11610
J. Phys. Chem. 1996, 100, 11610-11615
Matrix Infrared Spectra and ab Initio Calculations of the Nitrous Acid Complexes with N2 and CO Zofia Mielke,* Zdzisław Latajka, and Joanna Kołodziej Faculty of Chemistry, Wrocław UniVersity, Joliot-Curie 14, 50-383 Wrocław, Poland
Konstantin G. Tokhadze Institute of Physics, St. Petersburg UniVersity, Peterhof 198904 St. Petersburg, Russia ReceiVed: February 14, 1996; In Final Form: April 23, 1996X
The complexes formed by trans- and cis-HONO isomers with nitrogen and carbon monoxide have been observed and characterized in argon matrices. Six perturbed trans-HONO vibrations and four perturbed cisHONO vibrations were identified for both the N2 and CO complexes. The perturbation of the OH group vibrations proves that all four complexes are hydrogen bonded; the blue shifts of the CO vibrations in transand cis-HONO complexes as compared to CO monomer indicate the OC‚‚‚HONO structures in which carbon atoms are the acceptor sites. The strength of interaction, as evidenced by the perturbation of the OH vibrational modes of the nitrous acid, increases from cis- to trans-isomer and from nitrogen to carbon monoxide. Theoretical studies of the structure and spectral characteristics of the complexes formed between the two isomers of nitrous acid and nitrogen or carbon monoxide were carried out on the electron correlation level with the 6-31G(d,p) basis set. The binding energy and the calculated spectral parameters are in very good agreement with experimental data.
Introduction Weakly bound molecular dimers have been recently extensively investigated in the gas phase by radiofrequency, microwave, and high-resolution infrared laser-molecular beam spectroscopy.1 The matrix infrared spectra provide complementary information on bonding and structure of such systems.2 The complexes between hydrogen halides and simple molecules such as N2, CO, NO, and CO23-11 have been the subject of particular interest as the simplicity of these systems allows for both detailed spectroscopic analysis and application of refined ab initio calculations. In this paper we report the results of infrared matrix isolation and ab initio studies on the complexes of nitrous acid with nitrogen and carbon monoxide. Nitrogen, carbon monoxide, and nitrous acid are important atmospheric species. The interaction between nitrous acid and N2 or CO is of potential interest in connection with atmospheric modeling. Nitrous acid is unstable in the gas phase and occurs in complex equilibrium with the products of its decomposition.12 Despite these difficulties, it has been extensively studied in the vapor phase13-23 and low-temperature matrices24-27 and the infrared spectra of the two isomers of nitrous acid, trans-HONO and cis-HONO, are well understood. As part of a wider study of the molecular complexes of nitrous acid with a variety of molecules of atmospheric significance, we reported recently the results of ab initio and infrared matrix isolation studies of nitrous acid-ammonia complexes.28 This work presents an ab initio calculation and infrared study of argon-isolated trans-HONO and cis-HONO complexes with the nitrogen molecule and carbon monoxide. Experimental Section Matrix Isolation Studies. Gas mixtures were prepared by adding N2/Ar or CO/Ar mixtures of concentration varying in X
Abstract published in AdVance ACS Abstracts, June 1, 1996.
