Photolysis of the C2H4-HONO complex in low temperature matrices: Formation of 2-nitrosoethanol

Article abstract of DOI:10.1016/S0022-2860(03)00329-6

Photochemistry of C₂H₄?HONO complexes in argon matrices has been investigated using λ > 340 nm radiation of a medium pressure mercury arc. IR spectra were recorded after various periods of photolysis. The photolysis reaction was found to lead to formation of 2-nitrosoethanol that was identified for the first time. Three groups of bands showing different behavior during photolysis process and after matrix annealing were assigned to three different conformers of 2-nitrosoethanol molecule. The experimental data are supported by ab initio calculations.

Full text of DOI:10.1016/S0022-2860(03)00329-6

Journal of Molecular Structure 656 (2003) 321–332
www.elsevier.com/locate/molstruc
Photolysis of the C2H4–HONO complex in low temperature
matrices: formation of 2-nitrosoethanol
a
Adriana Olbert-Majkut , Zofia Mielke , Konstantin G. Tokhadze
a,
b
*
aFaculty of Chemistry, Wrocław University, Joliot-Curie 14, 50-383 Wrocław, Poland
bInstitute of Physics, St Petersburg University, Peterhof, 198504, St Petersburg, Russian Federation
Received 13 February 2003; accepted 25 February 2003
Dedicated to Professor Achim Mu¨ller on the occasion of his 65th birthday
Abstract
Photochemistry of C2H4· · ·HONO complexes in argon matrices has been investigated using l . 340 nm radiation of a
medium pressure mercury arc. IR spectra were recorded after various periods of photolysis. The photolysis reaction was found
to lead to formation of 2-nitrosoethanol that was identified for the first time. Three groups of bands showing different behavior
during photolysis process and after matrix annealing were assigned to three different conformers of 2-nitrosoethanol molecule.
The experimental data are supported by ab initio calculations.
q 2003 Elsevier B.V. All rights reserved.
Keywords: Infrared spectra; Ab initio calculations; Photochemistry; Rare-gas matrices; Conformers; Nitrous acid; Ethene; Nitrosoethanol
1. Introduction
mechanisms of the reactions of OH radical with
hydrocarbons [2].
Matrix isolation technique is very useful method in
obtaining direct spectroscopic information on the
reaction intermediates. The technique was recently
employed [3,4] to characterize the infrared absorption
spectrum of the 2-hydroxyethyl radical, HOCH2CH2
which is an intermediate product in OH þ ethene
reaction.
Hydrocarbons, nitrogen oxides (NO2, NO3), nitric
and nitrous acids are the major constituents of
polluted urban environments. The reactions of the
hydroxyl and (NO)x radicals with hydrocarbons are of
critical importance in the Earth’s troposphere. Con-
sequently, there has been a huge amount of research
carried out on the kinetics and mechanisms of the
reactions of OH and (NO)x with a wide range of
organic compunds [1]. In particular a large number of
work has been devoted to the study of kinetics and
We have studied recently the complexes between
ethene and nitrous acid isolated in an argon matrix [5].
It is well known that HONO photodissociates into OH
and NO radicals [6]. One may expect that the
photolysis of the C2H4· · ·HONO complex will lead
to formation of the OH and NO radicals in the
presence of the C2H4 molecule. The trapping of these
species in one matrix cage should favour the formation
*
Corresponding author. Tel.: þ48-71-3757475; fax: þ48-71-
3282348.
E-mail address: zm@wchuwr.chem.uni.wroc.pl (Z. Mielke).
0022-2860/03/$ - see front matter q 2003 Elsevier B.V. All rights reserved.
doi:10.1016/S0022-2860(03)00329-6

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A. Olbert-Majkut et al. / Journal of Molecular Structure 656 (2003) 321–332
of the product(s) of addition of the OH and/or NO
radicals to the ethene molecule. The similar studies of
the photochemistry of the OC· · ·HONO complexes
have shown that both HOCO radical and HOC(O)NO
molecule are the photoproducts of this reaction [7].
Herewith we report the results of the study of the
photochemistry of C2H4· · ·HONO complex in argon
matrices. The infrared matrix isolation studies are
supported by ab initio calculation of the structure and
vibrational frequencies of the formed photoproducts.
