A gas phase ab initio excited state geometry optimization study of thymine, cytosine and uracil

Molecular geometries of the nucleic acid bases thymine, cytosine and uracil in the ground and the lowest two singlet excited states were optimized using the ab initio approach employing the 4-31G basis set for all the atoms except the amino group of cytosine for which the 6-311+G^* basis set was used. The excited state calculations were performed employing configuration interaction involving singly excited configurations (CIS). Vibrational frequencies were computed in order to examine the nature of the stationary points on the potential energy surfaces obtained by geometry optimization. While the ground state geometries of uracil and thymine (except the methyl group hydrogens) are planar, the corresponding excited state geometries were found to be significantly nonplanar. In the case of cytosine, the amino group is pyramidal and the rest of the molecule is only slightly nonplanar in the ground state, but the excited state geometries are appreciably nonplanar. In particular, consequent to the S₂(n–π^*) excitation of cytosine, the amino group plane is strongly rotated. While thymine is stable in the S₂(π–π^*) excited state, uracil appears to be dissociative in the corresponding excited state.     

Exponent optimization for Π states of H2

The total energy of the lowest ¹ Π _u, ³ Π _u, ¹ Π _g, ³ Π _g states of H₂ was minimized in an extended Hartree-Fock procedure employing a double configuration wavefunction. The exponents of the basis functions 1s, 2s, 2pσ, 2pπ, 3dπ were optimized separately for all four states at various internuclear distances. The results show that the exponents are strongly state and distance dependent. They cannot be generally represented by atomic or equilibrium values. The details of the optimization process are presented.     

A geometry optimization benchmark using highly correlated wavefunctions (FCI and MRD-CI)

Summary Full configuration interaction (FCI) geometry optimizations have been performed for the X³B₁, a¹A₁, b¹B₁ and c¹A₁ electronic states of CH₂, the X²B₁ and A²A₁ electronic states of NH₂ and the X¹A₁ electronic state of BH₃ using a DZP basis set. The results are compared with those obtained using the MRD-CI method at different levels of theoretical treatment. The agreement between the geometrical parameters optimized with the FCI and MRD-CI methods is very good.     

Pseudopotential,MC-SCF and limited CI calculations on nickel-bis-dithiolene

The pseudopotential method is used to perform multiconfigurational (MC) SCF and full valence configuration interaction (CI) calculations on the dianion of nickel-bis-dithiolene, [Ni(S₂C₂H₂)₂]^2?. Both a planar (D_2h) and a near tetrahedral (D_2d) conformation are considered. Charge distributions are calculated, yielding a charge on Ni of 0.22 in the planar and 0.57 in the tetrahedral case. The experimental ground state, the planar ¹Ag state, approaches a 3d⁹ configuration on Ni (the d-population equals 8.87), while the nickel 3d-population of all other calculated states ranges from 8.35 to 8.69. The single d—d excitation spectrum for the planar complex is discussed, and the first transition is computed to be ¹Ag → ¹B_1g. Intramolecular rotation, i.e., one ligand ring rotating with respect to the other, is also discussed. The rotation is proposed to occur via a tetrahedral triplet intermediate since the relevant singlet tetrahedral state is estimated to lie more than 1 eV higher than the triplet intermediate. The discussion of the intramolecular rotation is made difficult by an error in the calculated singlet—triplet splittings. Possible causes of this error are pointed out.     

AB initio Calculations of the structure and spectra of chromium, molybdenum, and tungsten tetrafluorides. nontetrahedral molecular structure of WF4

The properties of MF₄ molecules (M = Cr, Mo, W) are investigated by the restricted Hartree-Fock method using Möller-Plesset second-order perturbation theory and by the second-order configuration interaction method using the multiconfigumtion wave function derived in a complete active space approximation, in wide bases complemented with polarization d and f functions. Relativistic effective potentials are used to describe the core electrons. For CrF₄ and MoF₄, the tetrahedral configuration of nuclei in the electronic state of³k₂ symmetry is energetically most favorable. In the WF4 molecule, the least-energy structure is a D_2h structure in the singlet state ¹A₁. The D_4h,(¹ A_1g) and T_d ( ³A₂) configurations in the WF₄ molecule are higher on the energy scale than the ground state by 4 and 6305 cm’1 and are saddle points. For all of the analyzed configurations of MF₄ molecules, the geometrical parameters, the vibrational spectra, and the energies of vertical electronic transitions are found. The chemical bonding is analyzed and a simple model is proposed to explain the variation of the relative energies of states in the series CrF₄→MoF₄→WF₄.     

