An extended geminal model calculation of the dissociation energy of the ground state of Li2

The dissociation energy of the ground state of the Li₂ molecule has been calculated by using an extended geminal model. The calculated result is 1.024 eV and the experimental value is 1.048 ± 0.012 eV. the discrepancy can at least partly be attributed to a deficiency in the orbital basis set.     

Electronically excited and ionized states of the chlorine molecule

Potentials curves for the ground and excited states of the chlorine molecules and its positive and negative ions have been calculated by means of the MRD-CI method. The standard AO basis employed consists of 74 functions including two atomic d and one set of s and p bond species, and the results at the corresponding full CI level are estimated for each state via a perturbation correction. Special emphasis is placed upon the treatment of Rydberg-valence mixing in this system, which phenomenon is found to be essential to the understanding of Cl₂ electronic absorption spectrum. All singlet states which correlate with the lowest dissociation limit plus many others which go to ionic Cl⁺+Cl^? or Rydberg Cl+Cl asymptotes are given explicit consideration. Among the triplet species of Cl₂ which dissociate into the ground state atoms only the ³Π_u state is not repulsive. The calculated D₀ value for the ground state is 2.455 eV compared to the experimental value of 2.475 eV, while the vertical ionization energy and electron affinity are found to be 11.48 and 2.38 eV respectively, also in very good agreement with the corresponding measured data of 11.50 and 2.51 ± 0.1 eV. In addition to Cl₂ laser line is confirmed to result from a ³Π_g → ³Π_u emission, whereby the calculated downward vertical transition energy of 4.86 eV fits in quite well with the known location of this line at 4.805 eV. The first two dipole-allowed transitions from the ground state of chlorine involve ¹Σ_u⁺ and ¹Π_u states which are calculated to be nearly isoenergetic, and these results also match very well with the location of the first absorption band in this spectrum. Finally quite similarly as in O₂ it is found that an avoided crossing between Rydberg and valences states produces a relatively steep potential well for an upper state (2 ¹Σ_u⁺), whose location concides with that of a second absorption band recently observed in synchrotron radiation studies.     

Role of the electric dipole moment in positron binding to the ground and excited states of the BeO molecule

Self-consistent-field and multireference single- and double-excitation configuration interaction (CI) calculations have been carried out for various electronic states of the beryllium oxide molecule and their positron-attached counterparts. Particular emphasis is placed on the correlation between the polarity of a given BeO state and the magnitude of the positron binding energy as the internuclear distance is varied. Potential curves are computed for all BeO states that correlate with the first three atomic limits for this system and good agreement is found between the experimental and calculated spectroscopic constants in all cases. The present level of CI treatment is known to underestimate the positron affinities of atoms by at least several tenths of an eV, and this fact needs to be taken into account in evaluating the results for positron binding to molecules. The lowest BeO excited states (3,1Pi) are not found to bind with a positron in the Franck-Condon region due to their comparatively small dipole moments caused by O to Be charge transfer relative to the X 1Sigma+ ground state, which in turn does have a fairly sizeable positron affinity. The situation changes significantly as dissociation proceeds, however, with both 4,2Pi and 2Sigma+ positronic states lying several tenths of an eV lower than their neutral counterparts over a broad range of internuclear distance.     

A CSF-based multi-reference coupled pair approximation III. An application to F2, As2 and As+2

Using the newly developed multi-reference coupled pair approximation program code, the adiabatic potential curves of the ground states of F ₂, As ₂ and As ₂ ⁺ were calculated. Computed spectroscopic constants of these molecules were found to be in good agreement with experimental values. The resulting binding energy of As ₂ (3.86 eV) was compared with the experimental value of 3.99 eV [15] and the best multi-reference configuration interaction value (3.58 eV) reported previously by the present authors. The calculated first adiabatic ionization potential of As ₂ (9.67 eV) was found to be in good agreement with the experimental result. Received: 5 July 1997 / Accepted: 27 August 1997     

