Franz A. Neugebauer

Electrochemical oxidation and structural changes of 5,6-dihydrobenzo[c]cinnolines

The location of energy minima on the conformational energy surface of molecules by computational methods (conformational searching) continues to play a key role in computer-assisted molecular modeling. Although a number of conformational search procedures have been devised over the past several years, new more efficient methods are urgently needed if molecules with increased complexity are to be treated in a quantitative manner. In this paper we describe a method, termed low-mode search (LMOD), which is based on eigenvector following (or mode following), for the exhaustive exploration of the potential energy hypersurface of molecules. It is particularly efficient at searching the conformational space of both cyclic and acyclic molecules, and we describe its effectiveness for a number of conformational search problems including acyclic, monocyclic, and bicyclic hydrocarbons and cyclic pentapeptides. No special treatment of rings in cyclic molecules is necessary, nor is it necessary to define rotatable bonds. LMOD generates structures “automatically” with minimum input from the user. We demonstrate that LMOD is one of the most efficient procedures yet devised for conformational searching of smallto medium-sized molecules.

Weak ferromagnetism and magnetically modulated microwave absorption at low magnetic fields in 1,3,5-triphenyl-6-oxoverdazyl

Abstract The magnetic susceptibility and microwave properties of powder samples of 1,3,5-triphenyl-6-oxoverdazyl were investigated between room temperature and 1.8 K. The high-temperature susceptibility follows a Curie-Weiss law with a Curie constant of 0.38(1) emu K mol −1 and a Weiss constant of −12(2) K. At low temperature we find 1D magnetic behaviour evidenced by a broad maximum in the susceptibility. Below 4.9 K weak ferromagnetism is detected which is attributed to a canting of the antiferromagnetic sublattices of about 0.13°. Strong nonresonant microwave absorption appears below 4 K which exhibits a hysteresis when measured in a small scanned external field. Possible origins of the weak ferromagnetism and the nonresonant microwave absorption are discussed.

PMR and DMR studies of 1,5-diphenylverdazyls

Abstract Using PMR and DMR the hfs H and D coupling constants for a series of substituted 1,5-diphenylverdazyls have been measured and assigned. The C-6 methylene proton resonance has been studied as a function of temperature. The results indicate a ring inversion in verdazyls which is slow on the NMR time scale. Furthermore, the gain in resolution is discussed obtained with PMR in DBNO, and with DMR.

Magnetic-properties of 1,4-dihydro-1,2,4-benzotriazin -4-yl radicals

Abstract Magnetic susceptibilities of 1,4-dihydro-1,3-diphenyl-1,2,4-benzotriazin-4-yl (DDB 1 ), 3-tert-butyl-1,4-dihydro-1-phenyl-1,2,4- benzotriazin-4-yl (BDPB 2 ) and 1-(4-chlorophenyl)-1,4-dihydro-3-phenyl-1,2,4-benzotriazin-4-yl (CDPB 3 ) radicals have been measured in the temperature range of 4.2-300 K. The susceptibility of DDB 1 and BDPB 2 follows the Curie-Weiss law with a negative Weiss constant of −2.2 and −0.3 K, respectively. The magnetic susceptibility of CDPB 3 , on the other hand, exhibits a broad maximum at 138 K. This susceptibility can be interpreted in terms of an alternating antiferromagnetic Heisenberg linear chain model with an exchange interaction of 2 J 1 / k =−220 K and an alternation parameter of α= J 2 / J 1 =0.3. The results of the X-ray structure analysis of CDPB 3 support the above explanation.

Isomeric Tetrakis(dimethylamino)naphthalenes: Syntheses, Structure-Dependence of Basicities, Crystal Structures, and Physical Properties

For comparison to the recently described 2,3,6,7-tetrakis(dimethylamino)naphthalene (1) the three isomers 2,3, and 4 were synthesized. The basicities of this group of isomers are strongly dependent upon the different mutual orientations of the pairs of dimethylamino substituents: only the isomers 3 and, partially, 4, both with dimethylamino groups in adjacent peri-positions of the naphthalene, are strong “proton sponges”. For the isomers 1 and 2 with the same number and kind of twofold dimethylamino substituents in neighbouring ortho-positions, however, no significant basicity increase is observed. To explain this difference between the two groups of isomers it is suggested that in the ortho-pairs of 1 and 2 the C-N bonds diverge considerably, leading to an increased N···N distance and consequently to less stable [N···H···N]+ hydrogen bonds in contrast to the parallel C-N bonds in the peri-substituted isomers 3 and 4. X-ray crystal structure analyses of the bases and of some of the salts derived therefrom were solved and are discussed. Cyclic voltammetry indicates that 1 to 4 are strong electron donors, reacting easily to radical cations or dications which with suitable acids have been obtained as salts.

