Ab initio calculation of molecular chiroptical properties

This review describes the first-principles calculation of chiroptical properties such as optical rotation, electronic and vibrational circular dichroism, and Raman optical activity. Recent years have witnessed a flurry of activity in this area, especially in the advancement of density-functional and coupled cluster methods, with two ultimate goals: the elucidation of the fundamental relationship between chiroptical properties and detailed molecular structure, and the development of a suite of computational tools for the assignment of the absolute configurations of chiral molecules. The underlying theory and the basic principles of such calculations are given for each property, and a number of representative applications are discussed.     

Ab initio studies on isomers of macropolyhedral borane ions [B20H18]n (n = 0, -2, -4)

Various isomers of macropolyhedral borane ions [B20H18]n (n = 0, -2, -4) are investigated by using the density functional theory methods at RB3LYP/6-31+G* and RB3LYP/6-31G* levels to obtain the optimized geometries, harmonic vibrational frequencies, electron structures, and the stability order. The calculated results show that optimized bond lengths are consistent with the available experimental values and the natural populations, taking [a2 -B20H18]4- (4) as an example, are also in agreement with NMR spectra. The calculated vibrational frequencies are all real, so all of these isomers could be stable, among which [a2 -B20H18]2- (3) and [a2 -B20H18]0 (7) are considered for the first time in this paper. On the basis of the contour maps of molecular orbitals, the delocalized characteristic of molecular orbitals and the possible redox mechanism of these ions are also discussed. Moreover, the analysis on counting of skeletal bonding electrons shows that the isomers (1)-(6) obey the electronic requirement predicted by the mno rule, whereas the newly predicted isomer (7) does not match the mno rule.     

Ab initio calculations and vibrational spectroscopy on the phenylenediamine isomers

Molecular orbital calculations at HF and MP2 levels have been performed using the 6-31G⁷ basis set for full geometry optimization of the phenylenediamine isomers. Our results show that only a transoid conformer is found for o-phenylenediamine, whereas cis and trans conformers exist for m- and p-phenylenediamine. Vibrational normal modes have been also analyzed for the gas phase and in chloroform solution, and compared with experimental data we have obtained using FTIR spectroscopy.     

Ab initio MO calculations on 1,2-dithietes and valence isomers

The valence isomerization of the 1,2-dithiete parent compound to the open-chain dithial was studied by CASSCF multiconfiguration methods including the CASPT2 perturbational treatment. The isomerization energy remains small at the highest level of theory. In agreement with Jonas and Frenking, the cyclic structure is only then preferred over the acyclic ones if f-functions on the sulfur atoms are considered. If they are included, the 1,2-dithiete is more stable by 3.8 kcal/mol and the barrier amounts to 24.9 kcal/mol at the CASPT2(8,8)/6-31G(2df)//MP2/6-31G* level of theory. According to MP2/6-31G* geometry optimizations, substitution of H by NH₂ and CH₃ reduces the stability of the 1,2-dithiete ring structures relative to the open-chain dithiocarbonyl structures, whereas the inverse holds for acceptor substitution by CN and CF₃. A higher stability of benzodithiete relative to ortho-dithiobenzoquinone is predicted at all employed levels of theory. This is in good agreement with conclusions drawn from experimental results. Whereas the experimental microwave geometry of 1,2-dithiete is well reproduced theoretically, the experimental electron diffraction geometry of 3,4-bis-(trifluoromethyl)-1,2-dithiete differs from the calculated one. © 1996 John Wiley & Sons, Inc.     

Ab initio SCF calculations on hydrogen bonded cresol isomers

Ab initio GAUSSIAN 80 calculations with two different basis sets (STO-3G and 4–31 G*) were performed on hydrogen bonded cresol isomers for comparison with experimental data from free jet fluorescence excitation spectroscopy. Form-cresol, the calculated barriers for hindered internal rotation of the OH-group and the CH₃-group are in good agreement with experiment. The calculations show the trans-linear configuration ofp-cresol·B-clusters (B = H₂O, CH₃OH) to be more stable than the all-planar configuration. This agrees with CI calculations and microwave spectroscopic investigations of the water dimer. Calculations of both the intermolecular stretch and bend frequencies ofp-cresol·B-clusters show little dependence on the all-planar or trans-linear configuration but a strong dependence on the choice of the basis set. With the minimal basis set STO-3G, the vibrational energies are generally too high. The agreement between the calculated vibrational frequencies from the 4–31 G* basis set and the experimental values is fair.     

