Molecular Orbital Configuration Research Articles

Conical Intersections in Solution: A QM/MM Study Using Floating Occupation Semiempirical Configuration Interaction Wave Functions

We describe a hybrid quantum mechanical/molecular mechanical (QM/MM) method implemented in the context of floating-occupation molecular orbital semiempirical configuration interaction wave functions. The QM/MM approximation is compared to a fully QM treatment, emphasizing its accuracy with respect to features important to photochemical mechanisms such as conical intersection geometries and topography. The methodology we describe allows the location and characterization of solution-phase conical intersections for the first time. This is demonstrated explicitly by application to several biologically relevant chromophores solvated in clusters of up to 150 water molecules. The effect of solvation on the photochemical energy landscapes of these molecules is investigated, and we note a tendency for conical intersections to become absolute excited state minima upon hydration.

Comparative electronic structure of a lanthanide and actinide diatomic oxide: Nd versus U

Using a modified version of the Alchemy electronic structure code and relativistic pseudopotentials, the electronic structure of the ground and low lying excited states of UO, NdO, and NdO+ have been calculated at the Hartree—Fock (HF) and multiconfiguration self-consistent field (MCSCF) levels of theory. Including results from an earlier study of UO+ this provides the information for a comparative analysis of a lanthanide and an actinide diatomic oxide. UO and NdO are both described formally as M+2O−2 and the cations as M+3O−2, but the HF and MCSCF calculations show that these systems are considerably less ionic due to large charge back-transfer in the π orbitals. The electronic states putatively arise from the ligand field (oxygen anion) perturbed f4, sf3, df3, sdf2, or s2f2 states of M+2 and f3, sf2 or df2 states of M+3. Molecular orbital results show a substantial stabilization of the sf3 or s2f2 configurations relative to the f4 or df3 configurations that are the even or odd parity ground states in the M+2 free ion. The compact f and d orbitals are more destabilized by the anion field than the diffuse s orbital. The ground states of the neutral species are dominated by orbitals arising from the M+2sf3 term, and all the potential energy curves arising from this configuration are similar, which allows an estimate of the vibrational frequencies for UO and NdO of 862 cm−1 and 836 cm−1, respectively. For NdO+ and UO+ the excitation energies for the Ω states were calculated with a valence configuration interaction method using ab initio effective spin—orbit operators to couple the molecular orbital configurations. The results for NdO+ are very comparable with the results for UO+, and show the vibrational and electronic states to be interleaved.

A note on Wiberg-type atomic valencies for a 3-electron 3-centre valence bond structure

An expression is deduced for a Wiberg-type atomic valence for the A-atom of the 3-electron 3-centre valence bond structure ▪ with the localised molecular orbital configuration ( y+ la) 1( a+ kb) 2. It is demonstrated that for l>0, the A-atom valence is less than unity. However for k≠1 and small kl, the value of this valence is larger than it is for the Lewis structure ▪ with the same value of k in the ( y) 1( a+ kb) 2 configuration for the latter structure. Elaboration of similar theory is also provided for the 4-electron 3-centre valence bond structure ▪ with a ( y) 1( y+ la) 1( a+ kb) 2 configuration, for which the Wiberg-type A-atom valence can exceed unity.

MC/AM1-SCI study of the effect of hydration shells on the nitrogen hyperfine coupling constant of the (CH3)2NO radical in aqueous solution

A combination of Monte Carlo (MC) simulation and semi-empirical AM1 molecular orbital (MO) singly-excited configuration interaction (SCI) calculation was applied to dimethyl nitroxide (DMNO) in aqueous solution and the solvent effect on the hyperfine coupling constant (HFCC) of nitrogen in DMNO was analyzed. During the MC simulation, 100 solution structures were picked up. For each solution structure, the DMNO–(H2O)n (n=1–40) clusters were cut out and the AM1-SCI calculation was applied to the cluster as a supermolecule. The HFCC of nitrogen was obtained by averaging the 100 solution structures. The H2O molecules included in the supermolecule were determined by two different types of selection and the contribution of H2O molecules to the HFCC was well elucidated in relation to the hydration shell structure.

The influence of Jahn—Teller interaction, vibronic coupling and spin—orbit splitting on the Hel excited photoelectron spectra of the boron trihalides

Hel excited photoelectron spectra of the boron trihalides have been recorded with improved resolution, and several new vibrational progressions have been observed in the spectra of BCl3, BBr3 and BI3. The spectra illustrate, as would be expected, that spin—orbit coupling becomes important in the heavier trihalides, and affects the photoelectron band structure strongly. Progressions involving excitation of the symmetric stretching vibrational mode are observed in all four molecules. Some of the new structure revealed in the photoelectron spectra of BCl3, BBr3 and BI3 is assigned, and the molecular orbital configurations in BBr3 and BI3 are discussed, taking into account existing theoretical predictions and the present experimental results.

