HF stretching-bending combination bands for small complexes in solid argon

Article abstract of DOI:10.1063/1.479895

Infrared spectra of HF (with DF substitution) in solid argon contain weak HF stretching-bending combination bands for N₂-HF, (HF)₂, and (HF)₃ that are higher than the sum of fundamentals. This is in accord with stronger hy

Full text of DOI:10.1063/1.479895

HF stretching–bending combination bands for small complexes in solid argon
Lester Andrews and Philip F. Souter
Citation: The Journal of Chemical Physics 111, 5995 (1999); doi: 10.1063/1.479895
View online: http://dx.doi.org/10.1063/1.479895
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JOURNAL OF CHEMICAL PHYSICS
VOLUME 111, NUMBER 13
1 OCTOBER 1999
HF stretching–bending combination bands for small complexes
in solid argon
Lester Andrews and Philip F. Souter
Department of Chemistry, University of Virginia, Charlottesville, Virginia 22901
͑Received 26 April 1999; accepted 9 July 1999͒
Infrared spectra of HF ͑with DF substitution͒ in solid argon contain weak HF stretching–bending
combination bands for N2–HF, (HF)2 , and (HF)3 that are higher than the sum of fundamentals.
This is in accord with stronger hydrogen bonds upon vibrational excitation of the H–F stretching
mode, as observed in the gas phase. Overtone bands were also observed for DF and (DF)2 in solid
argon. © 1999 American Institute of Physics. ͓S0021-9606͑99͒01537-8͔
I. INTRODUCTION
III. RESULTS AND DISCUSSION
The low-frequency intermolecular modes of the N2–HF
complex have been measured in combination with the sec-
ond overtone (3␯1) of the H–F stretching mode recently by
the Klemperer group.1 These workers noted a striking varia-
tion in the properties of Ar–HF and N2–HF with the H–F
bond vibrational excitation, predicted that the N2–HF bend-
Matrix infrared spectra for HF and DF in solid argon at
7–8 K are presented in Figs. 1, 2, and 3 in the overtone and
combination band regions for 1 h sample deposits, and the
new absorptions are listed in Table I. Infrared spectra in the
4000–400 cmϪ1 region matched spectra recorded earlier.14
These regions were also examined for the tracking of bands
in overtone, combination band, and fundamental regions in
experiments with different reagent concentrations.
ing mode observed at 328.6 cmϪ1 in combination with 3␯
1
would extrapolate to 259 cmϪ1 for the fundamental bending
mode, and noted agreement with the direct observation of
this bending mode at 262 cmϪ1 for the N2–HF complex in
solid argon.2 This suggests that a combination band should
be observable more than 262 cmϪ1 above the 3881 cmϪ1
H–F stretching fundamental for N2–HF in solid argon.2
Clearly, the N2–HF hydrogen bond is strengthened upon vi-
brational excitation of the H–F stretching mode.3 It will also
be of interest to see if the NwN and D–F stretching combi-
nation band can be observed in the N2–DF complex. The
N2–HF complex has been calculated at a high level of theory
and the bending mode fundamental predicted somewhat
higher at 369 cmϪ1.4 There has been a considerable interest
in the dissociation dynamics of this linear complex.5–8 Fur-
thermore, the increase in HF bending mode frequency in
combination bands has been observed for (HF)2 in the gas
phase9–13 and should apply to (HF)3 and higher clusters as
well. We report here an argon matrix investigation of the
combination band region to examine the increase in HF
bending modes in combination relative to the observed fun-
damental bands.
A. DF
Since the DF matrix spectra always contained some HF,
Fig. 1 contrasts spectra in the 5740–5420 cmϪ1 region with
HF and DF; the HF experiments were performed after DF
and also contained residual DF impurity. Figure 1͑a͒ for
0.5% HF in argon with very little DF present shows weak
bands at 5692 and 5618 cmϪ1. Figure 1͑b͒ with 2% HF and
approximately 0.5% DF reveals stronger 5692 and 5618
cmϪ1 bands and weak new 5679 and 5607 cmϪ1 features.
