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The HOO complexes with N2 and CO in argon matrices
Chemical Physics 262 (2000) 445±452
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The HOO complexes with N2 and CO in argon matrices Thomas Svensson, Bengt Nelander * Chemical Physics, Chemical Center, P.O. Box 124, S-22100 Lund, Sweden Received 24 July 2000
Abstract Complexes of the peroxy radical with N2 and CO have been investigated with FTIR spectroscopy. The complexes were produced by the addition of hydrogen atoms to a mixture of O2 and N2 or CO on the surface of growing argon matrices. Their spectra were identi®ed from concentration dependencies, H to D shifts and by photolysis experiments. Ó 2000 Elsevier Science B.V. All rights reserved.
1. Introduction Combinations of theory and experiments have made it possible to develop models, which allow calculations of intermolecular potentials for interacting pairs of closed shell molecules [1]. Considerably less is known about the interactions between radicals and closed shell molecules. Such interactions govern the relaxation of nascent, hot radicals and may modify their reactivity in solution. The odd electron may make the polarizability of radicals larger than that of closed shell molecules. Since calculations of model potential functions are often based on estimates of electrostatic, polarization and exchange repulsion contributions, the balance between the dierent types of interaction may be dierent when radicals are involved. It is therefore desirable to test interaction potential models on closed shell molecule-free radical interactions, to ®nd if they work also in
*
Corresponding author. Fax: +46-2224119. E-mail address: [email protected] (B. Nelander).
these cases. Very few experimental investigations of radical interactions exist. This paper extends an earlier, preliminary study of the interactions of the peroxy radical [2]. The HOO radical is involved in catalytic ozone depletion cycles in the stratosphere and in oxidizing processes in the troposphere [3]. The complexes between the peroxy radical and water, hydrogen chloride, chlorine and ammonia have been investigated in an earlier study [2]. The results indicated that the peroxy radical can act both as a hydrogen bond donor and as a hydrogen bond acceptor. The water peroxy radical complex was found to be cyclic in Ref. [2] in agreement with ab initio calculations of Aloisio and Francisco [4]. The binding energies of the complexes of the hydroperoxyl radical with formaldehyde, acetaldehyde and acetone are calculated to be about twice as large compared with the peroxy radical water complex [5]. N2 and CO are present in the atmosphere and therefore a study of the HOO complex with these species is of substantial interest. A few studies of the peroxy radical have been made [2,6,7]. Calculations have shown that CO and peroxy radicals do not react to form CO2 and OH
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T. Svensson, B. Nelander / Chemical Physics 262 (2000) 445±452
[8]. Our experiments show that the OC±HOO and N2 ±HOO complexes are formed at cryogenic temperatures. Small but varying amounts of H2 O and CO are always formed in the experiments, and we have therefore also studied the water±CO complex. Lundell and Rasanen [9] prepared matrix isolated water carbon monoxide complexes by photolysis of formic acid. They combined FTIR spectra and ab initio calculations to study their structure. The calculations showed that the HOH±CO isomer is the stable form of the complex, as was also found in earlier calculations. In argon matrices this was also the dominating isomer, but in xenon matrices signi®cant amounts of HOH±OC were observed. The HOH±CO complex has also been studied in argon matrices by codeposition of Ar/CO and Ar/ water mixtures by Givan et al. [10]. 2. Experimental section Hydrogen (AGA), deuterium (LÕAir liquide, 99.7% D), O2 (LÕAir Liquide 99.9995%), N2 (LÕAir Liquide 99.9995%), CO (Aga 4.7), Ar (LÕAir Liquide 99.9995%) were used as delivered. Deionized water was degassed before use. Gas mixtures were prepared by standard manometric techniques. In one volume a mixture of argon and oxygen and nitrogen or carbon monoxide was prepared. The Ar/O2 ratio was varied between 38 and 75 and the ratios Ar/N2 or Ar/CO between 150 and 750. From a second volume, a H2 /Ar or D2 /Ar mixture was passed through a microwave discharge in a quartz tube, excited by a microwave generator (Opthos MPG 4). The Ar/H2 and Ar/D2 ratios varied between 38 and 75. We did not observe any eect of H or D atoms on CO or N2 . In a few experiments water was added to CO. Then two volumes were prepared, one with Ar and water and a second with Ar and CO. The Ar/water and Ar/CO ratios were kept at 75 and 150 respectively. Nupro needle valves were used to regulate the ¯ows from the two volumes. The ¯ow rate was 10 mmol hÿ1 and the deposition time was 2 h. The matrices were deposited on a CsI window at 17 K, cooled by a Leybold RDK 10-320 closed cycle cryocooler. In some of the experiments the matrix
