Theoretical Study of Decomposition of Methanediol in Aqueous Solution

Rotamers of Methanediol: Composite Ab Initio Predictions of Structures, Frequencies, and Rovibrational Constants

Geminal diols are known to be important intermediates in atmospheric ozonolysis and the aerosol cycle. Recently, the simplest member of this class, methanediol, was interrogated in the gas phase with infrared spectroscopy. To aid in future spectroscopic investigations of methanediol, including in the interstellar medium, we report fundamental frequencies and rovibrational constants for the two rotamers of this molecule using ab initio composite methods along with vibrational perturbation theory. Sensitivity of the predictions to the level of theory and the treatment of anharmonic resonances are carefully assessed. The OH stretching harmonic frequencies of both rotamers are particularly sensitive to the level of theory. The CH stretches of the C_s rotamer are sensitive to the treatment of anharmonic resonances with VPT2-based effective Hamiltonian models. Equilibrium bond distances and harmonic frequencies are converged conservatively to within 0.0005 Å and 3 cm^-1, respectively. The effect of tunneling on the rotational constants is investigated with a 2D variational calculation, based on a relaxed hydroxyl torsional potential energy surface. Tunneling is found to be negligible in the lower energy C₂ rotamer but should modify the rotational constants of the C_s rotamer on the order of MHz, giving rise to rotational line splittings of the same order. The rovibrational constants of the C_s rotamer are dominated by hydroxyl torsional effects, and here we see evidence for the breakdown of vibrational perturbation theory.

Accurate Mass Identification of an Interfering Water Adduct and Strategies in Development and Validation of an LC-MS/MS Method for Quantification of MPI8, a Potent SARS-CoV-2 Main Protease Inhibitor, in Rat Plasma in Pharmacokinetic Studies

MPI8, a peptidyl aldehyde, is a potent antiviral agent against coronavirus. Due to unique tri-peptide bonds and the formyl functional group, the bioassay of MPI8 in plasma was challenged by a strong interference from water MPI8. Using QTOF LC-MS/MS, we identified MPI8•H2O as the major interference form that co-existed with MPI8 in aqueous and biological media. To avoid the resolution of MPI8 and MPI8•H2O observed on reverse phase columns, we found that a Kinetex hydrophilic interaction liquid chromatography (HILIC) column provided co-elution of both MPI8 and MPI8•H2O with a good single chromatographic peak and column retention of MPI8 which is suitable for quantification. Thus, a sensitive, specific, and reproducible LC-MS/MS method for the quantification of MPI8 in rat plasma was developed and validated using a triple QUAD LC-MS/MS. The chromatographic separation was achieved on a Kinetex HILIC column with a flow rate of 0.4 mL/min under gradient elution. The calibration curves were linear (r2 > 0.99) over MPI8 concentrations from 0.5−500 ng/mL. The accuracy and precision are within acceptable guidance levels. The mean matrix effect and recovery were 139% and 73%, respectively. No significant degradation of MPI8 occurred under the experimental conditions. The method was successfully applied to a pharmacokinetic study of MPI8 after administration of MPI8 sulfonate in rats.

Rapid preparation of gaseous methanediol (CH2(OH)2)

The simplest geminal diol CH₂(OH)₂ serves as an important precursor to form atmospheric formic acid. CH₂(OH)₂ vapour can be generated by the evaporation of an aqueous formaldehyde solution, prepared by dissolving paraformaldehyde under reflux. Its rovibrational feature at 980-1100 cm^-1 is consistent with the simulation and free of the intense interferences of H₂O and CH₂O.

Synthesis of methanediol [CH2(OH)2]: The simplest geminal diol

Methanediol [CH₂(OH)₂] represents a pivotal atmospheric volatile organic compound and plays a fundamental role in aerosol growth. Although sought for decades, methanediol has never been identified due to the inherent dehydration tendency of two adjacent hydroxyl groups (OH) at the same carbon atom. Here, we prepare and identify methanediol via processing of low-temperature ices followed by sublimation into the gas phase. These findings open up a concept to synthesize and characterize unstable geminal diols—critical organic transients in Earth’s atmosphere. The excited state dynamics of oxygen may also lead to methanediol in methanol-rich interstellar ices in cold molecular clouds, followed by sublimation in star-forming regions and prospective detection of these reactive intermediates in the gas phase by radiotelescopes.

