Rotational spectroscopic and ab initio studies of the Xe-H2O van der Waals dimer

An ab initio potential energy surface of the Xe-H(2)O van der Waals dimer was constructed at the coupled cluster level of theory with single, double, and pertubatively included triple excitations. For the Xe atom, the small-core pseudopotential and augmented correlation-consistent polarized valence quadruple-zeta (aug-cc-pVQZ-PP) basis set was used. Dunning's augmented correlation-consistent polarized valence triple-zeta (aug-cc-pVTZ) basis set was chosen for O and H atoms. Midbond functions were used to supplement the atom-centered basis sets. Rotational spectra of the Xe-H(2)O van der Waals dimer were recorded with a pulsed-nozzle Fourier transform microwave spectrometer. Rotational transitions within two internal rotor states, namely, the 0(00) and 1(01) states, were measured and assigned. Nuclear quadrupole hyperfine structures due to the (131)Xe (I = (3)/(2)), D (I = 1) and (17)O (I = (5)/(2)) nuclei were also observed and analyzed. Information about the molecular structure and the H(2)O angular motions was extracted from the spectroscopic results with the assistance of the ab initio potential.     

Ab initio investigation of the NH(X)-N2 van der Waals complex

The NH-N(2) van der Waals complex has been examined at the CCSD(T) level of theory using aug-cc-pVDZ and aug-cc-pVTZ basis sets. The full basis set superposition error correction was applied. Two minimum energy structures were located for the electronic ground state. The global minimum corresponds to a linear geometry of the complex (NH-N-N), with D(e)=236 cm(-1) and R(c.m.)=4.22 A. The secondary minimum corresponds to a T-shaped geometry of C(2v) symmetry, where the nitrogen atom of the H-N moiety points toward the center of mass of the N(2) unit, aligned with the a-inertial axis of the complex. The binding energy and R(c.m.) value for the secondary minimum were 144 cm(-1) and 3.63 A, respectively. This potential energy surface is consistent with the properties of matrix-isolated NH-N(2), and it is predicted that linear NH-N(2) will be a stable complex in the gas phase at low temperatures.     

An ab initio study of van der Waals potential energy parameters for silver clusters

We employ ab initio calculations of van der Waals complexes to study the potential energy parameters (C(6) coefficients) of van der Waals interactions for modeling of the adsorption of silver clusters on the graphite surface. Electronic structure calculations of the (Ag(2))(2), Ag(2)-H(2), and Ag(2)-C(6)H(6) complexes are performed using a coupled-cluster approach that includes single, double, and perturbative triple excitations (CCSD(T)), M?ller-Plesset second-order perturbation theory (MP2), and spin-component-scaled MP2 (SCS-MP2) methods. Using the atom pair approximation, the C(6) coefficients for silver-silver, silver-hydrogen, and silver-carbon atom systems are obtained after subtracting the energies of quadrupole-quadrupole interactions from the total electronic energy.     

Ozone-water 1:1 complexes O3-H2O: an ab initio study

Ab initio MO calculations have been carried out for the ozone-water 1:1 complexes in order to elucidate the structures and electronic state of the complexes. The QCISD calculations indicated that three structures are obtained as stable forms of O(3)-H(2)O. The most stable structure of O(3)-H(2)O has C(s) symmetry where the central oxygen of O(3) and all atoms of H(2)O are located on the molecular C(s) plane. The dipole of H(2)O orients toward the central oxygen atom of O(3) (i.e., dipole orientation form). The other two forms are cis and trans forms of O(3)-H(2)O where all atoms are located on the molecular plane, and a hydrogen of H(2)O binds to one of the terminal oxygen atoms of O(3) by a hydrogen bond. The binding energies of O(3) to H(2)O for dipole, cis, and trans forms are calculated to be 2.39, 2.27, and 2.30 kcal/mol, respectively, at the QCISD(T)/6-311++G(3df,3pd)//QCISD/6-311++G((d,p) level. The dipole orientation form is more stable in energy than the cis and trans forms. Rotational constants for the dipole orientation form are calculated to be A = 11.897, B = 4.177, and C = 3.318 GHz which are in good agreement with the experimental values (A = 11.961, B = 4.174, and C = 3.265 GHz). The electronic states of O(3)-H(2)O were discussed on the basis of theoretical results.     

