Large gas-solid structural differences in complexes of haloacetonitriles with boron trifluoride

Article abstract of DOI:10.1021/ic051491x

The structural properties of the singly halogenated derivatives of CH ₃CN-BF₃ (X-CH₂CN-BF₃: X = F, Cl, Br, I) have been investigated via single-crystal X-ray crystallography, solid-state infrared spectroscopy, and correlated electronic-structure theory. Taken together, these data illustrate large differences between the gas-phase and solid-state structures of these systems. Calculated gas-phase structures (B3PW91/aug-cc-pVTZ) of FCH₂CN-BF₃, ClCH ₂CN-BF₃, and BrCH₂CN-BF₃ indicate that the B-N dative bonds in these systems are quite weak, with distances of 2.422, 2.374, and 2.341 A, respectively. However, these distances, as well as other calculated structural parameters and normal-mode vibrational frequencies, indicate that the dative interactions do become slightly stronger in proceeding from F- to Br-CH₂CN-BF₃. In contrast, solid-state structures for FCH₂CN-BF₃, ClCH ₂CN-BF₃, and ICH₂CN-BF₃ from X-ray crystallography all have B-N distances that are quite short, about 1.65 A. Thus, the B-N distances of the F- and Cl-containing derivatives contract by over 0.7 A upon crystallization. Large shifts in the vibrational modes involving motions of the BF₃ subunit parallel these structural changes. An X-ray crystal structure could not be determined for BrCH ₂CN-BF₃(s), but the solid-state IR spectrum is consistent with those obtained previously for related complexes and suggests that the solid-state structure resembles those of the others, and in turn, implicates a large gas-solid structural difference for this species as well.

Full text of DOI:10.1021/ic051491x

Inorg. Chem. 2006, 45, 722−731
Large Gas−Solid Structural Differences in Complexes of
Haloacetonitriles with Boron Trifluoride
James A. Phillips,*,† Jason A. Halfen,*,† John P. Wrass,† Christopher C. Knutson,† and
Christopher J. Cramer‡
Department of Chemistry, UniVersity of Wisconsin-Eau Claire, 105 Garfield AVenue, Eau Claire,
Wisconsin 54701, and Department of Chemistry and Supercomputer Institute, UniVersity of
Minnesota, 207 Pleasant Street SE, Minneapolis, Minnesota 54455
Received September 1, 2005
The structural properties of the singly halogenated derivatives of CH3CN
have been investigated via single-crystal X-ray crystallography, solid-state infrared spectroscopy, and correlated
electronic structure theory. Taken together, these data illustrate large differences between the gas-phase and
solid-state structures of these systems. Calculated gas-phase structures (B3PW91/aug-cc-pVTZ) of FCH2CN BF3,
ClCH2CN BF3, and BrCH2CN BF3 indicate that the B N dative bonds in these systems are quite weak, with
−BF3 (X−CH2CN−BF3: X ) F, Cl, Br, I)
−
−
−
−
−
distances of 2.422, 2.374, and 2.341 Å, respectively. However, these distances, as well as other calculated structural
parameters and normal-mode vibrational frequencies, indicate that the dative interactions do become slightly stronger
in proceeding from F
−
to Br−CH2CN−BF3. In contrast, solid-state structures for FCH2CN−BF3, ClCH2CN−BF3, and
ICH2CN BF3 from X-ray crystallography all have B
−
−N distances that are quite short, about 1.65 Å. Thus, the B−N
distances of the F- and Cl-containing derivatives contract by over 0.7 Å upon crystallization. Large shifts in the
vibrational modes involving motions of the BF3 subunit parallel these structural changes. An X-ray crystal structure
could not be determined for BrCH2CN
previously for related complexes and suggests that the solid-state structure resembles those of the others, and in
turn, implicates a large gas solid structural difference for this species as well.
−BF3(s), but the solid-state IR spectrum is consistent with those obtained
−
1-13
I. Introduction
Most of these have focused upon CH3CN-BF3
and/or
HCN-BF3,1-2,8-9,13-22 and recent interest has stemmed from
the fact that the gas-phase structures of these species manifest
partial bonds, which seem to defy classification as purely
bonding or nonbonding interactions.1,2 For example, CH3CN-
The structure and bonding of nitrile-boron trifluoride
complexes has been the subject of numerous studies.1-23
* To whom correspondence should be addressed. Phone: 715-836-5399
(J.A.P.); 715-836-4360 (J.A.H.). Fax: 715-836-4979 (J.A.P.); 715-836-4979
(J.A.H.). E-mail: phillija@uwec.edu (J.A.P.); halfenja@uwec.edu (J.A.H.).
† University of Wisconsin-Eau Claire.
(12) Giesen, D. J.; Phillips, J. A. J. Phys Chem. A 2003, 107, 4009.
(13) Beattie, I. R.; Jones, P. J. Angew. Chem., Int. Ed. Engl. 1996, 35,
1527.
‡ University of Minnesota.
(1) Leopold, K. R., Canagaratna, M.; Phillips, J. A. Acc. Chem. Res. 1997,
30, 57.
(2) Leopold, K. R. In AdVances in Molecular Structure Research; Hargittai,
M., Hargittai, I., Eds.; JAI Press: Greenwich, CT, 1996; Vol. 2.
(3) Dvorak, M. A.; Ford, R. S.; Suenram, R. D.; Lovas, F. J.; Leopold,
K. R. J. Am. Chem. Soc. 1992, 114, 108.
(4) Swanson, B.; Shriver, D. F.; Ibers, J. A. Inorg. Chem. 1969, 8, 2183.
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Coerver, H. J.; Curran, C. J. Am. Chem. Soc. 1958, 80, 3522.
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K. R. J. Phys. Chem. 1993, 97, 10630.
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100, 6904.
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Comput. Sci. 1996, 36, 377. (b) Nxumalo, L. M.; Andrzejak, M.; Ford,
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(20) Fiacco, D. L.; Leopold, K. R. J. Phys. Chem. A 2003, 107, 2808.
(21) Venter, G.; Dillen, J. J. Phys. Chem. A 2004, 108, 8378.
(22) Phillips, J. A. and Cramer, C. J. J. Chem. Theor. Comput. 2005, 1,
827.
(23) Phillips, J. A.; Geisen, D. J.; Wells, N. P.; Halfen, J. A.; Knutson C.
C.; Wrass, J. P. J. Phys. Chem. A 2005, 109, 8199.
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722 Inorganic Chemistry, Vol. 45, No. 2, 2006
10.1021/ic051491x CCC: $33.50
© 2006 American Chemical Society
Published on Web 12/24/2005

Haloacetonitrile-BF3 Complexes
Table 1. Gas Phase Proton Affinities and Ionization Potentialsa
molecule
NH3
(CH3)3CCN
BrCH2CN
ClCH2CN
CH3CN
FCH2CN
NCCN
HCN
ionization potential (eV)
proton affinity (kJ/mol)
10.07
-
853.6
810.9
-
745.7
779.2
-
674.7
712.9
688.4
11.28
11.95
12.20
12.81
13.37
13.60
13.93
Figure 1. HOMOs for F-, Cl-, and Br-CH2CN (HF/6-311G*).