The structure of the SF6 molecule and the SF 6 − anion excited states

The electronic and geometric structures of the ground state and a number of excited states of the SF₆ molecule and the SF ₆ ^– anion have been calculated by the discrete-variation method of the local density-functionals. The anion was found to possess a number of states stable toward the outer electron detachment, and at least one excited state stable toward dissociation. The adiabatic electron affinity (EA) was determined as 3.46 eV at the highest level of theory. This result is correlated to the high EAs of the isovalent compound SeF₆ and TeF₆; however, it does not agree with the presently accepted experimental estimate of 1.0 ± 0.2 eV for the SF₆EA value. The basic anion configuration is octahedral with a S-F bond length of 1.717 Å. The calculated limit for the highest dissociation channel of the ground state SF ₆ ^– SF ₅ ^– + F is 1.5 eV lower than the minimum of the total energy of the neutral molecule; this is in good agreement with experimental estimates.Institute for Chemical Physics, Russian Academy of Sciences, 142432 Chernogolovka. Translated from Izvestiya Akademii Nauk, Seriya Khimicheskaya, No. 3, pp. 641–649, March, 1992.     

Ab initio MRD-CI study of the rydberg states of methylene

Potential curves have been calculated for the low-lying Rydberg states of CH₂ as well as for a number of its valence-shell species by employing the ab initio MRD-CI method. The first Rydberg transition is found to occur with a vertical energy of 6.38 eV (1b₁ → 3s), but the corresponding upper state is believed to be strongly predissociated since it correlates directly with the CH(²II) + H(²S_g) ground state fragments at lower energy. The assignment of the first observed Rydberg transition at 8.757 eV by Herzberg as 1b₁ → 3dπ is confirmed almost quantitatively in the calculations, while the corresponding minimum 1P value is computed to be 10.21 eV compared to the experimental result of 10.3 ± 0.1 eV. The dissociation energy of methylene in its ground state is calculated to be 4.47 eV, and this result also fits in well with experimental evidence, which determines a lower limit for this quantity of D₀ > 4.23 eV. Finally, it is found that none of the Rydberg states nor any of the higher-lying valence-shell species of methylene are of sufficiently low energy to play a significant role in the experimental determination of the ¹A₁-³B₁ splitting of this system.     

On the structure of negative ions formed by dissociative electron attachment by monochlorophenol molecules

The energetics of negative ion formation by resonant dissociative electron attachment by o-, m-, and p-chlorophenol molecules was studied. The structures of some fragment ions and their neutral partners were established. Hidden rearrangement processes leading to the formation of oxy anions by the detachment of chlorine atoms from molecular ions were found. The O—H bond dissociation energies for o-, m-, and p-chlorophenol molecules were 3.74±0.11, 3.72±0.17, and 3.94±0.11 eV, respectively.     

Measurement of the ground-state lambshift of hydrogenlike uranium at the electron cooler of the ESR

X-rays are emitted with the radiative recombination of free electrons in an electron cooler of a heavyion storage ring. Due to a small width of the X-ray lines, an observation angle close to 0° and an accurate determination of the ion velocity, the ground-state Lambshift of hydrogenlike uranium (470 ± 16) eV could be measured to an accuracy of 3.4%. A re-evaluation of a measurement of the 1s _1/2 Lambshift in hydrogenlike gold gave a new value of (202.3 ± 7.9) eV as compared to the former value of (212 ± 15) eV. The results are in excellent agreement with QED calculations and are more precise than any other measurements previously reported for a high-Z, hydrogenlike ion.     

