Observation and rovibrational analysis of the intermolecular NH3 libration band v91 of H3N-HCN

Article abstract of DOI:10.1021/jp044740w

The high-resolution far-infrared absorption spectrum of the gaseous molecular complex H₃N-HCN is recorded by means of static gas-phase Fourier transform far-infrared spectroscopy at 247 K, using a synchrotron radiation source. The spectrum contains distinct rotational structures which are assigned to the intermolecular NH₃ libration band v₉ ¹ (v_B) of the pyramidal H₃N-HCN complex. A rovibrational analysis based on a standard semirigid symmetric top molecule model yields the band origin of 260.03(10) cm^-1, together with values for the upper state rotational constant B′ and the upper state quartic centrifugal distortion constants D′ _J and D′ _JK. The values for the upper state spectroscopic constants indicate that the hydrogen bond in the H₃N-HCN complex is destabilized by 5% and elongates by 0.010 A upon excitation of a quantum of libration of the hydrogen bond acceptor molecule.

Full text of DOI:10.1021/jp044740w

J. Phys. Chem. A 2005, 109, 4459-4463
4459
Observation and Rovibrational Analysis of the Intermolecular NH3 Libration Band ν1 of
9
H3N-HCN
R. Wugt Larsen,*,† F. Hegelund,‡ and B. Nelander†
Chemical Center, Department of Chemical Physics, Lund UniVersity, P.O. Box 124, S-221 00 Lund, Sweden,
and Department of Chemistry, Aarhus UniVersity, Langelandsgade 140, DK-8000 Aarhus C, Denmark
ReceiVed: NoVember 18, 2004; In Final Form: March 8, 2005
The high-resolution far-infrared absorption spectrum of the gaseous molecular complex H3N-HCN is recorded
by means of static gas-phase Fourier transform far-infrared spectroscopy at 247 K, using a synchrotron radiation
source. The spectrum contains distinct rotational structures which are assigned to the intermolecular NH3
libration band ν1 (νB) of the pyramidal H3N-HCN complex. A rovibrational analysis based on a standard
9
semirigid symmetric top molecule model yields the band origin of 260.03(10) cm-1, together with values for
the upper state rotational constant B′ and the upper state quartic centrifugal distortion constants D′J and D′JK.
The values for the upper state spectroscopic constants indicate that the hydrogen bond in the H3N-HCN
complex is destabilized by 5% and elongates by 0.010 Å upon excitation of a quantum of libration of the
hydrogen bond acceptor molecule.
1. Introduction
energy surface for the H3N-HCN dimer is required in order to
achieve a detailed picture of the energetics and structural
High-resolution spectroscopic studies of weakly bound mo-
lecular complexes provide important information about the
intermolecular interactions involved in the transition from
isolated gaseous molecules to the formation of condensed
phases. The first high-resolution rotationally resolved infrared
studies of simple gaseous molecular complexes using static gas-
phase FTIR spectroscopy and conventional radiation sources
were published in the mid-1980s. In particular, a wide range of
vibrations of the linear prototype HCN-HF heterodimer was
observed and assigned by Bevan et al. [see ref 1 and references
therein]. Subsequently, the investigation of such species were
extended by slit supersonic jet techniques and laser radiation
sources leading to improved experimental sensitivity and
complementary rovibrational assignments owing to the much
lower rotational temperatures of supersonic jets. However, only
a limited number of studies have dealt with the important
intermolecular part of the vibrational spectrum for such mo-
lecular complexes. The reason appears to be that there exist
few suitable spectroscopic radiation sources in the far-infrared
spectral region where the floppy intermolecular vibrations of
molecular complexes are normally observed. In the present study
we use the highly brilliant source of synchrotron radiation
offered from the electron storage ring MAX-I at MAX-lab in
Lund for the study of the intermolecular NH3 libration band of
the weakly bound H3N-HCN dimer by means of high-
resolution static gas Fourier transform far-infrared spectroscopy
changes associated with this phase transition. The direct
observation and rovibrational analysis of intermolecular vibra-
tion bands of the H3N-HCN dimer is important for the
elucidation of the intermolecular potential energy surface away
from the equilibrium configuration.
