Theoretical study of thermal decomposition of azomethane

Summary.  Thermal one- and two-bond dissociation processes of cis- and trans-azomethane were studied by ab initio computation with DZP and TZ2P basis sets, using the d(N–C) bond lengths as the reaction coordinates. The geometries were optimized at the MP2 level, and the dissociation energies obtained exploiting a single-point, fourth-order M?ller–Plesset calculations [MP4SDTQ/TZ2P]. At this level of theory including zero-point energies, the trans-isomer is by 9.3 kcal/mol more stable than the cis-isomer. The results show that the energetically more favourable one-bond cleavage proceeds without transition state with the predicted bond dissociation energy D ₀ of 47.8 kcal/mol for trans-azomethane and 38.5 kcal/mol for cis-azomethane. With calculated barrier heights the unimolecular dissociation rate constants have been determined by means of the RRKM theory. The second-order saddle points localized for synchronous decomposition pathways lie 13 (trans)-23(cis) kcal/mol above the one-bond dissociation energies [MP2/DZP]. Received May 28, 1996/Final version received November 1, 1996 / Accepted November 1, 1996     

The C2H 3 + cation and its interaction with HF

The structure and stability of classical and bridged C₂H ₃ ⁺ is reinvestigated. The SCF and CEPA-PNO computations performed with flexibles andp basis sets including twod-sets on carbon confirm our previous results. We find the protonated acetylene structure to be more stable than the vinyl cation by 3.5–4 kcal/mol. The energy barrier for the interconversion of these two structures is at most a few tenths of a kcal/mol. The equilibrium SCF geometries of Weberet al. [15] are affected insignificantly by further optimization at the CEPA-PNO level. Several structures for the interaction of C₂H ₃ ⁺ with HF have been investigated at the SCF level. With our largest basis set which includes a complete set of polarization functions we find a remarkable levelling of the stabilities of most of the structures. In these cases the stabilization energy ΔE ranges from −10 to −13 kcal/mol.     

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.     

A DFT study of a novel oxime anticancer trans platinum complex: Monofunctional and bifunctional binding to purine bases

The first and second substitution reactions binding of the anticancer drug trans‐[Pt((CH₃)₂C?NOH)((CH₃)₂CHNH₂)Cl₂] to purine bases were studied computationally using a combination of density functional theory and isoelectric focusing polarized continuum model approach. Our calculations demonstrate that the trans monoaqua and diaqua reactant complexes (RCs) can generate either trans‐ or cis‐monoadducts via identical or very similar trans trigonal‐bipyramidal transition‐state structures. Furthermore, these monoadducts can subsequently close by coordination to the adjacent purine bases to form 1,2‐intrastrand Pt‐DNA adducts and eventually distort DNA in the same way as cisplatin. Thus, it is likely that the transplatin analogues have the same mechanism of anticancer activity as cisplatin. For the first substitutions, the activation free energies of monoaqua complexes are always lower than that of diaqua complexes. The lowest activation energy for monoaqua substitutions is 16.2 kcal/mol for guanine and 16.5 kcal/mol for adenine, whereas the lowest activation energy for diaqua substitutions is 17.1 kcal/mol for guanine and 25.9 kcal/mol for adenine. For the second substitutions, the lowest activation energy from trans‐monoadduct to trans‐diadduct is 19.1 kcal/mol for GG adduct and 20.7 kcal/mol for GA adduct, whereas the lowest activation energy from cis‐monoadduct to cis‐diadduct is 18.9 kcal/mol for GG adduct and 18.5 kcal/mol for GA adduct. In addition, the first and second substitutions prefer guanine over adenine, which is explained by the remarkable larger complexation energy for the initial RC in combination with lower activation energy for the guanine substitution. Overall, the hydrogen‐bonds play an important role in stabilizing these species of the first and second substitutions. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

A computational investigation on the photochemistry of the popular spin-trap agent N-tert-butyl-α-phenylnitrone (PBN) and thermal isomerization pathways of its photoproduct oxaziridine