3. Results
3.1. Infrared matrix isolation studies
3.1.1. Photolysis and annealing of HONO/Ar matrices
Before we studied photochemistry of HONO/
C2H4/Ar matrices a set of experiments had been
performed with HONO/Ar matrices of concentrations
varying in the range 1/800–1/300. The matrices were
photolyzed with l . 340 nm radiation at
12 K. Similarly as in HONO/N2 matrices, [7,9] also
in the photolyzed HONO/Ar matrices, the OH and NO
radicals were detected as the primary photolysis
products. The bands, assigned to the perturbed OH
radical and its complex with water, were observed at
3532.3 and 3498.5 cm21, respectively. The bands due
to NO and its complex with HONO were identified at
1871.5, and at 1882.5, 1880.8 cm21, respectively. The
observed frequencies are in accord with literature data
for those two radicals [10–16]. In addition to the
absorptions due to the OH and NO radicals, the bands
due to trans-HONO and cis-HONO molecules trapped
in unstable sites were also observed after matrix
photolysis (at 3564.5, 1686.7, 1259.6, 788.7 cm21 for
trans-HONO and at 843.6, 631.8 cm21 for cis-
HONO). The HONO molecule in unstable site is
most probably formed in recombination reaction of
OH and NO radicals, and is a secondary reaction
product. The other secondary reaction products of
HONO photolysis in an argon matrix involve H2O,
HO2, NO2, and N2O3 [9] molecules. All products were
identified by comparison of the studied spectra with
literature data reported for H2O, [17,18] HO2 [19]
NO2 [20,21] and N2O3 [22–24].
2. Experimental
Two different deposition setups were used to
obtain HONO/C2H4/Ar matrices; their detailed
description is given in our earlier report [7].
The C2H4/Ar mixtures were prepared by stan-
dard manometric techniques and their concen-
trations varied in the range 1/800–1/100. The
overall HONO/C2H4/Ar concentrations of the
studied matrices were estimated to be: 1=n=800
(n ¼ 1; 2; 4Þ, 1/1/400 and 1/1/300.
The gas mixtures were sprayed onto a gold-
plated copper mirror held at 20 K by a closed
helium refrigerator (Air Products, Displex 202A).
Infrared spectra were recorded of the matrices
maintained at ca. 12 K. After the infrared spectra of
the initial deposit have been recorded the sample
was subjected to the filtered radiation of a 200 W
medium pressure mercury arc (Philipps CS200W2).
A 5 cm water filter served to reduce the amount of
infrared radiation reaching the matrix and glass
long wavelength pass filter (Zeiss WG 345) was
applied to cut off the radiation with l , 340 nm:
The spectra were recorded at 0.5 cm21 resol-
ution in a reflection mode, with a Bruker 113v
FTIR spectrometer equipped with a liquid N2
cooled MCT detector.
3.1.2. Photolysis and annealing of HONO/C2H4/Ar
matrices
Experiments were performed for HONO/C2H4/Ar
matrices of concentration varying in the range
1/1/800–1/4/800 and, in addition, for matrices of
concentration 1/1/400 and 1/1/300. Three experiments
were also performed for each of the isotopically
substituted systems: HONO/C2D4/Ar and DONO/
C2H4/Ar. The matrices were photolyzed with l .
340 nm at 12 K. The spectra were registered after 10,
20, 30, 60 and 90 min. of matrix irradiation. After the
irradiation process was completed the samples were
annealed to 30 K for 10 min, recooled to 12 K and
NH4NO2 was prepared from (NH4)2SO4 and KNO2
according to Ref. [8]. A source of DONO was
ND4NO2 which was synthesised from (ND4)2SO4
and KNO2. (ND4)2SO4 was obtained by multiple
dissolution and crystallisation of ammonium sulphate
from D2O. C2H4 (99.5%, Aldrich Chemical Co.) and
C2D4 (99% Aldrich Chemical Co.) were used as
supplied by the companies.

A. Olbert-Majkut et al. / Journal of Molecular Structure 656 (2003) 321–332
323
Fig. 1. The n(OH) stretching region in the spectra of the matrices
HONO/C2H4/Ar ø 1/1/400: (a) the spectrum recorded directly after
matrix deposition; (b) after 30 min. photolysis at l . 340 nm and
(c) after matrix annealing to 30 K and recooling to 12 K.
