Conformational stability of peroxynitric acid molecule and the barriers to internal rotation about the O−O and N−O bonds

The structure of conformers and potential curves of the internal rotation (PCR) about the O?O and N?O bonds in peroxynitric acid (PNA) were calculated by the unrestricted Hartree-Fock-Roothaan method. The standard valence-split 6–31G and 6–31G^* basis sets were used. The presence of two maxima on every curve has been shown. To refine the values of barriers to the internal rotation in the regions of minima and maxima of PCR, calculations taking into account the electron correlation energy have been carried out at the second- and fourth-order Møller-Plesset level of perturbation theory (MP2 and MP4, respectively). At the MP4/6-61G^* level of approximation, the barriers to the rotation about the O?O bond are equal to 8.6 kJ mol^?1 and 14.7 kJ mol^?1, and both barriers to the rotation about the N?O bond are equal to 33.5 kJ mol^?1. The results are compared with those published for PCR in hydrogen peroxide and peroxynitric acid.     

Ab initio study of the ground state and conformational stability of peroxyacetyl nitrate in internal rotation about the peroxide bond

The molecular and electronic structure of the ground state of peroxyacetyl nitrate (PAN) was calculated by the unrestricted Hartree-Fock-Roothaan method with the use of the standard 3–21G and 6–31G basis set. The potential curve of the internal rotation about the peroxide bond of PAN was calculated with the 6–31G basis set. The curve contains two maxima. The ground state of PAN is characterized by a structure in which groups of atoms adjacent to the peroxide bond lie in planes that are perpendicular to each other (the dihedral angle ϱ(COON) is 89.9°). The calculated barriers to rotation are 19.6 and 66.8 kJ mol⁻¹. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 600–604, April, 1998.     

Cyclohexanone Dimethyl Acetals: A 13C NMR, Thermodynamic, and Ab Initio Study

The spatial structures of a number of mono- and disubstituted 1,1-dimethoxycyclohexanes (cyclohexanone dimethyl acetals) were studied by ¹³C NMR spectroscopy. In the monosubstituted acetals, substituents (Me, Et, i-Pr, and MeO) on C-2 are axially oriented, contrary to their normal, equatorial orientation on C-3 and C-4. Besides the spectroscopic study, the relative thermodynamic stabilities of the cis-trans isomers of a few 2,X-dialkyl (X = 3, 4, 5, or 6) derivatives of the parent cyclohexanone dimethyl acetal were determined by acid-catalyzed chemical equilibration in MeOH solution. In the most stable isomeric form, the 2-substituent is axial and the other equatorial. In the less stable isomer, both substituents are equatorial, excluding the cis-2,6-dimethyl derivative, where the ¹³C NMR shift data point to a predominance of the diaxial form. In general, the enthalpy difference between the isomeric forms is ca. 9 kJ mol^–1, while the entropy term favors the less stable isomer by 4 to 16 J K^–1 mol^–1. In the 2,6-dimethyl derivatives, however, the trans form is favored by only 0.8 kJ mol^–1 in G _m at 298.15 K. The main findings of the experimental work are in good agreement with ab initio calculations.     

Effect of substituents (NH2, OH, F) on structures and stability of lithofluorosilylenoids

Isomeric structures and energies of three kinds of lithofluorosilylenoids, R₂SiLiF (R = NH₂, OH, F) were studied using theab initio molecular orbital theory. The calculations show that thermal stability of the three-membered ring structures of these three kinds of silylenoids decreases in the order of substituents NH₂ > OH > F because of the conjugation between NH₂, OH or F and Si atom. The interaction of substituents R with Li atom makes R₂SiLiF have a structure with two Li-A-Si-F (A = N, O, F) four-membered rings, which is the most stable of the isomers of each of three kinds of silylenoids and whose stability decreases in the order of substituents F > OH > NH₂. Inductive effect of substituents influences the thermal stability of the linear structure of silylenoids.     