Figure 1͑c͒ for 0.25% DF and 0.1% HF shows the same
features. The spectrum in Fig. 1͑d͒ for 2% DF and 0.2% HF
reveals a stronger 5607 cmϪ1 band and new features at 5659,
5552, and 5435 cmϪ1. All spectra contain a 5753 cmϪ1 band
͑not shown͒ that clearly arises from an impurity in the HF
system.
In the DF spectrum, the two weak bands at 5679 and
5659 cmϪ1 ͑0.010 and 0.006 absorbance units, respectively͒
͓Fig. 1͑d͔͒ are overtones of the 2896 and 2877 cmϪ1 DF
R(0) and impurity-induced Q bands ͑0.74 and 0.84 absor-
bance units, not shown͒ in solid argon.14,15 The 5679 cmϪ1
band is stronger in the most dilute DF sample spectrum, Fig.
1͑c͒, but the 5659 cmϪ1 band is strong at higher concentra-
tions, Fig. 1͑d͒; the 2896 and 2877 cmϪ1 fundamental bands
follow these intensities as the Q bands exhibit intensities
dependent on other molecules present. The DF overtone
bands at 5679 and 5659 cmϪ1 in solid argon are each red-
shifted 63 cmϪ1 from the gas phase 5742.4 and 5721.8 cmϪ1
R(0) and Q(0) band positions.16 The fundamental bands at
2896 and 2877 cmϪ1 are red-shifted 32 and 30 cmϪ1, respec-
tively from the gas phase 2927.8 and 2906.7 cmϪ1 band
positions.16
II. EXPERIMENT
Matrix isolation experiments were performed for HF and
DF in argon using the same vacuum system described
previously2,14,15 with a colder 7–8 K substrate temperature
for better isolation and a Nicolet 550 instrument ͑6000–400
cmϪ1, 0.5 cmϪ1 resolution͒ with HgCdTe detector. A large
number of scans ͑4096͒ was required to give good signal-to-
noise at 5700 cmϪ1. Hydrogen fluoride ͑Matheson͒ was con-
densed and pumped; DF was prepared by reacting D2 with F2
and the DF formed was condensed and evacuated to remove
volatiles.14
0021-9606/99/111(13)/5995/4/$15.00
5995
© 1999 American Institute of Physics
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5996
J. Chem. Phys., Vol. 111, No. 13, 1 October 1999
L. Andrews and P. F. Souter
FIG. 1. Infrared spectra of HF and DF in solid argon at 7–8 K in the
5740–5420 cmϪ1 region. ͑a͒ 0.5% HF with 0.1% DF in argon, ͑b͒ 2% HF
with 0.5% DF in argon, ͑c͒ 0.25% DF in argon with 0.1% HF and ͑d͒ 2%
DF with 0.2% HF in argon.
B. „DF…2
The most dilute DF sample also reveals a new 5607
cmϪ1 band, which increases markedly in intensity relative to
DF as does the (DF)2 fundamental at 2804 cmϪ1 ͑not shown,
see Ref. 14͒ with increasing DF concentration. This band is
due to the first DF cluster, namely (DF)2 . The only overtone
FIG. 3. Infrared spectra of DF in solid argon at 7–8 K in the 3140–3000
cmϪ1 region. ͑a͒ 1% DF plus 3% N2 in argon, ͑b͒ 1% DF in argon, and ͑c͒
after annealing to 30 K.
observed to date for (HF)2 in the gas phase17 is 2␯
at
acceptor
7683 cmϪ1. A simple prediction for 2␯
of (DF)2 is to
acceptor
scale 7683 cmϪ1 by the ␯
fundamental ratio ͑2883/
acceptor
3931͒ for gas phase10,11,13,18 (DF)2 and (HF)2 , which gives
5635 cmϪ1, but this does not account for large changes in the
average angles, which will affect a simple coupled oscillator
model. Jensen, et al.19 predict the 2␯
mode for (HF)2
acceptor
at 7674 cmϪ1, near the observed value, and at 5664 cmϪ1 for
(DF)2 . The latter value for 2␯
of (DF)2 in the gas
acceptor
phase is 59 cmϪ1 higher than our observed 5607 cmϪ1 band,
and the 2883 cmϪ1 gas phase fundamental is 27 cmϪ1 higher
than the ␯
mode for (DF)2 in solid argon.20 Accord-
acceptor
ingly the 5607 cmϪ1 band is assigned to 2␯
of (DF)2 .
