The ionic hydrogen/deuterium bonds between diammoniumalkane dications and halide anions

Article abstract of DOI:10.1002/cplu.201300084

Halide-anion binding to 1,12-dodecanediammonium, tetramethyl-1,12- dodecanediammmonium, and tetramethyl-1,7-heptanediammonium has been investigated with infrared multiple-photon dissociation (IRMPD) spectroscopy in the 1000-2250cm^-1 spectral region and with theory. Both charged ammonium groups in these diammonium compounds interact with the halide anion resulting in an ionic hydrogen bond (IHB) stretching frequency outside of the spectral frequency range that can be measured with the free-electron laser (FEL). This frequency is shifted into the spectral range upon exchanging all of the labile hydrogen atoms with deuterium atoms, thus making measurement of the ionic deuterium bond (IDB) stretching frequency possible. The IDB stretching frequency shifts to higher values with increasing halide-anion size, methylation of the ammonium groups, and alkane chain length, consistent with the halide-anion-deuterium bond strength decreasing with decreasing gas-phase basicity of the halide anion and the increasing gas-phase basicity of the ammonium groups. The IDB stretching frequency also depends on the alkane chain length owing to constraints on the angle of the bonds between the halide anion and the two ammonium groups. There are additional bands in the IDB stretching feature in the IRMPD spectra, which are attributed to Fermi resonances and arise from coupling with overtone or combination bands that can be identified from theory and depend on the halide-anion identity and alkane chain length.

Full text of DOI:10.1002/cplu.201300084

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DOI: 10.1002/cplu.201300084
The Ionic Hydrogen/Deuterium Bonds between
Diammoniumalkane Dications and Halide Anions
[a]
[b, c]
[b]
[a]
Maria Demireva, Jos Oomens,
Giel Berden, and Evan R. Williams*
In memory of Detlef Schrçder
Halide-anion binding to 1,12-dodecanediammonium, tetra-
methyl-1,12-dodecanediammmonium, and tetramethyl-1,7-
heptanediammonium has been investigated with infrared mul-
shifts to higher values with increasing halide-anion size, meth-
ylation of the ammonium groups, and alkane chain length,
consistent with the halide-anion–deuterium bond strength de-
creasing with decreasing gas-phase basicity of the halide anion
and the increasing gas-phase basicity of the ammonium
groups. The IDB stretching frequency also depends on the
alkane chain length owing to constraints on the angle of the
bonds between the halide anion and the two ammonium
groups. There are additional bands in the IDB stretching fea-
ture in the IRMPD spectra, which are attributed to Fermi reso-
nances and arise from coupling with overtone or combination
bands that can be identified from theory and depend on the
halide-anion identity and alkane chain length.
tiple-photon dissociation (IRMPD) spectroscopy in the 1000–
À1
2
250 cm spectral region and with theory. Both charged am-
monium groups in these diammonium compounds interact
with the halide anion resulting in an ionic hydrogen bond
(IHB) stretching frequency outside of the spectral frequency
range that can be measured with the free-electron laser (FEL).
This frequency is shifted into the spectral range upon exchang-
ing all of the labile hydrogen atoms with deuterium atoms,
thus making measurement of the ionic deuterium bond (IDB)
stretching frequency possible. The IDB stretching frequency
Introduction
Ionic hydrogen bonds (IHBs) affect the physical properties and
reactivities of many molecular species, and these strong inter-
actions play an important role in ion solvation, nucleation,
acid–base chemistry, enzyme catalysis, and many other chemi-
IHBs in various isolated complexes have been investigated
extensively using both experimental and theoretical meth-
[1,2]
ods.
Thermochemical techniques, such as high-pressure
[3]
mass spectrometry and equilibrium measurements, guided-
[
1]
[4]
cal and biological processes. Understanding the nature of
these interactions is of fundamental importance, but IHBs can
be difficult to study in complex systems owing to many com-
peting effects. By investigating simple systems without com-
peting interactions in the gas phase, the IHBs can be probed
directly without interference from solvent molecules. Such
studies provide valuable information on the strengths, geome-
tries, and character of the IHBs, thereby giving insight into the
ion-beam mass spectrometry, and blackbody infrared radia-
[5]
tive dissociation, have played an important role in determin-
ing the factors that influence the strength and nature of IHBs
from the measured dissociation enthalpies and entropies of
hydrogen bonding in various gas-phase complexes. From
these experiments, the trends in binding energies have been
correlated with gas-phase basicities of the hydrogen-bond
donor or acceptor in the complexes and have been used to
obtain information on whether multiple hydrogen bonding,
partial charge transfer, and/or steric hindrance occurs in the
[1,2]
role of these interactions in more complex environments.
[6]
complexes. Infrared photodissociation (IRPD) spectroscopy
has emerged as a powerful and sensitive technique for directly
[a] M. Demireva, Prof. E. R. Williams
[7]
probing the structures of various gaseous ion complexes.
