Low-lying Excited Electronic States Research Articles

Theoretical study of MgNa+ ionic system: potential energy curve, vibrational levels, dipole moments, radiative lifetimes and laser-cooling analysis

An extensive ab-initio quantum chemistry study and a discussion of the possible laser cooling and formation of the heteronuclear ionic MgNa+ molecule is presented in this paper. Our work is based on the use of non–empirical pseudo-potentials for the Mg2+ and Na+ cores, large Gaussian basis sets, parametrized l–dependent polarization potentials, and full valence configuration interaction calculations. We present here adiabatic potential energy curves of the ground and 44 low-lying excited electronic states of 1,3Σ+, 1,3Π, 1,3Δ symmetries, and their spectroscopic parameters (Re, De, Te, ωe, ωexe, and Be). Furthermore, numerous ionic – neutral avoided crossings, specially between higher adjacent electronic states of 1,3Σ+, 1,3Π symmetries, are identified and interpreted. The energy separations between these avoided crossings are also calculated. The electric dipole moment curves have been computed for a wide range of inter-nuclear distances. The vibrational energies are obtained by solving the nuclear Schrodinger equation, using the calculated potential energy curves. Thereafter, spontaneous and black-body radiation induced transition rates are calculated to obtain the ground and excited states vibrational level lifetimes. It has been observed that the lifetimes of the ground state vibrational levels are of the order of a second, while those of the excited states, mainly the 21Σ+ state, have the order of a nanosecond.

Second-order nonlinear optical properties of [formula omitted]-shaped pyrazine derivatives

The linear and nonlinear optical (NLO) properties of a series of Λ-shaped derivatives containing a 4,5-dicyanopyrazine acceptor unit, N,N-dimethylamino donor groups and systematically enlarged π-conjugated linkers are investigated by means of UV/Visible and Hyper-Rayleigh scattering spectroscopies. Density functional theory calculations are also carried out to rationalize the magnitude and symmetry of the NLO responses. The results demonstrate that these compounds possess two low-lying excited electronic states close to each other in energy, which are accessible through one-photon optical transitions respectively polarized perpendicular and parallel to the two-fold molecular axis. These two states contribute additively to the first hyperpolarizability, which exhibits a dominant dipolar character. We also show that the NLO responses significantly deviate from Kleinman's index permutation symmetry.

The electronic structure of WS molecule below 21,500 cm−1

The low-lying electronic states of the tungsten monosulfide (WS) molecule have been investigated using laser-induced fluorescence excitation spectra and high-level ab initio calculations. 29 vibronic bands in total are experimentally observed in the range of 13,100 – 21,500 cm−1 and grouped into 8 electronic transition systems. The spectroscopic constants including the vibrational frequency, rotational constant and radiative lifetime in the low-lying excited electronic state are obtained by analysis of the rovibrational-resolved spectra. Seven lowest Ω sub-states that belong to the lowest three Λ-S states considering the spin-orbit effect, i.e. the ground state 3Σ−, first and second excited states 5Π and 3Φ, are identified through single-vibrational-level emission spectra. In addition, seven Ω = 1 excited sub-states in the visible energy range are identified, which are consecutively consistent with the ab initio calculation results of the sixth to twelfth Ω = 1 sub-states. The experimental and theoretical results on WS molecule may provide a benchmark to study the electronic structure of those molecules with strong spin-orbit coupling and electron-electron correlation.

Peculiarities of electronic structure and chemical bonding in iron and cobalt metal complexes of porphyrazine and tetra(1,2,5-thiadiazole)porphyrazine

The geometrical and electronic structures of iron and cobalt metal complexes of porphyrazine and tetra(1,2,5-thiadiazole)porphyrazine in ground and low-lying excited electronic states were determined by DFT calculations and the complete active space (CASSCF) method with following accounting dynamic correlation by multiconfigurational quasidegenerate second-order perturbation theory (MCQDPT2). A geometrical structure of D[Formula: see text] symmetry has been obtained for all four complexes. According to data obtained by the MCQDPT2 method, the complexes of cobalt and iron possess the ground states 2A[Formula: see text] and 3A[Formula: see text], respectively, and wave functions of the ground states have the form of a single determinant. It is shown that the crystal field theory (CFT) can be used to describe the sequence of electronic states of the investigated complexes. The nature of the bonds between metal atoms and nitrogen atoms has been described using the analysis of the electron density distribution in the frame of Bader’s quantum theory of atoms in molecule (QTAIM).

