Chemistry Reference
In-Depth Information
Table 4
Calculated harmonic vibrational frequencies and infrared intensities of the di-
halogens as obtained with different methods applying the aug-cc-pVTZ basis set
a
[35]
MP2
CCSD(T)
B3LYP
BH&HLYP
Exp.
Refs.
F
2
1003
916
1050
1158
917
[26]
800 (27)
b
ClF
771
787 (29)
854 (36)
786
[26]
Cl
2
573
542
537
580
560
[26]
BrF
693 (30)
672
676 (32)
728 (41)
671
BrCl
461 (1.5)
434
430 (1.5)
461 (2.0)
444
Br
2
341
322
316
338
325
a
Frequencies in cm
-1
, infrared intensities in km
/
mol
b
Intensities in parentheses
From these monomer properties alone it is not quite clear how the trends in
the interaction strength with a fixed given acceptor B will develop. For the X
2
molecules the polarizabilities rise in the series F
2
,Cl
2
,andBr
2
. The dipole
moments of the interhalogens XY decrease in the series ClF, BrF, and BrCl,
whereas the polarizabilities increase in this series.
In agreement with experiment, all XY
NH
3
complexes have the expected
C
3v
equilibrium structure, irrespective of the computational level chosen, and
in all XY
···
···
NH
3
complexes, the heavier halogen atom forms the halogen bond
Y
N.Thepresenceofthe
C
3v
symmetry is verified via vibrational frequency
analyses. Hence, the XY
···
N substructure is perfectly linear. Assuming stan-
dard van der Waals radii values of 1.6
A
for N, and 1.5, 1.8, 1.9, and 2.1
A
for
the halogens F, Cl, Br, and I, respectively, one arrives at intermolecular dis-
tances of 3.1, 3.4, 3.5, and 3.7A for the hypothetical intermolecular van der
Waals distances of the XY
···
NH
3
complexes. The experimental [7-11] and
calculated interatomic distances of all six XY
···
···
NH
3
complexes, as compiled
in Table 5, demonstrate conclusively that the Y
N distances are significantly
shorter by about 0.4-1.1A, thus fulfilling one criterion that resembles the be-
havior of hydrogen-bonded complexes. The comparison between optimized
structures obtained with and without CP correction shows once more that
theCPcorrectionsarestillsubstantial(0.02-0.07
A
) and nearly identical
for MP2 and CCSD(T), but are entirely negligible for both DFT variants.
The encouraging and amazingly good performance of the BH&HLYP method
for the halogen-bonded cases, judged by the agreement with CCSD(T) and
with experiment, has already been noted earlier by several other groups [20-
22, 27-30, 32, 34].
The calculated intramolecular XY distances and the lengthening relative
to the unperturbed monomers are given in Table 6. In all cases, the XY dis-
tances are elongated in the complex and these elongations range from about
0.01
A
in F
2
···
NH
3
to about 0.07
A
in BrF
···
···
NH
3
. With the exception of
F
2
···
NH
3
, all methods predict quite similar modifications of the XY distances
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