Department of Chemistry
University of California
Berkeley, CA 94720-1460 (USA)
Fax: (+1)(510)-642-7714
These spectroscopic studies can provide both detailed struc-
tural information as well as information about the potential
surface of the proton bond and coupling with other modes
that may occur. In an elegant and systematic vibrational pre-
dissociation spectroscopic study of proton-bound homo- and
heterodimers, Johnson and co-workers showed that the fre-
quency of the shared proton depends on the proton-affinity
difference between the two molecules, with higher frequencies
E-mail: williams@cchem.berkeley.edu
[
b] Prof. J. Oomens, Dr. G. Berden
Radboud University Nijmegen
Institute for Molecules and Materials, FELIX Facility
Toernooiveld 7, 6525ED Nijmegen (The Netherlands)
[
c] Prof. J. Oomens
Van’t Hoff Institute for Molecular Sciences
University of Amsterdam
[7a]
resulting from larger proton-affinity differences. This effect
was explained by a simple one-dimensional potential for the
shared proton motion, with homodimers that share the proton
equally having relatively flat potentials resulting in lower vibra-
Science Park 904, 1098XH Amsterdam (The Netherlands)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cplu.201300084.
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tional frequencies. As the proton-affinity difference between
the molecules increases, the proton becomes increasingly lo-
calized on one side of the complex resulting in a narrower po-
tential and a stronger bond with a higher frequency. The spec-
tral band for the shared proton is sharp for complexes with fre-
anion size, methylation of the ammonium groups, and with in-
creasing alkane chain length. Additional bands appearing in
the stretching feature arise from Fermi resonances, owing to
coupling of this stretch with overtone or combination bands,
assigned from calculated and experimental spectra. These
Fermi resonances are sensitive to the specific composition of
the halide-anion-bound diammonium complexes, and can be
effectively tuned in or out of resonance by changing the
halide anion or the length of the alkane chain.
À1
quencies above 2000 cm , whereas the peaks are broader and
contain multiplets when this band is at lower frequencies
[7a]
owing to coupling with other modes in these complexes.
The properties of IHBs have been extensively investigated in
complexes in which nitrogen or oxygen serve as proton ac-
ceptors, but there are fewer studies of IHBs in halide-anion
[2b]
complexes in which the halide anion is the proton acceptor.
Results and Discussion
Halide anions are important in biology, and recent results indi-
cate that they may be involved in stabilizing biomolecular
IRMPD spectra of halide-anion-bound diprotonated diamine
complexes
[
8]
structures by forming halogen-ionic bridges. Halide-anion
[
7g,h,9]
[7c–f,10]
binding to ammonia
or water
molecules has been
IRMPD spectra of diprotonated 1,12-dodecanediamine com-
À
investigated. These systems can serve as simple models for in-
vestigating the interactions between amine- or hydroxyl-group
hydrogen atoms in biomolecules and halide anions. The bind-
plexed with I , and tetramethyl-1,12-dodecanediamine com-
À
À
plexed with Cl or I were measured in the spectral frequency
À1
range of approximately 500–2250 cm (Figure 1, top) and
[
10]
[9]
À
À
ing energies of water and ammonia molecules to F , Cl ,
compared with their deuterated forms (Figure 1, bottom). The
IRMPD spectrum of diprotonated 1,12-dodecanediamine com-
À
À
Br , and I decrease with increasing halide-anion size, which is
consistent with weakening of the IHB as the gas-phase basicity
À
plexed with Br is essentially identical to that of the iodide-
[
9,10]
of the anion decreases with increasing anion size.
IRPD
[7g,h]
bound complex and is therefore not shown. In the IRMPD
spectrum of diprotonated 1,12-dodecanediamine complexed
[
7c–f]
spectroscopy of halide anions bound to water
or NH3
À
show that the IHB stretching frequency increases with increas-
ing halide-anion size, which is consistent with a decrease in
gas-phase basicity of the halide anion reported from the earlier
with I (Figure 1a), there are two relatively narrow and intense
À1
bands at approximately 1470 and approximately 1585 cm
À1
and a broad and weak feature at approximately 940 cm . The
[
9,10]
À1
binding-energy studies.
The IRPD spectroscopy experi-
band at approximately 1470 cm is attributed to CH bending
2
ments also indicate that coupling between the IHB stretches
and bending overtones occurs in the halide-anion-bound
water and ammonia complexes owing to the anharmonic
nature of the IHB, which results in Fermi resonances and the
modes, an assignment that is consistent with previous IRMPD
results for protonated tetramethylputrescine and tetramethyl-
[7l]
À1
1,3-propanediamine. The band at approximately 1585 cm is
assigned to NÀH bending modes, consistent with the absence
of this band in the IRMPD spectrum of deuterated 1,12-dodec-
[
7f–h]
appearance of additional spectral bands.
À
In addition to the gas-phase basicity of the proton donor,
IHBs in halide complexes are affected by the presence of
a charge and by the solvent. In isolation, a hydrogen bond is
anediammonium complexed with I , in which all amine hydro-
gen atoms have been exchanged for deuterium atoms (Fig-
ure 1a, bottom). A new band corresponding to the NÀD bend
À1
formed in ammonium chloride, that is, H N···HÀCl, whereas the
appears at lower frequency (ꢀ1130 cm ) as a result of the
3
+
À [11]
ionic species are formed in water, that is, NH4 and Cl . Cal-
culations indicate that only one water molecule is required to
induce the ionic rather than the hydrogen-bonded ammonium
heavier deuterium isotope (Figure 1a, bottom). The broad
À1
weaker feature at approximately 940 cm (Figure 1a, top) is
assigned to NÀH wagging modes, and this assignment is sup-
ported by the absence of intensity in this same spectral region
for the deuterated complex (Figure 1a, bottom).
