Full Paper
F-, Cl-, Br-, and I- derivatives, re-
spectively. These results show
the systematically smaller values
of charges of all halogen atoms
in HalC(CH3)3 compared with
those in HalC(NO ) . This is con-
2 3
Table 6. Distances [ꢁ] and angles [8] of some intra- and intermolecular interactions of 1–3 and ClC(NO ) .
[
12]
1
FC(NO
2
)
3
1 ClC(NO
2
)
3
2 BrC(NO
2
)
3
2 3
3 IC(NO )
XRD
GED
XRD
XRD
GED
XRD
[
a]
N···Ointra
2.637(3)
2.571(2)
2.547(2)
2.554(2)
2.518(9)
2.539(9)
2.504(11)
2.554(6)
2.542(6)
2.556(5)
2
2
.615(3)
.604(3)
2.611(7)
2.543(4)
2
3
sistent with the conclusions on
electronegativity made before.
The AIM basin volumes of hal-
ogen atoms in HalC(CH3)3 are
[
a]
Hal···Ointra
2.609(3)
2.939(2)
2.897(2)
2.903(2)
3.007(5)
2.984(7)
3.002(8)
3.158(4)
3.194(5)
3.204(4)
2
2
.563(3)
.616(3)
2.576(6)
2.980(4)
[
a]
Hal···Ointer
2.782(3)
141.0(2)
–
–
2.950(2)
172.8(1)
3.035(5)
165.3(2)
–
–
2.930(3)
172.8(2)
1
4
624,
3486,
4294,
and
CꢀHal···Ointer
3
937 Bohr , and in HalC(NO )
2
3
[
3
a] The shortest Hal···Ointra/inter contacts are given. For comparison: Sum of the van der Waals radii: r(N+O) =
.07 ꢁ, r(F+O) =2.99 ꢁ, r(Cl+O) =3.27 ꢁ, r(Br+O) =3.37 ꢁ, r(I+O) =3.50 ꢁ.
the corresponding basin vol-
umes are systematically larger
[
21]
(
Table 4). This indicates that in
HalC(NO2)3 the halogen atoms
does not necessarily indicate attractive interactions between
the oxygen and halogen atoms. Due to the particularly high
electron-withdrawing properties of the trinitromethyl moiety,
the CꢀHal bonds are shorter than usual, and consequently,
sterically overcrowded structures are observed.
are more diffuse. Also interesting is to compare basin volumes
of the central carbon atoms in both series of compounds.
Table 4 shows these quantities for the HalC(NO ) molecules.
2
3
The corresponding values for HalC(CH ) molecules are 36, 44,
3
3
3
44, and 47 Bohr . Thus, the carbon atoms are more compact in
all HalC(NO ) derivatives, in agreement with the concept of
The electron-withdrawing strength can be estimated using
the Taft parameter (s*), which reflects polar (inductive, field,
2
3
electronegativity and explaining the shortening of the HalꢀC
bonds in these molecules. Therefore, these bonds in 1–3, FꢀC
1.297(3) ꢁ (crystal), 1.300(11) ꢁ (gas phase); BrꢀC 1.853(5) ꢁ
[
22]
and resonance) effects. The trinitromethyl moiety has a very
[23]
large value of that polar substituent constant of s*=4.54,
compared with the smaller ones for fluorine (3.19), chlorine
(crystal), 1.869(6) ꢁ (gas phase); IꢀC 2.097(4) ꢁ (crystal), respec-
[
24]
3
(
2.94), bromine (2.80), and iodine (2.22). Consequently, con-
tively, are remarkably shorter than average HalꢀC
bond
sp
[19a]
ventional (electrostatic) properties of the halogen atoms are
expected to change under the action of the trinitromethyl
moiety, even the strongly electronegative fluorine atom.
Electronegativity can also be attributed to composition of s-
bonding orbitals. NBO calculations of BrC(NO ) have shown
lengths (FꢀC: 1.43 ꢁ, BrꢀC: 1.97 ꢁ, IꢀC: 2.16 ꢁ).