A Nonempirical Quantum-Chemical Study and Natural Bond Orbital Analysis of C6H5XCY3 Species (X = SO or SO2, Y = H or F)

The potential functions of internal rotation about the C² _sp—S bonds for C₆H₅XCY₃ species (X = SO or SO₂, Y = H or F) have been obtained at the MP2 (full)/6-31+G(d) level of ab initio theory. It is found that the spatial structures with the plane of C² _sp—S—C³ _sp bonds, which is near perpendicular to the benzene ring plane, are the energy-favourable conformations. The values of the rotational barrier about the C² _sp—S bond are equal to (kJ/mole): 21.2 (C₆H₅SOCH₃), 29.0 (C₆H₅SOCF₃), 20.4 (C₆H₅SO₂CH₃), and 28.2 (C₆H₅SO₂CF₃). On the basis of the Natural Bond Orbital (NBO) analysis results, it has been revealed that the double S=O bond is a strongly polarized covalent -bond, whereas -bond electrons practically are localized on the oxygen atom. The S=O bond order for aromatic sulfoxides and sulphones is mainly caused by hyperconjugational interactions according to the LP(O) ^*(S—C_ipso) and LP(O) ^*(S—C _Y ) mechanisms. In sulphones there is also the additional mechanism of hyperconjugational interactions such as LP(O^{1 *}(S—O²) and LP(O²) ^*(S—O¹). With the replacement of one hydrogen atom on the —XCY₃ group, the charge loss of the unsubstituted benzene molecule increases: —SOCH₃ < —SO₂CH₃ < — SOCF₃ < —SO₂CF₃. The substitution of the —CH₃ group for the —CF₃ group weakly influences the charge value on the sulfur atom but effects the acceptor characteristics of the substituent to a greater extent than the variation of the sulfur atom coordination.     

Structure and isomerization of heterapentalene compounds containing hypervalent bonds with central nitrogen,phosphorus, or arsenic atom

The possibility for a specific type of isomerization (electromorphism) to occur in conjugated bicyclic organic compounds containing Group V elements was studied by the ab initio (RHF/6-31G**, MP2(full)/6-31G**), and DFT (B3LYP/6-31G**) methods. Compounds 2 (X = N, P, As) were found to exist in a monocyclic planar form with intramolecular donor-acceptor N...O coordination (X = N) and as aromatic heterapentalene structures with hypervalent O--X--O bonds (X = P, As). According to calculations, no isomerization of planar heteroaromatic structures into pyramidal ones occurs. The strength of the O--X--O hypervalent bond and the aromaticity of heterapentalene structures 2 with ten -electrons increase on going from X = N to X = P. Correct estimation of these effects requires the inclusion of electron correlation.     

Na2O-P2O5系晶体微结构形态的拉曼光谱研究及其ab initio计算

本文采用激光拉曼光谱仪测量了磷酸钠二元系(1-x)Na₂O·xP₂O₅(x=0.25,0.33,0.50,1.0)几种晶体的拉曼光谱,比较并解释了随化学组成而变化微结构单元的拉曼振动模。同时用Gaussian 98W量子化学软件从头计算了这几种化合物的拉曼光谱。实验和计算均表明,磷酸盐晶体的基本结构单元为磷氧四面体[PO₄],并且晶体中磷氧四面体的伸缩模振动频率与连接中心磷原子的桥氧数密切相关,随桥氧数增加而升高。此外还解释了模拟图谱与实验谱差异的原因。     

Rotational isomerism involving an acetylenic carbon. Part 5: Restricted rotation about acetylenic axis in sterically crowded bis(1-phenyl-9-anthryl)ethynes

The title sterically crowded di-9-anthrylethyne derivative with 3,5-ⁱPr₂-phenyl groups at 1-position showed a barrier to rotation about the acetylenic axis of 18.0 kcal mol⁻¹ based on a dynamic NMR study, which is an extremely high value for acyclic diarylethynes. The mechanism of the dynamic stereochemistry and the substituent effect on the rotational barrier are discussed.     

Ab initio study of the complexes of first-row transition-metal ions with CH, CH2, and CH3

The geometries and bonding characteristics of the complexes of the first-row transition-metal ions with CH, CH₂ and CH₃ were investigated byab initio molecular orbital theory. MCH⁺ and MCH₂ ⁺ are linear and coplanar, respectively. Both of them are with obvious treble or double bond characteristics, but these multiple bonds are mostly “imperfect”. The calculated bond dissociation energies of , and are mostly close to the experimental values, and appear in similar periodic trends from Sc to Zn. Project supported by the National Natural Science Foundation of China (Grant No. 29170070).     

