Showing papers on "Ab initio quantum chemistry methods published in 1993"

Ab initio molecular dynamics for open-shell transition metals.

Abstract: We show that quantum-mechanical molecular-dynamics simulations in a finite-temperature local-density approximation based on the calculation of the electronic ground state and of the Hellmann-Feynman forces after each time step are feasible for liquid noble and transition metals. This is possible with the use of Vanderbilt-type ``ultrasoft'' pseudopotentials and efficient conjugate-gradient techniques for the determination of the electronic ground state. Results for liquid copper and vanadium are presented.

Ab initio energy-adjusted pseudopotentials for elements of groups 13-17

Abstract: Quasi-relativistic energy-adjusted ab initio pseudopotentials for the elements of groups 13–17 up to atomic number 53 (I) are presented together with corresponding energy-optimized valence basis sets. Test calculations for atomic excitation and ionization energies show the reliability of the derived pseudopotentials and basis sets.

Frozen density functional approach for ab initio calculations of solvated molecules

Abstract: A new density functional method for treatment of chemical processes in solution is presented. The method is based on freezing the electron density of the solvent molecules, while solving the eigenvalue problem for the solute Hamiltonian, which includes the effective potential of the solvent molecules. The method is developed and examined in the simple case of one solvent and one solute molecule. The results are encouraging and the deviation between the unfrozen and frozen treatments can be attributed to the kinetic energy functional used. The method can be implemented in ab initio calculations of solvation free energies, following a recent pseudopotential approach [Vaidehi et al., 19921.

Ab initio studies of cyclic water clusters (H2O)n, n=1–6. I. Optimal structures and vibrational spectra

Abstract: The optimal structures and harmonic vibrational frequencies of cyclic water clusters, (H2O)n, have been determined at the Hartree–Fock (for n=2–6) and second order perturbation theory (for n=2–4) levels of theory with an augmented correlation consistent double zeta basis set At the MP2 level this basis set yields very accurate results for the structure, dipole moment, and polarizability of the water monomer as well as results of comparable accuracy for the structure, binding energy, and harmonic vibrational frequencies of the water dimer The optimal structure of (H2O)4 and the harmonic frequencies of (H2O)3,4 are the first ones reported at a correlated level for these species Analysis of the structural trends reveals that the separation between neighboring oxygen atoms decreases exponentially with increasing cluster size The predicted R0(O–O) for the ring hexamer is less than 002 A shorter than the interoxygen separation in ice Ih Furthermore, the trends in the harmonic vibrational frequencies sugge

‘‘Ab initio’’ liquid water

Abstract: An ab initio molecular dynamics simulation of liquid water has been performed using density functional theory in the Kohn–Sham formulation and a plane wave basis set to determine the electronic structure and the forces at each time step. For an accurate description of the hydrogen bonding in the liquid, it was necessary to extend the exchange functional with a term that depends on the gradient of the electron density. A further important technical detail is that supersoft pseudopotentials were used to treat the valence orbitals of the oxygen atoms in a plane wave expansion. The structural and dynamical properties of the liquid were found to be in good agreement with experiment. The ab initio molecular dynamics also yields information on the electronic structure. The electronic feature of special interest is the lowest unoccupied molecular orbital (LUMO) of the liquid which is the state occupied by a thermalized excess electron in the conductive state. The main result of calculating the liquid LUMO is that it is a delocalized state distributed over interstitial space between the molecules with a significant admixture of the σ* orbitals of the individual water molecules.