The first observation of the dimer between NH3 and HCN in
the gas phase was reported by Jones et al.4 The low-resolution
IR absorption spectrum of a mixture of NH3 and HCN showed
a parallel band, which was assigned to the C≡N stretching
vibration for the hydrogen-bonded H3N-HCN complex. A
contour analysis of the band showed that the molecular complex
is a symmetric top. The first MW spectrum of the H3N-HCN
complex was measured by Fraser et al.6 using molecular beam
electric-resonance spectroscopy. This study confirmed the C3v
symmetry with the HCN subunit as the hydrogen bond donor
and yielded values for the spectroscopic ground-state constants.
The hydrogen bond length was estimated to 2.156(10) Å and
the harmonic value of the hydrogen bond stretching vibration
(νσ) to 141(3) cm-1 from the values of the ground-state
constants. The high-resolution infrared absorption spectrum of
the ν4 band (the NH3 umbrella vibration) of H3N-HCN was
obtained later by Fraser et al.7 by means of a microwave-
sideband CO2 laser and optothermal electric-resonance (EROS)
detection. The observed band origin of 1041.7 cm-1 for this
mode is blue shifted 91.8 cm-1 relative to the band origin of
the hypothetical inversion-free umbrella vibration of the NH3
monomer indicating a significant destabilization of the hydrogen
bond in the molecular complex upon vibrational excitation of
the ν4 mode. Smith et al.8 and Hilpert et al.9 have both reported
the high-resolution infrared absorption spectrum of the ν2 (νS)
band (the C-H stretching vibration) of H3N-HCN by means
of Fourier transform static gas-phase and molecular-beam EROS
spectroscopies, respectively. The band origin of the ν2 band is
red shifted by 200.8 cm-1 relative to the band origin for the
C-H stretching vibration of the HCN monomer indicating a
The ionic solid NH4CN consisting of NH4 and CN-
+
subunits, which dissociates in the gas phase into a pair of
covalent bound acid (HCN) and base (NH3) molecules, is a
prototype system and spectroscopic information is available for
the ionic solid,2,3 the hydrogen-bonded gaseous heterodimer
H3N-HCN,4-9 and the covalent subunits HCN10-12 and NH3.13
An accurate characterization of the intermolecular potential
* Address correspondence to this author.
† Lund University.
‡ Aarhus University.
10.1021/jp044740w CCC: $30.25 © 2005 American Chemical Society
Published on Web 05/03/2005

4460 J. Phys. Chem. A, Vol. 109, No. 20, 2005
Wugt Larsen et al.
significant strengthening of the hydrogen bond upon vibrational
excitation of the ν2 mode.
Bohn and Andrews5 observed a vibration band at 279 cm-1
in the far-infrared absorption spectrum of H3N-HCN in a solid
argon matrix, which was assigned to the intermolecular HCN
libration band. The results from high-level quantum chemical
calculations have later established that the observed band should
be assigned to the intermolecular NH3 libration mode. In the
present study we report the direct observation and rovibrational
analysis of the intermolecular NH3 libration band ν1 (νB) of the
9
H3N-HCN dimer.
2. Experimental Section
The present experiments were carried out at the infrared
beam-line at MAX-Lab at Lund University with use of a
temperature controlled 200-L static gas absorption cell interfaced
with a Bruker IFS 120 HR Fourier transform spectrometer (FTS)
described in refs 14-16. The absorption cell has a White type
multipass mirror system. The base length of the absorption cell
is 2.85 m and the optics provides a total optical path length of
ca. 91.2 m. The absorption cell is equipped with two sets of
3.2 mm thick CsI windows. 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 regulates the current supplied to the three
different resistive heaters welded to the outside of the inner cell
and maintained the cell temperature of 247 ( 0.25 K during
these experiments. This temperature is close to the condensation
point of HCN at the pressure used for the experiments.
