TY - JOUR A1 - Kleinpeter, Erich A1 - Neuvonen, Kari A1 - Neuvonen, Helmi A1 - Koch, Andreas T1 - NBO analysis of polar and steric effect using the axial-equatorial equilibrium of cyclohexyl acetates as a probe Y1 - 2011 SN - 2210-271X ER - TY - JOUR A1 - Kleinpeter, Erich A1 - Koch, Andreas T1 - The anisotropic effect of functional groups in 1H NMR spectra is the molecular response property of spatial NICS-the frozen conformational equilibria of 9-arylfluorenes N2 - Rotation about the single bond adjoining the aryl and fluorene moieties in 9-arylfluorenes can be frozen out on the NMR timescale if methyl groups are located at either one or both of the ortho positions of the aryl substituent. In the ground-state of these rotamers, the planes of the aryl and fluorene moieties are perpendicular to each other and the methyl protons are either shielded or deshielded, respectively, due to the ringe current effect of the fluorene moiety. This anisotropic effect on the 1H chemical shifts of the methyl protons has been quantified on the basis of through- space NMR shieldings (TSNMRS) and subsequently _______ compared with the experimentally observed chemical shift differences _____. In this context, the experimental anisotropic effects of functional groups in the !H NMR have proven to quantitatively be the molecular response property of theoretical spatial nucleus independent chemical shieldings (NICS). Differences between _______ and _____ were, for the first time, also quantified as arising from steric compression. Y1 - 2011 UR - http://www.sciencedirect.com/science/article/pii/S0040402011008507 (13.9.13) SN - 0040-4020 ER - TY - JOUR A1 - Kleinpeter, Erich A1 - Pihlaja, Kalevi A1 - Sinkkonen, Jari A1 - Stájer, Gezá A1 - Koch, Andreas T1 - 1-Oxo-1,3-dithiolanesùsynthesis and stereochemistry N2 - 1-Oxo-1,3-dithiolane (4) and its cis- andtrans-2-methyl (5,6), -4-methyl (7,8) and -5-methyl (9,10) derivatives were prepared by oxidizing the corresponding 1,3-dithiolanes (1-3) with NaIO4 in water. The oxides were purified and their isomers separated using thin layer chromatography. The structural characterization was carried out with 1H and 13C NMR spectroscopy and molecular modelling. The sulfoxides 4-6 and 8-10 attain two S(1) type envelopes (sometimes slightly distorted) the S=Oax envelope greatly dominating. Cis-4-methyl-1-oxo-1,3-dithiolane is a special case exhibiting both two closely related S=Oax (30 and 27%) as well as S=Oeq (21 and 22%) forms [S(1) and C(4) envelopes, respectively]. The relative energies of these conformations, the values of 1H-1H coupling constants and 1H and 13C chemical shifts were estimated by computational methods and they support well the conclusions based on the experimental data. Y1 - 2011 SN - 0749-1581 ER - TY - JOUR A1 - Kleinpeter, Erich A1 - Koch, Andreas T1 - Chelatoaromaticity-existing: yes or no? An answer given by spatial magnetic properties (through space NMR shieldings-TSNMRS) Y1 - 2011 SN - 1463-9076 ER - TY - JOUR A1 - Kolocouris, Antonios A1 - Zervos, Nikolaos A1 - De Proft, Frank A1 - Koch, Andreas T1 - Improper Hydrogen Bonded Cyclohexane C-H-ax center dot center dot center dot Y-ax Contacts theoretical predictions and experimental Evidence from H-1 NMR Spectroscopy of Suitable Axial Cyclohexane Models JF - The journal of organic chemistry N2 - C-H-ax center dot center dot center dot Y-ax are a textbook prototype of steric hindrance in organic chemistry. The nature of these contacts is investigated in this work. MP2/6-31+G(d,p) calculations predicted the presence of improper hydrogen bonded C-H-ax center dot center dot center dot Y-ax of different strength in substituted cyclohexane rings. To support the theoretical predictions with experimental evidence, several synthetic 2-substituted adamantane analogues (1-24) with suitable improper H-bonded C-H-ax center dot center dot center dot Y-ax contacts of different strength were used as models of a substituted cyclohexane ring. The H-1 NMR signal separation, Delta delta(gamma-CH2), within the cyclohexane ring gamma-CH(2)s is raised when