Transition State Of Symmetry Research Articles

Dinitrogen Difluoride Chemistry. Improved Syntheses ofcis- andtrans-N2F2, Synthesis and Characterization of N2F+Sn2F9−, Ordered Crystal Structure of N2F+Sb2F11−, High-Level Electronic Structure Calculations ofcis-N2F2,trans-N2F2, F2N═N, and N2F+, and Mechanism of thetrans−cisIsomerization of N2F2

N(2)F(+) salts are important precursors in the synthesis of N(5)(+) compounds, and better methods are reported for their larger scale production. A new, marginally stable N(2)F(+) salt, N(2)F(+)Sn(2)F(9)(-), was prepared and characterized. An ordered crystal structure was obtained for N(2)F(+)Sb(2)F(11)(-), resulting in the first observation of individual N[triple bond]N and N-F bond distances for N(2)F(+) in the solid phase. The observed N[triple bond]N and N-F bond distances of 1.089(9) and 1.257(8) A, respectively, are among the shortest experimentally observed N-N and N-F bonds. High-level electronic structure calculations at the CCSD(T) level with correlation-consistent basis sets extrapolated to the complete basis limit show that cis-N(2)F(2) is more stable than trans-N(2)F(2) by 1.4 kcal/mol at 298 K. The calculations also demonstrate that the lowest uncatalyzed pathway for the trans-cis isomerization of N(2)F(2) has a barrier of 60 kcal/mol and involves rotation about the N=N double bond. This barrier is substantially higher than the energy required for the dissociation of N(2)F(2) to N(2) and 2 F. Therefore, some of the N(2)F(2) dissociates before undergoing an uncatalyzed isomerization, with some of the dissociation products probably catalyzing the isomerization. Furthermore, it is shown that the trans-cis isomerization of N(2)F(2) is catalyzed by strong Lewis acids, involves a planar transition state of symmetry C(s), and yields a 9:1 equilibrium mixture of cis-N(2)F(2) and trans-N(2)F(2). Explanations are given for the increased reactivity of cis-N(2)F(2) with Lewis acids and the exclusive formation of cis-N(2)F(2) in the reaction of N(2)F(+) with F(-). The geometry and vibrational frequencies of the F(2)N=N isomer have also been calculated and imply strong contributions from ionic N(2)F(+) F(-) resonance structures, similar to those in F(3)NO and FNO.

Enantiomerization of bridged 1,1′‐binaphthyls

AbstractThe activation energy for the isomerization of 1,1′‐binaphthyl in which positions 2 and 2′ are bridged at by an —O—CH2—O— unit was calculated at various computational levels. AM1 gave good agreement with the experimental results. The transition‐state structure was found to be entirely different from that calculated for the non‐bridged parent compound: whereas the latter has C2 symmetry, the former has C s symmetry. The C s symmetry transition state for the non‐bridged parent compound was also located and found to be ca 6 kcal mol−1 higher than the C2 one. However, in the bridged compound, the inclusion of the bridge counterbalanced this by raising the energy of the ground state, leaving the activation energy essentially unchanged. The isomerization of optically active bridged 1,1′‐binaphthyls bearing linear polyphenyl rods of varying length, at positions 6 and 6′, was recently employed as a probe to gain information on the effect of rubber and glassy polymers on reaction rates. The model showed that the rod segments of these molecules traverse long distances in order to reach the transition state, which was consistent with a strong rod length dependence on racemization of the bridged binaphthyls in the glassy state. However, the present results demonstrate an unexpected twisting motion in the racemization process, suggesting that the appended oligophenyl rods are displaced to about half the distance previously expected. This may contribute in part to the experimental observation in the rubbery state where the microviscosity affects the racemization as a function of the appended rods far less than expected. AM1 results also gave reasonable agreement with the experimental ponderal effect consistent with the prior conclusion of force constant independence of rod length for twisting about the 1,1′ bonds. Copyright © 2002 John Wiley & Sons, Ltd.