Low temperature luminescence spectra of the d10s2 complexes Cs2MX6 (M = Se,Te and X = Cl,Br). The Jahn—Teller effect in the Γ−4(3T1u) excited state

The emission spectra of the title compounds in microcrystalline form have been measured at 10 K. The extensive vibrational progression in the e_g mode is indicative of a tetragonal Jahn—Teller distortion in the Γ^?₄(³T_1u) excited state. The vibronic coupling of a threefold electronic state with a doubly degenerate e_g mode (T—e coupling), linear in the nuclear coordinates, has been reinvestigated considering spin—orbit coupling up to second order perturbation on energy levels which result from an a¹_1gt¹_1u electron configuration. For an estimation of Jahn—Teller coupling constants, the intensity distributions in the progressions were compared with the theoretical line shape functions which were obtained from a model which also permits the determination of potential energy minima and vibrational fundamentals of the excited state. The unusually large increase in the e_g vibrational frequency compared to the ground state is due to Jahn-Teller forces which distort the potential surface, yielding steeper excited state energy curves.     

Excited states of gaseous ions. Transitions to and predissociation of the CΣu state of n2

A configuration interaction study of different electronic states of N⁺₂ has been performed. The position in energy and the relative intensity of the photoelectron bands of the ²Σ⁺_u states has been calculated and compared with experiment. The C²Σ⁺_u state is predissociated by a ⁴Π_u state, as previously supposed. However, owing to the attractive nature of the ⁴Π_u state a double crossing occurs. Several predissociation mechanisms of the C state can therefore take place; their lifetimes have been calculated.     

Ab initio interpretation of Hund's rule for the methylene molecule: Variational optimization of its molecular geometries and energy component analysis

Hund's spin‐multiplicity rule for the ground state of the methylene molecule CH₂ is interpreted by Hartree‐Fock (HF) and multi‐reference configuration interaction (MRCI) methods. The stabilization of the triplet ground state ( ³B₁) relative to the second singlet excited state ( ¹B₁) is ascribed to the greater electron‐nucleus attraction energy that is gained at the cost of increasing the electron‐electron repulsion energy and with the aid of a reduction in the nucleus‐nucleus repulsion energy. The highest spin‐multiplicity in the ground state of CH₂ is accompanied by a set of three characteristic features, i.e., elongation of the internuclear distances, reduction in the bond angle, and contraction of the valence electron density distribution around the nuclei involving expansion of the core electron density distribution. The present calculations fulfill the virial theorem to an accuracy of ?V/T = 2.000 for both HF and MRCI. Accordingly, the molecular geometries are optimized for each of the two states. The inclusion of correlation by MRCI method reduces the energy splitting between the two states by about 14%. The energy splitting is analyzed by the correlational virial theorem 2T^c + V^c = 0 to make a clear interpretation of the correlation effect.© 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2008     

Palladium complexes of a chiral P,C-chelating phosphino-(sulfinylmethyl)phosphonium ylide ligand

A new type of phosphino-phosphonium ylide ligand bearing a chiral sulfinyl center affords a P,C-chelated palladium(II) complex with a resolved asymmetric ylidic carbon atom. According to ³¹P NMR analysis of the crude material, the diastereoselectivity of the complexation at room temperature is ca. 7:1. In the crystal state, an X-ray diffraction analysis of one epimer reveals a quasi C₂-symmetric chloro-bridged dinuclear structure, where the (S) configuration of the sulfur atom induces a (S) configuration of the ylidic carbon atom. A in situ Pd(0) catalyst generated from the phosphino-ylide and Pd(PPh₃)₄ promotes allylic substitution of 3-acetoxy-1,3-diphenylpropene by sodium malonate in 70% yield and 5% e.e.     

