Open-shell Van Der Waals Research Articles

Assessment of local response dispersion method for open-shell systems

The performance of the local response dispersion (LRD) method has been assessed for open-shell systems. The LRD method provided reasonable dispersion coefficients and atomic polarizabilities. In open-shell van der Waals complexes, the LRD method was found to improve interaction energies and geometries. Furthermore, the LRD method combined with the long-range corrected exchange functional reproduced the magnetic coupling constants as well as interaction energies of radical dimers. We conclude that this combination has a potential for application to magnetic materials based on organic radicals.

Symmetry-adapted perturbation theory based on unrestricted Kohn-Sham orbitals for high-spin open-shell van der Waals complexes

Two open-shell formulations of the symmetry-adapted perturbation theory are presented. They are based on the spin-unrestricted Kohn-Sham (SAPT(UKS)) and unrestricted Hartree-Fock (SAPT(UHF)) descriptions of the monomers, respectively. The key reason behind development of SAPT(UKS) is that it is more compatible with density functional theory (DFT) compared to the previous formulation of open-shell SAPT based on spin-restricted Kohn-Sham method of Żuchowski et al. [J. Chem. Phys. 129, 084101 (2008)]. The performance of SAPT(UKS) and SAPT(UHF) is tested for the following open-shell van der Waals complexes: He···NH, H(2)O···HO(2), He···OH, Ar···OH, Ar···NO. The results show an excellent agreement between SAPT(UKS) and SAPT(ROKS). Furthermore, for the first time SAPT based on DFT is shown to be suitable for the treatment of interactions involving Π-state radicals (He···OH, Ar···OH, Ar···NO). In the interactions of transition metal dimers ((3)Σ(u)(+))Au(2) and ((13)Σ(g)(+))Cr(2) we show that SAPT is incompatible with the use of effective core potentials. The interaction energies of both systems expressed instead as supermolecular UHF interaction plus dispersion from SAPT(UKS) result in reasonably accurate potential curves.

Probing chemical dynamics with negative ions

Experiments are reviewed in which key problems in chemical dynamics are probed by experiments based on photodetachment and/or photoexcitation of negative ions. Examples include transition state spectroscopy of biomolecular reactions, spectroscopy of open shell van der Waals complexes, photodissociation of free radicals, and time-resolved dynamics in clusters. The experimental methods used in these investigations are described along with representative systems that have been studied.

Selection of a quantum-chemical method and basis set for optimization of the complex ion Cu(H2O)+

Calculations of the open-shell van der Waals complex Cu(H2O)+ were carried out at 15 theoretical levels basing on the DFT theory in its variants B3LYP and BLYP. Five types of basis function were used (3-21G*, 3-21G**, 6-31G(d,p), 6-311G(d,p) and CEP-4G) each combined with one (+) or two (++) diffuse functions. The aim of the research was to find out the most accurate combination of method and basis set which predicts structural, energetic and vibration parameters closest to the experimentally found ones. Several experimental parameters were used as reference values.

Microwave and millimeter-wave spectroscopy of the open-shell van der Waals complex Ar–HO2

Pure rotational transitions of a rare gas atom-reactive open-shell triatom van der Waals complex Ar-HO2 have been observed by Fourier transform microwave spectroscopy. The transitions observed are of a type with K(a) = 0 and 1. Furthermore, by monitoring the change of the free induction decay signal of the a-type transitions, b-type transitions have been observed by a double resonance technique in the region 18-49 GHz. All these transitions provide us precise molecular constants. The r0 structure of Ar-HO2 has been determined by fixing the structure of the HO2 monomer. The determined structure is planar and almost T shaped, where the argon atom is slightly shifted to the hydrogen atom of HO2. The experimental data supplemented by high-level ab initio calculations indicate that the van der Waals bond of Ar-HO2 is relatively rigid. On the other hand, effects on the unpaired electron distribution by the complex formation are found to be fairly small, since the fine and hyperfine constants of Ar-HO2 are well explained by those of the HO2 monomer.

