Conformational analysis of 1,2-difluoroethane, an ab-initio investigation

Contrary to expectation, the gauche conformer of 1,2-difluoroethane is more stable than the trans conformer in the gas phase. In order to understand the underlying causes of the “gauche effect”, a complete geometry relaxation was performed for the gauche and trans conformers of 1,2-difluoroethane with the 4-21G, 4-31G and 4-31G** basis sets. The 4-31G** optimized geometry of the gauche conformer compares well with the experimental values obtained from a number of electron-diffraction studies. A correction for the correlation energy, calculated by means of second-order Møller—Plesset perturbation theory with the 6-31G** basis set, proves to be essential to obtain a correct estimate of the energy difference between the gauche and trans conformers 1,2-difluoroethane.     

NMR spectra of 1,2-difluoroethane in nematic solvents

In 1,2-difluoroethane, the gauche conformation predominates strongly. This circumstance allows the determination of its relative geometry by NMR in liquid crystals, though certain assumptions have still to be made, e.g. neglect of intermediate conformations. The principal result of the structure determination is the value of 69–70° for the dihedral angle ∠FCCF, which is quite insensitive to variations of the other internal coordinates. The value is, within the limits of error, equal to that obtained by microwave spectroscopy, but deviates from results of electron diffraction measurements.It is impossible to describe the average orientation of 1,2-difluoroethane using less than three orientation parameters. This indicates that the interconversion of the two gauche rotamers is slow compared with the reorientational motion.In an attempt to explain the lower energy of the gauche conformation relative to the trans conformation, and the high value of the dihedral angle, ∠FCCF, some explorative empirical calculations were carried out. These could not, however, reproduce the experimental data.     

小螺桨烷的"弯键"研究

本文利用Hellmann-Feynman力的ab initio计算程序ABHF, 定义了小螺桨烷中弯键的张力、胁变能、键弯曲角等公式, 计算了[1.1.1]螺桨烷、[2.1.1]螺桨烷、[2.2.2]螺桨烷[2.2.2]螺浆烷的重垒力f^0、等效点电荷q^0、键的弯曲角α、张力f、键合力F、胁变能ε等。对这种具有较高键张力的分子进行了稳定性和成键特性等方面的研究, 计算及理论分析结果满意地解释了实验事实。     

MNDO calculations on hydrogen bonds. Modified function for core-core repulsion

The MNDO method has been modified to calculate the properties of the structures involving hydrogen bonds X···H—Y, X, Y = N, O and F. A new method (referred to as MNDO/H) has been tested by calculation of a wide range of molecular systems with weak and strong (ionic) hydrogen bonds. The results obtained are in good agreement with the experimental data. In the cases where direct comparisons are possible, the MNDO/H method seems to give more accurate values of hydrogen bond energy than the ab initio method using STO-4-31G basis set.     

Competition between HF And HCl molecular elimination in IRMPD of 1,2-dichloro-1,2-difluoroethane

The infrared multiple-photon decomposition of 1,2-dichloro-1,2-difluoroethane has been studied using time-resolved infrared emission. Vibrationally excited parent molecules dissociate competitively via two channels to form vibrationally excited HCl and HF, depending strongly on CO₂ laser fluence. The yield of HF elimination increases with fluence, while that of HCl elimination reaches a plateau and then decreases.     

Nucleophilic addition to C,C-double bonds. VI. Intramolecular addition of primary amines to C,C-double bonds induced by steric compression

The tricyclic olefinic primary amine 1 readily cyclizes to the tetracyclic secondary amine 2 at approximately 200° in protic as well as aprotic solvents although the C, C-double bond is not activated by electron-attracting groups. This unusual intramolecular addition is the consequence of the close proximity of the nucleophile and the double bond. For the synthesis of the sterically highly hindered amine 1 , the tricyclic oxime 4 was reduced with TiCl₃ to the remarkably stable imine 5 and the latter treated with AlH₃. On the other hand, reaction of 4 with AlH₃ yielded the pentacyclic aziridine 6 .     

