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Vibronic coupling in the first eight electronic excited states of Pentafluorobenzene (PFBz) is investigated in this article. In particular, the vibronic coupling between the optically bright ππ* and optically dark πσ* states of PFBz is considered. A model 8 × 8 diabatic Hamiltonian is constructed in terms of normal coordinate of vibrational modes using the standard vibronic coupling theory and symmetry selection rule. The Hamiltonian parameters are estimated with the aid of extensive ab initio quantum chemistry calculations. The topography of the first eight electronic excited states of PFBz is examined at length, and multiple multi-state conical intersections are established. The nuclear dynamics calculations on the coupled electronic surfaces are carried out from first principles by the wave packet propagation method. Theoretical results are found to be in good accord with the available experimental optical absorption spectrum of PFBz. [ABSTRACT FROM AUTHOR]

Carbon chains have been predicted to be potential carriers of diffuse interstellar band features in astrophysical observations. Motivated by numerous predictions, we set out to carry out extensive ab initio quantum chemistry calculations to establish the ground and excited electronic potential energy surfaces and their coupling surfaces for carbon chains containing an odd number of carbon atoms (C2n+1, n = 7–10). Vibronic coupling models are established with the aid of the calculated electronic energies to investigate nuclear dynamics from first principles. The latter are reported in Ghosh et al. [J. Chem. Phys. 151, 054304 (2019)]. The mentioned carbon chains possess a linear cumulenic structure at the equilibrium minimum of their electronic ground state, and an electronic excited state of the Σ u + 1 term appears to be extremely bright optically and absorbs in the visible region of the electromagnetic spectrum. Vertical excitation energy of this state decreases and transition dipole moment increases, and as a result, the oscillator strength of this state linearly increases with an increase of the chain length. There are states belonging to 1Πg, 1Πu, Σ g + 1 , 1Δg, and 1Δu terms, in the immediate vicinity of the Σ u + 1 state, which are optically dark but can gain intensity through vibronic coupling with the optically bright Σ u + 1 state. Construction of a coupling scheme considering the Renner-Teller coupling within the degenerate Π states and pseudo-Renner-Teller coupling between the Renner-Teller split component states as well as with the nondegenerate Σ states is another motivation of this work. The coupled-state Hamiltonian is constructed in a diabatic electronic basis in terms of the dimensionless normal coordinates of the vibrational modes of the carbon chains. Both Renner-Teller and pseudo-Renner-Teller types of couplings are included in the Hamiltonian. The theoretical results are discussed in relation to the experimental findings. [ABSTRACT FROM AUTHOR]

A theoretical study of the photoabsorption spectroscopy of hexafluorobenzene (HFBz) is presented in this paper. The chemical effect due to fluorine atom substitution on the electronic structure of benzene (Bz) saturates in HFBz. State- of-the-art quantum chemistry calculations are carried out to establish potential energy surfaces and coupling surfaces of five energetically low-lying electronic (two of them are orbitally degenerate) states of HFBz. Coupling of these electronic states caused by the Jahn-Teller (JT) and pseudo-Jahn-Teller (PJT) type of interactions are examined. The impact of these couplings on the nuclear dynamics of the participating electronic states is thoroughly investigated by quantum mechanical methods and the results are compared with those observed in the experiments. The complex structure of the S1 ← S0 absorption band is found to originate from a very strong nonadiabatic coupling of the S2 (of πσ* origin) and S1 (of ππ* origin) state. While S2 state is orbitally degenerate and JT active, the S1 state is nondegenerate. These states form energetically low-lying conical intersections (CIs) in HFBz. These CIs are found to be the mechanistic bottleneck of the observed low quantum yield of fluorescence emission, non overlapping absorption, and emission bands of HFBz and contribute to the spectral width. Justification is also provided for the observed two peaks in the second absorption (the unassigned 'c band') band of HFBz. The peaks observed in the third, fourth, and fifth absorption bands are also identified and assigned. [ABSTRACT FROM AUTHOR]

