The phenylnitrene rearrangement in the inner phase of a hemicarcerand

The photochemistry of phenyl azide 1 and 13C-labeled phenyl azide 13C-1 incarcerated inside a hemicarcerand 4 was investigated. Low-temperature photolysis of hemicarceplex 41 and 413C-1 yields incarcerated 1-azacyclohepta-1,2,4,6-tetraene 42 and 413C-2 (18-50%), respectively, which were characterized by low-temperature FT-IR and 1H NMR and 13C NMR spectroscopy. After correction for the hemicarcerand-induced upfield shift, the 13C chemical shifts of incarcerated 13C-2 compare very well (Deltadelta 相似文献   

Chemistry and properties of cycloheptatetraene in the inner phase of a hemicarcerand

Low-temperature photolysis of phenyldiazirine, incarcerated inside a hemicarcerand which is built from two cavitands connected by four butane-1,4-dioxy linker groups, yields transient phenylcarbene; this carbene then undergoes ring photochemical expansion to cycloheptatetraene in low yield. Competitively, the transiently formed phenylcarbene reacts with the surrounding hemicarcerand. The yield of the photochemical ring expansion was increased when the photolysis was carried out inside a partially deuterated hemicarcerand. Two insertion products resulting from an intramolecular phenylcarbene insertion into an acetal C-H(D) bond or an alpha-C-H bond of a butane-1,4-dioxy linker group have been isolated and characterized. The measured isotope effect for insertion into an acetal C-H(D) bond at 15.5 K is consistent with a reaction of singlet phenylcarbene. Incarcerated cycloheptatetraene is stable for a limited time at 100 degrees C and almost infinitely stable at room temperature in the absence of oxygen. NOESY experiments provide the distance ratio r21/r23 = 1.134 +/- 0.01 between protons H1-H2 and H2-H3 of cycloheptatetraene which is consistent with its twisted structure. Low-temperature photolysis of phenyldiazirine, incarcerated inside a chiral hemicarcerand which is built from two cavitands connected with three butane-1,4-dioxy and one (S,S)-2,3-O-isopropylidene-2,3-dihydroxybutane-1,4-dioxy linker group yields two diastereomeric cycloheptatetraene hemicarceplexes in a 2:3 ratio (30% total yield). Variable temperature 1H NMR studies provided a lower limit of deltaG++ = 19.6 kcalmol(-1) for the enantiomerization barrier of cycloheptatetraene. Incarcerated cycloheptatetraene reacts rapidly with oxygen to yield benzene and carbon dioxide via the 1,2-dioxaspiro[2,6]nona-4,6,8-triene intermediate. Different mechanisms for the formation of this spirodioxirane intermediate are discussed based on the measured rate of the oxygen addition. The activation parameters for the decarboxylation of the spirodioxirane have been measured in different bulk solvents. The free energy of activation shows very little solvent dependency. However. a strong propensity for enthalpy-entropy compensation due to a solvent reorganization that accompanies the reaction coordinate is observed.     

Photochemical and thermal reactions of intermediates in the phenylnitrene rearrangement inside a hemicarcerand

Broadband irradiation (lambda > 320 nm) of hemicarceplex H(.)1 between -74 degrees C and -84 degrees C, produces encapsulated didehydroazepine (2), triplet phenylnitrene ((3)PN), 2-azabicyclo[3.2.0]hepta-1,3,6-triene (6), and 4-azaspiro[2.4]hepta-1,4,6-triene (7). The highly strained anti-Bredt imine 6 is formed from 2 via a photochemical four-electron electrocyclization. Under the irradiation conditions, 6 rearranges further to azaspirene 7. In addition, 6 thermally rearranges to 7 via a 1,5-sigmatropic shift (DeltaG(267K) = 20.0 +/- 0.5 kcal/mol), yielding a final equilibrium composed of [7]/[6] = 5 at room temperature. The observation of a photochemical rearrangement of 2 to 6 contrasts earlier results of narrow band irradiations (lambda = 334 nm) of matrix-isolated 2, which gave (3)PN (Hayes, J. C.; Sheridan, R. S. J. Am. Chem. Soc. 1990, 112, 5879-5881). Encapsulated (3)PN is remarkably stable due to the prevention of its dimerization by the surrounding hemicarcerand. Above 255 K, it slowly decays with a rate constant k = 10(7.7+/-0.4) s(-1) x exp {(13300 +/- 500 cal/mol)/RT}. The isolation of substantial amounts of a hemicarcerand lacking one acetal spanner suggests that (3)PN decays preferentially by inserting into an inward-pointing acetal C-H bond of H.     

