Is the stereomutation of methane possible?

Large basis set ab initio calculations at correlated levels, including MP2, single reference, as well as multireference configuration interaction, carried out on the methane potential energy surface, have located and characterized a transition structure for stereomutation (one imaginary frequency). This structure is best described as a pyramidal complex between singlet methylene and a side-on hydrogen molecule with C_s symmetry. At the single reference CI level, it lies 105 kcal/mol above the methane T_d-ground state but is stable relative to dissociation into CH₂(¹A₁) and H₂ by 13 kcal/mol at 0 K (with harmonic zero point energy (ZPE) corrections for all structures). Dissociation of the transition state into triplet methylene and hydrogen also is endothermic (by 4 kcal/mol), but single bond rupture to give CH and H^. is 3 kcal/mol exothermic. Thus, it does not appear likely that methane can undergo stereomutation classically beneath the dissociation limit. Confirming earlier conclusions, side-on insertion of ¹A₁ CH₂ into H₂ in a perpendicular geometry occurs without activation energy. Planar (D_4h) methane (130.5 kcal/mol) has four imaginary frequencies. Two of these are degenerate and lead to equivalent planar C_2v structures with one three-center, two-electron bond and two two-electron bonds and two imaginary frequencies. The remaining imaginary frequencies of the D_4h form lead to tetrahedral (T_d) and pyramidal (C_4v) methane. The latter has three negative eigenvalues in the force-constant matrix; one of these leads to the T_d global minimum and the other to the C_s (parallel) stereomutation transition structure. Multireference CI calculations with a large atomic natural orbitals basis set produce similar results, with the electronic energy of the C_s stereomutation transition state 0.7 ± 0.5 kcal/mol higher than that of CH + H^. dissociation products, and a ZPE-corrected energy which is 5 ± 1 kcal/mol higher. Also considered are photochemical pathways for stereomutation and the possible effects of nuclear spin, inversion tunneling, and the parity-violating weak nuclear interaction on the possibility of an experimental detection of stereomutation in methane. © 1995 by John Wiley & Sons, Inc.     

Intramolecular Rearrangements of Metal β-Diketonates. Ab Initio Study of Sc(MDA)3 (MDA = C3H3O2)

The geometrical parameters, force constants, and vibrational spectra of the C_2v configuration of the Sc(MDA)₃ molecule were calculated in terms of second-order Möller–Plesset perturbation theory with inclusion of electron correlation. Calculations were carried out using effective pseudopotentials (for describing atomic cores) and double-zeta valence basis sets complemented with polarization functions. The C_2v structure corresponds to the first-order saddle point on the potential energy surface (PES) of the ground electronic state. The results of our previous calculations for D₃ and D_3h configurations were used to show that the C_2v and D_3h structures are the transition states of two intramolecular rearrangements, describing transitions between the PES minima corresponding to different equivalent geometrical D₃ configurations of metal -diketonate tris-complexes. In Sc(MDA)₃, the rearrangement that occurs via the C_2v configuration is energetically (9.4 kJ/mole) more favorable.     

Carbodications. I. The structures and energies of C4H isomers

At high levels of ab initio theory (6-31G*//4-31G), the most stable C₄H isomer is indicated to be the nonplanar cyclobutadiene dication ( 1a ); the planar form, 1b , is indicated to be 7.5 kcal/mol less stable. The second most stable C₄H isomer, the methylenecyclopropene dication, is indicated to prefer the perpendicular ( 2a ) over the planar ( 2b ) arrangement by 7 kcal/mol. The “anti van't Hoff” cyclo-(HB)₂C?CH₂ system ( 4 ), isoelectronic with 2 , also prefers the perpendicular conformation ( 4a ), and retains the C?C double bond. The linear butatriene dication ( 3 ) is the least stable C₄H species investigated. The perpendicular (D_2d) arrangement ( 3a ), permitting double allyl cationlike conjugation, is preferred over the planar D_2h form ( 3b ) by 26 kcal/mol. The heat of formation of the most stable form of C₄H, 1a , is estimated to be 623–640 kcal/mol. This species should be thermodynamically stable toward dissociation into smaller charged fragments.     

