Molecular structure of benzene

The molecular structure of benzene has been determined by combining the average distances obtained by the present electron diffraction study and the moments of inertia reported by Cabana et al. The following thermal average bond distances have been determined: r_g(C-C) = 1.399 ± 0.001 Å and r_g (C-H) = 1.101 ± 0.005 Å. The uncertainties represent the estimated limits of error. The C-H distance of this molecule is similar to vinyl C-H distances.     

Molecular structure of thiourea

The molecular structure of thiourea has been investigated under C(s), C(2), and C(2v) symmetry constraints. At the coupled-cluster level in conjunction with a triple-ζ basis set, only the C(2) conformer has been found to be a real minimum on the potential energy surface. Its equilibrium structure has therefore been accurately evaluated using both theoretical and experimental data. With respect to the former, high-level quantum-chemical calculations at the coupled-cluster level in conjunction with correlation-consistent basis sets ranging in size from triple- to quintuple-zeta have been carried out. Extrapolation to the complete basis-set limit as well as core-correlation effects and inclusion of full treatment of triple excitations in the cluster operator have been considered. On the basis of the vibrational ground-state rotational constants available for five isotopic species and the corresponding computed vibrational corrections, the semiexperimental equilibrium geometry of thiourea has also been determined for the first time.     

Molecular structure of proline

The molecular structures of the two lowest-energy conformers of proline, Pro-I and Pro-II, have been characterized by ab initio electronic structure computations. An extensive MP2/6-31G* quartic force field for Pro-I, containing 62,835 unique elements in the internal coordinate space, was computed to account for anharmonic vibrational effects, including total zero-point contributions to isotopomeric rotational constants. New re and improved r0 least-squares structural refinements were performed to determine the heavy-atom framework of Pro-I, based on experimentally measured (A. Lesarri, S. Mata, E. J. Cocinero, S. Blanco, J. C. Lopez, J. L. Alonso, Angew. Chem. 2002, 114, 4867; Angew. Chem. Int. Ed. 2002, 41, 4673) rotational constant sets of nine isotopomers and our ab initio data for structural constraints and zero-point vibrational (ZPV) shifts. Without the ab initio constraints, even the extensive set of empirical rotational constants cannot satisfactorily fix the molecular structure of the most stable conformer of proline, a 17-atom molecule with no symmetry. After imposing the ab initio constraints, excellent agreement between theory and experiment is found for the heavy-atom geometric framework, the root-mean-square (rms) residual of the empirical rotational constant fit being cut in half by adding ZPV corrections. The most significant disparity, about 0.07 A, between the empirical and the best ab initio structures, concerns the r(N...H) distance of the intramolecular hydrogen bond. Some of the experimental quartic centrifugal distortion constants assigned to Pro-II have been corrected based on data obtained from a theoretical force field.     

Molecular structure of liquid alcohols

A new thermodynamic treatment of continuous association is presented, where the various equilibria between i-mers are replaced by a single equilibrium between an OH groups in the bonded and the non-bonded states, linked in both cases to an indefinite ensemble of molecules. The treatment leads to an association constant K which differs from those considered in the theories of Kretschmer and Wiebe and of Wiehe and Bagley. For pure alcohols the association constant can be estimated from the vapor pressure of the alcohol and that of the homomorphous hydrocarbon. The fraction γ of free OH groups determined in this way is markedly smaller than those calculated from the other theories. For the normal alcohols the product $\text{K}\text{V}{}_{\mathrm{<mtext>A</mtext>}}$ is approximately constant at a given temperature, $\text{V}{}_{\mathrm{<mtext>A</mtext>}}$ being the molar volume. This can be expected from the increasing of the standard entropy of the non-bonded molecules when the molecular volume increases. For secondary and tertiary alcohols the product K$\text{V}{}_{\mathrm{<mtext>A</mtext>}}$ is significantly lower due to steric hindrances. However for all the alcohols considered here the enthalpy of the hydrogen bond remains nearly constant — δH being equal to 24.8 ± 2 kJ mol^?1.     

