(e.g. ionic contributions to the bonding).
CPMG sequence (Carr–Purcell–
Meiboom–Gill sequence) A sequence
of pulses used for spin-echo experiments in nuclear magnetic resonance
(NMR) in which the initial pulse is
90° followed by a series of 180°
pulses. A CPMG sequence is designed
so that the spin echoes die away exponentially with time. Spin–spin relaxation occurs characterized by a
time constant T 2 , which can be determined from the decay signal.
crack cocaine See cocaine.
cracking The process of breaking
down chemical compounds by heat.
The term is applied particularly to
the cracking of hydrocarbons in the
kerosine fraction obtained from *petroleum reÜning to give smaller hydrocarbon molecules and alkenes. It
is an important process, both as a
source of branched-chain hydrocarbons suitable for gasoline (for motor
fuel) and as a source of ethene and
other alkenes. Catalytic cracking is a
similar process in which a catalyst is
used to lower the temperature required and to modify the products
obtained.
cream of tartar See potassium hydrogentartrate.
creosote 1. (wood creosote) An almost colourless liquid mixture of
phenols obtained by distilling tar obtained by the destructive distillation
of wood. It is used medically as an
antiseptic and expectorant. 2. (coaltar creosote) A dark liquid mixture of
phenols and cresols obtained by distilling coal tar. It is used for preserving timber.
cresols See methylphenols.
CRG process (catalytic rich gas
process) An industrial process for
producing fuel gas from naphtha
and other hydrocarbon sources. It involves a nickel-based catalyst, pressures of up to 70 bar, and
temperatures between 250°C and
650°C depending on the feedstock.
The reactions are:
C n H 2n+2 + nH 2 O → nCO + (2n +1)H 2
CO + 3H 2 → CH 4 + H 2 O
CO + H 2 O → CO 2 + H 2
The result is a mixture of methane,
carbon monoxide, carbon dioxide,
and trace amounts of ethane and
other hydrocarbons. With partial carbon dioxide removal it is possible to
produce town gas with medium
caloriÜc value containing about 30%
CH 4 , 30% H 2 , and 2% CO. The process
can be used to produce SNG. In this
case there are multiple methanation
stages and complete removal of CO 2
to give a product containing about
98.5% CH 4 , 0.9% H 2 , and 0.1% CO.
cristobalite A mineral form of *silicon(IV) oxide, SiO 2 .
critical pressure The pressure of a
Ûuid in its *critical state; i.e. when it
is at its critical temperature and critical volume.
critical state The state of a Ûuid in
which the liquid and gas phases both
have the same density. The Ûuid is
then at its critical temperature, critical pressure, and critical volume.
critical temperature 1. The temperature above which a gas cannot
be liqueÜed by an increase of pressure. See also critical state. 2. See
transition point.
critical volume The volume of a
Üxed mass of a Ûuid in its *critical
state; i.e. when it is at its critical temperature and critical pressure. The
critical speciÜc volume is its volume
per unit mass in this state: in the
past this has often been called the
critical volume.
CPMG sequence
148
c
www.AzShimi.ir www.AzShimi.com
CPMG sequence (Carr–Purcell–
Meiboom–Gill sequence) A sequence
of pulses used for spin-echo experiments in nuclear magnetic resonance
(NMR) in which the initial pulse is
90° followed by a series of 180°
pulses. A CPMG sequence is designed
so that the spin echoes die away exponentially with time. Spin–spin relaxation occurs characterized by a
time constant T 2 , which can be determined from the decay signal.
crack cocaine See cocaine.
cracking The process of breaking
down chemical compounds by heat.
The term is applied particularly to
the cracking of hydrocarbons in the
kerosine fraction obtained from *petroleum reÜning to give smaller hydrocarbon molecules and alkenes. It
is an important process, both as a
source of branched-chain hydrocarbons suitable for gasoline (for motor
fuel) and as a source of ethene and
other alkenes. Catalytic cracking is a
similar process in which a catalyst is
used to lower the temperature required and to modify the products
obtained.
cream of tartar See potassium hydrogentartrate.
creosote 1. (wood creosote) An almost colourless liquid mixture of
phenols obtained by distilling tar obtained by the destructive distillation
of wood. It is used medically as an
antiseptic and expectorant. 2. (coaltar creosote) A dark liquid mixture of
phenols and cresols obtained by distilling coal tar. It is used for preserving timber.
cresols See methylphenols.
CRG process (catalytic rich gas
process) An industrial process for
producing fuel gas from naphtha
and other hydrocarbon sources. It involves a nickel-based catalyst, pressures of up to 70 bar, and
temperatures between 250°C and
650°C depending on the feedstock.
The reactions are:
C n H 2n+2 + nH 2 O → nCO + (2n +1)H 2
CO + 3H 2 → CH 4 + H 2 O
CO + H 2 O → CO 2 + H 2
The result is a mixture of methane,
carbon monoxide, carbon dioxide,
and trace amounts of ethane and
other hydrocarbons. With partial carbon dioxide removal it is possible to
produce town gas with medium
caloriÜc value containing about 30%
CH 4 , 30% H 2 , and 2% CO. The process
can be used to produce SNG. In this
case there are multiple methanation
stages and complete removal of CO 2
to give a product containing about
98.5% CH 4 , 0.9% H 2 , and 0.1% CO.
cristobalite A mineral form of *silicon(IV) oxide, SiO 2 .
critical pressure The pressure of a
Ûuid in its *critical state; i.e. when it
is at its critical temperature and critical volume.
critical state The state of a Ûuid in
which the liquid and gas phases both
have the same density. The Ûuid is
then at its critical temperature, critical pressure, and critical volume.
critical temperature 1. The temperature above which a gas cannot
be liqueÜed by an increase of pressure. See also critical state. 2. See
transition point.
critical volume The volume of a
Üxed mass of a Ûuid in its *critical
state; i.e. when it is at its critical temperature and critical pressure. The
critical speciÜc volume is its volume
per unit mass in this state: in the
past this has often been called the
critical volume.
CPMG sequence
148
c
www.AzShimi.ir www.AzShimi.com