A theoretical study of the relative stability of the isomeric forms of N2O3

The electronic structure, geometrical parameters and relative stability of the isomeric forms of N₂O₃ are analysed by means of ab initio calculations. Total energies of the different isomers are given. The energy difference between the most stable conformers of the symmetric N₂O₃ is 4.31 Kcal mol^–1 as provided by 6–31G basis set. The height of the rotational barrier determined by the ab initio technique is 7.12 kcal mol^–1.Member of the Carrera del Investigador CICPBA, R. Argentina.Member of the Carrera del Investigador CONICET, R. Argentina.Predoctoral fellow of CONICET, R. Argentina.     

Synthesis and structures of isomeric benzobisthiazoles

It is shown that the bases described in a number of papers and patents as 2,6-dimethyl-, 2,6-diamino-, and 2,6-dimercaptobenzo[1,2-d:4,3-d']bisthiazoles (isomers of linear structure) are actually 2,7-dimethyl-, 2,7-diamino-, and 2,7-dimercaptobenzo [1,2-d:6,5-d']bisthiazoles (isomers of angular structure). The synthesis of the hitherto unknown 2,6-dimethylbenzo[1,2-d:4,5-d'] bisthiazoles is described.     

Synthesis and structures of isomeric benzobisthiazoles

It is shown that the bases described in a number of papers and patents as 2,6-dimethyl-, 2,6-diamino-, and 2,6-dimercaptobenzo[1,2-d:4,3-d']bisthiazoles (isomers of linear structure) are actually 2,7-dimethyl-, 2,7-diamino-, and 2,7-dimercaptobenzo [1,2-d:6,5-d']bisthiazoles (isomers of angular structure). The synthesis of the hitherto unknown 2,6-dimethylbenzo[1,2-d:4,5-d'] bisthiazoles is described.     

Mass spectra and structures of isomeric phenylaminopyrazoles

The processes involved in the dissociative ionization of isomeric phenylaminopyrazoles under the influence of electron impact were studied. The pathways of fragmentation of the molecular ion (M⁺) were proved rigorously by means of the spectra of the metastable ions. The empirical compositions of the fragment ions were confirmed by the high-resolution mass spectra. It was established on the basis of the mass spectra of the amino-group-deuterated analogs that M⁺ exists exclusively in the amide from. A rearrangement leading to the formation of benzodiazepine cation radicals precedes fragmentation of M⁺. The elimination of an HCN particle in the first step of the fragmentation of M⁺ does not involve the amino group. The pK_a values are presented for all of the investigated phenylaminopyrazoles.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 10, pp. 1381–1388, October, 1978.     

Crystal and molecular structures of the isomeric dipeptides alpha-L-aspartyl-L-alanine and beta-L-aspartyl-L-alanine

The crystal and molecular structures of the alpha- and beta-L-Asp isomers of L-aspartyl-L-alanine have been determined at 120 K using 1226 and 1609 reflections (I greater than 2.5 sigma I), respectively. The space group for the alpha-isomer is P2(1), with cell parameters a = 4.788(1), b = 16.943(4), c = 5.807(1) A and beta = 107.55(2) degrees; final R factor 0.042. The space group for the beta-isomer is P2(1)2(1)2(1) with a = 4.845(1), b = 9.409(2) and c = 19.170(3) A; final R-factor 0.047. The two peptides crystallize as zwitterions with the side-chain acidic groups ionized. Each molecule adopts a trans configuration at the peptide bond with both carboxyl groups situated on the same side of the peptide plane. The geometries of the aspartyl moieties do, however, differ in the two structures. The peptide bond is significantly longer in the beta-isomer than in the alpha-isomer, with C-N 1.344(3) and 1.328(4) A, respectively. A very short intermolecular carboxyl...carboxyl hydrogen bond (O...O = 2.502(4) A) is observed in the crystals of the alpha-isomer.     

HAsO~2异构体结构、相对稳定性与体系势能面

在MP2/6-311++G(d,p)和QCISD(T)/6-311++G(3df,2p)(单点)水平下计算得到了包括9个异构体和10个过滤态的HAsO~2体系势能面。在势能面上，异构体cis-HOAsO(E1)的能量是最低的，其次是trans-HOAsO(E2)和HAsO(O)(C~2~V,E3)，能量分别比cis-HOAsO高13.15和192.74kJ/mol。根据体系的势能面，异构体E1，E3及cis-HOOAs(E6),trans-HOOAs(E5)具有一定的动力学稳定性，在实验中应该可以观测到。AsH和O~2反应的第一步产物将会异构化为具有较高动力学稳定性的异构体E3；而OH和AsO反应可直接生成E1。计算结果与HPO~2,HPS~2,HNO~2,HNS~2等价电子相同的分子的势能面进行了比较。     

