Potential energy profiles of the geometric isomerization and the thermal decomposition of diphosphene HP=PH in the ground and excited electronic states

Summary The geometric isomerization and the dehydrogenation of HP=PH in the ground and some low-lying excited states are investigated by theoretical calculations. The reaction paths are traced by either the CASSCF or UHF-SCF calculations using the 6-31G(d,p) basis functions, and the accompanying energy changes are calculated by the MRD-CI method employing the [5s3p1d]/[2s1p] basis functions. The barrier heights for the trans-to-cis isomerization, by the planar inversion and the nonplanar twisting, in the ground state are calculated to be 265 and 144 kJ/mol (with the vibrational zero-point energy corrections), respectively. The latter barrier is noticeably lower than the H-P and the P-P bond dissociation energies oftrans-HP=PH (¹A_g), which are 304 and 271 kJ/mol, respectively. The ground-state HP₂ radical (²A'), which is to be formed by the dehydrogenation of HP=PH, should suffer further decomposition into P₂ (¹ _g ⁺ ) and H with an activation energy of 139 kJ/mol. The lowest excited state of HP₂ is found to be a hydrogen-bridged 3-electron system (²A₂) having an isosceles triangle structure. It has proved to be formed by the dehydrogenation of the lowest excited singlet state (¹B) of HP=PH via a transition state which lies 194 kJ/mol above the¹B state. The excited HP₂ (²A₂) is state-correlated with P₂ (³_u)+H.     

Spin‐orbit ab initio and density functional theory investigation of bismuth monoboronyl,BiBO

The molecular properties of bismuth monoboronyl, BiBO, were investigated using high‐level ab initio and density functional theory calculations by including the effect of spin‐orbit coupling (SOC). SOC does not cause any change in the Bi? B bond length of BiBO, by contrast it causes significant elongation of the Bi? B bond of BiBO^?, by ～0.03 Å. The Bi? B bond length of BiBO^? that is calculated by considering SOC is almost identical to that of BiBO; this result is consistent with a recent experimental study. The term values of excited states of BiBO calculated by including SOC are in good agreement with the experimental results. One excited state which was not assigned in the previous experimental study is the Ω = 0⁺ state generated by strong SOC. In the theoretical calculations on molecules containing 6p‐block elements, including SOC is crucial for obtaining results that are consistent with the corresponding experimental results.     

On the low-lying states of TiC

The ground and low-lying excited states of TiC are investigated using a CASSCF—externally contracted Cl approach. The calculations yield a ³Σ⁺ ground state, but the ¹Σ⁺ state is only 780 cm^?1 higher and cannot be ruled out. The low-lying states have some triple bond character. The nature of the bonding and origin of the states are discussed.     

Coupled cluster investigation on the low-lying electronic states of CuCN and CuNC and the ground state barrier to isomerization

The observation of several metal cyanides and isocyanides in interstellar space has raised much interest these molecules. Optimum molecular structures, harmonic vibrational frequencies, and dipole moments of the ground electronic states (X1Sigma+), triplet excited states, and open shell singlet excited states of CuCN and CuNC were determined using different levels of nonrelativistic and scalar relativistic (Douglas-Kroll) [Ann. Phys. 82, 89 (1979)] coupled cluster theory in conjunction with atomic natural orbital basis sets and correlation consistent basis sets. For the relativistic computations the specially contracted correlation consistent Douglas-Kroll (DK) basis sets were used. Moreover, barriers to isomerization from CuCN to CuNC were computed. The predicted structures of the X1Sigma+ state for CuCN are re(Cu-C)=1.826 A and re(C-N)=1.167 A, at the most sophisticated level of theory, the scalar relativistic DK-CCSD(T)/cc-pVQZ(DK) method. These results are in excellent agreement with the experimentally determined Cu-C bond length of 1.829 A and C-N bond distance of 1.162 A. At the same level of theory, the zero-point corrected barrier to isomerization from CuCN to CuNC is estimated to be 14.7 kcal mol(-1), and the cyanide is more stable than the isocyanide by 11.5 kcal mol(-1). For both CuCN and CuNC the 3Sigma+ state is the lowest lying excited electronic state. At the DK-CCSD/cc-pVQZ(DK) level of theory, the energetic ordering of excited states of CuCN and CuNC is X1Sigma+相似文献   