S0022-3654(96)00452-2 CCC: $12.00
the range 1/800 to 1/50 into 1 l bulbs containing a small amount of solid NH4NO2. The thermal equilibrium, NH4NO2(s) S NH3(g) + HONO(g), provides partial pressures of p(HONO) ) p(NH3) ) 0.04 mbar at 298 K. The pressures of added N2/Ar or CO/Ar mixtures varied in the range 12-32 mbar, so the initial concentration of the overall mixture N2/HONO/NH3/Ar or CO/ HONO/NH3/Ar varied in the range m/1/1/300 to m/1/1/800, where m ) 1, 2, 4, and 6. The obtained mixtures were deposited onto a cold mirror through a 30 cm long, 5 mm o.d. stainless steel tube whose surface was saturated with HCl before the N2/ HONO/NH3/Ar or CO/HONO/NH3/Ar mixtures were passed through. In the spectra of matrices obtained in this way no absorptions (or weak ones) due to ammonia, hydrogen chloride, or their complexes were observed. In some experiments separate mixtures of CO/Ar or N2/Ar and HONO/NH3/Ar were prepared. In these experiments the two mixtures were sprayed simultaneously onto a mirror; the HONO/NH3/Ar was sprayed through the stainless steel tube whose surface was saturated with HCl. In three experiments the HONO/NH3/N2 mixtures of concentration 1/1/300, 1/1/500, and 1/1/800 were prepared by adding nitrogen gas into a bulb with solid NH4NO2, and the prepared mixtures were deposited onto a mirror through tubes saturated with HCl. These experiments allowed us to obtain the spectra of HONO in solid nitrogen; the spectra of HONO in solid argon were reported earlier.28 The gas mixtures were sprayed onto a gold-plated copper mirror held at 20 K by a closed cycle helium refrigerator (Air Products, Displex 202A). The prepared mixtures were deposited onto a cold mirror until the total pressure decreased about ca. 4 mbar, then a N2/Ar or CO/Ar mixture was added to the residual mixture to recover the initial pressure, and the deposition process was continued. The deposition was monitored by the infrared spectrum of the matrix. The approximate HONO/ Ar ratio varied during deposition in the range 1/800 to 1/700 for the most diluted mixture and in the range 1/300 to 1/200 for the most concentrated one, whereas the N2/Ar or CO/Ar © 1996 American Chemical Society
Nitrous Acid Complexes with N2 and CO
Figure 1. Infrared spectra in the the region of the ν(OH) stretching vibration of trans- and cis-HONO isomers. Spectra of matrices obtained by deposition of mixtures of initial concentration: HONO/N2/Ar ) 1/1/800, 1/4/800 (a, b); HONO/N2 ) 1/800 (c), and HONO/CO/Ar ) 1/1/800, 1/4/800 (d, e). Spectrum c is scaled by a factor of 0.4. The bands indicated by asterisks are due to the presence of N2, NH3, and H2O contaminants in the studied matrices.
Figure 2. Infrared spectra in the region of the ν(NdO) stretching vibration of trans- and cis-HONO isomers: (a-e) spectra of the same matrices as presented in Figure 1. Spectrum c is scaled by a factor of 0.4.
J. Phys. Chem., Vol. 100, No. 28, 1996 11611
Figure 4. Infrared spectra in the region of the ν(N-O) vibration of trans- and cis-HONO isomers: (a-e) spectra of the same matrices as presented in Figure 1. Spectrum c is scaled by a factor of 0.6.
Figure 5. Infrared spectra in the region of the δ(ONO) and τ(OH) vibrations of trans- and cis-HONO isomers: (a-e) spectra of the same matrices as presented in Figure 1. Spectrum c is scaled by a factor of 0.6.
NH4NO2 was prepared and purified according to ref 29. Computational Details. Ab initio calculations were carried out using the Gaussian-92 package of computer codes.30 The structures of considered complexes and the isolated monomers were fully optimized by using the second-order Møller-Plesset perturbation theory (MP2) with the 6-31G(d,p) basis set.31 Vibrational frequencies and intensities were computed both for the two monomers and for the complexes. Single-point calculations in which the valence electrons were correlated at the MP2 level were carried out with the 6-311+G(2d,2p) basis set32 for all species to evaluate the binding energy of the complex. Interaction energies were corrected by the Boys-Bernardi full counterpoise correction33 at both the SCF and MP2 levels. Results
Figure 3. Infrared spectra in the region of the δ(NOH) vibration of the trans-HONO isomer: (a-e) spectra of the same matrices as presented in Figure 1. Spectrum c is scaled by a factor of 0.4.
ratio was constant. The concentrations of the other mixtures varied during deposition in a similar way. In some experiments the deposited matrices were irradiated through a KBr window with the output of a 450 W Xe lamp (Oriel). Infrared spectra were recorded with the matrix maintained at ca. 11 K. The spectra were registered at 0.5 cm-1 resolution in a reflection mode, with a Bruker 113v FTIR spectrometer.
