Stephen F. Nelsen

Ab initio calculations on the intramolecular electron transfer rates of a bis(hydrazine) radical cation

Electron transfer (ET) rates of a charge localized (Class II) intervalence radical cation of a bis(hydrazine) are investigated theoretically. First, the intramolecular ET parameters, i.e., reorganization energy, electronic coupling, and effective frequency, are calculated using several ab initio approaches. And then, the extended Sumi-Marcus theory is employed to predict ET rates by using the parameters obtained. The results reveal that the rates of three isomers of [22/hex/22]+, oo+[22/hex/22]+, io +[22/hex/22]+, and oi+[22/hex/22]+, are agreement with the experiment quite well while the rate of isomer ii+[22/hex/22]+ is about 1000 times larger than those of the others. The validity of different ab initio approaches for this system is discussed.

Electrochemical oxidation of tetramethylhydrazine, 1,1-dimethyl-2,2-dibenzylhydrazine and tetrabenzyl-hydrazine

Abstract Cyclic voltammetric studies of the title hydrazines have shown that the first step of oxidation yields stable radical cations. Though the electron transfer reaction is somewhat slow and very dependent on surface condition when solid electrodes are used, the process is quite reversible at mercury electrodes. Chemical analyses of electrolysis solutions were used to supplement rotating ring-disk voltammetric studies of the second stage of oxidation. The initially formed dication undergoes rapid deprotonation and the protons so formed can react with incoming starting material reducing the currents observed below the two-electron level. Deprotonation of the dication at an α-carbon gives a postulated cationic intermediate which can undergo two reactions whose products were observed: dealkylation to form a hydrazone and deprotonation with N-N cleavage giving an imine.

Intramolecular π-Stacking Interactions of Bridged Bis-p-Phenylenediamine Radical Cations and Diradical Dications: Charge-Transfer versus Spin-Coupling†

In memory of Jay Kazuo KochiConsiderable work has been done on intermolecular p-stacking of radical and radical-ion systems using unlinkedexamples.Differentexamplesofsuchsystemsusuallydiffersogreatly in their DH8 and DS8 values for association thatdetermining details of the structural factors that change theirequilibrium constants for dimerization (where DS8 plays amajor role)

Solution and Solid-State Studies of Doubly Trimethylene-Bridged Tetraalkyl p-Phenylenediamine Diradical Dication Conformations

X-ray crystallographic structures are reported for 1(Me)(2+)(SbCl(6)(-))(2) x 2 CH(3)CN, 2(Et)(2+)(SbF(6)(-))(2) x 2 CH(3)CN x 2 CH(2)Cl(2), and 1(iPr)(2+)(SbF(6)(-))(2), which also contained unresolved solvent and is in a completely different conformation than the methyl- and ethyl-substituted compounds. A quite different structure of 1(Me)(2+)(SbF(6)(-))(2) than that previously published was obtained upon crystallizing it from a mixture rich in monocation. It does not contain close intramolecular PD(+),PD(+) contacts but has close intermolecular ones. Low temperature NMR spectra of 1(Me)(2+) and 1(Et)(2+) in 2:1 CD(3)OD/CD(3)CN showed that both contain three conformations of all-gauche NCCC unit material with close intramolecular PD(+),PD(+) contacts. In addition to the both PD(+) ring syn and anti material that had been seen in the crystal structure of 1(Me)(2+)(SbF(6)(-))(2) x 2 CH(3)CN published previously, an unsymmetrical conformation having one PD(+) ring syn and the other anti (abbreviated uns) was seen, and the relative amounts of these conformations were significantly different for 1(Me)(2+) and 1(Et)(2+). Calculations that correctly obtain the relative amounts of both the methyl- and ethyl-substituted material as well as changes in the optical spectra between 1(Me)(2+) and 1(Et)(2+), which contains much less of the uns conformation, are reported.

Electron transfer in bis(hydrazines), a critical test for application of the Marcus model

Electron transfer in the cations of bis(hydrazines), bridged by six different π‐systems (compounds 1–6) is studied using ab initio and density functional theory (DFT) methods. Due to ionization from an antibonding combination of the lone‐pair orbitals of the nitrogens in one of the hydrazine units, conjugation is introduced in the NN bond of that unit. This leads to a shortening of the NN bond distance and an increase of the planarity around the nitrogens. Due to steric hindrance, this causes an increase of the angle, called φ, between the lone‐pair orbital on the nitrogen attached to the bridge and the p‐orbital on the adjacent bridge carbon for the ionized unit in the charge localized, relaxed state of the molecule. This angle controls the magnitude of the electronic coupling. In the fully delocalized symmetric transition state of the ion, however, this angle is low for both units, due to the fact that the conjugation introduced at the ionized hydrazine unit is now shared between both units. An extended π‐system is formed including the orbitals of the hydrazine units and the bridge, which leads to a large electronic coupling. The electronic coupling derived by optical methods, corresponding to the structure of the relaxed, asymmetric cation with a large φ for the ionized unit, appears to be much smaller. We believe this is due to an approximate cosine dependence on φ of the coupling. The calculations carried out support these conclusions.

