Tetra-hydrides of the third-row transition elements: spin–orbit coupling effects on geometrical deformation in WH4 and OsH4

The dissociation energies of MH₄ (M =  La, Hf–Hg) were computed using full optimized reaction space (FORS) multi-configuration self-consistent field (MCSCF) and second-order multi-reference Møller–Plesset perturbation methods with the SBKJC basis sets augmented by a set of polarization functions (SBKJC(f,p)). It was shown that of the molecules examined, only four tetra-hydrides HfH₄, TaH₄, WH₄, and OsH₄ with T_d symmetry are lower in energy than the corresponding dissociation limits. For WH₄ and OsH₄, the potential energy surfaces from the D_4h to the T_d structure were explored from both theoretical calculations and symmetry arguments based on the pseudo-Jahn- Teller effect. As for WH₄, it is found that the ground state could be ³E_g, ³A_2g, or ³B_2g at the D_4h structure. The present calculations suggest that the ground state is ³E_g, and that this state is stabilized by the e_u deformation into a C_2v structure (³B₁) and then sequentially to the most stable T_d structure (³A₂). If the molecular system is promoted to the lowest ³B_2g state, the D_4h structure can directly deform into the most stable T_d structure along the b_2u vibrational mode. For OsH₄, the ground state (⁵B_1g) at the D_4h structure deforms into a D_2d structure and the resulting ⁵B₂ state strongly interacts with the lowest ³E and ¹A₁ states due to the spin-orbit couplings (SOCs). As a result, it was shown that the relativistic potential energy of the lowest spin-mixed state (ground state) monotonically decreases along the D_2d deformation path from the D_4h to the T_d structure.     

A theoretical comparison of the lowest-lying singlet and triplet states of H2CBe and of HCBeH

The low-lying singlet and triplet states of H₂CBe and HCBeH are examined using ab inito molecular orbital theory. In agreement with earlier results, the lowest-lying structure of H₂CBe has C_2v symmetry and is a triplet with one π electron (³ B₁). The results presented here suggest that the lowest-energy singlet structure is the (¹B₁) open-shell singlet, also with C_2v symmetry, at least 2.5 kcal/mol higher in energy. The singlet C_2v structure with two π electrons (¹A₁) is 15.9 kcal/mol higher than ³B₁. All of these structures are bound with respect to the ground state of methylene and the beryllium atom. In HCBeH, linear equilibrium geometries are found for the triplet (³Σ) and singlet (¹Δ) states. The triplet is more stable than the singlet (¹Δ) by 35.4 kcal/mol, and is only 2.9 kcal/mol higher in energy than triplet H₂ CBe. Since the transition structure connecting these two triplet molecules is found to be 50.2 kcal/mol higher in energy than H₂ CBe, both triplet equilibrium species might exist independently. The harmonic vibrational frequencies of all structures are also reported.     

Theory of spin triplet ground states ind 6 transition metal compounds and the effect of high-energy states on the nature of the ground state

The formation of spin triplet, quintet, and singlet ground states within the 3d ⁶ electron configuration is investigated inD _4h , andD _3d symmetries employing irreducible tensor operator methods. Significant differences in the possible ground states are encountered between a complete CI and spin-orbit interaction treatment and an approximate calculation within the cubic⁵ T ₂,¹A₁,³ T ₁, and³ T ₂ parents.     

Structure of the 1,3-dinitrobenzene dianion studied by multiconfigurational methods

The structure of the 1,3-dinitrobenzene dianion in the ground and lowest excited electron states was studied by quantum chemical methods. The dianion, unlike the radical anion, is characterized by the symmetrical structure in both the ground triplet (³B₁) and lowest excited singlet (¹A₁) states. The wave function of the singlet state has the biradical character to a great extent. The singlet-triplet splitting calculated by the CASSCF and MRMP2 methods is 6 and 2 kcal mol⁻¹, respectively. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1408–1410, July, 2007.     

An ab initio theoretical study of the optical spectrum of CF2

The ab initio calculation methods have been used to calculate the spectral and electronic characteristics of difluorocarbene in the ground electronic state (¹A₁), the lowest-lying singlet (¹B₁) and triplet (³B₁) states. The optimized equilibrium geometries, rotational constants, harmonic vibrational frequencies and energy gaps, electronic charges, dipole moments of these states have been computed with different basis sets. The calculated vibrational frequency of ³B₁ state (v₂=522 cm^?1) and the energy separation (2.26 eV) between ³B₁ and ¹A₁ states are in good agreement with the experimental results (519 cm^?1, 2.46 eV respectively). According to the calculations the previous assignment of vibrational symmetries of ¹B₁ state was corrected, and some experimentally undetermined vibrational frequencies were predicted.     

