Ab Initio Potential Surface Research Articles

Multiphoton Ion Pair Spectroscopy (MPIPS) with Ultrashort Laser Pulses for the H2 Molecule

Time-dependent Schrödinger equation, TDSE, simulations have been performed in order to prepare and study via MPIPS the evolution of vibrational wave packets on the ion pair electronic state potentials B''B1Sigma(u)(+) and Hh1Sigma(g)(+) of the H2 molecule. Using ab initio potential surfaces and transition moments, we present two- and three-photon excitation schemes with ultrashort pulses (tau <or= 10 fs) to prepare coherent superpositions of the two ion pair H(+)H(-) and H(-)H(+) states from the doorway B1Sigma(u)(+) state, which result from the strong radiative coupling between these two electronic states. The simulations are used to estimate the time evolution and recursion times of vibrational wave packets at large internuclear distances, usually not accessible by single-photon spectroscopy. Conditions for the localization of the ion pair states are proposed.

Hydrogen-bond assisted enormous broadening of infrared spectra of phenol-water cationic cluster: An ab initio mixed quantum-classical study

The infrared spectrum of phenol-water cationic cluster, [PhOH.H2O]+, taken by Sawamura et al. [J. Phys. Chem. 100, 8131 (1996)] is puzzling in that the peak due to the stretching mode of the phenolic OH (3657 cm-1 for a neutral monomer and 3524 cm-1 for PhOH.H2O) seemingly disappears and instead an extremely broad tail extending down to 2900 cm-1 is observed. The present authors theoretically ascribe this anomalous spectrum to an inhomogeneous broadening of the OH stretching peak caused by the hydrogen bond, the strength of which has been greatly enhanced by ionization of the phenyl ring. Indeed they estimate that the peak position is at 2300 cm-1 and the spectral width can become as wide as 1000 cm-1 at the cluster energy of 32 kcal/mol. This surprisingly wide broadening can be generic in hydrogen-bond systems, which in turn is useful to study the nature of the hydrogen-bond assisted dynamics in various systems such as those in DNA and proteins. To study the present system quantitatively, the authors have developed an ab initio mixed quantum-classical method, in which the nuclear motions on an adiabatic ab initio potential surface are treated such that only the OH stretching motion is described quantum mechanically, while all the other remaining modes are treated classically with on-the-fly scheme. This method includes the implementation of many numerical methodologies, which enables it to deal with a relatively large molecular system. With this theoretical method, the authors analyze the present anomalous broadening in a great detail. In particular, they suggest that one can extract direct information about the hydrogen-bond dynamics with respect to the clear correlation between the vibrational excitation energy of the OH stretching and intermolecular distance by means of a time-resolved infrared spectroscopy: Reflecting the slow and wide-range variation of the intermolecular distance of the relevant hydrogen bond, the time-resolved spectrum is predicted to vary (shift) largely covering the wide range of frequency domain. Thus, it is found that the short-time average along a selected trajectory sensitively reflects the change of the intermolecular distance. The authors also study the effect of internal energy on the hydrogen bonding and the OH spectrum.

Molecular dynamics simulation of the CH 4 and CH 4–H 2O systems up to 10 GPa and 2573 K

Based on our previous study of the intermolecular potential for pure H 2O and the strict evaluation of the competitive potential models for pure CH 4 and the ab initio fitting potential surface across CH 4–H 2O molecules in this study, we carried out more than two thousand molecular dynamics simulations for the PVTx properties of pure CH 4 and the CH 4–H 2O mixtures up to 2573 K and 10 GPa. Comparison of 1941 simulations with experimental PVT data for pure CH 4 shows an average deviation of 0.96% and a maximum deviation of 2.82%. The comparison of the results of 519 simulations of the mixtures with the experimental measurements reveals that the PVTx properties of the CH 4–H 2O mixtures generally agree with the extensive experimental data with an average deviation of 0.83% and 4% in maximum, which is equivalent to the experimental uncertainty. Moreover, the maximum deviation between the experimental data and the simulation results decreases to about 2% as temperature and pressure increase, indicating that the high accuracy of the simulation is well retained in the high temperature and pressure region. After the validation of the simulation method and the intermolecular potential models, we systematically simulated the PVTx properties of this binary system from 673 K and 0.05 GPa to 2573 K and 10 GPa. In order to integrate all the simulation results and the experimental data for the calculation of thermodynamic properties, an equation of state (EOS) is developed for the CH 4–H 2O system covering 673–2573 K and 0.01–10 GPa. Isochores for compositions <4 mol% CH 4 up to 773 K and 600 MPa are also determined in this paper. The program for the EOS can be downloaded from www.geochem-model.org/programs.htm.

