Benzene Cation Research Articles

Challenges in XUV Photochemistry Simulations: A Case Study on Ultrafast Fragmentation Dynamics of the Benzene Radical Cation.

The challenges of simulating extreme ultraviolet (XUV)-induced dissociation dynamics of organic molecules on a multitude of coupled potential energy surfaces are discussed for the prototypical photoionization of benzene. The prospects of Koopmans' theorem-based electronic structure calculations in combination with classical trajectories and Tully's fewest switches surface hopping are explored. It is found that a Koopmans' theorem-based approach overestimates the CH dissociation barrier and thus underestimates the fragmentation yield. However, the nonadiabatic population dynamics are in good agreement with previous approaches, indicating that the Koopmans' theorem based potentials are well described around the Franck-Condon point. This is explicitly tested for the ground state potential of the benzene cation employing CASPT2 calculations, for which very good agreement is found. This work highlights the need for efficient electronic structure approaches that can treat medium-sized organic molecules with a multitude of coupled excited states and several dissociation channels.

Few-femtosecond passage of conical intersections in the benzene cation

Observing the crucial first few femtoseconds of photochemical reactions requires tools typically not available in the femtochemistry toolkit. Such dynamics are now within reach with the instruments provided by attosecond science. Here, we apply experimental and theoretical methods to assess the ultrafast nonadiabatic vibronic processes in a prototypical complex system—the excited benzene cation. We use few-femtosecond duration extreme ultraviolet and visible/near-infrared laser pulses to prepare and probe excited cationic states and observe two relaxation timescales of 11 ± 3 fs and 110 ± 20 fs. These are interpreted in terms of population transfer via two sequential conical intersections. The experimental results are quantitatively compared with state-of-the-art multi-configuration time-dependent Hartree calculations showing convincing agreement in the timescales. By characterising one of the fastest internal conversion processes studied to date, we enter an extreme regime of ultrafast molecular dynamics, paving the way to tracking and controlling purely electronic dynamics in complex molecules.

Ab initio dynamics and photoionization mass spectrometry reveal ion–molecule pathways from ionized acetylene clusters to benzene cation

The growth mechanism of hydrocarbons in ionizing environments, such as the interstellar medium (ISM), and some combustion conditions remains incompletely understood. Ab initio molecular dynamics (AIMD) simulations and molecular beam vacuum-UV (VUV) photoionization mass spectrometry experiments were performed to understand the ion-molecule growth mechanism of small acetylene clusters (up to hexamers). A dramatic dependence of product distribution on the ionization conditions is demonstrated experimentally and understood from simulations. The products change from reactive fragmentation products in a higher temperature, higher density gas regime toward a very cold collision-free cluster regime that is dominated by products whose empirical formula is (C2H2) n+, just like ionized acetylene clusters. The fragmentation products result from reactive ion-molecule collisions in a comparatively higher pressure and temperature regime followed by unimolecular decomposition. The isolated ionized clusters display rich dynamics that contain bonded C4H4+ and C6H6+ structures solvated with one or more neutral acetylene molecules. Such species contain large amounts (>2 eV) of excess internal energy. The role of the solvent acetylene molecules is to affect the barrier crossing dynamics in the potential energy surface (PES) between (C2H2)n+ isomers and provide evaporative cooling to dissipate the excess internal energy and stabilize products including the aromatic ring of the benzene cation. Formation of the benzene cation is demonstrated in AIMD simulations of acetylene clusters with n > 3, as well as other metastable C6H6+ isomers. These results suggest a path for aromatic ring formation in cold acetylene-rich environments such as parts of the ISM.

