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shenaya

Member

Registered: 2019-10-16

Posts: 34

Predicting heat of vaporization and heat of sublimation

Dear All,

I'm calculating the heat of formation of some molecules using CBS-QB3 method. I have few questions.

1. When I'm finding enthalpy of formation of a radical ex: NH2 radical should I use unrestricted CBS-QB3?
2. As described in section  3.15 Quantitative analysis of molecular surface (12) in the manual heat of sublimation will be calculated.
My question is which basis set should use to calculate the Heat of sublimation parameters A and sigma.
CBS-QB3 or B3LYP/6-311++G(2df,2p)

Thanks and regards
Shen

sobereva

Tian Lu (Multiwfn developer)

From: Beijing

Registered: 2017-09-11

Posts: 1,624

Website

Re: Predicting heat of vaporization and heat of sublimation

1 You do not need to explicitly consider this problem, just use the default setting of Gaussian

2 If you intend to use the equation proposed in the JPCA paper to estimate Heat of sublimation, you should use exactly the same level as the authors to optimize your geometry and calculate the parameters defined based on ESP. Because the prediction equation contains three parameters a,b,c fitted at this level.

shenaya

Member

Registered: 2019-10-16

Posts: 34

Re: Predicting heat of vaporization and heat of sublimation

Hi

I have performed a few calculations

1. Overall surface area:         370.72232 Bohr^2  ( 103.81284 Angstrom^2)
2. Overall variance (sigma^2_tot):  0.00078572 a.u.^2 (   309.39126 (kcal/mol)^2)
3. Product of sigma^2_tot and nu:   0.00010927 a.u.^2 (   43.02795 (kcal/mol)^2)

As I understood these are the numbers I need to calculated HOS.
Is it correct?
A=103.81284
sigma^2_tot and nu =43.02795

Thanks a lot

sobereva

Tian Lu (Multiwfn developer)

From: Beijing

Registered: 2017-09-11

Posts: 1,624

Website

Re: Predicting heat of vaporization and heat of sublimation

Hi

I have performed a few calculations

1. Overall surface area:         370.72232 Bohr^2  ( 103.81284 Angstrom^2)
2. Overall variance (sigma^2_tot):  0.00078572 a.u.^2 (   309.39126 (kcal/mol)^2)
3. Product of sigma^2_tot and nu:   0.00010927 a.u.^2 (   43.02795 (kcal/mol)^2)

As I understood these are the numbers I need to calculated HOS.
Is it correct?
A=103.81284
sigma^2_tot and nu =43.02795

Thanks a lot

Correct

prerna

Member

Registered: 2021-05-22

Posts: 2

Re: Predicting heat of vaporization and heat of sublimation

If the experimental data is not available for novel synthesized energy compounds then how can one predict the heat of vaporization and sublimation using area, density and sigma nu2 values within the Politzer and Byrd and Rice approach without the availability of fitting parameters(alpha, beta, gamma-multiple regression)?

sobereva

Tian Lu (Multiwfn developer)

From: Beijing

Registered: 2017-09-11

Posts: 1,624

Website

Re: Predicting heat of vaporization and heat of sublimation

If the experimental data is not available for novel synthesized energy compounds then how can one predict the heat of vaporization and sublimation using area, density and sigma nu2 values within the Politzer and Byrd and Rice approach without the availability of fitting parameters(alpha, beta, gamma-multiple regression)?

If you do not have proper fitting parameters, of course you cannot directly evaluate the data by means of prediction equation. You should either fit the parameters yourself (using experimental data of analogous systems), or use molecular dynamics programs to calculate heat of vaporization and sublimation.