Ab initio calculation of the C/D ratio of magnetic circular dichroism

A procedure for calculating the magnetic circular dichroism C/D ratio from density functional theory calculations is discussed. The method is simplified considerably through the application of group theory and the irreducible-tensor method and only requires integrals of the magnetic dipole moment operator over a few orbitals and published tables of symmetry factors. The implementation of the method is tested through application to several small and medium-sized molecules.     

Ab initio relativistic calculation of the RbCs molecule

We apply the relativistic configuration-interaction valence-bond method to calculate various characteristics of the alkali-metal RbCs dimer. These include the electronic potentials and transition dipole moments between the ground and first excited states and permanent dipole moments of the X 1sigma+ and a 3sigma+ states of the ground configuration. In addition, we estimate the lifetime of the rovibrational levels of the X state due to blackbody radiation. These data can help experimentalists to optimize photoassociative formation of ultracold RbCs molecules and their longevity in a trap or in an optical lattice. Extended basis sets, constructed from Dirac-Fock and Sturm's orbitals, have been used to ensure convergence of our calculations. We compare our data with other theoretical and experimental results when they were available.     

Generating functions for the cage isomers of the C20n icosahedral fullerenes

The number of isomeric cages for the C_20n icosahedral fullerenes (Goldberg polyhedra), is given by the coefficient ofn ^–s in the expansion of the Dirichlet generating function (s)L[s, x(3)].When this coefficient is even, the cages occur as chiral pairs of point symmetry I; when odd, there is one structural isomer of point symmetry I _h , and the other isomers, if any, occur as chiral pairs. Asymptotic estimates are given for the number of isomers of each type.     

Ab initio derivative calculation of vibrational circular dichroism

The recent formalism of Stephens for the calculation of rotational strength in vibrational circular dichroism has been implemented, exploiting the analytical derivative technique for ab initio Hartree-Fock wavefunctions.     

Ab initio study of oxaphosphaalkyne P=C-OH and its isomers

Anab initio study on HP=C=O PC-OH isomerization has been performed at the 3-21G(d(P)), 4–31G(d(P)) and 6–31G** levels. In contrast to analogous transformation of 1-aza-3-phosphaallene, this process is endothermic in the gas phase. It proceedsvia a [1,3]H shift without an intermediate. The route, including two subsequent [1,2]H shifts, leads to phosphinidene.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 172–173, January, 1994.     

Ab initio characterization of C5

In this paper, the structure and spectroscopic parameters of the C5 cluster are determined using multiconfigurational quantum chemical methods as implemented in the MOLCAS software. A number of spectroscopic properties (band center positions, l-doubling parameters, and rotational constants) have been characterized. From the new results, the assignments of previous astrophysical observations [J. Goicoechea et al., Astrophys. J. 609, 225 (2004)] are discussed. A detailed exploration of the global potential energy surface confirms that C5 has a X1Sigmag+ linear isomer of prominent stability and, at least, three minimum energy structures showing singlet electronic ground states. Two of them are cyclic and one has a nonplanar geometry. Vertical and adiabatic electronic transitions and vibrational spectroscopic parameters are determined for the most stable linear isomer using multiconfigurational second order perturbation theory (CASPT2) using an active space containing 12 valence orbitals with 12 active electrons and extended ANO-type basis sets. The infrared spectrum has been analyzed from an anharmonic force field derived form the local surface, determined from the energies of a grid of 1350 geometries. The force field includes four coupling terms. The CASPT2 band center position of the nu7(piu) anharmonic fundamental has been calculated to be at 102 cm(-1), which validates the assignment to C5 of the pattern of bands centered at 102 cm(-1) observed with the ISO telescope.     

