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1.
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. 相似文献
2.
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. 相似文献
3.
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. 相似文献
4.
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 (D2h) and a near tetrahedral (D2d) 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 → 1B1g. 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. 相似文献
5.
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 D2h structure in the singlet state 1A1. The D4h,(1 A1g) and Td ( 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. 相似文献
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8.
Rainer Wernicke Hans Kupka Walther Ensslin Hans-Herbert Schmidtke 《Chemical physics》1980,47(2):235-244
The emission spectra of the title compounds in microcrystalline form have been measured at 10 K. The extensive vibrational progression in the eg mode is indicative of a tetragonal Jahn—Teller distortion in the Γ?4(3T1u) excited state. The vibronic coupling of a threefold electronic state with a doubly degenerate eg 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 a11gt11u 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 eg vibrational frequency compared to the ground state is due to Jahn-Teller forces which distort the potential surface, yielding steeper excited state energy curves. 相似文献
9.
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 4Πu state, as previously supposed. However, owing to the attractive nature of the 4Πu state a double crossing occurs. Several predissociation mechanisms of the C state can therefore take place; their lifetimes have been calculated. 相似文献
10.
Youhei Maruyama Kenta Hongo Masanori Tachikawa Yoshiyuki Kawazoe Hiroshi Yasuhara 《International journal of quantum chemistry》2008,108(4):731-743
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 2Tc + Vc = 0 to make a clear interpretation of the correlation effect.© 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2008 相似文献
11.
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. 相似文献
12.
I. Røeggen 《Chemical physics letters》1975,31(2):271-276
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 1Σg+ state of the B2 molecule. 相似文献
13.
L. Pellerito Rosalba Cefalú A. Gianguzza R. Barbieri 《Journal of organometallic chemistry》1974,70(2):303-308
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 NiII, and the coordination to tin involves three-coordinate phenolic oxygens. The environment of SnIV is judged to be octahedral in both types of compounds. A trans-R2, cis-Cl2 configuration is advanced for R2SnCl2NiSalen. 相似文献
14.
Ab initio potential energy surface and vibration‐rotation energy levels of silicon dicarbide,SiC2
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Jacek Koput 《Journal of computational chemistry》2016,37(26):2395-2402
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. 相似文献
15.
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. 相似文献
16.
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. 相似文献
17.
《Chemical physics letters》1987,140(5):476-482
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+(2Σu+) 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(3Σu+) state is also photodissociated directly at 308 nm producing electronically excited Ar atoms more energetic than the Ar(3p54p) configuration. 相似文献
18.
R.G. Hayes 《Chemical physics letters》1982,87(5):512-514
We have analyzed part of the M11,111 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. 相似文献
19.
Photophysical properties of the isomorphic emissive RNA nucleobase analogues and effect of water solution,ribose, and base pairing: A theoretical study
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Laibin Zhang Xiangmu Kong Mei Wang Mengmeng Zheng 《International journal of quantum chemistry》2017,117(13)
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. 相似文献
20.
A 1,2,4‐Triazole‐based Polynuclear Mixed‐valence MnIIMnIII Complex: Synthesis,Characterization, and Magnetic Property
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A mixed‐valence Mn complex {[MnIIMnIII(HL)2(4,4′‐bpy)(H2O)2] · (ClO4)(DMF)3(4,4′‐bpy)0.5}n ( 1 ) [H2L = 3‐(2‐phenol)‐5‐(pyridin‐2‐yl)‐1,2,4‐triazole] was synthesized and characterized by X‐ray single‐crystal structure analysis and magnetic susceptibility. Single‐crystal X‐ray analysis revealed that complex 1 has a dinuclear core, in which adjacent central MnIII atoms are linked by 4,4′‐bipyridine to form an infinite one‐dimensional (1D) molecular configuration. According to the Mn surrounding bond lengths and bond valence sum (BVS) calculations, we demonstrated that the Mn atom coordinated to the pyridine N atoms is in the +2 oxidation state, while another Mn atom coordinated to the phenolic oxygen atoms is in the +3 oxidation state. Magnetic susceptibility data of the complex 1 indicate that the ferromagnetic interaction dominates in this complex. 相似文献