Ab initio investigation of the CO-N2 quantum scattering: The collisional perturbation of the pure rotational R(0) line in CO.

We report fully quantum calculations of the collisional perturbation of a molecular line for a system that is relevant for Earth's atmosphere. We consider the N2-perturbed pure rotational R(0) line in CO. The results agree well with the available experimental data. This work constitutes a significant step toward populating the spectroscopic databases with ab initio collisional line-shape parameters for atmosphere-relevant systems. The calculations were performed using three different recently reported potential energy surfaces (PESs). We conclude that all three PESs lead to practically the same values of the pressure broadening coefficients.

Ab initio studies of the ground and first excited states of the Sr-H2 and Yb-H2 complexes.

Accurate intermolecular potential-energy surfaces (IPESs) for the ground and first excited states of the Sr-H2 and Yb-H2 complexes were calculated. After an extensive methodological study, the coupled cluster with single, double, and non-iterative triple excitation method with the Douglas-Kroll-Hess Hamiltonian and correlation-consistent basis sets of triple-ζ quality extended with 2 sets of diffuse functions and a set of midbond functions were chosen. The obtained ground-state IPESs are similar in both complexes, being relatively isotropic with two minima and two transition states (equivalent by symmetry). The global minima correspond to the collinear geometries with R = 5.45 and 5.10 Šand energies of -27.7 and -31.7 cm-1 for the Sr-H2 and Yb-H2 systems, respectively. The calculated surfaces for the Sr(3P)-H2 and Yb(3P)-H2 states are deeper and more anisotropic, and they exhibit similar patterns within both complexes. The deepest surfaces, where the singly occupied p-orbital of the metal atom is perpendicular to the intermolecular axis, are characterised by the global minima of ca. -2053 and -2260 cm-1 in the T-shape geometries at R = 2.41 and 2.29 Šfor Sr-H2 and Yb-H2, respectively. Additional calculations for the complexes of Sr and Yb with the He atom revealed a similar, strong dependence of the interaction energy on the orientation of the p-orbital in the Sr(3P)-He and Yb(3P)-He states.

Ab initio study of the CO-N2 complex: a new highly accurate intermolecular potential energy surface and rovibrational spectrum.

A new, highly accurate ab initio ground-state intermolecular potential-energy surface (IPES) for the CO-N2 complex is presented. Thousands of interaction energies calculated with the CCSD(T) method and Dunning's aug-cc-pVQZ basis set extended with midbond functions were fitted to an analytical function. The global minimum of the potential is characterized by an almost T-shaped structure and has an energy of -118.2 cm-1. The symmetry-adapted Lanczos algorithm was used to compute rovibrational energies (up to J = 20) on the new IPES. The RMSE with respect to experiment was found to be on the order of 0.038 cm-1 which confirms the very high accuracy of the potential. This level of agreement is among the best reported in the literature for weakly bound systems and considerably improves on those of previously published potentials.

Theoretical Study of the Pyridine-Helium van der Waals Complexes.

In this study we evaluate a high-level ab initio ground-state intermolecular potential-energy surface for the pyridine-He van der Waals complex, using the CCSD(T) method and Dunning's augmented correlation consistent polarized valence double-ζ basis set extended with a set of 3s3p2d1f1g midbond functions. The potential is characterized by two symmetric global minima of -93.2 cm(-1) that correspond to geometries where the distance between the helium atom and the pyridine center of mass is 3.105 Å and the angle with respect to the pyridine c rotational axis is 3.9°. Six local minima can be observed for geometries with the helium atom in the plane cotaining the pyridine molecule. To further analyze the nature of the intermolecular interactions in the complex, we use symmetry-adapted perturbation theory (SAPT). Additional consideration of the pyridine-He2 complex provides a better insight into many-body nonadditive contributions to intermolecular interactions in systems with more helium atoms.

Small and efficient basis sets for the evaluation of accurate interaction energies: aromatic molecule-argon ground-state intermolecular potentials and rovibrational states.

