An accurate fit of the potential energy surface of the BeHF system (original) (raw)
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An examination of the BEBO model with the results of ab initio calculations of a reaction series
The BEBO (bond energy-bond order) model of Johnston and Parr is examined with the results of ab initio MO calculations on a series of metathetic reactions which involve hydrogen transfer. Energies are calculated at the 6-31G**/PMF9=full//6- 31G** level while the bond orders sre estimsted using the 6-31G ** basis set with the geometry optimisation at the single configuration unrestricted Hartree-Fock frame. Our analysis reveals that the bond-order exponent in the BEBO theory is greater than unity for the reaction series and the entropy term becomes implicitly present in the BEBO model.
The BEBO (bond energy-bond order) model is further examined with the results of ab initio calculations on a series of metathetic reactions involving hydrogen transfer. Energies are calculated at the 6-31G**/PMP2=full/6-31G** level while the bond-orders are estimated using the 6-31G** basis set with the geometry optimisation at the single configuration unrestricted Hartree-Fock frame. Our analysis reveals that for all thermoneutral reactions where the making and breaking bonds are collinear in the transition state, the reduced distensions in the transition state structures are same and independent of the bond-order exponent in the BEBO model. For others such displacements increase with the square of the energy changes during reactions. The Bronsted coefficient is examined and is found to be neither strictly parallel to the position of the barrier in the reaction path nor a measure of the 'transition state' structure of the system
A study of the Be2+—H2O system by means of ab initio calculations
Chemical Physics Letters, 1986
The Be2+-Hz0 system has been taken as an example in order to investigate the changes induced in water molecules on coordination. Pronounced changes in water geometry and vibrational frequencies are found. Optimised geometries of Bez+-water complejres are presented.
International Journal of Quantum Chemistry, 1997
Ž Second-order density functional methods where the correlation energy depends on the . second-order density matrix and on a density functional are used to introduce the Ž . electron correlation in two-configuration direct minimization TCDM ab initio electronic Ž . energy calculations of three-dimensional potential energy surfaces PES . We analyzed the behavior of these methods in PES calculations by applying them to the Be q FH reaction. This system was studied also by the usual techniques, allowing a full comparison for the lowest 1 A X adiabatic state. In particular, we compared the results obtained using the TCDM and multiple reference single and double excitations configuration interaction Ž . Ž . MRDCI methods with the corresponding results obtained using the Colle᎐Salvetti CS Ž . and Moscardo᎐San Fabian MSF procedures, within the correlation factor method, usinǵá s starting point the TCDM results. We found that the CS and MSF results are in a good overall agreement with the more accurate ab initio results, including the heights of the saddle points and the transition state.
The Journal of Chemical Physics, 2002
The completely renormalized (CR) CCSD(T) method has been used to calculate the entire ground-state potential energy surface (PES) for the Be + HF reaction on a grid of nuclear geometries consisting of ∼3000 points. The cc-pVTZ and cc-pVQZ basis sets have been employed. In addition to the case of the Be atom approaching the HF molecule from the fluorine side and the case of the Be atom approaching HF from the hydrogen side, several values of the Be-F-H angle and the insertion of the Be atom between the H and F atoms of HF have been examined. The CR-CCSD(T) results have been compared with the results of CCSD(T) and multireference configuration interaction (MRCI) calculations. It has been demonstrated that the ground-state PES of the BeFH system obtained from the single-reference "black-box" CR-CCSD(T) calculations is in excellent agreement with the PES obtained from the expensive MRCI calculations, whereas the PES resulting from the standard CCSD(T) calculations is qualitatively incorrect and characterized by large errors relative to MRCI on the order of several electronvolts.