Ab initio valence bond calculations. VII. HF,HF+, and H2F+

Ab initio valence bond calculations for the ground and excited states of HF and HF⁺ are presented. Total energies, equilibrium geometries, dissociation energies, dipole moments, and spectroscopic constants for HF and HF⁺ have been calculated. The photoelectron spectrum of HF has been examined and interpreted by means of the valence bond formalism. The ground state of the protonated species H₂F⁺ has been investigated.     

IR laser manipulation of cis<-->trans isomerization of 2-naphthol and its hydrogen-bonded clusters

The cis<-->trans isomerization reaction has been carried out for 2-naphthol and its hydrogen (H) bonded clusters by infrared (IR) laser in the electronic excited state (S1) in supersonic jets. A specific isomer in the jet was pumped to the X-H stretching vibration in the S1 state, where X refers to C, O, or N atom, by using a stepwise UV-IR excitation, and the dispersed emission spectra of the excited species or generated fragments were observed. It was found that the isomerization occurs only in the H-bonded clusters but a bare molecule does not exhibit the isomerization in the examined energy region of Ev< or =3610 cm(-1), indicating a reduction of the isomerization barrier height upon the H bonding. The relative yield of the isomerization was observed as a function of internal energy. The isomerization yield was found to be very high at the low IR frequency excitation, and was rapidly reduced with the IR frequency due to the competition of the dissociation of the H bond within the isomer. Density-functional theory (DFT) and time-dependent DFT calculations were performed for estimating the barrier height of the isomerization for bare 2-naphthol and its cluster for electronic ground and excited states. The calculation showed that the isomerization barrier height is highly dependent on the electronic states. However, the reduction of the height upon the hydrogen bonding was not suggested at the level of our calculation.     

Quantumchemical calculations on the photochemistry of germacrene and germacrol. The exclusive role of the exocyclic double bond isomerization

The different photochemistry of the title compounds (reactions of the endocyclic 1,5-dlene moiety versus a photochemical [1,3]-OH shift) can be explained assuming an initial isomerization of the exocyclic double bond. MNDO/CI calculations of the potential energy curves and nonadiabatic couplings for the rotation of this bond showed that the 90° twisted conformation can easily be reached. For germacrol the lowest excited state has a zwitterionic character which is favourable for a planar photochemical [1,3]-OH shift. For germacrene, this polarized state is strongly coupled to two diradicalar states. In these twisted diradicalar states a redistribution of the charges in the endocyclic double bonds is found which is eminently suited for intramolecular bond formation.     

Relativistic all electron configuration interaction calculation of ground and excited states of the gold hydride molecule

We present relativistic configuration interaction calculations with the spin-free no-pair hamiltonian on the gold hydride molecule, treating the ground state as well as the eleven lowest excited states. The calculations provide a picture of the bonding in theX ¹Σ⁺ ground state consistent with previous work on this species using four-component spinors: compared to non-relativistic calculations, the dipole moment is reduced by a factor of two, hybridization (and thus participation ofd orbitals at the bonding) is greatly enhanced, the bond length is shortened by 20 pm, and the dissociation energy is increased by 50%. Comparison of the spin-averaged potential curves of the excited states with experiment suggests a reinterpretation of theC ¹Σ⁺ as the 0⁺ fine structure component of 2³Π and the prediction of a weakly bound³Σ⁺ state with weak transitions to the ground state in the range of 2.9–3.1 eV.     