acceptor
We cannot associate with confidence other features in the
more concentrated samples with (DF)2 for possible assign-
ment to other members of the 2␯ triad, which are predicted at
5723 and 5592 cmϪ1 by the ab initio calculations.19
The new 5435 cmϪ1 absorption exhibits the same profile
as the 3086, 3082 cmϪ1 band, which will be assigned below
to a (DF)3 combination band. This 5435 cmϪ1 band is likely
due to an overtone of (DF)3 , but we cannot make a specific
assignment.
C. N2–HF
Consider next the HF spectra in Fig. 2. The highest fre-
quency band at 4252 cmϪ1 corresponds to the gas phase
band12 near 4294 cmϪ1 for the ␯2ϩ␯ ͑hydrogen bonded
6
H–F stretchϩout-of-plane bend͒ of (HF)2 . The strongest
new band observed here at 4164 cmϪ1 is split into 4161.5
and 4165.5 cmϪ1 components. No counterpart of this band is
FIG. 2. Infrared spectra of HF in solid argon at 7–8 K in the 4270–4050
cmϪ1 region. ͑a͒ 0.5% HF in argon, ͑b͒ 2% HF in argon, and ͑c͒ after
annealing to 25 K.
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J. Chem. Phys., Vol. 111, No. 13, 1 October 1999
HF stretching-bending combination bands
5997
cmϪ1 from the nitrogen fundamental ͑2330.0 cmϪ1͒ in the
gas phase.21
TABLE I. Combination bands observed for N2–HF and (HF)n species in
solid argon at 7–8 K.
The extrapolation1 of the bending mode from the 328.6
HF
DF
Identification
cmϪ1 value in combination with 3 ␯ to 259 cmϪ1 suggests
1
¯
5692
5679
5659
5618
5607
5552
5435
5178
3118
3086, 3082
3072
3040
3029
3024
3014
¯
from HF sample
DF, R(0), 2␯
DF, impurity-induced Q, 2␯
from HF sample
that the argon matrix2 has only a small effect on this 262
cmϪ1 direct observation. Furthermore, this near agreement
between gas phase and argon matrix measurements shows
that the high level calculation4 does not adequately describe
this intermolecular motion. The intramolecular H–F stretch-
ing mode observed at 3918.5 cmϪ1 in the gas phase N2–HF
complex red shifts 36.7 to 3881.5 cmϪ1 in solid argon, which
is slightly less than the 41.9 cmϪ1 red shift in H–F funda-
mental itself in the gas phase ͑3961.4 cmϪ1͒ to the argon
matrix ͑3919.5 cmϪ1͒.6,14 The increase in H/D isotopic fre-
quency ratio for the fundamental bending mode, 262/214
ϭ1.224, and this bending mode in combination, 283/226
ϭ1.252, further illustrates the bending mode dependence on
HF ͑or DF͒ quantum number in the combination mode where
the hydrogen bond is stronger and stiffer. Finally, the low
H/D ratio for the HF bending fundamental suggests that this
large amplitude mode may also involve some N2–HF
stretching character as the H/D ratio for this bending mode in
Ar–HF is also low ͑1.164͒.22
¯
¯
¯
¯
(DF)2 , 2␯
1
¯
enhanced by N2
(DF)3 , overtone
¯
¯
N2–DF, ␯ ͑D–F stretch͒ϩ␯ ͑NwN stretch͒
1
2
4252
4184
4164
4143
4119
4109
4098
4077
4069
3963
3920
3881
3826
3702
2332
262
(HF)2 , ␯2ϩ␯
6
(HF)3 , ␯͑H–F stretch͒ϩ␯͑H–F bend͒
N2–HF, ␯ ͑H–F stretch͒ϩ␯͑H–F bend͒
1
(HF)n
(HF)n
(HF)n
(HF)n
(HF)2?