Stabile N-haltige Biradikale und Triradikale

Darstellung und Eigenschaften von kristallisierten freien Radikalen, die zwei und drei Verdazyl-Ringsysteme pro Molekul enthalten, werden beschrieben. Wahrend der Paramagnetismus der drei dargestellten Triradikale 3 ungepaarten Elektronen entsprach und bis 77° K unverandert blieb, fand sich unter den neun dargestellten Biradikalen eines, das beim Abkuhlen reversible diamagnetisch wurde. Die Absorptionsspektren und ESR-Spektren werden im Zusammenhang mit Strukturfragen diskutiert.

Intramolecular Electron Transfer between Terminal 1,4-Dimethoxybenzene Units in Radical Cations with a [2.2](1,4)Naphthalenophane, [2.2](1,4)Anthracenophane, and Pentacene Skeleton

Various radical cations, in which two terminal 1,4-dimethoxybenzene units are anellated to [2.2]paracyclophane (2b•+, 3b•+), [2.2](1,4)naphthalenophane (4d•+), and anthracene bridges (5•+), have been studied by ESR and ENDOR spectroscopy. In the syn- and anti-naphthalenophane radical cations 2b•+ and 3b•+ the delocalization of the unpaired electron over both π-moieties and the distinct difference between the first and second oxidation potentials, ΔE = E20 – E10, are evidence for a substantial intramolecular electronic interaction between the two electrophores. Extension of the bridge in 4d•+ and 5 by benzo anellation results in a localized radical cation. Strong intramolecular electronic interaction between the two electrophores is found in the 1,4,8,11-tetramethoxy-pentacene radical cation (5•+). The syntheses of 4d are described.

Spectroscopic investigations of the lowest triplet state of s-tetrazines

We report on the electronic and triplet state ODMR spectroscopy of s-tetrazme, 3,6-dimethyl-, 3,6-dimethoxy- and 3,6bis(methylthio)-s-tetrazines. Zero field splitting parameters were obtained by ODMR monitoring both the delayed fluorescence and the phosphorescence m neat single crystals. The S, and T, exciton electronic origins are reported. The zero field splitting tensor is nearly axially symmetric with 1 D 1 approximately 0.4 cm- ‘. This value is found to be in good agreement with calculattons based on magnetic dipole-dipole coupling in a ‘BS, (nx”) state. The calculations predict a positive value for D with the major axis passing through the C atoms at the 3 and 6 positions. 1. In~~ue~on

Spin densities in 1,3,5-triphenylverdazyl an NMR study

Abstract NMR spectra of 1,3,5-triarylverdazyls dissolved in DBNO yield all H hfs coupling constants and their assignment for 1,3,5-triphenylverdazyl. The experimental data are compared with McLachlan spin density calculations.

Intramolecular Electron Transfer between 2,5-Dimethoxy-1,4-phenylene Units in [n.n]Paracyclophane Radical Cations

A range of [n.n]paracyclophane radical cations (4·+−12·+), in which two 2,5-dimethoxy-1,4-phenylene units are connected by alkano bridges of varying length, have been studied by ESR and ENDOR spectroscopy. In the [2.2]- and [3.3]paracyclophane radical cations 4·+−6·+, 10·+ and 11·+ the delocalization of the unpaired electron over both π-moieties and the distinct difference between the first and second oxidation potentials, ΔE = E20 − E10, are evidence for a strong intramolecular electronic interaction between the two electrophores. The [5.5] and [7.7] species (8·+ and 9·+) are localized radical cations at low temperature (ca. 220 K). At room temperature, the higher molecular flexibility leads to a significant increase in the number of internal collisions between the electrophores, resulting in a fast (ESR time scale) intramolecular electron transfer. The intermediate [4.4]paracyclophane radical cations 7·+ and 12·+ are apparently also localized radical cations. The close interplanar distance between the two π-moieties, however, facilitates their mutual contacts. In 7·+, the intramolecular electron transfer becomes fast on the ESR time scale at room temperature; in 12·+ the transfer is fast over the temperature range 200−300 K.