Low-lying Excited Electronic States Research Articles

Electronic and structural effects in muscular relaxants: Riparin I and Riparin III

Two muscular relaxants (MR), methyl ethers of N-benzoyl tyramine (Riparin I) and N-(1,6-dihydroxybenzoyl) tyramine (Riparin III) were studied with X-ray diffraction (XRD), optical spectroscopy and semi-empirical calculations. The XRD data show that the C O group in Riparin I is 34° out of plane from the closest benzene ring. The abinitio calculations using RHF/6-31G* suggest that the dihydroxybenzoyl (DHB) group in Riparin III forms intramolecular hydrogen bond between the H of the O–H group and the O of the C O group, and gives a relatively planar structure between DHB group and C O group. The spectral analysis together with the excitation energy calculations using the ZINDO program showed that the absorption band at 309 nm in Riparin III, absence in Riparin I, is assigned to the π→π* transition associated mainly from the DHB. A long range charge-transfer (CT) transition, as the low-lying excited electronic state, S 1 has been calculated for both relaxants involving the non-bonding electron on carbonyl and/or amine group and the antibonding orbitals of aromatic ring. The planar structure of Riparin III and its extensive electron delocalization may have an important role on the pharmacological action mode and on the potency this MR as compared to those of Riparin I.

DFT and TD-DFT investigation of the ground and excited states for dinuclear and mononuclear copper(I) and silver(I) complexes of 3,5-bis(trifluoromethyl)pyrazole and related bis(pyrazolyl)borate

The ground-state and low-lying excited electronic states in mononuclear, {H 2B[3,5-(CF 3) 2Pz] 2M(2,4,6-Cn)} (M 1) and dinuclear {[3,5-(CF 3) 2Pz]M(2,4,6-Cn)} 2, (M 2) (Pz = pyrazole, Cn = collidine and M = Cu, Ag), are studied using DFT approach. Electronic properties are calculated using B3LYP, while excited singlet and triplet-states are examined using TD-B3LYP. All the calculated low-lying transitions are categorized as 1MLCT transitions. A good agreement was found between experimental spectra and predicted emission wavelengths ( λ em), the corresponding emissive states being assigned as 3MLCT for Cu 1 and Ag 2, 3MLLCT for Ag 1 and 3LLCT for Cu 2.

Theoretical studies on the ground and excited states of blue phosphorescent cyclometalated Ir(III) complexes having ancillary ligand

Abstract The ground state and low-lying excited electronic states in the Ir(III) complexes, fac -Ir(dfpp) 3 , Ir(dfpp) 2 (acac) and FIrpic, are studied using the density functional theory, where dfpp = 2-(2,4-difluorophenyl)pyridine, acac = acetylacetonate, pic = picolinic acid and fac = facial. Herein, the electronic properties of these molecules are studied using the B3LYP functional and the structural analysis of the optimized geometries are done in comparison with the structures of the dfpp ligand and Ir complexes. Excited triplet and singlet states are examined using the time-dependent density functional theory (TD-DFT). The calculated energies of triplets are 2.8 eV, 2.69 eV and 2.73 eV, in the order described above, which are all higher than that of fac -Ir(ppy) 3 with 2.60 eV, where ppy = 2-phenylpyridine. Nearly all of the low-lying excitations calculated in this study are categorized as metal-to-ligand charge-transfer (MLCT) transitions because HOMOs are strongly mixed between the π orbital of the dfpp ligand and the d orbital of the centric Ir atom.

Structure, energies, vibrational spectra and reactions of the boron–silicon cluster molecules B2Si, BSi2 and B2Si2

Boron–silicon clusters are produced in various semiconductor-related processes. The exact structure and properties of these clusters are of great practical interest. No previous theoretical studies of these neutral clusters were found, and few experimental studies. The molecules B2Si, BSi2 and B2Si2 tend to form rings as the lowest energy isomers. These rings all have strong π bonding interactions. For each of the formulas there are also energetically low-lying excited electronic states and structural isomers. Standard heats of formation are obtained theoretically. These can be compared to experiment for BSi and BSi2. The B3LYP method gives results in good agreement with experiment for these molecules. From theoretical heats of formation it is predicted that formation of B2Si2 from the smaller clusters will be thermodynamically favoured.