F3CCN
halogen substitution, and is thus the most likely bonding site
for hard Lewis acids. Nonetheless, the HOMOs do suggest
that the XCH2CN species are potentially bifunctional donors
and may coordinate to soft Lewis acids via the halogen.
Ultimately, it is clear that these compounds should show rich
coordination behavior.
While our long-term goal is to assess the effects of inert,
condensed-phase environments on the structural properties
of XCH2CN-BF3 complexes, at this point, we lack bench-
mark gas-phase or solid-state data for these systems to
compare with our forthcoming matrix isolation-IR results.
Here we present gas-phase structures and frequencies
obtained from density functional calculations, solid-state
structures from single-crystal X-ray crystallography, and
solid-state IR spectra for several of these complexes. These
results suggest, even in the absence of experimental, gas-
phase data, that very large structural changes do indeed occur
in these systems upon crystallization and that large vibra-
tional frequency shifts parallel these changes.
a From ref 26.
BF3 has a B-N distance of 2.01 Å and an N-B-F angle of
95.6° in the gas phase,3 and these values clearly lie between
24
those for fairly strong complexes such as H3N-BF3 and
“van der Waals complexes” such as N2-BF3.25 Yet another
remarkable feature of these complexes is that they undergo
dramatic structural changes upon crystallization. For HCN-
BF3, the gas-phase B-N distance is 2.47 Å,14 but it contracts
to 1.65 Å upon crystallization.15
Such large gas-solid structural differences raise the
question as to how a solvent or other bulk, condensed-phase
medium may affect these systems, and our work has been
concerned mainly with the effects of inert cryogenic matri-
ces.11,23 For CH3CN-BF3 in solid argon,11 a comparison of
the matrix-IR frequencies with those measured for the solid
and those calculated for the two minimum-energy gas-phase
structures (at the B3LYP/aug-cc-pvqz level)12 indicate that
the matrix does indeed induce a considerable contraction of
the B-N bond. At this point, we turn to singly halogenated
derivatives of CH3CN-BF3, i.e., X-CH2CN-BF3, where
X ) F, Cl, Br, and I. From a simple induction standpoint,
one would expect these species to be weaker electron donors
than CH3CN, in contrast to nitriles with larger organic
substituents, (e.g., (CH3)3CCN and C6H5CN). We have
recently found that (CH3)3CCN-BF3 and C6H5CN-BF3 have
dative bonds that are somewhat stronger than that in
CH3CN-BF3, and as a result, the condensed-phase effects
on their structural properties are somewhat less pronounced.23
Beyond simple inductive effects, the ionization energies
and proton affinities26 listed in Table 1 for select nitriles and
NH3 (as a strong-base reference point) impart a conflicting
picture of relative Lewis base strength among X-CH2CN
species. For example, the ionization energy of CH3CN lies
between those of FCH2CN and ClCH2CN, but the proton
affinity of ClCH2CN is actually much lower than that of
CH3CN. This effect stems from an increasing amount of
halogen p-orbital character of the highest occupied molecular
orbitals (HOMOs) as one proceeds from F- to BrCH2CN
(Figure 1). In turn, the ionization potentials decrease because
the halogen p orbitals are higher in energy (i.e., they have
less-negative energy values). However, the nitrile moiety still
bears a large partial negative partial charge, irrespective of
II. Experimental Section
Materials. FCH2CN (98%), ClCH2CN (99%), BrCH2CN (97%),
and ICH2CN (98%) were obtained from Aldrich and were subjected
to a few freeze-pump-thaw cycles prior to use. BF3 (99.5+%)
was also obtained from Aldrich, and N2 (standard purity) was
obtained from Praxair. Both gases were used without any additional
purification.
Preparation and Handling of Crystalline Samples. Solids were
prepared by first adding about 3-5 mL of liquid nitrile to a 50 or
100 mL Schlenk tube. The top of the tube was fitted with a rubber
septum. Gases were introduced through the septum via a small
hypodermic needle and were allowed to vent through a sidearm
(and into a fume hood) by slightly opening a Teflon stopcock.
Initially, the tube was purged with a steady flow of N2 for several
minutes to displace ambient air. Then, BF3 was slowly introduced
to the tube in a similar manner. In the case of the Br- and
I-containing derivatives, solids formed immediately at the bottom
of the tube, but for the F and Cl complexes, no obvious signs of
reaction were apparent aside from a slight warming. In any event,
the BF3 flow was continued for a few additional minutes to ensure
a slight excess, and it was clear (via white fumes from the sidearm)
that most of the nitrogen had been displaced. The excess BF3 was
allowed to remain in the tube, and the sample tube was sealed and
stored at reduced temperature for several days to allow for crystal
growth. For the I, Br, and Cl species, optimum crystal growth
occurred when the tubes were kept in a cold room at 4-5 °C. For
FCH2CN-BF3, slightly colder temperatures were found to be
optimal, approximately 0 °C. All the compounds were found to be
unstable and rather difficult to handle, especially FCH2CN-BF3
and ClCH2CN-BF3, as they were apt to decompose upon warming
and seemed to react with ambient water vapor. As a result, each
required special handling at low temperature to obtain and preserve
(24) Fujiang, D.; Fowler, P. W.; Legon, A. C. J. Chem. Soc., Chem.
Commun. 1995, 113.
(25) Janda, K. C.; Bernstien, L. S.; Steed, J. M.; Novick, S. E.; Klemperer,
W. J. Am. Chem. Soc. 1978, 100, 8074.
(26) The NIST Chemistry Webbook. http://webbook.nist.gov/chemistry
(accessed December 2004).
Inorganic Chemistry, Vol. 45, No. 2, 2006 723

Phillips et al.
