Ab Initio Potential Energy Research Articles

Theory of the photodissociation of ozone in the Hartley continuum: Potential energy surfaces, conical intersections, and photodissociation dynamics

Ab initio potential energy and transition dipole moment surfaces are presented for the five lowest singlet even symmetry electronic states of ozone. The surfaces are calculated using the complete active space self consistent field method followed by contracted multireference configuration interaction (MRCI) calculations. A slightly reduced augmented correlation consistent valence triple-zeta orbital basis set is used. The ground and excited state energies of the molecule have been computed at 9282 separate nuclear geometries. Cuts through the potential energy surfaces, which pass through the geometry of the minimum of the ground electronic state, show several closely avoided crossings. Close examination, and higher level calculations, very strongly suggests that some of these seemingly avoided crossings are in fact associated with non-symmetry related conical intersections. Diabatic potential energy and transition dipole moment surfaces are created from the computed ab initio adiabatic MRCI energies and transition dipole moments. The transition dipole moment connecting the ground electronic state to the diabatic B state surface is by far the strongest. Vibrational-rotational wavefunctions and energies are computed using the ground electronic state. The energy level separations compare well with experimentally determined values. The ground vibrational state wavefunction is then used, together with the diabatic B<--X transition dipole moment surface, to form an initial wavepacket. The analysis of the time-dependent quantum dynamics of this wavepacket provides the total and partial photodissociation cross sections for the system. Both the total absorption cross section and the predicted product quantum state distributions compare well with experimental observations. A discussion is also given as to how the observed alternation in product diatom rotational state populations might be explained.

An ab initio study of the potential energy surfaces for the collision between a Cs atom and an I2 molecule

For the Cs + I2 collision system, a systematic theoretical study is first reported using the ab initio method. Three of eight possible channels are considered. The nonadiabatic coupling between the covalent state and the ionic one is calculated from different angles, especially the T-shape collision. The complete ion-pair formation potential energy surfaces of the T-shape collision in two electronic states (ionic 2B2 state and covalent 2A1 state) and the reactive surface of the linear collision are constructed at the QCISD(T)/SDD level. The main features of potential energy surfaces, such as the minimum energy reaction path, the crossing radius (Rc), and energy minimum geometries, are analyzed. The cross section of this titled system is calculated based on the harpoon mechanism and compared with the available experimental data and those obtained for the M + I2 (M = Li, Na) systems.Key words: ab initio two-state potential energy surfaces, nonadiabatic coupling, ion-pair formation, cross section.

New Theoretical Line List for the B ′ 2 Σ + ← X 2 Σ + System of 24 MgH

Fully quantum-mechanical techniques have been applied to compute the complete line list for the B' 2Σ+ ← X 2Σ+ system of 24MgH. The list includes transition energies and oscillator strengths over the wavelength range 11,850-32,130 cm-1, for all possible allowed transitions from the ground electronic state vibrational levels v'' = 0-11. This list was computed using the best available ab initio potential energies and dipole transition moment function, with the former adjusted to account for experimental data. The agreement of the current calculations with previous theoretical results and the line list of Kurucz is discussed. Although spectroscopic accuracy for a particular line cannot be claimed for our calculations, comparison is made with the recent astronomical measurements of Wallace et al. A line list for pure rovibrational transitions in the X 2Σ+ state is also presented.

MRCI potential curve and analytical potential energy function of the ground state of H 2

The potential energy curve of the ground electronic state of H 2 has been calculated using valence internally contracted multireference configuration interaction calculations and augmented correlation consistent basis sets(aug-cc-pV6Z).These give bond dissociation energy D e=4.7446 eV, which is very close to other theoretical values in the literature. The ab initio potential energies have been fitted to analytical potential energy function (APEF) using a 5-parameter Murrell–Sorbie potential energy function. Based on the APEF, we have calculated the spectroscopic parameters of the state and compared with the experimental data available at present.

