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An overview is presented relating to the use of field-induced droplet ionization (FIDI) and sum-frequency generation (SFG) to unravel questions pertaining to interfacial structure in general and real-world processes in particular. The present contribution focuses on the study of air crude oil interfaces as addressed by a comparison of electrospray ionization spectra and interface-sensitive FIDI methodology. FIDI reveals clear trends toward the specific orientation of select crude components at the interface. We provide evidence that the determining factor for what places itself at the interface is the unpolar nature of air. SFG as a well-established technique shows the sensitivity toward molecular alignments, and molecular interactions for a few model polar and nonpolar systems. The implications of being able to address interfacial structure are discussed in the context of determining molecular orientation in sum-frequency generation experiments as model examples. The combined information identifies factors that play a major role in impacting macroscopic properties, such as interfacial tension, which is a key player in efforts that focus on enhanced oil recovery.

Precipitation of sparingly soluble salts is a common issue in water treatment processes and oil and gas production systems. Precipitation occurs when incompatible brine mix and operating parameters like pressure, pH, temperature, etc. change. This may lead to decreased production output, equipment failure, and a decline in process efficacy. Of the many different types of scales known to exist, the calcium sulfate scale is particularly challenging to deal with as CaSO4is practically insoluble in hydrochloric acid, which is used to remove other types of scales, such as carbonate scales. Although numerous initiatives to inhibit calcium sulfate precipitation have been documented in the literature, it remains largely an issue. This review offers a critical analysis of the state-of-the-art regarding calcium sulfate scaling, exposing its mechanisms of precipitation, inhibition, and methods for mitigating its effects. The limits of current technologies and further research areas are also highlighted in this review.

Surface-active components in the form of intrinsic emulsifiers have been studied, and their possible role as demulsifiers has been assessed. Polar components were extracted from crude oil via three isolation methods involving liquid/liquid extraction at pH 14, liquid/liquid extraction at pH 1, and column chromatography. All three isolates were found to improve separation within hours independent of their concentration. The material isolated from acidic extraction was found to be the most efficient. An expanded chemical analysis of the isolates involving Fourier transform infrared spectroscopy, UV−vis spectroscopy, liquid chromatography–mass spectrometry, and liquid chromatography–tandem mass spectrometry analysis was employed to identify the polar compounds in the three isolates. The most polar components were found to be naphthenic acids and saturated and nonsaturated noncyclic carboxylic acids. The least polar compounds were found to be nitrogen-containing aromatic bases such as pyridines, quinolines, pyrroles, indoles, indolines, and carbazoles. A mixture of compounds with polarities between the acids and bases were identified as phenols, pyrans, benzopyrans, naphthopyrans, benzonaphtholpyrans, dibenzopyrans, dinaphthopyrans, and compounds containing both nitrogen and oxygen atoms. The fact that so many compounds are found in each extract makes it difficult to speculate on exactly what types of molecules are significantly contributing positively to the emulsion breaking and separation process. The suggested polar molecules from this work have been sufficiently precise for a follow-up study to be realistic and promising, where the dominating polar molecules in each category are synthesized and subjected to separation experiments. Finally, interesting observations were made that relate to oil maturity and origin.

Dihydroazulenes are interesting because of their photoswitching behavior. While the ring-opening to vinylheptafulvalene (VHF) is light induced, the back reaction is known to proceed thermally. In the present paper, we show the first gas phase study of the ring-opening reaction of 2-phenyl-1,8a-dihydroazulene-1,1-dicarbonitrile (Ph-DHA) by means of time-resolved photoelectron spectroscopy which permits us to follow the ring-opening process. Moreover, we investigated s-trans-Ph-VHF in a series of transient absorption experiments, supported by ab initio computations, to understand the origin of the absence of light-induced ring-closure. The transient absorption results show a biexponential decay governed by a hitherto unknown state. This state is accessed within 1–2 ps and return to the ground state is probably driven through a cis–transisomerization about the exocyclic C1C2double bond. The rapid decrease in potential energy disfavors internal rotation to s-cis-Ph-VHF, the structure that would precede the ring-closure reaction.

Light harvesting via energy storage in azobenzene has been a key topic for decades and the process of energy distribution over the molecular degrees of freedom following photoexcitation remains to be understood. Dynamics of a photoexcited system can exhibit high degrees of nonergodicity when it is driven by just a few degrees of freedom. Typically, an internal conversion leads to the loss of such localization of dynamics as the intramolecular energy becomes statistically redistributed over all molecular degrees of freedom. Here, we present a unique case where the excitation energy remains localized even subsequent to internal conversion. Strong-field ionization is used to prepare cis- and trans-azobenzene radical cations on the D1surface with little excess energy at the equilibrium neutral geometry. These D1ions are preferably formed because in this case D1and D0switch place in the presence of the strong laser field. The postionization dynamics are dictated by the potential energy landscape. The D1surface is steep downhill along the cis/transisomerization coordinate and toward a common minimum shared by the two isomers in the region of D1/D0conical intersection. Coherent cis/transtorsional motion along this coordinate is manifested in the ion transients by a cosine modulation. In this scenario, D0becomes populated with molecules that are energized mainly along the cis–transisomerization coordinate, with the kinetic energy above the cis–transinterconversion barrier. These activated azobenzene molecules easily cycle back and forth along the D0surface and give rise to several periods of modulated signal before coherence is lost. This persistent localization of the internal energy during internal conversion is provided by the steep downhill potential energy surface, small initial internal energy content, and a strong hole–lone pair interaction that drives the molecule along the cis–transisomerization coordinate to facilitate the transition between the involved electronic states.