HCN is prepared by dropwise addition of diluted H2SO4 onto
KCN in vacuo and condensation of the gas evolved. The
hydrogen cyanide samples are dried by vacuum distillation
through a column containing the P2O5 drying agent. Impurities
of CO2 and (CN)2 are then removed from the samples by
fractional distillation. The partial pressures of HCN and NH3
in the absorption cell are 2 and 5 Torr, respectively
The radiation source is synchrotron radiation from the electron
storage ring MAX-I at MAX-lab. MAX-I is a 550 MeV electron
storage ring with a 250 mA maximum ring current and a mean
lifetime of 4 h. The transfer optics are described elsewhere.17
The CsI exit window from the electron storage ring is mounted
at Brewster’s angle to the horizontally polarized radiation. The
plane of polarization is then converted to vertical, since far-
infrared beam splitters are more effective with this polarization.
The electron storage ring is a high-brightness source of
broadband infrared radiation, covering the full far-infrared
spectral region. Synchrotron radiation is very close to a point
source and is very suitable for high-resolution infrared absorp-
tion spectroscopy. The radiation output from the electron storage
ring relative to the radiation output from a conventional globar
source is characterized in refs 17 and 18. The interferometer is
equipped with a 6 µm multilayer beam splitter that operates
well over the entire spectral range of 50-600 cm-1. The detector
is a liquid He cooled Si-bolometer operating at 1.7 K (Infrared
Laboratories, Inc.). The detector element is small, and allows
us to use high scanning speeds making the bolometer less
sensitive to motions of the electron beam. A cold band-pass
optical filter (0-370 cm-1) is mounted in the bolometer to
reduce the photonic noise level in the final 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 0.005 cm-1 (MOPD ) 180 cm). The sample
interferograms are transformed by using Mertz phase correction
Figure 1. The J-assignment of the assumed RQ0-branch in the ν19
band of H3N-HCN.
and boxcar apodization. A total scan collection time of ca. 12
h is achieved. Background interferograms of the evacuated
absorption cell are recorded with a resolution of 0.08 cm-1
.
These are transformed and interpolated onto a wavenumber grid
matching that of the sample spectra by using a zero-filling factor
of 16. This background resolution is appropriate for the
cancellation of the dominating interference fringes. The resulting
signal-to-noise ratio in the final absorbance spectra is about 30:1
for the most intense observed dimer transitions.
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 by comparing line
positions from H2O in the spectra with literature values reported
by Johns.19 The water lines appear in our spectra due to residual
water vapor in the evacuated interferometer tank. The accuracy
of these water line positions is estimated to be 0.0002 cm-1
.
Lists of line positions from the absorbance spectra are generated
with the Microcal Origin 7.0 software package (Microcal
Software, Inc.). The precision of the line positions reported in
the present study is estimated to be equal to the spectral
resolution, that is, 0.0050 cm-1. The observed FWHM line width
of ca. 0.01 cm-1 is dominated mainly by instrumental broaden-
ing and pressure broadening by the NH3 and HCN subunits.
3. Results and Analysis
The high-resolution far-infrared absorption spectrum of the
NH3/HCN mixture is very congested by the pure rotational
transitions of NH3 and the spectrum is fully saturated for 3-4
cm-1 wide spectral regions in ca. 20 cm-1 intervals. Several
distinct rotational structures are, however, observed in the
spectrum which do not appear in the spectra for the pure
monomeric species obtained with identical sample pressure,
sample temperature, and optical path length. The rotational
structure consists of at least two isolated Q-branches with origins
at 240.2 and 261.0 cm-1, the latter having more intensity than
the first. The Q-branches both degrade widely at lower wave-
numbers, and their subband origins are rather easy to locate.
We have not been able to observe accompanying P- and
R-branch transitions for reasons discussed above. This means
that we were not able to perform an unambiguous J-assignment
of the Q-branch lines by means of ground-state combination
differences based on the ground-state constants from ref 8.
However, from the shape of the Q-branches and the observed
subband origins we are able to propose reasonable J-assign-
ments. In Figure 1 the proposed J-assignment of the RQ0-branch
is shown. The appearances of the Q-branches are very similar

Intermolecular NH3 Libration Band of H3N-HCN
J. Phys. Chem. A, Vol. 109, No. 20, 2005 4461
TABLE 1: The Assigned Line Positions for the RQ0-Branch
and suggest that these belong to the same perpendicular band
of a symmetric top molecule. The Q-branches are therefore
for the ν1 Band of H3N-HCN
9
assigned to the intermolecular NH3 libration band ν1(E) (νB) of
obsd line
obsd - calcd/
9
H3N-HCN.