By evaluating a representative set of CCSD(T) ground state interaction energies for van der Waals dimers formed by aromatic molecules and the argon atom, we test the performance of the polarized basis sets of Sadlej et al. (J. Comput. Chem. 2005, 26, 145; Collect. Czech. Chem. Commun. 1988, 53, 1995) and the augmented polarization-consistent bases of Jensen (J. Chem. Phys. 2002, 117, 9234) in providing accurate intermolecular potentials for the benzene-, naphthalene-, and anthracene-argon complexes. The basis sets are extended by addition of midbond functions. As reference we consider CCSD(T) results obtained with Dunning's bases. For the benzene complex a systematic basis set study resulted in the selection of the (Z)Pol-33211 and the aug-pc-1-33321 bases to obtain the intermolecular potential energy surface. The interaction energy values and the shape of the CCSD(T)/(Z)Pol-33211 calculated potential are very close to the best available CCSD(T)/aug-cc-pVTZ-33211 potential with the former basis set being considerably smaller. The corresponding differences for the CCSD(T)/aug-pc-1-33321 potential are larger. In the case of the naphthalene-argon complex, following a similar study, we selected the (Z)Pol-3322 and aug-pc-1-333221 bases. The potentials show four symmetric absolute minima with energies of -483.2 cm(-1) for the (Z)Pol-3322 and -486.7 cm(-1) for the aug-pc-1-333221 basis set. To further check the performance of the selected basis sets, we evaluate intermolecular bound states of the complexes. The differences between calculated vibrational levels using the CCSD(T)/(Z)Pol-33211 and CCSD(T)/aug-cc-pVTZ-33211 benzene-argon potentials are small and for the lowest energy levels do not exceed 0.70 cm(-1). Such differences are substantially larger for the CCSD(T)/aug-pc-1-33321 calculated potential. For naphthalene-argon, bound state calculations demonstrate that the (Z)Pol-3322 and aug-pc-1-333221 potentials are of similar quality. The results show that these surfaces differ substantially from the available MP2/aug-cc-pVDZ potential. For the anthracene-argon complex it proved advantageous to calculate interaction energies by using the (Z)Pol and the aug-pc-1 basis sets, and we expect it to be increasingly so for complexes containing larger aromatic molecules.

A high-accuracy theoretical study of the CH(n)P systems n = 1-3.

We have performed high-level electronic structure computations on the most important species of the CH(n)P systems n = 1-3 to characterize them and provide reliable information about the equilibrium and vibrationally averaged molecular structures, rotational constants, vibrational frequencies (harmonic and anharmonic), formation enthalpies, and vertical excitation energies. Those chemical systems are intermediates for several important reactions and also prototypical phosphorus-carbon compounds; however, they are often elusive to experimental detection. The present results significantly complement their knowledge and can be used as an assessment of the experimental information when available. The explicitly correlated coupled-cluster RCCSD(T)-F12 method has been used for geometry optimizations and vibrational frequency calculations. Vibrational configuration interaction theory has been used to account for anharmonicity effects. Basis-set limit extrapolations have been carried out to determine accurate thermochemical quantities. Electronic excited states have been calculated with coupled-cluster approaches and also by means of the multireference configuration interaction method.

Ab initio ground- and excited-state intermolecular potential energy surfaces for the NO-Ne and NO-Ar van der Waals complexes.

The ground- [NO(X(2)Π)] and excited-state [NO(A(2)Σ(+))] intermolecular potential energy surfaces (IPESs) of the NO-Ne and NO-Ar van der Waals complexes are evaluated using the RCCSD(T) spin-restricted coupled cluster method and d-aug-cc-pVQZ basis set extended with a set of 3s3p2d1f1g midbond functions. These bases are selected from the results of a systematic basis-set convergence study carried out for the NO(A(2)Σ(+))-Ar state. We fit the interaction energies to analytic functions and compare the results to those previously available. The NO-Ar (NO-Ne) IPESs are characterized by absolute minima of -120 and -75 cm(-1) (-58 and -5 cm(-1)) at the ground and first excited state, respectively, located close to the T-shaped geometries for the ground states and at linear dispositions in the case of the excited states. The potentials are further used in the evaluation of the rovibrational spectra of the complexes, and the results are compared to those available in the literature.