Table 2. X-Ray Crystallographic Data for Haloacetonitrile-BF3 Complexesa
complex
empirical formula
fw
FCH2CN-BF3
C2H2BF4N
126.86
ClCH2CN-BF3
C2H2BClF3N
143.31
ICH2CN-BF3
C2H2BF3IN
234.76
cryst syst
space group
a (Å)
b (Å)
c (Å)
monoclinic
P21
5.173(1)
5.754(1)
7.927(2)
triclinic
P1h
monoclinic
P21/c
9.947(2)
8.240(1)
7.671(4)
7.499(1)
7.682(1)
9.275(1)
106.66(1)
90.39(1)
90.39(1)
519.2(2)
4
R (deg)
â (deg)
99.00(2)
108.96(2)
γ (deg)
V (Å3)
233.05(8)
2
594.6(3)
4
Z
T (K)
173(2)
1.808
173(2)
1.833
173(2)
2.622
calcd density (g‚cm-3
cryst size (mm)
)
0.40 × 0.35 × 0.10
0.62 × 0.30 × 0.05
0.20 × 0.20 × 0.04
absorption coefficient (mm-1
2θ max (deg)
)
0.220
0.680
5.338
50.02
899
806
681
49.86
1953
1804
1615
49.42
983
950
742
No. of reflns collected
No. of independent reflns
No. of reflns with I > 2σ(I)
No. of variables
73
146
73
R1 (wR2) [I > 2σ(I)] b
R1 (wR2) [all data]
GOF
0.0517 (0.1293)
0.0643 (0.1382)
1.078
0.0328 (0.0855)
0.0385 (0.0897)
1.088
0.0437 (0.1017)
0.0676 (0.1104)
1.055
difference peaks (e-‚Å-3
)
0.194, -0.216
0.448, -0.431
0.890, -0.887
a See Experimental Section for additional data collection, reduction, structure solution, and refinement details. b R1 ) ∑||Fo| - |Fc||/∑|Fo|; wR2 )
2
2
2
[∑[w(Fo -Fc )2]]1/2 where w ) 1/σ2(Fo ) + (aP)2 + bP.
appropriate samples for crystallographic characterization. A single
crystal of FCH2CN-BF3 was selected by immersing a vial
containing the solid in liquid nitrogen, transferring multiple potential
samples to the tip of a precooled spatula, and selecting an
appropriate single crystal while blanketing the spatula with a stream
of cold nitrogen gas. A single crystal of ClCH2CN-BF3 was
selected by examining the bulk sample using a locally designed
cold stage, cooled to a temperature below -70 °C with cold nitrogen
gas. Samples of ICH2CN-BF3 were somewhat less thermally
sensitive. A crystal of this complex was selected by examining the
bulk sample while covered by a generous film of chilled (0 °C)
heavyweight oil to protect the crystalline material from the
atmosphere. Crystalline samples of BrCH2CN-BF3 could also be
prepared and examined in anticipation of a crystallographic structure
determination, and several data sets were collected for this complex,
but an as-yet-unresolved twinning issue prevented successful
determination of the structure.
Crystallographic Data Collection, Structure Solution, and
Refinement. Crystals of each of the three complexes, selected as
described above, were mounted on the end of a thin-walled glass
capillary and transferred to a Bruker-Nonius MACH3S diffracto-
meter equipped with graphite-monochromated Mo KR radiation (λ
) 0.71073 Å) for data collections at -100 °C. Unit cell constants
were determined from a least squares refinement of the setting
angles of 25 high-angle, machine-centered reflections. Intensity data
were collected using the ω/2θ scan technique to a maximum 2θ
value of ∼50°. Corrections for crystal decay were not required.
For ClCH2CN-BF3 and ICH2CN-BF3, absorption corrections
based on azimuthal scans of several reflections were applied,
resulting in transmission factors between 1.0 and 0.8007 (ClCH2CN-
BF3) or 1.0 and 0.6896 (ICH2CN-BF3). The intensity data were
corrected for Lorentz and polarization effects and converted to
structure factors using the CrystalStructure suite of programs.27
Space groups for each complex were determined on the basis of
systematic absences and intensity statistics. Successful direct
methods solutions were calculated, which provided the positions
of most non-hydrogen atoms directly from the E-map. Remaining
atoms were located following several cycles of least squares
refinement and structure expansion using the SHELXTL suite of
programs.28 Hydrogen atoms were added geometrically. All non-
hydrogen atoms were refined with anisotropic displacement pa-
rameters. Carbon-bound hydrogen atoms were refined as riding
atoms with group isotropic displacement parameters fixed at 1.2
× U(eq) of the host carbon atom. The asymmetric units of
FCH2CN-BF3 and ICH2CN-BF3 contain a single formula unit of
each of the complex, while two crystallographically independent
complexes are found in the asymmetric unit of ClCH2CN-BF3. In
the final stages of refinement of ICH2CN-BF3, a number of low-
angle hk0 reflections were excluded from the refinement as the
Fo’s of these reflections were systematically larger than their Fc’s.
This behavior was ascribed to the presence of a small satellite crystal
that could not be mechanically separated from the larger specimen.
Relevant structure determination information for the complexes is
presented in Table 2. Selected interatomic distances and angles and
thermal ellipsoid representations of the refined structures are shown
below. Full details of the structure determinations, in CIF format,
are provided as Supporting Information.
Solid-State IR Spectra. Infrared spectra were collected using a
previously described matrix isolation-IR apparatus,11,23 which
consists of a Cryomech ST15 optical cryostat fitted with KBr
windows (both internal and external). Temperature was measured
and controlled using a Scientific Instruments 9600-1 temperature
controller and a single Si diode located at the end of the second
refrigeration stage. Spectra were collected with a Nicolet Avatar
360 FTIR, at 1.0 cm-1 resolution, and typically, 100 scans were
averaged for both sample and background data. The observed peak
maxima were reproducible to (4 cm-1. The cryostat apparatus is
assembled on a movable cart, which is wheeled into the spectrom-
eter sample compartment for each spectrum. As such, interference
from ambient water vapor and CO2 in the unpurged beam path can
be problematic in the case of weak absorbance signals.
(27) CrystalStructure, version 3.6.0; Rigaku Corporation and Rigaku/
MSC: The Woodlands TX, 2004.
(28) SHELXTL, version 6.10; Bruker AXS: Madison, WI, 2000.
724 Inorganic Chemistry, Vol. 45, No. 2, 2006

Haloacetonitrile-BF3 Complexes
Thin films of the bulk solids were prepared by first making a
dilute gas mixture containing nitrile, BF3, and argon on a vacuum
line. For the Cl- and F-containing species, mixtures containing
approximately 0.5% nitrile, 0.5% BF3, and 99% argon, were used,
though in some cases, a slight excess of nitrile was added to
minimize peaks due to unreacted BF3 (between 1450 and 1500
cm-1). For BrCH2CN-BF3, the mixtures were more dilute, typically
0.2% BrCH2CN and BF3, since the vapor pressure of BrCH2CN is
much lower that of FCH2CN and ClCH2CN. Films were deposited
directly onto the cold (120 K) KBr sample window by flowing the
gas mixtures into the cryostat vaccum chamber at a rate of roughly
5 mmol/h. Argon does not condense on the sample window at this
temperature; thus, only the nitrle and BF3 are deposited. Like
(CH3)3CCN-BF3 and C6H5CN-BF3,23 the spectra obtained closely
resemble low-temperature, solid-state IR spectra of CH3CN-BF3,6
indicating that the majority of the sample consists of nitrile-BF3
complexes, though the very bright vibrational bands of excess,
unreacted BF3 were also observed in some experiments.
for some MP2 calculations on FCH2CN-BF3, including those in
which we explored the relative stability of F-bonded isomeric
structures.