High-Spin Iron(VI), Low-Spin Ruthenium(VI), and Magnetically Bistable Osmium(VI) in Molecular Group 8 Nitrido Trifluorides NMF3

Pseudo-tetrahedral nitrido trifluorides N≡MF₃ (M=Fe, Ru, Os) and square pyramidal nitrido tetrafluorides N≡MF₄ (M=Ru, Os) were formed by free-metal-atom reactions with NF₃ and subsequently isolated in solid neon at 5 K. Their IR spectra were recorded and analyzed aided by quantum-chemical calculations. For a d² electron configuration of the N≡MF₃ compounds in C_3v symmetry, Hund's rule predict a high-spin ³A₂ ground state with two parallel spin electrons and two degenerate metal d(δ)-orbitals. The corresponding high-spin ³A₂ ground state was, however, only found for N≡FeF₃, the first experimentally verified neutral nitrido Fe^VI species. The valence-isoelectronic N≡RuF₃ and N≡OsF₃ adopt different angular distorted singlet structures. For N≡RuF₃, the triplet ³A₂ state is only 5 kJ mol⁻¹ higher in energy than the singlet ¹A′ ground state, and the magnetically bistable molecular N≡OsF₃ with two distorted near degenerate ¹A′ and ³A“ electronic states were experimentally detected at 5 K in solid neon.     

Ultraviolet Photodissociation Dynamics of m-Bromofluorobenzene at around 240 nm?

The photodissociation dynamics of m-bromofluorobenzene has been experimentally investigated at around 240 nm using the DC-slice velocity map imaging technique. The kinetic energy release spectra and the recoiling angular distributions of fragmented Br(²P_3/2) and Br(²P_1/2) atoms from photodissociation of m-bromofluorobenzene have been measured at different photolysis wavelengths around 240 nm. The experimental results indicate that two dissociation pathways via (pre-)dissociation of the two low-lying ¹ππ^* excited states dominate the production process of the ground state Br(²P_3/2) atoms. Because of the weak spin-orbit coupling effect among the low-lying triplet and singlet states, the spin-orbit excited Br(²P_1/2) atoms are mainly produced via singlet-triplet state coupling in the dissociation step. The similarity between the present results and that recently reported for o-bromofluorobenzene indicates that the substitution position of the fluorine atom does not significantly affect the UV photodissociation dynamics of bromofluorobenzenes.     

Theoretical study of the electronic spectra ofcis-1,3,5-hexatriene andcis-1,3-butadiene

Summary The electronic spectra forcis-1,3-butadiene andcis-1,3,5-hexatriene have been studied using multiconfiguration second-order perturbation theory (CASPT2) and extended ANO basis sets. The calculations comprise all singlet valence excited states below 8.0 eV, the first 3s, 3p, 3d Rydberg states, and the second 3s state. The four lowest triplet states were also studied. The resulting excitation energies forcis-hexatriene have been used in an assignment of the experimental spectrum, leading to a maximum deviation of 0.13 eV for the vertical transition energies. The calculations place the 1¹ B ₂ state 0.04 eV below the 2¹ A ₁ state. 16 excited states were studied incis-butadiene, using a CASPT2 optimized ground state geometry. The 1¹ B ₂ state was located at 5.58 eV, 0.46 eV below the 2¹ A ₁ state and 0.09 eV above the experimental value. No experimental assignments are available for the 15 other transitions. On leave from: Departmento de Quimica Física, Universidad de Valencia, Dr. Moliner 50, Burjassot, E-46100-Valencia, Spain     

The Low‐Lying Excited States of 2,2′‐Bithiophene: A Theoretical Analysis

The low‐energy regions of the singlet→singlet, singlet→triplet, and triplet→triplet electronic spectra of 2,2′‐bithiophene are studied using multiconfigurational second‐order perturbation theory (CASPT2) and extended atomic natural orbitals (ANO) basis sets. The computed vertical, adiabatic, and emission transition energies are in agreement with the available experimental data. The two lowest singlet excited states, 1¹B_u and 2¹B_u, are computed to be degenerate, a novel feature of the system to be borne in mind during the rationalization of its photophysics. As regards the observed high triplet quantum yield of the molecule, it is concluded that the triplet states 2³A_g and 2³B_u, separated about 0.4 eV from the two lowest singlet excited states, can be populated by intersystem crossing from nonplanar singlet states.     

