Observation and rovibrational analysis of the ν2 band of HCN-H35Cl

Article abstract of DOI:10.1016/j.chemphys.2004.10.026

The high-resolution infrared absorption spectrum of an equilibrium mixture of HCN and HCl in a static gas long-path absorption cell is recorded in the 2500-2900 cm^-1 spectral region at 205 K. The spectrum shows rovibrational structure which has the typical appearance of a parallel band of a linear molecule and is assigned to the intramolecular H-Cl stretching vibration band ν₂ of the linear HCN-H³⁵Cl heterodimer. The rovibrational analysis of the band yield a band origin ν₀ of 2779.0968(12) cm^-1 together with a value for the upper-state rotational constant B′ of 0.067722(2) cm^-1. The observed red shift of 107 cm^-1 for the ν₂ band of HCN-H ³⁵Cl relative to the H-Cl stretching vibration band of monomer H ³⁵Cl is in excellent agreement with results from the MP2/6-311++G** level of theory. The value of the upper-state rotational constant shows that the intermolecular hydrogen bond shortens by 0.022 A? upon intramolecular vibrational excitation of the ν₂ mode.

Full text of DOI:10.1016/j.chemphys.2004.10.026

Chemical Physics 310 (2005) 163–167
www.elsevier.com/locate/chemphys
Observation and rovibrational analysis of the m2 band of HCN–H35Cl
a,
b
a
R. Wugt Larsen *, F. Hegelund , B. Nelander
a
Chemical Center, Department of Chemical Physics, Lund University, P.O. Box 124, S-221 00 Lund, Sweden
b
Department of Chemistry, Aarhus University, Langelandsgade 140, DK- 8000 Aarhus C, Denmark
Received 11 August 2004; accepted 20 October 2004
Available online 6 November 2004
Abstract
The high-resolution infrared absorption spectrum of an equilibrium mixture of HCN and HCl in a static gas long-path absorp-
tion cell is recorded in the 2500–2900 cmꢀ1 spectral region at 205 K. The spectrum shows rovibrational structure which has the typ-
ical appearance of a parallel band of a linear molecule and is assigned to the intramolecular H–Cl stretching vibration band m2 of the
linear HCN–H35Cl heterodimer. The rovibrational analysis of the band yield a band origin m0 of 2779.0968(12) cmꢀ1 together with a
value for the upper-state rotational constant B0 of 0.067722(2) cmꢀ1. The observed red shift of 107 cmꢀ1 for the m2 band of HCN–
H35Cl relative to the H–Cl stretching vibration band of monomer H35Cl is in excellent agreement with results from the MP2/
6ꢀ311++G** level of theory. The value of the upper-state rotational constant shows that the intermolecular hydrogen bond short-
˚
ens by 0.022 A upon intramolecular vibrational excitation of the m2 mode.
Ó 2004 Elsevier B.V. All rights reserved.
1. Introduction
variety of gas-phase spectra of intra- and intermolecular
fundamental bands, overtone bands, hot bands and
combination bands is required.
Simple gaseous molecular complexes held together by
hydrogen bonds and van der Waals interactions are pro-
totype systems for detailed studies of intermolecular
interactions. A great deal of information about such
weakly bound species may be obtained from the obser-
vation and rovibrational analysis of high-resolution
infrared absorption gas-phase spectra of the species of
interest. The spectra may help to characterize the forces
acting between the two molecular species. The spectra
also yield information about the modification of the
force constants of the subunits by their incorporation
into the larger entity. In order to elucidate the intermo-
lecular potential energy surface for a molecular complex
over a wide range of geometries away from the equilib-
rium structure the observation and analysis of a wide
In the present work we investigate the gaseous hydro-
gen-bonded heterodimer of HCN and HCl. Despite the
wealth of spectroscopic information available for the
closely related prototype dimer HCN–HF (see [1] and
references therein) and to a lesser extent for (HCN)2
(see [2] and references therein) the spectroscopic infor-
mation reported in the literature for the molecular com-
plex HCN–HCl is rather limited.
The first observation of the gaseous molecular com-
plex between HCN and HCl was reported by Legon
et al. [3] who used the technique of Fourier transform
microwave (MW) spectroscopy conducted in a Fabry–
Perot cavity with a pulsed nozzle molecular source. In
this study accurate ground-state spectroscopic constants
were determined for the parent molecular complex and
three additional isotopologues containing 15N, 37Cl
and D. Interpretation of the spectroscopic constants
led to the conclusion that the most stable configuration
of the molecular complex of HCN and HCl is collinear
*
Corresponding author. Tel.: +46 46 22 24 064; fax: +46 46 22 24
710.
