## Quadrupole interaction and electric-field gradient tensor. 四極矩作用力 與 電場梯度張量 Contributor: Y. Millot

Home and Applets > Quadrupole Interaction > Definition > EFG Tensor

The page you are about to view requires Internet Explorer 6+ and MathPlayer, or Netscape7/Mozilla/Firefox and MathML fonts for the mathematical formulae using MathML 2.0.

 $\frac{\sqrt{\frac{3}{2}}}{\sqrt{{f\left(a+b\right)}^{2}}}\frac{\sqrt{3}}{2}$

If the two examples above are mathematically the same, your browser is correctly displaying MathML.

## Quadrupole interaction in a uniform space

Quadrupole nuclei with a half-integer spin larger than 1/2 possess a quadrupole moment Q, which interacts with the electric-field gradient (EFG) generated by their surroundings.

The coupling of Q (a property of the nucleus) with an EFG (a property of a sample) is called the quadrupole interaction (四極矩作用力).

### (A) Quadrupole interaction in Cartesian tensor representation 笛卡爾張量形式

Consider a free nucleus in a uniform space, that is, the three coordinate axes x, y, and z are equivalent. The Hamiltonian representing the quadrupole interaction of this nucleus, independent of the Cartesian coordinate frame, is defined by:

$ħ{H}_{Q}=\frac{\mathrm{eQ}}{6I\left(2I-1\right)}\sum _{j,k=x,y,z}{V}_{\mathrm{jk}}\left[\frac{3}{2}\left({I}_{j}{I}_{k}+{I}_{k}{I}_{j}\right)-I\left(I+1\right){\delta }_{\mathrm{jk}}\right]$
where Vjk are the Cartesian components of V, the EFG at the origin, which is a second-rank symmetrical tensor. In the principal-axis system of the EFG (XPAS, YPAS, ZPAS), V is diagonal:

$V=\left(\begin{array}{ccc}{V}_{XX}& 0& 0\\ 0& {V}_{YY}& 0\\ 0& 0& {V}_{ZZ}\end{array}\right)$
with the convention

$\mid {V}_{\mathrm{ZZ}}\mid \ge \mid {V}_{\mathrm{YY}}\mid \ge \mid {V}_{\mathrm{XX}}\mid$
However, another convention is also often used (for example in the NMR simulation program SIMPSON):

$\mid {V}_{\mathrm{ZZ}}\mid \ge \mid {V}_{\mathrm{XX}}\mid \ge \mid {V}_{\mathrm{YY}}\mid$

Furthermore, the Laplace equation, ${V}_{\mathrm{ZZ}}+{V}_{\mathrm{YY}}+{V}_{\mathrm{XX}}=0$, holds for V, because the electric field at the nucleus is produced by charges wholly external to the nucleus. Thus, only two independent parameters are required:

$\left\{\begin{array}{c}\mathrm{eq}={V}_{\mathrm{ZZ}}\\ \eta =\frac{{V}_{\mathrm{XX}}-{V}_{\mathrm{YY}}}{{V}_{\mathrm{ZZ}}}\end{array}\right\$
with $1\ge \eta \ge 0\text{,}$ the largest component (最大的梯度) and the asymmetry parameter (不對稱參數), respectively.

For SIMPSON, these two parameters become:

$\left\{\begin{array}{c}\mathrm{eq}={V}_{\mathrm{ZZ}}\\ \eta =\frac{{V}_{\mathrm{YY}}-{V}_{\mathrm{XX}}}{{V}_{\mathrm{ZZ}}}\end{array}\right\$
with $1\ge \eta \ge 0\text{.}$

In practice, the asymmetry parameter η defines the powder lineshape and eq is related to the linewidth.

The product of eq with eQ divided by Planck's constant is called the quadrupole coupling constant (Cq) 四極偶合常數.

In the principal-axis system of V, the quadrupole interaction takes the form:

$ħ{H}_{Q}=\frac{{e}^{2}\mathrm{qQ}}{4I\left(2I-1\right)}\left[3{I}_{Z}^{2}-I\left(I+1\right)+\eta \left({I}_{X}^{2}-{I}_{Y}^{2}\right)\right]$
In term of the operators I+ = IX + iIY and I- = IX - iIY, the quadrupole interaction becomes:

$ħ{H}_{Q}=\frac{{e}^{2}\mathrm{qQ}}{4I\left(2I-1\right)}\left[3{I}_{Z}^{2}-I\left(I+1\right)+\frac{1}{2}\eta \left({I}_{+}^{2}+{I}_{-}^{2}\right)\right]$

