NMR Studies of Mixed-Ligand Lanthanide Complexes in Solution: Pseudorotation and Ring Inversion of 18-Crown-6 Molecule in Cerium Subgroup Chelates

¹H and ¹³C NMR measurements are reported for the CDCl₃ and CD₂Cl₂ solutions of [La(NO₃)₃(18-crown-6)] (I), [Pr(NO₃)₃(18-crown-6)] (II) and [Ce(NO₃)₃(18-crown-6)] (III) complexes. Temperature dependencies of the ¹H NMR spectra of II have been analyzed using the dynamic NMR methods for multi-site exchange. Two types of conformational dynamic processes in II were identified (the first one with activation enthalpy ΔH ^‡=26 ± 4 kJ/mol is conditioned by interconversion of complex enantiomeric form and pseudorotation of macrocycle molecule upon the C ₂ symmetry axis, the second one with ΔH ^‡=46 ± 5 kJ/mol is conditioned by macrocycle molecule inversion). Studies of the values of the lanthanide-induced shifts revealed that the structure of complexes in solution is similar to that reported for the complex I in the crystal state.This revised version was published online in July 2005 with a corrected issue number.     

Thermochemistry of Amines: Experimental Standard Molar Enthalpies of Formation of N-Alkylated Piperidines

The standard molar enthalpies of formation _f H _m ^° (l) at the temperature T = 298.15 K were determined using combustion calorimetry for N-methylpiperidine (A), N-ethylpiperidine (B), N-propylpiperidine (C), N-butylpiperidine (D), N-cyclopentylpiperidine (E), N-cyclohexylpiperidine (F), and N-phenylpiperidine (G). The standard molar enthalpies of vaporization _l ^g H ^{m °} of these compounds were obtained from the temperature variation of the vapor pressure measured in a flow system. From these data the following standard molar enthalpies of formation in gaseous phase _f H _m ^° (g) were derived for: A –(61.39 ± 0.88); B –(88.1 ± 1.3); C –(105.81 ± 0.66); D –(126.2 ± 1.3); E ( –88.21 ± 0.75); F –(135.21 ± 0.94); G (70.3 ± 1.4) kJ · mol^–1. They are used to determine the strain enthalpies of the cyclic amines A–G. The N-alkylated piperidine rings have been found to be about strainless.     

Synthesis and thermal transformations of polyphosphosiloxane based on trimethyl phosphate and (3-aminopropyl)triethoxysilane

A method for the synthesis of polyphosphosiloxane by the thermal condensation of an equimolar mixture of trimethyl phosphate and (3-aminopropyl)triethoxysilane at 200 °C was developed. The reaction affords ethanol and polyphosphosiloxane-{Si(OEt)[(CH₂)₃NR¹R²]-O-P(O)(OMe)-O}_n-(R¹ = H, Me; R² = Me), whose composition and structure were confirmed by ¹H, ¹³C, and ³¹P NMR spectroscopy, IR spectroscopy, and elemental analysis. The scheme of polymerization involving the intermediate formation of methyl-and dimethylphosphoric acids and their condensation with ethoxysilanes was proposed. The calcination of the obtained polyphosphosiloxane in vacuo at 350 °C results in the elimination of the amino groups and alkoxide substituents, and a spatially cross-linked polymer is formed as an amorphous powder. Its further thermolysis at 600 and 1000 °C gives crystalline phosphosilicates Si₅O(PO₄)₆ or SiP₂O₇. Their amorphous and crystalline samples were characterized by IR spectroscopy, X-ray diffraction analysis, and solid-state ¹³C and ³¹P spectroscopy. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 11, pp. 2138–2148, November, 2007.     

Simple route to 3-(2-indolyl)-1-propanones via a furan recyclization reaction

A simple route to 1-R-3-(2-indolyl)-1-propanones has been elaborated based on recyclization of 2-(2-aminobenzyl)furan derivatives. Being a modification of the Reissert indole synthesis, our approach employs the furan ring as a source of carbonyl function. This approach is general and allows varying of substituents in aromatic ring as well as in 3-position of indole nucleus.     

