Ab initio CI study of the nitric oxide dimer (N2O2)

Configuration interaction (CI) studies of the ground and electronically excited states are reported for nitric oxide dimer (N₂O₂) in itscis equilibrium geometry. The lowest triplet state (³ B ₂) is found to lie only 0.43 eV above the ground state (¹ A ₁). The¹ A ₁ ¹ B ₁ transition is shown to be responsible for the rising absorption in the near infrared region observed experimentally. The transition of¹ A ₁¹ A ₂ calculated in the visible spectrum range of 701 nm (1.77 eV) is symmetry forbidden.     

Observation of X1A1 vinylidene by photoelectron spectroscopy of the C2H2− ion

Photoelectron spectra of the vinylidene anion (C₂H₂^?) show vibrational structure in X¹A₁ vinylidene up 12 kcal/ mol above the vibrational ground state. Analysis yields an EA(C₂H₂$\text{X}$¹ A₁) of 0.47 ± 0.02 eV, and frequencies for the CC stretch and HCH bend. Absence of the ³B₂ state in the photoelectron spectra indicates the ¹A₁-³B₂ splitting in vinylidene is ? 1.7 eV.     

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.     

Ground and low‐lying excited C2v states of FeO2—A challenge to computational methods

DFT (BPW91 and B3PW91), CCSD, CCSD(T), and MP2 geometry optimizations were performed on the lowest A₁, A₂, B₁, and B₂ singlet to septet states (a total of 16 states) of FeO₂, using in most cases the 6‐311+G(3df) basis set. With few exceptions, the different methods led to similar geometries. The lowest state is ³B₁ or ⁵B₂, depending on the method used. The quality of the various computational methods was tested by a comparison of vertical excitation energies with corresponding high‐level MRCI values. The best agreement with MRCI was found for BPW91 and B3PW91, with average deviations of about 0.2 eV. MRCI calculations were carried to extremely low configuration selection thresholds (0.025 μh) in order to find the lowest state of FeO₂. The full CI estimate (including Davidson correction and energy extrapolation to 0 μh) gave ³A₁ as the lowest state, followed by ⁵B₂ (0.03 eV) and ³B₁ (0.06 eV). Both ³B₁ and ³A₁ have geometries and vibrational frequencies consistent with the observed IR spectrum. The calculated adiabatic electron affinity of FeO₂ (from ³B₁ of FeO₂ to ⁴B₂ of FeO) is 2.46 eV with CCSD(T), and 2.22 eV with BPW91, compared with the experimental value of 2.36 eV. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009     

Multiphoton ionisation spectroscopy of H2S: a reinvestigation of the 1B1-1A1 band at 139.1 nm

The 3 + 1 multiphoton ionisation (MPI) spectrum of the ¹B₁-¹A₁ transition in H₂S at 139.1 nm has been recorded in both linearly and circularly polarised light. The rotational structure shows marked differences from that of the one-photon absorption spectrum. Properties of the excited state revealed through analysis of this structure include confirmation of its ¹B₁ character, refined values for its A, B and C rotational constants and the operation of an energy-dependent predissociation mechanism. It is shown that the third-rank tensor component of the transition operator dominates over the first-rank component in this MPI band. The orbital nature of this ¹B₁ excited state is considered.     

Theoretical study of the π→π* excited states of linear polyenes: The energy gap between 11Bu+ and 21Ag− states and their character

Multireference perturbation theory with complete active space self-consistent field (CASSCF) reference functions is applied to the study of the valence π→π* excited states of 1,3-butadiene, 1,3,5-hexatriene, 1,3,5,7-octatetraene, and 1,3,5,7,9-decapentaene. Our focus was put on determining the nature of the two lowest-lying singlet excited states, 1¹B_u⁺ and 2¹A_g⁻, and their ordering. The 1¹B_u⁺ state is a singly excited state with an ionic nature originating from the HOMO→LUMO one-electron transition while the covalent 2¹A_g⁻ state is the doubly excited state which comes mainly from the (HOMO)²→(LUMO)² transition. The active-space and basis-set effects are taken into account to estimate the excitation energies of larger polyenes. For butadiene, the 1¹B_u⁺ state is calculated to be slightly lower by 0.1 eV than the doubly excited 2¹A_g⁻ state at the ground-state equilibrium geometry. For hexatriene, our calculations predict the two states to be virtually degenerate. Octatetraene is the first polyene for which we predict that the 2¹A_g⁻ state is the lowest excited singlet state at the ground-state geometry. The present theory also indicates that the 2¹A_g⁻ state lies clearly below the 1¹B_u⁺ state in decapentaene with the energy gap of 0.4 eV. The 0–0 transition and the emission energies are also calculated using the planar C_2h relaxed excited-state geometries. The covalent 2¹A_g⁻ state is much more sensitive to the geometry variation than is the ionic 1¹B_u⁺ state, which places the 2¹A_g⁻ state significantly below the 1¹B_u⁺ state at the relaxed geometry. © 1998 John Wiley & Sons, Inc. Int J Quant Chem 66 : 157–175, 1998     