Interplay between the intramolecular hydrogen bonds and cation–π interactions in various complexes of salicylaldehyde, thiosalicylaldehyde and selenosalicylaldehyde with Li+, Na+, K+, Mg2+ and Ca2+ cations

ABSTRACTFor the first time, the mutual influences of the intramolecular hydrogen bond (IMHB) and cation–π interactions in various complexes of salicylaldehyde, thiosalicylaldehyde and selenosalicylaldehyde with Li+, Na+, K+, Mg2+ and Ca2+ cations were studied. First, the strength of IMHB and cation–π interactions of the mentioned complexes by energetic, geometrical, spectroscopic, topological and molecular orbital parameters was evaluated and compared with the corresponding results of benzene–cation complexes and salicylaldehyde analogues. The results show that the coexistence of IMHB and cation–π interactions increases the IMHB strength and decreases the cation–π interactions. Second, the significance of π–electron delocalisation (π–ED) within the resonance-assisted hydrogen bond (RAHB) unit and aromaticity of benzene ring in the studied complexes were estimated by using the harmonic oscillator model of aromaticity and compared with the respective amounts of references. The results indicated that the mentioned coupling decreases the π–ED of RAHB unit and aromaticity of the benzene ring. In addition, it was found that variations in the strength of the interactions, π–ED and aromaticity, depend on the charge-to-radius ratio of cations. Finally, the effects of replacement of O by S and Se atoms in both of the mentioned cases were explored.

Gas phase hydration of halogenated benzene cations. Is it hydrogen or halogen bonding?

Halogen bonding (XB) non-covalent interactions can be observed in compounds containing chlorine, bromine, or iodine which can form directed close contacts of the type R1-XY-R2, where the halogen X acts as a Lewis acid and Y can be any electron donor moiety including electron lone pairs on hetero atoms such as O and N, or π electrons in olefin double bonds and aromatic conjugated systems. In this work, we present the first evidence for the formation of ionic halogen bonds (IXBs) in the hydration of bromobenzene and iodobenzene radical cations in the gas phase. We present a combined thermochemical investigation using the mass-selected ion mobility (MSIM) technique and density functional theory (DFT) calculations of the stepwise hydration of the fluoro, chloro, bromo, and iodobenzene radical cations. The binding energy associated with the formation of an IXB in the hydration of the iodobenzene cation (11.2 kcal mol-1) is about 20% higher than the typical unconventional ionic hydrogen bond (IHB) of the CHδ+OH2 interaction. The formation of an IXB in the hydration of the iodobenzene cation involves a significant entropy loss (29 cal mol-1 K-1) resulting from the formation of a more ordered structure and a highly directional interaction between the oxygen lone pair of electrons of water and the electropositive region around the iodine atom of the iodobenzene cation. In comparison, the hydration of the fluorobenzene and chlorobenzene cations where IHBs are formed, -ΔS° = 18-21 cal mol-1 K-1 consistent with the formation of less ordered structures and loose interactions. The electrostatic potentials on the lowest energy structures of the hydrated halogenated benzene radical cations show clearly that the formation of an IXB is driven by a positively charged σ-hole on the external side of the halogen atom X along the C-X bond axis. The size of the σ-hole increases significantly in bromobenzene and iodobenzene radical cations which results in strong interaction potentials with the electron lone pairs of the oxygen atom of the water molecules and thus IXBs provide the most stable hydrated structures of the bromobenzene and iodobenzene radical cations. The results clearly distinguish the hydration behaviors resulting from the ionic hydrogen and halogen bonding interactions of fluorobenzene and iodobenzene cations, respectively, and establish the different bonding and structural features of the two interactions.

The Effect of Hydration on the Cation-π Interaction Between Benzene and Various Cations

The effect of hydration on cation-π interaction in M q+ B m W n (B= benzene; W= water; M q+= Na+, K+, Mg2+, Ca2+, Al3+, 0 ≤ n,m ≤ 4,1 ≤ m + n ≤ 4) complexes has been investigated using ab initio quantum chemical methods. Interaction energy values computed at the MP2 level of theory using the 6-31G(d,p) basis set reveal a qualitative trend in the relative affinity of different cations for benzene and water in these complexes. The π–cloud thickness values for benzene have also been estimated for these systems. Effect of hydration on cation-π interaction

