A study of interaction in the lowest singlet and triplet states of H2

The potential energy curves of the ground state and the first excited state of H₂ are examined in terms of the electronic force acting on each nucleus. The results reveal the detailed course of events that occur when two hydrogen atoms with parallel and antiparallel electron spins approach one another from a large internuclear separation.     

A theoretical study of the lowest-energy singlet and triplet electronic states p-iminophosphaalkyne and nitrilimine

The equilibrium geometries and fundamental vibrational frequencies of the two energetically lowest-lying electronic states of p-iminophosphaalkyne (HCPNH) and nitrilimine (HCNNH) were determined using a split-valence basis set with polarization functions on the heavy atoms. The most extensively correlated functions used in the geometry optimizations were of the Complete Active Space Self-Consistent Field (CASSCF) variety and included eight electrons distributed among seven active orbitals for HCPNH and six active orbitals for HCNNH. The effects on predicted geometry of the size of the CASSCF active space were investigated for HCPNH and are reported. Multi-Reference Configuration Interaction with Single and Double excitations calculations have been performed at the equilibrium geometries using larger basis sets to determine more accurately the relative energetics of the electronic states. It was found that nitrilimine has the expected singlet ground state, with the triplet lying 38.5 kcal mol⁻¹ higher in energy; however, p-iminophosphaalkyne has a triplet ground state, lying 9.7 kcal mol⁻¹ below the singlet.     

Molecular orbital theory of the hydrogen bond. VIII. Hydrogen bonding in H2OH2CO in relaxed singlet and triplet n → π* states

Ab initio SCF CI calculations with a minimal STO-3G basis set have been performed on the hydrogen bonded dimers in which H₂O is the proton donor to H₂CO in its relaxed singlet and triplet n→π^* states. Two dimers which are easily interconverted are found in the singet n→π^* state with hydrogen bond energies of 1.82 and 1.71 kcal/mole. The equilibrium dimer in the triplet state has a hydrogen bond energy of 2.97 kcal/mole. In both states, hydrogen bond formation occurs at the carbon atom. The structures of the dimers and the nature of the intermolecular surfaces in the regions of hydrogen bond formation are examined. Electron densities and distributions are also discussed.     

The behavior of the singlet and triplet spin states of methylene-bridged 1,8-naphthoquinodimethane with O2

It has been shown that thermolysis of naphthocyclopropane 8 yields the singlet 1,8-naphthoquinodimethane 7 by disrotatory ring opening. In fluid solution the singlet biradical prefers to ring close to regenerate the starting material rather than under a 1,2-hydrogen shift to yield phenalene (9). It has been demonstrated that the singlet 7 does not react with O₂ or undergo intersystem crossing to ground state triplet 7. It has also been shown by an intricate set of experiments that triplet 7 is produced on photolysis of 8 and that it reacts with O₂. Unfortunately, it was not possible to elucidate unequivocally the mechanism by which triplet 7 reacts with O₂.     

Bending potentials for H2O in the ground and the first six singlet excited states

MRD CI calculations are presented for the bending potentials of H₂O in the ground and first six singlet excited states. The results are discussed with reference to the H(²S) + OH(A ²Σ⁺) dissociation path of H₂O.     

Mechanism and rate constants of the CH2 + CH2CO reactions in triplet and singlet states: A theoretical study

Ab initio and density functional CCSD(T)-F12/cc-pVQZ-f12//B2PLYPD3/6-311G** calculations have been performed to unravel the reaction mechanism of triplet and singlet methylene CH₂ with ketene CH₂CO. The computed potential energy diagrams and molecular properties have been then utilized in Rice–Ramsperger–Kassel–Marcus-Master Equation (RRKM-ME) calculations of the reaction rate constants and product branching ratios combined with the use of nonadiabatic transition state theory for spin-forbidden triplet-singlet isomerization. The results indicate that the most important channels of the reaction of ketene with triplet methylene lead to the formation of the HCCO + CH₃ and C₂H₄ + CO products, where the former channel is preferable at higher temperatures from 1000 K and above. In the C₂H₄ + CO product pair, the ethylene molecule can be formed either adiabatically in the triplet electronic state or via triplet-singlet intersystem crossing in the singlet electronic state occurring in the vicinity of the CH₂COCH₂ intermediate or along the pathway of CO elimination from the initial CH₂CH₂CO complex. The predominant products of the reaction of ketene with singlet methylene have been shown to be C₂H₄ + CO. The formation of these products mostly proceeds via a well-skipping mechanism but at high pressures may to some extent involve collisional stabilization of the CH₃CHCO and cyclic CH₂COCH₂ intermediates followed by their thermal unimolecular decomposition. The calculated rate constants at different pressures from 0.01 to 100 atm have been fitted by the modified Arrhenius expressions in the temperature range of 300–3000 K, which are proposed for kinetic modeling of ketene reactions in combustion. © 2018 Wiley Periodicals, Inc.     