was irradiated with 266 nm radiation from a quadrupled Continuum NY 20 C YAG laser for 30 min. The N2 and CO peroxy radical complexes and free peroxy radicals were partially decomposed during the irradiation. Infrared spectra were recorded between 500 and 4000 cmÿ1 at 0.5 cmÿ1 resolution with a Bruker 113v FTIR spectrometer. 3. Assignment Nomenclature: In complexes of the type studied here, the intramolecular vibrations of the complex components retain their original character in the complex. Therefore, the perturbed ith fundamental of A in a complex with B will be denoted as mi (A± B). When the isotopic composition of B is immaterial, the peroxy radical will be denoted as perox and water as aq. 3.1. HOH±CO complex The method used to prepare peroxy radicals always produces signi®cant amounts of water [2]. Carbon monoxide tend to be present in small but varying amounts in experiments where part of the matrix gas has passed through a discharge. It was therefore necessary to study the water complex of carbon monoxide. The carbon monoxide complex in argon matrices has been studied by several groups [9,10] and we observe the same bands as they do. The results are collected in Table 1. The CO stretch of free CO was observed at 2138.6 cmÿ1 , with a shoulder at 2136.8 cmÿ1 due to (CO)2 [10±12]. The band at 2149.6 cmÿ1 with a satellite at 2148.8 cmÿ1 has been assigned to m(OC± HOH) [9,10]. The corresponding band of OC± DOD was observed at 2150.2 cmÿ1 [9,10]. The bands in the CO stretch region are shown in Fig. 1. In the H2 O m2 region, three new bands were observed, at 1596.0, 1605.5 (strongest) and at 1617.0 cmÿ1 . The band at 1596.0 cmÿ1 has been assigned to m2 (HOH±CO) [10]. It is disturbed by the formation of H2 O polymers close to this band [10]. The bands at 1605.5 and 1617.0 cmÿ1 have been assigned to (H2 O)2 ±CO [10]. Two signi®cant strong peaks in the H2 O m3 stretch region were
T. Svensson, B. Nelander / Chemical Physics 262 (2000) 445±452
447
Table 1 Observed bands of the complexes of water with CO in solid argon (cmÿ1 )
a
H2 Oa
D2 Oa
CO
HOH±CO
DOD±CO
H2 O(D2 O), m1
3638
2657.7
±
H2 O(D2 O), m2 H2 O(D2 O), m3
1589.1 3734.3
1174.6 2771.1
± ±
3622.8 3628.0 1596.0 3723.8
CO
±
±
2138.6
2149.6
2646.0 2650.0 1185.0 2753.3 2762.5 2150.2
Ref. [24].
Fig. 1. The CO stretching region: x-axis, cmÿ1 ; upper curve, an experiment with HOO; lower curve, with DOO. The curves have been shifted vertically for clarity. I: m(OC±HOO), II: m(OC±DOO), III: m(CO±HOO), IV: m(CO±DOO), V: CO, m, VI: m(OC±HOH), VII: m(OC±DOD).
observed at 3711.5 and 3723.8 cmÿ1 . The peak at 3723.8 cmÿ1 (strongest) was assigned to H2 O±CO and the peak at 3711.5 cmÿ1 to (H2 O)2 ±CO in Ref. [10]. 3.2. Peroxy radical complexes of N2 or CO The interaction between the peroxy radical and N2 or CO induced new strong bands near the vibrational fundamentals of HOO and CO or N2 . The bands were assigned to the peroxy radical complexes of N2 or CO. They are collected in Table 2. Traces of N2 and CO are normally present
in matrices and the bands due to their peroxy radical complexes had been observed as very weak satellites to the peroxy radical fundamentals in earlier experiments. Their intensities increased very dramatically when N2 or CO were deliberately added to the matrix. The free peroxy radical and the peroxy radical complexes of N2 and CO are all decomposed by 266 nm radiation. For a given radiation time, the decrease of the absorption bands was largest for the peroxy radical±CO complex and smallest for the peroxy radical±N2 complex. The free peroxy radical was decomposed at an intermediate rate.
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Table 2 Observed bands of the complexes of the peroxy radical with N2 and CO (cmÿ1 ) HOO, m1 HOO, m2 HOO, m3 HOO, 2m2 N2 CO, m HCl, m
HOOa
CO
HClb
N2 ±HOO
OC±HOO
HOO±HClb
3413.0 1388.9 1100.9 ± ± ± ±
± ± ± ± ± 2138.6 ±
± ± ± ± ± ± 2870.8
3408.6 1404.8 1104.7 2782.0 2331.6 ± ±
3342.5 1423.8 1108.3 2817.2 ± 2160.6 ±
3306.0 1425.2 1117.0 ± ± ± 2677.4
N2 ±DOO
OC±DOO
DOO±HClb
2524.9 1031.0 1127.4 2045.2 2332.0 ± ±
2477.2 1043.0 1132.2 2068.3 ± 2161.1 ±
2450.8 1053.4 ± ± ± ± 2682.0
DOOa DOO, m1 DOO, m2 DOO, m3 DOO, 2m2 N2 , m CO, m HCl, m a b
2530.2 1020.3 1122.9 ± ± ± ±
± ± ± ± ± 2138.6 ±
± ± ± ± ± ± 2870.8
Ref. [7]. Ref. [2].
Fig. 2. The HOO and DOO m1 stretching regions: x-axis, cmÿ1 ; upper curves, N2 added; lower curves, CO added; the curves have been shifted vertically for clarity. (a) I: m1 (OOH±N2 ), II: m1 (OOH±CO), III: HOO, m1 . The peak at approximately 3430 cmÿ1 is due to a complex between HOO and H2 O2 [26]. The peak at approximately 3360 cmÿ1 is due to HOOO [27]. (b) I: m1 (OOD±N2 ), II: m1 (OOD± CO), III: DOO, m1 . The peak at approximately 2540 cmÿ1 is due to a complex between DOO and H2 O2 [26].
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Fig. 3. The OH and OD bending regions: x-axis, cmÿ1 ; upper curves, N2 added; lower curves, CO added; the curves have been shifted vertically for clarity. (a) I: m2 (OOH±N2 ), II: m2 (OOH±CO), III: HOO, m2 . (b) I: m2 (OOD±N2 ), II: m2 (OOD±CO), III: DOO, m2 . The peak close to 1039 cmÿ1 is due to ozone.
