Weak hydrogen bonds in σ-1,4-difluorobenzene-ammonia: A rotational study

Article abstract of DOI:10.1016/j.cplett.2009.12.025

Structural and energetic features of the complex 1,4-difluorobenzene-ammonia have been investigate by rotational spectroscopy. The complex adopts a σ configuration, with the nitrogen of ammonia in the plane of the aromatic ring. Two weak hydrogen bonds, C-H?N and N-H?F, represent the linkages between ammonia and 1,4-difluorobenzene.

Full text of DOI:10.1016/j.cplett.2009.12.025

Chemical Physics Letters 485 (2010) 36–39
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Chemical Physics Letters
journal homepage: www.elsevier.com/locate/cplett
Weak hydrogen bonds in
r
-1,4-difluorobenzene-ammonia: A rotational study
*
Barbara M. Giuliano, Luca Evangelisti, Assimo Maris, Walther Caminati
Dipartimento di Chimica ‘G. Ciamician’ dell’Università, Via Selmi 2, I-40126 Bologna, Italy
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 6 November 2009
In final form 9 December 2009
Structural and energetic features of the complex 1,4-difluorobenzene-ammonia have been investigate by
rotational spectroscopy. The complex adopts a
r configuration, with the nitrogen of ammonia in the
plane of the aromatic ring. Two weak hydrogen bonds, C–Hꢀ ꢀ ꢀN and N–Hꢀ ꢀ ꢀF, represent the linkages
between ammonia and 1,4-difluorobenzene.
Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction
[18], where the water molecule is positioned above the benzene
plane.
Gas-phase studies on molecular complexes are able to elucidate
the nature of the intermolecular interactions, isolated from other
environmental effects. In particular, the analysis of their rotational
spectra gives the possibility of discriminating between different
molecular conformers and provides some estimates of the binding
energies and internal dynamics.
One of the most important non-covalent interactions – also be-
cause of its biological relevance – is hydrogen bonding (H-bond).
Many H-bond investigations have involved water, which has been
found to form relatively strong H-bonds with organic or biological
molecules, either as a proton donor or a proton acceptor [1–6].
In contrast with water, ammonia in its molecular complexes
with organic or biomolecules, frequently acts preferably as a pro-
ton acceptor forming an intermolecular H-bond through the nitro-
gen lone pair [7–11]. Indeed, in all the investigated systems where
ammonia could play the double proton donor/acceptor role,
F3CHꢀ ꢀ ꢀNH3 [7], CH3OHꢀ ꢀ ꢀNH3 [8], pyrroleꢀ ꢀ ꢀNH3 [9], tert-buta-
nolꢀ ꢀ ꢀNH3 [10] and glycidolꢀ ꢀ ꢀNH3 [11], only the forms where
NH3 has a basic behavior have been observed.
The insertion of electronegative substituents (such as F atoms)
in the aromatic ring could alter the geometry of the complex be-
cause a more classical (such as N–Hꢀ ꢀ ꢀF) H-bond could be favored
with respect to the N–Hꢀ ꢀ ꢀ
p bounding one. This is the case for the
water complexes with partially substituted fluorobenzenes, where
the water molecule lies on the plane of the benzene ring. Water
constitutes with the fluorobenzene molecule a pseudo six-member
ring, in which the principal interaction is between the fluorine
atom and one of the hydrogen atoms of the water molecule and
a secondary interaction is present involving the oxygen lone pair
and the hydrogen atom in the phenyl ring [19,20].
The interpretations of the spectroscopical evidences of com-
plexes of NH3 with fluoroderivatives of benzene, are often based
on ab initio calculations, and are frequently controversial. In the
case of fluorobenzeneꢀ ꢀ ꢀNH3, the results of one-color resonant
two-photon ionization (R2PI) and IR-vibrational predissociation
spectroscopy in the region of the NH stretches were interpreted
in terms of a out of plane (
onance enhanced multiphoton ionization (REMPI) spectroscopy
study indicated the complex to have a planar ( ) heavy atoms
p) complex [21], while a two-color res-
A few rotationally resolved electronic spectroscopy studies on
adducts of ammonia with larger molecules confirm this trend, such
as in the case of 1-naphtolꢀ ꢀ ꢀNH3 [12], hydroquinoneꢀ ꢀ ꢀNH3 [13],
and anilineꢀ ꢀ ꢀNH3 [14].
r
skeleton [22]. Regarding 1,4-difluorobenzene-ammonia (1,4-DFB-
NH3), the results of both techniques mentioned above were inter-
preted in terms of a p-complex [21,23].
In this Letter we present the results of a Fourier transform
microwave spectroscopic study of 1,4-DFB-NH3. Analysis of this
experimental investigation is found to be consistent with the most
In anisoleꢀ ꢀ ꢀNH3, where ammonia can play only the proton do-
nor role towards the ether oxygen, the NH3 group lies out of the
plane of the aromatic ring and it is bonded to anisole via three
weak, N–Hꢀ ꢀ ꢀO, CMe–Hꢀ ꢀ ꢀN and N–Hꢀ ꢀ ꢀ
p
contacts [15,16].
stable r-conformer as shown in Fig. 1. Also included are certain
intermolecular parameters.
The location of ammonia in anisole is reminiscent of the com-
plex benzene–ammonia, where the NH3 group points one of its
hydrogen atoms towards the benzene ring electronic
[17]. This structure is also very similar to that of benzene–water
p-cloud
2. Experimental details
A gaseous mixture of 1% anhydrous ammonia (99.99% pure, Sig-
ma–Aldrich) in Helium was passed over a sample of 1,4-DFB
(P99%, supplied by Aldrich and used without further purification)
* Corresponding author. Fax: +39 051 2099456.
E-mail address: walther.caminati@unibo.it (W. Caminati).
0009-2614/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.cplett.2009.12.025