Generation of 1O2 from O2 quenching of the lowest excited singlet and triplet states of 9,10-diphenylanthracene

Measurements of the quantum yield for photoperoxidation of 9,10-diphenylanthracene as a function of dissolved oxygen concentration in toluene at 20°C. are consistent with generation of ¹O₂ from both the lowest singlet and triplet states of the hydrocarbon.     

A comparison of singlet and triplet states for one- and two-dimensional graphene nanoribbons using multireference theory

This study examines the radical nature and spin symmetry of the ground state of the quasi-linear acene and two-dimensional periacene series. For this purpose, high-level ab initio calculations have been performed using the multireference averaged quadratic coupled cluster theory and the COLUMBUS program package. A reference space consisting of restricted and complete active spaces is taken for the π-conjugated space, correlating 16 electrons with 16 orbitals with the most pronounced open-shell character for the acenes and a complete active-space reference approach with eight electrons in eight orbitals for the periacenes. This reference space is used to construct the total configuration space by means of single and double excitations. By comparison with more extended calculations, it is shown that a focus on the π space with a 6-31G basis set is sufficient to describe the major features of the electronic character of these compounds. The present findings suggest that the ground state is a singlet for the smaller members of these series, but that for the larger ones, singlet and triplet states are quasi-degenerate. Both the acenes and periacenes exhibit significant polyradical character beyond the traditional diradical.     

The effect of the oxidative properties of [Mg(H2O)6] in the triplet and singlet states on the energetics of adenosine triphosphate cleavage

The interaction of the adenosine triphosphate (ATP) molecule (the ATP subsystem) with the magnesium complex [Mg(H₂O)₆]²⁺ (the Mg subsystem) in the singlet (S) and triplet (T) states in an aqueous medium mimicked by 78 water molecules was studied by the molecular dynamics (density functional theory) method MD DFT:B3LYP with the 6–31G** basis set at T = 310 K. Potential energy surfaces for the S (lowest-lying) and T (highest-lying) states are significantly separated in space. The Mg complex moves along these surfaces to approach either oxygen atoms of the γ-β phosphate groups (O1–O2) (S PES) or oxygen atoms of β-α phosphate groups (O2–O3) (T PES). Chelation of the γ-β β-α and phosphates yields, respectively, a stable low-energy complex ([Mg(H₂O)₄-(O1–O2)ATP]²⁻) and a metastable high-energy complex ([Mg(H₂O)₂-(O2–O3)ATP]²⁻), which differ in the number of water molecules surrounding the Mg atom. Crossing of two triplet PESs is accompanied by the formation of an unstable state characterized by redistribution of spins between the Mg and ATP subsystems. This state, sensitive to interaction with the ²⁵Mg nuclear spin, induces an unpaired electron spin, which initiates the ATP cleavage by the ion-radical mechanism, yielding a reactive radical ion of adenosine monophosphate (•AMP⁻), which was earlier found experimentally by the of chemically induced dynamic nuclear polarization (CIDNP) method. Biological aspects of the results obtained are discussed. Original Russian Text ? A.A. Tulub, V.E. Stefanov, 2009, published in Zhurnal Neorganicheskoi Khimii, 2009, Vol. 54, No. 7, pp. 1188–1195.     

Low-lying singlet and triplet electronic states of RhB

The low-lying XSigma+, a3Delta, A1Delta, b3Sigma+, B1Pi, c3Pi, C1Phi, D1Sigma+, E1Pi, d3Phi, and e3Pi electronic states of RhB have been investigated at the ab initio level, using the multistate multiconfigurational second-order perturbation (MS-CASPT2) theory, with extended atomic basis sets and inclusion of scalar relativistic effects. Among the eleven electronic states included in this work, only three (the X1Sigma+, D1Sigma+, and E1Pi states) have been investigated experimentally. Potential energy curves, spectroscopic constants, dipole moments, binding energies, and chemical bonding aspects are presented for all electronic states.     

Studies on the singlet and triplet states of unsaturated carbenes

Quantum chemical calculations and studies have been made on the different spin states of unsaturated carbenes X₂C?C: and X₂C?C?C: (X?H, Li, F). The geometries and relative stabilities of these carbenes with various groups X have been outlined. The ground states of unsaturated carbenes are all singlet. The energy splittings between the ground states and first excited states ΔE (¹A₁-³B₁) are generally within the value of 60 kcal/mol and change greatly with the electronegativities of groups X, but little with the sizes of the cumulidenes. The equilibrium conformations of ¹A₁ and ³B₁ are different.     