3.2.1. Peroxy radical complex of N2 The HO and DO stretching regions (Fig. 2). When N2 was added to HOO, a new intense band was observed at 3408.6 cmÿ1 which shifted to 2524.9 cmÿ1 with DOO. The band is assigned to m1 (OOH±N2 ). The HOO and DOO bending regions (Fig. 3). A new intense band was observed at 1404.8 cmÿ1 when N2 was added to HOO. It was assigned to m2 (OOH±N2 ). With DOO it was shifted to 1031.0 cmÿ1 . When the Ar/N2 ratio was increased, a band at 1408.6 cmÿ1 grew too rapidly for a 1:1 complex in H experiments. A corresponding band in D experiments was observed at 1036.7 cmÿ1 . We assign these bands to m2 (OOH±(N2 )m ) and m2 (OOD± (N2 )m ) (m P 2) respectively. The HOO and DOO m3 regions. A new intense band, blue shifted 3.8 cmÿ1 from HOO, was observed at 1104.7 cmÿ1 . It is assigned to m3 (OOH± (N2 )m ). With DOO the band was observed at 1127.4 cmÿ1 , blue shifted 4.5 cmÿ1 from the band of free DOO.
The N2 stretch region. m(N2 ±HOO) was observed at 2331.6 cmÿ1 and m(N2 ±DOO) at 2332.0 cmÿ1 . 3.2.2. Peroxy radical complex of CO The HO and DO stretching regions (Fig. 2). When CO was added to HOO, a new intense band was observed at 3342.5 cmÿ1 which shifted to 2477.2 cmÿ1 with DOO. The band is assigned to m1 (OOH±CO). The HOO and DOO bending regions (Fig. 3). In experiments with H atoms a new intense band was observed at 1423.8 cmÿ1 , with a small satellite at 1429.2 cmÿ1 , this band is assigned to m2 (OOH± CO). The corresponding band in experiments with D atoms was observed at 1043.0 cmÿ1 (small satellite at 1047.5 cmÿ1 ). The HOO and DOO m3 regions. In experiments with H atoms a new intense band was observed at 1108.3 cmÿ1 (m2 (OOH±CO)) with a small satellite at 1110.3 cmÿ1 , which shifted to 1132.2 cmÿ1 (m2 (OOD±CO)) in experiments with D atoms.
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T. Svensson, B. Nelander / Chemical Physics 262 (2000) 445±452
The CO stretch region (Fig. 1). An intense new band was observed at 2160.6 cmÿ1 in experiments with HOO, with a small satellite at 2161.4 cmÿ1 . The band was shifted from 0.5 cmÿ1 to 2161.1 cmÿ1 , with a small satellite at 2161.9 cmÿ1 , in experiments with DOO. We assign this band to the CO stretch in the complex between the peroxy radical and CO. A band at 2143.5 cmÿ1 has been assigned to a complex between CO and CO2 [11,12]. In our experiments this band is expected to be present, but rather weak since in CO experiments, CO2 is present as an impurity and the CO2 concentration is low. Nevertheless we observed a band here in OC±HOO and OC±DOO experiments. This band was to a large extent eliminated by 266 nm radiation, showing that it was not due to a CO±CO2 complex. We may have a contribution from the CO±CO2 complex but we believe that the major part of the absorption is due to a peroxy radical±CO complex. We tentatively assign the band to the CO±HOO (CO±DOO) complex, noting that Lundell et al [13]. have calculated a 15 cmÿ1 smaller CO shift for the CO±HOH complex, compared to the OC±HOH complex. We assign a weak band at 2140.7 cmÿ1 to m(OC±O3 ) [14]. 4. Discussion HOO and HCl are both expected to form hydrogen bonded complexes with proton acceptors. Structural data is available for a number of HCl complexes, it therefore seems of interest to com-
pare the frequency shifts of a set of HCl and HOO complexes (Table 3). HCl forms a linear complex with N2 [15]. The HCl stretch is shifted ÿ7:3 cmÿ1 in the complex. The OH stretching fundamental of HOO in its N2 complex is shifted ÿ4:4 cmÿ1 . The observation of a H to D shift of the NBN stretching vibration of the peroxy radical nitrogen complex clearly indicates that HOO forms a hydrogen bond to nitrogen. If we assume that the N2 ±HOO complex has an approximately linear hydrogen bond, with no signi®cant interaction between the terminal oxygen of the peroxy radical and the N2 molecule, this suggests that the peroxy radical forms somewhat weaker hydrogen bonds than HCl. Similarly the shifts of the HCl and OH fundamentals in the ammonia complexes are ÿ1499 cmÿ1 [16] and ÿ758:8 cmÿ1 [17] respectively. The back bonding from an ammonia hydrogen to the terminal oxygen of the peroxy radical is expected to be quite weak, and also in this case, the HCl shift is larger than the OH shift. We note that the shift of the OH stretching fundamental of HOO bound to nitrogen is slightly smaller than the OD shift of DOO bound to nitrogen. In normal cases, the ratio between the OH and OD shifts are approximately 1.33 as is observed for the CO and HCl complexes. The reason for the small shift is probably the extreme weakness of the complex. The observed shift is a vibrational average over the intermolecular vibrations. One of the intermolecular fundamentals of a peroxy radical complex is a libration of HOO (DOO) around the OO-axis. This libration
Table 3 Shifts of some selected HCl and HOO complexes (cmÿ1 ) HCl, m HOO, m1 HOO, m2 HOO, m3 OC, m NH3 , m2 a
