New view of the ICN A continuum using photoelectron spectroscopy of ICN-

Negative-ion photoelectron spectroscopy of ICN(-) (X??(2)Σ(+)) reveals transitions to the ground electronic state (X??(1)Σ(+)) of ICN as well as the first five excited states ((3)Π(2), (3)Π(1), Π(0(-) ) (3), Π(0(+) ) (3), and (1)Π(1)) that make up the ICN A continuum. By starting from the equilibrium geometry of the anion, photoelectron spectroscopy characterizes the electronic structure of ICN at an elongated I-C bond length of 2.65 A?. Because of this bond elongation, the lowest three excited states of ICN ((3)Π(2), (3)Π(1), and Π(0(-) ) (3)) are resolved for the first time in the photoelectron spectrum. In addition, the spectrum has a structured peak that arises from the frequently studied conical intersection between the Π(0(+) ) (3) and (1)Π(1) states. The assignment of the spectrum is aided by MR-SO-CISD calculations of the potential energy surfaces for the anion and neutral ICN electronic states, along with calculations of the vibrational levels supported by these states. Through thermochemical cycles involving spectrally narrow transitions to the excited states of ICN, we determine the electron affinity, EA(ICN), to be 1.34(5) (+0.04∕-0.02) eV and the anion dissociation energy, D(0)(X??(2)Σ(+) I-CN(-)), to be 0.83 (+0.04/-0.02) eV.     

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.     

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.     

Electronic structure of pyrazine: a study of the geometries in the lowest excited singlet π *← n and π*←π states

Electronic, excitation energies, charge distributions and geometries of pyrazine in the lowest excited singlet π^*←π and π^*←n states have been studied by the VE—PPP, CNDO/2 and CNDO/s-CI molecular orbital methods. Study of the change of geometry in the π^*←n excitation requires localization of the density matrices in the ground and excited states, and with the help of these σ-bond orders are defined. Charge distributions and bond orders in the lowest excited singlet π^*←π and π^*←n states are compared. Whereas in the lowest singlet π*←π excitation the pyrazine ring expands uniformly, in the case of the π^*←n excitation C-C bonds contract whereas C-N bonds elongate. The predictions of theory are in agreement with experimental results, showing that the method used can be employed to obtain reliably the trends of geometry changes following a π^*←π excitation of a molecule before a more complete theoretical or experimental study is performed.     

An ab initio study of electronic spectra and excited-state properties of 7-azaindole in vapour phase and aqueous solution

The geometries of 7-azaindole (7AI), its tautomer (7AT), and 7AI–H₂O and 7AT–H₂O complexes were optimised in the ground state and some low-lying singlet excited states using the 3-21G basis set. Differences of total energies of the optimised ground and excited states and the vertical excitation energies of these systems were used to explain the observed electronic spectra. Effect of solvation of these systems in bulk water was studied using the polarized continuum model (PCM). The mode of binding of a water molecule in the S₂(n–π^*) excited state of 7AI was found to be quite different from those in its ground and π–π^* excited states. It is shown that crossing of the lowest two singlet excited-state potential surfaces S₁(π–π^*) and S₂(n–π^*) of 7AI occurs in the vapour phase under geometry relaxation while on interaction with water, the S₂(n–π^*) excited state is raised up appreciably going even above the S₃(π–π^*) excited state. Ground- and excited-state molecular electrostatic potential mapping was carried out, which led to valuable information regarding the nature of excited states of the above-mentioned systems.     

Correlation effects in calculation of electronic states for model molecular systems

Multiply excited configurations have been taken into account in evaluation of singlet, triplet and quinlet state energies and the relevant intensities of electronic transitions for model molecular systems. The results of calculation point out the essential role of interelectron, for (Π,Π^*) triplets and quintets in particular.     

Theoretical electronic structure of the lowest-lying states of the YI molecule

CAS-SCF/MRCI calculations have been performed for 15 molecular states in the representation ^2S+1Λ^(+/−) (neglecting spin–orbit effects) for the molecule YI. The corresponding 33 molecular states in the representation Ω^(+/−) (including spin–orbit effects) have been calculated using a semi-empirical spin–orbit pseudopotential built up for yttrium. Calculated potential energy curves and spectroscopic constants are reported, to the best of our knowledge they are the first ones from ab initio methods for this molecule. Present results are compared to experimental accurate data available for the ground X¹Σ⁺ and 3 excited states (1)¹Π, (2)¹Σ⁺ and (2)¹Π.     

