Theoretical study of the barriers to internal rotation in nitrous acid

Nitrous acid, HONO, has been studied for three geometries by the ab initio LCAO SCF MO method with a basis of accurate gaussian atomic orbitals. The trans geometry is correctly predicted to be most stable, lying about 2 kcal/mole lower than the cis form, and 9 kcal/mole lower than the 90° form (experimental estimates being 0.4 and 11.6 kcal/mole, respectively). Population analysis, dipole moment components, and properties related to nuclear-nuclear and nuclear-electron potentials all show a partial breaking of the hydroxyl oxygen-nitrogen bond at 90° compared to cis and trans, as well as the effects of electronic rearrangement for nuclear screening in the high nuclear repulsion cis form. The cis to 90° barrier is dominated by the attractive components of the total energy, while the trans to 90° one is dominated by repulsive components, in agreement with our analysis and an earlier prediction by Allen.     

Ab initio study of hydrazoic acid

SCF and CI calculations were carried out on the ground^1A state of HN₃. The equilibrium geometry and vibration frequencies were computed. The results point to a planar structure (groupC _s) but to a non-linear (170 °) N-N-N conformation. The calculated vibration frequencies are in fair agreement with experimental assignments.The dissociation path of the molecule to NH and N₂ products was investigated and compared to the isoelectronic reaction of diazomethane. The dissociation energy of hydrazoic acid is estimated to be about –8 kcal/mole, with a potential barrier to dissociation of about 30 kcal/mole.Boursier IRSIA     

First separation and characterization of cis and trans 1,2-bisaryloxy perfluorocyclobutanes

4-Bromotrifluorovinyloxybenzene (4-Br-TFVE) undergoes cyclodimerization upon heating at 150 °C affording cis and trans isomers of 1,2-bis(4-bromophenoxy)hexafluorocylcobutane. Stereoisomers were separated by selective crystallization, confirmed by single crystal X-ray, and further characterized by NMR. Remarkable difference in the solid state structures include aryl to aryl dihedral angles of 12° for the trans isomer and 88° for the cis isomer. Polymers containing roughly equal amounts of cis and trans fused perfluorocyclobutyl (PFCB) rings should be expected to have low crystallinity due to the marked difference in the two unit cells as is observed.     

Ab initio MP2 calculations of the products of acetylene addition to HgCl2

Gas-phase reaction of acetylene with HgCl₂ resulting in -chlorovinylmercury derivatives and their interaction with Cl^– and I^– anions and KI molecule was studied by the ab initio MP2 method with the Dunning—Hay double zeta basis set and LanL pseudopotential for Hg, K, and I atoms. The reaction was shown to proceed via a -complex of acetylene and HgCl₂ (the calculated enthalpy of formation is –6.5 kcal mol^–1). According to calculations, the activation energy of formation of cis--chlorovinylmercury chloride from acetylene and HgCl₂ is 31 kcal mol^–1. Chloride and iodide anions and KI molecule are readily added to both cis- and trans-isomer of -chlorovinylmercury chloride to give stable species.     

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.     

Theoretical investigations of molecules composed only of fluorine,oxygen and nitrogen: determination of the equilibrium structures of FOOF, (NO)2 and FNNF and the transition state structure for FNNF cis-trans isomerization

The deficiencies of common ab initio methods for the reliable prediction of the equilibrium structures of compounds composed of only the fluorine, oxygen and nitrogen atoms are investigated. Specifically, the importance of using large one-particle basis sets with multiple sets of polarization functions has been studied. Additionally, the need for a set of f basis functions was investigated. Several different single reference electron correlation methods have been tested in order to determine whether it is possible for a single reference based method to be routinely used on such chemical systems. These electron correlation methods include second order Møller-Plesset perturbation theory (MP2), singles and doubles configuration interaction (CISD), the coupled pair functional (CPF) approach and singles and doubles coupled cluster (CCSD) theory. The molecular systems studied include difluoroperoxide (FOOF), the cis form of the NO dimer, cis and trans difluorodiazene (FNNF) and the transition state to interconversion of the cis and trans isomers of FNNF. To the best of our knowledge, this is the first time that the cis-trans isomerization transition state has been reported. At the highest level of theory employed, the equilibrium structures of cis and trans FNNF agree very well with the experimental structures. However, the barrier to interconversion is predicted to be 65 kcal/mole, which is substantially higher than the experimental activation energy of 32 kcal/mole. Potential sources of error are discussed. A new diagnostic method for determining a priori the reliability of single reference based electron correlation methods is suggested and discussed.Contribution CCQC No. 36     