N2 ±HCla
N2 ±HOOb
OC±HCla
OC±HOOb
(HCl)2 c
HOO±HCla
ClH±NH3 d
OOH±NH3 e
ÿ7.3 ± ± ± ± ±
± ÿ4.4 15.9 3.8 ± ±
ÿ55.5 ± ± ± 16.0 ±
± ÿ70.5 34.9 7.4 22.0 ±
ÿ52.8 ± ± ± ± ±
ÿ193.4 ÿ107 36.3 16.1 ± ±
ÿ1499 ± ± ± ± ±
± ÿ758.8 174.1 26 ± 120.5
Ref. [19]. Taken from Table 2. c Ref. [25]. d Ref. [16]. e Ref. [2]. b
T. Svensson, B. Nelander / Chemical Physics 262 (2000) 445±452
has a signi®cantly larger amplitude for the HOO complex than for the DOO complex. For the nitrogen complex the decrease of the shift due to vibrational averaging outweighs the mass shift. We note that for the N2 ±HCl complex, the ratio between the H to D shifts is 1.19 [18] while for the OC±HCl complex it is 1.34 [19]. In the gas phase HCl forms a linear complex with CO, with a hydrogen bond to the carbon atom [20]. An H to D shift of the CO fundamental of the peroxy radical complex indicates that the peroxy radical forms a hydrogen bond to CO. If we accept that the shift of the OH stretch of HOO is smaller than the shift of the HCl stretch, when they form complexes with the same compound and in the absence of additional interactions, we are forced to conclude that the structure of the HOO± CO complex diers from a simple linear hydrogen bonded structure, since the OH shift of this complex is 1.5 times more negative than the corresponding HCl shift. The shift of the CO stretch fundamental of the OC±HOO complex is 22.0 cmÿ1 and the corresponding shift of OC±HCl is 16.0 cmÿ1 , supporting the conclusion that the interaction energy is larger for the OC±HOO complex than for the OC±HCl complex. Since HOO is non-linear it can increase the interaction between its terminal oxygen and the CO oxygen signi®cantly without a serious distortion of the hydrogen bond. It should be noted that the stretching force constants for the relative motion of the centers of mass of the HCl complexes are 2.55 Nmÿ1 for the NN±HCl and 3.88 Nmÿ1 for OC±HCl [21]. The ratio of these is much smaller than the ratio of the HCl shifts. Also the estimated interaction energies ÿ0.00356 and ÿ0.00424 a.u. for the NN±HCl and OC±HCl complexes respectively are much closer than the HCl shifts for the two complexes [22]. This indicates that the correlation between the HCl complex shift and the dissociation energy of a complex is non-linear in this case. However the fact that the OH shift of OOH±CO is larger than the HCl shift of ClH±CO is still an indication of a dierence in complex structure. The comparison between the hydrogen chloride dimer and the HCl±HOO complex suggests the presence of an additional interaction in the peroxy radical complex. The HOO shift is larger than the
451
HCl shift of the proton donor of the dimer showing the presence of a hydrogen bond from HOO to HCl. Likewise the HCl stretch in the HCl±HOO complex is strongly shifted, almost as much as in the H2 O±HCl complex. Therefore a hydrogen bond from HCl to HOO must exist. The comparison between the water complexes of HOO and HCl suggests the need for a more quantitative theory. The water±HCl complex is known to be eectively Y shaped planar and with a low barrier separating two pyramidal energy minimum structures [23]. The water peroxy radical complex is cyclic [2,4]. Still, the HCl shift is larger than the HO shift. A possible explanation is that water forms relatively strong hydrogen bonds and therefore the complex may gain energy by deforming the hydrogen bond from the peroxy radical in order to strengthen the hydrogen bond from water. The resulting structure would then be a compromise between the two competing bonding requirements.
References [1] P.O. Astrand, G. Karlstr om, A. Engdahl, B. Nelander, J. Chem. Phys. 102 (1995) 3534. [2] B. Nelander, J. Phys. Chem. 101 (1997) 9092. [3] R.P. Wayne, Chemistry of the atmospheres, second ed., Oxford Science Publications, Oxford, 1991. [4] S. Aloisio, J.S. Francisco, J. Phys. Chem. A 102 (1998) 1899. [5] S. Aloisio, J.S. Francisco, J. Phys. Chem. A 104 (2000) 3211. [6] D.E. Milligan, M.E. Jacox, J. Chem. Phys. 38 (1963) 2627. [7] D.W. Smith, L. Andrews, J. Chem. Phys. 60 (1974) 81. [8] T.L. Allen, W.H. Fink, D.H. Volman, J. Photochem. Photobiol. 85 (1994) 201±205. [9] J. Lundell, M. R as anen, J. Chem. Phys. 99 (1995) 14301. [10] A. Givan, A. Loewenschuss, C.J. Nielsen, J. Chem. Soc. 92 (1996) 4927. [11] J. Lundell, M. R as anen, J. Chem. Phys. 97 (1993) 9657. [12] V. Raducu, D. Jasmin, R. Dahoo, P. Brosset, B. GauthierRoy, L. Abouaf-Marguin, J. Chem. Phys. 102 (1995) 9235. [13] J. Lundell, M. R as anen, J. Phys. Chem. 99 (1995) 14301. [14] V. Raducu, D. Jasmin, R. Dahoo, P. Brossel, B. GauthierRoy, L. Abouaf-Marguin, J. Chem. Phys. 101 (3) (1994) 1878. [15] R.S. Altman, M.D. Marshall, W. Klemperer, J. Chem. Phys. 79 (1) (1983) 57. [16] A.J. Barnes, T.R. Beech, Z. Mielke, J. Chem. Soc. Faraday 2 80 (1984) 455.