Ab initio and DFT study of the ground potential energy surface for the reaction
Chemical Physics Letters, 2001
An ab initio CASPT2//CASSCF and a DFT B3LYP study of the ground 3 A H potential energy surface (PES) for the title reaction have been carried out. Two reaction paths with cis-and trans-arrangements are described. Surface crossings are noted in both paths, while bifurcative behaviour is present in the energetically favoured cis-path. These phenomena have been analysed in more detail at the DFT level. The rate constant values have been calculated by means of the conventional transition state theory, including the contribution from the ground 3 A HH PES (reported elsewhere), yielding very good accordance with the experiment in the case of the CASPT2//CASSCF method. Ó
Influence of the potential energy surface on the reaction cross section: the K+ HF--> KF+ H system
Chemical physics, 1995
The effect of the potential energy surface on the K + HF-~ KF + H cross section has been studied using reasonable Sorbie-Murrell (bent saddle point) and LEPS (collinear saddle point) potential energy surfaces (PESs). Trajectory calculations for selected initial conditions (translational energies, rovibrational levels (o, J) of HF, as well as initial parallel or perpendicular alignments between the HF rotational angular momentum and the reactants relative velocity vectors) have been performed on these PESs to compare them with experiments. The Sorbie-Murrell and LEPS-4 PESs lead to steric effect ratio results quite close to the experimental ones, once the error margins are included. The results point towards a bent K-F-H saddle point although the PES is very isotropic. This could explain why experimental determinations lead to suggest a collinear saddle point. The K + HF-* KF + H reaction exhibits an enormous vibrational enhancement of reactivity with one quantum HF vibrational excitation, even at translational energies well above the HF(v = 0) threshold, where tunnelling effect contribution to reactivity can be neglected. This behaviour has not been reproduced in the trajectory calculations and no satisfactory explanation has been obtained for this fact. Nevertheless, the HF(v = 1)/HF(v-0) cross section ratio at translational energies not far from the HF(v = 0) threshold and the relative cross section for HF(v =0) have been satisfactorily described. In what regards rotation, the best theoretical results are those corresponding to the Sorbie-Murrell PES (the cross section increases with J), although important differences with experiment appear for the J = 0-3 interval at the lower translational energy values considered (0.54 and 0.77 eV).
Solvent Effect on the Potential Energy Surfaces of the F– + CH3CH2Br Reaction
The Journal of Physical Chemistry A, 2018
Although substantial works have been undertaken on reaction pathways involved in basepromoted elimination reactions and bimolecular nucleophilic substitution reaction of Fon CH CH 2 X (X=Cl, Br, I), the effect of solvents with varying dielectric constants on the stereochemistry of each of the reaction species involved across the reaction profile have not yet been clearly understood. The present investigation reports the effect of solvents on the potential energy surfaces (PES) and structures of the species appearing in the reaction pathway of Fwith bromoethane. The PESs in gas phase have been computed at MP2 level and CCSD(T) level. The performance of several hybrid density functional, such as B3LYP, M06, M06L, BHandH, X3LYP, M05, M05-2X, M06-2X have also been investigated towards describing the elimination and nucleophilic substitution reactions. With respect to MAE values and to make the computation cost effective, we have explored the implicit continuum solvent model, CPCM in solvents like cyclohexane, methanol acetonitrile, dimethyl sulphoxide and water. The reactant complexes proceed through the subsequent steps to produce fluoroethane as the substitution product and ethylene as one of the elimination products. For elimination reaction both syn and anti elimination have been explored. The calculated relatives energies values which are negative in gas phase are found to be positive in polar solvents since the point charge in the separated reactants are more stabilized than the dispersed charge in the transient complex, which has also been analysed through NBO analysis.
The Journal of Physical Chemistry, 1993
Ab initio molecular orbital theory has been used to calculate the exchange barriers for the reaction H + XH. The calculations employed large basis sets, and a variety of methods were used to calculate the correlation energy up through PMP4 and QCISD(T) (or CCSD(T)). The barriers to exchange at the QCISD(T) level, uncorrected for zero point effects, are 42.7,20.2, 13.8, and 6.4 kcal/mol for X = F, C1, Br, and I, respectively. The effect of basis set superposition was studied and found to be 0.4-2.2 kcal/mol depending on X and the basis set. These barriers are consistent with previous calculations which show large barriers instead of the low barriers expected from a number of experimental results. Rate constants and effective activation energies were calculated from transition-state theory and compared to experiment. The results suggest that the experiments need to be reinterpreted in terms of large activation energies instead of the small values used in many of the previous studies.
Quasi-chemical study of Be2+(aq) speciation
Chemical Physics Letters, 2003
Be 2+ (aq) hydrolysis can to lead to the formation of multi-beryllium clusters, but the thermodynamics of this process has not been resolved theoretically. We study the hydration state of an isolated Be 2+ ion using both the quasi-chemical theory of solutions and ab initio molecular dynamics. These studies confirm that Be 2+ (aq) is tetra-hydrated. The quasi-chemical approach is then applied to then the deprotonation of Be(H 2 O) 4 2+ to give BeOH(H 2 O) 3 +. The calculated pK a of 3.8 is in good agreement with the experimentally suggested value around 3.5. The calculated energetics for the formation of [Be • OH • Be] 3+ are then obtained in fair agreement with experiments.