@article{SeidlCarneiroTostesetal.2005, author = {Seidl, Peter Rudolf and Carneiro, J. W. D. and Tostes, J. G. R. and Koch, Andreas and Kleinpeter, Erich}, title = {Interpretation of conformational effects on 2-endo-norborneol by natural chemical shielding analysis}, issn = {1089-5639}, year = {2005}, abstract = {This paper represents an extension of our work on the H-1 and C-13 NMR chemical shifts of norbornane and 2-endo- norborneol. NCS-NBO analysis was employed to probe contributions of bond orbitals and orbitals of lone pairs to nuclear shielding in conformers of the alcohol generated by rotation of the C-O bond. Variations in H-1 and C-13 chemical shifts with the dihedral angle are discussed in terms of Lewis and non-Lewis partitioning and their respective importance is evaluated. In addition to hyperconjugation of the lone pair in a p orbital of oxygen that was previously reported, a sizable participation of the lone pair which is in an sp orbital is also observed and their combined effect dominates the carbon chemical shifts of the C-1-C-2-OH and C-3-C-2-OH fragments. Both lone pairs on oxygen also contribute to localized, though-space effects on nuclei in the vicinity, these effects answering for the largest deviations in hydrogen chemical shifts on rotation around the C-O bond. On the other hand, for conformers in which nonbonded repulsions lead to distortions in the molecular framework, variations in chemical shifts may be attributed to angular effects}, language = {en} } @article{NeuvonenNeuvonenKochetal.2005, author = {Neuvonen, Helmi and Neuvonen, Kari and Koch, Andreas and Kleinpeter, Erich}, title = {Ab initio study of the substituent effects on the relative stability of the E and Z conformers of phenyl esters : Stereoelectronic effects on the reactivity of the carbonyl group}, issn = {1089-5639}, year = {2005}, abstract = {Equilibria between the Z (tau(1) = 0 degrees) and E (tau(1) = 180 degrees) conformers of p-substituted phenyl acetates 4 and trifluoroacetates 5 (X = OMe, Me, H, Cl, CN, NO2) were studied by ab initio calculations at the HF/6-31G* and MP2/6-31G* levels of theory. The preference for the Z conformer, Delta E(HF), was calculated to be 5.36 kcal mol(-1) and 7.50 kcal mot(-1) for phenyl acetate and phenyl trifluoroacetate (i.e., with X = H), respectively. The increasing electron-withdrawing ability of the phenyl substituent X increases the preference of the Z conformer. An excellent correlation with a negative slope was observed for both series between Delta E of the E-Z equilibrium and the Hammett sigma constant. By using an appropriate isodesmic reaction, it was shown that electron-withdrawing substituents decrease the stability of both conformers, but the effect is higher with the E conformer. Electron-withdrawing phenyl substituents decrease the delocalization of the lone pair of the ether oxygen to the C=O antibonding orbital (n(O) -> pi*(C=O)) in both the E and Z forms and in both series studied; this effect is higher in the E conformer than in the Z conformer. The n(O) -> pi*(C=O) electron donation has a minimum value with tau(1) = 90 degrees and a maximum value with tau(1) = 90 degrees (the Z conformer), the value with tau(1) = 180 degrees (the E conformer) being between these two values, obviously due to steric hindrance. The effects of the phenyl substituents on the reactivity of the esters studied are discussed in terms of molecular orbital interactions. ED/EW substituents adjust the availability of the pi*(C=O) antibonding orbital to interact with the lone pair orbital of the attacking nucleophile and therefore affect the reactivity: EW substituents increase and ED substituents decrease it. Excellent correlations were observed between the rate coefficients of nucleophilic acyl substitutions and pi*(C=O) occupancies of the ester series 4 and}, language = {en} } @article{KleinpeterKochPihlaja2005, author = {Kleinpeter, Erich and Koch, Andreas and Pihlaja, Kalevi}, title = {Application of (1)J(C,H) coupling constants in conformational analysis}, issn = {0040-4020}, year = {2005}, abstract = {Conformational equilibria for a number of methyl substituted 1,3-dioxanes 1, 1,3-oxathianes 2 and 1,3-dithianes 3 were calculated at the HF and DFT levels of theory. In addition to the chair conformers also the energetically adjacent twist conformers were considered and the positions of the corresponding conformational equilibria estimated. On the basis of the global energy minima of conformers, participating in the conformational equilibria, the 1J(C,Hax,equ) coupling constants were calculated using the GIAO method and compared with the experimental values obtained from C-13, H- 1 coupled C-13 NMR spectra. The Perlin effect, the influence of the solvent and the suitability of this NMR parameter for assigning the conformational equilibria present are critically discussed. (c) 2005 Elsevier Ltd. All rights reserved}, language = {en} } @article{KlinkaImrichDaniheletal.2005, author = {Klinka, Karel D. and Imrich, Jan and Danihel, I. and Bohm, Stanislav and Kristian, Pavol and Harnul'akova, S. and Pihlaja, Kalevi and Koch, Andreas