Low-lying Excited Electronic States Research Articles

Theoretical investigation of the electronic structure of the Rhodium Halides molecules RhF and RhCl with dipole moment calculation

The current study involves an ab initio exploration of the ground and low-lying excited electronic states of the rhodium halide molecules RhF and RhCl using the complete active space self-consistent field (CASSCF) with multireference configuration interaction (MRCI+Q) method including single and double excitations and with Davidson corrections. We investigated the potential energy curves, the transition and permanent electric dipole moments, the electronic energy relative to the ground state Te, the harmonic frequency ωe, the internuclear distance Re, and the rotational constant Be corresponding to each of the bounded states. Our findings demonstrate good agreement with the available experimental data. Notably, this work represents the inaugural theoretical investigation of the excited states of RhF and RhCl molecules, identifying the ground state of both to be X3Π, as observed in the sole two experimental investigations.

Ab initio spectroscopic studies of AlF and AlCl molecules

In this work, we report the results from our spectroscopic study on AlF and AlCl molecules. We carry out detailed electronic structure calculations in both the molecules, including obtaining the potential energy surfaces of the [Formula: see text] ground electronic state and some of the low-lying excited electronic states belonging to [Formula: see text] and [Formula: see text] symmetries. This is followed by evaluating the spectroscopic constants and molecular properties such as electric dipole moments and electric quadrupole moments. Throughout, we employ the multi-reference configuration interaction method and work with high-quality quadruple zeta basis sets. Further, transition dipole moments between the ground electronic state and singlet excited states are also studied. The relevant vibrational parameters are computed by solving the vibrational Schrödinger equation. Subsequently, the vibrational energy spacings and transition dipole moments between the vibrational levels belonging to the same electronic states are used to evaluate the spontaneous and black-body radiation-induced transition rates, followed by computing the lifetimes. Finally, the energy differences between rotational levels belonging to different vibrational levels and within an electronic state as well as the Einstein coefficients are reported.

Identification of the interchromophore interaction in the electronic absorption and circular dichroism spectra of bis-phenanthrenes.

We characterize the low-lying excited electronic states of a series of bis-phenanthrenes using our newly developed diabatic scheme called the fragment particle-hole density (FPHD) method and calculate both the electronic absorption and circular dichroism (ECD) spectra using the time-dependent density functional theory (TDDFT) and the FPHD-based exciton model which couples intrachromophore local excitations (LEs) and the interchromophore charge-transfer excitations (CTEs). TDDFT treats each bis-phenanthrene as a single molecule while the mixed LE-CTE exciton model partitions the molecule into two phenanthrene-based aromatic moieties, and then applies the electronic coupling between the various quasi-diabatic states to cover the interactions. It is found that TDDFT and the mixed LE-CTE model reproduce all experimentally observed trends in the spectral profiles, and the hybridization between LE and CTE states is displayed differently in absorption and ECD spectral intensities, as it usually decreases the absorption maxima and affects the positive/negative extrema of the ECD irregularly. By comparing the results yielded by the LE-CTE model with and without the LE-CTE coupling, we identify the contribution of CTE on the main dipole-allowed transitions.

Vibrational and Dipolar Calculations of Mg-(Li, Na, K) Polar Molecules

In this present paper, high-level ab-initio calculations were well performed using multi-configuration self-consistent field, multi-reference configuration interaction and pseudo-potentials methods of diatomic polar molecules compound to magnesium and an alkali atom (Li, Na, K) essentially for the ground and different low-lying excited electronic states of both symmetries 2,4Σ+ and 2,4Π. In this context, adiabatic potential energy curves have been carefully derived as well as the spectroscopic parameters for the different states correlated up to {Mg (3s2)+Li (4 s)}, {Mg (3s2)+Na (4p)} and {Mg (3s2)+K (4d)} asymptotic limits for the studied systems. The obtained results are discussed and compared with the previous theoretical and experimental works which feature in general a good agreement. Afterward, vibrational levels and their spacings as well as the permanent and transition dipole moment curves were accurately calculated. Finally, the turning points (intersection of the potential energy curve with the considered vibrational levels) as well as the ro-vibrational parameter (Bv), were determined for the ground (X)2Σ+ and the first excited state (2)2Σ+. Thanks to their strong dipole-dipole interaction as well as their long coherence time, some bibliographical review have affirmed that these molecular systems can be considered firstly as perfect potential carriers for information in the field of molecular quantum computing and secondly as very promising candidates for certain experiments such as photo-association and laser cooling.

