Photodecomposition of N-hydroxyurea in argon matrices. FTIR and theoretical studies

Article abstract of DOI:10.1039/c2ra22694d

The photochemistry of N-hydroxyurea in solid argon has been investigated by FTIR and ab initio calculations. The irradiation of the NH₂CONHOH/Ar matrices with the full output of the Xe arc lamp leads to the formation of the HNCO-NH₂OH and N₂-H₂O-CO complexes. For the isocyanic acid-hydroxylamine complex, the spectra prove the existence of the hydrogen bonded structure with the NH group of HNCO attached to the oxygen atom of the NH₂OH molecule. Two structures were identified for the nitrogen-water-carbon monoxide complex. In the first one, water is hydrogen bonded to the carbon atom and interacts with the nitrogen atom through van der Waals forces. In the second structure, water serves as a proton donor toward the nitrogen and carbon atoms of N₂ and CO molecules, respectively. The identification of the products is confirmed by deuterium substitution and by MP2 calculations of the structure and vibrational spectra of the identified complexes.

Full text of DOI:10.1039/c2ra22694d

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PAPER
Photodecomposition of N-hydroxyurea in argon
matrices. FTIR and theoretical studies3
Cite this: RSC Advances, 2013, 3,
1922
Magdalena Sałdyka
The photochemistry of N-hydroxyurea in solid argon has been investigated by FTIR and ab initio
calculations. The irradiation of the NH2CONHOH/Ar matrices with the full output of the Xe arc lamp leads
to the formation of the HNCO–NH2OH and N2–H2O–CO complexes. For the isocyanic acid–hydroxylamine
complex, the spectra prove the existence of the hydrogen bonded structure with the NH group of HNCO
attached to the oxygen atom of the NH2OH molecule. Two structures were identified for the nitrogen–
water–carbon monoxide complex. In the first one, water is hydrogen bonded to the carbon atom and
interacts with the nitrogen atom through van der Waals forces. In the second structure, water serves as a
proton donor toward the nitrogen and carbon atoms of N2 and CO molecules, respectively. The
identification of the products is confirmed by deuterium substitution and by MP2 calculations of the
structure and vibrational spectra of the identified complexes.
Received 30th October 2012,
Accepted 21st November 2012
DOI: 10.1039/c2ra22694d
www.rsc.org/advances
the N–O bond. In 1960 Walling and Naglieri14 reported the
1. Introduction
photolytic
scission
of
the
N–O
bond
in
N-hydroxyurea (HU) was first synthesized in the 1860s.1 After it
was found to be active against a variety of tumors, HU was
shown to inhibit DNA synthesis2 by inhibiting ribonucleotide
reductase.3,4 As a result, HU has become standard therapy for
chronic myelogenous leukemia, polycythemia vera, and
myeloproliferative disorders.5 HU is considered an effective
systemic agent for the control of severe psoriasis6 and a
complement to radiation treatment in patients with carcinoma
of the cervix and in head and neck cancer.7 Recently, it was
reported that HU could stimulate fetal hemoglobin synthesis
and had been successfully used to treat sickle cell anemia.8 HU
is also an immunomodulatory agent that has been documen-
ted to enhance the antiretroviral activity of nucleoside reverse
transcriptase inhibitors in HIV.9,10 HU acts as a source of nitric
oxide, which plays an important role in the maintenance of
normal blood pressure and flow.11 Growing evidence indicates
the in vivo conversion of HU to nitric oxide in sickle cell
disease patients undergoing N-hydroxyurea therapy.12 Both
chemiluminescence detection experiments and EPR spectro-
scopy show the formation of NO and NO-related species from
N-hydroxyurea in humans.13
N,O-diacylbenzoylhydroxylamine. The formation of anilides
and amides after photoirradiation of N-substituted naphtha-
lenehydroxamic acids15 and N-phenylbenzenehydroxamic
acids,16,17 as well as 2-(arylthio)-benzenehydroxamic acids18,19
and 2-(aryloxy)benzenehydroxamic acids,20 is also consistent
with the initial homolysis of the N–O bond. Benzyl
N-methylbenzohydroxamate was suggested to undergo photo-
elimination in the singlet state through
a mechanism
involving N–O bond cleavage.21 In addition to the photolytic
N–O bond scission, other mechanisms of the photolysis of
hydroxamic acids have also been reported. The photoirradia-
tion of selected naphthalenehydroxamic acids22 suggests that
photoirradiation of these compounds leads to acylaminoxyl
radical generation and not to the scission of the N–O bond. It
has been suggested that acylaminoxyl radicals are primary
intermediates in the photochemical reaction of N-substituted
hydroxamic acids containing the hydroxyl groups.15,17
I have recently reported the isomerical and structural
properties of N-hydroxyurea.23 The importance of this mole-
cule for medical and pharmaceutical applications triggers
questions about the influence of UV-Vis irradiation on the
structure and photochemical properties of HU, especially in
view of the results of the recent biological studies showing the
N-hydroxyurea molecule as an important nitric oxide donor.11
In the recent paper of our research group,24 we showed that
the UV-Vis photodecomposition of the simplest representative
of hydroxamic acids, namely formohydroxamic acid
(HCONHOH), in low temperature matrices leads to the
HNCO + H2O and NH2OH + CO products, which suggests
two photodissociation channels proceeding through the
Since N-hydroxyurea is a representative of hydroxamic acids
(RCONHOH), presentation of some issues concerning photo-
chemistry of this class of compounds seems essential for this
study. As can be found in literature, photoirradiation of
hydroxamic acids may lead to an initial photolytic scission of
Faculty of Chemistry, University of Wrocław, F. Joliot-Curie 14, 50-383 Wrocław,
Poland. E-mail: magdalena.saldyka@chem.uni.wroc.pl
Electronic Supplementary Information (ESI) available. See DOI: 10.1039/
3
c2ra22694d
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min to 30 K (c). Fig. 2(d) and Fig. 4 present the spectra of the
ND2CONDOD/Ar matrix after 180 min irradiation.
scission of the N–O or C–N bonds. If the HU molecule exhibits
similar mechanisms of photodissociation to the formohy-
droxamic acid molecule, then the photodissociation of HU
should produce the HNCO–NH2OH and NH2NHOH–CO
complexes as photoproducts. In this paper, for the first time,
the photochemical behaviour of N-hydroxyurea in an argon
matrix is presented.
The spectra of N-hydroxyurea and its deuterated isotopomer
in solid argon have been previously reported.23 As discussed in
that paper, in the spectra recorded directly after matrix
deposition, strong bands due to 1Ea-HU and weak bands
due to 1Za-HU isomers were identified. Matrix irradiation
leads to a simultaneous decrease of the bands due to the HU
isomers and appearance of new product bands. All new bands
that appear in the spectra of photolysed matrices are assigned
to HU photodissociation products and are collected in Table 1.
