Anab initiostudy of the potential energy surface and spectrum of Ar–CO

Automated Construction of Potential Energy Surfaces Suitable to Describe van der Waals Complexes with Highly Excited Nascent Molecules: The Rotational Spectra of Ar-CS(v) and Ar-SiS(v)

Some reactions produce extremely hot nascent products which nevertheless can form sufficiently long-lived van der Waals (vdW) complexes-with atoms or molecules from a bath gas-as to be observed via microwave spectroscopy. Theoretical calculations of such unbound resonance states can be much more challenging than ordinary bound-state calculations depending on the approach employed. One encounters not just the floppy, and perhaps multiwelled potential energy surface (PES) characteristic of vdWs complexes, but in addition, one must contend with excitation of the intramolecular modes and its corresponding influence on the PES. Straightforward computation of the (resonance) rovibrational levels of interest, involves the added complication of the unbound nature of the wave function, often treated with techniques such as introducing a complex absorbing potential. Here, we have demonstrated that a simplified approach of making a series of vibrationally effective PESs for the intermolecular coordinates-one for each reaction product vibrational quantum number of interest-can produce vdW levels for the complex with spectroscopic accuracy. This requires constructing a series of appropriately weighted lower-dimensional PESs for which we use our freely available PES-fitting code AUTOSURF. The applications of this study are the Ar-CS and Ar-SiS complexes, which are isovalent to Ar-CO and Ar-SiO, the latter of which we considered in a previously reported study. Using a series of vibrationally effective PESs, rovibrational levels and predicted microwave transition frequencies for both complexes were computed variationally. A series of shifting rotational transition frequencies were also computed as a function of the diatom vibrational quantum number. The predicted transitions were used to guide and inform an experimental effort to make complementary observations. Comparisons are given for the transitions that are within the range of the spectrometer and were successfully recorded. Calculations of the rovibrational level pattern agree to within 0.2% with experimental measurements.

van der Waals potential energy surfaces from the exchange-hole dipole moment dispersion model

The potential energy surfaces (PESs) of 28 simple van der Waals complexes, each consisting of a rare-gas (Rg) atom interacting with a linear molecule, are calculated using the exchange-hole dipole moment (XDM) dispersion model in conjunction with three base density functionals (HFPBE, PW86PBE, and a commensurate hybrid functional). Results are compared with literature coupled-cluster reference data. The quality of the computed PESs is assessed based on the positions of the global minima and the corresponding binding energies. Only the hybrid functional is found to provide generally reliable PESs. Dispersion-corrected HFPBE strongly underestimates the equilibrium intermolecular separations and predicts different global minima than the reference PESs for Rg–HCl, Rg–HBr, and two of the Rg–HCN complexes. Analysis of the binding-energy errors reveals that the performance of HFPBE degrades as the size of the Rg atoms increase down the group, while the performance of PW86PBE is significantly worse for strongly-polar molecules. PW86PBE, and to a lesser extent the hybrid, strongly overbind Kr–HF due to charge-transfer error. Despite this, the XDM-corrected hybrid functional displays the best overall error statistics and provides binding energies to within ca. 10 cm–1 of the coupled-cluster reference data at a greatly reduced computational cost.

Three-dimensional potential energy surface of Ar-CO

A three-dimensional intermolecular potential energy surface of the Ar-CO complex has been determined by fitting most of the previously reported spectroscopic data, where observed transition frequencies by microwave, millimeter-wave, submillimeter-wave, and infrared spectroscopy were reproduced simultaneously within their experimental accuracies. A free rotor model Hamiltonian considering all the freedom of motions for an atom-diatom system was applied to calculate vibration-rotation energies. A three-dimensional potential energy surface obtained by ab initio calculations at the CCSD(T)-F12b/aug-cc-pV5Z level of theory was parameterized by a model function consisting of 46 parameters. They were used as initial values for the least-squares analysis of the experimental data. A total of 20 parameters were optimized to reproduce all the spectroscopic data.

