The structure and spectra of 1,1-bis(trimethylsilylethynyl)-cyclopropane: a priori calculated force field and vibrational effects

The molecular structure of 1,1-bis(trimethylsilylethynyl)cyclopropane has been studied by the gas electron diffraction method, by vibrational spectroscopic methods and by ab initio calculations at the RHF and MP2 levels. A scaled quantum-chemical force field was used for band assignment in the experimental IR (4000-100 cm⁻¹) and Raman (4000-200 cm⁻¹) spectra. The root-mean-square vibrational amplitudes and harmonic shrinkage corrections were calculated taking into account non-linear relations between Cartesian and internal vibrational coordinates at the level of first-order perturbation theory (h1) and with the use of the traditional scheme (h0).     

Far IR and Raman vibrational spectra of nitramines

Vibrational spectra of several nitramines in the long-wave region (50–450 cm⁻¹) were studied. The frequencies of intra- and intermolecular vibrations were separated and a tentative assignment of the frequencies of self-associative complexes was performed. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 11, pp. 2241–2244, November, 1998.     

Nonempirical quantum chemical calculation for nitramide,its chloro- and methyl-substituted derivatives. II. Structures and energies of transition states for internal rotation and inversion of the amino group

For five N-nitramines (H₂NNO₂, MeNHNO₂, ClNHNO₂, MeNClNO₂, Me₂NNO₂) using the program GAUSSIAN-90 we have carried out quantum chemical calculations by the restricted Hartree—Fock method, taking into account electron correlation by second-order Møller—Plesset perturbation theory in a standard 6–31G^* basis. In this paper, we consider the transition states for inversion of the amine nitrogen atom and rotation about the NN bond. We have obtained data on the changes in the geometric parameters during inversion and rotation. The changes in the NN bond length are especially significant they increase by 0.06–0.08 Å in the transition states for internal rotation compared with the equilibrium forms. We have calculated the barriers to inversion and internal rotation, the height of which strongly depends on the electronegativity of the substituents on the amine nitrogen atom. Estimates of the barriers to inversion lie within the range 0.4–6.0 kcal/mole while estimates of the barriers to rotation lie within the range 6–13 kcal/mole, which are 1.5–2 times lower than in amides and N-nitrosoamines.Moscow State University. Translated from Zhurnal Strukturnoi Khimii, Vol. 34, No. 1, pp. 12–19, January–February, 1993.     

Tautomeric and conformational stabilities of methylphosphonic dicyanide, methoxydicyanophosphine, and their isocyano analogs: an ab initio calculation

The relative energies and structural parameters of the equilibrium forms and the potential functions of internal rotation of methylphosphonic dicyanide, CH₃(=O)(CN)₂, methoxydicyanophosphine, CH₃OP(CN)₂, and their isocyano analogs, CH₃P(=O)(NC)₂ and CH₃OP(NC)₂, have been calculated at the RHF/6-31G* level. The total energy of the more stable oxo forms CH₃P(=O)(CN)₂ and CH₃P(=O)(NC)₂ are 10–20 kcal mol⁻¹ lower than the energies of the aci forms CH₃OP(CN)₂ and CH₃OP(NC)₂. The relative stabilities of the cyano and isocyano isomers are almost the same in the case of the oxo forms, but for the aci forms the energies of the cyano isomers are 8 kcal mol⁻¹ lower than those of the isocyano isomers. The potential curves for internal rotation in the aci forms are characterized by a deep minimum corresponding to the trans arrangement of the methyl group and the lone pair of electrons on the phosphorus atom. Two less pronounced minima are symmetrically situated with respect to relative maximum corresponding to the transition cis form. The potential curves of internal rotation in the oxo form possess three minima corresponding to staggered configurations of the methyl group and phosphorus atom bonds. The energy characteristics and geometrical parameters of the studied molecules are compared with known data for similar compounds.     

The geometry of the nitroguanyl fragment in the simplest nitroguanidine derivatives in the absence of intermolecular interactions: The gas electron diffraction data on 1,1,3,3-tetramethyl-2-nitroguanidine

The structures and force fields of the equilibrium forms of 2-nitroguanidine (1), 1,1,3,3-tetramethyl-2-nitroguanidine (2), and nitroguanyl azide (3) were determined in the MP2(full)/6-311G(3df, 2p) approximation; wagging-inversion motions of the N amine atoms were studied. The internal rotation potential function of the NO₂ group was calculated for 1. Similar functions for 1 and 2 were also obtained in the MP2(full)/6-311G(d, p) approximation. Direct one-dimensional problems for a nonrigid model were solved by the variational method, and the distribution of torsional levels was obtained. In the region of potential minimum, rotation in both molecules had the character of large-amplitude motions. For the first time, electron diffractions data were obtained at 100°C for molecule 2 without noticeable traces of substance decomposition. A structural r _e analysis was performed using the model of large-amplitude motions for characteristic NO₂ group torsional vibrations. Vibrational corrections to internuclear distances and mean amplitudes were calculated taking into account nonlinear kinematic effects using the force fields obtained in this work. The geometry of molecule 2 calculated in the MP2(full)/6-311G(3df, 2p) approximation well corresponds to the gas electron diffraction data. The parameters of molecule 2 in the crystalline phase, however, differ substantially from the parameters of the free molecule. This corresponds with the suggestion of the influence of intermolecular H-bonds involving the imine nitrogen atom and nitro groups oxygen atoms.     