Computational investigation on the low-lying photo-excited states of N-tert-butyl-α-phenylnitrone (PBN), a well-known spin-trap agent, has revealed its photo-product (oxaziridine) formation channel. The S₀-S₂ vertical excitation in PBN is subsequently followed by a non-radiative decay pathway through S₂/S₁ and S₀/S₁ conical intersections (CIs) with CNO-kinked structures, situated around 23 kcal/mol and 45 kcal/mol below the vertically excited S₂ state, respectively. The reverse photo-process of PBN formation involves photo-excitation of oxaziridine to its S₂ and S₃ photo-excited states. The forward photo-isomerization leads to the trans-oxaziridine with a backside CNO kink (trans-OX_B) while the reverse path studied by us, connects its front-side CNO-kinked analogue (trans-OX_F) with the PBN. Our search for the reverse thermal reaction paths from these two oxazirdines has led to their corresponding transition states, one at 35 kcal/mol and the other at 27 kcal/mol above trans-OX_F and trans-OX_B geometries, respectively. They lead to two different isomers (E and Z) of PBN which supports the reported nature of products from the trans-oxaziridine in this thermal reaction. The inversion path of the chiral nitrogen atom of this N-tert-butyl-oxaziridine (barrier 21 kcal/mol) has also been tracked. This reaction path has been compared with that of the N-methyl (barrier 30 kcal/mol) and N-acyl (barrier 10.5 kcal/mol) oxaziridine analogues.     

On the dimerization process of nitroso compounds

Summary Many organic C-nitroso compounds R-NO form stable dimers with a covalent NN bond. To gain insight into the dimerization reaction 2 R-NO (R-NO)₂ a theoretical study of the dimerization to atrans-form was performed using HNO as a model compound. Complete geometry optimizations were carried out at the HF, MP2 and QCISD levels using a 6–31G* basis. In the stationary points energies were calculated at the MP4(SDTQ) and QCISD(T) levels. For the equilibrium structure of the monomer and dimers stable RHF solutions were found, whereas for the TS UHF and UMPn calculations were applied. Extensive spin contamination was found in the UHF wavefunction, and projections up tos+4 were invoked. Relative energies were corrected for differences in ZPE. Calculations were made (a) for the least-motion path (C _2h symmetry) and (b) for a path with complete relaxation of all internal coordinates. Along the latter path a TS having virtuallyC _i symmetry was found. Along path (a) an activation energy of around 150 kcal/mol was predicted, in conformity with a symmetry forbidden reaction. On the relaxed path (b) the barrier to dimerization was estimated to be 10.7 kcal/mol at the MP4(SDTQ)//MP2 level, and 10.9 kcal/mol at the QCISD(T)//QCISD level. Unscaled ZPE corrections, calculated at the SCF level, changed these values to 12.7 and 12.9 kcal/mol, respectively. The reaction energy for the dimerization process is predicted to be – 17.2 kcal/mol at the MP4(SDTQ)//MP2 level corrected for ZPE. Calculations at the G1 level gave a corresponding value of – 16.4 kcal/mol. The equilibrium constant for the association to thetrans dimer is estimated to beK _p =259 atm, indicating that the dimer should be an observable species in the gas phase.     

Bowl-shaped hydrocarbons related to C60

Ab initio studies at the HF/6-31G* and B3LYP/6-31G* levels are reported for two bowl-shaped hydrocarbons related to C₆₀: C₃₀H₁₂ and C₃₆H₁₂, of C₃ and C_3v symmetry, respectively. The former has an approximate heat of formation of 211 kcal/mol. Bowl-to-bowl interconversion may occur through a planar (C_3h) form of ca. 64 kcal/mol greater energy having one imaginary vibrational frequency. The larger C₃₆H₁₂ bowl has a calculated ΔH°_f of 265 kcal/mol. Its HF/6-31G*, B3LYP/6-31G*, and MM3 bond lengths are in good agreement with a recent X-ray structure. Chemical shifts for both compounds calculated by the GIAO method are in good agreement with the measured NMR spectra. The observed ¹³C chemical shifts increase with the extent of pyramidalization. © 1998 John Wiley & Sons, Inc. J Comput Chem 19: 189–194, 1998     

Synthesis and reactions of a monosubstituted dithiirane 1-oxide, 3-(9-triptycyl)dithiirane 1-oxide

A 3-monosubstituted dithiirane 1-oxide, 3-(9-triptycyl)dithiirane 1-oxide, was prepared for the first time, by the reaction of (9-triptycyl)diazomethane and S₈O. The dithiirane 1-oxide was obtained as cis- and trans-isomers, and the structure of the trans-isomer was verified by X-ray crystallography. The cis-isomer isomerized gradually to the trans-isomer in solution. The divalent sulfur atom of the cis- and trans-dithiirane 1-oxides were removed on treatment with triphenylphosphine to give the corresponding Z- and E-sulfines, respectively. The reaction of the trans-dithiirane 1-oxide with (Ph₃P)₂Pt(C₂H₄) provided the (sulfenato-thiolato)Pt^II complex, and that with Lawesson's reagent yielded the 1,3,4,2-trithiaphospholane and 1,2,4,5,3-tetrathiaphosphorinane derivatives.     