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[17] A. Engdahl, B. Nelander, Chem. Phys. 249 (1999) 215. [18] L. Andrews, R.D. Hunt, J. Chem. Phys. 89 (1988) 3502. [19] J.P. Perchard, J. Cipriani, B. Silvi, D. Maillard, J. Mol. Struct. 100 (1983) 317. [20] P.D. Soper, A.C. Legon, W.H. Flygare, J. Chem. Phys. 74 (4) (1981) 2138. [21] Z. Kisiel, Pszcz olkowski, P.W. Fowler, A.C. Legon, Chem. Phys. Lett. 267 (3,4) (1997) 202.
[22] A.D. Buckingham, P.W. Fowler, Can. J. Chem. 63 (7) (1985) 2018. [23] A.C. Legon, L.C. Willoughby, Chem. Phys. Lett. 95 (1983) 449. [24] A. Engdahl, B. Nelander, J. Mol. Struct. 193 (1989) 101. [25] L. Andrews, R.T. Arlinghaus, G.L. Johnson, J. Chem. Phys. 78 (11) (1983) 6353. [26] B. Nelander, A. Engdahl, in preparation. [27] B. Nelander, A. Engdahl, T. Svensson, in preparation.
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The HOO complexes with N2 and CO in argon matrices Thomas Svensson, Bengt Nelander * Chemical Physics, Chemical Center, P.O. Box 124, S-22100 Lund, Sweden Received 24 July 2000
Abstract Complexes of the peroxy radical with N2 and CO have been investigated with FTIR spectroscopy. The complexes were produced by the addition of hydrogen atoms to a mixture of O2 and N2 or CO on the surface of growing argon matrices. Their spectra were identi®ed from concentration dependencies, H to D shifts and by photolysis experiments. Ó 2000 Elsevier Science B.V. All rights reserved.
1. Introduction Combinations of theory and experiments have made it possible to develop models, which allow calculations of intermolecular potentials for interacting pairs of closed shell molecules [1]. Considerably less is known about the interactions between radicals and closed shell molecules. Such interactions govern the relaxation of nascent, hot radicals and may modify their reactivity in solution. The odd electron may make the polarizability of radicals larger than that of closed shell molecules. Since calculations of model potential functions are often based on estimates of electrostatic, polarization and exchange repulsion contributions, the balance between the dierent types of interaction may be dierent when radicals are involved. It is therefore desirable to test interaction potential models on closed shell molecule-free radical interactions, to ®nd if they work also in
*
Corresponding author. Fax: +46-2224119. E-mail address: [email protected] (B. Nelander).
these cases. Very few experimental investigations of radical interactions exist. This paper extends an earlier, preliminary study of the interactions of the peroxy radical [2]. The HOO radical is involved in catalytic ozone depletion cycles in the stratosphere and in oxidizing processes in the troposphere [3]. The complexes between the peroxy radical and water, hydrogen chloride, chlorine and ammonia have been investigated in an earlier study [2]. The results indicated that the peroxy radical can act both as a hydrogen bond donor and as a hydrogen bond acceptor. The water peroxy radical complex was found to be cyclic in Ref. [2] in agreement with ab initio calculations of Aloisio and Francisco [4]. The binding energies of the complexes of the hydroperoxyl radical with formaldehyde, acetaldehyde and acetone are calculated to be about twice as large compared with the peroxy radical water complex [5]. N2 and CO are present in the atmosphere and therefore a study of the HOO complex with these species is of substantial interest. A few studies of the peroxy radical have been made [2,6,7]. Calculations have shown that CO and peroxy radicals do not react to form CO2 and OH
0301-0104/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 1 - 0 1 0 4 ( 0 0 ) 0 0 3 3 2 - 3
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T. Svensson, B. Nelander / Chemical Physics 262 (2000) 445±452
[8]. Our experiments show that the OC±HOO and N2 ±HOO complexes are formed at cryogenic temperatures. Small but varying amounts of H2 O and CO are always formed in the experiments, and we have therefore also studied the water±CO complex. Lundell and Rasanen [9] prepared matrix isolated water carbon monoxide complexes by photolysis of formic acid. They combined FTIR spectra and ab initio calculations to study their structure. The calculations showed that the HOH±CO isomer is the stable form of the complex, as was also found in earlier calculations. In argon matrices this was also the dominating isomer, but in xenon matrices signi®cant amounts of HOH±OC were observed. The HOH±CO complex has also been studied in argon matrices by codeposition of Ar/CO and Ar/ water mixtures by Givan et al. [10]. 