Non-empirical SCF and Cl Study of the ground and excited states of thioformaldehyde

Ab initio SCF and Cl calculations are reported for ground and various low-lying Rydberg and valence excited states of thioformaldehyde H₂CS. A double-zeta basis of near Hartree-Fock quality is employed in this work and the importance of polarization functions is also assessed. The calculations indicate uniformly larger CX bond lengths in this system than for H₂CO in the corresponding electronic states; they also lind potential minima for H₂CS non-planar nuclear conformations in the (n,π^*) and (π,π^*) excited states but in each case the calculated inversion barriers are seen to be smaller than those encountered in formaldehyde. The vertical transition energies to the various excited states studied are also found to be significantly smaller in H₂CS than in H₂CO but the order of electronic states is concluded to be virtually identical for the two systems. The lowest-lying excited states are the ^3,1(n,π^*) species calculated at 1.84 and 2.17 eV respectively; the first two allowed transitions are indicated to be the Rydberg species (n,s_R) and (n,px_R) at 5.83 and 6.62 eV. These are followed by the two allowed transitions σ → π^* and π → π^* at 7.51 and 7.92 eV respectively, both well below the first ionization limit in H₂CS. The much smaller splitting between the ^3,1(π,π^*) species in H₂CS than in H₂CO is attributed to the relatively diffuse charge distribution of the sulfur atom compared to that of oxygen.     

AB initio calculations of the dipole moments in low-lying electronic states of the ccn radical

The structures and dipole moments of the four low-lying electronic states (X²Π, A²Δ, B²Σ⁻ and C²Σ⁺) of the linear CCN radical are investigated by ab initio calculations at SDCI/DZP and TZP levels. For all the electronically excited states, the dipole moments are calculated to be ≈ 3.0 D. However, a significantly smaller dipole moment, ≈ 0.6 D, is predicted for the ground state. This result is consistent with the recent experiment by Suzuki, Saito and Hirota, where the MODR signals are observed for the A state CCN but not for the X state. Electronic correlation is important in determining both equilibrium bond lengths and dipole moments.     

The Early Life of a Peptide Cation-Radical. Ground and Excited-State Trajectories of Electron-Based Peptide Dissociations During the First 330 Femtoseconds

We report a new approach to investigating the mechanisms of fast peptide cation-radical dissociations based on an analysis of time-resolved reaction progress by Ehrenfest dynamics, as applied to an Ala-Arg cation-radical model system. Calculations of stationary points on the ground electronic state that were carried out with effective CCSD(T)/6-311++G(3df,2p) could not explain the experimental branching ratios for loss of a hydrogen atom, ammonia, and N–C_α bond dissociation in (AR + 2H)^+●. The Ehrenfest dynamics results indicate that the ground and low-lying excited electronic states of (AR + 2H)^+● follow different reaction courses in the first 330 femtoseconds after electron attachment. The ground (X) state undergoes competing loss of N-terminal ammonia and isomerization to an aminoketyl radical intermediate that depend on the vibrational energy of the charge-reduced ion. The A and B excited states involve electron capture in the Arg guanidine and carboxyl groups and are non-reactive on the short time scale. The C state is dissociative and progresses to a fast loss of an H atom from the Arg guanidine group. Analogous results were obtained by using the B3LYP and CAM-B3LYP density functionals for the excited state dynamics and including the universal M06-2X functional for ground electronic state calculations. The results of this Ehrenfest dynamics study indicate that reaction pathway branching into the various dissociation channels occurs in the early stages of electron attachment and is primarily determined by the electronic states being accessed. This represents a new paradigm for the discussion of peptide dissociations in electron based methods of mass spectrometry.     

Stairway to the conical intersection: a computational study of the retinal isomerization

The potential-energy surface of the first excited state of the 11-cis-retinal protonated Schiff base (PSB11) chromophore has been studied at the density functional theory (DFT) level using the time-dependent perturbation theory approach (TDDFT) in combination with Becke's three-parameter hybrid functional (B3LYP). The potential-energy curves for torsion motions around single and double bonds of the first excited state have also been studied at the coupled-cluster approximate singles and doubles (CC2) level. The corresponding potential-energy curves for the ground state have been calculated at the B3LYP DFT and second-order M?ller-Plesset (MP2) levels. The TDDFT study suggests that the electronic excitation initiates a turn of the beta-ionone ring around the C6-C7 bond. The torsion is propagating along the retinyl chain toward the cis to trans isomerization center at the C11=C12 double bond. The torsion twist of the C10-C11 single bond leads to a significant reduction in the deexcitation energy indicating that a conical intersection is being reached by an almost barrierless rotation around the C10-C11 single bond. The energy released when passing the conical intersection can assist the subsequent cis to trans isomerization of the C11=C12 double bond. The CC2 calculations also show that the torsion barrier for the twist of the retinyl C10-C11 single bond adjacent to the isomerization center almost vanishes for the excited state. Because of the reduced torsion barriers of the single bonds, the retinyl chain can easily deform in the excited state. Thus, the CC2 and TDDFT calculations suggest similar reaction pathways on the potential-energy surface of the excited state leading toward the conical intersection and resulting in a cis to trans isomerization of the retinal chromophore. According to the CC2 calculations the cis to trans isomerization mechanism does not involve any significant torsion motion of the beta-ionone ring.     