Accurate ab initio calculations of adiabatic energy and dipole moment: Prospects for the formation of cold Alkaline-Earth-Francium molecular ions ALKE-Fr+ (ALKE = Be, Mg, Ca, and Sr)

Alkaline-earth and alkali-metal mixtures have an electronic structure that is perfect for laser cooling. This makes them highly attractive for trapping and laser cooling experiments, allowing the formation of cold molecules. For this object, potential-energy curves and relevant spectroscopic parameters of the low-lying electronic excited states of 1,3Σ+, 1,3Π, and 1,3Δ symmetries of molecular-ion systems composed of alkaline-earth-ion and Francium alkali-metal-atom: ALKE-Fr+ (ALKE = Be, Mg, Ca and Sr), are determined using advanced theoretical technique in quantum chemistry, including a non-empirical pseudopotential, core-valence correlation, large Gaussian basis sets and Full Configuration Interaction (FCI). In order to obtain a more accurate understanding of the electronic structure of these systems, we also determined transition and permanent dipole moments and vibrational properties. Thereafter, the spontaneous and the black-body stimulated transition rates were determined and were employed to calculate lifetimes for all vibrational states of the ground electronic states 11Σ+ of molecular-ions under consideration. For the first and the second excited states, radiative lifetimes were investigated via the Franck–Condon approximation including bound-bound and bound-free transitions. High diagonal structure and large Franck Condon Factor (FCF) values f 00 = 0.987, f 11 = 0.959 and f 22 = 0.919 were obtained for the 11Π (v′ = 0, 1, 2)→ 11Σ+ (v = 0, 1, 2) transition making the BeFr+ system a good candidate for laser cooling. Furthermore, the current results could be used to investigate elastic scattering properties in cold-ion-atom collisions for the first excited states and may help the experimentalists for possible formation, spectroscopy, and photoassociation of cold ion-atom mixtures.

Vibrational Spectroscopy and Structural Analysis of V+(C2H6)n Clusters (n = 1-4).

The vibrational structure and binding motifs of vanadium cation-ethane clusters, V+(C2H6)n, for n = 1-4 are probed using infrared photodissociation spectroscopy in the C-H stretching region (2550-3100 cm-1). Comparison of spectra to scaled harmonic frequency spectra obtained using density functional theory suggests that ethane exhibits two primary binding motifs when interacting with the vanadium cation: an end-on η2 configuration and a side-on configuration. Determining the denticity of the side on isomer is complicated by the rotational motion of ethane, implying that structural analysis based solely on Born-Oppenheimer potential energy surface minimizations is insufficient and that a more sophisticated vibrationally adiabatic approach is necessary to interpret spectra. The lower-energy side-on configuration predominates in smaller clusters, but the end-on configuration becomes important for larger clusters as it helps to maintain a roughly square-planar geometry about the central vanadium. Proximate C-H bonds exhibit elongation and large red-shifts when compared to bare ethane, particularly in the case of the side-on isomer, demonstrating initial effects of C-H bond activation, which are underestimated by scaled harmonic frequency calculations. Tagging several of the clusters with argon and nitrogen results in nontrivial effects. The high binding energy of N2 can lead to the displacement of ethane from a side-on configuration into an end-on configuration. The presence of either one or two Ar or N2 can impact the overall symmetry of the cluster, which can alter the potential energy surface for ethane rotation in the side-on isomer and may affect the accessibility of low-lying electronic excited states of V+.

Iron(II) Complexes with Porphyrin and Tetrabenzoporphyrin: CASSCF/MCQDPT2 Study of the Electronic Structures and UV-Vis Spectra by sTD-DFT.

The geometry and electronic structures of iron(II) complexes with porphyrin (FeP) and tetrabenzoporphyrin (FeTBP) in ground and low-lying excited electronic states are determined by DFT (PBE0/def2-TZVP) calculations and the complete active space self-consistent field (CASSCF) method, followed by the multiconfigurational quasi-degenerate second-order perturbation theory (MCQDPT2) approach to determine the dynamic electron correlation. The minima on the potential energy surfaces (PESs) of the ground (3A2g) and low-lying, high-spin (5A1g) electronic states correspond to the planar structures of FeP and FeTBP with D4h symmetry. According to the results of the MCQDPT2 calculations, the wave functions of the 3A2g and 5A1g electronic states are single determinant. The electronic absorption (UV-Vis) spectra of FeP and FeTBP are simulated within the framework of the simplified time-dependent density functional theory (sTDDFT) approach with the use of the long-range corrected CAM-B3LYP function. The most intensive bands of the UV-Vis spectra of FeP and FeTBP occur in the Soret near-UV region of 370-390 nm.

