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Der Waals Radii (der + waal_radius)
Kinds of Der Waals Radii Selected AbstractsX-ray Crystal Structure Study of Sterically Congested DiphenyldiazomethanesEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 14 2004Takashi Iikubo Abstract X-ray crystallographic structures were determined for five diphenyldiazomethanes (DDMs) with various ortho substituents, namely, bis(2,4,6-trichlorophenyl)diazomethane, bis(2,4,6-tribromophenyl)diazomethane, bis(4- tert -butyl-2,6-dibromophenyl)diazomethane, (4- tert -butyl-2,6-dimethylphenyl)(2,4,6-tribromophenyl)diazomethane, and [4- tert -butyl-2,6-bis(trifluoromethyl)phenyl](4- tert -butyl-2,6-dibromophenyl)diazomethane, and for bis(4-bromophenyl)diazomethane, a DDM with no ortho substituents. The correlation between the structural parameters, the ESR zero-field splitting (ZFS) parameters and the lifetimes of the triplet diphenylcarbenes (DPCs) generated from these diazo compounds was examined. It is noted that as ortho substituents are introduced onto the phenyl rings of the DDMs, the interplanar angle between the two phenyl rings increases, while the angle of the diazo carbon changes very little. DDM 6 -N2, from which the longest-lived triplet carbene is generated, is shown to have the largest interplanar angle of the DDMs examined. The bond distances between the aromatic carbons and the ortho substituents as well as the van der Waals radii of the substituents were also examined. These studies do not provide a quantitative correlation between the structural parameters of the precursor DDMs and the ZFS parameters and lifetimes of their photoproducts, triplet DPCs, but show how steric shielding of the ortho substituents affects the structures and stabilities of triplet DPCs. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004) [source] Syntheses of optically active 2-(2-benzothiazolylimino)-heterazolidinesHETEROATOM CHEMISTRY, Issue 7 2001Alejandro Cruz The syntheses of compounds 4R, 5R-2-[2-benzothiazolylimino]-3-hydro-4-methyl-5-phenylthiazolidine (5a), 4S,5S-2-[2-benzothiazolylimino]-3,4-dimethyl-5-phenylthiazolidine (5b), 4R,5S-[2-benzothiazolylimino]-3-hydro-4-methyl-5-phenyloxazolidine (5c), 4R,5S-2-[2-benzothiazolyl-imino]-3,4-dimethyl-5-phenyloxazolidine (5d), and 4R,5S-2-[2-benzothiazolylimino]-1,3,4-trimethyl-5-phenylimidazolidine (5e) are reported. The stereochemistry of the reaction products and the X-ray diffraction analyses of compounds 5a,d are discussed. Compounds 5a,d present planar structures. We have found short distances between the thiazolidine S1 atom and the benzothiazole N9 atom for 5a and 5b (2.782 and 2.824 Å, respectively) and the distance between the oxazolidine O1 and the benzothiazole S7 for 5d (2.721 Å). These distances are shorter than the sum of the van der Waals radii. These distances, together with the coplanarity of the heterocycles, seem to indicate the existence of a weak coordination bonding from sp2 oxygen or nitrogen atoms towards the sulfur atom. © 2001 John Wiley & Sons, Inc. Heteroatom Chem 12:586,593, 2001 [source] Heuristic molecular lipophilicity potential (HMLP): Lipophilicity and hydrophilicity of amino acid side chainsJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 6 2006Qi-Shi Du Abstract Heuristic molecular lipophilicity potential (HMLP) is applied in the study of lipophilicity and hydrphilcity of 20 natural amino acids side chains. The HMLP parameters, surface area Si, lipophilic indices Li, and hydrophilic indices Hi of amino acid side chains are derived from lipophilicity potential L(r). The parameters are correlated with the experimental data of phase-transferring free energies of vapor-to-water, vapor-to-cyclohexane, vapor-to-octanol, cyclohexane-to-water, octanol-to-water, and cyclohexane-to-octanol through a linear free energy equation ,G°tr,i = b0 + b1Si+ + b2Si, + b3Li + b4Hi. For all above six phase-transfer free energies, the HMLP parameters of 20 amino acid side chains give good calculation results using linear free energy equation. HMLP is an ab initio quantum chemical approach and a structure-based technique. Except for atomic