Equilibrium Structure and Internal Rotation in B2F4 from Electron Diffraction and Spectroscopic Data and Quantum Chemical Calculations

Electron diffraction (ED) data for B2F4 recorded by Hedberg et al. over the temperature range −80 to +150°C have been used to obtain equilibrium geometry of this molecule in the framework of a large-amplitude motion model. The torsional coordinate has been adiabatically separated from the rest of vibrations. Two types of constraints applied to obtain ab initio torsional potential energy function (PEF) and the parameters of the geometry relaxation are discussed. The relations between anharmonic interaction force constants and the parameters of the geometry relaxation are briefly considered. Ab initio force constant matrices for rigid vibrational coordinates as well as large-amplitude torsional PEF have been scaled in the procedure of simultaneous fitting to the ED data and experimental vibrational frequencies. The resulting equilibrium geometry and potential function provided good fit to both ED and spectroscopic data. As expected, the results for the equilibrium geometry obtained from separate ED patterns recorded at different temperatures did not show noticeable temperature trend. The determined equilibrium structural parameters for B2F4 are: r e (B–B) = 1.719(4) A, r e (B–F) = 1.309(2) A, ∠BBF = 121.1(1)°. Uncertainties given in parentheses include three times standard deviation and a systematic error. The rotational barrier height was evaluated as 160(50) cm−1.

The Matrix Isolation Spectrum of the CH 2+ Ion

For CH 2 + molecular ions at 5 K we simulate the infrared absorption spectrum, and tabulate all strong absorption lines from 0 to 16 000 cm −1. We use ab initio potential energy, dipole moment, and transition moment surfaces in conjunction with our program system RENNER, which allows for the Renner–Teller effect and spin–orbit coupling in a full-dimensions calculation. This is done for the purpose of guiding our search for the matrix isolation spectrum; our attempts at finding this spectrum are also described.

Spectroscopy and dynamics of the isolated sp2 CH chromophore in trideuteroacetaldehyde CD3CHO as derived from extrapolated SDCI ab initio calculations

Ab initio CI potential energy (PES) and dipole moment (DMS) surfaces have been calculated with singles and doubles excitation configuration interaction (SDCI) for the 2-dimensional isolated sp2 CH chromophore subspace of trideuteroacetaldehyde (CD3CHO). Different extrapolation schemes to full-CI have been applied after the extrapolation to full-SDCI. Vibrational band centres and absolute intensities are determined variationally on six extrapolated surfaces. The band centres are analysed within the effective Hamiltonian model for isolated CH chromophores and the effective spectroscopic parameters are compared for the different extrapolation schemes. Time dependent population evolution for vibrational quantum motion with a pure CH stretching state being initially populated is calculated for some of the extrapolated surfaces within the effective Hamiltonian model.

Observation of a high lying van der Waals mode in the intermolecular potential of Ar-CO

A continuous two-dimensional jet expansion in combination with an infrared diode laser spectrometer has been used to record a new subband of the weakly bound complex Ar-CO in the 2168cm−1 region. Twenty-two transitions were assigned to a thus far unobserved K a = 0 van der Waals state at 36.765 cm−l above the ground state. This is the highest van der Waals mode of Ar-CO reported so far. These high lying energy levels will provide a very sensitive test for future ab initio and Semiempirical potential energy surfaces.

Vibrational and correlation effects on properties of water

Large basis set, ab initio potential energy and property surfaces of water have been used with quantum Monte Carlo vibrational analysis in the evaluation of the molecule's rotational constants, zero-point energy, and dipole moment. While there are clearly differences in vibrational state parameters due to including correlation effects, the vibrational averaging effect on rotational constants is very nearly additive with the correlation effect. This has implications for evaluation and estimation of properties of molecules in specific vibrational states.

An ab initio pair potential for Ne2 and the equilibrium properties of neon

Fourth-order Möller—Plesset quantum mechanical perturbation theory with all excitations up to quadruple included, MP4(SDTQ), is used to calculate an ab initio pair potential energy function for neon. This potential is in reasonable agreement with that obtained from experiment, having a well depth some 8.5% smaller but with a well position which agrees within experimental error. This pair potential is used to calculate the second virial coefficient, the room temperature isotherm and the compressibility factor for bulk neon. There is reasonable agreement with experimental values.