IV. Results and Discussion
Computational Structure Results. Initial B3LYP calcu-
lations indicated that all complexes were slightly bent about
the C-N-B linkage and, moreover, that the torsional degree
of freedom was extremely flat. Thus, to be systematic in
our search for the minimum-energy structures, we conducted
four parallel sets of optimizations, each starting with the
C-N-B angles set to 180° in either the eclipsed or staggered
conformation, and with or without enforcing Cs symmetry.
The staggered forms were found to be most stable at the
B3LYP/6-311+G** level, but by only a few calories per
mole (0.005 kcal/mol in the case of FCH2CN-BF3). Fur-
thermore, each complex was indeed bent slightly along the
C-N-B linkage by about 2°, so the conformers are not
strictly “eclipsed” or “staggered” in the formal sense.
Nonetheless, we retain these terms throughout for the sake
of simplicity.
At this point, we identified two reasons to be concerned
with these B3LYP results. First of all, B3LYP results for
CH3CN-BF3 lacked complete convergence with even larger
basis sets, up to aug-cc-pVQZ.12 Furthermore, a recent study
of amine-borane systems cast doubt on the performance of
B3LYP for B-N dative bonds.34 Thus, we conducted a
survey of various DFT methods (and MP2 with somewhat
smaller basis sets) on the equilibrium structure of staggered
FCH2CN-BF3 (Cs) under the presumption that some degree
of consistency would emerge. Table 3 lists equilibrium B-N
distances obtained in this survey, and overall, there is a
considerable degree of inconsistency among the various
methods. For the most part, however, B-N distances are a
few hundredths of an angstrom shorter when diffuse func-
tions are used (i.e., aug-cc-pVTZ), and this has been noted
in several previous instances for nitrile-BF3 systems.12,21-23
Regardless, in the absence of an experimental structure, it
was unclear which method was most reliable. To address
this ambiguity, we conducted (and recently completed) a
validation study on HCN-BF3,22 a similar and also fairly
weak complex for which an experimental structure has
been determined.14 In that work, we found that several hy-
brid DFT/HF methods (B3LYP, B3PW91, B98, and
mPW1PW91)29 yielded structures that agreed favorably with
experiment, with B3PW91 providing the best agreement,
even superior to MP2 (with the aug-cc-pVTZ basis set).
For the most part, the results in Table 3 track those
obtained for HCN-BF3,22 and thus, we chose to utilize
B3PW91/aug-cc-pVTZ as our preferred method for this
study. For completeness sake, we obtained B3LYP/aug-cc-
pVTZ and mPW1PW91/aug-cc-pVTZ results as well. Se-
lected structure parameters of both the eclipsed and staggered
conformers from all three methods are listed in Table 4, and
complete, minimum-energy B3PW91/aug-cc-pVTZ struc-
tures are displayed in Figure 2. For the F- and Cl-containing
III. Computational Methods
The HF/6-311G* orbitals displayed in Figure 1, as well as
preliminary B3LYP29 calculations on the X-CH2CN-BF3 (X )
F, Cl, Br) systems using Pople basis sets (6-31G*, 6-31+G*,
6-311G*, and 6-311+G**)29,30 were performed using SPARTAN
‘02 (Macintosh v 1.0.3, and Windows v 1.0.1).31 Aside from this
preliminary work, all other computations were performed using
Gaussian03,32 which enabled the use of several additional density
functional methods, larger basis sets, and tighter convergence
criteria for geometry optimizations. Version b.0.1 was used for MP2
and all density functional methods except O3LYP,29 for which
version c.0.3 was used. An ultrafine grid was employed for all DFT
calculations in Gaussian, and convergence criteria for optimizations
(except as noted) were set using the “opt)tight” option (which sets
the maximum and RMS forces to 1.5 × 10-5 and 1.0 × 10-5
hartrees/bohr, respectively, and the maximum and RMS displace-
ments to 6.0 × 10-5, and 4.0 × 10-5 bohr, respectively). The aug-
cc-pVTZ basis set29,33 was used for most Gaussian03 calculations,
although the cc-pVTZ basis set33 was also used for DFT calculations
on FCH2CN-BF3 to examine the effect of diffuse functions
(signified by “aug-“) on the structural results. The analogous
double-ú basis sets (cc-pVDZ, and aug-cc-pVDZ)33 were also used
(29) For a recent overview of density functional theory and other
computational methods and basis sets used in this work, see: Cramer,
C. J. Essentials of Computational Chemistry, 2nd ed.; John Wiley and
Sons: Sussex, 2004, and references therein.
(30) (a) Krishnan, R.; Binkley, J. S.; Seeger, R.; Pople, J. A. J. Chem.
Phys. 1980, 72, 650.
(31) SPARTAN ‘02; Wavefunction, Inc.: Irvine, CA, 2002.
(32) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin,
K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone,
V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G.
A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.;
Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai,
H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.;
Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R.
E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J.
W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.;
Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.;
Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari,
K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.;
Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.;
Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.;
Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.;
Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A.
Gaussian 03, revision B.03; Gaussian, Inc.: Wallingford, CT, 2004.
(33) Woon, D. E.; Dunning, T. H., Jr. J. Chem. Phys. 1993, 98, 1358.
(34) Gilbert, T. M. J. Phys. Chem. A. 2004, 108, 2550.
Inorganic Chemistry, Vol. 45, No. 2, 2006 725

Phillips et al.
Table 3. Equilibrium B-N Distances of FCH2CN-BF3 Obtained
on B3LYP/6-311+G**, but the energy difference is still only
a few calories per mole and arguably beyond the accuracy
of the computations. It is worth noting that torsional
frequencies obtained for the eclipsed forms (see below) are
all real, but in the case of ClCH2CN-BF3, the staggered
frequencies were found to be real as well. Effectively, these
complexes are best viewed as free rotors. Moreover, the
torsional conformation has very little impact on the structural
parameters with which we are mainly concerned (i.e., the
B-N distance and N-B-F angle).
We also attempted to locate a minimum-energy structure
of FCH2CN-BF3 in which the BF3 was bonded to the F
(i.e., NCCH2FsBF3). Using MP2, with double-ú basis sets,
a symmetric (Cs) structure was found that had an inter-
molecular B-F distance of about 2.4 Å. The F atoms of the
BF3 group were directed toward the H’s on FCH2CN,
suggestive of a very weak H-bonding interaction, at a
distance of about 2.75 Å. This structure was found to be 1.8
kcal/mol higher in energy than the N-bonded form (at the
MP2/aug-cc-pVDZ level). We also calculated the energy
difference between these structures at the MP2/aug-cc-pVTZ
level and found that the F-bonded structure was 2.6 kcal/
mol higher in energy. While DFT methods may be suspect
given this apparently weak F-B interaction, we note that
the energy difference between the analogous structures was
also 2.6 kcal/mol at the B3PW91/aug-cc-pVTZ level. (The
F-bonded structure was obtained using default convergence
criteria.) Given the energy differences and the observation
that all of the crystalline complexes are N-bonded (below),
we did not examine the Cl and Br derivatives for analogous
structures.