Molecular structure of dimethylsulfonium cyclopentadienylide

Conclusions The molecular structure of dimethylsulfonium cyclopentadienylide was established by the x-ray structure analysis method, and it was shown that the S atom is coplanar with the cyclopentadienyl ring, and that the S-C distance (cyclopentadienyl) is shorter than the S-C distances (methyl).Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 3, pp. 687–689, March, 1977.The authors express their gratitude to A. E. Kalinin for his assistance in formulating the results and valuable discussions, and to V. N. Setkina and A. Zh. Zhakaeva for supplying the study objects and their interest in the work.     

Molecular structure of cybotactic nematics

Fizika Science and Engineering Cooperative Firm. Translated from Zhurnal Strukturnoi Khimii, Vol. 33, No. 3, pp. 119–125, May–June, 1992.     

Molecular structure of carbene analogs

The geometrical parameters of all carbene analogs, halocarbenes, and the corresponding tetrahalides obtained from experimental data and high-level quantum-chemical calculations were collected. Trends in their variations are interpreted using the VSEPR model and by consideration of the HOMOs and LUMOs.     

Molecular structure of isolongifolene epoxide

An X-ray analysis on isolongifolene epoxlde definitely settles the dispute over the stereochemistry in favor of the $\text{endo}$-formulation.     

Molecular structure of ortho-nitrobenzyltriethylammonium bromide

It was established by x-ray crystallographic analysis that the molecular o-nitrobenzyltriethylammonium cation has an orthogonal orientation of the benzene ring and the vicinal C-N⁺ bond (=93.8°). The nitro group deviates from the plane of the aromatic ring by an angle of 26.9°. One of the ethyl groups at the quaternary nitrogen atom is in an ap-conformation (=176.2°); the other two are in sc-conformations, characterized by torsional angles 65.1 and 69.4°.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 8, pp. 1816–1818, August, 1991.     

Molecular structure of 1,2,4-triazole

The molecular structure of 1,2,4-triazole has been determined by gas phase electron diffraction. The intemuclear distances and bond angles were obtained by applying a least-squares analysis to the experimental intensity. The bond distances (r_g) and bond angles were N₁-N₂ = 1.380 ± 0.010 Å, N₂C₃ = 1.329 ± 0.009 Å, C₃-N₄ = 1.348 ± 0.009 Å, N₁-C₅ = 1.377 ± 0.004 Å, N₄C₅ = 1.305 Å (calculated value). N-H = 0.990 Å, C-H = 1.054 Å, ∠N₁N₂C₃ = 102.7± 0.5°, ∠N₂C₃N₄ = 113.8 ± 1.3°, ∠N₂N₁C₅ = 108.9 ± 0.8°, ∠H₁N₁N₂ = 110.9°, ∠H₂C₃N₄ = 119.2°, ∠H₃C₅N₁ = 131.0°, ∠C₃N₄C₅ = 105.7° (calculated value) and ∠N₄C₅N₁ = 108.7° (calculated value).     

Molecular structure of dicumyl peroxide

Conclusions It has been shown by x-ray diffraction analysis that the peroxide group in. dicumyl peroxide has a planar transoid conformation as in the crystals of its analogs (bis(triphen-ylmethyl) peroxide and di-tert-butyl peroxide); and so this conformation is the preferred one for peroxides with two tertiary hydrocarbon substituents. This conclusion is supported by the results of conformational calculations carried out using MNDO and molecular mechanics.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 4, pp. 791–795, April, 1988.     

Molecular structure and Bader's theory

 It is shown that a supposed catastrophe of Bader's theory of atoms in molecules, suggested by Cassam-chena? and Jayatilaka [Theor Chem Acc (2001) 105: 213] is merely a consequence of the approximate character of the adiabatic Born–Oppenheimer theory of molecular structure, and that nonadiabatic approaches could be in accordance with Bader's ideas. Received: 4 April 2001 / Accepted: 5 September 2001 / Published online: 3 June 2002     

Molecular and crystal structure of tetrahydroneosophoramine

The structure of tetrahydroneosophoramine has been studied by x-ray structural analysis. Rings A, B, and C have the chair conformation, and ring D the half-chair conformation. On the basis of a semiquantitative analysis it has been shown that the strains in the molecules of tetrahydroneosophoramine and of matrine are of the same order of magnitude.