2. Experimental section Hydrogen (AGA), deuterium (LÕAir liquide, 99.7% D), O2 (LÕAir Liquide 99.9995%), N2 (LÕAir Liquide 99.9995%), CO (Aga 4.7), Ar (LÕAir Liquide 99.9995%) were used as delivered. Deionized water was degassed before use. Gas mixtures were prepared by standard manometric techniques. In one volume a mixture of argon and oxygen and nitrogen or carbon monoxide was prepared. The Ar/O2 ratio was varied between 38 and 75 and the ratios Ar/N2 or Ar/CO between 150 and 750. From a second volume, a H2 /Ar or D2 /Ar mixture was passed through a microwave discharge in a quartz tube, excited by a microwave generator (Opthos MPG 4). The Ar/H2 and Ar/D2 ratios varied between 38 and 75. We did not observe any eect of H or D atoms on CO or N2 . In a few experiments water was added to CO. Then two volumes were prepared, one with Ar and water and a second with Ar and CO. The Ar/water and Ar/CO ratios were kept at 75 and 150 respectively. Nupro needle valves were used to regulate the ¯ows from the two volumes. The ¯ow rate was 10 mmol hÿ1 and the deposition time was 2 h. The matrices were deposited on a CsI window at 17 K, cooled by a Leybold RDK 10-320 closed cycle cryocooler. In some of the experiments the matrix
was irradiated with 266 nm radiation from a quadrupled Continuum NY 20 C YAG laser for 30 min. The N2 and CO peroxy radical complexes and free peroxy radicals were partially decomposed during the irradiation. Infrared spectra were recorded between 500 and 4000 cmÿ1 at 0.5 cmÿ1 resolution with a Bruker 113v FTIR spectrometer. 3. Assignment Nomenclature: In complexes of the type studied here, the intramolecular vibrations of the complex components retain their original character in the complex. Therefore, the perturbed ith fundamental of A in a complex with B will be denoted as mi (A± B). When the isotopic composition of B is immaterial, the peroxy radical will be denoted as perox and water as aq. 3.1. HOH±CO complex The method used to prepare peroxy radicals always produces signi®cant amounts of water [2]. Carbon monoxide tend to be present in small but varying amounts in experiments where part of the matrix gas has passed through a discharge. It was therefore necessary to study the water complex of carbon monoxide. The carbon monoxide complex in argon matrices has been studied by several groups [9,10] and we observe the same bands as they do. The results are collected in Table 1. The CO stretch of free CO was observed at 2138.6 cmÿ1 , with a shoulder at 2136.8 cmÿ1 due to (CO)2 [10±12]. The band at 2149.6 cmÿ1 with a satellite at 2148.8 cmÿ1 has been assigned to m(OC± HOH) [9,10]. The corresponding band of OC± DOD was observed at 2150.2 cmÿ1 [9,10]. The bands in the CO stretch region are shown in Fig. 1. In the H2 O m2 region, three new bands were observed, at 1596.0, 1605.5 (strongest) and at 1617.0 cmÿ1 . The band at 1596.0 cmÿ1 has been assigned to m2 (HOH±CO) [10]. It is disturbed by the formation of H2 O polymers close to this band [10]. The bands at 1605.5 and 1617.0 cmÿ1 have been assigned to (H2 O)2 ±CO [10]. Two signi®cant strong peaks in the H2 O m3 stretch region were
T. Svensson, B. Nelander / Chemical Physics 262 (2000) 445±452
447
Table 1 Observed bands of the complexes of water with CO in solid argon (cmÿ1 )
a
H2 Oa
D2 Oa
CO
HOH±CO
DOD±CO
H2 O(D2 O), m1
3638
2657.7
±
H2 O(D2 O), m2 H2 O(D2 O), m3
1589.1 3734.3
1174.6 2771.1
± ±
3622.8 3628.0 1596.0 3723.8
CO
±
±
2138.6
2149.6
2646.0 2650.0 1185.0 2753.3 2762.5 2150.2
Ref. [24].
Fig. 1. The CO stretching region: x-axis, cmÿ1 ; upper curve, an experiment with HOO; lower curve, with DOO. The curves have been shifted vertically for clarity. I: m(OC±HOO), II: m(OC±DOO), III: m(CO±HOO), IV: m(CO±DOO), V: CO, m, VI: m(OC±HOH), VII: m(OC±DOD).
observed at 3711.5 and 3723.8 cmÿ1 . The peak at 3723.8 cmÿ1 (strongest) was assigned to H2 O±CO and the peak at 3711.5 cmÿ1 to (H2 O)2 ±CO in Ref. [10]. 3.2. Peroxy radical complexes of N2 or CO The interaction between the peroxy radical and N2 or CO induced new strong bands near the vibrational fundamentals of HOO and CO or N2 . The bands were assigned to the peroxy radical complexes of N2 or CO. They are collected in Table 2. Traces of N2 and CO are normally present
in matrices and the bands due to their peroxy radical complexes had been observed as very weak satellites to the peroxy radical fundamentals in earlier experiments. Their intensities increased very dramatically when N2 or CO were deliberately added to the matrix. The free peroxy radical and the peroxy radical complexes of N2 and CO are all decomposed by 266 nm radiation. For a given radiation time, the decrease of the absorption bands was largest for the peroxy radical±CO complex and smallest for the peroxy radical±N2 complex. The free peroxy radical was decomposed at an intermediate rate.
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Table 2 Observed bands of the complexes of the peroxy radical with N2 and CO (cmÿ1 ) HOO, m1 HOO, m2 HOO, m3 HOO, 2m2 N2 CO, m HCl, m
HOOa
CO
HClb
N2 ±HOO
OC±HOO
HOO±HClb
3413.0 1388.9 1100.9 ± ± ± ±
± ± ± ± ± 2138.6 ±
± ± ± ± ± ± 2870.8
3408.6 1404.8 1104.7 2782.0 2331.6 ± ±
3342.5 1423.8 1108.3 2817.2 ± 2160.6 ±
3306.0 1425.2 1117.0 ± ± ± 2677.4
N2 ±DOO
OC±DOO
DOO±HClb
2524.9 1031.0 1127.4 2045.2 2332.0 ± ±
2477.2 1043.0 1132.2 2068.3 ± 2161.1 ±
2450.8 1053.4 ± ± ± ± 2682.0
DOOa DOO, m1 DOO, m2 DOO, m3 DOO, 2m2 N2 , m CO, m HCl, m a b
2530.2 1020.3 1122.9 ± ± ± ±
± ± ± ± ± 2138.6 ±
± ± ± ± ± ± 2870.8
Ref. [7]. Ref. [2].