For all of the gas-phase complexes, the B-N distances
are long and the N-B-F angles are just slightly larger than
90°, indicating that these complexes are fairly weakly bonded
in the gas phase. However, we neglected to calculate absolute
binding energies since neither DFT nor MP2 were found to
be accurate for HCN-BF3 compared to very highly cor-
related levels of electronic structure theory.22 Nevertheless,
there are slight yet very clear trends that suggest that the
dative bonds do become stronger in proceeding from
FCH2CN-BF3 to BrCH2CN-BF3, as would be expected on
the basis of simple inductive arguments. For example, the
(B3PW91/aug-cc-pVTZ) B-N distance is predicted to be
2.422 Å in FCH3CN-BF3, 2.374 Å in ClCH2CN-BF3, and
2.341 Å in BrCH2CN-BF3, and in turn, the N-B-F angles
open by a few tenths of a degree (on average) across the
three complexes. The B-F distances are only a few
thousandths of an angstrom longer than that calculated for
free BF3 at the B3PW91/aug-cc-pvVTZ level of theory,22
but they do increase very slightly across the series. Interest-
ingly, the degree of bend about the C-N-B linkage is
essentially identical among the three complexes.
Using Various Computational Methodsa
method
MP2
basis set
R(B-N) (Å)
cc-pVDZ
2.431
aug-cc-pVDZ
2.318
2.387
cc-pVTZ
Hybrid DFT Methods
B3LYP
B3PW91
MPW1K
mPW1PW91
O3LYP
cc-pVTZ
2.525
2.514
2.445
2.423
2.283
2.250
2.365
2.338
2.926
2.975
aug-cc-pVTZ
cc-pVTZ
aug-cc-pVTZ
cc-pVTZ
aug-cc-pVTZ
cc-pVTZ
aug-cc-pVTZ
cc-pVTZ
aug-cc-pVTZ
Pure DFT Methods
mPWPW91
BPW91
BLYP
cc-pVTZ
2.452
2.421
2.548
2.524
2.630
2.623
3.106
3.189
aug-cc-pVTZ
cc-pVTZ
aug-cc-pVTZ
cc-pVTZ
aug-cc-pVTZ
cc-pVTZ
OLYP
aug-cc-pVTZ
a These results are for the nearly staggered conformer (Cs symmetry),
which was ultimately found to be slightly higher in energy than the nearly
eclipsed form. For B3PW91 (see below), the B-N distance differs by less
than 0.001 Å between the two conformers. See text for discussion.
species, the B3LYP, B3PW91, and mPW1PW91 results
exactly parallel the HCN-BF3 results. Specifically, the
B3LYP results for the B-N distance are about 0.1 Å longer
and those from mPWPW91 and about 0.1 Å shorter. In the
case of BrCH2CN-BF3, the mPW1PW91/aug-cc-pVTZ bond
length is much shorter, 1.88 Å, but interestingly, the
mPW1PW91/cc-pvtz bond length is 2.34 Å (for either
conformer). This extreme sensitivity to diffuse functions, well
beyond that noted above, is reminiscent of CH3CN-BF3,12
in which the effect stems from a very flat B-N distance
potential. Though it only occurs for one of three functionals
for BrCH2CN-BF3, it may indicate that the B-N distance
potential is rather flat and worthy of a more detailed
examination in the future. For now, we conclude that the
minimum energy structure has a B-N distance near 2.34
Å.
As a whole, these complexes appear to be just a bit more
strongly bonded than HCN-BF3, yet much weaker than
CH3CN-BF3. This trend is at odds with the ionization
potential data (Table 1) but is consistent with proton affinity
data (though such data are only available for CH3CN and
ClCH2CN). This can be rationalized by recalling the char-
For each complex, the eclipsed form seems to be the
lowest in energy, in contrast to our initial assessment based
726 Inorganic Chemistry, Vol. 45, No. 2, 2006

Haloacetonitrile-BF3 Complexes
Table 4. Selected Structural Parameters and Relative Conformational Energies for F-, Cl-, and Br-CH2CN-BF3 from B3LYP, B3PW91, and
mPW1PW91 Calculations
methoda
conformer b
E (cal/mol)c
R(B-N) (Å)
<NBF′ (°) d
<NBF′′ (deg) d
<CNB (deg)
FCH2CN-BF3
2.513
2.514
2.422
2.423
B3LYP
eclipsed
staggered
eclipsed
staggered
eclipsed
staggered
0.0
0.7
0.0
1.4
0.0
2.0
93.4
92.6
94.1
93.4
94.7
94.0
93.0
93.4
93.7
94.1
94.3
94.7
178.2
178.0
178.2
178.1
178.2
178.0
B3PW91
mPW1PW91
2.337
2.338
ClCH2CN-BF3
2.487
B3LYP
eclipsed
staggered
eclipsed
staggered
eclipsed
staggered
0.0
2.9
0.0
3.7
0.0
4.6
93.6
93.0
94.6
93.9
95.5
94.9
93.2
93.5
94.1
94.4
93.2
95.4
178.2
178.3
178.2
178.2
178.2
178.2
2.488
2.374
2.375
2.260
B3PW91
mPW1PW91
2.262
BrCH2CN-BF3
2.473
B3LYP
eclipsed
staggered
eclipsed
staggered
eclipsed
staggered
0.0
2.6
0.0
46.6
0.0
194.4
93.8
93.1
94.8
94.3
100.4
99.9
93.3
93.6
94.5
94.7
100.1
100.3
178.2
178.4
178.3
178.3
178.6
174.2
2.473
2.341
2.343
1.879
B3PW91
mPW1PW91
1.881
a The aug-cc-pVTZ basis set was used for these results. b Approximate conformation. See text for discussion. c Energy relative to the lower-energy conformer.
d <NBF′ is the N-B-F angle in the plane of symmetry, <NBF′′ is the out-of-plane N-B-F angle.
slight. At the B3PW91/aug-cc-pVTZ level, the Mulliken
charge on the N in FCH2CN is -0.38, while that on the F is
-0.37. In proceeding to BrCH2CN, the amount of negative
charge on the N increases only slightly, to -0.41, while the
charge on the Br increases to +0.03. Considering that BF3
and H+ are both hard acids,35 it is reasonable that the
structural trends follow proton affinity data since BF3 prefers
coordination with the nitrogen, which appears to be more
favorable in terms of electrostatics. It is worth noting in this
context that Leopold and co-workers have argued (using data
for SO3 complexes) that it is key to examine the ionization
potential of the actual donor orbital (not necessarily the
HOMO) in attempts to correlate structural properties with
orbital energy gaps.36 However, we were unable to locate
any ionization potential data that would provide energies for
the lower-lying σ-CN-type orbitals. Moreover, like the π-CN-
type HOMOs, mixing between σCN and the halogen p orbitals
(especially F in this case) complicates any analysis of the
DFT orbital energies.