Understanding of matrix embedding: a theoretical spectroscopic study of CO interacting with Ar clusters, surfaces and matrices

Through benchmark studies, we explore the performance of PBE density functional theory, with and without Grimme's dispersion correction (DFT-D3), in predicting spectroscopic properties for molecules interacting with rare gas matrices.

Low-Temperature Rotational Relaxation of CO in Self-Collisions and in Collisions with Ne and He

The low-temperature rotational relaxation of CO in self-collisions and in collisions with the rare-gas atoms Ne and He has been investigated in supersonic expansions with a combination of resonance-enhanced multiphoton ionization (REMPI) spectroscopy and time-of-flight techniques. For the REMPI detection of CO, a novel 2 + 1' scheme has been employed through the A(1)Pi state of CO. From the measured data, average cross sections for rotational relaxation have been derived as a function of temperature in the range 5-100 K. For CO-Ne and CO-He, the relaxation cross sections grow, respectively, from values of approximately 20 and 7 A(2) at 100 K to values of approximately 65-70 and approximately 20 A(2) in the 5-20 K temperature range. The cross section for the relaxation of CO-CO grows from a value close to 40 A(2) at 100 K to a maximum of 60 A(2) at 20 K and then decreases again to 40 A(2) at 5 K. These results are qualitatively similar to those obtained previously with the same technique for N(2)-N(2), N(2)-Ne, and N(2)-He collisions, although in the low-temperature range (T < 20 K) the CO relaxation cross sections are significantly larger than those for N(2). Some discrepancies have been found between the present relaxation cross sections for CO-CO and CO-He and the values derived from electron-induced fluorescence experiments.

Accurate intermolecular ground state potential of the Ar-N2 van der Waals complex

After carrying out a systematic basis set convergence study, we evaluate several ground state potential energy surfaces of the Ar-N(2) van der Waals complex at the coupled cluster singles and doubles model including connected triples corrections. We use the aug-cc-pVXZ (X=5,Q,D) and the daug-cc-pVQZ basis sets augmented with a set of 3s3p2d1f1g (denoted 33211) and 3s3p2d2f1g (denoted 33221) midbond functions, respectively. aug-cc-pVTZ-33211 results were available in the literature. The aug-cc-pV5Z-33211 (daug-cc-pVQZ-33221) surface is characterized by a T-shaped minimum at R(e)=3.709 (3.701) A and of 99.01 (102.50) cm(-1), and a linear saddle point at 4.260 (4.257) A and D(e)=75.28 (79.73) cm(-1). These results are compared with the values provided by the semiempirical potentials available, and those of previous theoretical studies. The basis set convergence of the intermolecular potentials is also analyzed. From the potentials the rovibronic spectroscopic properties are determined. We study the basis set convergence of the rotational frequencies. The binding parameters that characterized the aug-cc-pVTZ-33211 surface are reasonable, but the surface is not good enough to evaluate the microwave spectra. The aug-cc-pVQZ-33211 basis set results considerably improve the triple zeta and are close to the aug-cc-pV5Z-33211. Considering the small differences between the quadruple and the quintuple zeta surfaces, the latter results can be expected to be close to convergence. At this level the differences with respect to the accurate experimental frequencies are in the order of 0.7%. In the case of the daug-cc-pVXZ-33211,33221 (X=5,Q,T,D) series, the convergence of the interaction energies with respect to basis set improvement is not so smooth. The errors in the frequencies obtained with the daug-cc-pVQZ-33221 basis set with respect to experiment are in the order of 0.4%.