Application of a least-squares method in configuration interaction calculations

A least-squares method is proposed for obtaining approximate solutions of large configuration interaction matrices. The method requires that only a small rectangular part and the diagonal of the interaction matrix need to be constructed. The method should be particularly useful as a supplement to MC SCF procedures. As a test example the method is applied to a 96 configuration calculation of the lowest ¹Σ_g⁺ state of the B₂ molecule.     

Complexes of organometallic compounds : XXXVII. Mössbauer and other studies on adducts of organotin(IV) chlorides with N,N′-ethylenebis(salicylideneiminato)-nickel(II)

Novel 1/1 adducts have been obtained from the complex N,N′-ethylenebis(salicylideneiminato)nickel(II) (NiSalen) with di- and mono-organotin(IV) chlorides, and their solid state configuration investigated by Mössbauer , IR and electronic spectroscopy and magnetic measurements. In coordinated NiSalen the square planar structure is maintained around Ni^II, and the coordination to tin involves three-coordinate phenolic oxygens. The environment of Sn^IV is judged to be octahedral in both types of compounds. A trans-R₂, cis-Cl₂ configuration is advanced for R₂SnCl₂NiSalen.     

Ab initio potential energy surface and vibration‐rotation energy levels of silicon dicarbide,SiC2

The accurate ground‐state potential energy surface of silicon dicarbide, SiC₂, has been determined from ab initio calculations using the coupled‐cluster approach. Results obtained with the conventional and explicitly correlated coupled‐cluster methods were compared. The core‐electron correlation, higher‐order valence‐electron correlation, and scalar relativistic effects were taken into account. The potential energy barrier to the linear SiCC configuration was predicted to be 1782 cm^?1. The vibration‐rotation energy levels of the SiC₂, ²⁹SiC₂, ³⁰SiC₂, and SiC¹³C isotopologues were calculated using a variational method. The experimental vibration‐rotation energy levels of the main isotopologue were reproduced to high accuracy. In particular, the experimental energy levels of the highly anharmonic vibrational ν₃ mode of SiC₂ were reproduced to within 6.7 cm^?1, up to as high as the v₃ = 16 state.     

Electronic structure and photoelectron spectrum Assignment of allene

In this paper a series of ab initio SCF and configuration calculations were reported forthe ground state and excited states X~2E, A~2E,~2B_2 and ~2A_1 of allene.For ground state X~2E Jahn-Teller distorsion was discussed and a twisted angle of 50° and a torsional barriers of 0.21—0.51 eVwere derived.Based on calculated results,the experimental photoelectron spectrum of allene has beenassigned.     

Theoretical study on the vertical electronic spectrum of carbonyl fluoride,F2CO

Ab initio self-consistent-field (SCF ) and configuration interaction (CI ) calculations on the ground and excited states of carbonyl fluoride (F₂CO) were carried out at its experimental ground-state equilibrium geometry. Vertical transition energies deduced from the CI results provide assignments for the electronic systems I–IV, experimentally observed by Workman and Duncan. The singlet excited state, ¹A₁ (π→π*), is found to be a mixed valence–Rydberg state and to he 1 to 1.2 eV above the suggested experimental value, irrespective of the choice of the basis used for the CI calculations.     

Laser-induced photoionization of Ar2(3Σu+) at 308 nm

Experimental evidence is provided to show that the Ar₂(³Σ⁺_u) excimer is photoionized by absorption of light at 308 nm. This direct photoionization of Ar₂(³Σ⁺_u) was used to measure the distribution of atomic states belonging to the Ar(3p⁵4p) electronic manifold produced in dissociative recombination of Ar₂⁺(²Σ_u⁺) at atmospheric pressure. It was found that electronically excited states, Ar(2p₂) and Ar(2p₁₀), accounted for 96% of the excited state population of the Ar(3p⁵4p) configuration produced in dissociative recombination. The Ar₂(³Σ_u⁺) state is also photodissociated directly at 308 nm producing electronically excited Ar atoms more energetic than the Ar(3p⁵4p) configuration.     