Towards an accurate molecular orbital theory for excited states: Ethene, butadiene, and hexatriene

Abstract: A newly proposed quantum chemical approach for ab initio calculations of electronic spectra of molecular systems is applied to the molecules ethene, trans‐1,3‐butadiene, and trans‐trans‐1,3,5‐hexatriene. The method has the aim of being accurate to better than 0.5 eV for excitation energies and is expected to provide structural and physical data for the excited states with good reliability. The approach is based on the complete active space (CAS) SCF method, which gives a proper description of the major features in the electronic structure of the excited state, independent of its complexity, accounts for all near degeneracy effects, and includes full orbital relaxation. Remaining dynamic electron correlation effects are in a subsequent step added using second order perturbation theory with the CASSCF wave function as the reference state. The approach is here tested in a calculation of the valence and Rydberg excited singlet and triplet states of the title molecules, using extended atomic natural orbital (ANO) basis sets. The ethene calculations comprised the two valence states plus all singlet and triplet Rydberg states of 3s, 3p, and 3d character, with errors in computed excitation energies smaller than 0.13 eV in all cases except the V state, for which the vertical excitation energy was about 0.4 eV too large. The two lowest triplet states and nine singlet states were studied in butadiene. The largest error (0.37 eV) was found for the 2 1Bu state. The two lowest triplet and seven lowest singlet states in hexatriene had excitation energies in error with less than 0.17 eV.

Hartree-Fock study of phase changes in ZnO at high pressure.

Abstract: The total energy of ZnO as a function of unit cell volume has been calculated for the zinc-blende, wurtzite, and rocksalt structures by the ab initio all-electron periodic Hartree-Fock linear-combination-of-atomic-orbitals method using a large Gaussian basis set that was variationally optimized for the solid state. Extensive convergence tests with respect to cutoffs of the real-space Coulomb and exchange series were carried out to ensure that the calculations were performed with sufficient precision. The calculated structural properties (equilibrium lattice constant, bulk modulus, etc.) of the wurtzite and rocksalt phases are in good agreement with experiment, as is the transition pressure between them (8.57 GPa versus 9--9.5 GPa experimentally), indicating that the method can reliably predict quite small energy differences between different densities or crystal structures of a nonmetallic solid. The calculated valence-band structure was also in satisfactory agreement with experiment. Detailed analysis of the charge-density distribution supports the expected picture of a transition from mixed ionic-covalent bonding in the tetrahedrally coordinated structures to predominantly ionic bonding in the high-pressure phase.

Ab initio molecular dynamics with variable cell shape: Application to MgSiO3.

Abstract: We report the development of an ab initio constant pressure extended molecular dynamics method with variable cell shape. This is a symmetry conserving method which allows for efficient structural searches and optimizations in spaces with preselected symmetry groups. We have used it to investigate ${\mathrm{MgSiO}}_{3}$, a perovskite, the marjor Earth-forming mineral phase which exists particularly in the lower mantle. We predict its structural behavior up to pressures which exceed the highest values reached in this region.

Evidence for site-sensitive screening of core holes at the Si and Ge(001) surface

Abstract: Typically surface core-level shifts (SCLS) of clean surfaces are explained in the initial-state model, thus ignoring the screening of the photon-induced hole We will show that this approach is not valid for the (001) surfaces of Si and Ge Using ab initio density-functional theory we calculate the SCLS from differences of total energies of slabs containing excited atoms at different positions at the surface and in the bulk Comparison with initial-state results reveals an enhanced screening at the surface, which is even remarkably different for the two atoms forming the surface dimer

Bare transition metal atoms in the gas phase: reactions of M, M+, and M2+ with hydrocarbons

Abstract: This Account focuses on gas-phase measurements and high quality ab initio calculations that are beginning to explain how metal atom electronic structure determines chemical reactivity The authors have an enormous body of basic chemical reactivity data for M[sup +] and a growing body of data for M[sup 2+] and neutral M Sophisticated experiments can control the kinetic energy and the electronic state of M[sup +] reactants One can study the reactivity of Fe[sup +] in the 3d[sup 6]4s, high-spin ground state, the 3d[sup 7], high-spin first excited state, or the 3d[sup 6]4s, low-spin second excited state The authors have learned to follow the elimination of H[sub 2] and C[sub 2]H[sub 6] from bimolecular Ni[sup +](n-butane) complexes in real time, on a 50-ns time scale In M[sup +] reactions, the authors can control the kinetic energy and the electronic state of the reactants A key advantage in interpreting these results is that one understands the electronic structure of the bare metal atom reactants very well Solution-phase chemists might well question the relevance of atomic species with genuine 1+ or 2+ charges and no ligands or solvent to the [open quotes]real world[close quotes] of organometallic chemistry Yet connections surely exist, as witnessedmore » by the fact that Rh and Ir atoms are unusually reactive in all phases Theoretical chemists are beginning to provide a conceptual framework that will unify seemingly diverse branches of experimental chemistry Of necessity, the ab initio quantum chemist works on model transition metal systems, draws experimental evidence from all available sources, and tries to abstract from the calculations what is robust and common to all phases Gas-phase metal atoms are idealized model systems well matched to the capabilities of modern theory Many new conceptual insights in the next 10 years will come from careful analysis of ab initio wave functions informed by incisive gas-phase experiments 30 refs, 5 figs, 1 tab« less