Interconversions,reactivities, and structures of isomeric hydrothiochromenes

2,4-Diphenyl-5,6,7,9-tetrahydrothiochromene was isomerized to 2,4-diphenyl-5,6-tetramethylene-4H-thiopyran in the presence of HCl and DCl. The relative thermodynamic stabilities of 2,4-diphenyl-5,6-tetramethylene-6H-thiopyran and the compound thus obtained were studied. The reactivities of the isomers of all three compounds in oxidation, disproportionation, and alkylation reactions are discussed. The structures of 2,4-diphenyl-5,6,7,9-tetrahydrothiochromene and 2,4-diphenyl-5,6-tetramethylene-4H-thiopyran were established by x-ray diffraction analysis.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 10, pp. 1342–1346, October, 1981.     

Experimental and theoretical investigation on the relative stability of the PdS2- and pyrite-type structures of PdSe2

Under ambient condition PdSe2 has the PdS2-type structure. The crystal structure of PdSe2 under pressure (up to 30 GPa) was investigated at room temperature by X-ray diffraction in an energy-dispersive configuration using a diamond anvil cell with a mixture of water/ethanol/methanol as a pressure transmitting medium. A reversible structural transition from the PdS2-type to the pyrite-type structure occurs around 10 GPa, and the applied pressure reduces the spacing between adjacent 2/proportional to [PdSe2] layers of the PdS2-type structure to form the three-dimensional lattice of the pyrite-type structure. First principles and extended Hückel electronic band structure calculations were carried out to confirm the observed pressure-induced structural changes. We also examined why the isoelectronic analogues NiSe2 and PtSe2 adopt structures different from the PdS2-type structure on the basis of qualitative electronic structure considerations.     

Structurally isomeric two pyridino macrocycles: complexation and structures

Pyridine-based, structurally isomeric two macrocyclic compounds displayed a remarkably different complexation nature. The cage-type compound has an ideal structure for spherical cations, especially for the NH₄⁺ ion, but the reaction with some transition metals and Ln³⁺ produced its protonated species. On the other hand, its isomer formed complexes with alkali metals and lanthanides (1:1) and also with transition metals. Some structures of the complexes were clarified by crystallographic analyses.     

On the relative stability of cycloalkanes

The relative stability of the cycloalkanesn=3 to 8 is investigated by means of semiempirical (CNDO) andab initio MO SCF calculations. It is found that the compensation of increasing nuclear repulsion by the gain of electronic energy reaches a maximum atn=6, leading thus to a maximum of stability forn=6, and that the usual qualitative models of hybridisation state, optimal bond angle and optimal hydrogen positions do not present sufficient explanations. The methods used, their limitations and basis set effects are discussed.     

The effects of the bulkiness of terminal chains on the stability of smectics deduced from the crystal structures of isomeric chiral biphenyl esters

Single crystal X-ray analyses have been performed on 4'-octyloxyphenyl 4'-[(S)1-methylheptyloxy]biphenyl-4-carboxylate (8*O-O8) and 4-[( S )-1-methylheptyloxy]phenyl 4'-octyloxybipheny-4-carboxylate (8O-O8*), with the phase sequences Cr-SmC*-Ch-I and Cr-SmX-SmC*-SmA-I, respectively. Both crystals have smectic-like layer structures, in which the molecular tilt angles are 10 and 30 , respectively. In the crystal phase of 8*O-O8, biphenyl moieties are twisted probably because of steric hindrance of the bulky chiral chains linked to them, whereas in 8O-O8* they are nearly planar due to the conjugation between the alkoxy oxygen atoms and the carbonyl groups. From comparison of the properties with those of other related compounds, it is concluded that the conformation of the biphenyl moieties are closely related to the stability of smectic phases.     

The isomeric structures of SiH2LiF

The structures of four isomeric forms of the model “silylenoid” SiH₂LiF have been investigated by ab initio molecular orbital theory. The two most stable forms are suggested to be the SiH₂Li⁺F^? ion pair and the H₂Si : FLi complex, analogous to the similar structures previously found for carbenoids. Two further species, a H₂Si : LiF complex and the “classical” (tetrahedral) SiH₂LiF, are also local minima on the potential energy surface, but are higher in energy.     

Discrimination of isomeric structures using information theoretic topological indices

Three newly defined information theoretic topological indices, namely “degree complexity (I^d),” “graph vertex complexity (H^V),” and “graph distance complexity (H^D)” along with three other information indices have been used to study their discriminating power of 45 trees and 19 monocyclic graphs. It is found that the newly defined indices have satisfactory discriminating power while H^D has been found to be the only index to discriminate all the graphs studied.     