Configuration interaction calculations for the N2 molecule and its three lowest dissociation limits

A series of multi-reference double-excitation CI calculations is reported for the N₂ molecule in its equilibrium conformation as well as for its three lowest dissociation limits ⁴S + ⁴S, ⁴S + ²D and ⁴S + ²P respectively. Special emphasis is placed upon satisfying the requirement that the molecular CI energies at large internuclear separations must converge to the same values as are obtained in an analogous treatment for the corresponding atomic limits. In the most extensive CI undertaken the lowest three dissociation limits are calculated to occur at 9.33, 12.08 and 13.39 eV respectively compared to the experimental values of 9.90, 12.29 and 13.48 eV. The advantages of employing bond-centered functions in obtaining the proper balance between the bound molecule and its dissociated atoms are underscored in the calculations.     

Superhalogen properties of PdFn (n=1–7) clusters using Quantum chemical method

In this work the interaction of Palladium (Pd) atom with Fluorine (F) has been studied using density functional theory. Up to seven F atoms are bound to a single Pd atom which results in increase of electron affinities of given molecule successively, reaching a peak value of 8.54 eV for PdF₇. By using HOMO–LUMO gap, molecular orbital analysis, binding energy of these clusters, we examined its stability and reactivity. It is found that energy required for dissociation of F₂ molecules are higher than energy required for dissociation of F atoms. The unusual properties brought about by involvement of inner shell 4d-electrons, which not only allow PdF_n clusters to belong to the class of superhalogens but also show that its valence can exceed the nominal value of 1.     

Mean Ionic Activity Coefficients and Dissociation Constant of HCl in the System HCl-H2SO4-H2O at 298 K

The mean ionic activity coefficients of HCl (_1±) in the system HCl-H₂SO₄-H₂O at 298 K were calculated by the Mikulin and MacKay-Perring methods and were then used for calculating the mixed thermo- dynamic dissociation constant of HCl (K _m). The mean value of the constant proved to be equal to that found previously for aqueous solution of HCl, and deviations from the mean value are most likely due to the fact that, when calculating K _m, incompleteness of dissociation of both electrolytes was neglected. The _1± values calculated by the MacKay-Perring and Mikulin methods virtually coincide, within the determination and calculation errors, with the published data. This result confirms the suitability of the previously suggested procedure for determining the strictly thermodynamic mixed dissociation constants from the experimental data on the vapor pressure in combination with the mean ionic activity coefficients.     

Solvent effects on the excited state properties of 2-aminopurine—a theoretical study by the ONIOM and Supramolecular method

In this paper, the micro-solvated effects on the lowest-energy vertical transition state and adiabatic excited states of 2-aminopurine (2Ap) were studied by Supramolecular method (B3LYP/6-31++G(d)) and ONIOM (B3LYP/6-31++G(d):PM3) method. The results are as follows: (1) In 2Ap molecule surrounded by six water molecules the pyramidalization of amino group in 2Ap almost disappears, and the hex-atomic ring is obviously buckled. The adiabatic lowest-energy valence excitation of gaseous 2Ap also causes the disappearance of amino pyramidalization. (2) The energy for lowest-energy singlet π→π^* vertical transition in water is predicted as 3.99 and 4.29 eV by Supramolecular and ONIOM method, respectively. Both values are in good agreement with the reported experimental result, 4.11 eV. The energy for the second lowest-energy n→π^* transition, 4.72 eV, by the Supramolecular method is obviously deviated from the reported experimental value 4.46 eV. The corresponding value given by the two-layer ONIOM method, 4.43 eV, is in good agreement with the experimental value. (3) The optimized energy of the fluorescent emission state (S₁ state) are 3.61 and 3.87 eV by Supramolecular and ONIOM methods, respectively. The calculated oscillator strengths, in both gas and water clusters, were compared with reported experimental and theoretical results. These results indicated that both Supramolecular and ONIOM methods, combined with TD DFT B3LYP/6311++G(d), can provide good results of calculating excited state and spectra properties of 2Ap in condensed phase. This fact encouraged us to extend our study to 2Ap-T base pair and its solvated model so as to obtain the spectra properties of 2Ap in real DNA environment.     

