Theoretical study of the vertical electron affinity and ionization potentials of C3

The electron affinity and first three ionization potentials of C₃ are calculated using the multiconfigurational SCF and configuration interaction methods and by Möller-Plesset perturbation theory. Whereas Koopmans' theorem and SCF calculations indicate that the first cation state is ²Π_u, upon inclusion of correlation effects both the ²Σ_u and ²Σ_g cation states are found to lie lower in energy. CI calculations indicate that the ground state (²Π_g) anion is stable by 1.74 eV. Allowing for the error in the calculated electron affinity of the carbon atom, C₃^? is estimated to be stable by 2.0 eV, in excellent agreement with the 2.05 eV value determined from recent photodetachment measurements. No excited anion states are found to be bound at the equilibrium geometry of the neutral molecule.     

Configuration interaction calculation of the potential curves for the C3 molecule in its ground and lowest-lying Πu states

Ab initio SCF and Cl calculations are reported for the C₃ molecule using a basis set of double-zeta plus polarization quality. Potential curves are obtained for the symmetric stretch and bending and antisymmetric stretch vibrational coordinates for the ground and 3σ_u → l π_g^3,1Π_u excited states of this system in order to calculate the intensity distributions for the associated electronic transitions. The calculated T₀ value for the ¹Π_u ← X?¹ ∑⁺_g transition of 3.03 eV is in quite good agreement with the location of the origin of the 4050 Å (3.06 eV) band system in C₃, confirming its previous assignment to this electronic transition; the lifetime of the ¹H_u upper state is also obtained in the CI treatment. A value of 2.04 eV is calculated for the corresponding ³Π_u ← X?¹ ∑⁺_g origin, which result in turn suggests that the weak feature starting at 2.10 eV (5900 Å) should be assigned thereto.     

GRECP/5e‐MRD‐CI calculation of the electronic structure of PbH

The correlation calculation of the electronic structure of PbH is carried out with the generalized relativistic effective core potential (GRECP) and multireference single‐ and double‐excitation configuration interaction (MRD‐CI) methods. The 22‐electron GRECP for Pb is used and the outer core 5s, 5p, and 5d pseudospinors are frozen using the level‐shift technique, so only five external electrons of PbH are correlated. A new configuration selection scheme with respect to the relativistic multireference states is employed in the framework of the MRD‐CI method. The [6, 4, 3, 2] correlation spin–orbit basis set is optimized in the coupled cluster calculations on the Pb atom using a recently proposed procedure, in which functions in the spin–orbital basis set are generated from calculations of different ionic states of the Pb atom and those functions are considered optimal that provide the stationary point for some energy functional. Spectroscopic constants for the two lowest‐lying electronic states of PbH (²Π_1/2, ²Π_3/2) are found to be in good agreement with the experimental data. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2002     

The structure of BO2 in its ground and low-lying excited states from ab initio SCF RHF AND CI calculations

Potential curves for the X²Π_g, A²Π_u, B²Σ⁺_u and C²Σ_g⁺ electronic states of BO₂ were calculated at ab initio SCF RHF and configuration interaction (CI) level. The results obtained are consistent with a linear molecular model for all states considered. The calculated structural parameters and transition energies are in good agreement with relevant experimental data.     

Analysis of chemical bonding in C2 using Dyson orbitals

Multireference configuration interaction wave functions with single and double excitations were calculated for the ¹Σ⁺_g ground state of the C₂ molecule and the excited states of C⁺₂ with symmetries ²Σ⁺_g, ²Σ^-_u, ²Π_u, and ²Π_g. The corresponding σ_g, σ_u, π_u, and π_g valence Dyson orbitals were calculated. Most of the density due to the valence electrons is accounted for by three σ_g, one σ_u, and one degenerate pair of π_u Dyson orbitals. Electron correlation plays an important role in the bond strength of C₂ by increasing the occupation of the σ_g valence orbitals and decreasing the occupation of the σ_u and π_u valence orbitals. © 1996 John Wiley & Sons, Inc.     

Low-lying states of cs22+

Using an MC SCF CI method, wavefunctions for the ground state ¹∑⁺_g and the excited states of the symmetries ¹∑⁺_g, ¹Π_g, and ¹Δ_g of the Cs²⁺₂ ionic system are generated. The potential curves for eleven ¹∑⁺_g twelve ¹Π_g, and six ¹Δ_g states are calculated. Results suggest a small charge-transfer cross section for the reaction CS⁺ + Cs⁺ → Cs CS²⁺.     