Syntheses and structures of isomeric diaminotriazinyl-substituted 2,2'-bipyridines and 1,10-phenanthrolines

Isomeric 2,2'-bipyridines 4a-6a and 1,10-phenanthrolines 7a-9a with two diaminotriazinyl (DAT) substituents were synthesized to explore their dual ability to direct association by the chelation of metals and the characteristic hydrogen bonding of DAT groups. Crystals of compounds 4a-6a and 7a-9a were grown under diverse conditions, and their structures were solved by X-ray crystallography. Analysis revealed multiple shared features analogous to those observed in the structures of simpler DAT-substituted pyridines 1-3. For example, the bipyridines and phenanthrolines favor flattened conformations except in the cases of compounds 8a and 9a, where the patterns of substitution prevent the DAT groups from lying in the plane of the phenanthroline core. As expected, the DAT groups form approximately coplanar hydrogen bonds according to standard motifs I-III, which play a key role in directing molecular organization. However, the structures of simple pyridines 1-3, which favor efficiently packed chains and sheets, differ predictably from those of bipyridines 4a-6a and phenanthrolines 7a-9a in two ways: (1) The larger number of DAT groups in compounds 4a-9a typically leads to complex three-dimensional networks held together by a larger number of hydrogen bonds per molecule, and (2) the need to respect multiple directional interactions prevents compounds 4a-9a from forming closely packed structures, and significant quantities of guests are included. Together, these observations confirm the effectiveness of incorporating special groups such as DAT within more complex molecular structures to control association according to reliable patterns. Bipyridines 4a-6a and phenanthrolines 7a-9a promise to be particularly rich sources of new supramolecular chemistry because they have well-defined molecular topologies and a dual ability to direct association by chelating metals and by engaging in multiple hydrogen bonds according to reliable patterns.     

Synthesis and crystal structures of two isomeric nitro-α-resorcylic acids

Nitration of α-resorcylic acid (3,5-dihydroxybenzoic acid) to 3,5-dihydroxy-2-nitrobenzoic acid (1) and 3,5-dihydroxy-4-nitrobenzoic acid (2), their separation, and crystallization is described. Crystal structures of 1, 1 · 3H₂O, and 2 were determined by X-ray single-crystal diffraction analysis and compared with DFT-calculated structures. An intramolecular hydrogen bond between the nitro group and o-hydroxyl group(s) occurs in all investigated structures. Intermolecular hydrogen bonds and π···π stacking interactions determine the molecular packing. Additionally, N···O interactions have a crucial role in arrangement of the molecules in the crystal structures of 1 and 2. DFT calculations in the gas phase and in the solution reveal that minor product 2 is a more stable regioisomer than 1, the major product in the nitration reaction.     

Relative stability of isomeric carbocations with migrating groups in the neighboring positions

The relative stabilities of isomeric 9,9-dimethyl-10-R- and 9-R-9,10-dimethylphenanthrenyl, as well as of 3-R-2,3-diphenylbutan-2-yl and 1-R-2-methyl-1,2-diphenylpropan-1-yl cations, were determined in terms of the density functional theory. The equilibrium isomer ratio was found to depend to an appreciable extent on steric requirements of the substituents.     

A nitrogen-14 NMR study of some oximes and their isomeric structures

The isomeric structures of oximes, nitrones and nitroso compounds may be readily differentiated on the basis of their nitrogen chemical shifts. The tautomeric equilibria present in oximenitroso systems are easily observed by means of ¹⁴N NMR spectroscopy. An explanation of the range of chemical shifts found within the oxime and nitrone groups of compounds is presented, based upon the average excitation energy approximation to the paramagnetic term in the nuclear screening tensor. The increase in screening of the nitrogen found upon protonation of an oxime is analogous to that observed for pyridine-type structures and their N-oxides.     

13C NMR spectra and three-dimensional structures of isomeric 3,4-substituted thiophans

The chemical shifts and the direct carbon-proton spin-spin coupling constants for a number of cis and trans isomers of 4-amino-substituted 3-hydroxy(acyloxy)-thiophans, the configurations of the substituents in which and the conformational states were previously established by an independent method, were studied. It was found that in the spectra of the cis isomers the signals of the vicinal ¹³C atoms, which bear the substituents, are shifted to strong field as compared with the trans isomers ( trans-cis 1.7–4.3 ppm). Conformational effects of the substituents on the chemical shifts of the ring ¹³C atoms were noted. It is shown that a relationship exists between the direct carbon-proton spin-spin coupling constants and the spatial orientations of the acyloxy and acylamino substituents for five-membered saturated rings.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 12, pp. 1612–1616, December, 1981.