New vibrational features are observed in the typical regions of
water absorptions (3700–3600 cm21 and 1650–1580 cm21),
isocyanic acid (HNCO) (2280–2200 cm21), carbon monoxide
(CO) (2160–2130 cm21) and hydroxylamine (NH2OH) (3630–
3600 cm21 and 1150–1100 cm21). The performed experiments
allowed us to distinguish three main groups of bands, labelled
as 1, 2a and 2b, which respond in different ways to the
photolysis of HU/Ar matrices and to annealing processes.
Matrix annealing leads to an increase of the bands of group 2a
and a disappearance of the bands due to group 2b, whereas
the bands assigned to group 1 decreased in intensity. The
absorptions attributed to group 1 appear at 3631.8, 3630.6,
3287.9, 3285.2, ca. 2264.0, 1613.4, 1148.3 and 898.0 cm21 in
the spectra of irradiated HU/Ar matrices. The bands due to
groups 2a and 2b are observed at 3728.4, 3722.0, 3638.0,
3624.8, 2148.9, 2148.5, 1598.7, 1595.0 cm21. As will be
discussed below, the bands due to group 1 can be assigned
with confidence to the HNCO–NH2OH complex and the bands
of group 2a and 2b to the N2–H2O–CO complexes.
In addition to the above mentioned band sets some medium
and weak intensity absorptions also appeared in the HU/Ar
matrix spectra after photolysis (see Table 1 and Fig. 1–4); their
assignment can be made on the basis of the literature data.
The bands at 3295.2, 574.5 and 2146.3, 1600.8, 1121.6 cm21
are due to the HNCO–H2O and NH2OH–CO complexes,
respectively.24 The absorption appearing at 2138.4 cm21 is
due to the CO monomer29 and the weak band at 2142.6 cm21
is attributed to the CO complex with NH radical.30 The very
weak absorptions at 2885.3, 2816.5, 1610.2, 1499.0 and 1173.8
cm21 are assigned to the H2CO–H2O complex and the 2796.0
cm21 band is due to the H2CO monomer, in agreement with
the literature data.31 In the HU-d4/Ar matrix spectra after
photolysis the very weak bands at 2693.4, 2689.5, 2688.2 and
1171.8 cm21 are attributed to the hydroxylamine-d3 mono-
mer.32 Three very weak absorptions at 2635.0, 2284.5 and
1077.6 cm21 are assigned to the DOCN molecule.33 The
2747.5, 2623.2, 2084.7, 1690.9, 1190.0 cm21 bands are due to
the D2CO–D2O complex and the 2071.3 cm21 band is assigned
to the D2CO monomer.31
2. Experimental
Infrared matrix isolation study
The studies were performed for NH2CONHOH/Ar and
ND2CONDOD/Ar samples. The solid sample of HU was placed
in a small electric oven assembled inside the cryostat. The
temperature of the oven was controlled by a DC regulated
power supply (NDN instruments). Matrices were prepared by
co-deposition of HU vapour coming out of the oven with a
large excess of argon onto the cold CsI window. The low
temperature was maintained by means of a closed cycle
helium refrigerator (ARS-2HW). FTIR spectra were recorded
between 4000–500 cm21 in transmission mode by means of a
Bruker 66 FTIR spectrometer with a resolution of 0.5 cm21 and
using a liquid N2 cooled MCT detector.
After the infrared spectra of the initially deposited matrices
were recorded, the samples were exposed to the full output
irradiation of a Xe arc lamp (Osram, XBO 450 W) for up to 300
min. A 5 cm water filter served to reduce the amount of
infrared radiation reaching the matrix.
A few experiments were performed for H2O/CO/Ar samples
prepared by the standard manometric technique; the concen-
tration of the gas mixture varied in the range from 1 : 1 : 1000
to 1 : 1 : 2000.
Computational details
All calculations were performed using the Gaussian09 code.25
Since most of the considered complexes are weakly bonded,
the structures were optimized in two steps at the MP2
computational level with the 6-311++G(2d,2p) and aug-cc-
pVTZ basis sets. First, standard geometry optimization of each
complex was carried out. Afterwards, all the structures found
were reoptimized with the counterpoise correction (CP).26,27
Both normal and CP-corrected harmonic wavenumbers and
intensities were obtained for the optimized complexes. The
anharmonic wavenumbers for the species identified in the
performed experiments were also calculated on the MP2/aug-
cc-pVTZ level. A potential energy distribution (PED) of the
normal modes was computed in terms of symmetry coordi-
nates with the GAR2PED program.28
Formation of the HNCO–NH2OH complex
The bands due to set 1 appeared in regions of the most intense
absorptions of isocyanic acid vibrations and in the vicinity of
the hydroxylamine monomer bands.
3. Results and discussion
Photolysis of HU/Ar and HU-d4/Ar matrices
Fig. 1–3(a–c) show the selected spectral regions of
NH2CONHOH isolated in solid argon, recorded directly after
matrix deposition (a), after matrix irradiation with the Xe arc
lamp for 180 min (b) and after annealing of matrix b for 10
The IR spectra of HNCO in low temperature matrices have
been reported.33 The HNCO monomer isolated in an argon
matrix is characterized by a very strong band of the NCO
antisymmetric stretching vibration at 2259.0 cm21 and weaker
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Fig. 1 The selected regions in the spectra of matrices: NH2CONHOH/Ar after matrix deposition at 15 K, spectrum measured at 9.5 K (a), after matrix irradiation with
the full output of Xe arc lamp for 180 min (b), and after annealing of matrix b for 10 min in 30 K, spectrum measured at 9.5 K (c).
bands due to the NH stretching and HNC and NCO bending at
3516.8, 3505.7 cm21 and at 769.8 and 573.7 cm21, respectively.
After deuterium substitution of the HNCO molecule, the NCO
antisymmetric stretching, the ND stretching and the DNC and
NCO bending vibrations appear at 2231.0, 2606.9 and at 578.6,
475.4 cm21, respectively.33 In the studied spectra the most
characteristic band within 1 band set is a broad absorption at
ca. 2264 cm21, in the region of nasNCO. After deuterium
substitution a narrow band at 2222.8 cm21 appears in this
region and is assigned to the perturbed NCO antisymmetric
stretching vibration of DNCO. The broad band at ca. 2234
cm21 is also observed in the vicinity of the nasNCO vibration of
Fig. 2 The 2295–2110 cm21 region in the spectra of the matrices (a, b, c) presented in Fig. 1. The spectrum of the ND2CONDOD/Ar matrix after matrix irradiation with
the full output of the Xe arc lamp for 180 min (d) is also presented.