Fig. 2. The n(OD) region in the spectra of matrices DONO/C2H4/-
Ar ø 1/1/400: (a) the spectrum recorded directly after matrix
deposition, (b) after 30 min. photolysis at l . 340 nm and (c) after
matrix annealing.
the spectra of the annealed matrices were recorded.
The spectra of the studied matrices are presented in
Figs. 1–4.
The intensities of the bands due to group IA increased
whereas the intensities of the bands due to group IB
decreased considerably after matrix annealing. The
bands of group II were less sensitive to annealing;
their intensities slightly decreased during annealing
process. Figs. 1–4 present different response of the
bands belonging to groups IA, IB and II on matrix
annealing. The bands due to groups IA and IB were
observed at 3639.6, 3648.0, 1561.9, 1225.6, 1040.0,
In the spectra recorded directly after matrix
deposition the absorptions due to trans- and cis-
HONO isomers, to C2H4 monomers and dimers and to
C2H4· · ·HONO-trans and C2H4· · ·HONO-cis com-
plexes were identified on the basis of literature data
and our earlier studies [5]. During photolysis the
bands due to ethene complexes with nitrous acid have
been diminishing rapidly, and simultaneously a set of
new bands have been growing in the spectra of
irradiated matrices.
770.0 cm21 and at 3657.0, 1555.4, 1048.5 cm21
,
respectively, in the spectra of HONO/C2H4/Ar
matrices. Their counterparts appeared at 3640.3,
3648.6, 1552.8, 1120.6, 1118.8, 1056.7, 887.9,
769.8 cm21 (IA) and at 3657.2, 1554.2, 1151.3,
1020.2 cm21 (IB) in the spectra of HONO/C2D4/Ar
matrices and at 2685.2, 2686.4, 1557.2, 876.8 cm-1
(IA); 2703.7, 2698.5, 1555.5 and 1057.4, 905.8 cm21
(IB) in the spectra of DONO/C2H4/Ar matrices. The
frequencies of the bands due to groups IA, IB and II are
collected in Table 1.
The performed experiments allowed us to charac-
terize three band sets (IA, IB and II) that appear after
irradiation of HONO/C2H4 matrices, in the presence
of C2H4 only, and which respond in different ways to
photolysis and annealing processes. The bands due to
groups IA and IB appeared in the spectra with
reasonable intensities after 10 min. of matrix
irradiation whereas the bands due to group II were
detectable only after 20 min of irradiation.

324
A. Olbert-Majkut et al. / Journal of Molecular Structure 656 (2003) 321–332
Fig. 3. (a, b, c) The NyO stretching region in the spectra of the same
matrices as presented in Fig. 1a–c. (d) The NyO stretching region
in the spectrum of the same matrix as presented in Fig. 2b.
Fig. 4. The C–O stretching region in the spectra of the same
matrices as presented in Fig. 3.
3.2. Computational details and results
conformers (A1–A4). The lowest energy conformer
corresponds to the S1 structure in which three torsion
angles, O1C1C2N, H1O1C1C2 and O2NC2C1, are
269.86, 72.69 and 4.28 corresponding to 2sc, þsc,
sp arrangement. This geometry favors the formation
of the hydrogen bond between the OH group and the
oxygen atom of the NyO group which stabilizes the
S1 conformer. The distance between H1 and O2 atoms
The full conformational search was undertaken by
DFT calculations using Gaussian 98 package of
computers code [25] employing the Becke, Lee,
Yang and Paar density functionals (B3LYP). The
structures of all conformers of 2-nitrosoethanol were
fully optimized using the 6-311þþG(2d,2p) basis set
[26,27].