van der Waals radii, there are no other empirical parameters used. The HMLP has clear physical and chemical meaning and provides useful lipophilic and hydrophilic parameters for the studies of proteins and peptides. © 2006 Wiley Periodicals, Inc. J Comput Chem 27: 685,692, 2006 [source] Intermolecular , -dimer of oxoverdazyl radicals with long-distance multicenter (2e/8c) bonding via nitrogen atomsJOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 5 2010Sergiy V. Rosokha Abstract The 1,5-dimethyl-6-oxoverdazyl radical's solid-state structure shows distinct , -dimeric units with close interplanar separations (3.10,Å) between the head-over-tail cofacial moieties and several interatomic contacts shorter than the sums of the van der Waals radii. Evaluations of the frontier orbitals of monomeric oxoverdazyl and its , -dimer reveal that interaction of the radical SOMOs (concentrated on the nitrogen atoms) leads to the formation of the supramolecular orbital involving four equivalent bonding (N,N) segments between two oxoverdazyl moieties. As such, this , -dimer represents a rare example of nitrogen-based multicenter (2e/8c) long-distance bonding and emphasizes the universal character of this phenomenon in organic (ion-) radical systems. Copyright © 2009 John Wiley & Sons, Ltd. [source] Unprecedented Association of [Mo6Bri7YiBra6]3, Cluster Units and [MoIIIBr6]3, Complexes: Synthesis, Crystal Structures, and Properties of the Double Salts Rb3[Mo6Bri7YiBra6](Rb3[MoBr6])3 (Y=Se, Te)CHEMISTRY - A EUROPEAN JOURNAL, Issue 24 2006Kaplan Kirakci Abstract The double salts Rb3[Mo6Bri7YiBra6](Rb3[MoBr6])3 (Y=Se, Te) result from the partial disproportionation of the Mo6Br12 octahedral-cluster-based bromide, in the presence of corresponding chalcogenides and RbBr salt (crystal data: Rb12[MoBr6]3[Mo6Bri7TeiBra6] (1), Pmm (No. 221), a=12.1558(2) Å, Z=1, R1=0.028; wR2=0.050; Rb12[MoBr6]3[Mo6Bri7SeiBra6] (2), Pmm, a=12.144(3) Å, Z=1, R1=0.028; wR2=0.050). The structures of 1 and 2 are built up from [MoIIIBr6]3, complexes and [Mo6Bri7YiBra6]3, cluster units characterised by a random distribution of seven bromine and one chalcogen ligands on all the eight inner positions that face cap the Mo6 clusters. Such a distribution implies a static orientational disorder of the [Mo6Bri7YiBra6]3, units around the origin of the unit cell. Close-packed anionic layers based on [MoIIIBr6]3, complexes and [Mo6Bri7YiBra6]3, cluster units are stacked in the sequence ABC. This arrangement leads to very short BraBra intercluster unit distances of 3.252 Å, much lower than the sum of the van der Waals radii (3.70 Å). The trivalent oxidation state of molybdenum in the Mo complexes and 24 valence electrons per Mo6 cluster have been confirmed by magnetic susceptibility measurements. Salts 1 and 2 constitute the first examples of structurally characterised bromides containing discrete [MoIIIBr6]3, complexes obtained by means of solid-state synthesis. [source] Test and modification of the van der Waals' radii employed in the default PCM modelINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 9 2008Wei-Hua Mu Abstract High level ab initio calculations at the B3LYP/6-311++G(d,p) and MP2(full)/6-311++G(d,p) levels employing PCM/UA0 model with different van der Waals' radii for the systems that contain lithium atoms have been carried out, in order to see if the van der Waal's radius for lithium atom employed in the default PCM/UA0 model is proper or not. Comparative analysis indicated that the van der Waals' radius for alkali metals, especially for lithium atom in the default PCM/UA0 model within the Gaussian 03 package, is too small, which causes erroneous redundant imaginary frequencies (RIFs) in the characterization of Li-containing compounds from moderate to big size. A new set of van der Waals' atomic radii based on QTAIM, proposed by Bader, was suggested for a better choice in the characterization of compounds containing alkali metals, for which it can effectively avoid the erroneous RIFs for corresponding geometries of these Li-containing systems. © 2008 Wiley Periodicals, Inc. Int. J. Quantum Chem, 2008 [source] | |||||||||||||||||||