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Fig. 3 The selected regions in the spectra of matrices (a, b, c) presented in Fig. 1. The spectrum of the H2O/CO/Ar = 1 : 1 : 2000 matrix is also presented (d).
the HNCO monomer. The annealing experiments show the
sensitivity of this band to the matrix temperature: the 2234
cm21 band becomes more intense while the 2264 cm21 one
decreases upon the matrix annealing to 30 K for several
minutes. This behaviour allows for the tentative assignment of
the 2234 cm21 band to the higher order HNCO–(NH2OH)n
complexes, it is most probably due to the HNCO–(NH2OH)2
complex. A similar pattern of absorptions in the region of the
nasNCO vibration was reported for the HNCO–NH3 system.34
The broad band at ca. 2269 cm21 was assigned to the HNCO–
NH3 complex, while the broad absorption at 2245 cm21 was
attributed to the HNCO–(NH3)2 complex.
The bands observed at 3287.9 and 3285.2 cm21 are ca. 225
cm21 shifted toward lower wavenumbers with respect to the
Fig. 4 The 2780–2435 cm21 region in the spectra of matrices: ND2CONDOD/Ar after matrix deposition at 15 K, spectrum measured at 9.5 K (a), after matrix
irradiation with the full output of the Xe arc lamp for 180 min (b), and after annealing of matrix b for 10 min in 30 K, spectrum measured at 9.5 K (c).
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Table 1 Observed absorptions (in cm21) after photolysis of N-hydroxyurea in solid argon
NH2CONHOH
ND2CONDOD
Species
Assignment
3728.4 (0.021)a
3722.0 (0.018)
3638.0 (0.015)
2765.5 (0.008)
2761.5 (0.004)
N2–H2O–CO, N2–D2O–CO (2b)
N2–H2O–CO, N2–D2O–CO (2a)
N2–H2O–CO (2b)
D2CO– D2O
ND2OD
ND2OD
ND2OD
HNCO–NH2OH, DNCO–ND2OD (1)
HNCO–NH2OH, DNCO–ND2OD (1)
N2–H2O–CO (2a)
nasH2O, nasD2O
nasH2O, nasD2O
nsH2O
nasD2O
nOD
nOD
nOD
nOH, nOD
nOH, nOD
nsH2O
2747.5 (0.004)
2693.4 (0.001)
2689.5 (0.002)
2688.2 (0.008)
2681.0 (0.001)
2679.0 (0.004)
3631.8 (0.006)
3630.6 (0.003)
3624.8 (0.004)
2642.0 (0.001)
2635.0 (0.002)
2623.2 (0.001)
2481.7 (0.014)
2473.8 (0.045)
2471.0 (0.057)
D2CO–HDO
DOCN
D2CO–D2O
HNCO–H2O, DNCO–D2O
HNCO–NH2OH, DNCO–ND2OD (1)
HNCO–NH2OH, DNCO–ND2OD (1)
H2CO–H2O
H2CO–H2O, D2CO–D2O
H2CO, D2CO
nsHDO
nOD
nsD2O
3295.2 (0.024)
3287.9 (0.062)
3285.2 (0.06)
2885.3 (0.001)
2816.5 (0.003)
2796.0 (0.002)
nNH, nND
nNH, nND
nNH, nND
nasCH2
nsCH2 , nsCD2
nsCH2, nsCD2
nCN
nasNCO
nasNCO
nasNCO
nCO
2084.7 (0.003)
2071.3
2284.5 (0.002)
2235 br (0.011)
2222.8 (0.13)
DOCN
DNCO–D2O
2264 br (0.65)
2234 br (0.3)
HNCO–NH2OH, DNCO–ND2OD (1)
(HNCO–NH2OH)n
CO + ?
N2–H2O–CO, N2–D2O–CO (2a)
N2–H2O–CO, N2–D2O–CO (2b)
NH2OH–CO, ND2OD–CO
CO–NH, CO–ND
2165.0 (0.002)
2149.8 (0.05)
2148.9 (0.03)
2146.8 (0.022)
2143.7
2148.9 (0.048)
2148.5 (0.026)
2146.3 (0.012)
2142.6
nCO
nCO
nCO
nCO
2138.4 br (0.16)
2138.4 br (0.11)
1690.9 (0.035)
1184.2
1190.0 (0.008)
1174.0
1179.3 (0.01)
1178.0 (0.01)
1171.8 (0.003)
CO
D2CO–H2O
nCO
nCLO
1613.4 (0.001)
1610.2 (0.001)
1600.8 (0.004)
1598.7 (0.012)
1595.0 (0.006)
HNCO–NH2OH, DNCO–ND2OD (1)
H2CO–H2O, D2CO–D2O
NH2OH–CO, ND2OD–CO
N2–H2O–CO, N2–D2O–CO (2b)
N2–H2O–CO, N2–D2O–CO (2a)
ND2OD
dNH2, dND2
dH2O, dD2O
dNH2, dND2
dH2O, dD2O
dH2O, dD2O
dND2
1499.0 (0.004)
1173.8 (0.001)
H2CO–H2O
H2CO–H2O
DOCN
HNCO–NH2OH, DNCO–ND2OD (1)
NH2OH–CO, ND2OD–CO
HNCO–NH2OH (1)
DNCO–ND2OD (1)
?
dCH2
vCH2
nC–O
vNH2, vND2
vNH2, vND2
dHNC
1077.6 (0.001)
931.5 (0.005)
922.8 (0.014)
1148.3 (0.014)
1121.6 (0.058)
898.0 (0.002)
816.5 (0.004)
850.0 (0.002)
nNO
574.5 (0.003)
HNCO–H2O
dNCO
a
Integrated intensities are given in parentheses.
NH stretch of the HNCO monomer, such a large shift can be
expected for HNCO serving as a proton donor in a hydrogen
bonded complex. The ND stretch in the complex is identified
at 2473.8 and 2471.0 cm21, showing a 134 cm21 red shift with
respect to nND in the DNCO molecule. A large perturbation of
the NH stretch in the complex is probably responsible for the
discrepancy between the deuterium shift of this vibration in
free and complexed HNCO (904 and 814 cm21, respectively).
The 898.0 cm21 band is 128 cm21 shifted toward higher
wavenumbers, as compared to the unperturbed HNC bending
vibration of the HNCO molecule. The position of this band
suggests that the 898.0 cm21 absorption corresponds to the
perturbed HNC bending vibration of the bonded HNCO
molecule. Unfortunately, no band corresponding to the
perturbed DNC bending vibration of the complexed DNCO
was identified.