˚
is equal to 2.36 A. The synclinal conformer, S2, is
The calculations predicted eight stationary points
on the potential energy surface. The conformers are
identified by a scheme based upon Newman projec-
tion and described by the values of the three torsion
angles, O1C1C2N, H1O1C1C2 and O2NC2C1, each of
them denoted as sp (synperiplanar), ap (antiperipla-
nar), sc (synclinal) or ac (anticlinal). Table 2 lists the
predicted geometrical parameters and energies of all
stable conformers of 2-nitrosoethanol molecule. In
Fig. 5 all stable conformers and the adopted atom
numbering system are presented.
computed to be 0.44 kcal/mol less stable than the S1
one. In the S2 conformer a weak hydrogen bond is
formed between the OH group and the nitrogen atom
of the NyO group. The calculated distance between
hydrogen atom of OH group and nitrogen atom is
˚
equal to 2.60 A. The S3 and S4 conformers are not
stabilized by the hydrogen bond and are 0.69 and
1.33 kcal/mol less stable than the S1 conformer.
The A1–A4 conformers showing antiperiplanar
arrangement of the CO and CN bonds are circa
2 kcal/mol less stable than the S1 conformer. Absence
of intramolecular hydrogen bond partially accounts
for the higher energy of these forms. The main
Four conformers that show the synclinal relations
of the CN and CO bonds (see S1–S4 conformers in
Fig. 5) are more stable than the four anticlinal

A. Olbert-Majkut et al. / Journal of Molecular Structure 656 (2003) 321–332
325
Table 1
The observed frequencies (cm21) of the three conformers (A1, A2, S1) of 2-nitrosoethanol molecule
HONO/C2H4
Exp.
DONO/C2H4
Exp.
HONO/C2D4
Exp.
Approximate descriptiona
Calc.
3847
Calc.
2801
A1
3639.6
3648.0
1561.9
1225.6
2685.2
2686.4
1557.2
3640.3
3648.6
1552.8
1120.6
1118.8
1056.7
887.9
n(OH), n(OD)
1640
1249
1640
864
n(NyO)
d(COH), d(COD)
1040.0
1050
892
1051
901
n(C–O) þ gG(CH2), n(C–O)
n(CN) þ d(CNO)
876.8
770.0
A2
769.8
3657.0
3836
1634
1049
3580
2703.7
2698.5
1555.5
905.8
2791
3657.2
n(OH), n(OD)
1555.4
1634
878
1554.2
1151.3
1020.2
n(N ¼ O)
d(COH), d(COD)
n(C–O) þ gG(CH2), n(C–O)
1048.5
S1
1057.4
1050
3580.6
2654.6
2643.1
1549.0
1376.1
898.4(?)
2787
3581.6
1544.3
1091.2
n(OH), n(OD)
1549.0
1383.6
1187.1
1633
1427
1237
1633
1424
844
n(N ¼ O)
d(CH2), d(CD2)
d(COH) þ gw(CH2)/g(CD2)
d(COD)
1019.6
1017.3
1074
1049
n(C–O)
The groups of bands corresponding to the three conformers A1, A2 and S1 are denoted in text as IA, IB and II, respectively.
The abbreviations used for description of CH2 group vibrations were adopted from reference 5.
a
difference between the A1 and A2 conformers is the
of the t(OH) vibration which is the most sensitive one
to hydrogen bond formation; the calculated t(OH)
frequency of S1 conformer is ca. 200 cm21 larger than
those calculated for A1 and A2 conformers.
value of the H1O1C1C2 torsion angle which is equal to
ca. 176 and 728, respectively, for the A1 and A2
conformers. The two conformers are characterized by
very close energy values; the A1 conformer is only ca.
0.02 kcal/mol less stable than the A2 one. The
difference in energy values between these two
conformers grows up to 0.18 kcal/mol when zero
point vibrational energy correction is taken into
account.