Mixed quantum-classical molecular dynamics simulation of vibrational relaxation of ions in an electrostatic field

The vibrational relaxation of ions in low-density gases under the action of an electrostatic field is reproduced through a molecular dynamics simulation method. The vibration is treated though quantum mechanics and the remaining degrees of freedom are considered classical. The procedure is tested through comparison against analytic results for a two-dimensional quantum model and by studying energy exchange during binary ion-atom collisions. Finally, the method has been applied successfully to the calculation of the mobility and the vibrational relaxation rate of O2+ in Kr as a function of the mean collision energy using a model interaction potential that reproduces the potential minimum of a previously known ab initio potential surface. The calculation of the steady mean vibrational motion of the ions in (flow) drift tubes seems straightforward, though at the expense of large amounts of computer time.

Dynamics of ionization of H2 by Ne*(P3) investigated by electron spectroscopy

The Penning ionization reaction Ne*(2p(5)3s 3P)+H2-->[NeH2]+ +e- has been studied in crossed supersonic molecular beams with electron-energy analysis at four collision energies E = 1.83, 2.50, 3.16, and 3.89 kcal/mol. The electron kinetic-energy spectra, which directly reflect the ionizing transition region, show resolved peaks assignable to v' = 0-4 of H2+. The vibrational populations deviate systematically from Franck-Condon behavior, suggesting that the discrete-continuum coupling increases with H2 bond stretching. Each peak displays both increasing breadth and increasing blueshift with increasing E, and the blueshift also increases with increasing v'. The first two properties are consistent with a predominantly repulsive excited-state potential-energy surface, while the last is speculated to be a reflection of the rHH dependence of the ionic surface. Quantum scattering calculations based on ab initio potential surfaces for the excited and ionic states in spherical and infinite-order-sudden rigid rotor approximations are in semiquantitative agreement with the measurements. Discrepancies suggest changes in the imaginary, absorptive part of the excited surface, which probably can be best effected by multiproperty fitting calculations.

Evolution of Structure in CH5+ and Its Deuterated Analogues

Diffusion Monte Carlo simulations are used to investigate the effects of deuteration on the fluxionality of CH(5)(+) or CD(5)(+), using an ab initio potential surface, developed by Jin, Braams, and Bowman [J. Phys. Chem. 2006, 110, 1569]. We find that partial deuteration quenches the fluxional behavior. The spectral consequences are also investigated. We find that, while CH(5)(+) and CD(5)(+) are nearly spherical tops, partial deuteration breaks the rotational symmetry and the mixed isotopologues are generally better characterized as symmetric tops. In addition, we investigate the effects of deuteration on the low-resolution vibrational spectrum and anticipate that signatures of this delocalization will be observable in the vibrational spectrum.

Equation of state of the H 2O, CO 2, and H 2O–CO 2 systems up to 10 GPa and 2573.15 K: Molecular dynamics simulations with ab initio potential surface

Based on our previous development of the molecular interaction potential for pure H 2O and CO 2 [Zhang, Z.G., Duan, Z.H. 2005a. Isothermal–isobaric molecular dynamics simulations of the PVT properties of water over wide range of temperatures and pressures. Phys. Earth Planet Interiors 149, 335–354; Zhang, Z.G., Duan, Z.H. 2005b. An optimized molecular potential for carbon dioxide. J. Chem. Phys. 122, 214507] and the ab initio potential surface across CO 2–H 2O molecules constructed in this study, we carried out more than one thousand molecular dynamics simulations of the PVTx properties of the CO 2–H 2O mixtures in the temperature–pressure range from 673.15 to 2573.15 K up to 10.0 GPa. Comparison with extensive experimental PVTx data indicates that the simulated results generally agree with experimental data within 2% in density, equivalent to experimental uncertainty. Even the data under the highest experimental temperature–pressure conditions (up to 1673 K and 1.94 GPa) are well predicted with the agreement within 1.0% in density, indicating that the high accuracy of the simulation is well retained as the temperature and pressure increase. The consistent and stable predictability of the simulation from low to high temperature–pressure and the fact that the molecular dynamics simulation resort to no experimental data but to ab initio molecular potential makes us convinced that the simulation results should be reliable up to at least 2573 K and 10 GPa with errors less than 2% in density. In order to integrate all the simulation results of this study and previous studies [Zhang and Duan, 2005a, 2005b] and the experimental data for the calculation of volumetric properties (volume, density, and excess volume), heat properties, and chemical properties (fugacity, activity, and possibly supercritical phase separation), an equation of state (EOS) is laboriously developed for the CO 2, H 2O, and CO 2–H 2O systems. This EOS reproduces all the experimental and simulated data covering a wide temperature and pressure range from 673.15 to 2573.15 K and from 0 to 10.0 GPa within experimental or simulation uncertainty.

