CC2 Methods Research Articles

First-Principles Simulations of One- and Two-Photon Absorption Band Shapes of the Bis(BF2) Core Complex

Motivated by the outstanding properties of bis(BF2) core complexes as fluorophore probes, we present a systematic computational study of their vibrationally resolved one- and two-photon absorption spectra in vacuum and in solution. Electronic and vibrational structure calculations were performed using the coupled cluster CC2 method and the Kohn-Sham formulation of density functional theory (DFT). A nonempirical estimation of the inhomogeneous broadening, accomplished using the polarizable embedding (PE) approaches combined with time-dependent DFT and CC2 methods, is used as a key ingredient of the computational protocol employed for simulations of the spectral features in solution. The inhomogeneous broadening is also determined based on the Marcus theory employing linear response and state-specific polarizable continuum model (PCM) methods. It is found that the polarizable embedding CC2 and the state-specific PCM are the most successful approaches for description of environmental broadening. For the 1(1)A(g) → 1(1)B(u) transition, the non-Condon effects can be safely neglected and a fair agreement between the simulated and experimental band shapes is found. In contrast, the shape of the vibrationally resolved band corresponding to the two-photon allowed 1(1)A(g) → 2(1)A(g) transition is largely dominated by non-Condon effects. A generalized few-level model was also employed to analyze the mechanism of the electronic two-photon 1(1)A(g) → 2(1)A(g) excitation. It was found that the most important optical channel involves the 1(1)B(u) excited state. Ramifications of the findings for general band shape modeling are briefly discussed.

Local NH-π interactions involving aromatic residues of proteins: influence of backbone conformation and ππ* excitation on the π H-bond strength, as revealed from studies of isolated model peptides.

Conformer-selective IR gas phase spectroscopy and high level quantum chemistry methods have been used to characterise the diversity of local NH-π interactions between the π ring of a phenylalanine aromatic residue and the nearby main chain amide groups. The study of model systems shows how the amide NH stretch vibrational features, in the 3410-3460 cm-1 frequency range, can be used to monitor the strength of these local π H-bonds, which is found to depend on both the backbone conformation and the aromatic side chain orientation. This is rationalized in terms of partial electron transfer between the π cloud and the main chain NH bonds, with the help of analysis tools based on Natural Bonding Orbitals and Non-Covalent Interactions plots. The experimental study, extended to the NH-π interactions when the Phe residue is excited in its first ππ* electronic state, also demonstrates the principle of the ππ* labelling technique, i.e. a selective labelling of those NH bonds in a peptide molecule that are in close contact with an aromatic ring, as an elegant tool for IR spectroscopic assignments. The validation of theoretical predictions against experimental data (frequency change upon excitation) eventually qualifies the use of the CC2 method for the description of the ππ* excited states of systems having a phenyl ring, both in terms of structure, vibrational modes and nature of excited states.

Assessment of Approximate Coupled-Cluster and Algebraic-Diagrammatic-Construction Methods for Ground- and Excited-State Reaction Paths and the Conical-Intersection Seam of a Retinal-Chromophore Model.

As a minimal model of the chromophore of rhodopsin proteins, the penta-2,4-dieniminium cation (PSB3) poses a challenging test system for the assessment of electronic-structure methods for the exploration of ground- and excited-state potential-energy surfaces, the topography of conical intersections, and the dimensionality (topology) of the branching space. Herein, we report on the performance of the approximate linear-response coupled-cluster method of second order (CC2) and the algebraic-diagrammatic-construction scheme of the polarization propagator of second and third orders (ADC(2) and ADC(3)). For the ADC(2) method, we considered both the strict and extended variants (ADC(2)-s and ADC(2)-x). For both CC2 and ADC methods, we also tested the spin-component-scaled (SCS) and spin-opposite-scaled (SOS) variants. We have explored several ground- and excited-state reaction paths, a circular path centered around the S1/S0 surface crossing, and a 2D scan of the potential-energy surfaces along the branching space. We find that the CC2 and ADC methods yield a different dimensionality of the intersection space. While the ADC methods yield a linear intersection topology, we find a conical intersection topology for the CC2 method. We present computational evidence showing that the linear-response CC2 method yields a surface crossing between the reference state and the first response state featuring characteristics that are expected for a true conical intersection. Finally, we test the performance of these methods for the approximate geometry optimization of the S1/S0 minimum-energy conical intersection and compare the geometries with available data from multireference methods. The present study provides new insight into the performance of linear-response CC2 and polarization-propagator ADC methods for molecular electronic spectroscopy and applications in computational photochemistry.