MM3 MODELING OF ALDOPENTOSE PYRANOSE RINGS

MM3 (version 1992, ?=3.0) was used to study the ring conformations of d-xylopyranose, d-lyxopyranose and d-arabinopyranose. The energy surfaces exhibit low-energy regions corresponding to chair and skew forms with high-energy barriers between these regions corresponding to envelope and half-chair forms. The lowest energy conformer is ⁴ C ₁ for α- and β-xylopyranose and α- and β-lyxopyranose, and the lowest energy conformer is ¹ C ₄ for α- and β-arabinopyranose. Only α-lyxopyranose exhibits a secondary low-energy region (¹ C ₄) within 1 kcal/mol of its global minimum. Overall, the results are in good agreement with NMR and crystallographic results. For many of these molecules, skew conformations are found with relatively low energies (2.5 to 4 kcal/mol above lowest energy chair form). The ² S _O and ¹ C ₄ conformers of crystalline benzoyl derivatives of xylopyranose are in secondary low-energy regions on the β-xylopyranose surface, within 3.8 kcal/mol of the global ⁴ C ₁ minimum.     

The mechanisms of substitution reactions in octahedral complexes from the point of view of ligand field theory

Two possible mechanisms for substitution reactions in octahedral complexes, ML₆, are discussed in terms of molecular orbital theory. Jorgensen's model with angular parameters is used to calculate the change in activation energy on forming complexes of the type ML₅ (D₃h symmetry), ML₅ (C_4v symmetry), and ML₇ D_5h symmetry). Analysis of the quantities obtained (Table 4) shows that high spin ML₆ octahedral complexes of metals with d³ or d⁸ electronic configurations, and low spin complexes with d⁶ electronic configurations are particularly stable. An S_N1 mechanism is, apparently, characteristic for complexes of metals with d¹ or d² electronic configurations. The formation of bonds facilitates the course of substitution reactions in octahedral complexes. The results we have obtained explain the available experimental material and permit us to make some predictions.     

Die Schwingungsspektren von Trimethylsilyl-alkyl-bzw.-phenyl-aminen

Zusammenfassung Die Infrarot- und die Raman-Spektren der Silylamine (CH₃)₃Si–NH–R (R=CH₃, C₂H₅ und C₆H₅) sowie der analogen N-deuterierten Verbindungen werden mitgeteilt und analysiert. Starke Kopplungen führen zu einer Mischung vonv SiN bei etwa 700 cm^–1 mit anderen Schwingungen des C₃Si–NHR-Skelettes.

---

The Infrared and Raman spectra of the silylamino compounds (CH₃)₃Si–NH–R (R=CH₃, C₂H₅, and C₆H₅) and the analogous N-deuterated species are reported and assigned. The SiN stretching mode at about 700 cm^–1 is strongly coupled with other vibrations of the molecules.

     

Bonding of acetylene to copper atom,dimer, and trimer

The bonding of acetylene to copper atom, dimer, and trimer was investigated with a Kohn–Sham density functional approach. Full geometry optimization yielded the equilibrium structures of various Cu_nC₂H₂ species. Gradient corrections were included in the calculation of binding energies (BE ). The Cu—C₂H₂ complex was found to have a C_s structure and a BE of 10 kcal/mol. Three isomers of Cu₂C₂H₂ have similar total energies: a C_2v end-bonded structure with a BE of 18 kcal/mol, and two 1,2-dicupro ethylene isomers—a cis form with a BE of 12 kcal/mol and a trans form with a BE of 15 kcal/mol. Two stable C_2v isomers of Cu₃C₂H₂ were found. In both isomers, the Cu₃ ring relaxes from its isosceles structure, with two short bonds (2.247 Å) and one long bond (2.478 Å), and adopts a nearly equilateral geometry. In one isomer of Cu₃C₂H₂, the acetylene is bonded to one apex of the Cu₃ ring with a BE of 29 kcal/mol. In the other, it is bonded to two copper atoms of one side of the Cu₃ ring with a BE of 33 kcal/mol. © 1994 John Wiley & Sons, Inc.     