Deuteration Effects on the Structure and Infrared Spectrum of CH5+

The CH5+ molecular ion is well-known for its large amplitude motions that lead to complete scrambling of the hydrogen atoms, even in the vibrational ground state. Experiments have been reported that probe the consequences of these large amplitude motions. We recently reported that quantum zero-point effects partially quench the scrambling when CH5+ is partially deuterated. Here, the consequences of this quantum localization are investigated through calculations of the low-resolution spectra of CH4D+, CHD4+, and CD5+. The spectra are obtained by convoluting stick spectra, evaluated for individual stationary points on an ab initio potential surface, multiplying them by Diffusion Monte Carlo ground state density at that stationary point, and taking the sum. The CH/D stretch regions of CH4D+ and CD5+ are red-shifted relative to CH5+, while the overall shape of the envelope remains unaffected. In contrast, for CHD4+, the shape of the spectral envelope in the CH/D stretch region differs from the other three isotopologs. These signatures of the quantum localization of the deuterium on the spectra are discussed.

Three-dimensional time-dependent wave-packet calculations of OBrO absorption spectra

The absorption spectra of the C(2A2) <-- X(2B1) transition of the OBrO molecule are calculated using three-dimensional time-dependent wave-packet method in Radau coordinates for a total angular momentum J=0. The wave packet is propagated using the split operator technique associated with fast Fourier transform. Employing the basis functions obtained by one-dimensional Fourier grid Hamiltonian method, the initial wave packet is calculated directly on the three-dimensional Fourier grid. The numerical model is characterized by simplicity and efficiency. The ab initio potential surfaces for the C(2A2) and X(2B1) states are used in the calculation. The calculated absorption spectra of the C(2A2) <-- X(2B1) transition of OBrO molecule agree well with the experimental results.

Supersonically cooled hydronium ions in a slit-jet discharge: High-resolution infrared spectroscopy and tunneling dynamics of HD2O+

Jet-cooled high-resolution infrared spectra of partially deuterated hydronium ion (HD2O+) in the O-H stretch region (nu3 band) are obtained for the first time, exploiting the high ion densities, long absorption path lengths, and concentration modulation capabilities of the slit-jet discharge spectrometer. Least-squares analysis with a Watson asymmetric top Hamiltonian yields rovibrational constants and provides high level tests of ab initio molecular structure predictions. Transitions out of both the lower (nu3(+)<--0(+)) and the upper (nu3(-)<--0(-)) tunneling levels, as well as transitions across the tunneling gap (nu3(-)<--0(+)) are observed. The nu3(-)<--0(+) transitions in HD2O+ acquire oscillator strength by loss of D(3h) symmetry, and permit both ground-state-[27.0318(72) cm(-1)] and excited-state-[17.7612(54) cm(-1)]-tunneling splittings to be determined to spectroscopic precision from a single rovibrational band. The splittings and band origins calculated with recent high level ab initio six-dimensional potential surface predictions for H3O+ and isotopomers [X. C. Huang, S. Carter, and J. M. Bowman, J. Chem. Phys. 118, 5431 (2003); T. Rajamaki, A. Miani, and L. Halonen, J. Chem. Phys. 118, 10929 (2003)] are in very good agreement with the current experimental results.

Theoretical study of anharmonic vibrational spectra of HNO 3, HNO 3–H 2O, HNO 4: Fundamental, overtone and combination excitations

Vibrational frequencies are computed for the fundamental, OH stretching overtone and combination transitions of HNO 3, HNO 4 and the HNO 3–H 2O complex. The frequencies are computed directly from ab initio MP2 potential surface points, using the correlation corrected vibrational self-consistent field (CC-VSCF) method, which includes anharmonic effects. The results are compared with experimental data. The computed fundamental transitions are in accord with experiment. The main improvement over the harmonic approximation is for the OH stretching frequencies. The OH overtone excitations (up to the 3rd overtone) of HNO 3, HNO 4 are also in good accord with experiment. For overtone levels near the dissociation threshold, the deviations from experiment are larger, as the VSCF method is unsatisfactory for the extremely large anharmonicities in these cases. Finally, very satisfactory results are obtained for the combination mode transitions. The main conclusions are (1) CC-VSCF seems to work well also for low overtone excitations and for combination transitions. (2) The MP2/TZP potential surfaces, used in the CC-VSCF calculations, are by the test of spectroscopy adequate for these species. The results are encouraging for VSCF calculations of larger, related systems such as HNO 3–(H 2O) n , n > 1.

