First-order Transition States Research Articles

Pushing 3c–4e Bonds to the Limit: A Coupled Cluster Study of Stepwise Fluorination of First-Row Atoms

To better understand why hypervalent F, O, N, C, and B compounds are rarely stable, we carried out a systematic study of 28 systems, including anionic, cationic, and neutral molecules, held together by covalent, hypervalent, and noncovalent bonds. Molecular geometries, frequencies, atomic charges, electrostatic potentials, energy and electron densities, Mayer bond orders, local stretching force constants, and bond strength orders (BSOs) were derived from high accuracy CCSD(T) calculations and utilized to compare the strength and nature of hypervalent bonds with other types of bonds. All hypervalent molecules studied in this work were found to be either first-order transition states or unstable to dissociation, with F3- and OF3- as the only exceptions. For several systems, we found that a weak noncovalent bonded complex is more stable than a hypervalent one, due to the high energetic cost to accommodate an extra ligand, which can surpass the stability gained by 3c-4e bonding.

Diffusion of interstitial species (H and O atoms) in fcc systems (Al, Cu, Co, Ni and Pd): Contribution of first and second order transition states

We present a discussion on the influence of high-order transition states on interstitial diffusion in fcc systems using first-principles calculations. In earlier works, only first-order transition states (1TS) were used to compute the diffusivity at the atomic-scale: the direct diffusion between tetrahedral (t) and octahedral (o) sites has been proposed to describe atomic-scale diffusion mechanisms. However, we show here that if this direct diffusion makes it possible to reproduce displacements remarkably well, neglecting higher-order transition states induces an underestimation of the diffusion coefficient at high temperature. We hereinafter revisit the diffusion coefficient of interstitial species in different fcc-systems. The effect of these configurations on atom diffusion in Al, Co, Cu, Ni and Pd, whose only stable sites are the tetrahedral and octahedral positions (H and O atoms) is thus discussed here. We show that if the correction is low, taking into account higher-transition states can modify the diffusivity values at high temperature.

Probing Intramolecular CH−π Interactions in o-Quinodimethane Adducts of [60]Fullerene Using Variable Temperature NMR

Several o-quinodimethane adducts of [60]fullerene were synthesized and their intramolecular aryl CH-fullerene π interactions were studied using variable temperature-NMR (VT-NMR). Evaluation of the rate constants associated with the first-order transition states for cyclohexene boat-to-boat inversions enables quantification of ΔG(‡) values for each inversion. A comparison between two constitutional isomers, only one of which is capable of intramolecular CH-π interactions, provides a lower limit of 0.95 kcal/mol for each aryl CH-fullerene π interaction.

NMR Spectroscopic Evidence for the Intermediacy of XeF3− in XeF2/F− Exchange, Attempted Syntheses and Thermochemistry of XeF3− Salts, and Theoretical Studies of the XeF3− Anion

The existence of the trifluoroxenate(II) anion, XeF(3)(-), had been postulated in a prior NMR study of the exchange between fluoride ion and XeF(2) in CH(3)CN solution. The enthalpy of activation for this exchange, ΔH(⧧), has now been determined by use of single selective inversion (19)F NMR spectroscopy to be 74.1 ± 5.0 kJ mol(-1) (0.18 M) and 56.9 ± 6.7 kJ mol(-1) (0.36 M) for equimolar amounts of [N(CH(3))(4)][F] and XeF(2) in CH(3)CN solvent. Although the XeF(3)(-) anion has been observed in the gas phase, attempts to prepare the Cs(+) and N(CH(3))(4)(+) salts of XeF(3)(-) have been unsuccessful, and are attributed to the low fluoride ion affinity of XeF(2) and fluoride ion solvation in CH(3)CN solution. The XeF(3)(-) anion would represent the first example of an AX(3)E(3) valence shell electron pair repulsion (VSEPR) arrangement of electron lone pair and bond pair domains. Fluorine-19 exchange between XeF(2) and the F(-) anion has also been probed computationally using coupled-cluster singles and doubles (CCSD) and density functional theory (DFT; PBE1PBE) methods. The energy-minimized geometry of the ground state shows that the F(-) anion is only weakly coordinated to XeF(2) (F(2)Xe---F(-); a distorted Y-shape possessing C(s) symmetry), while the XeF(3)(-) anion exists as a first-order transition state in the fluoride ion exchange mechanism, and is planar and Y-shaped (C(2v) symmetry). The molecular geometry and bonding of the XeF(3)(-) anion has been described and rationalized in terms of electron localization function (ELF) calculations, as well as the VSEPR model of molecular geometry. Quantum-chemical calculations, using the CCSD method and a continuum solvent model for CH(3)CN, accurately reproduced the transition-state enthalpy observed by (19)F NMR spectroscopy, and showed a negative but negligible enthalpy for the formation of the F(2)Xe---F(-) adduct in this medium.

An ab initio correlated study of the potential energy surface for the HOBr.H2O complex.