E-mail addresses: Rene.Wugt_Larsen@chemphys.lu.se, wugt@
larsen.dk (R. Wugt Larsen).
0301-0104/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.chemphys.2004.10.026

164
R. Wugt Larsen et al. / Chemical Physics 310 (2005) 163–167
at equilibrium, thus establishing the presence of a hydro-
gen bond to the N atom of the HCN molecule. Later
Bender et al. [4] observed and assigned the intramolecu-
lar C–H stretching vibration band m1 for HCN–H35Cl
using a static long-path absorption cell at 199 K inter-
faced with a tunable single-frequency color-center laser
spectrometer. Bender et al. also managed to make tenta-
point of HCN at the pressure used for the experiment.
The sample pressures of HCN and natural isotopic
HCl in the absorption cell during measurements are 1
and 5 hPa, respectively.
The spectra are recorded using a Si on CaF2 beam
splitter, a tungsten (W) black body radiator and a liquid
N2 cooled (77 K) InSb semiconductor detector. An opti-
cal band pass filter (2350–3050 cmꢀ1) is mounted in
front of the detector to reduce the photonic noise level
in the spectra. The FTS instrument resolution (RES) is
defined as RES = 0.9/(MOPD), where MOPD is the
maximum optical path difference in the interferometer.
Sample interferograms are recorded with a resolution
of 0.0055 cmꢀ1 (MOPD equal to 163.6 cm). The re-
corded sample interferograms are transformed using
Mertz phase correction and boxcar apodization. The to-
tal scan time is 9 h. Background interferograms of the
evacuated cell are recorded at low spectral resolution,
transformed and interpolated onto a wave number grid
matching that of the sample spectrum using appropriate
zero-filling factors.
The absolute wavenumber scale of the spectra is that
determined from a calibration of the internal FTS He–
Ne laser. The accuracy of this calibration is checked fur-
ther by comparing line positions from H2O reported in
[7]. Lists of line positions from the absorbance spectra
are generated using the Microcal Origin 7.0 software
package (Microcal software, Inc.). The precision of the
line positions is estimated to be slighty better than the
spectral resolution.
tive assignments of the
three hot bands
m1 þ m17 ꢀ m17; m1 þ 2m72 ꢀ 2m27 and m1 þ 3m37 ꢀ m73 where m71
is the doubly degenerate low-frequency intermolecular
bending vibration (libration of HCN). The banꢀd1origin
for the m17 mode was estimated to be 41(3) cm from
the value of the l-type doubling constant estimated by
Bender et al. The same authors estimated the band ori-
gin of the intermolecular hydrogen bond stretching
vibration m4 of HCN–HCl the to be 100(5) cmꢀ1 based
on the modified pseudodiatomic (MPD) approximation
developed by Millen [5]. The assignment of the m1 band
was later slightly modified and extended by Block and
Miller [6]. The small red shift of 2.5 cmꢀ1 for the band
origin for m1 of HCN–H35Cl relative to the C–H stretch-
ing vibration band of monomer HCN indicates that the
hydrogen bond is rather unaffected by m1 excitation.
In the present study we report the observation of the
intramolecular H–Cl stretching vibration band m2 of
HCN–H35Cl by means of static gas-phase Fourier trans-
form infrared spectroscopy at 205 K. The rovibrational
analysis of the band yields values for the band origin
and the upper-state rotational constant for the m2 level
which gives quantitative information on the excited-
state vibrational structure of the molecular complex.
3. Analysis and results
2. Experimental
The 2760–2810 cmꢀ1 region of the recorded high-res-
olution infrared absorption spectrum of the mixture of
HCN and HCl has a rather complicated appearance.
Strong and broad absorption lines from H35Cl and
H37Cl appear in the 2773–2777 cmꢀ1 (the P(6) transi-
tions) and 2796–2800 cmꢀ1 regions (the P(5) transi-
tions). In addition, numerous P-, Q- and R-branch
lines belonging to the m2 + m3 combination band of
HCN around 2805 cmꢀ1 are present in this spectral re-
gion making the identification of the weak lines in the
m2 band of the HCN–H35Cl complex rather difficult.