Experimental Spectra. Spectra of nitrogen/nitrous acid/ argon matrices showed a number of prominent new absorptions as compared to the spectra of nitrous acid isolated in argon. All the product absorptions appeared in the vicinity of the absorptions due to trans- and cis-HONO monomers. Figures 1a,b- 5a,b present the spectra of HONO/N2/Ar matrices of approximate concentrations 1/1/800(a) and 1/4/800(b) in the regions of the six HONO modes. In Figures 1c-5c the spectra of the HONO/N2 ) 1/800 matrix are presented. As can be seen in Figures 1a-5a, in the spectra of matrices at low N2 concentrations (N2/Ar ) 1/800) the new absorptions appeared at 3562.2, 3558.5, 1686.2, 1684.6, 1288.4, 1280.3, 805.3, 618.8,
11612 J. Phys. Chem., Vol. 100, No. 28, 1996
Mielke et al.
TABLE 1: Frequencies (cm-1) of the Perturbed HONO Vibrations in the Spectra of HONO/N2/Ar and HONO/CO/ Ar Matricesa HONO/N2/Ar freq assignt 3562.2 3558.5 3554.5 ≈3408
1:1, t 1:1, t 1:n, t 1:1, c
1686.2 1684.6 1683.6 1630.2 1628.8 1291.6 1288.4 1280.3 864.0 860.6 857.5 808.2 805.3
1:1, t 1:1, t 1:n, t 1:1, c 1:n, c 1:n, t 1:1, t 1:1, t 1:n, c 1:1, c 1:1, c 1:n, t 1:1, t
(666) ≈623 618.8 595.7 592.5
1:n, c 1:n, t 1:1, t 1:n, t 1:1, t
HONO/N2 freq
HONO/CO/Ar freq assignt 3498.9 3483.1
1:1, t 1:n, t
3407.3 c sh 3406.0 c 1681.3 t
3360.0
1:1, c
1681.0 1679.5
1:1, t 1:n, t
1630.2 c
1626.5
1:1, c
1296.9 t
1318.6 1312.6
1:n, t 1:1, t
865.2 c
875.9 872.0 870.6 820.9 817.9 815.0 (701.5) 635.0 629.6
1:n, c 1:1, c 1:1, 1:n, t 1:n, t 1:1, t 1:1, c 1:n, t 1:1, t
3551.8 t
815.0 t 659.4 c 625.9 t (623) t 582.1 t
a t, c the frequency corresponds to perturbed trans- or cis-HONO isomer, respectively.
and 592.5 cm-1 in the vicinity of trans-HONO absorptions and at 3408, 1630.2, 860.6, 857.5, and 666 cm-1 in the vicinity of cis-HONO bands. At higher N2 concentration the relative intensities of the new bands increased with respect to the bands of HONO monomers but remained constant with respect to each other. Additional bands also occurred at 3554.5, 1683.6, 1291.6, 808.2, 623, and 595.7 cm-1 close to trans-HONO bands and at 1628.8 and 864.0 cm-1 close to cis-HONO bands. The frequencies of all product absorptions observed in the spectra of HONO/N2/Ar matrices and the frequencies of HONO in a nitrogen matrix are collected in Table 1. The spectra obtained in the present work for nitrous acid isolated in a nitrogen matrix agree well with those previously reported.24,25 In the spectra of carbon monoxide/nitrous acid/argon matrices at low CO concentrations (CO/Ar ) 1/800) the product absorptions were observed at 3498.9, 1681.0, 1312.6, 815.0, and 629.6 cm-1 in the vicinity of trans-HONO absorptions and at 3360.0, 1626.5, 872.0, 870.6, and 701.5 cm-1 in the vicinity of cis-HONO bands. In addition, in the region of the ν(CdO) vibration two product bands occurred at 2161.8 and 2158.4 cm-1 on the high-frequency side of the CO monomer band. Irradiation of the matrix with a Xe lamp considerably decreased the 2161.8 cm-1 band and all the absorptions due to perturbed transHONO vibrations and left unchanged the 2158.4 cm-1 band and all the bands due to pertubed cis-HONO vibrations. So, irradiation of the CO/HONO/Ar matrices allowed us to differentiate the 2161.8, 2158.4 cm-1 bands and to assign the 2161.8 cm-1 absorption to the perturbed CO vibration in the CO‚‚‚HONO-trans complex and the 2158.4 cm-1 band to the corresponding vibration in the CO‚‚‚HONO-cis complex. The absorptions due to the perturbed HONO vibrations in trans and cis complexes are easily identified, as they appear in the vicinity of the bands due to trans- or cis-HONO monomers. The effect of radiation of different wavelength on nitrous acid isolated in solid nitrogen has been extensively studied by Pimentel and co-workers24,25 and by Shirk and co-workers.34,35
Figure 6. Optimized structures of the nitrous acid complexes with nitrogen and carbon monoxide.