Quantum chemical study of the electronic structure of NiCH2+ in its ground state and low-lying electronic excited states

The electronic structure of NiCH(2) (+), representative of transition metal carbene ions, is investigated by means of several methods of quantum chemistry. The relative stabilities of the four low-lying doublet electronic states ((2)A(1), (2)A(2), (2)B(1), and (2)B(2)) are determined at the coupled cluster singles and doubles level (CCSD) and triples level [CCSD(T) and CCSDT-3] with both a Hartree-Fock and density functional theory (Kohn-Sham) reference. The equation-of-motion coupled cluster for treatment of excited states in singles and doubles approximation (EOM-CCSD) is used to characterize the transition energies from the (2)A(1) electronic ground state to the low-lying doublet excited states. The (2)A(2) and (2)B(1) states are nearly degenerate, found to be separated by 940 cm(-1) at the EOM-CCSD level, in agreement with the CASSCF energy ordering. The (2)B(2) state is calculated to be higher in energy by more than 1.0 eV. The spin purity of the low-lying doublet and quadruplet states described by CCSD calculations based on the unrestricted open-shell Hartree-Fock reference is discussed.

Theoretical study of the ArH+ electronic states

Potential energy curves, permanent multipole and transition dipole moments were evaluated for the ground and low-lying excited electronic states of the ArH+ cation over a wide range of internuclear distance by the multireference averaged quadratic coupled cluster method (MR-AQCC). The electric dipole polarisability of the ground X 1sigma state was evaluated by the finite-field method. The permanent multipole moments and dipole polarisabilities corresponding to the ArH+ X 1sigma+ state were used to estimate quantum defect functions of the nonpenetrating d- and f-complex Rydberg states of neutral ArH molecule. The ground state dipole function and potential were tested by a simulation of intensity distributions in the rovibrational deltav = 1 bands, radiative lifetimes and rotational g-factors for the X 1sigma+ state.

Negative ion photoelectron spectroscopy of acridine molecular anion and its monohydrate

Negative ion photoelectron spectroscopy was employed to investigate the electronic structure of the acridine molecular anion and its monohydrated anion in the gas phase. Their adiabatic electron affinities were measured to be 0.896+/-0.010 and 1.18+/-0.05 eV, and the low-lying electronic excited states in both neutral acridine and in its monohydrate were revealed. The photoelectron spectra clearly exhibit the presence of low-lying singlet and triplet states having a (pi,pi*) configuration in an uncomplexed acridine molecule. Comparison of the photoelectron spectrum of acridine with that of anthracene shows that photodetachment processes into the excited states of (n,pi*) configuration have little intensity, implying a relatively large intramolecular structural relaxation in the (n,pi*) states.

Full configuration interaction potential energy curves for the X 1Σg+, B 1Δg, and B′ 1Σg+ states of C2: A challenge for approximate methods

The C(2) molecule exhibits unusual bonding and several low-lying excited electronic states, making the prediction of its potential energy curves a challenging test for quantum chemical methods. We report full configuration interaction results for the X (1)Sigma(g) (+), B (1)Delta(g), and B(') (1)Sigma(g) (+) states of C(2), which exactly solve the electronic Schrodinger equation within the space spanned by a 6-31G( *) basis set. Within the D(2h) subgroup used by most electronic structure programs, these states all have the same symmetry ((1)A(g)), and all three states become energetically close for interatomic distances beyond 1.5 A. The quality of several single-reference ab initio methods is assessed by comparison to the benchmark results. Unfortunately, even coupled-cluster theory through perturbative triples using an unrestricted Hartree-Fock reference exhibits large nonparallelity errors (>20 kcal mol(-1)) for the ground state. The excited states are not accurately modeled by any commonly used single-reference method, nor by configuration interaction including full quadruple substitutions. The present benchmarks will be helpful in assessing theoretical methods designed to break bonds in ground and excited electronic states.

A density functional theory study of hexafluoropropene: low-lying singlet excited states and primary photodissociation channel

Time-dependent density functional theory (TD-DFT) with different functional and related atomic basis sets, is applied to calculate the vertical transitions from the ground to the low-lying valence electronic excited states of hexafluoropropene in vacuum. The results are in satisfactory agreement with the recent high-resolution photoabsorption spectrum of hexafluoropropene in gas phase. The primary photodissociation channel was also studied and the binding energy of the weakest C–F bond of the molecule was determined.