Ab initio lattice dynamics of diamond

Abstract: We present a first-principles calculation of lattice dynamical properties of diamond. Our calculations have been performed using density-functional perturbation theory together with plane-wave expansion and nonlocal pseudopotentials. As a first step we have evaluated the equilibrium structure of diamond via the minimization of the total energy. Then, harmonic phonon dispersion curves and phonon eigenvectors have been evaluated within the linear-response framework. As a by-product of the calculation we have also obtained the internal-strain parameter. Furthermore, we have also tested the validity of the ab initio calculation for describing properties beyond the harmonic approximation. Using the quasiharmonic approximation we have calculated the thermal expansion coefficient and the mode Gr\"uneisen parameter dispersion curves. Where experimental data are available, good agreement is found with our theoretical predictions.

Study of methane, acetylene, ethene, and benzene using Kohn-Sham theory

Abstract: Self-consistent Kohn-Sham (KS) theory with local density approximation (LDA) and gradient-corrected exchange-correlation (BLYP) functionals are used to optimize the geometries and calculate the frequencies of methane, acetylene, ethylene, and benzene. Large basis sets are employed with accurate numerical quadrature. The predictions are compared with ab initio calculations and with experiment. KS (BLYP) bond lengths are a little long with the result that the stretching frequencies are too small, but good predictions are obtained for bending frequencies. In particular, the ω 14 B 2u mode of benzene is predicted to within 10 cm -1 , unlike all of the ab initio methods which apparently require multiconfiguration methodology

The structure of the water trimer from ab initio calculations

Abstract: The first fully optimized structure of the water trimer at the MP2 level of theory is reported. It corresponds to a cyclic chiral structure in which all O–O separations are equal to 2.80 A, the OαH...Oβ hydrogen bonds are nonlinear, and two of the terminal hydrogens lie on one side of the O–O–O plane and the third lies on the other. This structure is in qualitative agreement with that reported recently by Pugliano and Saykally [Science 257, 1937 (1992)]. However, the calculations predict the O–O separations to be substantially shorter than those used to fit the far‐infrared vibration–rotation–tunneling spectrum. Nonetheless, the computed structure reproduces the measured rotational constants of (D2O)3 ; the errors are ≤1% for A and B and 6% for C. An energy analysis yields a three‐body term of 2.3 kcal/mol (∼15% of De with respect to three isolated water molecules).

Efficient elimination of basis set superposition errors by the local correlation method: Accurate ab initio studies of the water dimer

Abstract: The water dimer has been studied by accurate ab initio calculations. The main purpose of the calculations was to investigate the magnitude of, and how to eliminate the basis set superposition errors at different levels of theory. At the Hartree–Fock level the superposition errors are insignificant with the largest basis sets, and the counterpoise method works well with all the basis sets used in this study. At the correlated level superposition errors are still significant even for very large basis sets, and the standard counterpoise technique leads to overcorrection. The most important result of the present study is that the local correlation methods gives essentially the correct result for the correlation contribution to the association energy even with modest basis sets. The association energy at the MP4(SDQ) level is predicted to be 4.8 kcal/mol. The correlation contribution to the association energy is 1.2 kcal/mol which can be decomposed into an attractive intermolecular contribution of 1.8 kcal/mol and a repulsive intramolecular contribution of 0.6 kcal/mol. Ionic terms contribute about 30% to the dispersion force at the equilibrium distance. If the effect of triple substitutions is taken into account the association energy is estimated to be around 5.1 kcal/mol.