Fig. 2. The HOO and DOO m1 stretching regions: x-axis, cmÿ1 ; upper curves, N2 added; lower curves, CO added; the curves have been shifted vertically for clarity. (a) I: m1 (OOH±N2 ), II: m1 (OOH±CO), III: HOO, m1 . The peak at approximately 3430 cmÿ1 is due to a complex between HOO and H2 O2 [26]. The peak at approximately 3360 cmÿ1 is due to HOOO [27]. (b) I: m1 (OOD±N2 ), II: m1 (OOD± CO), III: DOO, m1 . The peak at approximately 2540 cmÿ1 is due to a complex between DOO and H2 O2 [26].
T. Svensson, B. Nelander / Chemical Physics 262 (2000) 445±452
449
Fig. 3. The OH and OD bending regions: x-axis, cmÿ1 ; upper curves, N2 added; lower curves, CO added; the curves have been shifted vertically for clarity. (a) I: m2 (OOH±N2 ), II: m2 (OOH±CO), III: HOO, m2 . (b) I: m2 (OOD±N2 ), II: m2 (OOD±CO), III: DOO, m2 . The peak close to 1039 cmÿ1 is due to ozone.
3.2.1. Peroxy radical complex of N2 The HO and DO stretching regions (Fig. 2). When N2 was added to HOO, a new intense band was observed at 3408.6 cmÿ1 which shifted to 2524.9 cmÿ1 with DOO. The band is assigned to m1 (OOH±N2 ). The HOO and DOO bending regions (Fig. 3). A new intense band was observed at 1404.8 cmÿ1 when N2 was added to HOO. It was assigned to m2 (OOH±N2 ). With DOO it was shifted to 1031.0 cmÿ1 . When the Ar/N2 ratio was increased, a band at 1408.6 cmÿ1 grew too rapidly for a 1:1 complex in H experiments. A corresponding band in D experiments was observed at 1036.7 cmÿ1 . We assign these bands to m2 (OOH±(N2 )m ) and m2 (OOD± (N2 )m ) (m P 2) respectively. The HOO and DOO m3 regions. A new intense band, blue shifted 3.8 cmÿ1 from HOO, was observed at 1104.7 cmÿ1 . It is assigned to m3 (OOH± (N2 )m ). With DOO the band was observed at 1127.4 cmÿ1 , blue shifted 4.5 cmÿ1 from the band of free DOO.
The N2 stretch region. m(N2 ±HOO) was observed at 2331.6 cmÿ1 and m(N2 ±DOO) at 2332.0 cmÿ1 . 3.2.2. Peroxy radical complex of CO The HO and DO stretching regions (Fig. 2). When CO was added to HOO, a new intense band was observed at 3342.5 cmÿ1 which shifted to 2477.2 cmÿ1 with DOO. The band is assigned to m1 (OOH±CO). The HOO and DOO bending regions (Fig. 3). In experiments with H atoms a new intense band was observed at 1423.8 cmÿ1 , with a small satellite at 1429.2 cmÿ1 , this band is assigned to m2 (OOH± CO). The corresponding band in experiments with D atoms was observed at 1043.0 cmÿ1 (small satellite at 1047.5 cmÿ1 ). The HOO and DOO m3 regions. In experiments with H atoms a new intense band was observed at 1108.3 cmÿ1 (m2 (OOH±CO)) with a small satellite at 1110.3 cmÿ1 , which shifted to 1132.2 cmÿ1 (m2 (OOD±CO)) in experiments with D atoms.
