Search

In Bayesian learning, designs based on noninformative priors are appropriate when the user cannot confidently identify the data-generating distribution. While such learners cannot achieve the performance of those based on a well-matched subjective prior, they impart a robustness against poor prior selection. The uniform Dirichlet distribution is the true non-informative prior as it has full support over the space of candidate distributions; additionally, it leads to closed-form posteriors. This work applies such a prior to classification using the 0–1 loss, determines the optimal Bayes classifier and the corresponding minimum probability of error, and analyzes the results.

Obwohl es immer wieder aufsehenerregende Falle gibt, ist Betrug in der naturwissenschaftlichen Forschung wohl eher selten, auch wenn es vermutlich eine betrachtliche Dunkelziffer geben durfte. Fruher oder spater werden eklatante Beispiele unethischen Verhaltens oder Verstose gegen gute wissenschaftliche Praxis von den im System wirksamen Kontrollen aufgedeckt. Zur Verhutung derartiger Falle ist im Wissenschaftsbetrieb die Beachtung der strengen Leitlinien guter wissenschaftlicher Praxis erforderlich. Ein Ethikunterricht fur alle Studierenden und eine Verpflichtung von Wissenschaftlern zur Beachtung ethischer Grundsatze erscheint als eine unerlassliche Voraussetzung. Although repeatedly there are worrying cases, fraud in science is rather rare; however, probably there are a considerable number of unreported cases. Sooner or later blatant examples of unethical behavior are revealed by the controls in the system. The prevention of such situations requires compliance with strict ethical principles in the academia. An ethics education for all students and a commitment from scientists to observe ethical principles appears as an essential condition.

The electronic and geometrical structures of fluorocyclopropanes (1–12) have been analysed using DFT B3LYP calculations. A linear relationship, Δɛω=−0.172 Δr−0.171 (n=12, R=0.931), between Δɛω (in eV), the difference of the energies of the Walsh orbitals ωS and ωA, and Δr (in pm), the difference of vicinal and distal C–C bond lengths, is established. Correcting the orbital splitting by the basic value at Δr=0.00 pm, an even better linear correlation Δɛω eff=0.0720 Δr (n=12, R=0.984) is obtained. The results confirm the general applicability of the two-orbitals model for the relationship between geometrical and electronic structures for substituted cyclopropanes.

Die Beschaftigung mit Isomeriefragen hat im Laufe der Chemiegeschichte haufig zu Missverstandnissen, Verwirrung und Kontroversen gefuhrt, aber stets auch neue Impulse zur Weiterentwicklung der Strukturtheorie ausgelost, die Entwicklung neuer analytischer Methoden sowie die Darstellung neuer und neuartiger Verbindungen angeregt. Studies in the field of isomerism have often created misunderstandings, confusion and controversies, but have also led to new impulses for the development of structure theory, new analytical methods and synthesis of novel compounds. Problems of isomerism are present in nearly all fields of chemistry and have stimulated creation of new concepts and enriched the chemical nomenclature. Presently, probably the most ambitious studies in structure chemistry deal with the elucidation of conformational isomerism of large bio-molecules such as proteins in liquid and solution.

The electronic structures of pyrimidine (1) and its substituted derivatives 2−15 have been investigated by ultraviolet photoelectron spectroscopy and quantum chemical methods. The ionisation potentials corresponding to the π MOs π1-π3 and the two nN orbitals of the pyrimidine unit could be determined and assigned for 1−15. Multiple linear regression analyses of the IPs related to these orbitals with different substituent constants indicated that Hammett σp values performed well for all these IPs, whereas the resonance parameters R and R+ were satisfactory for π and poor for nN ionisations. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)

The molecular structures of four cyanocyclopropanes 1−4 have been investigated by DFT B3LYP calculations. PE spectra of trans- and cis-1,2-dicyanocyclopropane (3 and 4) have been measured and analysed by semiempirical PM3 and B3LYP calculations. For compounds 1−4 a linear relationship, ΔIPψ = 0.218 Δr, between ΔIPψ (in eV), the difference in the IPs associated with the Walsh orbitals ψS and ψA of the three-membered ring, and Δr (in pm), the difference of vicinal and distal C−C bond lengths, is established. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)