Theory of Vibrational Circular Dichroism and Infrared Absorption: Extension to Molecules with Low-Lying Excited Electronic States

A vibronic theory of unpolarized infrared absorption and vibrational circular dichroism (VCD) is presented that is valid in the limit of vibrational resonance with low-lying electronic states (LLESs). The theory is developed within the complete adiabatic vibronic coupling formalism that describes both the correlation of electron density to nuclear positions and the correlation of electron current density to nuclear velocities. It is found that additional terms contribute to the electric-dipole and magnetic-dipole transition moments that become zero in the limit where all excited electronic states in the molecule are much higher in energy relative to fundamental vibrational transitions. Two correction terms appear that are extensions of the lowest-order non-Born−Oppenheimer expansion term when vibronic detail is included in the energy denominators. One term is a resonance term with respect to the energy difference between the electronic and vibrational transitions, and the other is a nonresonance term. Und...

Does chlorine peroxide absorb below 250 nm?

Low-lying singlet and triplet electronic excited states of ClOOCl are presented. Calculations of the excitation energies and oscillator strengths are reported using excited state coupled cluster response methods, as well as the complete active space self-consistent field method with the full Breit-Pauli spin-orbit operator. These calculations predict that for ClOOCl there should be a weakly absorbing triplet state lying below the lowest absorbing singlet excited state. This state is predicted to have an absorption maximum at about 385 +/- 25 nm. This lowest triplet state is calculated to be dissociative and leads to ClOO+Cl.

A DFT Study of the Low-Lying Singlet Excited States of the All-Trans Peridinin in vacuo

The electronic properties of peridinin (Per) are investigated using density functional theory (DFT) with a time dependent (TD) treatment for transitions from the ground state to the low-lying electronic excited states. The use of a TD-DFT approach was first tested on the simpler β-carotene molecule in order to see the performances of different functionals. It turns out from the present study that a TD-DFT approach provides a rather good qualitative picture for the electronic properties of this system for which the use of highly correlated ab initio methods with the needed prescriptions, for example, a large enough basis set and/or active space, is still prevented by its large dimension. The vertical transitions of Per obtained in vacuo using TD-DFT are in good agreement with experimental data available in a nonpolar solvent like n-hexane. Vertical energies obtained for the minimum energy structure of Per were also refined, spanning the Per in vacuo conformational space using ab initio molecular dynamics o...

Near-infrared diode laser spectroscopy of free radicals

Spectroscopic results on the radicals HCSi, CCO, and FeC obtained by studying in detail energy level structures using 0.8 μm diode laser system are reported. Of these radicals, the CCO radical was investigated mainly using Fabry–Perot type diode lasers with inconvenient mode gaps in the early stage of our near-infrared diode laser spectroscopic study of free radicals, and on the other hand, the FeC and HCSi radicals were studied using an external cavity diode laser. For the FeC radical, which is an interesting radical composed of an iron atom having 3d electrons, information on spin–orbit interaction between the triplet electronic ground state and a low-lying singlet electronic excited state is reported somewhat in detail. For the HCSi and CCO radicals, spectral particularities produced by a Renner–Teller interaction and a spin–orbit interaction are described for their high-resolution spectroscopic interest.

The low-lying electronic excited states of NiCO.

Highly correlated coupled cluster methods with single and double excitations (CSSD) and CCSD with perturbative triple excitations were used to predict molecular structures and harmonic vibrational frequencies for the electronic ground state X 1Sigma+, and for the 3Delta, 3Sigma+, 3Phi, 1 3Pi, 2 3Pi, 1Sigma+, 1Delta, and 1Pi excited states of NiCO. The X 1Sigma+ ground state's geometry is for the first time compared with the recently determined experimental structure. The adiabatic excitation energies, vertical excitation energies, and dissociation energies of these excited states are predicted. The importance of pi and sigma bonding for the Ni-C bond is discussed based on the structures of excited states.

Femtosecond Time-Resolved Stimulated Raman Spectroscopy of the S2 (1Bu+) Excited State of β-Carotene

The electronic and vibrational structure of beta-carotene's early excited states are examined using femtosecond time-resolved stimulated Raman spectroscopy. The vibrational spectrum of the short-lived ( approximately 160 fs) second excited singlet state (S(2),1B(u) (+))of beta-carotene is obtained. Broad, resonantly enhanced vibrational features are observed at approximately 1100, 1300, and 1650 cm(-1) that decay with a time constant corresponding to the electronic lifetime of S(2). The temporal evolution of the vibrational spectra are consistent with significant population of only two low-lying excited electronic states (1B(u) (+) and 2A(g) (-)) in the ultrafast relaxation pathway of beta-carotene.