Ab initio ground state phenylacetylene-argon intermolecular potential energy surface and rovibrational spectrum.

We evaluate the phenylacetylene-argon intermolecular potential energy surface by fitting a representative number of ab initio interaction energies to an analytic function. These energies are calculated at a grid of intermolecular geometries, using the CCSD(T) method and the aug-cc-pVDZ basis set extended with a series of 3s3p2d1f1g midbond functions. The potential is characterized by two equivalent global minima where the Ar atom is located above and below the phenylacetylene plane at a distance of 3.5781 Å from the molecular center of mass and at an angle of 9.08° with respect to the axis perpendicular to the phenylacetylene plane and containing the center of mass. The calculated interaction energy is -418.9 cm(-1). To check further the potential, we obtain the rovibrational spectrum of the complex and the results are compared to the available experimental data.

Calculated nuclear magnetic resonance parameters for multiproton-exchange and nonbonded-hydrogen rotation processes in cyclic water clusters.

In this work we report, for the first time, calculations of nuclear magnetic resonance parameters for the processes of multiproton-exchange and nonbonded-proton rotations in small, cyclic water clusters. The simultaneous proton exchange induces a large decrease in the oxygen shielding constants in both clusters, with a mean value of -52.6 ppm for the water trimer and -50.1 ppm for the water tetramer. The (1(h))J(OH) coupling constant between an oxygen nucleus and exchanging proton decreases (in absolute value) along the path, changes sign, finally reaching a value of 5-7 Hz. The changes in the NMR parameters induced by the nonbonded proton rotations are smaller. The calculated dependencies of the intermolecular spin-spin coupling constants upon rotation reveal that the largest changes are expected for the couplings transmitted through the hydrogen bond between the rotating and neighboring molecule which acts as a proton donor. The symmetry-adapted perturbation theory (SAPT) interaction energy calculations for each dimer forming the water trimer have allowed us to relate a strength of interactions within pairs of water molecules with coupling constant values. The predicted maximal values of the interaction-energy terms (energetically unfavorable orientations of the constituent dimers) along paths correlate with the extremal values of the spin-spin coupling constants, which mostly correspond to the maximal couplings along pathways.

A computational study of the nuclear magnetic resonance parameters for double proton exchange pathways in the formamide-formic acid and formamide-formamidine complexes.

In this paper, we present density functional theory calculations to predict the NMR parameters for two model systems: the formamide-formic acid (FM...FA) and formamide-formamidine (FM...FI) complexes, where intermolecular double proton exchange occurs. For the first time, the NMR parameters have been calculated along the reaction paths of the proton transfers described by means of the intrinsic reaction coordinate (IRC) procedure. The most interesting one-bond spin-spin coupling constants, (1(h))J(XH), between migrating protons and heavier nuclei change character from intra- to intermolecular along the pathway. The maximal positive values of the reduced (1(h))K(XH) coupling constants correspond to the situation when they are intramolecular; they decrease along the path, change sign and reach small negative values, becoming intermolecular couplings. The differing character of the double proton exchange resulting from the synchronicity or asynchronicity of the process is reflected in the calculated NMR parameters. Surprisingly substantial values have been calculated for the six-bond intermolecular proton-proton (6h)J(HH) coupling constants between protons bound to the carbon atoms. A simple procedure consisting of removal of the proton(s) forming the hydrogen bonds has been employed to indicate an influence of hydrogen bonding on the intermolecular coupling constants. Some of the spin-spin coupling constants ((2h)J(XY)) are predominantly transmitted through hydrogen bonds and decrease with removal of the proton(s), while others ((4h)J(CC)) are less sensitive to the presence or absence of the protons of hydrogen bonding.

The water-nitric oxide intermolecular potential-energy surface revisited.