Solid-State Structures. The solid-state structures of
FCH2CN-BF3, ClCH2CN-BF3, and ICH2CN-BF3 were
determined by X-ray crystallography. In addition to confirm-
ing the atomic connectivity in these complexes, these
structure determinations allow us to probe potential structural
changes that occur upon deposition and characterize structural
trends that are concomitant with changes in the carbon-bound
halogen atom. The crystallographic results are summarized
Figure 2. Gas-phase (B3PW91/aug-cc-pVTZ) structures of FCH2CN-
in Table 2, while significant interatomic distances and angles
BF3 (a), ClCH2CN-BF3 (b), and BrCH2CN-BF3 (c).
are presented in Table 5. Full details of the structure
determinations, in CIF format, are provided as Supporting
Information.
Thermal ellipsoid representations of the three X-ray crystal
structures are presented in Figure 3. In each case, the
acteristics of the HOMOs, as discussed above and depicted
in Figure 1. That is, ionization potentials decrease from
F-CH2CN to Br-CH2CN but do so because the HOMOs
have more halogen character and less amplitude on the nitrile
moiety. As far as electrostatics are concerned, however, it
appears that the nitrogen bears more negative charge than
the halogens, even in FCH2CN, though the difference is very
(35) Pearson, R. G., Ed. Hard and Soft Acids and Bases; Dowden,
Hutchinson and Ross, Inc.: Stroudsburg, PA, 1973.
(36) Hunt, S. W.; Leopold, K. R. J. Phys. Chem. A 2001, 105, 5498.
Inorganic Chemistry, Vol. 45, No. 2, 2006 727

Phillips et al.
Table 5. Significant Interatomic Distances (Å) and Angles (deg) for Haloacetonitrile-BF3 Complexesa
complex
FCH2CN-BF3
ClCH2CN-BF3 (a)
ClCH2CN-BF3 (b)
ICH2CN-BF3
B-F(1)
1.340(6)
1.362(6)
1.354(6)
1.635(6)
1.130(5)
113.2(4)
113.5(4)
112.3(4)
105.7(4)
105.0(3)
106.2(3)
179.7(4)
1.358(3)
1.355(3)
1.364(3)
1.649(2)
1.132(3)
113.6(2)
112.3(2)
113.7(2)
105.9(2)
105.1(2)
105.3(2)
175.2(2)
1.345(3)
1.355(3)
1.354(3)
1.640(3)
1.137(3)
114.7(2)
112.5(2)
111.7(2)
105.7(2)
105.8(2)
105.5(2)
176.3(2)
1.374(12)
1.341(13)
1.356(13)
1.643(12)
1.124(11)
114.0(8)
112.4(9)
114.5(9)
103.3(8)
105.3(8)
106.1(7)
174.2(9)
B-F(2)
B-F(3)
B-N
N-C(1)
F(1)-B-F(2)
F(1)-B-F(3)
F(2)-B-F(3)
F(1)-B-N
F(2)-B-N
F(3)-B-N
B-N-C(1)
a Metrical details for each of the crystallographically independent molecules of ClCH2CN-BF3 are provided separately as molecule (a) and molecule (b).
B-N distance and N-B-F angles, there are also some subtle
structural changes that occur within the nitrile and BF3
subunits that are associated with shortening and strengthening
of the B-N bond. Specifically, the C-N distance contracts
by about 0.01 Å, an effect noted previously in CH3CN-
BF3,3 and the B-F bonds lengthen by about 0.03 Å (both
relative to the B3PW91/aug-cc-pVTZ structure). The C-N-B
linkage straightens upon crystallization, yielding a B-N-C
bond angle of 179.7(4)°. Furthermore, the staggered con-
former is observed exclusively in the solid-state (the F4-
C2‚‚‚B1-F3 torsion angle is 62.5°), and we observe no
evidence of any thermal averaging in the crystal used for
the structure determination.
Comparable gas-solid structural differences are also
apparent for ClCH2CN-BF3 (Figure 3b). The B-N distance
in this solid complex is 1.640(3) Å, which is 0.73 Å shorter
than in the calculated gas-phase structure, while the N-B-F
angles are 11.2° larger (on average) in the solid. As observed
in FCH2CN-BF3, some slight structural changes occur in
the nitrile and BF3 subunits of ClCH2CN-BF3 that reflect a
Figure 3. Views of the X-ray crystal structures of the haloacetonitrile-
BF3 complexes drawn at the 50% probability levels for all non-hydrogen
atoms.
stronger B-N interaction in the crystalline complex. Specif-
ically, the B-F distances increase by approximately 0.04
Å, while the C-N distance decreases by 0.01 Å. Finally,
each of the two crystallographically independent formula
units of ClCH2CN-BF3 is twisted away from idealized
eclipsed or staggered conformations: the Cl-C2‚‚‚B1-F3
torsion angle is 41.7° in one complex and 23.4° in the second
independent molecule.
haloacetonitrile-BF3 complexes are found with the Lewis
acidic BF3 unit bound to the nitrile nitrogen, even though
each haloacetonitrile features two potentially Lewis basic
sites. In general, the metrical parameters for the complexes
very closely resemble those previously determined for
CH3CN-BF3(s),4 HCN-BF3(s),15 and C6H5CN-BF3(s),23
with B-N distances near 1.65 Å and N-B-F angles
between 105° and 106°. Thus, introduction of the halogen
substituent does not cause dramatic changes in the solid-
state structural properties of the complexes relative to those
of previously examined nitrile-BF3 complexes.
Comparison of the calculated gas-phase and crystallo-
graphically determined solid-state structures of FCH2CN-
BF3 and ClCH2CN-BF3 reveals that major structural changes
occur upon deposition of the complexes from the vapor
phase. For FCH2CN-BF3 (Figure 3a) the solid-state B-N
distance is 1.635(6) Å, which is 0.78 Å shorter than in the
calculated gas-phase structure. Furthermore, the N-B-F
angles are 11.7° larger (on average) in the solid. These gas-
solid structural differences rival those of HCN-BF3,14,15 the
most extreme case in this class of compounds. Besides the
Inspection of the X-ray crystal structure of ICH2CN-BF3
(Figure 3c) reveals an extension of two structural trends that
exist within this group of three haloacetonitrile-BF3 com-
plexes. Specifically, the complex exists in the most nearly
eclipsed rotational conformation (the I-C2‚‚‚B1-F3 torsion
angle is 12.8°) and the B-N-C bond angle exhibits the
largest deviation from linearity among the three complexes.