Structures and Conformations of Non-Rigid Molecules

Abstract: From the beginnings of modern chemistry, molecular structure has been a lively area of research and speculation. For more than half a century spectroscopy and other methods have been available to characterize the structures and shapes of molecules, particularly those that are rigid. However, most molecules are at least to some degree non-rigid and this non-rigidity plays an important role in such diverse areas as biological activity, energy transfer, and chemical reactivity. In addition, the large-amplitude vibrations present in non-rigid molecules give rise to unusual low-energy vibrational level patterns which have a dramatic effect on the thermodynamic properties of these systems. Only in recent years has a coherent picture of the energetics and dynamics of the conformational changes inherent in non-rigid (and semi-rigid) molecules begun to emerge. Advances have been made in a number of different experimental areas: vibrational (infrared and Raman) spectroscopy, rotational (microwave) spectroscopy, electron diffraction, and, most recently, laser techniques probing both the ground and excited electronic states. Theoretically, the proliferation of powerful computers coupled with scientific insight has allowed both empirical and ab initio methods to increase our understanding of the forces responsible for the structures and energies of non-rigid systems. The development of theory (group theoretical methods and potential energy surfaces) to understand the unique characteristics of the spectra of these floppy molecules has also been necessary to reach our present level of understanding. The thirty chapters in this volume contributed by the key speakers at the Workshop are divided over the various areas. Both vibrational and rotational spectroscopy have been effective at determining the potential energy surfaces for non-rigid molecules, often in a complementary manner. Recent laser fluorescence work has extended these types of studies to electronic excited states. Electronic diffraction methods provide radial distribution functions from which both molecular structures and compositions of conformational mixtures can be found. Ab initio calculations have progressed substantially over the past few years, and, when carried out at a sufficiently high level, can accurately reproduce (or predict ahead of time) experimental findings. Much of the controversy of the ARW related to the question of when an ab initio is reliable. Since the computer programs are readily available, many poor calculations have been carried out. However, excellent results can be obtained from computations when properly done. A similar situation exists for experimental analyses. The complexities of non-rigid molecules are many, but major strides have been taken to understand their structures and conformational processes.

The structure of aniline by ab initio studies

Abstract: The structure of aniline has been studied by ab initio calculations. Complete geometry optimization of (1) the energy minimum structure and the transition states for (2) internal rotation and (3) inversion of the amino group were carried out at the SCF level using several different basis sets. For these three stationary geometries vibrational frequencies were calculated at the SCF/6-31G∗∗ level. The effect of electron correlation was estimated by single point MP4(SDQ) calculations using the 6-311G∗∗ basis set. To satisfactorily describe the conformation and orientation of the amino group a fully polarized (6-31G∗∗) basis set is required. It is predicted that the aniline molecule has a pyramidal amino group with an angle between the C-N bond and the NH2 plane of 42.3°. The angle between the C-N bond and the plane of the benzene ring is 2.0°. The barriers to inversion and internal rotation of the amino group are estimated to be 1.7 and 3,7 kcal mol− respectively.

Y-Conjugated compounds: the equilibrium geometries and electronic structures of guanidine, guanidinium cation, urea, and 1,1-diaminoethylene

Abstract: Ab initio calculations at the MP2/6-31G(d) level of theory predict that the equilibrium geometries of the Y-conjugated compounds guanidine (1), guanidinium cation (2), urea (5), and 1,1-diaminoethylene (6) are nonplanar. 1, 5, and 6 have energy minimum structures with strongly pyramidal amino groups. The equilibrium geometry of the guanidinium cation 2b has D 3 symmetry; the planar amino groups are rotated by ∼15 o out of the D 3h form 2a. The planar structure 2a becomes lower in energy than 2b when corrections are made for zero-point vibrational energies

