Data from GCA article "Thermodynamic property estimations for aqueous primary, secondary, and tertiary alkylamines, benzylamines, and their corresponding aminiums across temperature and pressure are validated by measurements from experiments"
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Abstract: On Earth and beyond, organic chemistry often occurs in the presence of water within environments that deviate drastically from ambient conditions (25°C and 1 bar). Accurately predicting aqueous organic reaction pathways is crucial toward understanding planetary scale processes such as the cycling of elements crucial for life (e.g., carbon and nitrogen). Advanced thermodynamic modeling can be utilized to determine the favorability of various organic reactions based on geologically relevant ranges of temperature and pressure, as well as compositional variables (e.g., pH). However, data that would otherwise allow for a diversity of organic compounds and environmental conditions to be modeled are sparse and rarely tested with experiments, particularly with regard to organic-nitrogen compounds. In this work, we develop a framework to estimate thermodynamic properties at ambient conditions that can then be extrapolated across ranges of temperature and pressure for aqueous primary, secondary, and tertiary amines and aminiums (protonated amines), specifically those structures containing linear alkyl chains and benzyl functional groups. We also performed hydrothermal experiments (250°C, ~40 bar) involving reactions of methylamines to test our resulting thermodynamic models, and we compare our models for other alkylamines and benzylamines to previous empirical measurements from the literature. Specifically, we use existing thermodynamic data along with our estimates at ambient conditions in combination with a variety of existing extrapolation methods related to the revised Helgeson-Kirkham-Flowers (HKF) equations of state to generate temperature- and pressure-dependent predictions of acid dissociation constants (i.e., pKa values) that strongly agree with previous empirical measurements. We use similar methods to predict product distributions for reactions involving primary, secondary, and tertiary amines/aminiums, as well as ammonia/ammonium and corresponding alcohols whose collective distributions depend on reversible substitution reactions. Our predictions are in good agreement with our experimental results involving the methylamine reaction system as well as previous experiments involving the benzylamine reaction system, for which we also produced thermodynamic estimates involving benzyl alcohol. The agreement between independent theoretical predictions and experimental measurements suggests that our estimated properties can be applied to modeling amine chemistry in other experimental and natural aqueous systems that range in temperature and pressure, providing new tools for planetary exploration.
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Thermodynamic Estimations: In this study, we first developed an estimation scheme for thermodynamic properties at 25°C and 1 bar for primary amines (R-NH2), secondary amines (R-NH-R), and tertiary amines (NR3), as well as their protonated forms (e.g., R-NH3+), where R represents a linear alkyl group containing at least one methylene group (CH3 [CH2]n, where whole numbers n ≥ 1, n ≤ 8). Next, we slightly modified those 25°C and 1 bar estimation strategies to accommodate other R groups, including methyl (CH3) and benzyl (C6H5-CH2) groups. We then used existing methods to calculate revised Helgeson-Kirkham-Flowers (HKF) equation of state parameters that allow for extrapolation of property values from 25°C and 1 bar across wide ranges of temperature (0 350°C) and pressure (1-5000 bar). Lastly, we compare thermodynamic predictions resulting from these estimation strategies to empirical data from experiments in previous literature as well as newly presented experimental data in the current study. Experimental Setup and Analysis: Experiments were performed by placing reaction vessels into tube furnaces that were preheated to 250°C (Psat of 40 bar), with spatial and temporal variations of ± 2.5°C. Ten experiments were performed with durations (i.e., heating times) up to 264 days to observe changing reactant and product abundances over time. Upon completion of experiments, the hot reaction vessels were submerged in room-temperature water to quench the reactions. The glass vessels were cracked open, and aqueous aliquots were removed and diluted by a factor of 200 for analysis of ammonium, methylaminium, dimethylaminium, and trimethylaminium total concentrations (i.e., sum of amine and aminium). These diluted samples were then analyzed on a Dionex DX-500 ion chromatograph, utilizing a Dionex CS16 column with 120 mmolal methanesulfonic acid eluent, and Dionex CDRS 600 suppressor with 18.2 MΩ deionized water regenerant. Product quantification was achieved via five-point linear standard calibration curves run with each ion chromatography sequence of experimental samples. Separate aqueous aliquots were removed from post-experimental solutions for the analysis of methanol concentrations. Methanol analysis was performed via gas chromatography (GC) utilizing an Alltech ECONO-CAP EC-Wax column and flame ionization detection (FID), along with MSD Chemstation software for GC-FID programming and peak integration. Methanol quantification was achieved via three-point linear standard calibration curves run with each GC-FID sequence of experimental samples. For all chromatography analyses, retention time matching with authentic standards was used for product identification; in some cases, standard compound addition to post-experimental solutions was also used.