The factors that influence the rotational conformations of
the three complexes are unclear. However, it may be that
observation of the three different rotomers in the X-ray
crystal structures of FCH2CN-BF3, ClCH2CN-BF3, and
ICH2CN-BF3 reflects the relatively flat torsional potential
discussed in the computational section above. The slight bend
of the B-N-C bond angle may result from intramolecular
or intermolecular effects. We note that ICH2CN-BF3 and
728 Inorganic Chemistry, Vol. 45, No. 2, 2006

Haloacetonitrile-BF3 Complexes
Table 6. Calculated Gas-Phase and Observed Solid-State Frequencies
for the BF3-Localized Vibrations and the C-N Stretching Mode in F-,
Cl-, and Br-CH2CN-BF3
frequencies (cm-1
gas phasea
solid
)
shift
description
(solid-gas)
FCH2CN-BF3
BF3 symmetric deformation (δs
)
608
655
854
1186
2375
+47
-17
BF
BF3 symmetric stretch (νs
)
871
1429b
2395c
BF
BF3 asymmetric stretch (νa
)
-243
BF
C-N stretch (νCN
)
-20c
ClCH2CN-BF3
BF3 symmetric deformation (δs
)
596
653
863
1200
2373
+57
-6
BF
BF3 symmetric stretch (νs
)
869
BF
BF3 asymmetric stretch (νa
)
1423b
2395c
-223
BF
Figure 4. I‚‚‚F intermolecular interactions in ICH2CN-BF3.
C-N stretch (νCN
)
-22c
BrCH2CN-BF3
ClCH2CN-BF3, which are more eclipsed than FCH2CN-
BF3, have B-N-C angles that are both slightly nonlinear,
with the BF3 unit tilted away from the carbon-bound halogen
atom. In addition, the magnitude of the deviation of
the B-N-C angle from linearity in ICH2CN-BF3 and
ClCH2CN-BF3 does appear to correlate with the progression
of the complexes toward a more eclipsed conformation. Thus,
one might conclude that the BF3 unit is bending away from
the intramolecular carbon-bound halogen atoms because
these relatively large atoms are impinging spatially upon a
BF3 fluorine atom (specifically, F(3)). However, the long
X‚‚‚F distances involved (X ) Cl, 5.39 Å; X ) I, 5.59 Å)
render this possible explanation unlikely. A more plausible
explanation is that weak intermolecular interactions cause
the B-N-C unit to flex slightly in the solid state. We have
previously identified weak F‚‚‚H interactions in the X-ray
crystal structure of C6H5CN-BF3 as being potentially
responsible for the slight bending of the B-N-C angle
(177.7°) in that crystalline compound. In the case of
ICH2CN-BF3, one intermolecular interaction that might tilt
the BF3 unit away from the CtN vector is a weak F‚‚‚I
interaction between two centrosymmetrically related com-
plexes (Figure 4). It may be that tilting of the BF3 unit away
from the neighboring complex’s iodine atom enhances the
electrostatic interaction between the electronegative fluorine
atom and the polarizable iodine atom while maintaining a
nearly linear disposition of the C-I‚‚‚F unit. Analogous
intermolecular BF3‚‚‚H interactions occur within centrosym-
metrically related pairs of ClCH2CN-BF3 complexes in the
solid state, leading to a slight bending of the CtN-B bond
(average ≈ 176° for the two crystallographically independent
complexes). However, a fundamentally different set of
intermolecular interactions exists in the unit cell of FCH2CN-
BF3. The lack of a crystallographic center of symmetry in
this solid complex causes the principle intermolecular BF3‚
‚‚H interactions to occur within infinite chains of complexes
propagating parallel to the crystallographic b axis. Similar
BF3‚‚‚H interactions occur between FCH2CN-BF3 com-
plexes related by the 21 axis, with these interactions occurring
parallel to the crystallographic c axis. However, none of these
intermolecular interactions occur within a closely associated,
centrosymmetric pair of FCH2CN-BF3 complexes, which
perhaps underlies the linearity of this complex’s CtN-B
bond angle.
BF3 symmetric deformation (δs
)
591
649
862
1189
2360
+58
-5
BF
BF3 symmetric stretch (νs
)
867
BF
BF3 asymmetric stretch (νa
)
1419b
2391c
-230
BF
C-N stretch (νCN
)
-31c
a Calculated using the B3PW91/aug-cc-pVTZ method. b The higher
frequency of two (A′ and A′′) forms of this vibration in Cs symmetry. The
other bands are 4-5 cm-1 lower for all three complexes. c The C-N stretch
would be expected to blue-sift as the B-N bond contracts.12 However, we
have shown previously22 that the B3PW91/aug-cc-pVTZ method gives
unreliable results for CN stretching frequencies, irrespective of any scaling
procedure. Thus, the gas-phase C-N stretching frequencies and, in turn,
the predicted gas-to-solid shifts are not considered to be reliable.
Frequencies. Complete sets of calculated gas-phase
frequencies and intensities (B3PW91/aug-cc-pVTZ) for the
eclipsed conformers of the 11B isotopomers of FCH2CN-
BF3, ClCH2CN-BF3, BrCH2CN-BF3, are included as Sup-
porting Information (Tables S1-S3, respectively). For the
strong, structurally sensitive modes that we observed in the
solid-state IR spectra, calculated frequencies are listed in
Table 6 with solid-state peak maxima and gas-to-solid shifts.
We note that the B3PW91/aug-cc-pVTZ method provided
excellent agreement with measured gas-phase bands of BF3,
as well as MP2/aug-cc-pVTZ frequencies for HCN-BF3.22
It is apparent that shifts in the BF3 asymmetric stretching
modes (ν12 and ν23 in each complex, or νaBF in general), as
well as the BF3 symmetric deformation, or “umbrella” mode
(ν7, or δs ) further support a slight increase in interaction
BF
strength in proceeding from FCH2CN-BF3 to BrCH2CN-
BF3. It has been noted previously12,18b that these modes shift
consistently in accord with B-N distance (or any other
structural property that parallels interaction strength). The
BF3 asymmetric stretch shifts monotonically to lower
frequencies as the B-N distance contracts. The calculated
(B3LYP/aug-cc-pVTZ) frequency of this mode for free BF3
(ν3) is 1457 cm-1,22 and those of the symmetric component
(A′ in CS) for the F-, Cl-, and Br-containing complexes are,
1429, 1423, and 1419 cm-1, respectively. The shift of the
umbrella mode is a bit peculiar; initially, it shifts to lower
frequencies as the B-N distance contracts,18b but at shorter
distances, the trend reverses and the band shifts abruptly to
higher frequencies.12 Among these fairly weak complexes,
however, there is a steady red shift in proceeding from
F-CH2CN-BF3 to Br-CH2CN-BF3: the calculated fre-
quencies are 608, 596, 591 cm-1, respectively. The BF3
symmetric stretch (ν8/ν9, or νs ) is predicted to shift very
BF
Inorganic Chemistry, Vol. 45, No. 2, 2006 729

Phillips et al.
little across this series of complexes, but the calculated
intensity of this mode, which is dipole-forbidden in free BF3,
does increase in proceeding from F to Br; the result of an
increase in nonplanar distortion of the BF3 subunit. The C-N
stretch (ν14, or νCN), which has often been the focus of
previous attempts to correlate structural properties with
frequency shifts in these systems,5,13 does not shift systemati-
cally across the series. However, a gas-to-solid red shift is
predicted (Table 6), which is not physically justifiable, and
must stem form an inaccurate computational prediction of
ν
CN. We have found previously that calculated frequencies
of this mode are somewhat unreliable with B3PW91/aug-
cc-pVTZ (and several other methods).22 Nonetheless, we
observe no significant shift among the calculated νCN
frequencies, and there may be a substituent effect among
free nitriles that is obscuring or counteracting a trend that
would parallel the degree of binding to BF3.