Ab initio calculations of H2-H2 potential surfaces near the van der Waals minimum

The interaction energy between two hydrogen molecules near the van der Waals minimum is computed, for four relative orientations, and for intermolecular distances ranging from 5.5 to 16 au. A partially optimized basis set limited to 52 independent gaussian functions was used throughout the energy calculations. A new method based on the polarized atomic orbital technique has been used to reduce subsequently the size of the CI calculations which makes this method tractable for heavier molecular systems than H₂-H₂.     

Microwave and ab initio studies of rare gas-methane van der Waals complexes

Rotational spectra of the weakly bound Kr-methane van der Waals complex were recorded using a pulsed molecular beam Fourier transform microwave spectrometer in the range from 3.5 to 18 GHz. Spectra of 25 isotopomers of Kr-methane were assigned and analyzed. For isotopomers containing CH4, 13CH4, and CD4, two sets of transitions with K = 0 and one with K = 1 were recorded, correlating to the j = 0, 1, and 2 rotational levels of free methane, respectively (j is the rotational angular momentum quantum number of the methane monomer). For isotopomers containing CH3D and CHD3, two K = 0 components were recorded, correlating to the j(k) = 0(0) and 1(1) rotational levels of free methane (k corresponds to the projection of j onto the C3 axis of CH3D and CHD3). The obtained spectroscopic results were used to derive van der Waals bond distance R, van der Waals stretching frequency nu(s), and the corresponding stretching force constant k(s). Nuclear spin statistical weights of individual states were obtained from molecular symmetry group analyses and were compared with the observed relative transition intensities. The tentatively assigned j = 2 transitions were more intense than predicted from symmetry considerations. This is attributed to a relatively large effective dipole moment of this state, supported by ab initio dipole moment calculations. Ab initio potential energy calculations of Kr-CH4 and Ar-CH4 were done at the coupled cluster level of theory, with single and double excitations and perturbative inclusion of triple excitations, using the aug-cc-pVTZ basis set supplemented with bond functions. The theoretical results show that the angular dynamics of the dimer does not change significantly when the binding partner of methane changes from Ar to Kr. The dipole moment of Ar-CH4 was calculated at various configurations, providing a qualitative explanation for the unsuccessful spectral searches for rotational transitions of Ar-CH4.     

Kinetic studies of the reactions of O2(b 1sigma(g)+) with several atmospheric molecules

Thermal rate coefficients for the removal (reaction + quenching) of O2(1sigma(g)+) by collision with several atmospheric molecules were determined to be as follows: O3, k3(210-370 K) = (3.63 +/- 0.86) x 10(-11) exp((-115 +/- 66)/T); H2O, k4(250-370 K) = (4.52 +/- 2.14) x 10(-12) exp((89 +/- 210)/T); N2, k5(210-370 K) = (2.03 +/- 0.30) x 10(-15) exp((37 +/- 40)/T); CO2, k6(298 K) = (3.39 +/- 0.36) x 10(-13); CH4, k7(298 K) = (1.08 +/- 0.11) x 10(-13); CO, k8(298 K) = (3.74 +/- 0.87) x 10(-15); all units in cm3 molecule(-1) s(-1). O2(1sigma(g)+) was produced by directly exciting ground-state O2(3sigma(g)-) with a 762 nm pulsed dye laser. The reaction of O2(1sigma(g)+) with O3 was used to produce O(3P), and temporal profiles of O(3P) were measured using VUV atomic resonance fluorescence in the presence of the reactant to determine the rate coefficients for removal of O2(1sigma(g)+). Our results are compared with previous values, where available, and the overall trend in the O2(1sigma(g)+) removal rate coefficients and the atmospheric implications of these rate coefficients are discussed. Additionally, an upper limit for the branching ratio of O2(1sigma(g)+) + CO to give O(3P) + CO2 was determined to be < or = 0.2% and this reaction channel is shown to be of negligible importance in the atmosphere.     