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.     

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     

(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.     

Theoretical evidence for bound electronic excited states of ozone

Extensive configuration interaction calculations (up to 1532 spin eigenfunctions) have been carried out on ozone with both minimal and extended bases. Vertical and adiabatic excitation energies to 14 excited states are reported, including seven states with vertical excitation energies less than 4 eV. Our calculations indicate that in addition to the ground state there are four other states of ozone (³B₂, ³A₂, ¹A₂ and ³B₁) bound with respect to dissociation to ground state O₂ and O (by 0.4, 0.3, 0.1 and 0.0 eV, respectively). With such small bonding energies, the current results cannot be said to show definitively (except perhaps for ³B₂) these four states to be bound with respect to O₂ + O. However, the theoretical evidence is sufficiently strong as to warrant careful experimental studies. Such bound excited electronic states could play important roles in the chemistry of the upper atmosphere and in the chemistry of oxygen discharge systems. One (or more) of these states may be responsible for the short-lived intermediate (‘ozone precursor’) recently observed in oxygen radiolysis.     

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.     

Kinetic studies on state-state coupling and collisional quenching of excited sulfur dioxide

The time-resolved total emission of SO₂ (B¹B₁→X¹A₁) at 354.9 nm were monitored following the direct excitation by a 266 nm laser pulse. A three-level model was proposed to deal with SO₂ (X¹A₁, A¹A₂, B¹B₁) system. From a kinetic treatment of these measurements, the coupling coefficient, ξ, and the relaxation time, τ, relating to the high vibronic levels of A¹A₂ and B¹B₁ states were first obtained. It is found that ξ and τ values keep basically constant, reflecting the characteristics of the studied system. In addition, the quenching rate constants of SO₂ (A¹A₂, B¹B₁) by some alkane and chloromethane molecules were measured at room temperature. The formation cross sections of complexes of SO₂ (A¹A₂, B¹B₁) and quenchers were calculated by means of a collision complex model. It is shown that the dependence of the formation cross section of complex on the number of C(SINGLEBOND)H or C(SINGLEBOND)Cl bonds is generally in agreement with that of the measured quenching cross section. © 1998 John Wiley & Sons, Inc. Int J Chem Kinet 30: 831–837, 1998     

Ab initio MRD-CI study of the rydberg states of methylene

Potential curves have been calculated for the low-lying Rydberg states of CH₂ as well as for a number of its valence-shell species by employing the ab initio MRD-CI method. The first Rydberg transition is found to occur with a vertical energy of 6.38 eV (1b₁ → 3s), but the corresponding upper state is believed to be strongly predissociated since it correlates directly with the CH(²II) + H(²S_g) ground state fragments at lower energy. The assignment of the first observed Rydberg transition at 8.757 eV by Herzberg as 1b₁ → 3dπ is confirmed almost quantitatively in the calculations, while the corresponding minimum 1P value is computed to be 10.21 eV compared to the experimental result of 10.3 ± 0.1 eV. The dissociation energy of methylene in its ground state is calculated to be 4.47 eV, and this result also fits in well with experimental evidence, which determines a lower limit for this quantity of D₀ > 4.23 eV. Finally, it is found that none of the Rydberg states nor any of the higher-lying valence-shell species of methylene are of sufficiently low energy to play a significant role in the experimental determination of the ¹A₁-³B₁ splitting of this system.     

