Photolysis of isothiocyanic acid HNCS in low-temperature matrices. Infrared detection of HSCN and HSNC isomers

Article abstract of DOI:10.1016/S0009-2614(01)01180-0

A combination of matrix isolation technique and Fourier transform infrared spectroscopy has been used to study the low-temperature photochemistry of isothiocyanic acid HNCS. Near UV photolysis with a medium-pressure mercury lamp of the matrix isolated HNCS/DNCS precursor in nitrogen and argon led to the formation of two unobserved, so far, isomers: thiocyanic acid HSCN and isothiofulminic acid HSNC and their deuterated analogs. Both species are formed as primary products presumably via two reaction channels: formation of the [HNCS] and hydrogen migration leading to the HSCN molecule and formation of the [HNCS] followed by rearrangement to HSCN and HSNC species.

Full text of DOI:10.1016/S0009-2614(01)01180-0

30 November 2001
Chemical Physics Letters 349 G2001) 227±234
www.elsevier.com/locate/cplett
Photolysis of isothiocyanic acid HNCS in low-temperature
matrices. Infrared detection of HSCN and HSNC isomers
*
Maria Wierzejewska , Zo®a Mielke
Faculty of Chemistry, University of Wrocøaw, Joliot-Curie 14, 50-383 Wrocøaw, Poland
Received 13 August 2001; in ®nal form 2 October 2001
Abstract
A combination of matrix isolation technique and Fourier transform infrared spectroscopy has been used to study the
low-temperature photochemistry of isothiocyanic acid HNCS. Near UV photolysis with a medium-pressure mercury
lamp of the matrix isolated HNCS/DNCS precursor in nitrogen and argon led to the formation of two unobserved, so
far, isomers: thiocyanic acid HSCN and isothiofulminic acid HSNC and their deuterated analogs. Both species are
formed as primary products presumably via two reaction channels: formation of the [HÁ Á ÁNCS] and hydrogen mi-
gration leading to the HSCN molecule and formation of the [HNCÁ Á ÁS] followed by rearrangement to HSCN and
HSNC species. Ó 2001 Elsevier Science B.V. All rights reserved.
1. Introduction
that isothiocyanic acid HNCS is the only hitherto
spectroscopically described isomer. Several papers
on infrared spectra of gaseous [13,14], solid and
isolated in argon matrix [14] isothiocyanic acid
have been published. Microwave studies of various
isotopic species of HNCS have been performed and
the structure of the molecule was evaluated [15,16].
Flash photolysis of HNCS was also reported [17].
Results of a detailed photoionization mass spec-
trometric studies are published as well [18].
The isothiocyanic acid HNCS is a sulfur analog
of the well-known isocyanic acid HNCO. Since the
latter species plays an important role both in at-
mospheric and combustion processes it has at-
tracted a considerable attention. The performed
theoretical and experimental studies concern the
structure, stability and photolytic behavior of
HNCO and its structural isomers [1±11]. Much less
is known so far about isothiocyanic acid HNCS
and its isomers. The early theoretical work of Bak
et al. [12] has predicted a stability of four isomers to
increase in the following order: HCNS, HSNC,
HSCN and HNCS. This trend agrees with the fact
In this Letter we report the infrared matrix
isolation studies of the UV photolysis of isothio-
cyanic acid HNCS in solid argon and nitrogen.
2. Experimental
* Corresponding author. Fax: +49-308-3853444.
E-mail address: mw@wchuwr.chem.uni.wroc.pl GM. Wie-
rzejewska).
Gaseous HNCS was prepared in a vacuum
line by treating KNCS kept at 0 °C with 60%
0009-2614/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved.
PII: S0009-2614G01)01180-0