The additional bands belonging to group 1 were found in
the regions of the hydroxylamine molecule vibrations. The IR
spectra of NH2OH isolated in argon and nitrogen matrices are
well known.32,35 In the studied spectra, the 3631.8 and 3630.6
cm21 bands are placed in the vicinity of the OH stretching
vibration of the NH2OH monomer (3634.7 cm21).32 These
bands appear at 2681.0 and 2679.0 cm21 after deuterium
substitution, showing the same 2951 cm21 deuterium shift
within experimental error, as observed in the NH2OH molecule
(H/D = 1.35). The 1613.4 cm21 band appears in the region of
the NH2 bending vibration of NH2OH (1598.9 cm21) and
shows a very similar deuterium shift to dNH2 in the monomer
(2429 cm21 shift is observed for the 1613.4 cm21 band and
2428 cm21 is reported for the 1598.9 cm21 absorption).32 The
1148.3 cm21 band is observed in the vicinity of the NH2
wagging vibration of the NH2OH molecule (1117.0 cm21). The
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1148.3 cm21 band exhibits a similar deuterium shift (2216.8
cm21) to the vNH2 mode of the NH2OH monomer (2202
cm21). The 816.5 cm21 absorption is identified in the region of
the nNO vibration of the ND2OD molecule. Unfortunately no
band corresponding to the perturbed NO stretch of NH2OH
was observed in the studied spectra.
The ab initio calculations demonstrate the stability of five
hydrogen bonded structures of the HNCO–NH2OH complex in
which the OH or NH2 group of hydroxylamine interacts with
the NH group or the oxygen atom of the HNCO molecule. In
the predicted 3 cm21 blue shift value. The 2264 cm21 band
shows the 8.2 cm21 red shift on deuteration, which is in
agreement with theoretical predictions (213 cm21). The lower
wavenumber of nasNCO in DNCO–ND2OD than in HNCO–
NH2OH could be due to a 25% contribution of the ND
stretching coordinate to the 2222.8 cm21 vibration (Table 2).
The band observed at 898.0 cm21 is 128 cm21 blue shifted, as
compared to the unperturbed HNC bending vibration of the
HNCO molecule, and this is in a good agreement with the
calculated shift value (+93 cm21). The normal coordinate
analysis indicates coupling between the dHNC, dNCO and
cNCO internal coordinates. As a result, the NCO bending
contributes to the 898.0 cm21 normal mode, to which the
largest contribution comes from dHNC. On deuteration, the
cNCO internal coordinate gives an additional 38% contribu-
tion to the calculated 711 cm21, vibration and this probably
causes a small 5 cm21 red shift of the dDNC in the complex
from the corresponding DNCO monomer absorption.
The identified hydroxylamine vibrations confirm the exis-
tence in matrices of the HNCO–NH2OH complex of the
structure shown in Fig. 5. The bands at 3631.8 and 3630.6
cm21 are assigned to the stretching vibration of the OH group,
which is only slightly perturbed after complex formation. The
observed shift of these bands with respect to the OH stretch of
the hydroxylamine monomer (ca. 24 cm21) is in good
agreement with the predicted shift value (29 cm21). The
observed perturbations of the other hydroxylamine vibrations
in both HNCO–NH2OH and DNCO–ND2OD complexes are also
in good agreement with the calculated ones.
Fig. S1, all optimized structures of the HNCO–NH OH system
3
2
are shown. The selected bond distances (in Å) and interaction
energies (in kJ mol21) are also presented. The optimized
geometrical parameters of the structures and the full set of
vibrational wavenumbers are collected in Tables S1 and S2 (ESI
). Comparison of the experimental spectra with the calculated
3
ones shows that the complex trapped in the matrix has the
structure presented in Fig. 5, with the HNCO molecule acting
as a proton donor toward the oxygen atom of hydroxylamine.
There is an additional interaction between the NH2 group of
NH2OH and the nitrogen atom of HNCO. It is worth
mentioning that this structure is the second one in the
calculated order of stability of five optimized complexes: it is
5.42 kJ mol21 less stable than the most stable structure
…
…
stabilized by the OH N and NH N hydrogen bonds.
…
The formation of the NH O bond between the NH group of
isocyanic acid and the oxygen atom of hydroxylamine is
reflected in the calculated spectra by strong perturbations of
the NH stretching and HNC bending vibrations of the HNCO
and OH out of plane bending mode of NH2OH. Table 2
presents the wavenumbers identified for the HNCO–NH2OH
and DNCO–ND2OD complexes in an argon matrix and
compares them with the calculated ones for the structure
shown in Fig. 5. The appearance of the NH stretch at 3287.9,
3285.2 cm21 provides very strong evidence for the formation of
As one can see in Table S2, all identified absorptions are
3
predicted to be the most intense ones for the HNCO–NH2OH
…
complex with NH O hydrogen bonding.
Formation of N2–H2O–CO complexes
The bands due to sets 2a and 2b appeared in the regions of the
stretching and bending water vibrations36 and in the vicinity
of the CO monomer band.29
…
the HNCO–NH2OH complex with a NH O hydrogen bond.
The observed red shift of these bands with respect to the
HNCO monomer absorption (ca. 2225 cm21) is in good
agreement with the calculated shift value for this complex
(2266 cm21). The deuterium counterparts of these bands
show a ca. 134 cm21 red shift with respect to the correspond-
ing DNCO monomer vibration, which is in agreement with the
calculated wavenumber shift (2186 cm21). The broad absorp-
tion at ca. 2264 cm21, assigned to the NCO antisymmetric
stretching vibration, is 5 cm21 shifted toward higher wave-
numbers with respect to the monomer band, which is close to
The 3728.4 (2b) and 3722.0 (2a) cm21 absorptions are placed
in the vicinity of the nas mode of the water molecule and show
small 6.6 and 13 cm21 red shifts from the H2O monomer
band. The deuterium counterparts of these bands are
identified at 2765.5 (2b) and 2761.5 (2a) cm21, showing almost
the same 2963 and 2961 cm21 deuterium shifts as observed
in the H2O molecule (964 cm21). The 3638.0 (2b) and 3624.8
(2a) cm21 bands appear in the vicinity of the nsH2O vibration.
Unfortunately their deuterium counterparts were not identi-
fied since they were expected in a complicated spectral region.
The 1598.7 (2b) and 1595.0 (2a) cm21 bands appearing in the
vicinity of the bending mode of water are shifted 9.7 and 6
cm21 toward higher wavenumbers, as compared to the
unperturbed dH2O vibration, and exhibit ca. 2419 and 2417
cm21 deuterium shifts, respectively.
Additionally, the bands appearing in the region of the CO
molecule vibration at 2148.9 (2a) and 2148.5 (2b) cm21 (shifted
to 2149.8 and 2148.9 cm21 after deuteration, respectively) also
show the behaviour of the bands of group 2a and 2b.
Decomposition of N-hydroxyurea leading to the formation of
H2O and CO, as indicated above, must also produce N2 and H2
species in the same matrix cage. Since the bands observed
Fig. 5 The MP2/aug-cc-pVTZ optimized structure of the HNCO–NH2OH complex
(the selected bond distances (in Å); interaction energies DECP = 227.31; DECP
ZPE
= 221.23 (in kJ mol21)).