Nonadiabatic bending dissociation in 16 valence electron system OCS

The speed, angular, and alignment distributions of S(1D2) atoms from the ultraviolet photodissociation of OCS have been measured by a photofragment imaging technique. From the excitation wavelength dependence of the scattering distribution of S(1D2), the excited states accessed by photoabsorption were assigned to the A′ Renner–Teller component of the 1Δ and the A″(1Σ−) states. It was found that the dissociation from the A′ state gives rise to high- and low-speed fragments, while the A″ state only provides the high-speed fragment. In order to elucidate the dissociation dynamics, in particular the bimodal speed distribution of S atoms, two-dimensional potential energy surfaces of OCS were calculated for the C–S stretch and bending coordinates by ab initio molecular orbital (MO) configuration interaction (CI) method. Conical intersections of 1Δ and 1Σ− with 1Π were found as adiabatic dissociation pathways. Wave packet calculations on these adiabatic surfaces, however, did not reproduce the low-speed component of S(1D2) fragments. The discrepancy regarding the slow S atoms was attributed to the dissociation induced by nonadiabatic transition from A′(1Δ) to A′(1Σ+) in the bending coordinate. This hypothesis was confirmed by wave packet calculations including nonadiabatic transitions. The slow recoil speed of S atoms in the nonadiabatic dissociation channel is due to more efficient conversion of bending energy into CO rotation than the adiabatic dissociation on the upper state surface. By analyzing the experimental data, taking into account the alignment of S(1D2) atoms, we determined the yield of the nonadiabatic transition from the A′(1Δ) to the ground states to be 0.31 in the dissociation at 223 nm. Our theoretical model has predicted a prominent structure in the absorption spectrum due to a Feshbach resonance in dissociation, while an action spectrum of jet-cooled OCS measured by monitoring S(1D2) exhibited only broad structure, indicating the limitation of our model calculations.

Analysis of the bound–free emission spectra from the E(0+) and f(0+) ion-pair states of ClF to obtain potentials for the ion-pair and repulsive valence states

The optical–optical double resonance method has been used to excite selected vibrational levels of the E(0+) and f(0+) ion-pair states of ClF. Subsequent bound–free transitions terminating on three repulsive valence states, Y(3Σ0+−), B1(3Π0+), and C(1Σ0++), have been analyzed. The bound–free continua have been modeled to obtain analytical expressions for the three 0+ repulsive potentials and to improve the description for the E(0+) and f(0+) state potentials. One of the repulsive states is Y(3Σ0+−), which is responsible for the predissociation of B(3Π0+). The transition dipole functions also have been assigned for the bound–free transitions. The molecular orbital configurations of the lower states are discussed. Finally, the influence of homogeneous interaction between the E(0+) and f(0+) states on the bound–free spectra from certain vibrational levels is discussed.

Theoretical model for the reaction mechanisms of singlet carbene analogs into unsaturated hydrocarbon and the origin of the activation barrier

The addition of singlet carbene analogs XR2 (X=C, Si, Ge, and Sn; R=H and F) into ethylene have been investigated by ab initio molecular orbital methods. The reaction mechanisms are two-step processes. The first step is the complex formation, the CX bond formation of one site, with the interaction between the empty pπ orbital of carbene analogs and the π orbital of ethylene, and the second is the other CX bond formation with biradical coupling (addition). The transition states correspond to the intermediate process between the first and the second steps. The localized molecular orbital (LMO) configuration analysis along the intrinsic reaction coordinate (IRC) path showed important singlet excitation character for the transition state. An increase in the energy barrier heights for the above systems is associated with a general increase in the singlet excitation energies of carbene analogs XR2. On the basis of the proposed reaction mechanism, the heats of formation for the treated reaction systems were estimated. © 1998 John Wiley & Sons, Inc. Int J Quant Chem 70: 291–302, 1998

Theoretical study of the potential energy surfaces and dynamics of CaNC/CaCN

Potential energy surfaces for the ground and two low-lying electronically excited states of CaNC/CaCN, are calculated using the ab initio molecular orbital (MO) configuration interaction (CI) method. The absorption and emission spectra of the system are computed by performing time-dependent quantum dynamical calculations on these surfaces. The most stable geometries for the two lowest lying 12Σ+ and 12Π electronic states correspond to the calcium isocyanide (CaNC) structure. These two states are characterized by ionic bonding and the potential energy curves along the bending coordinate are relatively isotropic. The result of our wave packet dynamics shows that the characteristics of the experimental spectra observed by the laser-induced fluorescence spectroscopy can be explained by the Renner–Teller splitting.

