Permutational symmetry, isotope effects, side crossing, and singlet-triplet splitting in anthracene⋅HeN (N=1, 2) clusters

Ab Initio Potentials for the Ground S0 and the First Electronically Excited Singlet S1 States of Benzene-Helium with Application to Tunneling Intermolecular Vibrational States

We present new ab initio intermolecular potential energy surfaces for the benzene-helium complex in its ground (S0) and first excited (S1) states. The coupled-cluster level of theory with single, double, and perturbative triple excitations, CCSD(T), was used to calculate the ground state potential. The excited state potential was obtained by adding the excitation energies S0 → S1 of the complex, calculated using the equation of motion approach EOM-CCSD, to the ground state potential interaction energies. Analytical potentials are constructed and applied to study the structural and vibrational dynamics of benzene-helium. The binding energies and equilibrium distances of the ground and excited states were found to be 89 cm-1, 3.14 Å and 77 cm-1, 3.20 Å, respectively. The calculated vibrational energy levels exhibit tunneling of He through the benzene plane and are in reasonable agreement with recently reported experimental values for both the ground and excited states [Hayashi, M.; Ohshima, Y. J. Phys. Chem. Lett. 2020, 11, 9745]. Prospects for the theoretical study of complexes with large aromatic molecules and He are also discussed.

Quantum Tunneling of a He Atom Above and Below a Benzene Ring

Van der Waals (vdW) complexes with helium atoms have deserved much attention for their intriguing quantum nature relevant to microscopic superfluidity. However, tunneling splitting, the clear signature of quantum delocalization of He atoms, has rarely been identified in any of the He-containing complexes. Here, UV excitation spectra of benzene-He were extensively examined with almost full rotational resolution to identify two weak vibronic bands with vibrational excitation energies of only ∼13 and ∼16 cm^-1. Each of rotational transitions appears to be split into doublets in the higher-frequency band. This splitting is attributed to quantum tunneling due to the delocalization of He spread over two minimum locations below and above the benzene ring. The magnitude of the tunneling splitting as well as the vibrational frequencies of the two vdW modes are compared with the reported theoretical prediction to quantitatively assess the intermolecular potential energy surfaces so far derived.

Fully quantal calculation of H2 translation-rotation states in (H2)4@5(12)6(4) clathrate sII inclusion compounds

The quantal translation-rotation (TR) states of the (p-H2)4@5(12)6(4) and (o-D2)4@5(12)6(4) hydrate clathrate sII inclusion compounds have been computed by nuclear-orbital/configuration-interaction methods. The model of these compounds in a rigid, high-symmetry 5(12)6(4) cage is treated in detail. The low-energy TR level structures of both isotopomers within this model are found to consist of states that can be readily described in terms of a small number of single-H2 and double-H2 excitation modes. The use of the high-symmetry results to facilitate the calculation and interpretation of (p-H2)4 and (o-D2)4 TR states in low-symmetry physically realizable 5(12)6(4) cages is also reported.

Nuclear-orbital/configuration-interaction study of coupled translation-rotation states in (H2)2@C70

The quantal translation-rotation states of two endohedral H(2) moieties in C(70) are computed by means of a nuclear-orbital/configuration-interaction method. H(2) "nuclear orbitals" are calculated as the translation-rotation eigenfunctions of one H(2) molecule interacting with C(70) and the mean field of the second H(2) molecule. Configurations are constructed as symmetrized bilinear products of these orbitals. These configurations are employed as the basis in which the matrix of the translation-rotation Hamiltonian of the cluster is computed and diagonalized. We show that this scheme allows for an efficient means to calculate the Hamiltonian matrix elements. We show that the configuration basis states represent excellent first approximations to the eigenstates of the species. Finally, we present results pertaining to the (H(2))(2)@C(70) low-energy translation-rotation level structure that can be understood in terms of a small number of H(2) excitation types.

Spectral shifts and helium configurations in 4He(N)-tetracene clusters

Spectral shifts of electronic transitions of tetracene in helium droplets are investigated in a theoretical study of (4)He(N)-tetracene clusters with 1 < or = N < or = 150. Utilizing a pairwise interaction for the S(0) state of tetracene with helium that is extended by semiempirical terms to construct a potential for the S(1) state of tetracene with helium, the spectral shift is calculated from path integral Monte Carlo calculations of the helium equilibrium properties with tetracene in the S(0) and S(1) states at T = 0 and at T = 0.625 K. The calculated spectral shifts are in quantitative agreement with available experimental measurements for small values of N (< or = 8) at T approximately 0.4 K and show qualitative agreement for larger N (10-20). The extrapolated value of the spectral shift in large droplets (N approximately 10(4)) is approximately 90% of the experimentally measured value. We find no evidence of multiple configurations of helium for any cluster size for either the S(0) or S(1) state of tetracene. These results suggest that the observed spectral splitting of electronic transitions of tetracene in large helium droplets is not due to the coexistence of static metastable helium densities, unlike the situation previously analyzed for the phthalocyanine molecule.

Exact, Born-Oppenheimer, and quantum-chemistry-like calculations in helium clusters doped with light molecules: The He2N2(X) system

Helium clusters doped with diatomic molecules, He(N)-BC, have been recently studied by means of a quantum-chemistry-like approach. The model treats He atoms as "electrons" and dopants as "nuclei" in standard electronic structure calculations. Due to the large mass difference between He atoms and electrons, and to the replacement of Coulomb interactions by intermolecular potentials, it is worth assessing up to what extent are the approximations involved in this model, i.e., decoupling of the BC rotation from the He-atom orbital angular momenta and Born-Oppenheimer separation of the BC stretch versus the He motions, accurate enough. These issues have been previously tackled elsewhere for the (4)He(2)-Br(2)(X) system, which contains a heavy dopant [Roncero et al., Int. J. Quantum Chem. 107, 2756 (2007)]. Here, we consider a similar cluster but with a much lighter dopant such as N(2)(X). Although the model does not provide the correct energy levels for the cluster, positions and intensities of the main detectable lines of the vibrotational Raman spectrum at low temperature are accurately reproduced.

