Showing papers on "Potential energy surface published in 1988"

The potential surface and stretching frequencies of X̃ 3B1 methylene (CH2) determined from experiment using the Morse oscillator‐rigid bender internal dynamics Hamiltonian

Abstract: The Morse oscillator‐rigid bender internal dynamics (MORBID) Hamiltonian [P. Jensen, J. Mol. Spectrosc. 128, 478 (1988)] has been used in a fitting to all extant rotation–vibration data for X 3B1 methylene CH2. This fitting leads to an improved determination of the potential energy surface, and in particular to reliable predictions for the stretching frequencies. We predict ν1=2992 cm−1 and ν3=3213 cm−1 for 12CH2, and we hope that the new predictions will encourage the experimental search for these weak fundamentals. In the MORBID approach the rotation–vibration energies are obtained from the potential energy surface in a purely variational calculation, and consequently the present work is an improvement over previous determinations of the CH2 potential energy surface from experiment that used the nonrigid bender formalism [see P. R. Bunker et al., J. Chem. Phys. 85, 3724 (1986), and references therein]; this latter approach treats the stretching vibrations by second order perturbation theory. A fitting ...

Adiabatic and diabatic potential energy surfaces for collisions of CN(X 2Σ+, A 2Π) with He

Abstract: The interaction potential energy surfaces for CN(X 2∑+, A 2∏)+He have been computed from ab initio MCSCF and MCSCF‐CI wave functions using an extensive basis set. In the presence of the He atom the two degenerate components of the CN 2∏ state split into wave functions of A’ and A‘ symmetry, and the symmetry of the 2∑+ state reduces to A’. The two adiabatic potentials for the A’ states are transformed to a diabatic basis, which yields a fourth potential energy surface V1, describing the collision‐induced electrostatic coupling between the two A’ states. The degree of mixing of the two diabatic A’ states has been determined by integration of the relevant nonadiabtic coupling matrix elements and, in a simpler method, from the coefficients of the MCSCF configurations. Both procedures yield virtually identical results. The nonadiabatic coupling matrix elements are strongly peaked near the CN bond distance at which the X 2∑+ and A 2∏ states cross in the isolated molecule. The diabatic coupling potential V1, however, is only weakly dependent on the CN bond distance, and decreases exponentially with the CN–He separation. Near the classical turning points for room temperature collisions the magnitude of V1 is approximately 50 cm−1. The V1 potential shows a bimodal character as a function of the collision angle θ. These results are discussed in connection with recent experiments of Dagdigian and co‐workers.

Vibrational spectrum and potential energy surface of the CH chromophore in CHD3

Abstract: The rovibrational spectrum of trideutero‐methane has been measured at resolutions mostly close to the Doppler limit on an interferometric Fourier transform spectrometer from the lowest fundamental vibration to high overtones of the CH stretching vibration (wave numbers from 900 to 12 000 cm−1). The CH chromophore spectrum is fully assigned and interpreted by means of the tridiagonal Fermi resonance Hamiltonian for the coupled CH stretching and bending vibrations. The Hamiltonian predicts and also fits the visible spectrum up to 19 000 cm−1 measured by Scherer et al., Perry et al., and Campargue et al. The effective tridiagonal Hamiltonian is derived ab initio by means of MRD‐CI and full CI calculations of the potential surface of methane, a variational vibrational calculation in a normal coordinate subspace of the coupled CH stretching and bending motions and an approximate similarity transformation to tridiagonal form. Fits of the experimental results by the tridiagonal and the variational Hamiltonian le...