Is acetylene radical anion with a trans–bent form observed in matrix experiment? An ab initio study

Based on extensive ab initio calculations of the hyperfine coupling constants of both hydrogen and carbon nuclei of three C2H−2 anions, it is confirmed that an acetylene radical anion has effectively been produced radiolytically in low temperature alkane matrices and it possesses a trans-bent form. The results are derived from different molecular orbital configuration interaction calculations (CIS, CISD, QCISD and MRD-CI) as well as density functional theory (DFT) employing extended basis sets. Both popular DFT methods (BLYP and B3LYP) show some difficulties in calculating the isotropic coupling constants of the carbon nuclei.

A Systematic Method for Determining Molecular Term Symbols for Diatomic Molecules Using Uncoupled-States Orbital Diagrams

A systematic method for determining molecular term symbols for diatomic molecules using uncoupled states orbital diagrams is described. Using this method, molecular terms symbols for a particular molecular orbital electron configuration are determined from the possible combinations of quantum numbers of the individual electrons. Two examples are included to illustrate the method,

Spin-orbit interactions between Rydberg states of O 2

Spin-orbit interactions between Rydberg states of O 2 have been studied by observing the photoabsorption spectrum in the 1100 – 1200 Å region. The dominant interactions are between states having the same primary molecular orbital configuration and these give rise to perturbations in rotational-level structures and the appearance of nominally-forbidden absorption bands. Three examples are given showing the effects of 3 II u − 1 II u and 3∑ u + - 3∑ u − interactions in the ground-state absorption spectrum.

Identification of the 4pσu 1Πu Rydberg state of O2

New high-resolution measurements of the O2 photoabsorption cross section in the 1147–1155 Å region are presented. The observation of three previously-unreported narrow lines in a band near 1148 Å has enabled unambiguous assignment of the band to a 1Πu←X3Σg− transition which gains strength through a spin-orbit interaction with the upper state of a 3Πu←X3Σg− transition appearing at 1153 Å. Other lines in the 1148 Å band are broadened by predissociation through a heterogeneous interaction with a nearly energetically coincident 1Δu level. The states, labelled h1Πu, F3Πu and i1Δu, are predominantly of 4pλ Rydberg molecular orbital configurations although there are indications that the Πu states interact strongly with valence states of the same symmetry.

A quantum chemical ab initio study of the superexchange coupling in binuclear oxygen-bridged Ni(II) complexes

Abstract Quantum chemical ab initio calculations have been performed for binuclear Ni(II) complexes of the form L 5 NiONiL 5 and L 4 NiO 2 NiL 4 , with either one linear oxygen bridge or with two equivalent oxygen bridges and NiONi angles between 80° and 110°. He-like model ligands L have been used instead of the macrocyclic ligands which are necessary in the experiments to stabilize the complexes. The spin coupling between the two Ni(II) ions is antiferromagnetic in the linear oxo-bridged complex, with an exchange integral J of about −30 cm −1 at the CASSCF level and about −80 cm −1 at the multi-reference CEPA level. In the bioxygen-bridged complex the coupling is antiferromagnetic for NiONi angles smaller than 85° and larger than 102° and weakly ferromagnetic in between with J ≈ 10 cm −1 . Comparison with experiment and an interpretation of the exchange coupling by means of perturbation theory in terms of covalent and ionic molecular orbital configurations is also given.

Novel intermolecular and intramolecular photochemical reactions initiated by excited state single electron transfer processes

Several examples of photoinduced electron transfer initiated cation radical carboncarbon bond fragmentation reactions are reported. 1,2-aminoalcohols and 1,2-diamines undergo analogous fragmentation reactions although they show quite different reactivities. In general, the cation radicals of 1,2-diamines fragment more efficiently than those of aminoalcohols and the reactions are not dependent on the basicity of the reduced acceptor counterion species. A systematic study of 1,2-diamines indicates that the quantum efficiencies of the reaction are highly dependent on the reaction conditions (solvent, type of sensitizer, cosensitization, etc.) and structural considerations of the molecules being studied. Higher quantum efficiencies can be obtained using a triplet sensitizer or cosensitization. The effects of molecular orbital alignment on the efficiency of fragmentation were analyzed by modifying the structures of the 1,2-diamines. Interestingly, higher quantum yields are observed when certain molecular orbital configurations are achieved. Similar conclusions are obtained from studies of the reactivity of “remote” aminoalcohols and diamines. In this report, we also demonstrate that the quantum efficiency of the cation radical fragmentation can be improved dramatically by utilizing an efficient fragmentable electron acceptor as sensitizer (such as carbon tetrachloride). A new concept of “double fragmentation” is proposed.