Solid-state IR spectra of FCH2CN-BF3, ClCH2CN-BF3,
and BrCH2CN-BF3 are displayed in Figure 5 (a-c, respec-
tively) together with simulated gas-phase spectra based on
the calculated frequencies (dashed lines). Band assignments
for the most prominent bands are listed directly on the plots,
while peak frequencies are listed with gas-phase data in Table
6. The solid-state spectra are all quite similar and are
dominated by four prominent peaks, very much like those
previously measured for CH3CN-BF3(s),6 (CH3)3CCN-
BF3(s),23 and C6H5CN-BF3(s).23 As such, we presume that
we can assign the dominant bands by comparison to the
previously recorded spectra and that peak frequencies cor-
respond to those of the 11B isotopomer, which comprises
81% of the sample. The three strongest bands in each
spectrum are the BF3 asymmetric stretch (νa ) near 1200
BF
cm-1, the BF3 symmetric stretch near (νBs F) 860 cm-1, and
the BF3 umbrella mode (δsBF) near 650 cm-1. The C-N
stretch (νCN) is also readily apparent and is observed near
2375 cm-1. In contrast, the gas-phase spectra feature only
two strong, prominent bands; the BF3 asymmetric stretch (
νaBF) and the “umbrella” mode (δBs F), and these are shifted
significantly from their solid-state counterparts, especially
in the case of the BF3 asymmetric stretching mode (νa ).
BF
These gas-to-solid shifts (aside from the C-N stretch as
discussed above) directly parallel the structural changes
predicted for FCH2CN-BF3 and ClCH2CN-BF3 on the basis
of an explicit examination of each modes’ structural depen-
dence CH3CN-BF3.12 For the BF3 symmetric stretching
mode, the gas-to-solid shifts are predicted to be quite small,
but the band intensity increases dramatically upon deposition.
These peaks are barely notable in the simulated gas-phase
spectra but are quite prominent in the solid-state spectra. This
stems from the increase in pyramidal distortion of the BF3
unit that accompanies the contraction of the B-N bond.
Moreover, though we lack a crystal structure for BrCH2CN-
BF3, the IR spectrum clearly indicates that the solid-state
structure of the complex resembles those for which the crystal
structures have been measured.4,15,23 Beyond the frequencies
themselves and the predicted gas-solid band shifts, the
characteristic relative intensity pattern of the peaks corre-
Figure 5. IR spectra of a bulk, solid samples of BF3 and (a) FCH2CN,
(b) ClCH2CN, and (c) BrCH2CN, co-deposited at 120 K. The dashed spectra
are simulations of the gas-phase spectra based on the calculated frequencies
of the respective complexes. Peak frequencies are listed in Table 6.
sponding to the three prominent BF3-localized modes
provides further evidence of this assessment. Thus, there is
clear evidence that BrCH2CN-BF3 also undergoes major
structural changes upon deposition, even in the absence of a
precise, crystallographically determined structure.
V. Summary and Conclusions
We have examined the gas-phase and solid-state structural
properties of the singly halogenated derivatives of CH3CN-
730 Inorganic Chemistry, Vol. 45, No. 2, 2006

Haloacetonitrile-BF3 Complexes
BF3 using density functional theory, crystallography, and IR
spectroscopy and found that these compounds undergo major
structural changes upon crystallization. Gas-phase structures
from B3PW91/aug-cc-pVTZ calculations exhibit rather long
B-N bond distances of 2.422, 2.374, and 2.341 Å, for
FCH2CN-BF3, ClCH2CN-BF3, and BrCH2CN-BF3, re-
spectively. These, as well as other structural properties,
indicate that the complexes have fairly weak dative bonds
in the gas phase but that the degree of interaction with BF3
(as inferred from computed structural properties) does
increase across the series from F- to Br-CH2CN-BF3.
Collectively, these complexes are bound perhaps a bit more
strongly than HCN-BF3 but much more weakly than
CH3CN-BF3. Both of these observations parallel proton
affinity values but not ionization potential data, at least as
far as HOMO orbital energies are concerned. Crystal
structures very much resemble those measured previously
for other nitrile-BF3 complexes,4,15,23 (aside from a slight
bend about the C-N-B linkage), as all have short B-N
bond distances of about 1.65 Å and N-B-F angles of 105-
106° (on average). For FCH2CN-BF3, the B-N distance is
is 1.635(6) Å, which is 0.78 Å shorter than the calculated
gas-phase result. For ClCH2CN-BF3, the B-N distance is
1.640(3) Å, which is 0.73 Å shorter than the gas phase result.
These changes are extreme, rivaling those observed previ-
ously for HCN-BF3.14,15 Calculated vibrational frequencies
for the gas-phase complexes show subtle trends in the BF3-
localized modes that parallel the slight structural differences
that occur among the F-, Cl-, and Br-containing complexes.
Solid-state IR spectra show major differences in these bands
for FCH2CN-BF3, ClCH2CN-BF3, and BrCH2CN-BF3.
While we did not obtain a crystal structure for the Br-
containing derivative, the general features of the solid-state
IR spectrum, including both frequencies, as well as a
characteristic intensity pattern, provide solid evidence that
the complex has a similar structure to other nitrile-BF3
complexes in the solid state and that a large gas-solid
structural difference occurs for this complex as well.
Acknowledgment. We gratefully acknowledge financial
support of this work from the Camille and Henry Dreyfus
Foundation (Henry Dreyfus Teacher-Scholar Awards to
J.A.P. and J.A.H.), the National Science Foundation (CHE-
0216058 and CHE-0407824 to J.A.P., CHE-0243951 to
J.A.H., and CHE-0203346 to C.J.C.), the donors of the ACS
Petroleum Research Fund (no. 37834-B6 to J.A.P.), and
Research Corporation (no. CC5622 to J.A.P.). Additional
support was provided by the Office of Research and
Sponsored Programs at the University of Wisconsin-Eau
Claire. Sabbatical support for J.A.P. was provided by the
NSF-RSEC (CHE-0113894) program at the University of
Minnesota.
Supporting Information Available: Crystallographic informa-
tion files; calculated gas-phase frequencies and band intensities
(Tables S1-S3). This material is available free of charge via the
Internet at http://pubs.acs.org.
IC051491X
Inorganic Chemistry, Vol. 45, No. 2, 2006 731