Some theoretical studies on hydrogen molecule capture by platinum atoms

Pseudopotential SCF-LCAO-MO and variational and perturbative Cl calculations were carried out for H₂ molecule capture by a single Pt atom with C_2v symmetry. A pseudopotential for the platinum atom including relativistic effects was used. Singlet and triplet states of the Pt-H₂ interaction having different representations of the mentioned C_2v symmetry were studied. The triplet ground state of Pt leads to two A₁ and B₂ states in which the metal atom cannot capture H₂; i.e., both have repulsive interaction energies. The electronic state responsible for the capture of H₂ is the closed-shell, singlet A₁ excited state. The equilibrium geometry of the system is reached with a broken H? H bond at a HPtH angle of about 100°. Additionally another shallower minimum for a singlet A₁ linear structure is observed. Specific predictions for the thermal and photochemical Pt + H₂ reactions that can be carried out under matrix isolation conditions are made.     

(all-E)-1,3,5,7-Octatetraene: Electron-Energy-Loss and Electron-Transmission Spectra

Electron-energy-loss and electron-transmission spectra of (all-E)-1,3,5,7-octatetraene were recorded with a trochoidal electron spectrometer. The energy-loss spectra reveal two triplet states at 1.73 and 3.25 eV (0,0-transitions), the UV-active 1¹B_u state at 4.40 eV, and a higher-lying singlet state at 6.04 eV. The 2¹A_g state recently reported to be the lowest excited singlet state in this polyene, could not be observed. This failure is probably de to a small excitation cross section under the scattering conditions used and the presence of the second triplet sate in the pertinent energy region. The electron-transmission spectrum revealed three resonances (i.e. short-lived anion states) at 1.5, 2.5, and 4.15 eV.     

Triplet Energy of 2, 2-Dimethylisoindene from Electron-Energy-Loss Spectroscopy and Photoinduced Triplet Energy Transfer

The excited electronic states of 2, 2-dimethylisoindene ( 1 ) have been studied by electron-energy-loss spectroscopy. Its vertical gas-phase triplet (1³B₂), and singlet (1¹B₂) excitation energies are 1.61 and 3.19 eV, respectively. The excited states are thus lowered by 0.49 eV and 1.21 eV, respectively, when compared to the corresponding states of (all-E)-octatetraene, which serves as a reference compound. These shifts are partially reproduced by ZINDO calculations. The spectra give no evidence for a 2¹A_g state below the 1¹B₂ state, but this lack of observation does not exclude its existence. The lowest triplet state T₁( 1 ) was further characterized by flash photolysis. T₁( 1 ) was observed as a transient intermediate, λ ≤ 350 nm, with a lifetime of 8 m?s in degassed hexane. The adiabatic excitation energy of T₁( 1 ) was bracketed to the range of 1.1 ± 0.1 eV by energy-transfer experiments. Relationships between the energies of the lowest excited singlet and triplet states of 1 and the lowest excited doublet state of its radical cation ${1}^{+\kern0pt {.}}$ – essentially a non-Koopmans' state – are discussed.     

The Role of the nπ* 1Au State in the Photoabsorption and Relaxation of Pyrazine

The geometric, energetic, and spectroscopic properties of the ground state and the lowest four singlet excited states of pyrazine have been studied by using DFT/TD‐DFT, CASSCF, CASPT2, and related quantum chemical calculations. The second singlet nπ* state, ¹A_u, which is conventionally regarded dark due to the dipole‐forbidden ¹A_u←¹A_g transition, has been investigated in detail. Our new simulation has shown that the state could be visible in the absorption spectrum by intensity borrowing from neighboring nπ* ¹B_3u and ππ* ¹B_2u states through vibronic coupling. The scans on potential‐energy surfaces further indicated that the ¹A_u state intersects with the ¹B_2u states near the equilibrium of the latter, thus implying its participation in the ultrafast relaxation process.     