Spatial structure of β-substituted alkoxyvinyl trifluoromethyl ketones

Spatial structure of six β-substituted enones, with common structure R₁O–CR₂CH–COCF₃, were R₁ = C₂H₅, R₂ = H (ETBO); R₁ = R₂ = CH₃ (TMPO); R₁ = C₂H₅, R₂ = C₆H₅ (ETPO); R₁ = C₂H₅, R₂ = 4- O₂NC₆H₄ (ETNO); R₁ = C₂H₅, R₂ = C(CH₃)₃ (ETDO) were investigated by ¹H and ¹⁹F NMR, infrared spectroscopy and AM1 calculations. NMR spectra revealed that enones (MBO), (ETBO) and (TMPO) are exclusively (3E) isomers, whereas in (ETPO), (ETNO) and especially in (ETDO) the percentage of (3Z) isomers is significant and depends on the nature of solvents. Conformational behaviour of studied enones are determined by the rotation around of CC double bond, C–C and C–O single bonds (correspondingly trifluoroacetyl and alkoxy groups), and (EZZ) conformer being the most stable in all cases. IR spectra revealed that with the exception of (ETDO) (EZZ) conformer is most populated in all cases. Bulky substituents like phenyl or tert-butyl group at β-position of enone result in the equilibrium mainly between (EZZ) and (ZZZ) forms, whereas β-hydrogen and β-methyl substituents determine the equilibrium between (EZZ) and (EEZ) or (EZE) conformers.     

Crystal structure and nontrivial magnetic properties of CuII binuclear complex based on 4-methyl-2,6-bis{[2-(4,6-dimethyl-pyrimidin-2-yl)hydrazono]methyl}phenol

1. Download high-res image (167KB)
2. Download full-size image

     

Intramolecular charge transfer with the planarized 4-aminobenzonitrile 1-tert-butyl-6-cyano-1,2,3,4-tetrahydroquinoline (NTC6)

Fast and efficient intramolecular charge transfer (ICT) and dual fluorescence is observed with the planarized aminobenzonitrile 1-tert-butyl-6-cyano-1,2,3,4-tetrahydroquinoline (NTC6) in a series of solvents from n-hexane to acetonitrile and methanol. Such a reaction does not take place for the related molecules with 1-isopropyl (NIC6) and 1-methyl (NMC6) groups, nor with the 1-alkyl-5-cyanoindolines with methyl (NMC5), isopropyl (NIC5), or tert-butyl (NTC5) substituents. For these molecules, a single fluorescence band from a locally excited (LE) state is found. The charge transfer reaction of NTC6 is favored by its relatively small energy gap DeltaE(S(1),S(2)), in accordance with the PICT model for ICT in aminobenzonitriles. For the ICT state of NTC6, a dipole moment of around 19 D is obtained from solvatochromic measurements, similar to micro(e)(ICT) = 17 D of 4-(dimethylamino)benzonitrile (DMABN). For NMC5, NIC5, NTC5, NMC6, and NIC6, a dipole moment of around 10 D is determined by solvatochromic analysis, the same as that of the LE state of DMABN. For NTC6 in diethyl ether at -70 degrees C, the forward ICT rate constant (1.3 x 10(11) s(-1)) is much smaller than that of the back reaction (5.9 x 10(9) s(-1)), showing that the equilibrium is on the ICT side. The results presented here make clear that ICT can very well take place with a planarized molecule such as NTC6, when DeltaE(S(1),S(2)) is sufficiently small, indicating that a perpendicular twist of the amino group relative to the rest of the molecule is not necessary for reaching an ICT state with a large dipole moment. The six-membered alicyclic ring in NMC6, for example, prevents ICT by increasing DeltaE(S(1),S(2)) relative to that of DMABN.     

Finite element investigation of the ground states of the helium trimers 4He3 and 4He2–3He

A three‐dimensional finite element method is applied to the ground states of the symmetric and asymmetric atomic helium trimers ⁴He₃ and ⁴He₂–³He. Three different He–He interaction potentials of hard‐core nature were studied. Two extrapolation procedures based on the convergence properties of the finite element method are investigated. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007