[
12]
chloride complex. Similarly, experiments show that adding
an extra electron results in proton transfer and formation of
+
À À
[13]
the negatively charged [NH4 Cl ] ionic species. The ionic
gaseous complex becomes more favorable with increasing
halide-anion size and increasing proton affinity of the ammoni-
The IRMPD spectra of diprotonated tetramethyl-1,12-dodeca-
À
À
nediamine with Cl or I attached (Figure 1b,c, top, respective-
ly) are similar to each other. The most intense band corre-
[
11,14]
um group by methylation.
sponding to CH bending modes in these spectra is also at ap-
2
À1
Diammoniumalkanes complexed with halide anions are simi-
lar to the neutral ammonium halide species, but can be readily
investigated by IRPD spectroscopy because there is a net posi-
tive charge. Herein, halide-anion binding to 1,12-dodecane-
diammonium, tetramethyl-1,12-dodecanediammmonium, and
tetramethyl-1,7-heptanediammonium is studied using infrared
multiple-photon dissociation (IRMPD) spectroscopy and theory.
Changes in the IHB stretching frequency as a function of
halide-anion size, methylation of the ammonium groups, and
alkane chain length are investigated. These results show that
the IHB stretching frequency increases with increasing halide-
proximately 1470 cm . However, the sharp bending NÀH
À1
band at approximately 1585 cm in the spectrum of 1,12-do-
À
decanediammonium with I attached is absent from these
spectra and instead there is a broader shoulder band at lower
À1
À1
frequency around 1380 cm . This 1380 cm shoulder band is
absent in the spectra of the deuterated complexes (Fig-
ure 1b,c, bottom, respectively), which indicates that this band
corresponds to the NÀH bending mode. There are two sharp
À1
peaks at approximately 945 and 1000 cm in the IRMPD spec-
tra of the chloride- and iodide-bound tetramethyl-1,12-dodeca-
nediammonium complexes. These bands are also observed in
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À
Figure 1. IRMPD spectra at 298 K of doubly charged protonated (top) or deuterated (bottom) (a) 1,12-dodecanediamine complexed with I , (b) tetramethyl-
À
À
1
,12-dodecanediamine complexed with Cl , and (c) tetramethyl-1,12-dodecanediamine complexed with I .
À
the spectra of protonated tetramethylputrescine and tetra-
with I , as shown in the Supporting Information. The IHB
stretching frequency for this diprotonated complex appears
[
7l]
methyl-1,3-propanediamine and correspond to CÀN stretches
between the nitrogen and the methyl groups. This assignment
is consistent with the absence of these sharp features in the
IRMPD spectrum of the diprotonated 1,12-dodecanediamine
iodide complex (Figure 1a, top). Both of these bands should be
in the IRMPD spectrum of deuterated tetramethyl-1,12-dodeca-
À1
around 2630 cm and upon deuteration this frequency shifts
À1
down to approximately 2030 cm .
IRMPD spectra of halide-anion-bound deuterated diammoni-
um complexes
À
nediammonium complexed with I (Figure 1c, bottom). The
absence of the CÀN stretches, which are calculated to be ap-
Deuteration shifts the frequencies of the halide-anion-bound
amine-hydrogen stretches into the spectral range of the FEL,
thus allowing direct observation of the ionic deuterium bond
(IDB) stretching frequency between the halide anions and the
ammonium groups. Therefore, IRMPD spectra of the deuterat-
ed forms of 1,12-dodecanediammonium, tetramethyl-1,12-do-
decanediammonium, and tetramethyl-1,7-heptanediammoni-
À1
proximately 1000 cm (see below), can be attributed to low
À1
laser power. The bands at approximately 945 and 1000 cm in
Figure 1b,c (top) were observed only after increasing the laser
power by a factor of two in this frequency range. This indicates
that absorption of multiple IR photons is required to produce
[
6b]
measureable fragmentation at these frequencies. The aver-
age number of absorbed photons can be estimated from the
measured dissociation extent by master equation modeling,
which takes into account the initial ion internal-energy distri-
bution and the microcanonical rate constants for radiative ab-
À
À
À
um complexed with Cl , Br , or I were measured and are
shown in Figure 2. The broad feature that appears in the fre-
À1
quency range 1600–2200 cm in all spectra corresponds to
the IDB stretches. There are clear differences in the frequency,
width, and number of identifiable peaks in the IDB stretching
[
15]
sorption and emission, as well as dissociation.