Sigma,pi, and delta wavefunction forms for the hydrogen molecule

Using variational Monte Carlo methods, we examine simple, explicitly‐correlated trial wavefunction forms for the X¹Σ, B¹Σ, a³Σ, b³Σ, I¹Π_g, C¹Π_u, i³Π_g, c³Π_u, J¹Δ_g, and j³Δ_g states of the hydrogen molecule. The energies produced by our best wavefunctions are slightly above the best values in the literature. When we combine our trial wavefunction forms with the generalized Feynman‐Kac path integral method, our results are in excellent agreement with the best nonrelativistic values for these systems except for the I¹Π_g state. Our best energy for this state, ?0.65951554(6), is lower by several microhartrees than that obtained by Wolniewicz [J Mol Spectrosc 1995, 169, 329]. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2010     

MRD-CI ground state geometry and vertical spectrum of N3

CI calculations have been carried out for the prediction of the ground state geometry and of the vertical spectrum of N₃. The first three states are ²Π_g, ⁴Π_u and ²Σ⁺_u. The C_∞v correlation diagram for the first dissociation limits is discussed by taking into account possible nonadiabatic pathways.     

Electronic states and nature of the chemical bond in the molecule CrC by all-electron ab initio calculations

All-electron ab initio Hartree–Fock (HF ), valence configuration interaction (CI ), and multiconfiguration self-consistent-field (CASSCF ) calculations have been applied to investigate the electronic states of the CrC molecule. The molecule is predicted as having four low-lying electronic states, ³∑^?, ⁵∑^?, ⁷∑^?, and ⁹∑^?, separated by an energy gap of 0.55 eV from the next higher-lying state, ¹∑^?, which is followed by the states ⁵Π and ⁷Π. The four lowest-lying electronic states are due to the coupling of the angular momenta of the ⁶S_g Cr⁺ ion with those of the ⁴S_u C^? anion. The chemical bond in the ³∑^? ground state can be viewed as a quadruple bond composed of two σ and two π bonds. One σ bond is due to the formation of a molecular orbital that is doubly occupied. The remaining bonds, i.e., one σ and two π bonds, arise from valence-bond couplings. The π bonds originate from the valence-bond couplings of the electrons in the C 2pπ orbitals with those in the Cr 3dπ orbitals. The σ bond originates from the valence-bond coupling of the C 2pσ electron with an electron in the Cr 4s, 4p hybrid that is polarized away from the C atom.     

Theoretical Characterisation of Phosphinyl Radicals and Their Magnetic Properties: g Matrix

The g matrices (g tensors) of various phosphinyl radicals (R₂P^.) were calculated using the DFT and multireference configuration interaction (MRCI) methods. The g matrices were distinctly dependent on the molecular structure of the radical. To thoroughly examine this dependence, the contributions from individual atoms and excited states were calculated. The former revealed the gain from the phosphorus atom to be preeminent unless P?O or P?S bonds are present in the radical molecule. The contributions owing to excited states arising from electronic transitions between doubly occupied molecular orbitals and the SOMO were clearly positive, as in the case of semiquinone and niroxide radicals. The transitions from the phosphorus lone pair were of paramount importance. Surprisingly, unlike for semiquinones and nitroxides, a significant negative contribution was observed from excitations from the SOMO to unoccupied molecular orbitals. For radicals with P?O bonds, this contribution to the g₂ component was dominant.     

Influence of donor–acceptor conjugation between thiophene‐phthalazinone structures containing Sp3 CN bond on the frontier orbital levels and optic–electronic properties

Herein, an electron‐deficient phthalazinone unit containing a unique Sp³ hybrid nitrogen atom as an acceptor to cut off the ether bond was presented and three Donor–Acceptor (D–A) conjugated fluoropolymers via Classical Aromatic Nucleophilic Displacement Polymerization (CANDP) method were successfully synthesized. These polymers exhibit excellent thermostability. The 5% weight loss temperature of PDT2TF under nitrogen atmosphere is high, up to 470 °C. This is due to that the D–A conjugation between thiophene‐phthalazinone units affects its resonance energy and stabilizes the thiophene‐phthalazinone structures. These three fluoropolymers show strong absorptions and fluorescence in visible light region both in solution and thin film states. The band gaps (E_g) of these polymers are narrower than that of their corresponding di‐NH capped monomers, owing to the good electronic communication properties of the Sp³ nitrogen atom in the phthalazinone unit. The E_g value of PDT3TF is decreased to 2.03 eV. These results indicate that phthalazinone unit is an efficient acceptor and could exhibit strong D–A effect with thiophene unit. Also, the Sp³ nitrogen atom in the phthalazinone shows good electronic wave function and charge transport properties. This facile CANDP synthetic method combined with Sp³ C? N bond linked electron‐deficient phthalazinone unit affords polymers with controllable photoelectric properties. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 54, 3470–3483     

Rydberg and valence-shell character as functions of internuclear distance in some excited states of CH,NH, H2, and N2

SCF MO computations have been carried out on several excited states of CH and NH in which the excited MO 4σ is a Rydberg or near-Rydberg MO at internuclear distances R near that (R_e) of equilibrium in the ground state, but becomes an antibonding valence-shell MO as R increases toward dissociation. For the lowest ³Π_g state of H₂ and the lowest ³Π_g and ³Π_u states of N₂ the extent of 3dπ Rydberg character in the excited MO as a function of R for some R values ? R_e is evaluated by SCF MO computations.     