When CO concentration in the matrix increased, additional absorptions occurred at 3483.1, 1679.5, 1318.6, 820.9, and 635.0 cm-1 close to trans-HONO bands and at 875.9 cm-1 close to cis-HONO bands. In matrices with higher CO concentration the relative intensities of these bands increased with respect to product absorptions observed in diluted CO matrices. The relative intensities of the two components at 872.0, 870.6 were not affected by CO concentration increase or by matrix annealing. Figures 1d,e-5d,e present the effect of an increase of CO concentration on the spectra of CO/HONO/Ar matrices; the frequencies of all product absorptions observed in the spectra of CO/HONO/Ar matrices are collected in Table 1. Ab Initio Calculations. Three structures of the nitrogennitrous acid system were taken into consideration in the calculations: two structures with trans- or cis-HONO isomers acting as proton donors and the third structure, trans-G, in which an oxygen atom of the trans-HONO molecule was the site of interaction. Only planar structures were taken into account. Recently performed ab initio calculations for the HCOOHN236, HCOOH-CO37 and for the CH3OH-N2, CH3OH-CO38 hydrogen-bonded complexes showed that nonplanar geometries for these complexes correspond to transition states on their potential energy surface. The structures corresponding to those optimized at the MP2 level with the 6-31G(d,p) basis set stationary points on the PES are shown in Figure 6. The fully optimized geometrical parameters of the trans, cis, and trans-G complexes are presented in Table 2; the best estimate of the binding energy at the MP2/6-311+G(2d,2p) level for the three complexes is also given. Four structures of carbon monoxide-nitrous acid system corresponding to those optimized at the MP2 level with the 6-31G(d,p) basis set stationary points on the PES are shown in Figure 6. Two cis and two trans forms were localized in which trans- or cis-HONO molecules are interacting with oxygen or carbon atoms of carbon monoxide. The fully optimized geometrical parameters of the four complexes are presented in Table 2; the best estimate of the binding energy at the MP2/ 6-311+G(2d,2p) level for the four complexes is also given. The calculated frequencies of the trans and cis N2‚‚‚HONO and OC‚‚‚HONO complexes are displayed in Table 3 and compared with experimental data.