Adiabatic ionization potential and electron affinity of formaldehyde

The adiabatic ionization potential and electron affinity for CH₂O have been calculated using high levels of ab initio molecular orbital theory. Harmonic vibrational frequencies and zero-point energies also have been predicted. At the CCSD(T)/6-311++G(3df,3pd) level of theory, the adiabatic ionization potential is calculated as 10.82 eV as compared to the experimental literature value of 10.8887±0.0030 eV. The electron affinity is calculated to be −0.96 eV, compared to the experimental literature value of −0.65±0.05 eV.     

Electron correlation and relativistic effects in xenon tetrafluoride

Ab initio all-electron fully relativistic Dirac–Fock self-consistent field and Dirac–Fock–Breit calculations are reported for the XeF₄ molecule at various internuclear distances assuming the experimental D_4h geometry with our recently developed relativistic universal Gaussian basis set. The nonrelativistic limit Hartree–Fock calculations were also performed for XeF₄ at various internuclear distances. The calculated relativistic correction to the total energy of molecule at the Dirac–Fock level is ～ ?5856 eV, whereas the magnetic part of the Breit correction to the electron-electron interaction is calculated as ～ 177 eV. The electron correlation effects were included in the nonrelativistic Hartree–Fock calculations using the second-order Møller-Plesset (MP 2) theory, and the calculated correlation energy for XeF₄ is ?71 eV. The basis-set superposition error (BSSE ) was estimated by using the counterpoise method for Xe and F. The inclusion of both the relativistic and electron correlation effects in the calculated total energies of F, Xe, and XeF₄ predicts the Xe—F bond length and dissociation energy of XeF₄ as 1.952 Å and 5.59 eV, respectively, which are in excellent agreement with the experimental values of 1.953 Å and 5.69 eV, respectively, for XeF₄. The contribution of the electron correlation and relativistic effects to the dissociation energy of XeF₄ is 8.11 and 0.05 eV, respectively. The Breit interaction, however, contributes only 0.02 eV to the dissociation energy of XeF₄. Electron correlation is most significant for the prediction of an accurate value of dissociation energy, whereas relativistic effects are very important for the prediction of spin-orbital splitting as well as the energies of the orbitals, especially the inner orbitals of XeF₄. © 1995 John Wiley & Sons, Inc.     

Vacuum ultraviolet (VUV) photoionization of small water clusters

Tunable vacuum ultraviolet (VUV) photoionization studies of water clusters are performed using 10-14 eV synchrotron radiation and analyzed by reflectron time-of-flight (TOF) mass spectrometry. Photoionization efficiency (PIE) curves for protonated water clusters (H2O)(n)H+ are measured with 50 meV energy resolution. The appearance energies of a series of protonated water clusters are determined from the photoionization threshold for clusters composed of up to 79 molecules. These appearance energies represent an upper limit of the adiabatic ionization energy of the corresponding parent neutral water cluster in the supersonic molecular beam. The experimental results show a sharp drop in the appearance energy for the small neutral water clusters (from 12.62 +/- 0.05 to 10.94 +/- 0.06 eV, for H2O and (H2O)4, respectively), followed by a gradual decrease for clusters up to (H2O)23 converging to a value of 10.6 eV (+/-0.2 eV). The dissociation energy to remove a water molecule from the corresponding neutral water cluster is derived through thermodynamic cycles utilizing the dissociation energies of protonated water clusters reported previously in the literature. The experimental results show a gradual decrease of the dissociation energy for removal of one water molecule for small neutral water clusters (3 相似文献   

Mass Spectrometric and Photoelectron Spectroscopic Studies of Zirconium Oxide Molecular and Cluster Anions

We present a preliminary report on our mass spectrometric and photoelectron spectroscopic studies of zirconium oxide molecular and cluster anions using a newly built laser vaporization/time-of-flight/magnetic bottle, negative ion photoelectron spectrometer. This work was motivated in part by evidence which suggests that zirconium dioxide catalyzes the radiolysis of interfacial water. We present our mass spectrometric observations of oxygen-rich zirconium oxide cluster anions and our photoelectron spectra of ZrO^– and ZrO ₂ ^– . From the photoelectron spectrum of ZrO^–, the adiabatic electron affinity of ZrO was determined to be 1.3±0.3 eV, and from this value, the dissociation energy of ZrO^– (into Zr and O^–) was found to be 7.8±0.3 eV. From the photoelectron spectrum of ZrO₂, the adiabatic electron affinity of ZrO₂ was determined to be 1.8±0.4 eV.     