(HF)2?
HF, R(0)
HF, impurity induced Q
N2–HF, ␯1(H–F stretch)
(HF)2 , ␯2(bound H–F stretch)
(HF)3 , cyclic, ␯donor(e)
N2–HF, ␯2(NwN stretch)
N2–HF, ␯͑H–F bend͒
¯
2896
2877
2846
2804
2717
2333
214
D. „HF…2
Comparison with the gas phase12 bands at 4294 cmϪ1 for
␯2ϩ␯6 , and at 3868 cmϪ1 for ␯ of (HF)2 with the ␯ band
2
2
observed up to 4200 cmϪ1 in the gas phase (HF)2 dimer
spectrum.12 Doping the Ar/HF sample with 1% N2 increased
the 4164 cmϪ1 combination band and the 3881 cmϪ1 funda-
mental N2–HF band2 by 4ϫ and decreased the 4252 cmϪ1
combination and 3826 cmϪ1 fundamental (HF)2 bands14 by
an order-of-magnitude. Figure 3 illustrates spectra for
Ar/DF/N2 and Ar/DF samples, which reveal the new combi-
nation band at 3072 cmϪ1. The 5930Ϯ20 cmϪ1 region re-
vealed no absorption above the noise level.
for (HF)2 in solid argon,14,15 indicates this combination band
assignment for the 4252 cmϪ1 solid argon band. Interest-
ingly, the differences 4294Ϫ3868ϭ426 cmϪ1 for the gas
phase and 4252Ϫ3826ϭ426 cmϪ1 for the argon matrix give
the same value for ␯ in combination with ␯2 . The ␯ out-
6
6
of-plane bending mode has been observed at 400 cmϪ1 in
solid argon, and the Kϭ1 state observed in the gas phase
near 400 cmϪ1 with an estimated 380 cmϪ1 fundamental
band origin.23,24 For (DF)2 the 3118 cmϪ1 argon matrix band
On the basis of the above evidence, the 4164 cmϪ1 band
is assigned to the H–F stretching and bending combination
band of the N2–HF complex and the deuterium counterpart
at 3072 cmϪ1, which has the same behavior, to the analogous
mode for the N2–DF complex. The differences 4164Ϫ3881
ϭ283 cmϪ1 for N2–HF and 3072Ϫ2846ϭ226 cmϪ1 for
N2–DF correspond to the HF͑DF͒ bending modes in combi-
is red-shifted from the gas phase13 ␯2ϩ␯ band at 3152
6
cmϪ1. The differences, 3152Ϫ2835ϭ317 cmϪ1 for the gas
phase and 3118Ϫ2804ϭ314 cmϪ1 for the argon matrix, are
nearly the same and the fundamental band is observed at 300
cmϪ1 in solid argon.14,15 So we see that the argon matrix has
little effect on the enhanced strength and rigidity of the hy-
drogen bond in HF–HF with excitation of ␯2 , the hydrogen-
bonded H–F stretching fundamental. Note also that the H/D
ratio for the bending mode in combination, 426/314
ϭ1.357, is higher than for the fundamental bending mode,
400/300ϭ1.333.
nation with ␯ and are higher than the bending mode funda-
1
mentals observed at 262 and 214 cmϪ1, respectively, in solid
argon.2 This follows from the Klemperer group observation
of the H–F bending mode 329 cmϪ1 above 3 ␯ in the com-
1
bination band and suggests that in combination with 2 ␯1 ,
the H–F bending mode should be near 305 cmϪ1
.