Photodetachment bands of anionic boron clusters, Bn (n = 4,5) are theoretically examined here. The model Hamiltonians developed through extensive ab initio quantum chemistry calculations in Paper I are employed for the required nuclear dynamics study. While the precise location of vibronic lines and progression of vibrational modes within a given electronic band is derived from time-independent quantum mechanical studies, the broadband spectral envelopes and the nonradiative decay rate of electronic states are calculated by propagating wave packets in a time-dependent quantum mechanical framework. The theoretical results are in good accord with the experiment to a large extent. The discrepancies between the two can be partly attributed to the inadequate energy resolution of the experimental results and also to the neglect of dynamic spin-orbit interactions and computational difficulty related with detachment channels involving multi-electron transitions in the theoretical formalism. [ABSTRACT FROM AUTHOR]

The electronic structure of energetically low-lying excited singlet states of fluorobenzene molecules is investigated here. Increasing fluorine substitution alters the nature of the excited electronic states and the so-called perfluoro effect is observed for penta- and hexafluorobenzene. Detailed quantum chemistry calculations are carried out at the equation-of-motion coupled-cluster singles and doubles level of theory to establish the potential energy surfaces of the low-lying electronic states of mono-, di- (ortho- and meta-), and pentafluorobenzene molecules. A sequence of low-energy conical intersections among the electronic potential energy surfaces is established. It is found that increasing fluorine substitution lowers the energy of the πσ* electronic state and leads to conical intersections between the S1 and S2 electronic states of pentafluorobenzene. Existence of numerous conical intersections among the excited electronic states of these molecules forms the mechanistic details underlying their nonradiative internal conversion. In particular, the slow and biexponential fluorescence emission in pentafluorobenzene is attributed to the existence of low-lying S1-S2 conical intersections. The electronic structure data are analyzed in detail and the coupling mechanism among various electronic excited states of mono-, di-, and pentafluorobenzene molecules is established. [ABSTRACT FROM AUTHOR]

The quantum wave packet dynamics of the title reaction within the coupled state approximation is examined here and initial state-selected reaction probabilities, integral reaction cross sections, and thermal rate constants are reported. The ab initio potential energy surface of the electronic ground state (1 2A″) of the system recently reported by Ho et al. [J. Chem. Phys., 119, 3063 (2003)] is employed in this investigation. All partial wave contributions up to the total angular momentum J=55 were necessary to obtain converged integral reaction cross sections up to a collision energy of 1.0 eV. Thermal rate constants are calculated from the reaction cross sections and compared with the available theoretical and experimental results. Typical resonances formed during the course of the reaction and elucidating the insertion type mechanism for the product formation are calculated. Vibrational energy levels supported by the deep well (∼5.5 eV) of the 1 2A″ potential energy surface of NH2 are also calculated for the total angular momentum J=0. A statistical analysis of the spacing between the adjacent levels of this energy spectrum is performed and the extent of irregularity in the spectral sequence is assessed. [ABSTRACT FROM AUTHOR]

Electronic and nuclear motions on intersecting potential energy surfaces are often intricately mixed and the spectrum can become very complex. Here we choose the strongly coupled Jahn-Teller system CH3F+ as a prototype example, and establish the importance of intermode coupling terms on multimode vibronic dynamics. The theoretical approach consists of a full second-order diabatic vibronic Hamiltonian, constructed from high-quality electronic structure calculations. Our results compare amazingly well with the experimental data. This highlights the success of the present theoretical approach in explaining the complex structure of vibronic spectra, ubiquitous in molecular systems. [ABSTRACT FROM AUTHOR]

The static and dynamic aspects of the Jahn-Teller (JT) interactions in the 3p(E′) and 3d(E″) Rydberg electronic states of H3 are analyzed theoretically. The static aspects are discussed based on recent ab initio quantum chemistry results, and the dynamic aspects are examined in terms of the vibronic spectra and nonradiative decay behavior of these states. The adiabatic potential-energy surfaces of these degenerate electronic states are derived from extensive ab initio calculations. The calculated adiabatic potential-energy surfaces are diabatized following our earlier study on this system in its 2p(E′) ground electronic state. The nuclear dynamics on the resulting conically intersecting manifold of electronic states is studied by a time-dependent wave-packet approach. Calculations are performed both for the uncoupled and coupled state situations in order to understand the importance of nonadiabatic interactions due to the JT conical intersections in these excited Rydberg electronic states. [ABSTRACT FROM AUTHOR]