228
M. Wierzejewska, Z. Mielke / Chemical Physics Letters 349 -2001) 227±234
H3PO4 ꢀD3PO4†. It was collected and shortly
stored in 1 l bulb at low Gless than 10 mbar)
pressure. Gaseous mixtures of isothiocyanic acid
with matrix gases GAr or N2) were prepared by
standard manometric techniques. Their concen-
tration ranged between 1/600 and 1/3000. The gas
mixtures were condensed onto gold-plated copper
mirror kept at 15 and 18 K for deposition of
argon and nitrogen matrices, respectively. The
temperature was maintained by means of a
closed cycle helium refrigerator GAir Products,
Displex 202A). Infrared spectra were recorded at
11 K in a re¯ection mode with a resolution of
0:5 cmÀ1 by means of a Bruker 113v FTIR
spectrometer equipped with a liquid N2 cooled
MCT detector.
peak. Another absorption is observed at 960.9
cmÀ1 with a weaker band at 965.9 cmÀ1. The
2580.2 and 960.9 cmÀ1 bands have their deute-
rium counterparts at 1873.6 and 688.5 cmÀ1, re-
spectively, in the spectra recorded in the DNCS
experiments. The other new absorptions observed
at 2182.3 and 2065.9 cmÀ1 in the spectra with
HNCS precursor are only slightly a€ected by
deuteration; the bands are 0.5 cmÀ1 blue and 0.5
cmÀ1 red shifted, respectively, in the spectrum of
deuterated sample. The relative intensities of the
four bands at 2580.2, 2182.3, 2065.9 and 960.9
cmÀ1 remain constant during prolonged photoly-
sis time but the 2065.9 cmÀ1 band decreases
slightly with respect to the other three bands after
matrix annealing. This behavior indicates that the
bands at 2580.2, 2182.3 and 960.9 cmÀ1 belong to
one product species while the 2065.9 cmÀ1 band
to another photoproduct.
A corresponding set of bands was observed in
argon matrices. The most intense bands after UV
irradiation of HNCS/Ar matrices appeared at
2182.3 and 2064.2 cmÀ1 GFig. 2a). They are
slightly shifted on deuteration as shown in Fig.
2b, the 2182.3 cmÀ1 band moves by 0.7 cmÀ1 to
higher wavenumbers while the 2064.2 cmÀ1 ab-
sorption is red shifted by 0.7 cmÀ1. Two addi-
tional weaker absorptions were localized in the
spectra of the irradiated HNCS/Ar matrices at
2581.0 and 959.7 cm1. Their counterparts in the
DNCS/Ar system were found at 1874.0 and 692.8
cmÀ1, respectively.
Wavenumbers of the new absorptions men-
tioned above, observed in N2 and Ar matri-
ces, together with the assignment proposed
in the following discussion are gathered in Table
1.
In some experiments weak and relatively
broad absorptions appeared in the spectra after
photolysis at 3217.7 cmÀ1 for HNCS/Ar, at 3287,
3224.0, 2050.3, 2029.0 cmÀ1 for HNCS/N2 and at
1964.6 cmÀ1 for DNCS/N2. The intensities of
these bands grow with time during photolysis
however they are not correlated one with an-
other or with the absorptions described above.
Probably they are due to some unidenti®ed
complexes formed between species present in the
matrix.
UV photolysis was performed by light from a
medium-pressure
mercury
lamp
GPhilipps
CS200W-2) equipped with water ®lter and glass
longwavelength pass ®lters GSchott-WG 360, 345,
335, 305, 295 and 280 nm).
3. Results
The results of the UV photolysis of the
isothiocyanic acid isolated in argon and nitro-
gen matrices will be presented. First, matri-
ces were subjected to the full output of the
mercury lamp. Additional experiments were
performed using glass longwavelength pass ®l-
ters to evaluate the threshold energy for the
photolytic reaction. Details of the initial spec-
tra, themselves of interest as well, will be the
subject of a separate report [19]. The spectra of
HNCS in solid argon are in good agreement
with those previously published by Durig and
Wertz [14].
When matrices containing HNCS/DNCS sus-
pended in nitrogen or argon are subjected to the
full output of the mercury lamp several new
bands grow following photolysis. In nitrogen
matrices four new absorptions appear for both
HNCS and DNCS precursors as presented in Fig.
1. The most intense absorption in the HNCS
experiments occurs at 2580.2 cmÀ1 and is ac-
companied by three much weaker features situ-
ated a few wavenumbers apart from the main

M. Wierzejewska, Z. Mielke / Chemical Physics Letters 349 -2001) 227±234
229
Fig. 1. New bands observed in the spectra recorded after 350 min UV photolysis of the matrix ꢀHNCS ‡ DNCS†=N2 ˆ 1=1500.
4. Discussion
lated in nitrogen and argon matrices, respectively,
to one of the theoretically predicted but unob-
served so far isomers of the HNCS molecule,
namely, to thiocyanic acid HSCN. This assignment
may be justi®ed by the following arguments:
The 2580.2 and 2581.0 cmÀ1 bands appearing
after photolysis of the HNCS/N2 and HNCS/Ar
4.1. Identi®cation of new species
We assigned three bands at 2580.2, 2182.3,
960.9 cmÀ1 and at 2581.0, 2182.3, 959.7 cmÀ1 ap-
pearing in the spectra of photolyzed HNCS iso-