Electron—density maps of bent bonds in non-cyclic molecules

Much structural evidence supports the crude notion that a chemical bond behaves much like a flexible spring and is readily bent by unbalanced 1 … 3 steric repulsions. Electron-density maps, derived experimentally (X-ray diffraction) or theoretically (extended-basis SCF calculations), show that such bending is expressed in a deformation-density peak slightly displaced off the bond axis. This is demonstrated in 2-cyanoguanidine, hydrazoic acid, cyanogen azide, formic acid, and diimide. Besides accounting for tilted methyl groups, as in methanol, and numerous similar violations of local symmetry, the flexible—spring model rationalizes a variety of conformational effects, such as the preferred syn-planar conformation of carboxylio acids, where the axis of internal rotation does not coincide with the internuclear vector.     

Aluminum foils: the contrasting characters of hyperconjugation and steric repulsion in aluminum dimetallocenes

The novel sandwich complex Cp2*Al2I2, which was recently synthesized by Minasian and Arnold, has been characterized using ab initio and density functional methods. A large family of related compounds was also investigated. Although a few Al(II)–Al(II) bonds are known, this is the first such bond to be supported by Cp-type ligands. In addition, in the remarkable Cp4*Al4 synthesis by Roesky, Cp2*Al2I2 is the Al(II) intermediate; Cp4*Al4 is important as a precursor to novel organoaluminum species. Halogen and ligand effects on the Al–Al bond in Cp2*Al2I2 were systematically explored by studying a series of 20 Cp2*Al2I2 derivatives using density functional theory with relativistic basis sets for the halogens. Comparison was made with the focal point treatment, which uses extrapolation to estimate the full configuration interaction and complete basis set limit energy. Torsional potential energy curves, natural population analyses, and enthalpies of hydrogenation were computed. Using the focal point approach, torsional barriers were computed with 0.05 kcal mol(–1) uncertainty. The interplay of steric and electronic effects on the torsional potential energy curves, enthalpies of dehydrogenation reactions, and geometries is discussed. In species with small ligands (R = H, Me), hyperconjugative effects determine the torsional landscape, whereas steric repulsions dominate in species with Cp* alkyl ligands. Species with Cp ligands represent an intermediate case, thus providing insight into how ligands modulate the structures and properties of small metal clusters.     

1,2-migration in rhodium(II) carbene transfer reaction: remarkable steric effect on migratory aptitude

A series of diazo carbonyl compounds bearing different substituents have been prepared in order to investigate the steric effect in 1,2-migration reaction of rhodium(II) carbene. Through the investigation on the diazo decomposition of these compounds with Rh2(OAc)4, it was found that the steric effect could dramatically influence the migratory aptitude. In many cases, the steric effect could override the inherent electronic effect of the substituent.     

IR and visible luminescence studies in the infrared multiphoton dissociation of 1,2-dibromo-1,1-difluoroethane

The infrared multiphoton dissociation of 1,2-dibromo-1,1-difluoroethane gives rise to IR and visible luminescence. Vibrationally excited parent molecules dissociate via two primary channels yielding bromine and vibrationally excited HBr. The strong visible emission observed between 350 to 750 nm has been assigned to electronically excited carbene CF₂Br H.     