Nitrous Acid Complexes with N2 and CO
J. Phys. Chem., Vol. 100, No. 28, 1996 11613
TABLE 2: Calculated Geometries of the Trans- and Cis-HONO Monomers and N2‚‚‚HONO and CO‚‚‚HONO Complexes at the MP2 Level with the 6-31G(d,p) Basis Seta,b trans-HONO monomer R(O2‚‚‚X)b r(N1-O1) r(N1-O2) r(O2-H) r(N2-N3) r(C-O3) θ(O1N1O2) θ(N1O2H) θ(N1O2N2) θ(O2N2N3) θ(O2HC) θ(HCO3) θ(O2HO3) θ(HO3C) φ(N1O2XX)d ∆EMP2
cis-HONO
N2-HONO
G-N2-HONO
OC-HONO
CO-HONO
3.200 1.200 1.416 0.972 1.130
3.349 1.197 1.426 0.974 1.131
3.194 1.202 1.411 0.974
2.984 1.199 1.417 0.972
110.54 101.60 97.57 177.72
110.20 101.88 100.54 70.75
1.149 110.65 101.74
1.152 110.48 101.68
1.197 1.425 0.972 1.131 1.151 110.30 101.69
monomer
N2-HONO 3.277 1.212 1.382 0.982 1.130
1.210 1.387 0.982 1.131 1.151 112.76 104.33
112.85 104.91 107.86 171.92
176.84 177.49 0.0 -2.56
0.0 -1.20
OC-HONO
CO-HONO
3.263 1.213 1.380 0.984
3.224 1.211 1.384 0.982
1.149 112.99 105.37
1.152 112.81 104.76
172.94 185.89 176.30 173.90 0.0 -2.05
0.0 -4.09
0.0 -2.23
177.19 194.19 0.0 -1.83
0.0 -3.53
a Values of the interaction energy, ∆E b MP2 , are calculated with 6-311+ G(2d,2p). Bond lengths are in angstroms; bond angles in degrees; interaction energies in kcal/mol. c X ) N2, C, O3. d XX ) N2N3, CO3, O3C.
TABLE 3: Comparison of Experimental and Calculated Frequenciesa (cm-1Å) for N2, CO, and HONO Monomers and N2-HONO, and OC-HONO Complexes trans-HONO monomer
HONO-N2
cis-HONO HONO-CO
monomer
expt
calc
expt
calc
expt
calc
expt
calc
3572.6 3568.5 1689.1 s 1688.0 1265.8 1263.9 800.4 s 796.6 608.7 549.4 s 548.2
3810.9
3562.2 s 3558.5 1686.2 s 1684.6 1288.4 1280.3 s 806.4 805.3 618.8 592.5
3805.2
3498.9
3759.3
3642.7
1651.8
1681.0
1648.5
3412.4 3410.7 1634.0 s 1632.8
1620.4
1337.5
1312.6
1360.2
886.7
817.9 s 815.0 629.6
900.3
2139b a
1657.8 1301.9 864.2 628.8 599.8 2174.9 2119.6
650.5 649.4
661.2 666.6
HONO- N2 expt
expt
calc
3408
3643.3
3360.0
3607.5
ν1 OH stretch
1630.2
1613.8
1626.5
1612.8
ν2 NdO stretch
1374.2
ν3 NOH bend
1346.4 853.1 s 850.2 638.4
2180.3
944.8 652.9 743.7
1362.6 860.6 857.5 s 666
2174.9 2161.8
2140.5
HONO-CO
calc
2139b
968.4
872.0 s 870.6
assignment
977.4
ν4 N-O stretch ν5 ONO bend ν6 OH torsion
662.1 780.6
701.5
666.5 815.0
2179.1 2119.6
2158.4
2139.1
ν NN ν CO
Calculated at the MP2 level with the 6-31G(d,p) basis set. b Dubost, H.; Abouaf-Marguin, L. Chem. Phys. Lett. 1972, 17, 269.