These weak lines are further obscured by the presence
of the HCN–H37Cl isotopologue and several hot bands
associated with the low-frequency intermolecular vibra-
tional levels for HCN–H35Cl.
The m2 band is a parallel type linear molecule band
and no Q-branch transitions are allowed. A search for
the less pronounced P- and R-branch series of the band
is therefore necessary to obtain the band origin and
other spectroscopic constants. In the 2780–2786 cmꢀ1
region of the spectrum a regular series of ca. 30 rela-
tively strong lines spaced approximately 0.17 cmꢀ1 are
HCN is prepared by dropwise addition of diluted
H2SO4 onto NaCN in vacuo and condensation of the
gaseous product. The HCN sample is dried by vacuum
distillation through a column containing the drying
agent P2O5. Impurities of CO2 and (CN)2 are removed
from the HCN sample by fractional distillation.
The spectra are recorded at the infrared beam-line at
MAX-lab, Lund University. The set up consists of a
200-L static absorption cell made of stainless steel which
is interfaced to a Bruker IFS 120 HR Fourier transform
spectrometer (FTS). The absorption cell has a White
type multireflection mirror system. The base length of
the cell is 2.85 m and the total optical path length in
the present experiments is 91.2 m. The cell temperature
is measured in the middle and at both ends of the inner
cell with standard Pt100 resistance thermometers. A
computer emulated PID temperature controller controls
the current supplied to the three different resistive heat-
ers welded to the outside of the inner cell and maintains
the cell temperature to 205 0.25 K during the experi-
ment. This temperature is close to the condensation

R. Wugt Larsen et al. / Chemical Physics 310 (2005) 163–167
165
cm1 region is a part of the R-branch of the m2 band
for HCN–H35Cl. The band origin of m2 is estimated to
be close to 2779 cmꢀ1 from the intensity distribution
within this R-branch.
No clear P-branch series is observed below 2779 cmꢀ1
.
This is partly because of overlap by the strong P(6) lines
of H35Cl and H37Cl which obscure the spectrum below
2777 cmꢀ1. Thus, it is difficult to establish the J-assign-
ment of the observed R-branch series by comparing with
P-branch lines. However, the use of ground-state combi-
nation differences (GSCDs) allows the identification of a
number of P-branch lines and a probable J-assignment
of the observed P- and R-branch lines is established.
Since the values of the ground-state constants B00 and
D0J0 of HCN–H35Cl have already been determined by Le-
gon et al. [3] the GSCDs R(J ꢀ 1)–P(J + 1) are used to
search for P-branch lines and at the same time to estab-
lish the J-assignment for the R-branch. This procedure is
facilitated by the use of computer-assisted Loomis–
Wood plots with lower-state energies calculated from
the ground-state constants and polynomial fits of
upper-state energies derived from R-branch wavenum-
bers with various assumed J-assignments. In this way a
series of 11 P-branch lines for which the GSCDs are con-
sistent with the corresponding R-branch lines is found.
The final J-assignment of part of the R-branch is shown
in Fig. 1 and the total number of assigned lines are listed
in Table 1. Since the GSCDs are rather sensitive to a
change in the J-assignment (even by one unit) the estab-
lished J-assignment seems reasonable even though the
number of assigned lines is rather small.
Fig. 1. The high-resolution infrared transmittance spectrum of HCN–
H35Cl showing part of the R-branch assignments for the m2 band.
identified (see Fig. 1). The intensity distribution for this
series is obscured slightly by the presence of the overlap-
ping HCN–H37Cl transitions and hot band transitions.
The series spacing is ca. 0.14 cmꢀ1 at lower wavenum-
bers and increases to ca. 0.19 cmꢀ1 around 2785 cmꢀ1
.