Improving the Accuracy of the Charge Transfer Integrals Obtained by Coupled Cluster Theory, MBPT(2), and TDDFT.

Theoretical modeling of the charge transport in organic materials in the diabatic representation requires an accurate evaluation of the charge transfer integrals. In this paper, we show that the coupled cluster and MBPT(2) approaches are the methods of choice for performing the benchmark calculations of this quantity, in contrast to some recently published results. We demonstrate that a proper treatment of the involved ionized states, achieved by applying the continuum-orbital strategy, reduces the error of the transfer integrals by one order of magnitude, which in the case of the CC2 method corresponds to a lowering of the mean relative unsigned error (MRUE) from 39.9 to 3.8%. Moreover, we extend the application of the continuum-orbital strategy to the TDDFT method, and show that it leads to a dramatic improvement of the description of ionized states compared to the conventional TDDFT approach, characterized by lowering of MRUE from 209.0 to 24.5%.

Spectroscopic signatures and structural motifs in isolated and hydrated serotonin: a computational study

The conformational landscapes of neutral serotonin characterized by MP2, CC2 and DFT methods. The Gph-out/anti conformation is found most stable.

Erratum to "Geometries and Vertical Excitation Energies in Retinal Analogues Resolved at the CASPT2 Level of Theory: Critical Assessment of the Performance of CASSCF, CC2, and DFT Methods".

A systematic investigation of structural properties and vertical excitation energies of a series of structurally modified 11-cis-retinal chromophores in vacuo was performed by means of multiconfigurational second-order perturbation theory (CASPT2). CASPT2-based geometries agree reasonably well with Moller–Plesset second-order perturbation theory (MP2), local second-order approximate coupled cluster singles and doubles (LCC2), and density functional theory (DFT) geometries, while the complete active space self-consistent field (CASSCF) method exaggerates dramatically the bond length pattern in the polyene chain. The quality of the resulting vertical excitation energies obtained by employing CASSCF, second-order approximate coupled cluster singles and doubles (CC2), LCC2, and time-dependent density functional theory (TD-DFT) approaches is assessed with respect to the CASPT2 data. We show that the commonly used CASSCF/CASPT2 approach works reasonably well in the case of vertical excitation energies of planar...

Accuracy of density functionals in the description of dispersion interactions and IR spectra of phosphates and phosphorylated compounds.

The performances of quantum chemistry methods (i.e., DFT and ab initio) in calculating the structural and vibrational properties of phosphates and phosphorylated compounds have been evaluated. Diethyl-phosphate, phosphonic acid, dihydrogen phosphate anion, phosphoric acid dimer and protonated glycylphosphotyrosine dipeptide were selected for our study. Geometry and harmonic frequency deviations were investigated, pointing out the contribution of dispersion interactions on diethyl-phosphate, [Gly-pTyr+H](+) and the phosphoric acid dimer. The B3LYP-D functional, followed by CC2 and MP2 methods, revealed significant accuracy for frequency calculations of the majority of the phosphorylated compounds in comparison with available experimental data. These investigations provide a guide to the accurate computation of phosphorylated biological compounds.

How Method-Dependent Are Calculated Differences between Vertical, Adiabatic, and 0–0 Excitation Energies?

Through a large number of benchmark studies, the performance of different quantum chemical methods in calculating vertical excitation energies is today quite well established. Furthermore, these efforts have in recent years been complemented by a few benchmarks focusing instead on adiabatic excitation energies. However, it is much less well established how calculated differences between vertical, adiabatic and 0-0 excitation energies vary between methods, which may be due to the cost of evaluating zero-point vibrational energy corrections for excited states. To fill this gap, we have calculated vertical, adiabatic, and 0-0 excitation energies for a benchmark set of molecules covering both organic and inorganic systems. Considering in total 96 excited states and using both TD-DFT with a variety of exchange-correlation functionals and the ab initio CIS and CC2 methods, it is found that while the vertical excitation energies obtained with the various methods show an average (over the 96 states) standard deviation of 0.39 eV, the corresponding standard deviations for the differences between vertical, adiabatic, and 0-0 excitation energies are much smaller: 0.10 (difference between adiabatic and vertical) and 0.02 eV (difference between 0-0 and adiabatic). These results provide a quantitative measure showing that the calculation of such quantities in photochemical modeling is well amenable to low-level methods. In addition, we also report on how these energy differences vary between chemical systems and assess the performance of TD-DFT, CIS, and CC2 in reproducing experimental 0-0 excitation energies.