Multiple ESIPT pathways originating from three-state conical intersections in tropolone.

Internal conversion decay dynamics associated with the potential energy surfaces of three low-lying singlet excited electronic states, S1 (ππ*, A'), S2 (ππ*, A'), and S3 (nπ*, A″), of tropolone are investigated theoretically. Energetic and spatial aspects of conical intersections of these electronic states are explored with the aid of the linear vibronic coupling approach. Symmetry selection rules suggest that non-totally symmetric modes would act as coupling modes between S1 and S3 as well as between S2 and S3. We found that the S1-S2 interstate coupling via totally symmetric modes is very weak. A diabatic vibronic Hamiltonian consisting of 32 vibrational degrees of freedom is constructed to simulate the photoinduced dynamics of S0 → S1 and S0 → S2 transitions. We observe a direct nonadiabatic population transfer from S1 to S3, bypassing S2, during the initial wavepacket propagation on S1. On the other hand, the initial wavepacket evolving on S2 would pass through the S2-S3 and S1-S3 conical intersections before reaching S1. The presence of multiple proton transfer channels on the S1-S2-S3 coupled potential energy surfaces of tropolone is analyzed. Our findings necessitate the treatment of proton tunneling dynamics of tropolone beyond the adiabatic symmetric double well potentials.

Role of the Propionic Acid Side-Chain of C-Phycocyanin Chromophores in the Excited States for the Photosynthesis Process

Abstract This paper focuses on a theoretical investigation of the peculiar properties of the chromophore in the C-phycocyanin (C-PC), phycocyanobilin (PCB). The scope is to unravel their key features upon light absorption and transmission occurring in natural photosynthesis. To this aim, by resorting to the time-dependent density functional theory (TDDFT) and natural bond orbital (NBO) methods, we compute the photoabsorption spectra and electronic properties of PCB, showing that three different orientations of the PCB in C-PC contribute to the nonhomogeneous broadening of the entire photoabsorption spectrum of C-PC. Furthermore, the photoabsorption peaks of PCB can undergo a shift up to 40 nm because of solvation effects. Further investigations on the competitive influence of the nearby aspartate residue and two propionic acids on the absorption spectra show that the latter play a significant role in realizing the different photo-response among the three isomers of PCB. In the low-lying electronic excited states, the π conjugated C-C bonds and the twisting angle of the pyrrole rings turn out to be affected. The NBO geometrical analyses of the bond lengths, interatomic angles, and dihedral angles evidenced that the intermolecular interactions of the propionic acid side chains play a crucial role in the determination of the excited state molecular conformations. These results indicate that the absorption spectra and the excited state structures of PCB are efficiently tuned during natural photosynthesis processes.

Electronic structure, spectroscopic properties, and bonding in a thermodynamically stable transition metal-containing diatomic dication: The case of ScS2+

Electronic states of ScS2+ were characterized by the SA-CASSCF-MRCI+Q/cc-pV5Z approach with the ground state thermodynamically stable. Low-lying excited electronic states turned out to be bound, all resulting from avoided crossings of states correlating diabatically to Sc2+ + S and those dissociating into Sc+ + S+. Although metastable, they have high and broad barriers and should be long-lived. Spectroscopic parameters, dipole moments and vibrationally averaged dipole moments calculated and the polarity of the states quantified. Transition moments and transition probabilities allow one to better identify transitions more easily accessible experimentally. Ion-dipole induced interactions and covalent bonding account for their stability.

