Sensitized fluorescence (A 1Π−X 1Σ+ and b 3Σ+−a 3Π) of 12C16O, 13C16O,and 12C18O by Kr(3P1) and Kr(1P1)

The Kr(³P₁) state was shown to excite preferentially the A ¹Π υ′ = 13 state of ¹³C¹⁶O and ¹²C¹⁸O by a resonance process. No resonance energy transfer was observed between Kr(³P₁) and ¹²C¹⁶O. The Kr(¹P₁) state was shown to excite the b ³Σ⁺ υ′ = O and υ′= 1 states of ¹²C¹⁶O, ¹³C¹³O by a non-resonance process.     

Angular correlation parameters for the electron impact excitation of the 21 P and 31 P states of helium using first-order many-body theory

A simplified form of the first-order many-body theory was used for the calculation of the coherence and correlation parameters for the excitation of the 2¹ P and 3¹ P states of helium. The excited state wave functions were calculated numerically in the fixed core Hartree-Fock(HF) approximation. The ground state wave function was used in the HF approximation. Scattering orbitals were calculated numerically in the static exchange approximation. Calculations were performed forE=29.6, 40, 50, 60 and 80 eV impact energies for the 2¹ P state, and forE=50 eV and 80 eV for the 3¹ P state. Results for the coherence and correlation parameters are compared with the experimental values obtained from electronphoton coincidence experiments, and with other theoretical results.     

Spin-orbit ab initio investigation of the photodissociation of C2H5Br

The photodissociation of ethyl bromide (C₂H₅Br) has been investigated by spin-orbit (SO) ab initio calculations. The vertical excitation energies of some excited states for C₂H₅Br were calculated. The potential energy curves of C₂H₅Br along the C–Br dissociation coordinate were calculated by multistate second-order multiconfigurational perturbation theory in conjunction with spin-orbit (SO) interaction through complete active space state interaction (MS-CASPT2/CASSI-SO). The calculated results clearly assigned the experimentally observed photodissociation channels leading to C₂H₅ + Br (²P_3/2) and C₂H₅ + Br*(²P_1/2).     

Self-consistent calculation of excited 3P state wave functions of atoms

A variation perturbation method in the Hartree–Fock scheme has been described to calculate excited ³P state wave functions of atoms. The starting wave functions are obtained from a study of the singularities in the dynamic polarizability calculation [1]. The 2³P, 3³P and 4³P states of He, Li⁺, Be²⁺, B³⁺ and C⁴⁺ are studied. The results obtained are in satisfactory agreement with experimental values and with other accurate theoretical estimates.     

Conformational study of the structure of free 12-thiacrown-4 and some of its cation metal complexes

Conformational search of 12-thiacrown-4, 12t4, was performed using the CONFLEX method and the MMFF94S force field whereby 156 conformations were predicted. Optimized geometries of the 156 predicted conformations were calculated at the HF, B3LYP, CAM-B3LYP, M06, M06L, M062x and M06HF levels using the 6-311G** basis set. The correlation energy was recovered at the MP2 level using the same 6-311G** basis set. Optimized geometries at the MP2/6-311G** level and G3MP2 energies were calculated for some of the low energy conformations. The D ₄ conformation was predicted to be the ground state conformation at all levels of theory considered in this work. Comparison between the dihedral angles of the predicted conformations indicated that for the stability of 12t4, a SCCS dihedral angle of 180° requirement is more important than a gauche CSCC dihedral angle requirement. Conformational search was performed also for the 12t4?CAg⁺, Bi³⁺, Cd²⁺, Cu⁺ and Sb³⁺ cation metal complexes using the CONFLEX method and the CAChe-augmented MM3 and MMFF94S force fields. Conformations with relative energies less than 10?kcal/mol at the MP2/6-31+G*//HF/6-31+G* level, with double zeta quality basis set on the metal cations, were considered for computations at the same levels as those used for free 12t4, using also the 6-311G** basis set. The cc-pVTZ-pp basis set was used for the metal cations. The predicted ground state conformations of the 12t4?CAg⁺, Bi³⁺, Cd²⁺, Cu⁺ and Sb³⁺ cation metal complexes are the C ₄, C ₄, C ₄, C _2v and C ₄ conformations, respectively. This is in agreement with the experimental X-ray data for the 12t4?CAg⁺ and Cd²⁺ cation metal complexes, but experimentally by X-ray, the 12t4?CBi³⁺ and Cu⁺ cation metal complexes have C _s and C ₄ structures, respectively.     