New ab initio potential energy surface for the (HOCO+-He) van der Waals complex

A three-dimensional potential energy surface has been calculated for the ground electronic state of the HOCO+-He system. The calculations were performed at the coupled electron pair approximation level with an extended basis set which ensures a balance between accuracy and feasability. The validity of the method and of the basis set was tested through calculations of the polarizability of the He atom and of the spectroscopic constants of the HOCO+ ion. The calculated potential energy surface has been fitted to a spherical harmonic expansion to facilitate calculations of rotational excitation of HOCO+ by collisions with He.     

Microwave and ab initio studies of the Xe-CH4 van der Waals complex

An ab initio potential-energy surface of the Xe-CH4 van der Waals complex was constructed at the coupled cluster level of theory with single, double, and perturbatively included triple excitations. The recently developed small-core pseudopotential and augmented correlation-consistent polarized valence quadruple-zeta basis set was used for the xenon atom and Dunning's augmented correlation-consistent polarized valence triple-zeta basis set for the other atoms. The basis sets were supplemented with bond functions. Dipole moments were also calculated at various configurations. Rotational spectra of the Xe-CH4 van der Waals complex were recorded using a pulsed-nozzle Fourier transform microwave spectrometer. The isotopomers studied include those of CH4,13CH4,CD4,CH3D, and CHD3 with the five most abundant Xe isotopes. Transitions within three internal rotor states, namely, the j=0,K=0; j=1,K=0; and j=2,K=1 states, were observed and assigned. Nuclear quadrupole hyperfine structures due to the presence of 131Xe(I=3/2) were detected and analyzed. It was found that the j=1,K=0 state is perturbed by a Coriolis interaction with a nearby j=1,K=1 state. For isotopomers containing CH3D and CHD3, the j=2 states are no longer metastable and could not be observed. The spectroscopic results were used to derive structural and dynamical information of the Xe-CH4 complex.     

用于计算机模拟的ab initio N2-H2O势

用6-31G^*^*基组对N2-H2O体系作了量子化学ab initio计算研究。采用模拟褪火(Simulated anneal)算法将所得的计算结果用于拟合一个适用于计算机模拟的解析势能函数。函数的形式为10-6的Lennard-Jones势加指数项校正。鉴于非极性溶质的排斥能对第一水合层的形成起着重要作用, 在拟合过程中对势能的排斥部分和吸引部分作了等权考虑。     

Approximate generation of full-dimensional ab initio van der Waals surfaces for high-resolution spectroscopy

A method for the generation of highly accurate, nearly-exact, full-dimensional interaction energy surfaces for weakly interacting subsystems is proposed. The method is based on the local expansion of the exact interaction energy surface in the Taylor series with respect to intramolecular coordinates. It is shown that without any significant loss of accuracy this expansion can be limited to a few low-order terms. This leads to significant savings in computations of the full-dimensional interaction energy surfaces. Also a method for the direct calculation of the interaction energy surface of reduced dimensionality, corresponding to averaging over the intramolecular vibrations, without explicit knowledge of the full-dimensional surface, is presented. The main ideas and computational features of the proposed scheme are comprehensively tested for the Ar-HF system.     

Semiclassical dynamics of the van der Waals states in O3(X1A1)

We present the analysis and the semiclassical quantization of the van der Waals states of ozone in the ground electronic state X1A1. Progressions of these states dominate the spectrum of O3 at threshold. Periodic orbits are used to perform assignment and quantization of the vibrational states. Semiclassical quantization is numerically accurate despite the fact that the classical phase space is chaotic while the nodal patterns of the quantum mechanical wave functions are regular. The lifetimes of recombination of the van der Waals states into the "normal" ozone are also discussed.     

Study of the Xe-NH3 van der Waals complex: high-resolution microwave spectra and ab initio calculations