Non-planarity in four-membered homo-cyclic compounds A4 (A = O, S, Se, Te, Po) and restoring their planarity: a study of the pseudo-Jahn–Teller effect

The bent D2d structures of the four-membered homo-cyclic compounds A4 (A = O, S, Se, Te, Po) were examined computationally to understand the pseudo-Jahn–Teller effect (PJTE). To do this, ab initio geometry optimizations and corresponding frequency calculations (at the MP2/cc-pVQZ-(PP) level of theory) show that all A4 compounds under-consideration are unstable in their planar (D4h) configuration. The ground state and six low-lying non-degenerate and degenerate electronic excited states were computed at the CASSCF (6,7)/cc-pVQZ-(PP) along the bending normal coordinate connecting the D4h and D2d geometries; these represent the adiabatic potential energy surfaces (APESs). Based on the APESs, the coupling between the ground state (1A1g) and the 1B2u excited state is demonstrated to be the reason for the planar structure bends from the high-symmetry D4h geometry into the lower-symmetry D2d stable equilibrium configuration. The solution to the PJTE (1A1g + 1B2u) ⊗ b2u problem is useful to answer the question of “how instability rises in A4 planar configuration?”. Although all A4 compounds in the series are non-planar with D4h symmetry, but geometrical optimizations and frequency calculations show that coordination of two noble gas cations (NG+ = He+, Ne+ and Ar+) above and below the σh plane of the A4 (A = O, S, Se) ring could restore ring planarity in (A4 NG)2+ complexes. The PJTE is also quenched in the A42+ (A = O, S, Se, Te, Po) cation and dication series, and planarity of the rings is also restored, i.e., the high-symmetry D4h structure becomes the equilibrium configuration.

Elastic and inelastic low-energy electron collisions with PH2, PF2 and PCl2 radicals

Ab initio scattering calculations for low-energy electron collisions with PH2, PF2, and PCl2 are performed using the R-matrix method in this study. The elastic integral, differential, and momentum transfer cross sections and the excitation cross sections from the ground state to the five low-lying electron excited states are presented in the energy region of 0.01–15 eV. The target states are represented by including correlations via a configuration interaction technique. The resonant states are detected and identified. The sensitivity of the results to changes in the theoretical model is checked by comparing predictions from a variety of approximations including static exchange, one state close-coupling (CC), and many-state CC approximations. The similarities and differences among the resonances of these three radicals are discussed.

Cross sections for vibronic excitation of CH+ by low-energy electron impact

A theoretical approach for the electron-impact vibronic excitation of molecular ions with low-lying excited electronic states is described. In this approach, the fixed-nuclear $R$-matrix method is employed to compute electron-ion scattering matrices in the Born-Oppenheimer approximation. A vibronic frame transformation and the closed-channel elimination procedure in a spirit of molecular quantum defect theory are employed to construct an energy-dependent scattering matrix describing interactions between vibronic channels of the target ion induced by the incident electron. The obtained scattering matrix accounts for Rydberg series of vibronic resonances in the collisional spectrum. The approach is applied to the ${\mathrm{CH}}^{+}$ ion of an astrophysical and technological interest. Cross sections for vibronic excitation for different combinations of initial and final vibronic states are computed. A good agreement between electronic-excitation cross sections, obtained using the quantum defect theory and in a direct $R$-matrix calculation, demonstrates that the present approach provides a reliable tool for determination of vibronic (de-)excitation cross sections for targets with low-energy electronic resonances. Such targets were difficult to treat theoretically using earlier methods.

Theoretical study on the low-lying electronic excited states and laser cooling feasibility of AuH molecule

To evaluate characteristics of the low-lying electronic states and the feasibility of the laser cooling of the AuH molecule, we investigate the potential energy curves (PECs), the vibrational and rotational characteristics of the ground and low-lying excited electronic states based on ab initio calculations. The PECs of the X1Σ+, A1Σ+, B1Σ+, a3Σ+, b3Σ+, e3Σ+, C1П, D1П, c3П and d3П states and the transition dipole moments among these states are calculated with the multi-reference configuration interaction method with Davidson correction and all-electron basis sets. Based on the obtained PECs, the Schrödinger equations of nuclear movement are solved to obtain the rotational and vibrational energy levels of the electronic states. The spectroscopic parameters are obtained by fitting the obtained rotational and vibrational levels with Dunham series expansion. The matrices of the Franck-Condon factors (FCFs) dependent on the vibrational quantum numbers are obtained. The feasibility of laser cooling is explored. The FCF of the D1П (v′ = 0) ↔ X1Σ+ (v″ = 0) transition is 0.9862, which implies that the AuH molecule is feasible but not the much better ones for laser cooling and more pumping lasers are needed. The optical laser cooling scheme is suggested and the Einstein coefficient and the recoil temperature are calculated to evaluate the effect of the cooling scheme.