Photodissociation dynamics of H2O and D2O in the first absorption band: A complete abinitio treatment

Abstract: We report a detailed theortical study of the photodissociation of H2O and D2O in the first absorption band (λ∼165 nm). The calculations are three dimensional and purely quantum mechanical. They include an ab initio potential energy surface for the A state and a calculated SCF dipole moment function for the X→A transition. The dynamical calculations are performed within the infinite‐order‐sudden approximation for the rotational degree of freedom of OH and the LHL approximation for the masses. The resulting vibrational–translational motion is then treated exactly in two dimensions using hyperspherical coordinates. This study does not include any adjustable parameters. The thermally averaged total absorption spectra for H2O and D2O agree perfectly with the experimental spectra. Even finer details such as the progression of ‘‘vibrational’’ structures are well reproduced. They are not induced by any selective absorption but can be explained on the basis of the A state potential energy surface and details o...

A realistic double many-body expansion (DMBE) potential energy surface for ground-state O3 from a multiproperty fit to ab initio calculations, and to experimental spectroscopic, inelastic scattering, and kinetic isotope thermal rate data

Abstract: A new single-valued potential energy surface is reported for the ground electronic state of ozone from the double many-body expansion (DMBE) method. The parameters appearing in the DMBE formalism are determined from a multiproperty analysis using ab initio energies, and experimental data from spectroscopic, incomplete total scattering cross section, and kinetic thermal rate measurements. Based on this new surface, thermal rate coefficients for the 18O + 16O2 → 18O16O + 16O isotope exchange reaction are also reported over the temperature range 300 < T < 2000 K from the quasi-classical trajectory method.

Bond breaking without barriers: Photofragmentation of ketene at the singlet threshold

Abstract: Ketene (CH2CO) in a supersonic free jet was photodissociated by a tunable pulsed laser in the frequency range just above the threshold for production of singlet methylene, CH2 (a 1A1). CH2 was detected by laser‐induced fluorescence (LIF). The appearance threshold and yield curve of individual 1CH2 rotational states were obtained by scanning the photolysis laser frequency with a fixed LIF probe laser frequency. The dissociation occurs on the ground electronic state potential energy surface. The threshold for CH2CO→1CH2+CO is found to be 30 116.2±0.4 cm−1. By varying the delay between the photolysis and probe pulses, a lower bound of 7×107 s−1 was set for the dissociation rate on the triplet surface at the singlet energy threshold. The yield curves, or photofragment excitation (PHOFEX) spectra, exhibit sharp steps spaced by the CO rotational term values. The experimental data provide a rigorous test of theoretical models of photofragment dynamics. The data clearly show that nuclear spin is conserved throug...

Conformational analysis and molecular dynamics simulations of maltose.

Abstract: Constrained conformational energy minimizations have been used to calculate an adiabatic (Φ, ψ) potential energy surface for the disaccharide β-maltose. The inclusion of molecular flexibility in the conformational energy analysis of the disaccharide was found to significantly lower the barriers to conformational transitions, as has been observed previously for other systems. Several low energy wells were identified on the adiabatic surface which differ in energy by small amounts and with low absolute barriers separating them, indicating the possibility of a non-negligible equilibrium population distribution in each well. If such a distribution of conformations existed in the physical system, the conformation observed by NMR NOE measurements would thus be a “virtual” conformation. Molecular dynamics simulations of the motions of this molecule in vacuum were also conducted and indicate that the rate of relaxation of the molecule to the adiabatic surface may be slower than the typical timescale of conformational fluctuations. This effect is apparently due to an unphysical persistence of hydrogen bond patterns in vacuum which does not occur in aqueous solution. Trajectories undergoing transitions between wells were calculated and the effects of such conformational transitions upon the ensemble mean structure, such as might be observed in an NMR experiment, were demonstrated.

