Pascals’ constants are numbers used in the evaluation of the magnetic susceptibilities

Magnetic susceptibility

In electromagnetism, the magnetic susceptibility \chi_m is a dimensionless proportionality constant that indicates the degree of magnetization of a material in response to an applied magnetic field...

 of coordination compounds. The magnetic susceptibility of a compound is the sum of the paramagnetic susceptibility associated with the unpaired electron

Unpaired electron

In chemistry, an unpaired electron is an electron that occupies an orbital of an atom singly, rather than as part of an electron pair. As the formation of electron pairs is often energetically favourable, either in the form of a chemical bond or as a lone pair, unpaired electrons are relatively...

s and the opposing diamagnetic

Diamagnetism

Diamagnetism is the property of an object which causes it to create a magnetic field in opposition to an externally applied magnetic field, thus causing a repulsive effect. Specifically, an external magnetic field alters the orbital velocity of electrons around their nuclei, thus changing the...

 susceptibility associated with electron pair

Electron pair

In chemistry, an electron pair consists of two electrons that occupy the same orbital but have opposite spins.Because electrons are fermions, the Pauli exclusion principle forbids these particles from having exactly the same quantum numbers. Therefore the only way to occupy the same orbital, i.e....

s. Typically, the paramagnetic susceptibility greatly exceeds in magnitude the diamagnetic susceptibility. Thus, the diamagnetic correction is not considered for many purposes. For more precise analyses, however, the diamagnetic corrections are calculated by summing the contributions from the components of the molecule. These group contributions are Pascal’s constants. This analysis assumes that these group contributions are identical in all molecules. In general, the magnitude of Pascal’s constants correlates with the number of electrons in the groups. Groups with extended pi-delocalization

Conjugated system

In chemistry, a conjugated system is a system of connected p-orbitals with delocalized electrons in compounds with alternating single and multiple bonds, which in general may lower the overall energy of the molecule and increase stability. Lone pairs, radicals or carbenium ions may be part of the...

 have larger diamagnetic corrections compared to related saturated ligands.

These correction factors were first described by Pascal in 1910. The values and the method of analysis have been revised several times.

Representative Pascal Constants

cation	value	anion	value
Li+
F–	-9.1
Na+
Cl–	-23.4
K+
Br–
Rb+
I–
Cs+
OH-
NH4+
CN-
Mg2+
NCS-
Fe2+
CO32–
Co2+
NO3–
Ni2+
SO42-
Cu2+
ClO3-
Zn2+
ClO4-
Cd2+
BrO3-
Hg2+
IO3-
Pb2+
IO4-

Representative Pascal Constants

ligands	value
H2O
NH3
pyridine Pyridine Pyridine is a basic heterocyclic organic compound with the chemical formula C5H5N. It is structurally related to benzene, with one C-H group replaced by a nitrogen atom...
ethylenediamine
bipyridyl
phenanthroline Phenanthroline Phenanthroline is a heterocyclic organic compound. As a bidentate ligand in coordination chemistry, it forms strong complexes with most metal ions...
acetylacetonate Acetylacetone Acetylacetone is an organic compound that famously exists in two tautomeric forms that rapidly interconvert. The less stable tautomer is a diketone formally named pentane-2,4-dione. The more common tautomer is the enol form. The pair of tautomers rapidly interconvert and are treated as a single...
glycinate Glycine Glycine is an organic compound with the formula NH2CH2COOH. Having a hydrogen substituent as its 'side chain', glycine is the smallest of the 20 amino acids commonly found in proteins. Its codons are GGU, GGC, GGA, GGG cf. the genetic code.Glycine is a colourless, sweet-tasting crystalline solid...
Triphenylphosphine Triphenylphosphine Triphenylphosphine is a common organophosphorus compound with the formula P3 - often abbreviated to PPh3 or Ph3P. It is widely used in the synthesis of organic and organometallic compounds. PPh3 exists as relatively air stable, colorless crystals at room temperature...

Representative Pascal Constants

In covalent compounds	value
H
B
C
C (ring)
N (open chain)
N (ring)
O
F
Cl
Br
I

With organic molecules corrections are made for bond types other than C-C and C-H single bonds.

Correction Constants

5.5

0.8

6.3

Bond type	value
C=C
C≡C
C=O
COOH
C≡N	0.0