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Table 2 Wavenumbers and wavenumber shifts (in cm21), calculated and observed for the HNCO–NH2OH complex in solid argona
…
…
NH2O(H) HNCO
ND2O(D) DNCO
Experimental
Calculated
Experimental
Calculated
n
Dn
n
Dn
n
Dn
n
Dn
Assignmentb,c
3631.8
3630.6
3287.9
3285.2
2264
24
3829
3436
2321
1673
1166
888
29
2266
+3
2681.0
2679.0
2474.0
2471.0
2222.8
25
2789
2546
2280
1212
844
711
939
28
2186
213
+6
nOH (100)
{nOD (100)}
nNH (100)
{nND (100)}
nasNCO (100)
{nasNCO (75), nND (25)}
dNH2 (88)
2225
+5
2134
28.2
+13.2
+16.5
—
1613.4
1148.3
898.0
—
+14.4
+31.3
+128
—
+9
1184.2
931.5
—
{dND2 (87)}
+14
+93
23
+9
vNH2 (66), dOH (26)
{vND2 (71), nNO (18)}
dHNC (51), dNCO (33)
{dDNC (40), cNCO (38), dNCO (11)}
nNO (73), vNH2 (19)
{nNO (61), dOD (38)}
25
26
932
816.5
22
a
b
In all calculations the MP2/aug-cc-pVTZ method was used. A potential energy distribution (PED) of the normal modes is presented; only
c
the PED elements larger than 15% are shown. The assignment for the DNCO–ND2OD complex is given in brackets.
after matrix photolysis are not located at wavenumbers
reported in literature for H2O–CO29 and H2O–N2 complexes,37
there is a high probability that the ternary N2–H2O–CO
complexes are formed. The question arises if the H2 molecule
stays in the argon cage or even in the vicinity, or does it
possess enough energy from the decomposition process to
leave the matrix cage. According to previous calculations
results, the interaction energy between H2 and each molecule
discussed does not exceed 2.4 kJ mol21 (ca. 0.9 kJ mol21 for
geometrical parameters for the three structures are collected
and in Tables 3 and S4 the calculated harmonic and
3
anharmonic wavenumbers are presented. The most stable
…
structure A, stabilized by the O–H C(O) bond and van der
Waals interaction between the N2 and H2O molecules, is 1.21
kJ mol21 more stable than structure B, in which water acts as a
…
…
proton donor forming the O–H C and O–H N hydrogen
bonds. Structure A is also 1.31 kJ mol21 more stable than
structure C, in which water serves as a proton donor toward
H2 N2,38 ca. 1.11 kJ mol21 for H2 CO,39 ca. 2.41 kJ mol21 for
the nitrogen molecule forming the O–H N(N) bond and the
…
…
…
H2 H2O40). This fact points to a spontaneous decomposition
…
van der Waals interaction between CO and H2O is also present.
…
The formation of the O–H C(O) hydrogen bonds in structures
of the aforementioned H2 complexes to the separate mono-
mers in solid argon at the temperatures used. Thus, taking
into considerations the N2 + H2O + CO + H2 system, H2 is not
involved as its fourth part and the final product would be the
ternary N2–H2O–CO complex.
A and B is reflected in a similar decrease of the calculated
nasH2O and nsH2O wavenumbers (Dncalc = 225 vs. 226 and 231
vs. 221 cm21, respectively) and an increase of the dH2O and
nCO wavenumbers (Dncalc = +7 vs. +10 and +13 vs. +11 cm21
,
…
respectively). The presence of the O–H N hydrogen bond in
structure C causes much less perturbation of the subunit
vibrations. The comparison of the calculated wavenumbers for
the three structures with absorptions identified in solid argon
indicates that A and B complex structures are present in the
matrix and they correspond to the 2a and 2b band sets,
respectively.
The performed calculations demonstrate the stability of
three structures of the N2–H2O–CO complex that are shown in
Fig. 6. The selected bond distances (in Å) and interaction
energies (in kJ mol21) are also presented. In Table S3, the
3
In relation to the experimental and theoretical data
concerning the H2O–CO system,29,41–43 the calculations at
the MP2/aug-cc-pVTZ level together with an argon matrix
spectrum of the H2O–CO complex were performed and the
received results showed good agreement with the previous
ones. Both the obtained theoretical wavenumber shifts with
respect to the H2O and CO monomers and the positions of the
complex bands in solid argon demonstrate that the wavenum-
ber values of H2O–CO are located between those assigned to
the N2–H2O–CO complex of structures A and B (see Table 3 and
Fig. 3). The literature data show that water vibrations in a
complex appear at 3723.5, 3627.8 and 1596.0 cm21 for H2O–
CO and at 2762.7, 2650.1 and 1185.0 cm21 for D2O–CO and the
Fig. 6 The MP2/aug-cc-pVTZ optimized structures of the N2–H2O–CO com-
plexes. The selected bond distances (in Å) and interaction energies DECP (in kJ
mol21) are also presented; the DECP
values (in kJ mol21) are given in
ZPE
parentheses.
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Table 3 Wavenumbers and wavenumber shifts (in cm21) calculated for the three structures of the N2–H2O–CO complexes and the corresponding values for the
complexes observed in an argon matrix.a The calculated and observed values for the H2O–CO complex are given for comparison
A
B
C
H2O–CO
Assignment
…
…
…
…
… …
OC (H)OH N2
N2 (H)OH CO
N2 HOH CO
n harm Dn n anh Dn obs
n harm Dn n anh Dn obs
n harm Dn
n anh
n harm Dn
n anh Dn obsb
3926
3794
1636
2123
2186
225 3749 213.0
231 3632 213.7
3925
3804
1638
2121
2189
226 3756 26.6
221 3648 20.5
+10 1580 +8.7
+11 2090 +10
3935
3810
1632
2115
2189
216 3757
215 3645
3932
3802
1635
2123
—
219 3755 211.5
223 3644 210.2
nasH2O
nsH2O
dH2O
nCO
+8
+13 2093 +10.4
+1 2146
1585 +5
+4
+5
+2
1584
2089
2150
+7
1584 +7
+13 2098 +11
—
+2
2151
—
—
—
nN2
a
b
In all calculations the MP2/aug-cc-pVTZ method was used. See ref. 29.
CO molecule perturbations are observed at 2149.4 cm21 and
2150.2 cm21 for both these systems,29,41 respectively.