Comparative OHD-RIKES Study of the Low-Frequency (0–250 cm–1) Vibrational Dynamics of Dibenzyl- and Monobenzyl-Substituted Imidazolium Ionic Liquids and Benzene/Dimethylimidazolium Mixtures

The low-frequency (0–450 cm–1) Kerr spectra of 1-benzyl-3-methylimidazolium bis[(trifluoromethane)sulfonyl]amide ([BnzC1im][NTf2]) and 1,3-dibenzylimidazolium bis[(trifluoromethane)sulfonyl]amide ([(Bnz)2im][NTf2]) and 2:1 and 1:1 mixtures of benzene in 1,3-dimethylimidazolium bis[(trifluoromethane)sulfonyl]amide ([C1C1im][NTf2]) were measured by the use of optical heterodyne-detected Raman-induced Kerr effect spectroscopy (OHD-RIKES). Surprisingly, the line-shape of the intermolecular part of the height-normalized Kerr spectrum is the same for both the 1:1 and 2:1 C6H6/[C1C1im][NTf2] mixtures, which may be a signature of the local liquid structure being characterized by π-stacked benzene–cation complexes. The intermolecular part of the Kerr spectrum of [BnzC1im][NTf2] is lower in intensity, higher in frequency, and broader than that of the 1:1 C6H6/[C1C1im][NTf2] mixture. Similarly, the intermolecular part of the Kerr spectrum of [(Bnz)2im][NTf2] is lower in intensity, higher in frequency, and broader th...

Imaging Charge Transfer in a Cation−π System: Velocity-Map Imaging of Ag+(benzene) Photodissociation

Ag(+)(benzene) complexes are generated in the gas phase by laser vaporization and mass selected in a time-of-flight spectrometer. UV laser excitation at either 355 or 266 nm results in dissociative charge transfer (DCT), leading to neutral silver atom and benzene cation products. Kinetic energy release in translationally hot benzene cations is detected using a new instrument designed for photofragment imaging of mass-selected ions. Velocity-map imaging and slice imaging techniques are employed. In addition to the expected translational energy release, DCT of Ag(+)(benzene) produces a distribution of internally hot benzene cations. Compared with experiments at 355 nm, 266 nm excitation produces only slightly higher translational excitation and a much greater fraction of internally hot benzene ions. The maximum kinetic energy release in the photodissociation sets an upper limit on the Ag(+)(benzene) dissociation energy of 32.8 (+1.4/-1.5) kcal/mol.

Fluorinated benzene cations

Abstract While some fluorinated benzene cations have been detected quite some time ago, usually by EPR spectroscopy, only in the last few years have their isolation and structural elucidation been advanced. The state of knowledge about di-cations is very limited.

Geometric Rotation of the Nuclear Gradient at a Conical Intersection: Extension to Complex Rotation of Diabatic States.

Nonadiabatic dynamics in the vicinity of conical intersections is of essential importance in photochemistry. It is well known that if the branching space is represented in polar coordinates, then for a geometry represented by angle θ, the corresponding adiabatic states are obtained from the diabatic states with the mixing angle θ/2. In an equivalent way, one can study the relation between the real rotation of diabatic states and the resulting nuclear gradient. In this work, we extend the concept to allow a complex rotation of diabatic states to form a nonstationary superposition of electronic states. Our main result is that this leads to an elliptical transformation of the effective potential energy surfaces; i.e., the magnitude of the initial nuclear gradient changes as well as its direction. We fully explore gradient changes that result from varying both θ and ϕ (the complex rotation angle) as a way of electronically controlling nuclear motion, through Ehrenfest dynamics simulations for benzene cation.