The potential energy surface (PES) for the HOBr.H(2)O complex has been investigated using second- and fourth-order Møller-Plesset perturbation theory (MP2, MP4) and coupled cluster theory with single and doubles excitations (CCSD), and a perturbative approximation of triple excitations (CCSD-T), correlated ab initio levels of theory employing basis sets of triple zeta quality with polarization and diffuse functions up to the 6-311++G(3dp,3df ) standard Pople's basis set. Six stationary points being three minima, two first-order transition state (TS) structures and one second-order TS were located on the PES. The global minimum syn and the anti equilibrium structure are virtually degenerated [DeltaE(ele-nuc) approximately 0.3 kcal mol(-1), CCSD-T/6-311++G(3df,3pd) value], with the third minima being approximately 4 kcal mol(-1) away. IRC analysis was performed to confirm the correct connectivity of the two first-order TS structures. The CCSD-T/6-311++G(3df,3pd)//MP2/6-311G(d,p) barrier for the syn<-->anti interconversion is 0.3 kcal mol(-1), indicating that a mixture of the syn and anti forms of the HOBr.H(2)O complex is likely to exist.

New insights into an old reaction. High-resolution x-ray powder diffraction of Wiberg's aminoalane intermediate.

In accordance with the procedure described by E. Wiberg, Me(3)Al-NH(3) was heated as a bulk material in inert atmosphere to give a colorless liquid which slowly loses methane. Close to the end of this elimination reaction, the melt crystallized to give a microcrystalline powder of (Me(2)AlNH(2))(x)(). The structure of this intermediate has been solved by the method of high-resolution X-ray powder diffraction. The compound crystallizes in the monoclinic space group C2/c with the cell parameters of a = 15.0047(6) A, b = 8.7500(2) A, c = 24.4702(8) A, and beta = 107.290(2) degrees, with eight trimers (Me(2)AlNH(2))(3) per unit cell. These trimers crystallize in a boat conformation in contrast to the known trimers of the same composition where a twist-boat conformation had been found by single crystal determination. Different conformers of (Me(2)AlNH(2))(3) have been investigated by theoretical methods (HF/6-31G(d), B3LYP/6-31G(d), B3LYP/6-311G(d,p), MP2(fc)/6-31G(d), and MP2(fc)/6-311G(d,p)). The twist-boat and the chair conformer correspond to minima at the potential energy surface, whereas the boat conformer corresponds to a first-order transition state (relative energies of 0.45-2.56 kJ/mol (boat) and 6.66-11.91 kJ/mol (chair)). Relaxed scans of the potential energy surface at the HF/6-31G(d) and B3LYP/6-31G(d) levels have shown that the boat conformer (C(s)() symmetry) connects two enantiomers of the twist-boat form (C(2) symmetry).

Nitrosodifluoroamine, F2N2O,

Nitrous oxide, N2O, was observed to react with F2 under UV irradiation at −196 °C to form initially F2N2O which in accord with earlier reports from the literature decomposes above −140 °C to form N2F4 and NO. The apparently different reaction behavior of N2O toward H radicals (→H2O + N2) and F radicals (→F2N2O → N2F4 + NO) can be accounted for by thermodynamic considerations [average bond energies (kcal mol-1): O−H (114.2) > N−H (93.3); N−F (66.4) > O−F (44.7)]. A reaction mechanism for the formation and decomposition of F2N2O from N2O and F2 (present study) and from N2F4 and NO has been suggested using valence bond considerations. The structure of F2N2O was fully optimized at the electron correlated MP2(FULL)/cc-pVTZ level of theory and was shown to possess C1 symmetry. A frequency analysis clearly shows the C1 structure to represent a true minimum (NIMAG = 0), whereas the earlier reported planar Cs structure was shown to represent a first-order transition state (NIMAG = 1).

Rotation/Inversion Study of the Amino Group in Ethylamine

Using the HF, MP2, and DFT methodologies in the GAUSSIAN 94 set of quantum chemistry codes with a 6-311G** basis set, we examine theoretically the internal rotation and inversion of the amino group in ethylamine. An analytical potential energy surface corresponding to energy as a function of φ, the dihedral angle of the lone-pair of electrons on the amino group, and τ, the angle between the amine plane and the C−N axis, for these motions is generated by fitting the energy determined at 109 geometries to sums of products of Bessel functions of the first kind, Jm, and cos(mφ). The surface depicts several special points: the minimum energy trans conformer, the two gauche conformers, the three first-order transition states for internal rotation of the amino group about the C−N bond, the two first-order transition states for the inversion of the amino group, and the three second-order transition states also for the inversion motion. The results of a related study on the internal rotation of the ammonium group...