Discussion Nitrogen-Nitrous Acid 1:1 Complexes. The set of bands observed in diluted matrices (at low N2 concentration) close to the trans-HONO absorptions can be assigned with confidence to the 1:1 N2‚‚‚HONO-trans complexes. Similarly, the set of bands in the vicinity of cis-HONO bands is attributed to the 1:1 N2‚‚‚HONO-cis complexes. The frequencies of the perturbed HONO vibrations in the 1:1 N2‚‚‚HONO-trans and N2‚‚‚HONO-cis complexes are presented in Table 3. As can be seen in Figures 1a,b-5a,b and in Table 3, most of the perturbed nitrous acid vibrations in trans and cis complexes exhibit matrix splitting. The two-site bands are typically a few wavenumbers apart; their relative intensities do not depend on N2 concentration in the matrix. Matrix annealing also didn’t affect the relative intensities of the two site components. As can be noted in Table 3, in both complexes large shifts are observed for the OH group vibrations. In the N2..HONOtrans complex the OH stretch is shifted ca. 10 cm-1 toward lower frequencies and the NOH in-plane bend and the NOH torsion are shifted ca. 23 and 43 cm-1, respectively, toward higher frequencies with respect to corresponding HONO-trans vibrations. In the N2‚‚‚HONO-cis complex the OH stretch is ca. 4 cm-1 red shifted and the NOH torsion is ca. 28 cm-1 blue shifted from the corresponding vibrations of the cis-HONO
monomer. The relatively large perturbations of the OH group vibrations in both complexes indicate that the OH group interacts directly with the N2 molecule, forming a weakly hydrogen bonded N2‚‚‚HONO-trans or N2‚‚‚HONO-cis complex. The OH group vibrations in the trans complex are more perturbed than in the cis one, suggesting higher interaction energy and stronger bonding in the trans complex. The two-site bands observed for the perturbed vibrations of trans- and cis-HONO isomers are assigned to one complex structure in which the nitrogen molecule is interacting directly with the hydrogen atom. The two-site components are a few wavenumbers apart, and both are shifted in the same direction (toward higher or lower wavenumbers) from the corresponding, unperturbed HONO vibrations, as expected for the complex trapped in two matrix cages of different geometries. In particular, the two components due to the perturbed OH stretch (3562.2, 3558.5 cm-1) are red shifted and those due to the perturbed NOH in-plane bend (1288.4, 1280.3 cm-1) are blue shifted from the corresponding trans-HONO vibrations. Such perturbation of the two modes is expected for the N2‚‚‚HONOtrans complex in which nitrogen is interacting with the hydrogen atom, whereas for the G-trans complex in which nitrogen is interacting with the O(H) atom one would expect red shifts for both the OH stretching and NOH in-plane bending vibrations.
11614 J. Phys. Chem., Vol. 100, No. 28, 1996 The results of ab initio calculations are in agreement with the experimental data. The trans complex is calculated to be more stable by about 0.33 kcal/mol than the cis one. In Table 3 the experimental frequencies of the N2‚‚‚HONO-trans and N2‚‚‚HONO-cis isomers are compared with the frequencies predicted by ab initio calculations. The calculations predict the 5.7 cm-1 red shift for the OH stretch and the 35.6, 49.6 cm-1 blue shifts for the NOH in-plane bend and OH torsion, respectively, in the N2‚‚‚HONO-trans complex as compared to the trans-HONO monomer. The shifts predicted by ab initio calculations match well with the experimental data. In the case of the N2‚‚‚HONO-cis complex the calculations predict negligible shift (within calculation error) for the ν(OH) stretch vibration and a 36.9 cm-1 blue shift for the OH torsion. This result is also in accord with the measured spectra, which show a small red shift for the OH stretch vibration (4 cm-1) and a distinct blue shift for the OH torsion (28 cm-1). The calculated frequency shifts for other HONO vibrations are slightly overestimated, but the directions of the shifts are properly predicted. Nitrogen has been found to interact strongly with a variety of molecules in argon matrices. In all the complexes studied the interaction is a specific one with an X-H group in the molecule.4,39 Recently the nitrogen-nitric acid complex in an argon matrix has been studied, and a similar structure was concluded from the infrared spectra in which nitrogen is interacting with an OH group.40 The frequencies of the OH group vibrations are less perturbed in nitrogen-nitrous acid complexes than in the nitrogen-nitric acid complex, indicating weaker interaction in nitrous acid complexes. The studied spectra of the N2-HONO system in argon gave no evidence for the existence of a 1:1 N2‚‚‚HONO complex of trans-G geometry. This may be due to much less stability of this complex as compared to hydrogen-bonded complexes. Carbon Monoxide-Nitrous Acid 1:1 Complexes. The behavior exhibited by carbon monoxide-nitrous acid mixtures in argon matrices is similar to that found for nitrogen-nitrous acid mixtures for the spectra to be assigned in an analogous manner. The product bands observed in matrices at low CO concentration are assigned to the 1:1 complexes of carbon monoxide with trans-HONO