transition
position/cm-1
cm-1
RQ0(62)
RQ0(61)
RQ0(59)
RQ0(58)
RQ0(57)
RQ0(55)
RQ0(54)
RQ0(53)
RQ0(52)
RQ0(50)
RQ0(49)
RQ0(48)
RQ0(46)
RQ0(44)
RQ0(43)
RQ0(42)
RQ0(41)
RQ0(40)
RQ0(38)
RQ0(37)
RQ0(36)
RQ0(35)
RQ0(34)
RQ0(33)
RQ0(32)
RQ0(31)
RQ0(30)
RQ0(29)
RQ0(28)
RQ0(27)
RQ0(26)
RQ0(25)
RQ0(24)
RQ0(23)
RQ0(22)
RQ0(21)
RQ0(20)
RQ0(19)
RQ0(18)
RQ0(17)
RQ0(16)
RQ0(15)
RQ0(14)
RQ0(13)
RQ0(10)
RQ0(9)
258.7822
258.8596
259.0031
259.0749
259.1466
259.2786
259.3446
259.4049
259.4680
259.5857
259.6460
259.7005
259.8095
259.9100
259.9616
260.0075
260.0535
260.0994
260.1855
260.2285
260.2687
260.3089
260.3462
260.3806
260.4179
260.4524
260.4839
260.5155
260.5471
260.5758
260.6044
260.6331
260.6590
260.6848
260.7078
260.7307
260.7537
260.7738
260.7939
260.8111
260.8283
260.8455
260.8599
260.8742
260.9115
260.9201
260.9316
-0.0004
0.0014
-0.0015
-0.0006
0.0019
0.0000
0.0013
-0.0017
-0.0002
-0.0017
0.0012
-0.0003
0.0009
-0.0010
0.0014
-0.0005
-0.0012
-0.0006
-0.0012
0.0004
0.0003
0.0015
0.0010
-0.0012
0.0007
0.0010
-0.0005
-0.0008
0.0000
-0.0009
-0.0006
0.0008
0.0004
0.0011
0.0001
0.0002
0.0013
0.0007
0.0012
The relative intensities strongly suggest that the 240.2 and
P
261.0 cm-1 structures constitute the expected strong Q3- and
RQ0-branches of the band, respectively, since subbands with K
) 3n have three times more intensity than other subbands owing
to the different nucleir spin statistical weights. We have
unsuccessfully searched the spectrum for the corresponding RQ3-
branch around 281 cm-1. This Q-branch may be obscured by
strong NH3 absorption lines. A total number of 77 observed
rovibrational transitions for the two Q-branches are fitted to a
standard semirigid symmetric top frequency expression20 with
a standard deviation of 0.00108 cm-1. In this fit the spectro-
scopic ground-state constants B′′, D′′J, and D′′JK are fixed to
the values given by Fraser et al.7 The values for the rotational
constant A and the quartic centrifugal distortion constant DK
cannot be determined from the present analysis. The assigned
rovibrational transitions for the RQ0- and PQ3-branches are listed
in Tables 1 and 2.
The determined values for the upper state spectroscopic
constants are listed in Table 3. The value obtained for the upper
state constant D′JK seems rather large. Both upper state constants
DJ′K and B′ are effective constants since these may include
contributions from the l-type doubling constant q9. This is
because the upper states V9 ) 1, l9 ) (1, K ) (1 of the RQ0-
transitions are influenced by l-type doubling. Since we only
R
P
observe two Q-branches, Q0 and Q3, we have no possibility
to separate the l-type contribution from D′ and B′ which may
JK
be seen from the conventional symmetric top frequency expres-
sions for these Q-branches:
RQ0(J) ) ν90 + (A - B - 2Aς9) +
(∆B + q9/2 - D′ )J(J + 1) - ∆DJJ2(J + 1)2 (1)
JK
-0.0002
-0.0005
0.0003
-0.0008
-0.0008
-0.0003
-0.0019
-0.0005
PQ3(J) ) ν09 + (A - B - 2Aς9) - 6(A - B - Aς9) +
(∆B - 4D′ + 9D′′ )J(J + 1) - ∆DJJ2(J + 1)2 (2)
JK
JK
In eqs 1 and 2 contributions from ∆A, ∆B, and DK are neglected
in the subband terms. Since we are forced to use a model
neglecting l-type doubling to obtain the values of the upper state
constants, the observed value of -4.643(6) × 10-4 cm-1 for
∆B includes contribution from this effect. In the following we
assume that this is of minor importance. The values of the
Coriolis coupling constant ú9 and the NH3 libration band origin
ν09 are estimated from the PQ3 and RQ0 subband origins at
240.1670(7) and 260.9674(4) cm-1, respectively. A value of
0.415(5) for ú9 and a band origin of 260.03(10) cm-1 for the ν91
mode are estimated by using the values of the ground-state
constants A′′ and B′′ reported in ref 8.