Theoretical rate constant calculations for O(3P) with saturated hydrocarbons

Theoretical rate constants have been calculated for O(³P) with five saturated hydrocarbons, CH₄, C₂H₆, C₃H₈, iso-C₄H₁₀, and neo-C₅H₁₂. The method of choice is bond energy–bond order (BEBO) with activated complex theory (ACT). Because the BEBO method is empirical, O(³P) + CH₄ is evaluated first, and the theoretical results are compared to more rigorous calculations and to the empirical transition state method. Comparisons are also made between predictions and experimental results. All of these comparisons show that the BEBO-ACT method gives results which are consistent with experiment and other theory. Because the method is successful, the other four cases are then considered. Ambiguity arises for the higher hydrocarbons from the problem of internal rotations in the activated complexes, and three cases are evaluated. Best agreement with experiment is obtained if the primary rotor(s) in the complexes are considered to be free. Predictions of rate constants are made from 500 to 2500 K. Throughout the discussion issues of theory which are common to any ACT calculation from any method of potential energy evaluation (LEP, LEPS, or ab initio quantum mechanics) are featured.     

Calculation of 2p 4(3 P)nl(l=s,d) Rydberg spectra of the fluorine isoelectronic sequence

The basic structure parameters of 2p ⁴(³ P)ns and 2p ⁴(³ P)nd (J=1/2, 3/2 and 5/2) Rydberg spectra for the fluorine isoelectronic sequence from FI to NiXX, as functions of effective nuclear charge, are calculated by using the eigenchannel quantum-defect theory. The results can be interpreted in terms of the variations of electrostatic and spin-orbit interactions along the sequence. A vast amount of basic atomic data can be obtained with these parameters as input. Some numerical examples are given, in which our results are in perfect agreement with experimental.     

Microwave spectrum of P14N and P15N: Spectroscopic constants and molecular structure

In the present work the J+1←J, with J=1–3, 6, 7, 10–16, rotational transitions of P¹⁴N and the J+1←J, with J=1, 2, 7, 8, 10–14, rotational transitions of P¹⁵N have been observed in the millimeter- and submillimeter-wave region. These measurements allowed us to improve the ground state rotational parameters of P¹⁴N as well as provide those of P¹⁵N for the first time. The present measurements have been combined with rotational and vibrorotational transitions available in the literature to yield the Dunham coefficients. The equilibrium structure has also been determined to a high accuracy. The experimental investigation has been supported by highly accurate ab initio computations.     

Quantal close-coupling calculation of the cross sections for the j 1 m 1 → j 2 m 2 transitions in the 4p 2P and 3d 2D excited states of Ca+ induced by collisions with helium

Cross sections for the j ₁ m ₁ → j ₂ m ₂ transitions in the resonance 4p ²P and metastable 3d ²D states of the singly charged calcium ion induced by collisions with the ground-state He atom have been calculated using the quantal close-coupling method. The calculations are based on the earlier obtained Ca⁺-He pseudopotential SCF potential energy curves. The calculated cross sections are discussed in the energy range from threshold to 1.5 eV. Satisfactory agreement with other theoretical results has been found for the 4p ²P state. However, relatively large discrepancy between theory and available experimental data still exists for both the Ca⁺ states.     

Vibrational spectra of the trihalocyclopropenium ions [C3X3]+ (X = Cl,Br or I)

Vibrational spectra are reported and assigned for the planar D_3h symmetry cyclopropenium cations [C₃X₃]⁺ (X= Cl, Br or I) from investigations of the compounds C₃Cl₃AlCl₄, C₃Cl₃GaCl₄, C₃Cl₃FeCl₄, C₃Cl₃SbCl₆, C₃Br₃AlBr₄ and C₃l₄, using conventional infrared and Raman spectroscopy and Fourier transform Raman spectroscopy. The symmetric C—X stretching modes of [C₃X₃]⁺ occur at 458, 269 and 180 cm⁻¹ and the ring-breathing modes at 1790, 1732 and ca. 1650 cm⁻¹ in [C₃Cl₃]⁺, [C₃Br₃]⁺ and [C₃I₃]⁺, respectively. A normal coordinate calculation is performed for [C₃Cl₃]⁺.     