Explorations on the multidimensional potential energy surface of a chiral stationary phase

The multidimensional potential energy surface for a soluble analog of a chiral phase is computed with MM2 and MNDO. Minimum energy conformations are located. The minimum energy reaction pathway between these forms is located, and the templating ability of these phases is described.     

Mapping the triplet potential energy surface of 1-methyl-8-nitronaphthalene

Spin-unrestricted calculations and time-dependent DFT were used to characterize structure and reactivity of 1-methyl-8-nitronaphthalene (1) in the triplet state. Four hybrid models (B3LYP, PBE0, MPW1K, BHLYP) with significantly different amount of the exact exchange were employed. The triplet potential energy surface of 1 was mapped by using the UB3LYP and UMPW1K techniques. Both hybrid models provided qualitatively consistent pictures for the potential energy landscape. Thirty-one stationary points, of which 15 were minima, were found at the UB3LYP level of theory. Three minima corresponding to the nitro form of 1 were located on the triplet surface; just one was found for the singlet ground state. Two reaction paths leading from 1 either to a nitrite-type intermediate (2) or to the aci-form (3) were characterized. For both paths, reaction products were of diradical nature. The lower activation energy was obtained for the triplet-state tautomerization affording 3. The ground state of triplet multiplicity was predicted for two isomers of the aci-form. The triplet diradical 3 is expected to react through the thermal population of a close-lying singlet excited state. The results are discussed in relation to mechanisms of photoinduced rearrangements of peri-substituted nitronaphthalenes that can be used to develop novel photolabile protecting groups.     

Mapping potential energy surfaces

A recently proposed dynamical method [A. Laio and M. Parrinello, Proc. Natl. Acad. Sci. U.S.A. 99, 12562 (2002)] allows us to globally sample the free energy surface. This approach uses a coarse-grained non-Markovian dynamics to bias microscopic atomic trajectories. After a sufficiently long simulation time, the global free energy surface can be reconstructed from the non-Markovian dynamics. Here we apply this scheme to study the T=0 free energy surface, i.e., the potential energy surface in coarse-grained space. We show that the accuracy of the reconstructed potential energy surface can be dramatically improved by a simple postprocessing procedure with only minor computational overhead. We illustrate this approach by conducting conformational analysis on a small organic molecule, demonstrating its superiority over traditional unbiased approaches in sampling potential energy surfaces in coarse-grained space.     

Surveying a potential energy surface by eigenvector-following

We have developed a method to search potential energy surfaces which avoids some of the difficulties associated with trapping in local minima. Steps are directly taken between minima using eigenvector-following. Exploration of this space by low temperature Metropolis Monte Carlo is a useful global optimisation tool. This method successfully finds the lowest energy icosahedral minima of Lennard- Jones clusters from random starting configurations, but cannot find the global minimum in a reasonable time for difficult cases such as the 38-atom Lennard-Jones cluster where the face-centred-cubic truncated octahedron is lowest in energy. However, by performing searches at higher temperatures, we have found a pathway between the truncated octahedron and the lowest energy icosahedral minima. Such a pathway may be illustrative of some of the structural transformations that are observed for supported metal clusters by electron microscopy.     