Thermochemical properties of the Fe-analogue of cryolite, Na3FeF6

Relative enthalpies for low-and high-temperature modifications of Na₃FeF₆ and for the Na₃FeF₆ melt have been measured by drop calorimetry in the temperature range 723–1318 K. Enthalpy of modification transition at 920 K, δ_trans H(Na₃FeF₆, 920 K) = (19 ± 3) kJ mol⁻¹ and enthalpy of fusion at the temperature of fusion 1255 K, δ_fusH(Na₃FeF₆, 1255 K) = (89 ± 3) kJ mol⁻¹ have been determined from the experimental data. Following heat capacities were obtained for the crystalline phases and for the melt, respectively: C _p(Na₃FeF₆, cr, α) = (294 ± 14) J (mol K)⁻¹, for 723 = T/K ≤ 920, C _p(Na₃FeF₆, cr, β) = (300 ± 11) J (mol K)⁻¹ for 920 ≤ T/K = 1233 and C _p(Na₃FeF₆, melt) = (275 ± 22) J (mol K)⁻¹ for 1258 ≤ T/K ≤ 1318. The obtained enthalpies indicate that melting of Na3FeF6 proceeds through a continuous series of temperature dependent equilibrium states, likely associated with the production of a solid solution.      

Conformational analysis,part 43. A theoretical and LIS/NMR investigation of the conformations of substituted benzamides

A refined Lanthanide‐Induced‐Shift Analysis (LISA) is used with molecular mechanics and ab initio calculations to investigate the conformations of benzamide ( 1 ), N‐methylbenzamide ( 2 ), N,N‐dimethylbenzamide ( 3 ) and the conformational equilibria of 2‐fluoro ( 4 ), 2‐chloro ( 5 ) and N‐methyl‐2‐methoxy benzamide ( 6 ). The amino group in 1 is planar in the crystal but is calculated to be pyramidal with the CO/phenyl torsional angle (ω) of 20–25°. The LISA analysis gave acceptable agreement factors (Rcryst ≤ 1%) for the ab initio geometries when ω was decreased to 0°, the other geometries were not as good. In 2 , the N‐methyl is coplanar with the carbonyl group in all the geometries. Good agreement was obtained for the RHF geometries, with ω 25°, the other geometries were only acceptable with increased values of ω. In 3 , good agreement for the RHF and PCModel geometries was found when ω was changed from the calculated values of 40° (RHF) and 90° (PCModel) to ca. 60°, the X‐ray and B3LYP geometries were not as good. The two substituted compounds 4 , 5 and 6 are interconverting between the cis (O,X) and trans (O,X) conformers. The more stable trans conformer is planar in 4 and 6 but the cis form non‐planar. Both the cis and trans conformers of 5 are non‐planar. There is an additional degree of freedom in 6 due to the 2‐methoxy group, which can be either planar or orthogonal to the phenyl ring in both conformers. The conformer ratios were obtained from the LISA analysis to give Ecis‐Etrans in 4 > 2.3 kcal/mol (CDCl₃) and 1.7 kcal/mol (CD₃CN), in 5 0.0 kcal/mol (CD₃CN) and in 6 > 2.5 kcal/mol (CDCl₃) and 2.0 kcal/mol (CD₃CN). These values were used with the observed versus calculated ¹H shifts to determine the conformer ratios and energies in DMSO solvent to give Ecis‐Etrans 1.1, ?0.1 and 1.8 kcal/mol for ( 4 ), ( 5 ) and ( 6 ). Comparison of the observed versus calculated conformer energies show that both the MM and ab initio calculations overestimate the NH..F hydrogen bond in ( 4 ) by ca. 2 kcal/mol. Copyright © 2015 John Wiley & Sons, Ltd.     

Computational studies of 1,2-dithiete and dithioglyoxal

Ab initio calculations are reported on the energies, geometries, vibrational frequencies, and ionization potentials of the H₂C₂S₂ isomers: 1,2-dithiete, cis-dithioglyoxal, and trans-dithioglyoxal. In contrast to most earlier computations, the results of this work indicate that 1,2-dithiete and cis-dithioglyoxal lie close in energy (within 3 kcal/mol) with the dithial more stable. Trans-dithioglyoxal is found to be 4.1 kcal/mol more stable than the cis isomer and faces a barrier to internal rotation of 5.5 kcal/mol. The predicted rotational constants for 1,2-dithiete agree within ～0.05 GHz with the experimentally observed values thus lending credence to the predictions for cis-dithioglyoxal (A_e 14.30683, B_e 2.46324, C_e 2.10143 GHz). Vibrational frequencies are given as potential aids to the identification of these molecules. 6-31G* ΔSCF calculations predict that at low energies (8 to 13 eV) the photoelectron spectra of 1,2-dithiete, cis-dithioglyoxal and trans-dithioglyoxal should be similar.     