Gas Electron Diffraction and Quantum-Mechanical Study of Dimethyloxalate

The geometrical structure and conformation of dimethyloxalate, CH₃OC(O)–C(O)OCH₃, have been studied by gas electron diffraction (GED) and quantum-chemical calculations (MP2 and B3LYP methods with 6-31G* and cc-pVTZ basis sets). The GED analysis with a dynamic model (T = 323 K) results in a mixture of two planar conformers, anti (C_2h symmetry) and syn (C_2v symmetry) orientation of the two C=O bonds. The energy difference between these conformers is 0.02(0.18) kcal/mol and barrier to internal rotation around the C–C bond is 0.44(0.41) kcal/mol. The CH₃ groups occupy synperiplanar positions with respect to the C=O bonds. The following main geometrical parameters for the anti conformer (Å and degrees) have been derived: r_g(C–C) = 1.532(3), r_g(C=O) = 1.203(2), r_g(C_sp3–O) = 1.436(3), r_g(C_sp2–O) = 1.333(3), (C_sp2–C_sp2–O) = 111.9(1.9), (C_sp2–O–C_sp3) = 116.3(1.6), (O–C= O) = 127.0(1.8).This paper is devoted to the 75th anniversary of gas electron diffraction method.     

Structure and stability of SiH+4

Ab initio calculations at the 4-31G level are carried out on the species SiH_n (n = 0 to 4) and the corresponding ions. SiH⁺₄ is found to distort from T_d to D_2d. C_2v, and C_3v, with the latter structure being the lowest in energy by 11 kcal/mole. Consistent with experimental mass spectroscopy, SiH⁺₄ is found to be much less stable to dissociation than CH⁺₄.     

Theoretical study of the H 5 + system

Ab initio calculations have been carried out for the ground state of H ₅ ⁺ in order to predict its equilibrium geometry, binding energy, enthalpy of formation, and the features of the H₂ · H ₃ ⁺ interaction at large and intermediate intermolecular distances. The extended basis set of Gaussian functions was carefully optimized to describe the various kinds of intermolecular interactions. Electron correlation was accounted for by means of CI calculations. Different from previous studies we find a D _2d equilibrium geometry with D _e = 7.4 kcal/mol and H ₃₀₀ ⁰ –8.7 kcal/mol. The potential surface turns out to be extremely shallow in the vicinity of the D _2d structure which results in a great mobility of the central nucleus at room temperature.     

Tin(II) Phthalocyaninate and Tin(IV) bis-Phthaloc Yaninate: A Quantum Chemical Study

The structural parameters of tin(II) phthalocyaninate PcSn and tin(IV) bis-phthalocyaninate Pc₂Sn as well as of their cations are determined by B3LYP/SDD and PBE0/SDD quantum chemical methods. The PcSn molecule is characterized by C_4v symmetry, and SnN bond lengths are 2.307/2.299 ? (B3LYP/PBE0). The Sn nucleus is by 1.11 ? (B3LYP, PBE0, single crystal X-ray diffraction analysis) higher than the plane of four neighboring nitrogen nuclei. The “hindered” configuration (D _4d symmetry) with a high (27–30 kcal/mole) internal rotation barrier corresponds to the Pc₂Sn energy minimum. The calculated equilibrium lengths of eight equivalent SnN bonds of 2.366/2.347 (B3LYP/PBE0) are similar to the average SnN bond length of 2.347 ? (single crystal X-ray diffraction). Vertical and adiabatic ionization potentials are calculated: I^v 6.40/6.48 eV, I^A 6.38/6.45 eV for PcSn and I^v 5.63/5.66 eV, I^A 5.60/5.63 eV for Pc₂Sn.     

Carbon–nitrogen bond dissociation energy values in C-nitrosocompounds

Measurements of the D(R? NO) bond strength in some C-nitrosocompounds have been made using an electron impact method. The appearance potential of the radical ion (R⁺) has been determined, the D(R? NO) bond energy being obtained from the relation The values obtained are: D(C₆H₅? NO) = 41 kcal/mole, D(t-C₄H₉? NO) = 34 kcal/mole, D(t-C₅H₁₁? NO) = 36 kcal/mole and D(i-C₃H₇? NO) = 36.5 kcal/mole. These values are in good agreement with the numerous estimations of Benson and coworkers and confirm that the C? N bond strength in C-nitrosocompounds is very much less than in nitrocompounds or in amines.     

The splitting of d-orbitals in less symmetrical molecules containing a cyclopentadienyl ligand

The splitting of d orbitals in an electrostatic field due to a cyclopentadienyl ring and four ligands arranged at the corners of a square is investigated. It is shown that, although the symmetry of the molecule is C _s, the effective symmetry for the d ¹ and d ² systems is G _4v. The linear combinations for the d ² system in C _1v symmetry are given and the matrix elements for the secular determinant are listed. The results may be applied to a discussion of the bonding in tricarbonyl--cyclopentadienylethyl-molybdenum, whose structure is related to the idealized model considered.

