کد مقاله کد نشریه سال انتشار مقاله انگلیسی نسخه تمام متن
216669 1426302 2009 6 صفحه PDF دانلود رایگان
عنوان انگلیسی مقاله ISI
Revision of standard molar enthalpies of formation of glycine and l-alanine in the gaseous phase on the basis of theoretical calculations
موضوعات مرتبط
مهندسی و علوم پایه مهندسی شیمی مهندسی شیمی (عمومی)
پیش نمایش صفحه اول مقاله
Revision of standard molar enthalpies of formation of glycine and l-alanine in the gaseous phase on the basis of theoretical calculations
چکیده انگلیسی

The standard molar enthalpies of formation of urea, glycine, and l-alanine in the gaseous phase at 298.15 K were calculated by the high-level Gaussian-3X method. The agreement with the available experimental data is very good for urea and glycine and, thus, supports the high accuracy of calculated values. A significant discrepancy between theoretical and experimental enthalpy of formation values for l-alanine provides a reason to reconsider the experimental data previously used to derive the standard molar enthalpy of formation of l-alanine in the gaseous phase at 298.15 K. To obtain a more reliable value of enthalpy of sublimation at 298.15 K, the heat capacity values of gaseous l-alanine were calculated by standard statistical thermodynamics formulae using molecular parameters determined from B3LYP/cc-pVTZ calculations. With the obtained value of Cp,m∘ (l-alanine, g, 298.15 K) = 112.6 ± 4.0 J · K−1 · mol−1 the original published experimental values of enthalpy of sublimation of l-alanine were readjusted to the reference temperature: ΔcrgHm (l-alanine, 298.15 K) = 135.2 ± 2.0 kJ · mol−1. This value, together with the experimental enthalpy of formation of solid l-alanine, ΔfHm∘ (l-alanine, cr, 298.15 K) = −560.0 ± 1.0 kJ · mol−1 [S.N. Ngauv, R. Sabbah, M. Laffitte, Thermochim. Acta 20 (1977) 371–380; I. Contineanu, D.I. Marchidan, Rev. Roum. Chim. 29 (1984) 43–48], gives a new value for the enthalpy of formation of l-alanine in the gaseous phase, ΔfHm∘ (l-alanine, g, 298.15 K) = −424.8 ± 2.0 kJ · mol−1, which is in good agreement with our theoretical G3X result, −427.6 ± 4.0 kJ · mol−1. The same procedure for glycine allowed us to improve the literature value of the enthalpy of formation for this compound, ΔfHm∘ (glycine, g, 298.15 K) = −393.7 ± 1.5 kJ · mol−1. As a result a set of self-consistent thermochemical data for glycine and l-alanine is proposed.

ناشر
Database: Elsevier - ScienceDirect (ساینس دایرکت)
Journal: The Journal of Chemical Thermodynamics - Volume 41, Issue 4, April 2009, Pages 433–438
نویسندگان
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