Paul Rademacher

Conformational analysis by photoelectron spectroscopy

The conformation of organic molecules may be determined by means of photoelectron (PE) spectroscopy if there are orbital interactions present which depend on a dihedral angle. Straightforward application of the method requires that all ionization bands of interest can be assigned unambiguously and that inductive effects and secondary orbital interactions do not occur or can be accounted for appropriately. In many cases, this method complements conventional methods of conformational analysis. PE conformational analysis is particularly suited to compounds containing vicinal lone pairs of ..pi.. systems. 105 references, 10 figures, 2 tables.

Photoelectron spectroscopic and computational study of the thermolysis of 1,2,3,6-tetrahydro-1,2,4,5-tetrazines

Abstract 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.

Structural chemistry of polycyclic heteroaromatic compounds. Part 4. Electronic structures of angular dithienopyridines

The Hel photoelectron spectra of nine isomeric [b,d]-annellated dithienopyridines (5–13) are reported and discussed including those of some closely related compounds such as phenanthridine (2), dithieno[3,4-a: 3′,4′-c]benzene (4) and its tetrahydro derivative 3. The ionization potentials are assigned to molecular orbitals using the results of MNDO, AM1 and PM3 calculations. The ability of PE spectroscopy as an analytical method in the analysis of the isomers 5–13 is investigated. From the spectra as well as from the theoretical results, the isomers can be divided into two groups, depending on the mode of annellation. The crystal and molecular structures of one member of each group (isomers 9 and 12) are determined by X-ray diffraction. The mode of annellation is also reflected in the bond lengths of the pyridine and thiophene rings.

Schwingungsspektren isotoper dimethylnitrosamine

Abstract Vibrational spectra of seven isotopic dimethylnitrosamines in different states of aggregation and solvents have been obtained. An assignment of the observed frequencies to the normal vibrations is given, which is based essentially on the isotopic frequency shifts.

Photoelectron Spectra, Electronic Structures, and Conformational Properties of (E)-Stilbene, Styrylthiophenes, and (Thienylethenyl)pyridines

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.

Conformational properties and electronic structure of tetrahydrotetrazines studied by photoelectron spectroscopy and quantum chemical calculations

Abstract 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.

Reactivity of Lewis acids towards nitriles; crystal structure and electron deformation density of C2N2·SbF5 and photoelectron spectrum of AsF5

The reactivity of nitriles and dinitriles HCN, C2N2, CCl2(CN)2 and CH2(CN)2 towards AsF5 and SbF5 has been investigated. The new compounds HCN·AsF51, HCN·SbF52, C2N2·AsF53, C2N2·SbF54, CH2(CN)2·AsF55, CH2(CN)2·(2AsF5)6 and CCl2(CN)2·AsF57 have been characterized by chemical analysis, IR, Raman, 1H, 19F and 14N NMR spectroscopy. The crystal structure of the first example of a cyanogen adduct C2N2·SbF54 has been determined. Compound 4 crystallizes in the orthorhombic space group C2221 with cell parameters a= 6.579(1), b= 16.245(1) and c= 6.389(1)A. The deformation density of the molecule was examined. The reactivities of HCN and C2N2 towards the strong Lewis acid AsF5 are discussed on the basis of the hard–soft acid–base principle. The hitherto unknown ionization potential of AsF5(15.57 eV) was obtained by photoelectron spectroscopy. A semiquantitative molecular orbital scheme for AsF5 is presented.

Chiral platinum (II) complexes with phosphorus derivatives of the amino acid l-proline. NMR spectroscopic and X-ray structure investigations of the cis influence of tertiary phosphorus ligands

Abstract 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.

Electronic structures and thermolyses of 2-tetrazenes

Abstract The electronic structures and gas phase thermolyses of the cyclic 2-tetrazenes 2 and 3 and of open chain 1,1,4,4-tetramethyl-2-tetrazene (1) have been studied by photoelectron spectroscopy. While the six-membered ring compound 2 yields 1-methylmethylenamine ( 6 ) and nitrogen as fragments, the seven-membered ring compound 3 is contracted to 1,2-dimethylpyrazolidine ( 11 ). The acyclic 2-tetrazene 1 prefers disproportionation to 6 and dimethylamine ( 7 ). Based on MNDO calculations the tonization potentials of 1 – 3 were assigned to molecular orbitals. Several conformations of 2 and 3 were calculated. Compound 2 shows a rigid boat conformation with equatorial methyl groups, while 3 can occupy several conformations of similiar energies. The different thermal decompositions of 1 – 3 are explained.

Rotation and inversion in nitrosamines

Abstract Geometry optimizations of the ground states as well as of the transition states for internal rotation and inversion have been performed by the semiempirical MNDO method for dimethyl nitrosamine (1), perfluordimethyl nitrosamine (2), N -nitroso aziridine (3), and N -nitroso azetidine (4). It was found that the potential barrier to internal rotation about the N-N bond is always of lower energy than that to inversion on the nitroso nitrogen. While the ground states tend to adopt structures which enable mesomerism, the lowest transition state is characterized by a pyramidal sp 3 -hybridized amino nitrogen. In accordance with experimental results the low barriers to rotation of 2 (7.96 kcal mol −1 ), 3 (3.38 kcal mol −1 ) and 4 (9.97 kcal mol −1 ) in comparison with 1 (12.54 kcal mol −1 ) indicate that in donor-acceptor molecules the transfer of charge can be limited by electronic and stereochemical effects. In particular, the equivalence of the α-methylene hydrogens which was observed in the NMR-spectrum of 3 is due to unhindered rotation and ring inveirsion.