Ultrafast excitation process in NaNH 3: a combined theoretical and experimental analysis

The conditions for spectroscopic studies of fast decaying excited states with femtosecond laser pulses are examined both experimentally and by quantum dynamical calculations. The sodium-ammonia complex (NaNH 3) acts as a model system which can be treated in quantum chemical calculations and is of chemical relevance for metal-molecular interaction in polar liquids. The state selective dynamics is studied in a two colour pump-probe experiment. The fundamental wavelength of a Ti:sapphire generated femtosecond laser pulse excites the NaNH 3 complex and the second harmonic is used for ionization. The calculations performed on ab initio potential energy and transition dipole moment surfaces show that for laser intensities with a Rabi cycle less than π the pump-probe process is a pure two-photon ionization process. Under these conditions the observed ion signal reflects directly the population of the excited state and the observed wavelength dependence resembles the vibrational spectrum of the NaNH 3 complex.

Rovibrational states of the 1A1 ground electronic state of Si 3

The rovibrational spectrum of the ground electronic state (1A1) of Si3 has been calculated using QCISD(T)_FC electronic model, with a large contracted basis set. The three-dimensional ab initio discrete potential energy surface was fitted with a 6th order power series expansion, which incorporated a Dunham expansion variable. The χ2 for the fit was 6·7 x 10-4 au. The force field was embedded in the Eckart–Watson Hamiltonian in order to obtain variationally the low-lying vibrational and rovibrational wavefunctions and eigenenergies. The differences between the anharmonic and harmonic calculations for the fundamental modes v 1(a1),v 2(a1),v 3(b2) are 11, 2 and 20 cm-1respectively. Although the vibrational separations observed in photodetachment studies of Si3 - are 337 ± 10 cm-1 501 ± 10 cm-1, which are in close agreement with the calculated values of 343 cm-11 (two-quanta excitation in the bend mode) and 498 cm-1 (one-quantum excitation in the asymmetric stretch mode), respectively, it would be anticipated that the Franck–Condon factor would be zero for the latter transition, and so it would not be observed in this experiment.

Measurement of the K a = 1 (v co = 1)stretching mode in Ar—CO using a Herriott multipass cell

A Herriott multipass cell has provided a significant increase in the sensitivity of our computer controlled diode laser spectrometer for the investigation of van der Waals clusters in a continuous slit jet expansion. The assignment was then possible of 57 new lines to the perpendicular transition from the (v co, j, K a,n) = (0,2,2,0) ground state of the Ar—CO van der Waals complex to the combination band of the CO fundamental vibration (v co = 1) and the K a = 1 (j = 1) stretching mode (1, 1, 1, 2). The band origin of 2159·88 cm-1 locates the energy of the (1,1,1,2) stretching mode at 26 ·19cm-1. This is the highest van der Waals mode observed in Ar—CO so far. An analysis of the rotational constants resulted in a level ordering of this state relative to the nearby (1, 3, 2, 1) state, which is in contradiction to existing ab initio calculations. The experimental results demonstrate that even these high lying van der Waals modes still show relatively regular behaviour, and therefore will provide a sensitive test for future ab initio and semiempirical potential energy surfaces.

Large amplitude vibrations in the HFClF complex

The intermolecular large amplitude motions of the HF...ClF complex were studied theoretically. The ab initio intermolecular potential energy surface, calculated at the MP2/DZ+(2d1f/2p1d)//HF/ DZP level of theory including BSSE correction, was used in the solution of the dynamical equations. The frequency for the van der Waals stretching and the frequency and the intensity of the HF...Cl bending were calculated. The frequency results were compared with reported theoretical and the experimental harmonic values. The harmonic stretching frequency calculated is 97.60 cm-1, which is in excellent agreement with the experimental harmonic value of 100 ± 2 cm-1. The frequency value calculated including anharmonicity is 89.38 cm-1. The calculated fundamental frequency of the HF...Cl bending vibration is 13.12 cm-1, while harmonic ab initio calculations predicted a value of 72 cm-1. This significant difference is due to tunneling motion through the HF...Cl angle, which has a barrier to internal rotation of 151 cm-1. The intensities of these transitions at two temperatures have also been calculated. At 10 K, only the fundamental transition is significant, whereas the hot band transition of 81.30 cm-1 between the third and fourth levels dominates the spectrum at 300 K.