An ab initio potential energy surface of the Xe-NH(3) van der Waals complex was constructed at the coupled cluster level of theory with single, double, and pertubatively included triple excitations. The small-core pseudopotential and augmented correlation-consistent polarized valence quadruple-zeta basis set was used for the Xe atom and Dunning's augmented correlation-consistent polarized valence triple-zeta basis set for the other atoms. The basis sets were supplemented with midbond functions. Rotational spectra of the Xe-NH(3) van der Waals complex were recorded using a pulsed-nozzle Fourier transform microwave spectrometer. Rotational transitions within two internal rotor states, namely, the Sigma0(0) and Pi1(1) (lower) states, were measured and assigned to the Xe-(14)NH(3) and Xe-(15)NH(3) isotopologues. For the deuterated isotopologues, only the Sigma0(0) states were observed. Two inversion components were observed for each state except for the "s" component of the Sigma0(0) state of the Xe-(14)NH(3) and Xe-(15)NH(3) isotopologues, which has a spin statistical weight of zero. Nuclear quadrupole hyperfine structures arising from the (14)N (nuclear spin angular momentum quantum number I=1) and (131)Xe (I=32) nuclei were detected and analyzed. The observed spectra suggest that the Pi1(1) (lower) state has lower energy than the unobserved Sigma1(1) state, in contrast to the case of Ar-NH(3).     

Reactivity enhancement of ultracold O(3P)+H2 collisions by van der Waals interactions

The role of van der Waals forces in O((3)P)+H(2)(upsilon=1,j=0) collisions is investigated theoretically at low and ultralow temperatures. Quantum scattering calculations have been performed for zero total angular momentum using the lowest London-Eyring-Polanyi-Sato double-polynomial (3)A(") potential-energy surface reported by [Rogers et al., J. Phys. Chem. A 104, 2308 (2000)] and its recent BMS1 and BMS2 extensions developed by [Brandao et al., J. Chem. Phys. 121, 8861 (2004)] which provide a more accurate treatment of the van der Waals interaction. Our calculations show that van der Waals forces strongly influence chemical reactivity at ultracold translational energies. The presence of a zero-energy resonance for the BMS1 surface is found to enhance reactivity in the ultracold regime and shift the Wigner threshold to lower temperatures.     

Five-dimensional ab initio potential energy surface and predicted infrared spectra of H2-CO2 van der Waals complexes

The authors present a new five-dimensional potential energy surface for H2-CO2 including the Q3 normal mode for the nu3 antisymmetric stretching vibration of the CO2 molecule. The potential energies were calculated using the supermolecular approach with the full counterpoise correction at the CCSD(T) level with an aug-cc-pVTZ basis set supplemented with bond functions. The global minimum is at two equivalent T-shaped coplanar configurations with a well depth of 219.68 cm-1. The rovibrational energy levels for four species of H2-CO2 (paraH2-, orthoH2-, paraD2-, and orthoD2-CO2) were calculated employing the discrete variable representation (DVR) for radial variables and finite basis representation (FBR) for angular variables and the Lanczos algorithm. Our calculations showed that the off-diagonal intra- and intermolecular vibrational coupling could be neglected, and separation of the intramolecular vibration by averaging the total Hamiltonian with the wave function of a specific vibrational state of CO2 should be a good approximation with high accuracy. The calculated band origin shift in the infrared spectra in the nu3 region of CO2 is -0.113 cm-1 for paraH2-CO2 and -0.099 cm-1 for orthoH2-CO2, which agrees well with the observed values of -0.198 and -0.096 cm-1. The calculated rovibrational spectra for H2-CO2 are consistent with the available experimental spectra. For D2-CO2, it is predicted that only a-type transitions occur for paraD2-CO2, while both a-type and b-type transitions are significant for orthoD2-CO2.     

Accurate ab initio determination of the van der Waals interaction in the X 2Σ+ ground state of LiHe

Accurate quantum-chemical ab initio calculations have been performed at the SCF and CEPA (coupled electron pair approximation) levels for the van der Waals interaction in the X ² Σ ⁺ ground state of LiHe. An extended basis set has been used and the counterpoise correction for the basis set superposition error (BSSE) has been applied. The calculated potential energy curve has a very shallow minimum at 11.56 a ₀ with a well depth of only 1.49 cm^?1. This is too small to allow for a bound vibrational level. The analysis of the results shows that the interaction mainly consists of the Pauli repulsion between Li(1s ²2s) and He (1s ²), which is decaying exponentially, and the attractive London dispersion energy. Van der Waals coefficients C₆, C₈, and C₁₀ have been determined by a least squares fit to the long-range part of the calculated potential curve.     