Theoretical characterization of the minimum energy path for the reaction H+O2→HO2*→HO+O

Abstract: The potential energy surface for the reaction H+O2→HO*2 →HO+O has been characterized in the vicinity of the minimum energy path using CASSCF/contracted CI calculations with a basis set which is triple zeta valence quality plus three sets of polarization functions CASSCF/CI calculations were carried out along the CCI minimum energy path The latter calculation shows essentially no barrier for addition of an H atom to O2, in agreement with predictions made in earlier studies The potential surface for recombination of OH and O is complicated by a crossing, at rOO ≈55a0, between the surface for electrostatic (OH dipole–O quadrupole) interaction and that for the formation of an O–O chemical bond This surface crossing results in a small (≈05 kcal/mol) barrier

On the dynamics of the associative desorption of H2

Abstract: The dynamics of activated associative desorption is discussed with particular reference to the system H2–Cu and to the partitioning of the energy released among the various product degrees of freedom. It is argued that a simple theory based on transition‐state concepts should hold for this system because the potential energy surface (PES) divides naturally into reactant and product regions, separated by a ‘‘seam’’ or ‘‘ridge’’ at which it is reasonable to assume a thermal distribution of desorbing trajectories. Using a PES constructed in accordance with available electronic structure calculations we consider the angular distributions and translational, vibrational, and rotational energy distributions of the desorbing molecules. It is shown that, whereas the rotational energy reflects the surface temperature, the vibrational energy is markedly enhanced because the energetically low‐lying regions of the ridge in the PES correspond to an H–H bond distance that is distended as compared with the gas‐phase equilibrium separation. The enhancement is found to be a strong function of the surface temperature. The translational energy, however, is found to be only very weakly dependent on the temperature. These results are discussed in connection with available data.

An analytical six‐dimensional potential energy surface for (HF)2 from ab initio calculations

Abstract: We have developed an analytical expression for the 6D potential energy surface of (HF)2 and fitted it to 1061 ab initio points covering an energy range of up to 25 000 cm−1 above equilibrium. The ab initio calculation used the coupled pair functional approach with an extended polarized basis set. We have adjusted 42 parameters (and constrained 7 others) in the fitting of the analytical function to the points; the standard deviation of the weighted fitting is 26.8 cm−1 . The minimum energy path from the linear saddle point (345 cm−1 above equilibrium), through the equilibrium point, to the C2h saddle point (332 cm−1 above equilibrium) has been determined, and several cuts through the potential are presented. We plan to use this potential surface in reaction path and close coupling calculations of rotation–vibration term values.

A theoretical study of the potential energy surface for the reaction OH+CO→CO2+H

Abstract: The mechanism of the reaction OH+CO→CO2+H is studied theoretically. The potential energy surface is constructed by using ab initio multiconfiguration (MC) SCF and configuration interaction (CI) calculations. The reaction surface Hamiltonian is derived to estimate the hydrogen tunneling probability. It is found that the first step is the trans addition of OH to CO and the trans–cis isomerization is the subsequent step to lead the hydrogen elimination from the HOCO. The rate constant is calculated by using the RRKM equation and the curved Arrhenius temperature dependence, experimentally observed, is obtained. We have found here that the effects of both the hydrogen tunneling from the HOCO to products and the backreaction from the HOCO to the reactants are essential to obtain this temperature dependence.

Correlated ab initio harmonic frequencies and infrared intensities for furan, pyrrole, and thiophene

Abstract: Equilibrium geometries, harmonic vibrational frequencies, and infrared intensities are calculated analytically at the second-order Moeller-Plesset level (MP2) with a DZP basis for the five-membered heterocyclic aromatics furan, pyrrole, and thiophene. The results are of an accuracy to show up misassignments in the original experimental interpretation of the spectra. They also give confidence that ab initio calculations including electron correlation and using flexible basis sets can describe accurately the quadratic part of the potential energy surface. For such systems, these ab initio studies will aid the spectroscopic determination of force constants.