Non-empirical calculations and basis set effects on the inversion geometry of the aniline molecule

The geometry of the amine group and the barrier to internal conversion in aniline have been studied by single-determinantab initio SCF calculations using several basis sets from minimal to double-zeta quality. The results obtained from different types and sizes of basis sets are discussed. Calculations performed with the two most flexible basis sets yield inversion barriers of 0.9–1.1 kcal/mole and angles of pyramidalization at the nitrogen atom of 38–39 ° which are in good agreement with the experimental results (1.3 kcal/ mole and 38 °). Orbital and overlap population analyses are performed and compared with the expected mesomeric and inductive effect. The calculated dipole moment 1.48–1.49 D also agrees with the experimental values (1.48–1.50 D).Dedicated to Professor O. E. Polansky, Mülheim/Ruhr, on the occasion of his 60th birthday.     

Mechanism of cyclopolymerization. Conformational analysis of cis-1,3-diisocyanatocyclohexane

In 1,3-disubstitued cyclohexanes, in general, the diaxial conformation of the cis isomer is, energetically, the least favored conformation. An interspacial electronic interaction in the ground state of a cis-1,3-disubstituted cyclohexane would be expected to increase the proportion of this conformer in the equilibrium mixture. Such an interaction would provide an energetically favorable pathway for cyclopolymerization. From nuclear magnetic resonance studies on cis-and trans-1,3-diisocyanatocyclohexane the conformational equilibrium in the cis isomer was determined. It is shown that cis-1,3-diisocyanatocyclohexane exists in the diequatorial conformation; this is taken as evidence that a ground-state interaction between isocyanato groups in this monomer, which readily cyclopolymerizes, is not a significant factor in the cyclopolymerization mechanism. The value of the free energy barrier, ΔG?, for trans-1,3-diisocyanatocyclohexane was calculated as 11.1 kcal/mole.     

Radiation-induced polymerization at high pressure of cis- and trans-1,3,3,3-tetrafluoropropene in bulk and with tetrafluoroethylene

The radiation-induced polymerization of cis- and trans-1,3,3,3-tetrafluoropropene in bulk and with tetrafluoroethylene was studied at pressures between 5000 and 15000 atm and temperatures between 21 and 100°C. At 10³ rad/hr the homopolymerization rates range from about 10^?4 to 1%/hr. The activation enthalpy and volume are about 8 kcal/mole (33 kJ/mole) and ?10 cm³/mole, respectively, for both isomers. The cis isomer polymerizes about twice as rapidly as the trans isomer. The latter freezes in the experimental range of temperature and pressure; the polymerization rate is very low in solid phase. Polymer intrinsic viscosities increase with polymerization pressure and decrease with polymerization temperature; the largest value obtained was 0.23 dl/g. In the copolymerizations all reactivity ratios favor incorporation of tetrafluoroethylene by factors of 6–16. The preference is stronger when the trans isomer is used.     

Conformation dependence of pKa’s of the chromophores from the purple asFP595 and yellow zFP538 fluorescent proteins

Two members of the green fluorescent protein family, the purple asFP595 and yellow zFP538 proteins, are perspective fluorescent markers for use in multicolor imaging and resonance energy-transfer applications. We report the results of quantum based calculations of the solution pK_a values for selected protonation sites of the denatured asFP595 and zFP538 chromophores in the trans- and cis-conformations in order to add in the interpretation of photo-physical properties of these proteins. The pK_a values were determined from the theromodynamic cycle based on B3LYP/6-311++G(2df, 2p) calculations of the gas phase free energies of the molecules and the B3LYP/6-311++G(d, p) calculations of solvation energies. The results show that the pK_a’s of the protonation sites of the chromophore from asFP595 noticeably depend on the isomer conformation (cis- or trans-), while those of zFP538 are much less sensitive to isomerization.     