and Kleinpeter, Erich}, title = {Configuration and E/Z interconversion mechanism of O(S)-allyl-S(O)-methyl-N-(acridin-9-yl)iminothiocarbonate}, issn = {0749-1581}, year = {2005}, abstract = {The configuration and dynamic behavior of O-allyl-S-methyl-N-(acridin-9-yl)iminothiocarbonate (1) and its S- allyl-O-methyl regioisomer (2) were studied using quantum chemical calculations and by applying a novel graphical method to scatter maps obtained from MD simulations for evaluation of an NOE-weighted internuclear distance (r(NOE)). Energy calculations indicated that the Z configuration was predominant for each compound and, further, this was supported both by the calculated chemical shifts and the rNOE. Both N-inversion- and rotation-type transition-state structures were also calculated for the E/Z isomerization process, the results indicating that the preferred interconversion mechanism for 1 is N-inversion, but contrastingly, interconversion via rotation is equally as probable as N-inversion for 2. This supports the notion that one or the other or both pathways can be active and each system needs to be assessed on a case- by-case basis. Copyright (c) 2005 John Wiley \& Sons, Ltd}, language = {en} } @article{HeydenreichKochSarodnicketal.2005, author = {Heydenreich, Matthias and Koch, Andreas and Sarodnick, Gerhard and Kleinpeter, Erich}, title = {Quinoxalines XIV : Synthesis, H-1, C-13, N-15 NMR spectroscopic, and quantum chemical study of 1H-pyrazolo[3,4- b]quinoxalines (flavazoles)}, issn = {0040-4020}, year = {2005}, abstract = {The synthesis of a series of 1H-pyrazolo[3,4-b]quinoxalines (flavazoles) by acylation, alkylation, halogenation, and aminomethylation of the parent compound is reported and their structure is investigated by H-1, C-13 and N-15 NMR spectroscopy. The restricted rotation about the partial C, N double bond of the N-acyl derivatives 7-10 is studied by dynamic NMR spectroscopy and the barriers to rotation are determined. In order to assign unequivocally the 15 N chemical shifts of N-4 and N-9, in case of 3-substituted flavazoles, exemplary the H-1, C-13, and N-15 NMR chemical shifts of 34, 35, and 39 are also theoretically calculated by quantum chemical methods [ab initio at different levels of theory (HF/6-3G* and B3LYP/6-31G*)]. (C) 2005 Elsevier Ltd. All rights reserved}, language = {en} } @article{NeuvonenFulopNeuvonenetal.2005, author = {Neuvonen, Kari and Fulop, Ferenc and Neuvonen, Helmi and Koch, Andreas and Kleinpeter, Erich and Pihlaja, Kalevi}, title = {Propagation of polar substituent effects in 1-(substituted phenyl)-6,7-dimethoxy-3,4-dihydro- and -1,2,3,4- tetrahydroisoquinolines as explained by resonance polarization concept}, year = {2005}, abstract = {Propagation of inductive and resonance effects of phenyl substituents within 1-(substituted phenyl)-6,7- dimethoxy-3,4-dihydro- and -1,2,3,4-tetrahydroisoquinolines were studied with the aid of C-13 and N-15 NMR chemical shifts and ab initio calculations. The substituent-induced changes in the chemical shift (SCS) were correlated with a dual substituent parameter equation. The contributions of conjugative (rho(R)) and nonconjugative effects (rho(F)) were analyzed, and mapping of the substituent-induced changes is given over the entire isoquinoline moiety for both series. The experimental results can be rationalized with the aid of the resonance polarization concept. This means the consideration of the substituent-sensitive balance of different resonance structures, i.e., electron delocalization, and the effect of the aromatic ring substituents on their relative contributions. With tetrahydroisoquinolines, the delocalization of the nitrogen lone pair (stereoelectronic effect) particularly contributes. Correlation analysis of the Mulliken atomic charges for the dihydroisoquinoline derivatives was also performed. The results support the concept of the substituent-sensitive polarization of the isoquinoline moiety even if the polarization pattern achieved via the NMR approach is not quite the same as that predicted by the computational charges. Previously the concepts of localized pi- polarization and extended polarization have been used to explain polar substituent effects within aromatic side-chain derivatives. We consider that the resonance polarization model effectively contributes to the understanding of the polar substituent effects}, language = {en} } @article{KlinkaBalentovaBernatetal.2006, author = {Klinka, Karel D. and Balentova, Eva and Bern{\´a}t, Juraj and Imrich, J{\´a}n and Vavrusov{\´a}, Martina and Pihlaja, Kalevi and Koch, Andreas and Kleinpeter, Erich and Kelling, Alexandra and Schilde, Uwe}, title = {Structural revision of products resulting from the reaction of methylhydrazine with acridin-9-yl isothiocyanate due to unexpected acridinyl migration And further reactions}, issn = {1551-7004}, year = {2006}, abstract = {The reaction of methyl acridin-9-ylthiosemicarbazide under basic conditions with methyl bromoacetate resulted in a 1,3-thiazolin-4-one structure as provided by X-ray crystallography. The structure forced a re-evaluation of the reactant methyl