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calculation of an oscillatory component of thermodynamic properties of n-alkanes, n-alkilnyh radicals, nitroalkanes and nitroalkilnyh radicals. Comparison of models

Approach of division of movements allows to refuse the decision of nuclear equation SHredingera for each connection, and to consider components of such movement in full energy as forward, rotary and oscillatory contributions, predstavyolennye in the form of temperature dependences.

Expressions for calculation of these summands (and also sizes of forward and rotary contributions) are described in previous razyodelah this chapter. In this partition on parities of statistical physics on a basis chayostot fluctuations the contribution of oscillatory making entropy, heat capacity, enthalpy and a free energy of Gibbs is calculated.

The oscillatory contribution to thermodynamic properties has been calculated, leaning on harmonious and angarmonicheskie frequencies, on the basis of several models:

In harmonious approach

- With limiting harmonious frequencies ugarm (),

- With the harmonious frequencies scaled on the equation of regression umaet JA^ugarm + b (BH

- With angarmonicheskimi frequencies Vanharm ()

- In angarmonicheskom approach with use ugarm and Vanharm (),

As it is noted above, in some cases the theoretical frequencies found with poyomoshchju VT2 QFFи answering nizkobarernomu internal braked vrashcheyoniju, appear imaginary. Such values at calculation in harmonious approach should be replaced on similar but, received of the decision torsionnogo uravneyonija SHredingera, or on scaled umaet. Calculation of the contribution to properties of the-alkanes, not representing such interest as the contribution to properties of radicals and the employee for posleyodujushchego comparisons with experiment, is made for T = 298 K.Vklady of the harmonious and scaled frequencies are calculated on the equations (V.5.3) ÷ (V.5.10), angarmonicheskih on (V.5.16) and (V.5.31) ÷ (V.5.38).

The made comparison with experiment of full theoretical values of properties yields an error considerably большую1 a kdzh/MOLE for enthalpy of formation and a free energy of Gibbs, and ae меаее1 Джхмоль-1хК-1 for entropy and heat capacity. It demands a rounding off of number of signs on these properties to the whole values. From methodical reasons oscillatory contributions are yielded to within one figure after a comma. Besides, isyopolzovanie harmonious frequencies at once deduces for limits of "chemical accuracy». The error of calculation of the contribution at use of the scaled harmonious frequencies twice is less, than for not scaled, however and it exceeds specified veliyochiny. It means that achievement of the declared accuracy generally (without hope of prospective reduction of regular errors) is possible only with the help very much resursoemkogo a method. And any harmonious model using angarmonicheskie, the scaled and harmonious frequencies, will yield an error, especially with growth of temperature and number of atoms. On the other hand, usual practice of definition of enthalpy of radicals is the estimation ΔjHo2

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A source: Turovtsev Vladimir Vladimirovich. Creation and application of a quantum mechanical model for calculating the thermodynamic properties of substances in a wide range of temperatures. Thesis for the degree of doctor of physico-mathematical sciences. Tver - 2014. 2014

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