Thermodynamic properties of 2–methylindole: Experimental and computational results for gas-phase entropy and enthalpy of formation☆
Introduction
This work is part of our continuing research [1], [2], [3], [4], [5], [6], [7], [8], [9], [10] into quantification of uncertainties for thermodynamic properties derived with computational methods. In previous studies, the focus has been on entropies for the ideal-gas state. In the present work, comparison of experimental and computed enthalpies of formation for the ideal-gas state is also considered.
Entropies and enthalpies of formation for the ideal-gas state can be derived with structural information and computational methods, as well as through appropriate combination of experimentally determined properties. These methods are independent, and their study allows for mutual validation through analysis of observed differences. Reliable ideal-gas properties have key roles in property predictions, thermodynamic-consistency analyses, constrained property extrapolations, and they form the basis of important equation-of-state formulations, which are expressed as deviations from the ideal-gas state [11]. As noted previously [3], the ability to derive ideal-gas properties solely from computational methods with reliable uncertainties would provide key values that are essentially unobtainable experimentally for many materials due to reasons such as high expense, high toxicity, or low stability.
This article describes thermodynamic property measurements for 2-methylindole (Chemical Abstracts registry number [95–20–5]). A summary of the experiments is provided in Table 1. Entropies for a wide range of temperatures (298.15 ≤ T/K ≤ 700) and the enthalpy of formation at T/K = 298.15 for the ideal-gas state are derived from the thermophysical property measurements. These are compared with values calculated independently with the methods of computational chemistry. This article follows our recent work on a series of methyl-substituted pyrroles [10], where excellent accord between experimental and computed ideal-gas entropies was obtained. The present work provides a further test of the efficacy of computations for molecules containing a pyrrolic ring with the addition of the fused phenyl ring in 2-methylindole. This work also serves as a precursor to research on 3-ring carbazole systems.
Section snippets
Materials
The sample of 2-methylindole used in this research was obtained by purification of a commercial product (Aldrich). Purification was carried out by the research group of Professor E. J. “Pete” Eisenbraun (retired) of Oklahoma State University. Commercial 2-methylindole (184 g) was passed through a column of basic alumina (3 cm wide by 5 cm long) contained in a Soxhlet apparatus using hexane as solvent under an argon atmosphere to yield a nearly colorless material. A deep red picrate was prepared
Heat capacities and properties of melting determined with adiabatic calorimetry
Crystals of 2-methylindole were prepared by slow cooling (∼1 mK⋅s–1) the sample to ∼3 K below Ttp, where the sample crystallized. Slow cooling was continued to ∼20 K below Ttp. Complete crystallization was achieved by reheating and maintaining the sample under adiabatic conditions in the partially melted state (∼20 percent liquid) until ordering of the crystals was complete, as evidenced by the absence of spontaneous warming. The sample of 2-methylindole warmed slowly for approximately 4 h,
Comparisons with literature densities and properties of melting
As noted in Section 3.3, only two values for the density of 2-methylindole in the liquid phase have been reported [43], [44], and these were used in the present research with the Riedel equation {Eq. (5)} to estimate densities required to evaluate enthalpies of vaporization with the Clapeyron equation {Eq. (6)}. The values reported by von Auwers and Susemihl [43] and Yokoyama et al. [44] are in mutual accord, as noted earlier, and are consistent with with the approximate value determined in
Conclusions
In our work on a variety of aromatic ring systems [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], we have shown that calculations performed at the B3LYP/6-31+G (d,p) model chemistry with the scale factor (0.975 ± 0.005) can be applied for computation of entropies in the ideal-gas state with relative expanded uncertainties (0.95 level of confidence) near 0.2 percent. Most recently, we extended this type of analysis to a series of single-ring pyrrolic compounds (pyrrole, 1-methylpyrrole,
Acknowledgments
We gratefully acknowledge the contributions of Stephen E. Knipmeyer in the d.s.c. studies, An (Andy) Nguyen in the vapor-pressure measurements, Norris K. Smith in the combustion calorimetric measurements, I. Alex Hossenlopp in vapor transfer of chemical samples in preparation for the physical property measurements, and William V. Steele for helpful guidance to these experimentalists. The authors acknowledge the financial support of the Office of Fossil Energy of the U.S. Department of Energy
References (83)
- et al.
J. Chem. Thermodyn.
(2007) - et al.
J. Chem. Thermodyn.
(2009) - et al.
J. Chem. Thermodyn.
(2010) - et al.
J. Chem. Thermodyn.
(2010) - et al.
J. Chem. Thermodyn.
(2012) - et al.
J. Chem. Thermodyn.
(2014) - et al.
J. Chem. Thermodyn.
(2015) - et al.
J. Chem. Thermodyn.
(2015) - et al.
J. Chem. Thermodyn.
(2016) - et al.
J. Chem. Thermodyn.
(2018)
J. Chem. Thermodyn.
J. Chem. Thermodyn.
J. Chem. Thermodyn.
J. Chem. Thermodyn.
J. Chem. Thermodyn.
J. Chem. Thermodyn.
Anal. Chim. Acta
Cryogenics
J. Chem. Thermodyn.
J. Chem. Thermodyn.
Spectrochim. Acta A: Mol. Biomol. Spectrosc.
J. Chem. Thermodyn.
J. Chem. Thermodyn.
M.J.S. Monte, M. J. S
J. Chem. Thermodyn.
J. Chem. Inf. Model.
Pure Appl. Chem.
J. Phys. Chem. Ref. Data
Metrologia
Pure Appl. Chem.
Rev. Sci. Instrum.
J. Phys. Chem. Ref. Data
Ebulliometric Measurements
J. Chem. Eng. Data
J. Phys. Chem. Ref. Data
J. Sci. Instrum.
J. Chem. Eng. Data
J. Chem. Eng. Data
J. Phys. Chem.
J. Phys. Chem.
Cited by (8)
Comprehensive thermodynamic study of substituted indoles/perhydro indoles as potential liquid organic hydrogen carrier system
2023, FuelCitation Excerpt :Experimental details and results of the transpiration method for indole derivatives are given in Table 1. Only few vapor pressure studies of methyl-indoles have been found in the literature [9,20,24]. In our recent work, [9] we studied vapor pressures over the crystal sample of 2–methyl-indole to validate the questionable sublimation thermodynamics of this compound.
Phase Transition Enthalpy Measurements of Organic Compounds. An Update of Sublimation, Vaporization, and Fusion Enthalpies from 2016 to 2021
2022, Journal of Physical and Chemical Reference DataReconciliation of Experimental and Computed Thermodynamic Properties for Methyl-Substituted 3-Ring Aromatics. Part 2: 3-Methylphenanthrene
2022, Journal of Chemical and Engineering DataReconciliation of Experimental and Computed Thermodynamic Properties for Methyl-Substituted 3-Ring Aromatics. Part 1: 9-Methylanthracene
2022, Journal of Chemical and Engineering DataThermochemical Properties and Dehydrogenation Thermodynamics of Indole Derivates
2020, Industrial and Engineering Chemistry ResearchMultireaction Approach to Quantum Thermochemistry
2020, Journal of Physical Chemistry A
- ☆
This article belongs to Virtual Special Issue: G. Kabo’s birthday issue.