E. „HF…3
A very weak 5178 cmϪ1 band (Aϭ0.002) appears with
added N2 and tracks with the 2333 cmϪ1 NwN fundamental
(Aϭ0.116) and very strong 2846 cmϪ1 D–F fundamental
(Aϭ5.7) of the N2–DF complex. The 5178 cmϪ1 band is
The 3702 cmϪ1 (HF)3 band is favored relative to the
3826 cmϪ1 (HF)2 band on increasing HF concentration in
argon and on annealing.14,15 Likewise the 4184 cmϪ1 combi-
nation band is favored over the 4252 cmϪ1 (HF)2 band so
the 4184 cmϪ1 band must be considered for (HF)3 . The
difference 4184Ϫ3702ϭ482 cmϪ1 is again just above the
bending mode observed in solid argon, at 446 cmϪ1 for the
degenerate in-plane mode of the trigonal-ring (HF)3
complex.14,15,25 Similarly, for the deuterium counterpart at
assigned to the ␯NNϩ␯ combination band for N2–DF and
DF
appears just 1 cmϪ1 below the sum of fundamentals. The DF
interaction with N2 is weak and the D–F, NwN vibrational
coupling is small; hence, a small difference is observed for
this mode in N2–DF ͑2332.8 cmϪ1͒ and N2–HF ͑2332.1
cmϪ1͒.2 These complex modes are blue-shifted only 2.1–2.8
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5998
J. Chem. Phys., Vol. 111, No. 13, 1 October 1999
L. Andrews and P. F. Souter
the gas phase1,4,11–13 occurs equally for the argon matrix iso-
lated species and for the cyclic-(HF)3 trimer as determined
from increase in HF bending frequencies from fundamental
to combination bands. This increase in combination with the
hydrogen-bonded H–F stretching fundamental is 21 cmϪ1
for the 262 cmϪ1 bending fundamental in N2–HF, 26 cmϪ1
for the 400 cmϪ1 out-of-plane bending mode in (HF)2 , and
36 cmϪ1 for the 446 cmϪ1 in-plane bending mode in (HF)3 .
The increased bending modes in the combination bands have
higher H/D isotopic ratios than the corresponding fundamen-
tal bands in the bending potential for the stronger hydrogen
bond in the combination mode, which follows from the ef-
fective intermolecular quantum number dependence for the
intermolecular bending mode.
FIG. 4. Plot of HF intermolecular bending modes for N2–HF, (HF)2 , and
(HF)3 complexes in solid argon as a function of effective H–F intramolecu-
lar quantum number (H–F)ϩ1/2 . The ϭ3 point is for the gas phase
N2–HF complex, Ref. 1.
͓v
͔
v
ACKNOWLEDGMENTS
We gratefully acknowledge NSF support for this work
and many very helpful correspondences with W. Klemperer.
3084 cmϪ1 ͑average of split band͒, the difference 3084
Ϫ2717ϭ367 cmϪ1 is just above the 347 cmϪ1 value for this
mode measured in solid argon at 347 cmϪ1. The H/D ratios
for the fundamental bending mode, 446/347ϭ1.285, and for
this bending mode in combination, 482/367ϭ1.313, again
show the increase with intramolecular quantum number for
this bending mode in the combination band where the hydro-
gen bond is strengthened.
1 S. N. Tsang, C.-C. Chuang, R. Mollaaghababa, W. Klemperer, and H.-C.
Chang, J. Chem. Phys. 105, 4385 ͑1996͒.
2 L. Andrews, B. J. Kelsall, and R. T. Arlinghaus, J. Chem. Phys. 79, 2488
͑1983͒; L. Andrews and S. R. Davis, ibid. 83, 4983 ͑1985͒.
3 S. N. Tsang, H.-C. Chuang, and W. Klemperer, J. Phys. Chem. 98, 7313
͑1994͒.
4 D. E. Woon, T. H. Dunning, Jr., and K. A. Peterson, J. Chem. Phys. 104,
5883 ͑1996͒.
The weaker absorptions at 4143–4098 cmϪ1 are prob-
ably due to combination bands in higher clusters, most likely
(HF)4 , but we cannot be more definite.
5 K. W. Jucks, Z. S. Huang, and R. E. Miller, J. Chem. Phys. 86, 1098
͑1987͒.
6 G. M. Lovejoy and D. J. Nesbitt, J. Chem. Phys. 86, 3151 ͑1987͒.
7 J. T. Farrell, Jr., O. Sneh, and D. J. Nesbitt, J. Phys. Chem. 98, 6069
͑1994͒.
