An Ab Initio Study of the Relative Stabilities and Molecular Structures of 3-Substituted 2,5-Dihydrofurans and 4-Substituted 2,3-Dihydrofurans

The geometry optimized structures and total energies of 3-substituted (R) 2,5-dihydrofurans (a) and their isomers, 4-substituted 2,3-dihydrofurans (b), have been determined by ab initio calculations at the MP2/6-31G^*//HF/6-31G^* level. The nature of the moiety R has a marked effect on the relative total energies of the isomeric forms: at the calculation level cited, the reaction enthalpies for the a b isomerization range from +4.7 kJ mol^–1 for R = MeO to –30.5 kJ mol^–1 for both R = COOMe and R = NO₂. The reaction enthalpies appear to be controlled by the electronic effect of R on the strength of p- conjugation in b. The a isomer has a planar ring, independent of R (excluding NH₂), whereas the planarity of b depends on the electronic nature of R: the 2,3-dihydrofuran ring is planar for both R = COOMe and R = NO₂, but nonplanar for less conjugation-enhancing substituents.     

Molecular Structure of ortho-Fluoronitrobenzene Studied by Gas Electron Diffraction and Ab Initio MO Calculations

The molecular structure of ortho-fluoronitrobenzene (o-FNB) has been investigated by gas-phase electron diffraction and ab initio MO calculations. The geometrical parameters and force fields of o-FNB were calculated by ab initio and DFT methods. The obtained force fields were used to calculate vibrational amplitudes required as input parameters in an electron diffraction analysis. Within the experimental error limits, the geometrical parameters obtained from the gas-phase electron diffraction analysis are mostly in agreement with the results obtained from the ab initio calculations. The main results are: the molecular geometry of o-FNB is nonplanar with a dihedral angle about C–N of 38(3)°. The r _g (C–F) bond is shortened to 1.307(13) Å in comparison with r _g (C–F) = 1.356(4) Å in C₆H₅F.     

Molecular Conformation of 2,2'-Dinitrodiphenylamine According to Dielectric Studies and Ab Initio Quantum Chemical Calculations

According to the results of ab initio calculations employing the HF/6-31G* approach, the isolated 2,2'-dinitrodiphenylamine molecule exists as an sc, sc conformer stabilized by a symmetric intramolecular bifurcated (three-center) hydrogen bond. In protophilic solvents (1,4-dioxane), the conformational equilibrium is also shifted in the direction of this rotational isomer.     

Crystal Structure and Quantum Chemical Ab Initio Calculations of Ibotenic Acid,an Excitatory Amino Acid Receptor Agonist

Ibotenic acid, a constituent of Amanita muscaria, is a potent NMDA receptor agonist. The structure of the ibotenic acid zwitterion monohydrate in the crystalline state has been determined by X-ray crystallography. The crystal structure has two ibotenic acid monohydrate formula units in the asymmetric unit and features an extensive hydrogen bond network. The ibotenic acid zwitter-ions in the crystalline state are compared to the in vacuo structure calculated by quantum chemical ab initio calculations at the HF/6-31+G* level and the effects of the hydrogen bond network on the structures in the solid state are discussed. The calculated potential energy curve with respect to side-chain orientation displays a single energy minimum. The conformations corresponding to the solid-state conformations are calculated to be ca. 2 kcal/mol higher in energy than the minimum-energy conformation in vacuo.     

Molecular Structures and Thermodynamic Stabilities of Cycloalkadienes and Their Methoxy Derivatives: An Ab Initio and DFT Study

The geometry-optimized molecular structures and total energies of 4- to 6-membered cycloalkadienes, and of a number of their monoand dimethoxy derivatives, have been calculated by ab initio (HF/6-31G^*, MP2/6-31G^*//HF/6-31G^*) and DFT (B3LYP/6-31G^*) methods. By comparison with available experimental data, the reliability of these computational methods for an estimation of the relative stabilities (enthalpies) of the isomeric forms of the title compounds was tested. The experimental enthalpies of isomerization proved to agree best with the respective theoretical data based on the mean of the HF/6-31G^* and B3LYP/6-31G^* energies. The theoretical calculations were then extended to several isomeric methoxy-substituted cycloalkadienes, for which no previous thermodynamic data exist. Some structural features of the title dienes were also discussed.     