On the validity of marcus relationship for dissociative electron transfer reactions in solution

Abstract In this paper Monte Carlo simulations of a dissociative electron transfer reaction in solution have been performed in order to test to what extent the deviation of Marcus' relationship for this kind of reactions can be attributed to the features of the solute internal potential energy. It is shown that Marcus' equation does not apply to this kind of dissociative electron transfer processes if the ab initio solute internal potential energies are used. Conversely, if a Morse curve and its repulsive part are employed to describe the solute internal potential energies, Marcus' relationship is recovered. Then, the problem is reduced to what extent the ab initio solute internal potential energies can be represented by Morse curves.

Laser Applications in Chemistry and Biology: Stimulation, Observation, and Manipulation

AbstractResults from three different types of laser‐based experiments are presented. Using translationally‐excited H atoms, the reaction dynamics of H(2S) + O2(3Σg−) → OH(2Π) + O(3P) was investigated. In order to study the influence of selective reagent translational excitation on the reactivity, absolute reaction cross sections and nascent OH product quantum‐state distributions were measured at different collision energies by means of the laser photolysis/laser‐induced fluorescence (LP/LIF) “pump‐probe” technique. The measured absolute reaction cross sections: σR(1.0eV) = (0.20 ± 0.08) Å2, σR(1.6 eV) = (0.58 ± 0.17) Å2, and σR(1.9 eV) = (0.35 ± 0.14) Å2 clearly confirm the presence of a pronounced maximum in the excitation function (Keßler, K.; Kleinermanns, K. J. Chem. Phys. 1992, 97: 374). The experimental results are compared with results from quasiclassical and recent 3D quantum scattering calculations on ab initio potential energy surfaces.In addition, results from experiments are reported in which two‐dimensional laser light sheet techniques were applied to investigate both the NO formation in a domestic natural gas burner and fuel mixing processes in combustion engines. Images of temperature and NO concentration distributions, as well as series of images of the fuel distribution, are presented.Finally, recent results of laser applications in biology are reported. A new concept in biodiagnostics using multiplex dyes, which have different characteristic fluorescence lifetimes but identical excitation and emission spectra, to distinguish different biomolecules has been developed. First experiments are described in which multiplex dyes in capillaries were identified by steady‐state fluorescence spectroscopy using a diode laser system. In combination with a pattern recognition technique, a detection limit of about 100 molecules per 25 picoliters detection volume was determined. The possible application in DNA sequencing is discussed. Results from laser‐induced cell fusion experiments, in which the laser nanoscalpel and photonic tweezers techniques were combined, are also presented.