or cis-HONO isomers according to their position in the vicinity of the absorptions of trans- or cis-HONO monomers. The frequencies of the perturbed HONO vibrations in trans and cis complexes are collected in Table 3. As can be seen in Figures 1-5, the absorptions due to the perturbed nitrous acid vibrations in carbon monoxide complexes are broader than those observed in the spectra of nitrogen complexes. This can be due to stronger interaction of nitrous acid with carbon monoxide than with nitrogen. The characteristic broadening of the band due to the XH stretch vibration, which is dependent on the hydrogen bond strength, is a characteristic feature of the hydrogen-bonded complexes.41 The observed shifts of the OH vibrations (measured with respect to the average frequency of the two site components) are -71.5 and +47.7 cm-1, for the OH stretch and NOH inplane bend in the trans complex and ca. -50 and +63.1 cm-1 for the OH stretch and NOH torsion in the cis complex. This is in reasonable agreement with the calculated frequency shifts (-51.6, +58.3 cm-1 for the ν(OH) stretch and NOH in-plane bend in the trans complex and -35.2, +71.3 cm-1 for the ν(OH) stretch and OH torsion in the cis complex; see Table 3). The larger perturbation of the ν(OH) stretch indicates stronger interaction with carbon monoxide in the trans complex than in the cis one. The frequencies of the CO stretching vibrations are shifted to higher wavenumbers in both trans and cis
Mielke et al. complexes, indicating that the interaction is of the type OC‚‚‚HONO. Infrared matrix isolation studies and gas phase studies of the complexes formed between carbon monoxide and hydrogen halides,3,6,9,11,42 water,43 or nitric acid40 prove that in the more stable carbon-attached complexes the perturbed CO stretching vibrations are shifted toward higher frequencies. The band observed at a higher wavenumber, 2161.8 cm-1, is assigned to the trans complex, as it strongly diminished after matrix irradiation like the bands due to perturbed trans-HONO vibrations. The band observed at 2158.4 cm-1, not affected by the irradiation, is assigned to the cis complex. Stronger perturbation of the CO vibration in the trans complex than in the cis one is in accord with stronger perturbation of the transHONO vibrations and confirms stronger interaction and higher stabilization energy for the trans complex. As can be seen in Table 3, the ab initio calculations predict the trans OC‚‚‚HONO complex to be more stable by 0.65 kcal/ mol than the cis OC‚‚‚HONO complex. The trans- and ciscarbon-attached OC‚‚‚HONO complexes are predicted to be more stable by 2.04 and 1.69 kcal/mol, respectively, than the trans- and cis-oxygen-attached CO‚‚‚HONO complexes. We did not identify in the studied matrices the oxygen-attached CO‚‚‚HONO complexes. This is probably due to their lower stability and to relatively low concentration of nitrous acid and its complexes in the studied matrices. The carbon monoxide complexes in which an oxygen atom is the interacting site are known to be formed in larger yield when the complex is generated directly in the matrix cage. Photolysis of formyl fluoride in argon gave a relatively large yield of the CO‚‚‚HF complex,9 and the photolysis of formic acid in argon and xenon led to identification of the CO‚‚‚HOH complex.43 However the observable concentration of the CO‚‚‚HF complex in an argon matrix was also obtained by deposition of the CO/HF/Ar gaseous mixture.6 Higher Order Aggregates. In the spectra of matrices at higher N2 or CO concentrations the absorptions assigned to the perturbed HONO modes in the 1:1 complexes are accompanied by additional bands. The relative intensities of these bands increase with respect to the corresponding 1:1 complex bands at higher N2 or CO concentration. This fact suggests that (N2)nHONO and (CO)nHONO aggregates of relatively well defined structure exist in the studied matrices. The aggregates are formed both by cis- and trans-HONO isomers. The HONO modes are slightly more perturbed in the aggregates than in the 1:1 complexes (see Table 1), but the particular modes are shifted in the same direction as for the 1:1 complexes. Conclusions Ab initio calculations performed for the nitrous acid-nitrogen system indicate that the trans- and cis-HONO isomers form with the nitrogen molecule weak molecular hydrogen-bonded complexes; the calculations carried out for the trans isomer demonstrate also the stability of the G-trans complex, in which an oxygen atom of the OH group is the acceptor site. The binding energy at the MP2 level with the 6-311+G(2d,2p) basis set is -2.56 and -2.05 kcal/mol for hydrogen-bonded N2‚‚‚HONO-trans and N2‚‚‚HONO-cis complexes, respectively, and -1.20 kcal/mol for the G-trans complex. Infrared matrix isolation studies confirm the formation of hydrogen-bonded N2‚‚‚HONO-trans and N2‚‚‚HONO-cis complexes. Six perturbed trans-HONO vibrations and four perturbed cis-HONO vibrations were identified, respectively, for the two complexes. The stronger perturbation of the trans-HONO vibrations indicates the higher interaction energy for the trans complex than for the cis one, in agreement with calculations.