Molecular Structure of an Isocytosine Analog: Combined X-ray Structural and Computational Study of 2-Diethylamino-6-methyl-5-n-propyl-4(3H)-pyrimidinone

The structure of 2-diethylamino-6-methyl-5-n-propyl-4(3H)-pyrimidinone has been studied by X-ray crystallography and quantum-chemical calculations. X-ray analysis established that 2-diethylamino-6-methyl-5-n-propyl-4(3H)-pyrimidinone exists exclusively as the lactam tautomer protonated at the N3 ring nitrogen in the solid state. Crystals of 2-diethylamino-6-methyl-5-n-propyl-4(3H)-pyrimidinone are monoclinic (space group P2₁/n); the unit-cell dimensions are: a = 11.0460(8) Å, b = 5.0064(4) Å, c = 22.8358(17) Å, = = 90°, = 90.521(1)°. In the crystal, molecules of 2-diethylamino-6-methyl-5-n-propyl-4(3H)-pyrimidinone are assembled in planar centrosymmetric dimers by strong resonance-assisted N—H···O intermolecular hydrogen bonds from the NH group of one molecule to the C=O of the adjacent molecule (N—H···O distance 2.804 Å). Bond distances and angles are generally similar to those reported for the corresponding tautomer of isocytosine and derivatives. Quantum-chemical calculations on 2-diethylamino-6-methyl-5-n-propyl-4(3H)-pyrimidinone are also reported in order to estimate the relative energies of the possible tautomeric forms; ab initio and DFT results predict the coexistence of the N3 and AH tautomers in the gas phase. There is excellent correspondence between the crystal and the HF/6-311G** or B3LYP/6-31G* calculated structures of the N3 lactam form; the largest deviations between the experimental and computed structures are mostly the effects of strong intermolecular H bonds in the crystal.     

Ab Initio Calculation of the Structure of 5-Chloro-1,2,4-triazole

Three tautomeric forms of 5-chloro-1,2,4-triazole were calculated using the Hartree-Fock (ab initio) and Meller-Plesset methods in the 6-31G(d) basis. The ³⁵Cl NQR parameters were calculated using the occupancies of the 3p-components of the valence p-orbitals of the chlorine atom. The structure of this triazole was derived from the data obtained.     

Conformational Study of Naringenin in the Isolated and Solvated States by Semiempirical and Ab Initio Methods

Naringenin is a natural widespread flavanone occurring in different foodstuffs that presents several important biological activities. Although its properties are well documented, its mechanisms of action are still controversial. The present article reports a conformational analysis of naringenin, using the semiempirical AM1 and ab initio methods, at the Hartree–Fock level of theory. The 3-21G, 3-21G*, 6-31G, and 6-31G** basis sets were used. The electron correlation effects were included through the Møller–Plesset second-order perturbation theory. The solvation of naringenin has been investigated through the standard SCRF, the supermolecule (SM), and the combined SM/SCRF models. The results have shown that there are two degenerate forms of naringenin, differing mainly by the orientation of a hydroxyl group (C4—OH). The energy barrier for the interconversion between them is ca. 6 kcal.mol^–1, suggesting some conjugation between the -system of the aromatic B ring and the hydroxyl group (C4—OH).     

The Synthesis and Chemistry of 8-Substituted Bicyclo[5.1.0]oct-1(8)-ene

8-Bromobicyclo[5.1.0]oct-1(8)-cne (7), an intermediate for the preparation of 8-substituted bicyclo[5.1.0]oct-1(8)-enes, was synthesized by denomination of 1,8,8-tribromobicyclo[5.1.0]octane (6). Compound 7 underwent bromo-lithium exchange followed by nucleophilic substitution reactions to generate bicyclo[5.1.0]oct-1(8)-ene (5), 8-methylbicyclo[5.1.0]oct-1(8)-ene (10), and 8-trimethylsilylbicyclo[5.1.0]oct-1(8)-ene (11). The bicyclic cyclopropenes 7, 5, 10, and 11 reacted with cyclopentadiene to form adducts 12, 13, 14, and 15, respectively. All of these Diels-Alder adducts are endo-exo isomers (endo-addition from the view of the cyclopropene and exo-addition from the view of the cyclooctene).     

Molecular Structure of tert-Butylazide: A Gas-Phase Electron Diffraction and Quantum Chemical Study

The molecular structure of tert-butylazide has been determined by gas-phase electron diffraction and quantum chemical calculations. The HF/6-31G* and B3LYP/6-31G** calculations yielded near C _s symmetry for the tert-butyl group, anti conformation of the (C)N—N bond with respect to one of the bonds, and an essentially free rotation around the bond with a 0.34 kcal/mol energy difference between syn and anti conformations of the CNNN moiety, the anti being the more stable form. The electron diffraction analysis was carried out by modeling a mixture of conformational isomers, generated by rotating the terminal nitrogen of the azide group, using a computed rotational potential. The data are consistent with C _s symmetry for the tert-butyl group. The bond, however, was found to be rotated out of the anti position, with respect to one of the bonds, by 12.5(12)°. The electron diffraction analysis yielded the following bond lengths (r _g), bond angles, and torsional angles: , .     