不饱和卡宾的单重态与三重态

对X2C=C:和X2C=C=C:(X=H,Li,F)进行了量子化学计算和研究,得到了不饱和卡宾单重态(^1A1)和三重态(^3B1)的几何构型及其相对稳定性随取代基X电负性变化的一般规律,结果表明,不饱和卡宾的基态都是单重态,第一激发态与基态的能量差ΔE(^1A1 - ^3B1)随X的电负性大小而变化,其值一般在60kcal.mol^-^1以内,累积烯基的大小对ΔE(^IA1-^3B1)的影响相对较小,^IA1和^3B1的平衡几何构型并不相同。     

Theoretical study of the excited singlet and triplet states of alloxan

The possibility of excited‐state protomeric shifts in the biologically important molecule, alloxan, is investigated. We have focused on the S₁ and T₁ excited states of alloxan and its hydroxy tautomers. Modifications brought in by excitation on the relative stabilities, activation barriers, and optimized geometries, computed at the MNDO, AM1, and PM3 levels of approximation, have been discussed for both excited electronic states. The absorption and fluorescence spectra for the three tautomers are also discussed. Results show significant changes in the geometries on excitation, although the changes are similar for the singlet and triplet excited states. Though the relative stability orders do not change, the 2‐hydroxy tautomer is stabilized, while the 4‐hydroxy tautomer gets destabilized on excitation. The excited states are (n,π*) states, involving the promotion of a nonbonding oxygen lone pair from the CO? CO? CO moiety, which explains why the oxygens of this group become less basic and the 4‐hydroxy tautomer gets destabilized on excitation. However, the activation barriers do not reduce significantly on excitation, and this precludes the possibility of ground‐ or excited‐state proton transfer in the gas phase. © 2001 John Wiley & Sons, Inc. Int J Quantum Chem, 2001     

Ab initio calculations on the singlet and triplet energy surfaces for CsiH2

Singlet and triplet calculations, including configuration interaction, are reported for H₂CSi, HCSiH and CSiH₂, and for the transition state on both surfaces.     

Two-dimensional fully numerical solutions of molecular dirac equations. Results for ground singlet states of H2 and HeH+

Two-dimensional fully numerical solutions of the Dirac-Fock problem are reported for the singlet ground states of He, H₂ and HeH⁺. The relativistic total energy R = 1.4 au for H₂ is -1.13364396 au and the relativistic correction is 1.439 × 10^?5 au.     

The ground and two lowest-lying singlet excited electronic states of copper hydroxide (CuOH)

Various ab initio methods, including self-consistent field (SCF), configuration interaction, coupled cluster (CC), and complete-active-space SCF (CASSCF), have been employed to study the electronic structure of copper hydroxide (CuOH). Geometries, total energies, dipole moments, harmonic vibrational frequencies, and zero-point vibrational energies are reported for the linear 1Sigma+ and 1Pi stationary points, and for the bent ground-state X 1A', and excited-states 2 1A' and 1 1A". Six different basis sets have been used in the study, Wachters/DZP being the smallest and QZVPP being the largest. The ground- and excited-state bending modes present imaginary frequencies for the linear stationary points, indicating that bent structures are more favorable. The effects of relativity for CuOH are important and have been considered using the Douglas-Kroll approach with cc-pVTZ/cc-pVTZ_DK and cc-pVQZ/cc-pVQZ_DK basis sets. The bent ground and two lowest-lying singlet excited states of the CuOH molecule are indeed energetically more stable than the corresponding linear structures. The optimized geometrical parameters for the X 1A' and 1 1A" states agree fairly well with available experimental values. However, the 2 1A' structure and rotational constants are in poor agreement with experiment, and we suggest that the latter are in error. The predicted adiabatic excitation energies are also inconsistent with the experimental values of 45.5 kcal mol(-1) for the 2 1A' state and 52.6 kcal mol(-1) for the 1 1A" state. The theoretical CC and CASSCF methods show lower adiabatic excitation energies for the 1 1A" state (53.1 kcal mol(-1)) than those for the corresponding 2 1A' state (57.6 kcal mol(-1)), suggesting that the 1 1A" state might be the first singlet excited state while the 2 1A' state might be the second singlet excited state.     

A quantum-mechanical study of the lone-pairs in H2O and H2S

It is shown from SCF-MO studies using localised orbitals that the angles between the lone-pairs in H₂O and H₂S are 115° and 127°, in agreement with the qualitative predictions of the Sidgwick-Powell theory.     

Structure and properties of H2CN,H2CC−, H2BO and H2CO+

Ab initio SCF—MO calculations are presented for H₂CN, H₂CC^?, H₂BO and H₂CO⁺, including geometry optimization. One-electron properties are presented and compared with experiment where possible, particularly ESR hyperfine data.