The far-infrared absorption spectrum of the NH3/H13CN
mixture contains one distinct Q-branch at 259.5 cm-1, which
is assigned to RQ0 for the ν91 band of H3N-H13CN. The
observed RQ0-transitions are not sufficient for a semirigid
symmetric top model fit. The observations are, however, fitted
to a conventional diatomic molecule model to make a tentative
J-assignment leading to an estimated subband origin of 259.52
cm-1. The band origin of the ν91 mode of H3N-H13CN is red
shifted by 1.44(14) cm-1 relative to H3N-H12CN assuming the
values of A, B, and ú9 are identical for the two isotopomers.
RQ0(8)
4. Discussion
The observed negative value of ∆B ) -4.643(6) × 10-4
cm-1 together with the simultaneous positive value of ∆DJ )
1.344(13) × 10-8 cm-1 indicate that the hydrogen bond in the
H3N-HCN dimer is destabilized upon excitation of the inter-
molecular NH3 libration mode. This observation agrees well
with the electrostatic viewpoint that the dominant contribution
to the hydrogen bond is the interaction between the electric
dipole moments of NH3 and HCN. The dipole-dipole interac-
tion is strongest in the pyramidal equilibrium configuration when
the electric dipole moments are aligned parallel to each other.
The observed destabilization of the hydrogen bond can be
explained by the angular dependence of the dipole-dipole
interaction. The electric dipole moment of NH3 is tilted away
from the intermolecular symmetry axis in the molecular complex
as the NH3 subunit carries out the librational motion and the
dipole-dipole interaction becomes more unfavorable owing to
the enhanced nonlinearity of the hydrogen bond. In the harmonic
oscillator approximation based on the NH3 libration band origin

4462 J. Phys. Chem. A, Vol. 109, No. 20, 2005
Wugt Larsen et al.
P
TABLE 2: The Assigned Line Positions for the Q3-Branch
TABLE 4: The Center of Mass Distance r, the Harmonic
Frequency ωσ, and Harmonic Force Constant kσ for the
Hydrogen Bond Stretching Vibration in the Ground State
and the Excited NH3 Libration State
for the ν1 Band of H3N-HCN
9
obsd line
obsd - calcd/
transition
position/cm-1
cm-1
r′′/Å
r′/Å
3.844
3.853
143
136
0.125
0.115
PQ3(47)
PQ3(46)
PQ3(45)
PQ3(43)
PQ3(42)
PQ3(41)
PQ3(39)
PQ3(37)
PQ3(36)
PQ3(35)
PQ3(34)
PQ3(33)
PQ3(32)
PQ3(31)
PQ3(29)
PQ3(28)
PQ3(27)
PQ3(26)
PQ3(25)
PQ3(23)
PQ3(22)
PQ3(21)
PQ3(20)
PQ3(18)
PQ3(17)
PQ3(16)
PQ3(15)
PQ3(14)
238.9494
239.0039
239.0555
239.1531
239.2019
239.2507
239.3396
239.4229
239.4630
239.5032
239.5405
239.5778
239.6151
239.6496
239.7127
239.7443
239.7730
239.8017
239.8304
239.8820
239.9050
239.9279
239.9509
239.9911
240.0112
240.0284
240.0427
240.0571
0.0002
0.0013
0.0009
-0.0017
-0.0010
0.0010
ω′′σ /cm-1
ωσ′ /cm-1
k′′σ /mdynÅ-1
k′σ/mdynÅ-1
0.0003
the symmetric top dimer and the subunits NH3 and HCN,
respectively, and µD is the reduced mass of H3N-HCN. The
values of the harmonic force constant for the stretching vibration
of the hydrogen bond in the vibrational ground state and the
excited NH3 libration state (V9 ) 1) are listed in Table 4 together
with values for the corresponding harmonic stretching frequen-
cies ωσ for the hydrogen bond stretching vibration. The value
of the force constant decreases by 5% upon intermolecular
vibrational excitation of the ν19 mode and thereby indicates that
the destabilization of the hydrogen bond is about 5% within
the harmonic oscillator approximation. The intermolecular
stretching vibration of the hydrogen bond in H3N-HCN is
highly anharmonic and the estimated values of the force
constants are therefore approximate numbers.