PE spectra of six chloro-( 1 – 6 ), four bromo-( 7 – 10 ) and two iodocyclopropanes ( 11 , 12 ) have been measured and analysed using semiempirical PM3 and DFT B3LYP calculations. The spectra are dominated by strong ionizations of electrons from the halogen atoms. The first IP of all chloro derivatives has a rather constant value of 10.22–10.32 eV. The same holds for the bromo (IP 1 : 9.90–9.94 eV) and the iodo derivatives (IP 1 : 9.13–9.15 eV). In the central region of the spectra, ionizations related to the Walsh orbitals ω A (IP: 11.2-14.2 eV) and ω S (IP: 11.6–13.9 eV), which interact to different extents with n orbitals of the halogens, are found. A linear relationship, ΔIP ω =0.375 Δ r +0.165, between ΔIP ω (in eV), the difference of the IPs related to the Walsh orbitals ω S and ω A of the three-membered ring, and Δ r (in pm), the difference of vicinal and distal C–C bond lengths, is established.

The molecular structure of the pale yellow crystals of 2-(2′,4′-dinitrobenzyl)pyridine (CH 2 form) and its photo induced ‘enamine’ NH tautomer (dark blue crystals) have been studied by means of vibrational spectra and ab initio calculations. The Raman spectrum of the photo-sensitive CH 2 form was registered by NIR FT-Raman spectroscopy by means of the Nd:YAG laser as an excitation source. Ab initio calculations have been performed for the CH 2 and NH tautomers at the Hartree–Fock level using a 6-21G** basis set. The theoretical geometrical parameters for the isolated 2-(2′,4′-dinitrobenzyl)pyridine molecule (CH 2 form) are close to the literature X-ray diffraction data. According to the theory the dihedral angle between the benzene and pyridine ring planes in the NH photo induced tautomer is about 46°, the ortho -nitro group is twisted about 25° towards the benzene ring plane, whereas the para -nitro group is coplanar to the benzene ring. The assignment of the fundamental vibration frequencies of both 2-(2′,4′-dinitrobenzyl)pyridine tautomers CH 2 and NH have been performed on the basis of Raman and infrared spectra and ab initio force field calculations. The computed frequencies are in coincidence with the registered ones; the mean deviations are between 23.7 and 28.5 cm −1 .

From the irradiation mixture of N -acetyl-2-styrylpyrroles ( 5a , b ) the deacylated ( 6a , b ), phototransposition ( 7a , b – 10a , b ), electrocyclisation ( 12 ), addition and reduction ( 11 , 13 and 14 ) and dimerisation products ( 15 ) have been detected. No intramolecular cycloaddition products have been observed.

The molecular and electronic structure of three pairs of diastereomeric quinuclidine derivatives (1-6) have been investigated by quantum chemical calculations and UV photoelectron (PE) spectroscopy. Compounds 1-6 each possess a hydroxymethyl group at C2 and an ethyl (1, 2), vinyl (3, 4) or ethynyl group (5, 6) at C5. The diastereomeric pairs differ only in the configuration of carbon C2 to which the hydroxymethyl group is attached. The 1-azabicyclo[2.2.2]octane cages of 1-6 are slightly twisted. The torsion angles show opposite signs in the pseudoenantiomeric pairs and vary only little with the degree of unsaturation in the C5 substituent. In all compounds the hydroxymethyl group forms an intramolecular hydrogen bond with the nitrogen atom of the bicycle in the gas phase (in contrast to the crystalline and liquid state). The PE spectra of the diastereomers are nearly identical. Compared to unsubstituted quinuclidine, the orbital of the nitrogen lone pair electrons n(N) is stabilized by 0.3-0.4 eV, which is a result of partially compensating contributions of the OH...N hydrogen bond and substituent effects. Among these a substantial through-bond interaction of the n(N) with a pi(CC) orbital is detected for 3-6. The marked differences of the properties of 1-6 in the gas phase versus solid state and solution are emphasised. In solution the ethynyl azabicyclics 5 and 6 are more polar than the vinyl analogues 3 and 4.