4. Discussion
4.1. Identification of the photoproduct—
2-nitrosoethanol
The harmonic vibrational frequencies computed at
the B3LYP levels of theory for the S1, A1 and A2
conformers of the CH2OHCH2NO and CH2ODCH2-
NO molecules are presented in Table 3. The
calculated IR spectra confirm the formation of
intramolecular hydrogen bond in S1 and S2 con-
formers. The n(OH) stretching frequency calculated
for S1 has slightly lower value than those calculated
for A1, A2 conformers. However this is the frequency
The most prominent bands for all three photo-
products (IA, IB and II) were identified in three
spectral regions, namely in the regions of the OH and
NyO stretching vibrations and in the 1250–750 cm21
spectral region in which the bands due to COH
bending þ C–O stretching þ CH2 wagging, rocking
and torsion vibrations appear. In the spectra of
HONO/C2H4/Ar matrices the bands due to OH

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A. Olbert-Majkut et al. / Journal of Molecular Structure 656 (2003) 321–332
stretches of the three photoproducts appeared at
3639.6 cm21 (IA), 3657.0 cm21 (IB) cm-1 and at
3580.6 (II) cm21. There is one additional band in this
region at 3648.0 cm21, whose origin is less clear. Due
to the fact that the band appears immediately after
photolysis and is placed very close to the 3639.6 cm21
IA band it is also assigned to the photoproduct IA
although it shows slightly different behavior after
matrix annealing (smaller increase of intensity) than
the 3639.6 cm21 band. No counterparts of this band
were found in other spectral regions. The deuterium
counterparts of the 3639.6, 3657.0 and 3580.6 cm21
bands are observed, respectively, at 2685.2 cm21 (IA),
2703.7, 2698.5 cm21 (IB) and at 2643.1 cm21 (II) in
the spectra of DONO/C2H4/Ar matrices which proves
their assignment to the OH, OD stretching vibrations.
The second prominent band of each of the three
photoproducts appears in the NyO stretching region.
The bands were identified at 1561.9, 1555.4 and at
1549.0 cm21, respectively, for the IA, IB and II
products in the spectra of HONO/C2H4/Ar matrices.
The characteristic feature of the 1549.0 cm21 band
(II) is its broadness. The three bands show little
sensitivity to deuterium substitution of both nitrous
acid and ethene molecule (Table 1) which suggests
their assignment to the NyO stretching vibration. In
the spectra of trans- and cis-nitrosomethanol mol-
ecule isolated in argon the NyO stretching vibration
was identified at 1555, 1559 cm21, respectively, and
in the spectra of 1-nitrosoethanol in solid argon the
NyO stretch was observed at 1552 cm21 [28,29].
The other characteristic bands of the three photo-
products appeared in the spectral region 1250–
1000 cm21. In the spectra of HONO/C2H4/Ar
matrices the bands due to IA product appeared at
1225.6, 1040.1 cm21 and in addition one band was
observed at 770.0 cm21. Only one band due to IB
product was identified in this region at 1048.5 cm21
and the bands due to the product II were observed at
1187.1, 1019.6, 1017.3 cm21. As one can see in
Table 1 the frequencies of the bands appearing in this
region are sensitive to deuterium substitution of both
C2H4 and HONO molecules.
The fact that all three photoproducts IA, IB, II
exhibit prominent bands due to the OH and NyO
stretching vibrations suggests that the main secondary
product of the photolysis of C2H4· · ·HONO complex
is 2-nitrosoethanol which is formed in double addition
u
u
D

Fig. 5. The optimized structures of the eight stable conformers of 2-nitrosoethanol molecule.

328
A. Olbert-Majkut et al. / Journal of Molecular Structure 656 (2003) 321–332
Table 3
The calculated frequencies and intensities of the experimentally observed conformers of 2-nitrosoethanol
S1
A1
A2
Approximate descriptiona
Freq (cm21
)
I [km/mol]
Freq (cm21
)
I [km/mol]
Freq (cm21
)
I (km/mol)
3829
3091
3063
3031
3025
1633
1494
1427
1414
1390
1347
1237
1183
1074
1026
940
42
16
8
3847
3150
3044
3029
3008
1640
1530
1463
1451
1322
1292
1249
1190
1050
1129
992
40
16
24
10
38
96
2
3836
3113
3092
3031
3024
1634
1516
1468
1407
1388
1315
1259
1185
1049
1094
984
38
21
2
n(OH)
nas (CH2)1
nas (CH2)2
ns (CH2)2
ns (CH2)1
n(NO)
7
23
21
96
2
33
71
4
d (CH2)1
d (CH2)2
38
25
5
8
6
7
49
4
g
w (CH2)1
9
gt (CH2)1
g
w (CH2)2
15
41
4
24
46
3
10
21
11
167
4
d(COH)
gt (CH2)2
n(CO)
gr (CH2)1
d(NCC)
d(ONC)
n(CN)
gr (CH2)2
t (OH)
d(OCC)
86
3
94
7
19
20
13
1
21
14
15
2
4
870
579
586
6
775
892
879
37
7
658
816
817
426
119
4
227
118
6
273
42
1
407
413
407
a
Upper index 1 or 2 indicates that the coordinate of the (CH2)1, (CH2)2 groups of the C1 or C2 atoms (Fig. 5) give the main contribution to the
vibration. The abbreviations used for the description of CH2 group vibrations were adopted from reference 35.
reaction of the OH and NO radicals to C2H4 molecule.