Unraveling the T1 formation in mono-arm styrylbenzene heteroanalogues

We present a theoretical investigation of the nonradiative relaxation dynamics associated with the low-lying excited electronic states of mono-arm styrylbenzene heteroanalogues. These molecules possess near-degenerate S 1 (bright π π ∗ ) and S 2 (dark n π ∗ ) states, enabling intersystem crossing via both states upon photoexcitation to S 1 . Singlet-triplet energy gaps and associated spin–orbit coupling parameters suggest favorable intersystem crossing via S 2 → T 5 . The nuclear wavepacket initiated at the Franck–Condon point of the receiver triplet state (T 5 ) decays rapidly to lower triplet states via accessible multiple conical intersections, indicating the ultrafast formation of T 1 in these molecules. Our findings show that these molecules would transfer energy to molecular oxygen via T 1 . • T 1 formation pathways of mono-arm styrylbenzene heteroanalogues are investigated. • TD-DFT calculations and SOC values reveal S 2 -T 5 as the triplet generating channel. • Conical intersections in the triplet manifold lead to ultrafast internal conversion.

Spectroscopy and photochemistry of ClSSO.

Sulfur-chlorine cycles play a role in the atmosphere of Venus. It is thought that many sulfur-chlorine bearing molecules could be present in Venus's atmosphere and play an important role in its chemical processes. The goal of this work is to provide new insight into the electronic structure and spectroscopy of the [Cl, S, S, O] molecular system. Eight isomers could be formed, but only three were found to be thermodynamically stable relative to the first dissociation limit. We spectroscopically characterized the two lowest energy stable isomers, C1-ClSSO and trans-ClSSO, using the accurate CCSD(T)-F12/aug-cc-pVTZ method. The dipole moments of the two lowest energy stable isomers are predicted to be 1.90 and 1.60 debye, respectively. The C1-ClSSO isomer is suitable for laser induced fluorescence detection since the lowest excited electronic states absorb in the visible, ∼610 nm, and near UV region, 330 nm. We mapped the evolution of the low-lying excited electronic states along the ClS, SS, and SO bond lengths to find that the production of ClS, SO, or S2O is plausible, whereas the production of ClS2 is not allowed.

Theoretical and Experimental Study of the Spectroscopy and Thermochemistry of UC+/0/.

A combination of high-level ab initio calculations and anion photoelectron detachment (PD) measurements is reported for the UC, UC-, and UC+ molecules. To better compare the theoretical values with the experimental photoelectron spectrum (PES), a value of 1.493 eV for the adiabatic electron affinity (AEA) of UC was calculated at the Feller-Peterson-Dixon (FPD) level. The lowest vertical detachment energy (VDE) is predicted to be 1.500 eV compared to the experimental value of 1.487 ± 0.035 eV. A shoulder to lower energy in the experimental PD spectrum with the 355 nm laser can be assigned to a combination of low-lying excited states of UC- and excited vibrational states. The VDEs calculated for the low-lying excited electronic states of UC at the SO-CASPT2 level are consistent with the observed additional electron binding energies at 1.990, 2.112, 2.316, and 3.760 eV. Potential energy curves for the Ω states and the associated spectroscopic properties are also reported. Compared to UN and UN+, the bond dissociation energy (BDE) of UC (411.3 kJ/mol) is predicted to be considerably lower. The natural bond orbitals (NBO) calculations show that the UC0/+/- molecules have a bond order of 2.5 with their ground-state configuration arising from changes in the oxidation state of the U atom in terms of the 7s orbital occupation: UC (5f27s1), UC- (5f27s2), and UC+ (5f27s0). The behavior of the UN and UC sequence of molecules and anions differs from the corresponding sequences for UO and UF.

Rationalizing the Fluorescence Behavior of Core-Substituted Naphthalene Diimides

We study the internal conversion (IC) and intersystem crossing (ISC) pathways of low-lying excited electronic states of three core-substituted naphthalene diimides (bNDI, yNDI, and gNDI) using wavepacket simulations within the linear vibronic coupling method. Our wavepacket simulations reproduce the experimental electronic absorption spectra very well. All molecules decay rapidly to S2 upon populating a higher dipole-allowed singlet excited-state. The S2 → S1 IC dynamics and singlet-triplet energy gap, spin-orbit coupling strength trends suggest a favorable S2 → T4 ISC in gNDI. The efficient ultrafast T4 formation and its decay to lower triplet states make gNDI nonfluorescent. Such triplet formation pathways are not operative in both bNDI and yNDI; hence, these molecules emit fluorescence from S1 after a slower S2 → S1 IC.

Accurate ab initio calculations of RaF electronic structure appeal to more laser-spectroscopical measurements.