the MP2/6-31+G(d,p) calculated parameters, reflecting the strength of the H-bonded C-H-ax center dot center dot center dot Y-ax contact, are increased. In molecules with enhanced improper H-bonded contacts C-H-ax center dot center dot center dot Y-ax, like those having sterically crowded contacts (Y-ax = t-Bu) or contacts including considerable electrostatic attractions (Y-ax = O-C or O=C) the calculated DFT steric energies of the gamma-axial hydrogens are considerably reduced reflecting their electron cloud compression. The results suggest that the proton H-ax electron cloud compression, caused by the C-H-ax center dot center dot center dot Y-ax contacts, and the resulting increase in Delta delta(gamma-CH2) value can be effected not just from van der Waals spheres compression, but more generally from electrostatic attraction forces and van der Waals repulsion, both of which are improper H-bonding components. Y1 - 2011 U6 - https://doi.org/10.1021/jo102353f SN - 0022-3263 VL - 76 IS - 11 SP - 4432 EP - 4443 PB - American Chemical Society CY - Washington ER - TY - JOUR A1 - Pihlaja, Kalevi A1 - Sinkkonen, Jari A1 - Stajer, Geza A1 - Koch, Andreas A1 - Kleinpeter, Erich T1 - 1-Oxo-1,3-dithiolanes - synthesis and stereochemistry JF - Magnetic resonance in chemistry N2 - 1-Oxo-1,3-dithiolane (4) and its cis- and trans-2-methyl (5,6), -4-methyl (7,8) and -5-methyl (9,10) derivatives were prepared by oxidizing the corresponding 1,3-dithiolanes (1-3) with NaIO(4) in water. The oxides were purified and their isomers separated using thin layer chromatography. The structural characterization was carried out with (1)H and (13)C NMR spectroscopy and molecular modelling. The sulfoxides 4-6 and 8-10 attain two S(1) type envelopes (sometimes slightly distorted) the S=O(ax) envelope greatly dominating. Cis-4-methyl-1-oxo-1,3-dithiolane is a special case exhibiting both two closely related S=O(ax) (30 and 27%) as well as S=O(eq) (21 and 22%) forms [S(1) and C(4) envelopes, respectively]. The relative energies of these conformations, the values of (1)H-(1)H coupling constants and (1)H and (13)C chemical shifts were estimated by computational methods and they support well the conclusions based on the experimental data. KW - NMR KW - (1)H NMR KW - (13)C NMR KW - sulfur heterocycles KW - conformational analysis KW - computational chemistry Y1 - 2011 U6 - https://doi.org/10.1002/mrc.2764 SN - 0749-1581 VL - 49 IS - 7 SP - 443 EP - 449 PB - Wiley-Blackwell CY - Malden ER - TY - JOUR A1 - Kleinpeter, Erich A1 - Koch, Andreas T1 - The anisotropic effect of functional groups in H-1 NMR spectra is the molecular response property of spatial NICS-the frozen conformational equilibria of 9-arylfluorenes JF - Tetrahedron N2 - Rotation about the single bond adjoining the aryl and fluorene moieties in 9-arylfluorenes can be frozen out on the NMR timescale if methyl groups are located at either one or both of the ortho positions of the aryl substituent. In the ground-state of these rotamers, the planes of the aryl and fluorene moieties are perpendicular to each other and the methyl substituents are consequently positioned either above the fluorene moiety or in-plane with it; thus, the methyl protons are either shielded or deshielded, respectively, due to the ring current effect of the fluorene moiety. This anisotropic effect on the H-1 chemical shifts of the methyl protons has been quantified on the basis of through-space NMR shieldings (TSNMRS) and subsequently Delta delta(calcd) compared with the experimentally observed chemical shift differences, Delta delta(exp). In this context, the experimental anisotropic effects of functional groups in the H-1 NMR have proven to quantitatively be the molecular response property of theoretical spatial nucleus independent chemical shieldings (NICS). Differences between Delta delta(calcd) and Delta delta(exp) were, for the first time, also quantified as arising from steric compression. KW - Through-space NMR shieldings (TSNMRS) KW - Spatial NICS KW - Anisotropic effect KW - Iso-chemical-shielding surfaces (ICSS) KW - 9-Arylfluorenes Y1 - 2011 U6 - https://doi.org/10.1016/j.tet.2011.06.005 SN - 0040-4020 VL - 67 IS - 32 SP - 5740 EP - 5743 PB - Elsevier CY - Oxford ER - TY - JOUR A1 - Neuvonen, Kari A1 - Neuvonen, Helmi A1 - Koch, Andreas A1 - Kleinpeter, Erich T1 - NBO analysis of polar and steric effect using the axial-equatorial equilibrium of cyclohexyl acetates as a probe JF - Computational and theoretical chemistry N2 - The proportion of the axial conformer increases in the ax reversible arrow eq equilibrium of cyclohexyl acetates (RCOOC(6)H(11), R reversible arrow Me, Et, iPr, tBu, CH(2)Cl, CHCl(2), CO(3). CH(2)Br, CHBr(2), CBr(3)) with the increasing size of the acyloxy substitution. The nature of this unexpected steric substituent effect, which is opposite to general stereochemical concepts, was studied by means of ab kiln MO method, accompanied by NBO and isodesmic calculations. NBO parameters seem to be good descriptors for quantitative prediction of the experimental Delta G degrees value of the title conformational equilibrium. The origin and propagation of the substituent effect of the polar substitutions (CH(2)Cl, CHCl(2), CCl(3), CH(2)Br, CHBr(2), CBr(3)) differ, however, from those of the pure alkyl (Me, Et, iPr, tBu) substitutions. The Delta G degrees value of the polar derivatives depends on the qC8 charges, on the occupation of the sigma(center dot)(C1-07) orbital and on the hyperconjugative pi(center dot)(c=O) -> sigma(center dot)(C10-X) and sigma(center dot)(C10-X) -> pi(center dot)(c=O) interactions. The substituent sensitivity of these NBC parameters for the two conformers differ to the effect that the ax reversible arrow eq equilibrium is shifted to the left side with increasing electron withdrawing character of the acyloxy group. The Delta G degrees values of the alkyl derivatives are interpreted in terms of the calculated dipole moments. The destabilization in the non-polar medium (the experimental Delta G degrees values used were measured in CD(2)Cl(2)) due to the enhanced dipolar character is more prominent in the case of the equatorial alkyl conformers. As the consequence, the ax reversible arrow eq equilibrium is shifted to the left despite the increasing size of the R group when going from Me to tBu substitution. KW - Substituent effects KW - Ab initio MO computations KW - Conformational equilibria KW - Cyclohexyl esters KW - NBO analysis Y1 - 2011 U6 - https://doi.org/10.1016/j.comptc.2010.12.033 SN - 2210-271X VL - 964 IS - 1-3 SP - 234 EP - 242 PB - Elsevier CY - Amsterdam ER - TY - JOUR A1 - Kleinpeter, Erich A1 - Koch, Andreas T1 - Chelatoaromaticity-existing: yes or no? An answer given by spatial magnetic properties (through space NMR shieldings-TSNMRS) JF - Physical chemistry, chemical physics : a journal of European Chemical Societies N2 - The spatial magnetic properties (through space NMR shieldings-TSNMRS) of metal complexes (with ligands such as acetylacetone, 3-hydroxy-pyran(4) one) and "metallobenzenes" have been calculated by the GIAO perturbation method and visualized as Iso-Chemical-Shielding Surfaces (ICSS) of various sizes and directions. The TSNMRS values, thus obtained, can be successfully employed to quantify and visualize partial aromaticity of the metallocyclic ring by comparison with the spatial magnetic properties of the corresponding non-complexed ligands in comparable structural and electronic situations, and benzene, respectively. Because anisotropy/ring current effects in H-1 NMR spectra proved to be the molecular response property of TSNMRS, the results obtained concerning partial "chelatoaromaticity" are experimentally ensured. Y1 - 2011 U6 - https://doi.org/10.1039/c1cp21942a SN - 1463-9076 SN - 1463-9084 VL - 13 IS - 46 SP - 20593 EP - 20601 PB - Royal Society of Chemistry CY - Cambridge ER - TY - JOUR A1 - Csütörtöki, Renáta A1 - Szatmári, István A1 - Koch, Andreas A1 - Heydenreich, Matthias A1 - Kleinpeter, Erich A1 - Fulop, Ferenc T1 - Synthesis and conformational analysis of new naphth[1,2-e][1,3]oxazino[3,4-c]quinazoline derivatives Y1 - 2011 SN - 0040-4020 ER -