Intermolecular bonding and vibrations of phenol⋅H2O (D2O)

Abstract: Extensive ab initio calculations of the phenol⋅H2O complex were performed at the Hartree–Fock level, using the 6‐31G(d,p) and 6‐311++G(d,p) basis sets. Fully energy‐minimized geometries were obtained for (a) the equilibrium structure, which has a translinear H bond and the H2O plane orthogonal to the phenol plane, similar to (H2O)2; (b) the lowest‐energy transition state structure, which is nonplanar (C1 symmetry) and has the H2O moiety rotated by ±90°. The calculated MP2/6‐311G++(d,p) binding energy including basis set superposition error corrections is 6.08 kcal/mol; the barrier for internal rotation around the H bond is only 0.4 kcal/mol. Intra‐ and intermolecular harmonic vibrational frequencies were calculated for a number of different isotopomers of phenol⋅H2O. Anharmonic intermolecular vibrational frequencies were computed for several intermolecular vibrations; anharmonic corrections are very large for the β2 intermolecular wag. Furthermore, the H2O torsion τ around the H‐bond axis, and the β2 mode are strongly anharmonically coupled, and a two‐dimensional τ/β2 potential energy surface was explored. The role of tunneling splitting due to the torsional mode is discussed and tunnel splittings are estimated for the calculated range of barriers. The theoretical studies were complemented by a detailed spectroscopic study of h‐phenol⋅H2O and d‐phenol⋅D2O employing two‐color resonance‐two‐photon ionization and dispersed fluorescence emission techniques, which extends earlier spectroscopic studies of this system. The β1 and β2 wags of both isotopomers in the S0 and S1 electronic states are newly assigned, as well as several other weaker transitions. Tunneling splittings due to the torsional mode may be important in the S0 state in conjunction with the excitation of the intermolecular σ and β2 modes.

Ab initio calculation of a global potential, vibrational energies, and wave functions for HCN/HNC, and a simulation of the (A-tilde)-(X-tilde) emission spectrum

Abstract: We present a potential energy surface for the HCN/HNC system which is a fit to extensive, high quality ab initio, coupled‐cluster calculations. The new surface is an improved version of one that was reported previously by us [J. A. Bentley, J. M. Bowman, B. Gazdy, T. J. Lee, and C. E. Dateo, Chem. Phys. Lett. 198, 563 (1992)]. Exact vibrational calculations of energies and wave functions of HCN, HNC, and delocalized states are done with the new potential using a new method, which combines a truncation/recoupling method in a finite basis representation procedure with a moveable basis to describe the significant bend–CH stretch correlation. All HCN and HNC states with energies below the energy of the first delocalized state are reported and characterized. All delocalized states up to 18 347 cm−1 above the HCN zero‐point energy and higher energy localized HCN states are also reported and characterized. Vibrational transition energies are compared with all available experimental data on HCN and HNC, including high CH‐overtone states up to 23 063 cm−1. We also report a simulation of the A–X stimulated emission pumping (SEP) spectrum, and compare the results to experiment. The simulation is performed within the Franck–Condon approximation, and makes use of 400 even‐bend wave functions for the ground electronic state, and a realistic vibrational wave function for the first excited bend state in the excited A state. The potential for the A state is slightly modified, relative to one implied by a previously reported force field, to improve agreement with the experimental fundamentals for the A state. In addition, the A‐state wave function is adjusted slightly to improve agreement with the SEP spectrum. We also report Franck–Condon factors for odd bending states of HCN, with one quantum of vibrational angular momentum, in order to compare with the recent assignment by Jonas, Yang, and Wodtke [J. Chem. Phys. 97, 2284 (1992)], based on axis‐switching arguments of a number of previously unassigned states in the SEP spectrum.

Electron attachment to uracil. Theoretical ab initio study

Abstract: Ab initio calculations indicate that the uracil molecule can bind an electron and form a stable dipole-bound anion. The adiabatic electron affinity of this process is estimated to be only 0.003 15 hartree (0.086 eV). This study shows that the crucial factor which allows one to determine the electron efficiency of a polar molecule such as uracil in an ab initio calculation is a proper selection of the basis set. Without very diffused orbitals located on the positive side of the molecular dipole, the stable electronic states of the anion can be completely missed. 8 refs., 1 fig., 1 tab.