(a-cleavage), and in the case of substituted hydroxamic acids a
scission of the N–O bond also takes place. Recently, the
mechanistic photochemistry of urea was studied by means of
ab initio calculations on the stationary and intersection
structures in the lowest three energy states,49 which gave
new insight into the mechanism proposed experimentally in
low temperature matrices upon VUV irradiation at wavelengths
of l > 160 nm.50 Based on these studies, the formation of the
HNCO–NH2OH and N2–H2O–CO complexes, as the major
photoproducts of HU, can be proposed. Scheme 1 gives the
mechanistic view, indicating the routes toward new species.
The deamination reaction (NH2CONHOH A HNCO + NH2OH)
may occur in the S0 state after a very fast S1 A S0 internal
conversion (like in the case of the urea molecule). It is a single
step reaction in which one hydrogen atom of the NH2 group
transfers to the nitrogen atom of the NHOH group and then
the C–N bond dissociates, forming HNCO and NH2OH and
finally the HNCO–NH2OH complex. The next possibility of
deamination is at first a scission of the N–O bond and
production of NH2C(O)NH + OH radicals. Then, the hot
NH2C(O)NH radical disintegrates into NH2 + C(O)NH, and the
HNCO + NH2OH products are formed. The other channel for
the NH2OH formation is a two step reaction in which at first
an isomerization reaction NH2CONHOH A NH2(HO)CLNOH
occurs, and then the OH radical is created and transferred to
the NH2 group and NH2OH forms. However, the last
mechanism seems unlikely, since the iminol tautomers of
HU are substantially less stable than the keto forms (40–85 kJ
Photodissociation of HU - formation of the HNCO–NH2OH
and N2–H2O–CO complexes
Analysis of the spectra of the NH2CONHOH/Ar matrices
irradiated with Xe arc lamp output proves the existence of
two photodissociation channels of NH2CONHOH under the
conditions of this experiment:
NH2CONHOH A HNCO + NH2OH
(1)
NH2CONHOH A (NH2NHOH + CO A N2H2 + H2O + CO)
A H2 + N2 + H2O + CO
(2)
The products of each photodissociation reaction are trapped
in the original precursor site in the matrix, resulting in the
formation of the HNCO–NH2OH and N2–H2O–CO complexes.
However, very weak absorptions due to the NH2OH, HNCO,
HOCN and CO monomers are also detectable. A cage exit
probability of small photo fragments is enhanced by an excess
of energy, which they have after decomposition of the
precursor, and depends on the interactive forces between the
fragment and the surrounding cage. The photon absorption
may induce even long-range migration of fluorine, oxygen and
hydrogen atoms, moving them several lattice sites away from
their origin. The barrier for the cage exit of the hydrogen atom
in rare gas solids is 1–1.5 eV (96–145 kJ mol21), which stays
within the range of the studied experiment.44,45 The NCO
radicals were not observed during photolysis of HNCO in an Ar
matrix with a medium pressure mercury lamp. However the
radicals were formed when the matrix was irradiated with VUV
radiation at 121.6 nm.30 This could be explained by the cage
exit of the hydrogen atom with a larger excess of energy. The
photodissociation of ozone in solid argon at 266 nm resulted
in recombination of O + O2 as a major pathway and the
decrease of ozone was due to a cage exit of oxygen atoms,
which depended on the temperature and irradiation power.46
The cage exit probability for an OH radical in 230 nm
photolysis of H2O2 was found to be 17% in Ar at 17 K.47
The photochemical studies of formohydroxamic acid24 and
formamide48 show that the nAp* excitation within the
carbonyl moiety in simple amide molecules leads to the
excited singlet state in which cleavage of the C–N bond occurs
mol21 51
and have not been identified experimentally so far.
)
Isourea, NH2(HO)CLNH, was observed upon VUV irradiation
of urea in argon and xenon matrices as an intermediate
product converted in a dehydration reaction to the NH2CN–
H2O complex.50 The excited S1 state can decay to the T1 state
via the S1 A T1 intersystem crossing and then the C–N bond
fission would create the NH2CO and NHOH radicals. The hot
NH2CO radical dissociates into NH2 + CO and then the NH2
and
NHOH
radicals
could
recombine
forming
N-hydroxyhydrazine, NH2NHOH. This very unstable molecule
was observed experimentally by Andrews et al.52 as the product
of the reaction between hydrazine and ozone during deposi-
tion in an argon matrix. Despite a very careful analysis of the
observed spectra, no bands expected in the regions of
N-hydroxyhydrazine vibrations were identified. This indicates
that the fast decomposition process proceeds: NH2NHOH A
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Scheme 1 Possible mechanism of the formation of HNCO + NH2OH and H2 + N2 + H2O + CO photoproducts from N-hydroxyurea.
(N2H2 + H2O) A N2 + H2 + H2O, and finally the N2, H2O and CO
molecules trapped in the same cage form the N2–H2O–CO
complexes (migration of hydrogen atoms in an argon matrix
was already discussed). The matrix isolated diazene molecule,
N2H2, was recognized to be one of the secondary products
during photolysis of urea50 or the NH3–CO2 complex,53 but
there are no absorptions that could be assigned to the N2H2
vibrations in the studied spectra.
nitrogen and carbon atoms as compared to urea, and affects
the excited states of the molecule. Second, the broad band
excitation of the HU molecule may also affect the efficiency of
the intersystem crossing to a triplet state.
Other photolysis products
The very weak absorptions due to the CO, hydroxylamine
(ND2OD) and cyanic acid (DOCN) monomers that appeared in
the spectra of irradiated matrices indicate that under the
conditions of the experiment a small number of these
molecules can escape from the parent cage. The escaping
molecules can form molecular complexes with other mole-
cules present in the matrix. The weak bands at 3295.2, 574.5
cm21 and 2146.3, 1600.8 and 1121.6 cm21 are attributed to the
HNCO–H2O and NH2OH–CO complexes, respectively.24
The very weak absorptions at 2635.0, 2284.5, 1077.6 cm21
are due to the DOCN molecule,33 and the band at 2142.6
(2143.7) cm21 is assigned to the NH–CO (ND–CO) complex.30
These species most probably originate from the HNCO
molecule formed on irradiation and having undergone further
reactions. The literature reports on the photoequilibrium
between HNCO and HOCN observed during irradiation with l
> 224 nm of the HNCO molecule isolated in solid argon.54 The
photodecomposition of HNCO and formation of the NH–CO
complexes were also observed.30
The performed study does not allow us to draw any
definitive conclusions concerning the electronic states that
lead to the observed reaction channels, as the matrices were
irradiated with the broad band irradiation of the Xe arc lamp,
and the excited electronic states of the HU molecule have not,
to my knowledge, been studied so far. The similarity of the
products formed during photolysis of N-hydroxyurea and urea
suggests comparable mechanisms of photodecomposition of
these molecules. The photodissociation process of
N-hydroxyurea gives HNCO + NH2OH and H2 + N2 + H2O +
CO products under the conditions of the experiment and
photodecomposition of urea (NH2CONH2) leads mainly to
HNCO + NH3 and N2H4 + CO products. If a similar mechanism
of photodissociation for these molecules is assumed and the
results of the high level calculations of photochemical path-
ways of urea are taken into consideration,49 then it follows that
the deamination reaction proceeds at the S0 state after fast S1
A S0 internal conversion and the C–N bond cleavage occurs in
the T1 state as a result of the S1 A T1 intersystem crossing.
