A
AAR See amino acid racemization.
AAS See atomic absorption spectroscopy.
abherent See release agent.
ab-initio calculation A method of
calculating atomic and molecular
structure directly from the Ürst principles of quantum mechanics, without using quantities derived from
experiment (such as ionization energies found by spectroscopy) as parameters. Ab-initio calculations require
a large amount of numerical computation; the amount of computing
time required increases rapidly as
the size of the atom or molecule increases. The development of computing power has enabled the properties
of both small and large molecules to
be calculated accurately, so that this
form of calculation can now replace
*semi-empirical calculations. Abinitio calculations can, for example,
be used to determine the bond
lengths and bond angles of molecules
by calculating the total energy of the
molecule for a variety of molecular
geometries and Ünding which conformation has the lowest energy.
absolute 1. Not dependent on or
relative to anything else, e.g. *absolute zero. 2. Denoting a temperature measured on an absolute scale,
a scale of temperature based on absolute zero. The usual absolute scale
now is that of thermodynamic *temperature; its unit, the kelvin, was formerly called the degree absolute (°A)
and is the same size as the degree
Celsius. In British engineering practice an absolute scale with Fahrenheit-size degrees has been used: this
is the Rankine scale.
absolute alcohol See ethanol.
absolute conÜguration A way of
denoting the absolute structure of an
optical isomer (see optical activity).
Two conventions are in use: The d–l
convention relates the structure of
the molecule to some reference molecule. In the case of sugars and similar compounds, the dextrorotatory
form of glyceraldehyde
(HOCH 2 CH(OH)CHO), 2,3-dihydroxypropanal) was used. The rule is as
follows. Write the structure of this
molecule down with the asymmetric
carbon in the centre, the –CHO
group at the top, the –OH on the
right, the –CH 2 OH at the bottom, and
the –H on the left. Now imagine that
the central carbon atom is at the centre of a tetrahedron with the four
groups at the corners and that the –H
and –OH come out of the paper and
the –CHO and –CH 2 OH groups go
into the paper. The resulting threedimensional structure was taken to
be that of d-glyceraldehyde and
called d-glyceraldehyde. Any compound that contains an asymmetric
carbon atom having this conÜguration belongs to the d-series. One having the opposite conÜguration
belongs to the l-series. It is important
to note that the preÜxes d- and l- do
not stand for dextrorotatory and
laevorotatory (i.e. they are not the
same as d- and l-). In fact the arbitrary
conÜguration assigned to d-glyceraldehyde is now known to be the correct one for the dextrorotatory form,
although this was not known at the
www.AzShimi.ir www.AzShimi.com
AAR See amino acid racemization.
AAS See atomic absorption spectroscopy.
abherent See release agent.
ab-initio calculation A method of
calculating atomic and molecular
structure directly from the Ürst principles of quantum mechanics, without using quantities derived from
experiment (such as ionization energies found by spectroscopy) as parameters. Ab-initio calculations require
a large amount of numerical computation; the amount of computing
time required increases rapidly as
the size of the atom or molecule increases. The development of computing power has enabled the properties
of both small and large molecules to
be calculated accurately, so that this
form of calculation can now replace
*semi-empirical calculations. Abinitio calculations can, for example,
be used to determine the bond
lengths and bond angles of molecules
by calculating the total energy of the
molecule for a variety of molecular
geometries and Ünding which conformation has the lowest energy.
absolute 1. Not dependent on or
relative to anything else, e.g. *absolute zero. 2. Denoting a temperature measured on an absolute scale,
a scale of temperature based on absolute zero. The usual absolute scale
now is that of thermodynamic *temperature; its unit, the kelvin, was formerly called the degree absolute (°A)
and is the same size as the degree
Celsius. In British engineering practice an absolute scale with Fahrenheit-size degrees has been used: this
is the Rankine scale.
absolute alcohol See ethanol.
absolute conÜguration A way of
denoting the absolute structure of an
optical isomer (see optical activity).
Two conventions are in use: The d–l
convention relates the structure of
the molecule to some reference molecule. In the case of sugars and similar compounds, the dextrorotatory
form of glyceraldehyde
(HOCH 2 CH(OH)CHO), 2,3-dihydroxypropanal) was used. The rule is as
follows. Write the structure of this
molecule down with the asymmetric
carbon in the centre, the –CHO
group at the top, the –OH on the
right, the –CH 2 OH at the bottom, and
the –H on the left. Now imagine that
the central carbon atom is at the centre of a tetrahedron with the four
groups at the corners and that the –H
and –OH come out of the paper and
the –CHO and –CH 2 OH groups go
into the paper. The resulting threedimensional structure was taken to
be that of d-glyceraldehyde and
called d-glyceraldehyde. Any compound that contains an asymmetric
carbon atom having this conÜguration belongs to the d-series. One having the opposite conÜguration
belongs to the l-series. It is important
to note that the preÜxes d- and l- do
not stand for dextrorotatory and
laevorotatory (i.e. they are not the
same as d- and l-). In fact the arbitrary
conÜguration assigned to d-glyceraldehyde is now known to be the correct one for the dextrorotatory form,
although this was not known at the
www.AzShimi.ir www.AzShimi.com