Ab initio calculation of maximum bond order hybrid orbitals

Summary The maximum bond order hybrid orbital (MBOHO) procedure is tested onab initio level by use of the density matrix in Löwdin orthogonalized atomic orbital basis. The direct MBOHO calculation based on the whole density matrix includes also the hybridization of the inner atomic orbitals, and the MBOHO calculation based on the valence orbital part of the density matrix considers only the hybridization of the valence atomic orbitals. The concrete MBOHO calculations based on theab initio calculation with STO-3G basis show that the components of the s atomic orbitals in MBOHOs and the maximum bond orders obtained from the two kinds of MBOHO calculations are very close to each other, and that the two kinds of MBOHOs all have the excellent correlativity with the nuclear spin-spin coupling constants.The project supported by National Natural Science Foundation of China and the Excellent Young University Teacher's Foundation of State Education Commission of China.     

Ab initio calculation on the H3 activated complex

A full configuration interaction treatment has been carried out for linear, symmetrical H₃ activated complex, with a limited basis set of Slater orbitals. A similar calculation is performed on H₂ in order to obtain an estimate of the activation energy of the reaction H+H₂=H₂+H. The variation of nonlinear parameters in the basis set is studied and the different behaviour of the H₃ and H₂ resulting wave functions examined. — A larger basis set is needed.

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Zusammenfassung Eine vollstÄndige CI-Behandlung wurde für einen linearen, symmetrischen, aktivierten H₃-Komplex mit einem begrenzten Basissatz von Slaterorbitalen durchgeführt. Eine Ähnliche Rechnung wird für H₂ ausgeführt, um eine AbschÄtzung der Aktivierungsenergie der Reaktion H+H₂=H₂+H zu erhalten. Die Variation von nichtlinearen Parametern im Basissatz wird untersucht und das verschiedenartige Verhalten der resultierenden H₃ und H₂ Wellenfunktion überprüft. — Ein grö\erer Basissatz wird benötigt!

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Résumé Interaction de configurations totale pour le complexe activé H₃ linéaire et symétrique, dans une base limitée d'orbitales de Slater. Un calcul analogue a été effectué sur H₂ afin d'évaluer l'énergie d'activation de la réaction H+H₂=H₂+H. La variation des paramétres non linéaires de la base est étudiée, et l'on examine le comportement différent des fonctions d'onde de H₃ et de H₂. Une base plus étendue serait nécessaire.

     

Ab initio Calculations on the thermal ring closure of acrolein

The electrocyclic reaction which interconverts acrolein and oxetene is investigated by ab initio calculations, using a 4-31G basis set. Like the transition state of the thermal ring closure of cisbutadiene, the transition state of this reaction shows a torsion of about 18 ° around the central bond, in spite of the absence of H-H repulsion. The resulting oxetene is 43 kcal/mol endothermic with regard to acrolein. The calculated activation energies for ring closure and -opening are 3.4 and 1.0 eV. The conformational result of the ring opening reaction is discussed.     

Ab initio calculation of the dipole polarizabilities of unsaturated hydrocarbons

Finite-field SCF MO calculations of polarizability are reported for ethene, benzene and naphthalene using three good gaussian basis sets Double-zeta wavefunctions yield values of α_LL and α_MM in agreement with experiment but the calculated α_NN is half the experimental value. Polarization functions when present preferentially improve α_NN. (L,M) and N refer to the molecular long, medium and normal axes, respectively.) STO-4G results are given for benzene, naphthalene, azulene, anthracene and phenanthrene, and the agreement with the experiment is reasonable after scaling the calculated α_LL and α_MM by a constant factor. The calculated α_MM are an order of magnitude too low.     

Ab initio calculation of optical rotation in (P)-(+)-[4]triangulane

Optical rotation, the angle through which plane-polarized light rotates when passed through an enantiomerically pure medium, plays a vital role in the determination of the absolute configurations of chiral molecules such as natural products. We describe new quantum mechanical methodology designed to assist in this endeavor by providing high-accuracy computational optical rotatory dispersion data for matching to experimental results. Comparison between theory and experiment for the rigid, helical molecule trispiro[2.0.0.2.1.1]nonane [also known as (P)-(+)-[4]triangulane], recently synthesized with enantiomeric purity, shows that the coupled cluster quantum chemical model provides superb agreement for optical rotation across a wide range of wavelengths (589-365 nm), with errors averaging only 1%.