Trapping of unstable conformations from thermal molecular beams in argon matrices: 1,2-difluoroethane and 1,3-butadiene,i.r. spectra and conformer equilibria

Ar matrix i.r. spectra of gauche and trans 1,2-difluoroethane -d₀ and -d₄, and of butadiene-1¹³C, -1,4-¹³C₂, and -d₆ generated from thermal molecular beams with Knudsen cell temperatures between 300 and 850 K are reported. It is shown that the conformers of the haloethane (gauche and trans) and the diene (trans and cis) are trapped in the matrix. For 1,2-difluoroethane the potential function to internal rotation is derived from the far i.r. gas spectrum. Basing on matrix spectra and data available on gas spectra and normal coordinate analysis approximations to the harmonic potential of the conformations of CH₂FCH₂F, CH₂ClCH₂Cl and CH₂:CHCH:CH₂ are derived. From van't Hoff plots of the concentration ratio of CH₂FCH₂F gauche and trans conformations the enthalpy ΔH(T₀)≈ 207(30) cm⁻¹ is derived. By comparison of the equilibrium constant of the gauche → cis conversion calculated from statistical thermodynamics with the observed concentration ratio it is shown that the trapping process of the thermal molecular beam in the matrix occurs with essentially no thermal relaxation of the unstable conformer.     

Hyperconjugation effect in substituted methyl boranes: an orbital deletion procedure analysis

The hyperconjugation effect in the substituted methyl boranes, XCH(2)BH(2) (X = H, CH(3), NH(2), PH(2), OH, SH, F, Cl, Br), has been quantitatively evaluated by using the orbital deletion procedure (ODP), where the p(pi) orbital on boron is deactivated. Except for the case of X = NH(2), which forms a three-membered ring, the magnitude of the hyperconjugative stabilization in all other substituted methylborane ranges from 6.8 to 3.4 kcal/mol. Significant structural changes are observed, particularly the shortening of the central B-C bond distance and the reducing of the corresponding XCB and HCB bond angles. In general, the strength of the hyperconjugative interaction between the occupied sigma(C-X) bond and the vacant p(pi) orbital on boron is correlated to the electronegativity of X, and the competition between the donation ability of the sigma(C-X) and the sigma(C-H) bonds determines the preference of the staggered or eclipsed structure as the energy minimum state. When the donation abilities of the C-X and C-H bonds are comparable, other factors such as electron correlation and steric effect may play elaborate roles in the geometrical propensity of the most stable structures.     

A steric deuterium isotope effect in 1,1,3,3-tetramethylcyclohexane

The equilibrium isotope effect (EIE) for the interconversion of the two chair isotopomers of 1-trideutero-1,3,3-trimethylcyclohexane was predicted using geometry and vibrational force constants derived from electronic structure theory at HF, B3LYP, and MP2 levels as input for the program THERMISTP. Agreement between theory and previously reported NMR results is very good (experimental K(eq) = 1.042 +/- 0.001 vs K(eq) = 1.0409 at MP2/6-311G* level, K(eq) = 1.0503 at HF/6-311G*, and K(eq) = 1.0417 at B3LYP/6-311G* level, all at 17 degrees C). In order to investigate the origin of this isotope effect, the calculated EIEs for the monodeuterated isotopomers were analyzed. It has been shown that the hydrogen atom on an axial methyl group which is unusually close to its counterpart on the other axial methyl is responsible for the large (steric) isotope effect in the compound studied.     

Trapping of unstable molecular conformations in argon matrices: Gauche- and trans-1,2-difluoroethane

Experiments are reported which describe the trapping of the unstable trans conformer of 1,2-difluoroethane in argon matrices. Assignments of the infrared spectra of the gauche and trans conformers are given. The latter is shown to have a 600 cal/mol higher free enthalpy.     

Metal coordination architectures of N-acyl-salicylhydrazides: The effect of metal ions and steric repulsion of ligands to their structures of polynuclear metal complexes

Three polynuclear transition metal complexes [Mn₈(DMF)₈(L1)₈] · 4DMF (1), [Mn₆(DMF)₆(L2)₆] · [Mn₆(DMF)₄(H₂O)₂(L2)₆] · 2DMF (2), [Cu₃(L3)₂(py)₂] (3) of the pentadentate ligands N-acyl-salicylhydrazides were synthesized and characterized, their crystal structures were investigated. The oxidation state and properties of the central metal ions are important in crystal structure formation, trivalent Mn(III) ion which easily form stable octahedral coordination metallamacrocycle complexes, metallacrowns 1 and 2 were obtained; while bivalent Cu(II) ion is easier to form square planar, trinuclear complexes 3 was obtained. The steric effect of the N-acyl side chains also plays an important role in the structures of these polynuclear complexes. The magnetic property of 1 was also investigated.     

