核独立化学位移

核独立化学位移（英語：Nucleus-independent chemical shift）是被使用得最广泛的衡量芳香性的指标。它的含义是在某个人为设定的不在原子核位置上的磁屏蔽值的负值，负值越大（即对磁场屏蔽越强）则芳香性越强。这个位置的设定有一定的任意性。NICS在使用之初，原文（JACS,118,6317）是将这个位置取在共轭环的几何中心，这个位置的NICS被称为NICS(0)。

随后有人认为取在环中心会将σ和π轨道的贡献叠加在一起而说不清楚，于是提出取在平面上方或下方1埃的位置，称为NICS(1)，这个指标体现的主要是π电子的贡献。后来有人更进一步认为，NICS值不该取各项同性值（磁屏蔽张量xx、yy、zz之和的平均），而应该只考察zz值（z轴垂直于环平面），这样最适合体现π芳香性，这被称为NICS(1)_zz。虽然NICS(1)_zz还是不可避免地稍微掺入了一点的σ轨道的影响，但这种影响几乎可以忽略，因此NICS(1)_zz很适合讨论π芳香性。^[1]

NICS值可以判断芳香性是否存在，不能决定芳香性的强弱，例如蒽分子中，中间的环比两端的NICS值更负（绝对值更大）但9,10位在大多反应中活性却更高。

NICS数据

System	NICS(0)	NICS(1)	NICS_zz(1)
${\displaystyle {\ce {Al3-}}}$	-35.8	-26.7	-30.6
${\displaystyle {\ce {Al4^2-}}}$	-34.5	-27.4	-54.9
${\displaystyle {\ce {Al2Si2}}}$	-11.7	-16.9	-35.5
${\displaystyle {\ce {As4^2-}}}$	8.5	4.1	21.9
${\displaystyle {\ce {As5-}}}$	-15.3(-20.5)	-14.6(-19.5)	-33.9
${\displaystyle {\ce {B4}}}$	-35.7	-9.1	4.8
${\displaystyle {\ce {B4+}}}$	-33.4	-6.6	1.9
${\displaystyle {\ce {B4-}}}$	-42.0	-19.2	-27.0
${\displaystyle {\ce {B5}}}$	-17.1	-14.1	-21.1
${\displaystyle {\ce {B5+}}}$	-36.2	-18.8	-24.3
${\displaystyle {\ce {B5-}}}$	-7.3	-13.8	-22.3
${\displaystyle {\ce {B6}}}$	-23.3	-1.9	8.8
${\displaystyle {\ce {B6^2-}}}$	-3.6(-4.2)	8.9(9.0)	29.0
${\displaystyle {\ce {B2Si2}}}$	-42.0	-29.1	-57.2
${\displaystyle {\ce {B2Si2+}}}$	-39.8	-21.3	-36.5
${\displaystyle {\ce {B2Si2^2+}}}$	-25.7	-0.7	19.4
${\displaystyle {\ce {B4Si3}}}$	-9.2	-2.8	8.3
${\displaystyle {\ce {B4Si2^2+}}}$	-13.3	-8.5	-15.7
${\displaystyle {\ce {Be5}}}$	13.1	5.8	-15.7
${\displaystyle {\ce {Be5-}}}$	-4.8	-4.6	-23.6
${\displaystyle {\ce {Be2Al3+}}}$	-24.5	-19.0	-32.1
${\displaystyle {\ce {Be3Al2+}}}$	-10.3	-8.4	-27.9
${\displaystyle {\ce {Be3Al2^2+}}}$	-0.8	-2.2	-25.4
${\displaystyle {\ce {Be4Al2^2+}}}$	13.0	10.0	12.3
${\displaystyle {\ce {Be2Si2}}}$	-22.4	-12.6	-8.2
${\displaystyle {\ce {Be2Si2-}}}$	-28.2	-19.8	-33.8
${\displaystyle {\ce {Be2Si2^2-}}}$	-33.5	-24.9	-53.9
${\displaystyle {\ce {Be3Si2}}}$	-30.3	-21.7	-36.8
${\displaystyle {\ce {C3B4}}}$	-22.6	-12.4	-19.3
${\displaystyle {\ce {C2N2+}}}$	-28.8	-14.6	-23.1
${\displaystyle {\ce {C3N+}}}$	-18.7	-18.7	-34.9
${\displaystyle {\ce {C3N2-}}}$	-14.0	-4.7	-11.7
${\displaystyle {\ce {C3N3^2+}}}$	-28.2	-15.7	-38.4
${\displaystyle {\ce {Ga3-}}}$	-27.3	-22.6	-25.1
${\displaystyle {\ce {Ga4^2-}}}$	-39.2	-29.9	-57.7
${\displaystyle {\ce {GeAl3-}}}$	-34.7	-28.5	-56.7
${\displaystyle {\ce {Mg3}}}$	-1.0(1.7)	-1.5(1.2)	-7.5
${\displaystyle {\ce {Mg3-}}}$	-0.2	-2.6	-14.6
${\displaystyle {\ce {Mg3^2-}}}$	-2.8	-4.0	-12.2
${\displaystyle {\ce {Mg4^2+}}}$	-10.7	-8.6	-25.7
${\displaystyle {\ce {Mg2Al3+}}}$	-4.8	-10.8	-46.3
${\displaystyle {\ce {Mg2Si2^2-}}}$	-24.1	-21.9	-49.1
${\displaystyle {\ce {Mg3Si2^2+}}}$	-33.4	-17.3	-16.8
${\displaystyle {\ce {N4^2-}}}$	4.0	-1.5	-8.6
${\displaystyle {\ce {N5-}}}$	-16.5	-16.5	-45.2
${\displaystyle {\ce {P4^2-}}}$	9.6	3.7	17.6
${\displaystyle {\ce {P5-}}}$	-16.6(-18.5,-22.0)	-15.8(-18.3)	-39.2
${\displaystyle {\ce {S4^2+}}}$	2.1(-10.6)	-0.5(-7.9)	9.0
${\displaystyle {\ce {S2N2}}}$	3.6(-2.6)	1.4(-4.7)	1.1
${\displaystyle {\ce {S4N3+}}}$	-10.0(-12.1)	-8.8(-10.7)	-21.0
${\displaystyle {\ce {S4N4^2+}}}$	-17.4(-20.6)	-14.7(-17.0)	-41.5
${\displaystyle {\ce {Se4^2+}}}$	-3.1(-9.8)	-2.2(-7.6)	6.9
${\displaystyle {\ce {Si3^2+}}}$	-28.0	-21.6	-30.5
${\displaystyle {\ce {Si4^2+}}}$	-42.0	-28.8	-58.0
${\displaystyle {\ce {SiAl3-}}}$	-33.9	-26.3	-52.5

• NICS(0)与NICS(1)数据存在多个不同来源，详见原文^[2]。

参考资料

1. 将核独立化学位移(NICS)分解为sigma和pi轨道的贡献, [2017-04-27], （原始内容于2019-07-03） 
2. Nucleus-independent chemical shift (NICS) profiles in a series of monocyclic planar inorganic compounds (PDF), [2017-04-29], （原始内容 (PDF)于2013-06-20）