The loss of a Cl atom from metastable CH2Cl2+in the mass-analyzed ion kinetic energy experiment is characterized by a borderline zero kinetic energy release and large kinetic isotope effects on chlorine and hydrogen. Ab initio calculations are employed to assist the interpretation in terms of a nonadiabatic reaction involving electronic predissociation of the electronically excited state 2A1and two-dimensional reaction dynamics. Strong curvature in the reaction coordinate leads to a bobsled effect that accounts for the low kinetic energy release. The kinetic isotope effects enter via the predissociation rate and are interpreted in terms of vibrational overlap integrals.

Sulfur substitution of carbonyl oxygen atoms of DNA/RNA nucleobases promotes ultrafast intersystem crossing and near-unity triplet yields that are being used for photodynamic therapy and structural-biology applications. Replacement of sulfur with selenium or tellurium should significantly red-shift the absorption spectra of the nucleobases without sacrificing the high triplet yields. Consequently, selenium/tellurium-substituted nucleobases are thought to facilitate treatment of deeper tissue carcinomas relative to the sulfur-substituted analogues, but their photodynamics are yet unexplored. In this contribution, the photochemical relaxation mechanism of 6-selenoguanine is elucidated and compared to that of the 6-thioguanine prodrug. Selenium substitution leads to a remarkable enhancement of the intersystem crossing lifetime both to and from the triplet manifold, resulting in an efficiently populated, yet short-lived triplet state. Surprisingly, the rate of triplet decay in 6-selenoguanine increases by 835-fold compared to that in 6-thioguanine. This appears to be an extreme manifestation of the classical heavy-atom effect in organic photochemistry, which challenges conventional wisdom.

The discovery that the Newman–Kwart rearrangement can be performed at room temperature by action of a simple and readily available oxidant, cerium ammonium nitrate, is described. The conditions give clean conversion when using electron-rich aromatic substrates, and the reactions are often quantitative. Computational studies support a reaction mechanism where the O-thiocarbamate is first oxidized to the radical cation, followed by nucleophilic attack by the ipsocarbon of the aromatic system.

Hybrid molecules involving subphthalocyanine and Buckminsterfullerene derivatives are interesting candidates as heavy metal free triplet sensitizers. Subphthalocyanine efficiently absorbs visible photons and transfer the singlet excited state energy to the Buckminsterfullerene where intersystem crossing produces triplet states in high yield. Thus, far the efficiency of the triplet-generating photophysics in these systems has been hampered by back energy transfer to the subphthalocyanine triplet state resulting in loss of excitation energy. Herein an efficient strategy is realized to avoid loss of triplet energy by back energy transfer. A hybrid molecule based on subphthalocyanine and Buckminsterfullerene is presented in which dispersion-induced π–π interactions result in a molecular geometry where highly efficient through-space singlet excited state energy transfer takes place in one direction, whereas energy flow in the opposite direction via the triplet manifold is blocked by lack of orbital overlap. The approach opens for a new class of heavy-metal-free triplet sensitizers of particular relevance to the fields of photodynamic therapy and noncoherent photon upconversion.

The thermally induced OBn→ SBnand OBn→ SeBnmigration reactions facilitate the rearrangement of O-benzyl thio- and selenocarbamates [BnOC(═X)NMe2] (X = S or Se) into their corresponding S-benzyl thio- and Se-benzyl selenocarbamates [BnXC(═O)NMe2] (X = S or Se). A series of substituted O-benzyl thio- and selenocarbamates were synthesized and rearranged in good yields of 33–88%. The reaction rates are higher for substrates with electron-donating groups in the 2 or 4 position of the aromatic ring, but the rearrangement also proceeds with electron-withdrawing substituents. The rearrangement follows first-order reaction kinetics and proceeds via a tight ion pair intermediate consisting of the benzylic carbocation and the thio- or selenocarbamate moiety. Computational studies support these findings.