The electronic and molecular structure of carbon clusters: C8 and C10

Summary The equilibrium geometries of C₈ and C₁₀ have been determined from electronic structure calculations, using a variety of correlated methods and large basis sets of atomic natural orbitals. For C₈, a cyclic form withC _4h symmetry (¹ A _g) and a linear, cumulene-like form (³ _g ^– ) are isoenergetic candidates for the electronic ground state. For C₁₀, the ground-state equilibrium structure is definitely monocyclic. Three different cyclic structures have been considered here, i.e. cumulenicD _10h , distorted-cumulenicD _5h and acetylenicD _5h . These are all essentially isoenergetic, and are about 50 kcal/mol below the linear³ _g ^– state. The choice of basis sets and methods used has a strong impact on the predicted ground-state structures.Dedicated to Prof. Klaus Ruedenberg     

Computer simulation of the structure of large molecules: IV. 2D polybuckminsterfullerenes and their boraza analogs with bisingle nitrogen-boron bonds

The equilibrium structure of the dimer (C₆₀)₂, clusters (C₆₀)₉, and (C₆₀)₇ simulating the poly-C₆₀ (VIII) and poly-C₆₀ (XII), as well as their boraza analogs with bisingle nitrogen-boron bonds was determined by the quantum-chemical methods B3LYP/6-31G and PBE0/6-31G. The boraza analogs are characterized by the positive values of calculated (B3LYP/PBE0) energies of the interpolyhedral bonds: 10/20, 35/74, and 54/109 kcal mol⁻¹ respectively per a pair of monomers. The PBE0 method predicts shorter bond than B3LYP, and, in accordance with calorimetric data, positive bonding energy of the polyhedra in (C₆₀)n.     

Photoinduced Isomerization of Potassium Polyfluoroalk‐1‐enyltrifluoroborates

The UV irradition of K [RCF=CFBF₃] [R = C₄F₉ (trans), C₂F₅ (cis), C₆F₁₃ (cis), Cl (cis/trans 1 : 1)] in acetone led to cis/trans‐isomerization with a final cis/trans composition 7 : 3. In the case of R = C₄H₉ (trans) or C₃F₇O (cis/trans 25 : 75) the photoisomerization was accompanied by a partial decomposition.     

Vibrational spectra,conformational stability,barriers to internal rotation,r0 structural parameters and ab initio calculations of fluoromethyl methyl ether

The far-infrared spectrum of gaseous fluoromethyl methyl ether, FCH₂OCH₃, along with three of the deuterium isotopes, has been recorded at a resolution of 0.10 cm^–1 in the 350 to 50 cm^–1 region. The fundamental asymmetric torsional and methyl torsional modes are extensively mixed and have been observed at 182 and 132 cm^–1, respectively, for the stablegauche conformer with the lower frequency band having several excited states falling to lower frequency. An estimate is given for the potential function governing the asymmetric rotation. On the basis of a one-dimensional model the barrier to internal rotation of the methyl moiety is determined to be 527±9 cm^–1 (1.51±0.03 kcal/mol). A complete assignment of the vibrational fundamentals for all four isotopic species observed from the infrared (3500 to 50 cm^–1) spectra of the gas and solid and from the Raman (3200 to 10 cm^–1) spectra of the gas, liquid, and solid is proposed. No evidence could be found in any of the spectra for the high-energytrans conformer. All of these data are compared to the corresponding quantities obtained from ab initio Hartree-Fock gradient calculations employing the 3-21G and 6-31G* basis sets along with the 6-31G* basis set with electron correlation at the MP2 level. Additionally, completer ₀ geometries have been determined from the previously reported microwave data and carbon-hydrogen distances determined from infrared studies. The heavy-atom structural parameters (distances in Å, angles in degrees) arer(C₁-F) = 1.395 ± 0.005;r(C₁-O) = 1.368 ± 0.007;r(C₂-O) = 1.426 ±0.003; FC₁O = 111.33 ± 0.25; C₁OC₂ = 113.50 ± 0.18 and dih FC₁OC₂ = 69.12 ± 0.26. All of these results are discussed and compared with the corresponding quantities obtained for some similar molecules.     

The Structure of Gaseous Carbon Tetraiodide from Electron Diffraction and All Carbon Iodides,CIn (n = 1–4), and Their Dimers,C2I2n (n = 1–3) from High-Level Computation. Any Other Carbon-Iodide Species in the Vapor?