On the ab initio determination of higher-order force constants at nonstationary reference geometries

Abstract: Several complementary analyses have been performed in an investigation of the use of reference geometric structures which are not stationary at a given level of theory in the prediction of improved equilibrium anharmonic molecular force fields. Diatomic paradigms for the procedure were established by constructing empirical potential energy functions for the nitrogen and fluorine molecules which not only reproduce the available Rydberg–Klein–Rees data but also provide reliable derivatives through fourth order for ranges of 0.4 A or greater around the equilibrium bond distance. For comparison, analogous curves were determined at the double‐ζ plus polarization (DZP) restricted Hartree–Fock (RHF) level of theory, and the quartic force fields for N2 and F2 were also obtained at the experimental re structures using a (8s5p3d2f1g) basis set and the coupled‐cluster singles and doubles method augmented by a perturbative contribution from connected triple excitations [CCSD(T)].The results substantiate the ability o...

Theoretical Study of the Electronic Spectra of Cyclopentadiene, Pyrrole, and Furan

Abstract: The electronic spectra of the title molecules have been studied using a newly proposed quantum chemical approach for ab initio calculations of dynamic electron correlation effects in molecular systems: multiconfigurational second-order perturbation theory (CASPT2). For cyclopentadiene and furan, the calculations comprise three valence excited singlet states and, in addition, the la2 - 3s, 3p, and 3d Rydberg states, thus providing a full assignment of the spectra in the energy range below 8.0 eV. For pyrrole, the 2bl - 3s. 3p, and 3d components of the Rydberg series have been added. The four lowest triplet states have also been studied in all three molecules. The computed excitation energies deviate from experiment by less than 0.17 eV in all cases where an assignment is possible. It is shown that the two main features in the spectra are caused by the valence excited states 1Bz (5.27, 5.92,6.04 eV) and 'A: (7.89, 7.46, 7.74 eV), where the calculated energies for the two states in the three molecules are given in parentheses. In addition, the 'A; state has been determined to appear near 6 eV in all three molecules. These results differ drastically from earlier theoretical predictions but are in agreement with experimental data. A number of new assignments of the Rydberg states are suggested.

Force field for simulations of 1,2-dimethoxyethane and poly(oxyethylene) based upon ab initio electronic structure calculations on model molecules

Abstract: The force field parameters for simulations of 1,2-dimethoxyethane (DME) and poly(oxyethylene) were determined mainly utilizing the geometries and energies of the conformational minima and low-lying barriers in DME and diethyl ether (DEE) as determined from ab initio electronic structure calculations. A set of partial atomic charges, used to describe electrostatic interactions, was parameterized based upon the dipole moment and the partial atomic charges for DEE as determined from ab initio electronic structure calculations. The force field parameters thus determined reproduce well the experimental second virial coefficients of gas-phase DEE vs temperature

Molecular electron density lego approach to molecule building

Abstract: A new method is presented for the construction of ab initio quality approximate electronic charge distributions for large molecules from charge distributions of small molecular fragments. This method is reminiscent to building structures using Lego blocks. The electronic density distribution calculated using the method is quantitatively shown to be very similar to that calculated for entire molecules using conventional ab initio packages with standard basis sets such as 6-31G ** , while requiring only a fraction of the computational time. The pre-calculated fuzzy electron distributions of base molecular fragments, stored in a data bank, are merged (rotated, translated, and subsequently added together) to calculate these approximate charge distributions for the molecule

Ab initio studies of electron transfer. 2. Pathway analysis for homologous organic spacers