Synthesis of the arachno-4-CB(8)H(12)(2)(-) monocarbaborane dianion and NMR/computational studies of the fluxional processes observed in the isoelectronic nine-vertex arachno anions: arachno-4-CB(8)H(12)(2)(-), arachno-4-CB(8)H(13)(-), and arachno-4-SB(8)H(11)(-).

The new monocarbaborane dianion, arachno-4-CB(8)H(12)(2)(-) has been synthesized from the reaction of arachno-4-CB(8)H(14) with 2 equiv of NaH in polar solvents. DFT/GIAO computations at the B3LYP/6-311G//B3LYP/6-311G level, in conjunction with 1D and 2D NMR spectroscopic studies, have confirmed that the dianion results from deprotonation of both the endo-CH and one bridging hydrogen of the parent arachno-4-CB(8)H(14). While the DFT calculations indicate that a C(1) symmetric structure is lowest in energy, the experimental solution NMR data are consistent with the dianion having a C(s)() symmetric structure, thus suggesting that it is fluxional in solution. Transition state calculations located a low-energy pathway with an activation energy of only 2.7 kcal/mol that allows the migration of the bridging hydrogen between the two enantiomeric forms of the dianion. The process can occur by a single-step, simple rotation through a transition state structure containing a -BH(2) group at the B7 boron. Averaging the calculated (11)B NMR chemical shifts of the resonances for those atoms in the static enantiomeric structures that become equivalent by this fluxional process then gives excellent agreement with the solution NMR data. Transition state calculations of the fluxional behavior previously observed for the isoelectronic arachno-4-CB(8)H(13)(-) and arachno-4-SB(8)H(11)(-) monoanions have likewise revealed related low-energy (0.3 and 5.0 kcal/mol, respectively) rearrangement mechanisms involving the simultaneous rotation of three hydrogens (two bridging and one -BH(2)) through a C(s)() symmetry transition state containing three -BH(2) groups.

Ab initio study on the rate constants of SiCl4 + H→SiCl3 + HCl

The reaction SiCl4 + H → SiCl3 + HCl is studied using an ab initio dynamic method. The direct chlorine abstraction of reaction processes ia a C3v symmetry transition state is calculated by the intrinsic reaction coordinate (IRC) method at the UMP2/6-311G(d,p) level. The forward and reverse barriers are refined by UMP4(SDTQ) and UQCISD(T) single point energy calculations using the same basis set of 6-311G(d,p). The forward barrier is calculated to be 24.34 kcal mol−1 at the UMP4(SDTQ) level and 22.62 kcal mol−1 at the UQCISD(T) level, while the reverse barrier is 22.46 and 21.90 kcal mol−1, respectively. The enthalpy of reaction is calculated to be 1.94 kcal mol−1 at the UMP4(SDTQ) level and 0.72 kcal mol−1 at the UQCISD(T) level. The forward rate constants in the temperature range 1500–1800 K are calculated by the conventional and variational transition state theory (CVT) with small-curvature tunneling (SCT) correction. The variational effect is important for the calculation of forward rate constants but the SCT correction is small. The CVT and CVT/SCT rate constants of the forward reaction at the UMP4(SDTQ) and UQCISD(T) levels are consistent with the lower limit of the experimental result. However, the theoretical activation energies in the temperature range 1500–1800 K, 28.43 kcal mol−1 at the UMP4(SDTQ) level and 27.10 kcal mol−1 at the UQCISD(T) level, are much higher than the experimental value, 9.54 ± 5.17 kcal mol−1. The theoretical temperature dependence of the rate constants differs from the experimental result.