Characterization of capillary inner surface conditions with streaming potential

Streaming potential is created when an electrolyte solution is forced to flow pass a charged surface. For an uncoated fused silica capillary, the streaming potential is measured between the inlet and outlet vials while applying a pressure across the capillary. The changes in streaming potential can be used to characterize the properties of the capillary inner surface. In this work, HCl, NaCl, and NaOH solutions ranging from 0.4 to 6 mM were used as the background electrolyte (BGE) at temperatures of 15 to 35 °C for the mesurements. The streaming potential decreases with the increase in BGE concentration, and the trend is amplified at higher temperatures. When buffer solutions in the pH range of 1.5 to 12.7 were used as the BGE, streaming potential was shown to be sensitive to changes in pH but reaches a maximum at around 9.5. At pH < 3.3, no streaming potentials were observed. The pH of zero surface charge (streaming potential equals 0) changes with temperature, and is measured to be 3.3 to 3.1 when the temperature is changed from 15 to 35°C. Zeta potentials can be calculated from the measured streaming potential, conductivity, and the solution viscosity. Surface charge densities were calculated in this work using the zeta potentials obtained. We demonstrated that capillary surface conditions can significantly change the streaming potential, and with three different solutions, we showed that analyte-dependent adsorption can be monitored and mitigated to improve the peak symmetry and migration times reproducibility.     

Intermolecular potential energy surface of Ar-NO

Rotational spectra of an open-shell complex, Ar-NO, in the electronic ground state have been analyzed by employing an analysis using a free-rotor model, where previously observed data by Mills et al. [J. Phys. Chem. 90, 3331 (1986); 90, 4961 (1986)] and additional transitions observed by Fourier-transform microwave spectroscopy in the present study are simultaneously analyzed with a standard deviation of the least-squares fit to be 27.5 kHz. A two-dimensional intermolecular potential energy surface for Ar-NO has been determined from the analysis. The determined potential energy surface is compared with those of Ar-OH and Ar-SH, which are also complexes containing an open-shell species with the 2Pi ground electronic state.     

The potential energy surface of the triazadiphosphole formation

The formal GaCl₃-assisted [3+2] cycloaddition of two (Me₃Si)₂N–N(SiMe₃)–PCl₂ molecules resulting in the formation of a triazadiphosphole has been studied by means of B3LYP/6-31G(d,p) computations. These calculations revealed a stepwise reaction mechanism starting from the disguised 1,3-dipole and dipolarophile (Me₃Si)₂N–N(SiMe₃)–PCl₂. Comparison of the potential energy surface for the formation of a triazadiphosphole in the presence and without a Lewis acid indicate, that addition of a Lewis acid such as GaCl₃ decreases the activation barriers to Me₃Si–Cl elimination, in accord with experiment. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

The He-LiH potential energy surface revisited. II. Rovibrational energy transfer on a three-dimensional surface

We use our rigid rotor He-LiH potential energy surface [B. K. Taylor and R. J. Hinde, J. Phys. Chem. 111, 973 (1999)] as a starting point to develop a three-dimensional potential surface that describes the interaction between He and a rotating and vibrating LiH molecule. We use a fully quantum treatment of the collision dynamics on the current potential surface to compute rovibrational state-to-state cross sections. We compute excitation and relaxation vibrational rate constants as a function of temperature by integrating these cross sections over a Maxwell-Boltzmann translational energy distribution and summing over Boltzmann-weighted initial rotational levels. The rate constants for vibrational excitation of LiH are very small for temperatures below 300 K. Rate constants for vibrational relaxation of excited LiH molecules, however, are several orders of magnitude larger and show very little temperature dependence, suggesting that the collisions that result in vibrational relaxation are governed by long-range attractive interactions.     