Five hydroxyethyl-2-tetrazenes (1 - 5) and their methyl ethers (6 - 10) have been synthesized and hydrogen bonding in these compounds has been investigated by theoretical and spectroscopic (IR, 1H NMR, 15N NMR) methods. The structure of 1,1,4,4-tetrakis(2-hydroxyethyl)- 2-tetrazene (4) was determined byX-ray diffraction analysis. Several conformationswith intramolecular hydrogen bonds were investigated by ab initio B3LYP as well as semiempirical SCF calculations. In all cases, conformers with OH- - -N hydrogen bonds with azo nitrogen atoms as acceptors (conformers A, B, C) are found as most stable. Incompounds with small or flexible N1- and N4-substituents R besides the hydroxyethyl group (3, 4), hydrogen bonds forming six-membered rings, with the R groups taking syn positions at the N1-N2 and N3-N4 bonds (conformer A), are preferred over those with seven-membered rings and R taking anti positions (conformer B). Steric interaction in the other compounds (1, 2, 5) leads to destabilization of conformers A and conformers B become more stable. A special case is presented by compound 4 which has only hydroxyethyl substituents on the 2-tetrazene unit. In the most stable conformer (4C) there are two OH- - -O and one OH- - -N hydrogen bonds. By IR solution measurements intra- and intermolecular hydrogen bonds could be distinguished. Association shifts Δδ measured by 1H NMR spectroscopy, indicate that the investigated compounds exhibit comparable association properties with intermolecular association clearly prevailing. 15N NMR spectra of compounds 1 - 10 in two solvents have been measured if solubility was sufficient. The data indicate that all nitrogen atoms of 1 - 5 participate in H bonding. In the crystalline state, molecules 4 adopt a conformation without intramolecular H bonds (4D) and are associated by intermolecular OH- - -O hydrogen bonds that form a three-dimensional network. An untypical decomposition pattern was discovered for benzyl derivatives 5 and 10.

Continuous flow pyrolysis (CFP) of thiophene (1), benzo[b]thiophene (2), and dibenzothiophene (3) was investigated. 1 and 2 were studied in the temperature range 500-1100°C. Up to about 20 products were identified by GC-MS and their temperature profiles determined. 3 was pyrolysed at 900°C. All together, more than 30 products were identified in the pyrolysates, the largest number was found for 2. A distinct tendency to form polycyclic aromatic hydrocarbons (PAHs) and sulfur-containing polycyclic hetarenes (PHAs), even with more than one sulfur atom, is observed. At temperatures below 800°C, mainly ‘oligomers’ of the educts arise by radical recombination reactions. Above 800°C, fragmentation of the skeleton becomes important, and products are generated from the starting molecules and their fragments. From 1, PHAs such as benzo[b]thiophene (2, 29%), dibenzothiophene (3, 11%), and thianthrene (22, 4%), and from 2, PHAs such as benzo[b]naphtho[2,1-d]thiophene (32, 13%) and [1]benzothieno[2,3-b][1]benzothiophene (21, 5%) are formed in highest amounts at 850–900°C. Minor amounts of highly annulated thiophenes such as triphenyleno[1,1a,1b,2-b,c,c]thiophene (28) and benzo[1,2-b:3,4-b′:6,5-b″]tri(benzo[b]thiophene) (36), are detected. Formation of PAHs such as benzene (4), naphthalene (5), phenanthrene (7), fluoranthene (8), and pyrene (9) increases rapidly above 850°C. Product formation is explained by annulation reactions of the educts with their fragments (PHAs) or by exclusive combination of fragments (PAHs). Benzo-annulation of thiophene increases the thermal stability.