Quantum‐mechanical scattering of H2 from metal surfaces: Diffraction and dissociative adsorption

Abstract: Some theoretical and experimental aspects of the interaction of H2 with metal surfaces are reviewed. A variety of model potential energy surfaces based on simple extensions to the Lennard‐Jones model are presented. These potentials include activation barriers which vary across the surface unit cell giving rise to a variety of physically interesting and realistic topologies. The quantum‐mechanical scattering of hydrogen and its isotopes from these potentials illustrate that it is possible, in a very simple way, to account for simultaneous sticking and diffraction at a single beam energy. The inclusion of tunneling in a realistic description of initial sticking coefficients is shown to be essential for these light gases. Finally, it is proposed that by inverting diffraction data, it should be possible to obtain key information regarding the form of the potential energy surface.

CH2I2 photodissociation: Dynamical modeling

Abstract: A full quantum mechanical calculation is carried out on the first excited state of CH2I2 to model the absorption and emission spectra and examine the photodissociation dynamics from a time dependent point of view The dissociation at 355 nm is direct in the sense that the wave packet does not revisit the Franck–Condon region The initial motion of the excited molecule is mainly along the CI2 symmetric stretch coordinate while simultaneously spreading in the antisymmetric stretch coordinate The molecule then dissociates along a C–I ‘‘local’’ mode; no I2 can be formed in this energy region Vibrationally hot CH2I radical in the C–I mode is predicted The model is in good agreement with available experimental results A simple and intuitive method is presented to construct model potential energy surfaces for two chromophore systems from the potential energy surface and information known for the corresponding single chromophore CH3I and CH2I2 are used as numerical illustrations

A classical path approach to reactive scattering. I. Use of hyperspherical coordinates

Abstract: A classical path method is described for treating reactive collisions in three‐body systems The key feature of the method is the use of hyperspherical coordinates, which allow the definition of classical and quantum subsystems in such a way as to be independent of the arrangement of the particles The ‘‘relative’’ motion and overall rotation are treated classically, while the ‘‘arrangement’’ and internal motion are treated quantum mechanically The method is applied to the collinear D+HH system on the PK2 potential energy surface These calculations show a great improvement over quasiclassical trajectory results in the vicinity of the apparent threshold of the reaction

Improved potential energy surface for He–CO2

Abstract: The anisotropic potential energy surface of He–CO2 is determined by the simultaneous analysis of newly measured high resolution total differential cross sections, differential energy loss spectra, new low temperature second virial coefficients, new diffusion, and viscosity data. The calculations are carried out in the infinite‐order‐sudden approximation. The repulsive anisotropy of the potential is determined from the rotationally inelastic cross sections and the quenching of the diffraction oscillations, while the absolute scale is fixed by the position of these oscillations. The second virial coefficient data are essentially sensitive to the general features of the spherical effective potential well. The transport data are then correctly predicted by this potential surface which differs both in the anisotropy and the spherical part from the recently derived multiproperty fit potential for this system.

An analytic representation of the six‐dimensional potential energy surface of hydrogen fluoride dimer

Abstract: We present a semiglobal potential energy surface for HF dimer that should be especially realistic in the vicinity of the two equivalent hydrogen‐bonded‐complex geometries and the transition state separating them. It is based on fits to 106 correlated calculations and 378 SCF calculations of the energy of the planar complex plus a separate fit to the empirical out‐of‐plane bending potential of Barton and Howard at a sequence of geometries along the minimum energy path for the degenerate rearrangement connecting the two minima.

A concerted triple dissociation—the photochemistry of s‐tetrazine

Abstract: The method of photofragment‐translational spectroscopy was used in a molecular beam study of the photochemistry of s‐tetrazine. Following 1B3u ←1Ag (S1←S0 excitation) or 1B2u ←1Ag (248 nm excitation), s‐tetrazine reverts to the highly vibrationally excited ground electronic state through internal conversion, then decomposes into 2HCN+N2 via concerted triple dissociation. By analyzing the experimental data with a simple model, it was found that the potential energy surface along the reaction coordinate after the transition state is very repulsive, leading to most of the available energy, on average 73.9% in the case of S1 ←S0 excitation, going into translation. The asymptotic angles between N2 and HCN with respect to the center of mass of s‐tetrazine were 117.2±0.5° and 114.4±0.5° for S1 ←S0 and 248 nm excitation, respectively.