Theoretical prediction on HAlS+ and HSAl+ cations using multiconfiguration second‐order perturbation theory

Some low‐lying states of the HAlS⁺ and HSAl⁺ cations have been studied for the first time by large‐scale theoretical calculations using three methods: complete active space self‐consistent field (CASSCF), complete active second‐order perturbation theory (CASPT2), and density functional theory Becke's three‐parameter hybrid function with the nonlocal correlation of Lee–Yang–Parr (B3LYP) with the contracted atomic natural orbital (ANO‐L) and cc‐pVTZ basis sets. The geometries of all stationary points along the potential energy surfaces (PESs) were optimized at the CASSCF/ANO‐L and B3LYP/cc‐pVTZ levels. The ground and the first excited states of linear HAlS⁺ are predicted to be X²Π and A²Σ⁺ states, respectively. For the linear HSAl⁺ structure, the first excited state is A²Σ⁺. The X²Π state of linear HSAl⁺ is a second‐order saddle point, because it has two imaginary frequencies. Two bent global minima M1 and M2 were found along the 1²A′ and 1²A″ PESs, respectively. The CASPT2/ANO‐L potential energy curves of isomerization reactions were calculated as a function of HAlS bond angle. According to our calculations, the ground‐state HAlS⁺ is linear, whereas the ground‐state HSAl⁺ is bent. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

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     

An ab initio investigation of the geometry,bonding and coupling constants of BF2

Comparative calculations, using five different basis sets of contracted Gaussian functions, of the geometry, bonding and hyperfine coupling constants of BF₂ are reported. The best calculation, using a near Hartree-Fock atomic basis, predicts a bond angle of 120° and a bond length of 2.50 a.u. (=1.32 Å) for the X ² A ₁ ground state. The geometries of three low-lying excited states are also presented.     

Overtone-induced dissociation and isomerization dynamics of the hydroxymethyl radical (CH2OH and CD2OH). I. A theoretical study

The dissociation of the hydroxymethyl radical, CH(2)OH, and its isotopolog, CD(2)OH, following the excitation of high OH stretch overtones is studied by quasi-classical molecular dynamics calculations using a global potential energy surface (PES) fitted to ab initio calculations. The PES includes CH(2)OH and CH(3)O minima, dissociation products, and all relevant barriers. Its analysis shows that the transition states for OH bond fission and isomerization are both very close in energy to the excited vibrational levels reached in recent experiments and involve significant geometry changes relative to the CH(2)OH equilibrium structure. The energies of key stationary points are refined using high-level electronic structure calculations. Vibrational energies and wavefunctions are computed by coupled anharmonic vibrational calculations. They show that high OH-stretch overtones are mixed with other modes. Consequently, trajectory calculations carried out at energies about ~3000 cm(-1) above the barriers reveal that despite initial excitation of the OH stretch, the direct OH bond fission is relatively slow (10 ps) and a considerable fraction of the radicals undergoes isomerization to the methoxy radical. The computed dissociation energies are: D(0)(CH(2)OH → CH(2)O + H) = 10,188 cm(-1), D(0)(CD(2)OH → CD(2)O + H) = 10,167 cm(-1), D(0)(CD(2)OH → CHDO + D) = 10,787 cm(-1). All are in excellent agreement with the experimental results. For CH(2)OH, the barriers for the direct OH bond fission and isomerization are: 14,205 and 13,839 cm(-1), respectively.     