Ab initio study of the formation of molecular complexes between Cl2 and C2H2

The intermolecular potential energy surface (PES) for the interaction between the Cl2 C2H2 molecules has been comprehensively investigated using ab initio methods, aiming to locate the possible stationary points. The calculations were performed with the double zeta plus double polarization (DZ2P), triple zeta plus polarization (TZP) and triple zeta plus double polarization (TZ2P) basis sets, including electron correlation at the second-order Moller-Plesset (MP2) level, and basis set superposition error correction. Six stationary points were located on the PES: a T-shaped form where one chlorine atom is attached to the acetylene triple bond (bπ–σ type), a parallel form, a slipped parallel form, a crossed form and an inclined and a symmetric inverse T-shaped forms, where the van der Waals bond is between one of the H atoms of the acetylene and the Cl–Cl bond. At the MP2/TZ2P//MP2/TZP level of calculation, only the T-shaped and the parallel forms are minimum energy structures, and their stabilization energies are 2·002 and 0·422 kcal mol-1 The two inverse T-shaped forms and the slipped parallel form are predicted to be first-order transition states at this level of calculation, and their stabilization energies are 0·709 kcal mol-1 for the inclined form, 0·694 kcal mol-1 for the symmetric form, and 0·624 kcal mol-1 for the slipped parallel form, which suggest that the intermolecular PES is very flat in this region. The crossed form is a second-order transition state, and it is stabilized by 0·390 kcal mol-1. The shifts of the intramolecular frequencies upon complexation are also discussed. The global minimum is the T-shaped bπ–aσ structure, and the geometry, the intermolecular stretching force constant and the charge redistribution on complex formation lead to a classification of the outer (weak) type according to Mulliken. These results show the weakness of this interaction, which is dominated by dispersion forces, characteristic of the complexes between molecules without a permanent electric dipole, such as C2H2 and Cl2.

Searching for the ylide structure. An ab initio study of the H 2O…CCl 2 complex

The potential energy surface (PES) for the H 2O…CCl 2 complex has been investigated at the self-consistent field level with a DZP basis set in order to search for stationary points and to verify the possible formation of an ylide species. Six stationary points were located on the PES, being one minimum, three first-order transition states (TS) and two second-order TS structures. A stable ylide species was not found, and an explanation for this is given based on electrostatic grounds. The unique minimum corresponds to an H-bond structure, with a dissociation energy of 845.5 cm −1, calculated including zero point energy correction, a more extended basis set, electron correlation effects at the MP2 level and taking into account basis set superposition errors employing the counterpoise method.

Theoretical probation of FClCO⋯Cl 2 molecular complex : Post Hartree-Fock studies on molecular structures, stabilities and vibrational harmonic frequencies

Potential energy surfaces of the weak FClCO⋯Cl 2 complex were studied using ab initio post Hartree-Fock theory at the MP2 and MP4 levels with 6-311G(d) and 6-311G(2d) basis sets. Two minimum energy conformations, first-order transition state and two second-order transition structures were found. The minimum energy forms (C and T) are separated by a small (less than 0.5 kcal mol ) barrier. At the MP2/6-311G(d) level their relative stabilities amount to −0.63 and −0.60 kcal mol for C and T conformers, respectively, where at the MP2/6-311G(2d) approximation stabilities of both forms rise to −1.09 and −1.21 kcal mol . The harmonic vibrational frequencies were calculated and compared with available experimental data.

The “true” minimum energy structure of H 3SiOHAlH 3. Implications for conformational preferences of bridged hydroxyl groups in zeolites

The true minimum energy conformation for H 3SiOHAlH 3 (a cluster model for bridged hydroxyl groups in zeolites) is obtained using ab initio molecular orbital methods at the Hartree—Fock and post Hartree—Fock levels. It is demonstrated that previous investigations have used a first-order transition state structure. Implications for the structure and location of bridged hydroxyl groups in zeolites is discussed.

The potential energy surface of H 4SiO 4

Three “true” minimum energy conformations and several transition state structures have been located on the potential energy surface for the H 4SiO 4 molecule, calculated at the SCF/3-21G level of theory. Geometry optimisation for each stationary point has been performed with inclusion of polarization functions using the STO/6-31G* basis set. At the HF/6-31G* level of theory three minimum energy structures all possessing C 1 point group symmetry are located. Only one symmetric stationary point, possessing D 2d symmetry, is located as a first-order transition state structure.

AM1 study of hydrogen bonded complexes of water

The failure of AM1 to reproduce the alleged experimental geometry of the water dimer is reported in this paper. A second-order transition state, instead of a minimum, is found at the experimental geometry with a linear hydrogen bond. It collapses to a first-order transition state with a bifurcated hydrogen bond (similar to the ab initio one) from which the true AM1 minimum is obtainable. This structure has a stabilization energy of −5.47 kcal mol −1 and shows three hydrogen bond interactions. This failure of the method is attributed to an exceedingly weak bending potential of the hydrogen bond within AM1. The conclusions reached in the study of the dimers are further tested by the calculation of a water trimer and a water tetramer and the linear and bifurcated hydrogen bonded complexes between H 2O and NH 4 +. It is concluded that although it fails in the case of a dimer, AM1 may be useful in the treatment of larger clusters of hydrogen bonded molecules. All calculations are contrasted with the results of ab initio ones.