A DFT study of H2O dissociation on metal‐precovered Fe (100) surface

The H₂O adsorption and dissociation on the Fe (100) surface with different precovered metals are studied by density functional theory. On both kinds of metal‐precovered surface, H₂O molecules prefer adsorb on hollow sites than bridge and top sites. The impurity energy difference is proportional to the adsorption energy, but the adsorbates are not sensitive to the adsorption orientation and height relative to the surface. The Hirshfeld charge analysis shows that water molecules act as an electron donor while the surface Fe atoms act as an electron acceptor. The rotation and dissociation of H₂O molecule occur on the Co‐ and Mn‐precovered surfaces. Some H₂O molecules are dissociated into OH and H groups. The energy barriers are about 0.5 to 1.0 eV, whose are consistence with the experimental data. H₂O molecules can be dissociated more easily at the top site on Co‐precovered surface 1 than that at bridge site on Mn‐precovered surface 2 because of the lower reaction barrier. The dispersion correction effects on the energies and adsorption configurations on Co‐precovered surface 1 were calculated by OBS + PW91. The dispersion contributions can improve a bit of the bond energy of adsorbates and weaken the hydrogen bond effect between adsorption molecules a little.     

Ab initio calculations of relativistic and electron correlation effects in polyatomics using the universal Gaussian basis set: XeF2

Ab initio accurate all-electron relativistic molecular orbital Dirac–Fock self-consistent field calculations are reported for the linear symmetric XeF₂ molecule at various internuclear distances with our recently developed relativistic universal Gaussian basis set. The nonrelativistic limit Hartree–Fock calculations were also performed for XeF₂ at various internuclear distances. The relativistic correction to the electronic energy of XeF₂ was calculated as ～ ?215 hartrees (?5850 eV) by using the Dirac–Fock method. The dominant magnetic part of the Breit interaction correction to the nonrelativistic interelectron Coulomb repulsion was included in our calculations by both the Dirac–Fock–Breit self-consistent field and perturbation methods. The calculated Breit correction is ～6.5 hartrees (177 eV) for XeF₂. The relativistic Dirac–Fock as well as the nonrelativistic HF wave functions predict XeF₂ to be unbound, due to neglect of electron correlation effects. These effects were incorporated for XeF₂ by using various ab initio post Hartree–Fock methods. The calculated dissociation energy obtained using the MP 2(full) method with our extensive basis set of 313 primitive Gaussians that included d and f polarization functions on Xe and F is 2.77 eV, whereas the experimental dissociation energy is 2.78 eV. The calculated correlation energy is ～ ?2 hartrees (?54 eV) at the predicted internuclear distance of 1.986 Å, which is in excellent agreement with the experimental Xe—F distance of 1.979 Å in XeF₂. In summary, electron correlation effects must be included in accurate ab initio calculations since it has been shown here that their inclusion is crucial for obtaining theoretical dissociation energy (D_e) close to experimental value for XeF₂. Furthermore, relativistic effects have been shown to make an extremely significant contribution to the total energy and orbital binding energies of XeF₂. © 1995 John Wiley & Sons, Inc.     

Accurate determination of the dissociation energies of alkali halides

Photofragmentation spectroscopy offers a possibility to determine the dissociation energy of diatomic molecules with an accuracy of about 0.01 eV, which is half an order of magnitude better than any other applicable method. In this letter the method will be compared with other known techniques and results will be given for NaBr, KBr, NaI ad KI.