四(对氯苄基)锡的合成、结构和量子化学研究

The tetra(p-chlorobenzyl)tin has been synthesized via reaction of p-chlorobenzyl chloride with Sn and struc-turally determined by X-ray diffraction method. The crystal belongs to orthorhombic space group Fdd2 with a=2.1243(6), b=2.2136(6), c=1.1242(3)nm,V=5.286(2)nm³, Z=8, D_x=1.560Mg·m^-3, μ(MoKα)=13.86cm^-1,F(000)=2480,find R=0.0331 for 2191 unique reflection [I >2σ(I)]. The bond lengths of Sn-C is 0.2167,0.2183nm,respectively.The tin atom has a distorted tetrahedral geometry. The study on title compound has been performed,with ab initio calculation by means of G98W package and taking Lanl2dz basis set.The sta-bilities of the compound,some frontier molecular orbital energies,the populations of the atomic net charges in compound and composition characteristics of some frontier molecular orbitals have been investigated. CCDC:180868.     

Novel Triplet Ground State Silylenes: H–N = C = Si, CN–N = C = Si, and MeO–N = C = Si at DFT Levels

Summary.  DFT calculations predict the existence of three new triplet ground state silylenes: [(imino)-methylene]silylene, [(cyanoimino)methylene]silylene, and [(methoxyimino)methylene]silylene, with CNSiX formula (X = H, CN, and OMe, respectively). Discrepancies are found between DFT and some ab initio results.     

Ab initio study of molecular structure and internal rotation in methyldicyanophosphine and methyldiisocyanophosphine. The Raman spectrum of methyldicyanophosphine and its interpretation with the use of scaling ofab initio force fields

The equilibrium geometric parameters and structures of transition states of internal rotation for the molecules of methyldicyanophospine MeP(CN)₂ and its isocyano analog MeP(NC)₂ were calculated by the RHF and MP2 methods with the 6–31G^* and 6–31G^** basis sets. At the MP2 level, the total energy of cyanide is −35 kcal mol⁻¹ lower than that of isocyanide and the barriers to internal rotation of methyl group for MeP(CN)₂ and MeP(NC)₂ are 2.2 and 2.7 kcal mol⁻¹, respectively. For both molecules, the one-dimensionalab initio potential functions of internal rotation approximated by a truncated Fourier series were used to determine the frequencies of torsional transitions by solving direct vibrational problems for a non-rigid model. The Raman spectrum of crystalline MeP(CN)₂ was recorded in the range 3500–50 cm⁻¹. The vibrational spectra of this compound were interpreted by scalingab initio force fields calculated by the RHF and MP2 method. The vibrational spectrum of methyldiisocyanophosphine was predicted with the use of the obtained scale factors. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 1703–1714, September, 1998.     

Ab initio investigation of tautomeric stability,molecular structure,and internal rotation of methylphosphonic dicyanide,methoxydicyanophosphine, and their isocyano analogs

The equilibrium geometric parameters and structures of the transition states of internal rotation for MeP(O)(CN)₂, McOP(CN)₂, and their isocyano analogs, MeP(O)(NC)₂ and MeOP(NC)₂, have been calculated by theab initio SCF method and with inclusion of electron correlation effects according to the second-order Muuller-Plesset perturbation theory (MP2). At both levels the 6-31G* basis set has been used. The estimation of relative stability of these tautomeric forms depends largely on the calculation level. The total energies of the cyanides calculated by the MP2 method are 25–30 kcal mol^–1 lower than those of the corresponding isocyanides. The oxo-tautomeric forms containing four-coordinate phosphorus are 15–25 kcal mol^–1 more stable than the three-coordinate phosphorus aci-derivatives. The internal rotation potential curves of the aci-forms are characterized by a deep minimum for thetrans-arrangement of the methoxy group and phosphorus lone electron pair. Two additional less clearly pronounced minima are located symmetrically on both sides of the weak maximum, which corresponds to thecis-arrangement. The equilibrium oxo-form structures have a staggered configuration of the methyl group with respect to the phosphorus atom bonds.Translated from izvestiyaAkademii Nauk. Seriya Khimicheskaya, No. 5, pp. 1104–1115, May, 1996.     

The Equilibrium C–Cl,C–Br,and C–I Bond Lengths from Ab Initio Calculations,Microwave and Infrared Spectroscopies,and Empirical Correlations

The equilibrium structures of a few small molecules containing halogens: methyl halides, cyanogen halides, haloethynes, and halocyanoethynes (from chloride to iodide) are reviewed and redetermined if necessary using high-level ab initio calculations and experimental data from microwave and infrared spectroscopies. The correlation of the r(C–X) (X=Cl, Br, I) bond length with other properties (electronegativity or another bond length) is analyzed. It is shown that simple empirical correlations may be used to make a reliable prediction of the r(C–X) bond length.     