A theoretical study on elementary building blocks for organic solar cells – Influence of a donor molecule on electronic spectrum of PCBM

Modifications in the electronic spectrum of [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) caused by noncovalent interactions with smaller molecules, like benzene, thiophene, pyridine, methanimine, and porphyrin were studied by means of time-dependent density functional theory (TD-DFT) with the CAM-B3LYP functional. The calculated excited states for the complex with the first four molecules have mostly local character within the PCBM unit, only slightly perturbed by the second molecule, e.g. red shift of the spectrum with respect to this of a free PCBM is observed. The states with a predominantly intermolecular charge-transfer character are absent within the first 20 states of complexes with PCBM, with the exception of the complexes with porphyrin, where charge-transfer states and states delocalized on both molecules were observed. In addition to this, the lowest excited states of the recently synthetized 25Th-cardo molecule, its dimer and trimer, as well as its complex with PCBM were calculated and analyzed. This complex has been recently studied experimentally by Iwan et al. (2013) as a potential candidate for a donor–acceptor system in the solar cells. It has been found that in this case some higher excited states possess a partial charge-transfer character from 25Th-cardo to PCBM. The reliability of CAM-B3LYP for complexes of two molecules with aromatic rings or conjugated double bonds has been examined on a smaller example of pyrene interacting with benzene by comparing of the TD-DFT results with those of CIS, local CC2, and local ADC(2) methods.

Ab Initio Studies on the Photophysics of Uric Acid and Its Monohydrates: Role of the Water Molecule

The photophysical behavior of three lowest-energy tautomers of uric acid and seven most stable isomers of uric acid monohydrate is comprehensively studied by ab initio calculations. Ground-state energies are calculated with the CCSD(T) method, while excitation and ionization energies as well as excited-state potential energy profiles of photoinduced processes are calculated with the CC2 method. For the (1)ππ* state, it is found that the excitation energy of the monohydrate cluster is significantly lower than that of isolated uric acid when the water molecule is hydrogen-bonded at a specific carbonyl group. The calculated excited-state potential energy profiles suggest that some monohydrate isomers can undergo a migration of the water molecule from one site to another site in the (1)ππ* state with a small energy barrier. It is also found for both uric acid and its monohydrate that nonradiative decay via the NH bond dissociation in the (1)πσ* state is likely to occur at higher excitation energies. On the basis of the computational results, possible mechanisms for the absence of specific isomers of uric acid monohydrate from the resonant two-photon ionization spectrum are discussed.

Low-lying excited states and nonradiative processes of 9-methyl-2-aminopurine.

The UV spectrum of the adenine analogue 9-methyl-2-aminopurine (9M-2AP) is investigated with one- and two-color resonant two-photon ionization spectroscopy at 0.3 and 0.05 cm(-1) resolution in a supersonic jet. The electronic origin at 32,252 cm(-1) exhibits methyl torsional subbands that originate from the 0A1'' (l = 0) and 1E(″) (l = ±1) torsional levels. These and further torsional bands that appear up to 00 (0)+230 cm(-1) allow to fit the threefold (V3) barriers of the torsional potentials as |V3''|=50 cm(-1) in the S0 and |V3'|=126 cm(-1) in the S1 state. Using the B3LYP density functional and correlated approximate second-order coupled cluster CC2 methods, the methyl orientation is calculated to be symmetric relative to the 2AP plane in both states, with barriers of V3''=20 cm(-1) and V3'=115 cm(-1). The 00 (0) rotational band contour is 75% in-plane (a/b) polarized, characteristic for a dominantly long-axis (1)ππ(*) excitation. The residual 25% c-axis polarization may indicate coupling of the (1)ππ(*) to the close-lying (1)nπ(*) state, calculated at 4.00 and 4.01 eV with the CC2 method. However, the CC2 calculated (1)nπ oscillator strength is only 6% of that of the (1)ππ(*) transition. The (1)ππ(*) vibronic spectrum is very complex, showing about 40 bands within the lowest 500 cm(-1). The methyl torsion and the low-frequency out-of-plane ν1' and ν2' vibrations are strongly coupled in the (1)ππ(*) state. This gives rise to many torsion-vibration combination bands built on out-of-plane fundamentals, which are without precedence in the (1)ππ(*) spectrum of 9H-2-aminopurine [S. Lobsiger, R. K. Sinha, M. Trachsel, and S. Leutwyler, J. Chem. Phys. 134, 114307 (2011)]. From the Lorentzian broadening needed to fit the 00 (0) contour of 9M-2AP, the (1)ππ(*) lifetime is τ ⩾ 120 ps, reflecting a rapid nonradiative transition.