Rabbit-ears hybrids, VSEPR sterics, and other orbital anachronisms

We describe the logical flaws, experimental contradictions, and unfortunate educational repercussions of common student misconceptions regarding the shapes and properties of lone pairs, inspired by overemphasis on “valence shell electron pair repulsion” (VSEPR) rationalizations in current freshman-level chemistry textbooks. VSEPR-style representations of orbital shape and size are shown to be fundamentally inconsistent with numerous lines of experimental and theoretical evidence, including quantum mechanical “symmetry” principles that are sometimes invoked in their defense. VSEPR-style conceptions thereby detract from more accurate introductory-level teaching of orbital hybridization and bonding principles, while also requiring wasteful “unlearning” as the student progresses to higher levels. We include specific suggestions for how VSEPR-style rationalizations of molecular structure can be replaced with more accurate conceptions of hybridization and its relationship to electronegativity and molecular geometry, in accordance both with Bents rule and the consistent features of modern wavefunctions as exhibited by natural bond orbital (NBO) analysis.

Effect of ortho substitution on the charge localization of dinitrobenzene radical anions.

Optical and electron paramagnetic resonance spectroscopies were used to study the radical anions of several m-dinitrobenzenes and p-dinitrobenzenes with substituents on ortho positions relative to the nitro groups. 1,4-Dinitrobenzene, 1,4-dimethyl-2,5-dinitrobenzene, and 2,5-dinitrobenzene-1,4-diamine radical anions are delocalized (class III) mixed valence species, but in the dinitrodurene radical anion the nitro groups are forced out of the ring plane due to the steric hindrance, which results in localization of the charge. The radical anions m-dinitrobenzene, 2,6-dinitrotoluene, and dinitromesitylene are all localized (class II) mixed valence species, as is common for m-dinitrobenzenes, and the rate of intramolecular electron transfer reaction strongly decreases with the number of methyl substituents. The same mechanism of rotation of the nitro groups out of the ring plane due to steric hindrance caused by neighboring methyl groups is also responsible for slowing the reaction. However, 2,6-dinitroaniline radical anion and 2,6-dinitrophenoxide radical dianion are charge-delocalized because the strong electron releasing amino and oxido groups increase the conjugation between the two charge-bearing units.

EPR and ENDOR Studies of Dimeric Paracyclophane Radical Cations and Dications Containing Tri- and Pentamethylene-Bridged p-Phenylene Diamine Units

Solution EPR and ENDOR studies on the radical cations of three dimeric p-phenylene diamine (PD)-based compounds, the tetraisopropyl-substituted bis-trimethylene-bridged [5,5]paracyclophane 1(iPr)(+) and its tetramethyl- and tetraisopropyl-substituted bis-pentamethylene-bridged [7,7]paracyclophane analogues 3(Me)(+) and 3(iPr)(+), showed that charge is localized on one PD(+) unit on the EPR time scale in all three compounds and determined the nitrogen splitting constants and several of the hydrogen splitting constants for these complex spectra. Rigid glass studies of the diradical dications of 1(iPr)(2+), 3(iPr)(2+), and its tetramethylene-bridged [6,6]paracyclophane analogue 2(iPr)(2+), all of which show significant amounts of thermally excited triplet at low temperature, demonstrated that 1(iPr)(2+) has a singlet ground state but the triplet lies only 0.07 kcal/mol higher in energy, and 3(iPr)(2+) has its triplet lying 0.05 kcal/mol higher in energy than its singlet.

Electron transfer reactions within σ- and π-bridged dinitrogen-centered intervalence radical cations

Publisher Summary This chapter explains the intramolecular electron transfer (ET) between the charge-bearing units (M groups) of symmetrical intervalence (IV) compounds. The classical two-state Hamiltonian has been widely used by scientists. It is now accepted that ET is a tunneling process that must be treated quantum mechanically. Changing the bridge for the σ-bridged compounds discussed in this chapter requires preparation of a different bis(azo) compound. Because methylene chloride is a solvent of low dielectric constant, ion pairing occurs detectably even for rather large cations and anions. Medium effects on ET reactions of metal complexes have been recently reviewed. Solvent effects have been uniquely important in ET studies. Marcus splits the vertical reorganization energy into solvation and internal components using dielectric continuum theory. Marcus pointed out that intrinsic rate constants (those for zero driving force reactions) are the significant ones to consider for intermolecular ET reactions, and he developed a cross-rate theory to determine them. Electron hopping is unquestionably the way that really long-distance ET is achieved in nature, and is currently a topic of great interest.

Crystal structures of two syn bent tetraalkylhydrazines, their radical cations, and a dication

Abstract The crystal structures of 2,7-diazatetracyclo[6.2.2.23,6.02.7]tetradec-4-ene, 2, its cation radical nitrate salt 2+, NO3-, 2,7-diazatetracyclo[6.2.2.23,6.03,7]tetradecane, 3, its dication dihexafluorophosphate salt 32+(PF6-)2, and a low quality structure of the monocation radical tosylate salt of 3 are reported and compared with MNDO calculations of these structures. Cations 2+ and 3+ are found to be significantly syn bent at nitrogen, and the dication 32+ has a longer N-N distance than its azo analogue, 2,3-diazabicyclo [2.2.2]oct-2-ene (11).