ANALELE ŞTIINŢIFICE ALE UNIVERSITĂŢII “AL. I. CUZA” IAŞI
Tomul III, s. Biofizică, Fizică medicală şi Fizica mediului 2007
STRUCTURAL ANALYSIS OF BROMOBENZENE
MOLECULE
Mihaela Dimitriu1,2, Liliana-Mihaela Ivan1, Dana Ortansa Dorohoi1
KEYWORDS: bromobenzene molecule, structure analysis, HyperChem.
A series of physicochemical properties of the bromobenzene molecule have been derived
from an analysis of its structure using the software package HYPERCHEM. Various
molecular physical and structural properties, such as the length of the carbon-carbon and/or
carbon-hydrogen bonds, the angles between the bonds, symmetry class, energies of the
ground and excited states, dipole moment and polarizability, have been obtained. The study
of the electronic vibration spectra allowed us to determine the energy level diagram, level
occupation numbers, oscillator strengths, and the potential energy surface.
1.
INTRODUCTION
HyperChem [1] - one of the most frequently used programs in physicschemistry- permits to build and to analyze various molecular structures (hydrocarbons,
alcohols, acids aminoacids, polymers) and to determine their physical and chemical
properties.
The analysis of the molecular structures made by HyperChem programme
The main steps to be followed in the general case are here described.
1. In order to build any type a molecule the following algorithm is used:
-
the atoms’ placement for the creation of a 2D-aspect of the molecule (for
the organic molecules the placement begins with the C-atoms);
the establishment of the bond type among the atoms;
the addition of the hydrogen atoms for completing the valence;
the realization of the 3D-aspect of the molecule by using the model build
option.
1
Faculty of Physics, “Al. I. Cuza” University, Iasi, Romania
2
Gr. Şc. „I. Holban”, Iasi, Romania
22
Mihaela Dimitriu, Liliana-Mihaela Ivan, Dana Ortansa Dorohoi
2. The physical properties that can be determined are:
-
the lengths of the bonds among various atoms;
the angles between the bonds;
some physical characteristics of the molecule; polarizability, electric
dipole moment and so on.
3. Bonding and antibonding orbitals permit the achievement of the energetic levels’
diagram. The electron distribution on the energetical levels and the allowed transitions
are also established.
4. From the analysis of the vibronic spectra one can determine: the degeneracy, the
spin multiplicity, the oscillator strength, the vibration modes.
5. Graphical representation of the electrostatic potential.
6. Potential (energy) surface: A multidimensional plot of the potential energy of a
molecular system, as a function of all variables (degrees of freedom), of the molecule
(for example bond, angles). The potential surface for a molecule with N atoms has
3N-6 independent degrees of freedom (three degrees reserved for rotation and three
reserved for translation of the whole molecular system). Potential energy surfaces are
usually diagrammed in only one or two dimensions. A 1D potential surface might
show bond energy versus length. A 2D potential surface might show a contour plot of
energy as a function of two torsion angles. Important features on a potential surface
are minima, which HyperChem can locate by geometry optimization and molecular
dynamics calculations, and saddle points, which represent structural transition states.
7. Symmetry class: One molecule is characterized by its affiliation to a point group of
symmetry, whose symmetry operations (rotations, reflections, inversion or identity) do
not modify the molecular configuration.
8. By definition, the transition frequency, ν , of the UV spectrum is given by
ν=
E f − Ei
h
(1)
and the oscillator strength is given by
2
⎛ 8π 2 m ⎞
(2)
f i → f = ⎜ 2 2 ⎟ ( E f − Ei ) d fi
⎝ 3e h ⎠
for the transition i → f , where i is the initial state and f is the final state and dfi is the
transition dipole moment, and h is the Planck constant. Here only the dipole moment
transition is considered and all the higher orders of transitions are ignored because of
the small probability of the higher order transitions. The oscillator strength is a
dimensionless value.
23
STRUCTURAL ANALISYS OF BROMOBENZENE…
2.
RESULTS
Bromobenzene is a molecule belonging to C2V point group [2, 3]. It has as
symmetry elements: the molecular plane
σ v (1) , the plane σ v ( 2 )
perpendicular on the
former containing C-Br bond, a C2 axis at the two planes intersection and the identity
E. All the corresponding symmetry operations do not change the spatial distribution of
the molecular atoms.
C2
(1)
σV
( 2)
Br
σV
Fig. 1: Structure and symmetry of bromobenzene.
Fig. 2: Electron distributions on the energetic levels.
24
Mihaela Dimitriu, Liliana-Mihaela Ivan, Dana Ortansa Dorohoi
Table 1: Length of interatomic bonds, angles between bonds, polarizability
and dipole moment of bromobenzene estimated by HyperChem.
Substance
Bond
length (Å)
Angle between
bonds
Polarizability
(10-24 cm3)
Dipole
moment
(D)
Bromobenzene
C-C:1,4
C-H: 1,08
C-B: 1,91
(C,C,C)=1200
(Cl,C,C)=1200
(H,C,C)=1200
13.06
1.56
The computed values of the bromobenzene polarizability and dipole moments
from table 1 are in good accordance with the values from literature [4].
Table 2: Computed wavelengths and oscillator strengths
of the bromobenzene electronic transitions.
Substance
Wavelength
(nm)
Oscillator
strength
Bromobenzene
309,57
305,66
302,77
280,32
204,87
203,75
0,003
0,003
0,002
0,004
1,073
0,959
The computed wavelengths are located in the ultraviolet range.
Table 3: The experimentally determined wavelengths and logarithm from the
extinction coefficients of the UV electron absorption transitions [5].
Substance
Solvent
λmax
logε
Bromobenzene
Alcohol
210
264
3.9
2.3
The data from tables 2 and 3 illustrate a good accordance between the
theoretical estimations by HyperChem and experimental values of the wavelengths in
the UV range.
Hydration energy is defined as the energy change accompanying the hydration
of a mole of bromobenzene in its gaseous state.
25
STRUCTURAL ANALISYS OF BROMOBENZENE…
Table 4: Volume, hydration energy, mass of bromobezene
molecule obtained by HyperChem.
Substance
Volume (Ǻ3)
Hydration energy
(kcal/mol)
Mass (amu)
Bromobenzene
394.27
- 1.69
157.01
Fig. 3: Contour map of the total charge (electron) density.
Fig. 4: The 1D potential (energy) surface.
26
Mihaela Dimitriu, Liliana-Mihaela Ivan, Dana Ortansa Dorohoi
2.
CONCLUSION
Even the results obtained by the computational methods refer to the isolated
molecules and the spectral information are given for the molecules included in the
condensed state, a good qualitative accordance between theory and experiments has
been observed, regarding the spectral properties of this molecule.
REFERENCES
1.
2.
3.
4.
5.
http://physics.nist.gov.
Eliaşevici M.A., Spectroscopia atomică şi moleculară, Ed. Academiei, Bucureşti, 1966.
Murgulescu I.G., Sahini V. Em., Introducere în chimia fizică, vol. I, 2, Ed. Academiei, Bucureşti,
1978.
Lideed D.R., Handbook of Chemistry and Physics, 74th Edition, 1993-1994.
Hirayama K., Handbook of ultraviolet and visible absorption spectra of organic compounds,
Plenum Press Data Divison, New York, 1967.