Quantum chemistry studies of electronically excited nitrobenzene, TNA, and TNT

The electronic excitation energies and excited-state potential energy surfaces of nitrobenzene, 2,4,6-trinitroaniline (TNA), and 2,4,6-trinitrotoluene (TNT) are calculated using time-dependent density functional theory and multiconfigurational ab initio methods. We describe the geometrical and energetic character of excited-state minima, reaction coordinates, and nonadiabatic regions in these systems. In addition, the potential energy surfaces for the lowest two singlet (S(0) and S(1)) and lowest two triplet (T(1) and T(2)) electronic states are investigated, with particular emphasis on the S(1) relaxation pathway and the nonadiabatic region leading to radiationless decay of S(1) population. In nitrobenzene, relaxation on S(1) occurs by out-of-plane rotation and pyramidalization of the nitro group. Radiationless decay can take place through a nonadiabatic region, which, at the TD-DFT level, is characterized by near-degeneracy of three electronic states, namely, S(1), S(0), and T(2). Moreover, spin-orbit coupling constants for the S(0)/T(2) and S(1)/T(2) electronic state pairs were calculated to be as high as 60 cm(-1) in this region. Our results suggest that the S(1) population should quench primarily to the T(2) state. This finding is in support of recent experimental results and sheds light on the photochemistry of heavier nitroarenes. In TNT and TNA, the dominant pathway for relaxation on S(1) is through geometric distortions, similar to that found for nitrobenzene, of a single ortho-substituted NO(2). The two singlet and lowest two triplet electronic states are qualitatively similar to those of nitrobenzene along a minimal S(1) energy pathway.     

To the question on radiationless transitions in electronically excited zones of molecular crystals

A model is proposed describing the dynamics of radiationless transitions in the energy zones corresponding to excited electronic states in molecular crystals. In this model, the migration effect is explicitly reflected, which initially appears under pulse excitation of the systems with a periodical structure of a density distribution of states in separate parts of molecular crystals.     

Yield of electronically excited CN molecules from the dissociative recombination of HNC+ with electrons

The authors have studied CN(B-X) and CN(A-X) emissions produced by the dissociative recombination of HNC+ ions with thermal electrons in a flowing afterglow experiment. A separate drift tube study showed that the reaction Ar(+)+HCN, the precursor reaction used in the flow-tube experiment, produces predominantly HNC+ rather than the more energetic HCN+ isomer. Models simulating the ion-chemical processes, diffusion, and gas mixing in the afterglow plasma were fitted to observed position dependent CN(A-X) and CN(B-X) band intensities. Absolute yields of CN(B) and CN(A) were then obtained by comparing the CN band intensities to those of CO bands produced by recombination of CO(2) (+) ions. It was concluded that the 300 K recombination coefficient of HNC+ is close to 2 x 10(-7) cm(3) s(-1), that CN(B) is formed with a yield of 0.22+/-0.08 and CN(A) with a yield of 0.14+/-0.05. By comparison to synthetic spectra, the rotational temperature of CN(B) was estimated to be approximately 2500 K. It was also found that recombination produces CN(B) and CN(A) with far greater vibrational excitation than would be expected from the "impulse model" of Bates [Mon. Not. R. Astron. Soc. 263, 369 (1993)].     

On the electronically excited states of uracil

Vertical excitation energies in uracil in the gas phase and in water solution are investigated by the equation-of-motion coupled-cluster and multireference configuration interaction methods. Basis set effects are found to be important for converged results. The analysis of electronic wave functions reveals that the lowest singlet states are predominantly of a singly excited character and are therefore well described by single-reference equation-of-motion methods augmented by a perturbative triples correction to account for dynamical correlation.Our best estimates for the vertical excitation energies for the lowest singlet n --> pi* and pi --> pi* are 5.0 +/- 0.1 eV and 5.3 +/- 0.1 eV, respectively. The solvent effects for these states are estimated to be +0.5 eV and +/- 0.1 eV, respectively. We attribute the difference between the computed vertical excitations and the maximum of the experimental absorption to strong vibronic interaction between the lowest A" and A' states leading to intensity borrowing by the forbidden transition.     

N-dealkylation of electronically excited coumarins by nitrobenzenes

When excited (monochromatically or with λ ⩾ 280 nm) in benzene solution in the presence of nitroaromatic acceptors, several 7-dialkylamino-coumarins undergo moderately efficient mono-N-dealkylation to form 7-alkylaminocoumarins, accompanied by the reduction of the nitro function in the acceptor. For 7-(N,N-diethylamino)-4-methylcoumarin (1a) a linear plot of ø_P⁻¹ versus the reciprocal of the concentration of the starting material suggests that two molecules of 1a are involved in the dealkylation of irradiated 1a by ground state 3-chloronitrobenzene. When the concentration of 1a is kept constant but that of the acceptor is varied, ø_P goes through a maximum at intermediate concentrations. Furthermore, ø_P is found to be larger for weaker acceptors (more negative reduction potential) than for stronger acceptors (less negative reduction potential). The rigid dye Coumarin 102 does not show any analogous decomposition. All results are interpreted in terms of the interference of the nitroaromatic with the self-quenching mechanism of coumarin N-dealkylation put forward recently by Jones and coworkers.     