However, this is still a possible mechanism of photodecompo-
sition of HU and, based on the experimental results, ab initio
calculations of the ground and excited states are necessary to
improve the reaction pathways. There are two factors that
might have a significant influence on the obtained results.
First, the presence of the oxygen atom of the OH group in the
HU molecule changes the electron density distribution on
On the basis of the literature data,31 the very weak
absorptions at 2885.3, 2816.5, 1610.2, 1499.0 and 1173.8
cm21 are assigned to the H2CO–H2O complex and the 2796.0
cm21 band is due to the H2CO monomer. The formation of
H2CO and H2CO–H2O during UV-Vis irradiation of HU needs
some comment. Formaldehyde was not identified as
a
photodissociation product of simple hydroxamic acids, but
the molecule was formed in photodecomposition reactions of
urea, as the secondary species originated from the primary
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CO–N2H4 complex.50 It was suggested that H2CO + N2H2
products arise from the radical pair HCO + N2H3 produced
during excitation of the CO–N2H4 complex; the formation of
the HCO radical was also confirmed. In the case of HU
photolysis, a much more possible mechanism leading to the
H2CO formation seems to be the hydrogenation of CO. The C–
N bond cleavage of the N-hydroxyurea molecule creates the
NH2CO and NHOH radicals that undergo further photoche-
mical reactions, resulting in CO and most probably NH2NHOH
(as discussed above). Hydrogen atoms formed in a decom-
position process of NH2NHOH (together with the identified N2
and H2O molecules) react with CO molecules present in the
same matrix cage, and finally H2CO is produced. In this
manner, the formation of H2CO becomes indirect proof of
hydrogen migration in the studied argon matrices. Recently,
the successive hydrogenation of CO was studied by co-
injection of CO molecules and H atoms at 3 and 10 K.55 The
obtained results showed that in the CO + H reaction, H2CO
and CH3OH, together with HCO and CH3O species were
formed.
products from a photodecomposition process of HU, in
agreement with the proposed photochemical pathways.
Neither the deamination reaction nor the C–N bond cleavage
lead to nitroxide radical or C-nitrosoformamide. The per-
formed experimental studies clearly show the absence of a
photodissociation
channel,
converting
the
isolated
N-hydroxyurea molecule to NO or HNO species.
Acknowledgements
The author gratefully acknowledges a grant of computer time
from the Wrocław Center for Networking and
Supercomputing.
References
1 W. F. C. Dresler and R. Stein, Justus Liebigs Ann. Chem.,
1869, 150, 242.
2 J. W. Yarbro, B. J. Kennedy and C. P. Barnum, Proc. Natl.
Acad. Sci. U. S. A., 1965, 53, 1033.
3 I. H. Krakoff, N. C. Brown and P. Reichard, Cancer. Res.,
1968, 28, 1559.
4. Conclusions
4 S. Nyholm, L. Thelander and A. Graeslund, Biochemistry,
1993, 32, 11569.
5 A. S. Boyd and K. H. Neldner, J. Am. Acad. Dermatol., 1991,
25, 518.
6 A. M. Layton, R. A. Sheehan-Dare, M. J. D. Goodfield and J.
A. Cotterill, Br. J. Dermatol., 1989, 121, 647.
7 M. Piver, M. Khalil and L. J. Emrich, J. Surg. Oncol., 1989,
42, 120.
8 M. H. Steinberg, F. Barton, O. Castro, C. H. Pegelow, S.
K. Ballas, A. Kutlar, E. Orringer, R. Bellevue, N. Olivieri,
J. Eckman, M. Varma, G. Ramirez, B. Adler, W. Smith,
T. Carlos, K. Ataga, L. DeCastro, C. Bigelow,
Y. Saunthararajah, M. Telfer, E. Vichinsky, S. Claster,
S. Shurin, K. Bridges, M. Waclawiw, D. Bonds and
M. Terrin, JAMA, J. Am. Med. Assoc., 2003, 289, 1645.
9 F. Lori, A. Malykh, A. Cara, D. Sun, J. N. Weinstein,
J. Lisziewicz and R. C. Gallo, Science, 1994, 266, 801.
10 M. A. Gibbs and S. J. Sorensen, Ann. Pharmacother., 2000,
34, 89.
The irradiation of N-hydroxyurea isolated in an argon matrix
with the full output of the Xe arc lamp leads to HNCO +
NH2OH and H2 + N2 + H2O + CO product species, which
suggests two photodissociation pathways. The deamination
reaction, resulting in hydroxylamine formation, can proceed at
the S0 state after fast S1 A S0 internal conversion, whereas the
C–N bond cleavage, in which the radicals are formed, can
occur in the T1 state as a result of S1 A T1 intersystem
crossing. The photolysis products trapped in the same cage
…
…
…
…
…
form NH2O(H) HNCO, N2 (H)OH CO and N2 HOH CO
complexes. The calculations at the MP2/aug-cc-pVTZ level
demonstrate the stability of five structures for the isocyanic
acid–hydroxylamine complex and three structures for the
nitrogen–water–carbon monoxide complex. The calculations
…
predict the NH2O(H) HNCO complex with HNCO hydrogen
bonded to the oxygen atom of the hydroxylamine molecule to
be 5.42 kJ mol21 less stable than the most stable structure in
…
…
…
which the OH N and NH N hydrogen bonds are present. In
11 S. B. King, Free Radical Biol. Med., 2004, 37, 737.
12 M. T. Gladwin and A. N. Schechter, Semin. Hematol., 2001,
38, 333.
…
turn, the most stable N2 (H)OH CO complex stabilized by
the O–H C(O) hydrogen bond and van der Waals interaction
…
between the N2 and H2O molecules is calculated to be 1.21 kJ
13 J. Huang, E. Sommers, D. B. Kim-Shapiro and S. B. King, J.
Am. Chem. Soc., 2002, 124, 3473.
14 C. Walling and A. N. Naglieri, J. Am. Chem. Soc., 1960, 82,
1820.
15 E. Lipczynska-Kochany and J. Kochany, J. Photochem.
Photobiol., A, 1988, 44, 317.
16 E. Lipczynska-Kochany, H. Iwamura and J. Kochany,
Monatsh. Chem., 1987, 118, 1345.
17 E. Lipczynska-Kochany and J. Kochany, J. Photochem., 1987,
38, 331.