Accurate intermolecular ground state potential of the Ne-HCl van der Waals complex

From an accurate ground state intermolecular potential energy surface we evaluate the rovibrational spectrum of the Ne-HCl van der Waals complex. The intermolecular potential is obtained by fitting a considerable number of interaction energies obtained at the coupled cluster singles and doubles including connected triple excitations level and with the augmented correlation consistent polarized valence quintuple zeta basis set extended with a set of 3s3p2d1f1g midbond functions. This basis set is selected after a systematic basis set study carried out at geometries close to those of the three main surface stationary points. The surface is characterized by two linear minima, i.e. Ne-ClH and Ne-HCl, with distances from the Ne atom to the HCl center of mass of 3.398 and 3.833 angstroms, respectively; and binding energies of -65.10 and -66.85 cm(-1), respectively. These results agree well with the experimental data available in contrast to previous theoretical results. The rovibrational spectra calculated for the different isotopic species are also compared to the experiments.

Accurate intermolecular ground state potential of the Ne–N2 van der Waals complex

Ab initio ground state potential energy surfaces are obtained from interaction energies calculated with the coupled cluster singles and doubles model including connected triples corrections [CCSD(T)] and the aug-cc-pVXZ (X=5,Q,T,D) basis sets augmented with two different sets of midbond functions (denoted 33221 and 33211). The aug-cc-pV5Z-33221 surface is characterized by a T-shaped 49.5 cm(-1) minimum at Re=3.38 Angstroms and a linear saddle point at 3.95 Angstroms with De=36.6 cm(-1). These results agree well with the values provided by the accurate semiempirical potentials available. The rovibronic spectroscopic properties are determined and compared to the available experimental data and previous theoretical results. We study the basis set convergence of the intermolecular potentials and the rotational frequencies. The aug-cc-pVTZ basis sets provide reasonable binding parameters, but seem not to be converged enough for the evaluation of the microwave spectra. The aug-cc-pVQZ basis sets considerably improve the triple zeta results. The differences between the results obtained with the aug-cc-pVTZ-33221 basis set surface and those with the aug-cc-pVQZ-33221 are smaller than those of the corresponding bases with the set of 33211 midbond functions. The aug-cc-pVQZ surfaces are close to the aug-cc-pV5Z, that are expected to be close to convergence. With our best surfaces the errors in the frequencies with respect to the accurate experimental results go down to 0.6%.

Fluorobenzene–argon ground-state intermolecular potential energy surface

The ground-state intermolecular potential energy surface for the fluorobenzene-argon van der Waals complex is evaluated using the coupled-cluster singles and doubles including connected triple excitations model, with the augmented correlation consistent polarized valence double-zeta basis set extended with a set of 3s3p2d1f1g midbond functions. In the surface minima the Ar atom is located above and below the fluorobenzene plane at a distance of 3.562 A from the fluorobenzene center of mass and at an angle of 6.33 degrees with respect to the axis perpendicular to the fluorobenzene plane. The corresponding binding energy is 391.1 cm(-1). Both these results and the eigenvalues obtained from the potential compare well with the experimental data available.

Fourier transform microwave spectroscopy of the Rg–SH(2Πi) complexes (Rg:Ne, Kr): Determination of the intermolecular potential energy surfaces

Pure rotational spectra of Ne-SH and Kr-SH have been studied by Fourier transform microwave spectroscopy. R-branch transitions in the lower-spin component (Omega=3/2) corresponding to a linear (2)Pi(i) radical were observed for J(")=1.5-4.5 in the region 11-25 GHz for Ne-SH and for J(")=1.5-6.5 in the region 5-20 GHz for Kr-SH, respectively, with parity doublings and hyperfine splittings associated with the H nucleus. Although the spectral pattern of Kr-SH is relatively regular, that of Ne-SH is irregular with the J dependence of the parity doublings quite different from other Rg-SH or Ar-OH complexes. Two-dimensional intermolecular potential energy surfaces (IPSs) for both of the species have been determined from the least-squares fittings of the observed rotational transitions utilizing results of high-level ab initio calculations. These IPSs reproduce the observed transition frequencies within the experimental error and provide accurate knowledge on the intermolecular interaction and internal dynamics. Systematic comparisons of Rg-SH complexes have clarified various features of this series of complexes.