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Experimental Determination of a Single Atom Ground State Orbital through Hyperfine Anisotropy.

ABSTRACT: Historically, electron spin resonance (ESR) has provided excellent insight into the electronic, magnetic, and chemical structure of samples hosting spin centers. In particular, the hyperfine interaction between the electron and the nuclear spins yields valuable structural information about these centers. In recent years, the combination of ESR and scanning tunneling microscopy (ESR-STM) has allowed to acquire such information about individual spin centers of magnetic atoms bound atop a surface, while additionally providing spatial information about the binding site. Here, we conduct a full angle-dependent investigation of the hyperfine splitting for individual hydrogenated titanium atoms on MgO/Ag(001) by measurements in a vector magnetic field. We observe strong anisotropy in both the g factor and the hyperfine tensor. Combining the results of the hyperfine splitting with the symmetry properties of the binding site obtained from STM images and a basic point charge model allows us to predict the shape of the electronic ground state configuration of the titanium atom. Relying on experimental values only, this method paves the way for a new protocol for electronic structure analysis for spin centers on surfaces.

SUBMITTER: Farinacci L

PROVIDER: S-EPMC9650725 | biostudies-literature | 2022 Nov

REPOSITORIES: biostudies-literature

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Experimental Determination of a Single Atom Ground State Orbital through Hyperfine Anisotropy.

Farinacci Laëtitia L Veldman Lukas M LM Willke Philip P Otte Sander S

Nano letters 20221028 21

Historically, electron spin resonance (ESR) has provided excellent insight into the electronic, magnetic, and chemical structure of samples hosting spin centers. In particular, the hyperfine interaction between the electron and the nuclear spins yields valuable structural information about these centers. In recent years, the combination of ESR and scanning tunneling microscopy (ESR-STM) has allowed to acquire such information about individual spin centers of magnetic atoms bound atop a surface, ...[more]

PMID: 36305860

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Project description:Reported herein are thermochemical studies of hydrogen atom transfer (HAT) reactions involving transition metal H-atom donors M(II)LH and oxyl radicals. [Fe(II)(H(2)bip)(3)](2+), [Fe(II)(H(2)bim)(3)](2+), [Co(II)(H(2)bim)(3)](2+), and Ru(II)(acac)(2)(py-imH) [H(2)bip = 2,2'-bi-1,4,5,6-tetrahydropyrimidine, H(2)bim = 2,2'-bi-imidazoline, acac = 2,4-pentandionato, py-imH = 2-(2'-pyridyl)imidazole)] each react with TEMPO (2,2,6,6-tetramethyl-1-piperidinoxyl) or (t)Bu(3)PhO(*) (2,4,6-tri-tert-butylphenoxyl) to give the deprotonated, oxidized metal complex M(III)L and TEMPOH or (t)Bu(3)PhOH. Solution equilibrium measurements for the reaction of [Co(II)(H(2)bim)(3)](2+) with TEMPO show a large, negative ground-state entropy for hydrogen atom transfer, -41 +/- 2 cal mol(-1) K(-1). This is even more negative than the DeltaS(o)(HAT) = -30 +/- 2 cal mol(-1) K(-1) for the two iron complexes and the DeltaS(o)(HAT) for Ru(II)(acac)(2)(py-imH) + TEMPO, 4.9 +/- 1.1 cal mol(-1) K(-1), as reported earlier. Calorimetric measurements quantitatively confirm the enthalpy of reaction for [Fe(II)(H(2)bip)(3)](2+) + TEMPO, thus also confirming DeltaS(o)(HAT). Calorimetry on TEMPOH + (t)Bu(3)PhO(*) gives DeltaH(o)(HAT) = -11.2 +/- 0.5 kcal mol(-1) which matches the enthalpy predicted from the difference in literature solution BDEs. A brief evaluation of the literature thermochemistry of TEMPOH and (t)Bu(3)PhOH supports the common assumption that DeltaS(o)(HAT) approximately 0 for HAT reactions of organic and small gas-phase molecules. However, this assumption does not hold for transition metal based HAT reactions. The trend in magnitude of |DeltaS(o)(HAT)| for reactions with TEMPO, Ru(II)(acac)(2)(py-imH) << [Fe(II)(H(2)bip)(3)](2+) = [Fe(II)(H(2)bim)(3)](2+) < [Co(II)(H(2)bim)(3)](2+), is surprisingly well predicted by the trends for electron transfer half-reaction entropies, DeltaS(o)(ET), in aprotic solvents. This is because both DeltaS(o)(ET) and DeltaS(o)(HAT) have substantial contributions from vibrational entropy, which varies significantly with the metal center involved. The close connection between DeltaS(o)(HAT) and DeltaS(o)(ET) provides an important link between these two fields and provides a starting point from which to predict which HAT systems will have important ground-state entropy effects.

Project description:Reported herein are the hydrogen atom transfer (HAT) reactions of two closely related dicationic iron tris(alpha-diimine) complexes. FeII(H2bip) (iron(II) tris[2,2'-bi-1,4,5,6-tetrahydropyrimidine]diperchlorate) and FeII(H2bim) (iron(II) tris[2,2'-bi-2-imidazoline]diperchlorate) both transfer H* to TEMPO (2,2,6,6-tetramethyl-1-piperidinoxyl) to yield the hydroxylamine, TEMPO-H, and the respective deprotonated iron(III) species, FeIII(Hbip) or FeIII(Hbim). The ground-state thermodynamic parameters in MeCN were determined for both systems using both static and kinetic measurements. For FeII(H2bip) + TEMPO, DeltaG degrees = -0.3 +/- 0.2 kcal mol-1, DeltaH degrees = -9.4 +/- 0.6 kcal mol-1, and DeltaS degrees = -30 +/- 2 cal mol-1 K-1. For FeII(H2bim) + TEMPO, DeltaG degrees = 5.0 +/- 0.2 kcal mol-1, DeltaH degrees = -4.1 +/- 0.9 kcal mol-1, and DeltaS degrees = -30 +/- 3 cal mol-1 K-1. The large entropy changes for these reactions, |TDeltaS degrees | = 9 kcal mol-1 at 298 K, are exceptions to the traditional assumption that DeltaS degrees approximately 0 for simple HAT reactions. Various studies indicate that hydrogen bonding, solvent effects, ion pairing, and iron spin equilibria do not make major contributions to the observed DeltaS degrees HAT. Instead, this effect arises primarily from changes in vibrational entropy upon oxidation of the iron center. Measurement of the electron-transfer half-reaction entropy, |DeltaS degrees Fe(H2bim)/ET| = 29 +/- 3 cal mol-1 K-1, is consistent with a vibrational origin. This conclusion is supported by UHF/6-31G* calculations on the simplified reaction [FeII(H2N=CHCH=NH2)2(H2bim)]2+...ONH2 left arrow over right arrow [FeII(H2N=CHCH=NH2)2(Hbim)]2+...HONH2. The discovery that DeltaS degrees HAT can deviate significantly from zero has important implications on the study of HAT and proton-coupled electron-transfer (PCET) reactions. For instance, these results indicate that free energies, rather than enthalpies, should be used to estimate the driving force for HAT when transition-metal centers are involved.

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Experimental Determination of a Single Atom Ground State Orbital through Hyperfine Anisotropy.

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Experimental Determination of a Single Atom Ground State Orbital through Hyperfine Anisotropy.

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