Rotational rainbows in the vibrational predissociation of ICl–He complexes

Abstract: Strongly peaked ICl fragment rotational distributions are observed following vibrational predissociation of ICl–He B state complexes containing two or three quanta of ICl vibrational excitation. The nascent rotational distributions of the ICl product exhibit two distinct maxima, occurring at j=7 and j=16. A theoretical analysis demonstrates that the two maxima are due to rotational rainbows, arising from the He atom scattering off of the I and Cl ends of the ICl molecule. The vibrational predissociation of ICl–He B(vB=2) is simulated with a semiclassical scattering theory which is analogous to that developed by Schinke [J. Chem. Phys. 85, 5049 (1986)] for direct photodissociation. Vibrational predissociation is modeled as a rotationally inelastic ‘‘half‐collision,’’ following deactivation of the ICl vibration. The final rotational angular momentum of the ICl fragments is determined from exact classical trajectories and in a sudden limit on a model potential energy surface. The calculated ICl product rotat...

Quasiclassical trajectory calculations of the thermal rate coefficient for the oxygen atom + hydroxyl .fwdarw. oxygen + hydrogen atom reaction on realistic double many-body expansion potential energy surfaces for ground-state hydroperoxy

Abstract: Quasi-classical trajectory calculations of the thermal rate coefficient for the title reaction have been carried out over the temperature range 250 less than or equal to T less than or equal to 2500 K by using two recently reported DMBE potential energy surfaces for the ground state of the hydroperoxyl radical. The results are compared with each other and with experiment. The agreement is good. The authors results support previous theoretical calculations by Miller on the Melius-Blint potential energy surface in that nonstatistical recrossing effects are very important. For the DMBE II (DMBE III) potential energy surface, these nonstatistical corrections are found to increase from a factor of about 1.2 (1.4) at 250 K about 2.1 (2.5) at 2500 K. However, they are considerably smaller than the nonstatistical corrections reported by Miller (factors of about 2.2 and 3.3 at the above temperatures). Although due, of course, to topographical differences between the DMBE and Melius-Blint potential energy surfaces, such discrepancy stems also from the different definitions used for HO/sub 2/* complex in the simple chemical model O + OH /r reversible/ HO/sub 2/* ..-->.. O/sub 2/ + H.

Calculated rotational and rovibrational transitions in the spectrum of H3(

Abstract: New calculations of line strengths and transition frequencies for the forbidden rotational spectrum and the nu2 fundamental rovibrational band of H3(+) are presented. These first principles calculations use the highly accurate ab initio electronic potential energy surface of Meyer, Botschwina, and Burton (1986), which has previously been shown by the authors to give rovibrational transition frequencies, rotational constants and vibrational fundamentals of spectroscopic accuracy. The line strengths calculated for the pure rotational spectrum in the vibrational ground state are in general agreement with those previously predicted by Pan and Oka (1986), although individual line strengths may differ by as much as 50 percent. The data are presented to assist with the interstellar detection of this astrophysically important molecular ion. 25 references.

Accurate collision-induced line-coupling parameters for the fundamental band of CO in He - Close coupling and coupled states scattering calculations

Abstract: The first accurate theoretical values for off-diagonal (i.e., line-coupling) pressure-broadening cross sections are presented. Calculations were done for CO perturbed by He at thermal collision energies using an accurate ab initio potential energy surface. Converged close coupling, i.e., numerically exact values, were obtained for coupling to the R(0) and R(2) lines. These were used to test the coupled states (CS) and infinite order sudden (IOS) approximate scattering methods. CS was found to be of quantitative accuracy (a few percent) and has been used to obtain coupling values for lines to R(10). IOS values are less accurate, but, owing to their simplicity, may nonetheless prove useful as has been recently demonstrated.