Infrared Vibrational Autodetachment Spectroscopy of Microsolvated Benzonitrile Radical Anions

Simulation Investigation of Bulk and Surface Properties of Liquid Benzonitrile: Ring Stacking-Assessment and Deconvolution

The content and the molecular dynamics (MD) simulation analysis here are inspired by our recent ab initio calculation on benzonitrile (BZN), whereas the present results are to expand and develop macroscopic documentation involving data verification. MD simulations of the bulk liquid BZN in the range of 293-323 K unravel the hydrogen bond (-C≡N···H) formation with strength in the order of ortho-H ≫ meta-H ∼> para-H. The possibility for ortho-Hs to get involved in the formation of two bonds simultaneously confirms each having σ- and π-bonding features. Accordingly, we used vast efforts for structural analysis particularly based on the deconvolution of the corresponding complex correlation functions. Specific angle-dependent correlation functions led to the recognition of the molecular stacking with a strict anti-parallel orientation. The in-plane dimer and trimer also take part in the structural recognition. A singularity, found in the trend of the simulated temperature-dependent viscosity and diffusion coefficient of liquid BZN, is centered at about 313 K and quite fascinatingly emulates the reported experiment viscosity. An interplay between a small change in the trend of density and a large change in the corresponding viscosity is a key factor in supporting the singularity. Deconvolution of the simulation results allows attributing the singularity to structural alteration involving H-bonding of different types and extent. Approaching the range of 308-313 K, an alteration between hydrogen bond formation involving mostly ortho-Hs and mixed ortho-Hs + meta-H is possible and supports the singularity.

Vibrational Spectroscopy of Benzonitrile-(Water)1-2 Clusters in Helium Droplets

Polycyclic aromatic hydrocarbons are considered as primary carriers of the unidentified interstellar bands. The recent discovery of the first interstellar aromatic molecule, benzonitrile (C₆H₅CN), suggests a repository of aromatic hydrocarbons in the outer earth environment. Herein, we report an infrared (IR) study of benzonitrile-(D₂O)_n clusters using mass-selective detection in helium nanodroplets. In this work, we use isotopically substituted water, D₂O, instead of H₂O because of our restricted IR frequency range (2565-3100 cm^-1). A comparison of the experimental and predicted spectra computed at the MP2/6-311++G(d,p) level of benzonitrile-(water)_1-2 clusters reveals the formation of a unique local minimum structure, which was not detected in previous gas-phase molecular beam experiments. Here, the solvent water forms a nearly linear hydrogen bond (H-bond) with the nitrile nitrogen of benzonitrile, while the previously reported most stable cyclic H-bonded isomer is not observed. This can be rationalized by the stepwise aggregation process of precooled monomers. The addition of a second water molecule results in the formation of two different isomers. In one of the observed isomers, a H-bonded water chain binds linearly to the nitrile nitrogen similar to the monohydrated benzonitrile-water complex. In the other observed isomer, the water dimer forms a ring-type structure, where a H-bonded water dimer simultaneously interacts with the nitrile nitrogen and the adjacent ortho CH group. Finally, we compare the water-binding motif in the neutral benzonitrile-water complex with the corresponding positively and negatively charged benzonitrile-water monohydrates to comprehend the charge-induced alteration of the solvent binding motif.

Interpreting the physical background of empirical solvent polarity via photodetachment spectroscopy of microsolvated aromatic ketyl anions

The physical background of empirical solvent polarity is explored in regard to trends in solute-solvent intermolecular potential energy functions. Aromatic ketyl anions, benzophenone, and 9-fluorenone radical anions, are chosen for a model solute molecule showing solvatochromic behavior similar to betaine-30 dye, which provides the most established solvent polarity scale, E(T)(30). Common features among the ketyl anions and betaine-30 were examined with quantum chemical calculations for the electronic states and solvation structure. Vertical photodetachment and photoabsorption energies were determined for the ketyl anions microsolvated with a single solvent molecule by measuring photoelectron spectra as well as photodetachment excitation spectra for several aprotic and protic solvents. The spectroscopic data were analyzed through quantum chemical calculations based on density functional theory, and their relationship with the characteristics of intermolecular potential energies was considered. As a result, the typical solvent polarity parameter can be interpreted to reflect essentially the gradient of a potential energy function (namely, the strength of force) between a negative charge and the solvent molecules in the attractive region. A large polarity for protic solvents is attributed to an effective interaction of a proton-like hydrogen atom with the negative charge in a short-range.

Stepwise Solvatochromism of Ketyl Anions in the Gas Phase: Photodetachment Excitation Spectroscopy of Benzophenone and Acetophenone Radical Anions Microsolvated with Methanol

Electronic absorption spectra of bare and methanol-solvated radical anions of benzophenone ((C6H5)2CO) and acetophenone ((C6H5)CH3CO) were measured by monitoring the photodetachment efficiency in the gas phase. Strong absorption bands due to autodetachment after transitions to bound excited states were observed. Stepwise spectral shifts approaching the limit of the condensed phase spectra were found to occur as the cluster size increases. In the case of benzophenone radical anion, the solvation of two methanol molecules exhibits the near convergence to the limit, representing the full coordination with the solvent molecules around the carbonyl group. For the acetophenone case, the coordination number was not apparently determined because of their relatively small shifts. Relationships between hydrogen bonding and electronic structure are analyzed for the spectral shifts with the aid of calculations based on density functional theory. The calculational results show that the coordination angle of the solvent molecule is affected mostly by steric hindrance around the carbonyl group, and that there is no evidence for reorientation due to specific hydrogen bonding interaction with the singly occupied orbital, which has been formerly persisted for an interpretation of the transient absorption following pulse radiolysis in alcoholic solutions. An alternative possibility involving deformation with respect to intramolecular coordinates is discussed.