Thermodynamic properties of 2–methylindole: Experimental and computational results for gas-phase entropy and enthalpy of formation

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Highlights

  • Heat capacities were measured for the temperatures < (T/K) < 700 for 2-methylindole.

  • The enthalpy of formation of 2-methylindole was determined experimentally.

  • The critical temperature was determined experimentally for 2-methylindole.

  • Vapor pressures are reported for the temperature range 340 < (T/K) < 595.

  • Ideal-gas properties through experiment and computation are in accord.

Abstract

Measurements leading to the calculation of thermodynamic properties in the ideal-gas state for 2-methylindole (Chemical Abstracts registry number [95–20–5]) are reported. Experimental methods were adiabatic heat-capacity calorimetry, differential scanning calorimetry (d.s.c.), comparative ebulliometry, inclined-piston manometry, and oxygen bomb calorimetry. The critical temperature of 2-methylindole was determined experimentally with d.s.c. Molar thermodynamic functions for the condensed and ideal-gas states were derived from the experimental results. Statistical calculations were performed based on molecular geometry optimization and vibrational frequencies using B3LYP hybrid density functional theory with the def2-TZVPPD basis set. Excellent accord between computed and experimentally-derived ideal-gas entropies is shown. The enthalpy of formation for 2-methylindole in the gas phase was computed with an atomization-based protocol described recently, and excellent agreement with the experimental values is seen. The experimental literature for enthalpies of formation in the gas phase for 1- and 2-ring pyrrollic compounds is reviewed, and comparisons with computed values further support the findings here. All experimental results are compared with property values reported in the literature, where possible.

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)

  • R.D. Chirico et al.

    J. Chem. Thermodyn.

    (2007)
  • R.D. Chirico et al.

    J. Chem. Thermodyn.

    (2009)
  • R.D. Chirico et al.

    J. Chem. Thermodyn.

    (2010)
  • R.D. Chirico et al.

    J. Chem. Thermodyn.

    (2010)
  • R.D. Chirico et al.

    J. Chem. Thermodyn.

    (2012)
  • R.D. Chirico et al.

    J. Chem. Thermodyn.

    (2014)
  • R.D. Chirico et al.

    J. Chem. Thermodyn.

    (2015)
  • R.D. Chirico et al.

    J. Chem. Thermodyn.

    (2015)
  • R.D. Chirico et al.

    J. Chem. Thermodyn.

    (2016)
  • R.D. Chirico et al.

    J. Chem. Thermodyn.

    (2018)
  • W.V. Steele et al.

    J. Chem. Thermodyn.

    (2003)
  • W.V. Steele et al.

    J. Chem. Thermodyn.

    (1988)
  • S.C. Mraw et al.

    J. Chem. Thermodyn.

    (1979)
  • W.V. Steele

    J. Chem. Thermodyn.

    (1995)
  • W.V. Steele et al.

    J. Chem. Thermodyn.

    (2004)
  • N.K. Smith et al.

    J. Chem. Thermodyn.

    (1980)
  • J.P. McCullough et al.

    Anal. Chim. Acta

    (1957)
  • W. Wagner

    Cryogenics

    (1973)
  • J.L. Hales et al.

    J. Chem. Thermodyn.

    (1972)
  • R.D. Chirico et al.

    J. Chem. Thermodyn.

    (1990)
  • S.A. Popoola

    Spectrochim. Acta A: Mol. Biomol. Spectrosc.

    (2018)
  • M.A.V. Ribeiro da Silva et al.

    J. Chem. Thermodyn.

    (2009)
  • R.D. Chirico et al.

    J. Chem. Thermodyn.

    (1999)
  • A.R.R.P. Almeida

    M.J.S. Monte, M. J. S

    J. Chem. Thermodyn.

    (2014)
  • V. Diky et al.

    J. Chem. Inf. Model.

    (2007)
  • J. Meija et al.

    Pure Appl. Chem.

    (2016)
  • P.J. Mohr et al.

    J. Phys. Chem. Ref. Data

    (2016)
  • Comité International des Poids et Mesures

    Metrologia

    (1969)
  • R.N. Goldberg et al.

    Pure Appl. Chem.

    (1992)
  • F.L. McCrackin et al.

    Rev. Sci. Instrum.

    (1975)
  • D.G. Archer

    J. Phys. Chem. Ref. Data

    (1993)
  • W.V. Steele, R.D. Chirico, S.E. Knipmeyer, N.K. Smith, Report NIPER-360, December 1988. Published by DOE Fossil Energy,...
  • W. Swietoslawski

    Ebulliometric Measurements

    (1945)
  • A.G. Osborn et al.

    J. Chem. Eng. Data

    (1966)
  • W. Wagner et al.

    J. Phys. Chem. Ref. Data

    (2002)
  • D.R. Douslin, J.P. McCullough, US Bureau of Mines. Report of Investigation 6149, 1963, p....
  • D.R. Douslin et al.

    J. Sci. Instrum.

    (1965)
  • W.D. Good et al.

    J. Chem. Eng. Data

    (1970)
  • W.D. Good

    J. Chem. Eng. Data

    (1972)
  • W.D. Good et al.

    J. Phys. Chem.

    (1956)
  • W.D. Good et al.

    J. Phys. Chem.

    (1959)
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