Photophysics of a Schiff base: theoretical exploration of the excited-state deactivation mechanisms of N-salicilydenemethylfurylamine (SMFA)

Excited state reaction coordinates and the consequent energy profiles of a new Schiff base, N-salicilydenemethylfurylamine (SMFA), have been investigated with the CC2 method, which is a simplified version of singles-and-doubles coupled cluster theory. The potential energy profiles of the ground and the lowest excited singlet state are calculated. In contrast to the ground state, the excited state potential energy profile shows a barrier-less dissociation pattern along the O-H stretching coordinate which verifies the proton transfer reaction at the S1 (ππ*) state. The calculations indicate two S1/S0 conical intersections (CIs) which provide non-adiabatic gates for radiation-less decay to the ground state. At the CIs, two barrier-free reaction coordinates direct the excited system to the ground state of enol-type minimum. According to calculation results, a trans-keto type structure obtained from photoexcitation of the enol, can be responsible for the photochromoic effect of SMFA. Furthermore, our results confirm the suggestion that aromatic Schiff bases are potential candidates for optically driven molecular switches.

Ground- and Excited-State Geometry Optimization of Small Organic Molecules with Quantum Monte Carlo.

We present a comparative study of the geometry optimization in the gas phase of acrolein, acetone, methylenecyclopropene, and the propenoic acid anion with special emphasis on their excited-state structures, using quantum Monte Carlo (QMC), multireference perturbation theory (CASPT2 and NEVPT2), second-order approximate coupled cluster (CC2), and time-dependent density functional theory (TDDFT). We find that, for all molecules, the geometries optimized with QMC in its simplest variational (VMC) flavor are in very good agreement with the perturbation results both in the ground and the excited states of either n → π* or π → π* character. Furthermore, the quality of the QMC structures is superior to those obtained with the CC2 method, which overestimates the CO bond in all n → π* excitations, or to the symmetry-adapted-cluster configuration interaction (SAC-CI) approach, which gives a poorer description of the CC bonds in the excited states. Finally, the spread in the TDDFT structures obtained with several current exchange-correlation functionals is large and does not reveal a clear relation between the defining features of the functionals and the quality of the optimized structures. In summary, our findings demonstrate the good performance of QMC in optimizing the geometries of these molecules, also in cases where other correlated or TDDFT approaches are inaccurate, and indicate that the method represents a robust reference approach for future structural studies also of larger systems.

Geometries and Vertical Excitation Energies in Retinal Analogues Resolved at the CASPT2 Level of Theory: Critical Assessment of the Performance of CASSCF, CC2, and DFT Methods.

A systematic investigation of structural properties and vertical excitation energies of a series of structurally modified 11-cis-retinal chromophores in vacuo was performed by means of multiconfigurational second-order perturbation theory (CASPT2). CASPT2-based geometries agree reasonably well with Møller-Plesset second-order perturbation theory (MP2), local second-order approximate coupled cluster singles and doubles (LCC2), and density functional theory (DFT) geometries, while the complete active space self-consistent field (CASSCF) method exaggerates dramatically the bond length pattern in the polyene chain. The quality of the resulting vertical excitation energies obtained by employing CASSCF, second-order approximate coupled cluster singles and doubles (CC2), LCC2, and time-dependent density functional theory (TD-DFT) approaches is assessed with respect to the CASPT2 data. We show that the commonly used CASSCF/CASPT2 approach works reasonably well in the case of vertical excitation energies of planar structures, but lack of dynamic correlation leads to large errors in energetics for strongly strained structures. For example, the highly twisted conformers of 9,10-dimethyl and 9,10,13-trimethyl species are found as global minima at the CASSCF level, whereas they turn almost planar at the CASPT2, MP2, LCC2, and DFT levels of theory. The CC2 method has shown a remarkable performance, manifested by a maximum deviation of 0.05 eV from the reference CASPT2 results, whereas the local version of CC2 seems to fail to describe the charge-transfer character of the S0 → S1 transitions correctly. We believe that our CASPT2 benchmark set will provide a reference that can be utilized for validation and development studies on 11-cis-retinal protonated Schiff base chromophore analogues.