Geminal diols—organic molecules carrying two hydroxyl groups at the same carbon atom—have been recognized as key reactive intermediates by the physical (organic) chemistry and atmospheric science communities as fundamental transients in the aerosol cycle and in the atmospheric ozonolysis reaction sequence. Anticipating short lifetimes and their tendency to fragment to water plus the aldehyde or ketone, free geminal diols represent one of the most elusive classes of organic reactive intermediates. Here, we afford an exceptional glance into the preparation of the previously elusive methanediol [CH₂(OH)₂] transient—the simplest geminal diol—via energetic processing of low-temperature methanol–oxygen ices. Methanediol was identified in the gas phase upon sublimation via isomer-selective photoionization reflectron time-of-flight mass spectrometry combined with isotopic substitution studies. Electronic structure calculations reveal that methanediol is formed via excited state dynamics through insertion of electronically excited atomic oxygen into a carbon–hydrogen bond of the methyl group of methanol followed by stabilization in the icy matrix. The first preparation and detection of methanediol demonstrates its gas-phase stability as supported by a significant barrier hindering unimolecular decomposition to formaldehyde and water. These findings advance our perception of the fundamental chemistry and chemical bonding of geminal diols and signify their role as an efficient sink of aldehydes and ketones in atmospheric environments eventually coupling the atmospheric chemistry of geminal diols and Criegee intermediates.

Acid–Base Catalytic Effects on Reduction of Methanol in Hot Water

We have performed a number of quantum chemical simulations to examine the reduction process of methanol in hot water. Methanol is converted into a methane by capturing a hydrogen molecule and leaving a water molecule behind. The required energy for the reduction is too high to proceed in the gas phase. The energy barrier for the reduction of methanol is reduced by the catalytic effect of water molecules when we consider the reduction in aqueous solution. However, the calculated reduction rate is still much slower than that found experimentally. The ion product of water tends to increase in hot water, even though it eventually decreases at the high temperature of supercritical water. It is valuable to consider the acid–base catalytic effects on the reduction of methanol in hot water. The significant reduction of the energy barrier is accomplished by the acid–base catalytic effects due to hydronium or hydroxyde. Mean collision time between a hydronium and a methanol in hot water is shorter than the reduction time, during which a methanol is converted into a methane. The calculated reduction rate with the acid–base catalytic effects agrees well with that determined by laboratory experiments. The present study reveals a crucial role of the acid–base catalytic effects on reactions in hot water.

Primary Formation Path of Formaldehyde in Hydrothermal Vents

Formaldehyde is abundant in the universe and one of the fundamental molecules for life. Hydrothermal vents produce a substantial amount of hydrogen molecules by serpentinization and promote reductive reactions of single carbon compounds. The abundance of formaldehyde is expected to be low due to the high Gibbs free energy in hydrothermal vents. We consider two competing formation pathways of formaldehyde: (1) the reduction of CO by H₂ and (2) the reduction of HCOOH by H₂ to form a methanediol, followed by the dehydration of the methanediol. We performed a number of quantum chemical simulations to examine the formation of formaldehyde in the gas phase as well as in aqueous solution. The energy barrier is significantly reduced by the catalytic effect of water molecules in aqueous solution and becomes lowest when a water cluster consisted of 5 water molecules catalyzes the reduction. The energy barrier to form a methanediol by the reduction of HCOOH is lower by 17.5 kcal/mol than that to form a formaldehyde by the reduction of CO. Considering the low energy barrier to dehydrate methanediol, the primary pathway to form formaldehyde in hydrothermal vents is concluded to be the reduction of HCOOH by H₂, followed by the dehydration of methanediol.

Origin of Acid-Base Catalytic Effects on Formaldehyde Hydration

The mechanisms of hydronium- and hydroxide-catalyzed formaldehyde hydrations were investigated by quantum mechanical/molecular mechanical molecular dynamics in combination with flexible coordinates. A stepwise bimolecular and a concerted termolecular mechanism were found with a hydronium catalyst. The latter is more favorable and better consistent with experiment. Structurally, a dipole-bound species initially arranges the nucleophile in a favorable configuration for both routes, significantly enhancing the reactive collisions. On the one hand, the hydronium catalyst also plays a role of a reactant in the bimolecular path. On the other hand, only a stepwise mechanism was found with a hydroxide catalyst. Overall, hydroxide is a stronger catalyst than a hydronium when it is in contact distance with formaldehyde.