230
M. Wierzejewska, Z. Mielke / Chemical Physics Letters 349 -2001) 227±234
the thionitrous acid isolated in argon matrices and
observed the SH stretching mode at 2613.0 and
2566.0 cmÀ1 for trans and cis HSNO isomers, re-
spectively.
The 960.9 and 959.7 cmÀ1 absorptions growing
up during irradiation of the HNCS/N2 and HNCS/
Ar matrices are in the region anticipated for the
HSC bending mode. These bands show a large H/D
isotopic ratio of 1.38 Gthe mGDSC) occurs at 688.5
and 692.8 cmÀ1 in N2 and Ar, respectively),
therefore H Gor D) motion must be dominant.
Bohn et al. [20] have identi®ed the HSC bending
vibration in argon isolated HSCS species at 941.4
cmÀ1 Gat 713.8 cmÀ1 for DSCS, respectively), and a
corresponding mode for CH3SCH2SH in solid Ar
was reported by Li and Li [22] at 928 cmÀ1. We
assign the 960.9, 959.7 cmÀ1 and 688.5, 692.8 cmÀ1
features to the mGHSC) and mGDSC) bending modes
in the thiocyanic acid HSCN/DSCN, respectively,
in solid N2 and Ar.
Table 2 gathers the frequencies of the mGSH)
stretching vibrations of various molecules isolated
in argon matrices. The corresponding bending
modes are given as well, whenever identi®ed. The
positions of the absorptions attributed to the
HSCN in the studied matrices ®t very well to this
list.
Fig. 2. mGCN) and mGNC) regions of the spectra recorded: Ga)
after 290 min of the UV photolysis of the matrices,
HNCS=Ar ˆ 1=2000; Gb) after 260 min of the UV photolysis of
the matrices, ꢀHNCS ‡ DNCS†=Ar ˆ 1=2000.
matrices are situated in the range suitable for an
SH stretching mode. Their counterparts in the
DNCS/N2 and DNCS/Ar systems, occurring re-
spectively at, 1873.6 and 1874.0 cmÀ1, show H/D
isotopic ratio equal to 1.38 as expected for this
type of mode. These features are assigned to the
mGSH) and mGSD) modes of HSCN and DSCN
acids. The above assignment ®nds support in the
literature. Bohn et al. [20] have studied the co-
condensation reaction products of H atoms with
CS2 in solid argon. They identi®ed the HSCS
radical as the major product. Isotopic data used
for this system allowed to localize the SH
stretching mode at 2527.5 cmÀ1 and its SD coun-
terpart was found at 1805.3 cmÀ1. In turn Moller
et al. [21] performed an infrared investigation on
The 2182.3 cmÀ1 absorption is assigned to the
mGCN) stretching mode in thiocyanic acid HSCN.
This band is situated in the region characteristic
for CN group [27±29]. For instance, Fraenkel and
Haas [27] reported the CN stretching mode for
ICN molecule in argon at 2171 cmÀ1. The deute-
rium shift for the 2182.3 band is equal to 0.7 cmÀ1
and demonstrates that the species of interest con-
Table 1
New bands observed after photolysis of the HNCS/DNCS/N2 and HNCS/DNCS/Ar matrices GcmÀ1
)
HNCS/N2
DNCS/N2
HNCS/Ar
DNCS/Ar
Assignment
2580.9 sh
2580.2 G5.9)
2577.9
1874.3 sh
1873.6
1870.4
1869.4
2183.8
2065.4
688.5
S-H, S-D stretch in HSCN, DSCN
2581.0 G0.4)
1874.0
2574.6
2182.3 G1.0)
2065.9
965.9
2182.3 G1.0)
2064.2
2183.0
2063.5
692.8
CN stretch in HSCN, DSCN
NC stretch in HSNC, DSNC
HSC, DSC bend in HSCN, DSCN
960.9 G1.1)
687.8
685.5
959.7 G0.15)

