sifat molekul
DESCRIPTION
Farmasi FisikaTRANSCRIPT
Molecules of drug
Almost all of drugs are organic
There more than 2 atoms (C, H, O, N, etc.)
All atoms has a different charges
Between atoms there is binding
Atom bonds at molecule influences by
their charges
Their bonds make a unique properties
Potential Energy
(Coulomb Law)
Where is :
U(r) = potential energy for r meter
distances between atoms, Joule
q1 and q2 = charges different atoms,
coulomb
ε0 = permitivity constants,
8,85410-12 Coulomb2N-1m-2
r = distance between atoms
r
qqrU
0
21
4
Effect of Potential Energy
Both atoms have different
vibration (detected by IR) :
Contracting (attractive)
Stretching (repulsive)
Dipole Moment
Dipole moment depends on the variation in distribution of electrons along the bond, and also its length, which is why stretching a bond can change its dipole moment.
For bonds between unlike atoms, the larger the difference in electronegativity, the greater the dipole moment, and the more it changes when stretched.
For identical atoms (C=C, for example) the dipole moment, and its capacity to change with stretching, is much smaller.
Stretching frequencies for symmetrical molecules are measured using Raman spectra.
Polar or Non Polar
Fig. 4—2. Vectorial nature of permanent
dipole moments for (a) water, (b) carbon
dioxide, and (c) a peptide bond. The
distance r is given by the dashed line for
each molecule. The arrow represents
the conventional direction that a dipole
moment vector is drawn, from
negative to positive.
The symmetry of the molecule can also be associated with its dipole moment, which is
observed with carbon dioxide (no net dipole) in Figure 4—2. Likewise, benzene and
pdichlorobenzene are symmetric planar molecules and have dipole moments of zero.
Meta and ortho derivatives of benzene, however, are not symmetric and have significant
dipole moments, as listed in Table 4—3.
Dielectric Constant & Polarization
parallel conducting plates
(Figure 4—3)
C = q/V C: capasitansy (farad); q: electrocharge(coulomb); V: voltage (volt)
= Cx/C0
: Dielectric constants;
Cx : condensor capasitans with x agent;
C0 : standard capasitans (vakum)
The dielectric constant is a measure of the
ability of molecule to resist charge
separation. If the ratios of the
capacitances are close, then there is
greater resistance to a charge separation.
Dielectric Constant & Polarization
The concepts of polarity and dielectric constant :
Placing a molecule in an electric field is one way to induce a
dipole. Consider two parallel conducting plates, such as the
plates of an electric condenser, which are separated by some
medium across a distance r, and apply a potential across the
plates (Figure 4—3).
Electricity will flow from the left plate to the right plate through
the battery until the potential difference of the plates equals that
of the battery supplying the initial potential difference.
When non polar agent in suitable solvent has been placed
between 2 charges capasitor, the non polar molecule able
to inducted polarization/ inducted dipole.
There have separate electrocharges for a while. Electron &
nuclei of atom move to others temporary.
This temporary condition called inducted polarization
power, p which has equal with electrocity field force
This power describe how that molecule or atom polarized
by external force such as electricity, magnetic or ray of light
Dielectric Constant & Polarization
From electromagnetic theory, Clausius—Mossotti equation :
pn
3
4
2
1
M: Molecule weight, n : molecule number in
such of volume; N: Avogadro=6,023X1023 mol-1.
Pi: Inducted Polarized Molar, represent of
inducted dipole moment per mol of nonpolar
agent if there placed on condensator electricity
force field, V/m (volt per meter) = 1.
Dielectric Constant & Polarization
Molekul p x 1024 cm3
H2O
N2
HCl
HBr
HI
HCN
1,68
1,79
3,01
3,5
5,6
5,9
Polarizabilities
Clausius-Massotti equation :
ip
pPN
nMM
3
4
3
4
2
1-
Chloroform (=4,8) has molecule weight 119
g/mol and density 1,43 g/cm3 at 250 . How
much inducted polarization molar power of that
agent (polarizability) ?
Answer :
Dielectric Constant & Polarization
question sample
mol/cm 5,4643,1
119
28,4
18,4
2
1- P 3
i
M
MPi
2
1-
Permanent Dipole
Polar molecules have permanent dipoles.
The separation of positively and negatively charged
regions can be permanent, and the molecule will
possess a permanent dipole moment, .
MPPP i
2
10
P0 , is the orientation polarization of the permanent dipoles,= 4N2/9kT;
k, Boltzman constant= 1.38X10-23 J0K-1
Slop A= 4N2/9k T
APP i
1
The total molar polarization, P. is the
sum of induction and permanent dipole
effects:
Because P0 depends on the
temperature, T :
The importance of dipole interactions should not be
underestimated.
For ionic solutes and nonpolar solvents, ion— induced dipole
interactions have an essential role in solubility phenomena .
For drug—receptor binding, dipole— dipole interactions are
essential noncovalent forces that contribute to enhance the
pharmacologic effect.
For solids composed of molecules with permanent dipole
moments, the dipole interactions contribute to the crystalline
arrangement and overall structural nature of the solid. For
instance, water molecules in ice crystals are organized
through their dipole forces.
CONCLUSION