-0.0010
-0.0013
-0.0003
-0.0009
-0.0003
0.0014
0.0015
-0.0006
0.0002
-0.0009
-0.0007
0.0006
0.0007
-0.0004
-0.0004
0.0006
0.0003
0.0017
0.0014
-0.0008
-0.0019
The elongation of the hydrogen bond can be estimated
according to the expression for the intermolecular center of mass
distance r in eq 4:
h/8π2cBD - INH - IHCN
3
TABLE 3: Spectroscopic Constants for the Intermolecular
r )
(4)
NH3 Libration Band ν1 of the H3N-HCN Heterodimer
µD
x
9
ν0/cm-1
B′′/cm-1
B′/cm-1
260.03(10)a
0.10062809b
0.1001638(6)a
1.8446b
1.9814(13)a
1.3896b
4.250(13)c
77
where BD is the rotational constant of the dimer, INH and IHCN
3
are the moments of inertia of the NH3 and HCN subunits,
respectively, and µD is the reduced mass. The values of the
center of mass distance for the ground and excited NH3 libration
state are listed in Table 4. The hydrogen bond thus appears to
elongate by 0.010 Å upon intermolecular vibrational excitation
of the ν19 mode.
107D′′/cm-1
J
107D′/cm-1
J
105D′′ /cm-1
JK
105D′ /cm-1
JK
Nd
σe/cm-1
0.00108
The destabilization of the hydrogen bond in the symmetric
top complex H3N-HCN upon intermolecular vibrational excita-
tion of the high-wavenumber intermolecular bending vibration
νB seems to be less pronounced than that observed for the linear
complexes OC-HCl,14 HCN-HCl,16 and HCN-HF22 where
the destabilization of the hydrogen bond proves to be at least
20% and even 30% for OC-HCl. The librational motion of
the acceptor subunit NH3 affects the strength of the hydrogen
bond in the H3N-HCN dimer less than the librational motion
of the donor subunits HCl and HF affects the strengths of the
hydrogen bond in the OC-HCl, HCN-HCl, and HCN-HF
dimers. The reason seems to be that the NH3 librational motion
mostly involves displacements of the H atoms whereas the
displacement of the N atom is rather small. The lone pair on
the N atom therefore changes direction in the course of the
libration but the interaction between the acceptor atom N and
the H atom in the donor molecule does not change much. The
librational motion of the donor subunit HCl or HF, however,
involves a rather large displacement of the H atom in the donor
molecule and the interaction between the acceptor atom (the C
atom in OC-HCl and the N atom in HCN-HCl and HCN-
HF) and the displaced H atom weakens significantly.
a Uncertainties quoted are one standard error. b Reference 8. c In-
fluenced by l-type doubling (see text). d Number of observations in fit.
e Standard deviation of fit.
and the rotational constant for NH3 the bending amplitude θNH
3
increases from 15.8° in the ground state to 22.4° upon excitation
of the NH3 libration mode.