M. Wierzejewska, Z. Mielke / Chemical Physics Letters 349 -2001) 227±234
231
Table 2
The S-H/S-D stretching and HSC/DSC or HSN bending frequencies of di€erent SH containing molecules isolated in argon matrices
Molecule
S-H/mS-D stretch GcmÀ1
)
HSC/DSC bend GcmÀ1
)
Reference
HSCS/DSCS
H2BSH
2527.5/1805.3
2574
2575
941.4/713.8
[20]
[23]
H±C B C±SH
HSCN/DSCN
HSCH2CH2SH
C2H5SH
[24]
2581.0/1875.0
2586
2597 Gtrans), 2600 Ggauche)
959.5/692.0
This work
[22]
[25]
869.8
928
CH3SCH2SH
CH3SH
2603
[22]
[25]
2603.2
2613.0
2566.0
799.8
877.5/724.0
858.5/715.0
trans-HSNO
cis-HSNO
[21,26]
[21,26]
tains hydrogen atom that is directly bonded nei-
ther to N nor to C atom. For example, in HNCS,
HCN molecules, in which hydrogen atom is di-
rectly bonded to CN group, the deuterium shift of
the CN stretch frequency is equal to 42.2 and 164
cmÀ1 [30], respectively.
sorptions appearing at ca. 2580 and 960 cmÀ1 to
the thiocyanic acid HSCN Gand not due to the
isothiofulminic acid HSNC) on the basis of the
following arguments:
1. Fraenkel and Haas [27] have pointed out that
the transition dipole moment of the NC stretch
in INC is much larger than in ICN molecule. If
analogy holds for the HSCN, HSNC molecules,
then the concentration of HSCN in the studied
matrices is much larger than that of HSNC in
spite of comparable intensities of the mGCN)
and mGNC) absorptions.
2. The above conclusion is in agreement with re-
cent ab initio calculations performed for
HNCS and its isomers [31]. The results predict
40 times smaller intensity for the CN stretching
mode in HSCN than for the NC stretching
mode in HSNC while intensities calculated
for other modes are of the same order of mag-
nitude.
Weaker bands observed in addition to main
peaks in the SH stretching and H±SC bending
regions in nitrogen matrices Gsee Table 1) are as-
signed to the multiple trapping site e€ect and not
to the corresponding modes in HSNC molecule.
It is worth mentioning that there are apparent
variations in the relative intensities of the CN and
SH stretching and HSC bending modes of the
HSCN isomer depending on whether argon or
nitrogen is used as matrix material. This e€ect is
also shown in Table 1 where the relative intensities
for these modes are given in parenthesis. The ab-
sorptions due to the SH stretching and HSC
bending modes in nitrogen spectra are much
The remaining band showing up after photol-
ysis at 2065.9 cmÀ1 Gin nitrogen spectra) and at
2064.2 cmÀ1 Gin argon spectra) is attributed to the
NC stretching mode in the third undetected so far
isomer of isothiocyanic acid: isothiofulminic acid
HSNC. A small deuterium shift observed for this
absorption G0.5 cm1 in N2 and 0.7 cmÀ1 in Ar) is in
agreement with the proposed assignment. The
mGNC) mode was reported in the literature to ap-
pear in the 2030±2120 cmÀ1 region [27,28]. Studies
of various XCN±XNC pairs [27±29] revealed that
the frequency di€erence between mGCN) and mGNC)
stretching modes ranged between 110 and 200
cmÀ1. In the present study the di€erence between
mGCN) in HSCN and mGNC) in HSNC is equal to
116.4/118.1 cmÀ1 in solid nitrogen/argon, respec-
tively.
The mGNC) absorption is the only one identi-
®ed for the isothiofulminic acid HSNC in the
present study. The attempts to localize other
modes of this isomer were unsuccessful which is
probably due to the small yield of this species in
the performed experiments and to small ab-
sorption coecient of the other bands. It is
worth noting that the SH stretching and HSC
GHSN) bending modes are expected to be situ-
ated in the same regions for both HSCN and
HSNC isomers. However, we assigned the ab-