Crystal-field splitting in the M11,111 spectra of Co(II) compounds

We have analyzed part of the M_11,111 absorption spectra of the compounds CoF₂, CoCl₂, CoBr₂, Co(acac)₂ as due to the crystal-field splitting of a ⁴G state arising from the configuration 3p⁵3d⁸.     

Photophysical properties of the isomorphic emissive RNA nucleobase analogues and effect of water solution,ribose, and base pairing: A theoretical study

In the present work, a comprehensive theoretical investigation on the excited state properties of the isomorphic emissive RNA nucleobase analogues, namely ^tzA, ^tzG, ^tzC, and ^tzU, was performed. Vertical transition energies are determined with the time‐dependent density functional theory method at both the B3LYP and CAM‐B3LYP levels using the 6‐311++G(d,p) basis set. The nature of the low‐lying singlet excited states is discussed and the results are compared with the findings from experiment and those for thieno analogues and natural bases. In gas phase, it was found that the S₁ state is ππ* in nature for all the tz‐bases except for ^tzA, for which the S₁ state is predicted to be nπ* in nature with the ππ* state being the S₂. While in water solution, the S₁ state for all tz‐bases are predicted to be ππ* dominated by the configuration HOMO→LUMO. Compared with natural bases, the lowest ππ* states are about 0.85–1.22 eV red‐shifted. When compared with thieno analogues, it is interesting to note that the S₁ state (ππ*) transition energies of the two counterparts from the two alphabets are nearly equal due to the very little differences of their HOMO‐LUMO gaps. In addition, it was found that the hydration + PCM model can perfectly reproduce the photophysical properties of the tz‐bases since the calculated excitation maxima and fluorescence are in good agreement with the experimental data. The microenvironment effects of linking to ribose, base pairing, and further hydration of base pairs were also studied.     

Electronic state of Fe used as Mössbauer probe in the perovskites LaMO3 (M=Ni and Cu)

For the first time a comparative study of rhombohedral LaNiO₃ and LaCuO₃ oxides, using ⁵⁷Fe Mössbauer probe spectroscopy (1% atomic rate), has been carried out. In spite of the fact that both oxides are characterized by similar crystal structure and metallic properties, the behavior of ⁵⁷Fe probe atoms in such lattices appears essentially different. In the case of LaNi_0.99Fe_0.01O₃, the observed isomer shift (δ) value corresponds to Fe³⁺ (3d⁵) cations in high-spin state located in an oxygen octahedral surrounding. In contrast, for the LaCu_0.99Fe_0.01O₃, the obtained δ value is comparable to that characterizing the formally tetravalent high-spin Fe⁴⁺(3d⁴) cations in octahedral coordination within Fe(IV) perovskite-like ferrates. To explain such a difference, an approach based on the qualitative energy diagrams analysis and the calculations within the cluster configuration interaction method have been developed. It was shown that in the case of LaNi_0.99Fe_0.01O₃, electronic state of nickel is dominated by the d⁷ configuration corresponding to the formal ionic “Ni³⁺-O²⁻” state. On the other hand, in the case of LaCu_0.99Fe_0.01O₃ a large amount of charge is transferred via Cu-O bonds from the O:2p bands to the Cu:3d orbitals and the ground state is dominated by the d⁹L configuration (“Cu²⁺−O” state). The dominant d⁹L ground state for the (CuO₆) sublattice induces in the environment of the ⁵⁷Fe probe cations a charge transfer Fe³⁺+O⁻(L)→Fe⁴⁺+O²⁻, which transforms “Fe³⁺” into “Fe⁴⁺” state. The analysis of the isomer shift value for the formally “Fe⁴⁺” ions in perovskite-like oxides clearly proved a drastic influence of the 4s iron orbitals population on the Fe−O bonds character.