Theoretical studies of cyclometalated phenylpyrazol Ir(III) complex using density functional theory

The ground state and low-lying excited electronic states in the Ir(III) complex fac -Ir(ppz) 3 are studied using density functional theory where ppz=phenylprazol and fac =facial. We reported before that this complex was pure blue phosphorescent material with about 450 nm of electroluminescent peak. Herein, the electronic properties of the molecule are studied using the B3LYP functional and the structure analysis of the optimized geometries are processed in comparison with structures of ppz ligand and fac -Ir(ppy) 3 . Excited triplet and singlet states are examined using time-dependent density functional theory (TD-DFT). The calculated energies of the lowest triplet and singlet states are 3.14 and 3.56 eV. All of the low-lying transitions calculated in this study are categorized as metal-to-ligand charge-transfer (MLCT) transitions because the metal orbitals involved in the transitions have significant admixture of ligand π orbitals with amount of metal 5d character of three highest occupied molecular orbitals which are 50.2%, 47.8% and 47.8%, respectively. From the analysis of absorption spectrum of this complex, it was found that the singlets and triplets calculated in this study are distributed in the range of 270–350 and 340–410 nm in absorption spectrum.

Ground and Low-Lying Excited Electronic States of Difluorodiazirine

Ground and low-lying excited electronic states of difluorodiazirine (F2CN2) were investigated using the recently revised, multireference, second-order generalized Van Vleck perturbation theory (GVVPT2). Initial studies on the ground state were carried out with the single-reference Moller−Plesset second order perturbation theory (MP2), quadratic configuration interaction with single and double excitation (QCISD), and coupled cluster with single, double, perturbative triple excitations (CCSD(T)) methods using several correlation-consistent atomic basis sets. MP2 and QCISD did not give semiquantitatively correct descriptions of the ground state of the molecule. For the ground state, GVVPT2 results are in close agreement with CCSD(T) but lie slightly closer to the experimental results. A balanced treatment of nondynamic and dynamic correlation effects has been shown to be of crucial importance for correct description of the system. Equilibrium geometries, excitation energies, and thermodynamic stabilities of ...

TD-DFT Studies of Electronic Structures in Cyclometalated Phenylpyrazol Ir(III) Complexes: Fac-Ir(ppz) 3 and Ir(ppz) 2(acac)

The ground state and low-lying excited electronic states of two related Ir(III)complexes fac-Ir(ppz)3(1)and Ir(ppz)2(acac)(2)are studied using density functional theory where ppz = phenylpyrazol, acac = acetylacetonate and fac = facial. The complex 1 is a blue phosphorescent material with about 450 nm of electroluminescent peak. The ground states of these complexes are studied using Becke's three parameter hybrid functional with Perdew 86(B3LYP)and the structure analysis of the optimized geometries are processed. Excited triplet and singlet states are examined using time-dependent density functional theory. Every low-lying transition for the former molecule and 6 lowest singlet and triplet excitations for the latter molecule are categorized as metal-to-ligand charge-transfer transitions, because the metal orbitals involved in the transitions have significant admixture of ligandπcharacter with amount of metal 5d character in the occupied molecular orbitals related to those transitions. Through the comparison of the calculated excitons with experimental absorption spectra, each peak was examined its originality.

Multiconfigurational self-consistent field linear response for the polarizable continuum model: Theory and application to ground and excited-state polarizabilities of para-nitroaniline in solution

This paper presents the linear response (LR) function for a multiconfigurational self-consistent field (MCSCF) molecular wave function for the integral equation formalism version of the polarizable continuum model (PCM). Both equilibrium and nonequilibrium PCM solvation schemes are described: The nonequilibrium scheme is applied to the calculation of excited state wave functions (Franck–Condon states) and/or of dynamic response properties. An important characteristic of the LR-PCM-MCSCF theory is the explicit inclusion of the effects of solvent dynamics, and this allows us to treat a large variety of time-dependent phenomena. Here, in particular, the theory is applied to the study of the solvent effect on transition energies and on static and dynamic polarizabilities of para-nitroaniline (pNA). The study of the polarizability dispersion of pNA is performed for the ground state and for low-lying electronic excited states including the charge transfer state. We compare our results with available experimental and theoretical data.

Studies on Electronic Charge of the Hydrogen Bond Proton in Model Molecular Systems

The population analysis of the hydrogen bond atoms was analyzed within the different basis sets for model molecular systems for the ground and low-lying excited electronic states. The Mulliken, Lőwdin and Hirshfeld methods were used in our investigations. It has been shown that normally the proton is transferred, however, in some excited electronic states the hydrogen atom displacement might be responsible for the tautomeric interconversion.