Recently, a breakthrough has been achieved in laser-spectroscopic studies of short-lived radioactive compounds with the first measurements of the radium monofluoride molecule (RaF) UV/vis spectra. We report results from high-accuracy ab initio calculations of the RaF electronic structure for ground and low-lying excited electronic states. Two different methods agree excellently with experimental excitation energies from the electronic ground state to the 2Π1/2 and 2Π3/2 states, but lead consistently and unambiguously to deviations from experimental-based adiabatic transition energy estimates for the 2Σ1/2 excited electronic state, and show that more measurements are needed to clarify spectroscopic assignment of the 2Δ state.

Theoretical Study of Light-Induced Crosslinking Reaction Between Pyrimidine DNA Bases and Aromatic Amino Acids

Low-lying electronic excited states and their relaxation pathways as well as energetics of the crosslinking reaction between uracil as a model system for pyrimidine-type building blocks of DNA and RNA and benzene as a model system for aromatic groups of tyrosine (Tyr) and phenylalanine (Phe) amino acids have been studied in the framework of density functional theory. The equilibrium geometries of the ground and electronic excited states as well as the crossing points between the potential energy surfaces of the uracil–benzene complex were computed. Based on these results, different relaxation pathways of the electronic excited states that lead to either back to the initial geometry configuration or the dimerization between the six-membered rings of the uracil–benzene complex have been identified, and the energetic conditions for their occurrence are discussed. It can be concluded that the DNA–protein crosslinking reaction can be induced by the external electromagnetic field via the dimerization reaction between the six-membered rings of the uracil–benzene pair at the electronic excited-state level of the complex. In the case of the uracil–phenol complex, the configuration of the cyclic adduct (dimerized) conformation is less likely to be formed.

Temperature-dependent direct photodissociation cross sections and rates of AlCl

ABSTRACT The photodissociation process of aluminium monochloride (AlCl) plays an important role in modelling the chemistry of the circumstellar envelope. In this work, direct photodissociation cross sections of AlCl have been computed for transitions from the ground X1Σ+ state to six low-lying excited electronic states by using ab initio potential energy curves and transition dipole moments, which are obtained by the internally contracted multireference configuration-interaction method with Davidson correction and the aug-cc-pV6Z basis set. State-resolved cross sections for transitions from 38 958 rovibrational levels (υ″ ≤ 100, J″ ≤ 400) of the ground X1Σ+ state have been obtained for photon wavelengths from 500 Å to the dissociation threshold. Photodissociation cross sections in local thermal equilibrium are evaluated for gas temperatures from 500 to 10 000 K. Using the computed cross sections, temperature-dependent photodissociation rates of AlCl in the interstellar and blackbody radiation fields are determined. The results can be applied to the investigation of the chemical evolution of Al in the envelope of carbon-rich and oxygen-rich stars.

Cover Feature: Near‐Infrared Spectrum of the First Excited State of Au2+ (Chem. Eur. J. 61/2021)

Gold nanoclusters have surprising and interesting optical and catalytic properties. Understanding of the electronic structure of the prototypical Au2+ dimer gives us the tools we need to understand more complex systems that are dominated by the strong relativistic effects and spin–orbit coupling of gold. The first optical spectrum of the low-lying excited electronic state of cold Au2+ obtained by Ar tagging is exceptionally well resolved, and its surprisingly simple vibronic structure provides detailed information about structure and bonding in the ground and first excited states. More information can be found in the Communication by M. Förstel, O. Dopfer et al. (DOI: 10.1002/chem.202102542).

Carbon Clusters: Thermochemistry and Electronic Structure at High Temperatures.

This paper studies the thermochemistry and electronic structure of small carbon clusters and hydrocarbons, which are major constituents of pyrolysis gases released into the boundary layer of ablating heat shields. Our focus lies on clusters of up to four carbon atoms. Among other molecules, thermochemistry data for molecules such as C3H and C4H have been determined using the Weizmann-1 (W1) method. These molecules have very limited thermochemistry data recorded in the literature, thereby necessitating new and accurate computations of required properties such as electronic energies of low-lying states, heats of formation, harmonic frequencies, and rotational constants. A study of electronically excited states of these molecules computed using the equations of motion coupled cluster singles doubles method revealed C4 and C4H to be potential sources of radiation absorption in the boundary layer. The excited electronic states of interest are studied further to obtain their optimum geometries, rotational constants, and vibrational frequencies. Moreover, we also study the effect of low-lying excited electronic states on the partition function to assess their effect on the thermodynamics of these pyrolysis gases in the high-temperature regime. Neglecting the excited electronic states records a maximum difference of 12% in the computed specific heat capacity values, Cp values. Finally, comparisons of the equilibrium mole fractions obtained using the thermodynamics computed in this paper with the existing state-of-the-art tables used for hypersonic applications (e.g., JANAF and Gurvich Tables) show an order of magnitude difference in the mixture compositions. It is shown that the rhombic isomer of C4 (1Ag), which is energetically close to the ground state (3Σg-) and usually neglected in composition calculations, contributes to a 28% increase in the equilibrium mole fraction of the C4 molecule.