Theoretical studies on structures and electronic spectra of linear carbon chains C2nH+ (n = 1−5)

The density functional theory (DFT) and the complete active space self‐consistent‐field (CASSCF) method have been used for full geometry optimization of carbon chains C_2nH⁺ (n = 1–5) in their ground states and selected excited states, respectively. Calculations show that C_2nH⁺ (n = 1–5) have stable linear structures with the ground state of X³Π for C₂H⁺ or X³Σ^? for other species. The excited‐state properties of C_2nH⁺ have been investigated by the multiconfigurational second‐order perturbation theory (CASPT2), and predicted vertical excitation energies show good agreement with the available experimental values. On the basis of our calculations, the unsolved observed bands in previous experiments have been interpreted. CASSCF/CASPT2 calculations also have been used to explore the vertical emission energy of selected low‐lying states in C_2nH⁺ (n = 1–5). Present results indicate that the predicted vertical excitation and emission energies of C_2nH⁺ have similar size dependences, and they gradually decrease as the chain size increases. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009     

Structures and stabilities of [C3H2]+ ˙ and [C3H2]2+ ions

Ab initio molecular orbital theory using basis sets up to 6-311G^{* *}, with electron correlation incorporated via configuration interaction calculations with single and double substitutions, has been used to study the structures and energies of the C₃H₂ monocation and dication. In agreement with recent experimental observations, we find evidence for stable cyclic and linear isomers of [C₃H₂]^{+ ˙}. The cyclic structure (, a) represents the global minimum on the [C₃H₂]^{+ ˙} potential energy surface. The linear isomer (, b) lies somewhat higher in energy, 53 kJ mol^?1 above a. The calculated heat of formation for [HCCCH]^{+ ˙} (1369 kJ mol^?1) is in good agreement with a recent experimental value (1377 kJ mol^?1). For the [C₃H₂]²⁺ dication, the lowest energy isomer corresponds to the linear [HCCCH]²⁺ singlet (h). Other singlet and triplet isomers are found not to be competitive in energy. The [HCCCH]²⁺ dication (h) is calculated to be thermodynamically stable with respect to deprotonation and with respect to C? C cleavage into CCH⁺ + CH⁺. The predicted stability is consistent with the frequent observation of [C₃H₂]²⁺ in mass spectrometric experiments. Comparison of our calculated ionization energies for the process [C₃H₂]^{+ ˙} → [C₃H₂]²⁺ with the Q_min values derived from charge-stripping experiments suggests that the ionization is accompanied by a significant change in structure.     

Theoretical studies on ferromagnetic behavior of [Cr(C5(CH3)5)2]+[TCNE]− and [Mn(C5(CH3)5)2]+[TCNQ]−

By using broken-symmetry hybrid-DFT (UB3LYP and UB2LYP) calculation, the effective exchange integrals (J values) of [Cr(C₅(CH₃)₅)₂]⁺[TCNE]⁻[Cr(C₅(CH₃)₅)₂]⁺ and [Mn(C₅(CH₃)₅)₂]⁺[TCNQ]⁻[Mn(C₅(CH₃)₅)₂]⁺ were determined theoretically. Those calculated models were reduced to 3-spin-sites models from X-ray crystallographic data of charge transfer 3D crystal. The calculated results showed that effective exchange integrals were positive and the signs of spin densities on the cyclopentadienyl rings were negative. These results supported the so-called McConnell I mechanism for ferromagnetism proposed by Kollmar et al. and our previous calculations. Natural orbital analysis made it clear that the orbital overlap between SOMO on metals and SOMO on TCNE or TCNQ cations was nearly zero. These results indicated that orbital orthogonality was an important key factor for explaining the ferromagnetism of those systems.     

Jahn—Teller effects in the mindo approximation: structures of the molecular cations of methane and the chloromethanes

The MINDO/3 technique gives geometries for (CH₄)⁺, (CCl₄)⁺ and the intermediate ions (CH_nCl_{4 ? n})⁺ (n = 1, 2, 3) which have symmetries in precise accord with the predictions of the Jahn—Teller effect. The ground state of (CH₄)⁺ has D_2d symmetry, with a C_3v structure ca. 45.6 kJ mol^?1 higher. (CCl₄)⁺ has a C_2v ground state, with a D_2d structure ca. 144 kJ mol^?1 higher: no bound state of C_3v symmetry could be found. (CH₃Cl)⁺ and (CHCl₃)⁺ both have C_s symmetry, and (CH₂Cl₂)⁺ has C_2v symmetry. The analogous fluoro ions are discussed briefly.     

Synthesis of molecular magnetics based on cobalt trisoxalate. Structural and magnetic properties of NBun 4[MnIICoIII(C2O4)3]

Bimetallic oxalate-bridged complexes Q[M^IICo(C₂O₄)₃] (Q=Me₄N⁺, Buⁿ ₄P⁺; M^II=Mn, Co, Ni, Cu, Zn) were synthesized. Single crystals of [NBuⁿ ₄][Mn^IICo^III(C₂O₄)₃] were studied by XRD. Unit cell parametersa=b=9.242(3) Å,c=54.524(13) Å; space groupR3c. Magnetic measurements indicate the absence of a magnetic phase transition up to the temperature of liquid helium. The XRD data confirm the presence of Co^III ions with a low-spin configuration in the crystal.     