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T. Svensson, B. Nelander / Chemical Physics 262 (2000) 445±452
The CO stretch region (Fig. 1). An intense new band was observed at 2160.6 cmÿ1 in experiments with HOO, with a small satellite at 2161.4 cmÿ1 . The band was shifted from 0.5 cmÿ1 to 2161.1 cmÿ1 , with a small satellite at 2161.9 cmÿ1 , in experiments with DOO. We assign this band to the CO stretch in the complex between the peroxy radical and CO. A band at 2143.5 cmÿ1 has been assigned to a complex between CO and CO2 [11,12]. In our experiments this band is expected to be present, but rather weak since in CO experiments, CO2 is present as an impurity and the CO2 concentration is low. Nevertheless we observed a band here in OC±HOO and OC±DOO experiments. This band was to a large extent eliminated by 266 nm radiation, showing that it was not due to a CO±CO2 complex. We may have a contribution from the CO±CO2 complex but we believe that the major part of the absorption is due to a peroxy radical±CO complex. We tentatively assign the band to the CO±HOO (CO±DOO) complex, noting that Lundell et al [13]. have calculated a 15 cmÿ1 smaller CO shift for the CO±HOH complex, compared to the OC±HOH complex. We assign a weak band at 2140.7 cmÿ1 to m(OC±O3 ) [14]. 4. Discussion HOO and HCl are both expected to form hydrogen bonded complexes with proton acceptors. Structural data is available for a number of HCl complexes, it therefore seems of interest to com-
pare the frequency shifts of a set of HCl and HOO complexes (Table 3). HCl forms a linear complex with N2 [15]. The HCl stretch is shifted ÿ7:3 cmÿ1 in the complex. The OH stretching fundamental of HOO in its N2 complex is shifted ÿ4:4 cmÿ1 . The observation of a H to D shift of the NBN stretching vibration of the peroxy radical nitrogen complex clearly indicates that HOO forms a hydrogen bond to nitrogen. If we assume that the N2 ±HOO complex has an approximately linear hydrogen bond, with no signi®cant interaction between the terminal oxygen of the peroxy radical and the N2 molecule, this suggests that the peroxy radical forms somewhat weaker hydrogen bonds than HCl. Similarly the shifts of the HCl and OH fundamentals in the ammonia complexes are ÿ1499 cmÿ1 [16] and ÿ758:8 cmÿ1 [17] respectively. The back bonding from an ammonia hydrogen to the terminal oxygen of the peroxy radical is expected to be quite weak, and also in this case, the HCl shift is larger than the OH shift. We note that the shift of the OH stretching fundamental of HOO bound to nitrogen is slightly smaller than the OD shift of DOO bound to nitrogen. In normal cases, the ratio between the OH and OD shifts are approximately 1.33 as is observed for the CO and HCl complexes. The reason for the small shift is probably the extreme weakness of the complex. The observed shift is a vibrational average over the intermolecular vibrations. One of the intermolecular fundamentals of a peroxy radical complex is a libration of HOO (DOO) around the OO-axis. This libration
Table 3 Shifts of some selected HCl and HOO complexes (cmÿ1 ) HCl, m HOO, m1 HOO, m2 HOO, m3 OC, m NH3 , m2 a
N2 ±HCla
N2 ±HOOb
OC±HCla
OC±HOOb
(HCl)2 c
HOO±HCla
ClH±NH3 d
OOH±NH3 e
ÿ7.3 ± ± ± ± ±
± ÿ4.4 15.9 3.8 ± ±
ÿ55.5 ± ± ± 16.0 ±
± ÿ70.5 34.9 7.4 22.0 ±
ÿ52.8 ± ± ± ± ±
ÿ193.4 ÿ107 36.3 16.1 ± ±
ÿ1499 ± ± ± ± ±
± ÿ758.8 174.1 26 ± 120.5
Ref. [19]. Taken from Table 2. c Ref. [25]. d Ref. [16]. e Ref. [2]. b
T. Svensson, B. Nelander / Chemical Physics 262 (2000) 445±452
has a signi®cantly larger amplitude for the HOO complex than for the DOO complex. For the nitrogen complex the decrease of the shift due to vibrational averaging outweighs the mass shift. We note that for the N2 ±HCl complex, the ratio between the H to D shifts is 1.19 [18] while for the OC±HCl complex it is 1.34 [19]. In the gas phase HCl forms a linear complex with CO, with a hydrogen bond to the carbon atom [20]. An H to D shift of the CO fundamental of the peroxy radical complex indicates that the peroxy radical forms a hydrogen bond to CO. If we accept that the shift of the OH stretch of HOO is smaller than the shift of the HCl stretch, when they form complexes with the same compound and in the absence of additional interactions, we are forced to conclude that the structure of the HOO± CO complex diers from a simple linear hydrogen bonded structure, since the OH shift of this complex is 1.5 times more negative than the corresponding HCl shift. The shift of the CO stretch fundamental of the OC±HOO complex is 22.0 cmÿ1 and the corresponding shift of OC±HCl is 16.0 cmÿ1 , supporting the conclusion that the interaction energy is larger for the OC±HOO complex than for the OC±HCl complex. Since HOO is non-linear it can increase the interaction between its terminal oxygen and the CO oxygen signi®cantly without a serious distortion of the hydrogen bond. It should be noted that the stretching force constants for the relative motion of the centers of mass of the HCl complexes are 2.55 Nmÿ1 for the NN±HCl and 3.88 Nmÿ1 for OC±HCl [21]. The ratio of these is much smaller than the ratio of the HCl shifts. Also the estimated interaction energies ÿ0.00356 and ÿ0.00424 a.u. for the NN±HCl and OC±HCl complexes respectively are much closer than the HCl shifts for the two complexes [22]. This indicates that the correlation between the HCl complex shift and the dissociation energy of a complex is non-linear in this case. However the fact that the OH shift of OOH±CO is larger than the HCl shift of ClH±CO is still an indication of a dierence in complex structure. The comparison between the hydrogen chloride dimer and the HCl±HOO complex suggests the presence of an additional interaction in the peroxy radical complex. The HOO shift is larger than the
451
HCl shift of the proton donor of the dimer showing the presence of a hydrogen bond from HOO to HCl. Likewise the HCl stretch in the HCl±HOO complex is strongly shifted, almost as much as in the H2 O±HCl complex. Therefore a hydrogen bond from HCl to HOO must exist. The comparison between the water complexes of HOO and HCl suggests the need for a more quantitative theory. The water±HCl complex is known to be eectively Y shaped planar and with a low barrier separating two pyramidal energy minimum structures [23]. The water peroxy radical complex is cyclic [2,4]. Still, the HCl shift is larger than the HO shift. A possible explanation is that water forms relatively strong hydrogen bonds and therefore the complex may gain energy by deforming the hydrogen bond from the peroxy radical in order to strengthen the hydrogen bond from water. The resulting structure would then be a compromise between the two competing bonding requirements.
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