Density functional theory study of the multimode Jahn–Teller problem in the open-shell corannulenes and coronenes

Analysis of the multimode Jahn–Teller (JT) distortion in anions and cations of corannulene and coronene is presented, combining multideterminantal-DFT (MD-DFT) and Intrinsic Distortion Path (IDP) methods. The JT distortion is derived from the sum of contributions of all possible JT active normal modes. The CC stretching modes play the most important role in the stabilization of the systems, in all investigated species, pushing the nuclei toward the minima on the potential energy surface. The further inspection of the IDP revealed that the relaxation of the geometry arrives in the final part of the path and is encountered by the softest vibrational modes. The same trend was observed in cyclopentadienyl radical, benzene anion, benzene cation and fullerene ions. This gives microscopic origin into the mechanism of the distortion and provides general behavior of the JT effect in these similar molecules. Moreover, MD-DFT and IDP, as fast and fully non-empirical approaches, can be considered as a reliable tool for better understanding of the JT effect.

Electron dynamics upon ionization: control of the timescale through chemical substitution and effect of nuclear motion.

Photoionization can generate a non-stationary electronic state, which leads to coupled electron-nuclear dynamics in molecules. In this article, we choose benzene cation as a prototype because vertical ionization of the neutral species leads to a Jahn-Teller degeneracy between ground and first excited states of the cation. Starting with equal populations of ground and first excited states, there is no electron dynamics in this case. However, if we add methyl substituents that break symmetry but do not radically alter the electronic structure, we see charge migration: oscillations in the spin density that we can correlate with particular localized electronic structures, with a period depending on the gap between the states initially populated. We have also investigated the effect of nuclear motion on electron dynamics using a complete active space self-consistent field (CASSCF) implementation of the Ehrenfest method, most previous theoretical studies of electron dynamics having been carried out with fixed nuclei. In toluene cation for instance, simulations where the nuclei are allowed to move show significant differences in the electron dynamics after 3 fs, compared to simulations with fixed nuclei.

Dicationic organic salts: gelators for ionic liquids.

Diimidazolium and dipyrrolidinium organic salts were tested for their ability to gel both organic solvents and ionic liquids. Organic salts containing 1-(1-imidazolylmethyl)-3,5-di-(3'-octylimidazolylmethyl)-benzene and 1-(N-pyrrolidylmethyl)-3,5-di-(N,N-octylpyrrolidylmethyl)-benzene cations were used. In addition to the simple bromide anion, also dianions having a naphthalene core such as 1,5- and 2,6-naphthalenedisulfonate and 2,6-naphthalenedicarboxylate were taken into account. Gelation tests demonstrated that organic salts used were able to harden ionic liquids. The materials obtained were investigated for their thermal stability and also for electric conductivity properties using micro-DSC and dielectric spectroscopy. Furthermore, the opacity of some gel phases was monitored using UV-vis measurements. To obtain information about the gelation mechanism, gel phase formation was studied as a function of time by means of resonance light scattering investigation. Finally, the ability of materials to respond to external stimuli such as magnetic stirring or ultrasound irradiation was also analyzed. Data collected show that different relationships exist among the gelator and the ionic liquid structure, determining the properties of materials and their possible applications.

Task specific dicationic ionic liquids: recyclable reaction media for the mononuclear rearrangement of heterocycles.

Nine functionalized dicationic ionic liquids were used to study the base catalyzed mononuclear rearrangement of (Z)-phenylhydrazone of 3-benzoyl-5-phenyl-1,2,4-oxadiazole into the corresponding triazole at 363 K. Ionic liquids were characterized by the presence of 1-(1-imidazolylmethyl)-3,5-di[1-(3'-octylimidazolylmethyl)]benzene cation, bearing on the structure a neutral imidazole unit as basic functionality. Besides anions generally used for monocationic ionic liquids, such as [Br(-)], [BF4(-)], and [NTf2(-)], also dianions having a rigid aromatic spacer such as 1,4-benzenedicarboxylate, 2,6-naphthalenedicarboxylate, and 1,5- and 2,6-naphthalenedisulfonate were taken into account. Furthermore, to have information about the effect deriving from the spacer rigidity also dianions such as 1,4-butanedicarboxylate and 1,6-hexanedicarboxylate were used. The basic strength of dicationic ionic liquids was determined using the Hammett indicator method. Data collected gave evidence that the outcome of the target reaction was affected by the nature of interactions operating between cation and anion of the ionic liquid used. Quite interestingly, solvent systems used had a good recyclability. Indeed, in some cases, they were reused for four cycles without significant loss in yield.