Nitrous Acid Complexes with N2 and CO Ab initio calculations carried out for the nitrous acid-carbon monoxide system indicate formation of four stable complexes in which the trans- or cis-HONO isomers are attached to carbon or oxygen atoms of carbon monoxide. The binding energy at the MP2 level with the 6-311+G(2d,2p) basis set is -4.09, -2.05 kcal/mol for the carbon- and oxygen-attached trans isomer, respectively, and -3.53, -1.83 kcal/mol for carbonand oxygen-attached cis isomer. Infrared matrix isolation studies led to the identification of OC‚‚‚HONO-trans and OC‚‚‚HONO-cis complexes. Five perturbed trans-HONO vibrations and four perturbed cis-HONO vibrations were identified for the trans- and cis-HONO complexes; the blue shifts of CO vibrations in both complexes prove the OC‚‚‚HONO types of geometries. The spectra confirm the higher interaction energy for the trans complex, in accord with calculations. As presented in Table 2 and Figure 6, the HONO complexes with nitrogen and carbon monoxide are characterized by a nearly linear hydrogen bond between the N2 or CO subunits and the OH bond of HONO. A similar geometry of the hydrogen bond was found for the HCOOH‚‚‚N2 and HCOOH‚‚‚CO complexes.36,37 The calculated spectra of the N2‚‚‚HONO-trans, N2‚‚‚HONOcis and OC‚‚‚HONO-trans, OC‚‚‚HONO-cis complexes reproduce well the frequency values and frequency shifts in the measured spectra. Acknowledgment. Z.M. gratefully acknowledges financial support from the Polish State Committee for Scientific Research (Grant KBN No. 0848 91 01). The authors thank an anonymous reviewer for helpful comments and remarks. References and Notes (1) Bevan, J. W. In Structure and Dynamics of Weakly Bound Molecular Complexes; Weber, A., Ed.; Reidel: Dordrecht, 1987; p 149. (2) Andrews, L. In Chemistry and Physics of Matrix Isolated Species; Andrews, L., Moskovits, M., Eds.; North Holland: Amsterdam, Oxford, New York, Tokyo, 1989; p 15. (3) Barnes, A. J.; Hallam, H. E.; Scrimshaw, G. E. Trans. Faraday Soc. 1969, 65, 3172. (4) Maillard, D.; Schriver, A.; Perchard, J. P.; Girardet, C.; Robert, D. J. Chem. Phys. 1977, 67, 650. (5) Andrews, L.; Johnson, G. L. J. Chem. Phys. 1982, 76, 2875. (6) Andrews, L.; Arlinghaus, R. T.; Johnson, G. L. J. Chem. Phys. 1983, 78, 6347. (7) Andrews, L.; Kelsall, B. J.; Arlinghaus, R. T. J. Chem. Phys. 1983, 79, 2488. (8) Davies, S. R.; Andrews, L.; Trindle, C. O. J. Chem. Phys. 1987, 86, 6027. (9) Schatte, G.; Willner, H.; Hoge, D.; Kno¨zinger, E.; Schrems, O. J. Phys. Chem. 1989, 93, 6025.
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