The effect of destabilization can be quantified approximately
in terms of the harmonic force constant kσ for the stretching
vibration of the intermolecular hydrogen bond νσ since the
harmonic force constant for this mode is directly correlated to
the dissociation energy of the molecular complex. Millen21 has
developed an expression for the harmonic force constant for
the intermolecular stretching vibration of the hydrogen bond in
symmetric top dimers based on a modified pseudodiatomic
approximation. The harmonic force constant is related to the
rotational constant BD and the quartic centrifugal distortion
constant DJ for the symmetric top dimer according to eq 3
assuming that the dominating contribution to the centrifugal
distortion of the center of mass distance comes from the
hydrogen bond stretching vibration:
16π2µDBD
BD
BD
kσ )
1 -
-
(3)
5. Conclusions
(
)
DJ
B
BHCN
NH3
The high-resolution far-infrared absorption spectrum for the
BD, BNH , and BHCN are the values of the rotational constant of
intermolecular NH3 libration band ν1 (νB) of the pyramidal
3
9

Intermolecular NH3 Libration Band of H3N-HCN
J. Phys. Chem. A, Vol. 109, No. 20, 2005 4463
(5) Bohn, R. B.; Andrews, L. J. Phys. Chem. 1989, 93, 3974.
(6) Fraser, G. T.; Leopold, K. R.; Nelson, D. D., Jr.; Tung, A.;
Klemperer, W. J. Chem. Phys. 1984, 80, 3073.
(7) Fraser, G. T.; Pine, A. S.; Kreiner, W. A.; Suenram, R. D. Chem.
Phys. 1991, 156, 523.
(8) Smith, I. W. M.; Yarwood, G. Mol. Phys. 1988, 64, 627.
(9) Hilpert, G.; Fraser, G. T.; Pine, A. S. J. Chem. Phys. 1996, 105,
6183.
molecular complex H3N-HCN is recorded by means of static
gas-phase Fourier transform far-infrared spectroscopy at 247
K and a synchrotron radiation source. A rovibrational analysis
based on a standard semirigid symmetric top molecule model
yields the band origin of 260.03(10) cm-1, together with values
for the upper state rotational constant B′ and the upper state
quartic centrifugal distortion constants D′ and D′ . The values
J
JK
(10) Winnewisser, G.; Maki, A. G.; Johnson, D. R. J. Mol. Spectrosc.
for the upper state spectroscopic constants indicate that the
hydrogen bond in the H3N-HCN complex is destabilized by
5% and is elongated by 0.010 Å upon excitation of a quantum
of libration of the hydrogen bond acceptor molecule.
1971, 39, 149.
(11) Hyde, G. E.; Hornig, D. F. J. Chem. Phys. 1952, 20, 647.
(12) Bendsen, J.; Edwards, H. G. M. J. Raman Spectrosc. 1974, 2, 407.
(13) Sˇpirko, V.; Kraemer, W. P. J. Mol. Spectrosc. 1989, 133, 331.
(14) Wugt Larsen, R.; Hegelund, F.; Nelander, B. J. Phys. Chem. A
2004, 108, 1524.
(15) Wugt Larsen, R.; Hegelund, F.; Nelander, B. Mol. Phys. 2004, 102,
1743.
(16) Wugt Larsen, R.; Hegelund, F.; Nelander, B. Phys. Chem. Chem.
Phys. 2005, 7, 1953.
(17) Nelander, B. Vib. Spectrosc. 1995, 9, 29.
(18) Johnson, M. S.; Nelander, B. NuoVo Cimento 1998, 20, 449.
(19) Johns, J. W. C. J. Opt. Soc. Am. B 1985, 2, 1340.
(20) Duxbury, D. Infrared Vibration-Rotation Spectroscopy; Wiley:
Chichester, UK, 2000.
(21) Millen, D. J. Can. J. Chem. 1985, 63, 1477.
(22) Wofford, B. A.; Jackson, M. W.; Bevan, J. W. J. Chem. Phys. 1986,
84, 6115.
Acknowledgment. The running costs of the infrared beam-
line at MAX-lab are supplied by Vetenskapsrådet.
References and Notes
(1) McIntosh, A.; Gallegos, A. M.; Lucchese, R. R.; Bevan, J. W. J.
Chem. Phys. 1997, 107, 8327.
(2) Harvey, K. B.; McQuaker, N. R. J. Chem. Phys. 1971, 55, 4396.
(3) Kanesaka, I.; Kawahara, H.; Kiyokawa, Y.; Tsukamoto, M.; Kawai,
K. J. Raman Spectrosc. 1984, 15, 327.
(4) Jones, W. J.; Seel, R. M.; Sheppard, N. Spectrochim. Acta A 1969,
25, 385.