Growth kinetics and formation mechanisms of complex organics by sequential reactions of acetylene with ionized aromatics

In this work, we present direct experimental evidence for the formation of covalently-bonded organic ions by gas phase reactions of acetylene with the benzonitrile, phenylacetylene, and styrene radical cations, and compare these results with the previously studied acetylene reactions with the benzene, phenyl, pyridine, and pyrimidine cations. A very wide range of reaction rates has been found ranging from very slow reactions with energy barriers as in the case of benzene and styrene radical cations to reactions occurring at the collision rate as in the case of the pyrimidinium cation. In all cases, synthetic channels with increased numbers of carbon atoms are found in all the observed reactions. The resulting covalently-bonded precursor ions can undergo further growth to form polycyclic aromatic hydrocarbon ions including nitrogen-containing PAH ions. The observed chemistry under a wide range of experimental conditions, including extreme temperatures as high as 650K, strongly implies a series of ion–molecule reactions that can lead from a simple molecule such as acetylene to polycyclic complex organic ions. The product ions resulting from the sequential reactions of acetylene can, in turn be neutralized by charge transfer or recombination, to produce large neutral PAHs. Therefore, ion–molecule reactions can contribute to the formation of higher hydrocarbons and PAHs in ionizing environments. The observed reactions could explain the possible formation of complex organics under a wide range of temperatures and pressures in astrochemical environments such as the nitrogen-rich Titan's atmosphere.

The second-order Ehrenfest method

This article describes the Ehrenfest method and our second-order implementation (with approximate gradient and Hessian) within a CASSCF formalism. We demonstrate that the second-order implementation with the predictor–corrector integration method improves the accuracy of the simulation significantly in terms of energy conservation. Although the method is general and can be used to study any coupled electron–nuclear dynamics, we apply it to investigate charge migration upon ionization of small organic molecules, focusing on benzene cation. Using this approach, we can study the evolution of a non-stationary electronic wavefunction for fixed atomic nuclei, and where the nuclei are allowed to move, to investigate the interplay between them for the first time. Analysis methods for the interpretation of the electronic and nuclear dynamics are suggested: we monitor the electronic dynamics by calculating the spin density of the system as a function of time.

Molecular simulation study on Hofmeister cations and the aqueous solubility of benzene.

We study the ion-specific salting-out process of benzene in aqueous alkali chloride solutions using Kirkwood-Buff (KB) theory of solutions and molecular dynamics simulations with different empirical force field models for the ions and benzene. Despite inaccuracies in the force fields, the simulations indicate that the decrease of the Setchenow salting-out coefficient for the series NaCl > KCl > RbCl > CsCl is determined by direct benzene-cation correlations, with the larger cations showing weak interactions with benzene. Although ion-specific aqueous solubilities of benzene may be affected by indirect ion-ion, ion-water, and water-water correlations, too, these correlations are found to be unimportant, with little to no effect on the Setchenow salting-out coefficients of the various salts. We further considered LiCl, which is experimentally known to be a weaker salting-out agent than NaCl and KCl and, therefore, ranks at an unusual position within the Hofmeister cation series. The simulations indicate that hydrated Li(+) ions can take part of the benzene hydration shell while the other cations are repelled by it. This causes weaker Li(+) exclusion around the solute and a resulting, weaker salting-out propensity of LiCl compared to that of the other salts. Removing benzene-water and benzene-salt electrostatic interactions in the simulations does not affect this mechanism, which may therefore also explain the smaller effect of LiCl, as compared to that of NaCl or KCl, on aqueous solvation and hydrophobic interaction of nonpolar molecules.