Vibrational deactivation of F2(ν=1,j=0) by 1H at very low energy

Abstract We present close coupling calculations of the vibrational quenching cross-sections and rate constants for the F2(ν=1,j=0) molecule colliding with 1H atoms. The open-shell van der Waals complex H–F2 is studied using the internally contracted multireference configuration interaction method and an extensive basis set. The potential energy surface describing the H–F2 complex is obtained by fitting the ab initio points using the reproducing kernel Hilbert space method (RKHS). As for the closed shell He–F2 complex, we find that this surface exhibits two minima, the well associated with the linear intermediate being deeper than the one associated with the T-shape one. The quenching cross-section exhibits an unusual peak at very low energy which we suggest to be due to the presence of the reactive channel. The similarities and differences with the He–F2 system are also discussed.

Potential energy surfaces and bound states for the open-shell van der Waals cluster Br–HF

Semiempirical potential energy surfaces for the lowest three electronic states of the open-shell complex Br–HF are constructed, based on existing empirical potentials for Kr–HF and Kr–Ne and coupled-cluster electronic structure calculations for Br–Ne. Coupled cluster calculations are also described for He–F, Ne–F and Ar–F. Electrostatic interactions that arise from the quadrupole of the Br atom and the permanent multipoles of HF are also included in the Br–HF surfaces. The well depth of the lowest adiabatic surface is found to be 670 cm−1 at a linear equilibrium geometry. The results of helicity decoupled and full close-coupling calculations of the bound states of the complex are also described. The ground state, with total angular momentum projection quantum number |P|=3/2, is found 435 cm−1 below dissociation to Br (2P3/2)+HF (j=0). The lowest-frequency intermolecular bending and stretching vibrations are predicted around 145 and 211 cm−1, respectively. Parity splittings are found to be extremely small for bound states with projection quantum number |P|=3/2. The relevance of the results to recently recorded spectra of Br–HF is discussed.

Microwave spectrum, structure, and internal motion of the open-shell van der Waals complex Ar–ClO2

The rotational spectrum of the open-shell complex argon–chlorine dioxide has been measured between 5–24 GHz with a pulsed nozzle Fourier transform microwave spectrometer. This complex displays a- and c-type spectra, the latter of which is shifted by internal motion of the ClO2 unit. Structure information has been gained from the analysis of the rotational constants and of the fine and hyperfine structure parameters of the Ar–35ClO2 and Ar–37ClO2 complexes. Changes of the electronic structure of ClO2 upon complexation are shown to be very small. The observed internal motion effect was analyzed with a model of an internal rotation of the ClO2 subunit around its a axis. In addition, the structure of the complex and the barrier hindering internal motion has also been explored by ab initio calculations. The results of the analysis of the structure and internal motion of Ar–ClO2 were compared with those of the similar argon complexes of SO2, O3, and NO2.

On the strongly bound B 3Π state of the CAr van der Waals complex: Bonding and predissociation

For the open shell van der Waals molecule, CAr, the potential energy curves (PECs) for the B 3Π and 1 5Σ− states, the B 3Π–1 5Σ− spin–orbit coupling and the B 3Π fine structure splitting are determined using multireference configuration interaction wave functions as large as ∼8 million configuration state functions. The B 3Π state is strongly bound, with De=5100 cm−1. Re(B 3Π)=3.7a0 and is considerably shorter than Re(X 3Σ−)=6.07 a0. The PEC for the repulsive 1 5Σ− state crosses that of the B 3Π state at Rx(1 5Σ−,B 3Π)=3.31a0 leading to spin–orbit induced predissociation. The B 3Π–1 5Σ− spin–orbit coupling is the result of valence-Ryberg mixing in the B 3Π state and is considerably enhanced by the heavy atom effect. The heavy atom effect is also reflected in a marked decrease in the fine structure splitting of the B 3Π state with increasing vibrational level. The implications of these results for using CAr(B 3Π) in laser induced fluorescence detection of CAr(X 3Σ−,v) are discussed.

Zero electron kinetic energy spectroscopy of the KrBr−, XeBr−, and KrCl− anions

Three rare-gas halide (RgX−) anions, KrBr−, XeBr−, and KrCl−, and the corresponding neutral, open-shell van der Waals complexes are studied with anion zero electron kinetic energy spectroscopy. The spectra for each system reveal well-resolved progressions in the low frequency vibrations of the anion and one or more of the three neutral electronic states accessed by photodetachment, providing a detailed spectroscopic probe of the Rg–X− and Rg–X interaction potentials. In the case of KrBr−, transitions to all three of the “covalent” neutral electronic states (the X1/2, I3/2, and II1/2 states) were observed. For XeBr−, transitions to the X1/2 and II1/2 neutral states were observed. For KrCl−, only the X1/2 state could be studied. From our data, we construct model potentials for the anion and each observed neutral state, and these are compared with other experimental and theoretical potentials.