Theoretical study of the quadrupole-bound anion (BeO) 2−

The global minimum on the potential energy surface of (BeO) 2 −, calculated at the coupled-cluster level of theory with single, double, and non-iterative triple excitations, corresponds to a rhombic D 2h structure, which may be considered as a quadrupole-bound anion. The rhombic isomer is separated by a C s symmetry transition state from the linear isomer and the energy barrier amounts to 15.4 kcal/mol. The energies of the D 2h and C ∞v anionic isomers differ by 6.2 kcal/mol. The vertical electron detachment energies amount to 8903 and 23847 cm −1 (1.10 and 2.96 eV) for the rhombic and linear anion, respectively.

An ab initio molecular orbital study of the reaction NH 2+NO → H 2+N 2O

Potential energy surface of the reaction NH 2+NO → H 2+N 2O has been studied at several high levels of ab initio molecular orbital theory. The reaction pathway involves initially the formation without a barrier of a twisted non-planar H 2N–NO nitrosamine intermediate, and a C s symmetry transition state, followed by a dihydrogen H 2 elimination to form the products. The reaction path bifurcates before the transition state. At MP4(SDTQ)/6-311G(2d,p)//CASSCF/6-31G(d,p) level of theory, the reaction barrier for this path is found to be +33.7 kcal/mol.

Second‐order Jahn–Teller instability and the activation energy for Al+(1S) + H2 → AlH+(2∑+) + H

AbstractThe interaction of Al+ (1S) ions with H2 on the lowest electronic energy surface is studied using ab initio electronic structure methods. A Cs symmetry transition state is located and found to have the geometry of a product AlH+ ion loosely bound to a H atom, consistent with the Hammond postulate for this endothermic reaction. Locating this transition state, beginning at geometries that characterize vibrationally cold H2 and translationally hot Al+, posed special challenges to the commonly used “hill‐climbing” algorithm because of regions of geometrical instability along the path thus generated. This instability was found to be a result of second‐order Jahn–Teller coupling with a low‐lying 1B2 electronic state. In addition to these primary findings, a weakly bound T‐shaped Al+ ——— H2 C2v van der Waals complex is found that lies only 242 cm−1 below the Al+ and H2 asymptote, with HH internuclear separation only slightly distorted from the equilibrium bond distance of H2 and AlH distance (3.5 Å) much longer than the covalent bond length in AlH+ (1.6 Å). The locally stable but thermodynamically unstable linear HAlH+ (1∑) species and, of course, the H + AlH+(2∑+) reaction products have also been identified as critical points on the ground‐state surface. Where known, the geometries and energies that we calculte agree well with experimental data. © 1993 John Wiley & Sons, Inc.

An ab initio study of the structure and intramolecular proton transfer in tropolone

An ab initio study of tropolone was carried out at the 3-21G level, with full optimization of the geometry, and the intramolecular proton transfer in the ground state was analysed. The two equivalent Cs structures can be converted to each other via a C2v symmetry transition state with a double minimum potential. The splitting of the ground vibrational state generated by proton tunneling was determined by procedures of variable complexity and found to be in the range Δ = 0.1–0.15 cm−1, i.e., clearly smaller than experimental predictions. Key words: tropolone, intramolecular hydrogen bond, proton transfer, ab initio calculations, molecular structure.

A potential energy surface for the ground state of formaldehyde, H2CO(1A1)

A model potential energy function for the ground state of H2CO has been derived which covers the whole space of the six internal coordinates. This potential reproduces the experimental energy, geometry and quadratic force field of formaldehyde, and dissociates correctly to all possible atom, diatom and triatom fragments. Thus there are good reasons for believing it to be close to the true potential energy surface except in regions where both hydrogen atoms are close to the oxygen. It leads to the prediction that there should be a metastable singlet hydroxycarbene HCOH which has a planar trans structure and an energy of 2·31 eV above that of equilibrium formaldehyde. The reaction path for dissociation into H2 + CO is predicted to pass through a low symmetry transition state with an activation energy of 4·8 eV. Both of these predictions are in good agreement with recently published ab initio calculations.