The reaction of the benzene cation with acetylenes for the growth of PAHs in the interstellar medium

We investigated the formation of the naphthalene cation from the successive addition of two acetylenes on the benzene cation. A DFT approach was used for that purpose. The entry channel corresponding to the addition of the acetylenes is the energetically highest part of the entire reaction pathway. It was recalculated using the ab initio W1-F12 and G4 thermochemical protocols. At these levels, the energetically highest point in the entry channel is shown to be just below the energy of the free reactants. This Letter shows that the formation of the naphthalene cation from benzene and acetylenes should proceed in space.

Substituent Effects on Noncovalent Bonds: Complexes of Ionized Benzene Derivatives with Hydrogen Cyanide

Here, we report the first experimental and computational study of the noncovalent binding energies and structures of ionized benzenes containing electron-withdrawing substituents solvated by one to four HCN molecules. Measured by ion mobility mass spectrometric equilibrium studies, the bond dissociation enthalpies of the first HCN molecule to the fluorobenzene (C6H5F(•+)), 1,4-difluorobenzene (C6H4F2(•+)), and benzonitrile (C6H5CN(•+)) ions (11.2, 11.2, and 9.2 kcal/mol, respectively) are similar to those of HCN with the benzene (C6H6(•+)) and phenyacetylene (C6H5CCH(•+)) radical cations (9.2 and 10.5 kcal/mol, respectively). DFT calculations at the B3LYP/6-311++G(d,p) level show that HCN can form in-plane hydrogen bonds to ring hydrogens, or bind electrostatically to positively charged carbon centers in the ring. The electron-withdrawing substituents increase the bond energy by increasing the partial positive charge on the ring hydrogens that form CH(δ+)---NCH bonds, and by creating a π hole, as evidenced by positive charge centers on the fluorinated ring carbons for electrostatically bonded isomers. In the complexes of benzonitrile(•+), similar to benzene(•+), hydrogen bonded planar isomers have the lowest energy. In the complexes of (fluorinated benzene)(•+), the lowest energy isomers are electrostatically bonded where HCN is perpendicular to the ring and its dipole points to a positively charged ring carbon. However, in all cases the planar hydrogen-bonded and vertical electrostatic isomers have similar binding energies within 1 kcal/mol, although HCN interacts with different sites of the ionized benzenes in these isomers, suggesting that the observed cluster populations are mixtures of the planar and vertical isomers. Further HCN molecules can bind directly to unoccupied ring CH hydrogens or bind to the first-shell HCN molecules to form linear HCN---HCN--- hydrogen bonded chains. The binding energies decrease stepwise to about 6-7 kcal/mol by 4 HCN molecules, approaching the macroscopic enthalpy of vaporization of liquid HCN (6.0 kcal/mol).

Halogenated Benzene Cation Radicals

AbstractThe benzene cation radicals [C6F5‐CF3]+, [1,4‐C6F4(CF3)2]+, [1,2,4,5‐C6H2Cl4]+, [C6F5‐C6F5]+, and [C6Br6]+ have been obtained as salts with fluoroarsenic and fluoroantimony counterions. Their structures have been obtained by single‐crystal structure determinations, and supported by ESR measurements and DFT calculations. [C6F5‐CF3]+, [1,4‐C6F4(CF3)2]+, and [1,2,4,5‐C6H2Cl4]+ have the predicted bisallyl type structure. [C6F5‐C6F5]+ can be described as having a quinoid form with a strengthened central C–C bond and a smaller dihedral angle as compared to the neutral C6F5‐C6F5. [C6Br6]+ is the missing link between [C6F6]+, [C6Cl6]+, and [C6I6]+. It has a Jahn–Teller distortion, though weaker than [C6F6]+ and [C6Cl6]+, in contrast to [C6I6]+, which remains in a regular hexagon structure.