Jahn-Teller effect versus spin-orbit coupling: The structure of the free molybdenum pentafluoride molecule

The complete active space self-consistent field (CASSCF) and multiconfigurational quasi-degenerate second-order perturbation (MCQDPT2) calculations have been used to study the ground and low-lying excited electronic states of MoF5. Scalar-relativistic effects and spin-orbit coupling (SOC) have been taken into account employing the third-order Douglas-Kroll-Hess (DKH) Hamiltonian and full Breit-Pauli operator, respectively. The optimization and the calculation of the Hessian matrix have been performed by non-routine procedure employing the symmetry coordinates for all considered (D3h, C2v and C4v) geometrical configurations. The trigonal bipyramidal D3h structure of molybdenum pentafluoride MoF5 possesses two-fold degenerate ground first excited electronic orbital state and undergoes Jahn-Teller distortion to the C2v configurations. Accounting of SOC quenches the JT effect. As a result the D3h structure of MoF5 is no longer the conical intersection of two adiabatic potential energy surfaces (APES) but two separated APES corresponding to ground (12E1/2) and nearest excited (12E3/2) spin-orbit states are appeared instead. The D3h trigonal bipyramidal structure in the ground spin-orbit state corresponds to the 2-nd order saddle point on APES and lies above the minima (C2v structure, 12E1/2 state) by 244 cm−1. The warping barrier (another C2v structure) in the trough of APES and the Berry pseudorotation barrier (C4v structure, ground spin-orbit state 2E3/2) are 19 and 310 cm−1, respectively. Therefore MoF5 molecule is characterized in the gas phase by a large-amplitude motion of fluorine atoms in the equatorial plane and non-rigid intramolecular rearrangement by Berry mechanism. Despite such non-rigid nuclei dynamic, the calculated harmonic frequencies of MoF5 molecule do not contradict to available experimental IR spectrum. The repeat refinement of MoF5 gas electron diffraction data has been carried out using static and pseudoconformer models based on the SO-MCQDPT2 results. Corresponding to only extreme points on APES the configurations have been included into the pseudoconformer model. The both models have been fitted to the experimental data by the same quality.

Singlet fission in spiroconjugated dimers.

Spiroconjugation results in a unique arrangement of conjugated fragments providing a novel way to chemically connect chromophoric units and control their electronic interaction, which is a key factor for the viability of the singlet fission photophysical reaction. In this study, we computationally explore the possibility of intramolecular singlet fission in spiroconjugated dimers by characterizing the nature of the low-lying excited electronic states, evaluating the magnitude of interstate couplings, describing possible singlet fission mechanisms, and investigating the potential role of low and high frequency vibrational modes in the exciton fission process. The spiro linkage of organic chromophores with the proper excited singlet and triplet energies favors the presence of low-lying charge resonance states, which play a major role in the formation of the triplet pair state. Overall, our results suggest that spiroconjugated dimers are potentially good candidates to efficiently generate independent triplet states through singlet fission.

The Influence of the Electronic Structure Method on Intersystem Crossing Dynamics. The Case of Thioformaldehyde.