+
+
À
The ionic hydrogen bond (IHB) stretching frequency be-
tween the halide anion and the protonated amine groups is
outside of the spectral frequency range in which IRMPD can
be obtained with the free-electron laser (FEL) and is absent in
the spectra of the iodide-bound diprotonated 1,12-dodecane-
diamine and the chloride-bound diprotonated tetramethyl-
features for these spectra. For D N(CH ) ND with Cl at-
3 2 12 3
tached (Figure 2a, top), there are three bands in the IDB fea-
À1
ture. The most intense band is at approximately 1790 cm
À1
with shoulder peaks at approximately 1680 and 1900 cm . For
the bromide-bound complex (Figure 2a, middle), the IDB fea-
ture consists of two well-resolved bands at approximately
À1
1,12-dodecanediamine complexes in Figure 1a,b (top), respec-
1810 and 1930 cm and an unresolved shoulder at approxi-
À1
tively. However, this stretching frequency is shifted down into
the spectral range for the deuterated complexes and corre-
mately 1870 cm . There is also a peak at approximately
À1
1810 cm for the iodide-bound complex (Figure 2a, bottom),
sponds to the broad feature that appears around 1750 to
but it has significantly lower intensity than the band at approx-
À1
À1
2250 cm in Figure 1a,b (bottom). The corresponding IHB
imately 2000 cm .
+
stretching features for the protonated complexes are expected
The IRMPD spectra of the halide-anion bound D(CH ) N-
3
2
À1
+
+
+
to appear around 2350 to 2700 cm , based on IRMPD results
(CH ) N(CH ) D and D(CH ) N(CH ) N(CH ) D complexes
2 12 3 2 3 2 2 7 3 2
for diprotonated tetramethyl-1,7-heptanediamine complexed
shown in Figure 2b,c are similar for a given anion with respect
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Figure 2. IRMPD spectra at 298 K of deuterated (a) 1,12-dodecanediammonium (b) tetramethyl-1,12-dodecanediammonium, and (c) tetramethyl-1,7-heptane-
À
À
À
diammonium complexed with halide anion Cl (top), Br (middle), or I (bottom).
to width and the number of distinct bands in the IDB stretch-
ing feature. For example, the spectra for the chloride-bound
complexes have three distinct peaks in this broad feature at
These calculations show that increasing the binding angle by
208 and the bond lengths by 0.1 ꢁ results in an approximately
À1
40 and 100 cm increase in the IDB stretching frequency, re-
about the same frequencies of approximately 1850, 1910, and
spectively. This spread in binding angles and bond lengths is
consistent with that found for the structures obtained from ini-
À1
1970 cm . For the bromide-bound complexes (Figure 2b,c,
À
À
middle), the IDB stretching feature has an intense band around
tial Monte Carlo conformational searches for Cl - and Br -
bound diammoniumalkane complexes, which have standard
deviations less than 88 and 0.09 ꢁ for the binding angle and
bond lengths, respectively. Thus, a distribution of structures
with a small continuous range of binding angles and bond
lengths are likely present in the experiments. Contributions of
these structures to the IRMPD spectra should result in a broad-
ening of the IDB band as observed. However, because these
structures have a continuum of bond lengths and angles, they
should not cause the multiple peaks in this feature that occur
in many of these spectra. Thus, these distinct IDB peaks are
most likely the result of Fermi resonances. The effects of the
binding angle and bond lengths on the IDB stretching fre-
quency and on likely conformers are discussed further in the
Supporting Information.
À1
2
000 cm and two shoulder peaks at approximately 1850 and
À1
1
910 cm . However, these spectra differ in the intensities of
the two shoulder peaks relative to the most intense band.
There is a more significant difference in the IRMPD spectra be-
tween the iodide-containing complexes. The IDB stretching
À1
feature at approximately 2040 cm in the IRMPD spectrum of
+
+
the iodide-bound D(CH ) N(CH ) N(CH ) D complex (Fig-
3
2
2 12
3 2
ure 2b, bottom) is relatively narrow. This band does not have
any lower frequency shoulder peaks but it does have a higher
frequency shoulder. This contrasts with the same feature for
+
+
the iodide-bound D(CH ) N(CH ) N(CH ) D complex (Fig-
3
2
2 7
3 2
À1
ure 2c, bottom), which has a band at approximately 2020 cm
with a higher frequency shoulder and a lower-intensity should-
À1
er band at approximately 1910 cm .
High-intensity IHB stretches can couple with weaker over-
tone and combination bands of similar energy to and the
same symmetry as the IHB stretch and result in Fermi resonan-
Fermi resonance
[16]
To determine whether the multiplets in the IDB stretching fre-
quencies might arise from different conformers of similar
energy, quantum mechanical geometry optimization and fre-
quency calculations on tetramethyl-1,7-heptanediammonium
ces. For example, Fermi resonances in IRPD spectra were re-
À [7f]
ported for the IHB stretch between H O and Br , and be-
2
À
À [7g]
tween NH and I , and 3NH and Br . The IHB stretch in
3
3
these complexes couples with a weaker bending overtone
mode and “lends” intensity to the weaker band, which results
À
complexed with Cl were performed by systematically varying
[7f]
the halide-anion bond lengths and binding angles (see the
Supporting Information). Results from these calculations indi-
cate that structures with halide-anion deuterium bond lengths
between 1.9 and 2.0 ꢁ and halide-anion binding angles be-
tween 120 and 1408 have relative Gibbs free energies at 298 K
in a doublet with peaks of similar intensity. In our experi-
+
+
ments, the IRMPD spectrum of D N(CH ) ND complexed
3
2 12
3
À
with Br (Figure 2a, middle) contains a splitting in the IDB
stretching feature into mainly two bands with comparable in-
À1
tensities at approximately 1810 and 1930 cm , which suggests
À1
that are within 1.5 kcalmol of the lowest-energy structure.