The presence of the bands characteristic of the three
photoproducts in the COH bending and C–O stretch-
ing regions is in accord with this conclusion. The 2-
nitrosoethanol molecule has not been identified so far
and its spectra are not known. However there is report
on the infrared spectra of the cis-nitrosoethanol in
solid argon (the most stable conformer of nitrosoetha-
nol) which is formed in light induced rearrangement
reaction of the nitrosooxyethane [28]. The compari-
son of the spectra of the three photoproducts IA, IB and
II with the spectra of cis-nitrosoethanol excludes the
latter molecule as a possible product of the photolysis
of the C2H4· · ·HONO complex. So, we identify the
three photoproducts, IA, IB and II as the three
conformers of the 2-nitrosoethanol molecule. The
assignment is confirmed by ab initio calculations of
the geometry and vibrational frequencies of all stable
isomers of the 2-nitrosoethanol molecule as will be
discussed below.
4.2. Assignment of the observed bands in terms
of 2-nitrosoethanol conformers
As discussed earlier the calculations predicted eight
stable conformers of 2-nitrosoethanol molecule. The
most stable one is the synclinal conformer S1 with
intramolecular hydrogen bond between OH group and
oxygenatomoftheNOgroup.Thenextonewithrespect
to stability is synclinal conformer S2 (with intramole-
cular hydrogen bond between OH group and nitrogen
atom)whichiscalculatedtobe0.44 kcal/mollessstable
than the S1 conformer. The most stable antiperiplanar
conformer A1 is calculated to be 1.99 kcal/mol less
stable than the S1 conformer but there is very small
energy difference in the stability of the four antiper-
iplanar conformers (Table 2).
The calculations predict six modes of every stable
conformer to give rise to much more intense
absorptions than all the other modes. These modes
can be approximately described as the OH and NyO

A. Olbert-Majkut et al. / Journal of Molecular Structure 656 (2003) 321–332
329
stretching, CH2 deformation, COH in plane bending,
C–O stretching and OH torsion. There are relatively
small differences between the calculated vibrational
spectra of the eight conformers (compare the
vibrational frequencies of the S1, A1 and A2
conformers in Table 3) which is the reason why the
comparison of the calculated frequencies of the eight
conformers with the experimental spectra of the three
photoproducts did not allow us to draw definitive
conclusions about the geometry of the three photo-
products. However, some suggestions concerning the
geometries of the three photoproducts can be made on
the basis of the experimental results.
at 3657.6, 3662.2 cm21, respectively [30]. In the
spectra of 2-nitroethanol, CH2OHCH2NO2, in an
argon matrix the free or trans n(OH) band is at
3660 cm21 and the bonded or gauche n(OH) band
at 3614 cm21 [31]. In turn, in the spectra of the
most stable ethylene glycol conformer isolated in
an argon matrix the free and bonded n(OH) modes
occur at 3667 and 3626 cm21, respectively. [32]
The above data clearly indicate that in the species
II, characterized by the 3580.6 cm21 n(OH) stretch-
ing frequency, the OH group is involved in the
intramolecular hydrogen bond. So, we tentatively
identify the species II as the most stable 2-
nitrosoethanol conformer with intramolecular
hydrogen bond i.e. synclinal conformer S1. The
3580.6 cm21 band is not sensitive to substitution of
C2H4 by C2D4; in the spectra of CH2ODCH2NO
two bands due to product II are observed at 2654.6,
2643.1 cm21. The other bands of species II are not
helpful in determining its conformation. The NyO
stretching vibration occurs at circa 1549 cm21 in
the spectra of both CH2OHCH2NO and CH2
ODCH2NO and is shifted slightly toward lower
frequencies, to 1544.3 cm21, in the spectra of
CD2OHCD2NO. The absorption observed at
1383.6 cm21 in the spectra of CH2OHCH2NO (II)
is shifted to 1376.1 cm21 in the spectra of
CH2ODCH2NO (II) which suggests its assignment
to CH2 deformation mode. The 1187.1 cm21 band
characteristic for CH2OHCH2NO (II) is shifted to
1091.2, 898.4 cm21 in the spectra of CD2OHCD2
NO, CH2ODCH2NO (II), respectively. The sensi-
tivity of the 1187.1 cm21 absorption to deuterium
substitution of both CH2 and OH groups indicate
that the coupled CH2 rocking and torsion and COH
bending vibration may give rise to its intensity. The
contribution of the C–O stretching cannot be
excluded. The doublet observed at 1019.6,
1017.3 cm21 is tentatively assigned to the CO
stretching mode.