B.M. Giuliano et al. / Chemical Physics Letters 485 (2010) 36–39
37
The measured lines exhibit the hyperfine structure due to the
interaction between the nuclear quadrupole moment of the 14N
nucleus and the rotational angular momentum J, as shown in
Fig. 3. The following coupling scheme,
F ¼ J þ I
ð1Þ
has been adopted in the fitting procedure, where I is the nuclear
spin and F represents the total angular momentum.
No line splitting ascribed to an internal rotation motion has
been observed.
All measured lines (deposited in Supplementary material) were
fitted with the Watson’s semirigid Hamiltonian in the S reduction
and Ir representation [30] using the Pickett’s suite of programs SPFIT
[31]. The derived spectroscopic parameters are reported in Table 2.
Fig. 1. Sketch of the most stable conformer (r) of 1,4-DFB-NH3.
5. Conformation and structure
at room temperature, and expanded through a solenoid valve
(General Valve series 9) into the Fabry–Perot resonator chamber.
The backing pressure was kept at 3.5 MPa in order to reach a con-
centration of about 3% of p-DFB in the gas mixture prior to the
expansion.
The microwave spectra of the complex has been recorded in the
frequency range 6–18 GHz using a COBRA version [24] of a Balle–
Flygare type [25] molecular beam Fourier transform microwave
spectrometer already described elsewhere [26], updated with the
The inertial defect calculated from the experimental rotational
constants is
D
= ꢁ0.930 uÅ2, quite smaller (in absolute value) than
the value expected for two out of plane ammonia hydrogens
(ꢂꢁ2.7 uÅ2, [32]). This small value is probably due to an almost
free rotation of the NH3 group, and it will be discussed in the next
section, but it is only compatible with a planar configuration of the
heavy atoms of the complex. Also the values of the quadrupole
coupling constants correlate much better with the ab initio values
of the
r-complex. There are, however, considerable discrepancies
F
TMW++ set of programs [27].
between the experimental and ab initio values of Tables 1 and 2.
This kind of difference has been observed in other cases. In some
complexes of NH3 [7,8,10] having an almost linear X–Hꢀ ꢀ ꢀN H-bond
nearly co-linear with the a-axis, it has been interpreted as a reduc-
tion that arises from zero point angular oscillations of the NH3
moiety relative to the a-axis of the complex. In 1,4-DFB-NH3, such
an approach is not possible because the C–Hꢀ ꢀ ꢀN linkage is not co-
linear with the a-axis. Alternatively, such a reduction has been ex-
plained in terms of charge transfer for strongly bound complexes,
but this seems not to be our case.
After these preliminary considerations, we can calculate the
effective values of the intermolecular structural parameters, repro-
duce the experimental values of the rotational constants, obtaining
a partial effective r0 structure, while keeping the geometry of the
monomers fixed to the calculated values in the complex. The de-
fined intermolecular parameters are shown in Fig. 1. The corre-