Novel magnetic interactions in the open-shell NO–HF complex

The J ′=3/2 (f)− J ″=1/2 (f) rotational transition of the open-shell van der Waals complex NO–HF has been observed using a Fourier transform microwave spectrometer. The transition shows a rich hyperfine structure due to coupling of the NO electronic spin and orbital angular momenta with the N nucleus and also with both the H and F nuclei in a novel intermolecular magnetic hyperfine interaction. The N hyperfine coupling reveals the splitting of the Π state degeneracy into A ′ and A ″ states, with the A ′ state lower in energy.

Theory of rotational energy levels of open-shell complexes containing the O2 molecule

A new effective Hamiltonian is presented for the analysis of the high-resolution spectra of open-shell van der Waals complexes containing the O2 molecule. The effects of electron spin are included but the complications of nuclear spin and resultant nuclear spin splitting are neglected. The Hamiltonian is composed of the rotational, centrifugal distortion, and spin–spin interaction terms. The resulting energy levels are divided into two well-separated groups and the pattern is a complicated function of θ(the angle that the O2 molecule makes with the principal a axis of the complex)and φ (the azimuthal angle of the O2 out of the plane defined by the a and b axes of the complex). This model has been successfully applied to analyze the high-resolution spectrum of O2–N2O in the region of the N2O monomer ν3 vibrational band, which will be presented in a separate paper.

New and Unusual Bonding in Open Shell van der Waals Molecules Revealed by the Heavy Atom Effect: The Case of BAr

Calculations are reported for the excited 14Π and C2Δ electronic states of BAr and the (nominally 2s2p2) 4P and 2D states of atomic boron. It was found necessary to extend an augmented valence triple-zeta orbital basis set to correctly describe the C2Δ state. This enhancement is necessitated by the near degeneracy of the valence 2s2p2 2D and Rydberg 2s23d 2D electron configurations in atomic boron, both of which are required to describe BAr C2Δ. Unexpectedly strong bonding in the C2Δ state is found. This bonding is shown to have its root in a type of coordinate covalency, dative bonding, that is sensitive to electron correlation effects and is reflected in a large external heavy atom effect in the spin−orbit coupling of the 14Π and C2Δ states.

Open-shell van der Waals complexes of the coinage metals: Cu…H 2O, Ag…H 2O, Cu…H 2S, and Ag…H 2S

The interaction potential curves for complexes between the coinage metal atoms (Me ≡ Cu, Ag) and H 2X (X ≡ O, S) molecules are calculated for three selected configurations. Two of them represent H-bonded structures while the third corresponds to direct interactions between Me and the lone pair carrier of XH 2. Most calculations have been carried out at the level of the restricted open-shell Hartree-Fock (ROHF) formalism followed by a perturbation treatment of the electron correlation contribution (CASPT2). For some selected points the ROHF-based spin-adapted coupled cluster method (ROHF SA CCSD) has been used. The relativistic effects are accounted for through the mass-velocity and Darwin (MVD) corrections. We found remarkable differences in the size and the sign of MVD corrections to interaction energies for different configurations. For all structures bonded through the hydrogen of H 2X (H-bonded structures) the ROHF interaction potentials turn out to be essentially repulsive, the attractive force due to induction effects being negligible in comparison with the valence repulsion for all but very large intersystem distances. However, the direct Me…O interaction produces a significant minimum even at the level of the ROHF approximation. At the correlated levels of approximations all structures of all systems are attractive with binding energies of the order of 1 kcal mol −1. In the case of the water molecule complexes, lone pair interaction is favoured so that the Me…OH 2 bonding resembles the usual H-bonds with the Me atom playing the role of the electron acceptor. It is suggested that the Me-bonding in systems of the form Me…OH 2 may represent the coinage metal counterparts of the H-bonding. On the contrary, H-bonding is found to give the most stable structures for the Me…H 2S complexes, the interaction energies being slightly higher than those for the corresponding complexes with the water molecule.

