Roman M. Balabin

For the last quarter of century, the topic of chemistry of radical cations (RACs) is expanding at an extreme rate. Produced via single-electron transfer by photo-redox or electrochemical methods, RACs attract attention of organic chemists due to the diversity of possible chemical rearrangements. Thorough theoretical description of RACs is a key to better understanding their properties and predicting chemical behaviors. Unfortunately, the currently available quantum chemical methods were mostly parameterized on the ground-state neutral closed-shell system. As a result, their behavior in RACs remains hard to validate. Our literature analysis indicates that the accuracy of many commonly used methods is rather low, thus putting the chemical interpretation under question. Here we explain the methodology that could be used to evaluate the reliability (and modification, if necessary) of the density functional theory methods to produce realistic models of chemical rearrangements involving RACs. We used water dimer radical cations (WDRACs) as a model system. Recent experimental studies have shown that WRDACs display rather unexpected reactivity toward inert molecules, including catalytic-free nitrogen fixation, or ambient methane-to-methanol conversion. We demonstrate that the properly chosen broadly available models show reasonable agreement with experimental data and help to visualize excited-state chemistry behind RAC-transformations.

Methane combustion contributes 30% to global warming, making its conversion to cleaner chemicals a critical challenge. Here, we report a catalyst-free ambient conversion of methane to methanol with a remarkable yield exceeding 650 millimoles per hour and a high conversion rate of ~35%, achieved using in situ–generated water dimer radical cations, ( H 2 O ) 2 + · , in a specially designed batch reactor operating under mild conditions. In a single run, the reactor was typically operated for 10 minutes. The atom efficiency for hydrogen in our approach for methanol synthesis is ~67%. Hydrogen gas is the by-product, produced at a rate of over 2 millimoles per hour. Compared to the previously reported catalytic methods, our production yields are up by between 10 ² and 10 ⁶ times depending on the literature method. This breakthrough enables clean, energy-efficient methane conversion without unwanted oxidation by-products, offering substantial environmental and economic benefits beyond combustion.

Hydroxyl quaternization through the direct oxidation reaction of aromatic amines, facilitated by water radical cations within water microdroplets, is explored.

The growth and sustainable development of humanity is heavily dependent upon molecular nitrogen (N₂) fixation. Herein we discover ambient catalyst-free disproportionation of N₂ by water plasma which occurs via the distinctive HONH-HNOH^+• intermediate to yield economically valuable nitroxyl (HNO) and hydroxylamine (NH₂OH) products. Calculations suggest that the reaction is prompted by the coordination of electronically excited N₂ with water dimer radical cation, (H₂O)₂^+•, in its two-center-three-electron configuration. The reaction products are collected in a 76-needle array discharge reactor with product yields of 1.14 μg cm^–2 h^–1 for NH₂OH and 0.37 μg cm^–2 h^–1 for HNO. Potential applications of these compounds are demonstrated to make ammonia (for NH₂OH), as well as to chemically react and convert cysteine, and serve as a neuroprotective agent (for HNO). The conversion of N₂ into HNO and NH₂OH by water plasma could offer great profitability and reduction of polluting emissions, thus giving an entirely look and perspectives to the problem of green N₂ fixation.

A joint community effort to critically evaluate quantum chemical approaches to the prediction of vibrational shifts of hydrates in the gas phase.

The growth and sustainable development of humanity is heavily dependent upon the process of fixing nitrogen (N2) to ammonia (NH3). However, the currently adopted methods are associated with severe environmental hazards and tremendous energy costs, which limit their sustainability and profitability. Herein we discovered a catalyst-free disproportionation reaction of N2 by water dimer radical cation, (H2O)2+•, which occurs under mild ambient conditions via distinctive HONH-HNOH+• intermediate to yield economically valuable nitroxyl (HNO) and hydroxylamine (NH2OH) products, in alternative to NH3. Calculations suggest that the reaction is prompted by the coordination of electronically excited N2 with (H2O)2+• in its two-center-three-electron (2c-3e) configuration. Subsequent excited-state double proton transfer leads to one-step water addition to N2. The ambient fixation of N2 into HNO and NH2OH with high selectivity offers great profitability and total avoidance of polluting emissions, such as CO2 or NOy, thus giving an entirely new look and perspectives to the problem of green N2 fixation.