sponding fitted values for the RHꢀ ꢀ ꢀN distance and the h angle are
reported in Table 3 together with the values from the equilibrium
structure. From the derived structure the intermolecular distance
C–Fꢀ ꢀ ꢀH–N has been calculated to be 2.476 Å.
The estimated accuracy of the measured frequencies is about
2 kHz and the resolving power is about 7 kHz.
3. Theoretical calculations
Previously available ab initio calculations for 1,4-DFB-NH3 [20]
found two different stable structures (
r and p shapes) of the com-
plex, but the spectroscopic constants relevant to the rotational
spectrum were not reported. For this reason, we optimized the
**
two available structures at the MP2/6-311++G level of calcula-
tion using the GAUSSIAN03 software package [28]. We obtained a
slightly different conformation for the
quencies in the harmonic approximation indicated that both our
and conformations are energy minima. All binding energies
p species. Vibrational fre-
r
p
were corrected for basis set superposition errors (BSSE) using the
counterpoise procedure [29]. The calculated rotational constants,
dipole moment components, quadrupole coupling constants and
energy differences are shown in Table 1. In addition, the theoretical
value of the V3 barrier to internal rotation of the ammonia moiety
is reported for conformer
r. Furthermore, the dissociation energy
was evaluated considering the ZPE correction and the counterpoise
procedure. All these values are reported in Table 1.
The ab initio calculated electronic density shows (see Fig. 2) a
somehow ‘cooperative’ effect of the two (N–Hꢀ ꢀ ꢀF and C–Hꢀ ꢀ ꢀN)
intermolecular bonds, but it indicates the C–Hꢀ ꢀ ꢀN interaction to
be the stronger one.
6. Internal dynamics and dissociation energy
Aspects of internal dynamics of the complex can be obtained
from the experimental evidences.
The inertial defect provides the possibility to estimate the inter-
nal rotation barrier. As discussed above, to the only out of plane
atoms, two ammonia hydrogens, corresponds an inertial defect
D
ꢂ ꢁ2.8 uÅ2. The decrease to the observed value of ꢁ0.930 uÅ2
4. Rotational spectra
is attributable to the corrections of the ground state rotational con-
stants, with respect to the rigid limit, according to:
According to the calculated values of rotational constants, of the
dipole moment components and of the relative intensities of the
predicted rotational transitions in our frequency range, the first
search has been targeted to the low J, la R-type transitions. After
a first scan, the 60,6 50,5, 61,6 51,5 and 61,5 51,4 transitions
have been assigned. Subsequently more la and some weaker lb
R-type transitions have been measured. No lc type transitions
have been detected, according to the co-planarity of the ammonia
N atom with the aromatic ring.
A00 ¼ Ar þ Wð020ÞF
q
a2;
B00 ¼ Br þ Wð020ÞF
C00 ¼ Cr;
q
b2;
ð2Þ
where Ar, Br and Cr are the ‘rigid’ rotational constants in the limit of
the very high barrier. The Wð020Þ are the Hersbach’s barrier-dependent
perturbation sums relative to the A sublevels of the ground state

