NI + 1 N = # equivalent magnetic nuclei E M I = -½ M I = ½ M I = -½ M s = Magnetic Field (H)  E = h

Gas Inlet Liquid He Cooled Cold Shield 77 K Copper Rod 2 K ESR Cavity Magnets Neon Atom } - Analyte Radicals Under high vacuum – Torr

Neon Matrix Temp = 2.0 K Ne/H 2 = 5,000 : 1 Expected Spectrum two equivalent H hyperfine = triplet Simplest Molecule! A ┴ = 851 MHz A ‌ ‌‌ = 940 MHz g ┴ = g ‌‌ ‌‌ = Observed Spectrum outer lines of triplet (304.4 G) center of triplet obscured at g e Nuclear Spin (I) H = ½

Neon Matrix Dep. Temp. = 2 K Ne : (H 2 +HD) = 500 : 1 H 2 : HD = 3 : Magnetic Field (gauss) Observed Spectrum Large triplet (33.1 G) of small triplets (8.0 G) H atom H2D2+H2D2+ H4+H

2 K Magnetic Field (gauss) Moyano, G.; Pearson, D.; Collins, M. J. Chem. Phys. 2004, 121, Nuclear Spin Statistics Fermi Principle Only able to observe anti-symmetric lines Resultant Pattern 1 Doublet Neon Matrix Dep. Temp. = 2.0 K Ne : H 2 = 500 : 1 Expected Hyperfine Pattern Symmetric Lines Anti-Symmetric 1 1 2

Expected Hyperfine Pattern 1331 Symmetric Lines Anti-Symmetric 22 Nuclear Spin Statistics Fermi Principle Only able to observe anti-symmetric lines Resultant Pattern Doublet Magnetic Field (gauss) K 10 K Neon Matrix Dep. Temp = 2.0 K Ne : H 2 = 500 : 1

Magnetic Field 7 K Neon Matrix Dep. Temp = 2.0 K Ne : D 2 = 200 : 1

8.3 G 5.2 G 78 G G 512 G G G 7.8 G H2H2 D2D2 HD D atom H atom D atom Sample Gas H4+H4+ D4+D4+ H3D+H3D+ Proposed Cluster

Dihydrogen cation (H 2 + ): First observation in rare gas matrix (very reactive) Requires extremely dilute matrix (Ne : H 2 = 5,000 : 1) and low temperature (2.0 K) H 4 + isotopomers = structure dependent on hydrogen isotopes present and matrix temperature H 4 + temperature dependence: H 4 + = only observed after 2.0 K deposition, but survives up to 9 K H 4 + appears to undergo structural changes as the thermal energy in the matrix increases By invoking nuclear spin statistics, the spectral patterns observed for H 4 + can be explained

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