Abstract: Ab initio calculations of transfer integrals (T[sub DA]) for long-range [pi]- and [sigma]-type electron transfer through saturated spacers linking donor (D) and acceptor (A) groups in several radical anion and cation systems have been carried out and analyzed in terms of additive superexchange models. The sensitivity of calculated results to orbital basis and wave function type has been carefully examined. The one-electron Koopmans' theorem approach, based on the neutral triplet diradical parent state and employing the split valence 3-21 G basis, provides generally reliable results. The minimal STO-3G basis is quantitatively useful in some cases, where it facilitates a compact perturbative analysis of the coupling in terms of competing pathways. For trans alkyl spacers containing one to seven CC bonds, reasonably good global exponential fits are obtained for fall-off of T[sub DA] with D-A separation even though the relative importance of different pathway types (hole, electron, and hybrid) changes markedly over the range of spacers. The calculated decay coefficients [beta][sub r] cover a range of approximately 0.5-0.9 A[sup [minus]1], being systematically greater for radical anions than for cations (by a factor of 1.1-1.7, depending on wavefunction type), consistent with similar findings by Jordan and Paddon-Row for radical cation and anionmore » systems involving norbornyl spacer groups. The behavior of calculated transfer integrals for spacers composed of bicyclo [1.1.1] pentane (bcp) and bicyclo [2.2.2] octane (bco) units is complex, including cases of nonmonotonic variation with number of units (1, 2, or 3), and global exponential fits were possible only for [pi]-transfer through the bcp spacers. 53 refs., 4 figs., 10 tabs.« less

Geometric structure and vibrational spectrum of tetrahydrofuran

Abstract: Ab initio calculations for the symmetric C 2 , C s , and C 2v forms and for two unsymmetric C 1 forms of tetrahydrofuran were performed both at the SCF level and accounting for electron correlation by full second-order MOller-Plesset perturbative treatment. The standard STO-3G, 6-31G, and 6-31G ** bases and a 6-31G * basis with different exponents for polarization functions on oxygen and carbons were applied to a complete optimization of the geometrical parameters within each given molecular symmetry

Electronic and vibrational spectra of matrix isolated anthracene radical cations - Experimental and theoretical aspects

Abstract: The IR vibrational and visible/UV electronic absorption spectra of the anthracene cation, An(+), were studied experimentally, in argon matrices at 12 K, as well as theoretically, using ab initio calculations for the vibrational modes and enhanced semiempirical methods with configuration interaction for the electronic spectra. It was found that both approaches predicted well the observed photoelectron spectrum. The theoretical IR intensities showed some remarkable differences between neutral and ionized species (for example, the CH in-plane bending modes and CC in-plane stretching vibrations were predicted to increase by several orders of magnitude upon ionization). Likewise, estimated experimental IR intensities showed a significant increase in the cation band intensities over the neutrals. The implication of these findings for the hypothesis that polycyclic aromatic hydrocarbon cations are responsible for the unidentified IR emission bands from interstellar space is discussed.

Ab initio theoretical predictions of C28, C28H4, C28F4, (Ti@C28)H4, and M@C28 (M=Mg, Al, Si, S, Ca, Sc, Ti, Ge, Zr, and Sn)

Abstract: Recent experiments have demonstrated that C28 is the smallest fullerene cage that successfully traps elements in its inside. In this work, we have studied the electronic structures, equilibrium geometries, and binding energies of the title molecules at the self‐consistent field (SCF) Hartree–Fock level of theory employing basis sets of double‐zeta quality. The empty C28 fullerene is found to have a 5A2 open‐shell ground state and behaves as a sort of hollow superatom with an effective valence of 4, both toward the outside and inside of the carbon cage. The theoretical evidence suggests that C28H4 and C28F4 should be stable molecules. The possibility of simultaneous bonding from the inside and outside of the C28 shell, as in (Ti@C28)H4, is also explored. Our calculations show that the binding energy of the M@C28 species is a good indicator of the success in experimentally trapping the metal atoms (M) inside the fullerene cage. Based on these results, we propose that elements with electronegativities smaller than 1.54 should form endohedral fullerenes larger than a minimum size which depends on the ionic radius of the trapped atom. This qualitative model, correctly reproduces the available experimental evidence on endohedral fullerenes.