The ability of different electronic structure methods to correctly describe intersystem crossing dynamics is evaluated, using thioformaldehyde as a test case. Mischievously, all methods considered-ranging from the multireference methods MRCISD, MS-CASPT2, or SA-CASSCF, to the single-reference methods ADC(2), CC2, and TDDFT in different flavors-provide the same state ordering and energies of the low-lying singlet and triplet electronic excited states within an acceptable error of 0.2-0.3 eV. However, the outcome of the nonadiabatic simulations after excitation to the lowest S1 (1 nπ*) state are dramatically different. While MS-CASPT2, ADC(2), BP86, and PBE do not transfer population to the triplet states within 500 fs-consistent with experimental evidence-SA-CASSCF, B3LYP, and BHHLYP predict intersystem crossing yields between 3% and 21% within the same time. The different excited state dynamics can be rationalized by inspecting potential energy profiles along the C-S bond stretch mode and single-triplet energy gaps. It is found that already at a C-S bond length of 1.9 Å, all the single-reference methods struggle to describe the correct asymptotic behavior of the potentials. Moreover, some methods, including SA-CASSCF, obtain incorrect 1 nπ*-3 ππ* energy gaps, leading to compensation of errors (ADC(2), BP86, PBE), or wrong dynamics (SA-CASSCF, B3LYP, BHHLYP). Only the accurate MRCISD and MS-CASPT2 methods are able to describe the C-S bond correctly and thus able to deliver the correct potential energy surfaces and dynamics for the right reason. A correlation with the amount of Hartree-Fock exchange in the density functional and the ease to access the 3 ππ* state from the 1 nπ* are able to explain the different behavior observed for GGA and hybrid functionals. It is thus illustrated that even in the case of a simple molecule, like CH2S, the sole assessment of vertical excitation energies as reliability predictors for nonadiabatic dynamics is inadequate. The reason is that ISC does not occur at the FC geometry, but rather at distorted geometries where the singlet-triplet gaps become small. Hence, a characterization of the potential energy surfaces beyond the Franck-Condon region is mandatory.

Electron correlation effects in enhanced ionization of diatomic molecules in near-infrared fields

We investigate electron correlation effects in internuclear-distance-dependent enhanced ionization of $\mathrm{H}_2$, $\mathrm{LiH}$, and $\mathrm{HF}$ molecules by intense near-infrared laser pulses using a 3D description of the systems with the time-dependent generalized-active-space configuration-interaction method. This method systematically incorporates electron-electron correlation of the quantum many-electron system under consideration. Our correlated description of diatomic molecules shows that enhanced ionization occurs at certain critical internuclear separations and electron correlation systematically improves the ionization probability in this process until convergence is reached. We demonstrate the failure of the single-active-electron and the configuration-interaction singles approximations to produce the correct internuclear position and probability of the strong-field enhanced-ionization process. We elucidate the role of low-lying electronic excited states in the enhanced ionization process of diatomic molecules. There is clear evidence that an accurate description of low-lying electronically excited states is important to describe the non-perturbative enhanced ionization phenomenon in the ultrashort intense near infrared laser pulses.

Superatomic nature of alkaline earth metal-water complexes: the cases of Be(H2O) and Mg(H2O).

Beryllium- and magnesium-water complexes are shown to accommodate peripheral electrons around their Be2+(H2O)4 and Mg2+(H2O)6 cores in hydrogenic type orbitals. The lowest energy state of these tetra- and hexa-coordinated complexes possess one (cationic species) and two electrons (neutral species) in a pseudo spherical s-orbital, and populate p-, d-, f-, and g-type orbitals in their low-lying excited electronic states. High level quantum chemical calculations are performed to study the electronic structure of these complexes belonging to the category of solvated electrons precursors (SEPs). The observed Aufbau principle is in harmony with the previously introduced series for metal-ammonia complexes. In the current study we are able to expand the previously proposed shell model of SEPs beyond the 2d level. The observed shell model for Mg(H2O)6+ is found to be 1s, 1p, 1d, 2s, 2p, 1f, 2d, 3s, and 1g. The stability of the Be(H2O)0/+m and Mg(H2O)0/+n systems with m = 1-4 and n = 1-6 is also examined and the higher stability of metal-ammonia SEPs over metal-water SEPs is explained in terms of the metal-ligand binding energies, hydrogen bonding, and the activation energy barrier leading to H2 release.

Theoretical study on the low-lying excited electronic states and laser cooling feasibility of CuH molecule.