Wave function delocalization and large-amplitude vibrations of helium on corrugated aromatic microsurfaces: tetracene.He and pentacene.He van der Waals complexes

We report accurate quantum three-dimensional calculations of highly excited intermolecular vibrational states of the van der Waals (vdW) complexes tetracene.He and pentacene.He in the S1 excited electronic state. The aromatic molecules were taken to be rigid and the intermolecular potential energy surfaces (IPESs) were modeled as a sum of atom-atom Lennard-Jones pair potentials. The IPESs are corrugated in the direction of the long (x) axis of the aromatic molecules, due to the presence of the symmetrically equivalent global double minimum for tetracene.He, and a triple minimum (central global minimum and two equivalent local minima) for pentacene.He, on each side of the aromatic plane. Both IPESs have two additional minor equivalent local minima further away from the center of the molecule. The vdW vibrational states analyzed in this work cover about 80% of the well depths of the IPESs. The mode coupling is generally weak for those states whose out-of-plane (z) mode is unexcited. However, the z-mode fundamental is strongly coupled to the short-axis (y) in-plane mode, so that the pure z-mode excitation could not be identified. The He atom exhibits large in-plane spatial delocalizaton already in the ground vdW vibrational state, which increases rapidly upon the excitation of the in-plane x and y modes, with little hindrance by the corrugation of the aromatic microsurfaces. For the vdW vibrational energies considered, the He atom spatial delocalization reaches Deltax and Deltay values of approximately 5 and 4 A, respectively, and is limited only by the finite size of the aromatic substrates. Side-crossing delocalization of the wave functions on both sides of the molecular plane is found at excitation energies >30 cm(-1), giving rise to the energy splittings of the pairs of states symmetric/antisymmetric with respect to the aromatic plane; the splittings show strong vdW vibrational mode specificity.

Multiple solvation configurations around phthalocyanine in helium droplets

Recent measurements of the emission spectrum of phthalocyanine solvated in superfluid helium nanodroplets exhibit a constant 10.3 cm(-1) splitting of each emission line relative to the absorption spectrum. This splitting has been attributed to two distinct helium environments near the surface of the phthalocyanine molecule. Rigid-body path-integral Monte Carlo provides a means of investigating the origin of the splitting on a detailed microscopic level. Path-integral Monte Carlo simulations of 4He(N)-phthalocyanine at 0.625 K with N ranging from 24 to 150 show two distinct helium configurations. One configuration is commensurate with the molecular substrate and the other is a triangular lattice. We investigate the energetics of these two configurations and use a method for calculating electronic spectral shifts for aromatic molecule-rare-gas clusters due to dispersive interactions to estimate the spectral splitting that would arise from the two helium configurations seen for N=150. The results are in reasonable agreement with the experimentally measured splitting, supporting the existence of two distinct local helium environments near the surface of the molecule in the nanodroplets.

Solvent configuration-interaction calculations of intermolecular states in molecule-(atom)N clusters: application to Br2-4HeN

We describe variational calculations of J=0 intermolecular states in Br(2)-(4)He(N) clusters. The method employed is analogous to configuration-interaction calculations in electronic-structure work and relies on the ability to express the intermolecular Hamiltonian H(v) as a sum of one- and two-body terms. A basis set is built up from solutions to the Schrödinger equation in which only the one-body terms of H(v) are included. These configurations are products of N=1 eigenstates. The matrix of H(v) in a symmetry-adapted configuration basis is then computed, the two-body terms of H(v) serving to couple different configurations. This computation involves integrals of dimension five or less. Filter diagonalization is then used to obtain energies and eigenfunctions within a selected energy range. Results on clusters having N=2-5 are reported.

REFLECTIONS ON PHYSICAL CHEMISTRY: Science and Scientists

▪ Abstract  This is the story of a young person who grew up in Tel-Aviv during the period of the establishment of the State of Israel and was inspired to become a physical chemist by the cultural environment, by the excellent high-school education, and by having been trained by some outstanding scientists at the Hebrew University of Jerusalem and, subsequently, by the intellectual environment and high-quality scientific endeavor at the University of Chicago. Since serving as the first chairman of the Chemistry Department of the newly formed Tel-Aviv University he has been immersed in research, in the training of young scientists, and in intensive and extensive international scientific collaboration. Together with the members of his “scientific family” he has explored the phenomena of energy acquisition, storage and disposal and structure-dynamics-function relations in large molecules, condensed phase, clusters and biomolecules, and is looking forward to many future adventures in physical chemistry.

“What to leave out and what to put in? That's the problem.” Hugh Lofting, Doctor Dolittle's Zoo, 1925

Raman spectra of (He)N-Br2(X) clusters: The role of boson/fermion statistics in a quantum solvent

The aim of this paper is to elucidate the role played by the bosonic/fermionic character of N He atoms solvating a Br2(X) molecule. To this end, an adiabatic model in the molecular stretching coordinate is assumed and the ground energy levels of the complexes are searched by means of Hartree (or Hartree-Fock) Quantum Chemistry calculations for 4He (or 3He) solvent atoms. Simulations of vib-rotational Raman spectra point at the spin multiplicity as the main feature responsible for the drastic difference in the rotational structures of molecules embedded in boson or fermion helium drops as already observed by the experiments of Grebenev et al. [S. Grebenev, J. P. Toennies, and A. F. Vilesov, Science 279 (1998) 2083].