Construction and applications of symmetrized valence bond wave functions

A method constructing symmetry-adapted bonded Young tableau bases is proposed, based on the symmetry properties of bonded tableaus and the projection operator associated with a point group. Several examples including the ground states and π excited states of O₃⁻, O₃, O₃⁺, and C₃⁻ are shown for instruction to construct the symmetrized valence bond (VB) wave function. Excitation energies of transitions from the ground states to π excited states of O₃⁻, C₃H₅, and C₃⁻ are calculated with an optimized symmetrized valence bond wave function in the σ–π separation approximation. Good agreement between the VB and experimental excitation energies is observed. The bonding features of the ground state and the first π excited singlet and triplet states for S₃ are discussed according to bonding populations from VB calculations. Both the singlet-biradical and the dipole structures have significant contributions to the ground state X ¹A₁ of S₃, while the excited state 1 ¹B₂ is essentially composed of the dipole structures, and the 1 ³B₂ excited state is comprised from a triplet-biradical structure. © 1998 John Wiley & Sons, Inc. Int J Quant Chem 66 : 1–7, 1998     

Ab initio study on the reaction of Sc+ + CH4 → Sc+(SINGLE BOND)CH2 + H2

An ab initio study on the reaction of the ground state (³D) and the excited state (¹D) of Sc⁺ with methane was performed. Reaction channels on the singlet and triplet potential surface (PES) and the reaction mechanism are examined and discussed. Three regions of the potential surface was studied: the molecular complex, the C(SINGLE BOND)H insertion products, and the transition states for the reaction. Comparisons between singlet and triplet PESs show that the excited state (¹D) of Sc⁺ has more reactivity with methane than does the ground state (³D) due to the spin quantum number conservation with the more stable insertion intermediate. © 1997 John Wiley & Sons, Inc.     

Vertical proton affinities of CH2O and CH2OH+ in their ground singlet,excited triplet and ionized doublet states

Vertical proton affinities were calculated with closed and open shell direct SCF-MO methods for the ground, excited triplet and ionized doublet states of CH₂O and CH₂OH⁺.The computed gas phase basicity of CH₂O follows the order: CH₂O(¹ A ₁) > CH₂O*(³ A ₁ or ³ A ₂) > CH₂O⁺(² B ₂ or ² B ₁).     

The electronic states of 1,2,5-oxadiazole studied by VUV absorption spectroscopy and CI, CCSD(T) and DFT methods

The 1,2,5-oxadiazole VUV absorption spectrum in the range 5–11.5 eV, shows broad bands centred near 6.2, 7.1, 8.3, 8.8, 10.6 and 11.3 eV. Rydberg states associated with three ionisation energies (IE) were identified in the complex fine structure above 8.7 eV. Electronic vertical excitation energies for singlet and triplet valence, and Rydberg states were computed using ab initio multi-reference multi-root CI methods. There is generally a good correlation between the envelope of the theoretical intensities and the experimental spectrum. The nature of the more intense calculated Rydberg states, and positions of the main valence and Rydberg bands are discussed. The lowest triplet, singlet and Rydberg 3s excited states have equilibrium structures that are non-planar with C_S symmetry, in a chair-like orientation where the O and H atoms lie out of the NCCN plane. This finding is consistent with the doubling of the low energy UV spectral lines [B.J. Forrest, A.W. Richardson, Can. J. Chem., 50 (1972) 2088].The nearly degenerate IE of the UV-photoelectron spectrum (UV–PES, Palmer et al. 1977) makes analysis of the VUV spectrum difficult, leading to the necessity for reinvestigation. Vertical studies (IE_V) using CI, Tamm–Dancoff (TDA) and Green’s Function (GF) methods all gave similar results, with near degeneracy of the first 3IE_V confirming the earlier study. Studies of the adiabatic IE (IE_A) using CCSD(T) and B3LYP methods, showed the energy sequence ²A₂ < ²B₁ < ²B₂, but these states are all saddle points, in contrast to the 4th state (²A₁) which is a minimum. In contrast, MP2 study of the ²B₂ state showed a minimum, with only two saddle points.Complete minima were found after minor twisting of the structures. The lowest energy cationic state is ²A^″ (C_S), which closely resembles the ²B₂ state. The O–N–C–C skeleton is twisted by 8°. The corresponding ²A^′ state (C_S) is effectively identical to the ²B₁ state. Attempts to find minima for other symmetry states were unsuccessful.     

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.     

Calculations on 1,8-naphthoquinone predict that the ground state of this diradical is a singlet

(12/12)CASPT2, (16/14)CASPT2, B3LYP, and CCSD(T) calculations have been carried out on 1,8-Naphthoquinone (1,8- NQ ), to predict the low-lying electronic states and their relative energies in this non-Kekulé quinone diradical. CASPT2 predicts a ¹A₁ ground state, with three other electronic states—³B₂, ³B₁, and ¹B₁—within about 10 kcal/mol of the ground state in energy. On the basis of the results of these calculations, it is predicted that NIPES experiments on 1,8- NQ ^•– will find that 1,8- NQ is a diradical with a singlet ground state. © 2018 Wiley Periodicals, Inc.