Study of the ArBr−, ArI−, and KrI− anions and the corresponding neutral van der Waals complexes by anion zero electron kinetic energy spectroscopy

Three rare gas halide (RgX−) anions, ArBr−, ArI−, and KrI−, and the corresponding open-shell van der Waals complexes, ArBr, ArI, and KrI, were studied with anion zero electron kinetic energy (ZEKE) spectroscopy. Photodetachment of each anion accesses the three lowest-lying electronic states (the X1/2, I3/2, and II1/2 states) of the neutral complex. The spectra for each system reveal well-resolved progressions in the low frequency vibrations of the anion and the three neutral electronic states, providing a detailed spectroscopic probe of the Rg⋅X− and Rg⋅X interaction potentials. The line shapes observed in the ZEKE spectra are analyzed in terms of the line strengths of the underlying rotational transitions. From our data, we construct the potential energy curve for each neutral state as well as for the anion, and these interaction potentials are compared to potentials obtained from scattering and ion mobility experiments.

The microwave spectrum of the open-shell complex ArNO 2

The 5 05←4 04, 4 04←3 03, 3 03←2 02 and 2 02←1 01 rotational transitions of the open-shell van der Waals complex ArNO 2 have been observed using a Fourier-transform microwave spectrometer. The complex was produced by supersonic expansion of NO 2 gas in argon. The spectrum showed fine, magnetic hyperfine and electric quadrupole structure. The data have been fitted to a semi- rigid Hamiltonian to determine the molecular parameters for the K itA=0 state. We discuss the implications of these derived parameters for the electronic and geometric stucture of the complex.

Parity doubling in open-shell van der Waals complexes

In open-shell van der Waals complexes containing 2Π diatomic molecules, the splitting between states with the same angular momentum quantum numbers but different parity is sensitively dependent on the intermolecular potential. The splitting for states with angular momentum projection quantum number P = ± 1 2 has been investigated previously by Green and Lester, and shown to arise in third-order perturbation theory. We use the same model to calculate the splitting for states with P = ± 3 2 , which arises in fifth order. A formula previously used to estimate the ratio of the parity splittings in P = ± 1 2 and ± 3 2 states (Ohshima et al.) is shown to be incomplete, casting doubt on the resulting claim of compatibility between the measurements of these two splittings.

A perturbation theory guide to open-shell complexes: OH–Ar(X 2Π)

Perturbation theory is used to understand the experimentally observed stimulated emission spectra of OH–Ar(X 2Π). A useful zero-order Hamiltonian for an open-shell van der Waals complex is presented, and the most important perturbation terms are identified: rotational decoupling ( j⋅s), Renner–Teller coupling (V̂2), and a Coriolis interaction (J⋅j). This treatment reveals those parts of the Hamiltonian which are responsible for various unusual features in the spectra of open-shell complexes, such as the large parity splittings in certain vibrational bands and spin–orbit–induced predissociation of the OH–Ar(X 2Π) complex. The magnitude of the parity splitting is shown to be directly proportional to the change in the intermolecular potential when the odd electron in the free radical lies in or out of the O–H–Ar plane, the A′ and A″ surfaces. The measured splitting is used to infer the magnitude of the difference between the A′ and A″ potential-energy surfaces (∼12 cm−1) in the region sampled by the first excited bend.

Observation of the pure rotational spectra of the ArOH and ArOD complexes by a Fourier-transform microwave spectrometer

Pure rotational spectra of open-shell van der Waals complexes, ArOH and ArOD, have been observed for the first time by a pulsed-nozzle Fourier-transform microwave spectrometer. The complexes were produced in a pulsed electronic discharge in a water/Ar mixture, with subsequent expansion of the discharge products into a supersonic jet. The observed J =5/2–3/2 transitions for both the species showed P-type doublings, as well as magnetic hyperfine splittings due to the proton or deuterium nuclear spin. The hyperfine coupling constants of the complexes are much smaller than those of free OH or OD, showing that the complex is undergoing a large amplitude motion in the ground vibronic state with an average amplitude exceeding 50°.