Modern analytical chemistry of industrial products is in need of rapid, robust, and cheap analytical methods to continuously monitor product quality parameters. For this reason, spectroscopic methods are often used to control the quality of industrial products in an on-line/in-line regime. Vibrational spectroscopy, including mid-infrared (MIR), Raman, and near-infrared (NIR), is one of the best ways to obtain information about the chemical structures and the quality coefficients of multicomponent mixtures. Together with chemometric algorithms and multivariate data analysis (MDA) methods, which were especially created for the analysis of complicated, noisy, and overlapping signals, NIR spectroscopy shows great results in terms of its accuracy, including classical prediction error, RMSEP. However, it is unclear whether the combined NIR + MDA methods are capable of dealing with much more complex interpolation or extrapolation problems that are inevitably present in real-world applications. In the current study, we try to make a rather general comparison of linear, such as partial least squares or projection to latent structures (PLS); “quasi-nonlinear”, such as the polynomial version of PLS (Poly-PLS); and intrinsically non-linear, such as artificial neural networks (ANNs), support vector regression (SVR), and least-squares support vector machines (LS-SVM/LSSVM), regression methods in terms of their robustness. As a measure of robustness, we will try to estimate their accuracy when solving interpolation and extrapolation problems. Petroleum and biofuel (biodiesel) systems were chosen as representative examples of real-world samples. Six very different chemical systems that differed in complexity, composition, structure, and properties were studied; these systems were gasoline, ethanol–gasoline biofuel, diesel fuel, aromatic solutions of petroleum macromolecules, petroleum resins in benzene, and biodiesel. Eighteen different sample sets were used in total. General conclusions are made about the applicability of ANN- and SVM-based regression tools in the modern analytical chemistry. The effectiveness of different multivariate algorithms is different when going from classical accuracy to robustness. Neural networks, which are capable of producing very accurate results with respect to classical RMSEP, are not able to solve interpolation problems or, especially, extrapolation problems. The chemometric methods that are based on the support vector machine (SVM) ideology are capable of solving both classical regression and interpolation/extrapolation tasks.

We present a combined experimental and theoretical study to determine the gas-phase basicity of several relatively small organic molecules that are used as matrices in MALDI: alpha-cyano-4-hydroxycynnamic acid (αCHCA), ferulic acid (FA), 3-hydroxypicolinic acid (HPA), sinapinic acid (SA), and 2,4,6-trihydroxyacetophenone (THAP). Our simple and fast experimental approach utilizes the ability of an electrosonic spray ionization (ESSI) source to produce high densities of gas-phase ions at ambient conditions, so that their gas-phase thermochemical properties can be probed in a bracketing-like experiment at atmospheric pressure, with no need for high vacuum. The reaction between the analyte and reference bases is monitored by a commercial mass spectrometer. We determined the following GB values of the MALDI matrices studied: 908.1 ± 0.8 (αCHCA), 863.8 ± 8.0 (FA), 876.3 ± 10.2 (HPA), 867.4 ± 1.4 (SA), and 864.2 ± 7.6 (THAP) kJ mol⁻¹. A good agreement between the experimentally measured and computationally predicted (DFT-B3LYP, G4MP2) gas-phase basicities for the studied compounds is found, except for αCHCA for which the calculations resulted in approximately 80 kJ mol⁻¹ lower GB. We attribute the observed discrepancy between the measured and calculated values to a gas-phase rearrangement of αCHCA into a heterocyclic aromatic isomer, 7-hydroxyquinoline-3-carboxylic acid (HQA). HQA is thermodynamically more stable and features higher GB than that of αCHCA (calculated at approximately 920 kJ mol⁻¹). The gas-phase basicity values of THAP and HPA are reported for the first time.

High-level ab initio electronic structure calculations up to the CCSD(T) theory level, including extrapolations to the complete basis set (CBS) limit, resulted in high precision energetics of the tautomeric equilibrium in 2-substituted acetaldehydes (XH₂C-CHO). The CCSD(T)/CBS relative energies of the tautomers were estimated using CCSD(T)/aug-cc-pVTZ, MP3/aug-cc-pVQZ, and MP2/aug-cc-pV5Z calculations with MP2/aug-cc-pVTZ geometries. The relative enol (XHC = CHOH) stabilities (ΔE _{e,CCSD(T)/CBS}) were found to be 5.98 ± 0.17, −1.67 ± 0.82, 7.64 ± 0.21, 8.39 ± 0.31, 2.82 ± 0.52, 10.27 ± 0.39, 9.12 ± 0.18, 5.47 ± 0.53, 7.50 ± 0.43, 10.12 ± 0.51, 8.49 ± 0.33, and 6.19 ± 0.18 kcal mol⁻¹ for X = BeH, BH₂, CH₃, Cl, CN, F, H, NC, NH₂, OCH₃, OH, and SH, respectively. Inconsistencies between the results of complex/composite energy computations methods Gn/CBS (G2, G3, CBS-4M, and CBS-QB3) and high-level ab initio methods (CCSD(T)/CBS and MP2/CBS) were found. DFT/aug-cc-pVTZ results with B3LYP, PBE0 (PBE1PBE), TPSS, and BMK density functionals were close to the CCSD(T)/CBS levels (MAD = 1.04 kcal mol⁻¹).