F. Bending mode correlations
8 R. J. Bemish, E. J. Bohac, M. Wu, and R. E. Miller, J. Chem. Phys. 101,
9457 ͑1994͒.
The HF ͑and DF͒ intermolecular bending modes in com-
bination bands observed here in solid argon are plotted with
their fundamental bands observed earlier in solid argon2,14,15
against effective intramolecular HF ͑and DF͒ quantum num-
ber for N2–HF, (HF)2 , and (HF)3 in Fig. 4. The mass de-
pendent relationship with quantum number is clearly seen
from the increased slopes in each H/D pair. The gas phase
9 E. J. Bohac and R. E. Miller, J. Chem. Phys. 99, 1537 ͑1993͒.
10 D. T. Anderson, S. Davis, and D. J. Nesbitt, J. Chem. Phys. 104, 6225
͑1996͒.
11 S. Davis, D. T. Anderson, J. T. Farrell, Jr., and D. J. Nesbitt, J. Chem.
Phys. 104, 8197 ͑1996͒.
12 D. T. Anderson, S. Davis, and D. J. Nesbitt, J. Chem. Phys. 105, 4488
͑1996͒.
13 S. Davis, D. T. Anderson, and D. J. Nesbitt, J. Chem. Phys. 105, 6645
͑1996͒.
value for N2–HF, 328.6 cmϪ1 at ϭ3 from the Klemperer
v
group,1 is only 3 cmϪ1 above our linear extrapolation ͑325.5
cmϪ1͒, which suggests that the matrix has only a small effect
on this intermolecular bending mode and that the depen-
dence on intramolecular quantum number is nearly linear.
We note that the square roots of the slopes of these plots
͑1.323, 1.363, 1.342, respectively, for N2–HF, (HF)2 ,
(HF)3) approach the square root of the HF/DF reduced mass
ratio ͑1.379͒ more closely than the equilibrium ͑intercept͒
ratios ͑1.212, 1.321, 1.270͒. This suggests that these modes
attain more pure HF bending character with increasing in-
tramolecular HF stretching quantum number in the combina-
tion bands.
14 L. Andrews and G. L. Johnson, J. Phys. Chem. 88, 425 ͑1988͒; L. An-
drews, ibid. 88, 2940 ͑1984͒.
15 L. Andrews, S. R. Davis, and R. D. Hunt, Mol. Phys. 77, 993 ͑1992͒.
16 R. N. Spanbauer, K. N. Rao, and L. H. Jones, J. Mol. Spectrosc. 16, 100
͑1965͒.
17 M. A. Suhm, J. T. Farrell, Jr., A. McIlroy, and D. J. Nesbitt, J. Chem.
Phys. 97, 5341 ͑1992͒.
18 A. S. Pine and W. J. Lafferty, J. Chem. Phys. 78, 2154 ͑1983͒.
19 P. Jensen, P. R. Banker, A. Karpfen, M. Kofranek, and H. Lischka, J.
Chem. Phys. 93, 6266 ͑1990͒.
20 A 2856 cmϪ1 value for
of (DF)2 in solid argon is deduced from
vacceptor
the 2873 cmϪ1 observation, Ref. 15, of this mode in solid neon.
21 K. P. Huber and G. Herzberg, Constants of Diatomic Molecules ͑Van
Nostrand, New York, 1979͒.
22 J. M. Hutson, J. Chem. Phys. 96, 6752 ͑1992͒.
23 K. von Puttkamer, M. Quack, and M. A. Suhm, Mol. Phys. 65, 1025
͑1988͒.
IV. CONCLUSIONS
24 M. Quack and M. A. Suhm, Theor. Chim. Acta 93, 61 ͑1996͒.
25 K. D. Kolenbrander, C. E. Dykstra, and J. M. Lisy, J. Chem. Phys. 88,
5995 ͑1988͒.
The ‘‘stiffening’’ of the hydrogen bond in N2–HF and
HF–HF with bonded-HF vibrational excitation observed in
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