Tracking the early nonadiabatic events of ESIPT process in 2-acetylindan-1,3-dione by quantum wavepacket dynamics

Spectroscopy and dynamics associated with the three low-lying singlet excited electronic states of 2-acetylindan-1,3-dione (AID) are studied theoretically to interpret the early events of the excited-state intramolecular proton transfer process (ESIPT). Upon populating “bright” S3, AID decays to “dark” lower electronic states (S2 and S1) on a timescale of about 100 fs. Nuclear densities associated with the OH vibrations obtained from wavepacket calculations reveal their participation in the S3→ S2/S1 internal conversion. We show that AID requires very minimal distortions along these vibrations to access the conical intersections located near to the Franck–Condon (FC) point. Our findings demonstrate that the ultrafast nonadiabatic dynamics taking place within the FC zone make the assignment of ESIPT pathways and associated timescales in AID a challenging task.

Molecular Structure, Thermodynamic and Spectral Characteristics of Metal-Free and Nickel Complex of Tetrakis(1,2,5-thiadiazolo)porphyrazine.

The Knudsen effusion method with mass spectrometric control of the vapor composition was used to study the possibility of a congruent transition to the gas phase and to estimate the enthalpy of sublimation of metal-free tetrakis(1,2,5-thiadiazolo)porphyrazine and its nickel complex (H2TTDPz and NiTTDPz, respectively). The geometrical and electronic structure of H2TTDPz and NiTTDPz in ground and low-lying excited electronic states were determined by DFT calculations. The electronic structure of NiTTDPz was studied by the complete active space (CASSCF) method, following accounting dynamic correlation by multiconfigurational quasi-degenerate second-order perturbation theory (MCQDPT2). A geometrical structure of D2h and D4h symmetry was obtained for H2TTDPz and NiTTDPz, respectively. According to data obtained by the MCQDPT2 method, the nickel complex possesses the ground state 1A1g, and the wave function of the ground state has the form of a single determinant. Electronic absorption and vibrational (IR and resonance Raman) spectra of H2TTDPz and NiTTDPz were studied experimentally and simulated theoretically.

Energy transfer along Müller cell intermediate filaments isolated from porcine retina: II. Excitons at 2500 cm−1 produced by ADH1A upon hydrolysis of one ATP molecule

Low-energy (IR ~2500 cm−1) exciton transfer was explored in Müller cell (MC) intermediate filaments (IFs) isolated from porcine retina and filling a capillary matrix. IR excitons were generated either by absorption of IR radiation at 4 μm, or by ATP hydrolysis energy transfer to the MC IFs. ATP hydrolysis was catalyzed by human alcohol dehydrogenase (ADH1A) in ADH1A…NAD+…IF complexes. Each exciton was generated by hydrolysis of one ATP molecule. Exciton energy was independent on ATP concentration. The emission spectra of IR excitons arriving to the other side of the capillary matrix were dependent on the exciton generation method. Time-resolved experiments with direct exciton generation by pulsed IR radiation allowed to measure exciton travel velocity, along with the exciton emission time. The results obtained support new ideas on ATP energy transfer along protein filaments in vivo. These unique experimental results provide for detailed understanding of the electronic state structure of the MC IFs. This study reports that MC IFs isolated from porcine retina have a long-living low-lying electronic excited state at 2500 cm−1 above the ground state.

Searching for Monomeric Nickel Tetrafluoride: Unravelling Infrared Matrix Isolation Spectra of Higher Nickel Fluorides.

Binary transition metal fluorides are textbook examples combining complex electronic features with most fundamental molecular structures. High‐valent nickel fluorides are among the strongest known fluorinating and oxidizing agents, but there is a lack of experimental structural and spectroscopic investigations on molecular NiF3 or NiF4. Apart from their demanding synthesis, also their quantum‐chemical description is difficult due to their open shell nature and low‐lying excited electronic states. Distorted tetrahedral NiF4 (D 2d) and trigonal planar NiF3 (D 3h) molecules were produced by thermal evaporation and laser ablation of nickel atoms in a fluorine/noble gas mixture and spectroscopically identified by a joint matrix‐isolation and quantum‐chemical study. Their vibrational band positions provide detailed insights into their molecular structures.