Search results for "init"

(1Z,3Z)-1,4-Di(pyridin-2-yl)buta-1,3-diene-2,3-diol:  The Planar Highly Conjugated Symmetrical Enediol with Multiple Intramolecular Hydrogen Bonds

2002

1H, (13)C, and (15)N NMR spectral data show that in chloroform solution (1Z,3Z)-1,4-di(pyridin-2-yl)buta-1,3-diene-2,3-diol, OO, is in ca. 9:1 equilibrium with (3Z)-3-hydroxy-1,4-di(pyridin-2-yl)but-3-en-2-one, OK, while no 1,4-di(pyridin-2-yl)-2,3-butanedione, KK, was detected. The species present in the tautomeric mixture were identified by comparing their experimental chemical shifts with those known for similar compounds as well as with the theoretically calculated (GIAO-HF/DFT) values. Ab initio calculations show that OK and especially the highly conjugated OO forms are preferred in the tautomeric mixtures both in vacuo and in chloroform solution. Comparison of experimental (Arrhenius)…

Substituent and temperature controlled tautomerism of 2-phenacylpyridine: the hydrogen bond as a configurational lock of (Z )-2-(2-hydroxy-2-phenylvi…

2000

2-Phenacylpyridines substituted in the benzene ring are in equilibrium with (Z)-2-(2-hydroxy-2-phenylvinyl)pyridines when dissolved in chloroform. The substituent affects significantly the tautomeric equilibrium [the amount of the enolimine form stabilized by the intramolecular hydrogen bond is 1 and 92% for R = p-N(CH2)4 and p-NO2, respectively]. The negative logarithm of the tautomeric equilibrium constant, KT, is linearly dependent on the Hammett σ substituent constants. The dependence of KTvs. temperature is exponential in character: the more electron-withdrawing is the substituent, the more distinct is the influence of temperature. Unexpectedly, the tautomer present in the crystalline …

Ab initio molecular orbital study of the substituent effect on phosphine–borane complexes

1998

Abstract Ab initio molecular orbital calculations have been used to study the substituent effect on H 3 BPH n Me 3− n and Me 3− n H n BPH 3 ( n =0–3) phosphine–borane complexes. The ab initio results show that successive methyl substitution on the phosphine favours complex formation, contrary to successive methyl substitution on the borane. The natural bond orbitals partitioning scheme suggests that, in general, there is no correlation between the charge transfer and the complexation energies. It also shows the shortening of the P–H and P–C bond lengths, upon complexation, is due to the increasing `s' character of these bonds.

Influence of long chain branching on the elongational behaviour of different polyethylenes and their blends

1986

Elongational data on a series of polyethylenes with different structure and on their blends are presented in order to correlate the extensional behaviour with the long chain branching degree.

Thermodynamic theory of finite thickness of polyethylene crystals

1962

An Accurate Quartic Force Field and Fundamental Frequencies for the Ozonide Anion: A Rare Positive Anharmonicity for the Antisymmetric Stretch

2003

The CCSD(T) method has been used to compute a highly accurate quartic force field and fundamental frequencies for all 16O and 18O isotopomers of the ozonide anion. The CCSD and CASPT2 methods have also been used to verify the reliability of the CCSD(T) fundamental frequencies. The computed fundamental frequencies are in agreement with gas-phase experiments, but disagree with matrix isolation experiments for the antisymmetric stretch, ν3. CASPT2 calculations show that the antisymmetric part of the O3- potential surface is sensitive to the external environment. It is concluded that the antisymmetric stretch exhibits a significant matrix shift in the matrix isolation experiments and that the m…

Crystal structure and reactivity of 2-chloro-3,5-dinitrothiophene and of 2-phenylsulphonyl-3,5-dinitrothiophene with nucleophiles

1988

The geometry obtained by a crystal structure determination of the title compounds, as compared with that of the analogous benzene derivatives, along with results of ab initio calculations, is used to interpret the different SNAr reactivities in some thiophene and benzene compounds. The smaller rotation of the nitro groups with respect to the aromatic rings observed in thiophene derivatives should be considered a relevant factor in the higher reactivity in the thiophene series.

Potential energy surface for the decomposition of mandelic acid

1997

Abstract The decomposition of mandelic acid has been studied by ab initio calculations at the MP2/6-31G ∗∗ level. Three competitive reaction pathways have been characterized, two are stepwise processes with the formation of an α-lactone intermediate, achieved by the nucleophilic attack of either the carbonylic oxygen atom (mechanism A) or the hydroxylic oxygen atom (mechanism B) of the carboxyl group, followed by a ring opening; the third pathway (mechanism C) is a one-step process. The calculated rate coefficient agrees with experimental data. The decomposition is energetically favourable along mechanism A.

Der nachweis der primärreaktion bei der autoxydation des tetralins

1964

Die Autoxydation des Tetralins wurde bei kleinen Umsatzen durch polarographische oder kolorimetrische Bestimmung des Peroxidgehaltes verfolgt und eine Anfangsgeschwindigkeit gefunden, die groser ist, als sie nach dem Peroxidgehalt des Tetralins sein sollte. Diese zusatzliche Reaktionsgeschwindigkeit ist die der Primarreaktion der Autoxydation. Dabei bilden sich aus Tetralin und molekularem Sauerstoff Radikale. Diese Primarreaktion ist nur dann beobachtbar, wenn der Peroxidgehalt des Tetralins etwa 10−5 Mol/l betragt oder kleiner ist. The autoxidation of tetraline was measured to a low degree of conversion by polarographic or colorimetric determination of tetraline hydroperoxide. The extrapo…

Chemoselective reactions of N1-methyl-2-hydroxy-3-methylamino-3-phenylpropanamide with electrophiles. Synthesis of chiral hexahydro-4-pyrimidinones a…

2002

Abstract The reactivity of (2S,3S)-N1-methyl-2-hydroxy-3-methylamino-3-phenylpropanamide 1 , containing three nucleophilic centres has been studied against dihaloalkanes and aldehydes. Hexahydro-4-pyrimidinones or oxazolidines were obtained chemoselectively. Experimental results were explained by ‘ab initio’ calculations.