Theoretical Characterization of the Trends in Halogen Bonding - Halogen Bonding: Fundamentals and Applications - page 9

thus following neither purely the trends in XY dipole moments nor in XY po-
larizabilities. Overall, the interaction energies follow the order F 2 ,Cl 2 ,Br 2 ,
BrCl, ClF, and BrF. The calculated dipole moments and the dipole moment
enhancements relative to the sum of monomer dipole moments, displayed in
Table 8, show a somewhat different trend. increases in the order F 2 ,Cl 2 ,

ClF, Br 2 , BrCl, and BrF and varies from about 0.4 DinF 2 ···

∆ µ

NH 3 to about 3.6 D

in BrF NH 3 . These quantities are probably also a more reliable guideline

to the relative importance of charge transfer than the notoriously basis set
dependent atomic charges.
Finally, MP2 and DFT calculated intramolecular X-Y stretching frequen-
cies, their red shifts relative to the monomers, and the intermolecular Y

···

N
stretching frequencies are collected in Table 9, together with the correspond-
ing infrared intensities. Although there are no experimental gas-phase data
available, the predictions can be considered as reliable. The MP2 calcu-
latedredshiftsoftheX-Ystretchingmodeareintherangeof-37 cm -1
for Br 2 ···

···

NH 3 to - 174 cm -1 NH 3 . Their infrared intensities are

quite large. Comparable intensities are also predicted for the intermolecular
stretching vibrations.

for ClF ···

Table 9 ···

N stretching frequencies, frequency shifts,
and infrared intensities of the XY ··· NH 3 complexes as obtained with different methods
applying the aug-cc-pVTZ basis set a

Calculated harmonic X-Y and X

XY MP2 B3LYP BH&HLYP Refs.

F 2 ν F-F 932 (10) 631 (328) 1109 (48) [26]

∆ ν F-F - 106 - 419 - 49

ν F-N 96 (2) 191 (132) 89 (2)

ClF ν Cl-F 626 (232) 607 (249) 711 (241) [26, 28]

∆ ν Cl - F - 174 - 180 - 144

ν Cl-N 237 (104) 253 (66) 206 (57)

Cl 2 ν Cl-Cl 505 (45) 435 (90) 525 (42) [26]

∆ ν Cl-Cl -68 -102 -55

ν Cl-N 138 (34) 164 (57) 130 (23)

BrF ν Br-F 598 (133) 554 (164) 618 (181)

∆ ν Br-F

- 95 - 122 - 110

ν Br-N 247 (58) 258 (52) 237 (58)

BrCl ν Br-Cl 393 (48) 359 (56) 405 (52)

∆ ν Br-Cl -68 -71 -56

ν Br-N 186 (87) 195 (70) 164 (57)

Br 2 ν Br-Br 294 (23) 266 (29) 301 (23)

∆ ν Br-Br -37 -50 -28

ν Br-N 168 (76) 174 (65) 145 (47)

a Frequencies and shifts relative to the XY monomers in cm -1 , infrared intensities in
km

/ mol in parentheses