Potential energy surfaces of some low-lying states of fluoroformyl radical FCO

Potential energy surfaces for electronic ground and four excited states of the fluoroformyl radial, FCO, were calculated with the ab initio molecular orbital (MO) configuration interaction (CI) method. The electronic transition related with progressions I and II in the photoabsorption spectra observed by Jacox [J. Mol. Spectry.80 (1980) 257] was found to be the 1 2A″(1 2Π) ← 1 2A' transition, and the pathway of the photodissociation, FCO (1 2A') + hv→ CO ( 1Σ +) + F( 2P), with a threshold near 280 nm, was clarified.

Etude théorique de la dissociation de NO par attachement électronique — courbes de potential ab initio de différents états excités de NO −

Potential curves of the first excited states of NO and NO − were produced using molecular orbital configuration interaction (MO-CI) ab initio methods. Electron attachment dissociations were explained using these curves. The most important dissociation observed, N( 2D) + O-( 2P), seems to involve π states (singlet and triplet) or a 1Δ state. Widths of the resonance states 3σ −, 1Δ and 1σ + were calculated.

Structures and electronic states of ion-pair complexes formed between a carbonyl compound and a sodium atom. An ab initio molecular orbital and MRSDCI study

The geometric structure and electronic state of ion pairs, formed by an electron transfer from an alkali metal atom to the CO group of a carbonyl compound, have been determined using ab initio molecular orbital (MO) and multireference single and double excitation configuration interaction (MRSDCI) methods. As model systems of the ion pair, acetone–Na (AT–Na) and formaldehyde–Na (FA–Na) systems were chosen. The geometry optimizations of the FA–Na ion pair gave two structures as the stable form. One is a linear form with a CO—Na angle of 168.7° and the other is the π-form with an angle of 87.6°. The total energies of both ion pairs are similar, although the energy calculation shows that the π form is slightly more stable than the linear form. On the other hand, the geometry optimization of the AT–Na ion pair gave only a linear form for the stable structure. The MRSDCI wavefunctions of the ion pairs indicate that the interaction between the alkali metal and the carbonyl group is composed of an attractive Coulomb force [CO–⋯Na+] at the ground state, whereas the nature of the bonding is determined by the weakly attractive van der Waals force [CO⋯Na] at the first excited state. From an analysis of the excited-state wavefunctions, the first absorption band of the ion pairs is assigned to the charge-transfer (CT) transition expressed by π*CO→ Na(3s). The solvent effects on the spin density of the carbonyl oxygen in a 2-methyltetrahydrofuran (MTHF) matrix were also discussed on the basis of the fractional charge model.

Spectroscopy and electronic structure of ion-pair states

Ion-pair states are a distinctive sub-set of the valence states of molecules. In a few cases they constitute the ground electronic state, but more usually their term values lie close to the lowest Rydberg states. Their unusual spectroscopic properties and the techniques used to access them are discussed with particular reference to the diatomic halogens. Their electronic structure is best approached from two complementary points of view, as a single molecular orbital configuration or the product of separated ion configurations, according to the ionic separation. The consequences of this dual character for absorption and fluorescence spectroscopy are outlined. Finally, the state-specific coupling of Rydberg and ion-pair states is surveyed.

Characterization of excited electronic states of naphthalene by resonance Raman and hyper-Raman scattering

The first resonance Raman and hyper-Raman scattering from naphthalene are reported. Fourth harmonic of a mode-locked Nd:YAG laser is used to resonantly excite the 1B1u+ transition, producing Raman spectra that confirm the dominance of the vibronically active ν28 (b3g) mode and the Franck–Condon active ag modes, ν5 and ν3. A synchronously pumped stilbene dye laser and its second harmonic are employed as the excitation sources for hyper-Raman and Raman scattering from the overlapping 1B2u+ and 1Ag− states. The Raman spectra indicate that the equilibrium geometry of naphthalene is distorted primarily along ν5, ν8, and ν7 normal coordinates upon excitation to 1B2u+. The hyper-Raman spectrum shows that ν25 (b2u) is the mode principally responsible for vibronic coupling between the 1Ag− and 1B2u+ states. The results demonstrate the advantageous features of resonance hyper-Raman scattering for the case of overlapping one- and two-photon allowed transitions. Calculations based on simple molecular orbital configurations are shown to qualitatively agree with the experimental results.