A value of the ground-state rotational constant B00 of
0.066999175(7) cmꢀ1 for HCN–H35Cl was determined
from the MW study by Legon et al. [3]. It therefore
seems reasonable to assign the observed line series to
the m2 band of this molecular complex. The change of
the rotational constant is expected to be positive owing
to the expected increased electrostatic interaction in the
excited intramolecular vibrational state m2 = 1 as proved
by the red shift of the H–Cl stretching vibration in the
molecular complex. This means that the R-branch spac-
ing should increase with increasing values of J while the
P-branch should form a band head. We therefore con-
clude that the series of lines observed in the 2780–2786
The assigned P- and R-branch lines are fitted to the
conventional linear molecule rotational-vibrational en-
ergy expression. We consider the quality and limited
number of transitions not to warrant incorporation of
the upper-state quartic centrifugal distortion constant
Table 1
The assigned rovibrational transitions for the m2 band of HCN–H35Cl
Transition
Obs. line position (cmꢀ1
)
Obs. calc. (cmꢀ1
)
Transition
Obs. line position (cmꢀ1
)
Obs. calc. (cmꢀ1
)
P(15)
P(14)
P(13)
P(11)
P(10)
P(9)
P(8)
P(3)
P(1)
R(2)
R(3)
R(5)
R(6)
R(7)
R(10)
R(11)
R(15)
R(16)
2777.236
2777.349
2777.469
2777.702
2777.823
2777.947
2778.072
2778.707
2778.968
2779.514
2779.655
2779.932
2780.081
2780.225
2780.663
2780.816
2781.431
2781.591
ꢀ0.004
ꢀ0.005
0.001
R(17)
R(18)
R(19)
R(20)
R(21)
R(22)
R(23)
R(24)
R(25)
R(27)
R(28)
R(29)
R(30)
R(31)
R(32)
R(34)
R(37)
R(38)
2781.748
2781.909
2782.069
2782.234
2782.403
2782.572
2782.732
2782.901
2783.078
2783.419
2783.596
2783.777
2783.954
2784.131
2784.315
2784.681
2785.243
2785.432
ꢀ0.005
ꢀ0.005
ꢀ0.007
ꢀ0.006
ꢀ0.002
ꢀ0.001
ꢀ0.008
ꢀ0.008
ꢀ0.002
ꢀ0.006
ꢀ0.004
0.001
ꢀ0.001
0.001
0.004
0.006
0.008
0.005
0.007
0.008
0.001
0.006
0.000
0.005
ꢀ0.002
ꢀ0.002
ꢀ0.001
ꢀ0.005
ꢀ0.002
0.002
0.003
0.009
0.009

166
R. Wugt Larsen et al. / Chemical Physics 310 (2005) 163–167
Table 2
shift, however, agrees well with the theoretical value
of ꢀ111 cmꢀ1 predicted by Arau´jo et al. [10] using
the MP2/6ꢀ311++G** level of theory [11]. However,
the computed vibrational frequencies reported by
Arau´jo et al. were not scaled to take into account
anharmonicity effects, basis set truncation and method-
ology. A scaling factor of 0.96 as suggested by Scott
and Radom [12] gives a theoretical red shift of 107
cmꢀ1 in perfect agreement with our experimental find-
ings. The total electronic binding energy for the HCN–
HCl dimer, corrected for zero-point energy and basis
set superposition error, was predicted to be ꢀ1362
cmꢀ1 by Arau´jo et al. [10] using the MP2/
6ꢀ311++G** level of theory. The hydrogen bond
interaction thus appears to strengthen by nearly 8%
upon intramolecular vibrational excitation of the m2
mode.
The observed positive change of DB in the excited
state m2 = 1 relative to the ground state confirms the
stronger hydrogen bonding interaction upon excitation
of the H–Cl stretching vibration of HCN–H35Cl. In con-
trast to the case for m1 excitation a rather small value of
the vibration–rotation interaction constant a2 is ex-
pected for the m2 mode since the oscillating hydrogen
atom is close to the center of mass of the molecular com-
plex. The center of mass separation r between the sub-
units is given by Eq. (1)
Spectroscopic constants (in cmꢀ1 ) obtained from the m2 band of
HCN–H35Cla
m0
2779.0968(12)b
0.0007225(19)
36
DB
Nc
rd
0.0051
a
In the fit the ground-state constants and the upper-state centrif-
ugal distortion constant are fixed to the MW values [3]:
B00 ¼ 0:066999175 cmꢀ1 and DJ00 ¼ 1:136 ꢁ 10ꢀ7 cmꢀ1
.
b
The quoted uncertainties are one standard deviation.
Number of fitted observations.