In continuation to our earlier work [Sarkar et al., Mol. Phys. 2015, 113, 3073-3084], we present here the vibronic structure and nonradiative decay dynamics of energetically low-lying electronic states of CH2F2+. Interesting comparison is made with the results obtained for the deuterated isotopomer CD2F2+. The results are compared with both broad band as well as high-resolution experimental spectroscopy data available in the literature. The progression of vibrational modes in the spectra is identified, assigned and discussed in relation to the assignments available in the literature. Both time-independent and time-dependent quantum mechanical methods are used to carry out nuclear dynamics calculations. The underlying potential energy surfaces and their coupling surfaces are constructed by rigorous ab initio quantum chemistry calculations. [ABSTRACT FROM AUTHOR]

We report multimode vibronic coupling of the energetically low-lying electronic states of phenol and pentafluorophenol in this article. First principles nuclear dynamics calculations are carried out to elucidate the optical absorption spectrum of both of the molecules. This is motivated by the recent experimental measurements [S. Karmakar et al., J. Chem. Phys. 142, 184303 (2015)] on these systems. Diabatic vibronic coupling models are developed with the aid of adiabatic electronic energies calculated ab initio by the equation of motion coupled cluster quantum chemistry method. A nuclear dynamics study on the constructed electronic states is carried out by both the time-independent and time-dependent quantum mechanical methods. It is found that the nature of low-energy πσ* transition changes, and in pentafluorophenol the energy of the first two 1πσ* states, is lowered by about half an eV (vertically, relative to those in phenol), and they become energetically close to the optically bright first excited 1ππ* (S1) state. This results in strong vibronic coupling and multiple multi-state conical intersections among the ππ* and πσ* electronic states of pentafluorophenol. The impact of associated nonadiabatic effects on the vibronic structure and dynamics of the 1ππ* state is examined at length. The structured vibronic band of phenol becomes structureless in pentafluorophenol. The theoretical results are found to be in good accord with the experimental finding at both high energy resolution and low energy resolution. [ABSTRACT FROM AUTHOR]

Radicalcations of polycyclic aromatic hydrocarbons have been postulatedto be molecular carriers of diffuse spectroscopic features observedin the interstellar medium. Several important observations made bystellar and laboratory spectroscopists motivated us to undertake adetailed theoretical study attempting to validate the recorded data.In continuation of our work on this subject, we here focus on a detailedtheoretical study of the doublet ground (X̃) and low-lying excited (Ã, B̃, and C̃) electronic states of the radicalcation of phenanthrene, pyrene, and acenaphthene molecule. A multistateand multimode theoretical model in a diabatic electronic basis isdeveloped here through extensive ab initio quantum chemistry calculations.Employing this model, first-principles nuclear dynamics calculationsare carried out to unravel the spectral assignment, time-dependentdynamics, and photostability of the mentioned electronic states ofthe radical cations. The theoretical results compare well with theobserved experimental data. [ABSTRACT FROM AUTHOR]

Abstract: Static and dynamic aspects of the Jahn–Teller (JT) and pseudo-Jahn–Teller (PJT) interactions between the ground and first excited electronic states of the methyl cyanide radical cation are theoretically investigated here. The latter involves construction of a theoretical model by ab initio computation of electronic potential energy surfaces and their coupling surfaces and simulation of the nuclear dynamics employing time-independent and time-dependent quantum mechanical methods. The present system represents yet another example belonging to the (E + A)⊗ e JT–PJT family, with common JT and PJT active degenerate (e) vibrational modes. The theoretical results are found to be in very good accord with the recent experimental data revealing that the JT interactions are particularly weak in the ground electronic manifold of methyl cyanide radical cation, On the other hand, the PJT interactions of this ground electronic manifold with the first excited electronic state of the radical cation are stronger which cause an increase of the spectral line density. The effect of deuteration on the JT–PJT dynamics of the methyl cyanide radical cation is also discussed. [Copyright &y& Elsevier]