Quantum Scattering Calculations of the H + O2 → O + OH Reaction

We report quantum mechanical calculations of the H + O2 → OH + O reaction for total angular momentum quantum number J = 0 in the absence of recombination. Our calculations employ two different potential energy surfacesthe widely used semiempirical double many-body expansion (DMBE IV) potential surface of Pastrana et al. (J. Phys. Chem. 1990, 94, 8073) and an ab initio potential surface by Troe and Ushakov (J. Chem. Phys. 2001, 115, 3621) which is yet to be tested against detailed quantum dynamics calculations. We explore the sensitivity of the dynamics to details of the potential energy surface. The reaction is dominated by narrow resonances due to the formation of the HO2 radical, and results are sensitive to details of the potential energy surfaces. Thermal rate coefficients evaluated using a J-shifting approximation differ by about 50% on the two potential surfaces. Calculations show that, within the J-shifting approximation, the Troe−Ushakov potential surface yields rate coefficients that are in better agreement with experiments.

Spectroscopy of the OC-HF hydrogen-bonded complex at vHF=3.

The v(HF)=3 levels of the linear OC-HF complex are observed in the range of 10,800-11,500 cm(-1) using intracavity Ti-sapphire laser-induced fluorescence. The vibrational predissociation linewidths of both (30000) and (3001(1)0) states exceed 5 GHz; thus, the measured spectra are not rotationally resolvable. Under the assumption that these levels are not strongly perturbed, the rotational constants of the two levels are determined to be 0.1100(1) cm(-1) for (30000), 0.1081(1), and 0.1065(1) cm(-1) for f and e sublevels of (3001(1)0), respectively, through band contour fitting. The (30000)<--(00000) band origin is at 10,894.46(1) cm(-1), showing a HF wave number redshift of 478.3 cm(-1). The 4.07 redshift ratio of v(HF)=3 to that of v(HF)=1 indicates a significantly nonlinear increase of the intermolecular interaction energy through HF valence excitation. An ab initio interaction potential surface for HF valence coordinates varying from 0.8 to 1.25 A is used to examine vibrational dynamics. The HF valence vibration v(1) is treated perturbatively, showing that the vibrational redshifts are determined essentially in first order with only a very small second-order contribution. The (3001(1)0)<--(00000) combination transition is observed with the band origin at 11,432.66(1) cm(-1), giving the HF intermolecular bending mode to be 538.2 cm(-1). The high frequency of this vibration, compared to that in similar HF complexes, shows the strong angular anisotropy of the intermolecular interaction potential of OC-HF with respect to the HF subunit. The lifetime of the (3001(1)0) level increases to 28 ps from 14 ps for (30000).

Hydrogen Molecule Clusters

Ab initio calculations at the MP2 and coupled cluster levels have been performed for small clusters of hydrogen molecules. A concise model interaction potential was constructed on the basis of ab initio potential surface points for the dimer and on comparison with ab initio results for the trimer and tetramer. The model was used to investigate key features of cluster growth, starting with the aggregation energies for clusters with up to 10 molecules. Fully anharmonic zero-point vibrational energies were evaluated for these clusters and were found to be a sizable fraction of the well depth for small clusters. Calculations on larger clusters were used to follow the approach to regular structures in large aggregations, the energetics of cluster growth, and the internal rotation potential for hydrogen molecules embedded in a large cluster. It was found that the essentially free rotation involves concerted reorientation of hydrogen molecules in the first surrounding shell.

Vibrational dynamics of SCCl2 from the zero point to the first dissociation limitElectronic supplementary information (ESI) available: Potential constants (table) conversion matrix from normal coordinates to Cartesian coordinates (table) and correlation between two normal coordinate force fields (figure). See http://www.rsc.org/suppdata/cp/b4/b403114h/