232
M. Wierzejewska, Z. Mielke / Chemical Physics Letters 349 -2001) 227±234
stronger than their counterparts in argon. This
may be due to the speci®c hydrogen bond inter-
action between HSCN and nitrogen cage.
4.2. HNCS photolysis and isomerization
The ¯ash photolysis studies in the near UV [17]
demonstrate that there are two dissociation chan-
nels for HNCS in the gas phase:
HNCS ! NCS ‡ H
ꢀ1†
ꢀ2†
3
HNCS ! HNC ‡ Sꢀ P†
accompanied by a fast secondary process:
Fig. 3. A plot of the normalized integrated intensities of the
1981.8 cmÀ1 GmGCN) of HNCS) ꢀÆ, 2182.3 cmÀ1 GmGCN) of
HSCN) Gꢁ) and 2064.2 cmÀ1 GmGNC) of HSNC) Gꢂ) bands
versus the photolysis time for a single experiment in argon
matrix.
3
À
3
Sꢀ P† ‡ HNCS ! S2ꢀX R † ‡ HNC
Upon irradiating HNCS with vacuum ultravi-
olet light [32] the third channel of dissociation is
available according to the reactions:
The mechanism of HSCN and HSNC forma-
tion is of considerable interest. Taking into ac-
count two primary processes which occur in the
gas phase Greactions 1 and 2) one may consider
two di€erent ways of the HNCS isomerization on
near UV irradiation in matrices.
The ®rst path may proceed via either forma-
tion of [HÁ Á ÁNCS] pairs as observed in the gas
phase or via a cyclic transition state [35]. In both
cases the attachment of H atom to the sulfur
atom of the NCS would lead to formation of
HSCN and the H attachment to the nitrogen
atom would reproduce HNCS. The formation of
the isothiofulminic acid HSNC would require a
rearrangement of the NCS species to CNS. The
®nal products of the latter process are expected to
be both HSNC and HCNS which are formed
when hydrogen atom is attached to sulfur or
carbon atoms, respectively.
The energy di€erence between HNCS/HSCN
isomers was calculated to be ca. 21 kJ/mol [12].
However, the transition state for the 1,3 hydride
transfer in the HNCS/HSCN system was found to
be over 250 kJ/mol higher in energy than HSCN
[35]. Even if the latter value is overestimated it is
much too high to make the HNCS/HSCN iso-
merization energetically possible with the 300 nm
radiation used. The second possibility ± the for-
mation of the [HÁ Á ÁNCS] pairs seems to be more
HNCS ! NHꢀX3RÀ† ‡ CSꢀA1P†
ꢀ3†
1
CSꢀA P† ! CSꢀX1R †
‡
It is known that photolysis of molecules in
matrices di€ers from that in the gas phase. One of
the factors in¯uencing the photolysis in matrices is
cage e€ect which is responsible for trapping the
produced photofragments and preventing them
from being separated [9,33,34].
Fig. 3 presents a plot of the absorbance of the
1981.8 cmÀ1 band of HNCS, the 2182.3 cmÀ1 band
of HSCN and the 2064.2 cmÀ1 band of HSNC as a
function of the photolysis time during a single
experiment in argon matrix. The intensities have
been normalized to their own maximum absor-
bances within the experiment. Our results show
that both HSCN and HSNC are the primary
products of HNCS photodissociation with near
UV light irradiation. The back reaction must not
be of importance in the studied matrices since
HNCS has not been reproduced during photolysis
Gopposite to that what was observed by Jacox and
Milligan [1] for HNCO and HOCN species). Ac-
cording to Fig. 3 after prolonged photolysis there
was a slight decrease of concentration of HSCN
and HSNC photoproducts. However, no increase
of the HNCS precursor was noticed neither sec-
ondary products were identi®ed.

M. Wierzejewska, Z. Mielke / Chemical Physics Letters 349 -2001) 227±234
233
3. Analysis of the band positions and their isoto-
pic H/D shifts allowed us to conclude that near
UV photolysis led to the formation of two un-
observed so far isomers: thiocyanic acid HSCN
and isothiofulminic acid HSNC and their deu-
terated analogs.
4. The two isomers are probably formed via two
reaction channels:
· formation of the [HÁ Á ÁNCS] and hydrogen
migration leading to the HSCN molecule;
· formation of the [HNCÁ Á ÁS] followed by rear-
rangement to HSCN and HSNC species.
probable. Mechanism via pre-dissociation into
[HÁ Á ÁCNO] pairs was suggested by Bondybey et al.
[3] for fulminic acid isomerization.
As stated above the formation of HSNC via
rearrangement of NCS into CNS should be ac-
companied by HCNS formation. The latter isomer
has not been identi®ed in the studied matrices
though it is predicted to have comparable stability
to HSNC isomer and large intensities of the in-
frared absorptions [31]. The absence of HCNS
isomer rather excludes the formation of the HSNC
species via rearrangement of CNS.
The second dissociation channel that is ob-
served in the gas phase, HNCS ! HNC ‡ SG3P),
may be responsible for the formation of both
HSCN and HSNC isomers in the matrices. The
reaction may proceed via [HNCÁ Á ÁS] pairs fol-
lowed by hydrogen migration and formation of
[NCÁ Á ÁSH] pairs which recombine to form HSCN
or HSNC isomers.
Acknowledgements
Prof. Konstantin Tokhadze is thanked for his
help during the photolysis set-up arrangement.
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2. Several new absorptions grew in the regions of
SH, SD, CN stretching and HSC, DSC bending
modes.

234
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