Proton transfer in phenol–amine complexes: Phenol electronic effects on free energy profile in solution

Free energy profiles for the proton transfer reactions in hydrogen‐bonded complex of phenol with trimethylamine in methyl chloride solvent are studied with the reference interaction site model self‐consistent field method. The reactions in both the electronic ground and excited states are considered. The second‐order Møller‐Plesset perturbation (MP) theory or the second‐order multireference MP theory is used to evaluate the effect of the dynamical electron correlation on the free energy profiles. The free energy surface in the ground state shows a discrepancy with the experimental results for the related hydrogen‐bonded complexes. To resolve this discrepancy, the effects of chloro‐substitutions in phenol are examined, and its importance in stabilizing the ionic form is discussed. The temperature effect is also studied. In contrast to the ground state, the ππ^* excited state of phenol–trimethylamine complex exhibits the proton transfer reaction with a low barrier. The reaction is almost thermoneutral. This is attributed to the reduction of proton affinity of phenol by the ππ^* electronic excitation. We further examine the possibility of the electron–proton–coupled transfer in the ππ^* state through the surface crossing with the charge transfer type πσ^* state. © 2010 Wiley Periodicals, Inc. J Comput Chem, 2010     

Excited state properties from the equations of motion method. Application of the MCTDHF-MCRPA to the dipole moments and oscillator strengths of the A 1Π, a′3Π, a′3Σ+ and d3Δ low-lying valence states of CO

By examining the exact operator O_λ⁺ which is the solution of the equations of motion-Green's function method, we rederive expressions for non-reference (usually excited) state properties. Hence, additional useful information such as state expectation values, oscillator strengths, and frequency dependent and independent polarizabilities may be easily obtained from an equation of motion-Green's function calculation. With the multiconfigurational random phase approximation (MCRPA), which is equivalent to the multiconfigurational time dependent Hartree-Fock (MCTDHF), excitation energies, oscillator strengths, and excitation operators from the ground states are obtained for the low-lying valence (under 10 eV above the ground state) states of CO at the experimental ground state equilibrium geometry. We apply these techniques to obtain the excited state dipole moments for and oscillator strengths between the A ¹Π, a ³Π, a′ ³Σ⁺, and d ³Δ states of CO and compare our results to other calculations and experiments.     

Two-photon absorption spectroscopy of the Na2 molecule in the 4 eV region

By means of two-photon sequential absorption via real intermediate rotational levels of the A state, vibrational levels of three new excited electronic states of the sodium dimer have been observed in the 4 eV region. These states are identified as F¹ ∑⁺_g, G¹ Π_g and H¹ Π_g. Their vibrational and rotational constants have been determined.     

The orbital description of the potential energy curves and properties of the lower excited states of the BH molecule

The electronic wavefunctions for the ground (X¹ Σ⁺) and the low-lying excited states (a³Π, A¹Π, ³Σ⁺) of the BH molecule have been calculated as a function of internuclear distance using the ab initio generalized valence bond method (GVB) with optimization of spin coupling (SOGI). The potential curve of the A¹Π state in the zero rotational level is found to have a hump of 0.150 eV at R = 3.8₉a_o (experimentally a hump of unknown size is found at 3.9 ± 0.4 a₀); a smaller hump at larger R (0.02 eV at R = 4.9₂a₀) is also found for the calculated a³Π state. The presence of such humps is found to result from the recoupling of orbitals that must occur as R is decreased from ∞ to R_e and is comparable in origin to the activation barrier in a radical exchange reaction (e.g., H₂ + D ? HD + H). The calculated binding energies of the BH states are 3.272 eV (X¹ Σ⁺), 2.216 eV (a³ Π), and 0.502 eV (A¹ Π). The ³Σ⁺ state is unbound although it does exhibit a small unbound minimum. The dipole moment, quadrupole moment, and electric field gradient are calculated as a funtion of R. The shapes of the potential curves and the properties are interpreted in terms of simple qualitative considerations of the GVB orbitals.     

Velocity dependence of vibrational branching ratios in electronic energy transfer collisions of metastable argon atoms with nitrogen

Measurements have been made of the vibrational branching ratio (υ′=0)/(υ′=1) in N^*₂ (C³Π_u) formed in electronic energy transfer collisions between argon metastable atoms and ground state nitrogen molecules, using crossed molecular beams. In the relative collision energy range, 0.08–0.20 eV, this ratio is 3.5±0.2.     

Ab initio SCF CL study of the electronic spectrum of nitroso-methane

Configuration interaction studies of ground, n_ → π^*, n₊ → π^*, and π → π^* electronically excited states are reported for nitroso-methane in its eclipsed equilibrium geometry. The first (n_ → π^*) and the second (n⁺ → π^*) singlet states are calculated at 2.17 and 7.14 eV. it is shown that a significant delocalization of the nonbonding orbitals on the nitrogen and oxygen is responsible for the large energy gap between these two states. The two lowest triplet states occur at 1.29 and 5.39 eV and are of n_ → π^* and π → π^* origin.     