that a Fermi resonance occurs from coupling with a combina-
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tion or overtone band. In contrast, the IDB stretching feature
halide-anion identity. The NÀD bending mode at approximate-
+
+
À
À1
for D(CH ) N(CH ) N(CH ) D complexed with I (Figure 2b,
ly 1150 cm in the calculated spectra is consistent with the as-
3
2
2 12
3 2
À1
bottom) has no lower frequency shoulder bands, which indi-
cates that the IDB stretch in this complex does not couple
with other bands. The differences in the experimental IRMPD
spectra between methylated and nonmethylated diammonium
compounds complexed with halide anions can be attributed
to differences in the types of weak bands that couple with the
IDB stretch.
signment made for the band at approximately 1130 cm in
the experimental spectrum of the deuterated iodide-bound
complex in Figure 1. The NÀH wagging modes in the experi-
mental IRMPD spectrum for the nondeuterated iodide-bound
À1
complex occur around 940 cm . Deuterium exchange results
in the corresponding NÀD wagging frequencies appearing at
lower frequencies. These NÀD wagging modes were not ob-
served in the experimental spectra of the deuterated com-
plexes, likely a result of the bands appearing at frequencies
Nonmethylated diammonium halide-anion complexes
À1
below 800 cm .
Electronic structure calculations were used to obtain harmonic
frequencies and IR intensities to aid in identifying the combi-
nation or overtone bands (calculated as the sum of the scaled
harmonic frequencies) responsible for the Fermi resonances
observed in the experimental IRMPD spectra of the halide-
anion-bound diammonium complexes. Experimental IRMPD
spectra along with calculated IR spectra are shown in Fig-
The results from calculations and experimental data suggest
two possibilities for weak overtone or combination bands that
could couple with the IDB stretches in the halide-anion-bound
+
+
D N(CH ) ND complexes. The first possibility is an overtone
3
2 12
3
from the NÀD wagging modes that is calculated to be be-
À1
tween approximately 1360 and 1560 cm (calculated as the
sum of the scaled harmonic frequencies). The second possibili-
ty is a combination band that results from the NÀD wagging
and NÀD bending modes with a frequency of approximately
+
+
À
À
À
ure 3a–c for D N(CH ) ND complexed with Cl , Br , and I ,
3
2 12
3
respectively. The two distinct bands in the IDB stretching fea-
À
À
À1
ture for the Br and I complexes indicate that only one com-
bination or overtone band couples appreciably with the IDB
stretch. However, the IDB feature for the chlorinated complex
contains three bands, which indicates that coupling occurs
with an additional weaker overtone or combination band. The
1870 cm . Thus, the peak observed at approximately
À1
1650 cm for the chloride-bound complex could arise from
coupling between the IDB stretch and the overtone of the NÀ
À1
D wagging modes. The band at approximately 1900 cm cor-
responds to the IDB stretch and the most intense band at ap-
+
+
À
À1
calculated IR spectra for D N(CH ) ND complexed with Cl ,
proximately 1790 cm could be the result of coupling be-
3
2 12
3
À
À
Br , or I are very similar. The most notable difference is the
tween the IDB stretch and the NÀD wagging and bending
combination band. Because the IDB stretch for the bromide-
and iodide-bound complexes at approximately 1930 and
À1
IDB stretch, which shifts from approximately 1900 cm for the
À1
chloride-bound complex to approximately 2000 cm for the
À1
iodide-bound complex. The bands at approximately 720 and
2000 cm , respectively, shifts to higher frequencies as a func-
À1
1
150 cm are the NÀD wagging and bending modes, respec-
tion of the halide-anion size, coupling with the higher-frequen-
cy combination band is observed mainly in the IRMPD spectra
tively. The frequencies for these bands are insensitive to the
À
À
Figure 3. Experimental IRMPD spectra (top) and calculated IR spectra (bottom) of deuterated 1,12-dodecanediammonium complexed with (a) Cl , (b) Br , or
À
(
c) I . The ionic deuterium bond stretching feature in the experimental spectra is deconvolved by Lorentzian peak functions. The number of Lorentzian func-
2
tions used depends on the number of well-resolved bands contributing to this feature and the quality of the fits is given by the R values.