Using an improved sampling arrangement we observed the FT Raman spectra of the different phases of a 'jumping crystal', an inositol derivative. The phase transition produced – as consequences of large changes of the unit cell constants – changes in frequency and intensity mainly of CH deformation vibrations. Photochemical reactions, usually produced with light quanta in the visible range, are not activated with the quanta from the Nd:YAG laser at 1064 nm. The Raman spectra of the 'dark' form of a dinitrobenzyl pyridine and afterwards the 'light' form, the product of its illumination in the visible range, were recorded. We could not observe changes of most bands, especially not of the NO2-vibrations; however, a new strong band appeared at 1253 cm–1, which may be due to the expected NH-photo-isomer. Genuine gemstones and fakes can be unambiguously identified by FT Raman spectroscopy. This is especially useful for the stones whose physical properties are quite similar to those of diamonds – moissanite and zirconia. The quality of diamonds can be estimated from relative band intensities; however, this is not in complete agreement with the internationally accepted visual qualification. Synthetic diamonds produced by CVD (chemical vapor deposition) show remarkable differences from natural ones in their FT-Raman spectra.

The gas phase He(I) photoelectron (PE) spectra of several unsaturated alcohols (1–11) have been measured and analysed with respect to intramolecular OH⋯π hydrogen bonding. Evidence for such a hydrogen bond has been detected in the spectra of 2-allylphenol (1) and 2-phenylethan-1-ol (3). 1 exists as a conformational mixture of a hydrogen bonded form 1a and an open form 1b in a composition of roughly 2:1. A strong ionization band (IPv=10.01 eV; where IPv is the vertical ionization potential) is assigned to the ethylenic C C double bond in the major conformer (1a) and a weak band (IPv=9.72 eV) to that of the minor conformer (1b). The latter IP coincides with the corresponding ionization of allylbenzene. In the series of ω-phenylalkan-1-ols, compound 3 exhibits an unusually low nπ(O) ionization indicating hydrogen bonding between the OH group and the π electron system of the phenyl ring. The higher homologs 4 and 5 prefer ‘open’ conformations without such interaction. The PE spectra of alkenols such as but-3-en-1-ol (7) and pent-4-en-1-ol (8) as well as of alkynols such as but-3-yn-1-ol (10) and pent-4-yn-1-ol (11) are consistent with OH⋯π hydrogen bonded conformers. The methanol/ethylene hetero-dimer has a T-shaped structure, as indicated by B3LYP/6-311++G(d) calculations, with a binding energy of 5.65 kJ mol−1.

Synthesis, structure, reactivity, and theoretical studies of a new type of Ni triple-decker complexes are reported. A prominent feature of its molecular structure is the (η3:η3)-bis(enyl) coordination of two [(η5-Me4EtC5)Ni] fragments to the central arene ring of the polycondensed π-perimeter decacyclene. The central arene ring of decacyclene displays a distinct chair conformation with significant tilt angles of 25.5 and 26.1°. The three naphthalene units are twisted in a propeller-like fashion with torsion angles of 16°, 16.6°, and 19°, which is twice the twisting observed for the free decacyclene ligand. According to Extended Huckel Molecular Orbital (EHMO) calculations, a qualitative measure for arene lability in the title compound and a number of related arene-bridged metal complexes can be attributed to a subsequent filling of metal−arene antibonding orbitals. The experimentally observed reactivities of π-arene exchange in these complexes are reproduced nicely by the EHMO calculations. The title compound offers access to a number of organonickel chalcogenato complexes under mild conditions by cleavage of the S−S, Se−Se, or Te−Te bonds of diorganyl dichalcogenides R−E−E−R. The title compound is also capable of C−H activation of azobenzene and [(dimethylamino)methyl]ferrocene resulting in orthometallated complexes containing five-membered metallacycles. PES and semi-empirical ZINDO/S calculations were used to study the product of the orthometallation of the title compound with azobenzene. These studies reveal that the HOMO of the metallacycle (IE = 6.45 eV) has dominant Ni−Cp and Ni−Caryl π-antibonding character.