To evaluate the feasibility of the laser cooling of CuH molecule, we investigate the electronic properties, the vibrational and rotational characteristics of the molecule based on the multi-reference configuration interaction method with all-electron basis sets. The potential energy curves (PECs) of X1Σ+, A1Σ+, B1Σ+, a3Σ+, b3Σ+, e3Σ+, C1П, D1П, c3П and d3П states and the transition dipole moments between these states are calculated. The Schrödinger equation of nuclear movement is solved for each electronic state to obtain the rotational and vibration energy levels. The spectroscopic parameters are calculated based on the fitted analytical function from the PECs. The present results are in good agreement with the theoretical and experimental values available in the literature. The optical scheme of the laser cooling for CuH molecule is constructed with A1Σ+ ↔ X1Σ+ as the close-loop transition. Three lasers are necessary in each direction to maintain enough scattering photons because of the limited Franck-Condon factor of 0.78. The wavelengths of the pumping lasers are determined. The recoil temperature is 1.72 μk, which is the expected temperature to be reached through the method of the cooling below the doppler limit.

Electronic excited states of mixed sandwich complexes, (η7-C7H7)(η5-C5H5)M (M = V, Cr): Investigation with time-dependent density functional theory

The nature and energies of the low-lying electronic excited states of vanadium and chromium cycloheptatrienyl cyclopentadienyl complexes are determined on the basis of time-dependent density functional theory (TD DFT) calculations compared with the experimental gas-phase and condensed-phase absorption spectra. Both intravalency and Rydberg electronic excitations are taken into account. It is demonstrated that TD DFT can predict correctly the absorption band positions and describe well the influence of the metal atom and the ligand on the electronic transition energies. On the basis of comparison with the experimental transition energies the best combinations of the functional and basis set are chosen. For the mixed sandwiches, the “pure” BPW91 and hybrid B3LYP functionals are found to perform much better than the range-separated CAM-B3LYP and WB97XD functionals. The TD DFT calculations suggest new assignments for the Rydberg 4d peak in the gas-phase spectrum of the chromium complex and for the long-wavelength absorption band in the solution spectrum of the vanadium derivative.

Experimentally Calibrated Analysis of the Electronic Structure of CuO+ : Implications for Reactivity.

The CuO+ core is a central motif of reactive intermediates in copper-catalysed oxidations occurring in nature. The high reactivity of CuO+ stems from a weak bonding between the atoms, which cannot be described by a simple classical model. To obtain the correct picture, we have investigated the acetonitrile-ligated CuO+ ion using neon-tagging photodissociation spectroscopy at 5 K. The spectra feature complex vibronic absorption progressions in NIR and visible regions. Employing Franck-Condon analyses, we derived low-lying triplet potential energy surfaces that were further correlated with multireference calculations. This provided insight into the ground and low-lying excited electronic states of the CuO+ unit and elucidated how these states are perturbed by the change in ligation. Thus, we show that the bare CuO+ ion has prevailingly a copper(I)-biradical oxygen character. Increasing the number of ligands coordinated to copper changes the CuO+ character towards the copper(II)-oxyl radical structure.

Calculations of electronic properties and vibrational parameters of alkaline-earth lithides: MgLi+ and CaLi+

ABSTRACTThe electronic ground states of MgLi and CaLi molecular ions are investigated for their spectroscopic constants and properties such as the dipole - and quadrupole moments, and static dipole polarisabilities. The quadrupole moments and the static dipole polarisabilities for these ions have been calculated and reported here, for the first time. The maximum possible error bars, arising due to the finite basis set and the exclusion of higher correlation effects beyond partial triples, are quoted for reliability. Further, the adiabatic effects such as diagonal Born-Oppenheimer corrections are also calculated for these molecules. The vibrational energies, the wavefunctions, and the relevant vibrational parameters are obtained by solving the vibrational Schrödinger equation using the potential energy curve and the permanent dipole moment curve of the molecular electronic ground state. Thereafter, spontaneous and black-body radiation induced transition rates are calculated to obtain the lifetimes of the vibrational states. The lifetime of rovibronic ground state for MgLi, at room temperature, is found to be and for CaLi it is . It has been observed that the lifetime of the highly excited vibrational state is several times larger than (comparable to) that of the vibrational ground state of MgLi (CaLi). In addition, a few low-lying electronic excited states of Σ and Π symmetries have been investigated for their electronic and vibrational properties, using EOM-CCSD method together with the QZ basis sets.