Chemistry Reference
In-Depth Information
Table 18 Analysis of the ED at the BCP of the terminal CH bond (BCP
A
). EDs are given in
(e/
˚
3
), Laplacians in (e/
˚
5
), and the contributions in percent
r BCP
ð
Þ
r
2
r
ð
BCP
Þ
Source function contributions to the ED at the BCP
Adjacent
a
One bond
b
Two bonds
b
Three or more bonds
b
(a)
Structures fully optimized
Methane
1.86
22.39
90
10
-
-
Ethane
1.87
22.45
89
9
3
-
Propane
1.87
22.43
89
8
2
1
Butane
1.87
22.41
89
8
2
1
Pentane
1.87
22.42
89
8
2
1
22.41
Hexane
1.87
89
8
2
1
22.41
Heptane
1.87
89
8
2
1
Octane
1.87
22.41
89
8
2
1
Nonane
1.87
22.41
89
8
2
1
Decane
1.86
22.40
89
8
2
1
(b)
Truncated systems at the geometry of decane, saturated with hydrogen
Methane
1.87
22.79
92
8
-
-
Ethane
1.86
22.34
89
9
2
-
Propane
1.86
22.41
89
8
2
1
Butane
1.87
22.42
89
8
2
1
Pentane
1.87
22.41
89
8
2
1
Hexane
1.87
22.41
89
8
2
1
Heptane
1.87
22.40
89
8
2
1
Octane
1.87
22.40
89
8
2
1
Nonane
1.86
22.40
89
8
2
1
Decane
22.40 89 8 2 1
(c)
Truncated systems at the geometry of decane saturated with hydrogen, missing atoms replaced
by point charges
Methane
1.86
1.88
22.99
92
8
-
-
Ethane
1.86
22.43
89
9
2
-
Propane
1.87
22.49
89
8
2
1
Butane
1.87
22.48
89
8
2
1
Pentane
1.87
22.46
89
8
2
1
Hexane
1.87
22.44
89
8
2
1
Heptane
1.87
22.43
89
8
2
1
Octane
1.87
22.42
89
8
2
1
22.43
Nonane
1.87
89
8
2
1
22.40
Decane
1.86
89
8
2
1
a
Sum of contributions of the atoms which form the bond
b
Sum of contributions of the atoms being x covalent bonds away
To differentiate between direct effects and the influence of the geometry, we
took decane and successively shortened the chain by replacing the terminal methyl
group by a hydrogen atom. The molecular structure was not optimized but frozen at
the decane geometry. The results are also collected in Tables
18b
and
19b
.As
shown in these tables, the ED at the terminal C-H bond does not change at all.
Furthermore, for chains longer than ethane, also the major contributions stay nearly
constant. A small variation is only observed between ethane and methane. Due to
the substitution of the alkyl chain by a hydrogen atom, the terminal CH bond is
artificially elongated, whereby the contribution of this hydrogen atom is lowered by