Ab initio and density functional studies of cooperative hydrogen bonding in calix[4]-and calix[6]arenes

The cone conformation of C ₄ symmetry is shown by the Hartree-Fock method (3-21G basis) to be the predominant conformer of calix[4]arene; the compressed cone of C ₂ symmetry is the major conformer of calix[6]arene. Using quantum chemical methods we calculated hydrogen bond cleavage energies for calix[4]-(ab initio and density functional methods) and calix[6]arene (ab initio), and also for the complex of calix[4]arene with carbon disulfide. These energies along with structural data point to the cooperative effect of hydrogen bonds. The results of these studies provided an explanation to the greater conformational lability of calix[6]arene compared with calix[4]arene molecules. It is also predicted that the nucleophilic substitution reaction involving calix[6]arene in the presence of weak bases and in aprotic solvents, as well as in the gas phase, will occur via diastereomeric transition states.     

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.     

p-t-Butylcalix[4]arene tetra-acetamide: a new strong receptor for alkali cations [1]

From the reaction ofp-t-butylcalix[4]arene with -chloro-N,N-diethyl acetamide a new lipophilic ether-amide ligand (2) has been obtained in high yield. Solution studies show (2) to be a very strong cation receptor for alkali cations, especially sodium and potassium. The X-ray crystal structure determination of the free ligand (2) and two potassium complexes (KI and KSCN) shows the calix[4]arene in a fixed cone structure and the cation completely encapsulated in a polar cavity of eight oxygen atoms. Supplementary Data relating to this article are deposited with the British Library as supplementary publication No. SUP 82059 (57 pages).Presented at the Fourth International Symposium on Inclusion Phenomena and the Third International Symposium on Cyclodextrins, Lancaster, U.K., 20–25 July 1986.     

Spectra and Structure of Silicon-Containing Compounds. XXV. Raman and Infrared Spectra,r 0 Structural Parameters,Vibrational Assignment,and Ab Initio Calculations of Ethyl Chlorosilane-Si-d2

The infrared (3200 to 400 cm^–1) spectra of gaseous and solid and Raman (3200 to 20 cm^–1) spectra of liquid and solid ethyl chlorosilane-Si-d₂, CH₃CH₂SiD₂Cl, have been recorded. Both the gauche and trans conformers have been identified in the fluid phases, but only the gauche conformer remains in the solid phase. Variable temperature (–105 to –150°C) studies of the infrared spectra of CH₃CH₂SiH₂Cl dissolved in liquid krypton have been carried out. From these data, the enthalpy difference has been determined to be 78±11 cm^–1 (0.93±0.13 kJ/mol), with the gauche conformer the more stable form. Utilizing the frequencies of the silicon-hydrogen stretches, from the chlorosilane-Si-d isotopomer, Si—H bond distances of 1.481 and 1.480 Å have been obtained for the gauche conformer and 1.481 Å for the trans conformer. Complete vibrational assignments are proposed for both isotopomers which are consistent with the predicted frequencies utilizing the force constants from ab initio MP2/6-31G(d) calculations. Both the infrared intensities and the Raman activities and depolarization values have been obtained from the ab initio calculations. Complete equilibrium geometries have been determined by ab initio calculations employing the 6-31(d), 6-311++G(d,p), and 6-311+G(2d,2p) basis sets with full electron correlation by the Moller–Plesset (MP) perturbation method to second order. Continuing the previously reported rotational constants from five different isotopomers and the ab initio predicted structural parameters, adjusted r ₀ parameters have been calculated, which are compared to the corresponding r _s parameters. The results are discussed and the theoretical values are compared to the experimental values when appropriate.Taken in part from the dissertation of Y. E. Nashed, which will be submitted to the Department of Chemistry in partial fulfillment of the Ph.D. degree     

Stereoelectronic Control of Additions of Vinylmagnesium Bromide to 1-Methyland 3-Methylpentacyclo[5.4.0.02,6.03,10.05.9]undecane-8,11-diones and to 1-Methylhexacyclo[10.2.1.0.02,11.04,9.04,14.09,13]pentadeca-5,7-diene-3,10-dione

Grignard addition of excess vinylmagnesium bromide to 1-methyland 3-methylpentacyclo[5.4.0.0^2,6.0^3,10.0^5,9]undecane-8,11-diones (1a and 1b, respectively) and to 1-methylhexacyclo[10.2.1.0^2,11.0^4,9.0^4,14.0^9,13]pentadeca-5,7-diene-3,10-dione (4) each proceed regiospecifically to afford a single hemiketal adduct (i.e., 2a, 3a, and 5a, respectively). The structure of each of the three reaction products was established unequivocally via application of X-ray crystallographic methods. Relative energies for the model transition states obtained from geometry optimizations at the Hartree–Fock level of theory in basis set 3-21G(d) indicate that these reactions are kinetically controlled.     

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.     