A gas phase ab initio excited state geometry optimization study of thymine, cytosine and uracil

Molecular geometries of the nucleic acid bases thymine, cytosine and uracil in the ground and the lowest two singlet excited states were optimized using the ab initio approach employing the 4-31G basis set for all the atoms except the amino group of cytosine for which the 6-311+G^* basis set was used. The excited state calculations were performed employing configuration interaction involving singly excited configurations (CIS). Vibrational frequencies were computed in order to examine the nature of the stationary points on the potential energy surfaces obtained by geometry optimization. While the ground state geometries of uracil and thymine (except the methyl group hydrogens) are planar, the corresponding excited state geometries were found to be significantly nonplanar. In the case of cytosine, the amino group is pyramidal and the rest of the molecule is only slightly nonplanar in the ground state, but the excited state geometries are appreciably nonplanar. In particular, consequent to the S₂(n–π^*) excitation of cytosine, the amino group plane is strongly rotated. While thymine is stable in the S₂(π–π^*) excited state, uracil appears to be dissociative in the corresponding excited state.     

Excited state dipole moments and geometries of the bases of nucleic acids

CNDO/s-CI and VE-PPP methods have been employed to calculate the dipole moments of the bases of nucleic acids in the ground and excited states. A component analysis in terms of μ_hyb^(σ), μ_ch and μ_π has been done using the CNDO/s-CI method and these results have been compared with those obtained by the CNDO/2 and IEHT methods. It is observed that while the CNDO/2 and CNDO/s-CI methods give almost the same total dipole moments, component-wise their predictions are very different.Dipole moments of the molecules have also been studied for the lowest excited singlet and triplet π^* ← π states. It is observed that the conventional method of calculating dipole moments using changes of only the net charges in the excited state does not give correct results for uracil and thymine, for which experimental results are available. Considering deformed non-planar excited state geometries for these molecules, the observed excited state dipole moments have been explained. A method has been suggested to include the effects of non-planarity while calculating the properties of a complex molecule in a π^* ← π excited state. For adenine, guanine and cytosine, the excited state dipole moments are found to be smaller than the ground state values.     

A theoretical study of photoinduced electron transfer between tetracyanoethylene and arene

 Ab initio calculations have been performed to investigate the state transition in photoinduced electron transfer reactions between tetracyanoethylene and biphenyl as well as naphthalene. Face-to-face conformations of electron donor–acceptor (EDA) complexes were selected for this purpose. The geometries of the EDA complexes were determined by using the isolated optimized geometries of the donor and the acceptor to search for the maximum stabilization energy along the center-to-center distance. The correction of interaction energies for basis set superposition error was considered by using counterpoise methods. The ground and excited states of the EDA complexes were optimized with complete-active-space self-consistent-field calculations. The theoretical study of the ground state and excited states of the EDA complex in this work reveals that the S₁ and S₂ states of the EDA complexes are charge–transfer (CT) excited states, and CT absorption which corresponds to the S₀→S₁ and S₀→S₂ transitions arise from π−π^* excitation. On the basis of an Onsager model, CT absorption in dichloromethane was investigated by considering the solvent reorganization energy. Detailed discussions on the excited state and on the CT absorptions were made. Received: 30 April 2001 / Accepted: 18 October 2001 / Published online: 9 January 2002     

On the lower excited electronic states of biacetyl

An ab initio SCF and CI study has been carried out for the ground and electronically excited states of biacetyl (CH₃COCOCH₃). The second absorption band in the 4.40 eV region has been assigned to a ¹A_g → ¹B_g nπ^* transition The character of the lower-lying states has been analyzed in terms of the CI wavefunctions.     

Molecular orbital studies of the spectral and structural properties of chlorosilylidyne,SiCl

Si-Cl bond distances for the X̃² Π ground and low lying b̃⁴ Σ, ò Σ and B̃'² Δ excited states of SiCl have been optimized at the SCF and CI level with 6–31G basis set. Optimized bond distances are in good agreement with experimental values. Computed electronic excitation energies for the X̃²Π-Ã²Σ and X̃²Π-B̃'² Δ transitions compare well with the observed spectrum. The calculated harmonic vibrational frequency for the ground state, 525.2 cm⁻¹, also agrees with the experimental value 535.6 cm⁻¹.     

Theoretical study of electronic structure of rhodium mononitride and interpretation of experimental spectra

Potential energy curves of 22 electronic states of RhN have been calculated by the complete active space second‐order perturbation theory method. The X¹Σ₀⁺ is assigned as the ground state, and the first excited state a³Π₀+ is 978 cm^?1 higher. The ¹Δ(I) and B¹Σ⁺ states are located at 9521 and 13,046 cm^?1 above the ground state, respectively. The B¹Σ⁺ state should be the excited state located 12,300 cm^?1 above the ground state in the experimental study. Moreover, two excited states, C¹Π and b³Σ⁺, are found 14,963 and 15,082 cm^?1 above the X¹Σ⁺ state, respectively. The transition C¹Π₁–X¹Σ₀⁺ may contribute to the experimentally observed bands headed at 15,071 cm^?1. There are two excited states, D¹Δ and E¹Σ⁺, situate at 20,715 and 23,145 cm^?1 above the X¹Σ⁺ state. The visible bands near 20,000 cm^?1 could be generated by the electronic transitions D¹Δ₂–a³Π₁ and E¹Σ⁺₀–X¹Σ⁺₀ because of the spin–orbit coupling effect. © 2013 Wiley Periodicals, Inc.