Zinc, cadmium, and mercury 1,2-benzenedithiolates with intramolecular NH...S hydrogen bonds

Mononuclear Zn, Cd, and Hg 1,2-benzenedithiolates with intramolecular NH...S hydrogen bonds, [M(II){1,2-S2-3,6-(RCONH)2C6H2}2](2-) (R = CH 3, t-Bu; M = Zn, Cd, Hg), were synthesized and characterized by X-ray analysis and spectral measurements. The presence of intramolecular NH...S hydrogen bonds was established by the IR spectra. (199)Hg and (113)Cd nuclear magnetic resonance showed a stabilized four-thiolate coordinated structure and suggested the influence of the NH...S hydrogen bonds to ppi(Hg)-ppi(S) interactions. The NH stretching bands show that the NH...S hydrogen bonds in Cd and Hg complexes are stronger than those in the corresponding Zn complex. These results are supported by theoretical calculations. The experimental and theoretical results suggested that the NH...S hydrogen bond influences the efficient capture of toxic Cd and Hg ions by metallothioneins.     

Intramolecular halogen bonds in 1,2-aryldiyne molecules: a theoretical study

Intramolecular halogen bonds have been the subject of several current experimental and theoretical studies. In this work, intramolecular halogen bonds in a series of 1,2-aryldiyne molecules were investigated using density functional theory calculations at the M06-2x level of theory. For comparison, some dimeric complexes between halogenated aryldiynes and quinolinyl compounds were also considered. The calculated interatomic distances and interaction angles of intramolecular halogen bonds compare fairly well with those determined experimentally, and the triangle motifs retain almost perfectly planar in all the studied molecules. Many of the well-known properties of conventional halogen bonds are reproduced in intramolecular halogen bonds: the interaction strength tends to increase with the enlargement of the atomic radius of halogens (I > Br > Cl); the attachment of electron-withdrawing moieties to halogens leads to much stronger intramolecular halogen bonds; the X···N (quinolinyl) interactions are stronger than the X···O (carbonyl) halogen bonds. On the basis of the shorter interatomic distances and the larger values of electron densities at the bond critical points, intramolecular halogen bonds become stronger in strength than corresponding intermolecular halogen bonds. However, these interactions have similar structural, energetic, atoms in molecules (AIM), and noncovalent interaction index (NCI) characteristics to traditional halogen bonds. Therefore, these interactions can be recognized as halogen bonds that are primarily electrostatic in nature. Particularly, the formation of intramolecular halogen bonds gives rise to the essential coplanarity of the molecules, whereas the two subunits in the dimeric complexes deviate from planarity to a large degree. In addition, a small number of crystal structures containing intramolecular halogen bonds were retrieved from the Cambridge Structural Database (CSD), to provide more insights into these interactions in crystals. This work not only will extend the knowledge of noncovalent interactions involving halogens as electrophilic centers but also could be very useful in molecular design and synthetic chemistry.     

Role of steric factors in formation of O—H...M hydrogen bonds with metallocenes

Molecular mechanics calculations of geometric parameters and energies of molecular complexes with a O-H...M hydrogen bond have been performed for osmocene and decamethylosmocene with three proton donors. The results of calculations demonstrated that when rings are methylated, steric hindrances to formation of this hydrogen bond increase. This is the reason for anomalously low formation constants of H-bonded Cp ₂ ^* M molecular associates compared to CP₂M associates.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 1925–1927, October, 1995.The work was supported by the Russian Foundation for Basic Research (Project No. 93-03-4610) and the International Science Foundation (Grant MP5 000).