The two lowest energy intermolecular potential-energy surfaces (IPESs) of the water-nitric oxide complex are evaluated using the spin-restricted coupled-cluster R-CCSD(T) model and the augmented correlation-consistent polarized-valence triple-zeta basis set extended with a set of the 3s3p2d1f1g midbond functions. A detailed characterization of the IPESs for both the (2)A(') and (2)A(") electronic states in the C(s)-symmetry configurations of the complex is performed. The global minimum for the (2)A(') state represented by the lowest energy of -461.8 cm(-1) is deeper than the global minimum in the (2)A(") state with an energy of -435.2 cm(-1). To explore the physics of the interaction an open-shell implementation of the symmetry-adapted perturbation theory is employed and the results are analyzed as a function of the intermolecular parameters. The electrostatic term shows the strongest geometric anisotropy, while the exchange, induction, and dispersion contributions mostly depend on the intermolecular distance. The energy separation between the (2)A(') and (2)A(") states is largely dominated by electrostatic contribution for long intermolecular distances. In the region of short intermolecular distances the exchange part is as important as the electrostatic one and the induction and dispersion effects are also substantial.

Symmetry-Adapted Perturbation-Theory Interaction-Energy Decomposition for Hydrogen-Bonded and Stacking Structures.

This letter reports the computational ab initio studies on the stacked and hydrogen-bonded geometries of the uracil dimer and pyrimidine···p-benzoquinone complex with a special regard to the ratios of different interaction-energy terms calculated by means of the symmetry-adapted perturbation theory (SAPT). In the hydrogen-bonded systems the absolute value of the dispersion term constitutes approximately half of the absolute value of the total SAPT0 interaction energy, while in the stacking complexes the ratio of the dispersion to the total interaction energy is much larger, ca. 1.2-2.0. Our SAPT results are compared with the DFT-SAPT results published recently by the Hobza group (J. Chem. Phys. 2007, 127, 075104), and the role of the dispersion contribution in stacking and hydrogen-bonded arrangements is discussed. The methodological part of this letter presents the influence of counterpoise corrections in the optimization procedure on the geometries of the systems and the calculated SAPT contributions.

The properties of weak and strong dihydrogen-bonded D-H...H-A complexes.

The properties of six dihydrogen-bonded (DHB) dimers with the BeH2 molecule as a proton acceptor were calculated by MP2, CCSD(T) and B3LYP methods. The structural, energetic and spectroscopic parameters are presented and analyzed in terms of their possible correlation with the interaction energy and the intermolecular H...H separation. The symmetry-adapted perturbation theory (SAPT) calculations were performed to gain more insight into the nature of the H...H interactions. The studied complexes are divided into three groups based on the calculated intermolecular distances and the interaction energies which range from approximately -1 to -42 kJ mol(-1). The analysis of the interaction energy components indicates that, in contrast to conventional hydrogen bonds, the induction energy is the most important term in the BeH2NH4+ complex. On the other hand, there is no sharp boundary between the DHB complexes classified as hydrogen bonded and van der Waals systems. The complexation-induced changes in vibrational frequencies and in proton shielding constants show a relationship with the interaction energy. The values of the 2hJXH and 3hJBeX coupling constants correlate well with the interaction energy and with the intermolecular distance.

Theoretical studies of nuclear magnetic resonance parameters for the proton-exchange pathways in porphyrin and porphycene.

The nuclear magnetic resonance (NMR) parameters in porphyrin and porphycene have been calculated to investigate their changes during the process of proton exchange, using density-functional theory (DFT) for both the spin-spin coupling constants and the shielding constants. In addition, in calculations on the smaller 1,3-bis(arylimino)isoindoline molecule, we have tested the performance of our computational approach against experimental data. The calculated nuclear spin-spin coupling constants and shielding constants have been analyzed as functions of the progress of the proton transfer between two nitrogen atoms. The one-bond couplings between proton and nitrogen, dominated by the Fermi-contact term, decay steeply as the internuclear distance increases. The small changes in the intramolecular J(HH) coupling between two inner protons are mainly determined by the sum of relatively large spin-orbit terms. The isotropic shielding constant shows a strong deshielding of the nitrogen nuclei as the proton migrates away. Both the isotropic shielding of the exchanged protons and the shielding anisotropy exhibit a minimum close to the transition states.