Dissociation of toluene cation: a new potential energy surface

The potential energy surface (PES) for the formation of tropylium and benzylium ions from toluene cation (1) has been explored theoretically. Quantum chemical calculations at the B3LYP/6-311++G and G3//B3LYP levels were performed. A pathway to form o-isotoluene (5-methylene-1,3-cyclohexadiene) cation (5) from 1 was found. The isomerization occurs by two consecutive 1,2-H shifts from CH(3) to the ortho position of the aromatic ring via a distonic benzenium cation (2), which is also an intermediate in the well-known isomerization of 1 to cycloheptatriene cation (4). Since the barrier for the formation of 2 is the highest in the two isomerization pathways, 1, 4, and 5 are interconvertible energetically prior to dissociation. The benzylium ion can be produced via 5 as well as from 1 and the tropylium ion via 4. Rice-Ramsperger-Kassel-Marcus model calculations were carried out based on the obtained PES. The result agrees with previous experimental observations. From a theoretical analysis of kinetics of the isomerizations and dissociations, we suggest that 5 plays an important role in the formation of C(7)H(7)(+) from 1.     

Fluorobenzene-argon ground-state intermolecular potential energy surface

The ground-state intermolecular potential energy surface for the fluorobenzene-argon van der Waals complex is evaluated using the coupled-cluster singles and doubles including connected triple excitations model, with the augmented correlation consistent polarized valence double-zeta basis set extended with a set of 3s3p2d1f1g midbond functions. In the surface minima the Ar atom is located above and below the fluorobenzene plane at a distance of 3.562 A from the fluorobenzene center of mass and at an angle of 6.33 degrees with respect to the axis perpendicular to the fluorobenzene plane. The corresponding binding energy is 391.1 cm(-1). Both these results and the eigenvalues obtained from the potential compare well with the experimental data available.     

Nitromethane dimer potential energy surface studies

The interaction energy of several conformations of the nitromethane dimer is investigated at the SCF level. The dispersion energy and counterpoise correction are computed for certain relative orientations of the monomers. Fourth-order many body perturbation theory SDQ-MBPT(4) energies are reported for selected points. Double zeta and double zeta plus polarization basis sets were used. All calculations were done with the monomer fixed at the isolated monomer geometry. Interaction energies as large as 6 kcal/mol are found at minima of hydrogen bonding orientations.     

Stationary points on the aminomethanol potential energy surface

Summary In this work we study surface fitting equations for a rigid rotor model of aminomethanol. The energies were obtained from the GAUSSIAN88 package using 3-21G bases and fitted on a least square equation, thus generating a Fourier series expansion of the energy as a function of two dihedral angles. The dihedral angles chosen are those that represent rotation around the C-O and N-C axes in the first case, and rotation around C-O and inversion around the amino group in the second case. Results indicate that the hydroxyl hydrogen is subject to almost free rotation around the C-O axis. Further fully relaxed 6-31G* calculations were performed in order to qualify the results obtained for the rigid rotor model.     

On the OCS2 Singlet potential energy surface

The reaction between atomic oxygen and carbon disulfide is predicted to lead to at least two primary products, which are the dithiiranone ( 1 ) and the oxathiirane-thione ( 2 ) and/or the carbon disulfide S-oxide ( 4 ). The possible intramolecular equilibria 1 ? 2, 1 ? 3, 2 ? 4 , and 2 ? 5 as well as the fragmentations of the possible intermediates 1 – 5 have been studied theoretically within the semiempirical CNDO /B framework as conceivable ground-state reactions. On the basis of MO correlations and potential energy changes along the reaction paths, supplementary with previously reported experimental data, the single molecular transformations and the eventual product formations are discussed.     

Reaction path as a gradient line on a potential energy surface

The reaction path is shown to be always a gradient line on a potential energy surface (PES ) of a molecule. The properties of gradient lines on the PES are elucidated. Correct symmetry conservation rules along the gradient line are derived. The behavior of the gradient line on a PES with different topologies are considered. © 1994 John Wiley & Sons, Inc.     

Calculation of the potential energy surface of a polyatomic molecule in an adiabatic approximation

V. I. Vernadskii Institute of Geochemistry and Analytical Chemistry, Russian Academy of Sciences. Translated fromZhurnal Strukturnoi Khimii, Vol. 36, No. 6, pp. 1142–1144, November–December, 1995