Visual pigments. V. Ground and excited-state acid dissociation constants of protonated all-trans retinal schiff base and correlation with theory.

Abstract— The acid dissociation constants of protonated all-trans retinal Schiff base (SB-H⁺) in a 50% water-methanol solution at 0°C is 6–95 for the ground state and nominally 16–65 for the first excited singlet state, with a potential range of ? 12–21. These values are in qualitative agreement with the results of semiempirical MO calculations, which indicate that the total charge density on nitrogen is greater in the first excited singlet than in the ground state (Q_N* > Q_N). However, pertinent to vision, CNDO/2 calculations on all-trans and 11-cis Schiff base and SB-H⁺ indicate that, for torsional angles of approximately 80–100° around the 11–12 double bond, Q_N* < Q_N. This result suggests that it may be possible for the proton to come off the imine nitrogen during isomerization from 11 -cis SB-H⁺ to all-trans SB-H⁺. The potential consequence of this during isomerization of rhodopsin is the initiation of unfolding of the protein opsin.     

Electronic states and nature of bonding in the molecule YN by all-electron ab initio CASSCF calculations

In the present work, we present results of all-electron ab initio CASSCF calculations of nine electronic states of the molecule YN. Also reported are the spectroscopic constants derived on the basis of the calculated potential energies. The predicted electronic ground state is ¹∑⁺, and this state is found to be separated from the excited states ³∑⁺, ³Π, and ¹Π by 5177, 9290, and 9915 cm^?1, respectively. The chemical bond in the YN molecule is polar with charge transfer from Y to N, giving rise to a dipole moment of 8.19 Debye at 3.3 au in the ¹∑⁺ ground state is basically a double bond composed of two π bonds. The dissociation energy of the YN molecule has been derived as 4.59 eV. © 1993 John Wiley & Sons, Inc.     

Hydrogen bonding and solvatochromatic shift of the lowest 1(n, pi*) excitation of s-tetrazine in its hydrated clusters and dilute solutions

Hydrogen bonding involving azine and its derivatives such as nucleic bases is very important for understanding the structure and function of biological systems. In this work, we have investigated the hydrogen bonding structures of the hydrated cluster and dilute aqueous solution of s-tetrazine using computer simulation techniques, and evaluated the absorption and fluorescence shifts of the lowest 1(n, pi*) excitation of s-tetrazine solution using our solvent shift method. For the s-tetrazine-water cluster, a linear orthodox hydrogen bond arrangement is predicted in both ground and excited states with small structural and energetic differences, and a bifurcated hydrogen bond isomerization is anticipated. Further, ab initio calculations have verified these conformations. For the s-tetrazine-water solution, a mixture of two hydrogen bonding arrangements is found to be in both ground and excited states, resulting in small magnitudes of absorption and fluorescence solvent shifts. This finalizes our series investigation of hydrogen bonding and solvent shifts of dilute azines in water.     

Theoretical study of the desorption of neutral and ionic alkali metal atoms from the excited Li+(H2O)n = 1-4 and Na+(H2O)n = 1-4 cluster models: Electronic excitation charge transfer

In this work, the potential energy curves of several low-lying excited states of M⁺(H₂O)_{n = 1-4} (M = Li and Na) clusters with one M─O bond, related to the stretching of their M─O bond, were calculated in the gas phase. The time-dependent density functional theory and direct-symmetry-adapted cluster-configuration interaction were used in this study separately. Theoretical calculations showed that the charge transfer occurred between M⁺ and (H₂O)_n in the excited clusters so that the neutral metal atom was obtained at the dissociation limit of the potential curves. The excited potential curves of clusters were also calculated in the presence of the electrostatic field of water (EFW), and it was found that the charge transfer was blocked in the presence of EFW. The effect of the size of the (H₂O)_n cluster on the shape of the excited potential curves was investigated to observe how the M─O bond was affected in the excited states depending on the (H₂O)_n size. It was found that the increase in the size of the (H₂O)_n cluster increased the number of bonding excited potential curves. The difference between the electron density of the excited and ground electronic states was calculated to see how the charge transfer was affected by the size of the (H₂O)_n cluster.