Analysis of the stability of finite subspaces in density functional theory

We have studied photodissociation of the A state of the H₂S⁺ ion using the quantum-chemical CAS methods, and the 1² A″ (X ² B ₁) and 1⁴ A″ states are involved in photodissociation of the 1² A′ (A ² A ₁) state (the electronic states in dissociation were studied in the C _s symmetry). The CASPT2 S-loss dissociation potential energy curve (PEC) calculations indicate that the 1² A″ and 1² A′ states correlate with the second limit [H₂ + S⁺(² D)] while the 1⁴ A″ state correlates with the first limit [H₂ + S⁺(⁴S)] and that there are a transition state and a local minimum along the 1² A′ PEC and the repulsive 1⁴ A″ PEC crosses the 1² A″ and 1² A′ PECs. The CASPT2 H-loss dissociation PEC calculations indicate that the 1² A″ and 1⁴ A″ states correlate with the first limit [HS⁺(X ³Σ^?) + H] while the 1² A′ state correlates with the second limit [HS⁺(a ¹Δ) + H] and that the repulsive 1⁴ A″ PEC crosses the 1² A′ PEC. For the crossing doublet and quartet states in pairs, we performed CASSCF minimum energy crossing point (MECP) calculations, and the CASSCF spin-orbit couplings and CASPT2 energies at the MECP geometries were calculated. We examined the two previously proposed mechanisms (mechanisms I and II) for dissociation of the A state to the S⁺ ion, based on our calculation results. We suggest processes for dissociation of the A state to the S⁺ ion (processes I and II, based on mechanisms I and II, respectively) and to the SH⁺ ion (process III) and conclude that photodissociation of the A state mainly leads to the S⁺ ion via the most energetically favorable process II: A ² A ₁ (1² A′) (2.38 eV) → barrier at the linearity (2.96 eV) → X ² B ₁ (1² A″) (0.0 eV) → the 1² A″/1⁴ A″ MECP (3.50 eV, large spin-orbit coupling) → H₂ $ (X^{ 1} \Upsigma_{\text{g}}^{ + } ) $  + S⁺(⁴S) (2.92 eV) (the CASPT2 relative energy values to X ² B ₁ are given in parentheses and the largest value is 3.50 eV at the MECP).     

The X˜1A1, a˜3B1, a˜1B˜1, and B˜1A1 electronic states of SiH2

Four electronically low-lying states of silylene (SiH₂) have been studied systematically using high level ab initio electronic structure theory. Self-consistent field (SCF), two-configuration (TC) SCF, complete active space (CAS) SCF, configuration interaction with single and double excitations (CISD), and CASSCF second-order (SO) CI levels of theory were employed with eight distinct basis sets. The zeroth-order wave functions of the ground (X˜ ¹A₁ or 1 ¹A₁) and B˜ ¹A₁ (or 2 ¹A₁) excited states are appropriately described by the first and second eigenvectors of the TCSCF secular equations. The TCSCF-CISD, CASSCF, and CASSCF-SOCI wave functions for the B˜ ¹A₁ (or 2 ¹A₁) state were obtained by following the second root of the CISD, CASSCF, and SOCI Hamiltonian matrices. At the highest level of theory, the CASSCF-SOCI method with the triple zeta plus triple polarization augmented with two sets of higher angular momentum functions and two sets of diffuse functions basis set [TZ3P(2f,2d)+2diff], the energy separation (T₀) between the ground (X˜ ¹A₁) and first excited (a˜ ³B₁) states is determined to be 20.5 kcal/mol (0.890eV,7180cm⁻¹), which is in excellent agreement with the experimental T₀ value of 21.0 kcal/mol (0.910eV,7340cm⁻¹). With the same method the T₀ value for the a˜ ¹B₁−X˜ ¹A₁ separation is predicted to be 45.1 kcal/mol (1.957 eV,15780 cm⁻¹), which is also in fine agreement with the experimental value of 44.4 kcal/mol (1.925 eV,15530 cm⁻¹). The T₀ value for the B˜ ¹A₁−X˜ ¹A₁ separation is determined to be 79.6 kcal/mol (3.452 eV,27 840 cm⁻¹). After comparison of theoretical and experimental T₀ values for the a˜ ³B₁ and a˜ ¹B₁ states and previous studies, error bars for the B˜ ¹A₁ state are estimated to be ±1.5 kcal/mol (±525 cm⁻¹). The predicted geometry of the B˜ ¹A₁ state is r_e(SiH)=1.458 and θ_e=162.3^∘. The physical properties including harmonic vibrational frequencies of the B˜ ¹A₁ state are newly determined. Received: 10 March 1997 / Accepted: 2 April 1997     

