chemia - Studia

RALUCA O. POP, MIHAI MEDELEANU, MIRCEA MRACEC
The same explanation seems to be appropriate for the values obtained
for the shielding increments. The results presented in Table 1 show that all the
heteroanalogues of benzene have positive shielding increments, revealing
a possible aromatic character in all of the cases. The largest values of both
NICS(2.5) index and Δσ are obtained in case of heterobenzenes (λ 3 -P) 6 ,
(λ 3 -P-λ 3 -As) 3 and (λ 3 -As) 6 .
Shielding increments have also been computed at three different
steps in x and y directions (0.5 Å, 1.0 Å, 1.5 Å); the results are presented in
Table 3 and Table 4.
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Table 3. Shielding increments Δσ (ppm) computed in three different points
along the x axis (B3LYP/6-31G level)
Heterobenzene Δσ (x=0.5) (ppm) Δσ (x=1.0) (ppm) Δσ (x=1.5) (ppm)
C 6 H 6 0.974 0.938 0.896
(CH-λ 3 -N) 3 3.444 2.789 2.056
(CH-λ 3 -P) 3 4.147 3.385 2.463
(CH-λ 3 -As) 3 4.173 3.381 2.446
(λ 3 -N) 6 4.076 3.373 2.520
(λ 3 -N-λ 3 -P) 3 2.513 2.162 1.678
(λ 3 -N-λ 3 -As) 3 2.232 1.902 1.467
(λ 3 -P) 6 5.925 5.325 4.509
(λ 3 -P-λ 3 -As) 3 6.215 5.600 4.778
(λ 3 -As) 6 6.690 6.315 5.453
Table 4. Shielding increments Δσ (ppm) computed at three different points
along the y axis (B3LYP/6-31G level)
Heterobenzene Δσ (y=0.5) (ppm) Δσ (y=1.0) (ppm) Δσ (y=1.5) (ppm)
C 6 H 6 1.009 0.936 0.898
(CH-λ 3 -N) 3 3.445 2.804 2.065
(CH-λ 3 -P) 3 4.158 3.456 2.573
(CH-λ 3 -As) 3 4.188 3.472 2.601
(λ 3 -N) 6 4.076 3.375 2.521
(λ 3 -N-λ 3 -P) 3 2.505 2.119 1.579
(λ 3 -N-λ 3 -As) 3 2.224 1.867 1.391
(λ 3 -P) 6 5.925 5.308 4.395
(λ 3 -P-λ 3 -As) 3 6.219 5.591 4.670
(λ 3 -As) 6 6.961 6.300 5.323
The results presented in Table 2 and Table 3 show insignificant
differences between the values of the shielding increments computed along
x and y axes and this a possible consequence of the symmetry of the studied
heterobenzenes.