Charge exchange of O2+ with Cs: spectroscopy and predissociation pathways for the Πg Rydberg states of O2

Electron capture by keV O₂⁺ (X₂Π_g) ions from Cs atoms yields predominantly the ¹Π_g and ³Π_g Rydberg states of O₂ which subsequently predissociate. Through the use oftranslational spectroscopy on the neutral product atoms, we have located a number of vibrational levels of these states and determined the dissociation channels. Furthermore we have observed competition between diabatic and adiabatic behaviour in the dissociating channels.     

Non-orthonormal basis calculations of the dipole transition moment for the phillips system (A 1Πu → X 1Σ+g) in C2. Theoretical lifetime of the A 1Πu state

Non-orthonormal basis calculations have been carried out on the A ¹Π_u → X ¹Σ⁺_g transition moment of C₂ as a function of CC internuclear distance. Two types of AO basis set and different sizes of CI vectors were employed in order to examine the convergence of the theoretical results. Published ab initio potential energy curves and the transition moments of the present work were combined to calculate radiative lifetimes of vibrational levels of the A ¹Π_u state. The resulting lifetimes are in agreement with other theoretical values, obtained with orthonormal basis calculations, which however, are significantly lower than those of the most recent experiments.     

Exponent optimization for Π states of H2

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.     

Detection of nitrogen rotational distributions by resonant 2 + 2 multiphoton ionization through the a1Πg state

Characterization of laser 2 + 2 multiphoton ionization of nitrogen to obtain rotational state distributions has been investigated via the resonant two-photon transition a ¹Π_g(ν = 1) ← X ¹Σ_g(ν = 0). For room-temperature nitrogen, the spectral intensities and state distribution are directly related and give rotational temperatures of 290 ± 20 K. For power densities of 3 GW/cm², the ionization probability is 1 × 10⁻⁵ per N₂ molecule per average rotational state.     

Charge and spin correlation structures of conjugated π systems: Analysis of full CI wave functions of the PPP hamiltonian

An analysis of the electronic correlation structures by means of the charge and spin correlation functions is carried out for full CI wave functions of four, five, and six membered conjugated π systems described by the Pariser–Parr–Pople Hamiltonian. The low-lying states of these systems are classified as covalent (CV ) and ionic (IN ) states depending on whether the probability of finding two electrons simultaneously at the same position is small or large. It is found that many of excited CV states, the typical ones of which are the 2¹A_g state of linear π systems, have stronger CV character than the ground CV state, and their spin coupling structures are different from each other as well as from that of the ground CV state. The spin coupling structure in the ground CV state has an “antiferromagnetic” spin arrangement in favor of antiparallel coupling between nearest neighbor spins while in excited CV states the extent of the antiparallel spin coupling between nearest neighbor sites is decreased. IN states, which are less common for low-lying states than CV ones, are also found to have characteristic modulations in the charge correlation. In particular, the charge correlations in the lowest singlet IN states, 1¹B_u of linear π systems, 1¹B_2g of cyclobutadiene and 1¹B_1U of benzene, are alternating.     

Potential energy and dipole moment of the Na2+ ionic molecule

The potential energy curves of 26 electronic states of ²Σ⁺_{g, u}, ²Π_{g, u}, and ²Δ_{g, u} symmetries of the alkali dimer Na₂⁺, dissociating up to Na(4d) + Na⁺, are investigated using an ab initio approach involving a nonempirical pseudopotential for the Na⁺(1s²2s²2p⁶) core and core‐valence correlation corrections. Furthermore, the transition dipole functions between many electronic states and vibrational energy spacings are presented. The spectroscopic constants of these electronic states are extracted and compared with the available theoretical and experimental results. A very good agreement is observed, especially, for the ground and the first excited states. However, the comparison between our study and the model potential (MP) calculations (Magnier and Masnnou‐Seeuws Mol. Phys. 1996, 89, 711) for several states has shown a clear disagreement. The MP well depths of the 3‐4²Σ⁺_g, 1²Π_g, 3‐4²Π_g, and 2²Π_u electronic states are largely underestimated. In addition, the 5‐7²Σ⁺_g, 3‐7²Σ⁺_u, 2²Π_g, 4²Π_g, and 1‐2²Δ_u MP electronic states are repulsive, although in this work, they are attractive with potential well depths of some hundreds of cm^?1. The data presented in this study are very useful for studies on ion–atom interaction and cold collision in the presence of electromagnetic fields. © 2013 Wiley Periodicals, Inc.     

Vibrational analysis of N2(B 3Πg and C 3Πu) and CO(X) excited in N2 discharge and post discharge

A spectroscopic characterization of a N₂ radiofrequency discharge and N₂CO post discharge has been performed. The relative vibrational distribution of the excited B ³Π_g and C ³Π_u states of nitrogen and their correlation with the ground state have been analyzed. The analysis confirms the importance of the metastable molecules. N₂(A ³Σ⁺_u), in affecting the vibrational distribution of nitrogen in its ground state in the discharge and post discharge. The vibrational analysis of the CO ground state, excited in the post discharge by vibrationally excited N₂ molecules, confirms the high degree of vibrational non-equilibrium in the ground state of nitrogen, in the presence of a low first-level vibrational temperature.