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À1
of these complexes. In addition to these identified combina-
tion and overtone bands, there are likely other modes that
could couple and contribute to the broadness and complexi-
ties in this feature as well.
be approximately 1000 cm . Both the CÀN stretches and NÀD
bends are missing from the experimental spectrum of the deu-
terated iodide complex in Figure 4c (top) likely because of in-
sufficient laser power to dissociate the complex. Nonetheless,
the complexities in the IDB stretching features observed for
À
À
+
+
+
the Cl - and Br -bound D(CH ) N(CH ) N(CH ) D and D-
3
2
2 12
3 2
+
(
CH ) N(CH ) N(CH ) D complexes can be explained by cou-
3 2 2 7 3 2
Methylated diammonium halide-anion complexes
pling between the IDB stretch and the overtones of the CÀN
stretches and NÀD bends that result in Fermi resonances,
which give rise to the two shoulder bands at approximately
The IDB stretching features in the experimental IRMPD spectra
+
+
of the halide-anion-bound D(CH ) N(CH ) N(CH ) D and
3
2
2 12
3 2
+
+
D(CH ) N(CH ) N(CH ) D complexes differ from those of
À1
3
2
2 7
+
3 2
1850 and 1900 cm .
+
D N(CH ) ND . Methylation of the amine groups alters the
3
2 12
3
NÀD wagging and bending frequencies, which are likely re-
sponsible for the Fermi resonances observed for
IDB stretching frequency
+
+
À
À
À
D N(CH ) ND complexed with Cl , Br , or I . Possible over-
3
2 12
3
tone or combination bands for the methylated diammonium
halide-anion complexes that might be responsible for the mul-
tiple bands in the IDB stretch are identified from computa-
tions. Calculated and experimental IRMPD spectra are shown
The IRMPD spectra indicate that the IDB stretching frequency
depends on the halide anion, chain length, and methylation of
the amine groups. The IDB stretching frequencies were ob-
tained from the experimental IRMPD spectra by deconvolving
the IDB stretching feature into the minimum number of Lor-
entzian functions necessary to obtain a good fit with an
+
in Figures 4 and 5 for halide-anion-bound D(CH ) N(CH ) N-
3
2
2 12
+
+
+
(
CH ) D and D(CH ) N(CH ) N(CH ) D complexes, respec-
3 2 3 2 2 7 3 2
2
tively. Calculated harmonic frequencies at approximately 930
R value of at least 0.84. These Lorentzian fits were overlaid
À1
and 1000 cm correspond to CÀN stretches between the
with the experimental IRMPD spectra in Figures 3–5 for halide-
+
+
+
+
methyl groups and the amine nitrogen with contributions
from NÀD bending modes and are consistent with the assign-
ment of CÀN stretches for the two sharp bands at approxi-
anion-bound
D N(CH ) ND
,
D(CH ) N(CH ) N(CH ) D ,
3 2 2 12 3 2
3
2 12
3
+
+
and D(CH ) N(CH ) N(CH ) D complexes, respectively. Cou-
3
2
2 7
3 2
pling with overtone and combination bands will perturb the
frequency of the IDB stretch and shift it from the unperturbed
À1
mately 945 and 1000 cm in the experimental IRMPD spectra
[7e,g,17]
of the chloride- and iodide-bound diprotonated tetramethyl-
frequency value.
Thus, the observed IDB stretching fre-
1
,12-dodecanediamine complexes in Figure 1b,c, respectively.
quencies obtained from the deconvolution will likely deviate
somewhat from the unperturbed frequencies in the absence of
these resonances.
À1
The broader shoulder at 1380 cm from NÀH bending modes
in the IRMPD spectra of the chloride- and iodide-bound dipro-
tonated tetramethyl-1,12-dodecanediamine complexes (Fig-
ure 1b,c, top) is expected to shift to lower frequency upon
deuteration owing to the heavier isotope and is calculated to
The IDB stretching frequencies obtained from the Lorentzian
fits as a function of gas-phase basicity of the halide anion for
+
+
+
+
+
D N(CH ) ND
,
D(CH ) N(CH ) N(CH ) D , and D(CH ) N-
3 2 2 12 3 2 3 2
3
2 12
3
À
Figure 4. Experimental IRMPD spectra (top) and calculated IR spectra (bottom) of deuterated tetramethyl-1,12-dodecanediammonium complexed with (a) Cl ,
À
À
(
b) Br , or (c) I . The ionic deuterium bond stretching feature in the experimental spectra is deconvolved by Lorentzian peak functions. The number of Lorent-
2
zian functions used depends on the number of well-resolved bands contributing to the feature and the quality of the fits is given by the R values.
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À
Figure 5. Experimental IRMPD spectra (top) and calculated IR spectra (bottom) of deuterated tetramethyl-1,7-heptanediammonium complexed with (a) Cl ,
À
À
(
b) Br , or (c) I . The ionic deuterium bond stretching feature in the experimental spectra is deconvolved by Lorentzian peak functions. The number of Lorent-
2
zian functions used depends on the number of well-resolved bands contributing to this feature and the quality of the fits is given by the R values.
and the deuterium atoms on the amine groups, thus shifting
the IDB stretching frequency to higher values.