acridin-9-ylthiosemicarbazide, originally thought to be 2-methyl 4-acridin-9-ylthiosemicarbazide based on synthetic expectations, but which when examined by X-ray crystallography was found to be in fact the isomeric 2- methyl 1-acridin-9-ylthiosemicarbazide resulting from rearrangement via a spiro form which it is in equilibrium with in solution. The product resulting from reaction with methyl iodide was also studied and the previously reported semicarbazide produced by reaction with MNO was re-examined. In both cases, the 1,2 isomer rather than the 2,4 isomer was found to be present based on the sign of the 3JCH3,N11 coupling. Full characterization of the compounds was rendered by 1H, 13C, and 15N solution-state NMR, and in the solid state, by both 13C and 15N NMR.}, language = {en} } @article{ShainyanUshakovMeshcheryakovetal.2007, author = {Shainyan, Bagrat A. and Ushakov, Igor A. and Meshcheryakov, Vladimir I. and Schilde, Uwe and Koch, Andreas and Kleinpeter, Erich}, title = {The stereodynamics of 3,5-bis(trifluoromethylsulfonyl)-1,3,5-oxadiazinane and 1,3,5- tris(trifluoromethylsulfonyl)-1,3,5-triazinane- an experimental and theoretical study}, doi = {10.1016/j.tet.2007.09.041}, year = {2007}, abstract = {Multinuclear dynamic NMR spectroscopy of 3,5-bis(trifluoromethylsulfonyl)-1,3,5-oxadiazinane (3) revealed the existence of two conformers with differently oriented CF3 groups with respect to the ring, and two dynamic processes: ring inversion and restricted rotation about the N-S bond. Two transition states connecting the two conformers and corresponding to clockwise and counterclockwise rotations about the N-S bond were found; the calculated activation barriers of about 12 kcal/mol are in excellent agreement with those measured experimentally for the related molecule 1,3,5-tris(trifluoromethylsulfonyl)-1,3,5-triazinane (1). X-ray analysis proved the existence of the symmetric isomer of 3, which is the minor isomer in solutions but the only one in the crystal due to packing effects. The normal Perlin effect (JCHax < JCHeq)observed for 2(6)-CH2 in 3, whereas the reversed Perlin effect was found for the 4-CH2 group in 3 as well as for all CH2 groups in 1 both experimentally and theoretically. The latter effect in compounds 1, 3, and 1- (methylsulfonyl)-3,5-bis(trifluoromethylsulfonyl)-1,3,5-triazinane (2) can be considered as a genuine reverse Perlin effect since larger values of 1JCH are observed for longer C-H bonds.}, language = {en} } @article{KleinpeterKochMikhovaetal.2008, author = {Kleinpeter, Erich and Koch, Andreas and Mikhova, Bozhana and Stamboliyska, Bistra A. and Kolev, Tsonko M.}, title = {Quantification of the push-pull character of the isophorone chromophore as a measure of molecular hyperpolarizability for NLO applications}, issn = {0040-4039}, doi = {10.1016/j.tetlet.2007.12.107}, year = {2008}, abstract = {The push-pull character of a series of para-phenyl substituted isophorone chromophores has been quantified by the 13C chemical shift difference of the three conjugated partial C=C double bonds and the quotient of the occupations of both the bonding and anti-bonding orbitals of these C=C double bonds as well. The correlations of the two push-pull quantifying parameters, and to the corresponding bond lengths, strongly recommend ;*c=c/ ;c=c as the general parameter to estimate charge alternation and as a very useful indication of the molecular hyperpolarizabilities for NLO application of the compounds studied.}, language = {en} } @article{NeuvonenFulopNeuvonenetal.2008, author = {Neuvonen, Helmi and Fulop, Ferenc and Neuvonen, Kari and Koch, Andreas and Kleinpeter, Erich}, title = {Electronic effects of heterocyclic ring systems as evaluated with the aid of 13C and 15N NMR chemical shifts and NBO analysis}, doi = {10.1002/Poc.1271}, year = {2008}, abstract = {The electronic effects of the 5- and 6-membered heterocyclic rings on the C=N-N unit of five different hydrazone derivatives of pyridine-2-, -3- and -4-carbaldehydes, pyrrole-2-carbaldehyde, furan-2- and -3-carbaldehydes and thiophene-2- and -3-carbaldehydes have been studied with the aid of 13C and 15N NMR measurements together with the natural bond orbital (NBO) analysis. As model compounds are used the corresponding substituted benzaldehyde derivatives. The polarization of the C=N unit of the hydrazone functionality of the heteroaryl derivatives occurs in an analogous manner with that of phenyl derivatives. The electron-withdrawing heteroaryl groups destabilize and the electron-donating groups stabilize the positive charge development at the CN carbon while the effect on the negative charge development is opposite. The 15N NMR chemical shift of the C=N and C=N-N nitrogens and the NBO charges at C=N-N unit can be correlated with the replacement substituent constants of the heteroaryl groups. 13C NMR shifts of the C=N carbon of N,N- dialkylhydrazones of the heteroarenecarbaldehydes can be correlated with a dual parameter equation possessing the polar substituent constant ;* of the heteroaryl group and the electronegativity of the heteroatom as variables.}, language = {en} }