---

Zusammenfassung Bei der Aufspaltung der d-ZustÄnde in einem elektrostatischen Feld, welches erstens von einem Cyclopentadienyl-Ring und zweitens von vier an den Ecken eines Quadrates angeordneten Liganden hervorgerufen wird, ist die effektive Symmetrie D _4v — trotz der C _s-Symmetrie des Moleküls. Für das d ²-System werden die Linearkombinationen der Slaterdeterminanten und die Elemente der Störmatrix angegeben. Das Ganze lÄ\t sich z. B. auf die Diskussion von Tricarbonyl--cyclopentadienylÄthylmolybdÄn anwenden, dessen Struktur der des behandelten Modells sehr Ähnlich ist.

---

Résumé La décomposition des orbitales d dans le champ d'un groupe cyclopentadiényl et de quatre ligandes arrangés en carré est étudiée. Pour les systèmes d ¹ et d ², la symétrie effective est C _4v , celle de la molécule n'étant que C _s. Pour d ², on donne les combinaisons linéaires adapté à la symétrie C _4v , et la matrice des énergie perturbatrices. Les résultats s'appliquent à la discussion du cyclopentadiényl-éthyl-molybdène tricarbonyle dont la structure ressemble le modèle étudié.

---

---

On leave from: Institut Rudjer Bokovi, Zagreb, Yugoslavia     

The N4 molecule has an open-chain triplet C2h structure

The lowest-energy N₄ is computed ab initio to be the planar C_2h(³B_u) open-chain structure 13 . The open-chain N₄ singlet-state structures dissociate on geometry optimization. The tetraazatetrahedrane T_d structure 1 and the tetrazete D_2h structure 2 are minima at MP 2/6-31G *. However, both are higher in energy than 13 (24.1 and 21.2 Kcal/mol [UQCISD ) (T )(full)/6-311+G *//MP 2/6-31G * + ZPE (MP 2/6-31G )*, respectively]. The energy of 13 is 157.5 kcal/mol higher than that of two N₂(¹∑ molecules [UQCISD (T )(Full)/6-311+G *//MP 2/6-31G *] © 1993 John Wiley & Sons, Inc.     

A theoretical study of the relative affinities of an aliphatic and an aromatic bisguanylhydrazone for the minor groove of double-stranded (dA-dT) n oligomers

The nonintercalative binding of an aliphatic and an aromatic bisguanylhydrazone (BGH) to the minor groove of double-stranded (dA-dT) _n oligomers is investigated by means of theoretical computations. The preferred binding arrangements of both BGHs are stabilized by a number of H-bonding interactions with sites O₂(T), N₃(A) and o₁ on the two strands, and require limited conformational rearrangements of the BGHs around their C-C single bonds. The intermolecular interaction energy is larger with the aliphatic BGH than with the aromatic one. The energy difference is, however, considerably reduced when the oligomer is lengthened: it passes from 16.1 kcal/mole at the heptamer level, to 7.9 kcal/mole at the undecamer level and to 4.6 kcal/mole when each strand of the undecamer is flanked with a complementary complete helical turn of phosphates, on both the 3 and 5 termini.The interaction energies of the BGHs with water molecules in the first hydration shell are, however, also larger with the aliphatic BGH, than with the aromatic BGH. This energy difference is further enhanced when one considers also the water molecules in the second shell. It becomes greater than the difference in the interaction energy of the two BGHs with (dA-dT) _n for large values of n. When the dehydration energy of BGHs is taken into account the overall energy balance is then more favorable for the interaction of the aromatic than of the aliphatic BGH with the polynucleotide. This last conclusion is in agreement with experimental results.     