Quantum state-selected dynamics of C(3P) + OH (X2Π) → CO(a3Π) + H (2S) reaction on its first excited electronic potential energy surface (12A″) is examined here using a time-dependent wave packet propagation approach. All partial wave contributions for the total angular momentum, J = 0-95, are included to obtain the converged cross sections and initial state-selected rate constants in the temperature range of 10-500 K. The reaction probability, as a function of collision energy, exhibits dense oscillatory structures owing to the formation of resonances during collision. These resonance structures also persist in reaction cross sections. The effect of reagent rotational and vibrational excitation on the dynamical attributes is examined and discussed. Reagent rotational excitation decreases the reactivity whereas, vibrational excitation of the reagent has minor effects on the reactivity. The results presented here are in good accord with those obtained using the time-independent quantum mechanical and quasi-classical trajectory methods. [ABSTRACT FROM AUTHOR]

Abstract The photodetachment spectroscopy of CB 9 - is theoretically studied in this paper. Extensive ab initio quantum chemistry calculations are carried out to construct the potential energy surfaces of the electronic ground and excited states of CB 9. With the aid of these calculated adiabatic electronic energies, a vibronic coupling model is developed in a diabatic electronic basis and in terms of normal coordinates of vibrational modes following the standard vibronic coupling theory originally developed by Köppel et al. (1984). Employing the developed diabatic electronic model, first principles nuclear dynamics study is carried out to calculate the vibronic structure of photodetachment bands of CB 9 - . A systematic study is performed to elucidate the impact of electronic nonadiabatic effects on the photodetachment bands. The vibronic structure of the latter is assigned in terms of excitation of vibrational modes and the results are compared with the experimental findings. The internal conversion dynamics through conical intersections of electronic states is examined by analyzing the time-dependent electronic populations. The theoretical results are found to be in good agreement with the experimental data. The detailed theoretical study carried out here unambiguously supports the prediction of the global minimum structure of CB 9 - . [ABSTRACT FROM AUTHOR]

Abstract: In this paper we examine the Jahn–Teller (JT) and pseudo-Jahn–Teller (PJT) effects in the spiropentane radical cation (SP+) by an ab initio quantum dynamical method. Spiropentane (SP) possesses D 2d symmetry at its equilibrium configuration. The two low-lying electronic states of SP+ belong to and symmetry, respectively. SP+ in the degenerate state is susceptible to JT distortions along the vibrational modes of b symmetry. The state of SP+ is vertically ∼0.51eV spaced from its state. Symmetry rule allows a coupling of the and states via the degenerate e vibrational modes. This is termed as the (B + E ⊗ b)⊗ e JT and PJT effects revealing the symmetry of the electronic states and the coupling vibrational modes. The theoretical findings establish significant impact of the JT and PJT coupling in the observed complex structure of the bands of SP+. [ABSTRACT FROM AUTHOR]

Vibronic interactions in the two energetically lowest electronic states (X̃²Πg-òΠu) of the diacetylene radical cation (C4H2•+) are theoretically examined here. The spectroscopy of these two electronic states of C4H2•+ has been a subject of considerable interest and measured in the laboratory by various groups. Inspired by numerous experimental data, we attempt here a detailed investigation of vibronic interactions within and between the doubly degenerate Π electronic states and their impact on the vibronic structure of each state. A vibronic coupling model is constructed in a diabatic electronic basis and with the aid of ab initio quantum chemistry calculations. The vibronic structures of the electronic states are calculated by time-independent and time-dependent quantum mechanical methods. The progression of vibrational modes in the vibronic band is identified, assigned, and compared with the literature data. The nonradiative internal conversion dynamics is also examined and discussed. [ABSTRACT FROM AUTHOR]