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M.S. Lowenthal et al. / Spectrochimica Acta Part A 58 (2002) 73–78
Isocyanic acid, the most stable isomer of cyanic
IR spectra were recorded on a Perkin-Elmer
GX FTIR instrument with a resolution of 2.0
cm−1. Low temperature spectra were recorded by
depositing the sample on a KRS-5 substrate at-
tached to the cold finger of a helium closed cycle
cryo-cooler (Air Products Displex model 202).
Several depositions of 1 ml gas sample at 10 torr
resulted in clear films indicated by the production
of interference fringes. Details of the experimental
set-up are given in several previous publications
from our laboratory. Three films of different
thickness were employed in our investigations.
Spectra of a 1.6 mm thick film of solid HNCO,
deposited from the gas phase onto a substrate at
20 K, along with an overlay for the same sample
warmed to 145 K, are reproduced in Fig. 1.
acid, has been investigated by infrared spec-
troscopy as a gas and as a matrix-isolated species
[8–12]. The data on the gas phase vibrational
frequencies of HNCO is complete, but the IR
spectrum of the solid phase (which is fundamental
to the characterization of the interstellar species)
is not known except for a recent communication
[10] which identifies three of the six vibrational
modes of solid HNCO. In this report, we present
the infrared spectrum of a condensed thin film of
HNCO from 4000 to 400 cm−1 at low tempera-
ture (20–145 K). For absolute intensity determi-
nation, the real part of the refractive index and
the thickness of the film are determined from the
interference fringes overlaying the absorption
bands.
3. Discussion
2. Experimental
3.1. Band assignments
A number of methods reported in the published
literature were attempted to synthesize HNCO.
Reaction of sodium cyanate (NaOCN) with an
acid (concentrated as well as dilute) produced
relatively large amounts of CO2 in addition to the
desired product, HNCO. The vapor pressures of
CO2 and HNCO are similar, and therefore purifi-
cation of the compound by vacuum distillation
was unsuccessful. Production of HNCO by ther-
mal decomposition of isocyanuric acid (a trimer
of HNCO) also produced decomposition of the
sample and low product yield. The best yield of
the pure compound was obtained by reaction of
NaOCN with HCl gas. NaOCN (Aldrich Chemi-
cal Co., 96%) was contained in a flask connected
to a vacuum manifold that was then evacuated.
Hydrogen chloride gas (Aldrich Chemical Co.,
99%) was added to the manifold at a measured
pressure and was then frozen onto the NaOCN
solid by immersing the NaOCN-containing flask
into a liquid nitrogen bath. On repeated warming
and freezing of the sample, a reaction resulted
yielding almost pure HNCO with less than 0.1%
CO2. The sample was then transferred to a collec-
tion tube and kept at liquid nitrogen temperature
until it was deposited on a cold substrate for
spectroscopic evaluation.
Ab initio quantum mechanical calculations at
the MP2/6-31G** level for HNCO [8] predict a
planar structure where the HNC and NCO bond
angles are 124.7 and 187.4°, respectively. The six
vibrational modes of the molecule correspond to
three in plane stretches, two in plane bends and
one out of plane bend. Reported infrared frequen-
cies for the gas phase and matrix isolated species
[8] are given in Table 1. Along with the gas phase
frequencies, the vibrational frequency data we
measured for the thin film of solid HNCO, and
the results of ab initio calculations of the har-
monic frequencies of the molecule are reported. A
recent report on the infrared spectrum of interstel-
lar ices containing cyanic acid isomers [10] iden-
tifies three of the six modes for solid HNCO. In
terms of comparison with the theoretical calcula-
tions, it is to be stated that the calculated frequen-
cies correspond to harmonic force field derived
from the ab initio evaluated potential function. In
matrix isolated species, there may be solvent-like
induced frequency shifts, and in the solid phase
(our studies) there is an additional effect due to
hydrogen bonding which is not included in the
theoretical calculations. Thus, the theoretically
derived vibrational frequency data is useful pri-