Theoretical study of the electronic structure of diazomethane

The least-energy dissociation path of the ground state of CH₂N₂ was determined fromab initio calculations using in a complementary way basis sets of minimal size (STO-3G) and double-zeta (DZ) quality. The results indicate that the least-energy point of attack of the N₂ molecule on CH₂ (¹ A ₁) is roughly perpendicular to the molecular plane (93 °), the C and N atoms being almost co-linear (angle C-N-N203 ° with outermost N atom pointing away from CH₂). The potential barrier of 1.2 eV found previously on theC _2v dissociation path, disappears completely along the least-energy dissociation path (point groupC _s (out-of-plane)). These findings corroborate the Woodward-Hoffman rules for this process since the outermost orbitals of the two intersecting states found in point groupC _2v (...2b ₁ and ...8a ₁) both correlate to the same irreducible representation (10á) in point groupC _s (out-of-plane).Larger basis set calculations (DZ + polarization functions on all centers, 3d _c and 3d _N developed here), were also carried out on CH₂N₂ (¹ A ₁,³ A ₂ and¹ A ₂) at the¹ A ₁ equilibrium geometry and on CH₂ (³ B ₁) and N₂ (¹ _g ⁺ ) at their respective equilibrium geometries. These calculations, together with consideration of correlation energy differences, yieldD ₀ ⁰ (CH₂N₂,¹ A ₁) = 19 kcal/mole and vertical excitation energies of 67 and 73 kcal/mole for the³ A ₂ and¹ A ₂ states respectively. The latter value is in good agreement with the measured experimental value: 72.4 kcal/mole corresponding to the maximum of intensity in the¹ A ₂¹ A ₁ absorption band.     

Low-frequency dielectric behavior of poly(vinylidene fluoride)

An explicit mechanism is described for the anomalous increase in dielectric constant and dielectric loss at low frequencies and high temperatures for poly(vinylidene fluoride) containing ionic impurities. Relations are proposed for the ionic contributions, ε_i″ and ε_i″, to the dielectric constant and dielectric loss: where v₀ and D₀ are the concentration and the diffusion coefficient of the mobile ions at infinite temperature, q is the charge of an ion (in cgs electrostatic units), l is the distance between electrodes, k is the Boltzmann constant, T is the absolute temperature, E_d is the apparent activation energy for diffusion of the ions, and W is the dissociation energy of the ionic impurities. From the slopes of curves of log εT′ versus 1/T and log ε″T versus 1/T for poly(vinylidene fluoride), energies E_d = 34 kcal/mole and W = 342 kcal/mole were obtained.     

Analytic matrix elements of the dipole moment and Herman-Wallis coefficients

Perturbation theory proves to be a powerful approach to obtain in analytic form both vibration-rotational energies and matrix elements of the dipole moment of diatomic molecules in terms of the expansion parameter = 2B _e/gw_e,B _e and _e being, respectively, the equilibrium rotational and harmonic vibrational spectral parameters. A systematic and efficient algorithm has been developed to execute such calculations with sufficient accuracy for most physical applications when the potential-energy function is accurately represented in the Dunham form. The method also provides analytic expressions of the Herman-Wallis coefficientsC _v ^v andD _v ^v for the vibration-rotational overtone bandsv ¹v for diatomic molecules in¹ electronic states.     

Ab initio predictions of the molecular structures and harmonic vibrational frequencies of Ga2O2 isomers

The potential energy surface of Ga₂O₂ is examined at the SCF and MP2 levels employing basis set of triple- plus double polarization quality. Four stationary points located at the SCF level are characterized via their Hessian index. Electron correlation is important for the energy ordering and splitting of the isomers. For example, two minimum energy structures, a cyclicD _2h form and a linear Ga-O-Ga-O, separated by 25.69 kcal/mol at the SCF level have an energy difference of only 1.70 kcal/mol at the MP2 levels. Our computed harmonic vibrational frequency at 962 cm^–1 for the Ga-O-Ga-O minimum structure in in good agreement with the experimental predicted value of 967 cm^–1.     

Thermal reactions of ethane-d6–acetylene and D2–acetylene mixtures behind shock waves

A study of the thermal decomposition of an acetylene–ethane-d₆ mixture indicates that the rate constant for hydrogen abstraction from acetylene by methyl is more than 20 times less than for abstraction from ethane. Isotopic exchange is initiated by a rapid reaction between product D atoms and C₂H₂. A series of experiments involving the reactions of a D₂–acetylene mixture indicated that a molecular exchange process was also occurring, and it was shown that d[C₂HD]/dt = k[D₂]^0.7[C₂H₂]^0.3, effective activation energy = 15.8 kcal/mol. This mechanism made an insignificant contribution to isotope exchange in C₂H₂–C₂D₆ mixtures.