There is also a blueshift in the IDB stretching frequency as
a function of halide-anion size for the methylated diammoni-
+
+
+
um compounds, D(CH ) N(CH ) N(CH ) D and D(CH ) N-
3
2
2 12
3 2
3 2
+
À1
(
CH ) N(CH ) D . However, the shift (ꢀ25 cm ) to higher fre-
2
7
3 2
quencies per halogen is lower than for the nonmethylated
+
+
D N(CH ) ND complexes. This is due to the higher gas-
3
2 12
3
phase basicity of the amine group as a result of methylation,
which decreases the bond strength between the halide anion
and the deuterium atoms, and thereby reduces the effect of
halide-anion gas-phase basicity on the IDB stretching frequen-
cy. This is also consistent with the overall higher IDB stretching
frequencies observed for the methylated versus the nonmethy-
lated diammonium halide-anion-bound complexes. Results for
+
+
+
+
D(CH ) N(CH ) N(CH ) D
and
D(CH ) N(CH ) N(CH ) D
3 2 2 7 3 2
3
2
2 12
3 2
Figure 6. The measured IDB stretching frequency obtained by deconvolving
the experimental spectra with Lorentzian functions (see Figures 3–5) for
deuterated 1,12-dodecanediammonium (squares), tetramethyl-1,12-dodeca-
nediammonium (circles), and tetramethyl-1,7-heptanediammonium (trian-
gles) as a function of gas-phase basicity of the halide anion. Error bars indi-
cate the standard error in the IDB stretching frequency from the Lorentzian
fit. Data are fit with lines as a guide.
show that the IDB stretching frequency also depends on the
chain length separating the two charged amine groups. The
IDB stretching frequencies of the halide-anion-bound com-
plexes with the shorter (seven carbon) alkane chain are lower
than those for the complexes with the longer (twelve carbon)
+
À
alkane chain. This likely results from differences in the D -X -
+
+
D
angle, which is more constrained for D(CH ) N(CH ) N-
3 2 2 7
+
+
+
+
+
+
(
CH ) N(CH ) D are shown in Figure 6. For D N(CH ) ND
,
(CH ) D than D(CH ) N(CH ) N(CH ) D because of the
3 2 3 2 2 12 3 2
2
7
3 2
3
2 12
3
À1
À
the IDB stretching frequency shifts from 1904 cm for Cl to
shorter alkane chain separating the two charged amine
groups. The flexible longer alkane chain allows for a nearly
linear binding angle between the halide anion and the amine
deuterium atoms, which maximizes the distance between the
two positively charged amine groups and results in stronger
amine–deuterium bonds and a higher IDB stretching frequen-
cy. This is supported by calculations (see the Supporting Infor-
mation), which indicate that structures obtained with Monte
Carlo conformational searching for tetramethyl-1,12-dodecane-
À1
À
1
992 cm for I . The blueshift in the frequency of the IDB
À1
stretch with increasing halide-anion size is about 50 cm per
À
À
halogen from Cl to I , which is similar to results for halide-
[
7g]
anion-bound NH3 complexes,
but less than the blueshift
À1
(
ꢀ130 cm ) observed in the IHB stretching frequency per hal-
[
7e]
ogen for halide-anion-bound H O complexes. A decrease in
2
the gas-phase basicity of the halide anion results in a decrease
[
7e,g,9,10]
in the strength of the bond
between the halide anion
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diammonium halide-anion complexes have a binding angle of
approximately 1608 compared with approximately 1258 for tet-
ramethyl-1,7-heptanediammonium halide-anion complexes.
plexes, in which the protons reside closer to the amine groups
rather than the halide anion. This is consistent with results
showing that addition of an extra electron to gaseous ammo-
+
nium chloride induces proton transfer to the NH group, [NH
3
4
À À
Cl ] . Unlike the neutral ammonium chloride complex, dia-
mmonium compounds, such as those investigated here, con-
tain two protonated amine groups and form a singly charged
species when complexed to a halide anion, which makes them
amenable for study with IRMPD spectroscopy. Thus, diammoni-
um compounds for which the two ammonium groups are sep-
arated by a rigid linker that prevents these groups from both
interacting with the halide anion could be used in future
IR(M)PD experiments to characterize the hydrogen bond ex-
pected to form in neutral ammonium chloride complexes in
the gas phase.
Effects of the halide anion and chain length on the Fermi
resonance
A large halide-anion size results in reduced coupling between
the IDB stretch and the weaker overtone or combination
+
+
bands as shown for the iodide-bound D N(CH ) ND (Fig-
3
2 12
3
+
+
ure 3c) and D(CH ) N(CH ) N(CH ) D (Figure 5c) complexes,
3
2
2 7
3 2
and this Fermi resonance is eliminated for the iodide-bound
+
+
D(CH ) N(CH ) N(CH ) D complex as indicated by the rela-
3
2
2 12
3 2
tively narrow IDB stretching feature in the experimental IRMPD
spectrum. These results indicate that the increase in the IDB
stretching frequency with increasing halide-anion size can ef-
fectively “tune” the IDB stretching frequency out of resonance
with the weaker overtone or combination bands in these com-
Recent studies indicate that halide anions are involved in
stabilizing the structures of biomolecules by forming halogen-
[8]
ionic bridges. Thus, these diammonium compounds, in which
the alkane chain length is varied, can serve as simple models
for studying and providing insight into the interactions be-
tween halide anions and positively charged functional groups
in biomolecules, such as the basic residues in proteins.