Continuous flow pyrolysis (CFP) of furan (1), benzo[b]furan (2) and dibenzofuran (3) has been investigated at 900°C. Complete decomposition of 1 and 2 was observed, while 3 proved to be more temperature resistant. In the pyrolysates of 1–3, up to 22 different products were identified by GC-MS, 12 of these are identical. Mainly polycyclic aromatic hydrocarbons (PAHs), such as naphthalene (11) (6–28%), phenanthrene (17) (2–25%), biphenyl (22) (1–11%), pyrene (18) (1–10%), benzopyrene (19) (1–10%), fluoranthene (20) (3–9%), triphenylene (21) (3–9%), acenaphthylene (13) (1–6%), anthracene (23) (1–6%) and fluorene (12) (1–3%), were obtained. Only from 3, small amounts of larger oxygen containing polycyclic hetarenes (PHAs), such as benzo[b]naphtho[2,3-d]furan (37) and benzo[1,2-b:4,5-b′]bis[1]benzofuran (36), are generated as additional products besides PAHs.

The UV photoelectron spectra of two isomeric styrylthiophenes (5, 6) and six isomeric (thienylethenyl)pyridines (7−12) have been recorded and analyzed, making use of DFT Becke3LYP calculations. The spectra were compared with those of (E)-stilbene (1e) and the isomeric styrylpyridines (2−4), which are π-isoelectronic with 5−12. Using quantum chemical analyses, planar molecular structures were obtained for all these compounds. However, from the separation ΔIP of the ionization bands associated with the π7 and π3 MOs, it is possible to make a distinction between planar and twisted molecular structures. Accordingly in these compounds, 2-substituted pyridine rings and 3-substituted thiophene rings are nearly untwisted, whereas phenyl rings, 3- and 4-substituted pyridine, and 2-substituted thiophene rings are twisted to an extent similar to that in 1e. The apparent distortion of the molecules is probably caused by torsional vibrations, so that twisted average geometries correspond to planar equilibrium structures. The B3LYP data permit detailed conclusions to be drawn with regard to the conformational preferences of 2- and 3-substituted thiophene and pyridine rings in heterocyclic analogues of 1e, as well as the to relative stabilities of isomers 7−12. The results clearly indicate that PE spectroscopy is a powerful method for analysis of conformational properties of stilbene-like molecules.

The flash vacuum pyrolysis of 3,3,5,5-tetramethylpyrazolin-4-one (4) with subsequent trapping of the products in argon at 10 K results in the formation of a complex product mixture with tetramethylcyclopropanone (5) as one of the minor constituents. The 193 nm photochemistry of matrix-isolated 4 is much cleaner, yielding cyclopropanone 5 and allene oxide 9 as the major products. The reaction of tetramethylallene 13 with oxygen atoms yields the same product mixture of 5 and 9 and thus provides an independent route to these species. The experimental IR spectra of 5 and 9 are in good agreement with the results from DFT calculations.

Oxidative dimerization of a mixture of the thiolanone 5 and the dithiolanone 4 yields 2c as chromophore of thioindigo 1c, and 6 and 7 as chromophores of still unknown indigo dyes. As compared to 2c, the first ionization potential of the new indigo chromophore 7 is higher, but the calculated HOMO-LUMO splitting is lower. Accordingly, its longest wavelength absorption maximum surpasses that of 2c distinctly.

3-Methoxy-7α-phenyl-1,5-dimethyl-3-borabicyclo[3.3.1]nonane 5 in solution at room temperature exists in the double chair conformation, as shown by NMR studies. Increasing the temperature leads to an increase in the population of the chair–boat conformation. At decreased temperature hindered rotation around the BO bond is observed for 5. Dissolving 5 in deuteropyridine leads to the reversible formation of complex 6, which exists in the chair–boat conformation. The chair–boat conformation is also the most stable one for chelate compound 7 with a tetracoordinated boron atom.

The bicyclic dioximes 4–6 and dimethoximes 7–9, which contain the functional groups in opposite positions of bridged eight-membered rings, were synthesized. Their conformational properties and transannular interactions were investigated by spectroscopic (PE, 13C NMR) and theoretical (MMX, AM1, ab initio HF, and B3LYP) methods. While in the 3,7-disubstituted bicyclo[3.3.1]nonane derivatives 5 and 8 the eight-membered ring has a CC conformation favourable for through-space interactions of the π(CN) orbitals, in the bicyclo[3.3.0]octane derivatives 4 and 7 as well as the 2,6-disubstituted bicyclo[3.3.1]nonanes 6 and 9 the functional groups are in geometric orientations that are unfavourable for such interactions. Through-space orbital interactions in the molecules with favourable conformations lead to a splitting of the π(CN) MOs of 0.4–0.6 eV.