The equilibrium molecular structure of 3-cyanopyridine according to gas-phase electron diffraction and microwave data and the results of quantum-chemical calculations

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Gas-phase electron diffraction study of cyclic dimer of dimethylphosphinic acid (Me2P(O)OH)2 using quantum chemical data and a priori force field

The molecular structure of the hydrogen bonded cyclic dimer of dimethylphosphinic acid (Me₂P(O)OH)₂ was determined by gas-phase electron diffraction (GED) at 433 K. The presence of monomer cannot be determined at this temperature within the error limits for the GED method. Structural analysis was performed with consideration of non-linear kinematic effects at the first-order level of perturbation theory (h1). The vibrational characteristics of internuclear distances were calculated from a priori scaled quantum chemical (RHF/6-311G**) force field. The analysis aided by a constraint based on the RHF/6-311G** calculations yielded the following r_h1-parameters of the C₂-symmetry dimer configuration: PO 1.497(3); P-O 1.573(4); P-C 1.806(1) and 1.811(1) Å; (C-H)_av. 1.109(3) Å; ∠O-PO 120(1)°. Unlike PO and P-O bonds, whose lengths in the gas phase and in the solid state differ insignificantly, the -O?O distance in the gas phase (r_h1 2.81(4) Å) is considerably longer than in the solid state (r_α 2.48(2) Å). The latter is in accordance with the conclusion based on the IR spectra that transition from gas to a solid sample leads to strengthening of the H-bonds. Due to its small contribution to the diffraction pattern, the donor O-H bond length (r_h1 0.99(1) Å) was forcedly bound up with the parameters of C-H bonds. With this assumption, the other geometrical parameters characterizing the H-bond fragment have the following r_h1 values: O?H 1.84(4) Å, ∠-OHO164(6)°, and ∠P-O-H 117(4)°. Conformational flexibility of the non-planar eight-atom ring of the dimer is experimentally verified by absence of any apparent peaks of the f(r) curve at the r-region of more than 4.2 Å.     

A quantum-chemical and gas phase electron diffraction study of the structure of formylphosphine and acetyldimethylphosphine

The equilibrium geometry and energy and structural changes accompanying inversion motion in the phosphorus fragment and acyl group internal rotation in the H₂PCHO and Me₂PCMeO molecules were calculated by the MP2(full) method using basis sets from 6–31G(d, p) to 6–311G(3df, 2p). The structure of Me₂PCMeO was determined by electron diffraction using the dynamic model of acyl group internal rotation based on the quantum-chemical potential function of torsional motion. Acylphosphines have amide-type equilibrium conformations with acyl groups rotated through ∼100° from their orientation in the C=O/PX₂ anti- form, where X = H, Me (C ₁ symmetry). Considerable pyramidality of the phosphorus fragment distinguishes the equilibrium structures of acylphosphines from amides with a planar molecular frame (C _s symmetry). The r _h1 geometric parameters of the Me₂PCMeO molecule determined by electron diffraction closely agree with quantum-chemical estimates for the equilibrium configuration.     

Interpretation of the vibrational spectra of nitramines on the basis ofab initio calculations

It is shown that, unlike conventional methods of vibrational spectroscopy, the use ofab initia harmonic force fields, IR and Raman intensities, and depolarizations makes possible a rigorous interpretation of the experimental spectra of the simplest aliphatic nitramines (CH₃)₂NNO₂, CH₃NHNO₂, H₂NNO₂, and their isotopomers. The scale factors, which were introduced for each compound to remove the systematic errors of the SCF MO LCAO calculation by fitting the parameters to the observed frequencies, were mutually adjusted during the solution of the inverse vibrational problem. The set of transferable scale factors established in this work can be used directly to analyze spectra of larger molecules. Some common patterns of the force fields and vibrational spectra of nitramines are discussed.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya No. 12, pp. 2106–2117, December, 1994.The authors are grateful to the Russian Foundation for Basic Research (Project No. 93-03-4410) and to the Robert A. Welch Foundation for financial support of the parts of this work performed at the Department of Chemistry of Moscow State University and at the University of Texas at Austin, USA. The authors also acknowledge the support by the Scientific Technical Program Universities of Russia.     

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.