2. Introduction
Running frequency calculations using Gaussian09, frequency
calculations can serve a number of different purposes:
To predict the IR and Raman spectra of molecules (frequencies and
intensities). (Content 1)
To compute force constants for a geometry optimization. (Content 2)
To identify the nature of stationary points on the potential energy
surface. (Content 3)
To compute zero-point vibration and thermal energy corrections to
total energies as well as other thermodynamic quantities of interest
such and the enthalpy and entropy of the system (Content 4)
5. Chú ý: Các file .com tương ứng F2.1, F3.1 đều không thể
opt trên Gaussian09 do đó chỉ thực hiện được với file
F1.1.
6. Object: File .com of C6H6 + H2
# RHF/6-31G(d) Freq Pop=full Opt
Benzen Hydro 1
0 1
C 1.2016810000 0.3925400000 -0.0654360000
C 0.3261360000 0.5145560000 -1.7171510000
C -1.5466010000 0.5931780000 -1.7782590000
C -2.5348880000 0.5494680000 -0.1884670000
C -1.6557910000 0.4277120000 1.4555860000
C 0.2085270000 0.3493880000 1.5174730000
H -0.6431300000 1.0452900000 -0.1023190000
H -0.6893590000 -0.1027310000 -0.1626210000
H 2.2809920000 0.3465040000 -0.0164380000
H 0.9048330000 0.5393040000 -2.6319480000
H -2.0625750000 0.6638000000 -2.7271990000
H -3.6145720000 0.5941180000 -0.2107910000
H -2.2241290000 0.4023970000 2.3732440000
H 0.7087820000 0.2791060000 2.4720000000
12. Content 3: Stationary Points
• There are two pieces of information from the output which are critical
to characterizing a stationary point:
The number of imaginary frequencies.
The normal mode corresponding to the imaginary frequency.
lmanginary Frequencies are listed in the output of a frequency calculation
as negative numbers. By definition, a structure which has n imaginary
frequencies is an nth order saddle point
13. Content 3: Stationary Points
From: Exploring Chemistry with Electronic Structure Methods - James B. Foresman
Aleen Frisch
15. Content 4: The Zero-point and Thermal Energy-corrected
The zero-point energy is a correction to the electronic energy of the molecule or hamal mayy cdon. to account for
the effects of molecular vibrations which persist even at 0 K.
From: Exploring Chemistry with Electronic Structure Methods - James B. Foresman
Aleen Frisch
16. F1.3 F2.3 F3.3
Zero-point vibrational energy 345650.2 (Joules/Mol)
82.61238 (Kcal/Mol)
238397.0 (Joules/Mol)
56.97825 (Kcal/Mol)
237582.3 (Joules/Mol)
56.78352 (Kcal/Mol)
Zero-point correction 0.131651 (Hartree/Particle) 0.090801 (Hartree/Particle) 0.090490 (Hartree/Particle)
Thermal correction to Energy 0.136509 0.097207 0.096498
Thermal correction to Enthalpy 0.137454 0.098151 0.097443
Thermal correction to Gibbs Free
Energy
0.103571 0.061294 0.061095
(E0) Sum of electronic and zero-point
Energies
-231.700246 -193.443437 -230.767603
(E) Sum of electronic and thermal
Energies
-231.695388 -193.437031 -230.761595
(H) Sum of electronic and thermal
Enthalpies
-231.694444 -193.436087 -230.760651
(G) Sum of electronic and thermal
Free Energies
-231.728327 -193.472944 -230.796998
Content 4: The Zero-point and Thermal Energy-corrected
17. Nhận xét: Mặc dù không thể opt 2 file F2 và F3 xong có
thể tạm nhận xét năng lượng E0 của F1 là bé hơn so với
trường hợp E0 khi chưa opt. Và số điểm ảo của tần số
cũng không có => Minimum => Có xu hướng về cấu trúc
này trong tự nhiên.
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