First, the obtained experimental data indicate
that the IA, IB and II products correspond to three
different conformers and are not due to one
conformer trapped in various matrix sites. The
frequencies belonging to IA, IB and II products
show relatively large differences in isotopic shifts
after deuterium substitution of ethene or nitrous
acid molecules which is not expected for a
molecule trapped in sites of different geometries.
For example the band due to the NyO stretching
mode appears at 1561.9 (IA), 1555.4 (IB) and at
1549.5 cm-1 (II) in the spectra of irradiated
HONO/C2H4/Ar matrices and is shifted, respect-
ively, 9.1, 1.2, 5.2 cm21 toward lower frequencies
when C2H4 is replaced by C2D4 (Table 1). Site
effect cannot account for such, relatively large
differences, in isotopic shifts of the NyO stretching
absorption. The three products exhibit also slightly
different spectral pattern in the 1100–1000 cm21
region (Fig. 4, Table 1) which also indicates that
they correspond to different conformers.
Some information on the conformers produced
by the photochemical reaction is provided by the
n(OH) stretching frequencies of the three species.
For species II the n(OH) stretching frequency is
equal to 3580.6 cm21 which suggests that the OH
group in species II is involved in intramolecular
hydrogen bond. The OH stretching frequencies of
the nonbonded, free OH group in the spectra of
ethanol and its derivatives occur above 3600 cm21
whereas those of the bonded OH groups may
appear above or below 3600 cm21 depending on
the strength of interaction. For example in the
spectra of trans and gauche ethanol isolated in an
argon matrix the n(OH) stretching vibration appears
The frequency values of the OH stretches of the
other two photoproducts, (3639.6, IA) and
3657.0 cm21, IB), suggest that the corresponding
vibrations are due to the conformers with free,
nonbonded OH groups. Additional information
concerning their geometry is provided by matrix
annealing; the bands due to the product IA increase
whereas the bands due to IB product strongly

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A. Olbert-Majkut et al. / Journal of Molecular Structure 656 (2003) 321–332
decrease when the matrix is annealed to
30 K. Absence of intramolecular hydrogen bond
suggests that the species characterized by the bands
belonging to groups IA, IB have antiperiplanar
structure A. The four A conformers are calculated
to be within 1:99 2 2:47 kcal=mol less stable than
the most stable conformer S1. It is well known that
photochemical reactions carried out in low tem-
perature matrices may lead to population of less
stable reaction intermediates and products. For
example the irradiation of the CH2O· · ·HNO
complex may lead either to formation of less
stable trans conformer of nitrosomethanol or more
stable cis conformer depending on the wavelength
of radiation. [28] The two photoproducts IA and IB
are tentatively assigned to two antiperiplanar
conformers A1 and A2. The calculations predict
very small energy difference between these two
conformers which is equal to 0.18 kcal/mol when
zero point vibrational energy correction is taken
into account. The conformer A2 is converted into
A1 by rotating OH group around C–O bond which
requires small activation energy. This fact might
explain why the bands due to IB conformer
decrease whereas the bands due to IA increase
after matrix annealing.
of C2H4 and OH groups indicates that CH2 group
coordinates (rocking and/or torsion) and COH
bending also contribute to the mode giving rise to
this band.