The geometry of a series of carbon iodides have been determined, CI₄ by gas-phase electron diffraction and CI _n (n = 1–4) and C₂I_2n (n = 1–3) by high-level quantum chemical calulations. The bond length of the tetrahedral CI₄ molecule from electron diffraction is (r _g):2.157(10) Å. The indication of about 20% I₂ in the vapor suggests partial decomposition and it has been thoroughly investigated what other carbon iodide species might be present beside CI₄. There is no appreciable amount of either of the dimeric species in the vapor phase, in spite of the suggestion from thermodynamics. On the other hand, the electron diffraction data are compatible with the presence of about 18% of either of the monomeric free radicals, CI₃ or CI₂, beside CI₄ and I₂. Possible reasons for these observations are discussed. Our correlated level computations, in agreement with other high level computations, found the singlet ¹A₁ state to be the ground state for CI₂. This is in contrast with a recent photoelectron spectroscopic study according to which the triplet state is the ground state though with a large margin of error (1 ± 3 kcal/mol energy difference). The computed singlet-triplet separation strongly depends on the level of the computation, but it is at least 9 kcal/mol. Geometrical parameters, singlet-triplet separations, and dipole moments have been calculated for the CX₂ series (X = F, Cl, Br, I, H) and their variations are discussed. The thermodynamic stability of different carbon iodide species has also been investigated.     

Efficient Protocol for Stereoselective Epoxidation of Cinnamic Esters Using TsNBr2

An efficient method has been developed for the synthesis of epoxide from cinnamic esters without any catalyst. The reaction was performed in CH₃CN–water (4:1) using N,N-dibromo-p-toluenesulfonamide (TsNBr₂) in alkaline conditions. This procedure can be utilized for stereoselective synthesis of epoxides from cinnamic esters in excellent yield in a shorter reaction time with exclusive formation of the trans-isomer. The method was further extended successfully for styrenes.     

Is dodecahedral P20 special?

Summary The laboratory study of phosphorus clusters by laser-based mass spectrometric methods indicates, tentatively, that P ₊ ²¹ may be special. A plausible interpretation might place a P⁺ ion interior to a dodecahedral P₂₀ molecule.Ab initio quantum mechanical methods have been applied to the P₂₀ molecule using contracted gaussian basis sets as large as (9s 6p 4d 3f) on each phosphorus atom. At the highest level of theory, dodecahedral P₂₀ is predicted to lie 23 kcal/mol above five separated P₄ molecules.Dedicated to Professor Werner Kutzelnigg     

Dissociation of diimide

Parts of the potential energy surface of N₂H₂ have been studied using CASSCF- and contracted CI-methods. Of particular interest was the concerted dissociation of cis- and trans-diimide into N₂ and H₂, since the trans-dissociation is symmetry allowed and the cis-dissociation forbidden. Three different saddle points were located, of which only one, of C ₂- symmetry, is a true transition state. Elaborate numerical gradient methods using exact Hessians and update procedures had to be used to find these saddle points on the unexpectedly complex N₂H₂-surface. The barrier height with respect to trans-diimide is 61 kcal/mol after vibration correction. Since this energy is higher than the barrier for interconversion, cis- and trans-diimide have the same transition state. It is further found that diimide preferably dissociates stepwise, by losing one hydrogen at a time, rather than in a concerted way. This conclusion is drawn basically because the geometry of the transition state for the concerted dissociation has a very long H-H distance of 5.6 a.u. The N-H bond energy in trans-diimide is 56 kcal/mol after vibration correction.     

Electronic structure and the hydrogen-shift isomerization of hydrogen nitryl HNO2

Summary Electronic structure of hydrogen nitryl HNO₂, a yet not identified entity, and the path of its possible isomerization totrans-HONO have been investigated byab initio SCF and MRD-CI computations using the 6-31G** basis set. HNO₂ isC _2v -symmetric and its ground state (¹ A ₁) is less stable thantrans-HONO by 66 kJ/mol (with the SCF vibrational zero-point energy correction). The lowest two excited singlet states (¹ A ₂ and¹ B ₁) are nearly degenerate, their vertical excitation energies being predicted to be 4.8 eV. The isomerization path is traced by the CASSCF procedure and the activation barrier height is evaluated by the CI treatment. HNO₂ in its ground state isomerizes totrans-HONO by maintaining the planar (C _s-symmetric) structure. The activation energy is calculated to be 171 kJ/mol, which is clearly lower than the calculated H-N bond energy (253 kJ/mol). The transition state seems to be more adequately described as an interacting system of proton and the nitrite anion rather than as a pair of two fragment radicals.