Gas-phase equilibrium structures determined by GED allow
a direct comparison with theoretically calculated structures. In-
terestingly, the previously predicted length of the CꢀF bond in
[
9b]
FC(NO ) (1.305 ꢁ on the B3LYP/6-311+G(d,p) level) agrees
2
3
2 3
that s(BrꢀC) can be described as being composed of
well with the value measured in this work at 1.300(11) ꢁ. How-
ever, the same combination of DFT functional and basis set
6.7
2.8
0
0
.68(sp ) +0.73(sp ) . The polarization coefficients 0.73>
Br C
[9b]
.68 can serve as indicators of electronegativity and show that
gives overestimated lengths for the CꢀBr bond (1.894 ꢁ)
in 2 bromine is less electronegative than the carbon atom due
to the nitro-substituent effect. For comparison we have per-
formed similar calculations for the BrC(CH ) molecule. In this
versus the experimental value of 1.869(6) ꢁ. The corresponding
thermally averaged r value found in this work (1.876(6) ꢁ) is
a
also slightly smaller, in fact, almost equal to the r values from
3
3
a
[10a,c]
case the inverted picture was obtained: The composition of
previous GED measurements (1.885(9) and 1.894(9) ꢁ).
6
.1
5.7
the s(BrꢀC) orbital was 0.73(sp ) +0.68(sp ) , indicating
Concerning the Hal···O contacts in HalC(NO ) according to
Br
C
2 3
a normal situation in which bromine is more negative than the
the simplest model, the calculated AIM charges, predict repul-
carbon atom. The same relationship has been found for s(Halꢀ sion for the pair F···O and attraction for the pairs Br···O, and
C) orbital composition in the I-, but not for the Cl-, and espe-
cially for the F- derivatives. Nevertheless, the trend of decreas-
ing halogen electronegativity with respect to that of the
carbon atom upon transition from HalꢀC(CH ) to HalꢀC(NO )
I···O. The Cl···O contact is on the border between repulsion
and attraction due to the almost neutral charge of the chlorine
atom (0.01 e). Indeed, the E(Hal,O) energies (Table 4) from the
more advanced IQA theory show that the F···O contacts desta-
bilize the molecule, the Cl···O contacts play only a minor role,
whereas the Br···O and I···O should stabilize the respective mol-
ecules. The opposite trend is observed for the Hal···N contacts,
in which F···N stabilizes the molecule, whereas Br···N and I···N
are destabilizing. The Cl···N contacts show only very weak in-
teractions as in the case of the Cl···O contacts. Thus, the short-
ening of the all HalꢀC bonds in each of HalC(NO ) cannot be
3
3
2 3
was observed for all types of halogen atoms. Another observa-
tion is the much higher s contribution in the carbon hybrid to
2
.8
5.7
the BrꢀC bond in BrC(NO ) (sp ) than in BrC(CH ) (sp ),
2
3
3 3
again explaining the shorter bond in the first. It may thus be
concluded that electronegativity plays an important role in
shortening of the HalꢀC bonds in the HalC(NO ) molecules.
2
3
QTAIM calculations support this view by comparing the
charges and basin volumes of halogen atoms in HalC(NO2)3
and HalC(CH ) molecules. The values for HalC(NO ) are given
2
3
explained by a single model based on the analysis of intramo-
lecular contacts of atoms.
3
3
2 3
in Table 4. The AIM charges of the halogen atoms in HalC(CH3)3
Previously performed calculations showed the surfaces of
the halogen atoms in Fꢀ, Clꢀ and BrꢀC(NO ) to be entirely
molecules are ꢀ0.69, ꢀ0.31, ꢀ0.20, and ꢀ0.07 e in the series of
2
3
Chem. Eur. J. 2014, 20, 1 – 13
7
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&
&
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