The Cs(7P)+H2→CsH+H reaction. II. Rotationally resolved total cross sections

The Cs(7P)+H2→CsH(X 1Σ+)(v″=0,J″)+H harpooning reaction has been studied in a crossed-beam experiment, with electronic excitation of Cs atoms and laser-induced fluorescence detection of CsH products performed in a coherent saturation regime. Measurements of reactive cross sections have been achieved as a function of two parameters: The collision energy Ec (0.015≤Ec≤0.15 eV) and the rotational quantum number J″ of CsH products (0≤J″≤16, in v″=0). For Ec=0.09 eV, the total reactive cross sections corresponding to the two 7P fine structure levels of Cs are σ(Cs(7P1/2)+H2)=4.2×10−16 cm2 and σ(Cs(7P3/2)+H2)=10−16 cm2. These cross sections are much smaller than expected for a harpooning reaction. This is interpreted from ab initio potential energy and quantal dynamics calculations which show that the efficiency of the photochemical reaction (Cs atoms in the 7P state) is limited by the quenching processes associated with the underlying potential energy surfaces connected with Cs atoms in the 6S, 6P, 5D, and 7S states. From the present measurements, one deduces that the rate of these quenching processes is larger than 90%. The rotational distributions of products in v″=0 measured at two collision energies (Ec=0.045 and 0.09 eV) are close to ‘‘prior’’ statistical distributions, with the same ‘‘surprisal.’’ The energy dependence of rotationally resolved cross sections (J″ fixed, Ec variable) confirms the reaction thresholds and shows that there is no potential barrier higher than 0.015 eV on the entrance valley and the exit valley of the reactive system; it is in agreement with the dependence predicted by hemiquantal dynamical calculations performed in the frame of a harpooning model. The uncertainty on the measurement of absolute cross sections is larger than 70%, but of the order of 20% for relative cross sections.

Calculated rotational and rovibrational spectra of D 2S and HDS

Rovibrational energy levels, transition frequencies, and linestrengths are computed variationally for the sulfur hydrides D 2S and HDS, using ab initio potential energy and dipole surfaces. Wave-numbers for the pure rotational transitions agree to within 0.2 cm −1 of the experimental lines. For the fundamental vibrational transitions, the band origins for D 2S are 860.4, 1900.6, and 1912.0 cm −1 for ν 2, ν 1, and ν 3, respectively, compared with the corresponding experimental values of 855.4, 1896.4, and 1910.2 cm −1. For HDS, we compute ν 2 to be 1039.4 cm −1, compared with the experimental value of 1032.7 cm −1. The relative merits of local and normal mode descriptions for the overtone stretching band origins are discussed. Our results confirm the local mode nature of the H 2S, D 2S, and HDS system.

A theoretical study of transition state spectroscopy: Laser dressed potential energy surface and surface hopping trajectory calculations on K+NaCl and Na+KCl

Spectroscopy during the chemical reactions, K+NaCl and its reverse, has been studied by surface hopping trajectory calculations. Laser absorption and emission processes are modeled as the transitions between the laser-dressed ground and excited state potential energy surfaces (PESs), which are constructed from ab initio potential energy and transition dipole functions. The theoretical excitation spectrum measured by Na–D emission intensity as a function of laser wavelength agrees qualitatively with the experiment by Maguire et al. [J. Chem. Phys. 85, 844 (1986)]. The excitation spectrum is found to be very different from the absorption spectrum, because only a small portion of excited trajectories reach the Na* product due to the endothermicity of the excited state reaction. Therefore the excitation spectrum reflects only the excited state dynamics but not the transition state spectroscopy. The laser wavelength dependence of the spectra is well explained by a characteristic shift of the crossing seam accompanied with changes in laser wavelength. We have also predicted the absorption and excitation spectra for the reverse reaction and found that in this case the intensity of the product emission as a function of laser wavelength reflects to a large extent the true transition state spectroscopy.

Isotope Effects and Concerted Two-Hydron-Transfer between Formamidine Molecules

With the intention to assist mechanistic studies of two-hydron-transfer reactions using 1H/2H isotope effects we have carried out theoretical studies of the two-hydron-transfer reaction between formamidine molecules. The ab initio STO-3G potential energy surface has been calculated and all stationary points characterized by their force constant matrices. With this basis set the two hydrons are transferred concertedly and synchronously. The transition state (TS) has D2h symmetry. 1H/2H isotope effects were calculated using the Bebovib-IV program. Our results deviated from predictions made from common simple vibrational models. The isotope effects are dominated by the zero-pointenergy contribution. The effect due to the stretching vibrations of the initial state is to a small extent counteracted by stretching of the TS. The bending vibrations appear to play a dominant role in reducing the magnitude of the isotope effect for the two-hydron-transfer reaction to a value expected for a one-hydron-transfer reaction.