CNDO calculations of N-methyl substituted acrylamides

CNDO calculation is made for N-methyl acrylamide (both incis andtrans configurations) and N,N-dimethyl acrylamide. The charges, bond orders and dipole moments are discussed and compared with those of acrylamide. Thetrans form of N-methyl acrylamide is found to be more stable thancis isomer by 4.5 kcal/mole.     

Quantum chemical studies on electrophilic addition

The geometries of the 2-aminoethyl cation and the isomeric protonated aziridine have been optimized using ab initio molecular orbital calculations employing the split-valence shell 4-31G basis set. The protonated aziridine is computed to be the more stable ion by 46.5 kcal/mole (4-31G level) and 44.9 kcal/mole (double-zeta basis set). The profile to interconversion is found to have a barrier of less than 15 kcal/mole (relative to the 2-aminoethyl cation) and this profile is compared with those computed for the similar ions XCH₂CH ₂ ⁺ where X=OH, F, SH and Cl.     

Synthesis and molecular structure of 3-bromo-trans-2,6-diallyl-Δ3-piperideine hydrochloride

The structure of 3-bromo-trans-2,6-diallyl-³-piperideine hydrochloride was determined by X-ray diffraction analysis. The corresponding base was prepared by reductive diallylation of 3-bromopyridine with triallylborane. 3-Bromo-cis-2,6-diallyl-₃-piperideine3 was prepared by heatingtrans-isomer2 with triallylborane at 130 °C followed by deboronation of the resulting aminoborane with a solution of sodium hydroxide.Translated fromIzyestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 705–707, April, 1994.This work was carried out with the financial support of the Russian Foundation for Basic Research (project No 93-03-18193).     

Singlet Nitric Oxide Dimer Revisited: An Unexpected High Stability Isomer

A new form of the asymmetric cis isomer of the nitric oxide dimer, a cyclic parallelogram, has been found and characterized as the second most stable conformation by ab initio computational methods. Optimum equilibrium geometries, energetics, and harmonic frequencies are computed for the cyclic structure and the more common symmetric cis and trans isomers at the levels of second-order perturbation theory (MP2) and coupled-cluster theory limited to double excitations (CCD) using the 6-311G(2d) and 6-311+G(2d) basis sets. Single-point, more highly correlated calculations are then performed at these CCD optimum geometries. In addition, second-order geometries and frequencies are computed using the larger 6-311G(3df) basis, and single-point MP2 calculations are performed at these geometries using the Aug-cc-pVTZ and Aug-cc-pVQZ basis sets. At the highest levels of theory considered, the cyclic isomer lies within 17 kJ/mol of the symmetric cis global minimum and is more stable than the often-studied symmetric trans conformation.     

Reductivetrans-2,6-diallylation of pyridines with allylboranes. Synthesis oftrans- andcis-2,6-diallyl-1,2,5,6-tetrahydropyridines and their deuterated derivatives

The reductivetrans-2,6-diallylation of pyridines with triallyl- and allyl(dialkyl)boranes has been discovered. Heating (40–100 °C) of pyridine, deuteropyridine, or 3-bromopyridine complexes with triallylborane in the presence of alcohols (ROH or CH₃OD), water, or Et₂NH results in the respectivetrans-2,6-diaIlyl-1,2,5,6-tetrahydropyridines (2,3,22, or25) in 20–97 % yields. A preparative method for the isomerization oftrans-2,6-diallyl compounds2 and25 into the respectivecis-isomers4 and28 by heating them with triallyl- or allyl(dialkyl)boranes (125–150 °C) has been suggested. The hydrogenation oftrans- orcis-2,6-diallyl-1,2,5,6-tetrahydropyridines gavetrans- orcis-2,6-dipropylpiperidines, respectively. Thecis- andtrans-configurations of compounds2 and4 were established by analyzing the NMR spectra ofN-benzyl (7 and13) andN,N-dimethyl (6 and 14) derivatives of piperidine derivatives5 and8. A possible mechanism for the reductive diallylation of pyridines has been discussed.Translated fromIzyestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 693–704, April, 1994.This study was financially supported by the Russian Foundation for Basic Research (Project 93-03-18193).     