Standard deviation of fit (cmꢀ1 ).
c
d
in the fit. In the fit the ground-state constants and the
upper-state value for the centrifugal distortion constant
are therefore fixed to the MW values [3] The fitted values
for the band origin and the upper-state rotational con-
stant are listed in Table 2. A total number of 36 lines
are included in the fit. A few obviously overlapped P-
branch lines and the highest R-branch lines (J > 39)
are excluded from the fit since these lines seem to be per-
turbed by levels at higher energy. It is not possible to
trace the perturbed R-branch with confidence beyond
J = 42 since no confirmation of the assignment by
GSCDs by P-branch lines is possible because of the
overlap by HCl lines in the P-branch. A band head in
the P-branch is predicted from the constants in Table
1 for J = 69,70 at 2773.75 cmꢀ1. However, this region
is also overlapped by strong HCl lines and therefore
cannot be verified. Neither absorption lines in the m2
band of HCN–H37Cl nor absorption lines for any hot
band series of the parent isotopologue are possible to as-
sign from the present spectra.
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
h
8p2cB ꢀ IHCN ꢀ IHCl
r ¼
;
ð1Þ
l
where B is the rotational constant of the molecular com-
plex, IHCN and IHCN are the moments of inertia of the
HCN and HCl subunits and l is the reduced mass of
the molecular complex. The center of mass separation
for the excited vibrational state m2 = 1 in the dimer is
estimated using the moment of inertia for the excited
vibrational state in HCl taken from [13] in order to dis-
tinguish between the elongation of the HCl bond and
the intermolecular hydrogen bond. The hydrogen bond
4. Discussion
The fact that the band origin of the m2 band of
HCN–HCl is shifted to the red with ca. 107 cmꢀ1 rel-
ative to the H–Cl stretching vibration band origin of
monomer HCl shows that the m2 mode of the molecular
complex is significantly influenced by the formation of
the intermolecular hydrogen bond. The negative value
for the shift indicates that HCN–HCl is more strongly
bound in the excited intramolecular vibrational state
v2 = 1. The absolute value of the frequency shift gives
directly a quantitative measure of the enhanced dissoci-
ation energy of the molecular complex upon vibra-
tional excitation of m2 [8]. The present result deviates
somewhat from the experimental value of ꢀ80(5)
cmꢀ1 reported by Thomas and Thompson [9] but the
quoted uncertainty of the latter result may be underes-
timated owing to extended band overlaps between
HCN–HCl, HCN and HCl (see above) and pressure
broadening effects. The present observed frequency
˚
length shortens by ca. 0.022 A relative to the ground
state upon m2 excitation.
The tendency to form a stronger hydrogen bond
upon intramolecular vibrational excitation of the
H–X mode of the hydrogen bond donor has been ob-
served in a variety of bimolecular complexes of HCl
[14–16]. The effect is mainly associated with the in-
crease of the electrostatic interaction between the two
molecular species upon intramolecular vibrational exci-
tation of the HCl subunit. The dominant electrostatic
interaction in the linear HCN–HCl dimer is obviously
the dipole–dipole interaction. In the m2 = 1 excited
intramolecular state of HCN–HCl the dipole moment
of the HCl subunit increases and thereby the dipole–di-
pole interaction is strengthened in the molecular com-
plex. It should be noted that the vibrational

R. Wugt Larsen et al. / Chemical Physics 310 (2005) 163–167
167
dependences of higher order multipole moments and
Acknowledgments
The running costs of the infrared beam-line at
the change in the dispersion forces upon vibrational
excitation contribute to the strengthening of the hydro-
gen bond interaction as well. These contributions are,
however, assumed to be less important.
˚
MAX-lab are supplied by the Swedish Vetenskapsradet.
R. Wugt Larsen acknowledges receipt of a three months
stipend from MAX-lab and a travel grant from Nordic
Network of Chemical Kinetics (NoNeCK).
5. Conclusions
References
The high-resolution infrared absorption spectrum of
the m2 band of the linear HCN–HCl dimer is recorded
by means of static gas-phase Fourier transform infrared
spectroscopy at 205 K. A number of 36 rovibrational
transitions have been identified for the m2 band and a
rovibrational analysis of the rotational structure yields
a band origin m0 of 2779.0968(12) cmꢀ1 together a value
of 0.0007225(19) cmꢀ1 for the vibration–rotation inter-
action constant a2 The observed red shift of 107 cmꢀ1
for the H–Cl stretching vibration band of HCN–H35Cl
relative to the H–Cl stretching vibration band of mono-
mer H35Cl together with the positive value of DB indi-
cate that the hydrogen bond becomes significantly
stronger upon intramolecular vibrational excitation of
the H–Cl stretching mode m2 The upper-state value of
the rotational constant shows that the intermolecular
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