We report the He(I) photoelectron spectrum of 3,5,7-trimethyl-1-azaadamantan-2-one (20). Ionization potentials have been assigned to molecular orbitals on the basis of quantum chemical calculations. Compared with other lactams (1–19), the first ionization, IP[n(N)], of 20 has an usually low energy, while the second, IP[n(O)], appears at a “normal” value. For undistorted lactams, there is a linear correlation of IP[n(O)] values and normalized carbonyl frequencies ν120(CO): the data for 20 fall well off this correlation. The deformations of the amide linkage in 20, in the essentially undistorted δ-lactam 9, the moderately distorted α-lactam 17 and in the bicyclic lactam 19 were investigated by B3LYP/6-31+G* calculations. The deviation from the linear IP[n(O)]/ν120(CO) correlation can be related to the overall distortion parameter θ (defined as the sum of the absolute values of the C–N torsional angle τ and the out-of-plane bending deformations at the amino and the carbonyl group χN and χC, respectively). This allows θ for amides and lactams to be estimated from the observed IP[n(O)] and ν(CO) values.

Protection of the hydroxy group of the medium-ring hydroxy ketone, 5-hydroxycyclooctanone ( 1HK), can be achieved with an alcohol and hydrochloric acid, although the substrate predominately exists as the transannular hemiacetal 1HA. The reaction is explained by a mechanism, which includes a 1,5-hydride shift as an essential step.

Gas phase thermolyses of 3,6-dimethyl, -diethyl and -di-n-propyl substituted 1,2,3,6-tetrahydro-1,2,4,5-tetrazines 1a–1c yielded the corresponding N-unsubstituted imines 2a–2c. The He(I) photoelectron spectra of propanimine (2b) and butanimine (2c) are presented for the first time. First ionization potentials and orbital energies of the imines as determined with B3LYP/6-31+G*//HF/6-31+G* reproduce the experimental data. Ionization bands arising from N,N′-unsubstituted diazetidines 3a–3c could not be detected in the pyrolysis spectra of 1a–1c. Calculations for the extrusion of nitrogen from tetrahydrotetrazine 1a resulted in a transition state for a [2+2+2]-cycloreversion that leads to the imine 2a as product.

The spatial and electronic structure of the parent 2-tetrazene and its tetramethyl- and ethyl derivatives are examined by an ab initio approach. It is shown by using the MP2(fc)/6-311+G ∗∗ //HF/6-31G ∗ + ZPVE (HF/6-32G ∗ ) model that alkylated tetrazenes exhibit strong intrinsic basicity as evidenced by their high absolute proton affinities. The latter is due to the appreciable electron density concentration at the terminal nitrogen atoms, the increased II-electron delocalization in conjugate acids and the electron density relaxation effect occurring upon protonation.

Two members of previously unknown hydroxyalkyl-2-tetrazenes (1a, 2a) have been synthesized and hydrogen bonding of these novel difunctional compounds has been investigated by spectroscopic (IR, 1H NMR, 15N NMR) and theoretical methods. The structures of 1a and its bis(trimethylsilyl) derivative 1b were determined by X-ray analysis. In the crystalline state, molecules 1a are associated by O–H···O hydrogen bonds that form a three-dimensional network. Ab initio HF and DFT as well as semiempirical SCF calculations show that O–H···N hydrogen bonds of 2-tetrazenes are medium strong. The δ-15N data and the quantum chemical calculations indicate that the amino nitrogen atoms of a 2-tetrazene are involved in intermolecular hydrogen bonding to a larger extent than the azo nitrogen atoms; the corresponding energy difference of the two types of hydrogen bonds is about 3 kJ mol−1. The hydrogen bonds can either stabilize or destabilize 2-tetrazenes thermodynamically depending on which nitrogen atoms are involved. Complexation of 1,1,4,4-tetramethyl-2-tetrazene with methanol is accompanied by only minor changes in geometric parameters whereas systematic effects on the electronic structure are more distinct. Transition states for N–N bond cleavage are stabilized to a larger extent making such compounds rather sensitive for thermal decomposition.