The first study of pseudo‐bimolecular cycloaddition reaction dynamics in the gas phase is presented. We used femtosecond time‐resolved photoelectron spectroscopy (TRPES) to study the [2+2] photocycloaddition in the model system pseudo‐gem‐divinyl[2.2]paracyclophane. From X‐ray crystal diffraction measurements we found that the ground‐state molecule can exist in two conformers; a reactive one in which the vinyl groups are immediately situated for [2+2] cycloaddition and a nonreactive conformer in which they point in opposite directions. From the measured S1lifetimes we assigned a clear relation between the conformation and the excited‐state reactivity; the reactive conformer has a lifetime of 13 ps, populating the ground state through a conical intersection leading to [2+2] cycloaddition, whereas the nonreactive conformer has a lifetime of 400 ps. Ab initio calculations were performed to locate the relevant conical intersection (CI) and calculate an excited‐state [2+2] cycloaddition reaction path. The interpretation of the results is supported by experimental results on the similar but nonreactive pseudo‐para‐divinyl[2.2]paracyclophane, which has a lifetime of more than 500 ps in the S1state.

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The high-level ab initio procedures G2(+) and G2(+)[ECP(S)]have been employed in an investigation of SN2 reactions at neutral tri-coordinated phosphorus. The process has been modeled by identity and nonidentity substitution reactions involving a series of halophosphines PH2-X and halide ions. We find that the reaction proceeds without an intervening barrier by way of a tetra-coordinated phosphorus anion intermediate (X-PH2-Y−). This contrasts with the corresponding process for the carbon and nitrogen analogues, where the tetra-coordinated species is a transition structure. The threshold for inversion of the phosphorus intermediate is found to lie below the reaction energy in the cases where F−is the leaving group, but above it in all the other cases. The SN2 reaction will therefore lead to racemization when F−is expelled. In the other cases, it is possible in principle that the reaction can be controlled to proceed with inversion, but because of the relatively low barriers for inversion, this is not a very likely outcome. We predict that the tetra-coordinated intermediate should be detectable, if not isolable, and that the SN2 reaction at neutral tri-coordinated phosphorus is exothermic when the reactant halide ion is more electronegative than the product halide ion, and endothermic when the reverse applies.

The ligand exchange reactions between mono‐adducts of the sulfenium ion ([SH−X]+) and either unsaturated hydrocarbons or lone‐pair donors have been investigated computationally at the G2 level. The mono‐adducts react with acetylene or ethylene to form a thiiranium or a thiirenium ion, in most cases without an overall barrier. In the reactions involving lone‐pair donors, the original lone‐pair donor is expelled from the [SH−X]+mono‐adduct with the formation of a new mono‐adduct. The reaction proceeds in this case via an intermediate di‐adduct. Both the hydrocarbon and the lone‐pair donor attack the mono‐adduct with the relevant orbitals aligned in a near‐collinear fashion, as was also the case for previously investigated reactions involving PH2+and Cl+. The reaction energies and the binding energies of the intermediate complexes in the exchange reactions are primarily determined by the electronegativities of the lone‐pair donors. The thermochemical data can be rationalized within the framework of qualitative molecular orbital theory, and the results are compared with our previous findings for the corresponding reactions involving PH2+and Cl+.

The chemistry of mono-adducts ([Cl–X]+) between Cl+and a Lewis base (X = NH3, H2O, HF, PH3, H2S or HCl) has been investigated using ab initiomolecular orbital calculations at the G2 level. The reactions of such mono-adducts with additional Lewis bases (Y) are found to give [Y–Cl]+plus X, generally without an intermediate barrier, via a bis-adduct [Y–Cl–X]+. The binding energies of the bis-adduct and the reaction energies are related to the donor properties of the Lewis bases. The reactions between the mono-adducts [Cl–X]+and the π-donors ethylene and acetylene yield chloriranium and chlorirenium ions, respectively. These reactions proceed via complexes that resemble either the reactants or products depending on the sign of the reaction energy, the latter in turn being determined by the donor ability of the Lewis base. Results for the chlorine systems are compared with those for the corresponding phosphorus systems investigated previously.

Pi‐ligand exchange is predicted to be experimentally feasiblefor thiirenium and thiiranium ions by a G2 ab initio investigation (see figure). The reaction is also predicted to be stereospecific with respect to the configuration at sulfur. Comparisons are made with the analogous phosphorus systems.

The dissociation of metastable &z.rad;CH₂NH₂CH₃+ ions into CH₃&z.rad; and CH₂&z.dbd6;NH₂+ is accompanied by a remarkably high translational energy release, demonstrating that the addition of a methyl radical to the N-terminus of CH₂&z.dbd6;NH₂+ has an appreciable energy barrier. Isotope labelling shows the reaction to be highly specific. Alkyl radical loss from a number of substituted α-distonic ions was also examined; the kinetic energy release accompanying loss of ethyl radicals from N-ethyl α-distonic ions, CH₃CH₂&z.Nplus;H₂&z.Cdot;HR, varies with the nature of R. The energy barrier for dissociation and the relative energies of the five C₂H₇N&z.rad;+ isomers was examined with the G2(MP2) and CBS-Q composite theoretical methods.

A collinear approachto the sulfur atom of thiiranium and thiirenium ions (see scheme) is predicted by theoretical calculations for both σ and π nucleophiles.