Ab-initio calculations on the 1-imidazolyl radical

Ab-initio calculations on the 1-imidazolyl radical with a minimal basis set were performed. The ground state symmetry is calculated to be ²B₁(π); the first excited state is ²A₂(π) and lies at 0.359 eV. The σ-state shows a double minimum potential having its well 0.626 eV above the C_2v ground state minimum. The height of the inner hill, peaking at C_2v symmetry, is 0.612 eV; the state is of B₂ character. The reason for the distortion is discussed.     

The potential energy surfaces of the ground and excited states of carbon dioxide molecule

Potential energy surfaces (PESs) of the ¹A_l(¹Σ _g ⁺ ), ¹B₂ and ³B₂ electronic states of CO₂ have been computed as a function of the two bond distances and the bond angle. The calculations were based on the complete active space self consistent field (CASSCF) and multiconfigurational second-order perturbation theory (CASPT2) electronic structure models. From our calculations no crossing point between ¹B₂ and ³B₂ states was found, but there is a crossing point located between ¹B₂ and ³A₂ state on the PESs. The energy of the crossing point is lie 0.23 eV above the CO + O (³P), which is in agreement with the value of 0.27 eV on the experiment. This implies that the mechanism of the recombination of an oxygen atom with a carbon monoxide molecule: CO(X ¹Σ⁺, ν) + O(³P)→³CO₂*→¹CO₂*→CO(X ¹Σ⁺, ν = 0) + O(¹ D) may occur through the ³A₂ state crossing the ¹B₂ state. The equilibrium geometries and adiabatic excitation energies of ^1,3B₂, ^1,3A₂ states of CO₂ were reported and discussed in this paper, too.     

New details in the polarized spectrum of naphthalene by means of linear dichroism studies in oriented polymer matrices

Linear dichroism (LD) spectra are presented for naphthalene oriented in stretched polyethylene and polypropylene matrices at 77 K and 296 K. From the calculated spectrum LD(λ)/A(λ), where A(λ) is the corrected absorbance spectrum of the sample by unpolarized light, orientational parameters are calculated and component spectra, 235–315 nm, are resolved corresponding to polarization parallel to the long (B_3u = x) and the short (B_2u = y) axes in the molecular plane (D_2h). The orientational parameters indicate different orientational mechanisms in polyethylene and polypropylene, but the resolving procedure yields mainly identical component spectra. It is suggested that the polarization (B_3u) predominating in the 245–275 nm region isdue to a B_1g vibronic perturbation of the ¹B_2u state.     

Dipole moment of cis-glyoxal in the 1B1 electronic state from static and modulated stark spectroscopy

The Stark effect on the ¹B₁ - ¹A₁ transition of cis-glyoxal near 487.5 nm is investigated. The effect arises from interaction between appropriate pairs of accidentally degenerate rotational levels. By using static and modulated electric fiels a value of 2.3 ± 0.5 D for the dipole moment along the b-axis of the molecule in the ¹B₁ state is obtained.     

Low energy,variable angle electron-impact excitation of 1,3,5-hexatriene

A mixture of cis and trans 1,3,5-hexatriene has been studied by electron impact at incident electron energies of 20 eV, 40 eV, 50 eV, and 70 eV, at scattering angles from 0° to 80°, and with effective energy resolutions in the range from 0.05 eV to 0.15 eV. Singlet → triplet transitions with maximum intensities at 2.61 eV and 4.11 eV are observed. The lowest energy spin-allowed excitation which can be detected is the electric dipole-allowed $\text{X}$¹ A_g → 1 ¹B_u transition (in the notation appropriate for the trans isomer). No evidence has been found for a spin-allowed but symmetry-forbidden $\text{X}$¹ A_g → 2 ¹A_g excitation in the vicinity of 4.4 eV transition energy. Many other spin-allowed excitations are observed in the 6–11 eV energy-loss region, and the correlation between these features and those observed in high resolution ultraviolet absorption spectra and other electron-impact spectra is discussed.