plexes. This effect has been demonstrated previously for the
À
Cl -bound water complex by the sequential addition of CCl ,
4
which shifted the IHB stretch to higher frequencies and
“
tuned” this stretch through a Fermi resonance interaction
[
17]
with the bending overtone. Because the IDB stretching fre-
quency depends on the alkane chain length between the two
ammonium groups in these complexes, Fermi resonances can
also be effectively “tuned” in and out by changing the alkane
Experimental Section
+
Experiments
chain length. This is indicated for the iodide D(CH ) N(CH ) N-
3
2
2 7
+
+
+
(
CH ) D and D(CH ) N(CH ) N(CH ) D complexes, where
IRMPD experiments were performed using a 4.7 T Fourier-trans-
form ion cyclotron resonance mass spectrometer coupled to
a free-electron laser (FEL) that produces intense tunable IR radia-
tion. Both the experimental procedures and apparatus have
been described in detail elsewhere. Ions were formed by electro-
spray ionization from 70:30 methanol/water or 60:40 deuterated
water/methanol containing approximately 2 mm diamino com-
pound with approximately 2 mm of the corresponding HX acid, in
which X=Cl, Br, or I. The precursor-ion complexes of interest were
isolated by ejecting all other ions from the cell using stored wave-
3
2
3 2
2 12
3 2
there is still some coupling observed between the IDB stretch
and the weak overtone in the diammonium complex with the
shorter alkane chain, but there is no Fermi resonance observed
for the complex with the longer alkane chain.
[
18]
[19]
Conclusion
À1
IRMPD spectra in the 1000–2250 cm region were measured
[20]
for 1,12-dodecanediammonium, tetramethyl-1,12-dodecane-
diammmonium, and tetramethyl-1,7-heptanediammonium
form inverse Fourier-transform excitation and were subsequently
irradiated for 2–5 s with tunable radiation from the FEL. An IRMPD
spectrum for diprotonated tetramethyl-1,7-heptanediamine com-
À
À
À
complexed with Cl , Br , or I . The IDB stretching frequency
shifts with halide-anion size, methylation of the ammonium
groups, and alkane chain length, a result related to the gas-
phase basicities of the halide anions and the ammonium
groups, and the constraints in the binding angle between the
halide anion and the two ammonium groups. The IDB stretch-
ing frequencies in these complexes couple with weaker over-
tone or combination bands that give rise to Fermi resonances,
which can be tuned in or out of resonance by changing the
halide anion or the alkane chain length. The results obtained
for the deuterated halide-anion-bound diammonium com-
plexes should apply to the corresponding nondeuterated com-
plexes as well.
À
plexed with I in the frequency range between 2500 and
À1
3
100 cm was measured using a tunable optical parametric oscil-
lator/amplifier (OPO/OPA) system (LaserVision, Bellevue, WA) with
this same mass spectrometer and irradiation time of 4.5 s. The
major dissociation pathway observed in the IRMPD experiments
was the loss of HX. The IRMPD intensities in the spectra were ob-
tained from the IRMPD yield determined from the precursor- and
product-ion abundances, and were corrected for the frequency-de-
[18,21]
pendent laser power.
All reagents were obtained from Sigma–Aldrich (Zwijndrecht, Neth-
erlands) and were used without further purification. Methylation of
1,7-heptanediamine and 1,12-dodecanediamine to produce tetra-
methyl-1,7-heptanediamine and tetramethyl-1,12-dodecanedia-
mine, respectively, was accomplished using a previously reported
In gaseous ammonium chloride complexes, the proton re-
sides closer to the halide anion, that is, forming HÀCl rather
[22]
procedure. Briefly, the methylated diamines (ꢀ0.5 g) were pre-
pared by mixing the diamine (0.5 g), 3m sulfuric acid (1.4 mL), 40%
aqueous formaldehyde (2.5 mL), and slow addition of sodium bor-
ohydride (ꢀ0.6 g) to a glass flask placed in an ice bath. The mix-
ture was allowed to react for 5–10 min, after which concentrated
sulfuric acid was carefully added. Impurities were extracted three
+
than to the NH group to form NH , even though the ionic
3
4
[
11]
species are formed in aqueous solution. Our IRMPD results
indicate that binding of two ammonium groups to a halide
anion in the gas phase favors formation of the ionic com-
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times with diethyl ether (2 mL), and the aqueous layer was subse-
quently made basic with an excess amount of KOH. Three more ex-
tractions with diethyl ether (2 mL) followed, after which the methy-
lated diamines remained in the ether.
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À1
ent structures that were within 12 kcalmol of each other were
found for these respective ions. For a given diammoniumalkane
and halide anion, the structures differ mainly in the conformation
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the two charged amine groups (see the Supporting Information
for a more detailed discussion). Starting structures for complexes
À
with I attached were obtained by halide-anion substitution into
the brominated structures. Quantum mechanical geometry optimi-
zation of selected low-energy structures from the conformational
search was carried out in Q-Chem 3.2 at the B3LYP/6-31+ +G**
[23]
level of theory using the CRENBL basis set and effective core po-
À
À [24]
tential for Br and I .
2
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the line shapes were approximated with Lorentzian functions
1
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[
Keywords: anions · Fermi resonance · halides · ionic hydrogen
bonds · IR spectroscopy
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Received: March 8, 2013
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