The bands due to IA species which is identified
tentatively as A1 conformer appear in the vicinity
of the absorptions due to IB (except the lowest
frequency band at ca. 770 cm21) and are assigned
to the same modes. So, the 3639.6, 1561.9 and
1040.0 cm21 bands observed in the spectra of
CH2OHCH2NO, IA are assigned, respectively, to
the OH and NyO stretching and to the coupled C–
O stretching and CH2 deformation coordinates
(rocking and/or torsion). The 1225.6 cm21 band
appearing in the spectra of CH2OHCH2NO has its
counterparts at 1120.6, 1118.8 cm21 in the spectra
of CD2OHCD2NO; no counterpart of this band was
found in the spectra of CH2ODCH2NO. The
1120.6, 118.8 cm21 (IA) bands lie in the vicinity
of the 1151.3 cm21 (IB) band and are also assigned
to the COH bending mode to which may contribute
CH2 rocking and/or torsion coordinates. The band
observed at 887.9 cm21 in the spectra of CD2OH-
CD2NO is tentatively assigned to the coupled C–N
stretching and CNO bending coordinates.
The 3657.0 cm21 band observed for the CH2
OHCH2NO, IB molecule is not sensitive to
substitution of C2H4 by C2D4 and is shifted to
2698.5 cm21, with isotopic shift ratio 1.35, when
OH group is replaced by OD. The NyO stretching
vibration is identified at 1555.4 cm21 for CH2
OHCH2NO and at 1554.2 cm21 for CD2OHCD2
NO. The counterpart of the 1151.3 cm21 absorption
characteristic for the CD2OHCD2NO species was
found to appear at 905.8 cm21 in the spectra of
CH2ODCH2NO, however no counterpart of this
band was found in the spectra of CH2OHCH2NO.
The large sensitivity of this band to substitution of
OH by OD suggests its assignment to COH
bending vibration. The CH2 rocking and torsion
may also contribute to this mode. The 1048.5 cm21
band observed in the spectra of hydrogenated
molecule is shifted to 1020.2, 1057.4 cm21 in the
spectra of CD2OHCD2NO, CH2ODCH2NO mol-
ecules, respectively. The band appears in the region
characteristic for C–O stretching vibration but its
relatively strong sensitivity to deuterium substitution
4.3. Matrix chemistry
The obtained spectra prove that exposure of the
C2H4· · ·HONO complex trapped in a matrix cage to
l . 340 nm ðE , 84 kcal=molÞ radiation leads to
formation of 2-nitrosoethanol molecule. The pri-
mary
photodissociation
products
of
the
C2H4· · ·HONO complex are OH, NO radicals and
C2H4 molecule. As was already discussed in our
earlier paper [7] the OH and NO radicals are
generated in their ground electronic states with
some excess of vibrational and rotational energy.
Both radicals add to the double bond of ethylene to
form 2-nitrosoethanol. It is interesting to compare
the photochemistry of the C2H4· · ·HONO and
CO· · ·HONO systems, the latter one have been
studied recently [7]. The photolysis of the
CO· · ·HONO complex trapped in a matrix cage
leads to formation of trans and cis-HOCO radicals
(single addition products) and trans and cis-
HOC(O)NO molecules (double addition products).

A. Olbert-Majkut et al. / Journal of Molecular Structure 656 (2003) 321–332
331
The relative yield of the HOCO radicals and
Acknowledgements
HOC(O)NO molecules was dependent on the
conditions of the experiment (temperature of
deposition, matrix temperature during photolysis
process) however both species were always
detected. In contrast with the CO· · ·HONO system
after photolysis of the C2H4· · ·HONO complex only
2-nitrosoethanol molecule, which is the product of
double addition reaction, was detected. The spectra
of 2-hydroxyethyl radical are well known, [3,4] no
band characteristic for this radical was detected in
our spectra. There are two possible reasons which
could explain different behavior of these two
systems. First, the 2-hydroxyethyl radical, CH2
OHCH2, is more reactive than hydroxy formyl
radical, HOCO, so, once it is formed it reacts
readily with the NO radical present in the same
matrix cage to form 2-nitrosoethanol molecule.
Second, one perhaps should take into account the
possibility that the C2H4· · ·HONO complex does
not dissociate completely and as a transition state a
cyclic intermediate is formed.
The authors gratefully acknowledge financial
support from the Polish State Committee for Scientific
Research (grant KBN no 5439611). A.O.M. gratefully
acknowledges the Wrocław Supercomputer Center for
providing the computer time and facilities.
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