Red-light absorption and fluorescence of phytochrome chromophores: A comparative theoretical study

Currently, much experimental effort is being invested in the engineering of phytochromes, a large superfamily of photoreceptor proteins, into fluorescent proteins suitable for bioimaging in the near-infrared regime. In this work, we gain insight into the potential of computational methods to contribute to this development by investigating how well representative quantum chemical methods reproduce recently recorded red-light absorption and emission maxima of synthetic derivatives of the bilin chromophores of phytochromes. Focusing on the performance of time-dependent density functional theory but using also the ab initio CIS(D), CC2 and CASPT2 methods, we explore how various methodological considerations influence computed spectra and find, somewhat surprisingly, that density functionals lacking exact exchange reproduce the experimental measurements with smaller errors than functionals that include exact exchange. Thus, for the important class of chromophores that bilins constitute, the widely established trend that hybrid functionals give more accurate excitation energies than pure functionals does not apply.

Electro-optical properties from CC2 Calculations: A comparison between theoretical and experimental results

The coupled cluster CC2 method has been employed for the computation of dipole moments and static first order hyperpolarizability of a large size push–pull molecule with an extended π electron system. The results are critically compared with the outcomes of electro-optical absorption measurements. CC2 ground and excited state dipole moments are slightly overestimated with respect to the experimental ones, but their differences are well reproduced; good agreement has also been found for absorption wavelengths. Sum of state computations of the static first hyperpolarizability highlight the importance of at least two excited states.

Excited-State Structure and Dynamics of Keto–Amino Cytosine: The 1ππ* State Is Nonplanar and Its Radiationless Decay Is Not Ultrafast

We have measured the mass- and tautomer-specific S0 → S1 vibronic spectra and S1 state lifetimes of the keto–amino tautomer of cytosine cooled in supersonic jets, using two-color resonant two-photon ionization (R2PI) spectroscopy at 0.05 cm(–1) resolution. The rotational contours of the 0(0)(0) band and nine vibronic bands up to +437 cm(–1) are polarized in the pyrimidinone plane, proving that the electronic excitation is to a 1ππ* state. All vibronic excitations up to +437 cm(–1) are overtone and combination bands of the low-frequency out-of-plane ν1′ (butterfly), ν2′ (boat), and ν3′ (H–N–C6–H twist) vibrations. UV vibronic spectrum simulations based on approximate second-order coupled-cluster (CC2) calculations of the ground and 1ππ* states are in good agreement with the experimental R2PI spectrum, but only if the calculated ν1′ and ν2′ frequencies are reduced by a factor of 4 and anharmonicity is included. Together with the high intensity of the ν1′ and ν2′ overtone vibronic excitations, this implies that the 1ππ* potential energy surface is much softer and much more anharmonic in the out-of-plane directions than predicted by the CC2 method. The 1ππ* state lifetime is determined from the Lorentzian line broadening necessary to reproduce the rotational band contours: at the 0(0)(0) band it is τ = 44 ps, remains at τ = 35–45 ps up to +205 cm(–1), and decreases to 25–30 ps up to +437 cm(–1). These lifetimes are 20–40 times longer than the 0.5–1.5 ps lifetimes previously measured with femtosecond pump–probe techniques at higher vibrational energies (1500–3800 cm(–1)). Thus, the nonradiative relaxation rate of keto–amino cytosine close to the 1ππ* state minimum is k(nr) 2.5 × 10(10) s(–1), much smaller than at higher energies. An additional nonradiative decay channel opens at +500 cm(–1) excess energy. Since high overtone bands of ν1′ and ν2′ are observed in the R2PI spectrum but only a single weak 2ν3′ band, we propose that ν3′ is a promoting mode for nonradiative decay, consistent with the observation that the ν3′ normal-mode eigenvector points toward the “C6-puckered” conical intersection geometry.