The mononuclear complexes Pt(P-P′)Cl2 1–3, where P-P′ are the chiral ligands (S)-1-diphenylphosphino-2-(diphenylpho-phinomethyl)pyrrolidine (S-promephos), (S,S′)-1,1′-bis(diphenylphosphino)-2,2′-bipyrrolidine (S-bipyrphos) and (S)-1-diphenylphosphino-2-(diphenylphosphinoxymethyl)pyrrolidine (S-prolophos), were synthesized and investigated by NMR spectroscopy and X-ray structure analysis. The cis influence on the platinum aminophosphine bond in 1–3 is characterized by the coupling constant 1J(Pt-Pa) which is dominated by the Fermi contact term and varies mainly with the electron density in the platinum 6s valence orbital. The magnitude of the cis influence is too small to be detected by X-ray analysis because Pt-P bond lengths vary only by about 1 pm. Complex 1 crystallizes in two different molecular geometries, the six-membered chelate ring adopts either a half-boat or a half-chair conformation. The seven-membered platinum chelate 2 has a boat form. All conformations are fixed by the square planar platinum environment and the fused l -proline molecules. In both compounds the phenyl groups occupy isoclinal positions but not alternating axial/edge and equatorial/face arrangements.

The photoelectron (PE) spectra of tetrahydro-1,2,3,4-tetrazines 1 and 2 and tetrahydro-1,2,4,5-tetrazines 3–5 have been recorded and their conformations have been investigated by ab initio SCF calculations. While v-tetrazine2 is planar, tetrazines 1 and 3–5 each possess two low-energy conformations, according to ab initio HF and Becke3LYP methods. Attempts to assign ionization potentials to molecular orbitals obtained by semiempirical PM3 calculations indicate that this method is not suited for the compounds studied. Best results were obtained when the ab initio hybrid method Becke3LYP of the density functional theory was employed. Two conformers of 1 and 3–5 are present in the gas phase and their PE spectra are superimposed one upon the other. For v-tetrazine1, ionizations arising from half-chair and unsymmetrical boat conformers have similar energies and cannot be separated in the PE spectrum. For s-tetrazine3, on the other hand, the spectrum clearly shows different ionizations of both half-chairs, 3ee and 3ae.

N,N′-Diisopropyl-substituted 1,3,4-oxadiazolidine3c, 1,3,4-thiadiazolidine6, 4,5-benzo-1,2,3,6-tetrahydropyridazine8, and new 3,4-dialkyl-1,3,4-oxadiazolidines9–14 were synthesized and studied. The configurational stability of the N atoms does not change on going from compound3c to itsS-analog6 and decreases in the case of pyridazine8. 3,4-Di-tert-alkyl-1,3,4-oxadiazolidines3d and11–14 were found to be promising objectes for optical resolution.

The photoelectron spectra of carbazole ( 1 ), 9-methylcarbazole ( 2 ), benzo- and dibenzocarbazoles ( 3–7, 15 ), naphthocarbazoles ( 8, 9 ), benzonaphthocarbazoles ( 10–12 ), and dinaphthocarbazole ( 14 ) are reported. Based on HAM/3 and AM1 calculations, the ionization potentials are assigned to molecular orbitals. Close relationship of the electronic structures of the anellated carbazoles and the corresponding π-isoelectronic arenes is observed; ionization potentials for most π-electrons deviate by less than 0.2 eV. Analysis of the PE spectra of isomeric carbazoles leads to an extension of Clar's aromatic sextet model to hetarenes, which can explain the correlation between relative stabilities and first ionization potentials.

Bird aromaticity indices, based on ab initio (6-31G∗) calculated structure parameters, have been calculated for nine isomeric angular dithienopyridines 1–9. Depending on the mode of anellation, the three rings of the tricyclic molecules show varying aromaticities and a grouping of the isomers is possible. The aromaticity index V6 of the central pyridine ring is linearly correlated with the stability of the isomers, whereas the indices V565 of the tricycle and V12 of the perimeter correlate linearly with their energy.