The study of cis - and trans -2 butene using mass spectrometry

Using mass spectrometric technique, the effect of geometrical isomerism on the first and higher appearance energy values for C₄H₃ ⁺, C₄H₇ ⁺ and C₃H,₃ ⁺ ions obtained from cis-2-butene andtrans-2-butene is reported. The structure in the ionization efficiency curves (studied for 9 eV above threshold) for the same ions obtained from the two isomers is reported and compared. It is believed that at threshold C₄H₇ ⁺ fragment is formed from the two isomers as methallyl ion. For C₃H₃ ⁺ fragment formed from the cw-isomer at threshold the proposed structure is the propargyl ion with ΔH_f equal to 279-4 kcal/mole while for that ion obtained fromtransisomer the proposed structure is the allenyl ion with ΔH_f equal to 296.6 kcal/mole.     

An ab initio study of the electronic structure of diimide

Ab initio calculations on the structure and geometry of the three isomers of N₂H₂ (trans-diimide, cis-diimide, and 1,1-dihydrodiazine) were performed both on HF and CI level using gaussian basis sets with polarization functions. The trans and cis isomers have singlet ground states; the trans isomer is found to be lower in energy than the cis isomer by 6.9 kcal/mol (HF) and 5.8 kcal/mol (CI), respectively. The barrier for the trans-cis isomerization is predicted to be 56 (HF) and 55 (CI) kcal/mol. H₂ N=N has a triplet ground state with a non-planar equilibrium geometry and a rather long NN bond of 1.34 Å. Its lowest singlet state, however, is planar with an NN double bond of 1.22 Å; it is found to lie about 3 kcal/mol above the triplet and 26 kcal/mol above the singlet ground state of trans-diimide.     

The photoisomerization of <Emphasis Type="Italic">trans</Emphasis>-stilbene in Triton X-100/<Emphasis Type="Italic">n</Emphasis>-C<Subscript>5</Subscript>H<Subscript>11</Subscript>OH/H<Subscript>2</Subscript>O microemulsions

Different from organic solvent, self-assemblies are environment friendly media, and moreover, if they are used as micro-reactor, many meaningful and exciting results may be obtained. In this paper, we investigated the photoisomerization of trans-stilbene in Triton X-100/n-C₅H₁₁OH/H₂O microemulsions (a kind of self-assemblies) with different compositions and structures to establish the relationship between the reaction yields and the compositions and structures of microemulsions. The results show that the yield of cis-stilbene is increased with the increase of water content or with the decrease of Triton X-100 content, and oil in water (O/W) structure is the best structure for this reaction, which makes it possible to decrease the pollution of environment by organic solvent always used in organic reactions.     

Structures and NMR Parameters of 1,2-Diphenylcyclobutanes

A new reaction scheme for obtaining cis and trans 1,2-diphenylcyclobutane is described. Using ¹H-NMR at 600 MHz, full spectral assignment was made for both isomers, obtaining all J coupling constants and chemical shifts. NMR results on cis and trans 1,2-diphenylcyclobutane are compared with the vicinal coupling constants obtained by the Barfield–Smith equations from the literature internal and dihedral angles of cyclobutane. In the trans isomer, in agreement with previous results on halo-cyclobutanes, the conformation with the phenyls in the pseudo-di-equatorial position is strongly preferred. On the contrary, the cis isomer fluctuates between the two equivalent conformations: phenyl pseudo-axial and pseudo-equatorial.     

On the use of electrostatic potential derived charges in molecular mechanics force fields. The relative solvation free energy of cis- and trans-N-methyl-acetamide

We have carried out free energy perturbation calculations on the relative solvation free energy of cis- and trans-N-methyl-acetamide (NMA). Experimentally, the solvation free energy difference has been found to be near zero. Using 6-31G* ab initio electrostatic potential derived charges for both the cis and trans conformations, we calculate a solvation free energy difference of 0.1 ± 0.1 kcal/mol. Using the 6-31G* charges derived for the trans conformation for both the cis and trans models leads to a solvation free energy difference of 0.9 ± 0.1 kcal/mol, compared to the value of 2.2 kcal/mol determined for the OPLS model for trans-NMA.