The Molecular Structure and the Puckering Potential Function of Octamethylcyclotetrasilane,Si4Me8, Determined by Gas Electron Diffraction and Relaxation Constraints from Ab Initio Calculations

Gas electron diffraction data are applied to determine the geometrical parameters of the octamethylcyclotetrasilane molecule using a dynamic model in which the ring puckering is treated as a large amplitude motion. The structural parameters and parameters of the potential function were refined, taking into account the relaxation of the molecular geometry estimated from ab initio calculations at the Hartree–Fock level of theory using a 6-311G** basis set. The potential function has been described as V() = V ₀[(/ _e )² – 1]² with V ₀ = 1.0 ± 0.5 kcal/mol and _e = 28.3 ± 1.9°, where is the puckering angle of the ring. The geometric parameters at the minimum of V() (r _a in Å, in degrees and errors given as three times the standard deviations including a scale error) are as follows: r(Si—C)_av = 1.894(3), r(Si—Si) = 2.363(3), r(C—H) = 1.104(3), CSiC = 109.5(6), SiSiSi = 88.2(2), SiCH = 111.7(6), C = 4.1, where the tilt C was estimated from ab initio constraints. The structural parameters are compared with those obtained for related compounds.     

Conformational Analysis of Planar Enol Rotamers of 2,4-Pentanedione: an Ab Initio Study

All conformations among different planar enol conformers (rotamers) of 2,4-pentanedione were studied by means of the Hartree-Fock method using the STO-3G** basis set. The calculations were carried out with the Gaussian-98 program. For each conformation, stationary points with the highest energy on the energy curve were found graphically. Several conformations have low energy barriers and correspond to rotations around single bonds. They describe the spatial motion of only one (in most cases, hydrogen) atom or a small molecular fragment. All low energy barriers are in the interval 13-59 kJ·mol^-1. As would be expected, the lowest energy barrier is exhibited by the conformation that leads to the formation of an enol rotamer having an intramolecular H-bond (so-called -shaped form). On the other hand, conformations in which rotation around a bond leads to a break of the intramolecular hydrogen bond have the highest energy barriers. Conformations in which rotation occurs around the double bond have high energy barriers. The influence of the solvents CHCl₃ and CH₃CN on the intramolecular H-bond has also been studied by means of IPCM at the HF/6-31G** level.     

Electron density distribution in molecules of 4-aryl-substituted [2.2]paracyclophanes and regioselectivity of their complexation with Cr(CO)3

Charges on carbon atoms in the molecules of 4-aryl-substituted [2.2]paracyclophanes were estimated and the role of charge control as a kinetic factor in regioselectivity of their complexation with (NH₃)₃Cr(CO)₃ was investigated using electron density distribution analysis by the Bader, Weinhold-Reed (NPA), and Mulliken methods. The most plausible picture of the electron density distribution in substituted [2.2]paracyclophanes was obtained by the Bader method and compared with experimental data on the yields of reaction products. Topological analysis of the electron density distribution in the [2.2]paracyclophane molecule by the Bader method confirmed the existence of a weak antibonding interaction between the stacked benzene rings. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 441–444, March, 1999.     

Molecular Structure and Conformation of p-Bis(trimethylsilyl)benzene: A Study by Gas-Phase Electron Diffraction and Theoretical Calculations

The molecular structure and conformation of p-bis(trimethylsilyl)benzene have been investigated by gas-phase electron diffraction, ab initio MO calculations at the HF/6-31G*, MP2(f.c.)/6-31G*, and B3LYP/6-31G* levels, and MM3 molecular mechanics calculations. The calculations indicate the syn- and anti-coplanar conformations, with two bonds in the plane of the benzene ring, to be energy minima. The perpendicular conformations, with two bonds in a plane orthogonal to the ring plane, are transition states. The two coplanar conformers have nearly the same energy with a low interconversion barrier, 0.3–0.5 kJ mol^–1. The calculated lengths of the and bonds differ by only a few thousandths of an angstrom, in agreement with electron diffraction results from molecules containing either or bonds. The geometrical distortion of the benzene ring in p-bis(trimethylsilyl)-benzene may be described by superimposing independent distortions from each of the two SiMe₃ groups. The electron diffraction intensities from a previous study (Rozsondai, B.; Zelei, B.; Hargittai, I. J. Mol. Struct. 1982, 95, 187) have been reanalyzed, imposing constraints from the theoretical calculations, and using a model based on a 1:1 mixture of the two coplanar conformers. The effective torsion angles of the SiMe₃ groups may indicate nearly free rotation. Important geometrical parameters from the present electron diffraction analysis are , and . While the mean bond lengths are virtually the same from the previous and present analyses, the new ipso angle is in better agreement with the MO calculations [HF, 116.9° MP2(f.c.), 117.1° B3LYP, 116.9°].