Yield of electronically excited N2 molecules from the dissociative recombination of N2H+ with e-

Quantitative spectroscopic observations of the N2 first positive band system (N2(B 3Pig-A 3Sigmau+))/electron in a recombining N2H+ flowing-afterglow plasma indicate that a substantial fraction of the product N2 molecules are formed in one or more of the low-lying triplet states, B 3Pig, A 3Sigmau+, and W 3Deltau. The total measured N2(B-A) emission intensity from N2(B,v' > or = 1) is equivalent to a yield of (19 +/- 8)%. The effect of rapid collision-induced transitions between states of the triplet manifold is discussed..     

Hydrogen bonding in electronically excited states of molecules

Theoretical Chemistry Accounts - CNDO/2 calculations show that hydrogen bonds in the electronically excited states of +H2O and +HOCH3 systems are slightly weaker than in the ground states. The...     

Multi-reference state-universal coupled-cluster approaches to electronically excited states

The multi-reference (MR), general model space (GMS), state-universal (SU), coupled-cluster (CC) method with singles and doubles (GMS-SU-CCSD), as well as its triple-corrected versions GMS-SU-CCSD(T), are employed to assess their ability to describe low-lying excited states of various molecules, with an emphasis on a simultaneous handling of several states of the same symmetry species. A special attention is given to the role of the so-called C-conditions that account for non-vanishing internal cluster amplitudes when relying on an incomplete GMS, as well as to the choice of suitable model spaces and a perturbative account of secondary triples. The ambiguities arising when using large basis sets are also pointed out. To achieve a general assessment of the potential of the GMS-type SU-CC approaches, the vertical excitation energies of several species, including the challenging BN diatomic as well as larger systems, namely formaldehyde, trans-butadiene, formamide, and benzene are considered. These results are compared with those provided by the equation-of-motion EOM-CCSD method and, whenever available, the density functional theory results and experimental data. These comparisons clearly demonstrate the usefulness of GMS-type MR-CC approaches.     

Supersonic expansion cooling of electronically excited OH radicals

Supersonic nozzle cooling of excited-state melecular radicals produced in a corona discharge is demonstrated and investigated using hydroxyl radicals. Rotational temperatures of 11 ± 2 K and vibrational temperatures of 3400 ± 300 K are typically observed in the OH A ²Σ⁺ state.     

Reactions of electronically excited NH(aΔ) with ethane

Reactions of excited NH(a¹Δ) with C₂H₆ have been investigated. Reactions proceed via three pathways: (a) insertion of NH (a¹Δ) into a C-H bond, (b) abstraction of H, and (c) electro me quenching of NH (a¹Δ). From the pressure dependence of yields of the insertion product (C₂H₅NH₂) and fragmentation products (C₂H₅ and CH₃, the branching ratios were determined to be 0.6 for (a), 0.1 for (b) and 0.3 for (c).     

Comments on heteronuclear electronically excited rare gas diatomic molecules

Experimental evidence for the stability of the electronically excited (KrAr)^* molecule is presented from the study of the vacuum ultraviolet luminescence spectra of Kr/Ar gaseous, liquid and solid mixtures.     

Neutralization of ions at an electronically excited semiconductor surface

Neutralization of positive ions colliding with a semiconductor surface is studied. It is shown that the neutralization probability can be significantly enhanced if the surface exposed to the impinging ions is electronically excited. The basic reason behind this is that the impinging ions have easier access to the excited surface electrons than to bulk electrons.     

The role of electronically excited states in recombination reactions

Methods are described for including the participation of bound electronically excited states in calculations on radical recombination reactions. These methods are illustrated by applying them to the reactions For O₂, accurate ab initio potentials are used in calculations which show that the electronic degeneracy and long-range part of the potential are likely to be crucial in determining the contribution of a given electronic state to the overall reaction, as long as the state is not so weakly bound that it dissociates thermally before being electronically quenched. Weak collision effects are allowed for using a Monte Carlo technique and an assumed exponential form for the distribution of energies transferred in collisions with a third body. For larger systems it is evident that the role of bound excited states in the low-pressure regime falls rapidly as the size of the system increases. As the high-pressure limit is approached, however, the contribution of excited states is likely to come close to that expected simply on the basis of electronic degeneracy.     

Collisionless infrared multiphoton production of electronically excited parent molecules

In the absence of collisions chromyl chloride (CrO₂Cl₂) shows orange fluorescence when the parent molecule is irradiated by the focused output Time-resolved fluorescence is investigated over the pressure range 5 × 10^?4 to 15 torr. As the pressure is increased, a secondary collision-ind We interpret the collisionless part of the emission to arise from infrared transitions between the high vibrational levels of the ground state and the     

State-to-state dissociation rate coefficients in electronically excited diatomic gases

In the present Letter, state dependent dissociation rate coefficients in diatomic gases with non-equilibrium vibrational and electronic excitation and chemical reactions are studied. A widely used Treanor–Marrone model is generalized to take into account state-to-state vibrational and electronic distributions. The influence of electronic excitation on the rate of dissociation from various electronic states of CO molecules is estimated.