Which NICS Aromaticity Index for Planar π Rings Is Best for the Triplet State?
Abstract
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ISE(kcal/mol)
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NICS(0)zz(ppm)
Magnetic properties can reflect aromaticity best, especially nucleus-independent chemical
shifts (NICS). On the hand, the isomerization stabilization energy (ISE) relates well to
aromaticity. Consequently we drew graphs according to NICS and ISE, then we concluded
that NICS(0)zz is better than NICS(1)zz, which is the best indicator in ground state.
INTRODUCTION
Aromaticity, one of the most concepts in organic chemistry, though people have done vast
researches, no one can assign a meaning to it.1,3 Aromaticity is virtual property, can’t be
measured directly. However, due to aromatic molecules have special features, aromaticity
can be evaluated structural1, energetic2, electronic3, and magnetic4-5 criteria. Among the
above methods, magnetic criteria is the best one, for the reason that magnetic properties
are derived from induced π ring currents caused by cyclic electron delocalization. There are
several magnetic criteria have been developed, none of them is perfect, they all have
drawbacks separately. By contrast, nucleus-independent chemical shifts (NICS) manifests
aromaticity best.4
In 1996, NICS is proposed by Schleyer,6 as it can evaluate aromaticity and antiaromaticity
of wide range molecules, also it doesn’t need reference, most importantly, it correlates well
with other criteria. In 2006, Schleyer5 reported that NICS (0)πzz was the best and most reliable
aromaticity index, besides it was linear with aromatic stabilization energies (ASE). While NICS
is used as an evaluation of aromaticity or antiaromaticity for molecules in ground state, few
reports on the aromaticity of molecules evaluated by NICS have been delivered.
Last year, our team7 reported that methyl-methylene isomerization stabilization energy
(ISEⅠ), which was applied to evaluated aromaticity by Pühlhofer8 in 2002, correlated well
with NICS(1)zz in T1 state. Additionally we adopted another method, indene-isoindene
isomerization stabilization energy (ISEⅡ) to investigate the aromaticity of [4n]annulenes,
these values were correlated well with NICS(1)zz in T1 state.9 However, we didn’t describe
other NICS indices’ performance. Herein, we will make a supplement (Figure 1, 2).
Apparently NICS(1)zz is the best one for these monocyclic species with 4n π-electrons.
As previous works were focused on [4n]annulenes, we weren’t aware that ISEⅠ was
applicable to the evaluate aromaticity of five membered heterocycles. Now we report it here.
The homodesmotic reaction (Scheme 1) was used to calculate the ISEⅠ.
Table1. NICS aromaticity indexes (in ppm) at the ring centers and 1 Å above for five membered
monoheterocyles at the PW91/IGLO-Ⅲ//B3LYP/6-311+G** level.
NICS(1)zz
NICS(0)zz
NICS(1)iso NICS(0)iso
ISEⅠ
ISEⅡ
66.9
100.9
21.8
30
16.9
15.5
-21.1
-4.1
-8.8
-1.4
-14.5
-14.1
-32.7
-26.1
-10
-10.6
-24.6
-19.3
-32.4
-29.3
-10.9
-11.1
-24.7
-17.0
2.3
28.4
0.1
3.2
0.6
-2.1
-25.1
-3.2
-10.3
-2.5
-22.5
-20.9
-20.2
-3.1
-8.6
-3.7
-13.7
-17.1
-20.3
0.3
-7.7
0.2
-16.8
-20.1
-17.2
22.8
-6.8
-2.1
-16.4
-16.2
-
17.8
-
12.1
Scheme 1.
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R² = 0.8873
ISEⅠ(b)
ISEⅠ(a)
R² = 0.9208
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NICS(1)zz
NICS(0)zz
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R² = 0.9008 /通用格式
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R² = 0.8758
ISEⅠ(d)
ISEⅠ(c)
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NICS(1)iso
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NICS(0)iso
Figure 1
Uncategorized References
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