38
B.M. Giuliano et al. / Chemical Physics Letters 485 (2010) 36–39
Table 1
Calculated spectroscopic parameters of 1,4-DFB-NH3.
r
p
A (MHz)
B (MHz)
C (MHz)
la (D)
2974
818
644
1.9
1935
1125
933
0.2
lb (D)
0.4
0.8
lc (D)
0.0
1.3
vaa (MHz)
vbb (MHz)
vcc (MHz)
ꢁ2.92
0.82
2.10
0a
1.95
0.86
ꢁ2.81
4.5
12.8
3.5
9.7
6.4
4.9
D
E (kJ molꢁ1
)
De (kJ molꢁ1
)
17.3
b
D
E0 (kJ molꢁ1
0a
D0 (kJ molꢁ1
)
13.1
0a
11.3
b
D
ECP (kJ molꢁ1
)
b
DC0P (kJ molꢁ1
)
V3 (cmꢁ1
)
130
a
The absolute values of the energy, of the zero point corrected energy and of the counterpoise corrected energy of the
r
species are ꢁ486.160410, ꢁ486.041197 and
ꢁ486.158130 Ha, respectively.
De, D0 and DC0P are the equilibrium, zero point corrected, and counterpoise corrected dissociation energies, respectively.
b
[33] and
the observed change in
q
g = kgI /Ig. A value of V3 ꢂ 60 cmꢁ1 would correlate with
a
D with respect to the high barrier case.
However, since the ammonia group out of plane vibrations would
also give a negative contribution to the effective inertial defect so,
probably the V3 value is actually much lower. This result is in agree-
ment with the fact that we did not observe E-species transitions
nearby the A-species ones. The experimental evidence also indicates
that the ab initio value of the barrier (V3 = 130 cmꢁ1
) is
overestimated.
The hydrogen bond stretching force constant can be estimated
using the pseudo diatomic approximation which considers the
two molecular subunits of the complex as two rigid parts. This
model can be applied if the intermolecular stretching motion, di-
rected along the hydrogen bond mean direction, is almost parallel
to the a-axis of the complex.
By assuming a Lennard-Jones type potential, the zero point dis-
sociation energy of the complex can be derived applying the
approximate expressions [34]:
Fig. 3. Recorded 70,7–60,6 transitions of the complex. The 14N hyperfine structure
(the F0–F00 component lines are indicated) agrees with the quadrupole coupling
constants of the
r form.
2
2
2
ks ¼ 16p4
ð
lARCMÞ ½4B4 þ 4C4 ꢁ ðBA ꢁ CAÞ ðBA þ CAÞ ꢃ=ðhDJÞ;
ð3Þ
ð4Þ
Table 2
A
A
Spectroscopic parameters (S reduction, Ir representation) of
r-1,4-DFB-NH3.
ED ¼ 1=72ksR2
;
CM
A (MHz) 3048.228 (2)a DJ (kHz)
B (MHz)
C (MHz)
0.149 (1) vaa (MHz) ꢁ2.001 (7)
808.1610 (3) DJK (kHz) [0]b
639.5514 (2) DK (kHz)
d1 (kHz)
vbb (MHz)
vcc (MHz)
0.437 (7)
1.564 (7)
2
20.6 (4)
ꢁ0.0379 (6)
r
(kHz)
D
(uÅ2)
ꢁ0.930
d2 (kHz)
ꢁ0.51 (5)
Nc
76
a
b
c
Error in parentheses in units of the last digit.
Fixed to zero because undetermined in the fit.
Number of lines in the fit.
Table 3
**
r0 and re (MP2CP/6-311++G ) values of the intermolecular parameters.
r0
re
RN–H (Å)
h (°)
RH–F (Å)
2.48 (1)a
145.8 (1)
2.469b
2.427
148.0
2.514
a
Fig. 2. The electronic density of
interaction is stronger than the N–Hꢀ ꢀ ꢀF one.
r
-1,4-DFBꢀ ꢀ ꢀNH3 suggests that the C–Hꢀ ꢀ ꢀN
Error in parentheses in units of the last digit.
Derived from the other two parameters r0 structure.
b

B.M. Giuliano et al. / Chemical Physics Letters 485 (2010) 36–39
39
where BA and CA are the rotational constants of the adduct; DJ the
Appendix A. Supplementary material
centrifugal distortion constant; lA is the reduced mass of the com-
plex and RCM is the distance between the centers of mass of the two
subunits. From the partial r0 structure a RCM value of 3.792 Å is de-
rived, the corresponding ks is 5.1 N/m and ED results in a value of
10.1 kJ molꢁ1. This value is similar to the values reported for other
complexes of ammonia [7–11] and in a quite good agreement with
the ab initio DC0P value of Table 1.
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.cplett.2009.12.025.
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We thank the University of Bologna for financial support.