18 P. N. Chhaya, M. M. Nimbalkar and B. D. Hosangadi,
Tetrahedron Lett., 1983, 24, 5551.
19 B. D. Hosangadi, P. N. Chhaya, M.M. Nimbalkar and N.
R. Patel, Tetrahedron, 1987, 43, 5375.
mol21 more stable than the N2 HOH CO complex, with
water acting as a proton donor toward the nitrogen and carbon
atoms of N2 and CO molecules, respectively.
…
…
Since N-hydroxyurea demonstrates a wide spectrum of
biomedical therapeutic applications, acting mainly as a nitric
oxide and nitric oxide-related donor, it was interesting to see
the influence of UV-Vis irradiation on the NO (or HNO)
formation mechanism from the isolated HU molecule. In a
recent review,56 N-hydroxyurea was reported to release nitric
oxide (or nitroxyl) via oxidation reactions, proceeding through
the nitroxide radical or C-nitrosoformamide as intermediates.
A careful spectral analysis excluded formation of these
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RSC Adv., 2013, 3, 1922–1932 | 1931

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Paper
RSC Advances
20 D. Hosangadi, M. M. Nimbalkar and N. R. Patel,
Tetrahedron, 1988, 44, 5857.
34 S. Raunier, T. Chiavassa, F. Marinelli, A. Allouche and J.-
P. Aycard, J. Phys. Chem. A, 2003, 107, 9335.
21 J. E. Johnson, M. Arfan, R. Hodzi, L. R. Caswell and
S. Rasmussen, Photochem. Photobiol., 1990, 51, 139.
35 G. A. Yeo and T. A. Ford, J. Mol. Struct., 1990, 217, 307.
36 A. Engdahl and B. Nelander, J. Mol. Struct., 1989, 193, 101.
37 S. Coussan, A. Loutellier, J. P. Perchard, S. Racine and
Y. Bouteiller, J. Mol. Struct., 1998, 471, 37.
´
22 E. Lipczynska-Kochany, Chem. Rev., 1991, 91, 477.
23 M. Sałdyka, Phys. Chem. Chem. Phys., 2010, 12, 15111.
24 M. Sałdyka and Z. Mielke, Phys. Chem. Chem. Phys., 2003, 5,
4790.
´
38 P. Hobza and R. Zahradnık, Chem. Rev., 1988, 88, 871.
39 P. Jankowski and K. Szalewicz, J. Chem. Phys., 2005, 123,
104301.
40 J. Sadlej, B. Rowland, J. P. Devlin and V. Buch, J. Chem.
Phys., 1995, 102, 4804.
41 T. Svensson and B. Nelander, Chem. Phys., 2000, 262, 445.
42 J. Lundell, J. Phys. Chem., 1995, 99, 14290.
43 J. Langlet, J. Caillet and M. Allavena, J. Mol. Struct., 1998,
450, 69.
44 K. H. Godderz, N. Schwentner and M. Chergui, J. Chem.
Phys., 1996, 105, 451.
25 M. J. Frisch, G. W. Trucks, H. B. Schlegel, G.E. Scuseria, M.
A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone,
B. Mennucci, G. A. Petersson, H. Nakatsuji, M. Caricato,
X. Li, H.P. Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng, J.
L. Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda,
J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao,
H. Nakai, T. Vreven, J. A. Montgomery Jr., J. E. Peralta,
F. Ogliaro, M. Bearpark, J. J. Heyd, E. Brothers, K. N. Kudin,
V. N. Staroverov, T. Keith, R. Kobayashi, J. Normand,
K. Raghavachari, A. Rendell, J. C. Burant, S. S. Iyengar,
J. Tomasi, M. Cossi, N. Rega, J. M. Millam, M. Klene, J.
E. Knox, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo,
R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin,
R. Cammi, C. Pomelli, J. W. Ochterski, R. L. Martin,
K. Morokuma, V. G. Zakrzewski, G. A. Voth, P. Salvador, J.
J. Dannenberg, S. Dapprich, A. D. Daniels, O. Farkas, J.
B. Foresman, J. V.Ortiz and J. Cioslowskiand D. J. Fox,
GAUSSIAN 09 (Revision B.01), Gaussian, Inc., Wallingford
CT, 2010.
26 S. F. Boys and F. Bernardi, Mol. Phys., 1970, 19, 553.
27 J. S. Simon, M. Duran and J. J. Dannenberg, J. Chem. Phys.,
1996, 105, 11024.
28 J. M. L. Martin and C. Van Alsenoy, GAR2PED, University of
Antwerp, Antwerpen, Belgium, 1995.
29 H. Abe and K. M. T. Yamada, J. Chem. Phys., 2001, 114,
6134.
30 D. E. Milligan and M. E. Jacox, J. Chem. Phys., 1967, 47,
5157.
45 V. A. Apkarian and N. Schwentner, Chem. Rev., 1999, 99,
1481.
46 M. Bahou, L. Schriver-Mazzuoli and A. Schriver, J. Chem.
Phys., 1999, 110, 8636.
47 L. Khriachtchev, M. Pettersson, S. Tuominen and
¨ ¨
M. Rasanen, J. Chem. Phys., 1997, 107, 7252.
¨ ¨
48 J. Lundell, M. Krajewska and M. Rasanen, J. Phys. Chem. A,
1998, 102, 6643.
49 G. Cui, L. Ding, F. Feng, Y. Liu and W. Fang, J. Chem. Phys.,
2010, 132, 194308.
50 F. Duvernay, T. Chiavassa, F. Borget and J.-P. Aycard, J.
Phys. Chem. A, 2005, 109, 6008.
51 M. Remko, P. D. Lyne and W. G. Richards, Phys. Chem.
Chem. Phys., 1991, 1, 5353.
52 R. Lascola, R. Withnall and L. Andrews, Inorg. Chem., 1988,
27, 642.
´
53 J. B. Bossa, F. Duvernay, P. Theule, F. Borget and
T. Chiavassa, Chem. Phys., 2008, 354, 211.
54 M. E. Jacox and D. E. Milligan, J. Chem. Phys., 1964, 40,
2457.
31 B. Nelander, Chem. Phys., 1992, 159, 281.
32 R. Withnall and L. Andrews, J. Phys. Chem., 1988, 92, 2155.
33 J. H. Teles, G. Maier, B. A. Hess Jr., L. J. Schaad,
M. Winnewisser and B. P. Winnewiser, Chem. Ber., 1989,
122, 753.
´
55 C. Pirim, L. Krim, C. Laffon, Ph. Parent, F. Pauzat, J. Pilme
and Y. Ellinger, J. Phys. Chem. A, 2010, 114, 3320.
56 S. B. King, Curr. Top. Med. Chem., 2005, 5, 665.
1932 | RSC Adv., 2013, 3, 1922–1932
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