A new ab initio interaction energy surface and high-resolution spectra of the H2-CO van der Waals complex

A new four-dimensional intermolecular potential-energy surface for the H(2)-CO complex is presented. The ab initio points have been computed on a five-dimensional grid including the dependence on the H-H separation (the C-O separation was fixed). The surface has then been obtained by averaging over the intramolecular vibration of H(2). The coupled-cluster supermolecular method with single, double, and noniterative triple excitations has been used to calculate the interaction energy. The correlation part of the interaction energy has been obtained from extrapolations based on calculations in a series of basis sets. An analytical fit of the ab initio potential-energy surface has the global minimum of -93.049 cm(-1) at the intermolecular separation of 7.92 bohr for the linear geometry with the C atom pointing toward the H(2) molecule. For the other linear geometry, with the O atom pointing toward H(2), the local minimum of -72.741 cm(-1) has been found for the intermolecular separation of 7.17 bohr. The potential has been used to calculate the rovibrational energy levels of the para-H(2)-CO complex. The results agree very well with those observed by McKellar [A. R. W. McKellar J. Chem. Phys. 108, 1811 (1998)]: the discrepancies are smaller than 0.1 cm(-1). The calculated dissociation energy is equal to 19.527 cm(-1) and significantly smaller than the value of 22 cm(-1) estimated from the experiment. Predictions of rovibrational energy levels for ortho-H(2)-CO have also been done and can serve as a guidance to assign recorded experimental spectra. The interaction second virial coefficient has been calculated and compared with the experimental data.     

Theoretical studies of the N2O van der Waals dimer: ab initio potential energy surface, intermolecular vibrations and rotational transition frequencies

Theoretical studies of the potential energy surface and bound states were performed for the N(2)O dimer. A four-dimensional intermolecular potential energy surface (PES) was constructed at the CCSD(T) level with aug-cc-pVTZ basis set supplemented with bond functions. Three co-planar local minima were found on this surface. They correspond to a nonpolar isomer with slipped-antiparallel planar structure and two equivalent polar isomers with slipped-parallel planar structures. The nonpolar isomer is energetically more stable than the polar ones by 162 cm(-1). To assign the fundamental vibrational frequencies for both isomers, more than 150 vibrational bound states were calculated based on this PES. The orientation of the nodal surface of the wave functions plays an important role in the assignment of disrotation and conrotation vibrational modes. The calculated vibrational frequencies are in good agreement with the available experimental data. We have also found a quantum tunneling effect between the two equivalent polar structures in the higher vibrational excited states. Rotational transition frequencies of the polar structure were also calculated. The accuracy of the PES is validated by the good agreement between theoretical and experimental results for the transition frequencies and spectroscopic parameters.     

Additive intermolecular potentials from ab initio calculations: trends in Rg2–dihalogen van der Waals trimers

In the present study, the validity of the pairwise additivity of the interactions, derived from the Rg₂ and Rg-dihalogen CCSD(T) potentials, is investigated by means of ab initio electronic structure and quantum-mechanical calculations. The topology of the potential surfaces of three different types of Rg₂–dihalogen vdW complexes is studied and general trends within the Rg₂–dihalogen family are discussed. Calculations of vibrational energies, including all five intermolecular degrees of freedom, are performed on such pairwise-additive potentials. The results are compared with experimental data from high-resolution spectroscopy, and provide further information on the additivity of the intermolecular forces for the He₂-dihalogen trimers. Contribution to the Serafin Fraga Memorial Issue.     

Ab initio characterization of the Mg-HF van der Waals complex

The equilibrium structure and the three-dimensional potential energy surface of the Mg-HF van der Waals complex in its ground electronic state have been determined from accurate ab initio calculations using the coupled-cluster method, CCSD(T), in conjunction with the basis sets of triple- through quintuple-zeta quality. The core-electron correlation, high-order valence-electron correlation, and scalar relativistic effects were investigated. The Mg-HF complex was confirmed to be linear at equilibrium, with a vibrationless dissociation energy (into Mg and HF) D(e) of 280 cm(-1). The vibration-rotation energy levels of two isotopologues, (24)Mg-HF and (24)Mg-DF, were predicted using the variational method. The predicted spectroscopic constants can be useful in a further analysis of high-resolution vibration-rotation spectra of the Mg-HF complex.