The molecular and electronic structure of radical cations derived from tetramethyltin and hexamethylditin: An SCF-MO study

Molecular geometries and energies have been calculated, using the semi-empirical MNDO method for the closed-shell species SnMe₄, Sn₂Me₆, and (SnMe₃)⁺; and using the UHF-MNDO method for the radicals (SnMe₄)⁺, (Sn₂Me₆)⁺ and SnMe₃^.. The radical cation (SnMe₄)⁺ is calculated to have C_3v skeletal symmetry, with a C_2v isomer some 15 kJ mol^?1 higher in energy. The dinuclear radical cation (Sn₂Me₆)⁺ is calculated to be a σ(SnSn) radical, of D_3d skeletal symmetry: although the calculated Sn(5s) spin density is extremely low, the tin atoms are far from planarity. Calculated spin densities are compared with experimental hyperfine couplings.     

Fragmentation Mechanism of White Phosphorus: A Theoretical Insight into Multiple Cleavage/Formation of P−P and P−C Bonds

Molecular-level understanding of metal-mediated white phosphorus (P₄) activation is meaningful but challenging because of its direct relevance to the conversion of P₄ into useful organophosphorus compounds as well as the complicated and unforeseeable cleavage process of P−P bonds. The related study, however, has still rarely been achieved to date. Here, a theoretical insight into the step-by-step process of three P−P bond cleavage/four P−C bond formation for [P₃+P₁]-fragmentation of P₄ mediated by lutetacyclopentadienes is reported. The unique charge-separated intermediate and the intermolecular cooperation between two lutetacyclopentadienes play a vital role in the subsequent P−P/P−C bond breaking/forming. It is found that, although the first P−C formation is involved in the assembly of the cyclo-P₃ [R₄C₄P₃]⁻ unit, the construction of the aromatic five-membered P₁ heterocycle [R₄C₄P]⁻ is completed prior to the cyclo-P₃ formation. The reaction mechanism has been carefully elucidated by analyses of the geometric structure, frontier molecular orbitals, bond index, and natural charge, which greatly broaden and enrich the general knowledge of the direct functionalization of P₄.     

Cadmium (5 1 P 1 → 5 3 P J) excitation transfer and 5 3 P J intramultiplet relaxation by collisions with molecular gases

Collisional 5 ¹ P ₁ → 5 ³ P _J spin changing fine structure transfer as well as 5 ³ P _J intramultiplet mixing induced by various molecular gases (H₂, D₂, N₂, CO, CO₂, CH₄, C₂H₆, C₃H₈, C₂H₄) have been investigated using a combined method of fluorescence and absorption spectroscopy. After pulsed optical excitation of the Cd(5 ¹ P ₁) level the time dependence of the population densities has been measured both for the Cd(5 ¹ P ₁) level as well as the three collisionally populated Cd(5 ³ P _J) levels. By analyzing the signal curves at different molecular gas pressures not only the ratios of 5 ¹ P ₁ → 5 ³ P _J population transfer cross sections but also the Cd(5 ¹ P _J), Cd(5 ³ P _J) quenching cross sections and the Cd(5 ³ P _J) intramultiplet population transfer cross sections have been obtained.     

Dissociative charge exchange of C3F6

Charge exchange of neutral C₃F₆ by a variety of atomic and molecular ions in the 1 to 25 eV range of collision energies is used to characterize the energies associated with formation of [C₃F₆]⁺˙. The internal energy of the nascent [C₃F₆]⁺˙ ion, assessed by observing the degree to which it fragments, increases with the recombination energy of the charge-exchange reagent. The existence of excited states of the reagent ions is identified from the fragmentation behaviour of [C₃F₆]⁺˙ in the cases of [CS₂]⁺˙, NO⁺, O₂⁺˙, [NH₃]⁺˙ and possibly [CH₄]⁺˙. In addition, the data confirm that the [C₃F₆]⁺˙ parent ion fragments from both the ground state and a long-lived isolated electronic state. The latter is populated by near-resonant charge transfer. Translational excitation contributes relatively little to internal excitation of the charge-exchanged product ion and even less in the case of the isolated state.     

Semi-empirical Xα calculations in the vibronic framework. C2H+4 and H2O+ geometries

Linear electronic-vibrational coupling constants and equilibrium geometries of the ²B₁, ²A₁, and ²B₂ states of H₂O⁺ and the ²B_3u and ²B_3g states of C₂H⁺₄ are calculated with a semi-empirical Xα theory and compared to Hartree-Fock and experimental results.