Carbon X-ray absorption spectra of fluoroethenes and acetone: A study at the coupled cluster, density functional, and static-exchange levels of theory

Near carbon K-edge X-ray absorption fine structure spectra of a series of fluorine-substituted ethenes and acetone have been studied using coupled cluster and density functional theory (DFT) polarization propagator methods, as well as the static-exchange (STEX) approach. With the complex polarization propagator (CPP) implemented in coupled cluster theory, relaxation effects following the excitation of core electrons are accounted for in terms of electron correlation, enabling a systematic convergence of these effects with respect to electron excitations in the cluster operator. Coupled cluster results have been used as benchmarks for the assessment of propagator methods in DFT as well as the state-specific static-exchange approach. Calculations on ethene and 1,1-difluoroethene illustrate the possibility of using nonrelativistic coupled cluster singles and doubles (CCSD) with additional effects of electron correlation and relativity added as scalar shifts in energetics. It has been demonstrated that CPP spectra obtained with coupled cluster singles and approximate doubles (CC2), CCSD, and DFT (with a Coulomb attenuated exchange-correlation functional) yield excellent predictions of chemical shifts for vinylfluoride, 1,1-difluoroethene, trifluoroethene, as well as good spectral features for acetone in the case of CCSD and DFT. Following this, CPP-DFT is considered to be a viable option for the calculation of X-ray absorption spectra of larger π-conjugated systems, and CC2 is deemed applicable for chemical shifts but not for studies of fine structure features. The CCSD method as well as the more approximate CC2 method are shown to yield spectral features relating to π∗-resonances in good agreement with experiment, not only for the aforementioned molecules but also for ethene, cis-1,2-difluoroethene, and tetrafluoroethene. The STEX approach is shown to underestimate π∗-peak separations due to spectral compressions, a characteristic which is inherent to this method.

Singlet oxygen generation versus O–O homolysis in phenyl-substituted anthracene endoperoxides investigated by RASPT2, CASPT2, CC2, and TD-DFT methods

The electronic excited states corresponding to singlet oxygen generation versus O–O splitting in o-fluorine-phenyl-9-anthracene-9,10-endoperoxide 1 and its 9,10-bisarylanthracene analog 2 have been investigated using theoretical methods. In the case of the smaller endoperoxide 1, the recently developed second-order perturbation theory restricted active space (RASPT2) method has been employed and the results are compared to those from the complete active space (CASPT2), second-order approximated coupled cluster (CC2), and time-dependent density functional theory (TD-DFT) approaches. In addition to the vertical excited states, the photochemical path leading to homolytic O–O dissociation has been computed. This process is governed by a point, where four singlet and four triplet states are almost degenerate and show substantial spin-orbit coupling. The results obtained with RASPT2 indicate that the S 1 state is of π oo * σ oo * character, corresponding to the O–O homolytic dissociation, while higher excited states S n (n ≥ 2) correspond to local and charge transfer excitations and should be correlated to the generation of singlet molecular oxygen. A similar photochemical picture is obtained with CASPT2, although two different active spaces are required to describe different parts of the spectrum. The calculations carried out with CC2 as well as the functionals CAM-B3LYP and the B3LYP(32) containing 32 % of exact exchange show good agreement with the RASPT2 energies, but present a strong mixing of π oo * σ oo * and π oo * π an * excitations in the lowest S 1 state, contradicting the assignment of RASPT2/CASPT2. The use of BP86 is strongly discouraged since it misplaces a large number of charge transfer states below the π oo * σ oo * state. The excited states of 2, calculated with B3LYP(32) are very similar to those of 1, leading to the conclusion that both endoperoxides should show a similar photochemistry, that is, the O–O cleavage seems to be partially quenched and singlet oxygen generation is enhanced, in comparison with the parent compound, anthracene-9,10-endoperoxide. The different electronic excited states of o-fluorine-phenyl-9-anthracene-9,10-endoperoxide have been benchmarked with RASPT2. The lowest excited state corresponds to the homolytic O–O dissociation and higher excited states are connected to singlet oxygen generation.

The magnetic circular dichroism (MCD) and absorption studies of 1,8-naphthalimide. The theoretical analysis in terms of density functional (DF) and coupled cluster (CC) theories

The vibronic coupling theory based on the Franck–Condon (FC) approach was applied to study the absorption and magnetic circular dichroism (MCD) spectra of 1,8-naphthalimide in the excitation region corresponding to the low-energy 1A1→2A1 and 1A1→1B1 transitions. The computations carried out in terms of PBE0 and CC2 methods have shown that the vertical excitation energies of 2A1 and 1B1 states and Franck–Condon parameters from CC2/aug-cc-pVDZ computation lead to a very good agreement with the experiment. MCD analysis suggests that the vibronic coupling effects between 2A1 and 1B1 states may be responsible for observed minor discrepancies between the theoretical and empirical MCD spectra.