Over one thousand ← vibronic transitions of the SC35Cl2 and SC35Cl37Cl isotopomers, originating from eight state vibrational levels, have been measured by dispersed fluorescence spectroscopy and stimulated emission pumping. The observed ground state vibrational energy levels involve all six vibrational modes, and have up to 28 quanta of combined excitation. The energy window extends from the fundamental vibrations to beyond the molecular dissociation limit near E/h = 600 THz. 375 ground state vibrational levels of SC35Cl2 have been assigned and fitted to an effective Hamiltonian. 35–37 chlorine isotopomer splittings are regular except for a few resonances when v1 ≥ 7 and v4 ≥ 6. Regular progressions up to 600 THz indicate the conservation of at least two good vibrational quantum numbers at the SCCl–Cl and SC–Cl2 dissociation limits. A subset of over 100 transitions up to E/h = 240 THz was used to refine a 6-D ab initio potential surface represented in bond distance coordinates. This surface, together with a previously computed state potential surface and transition dipole surface, also reproduces the spectral intensities of the ν1–ν4 polyads up to E/h = 240 THz. SCCl2 is an excellent model system for intramolecular vibrational redistribution (IVR) among skeletal vibrations ranging from isolated resonances to extensively mixed quantum states. The newly extended spectra show that half of the 6-D quantum state space remains highly structured up to the first dissociation limit near 600 THz, resulting in sub-exponential IVR dynamics except at short times.

The infrared spectrum of acetylene–HF in helium nanodroplets

Rotationally resolved infrared spectra are reported for the T-shaped acetylene–HF complex in helium nanodroplets, corresponding to excitation of both the H–F and asymmetric C–H stretches. In comparison to the isolated complex, rotation about the A-axis is relatively unhindered by the helium, while overall rotation is slower by a factor of approximately two. An ab initio potential surface is also reported for this system. Although the bent complex, with acetylene acting as a hydrogen donor, is determined to be a local minimum, only the T-shaped isomer is experimentally observed, indicating that the barrier is too small to prevent the metastable bent structure from rearranging to the global minimum.

Structure and stability of [C2H4N]+ singlet‐state cations: Comparison between DFT and high‐level ab initio calculations

AbstractThe stationary points of the [C2H4N]+ singlet potential energy surface have been characterized by means of B3LYP/6‐311+G(3df,2p) and G2(MP2) approaches. A comparison between both sets of results shows that, although both methods yield similar values for the relative stability of the local minima, they differ significantly regarding the stability of some transition states, in particular those corresponding to 1,3‐H shift processes. This implies that the dynamics of the corresponding reaction may exhibit substantial differences if based on B3LYP or high‐level ab initio potential surfaces. The attachment of N+ to the CC bond of ethylene is preceded by a charge transfer and, as a consequence, a symmetry breaking problem arises. Our results show that inclusion of high‐order correlation contributions through the participation of triple excitations may be crucial. The B3LYP method is not a good alternative to treat this problem due to the spin contamination of the unrestricted treatment. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2003

Theoretical studies of rotational and spin–orbit predissociation of the Ne·XH (X̃ 2Π, X = O,S) van der Waals complexes.

The rotational and spin–orbit predissociation dynamics of Ne·XH ( 2Π, X = O,S) complexes is investigated using the stabilization method of Mandelshtam et al. (Phys. Rev. A, 1994, 50, 2764) with ab initio potential surfaces. The calculations show large variations in the lifetimes of the Ne·XH complexes within narrow energy ranges. In particular, we found that the calculated lifetimes displayed strong dependencies on the parity of the state and on the projection of the angular momentum onto the internuclear axis. Comparisons with available experimental resonance energies and lifetimes are made and the results are found to be in good agreement.

An Experimental Investigation of Collisions of NH 3 with Para‐H 2 at the Temperatures of Cold Molecular Clouds

Experimentally measured cross sections for collisional broadening of ammonia by J = 0, para-H2 are presented for the (J,K) = (1, 1),(2, 2), and (3, 3) NH3 inversion transitions at temperatures from 12.5 to 40 K. The cross sections were obtained in a quasi-equilibrium environment utilizing the collisional cooling technique. These are the first laboratory measurements of interactions between NH3 and J = 0, para-H2 at the temperatures of cold molecular clouds. The measured cross sections are compared to theoretical predictions using two existing ab initio NH3-H2 potential surfaces and to previously measured He broadening cross sections. In comparison to He broadening cross sections at these temperatures, the para-H2 results are up to 4 times larger. This is in contrast to 300 K experimental results where para-H2 cross sections are only 50% larger than He.

State-to-State Differential Cross Sections by Velocity Mapping for Rotational Excitation of CO by Ne

State-to-state differential cross sections for rotational excitation of CO by Ne were determined in a crossed beam experiment at 511 cm-1. Scattered CO molecules were ionized with 2+1 resonance enhanced multiphoton ionization and detected with velocity mapping. Cross sections were determined for most of the energetically allowed final states of CO. The results were compared with predictions from two Ne−CO ab initio potential surfaces. The CCSD(T) surface of McBane and Cybulski [J. Chem. Phys. 1999, 110, 11734] predicts the positions of the rotational rainbow maxima more accurately than does the surface of Moszynski et al. [J. Phys. Chem. A 1997, 101, 4690].