### (B) Quadrupole interaction in spherical tensor representation 球張量形式

The passage from one coordinate frame to another is more conveniently realized if the quadrupole interaction of a free nucleus is expressed as a product of second-rank irreducible spherical tensors:

${H}_{Q}=\frac{\mathrm{eQ}}{2I\left(2I-1\right)ħ}\sum _{q=-2}^{2}{\left(-1\right)}^{q}{V}_{\left(2,-q\right)}{T}^{\left(2,q\right)}$

In any Cartesian coordinate frame, the spherical tensor and Cartesian tensor components of V and T are related by:

 ${V}_{\left(2,0\right)}=\frac{\sqrt{6}}{2}{V}_{\mathrm{zz}}$ ${T}^{\left(2,0\right)}=\frac{\sqrt{6}}{6}\left\{3{I}_{z}^{2}-I\left(I+1\right)\right\}$ ${V}_{\left(2,1\right)}=-{V}_{\mathrm{xz}}-i{V}_{\mathrm{yz}}$ ${T}^{\left(2,1\right)}=-\frac{1}{2}\left({I}_{z}{I}_{+}+{I}_{+}{I}_{z}\right)=-\frac{1}{2}{I}_{+}\left(2{I}_{z}+1\right)$ ${V}_{\left(2,-1\right)}={V}_{\mathrm{xz}}-i{V}_{\mathrm{yz}}$ ${T}^{\left(2,-1\right)}=\frac{1}{2}\left({I}_{z}{I}_{-}+{I}_{-}{I}_{z}\right)=\frac{1}{2}{I}_{-}\left(2{I}_{z}-1\right)$ ${V}_{\left(2,2\right)}=\frac{1}{2}\left({V}_{\mathrm{xx}}-{V}_{\mathrm{yy}}\right)+i{V}_{\mathrm{xy}}$ ${T}^{\left(2,2\right)}=\frac{1}{2}{I}_{+}^{2}$ ${V}_{\left(2,-2\right)}=\frac{1}{2}\left({V}_{\mathrm{xx}}-{V}_{\mathrm{yy}}\right)-i{V}_{\mathrm{xy}}$ ${T}^{\left(2,-2\right)}=\frac{1}{2}{I}_{-}^{2}$

with I+ = Ix + iIy and I- = Ix - iIy. These two operators are different from those used previously despite the same notation. The numerical factors in the components of V and T depend on the authors. The spherical tensor representation of the quadrupole interaction becomes:

${H}_{Q}=\frac{\mathrm{eQ}}{2I\left(2I-1\right)ħ}\left\{\frac{\sqrt{6}}{6}\left[3{I}_{z}^{2}-I\left(I+1\right)\right]{V}_{\left(2,0\right)}$
$+\frac{1}{2}{I}_{+}\left(2{I}_{z}+1\right){V}_{\left(2,-1\right)}-\frac{1}{2}{I}_{-}\left(2{I}_{z}-1\right){V}_{\left(2,1\right)}$
$+\frac{1}{2}{I}_{+}^{2}{V}_{\left(2,-2\right)}+\frac{1}{2}{I}_{-}^{2}{V}_{\left(2,2\right)}\right\}$

Expressing the latter Hamiltonian in the principal-axis system of the EFG tensor and comparing the result with the last expression of quadrupole interaction in Cartesian tensor section yield the spherical tensor components of V in the principal-axis system:

${V}_{\left(2,0\right)}^{\mathrm{PAS}}=\sqrt{\frac{3}{2}}\mathrm{eq},{V}_{\left(2,±1\right)}^{\mathrm{PAS}}=0,{V}_{\left(2,±2\right)}^{\mathrm{PAS}}=\frac{1}{2}\mathrm{eq}\eta$

### (D) Mathematica-5 notebook

(top)

[Contact me] - Last updated:
Solid-state NMR bibliography for
Aluminum-27
Antimony-121/123
Arsenic-75
Barium-135/137
Beryllium-9
Bismuth-209
Boron-11
Bromine-79/81
Calcium-43
Cesium-133
Chlorine-35/37
Chromium-53
Cobalt-59
Copper-63/65
Deuterium-2
Gallium-69/71
Germanium-73
Gold-197
Hafnium-177/179
Indium-113/115
Iodine-127
Iridium-191/193
Krypton-83
Lanthanum-139
Lithium-7
Magnesium-25
Manganese-55
Mercury-201
Molybdenum-95/97
Neon-21
Nickel-61
Niobium-93
Nitrogen-14
Osmium-189
Oxygen-17
Potassium-39/41
Rhenium-185/187
Rubidium-85/87
Ruthenium-99/101
Scandium-45
Sodium-23
Strontium-87
Sulfur-33
Tantalum-181
Titanium-47/49