Salicylaldehyde Benzoyl Hydrazone: Isomerization Due to Water. A Structural Analysis Using a Combination of NMR,IR, and Theoretical Investigations

Solutions of salicylaldehyde benzoyl hydrazone (SBH) in mixed DMSO/H₂O solvents have been studied by IR and NMR spectroscopy. Adding water to dilute solutions in DMSO changes the major conformation from the fully extended E isomer denoted B2E to another E isomer, B1E, where the carbonyl is associated with water. A minor Z form, B2Z, is also detected both in DMSO and in the mixed solvent. Near saturation in the mixed solvent, the major species has an imidinol function and the ratio B2Z/B1E increases. Raising the pH also favors the Z configuration. According to quantum mechanical calculations, the B2Z imidinol or monodeprotonated forms are stabilized by several kcal-mol^–1 with respect to their B1E and B2E counterparts, whereas the reverse is true for free or hydrated ketones. Free B2E ketone is calculated to be slightly more stable than its B1E counterpart, but the specific association with water reverses the situation. The balance between B1E, B2E, and B2Z forms appears to depend mainly on steric strain release and on hydrogen bond formation involving either both nitrogens, N₁ and N₂, of the hydrazone or at least one of them, N₁ for the E configuration or N₂ for the Z one. Calculation indicates that the first deprotonation occurs at the NH site for the B1E and B2E forms, but at the OH phenol site for B2Z.     

Ab initio studies on vibrational spectra of XSO2NCO (X = F,Cl): harmonic force fields and frequency assignments

The harmonic vibrational force fields and the IR spectrum of XSO₂NCO (X= F, C1) molecules have been studied usingab initio HF/SCF method with the 6-31G’ basis set. Theab initio harmonic force fields are scaled empirically using the scaled quantum mechanical (SQM) method of Pulay. A set of scale factors are optimized by the least-squares fitting to the experimental frequencies of FSO₂NCO and then are transferred to CISO₂NCO to give ana priori prediction of its fundamental frequencies. The average deviations between the theoretical frequencies and the experimental values for FSO₂NCO and C1SO₂NCO are 3 and 5 cm^-1, respectively. The assignments of the fundamentals for these two molecules are also made atcording to the potential energy distributions and theab initio IR intensities Project supported by the National Natural Science Foundation of China (Grant No. 29673029)     

Anab initio MO study on the direct and migratory reaction mechanism for the formation of HF in H + CIF system

Anab initio MO method has been used to calculate the potential energy surface for the formation of HF when H reacts with C1F. The various transition states possibly related to forming HF were optimized. An IRC calculation starting from the collinear F-centered transition state was performed and energies of a series of points with bending configurations were also calculated to determine the direct reaction path leading to HF product. Another IRC calculation starting from the H-centered transition state was run to determine the migratory path to forming HF. By doing so, the Polanyi’s assumption that it would involve the direct and a migratory mechanisms for the formation of HF when H reacts with CIF has been verified theoretically. Project supported by the National Natural Science Foundation of China.     

Conformational flexibility and aromaticity of azanaphthalenes

Conformational flexibility of naphthalene and its different mono-and diaza analogues has been studied by MP2/6-31G(d, p) quantum chemical computations. It is shown that all molecules possess a considerable conformational flexibility. The out-of-plane deformation energy of both a bicyclic π-system on the whole and separate rings directly depends on the degree of aromaticity of the conjugated system. Effects of the amount and arrangement of nitrogen atoms on conformational features and aromaticity of the molecules considered were analyzed.     

A Quantum Chemical Study of N14 Cluster

Ab initio molecular orbital theory and density functional theory have been employed to study N₁₄ cluster with low spin at the HF/6-31G*, B3LYP/6-31G*, B3PW91/6-31G*, BP86/6-31G*, and BHLYP/6-31G* levels of theory. Twelve isomers were studied, including one previously investigated cage molecule. The most stable isomer of N₁₄ is a C _2h -symmetric molecule that contains two separated five-membered nitrogen rings connected by a —N=N—N=N— bridge. The second, third, and fifth most stable isomers each have one five-membered nitrogen ring. The theoretical results suggest that the five-membered nitrogen ring gives rise to a particularly stable structural unit, and the more side chains that the five-membered nitrogen ring links with, the less stable the structure will become.