J. Phys. Chem. 1996, 100, 539-545
539
Spectroscopic and Theoretical Studies of the Complexes between Nitrous Acid and Ammonia Zofia Mielke,* Konstantin G. Tokhadze,† and Zdzisław Latajka Institute of Chemistry, Wrocław UniVersity, Joliot-Curie 14, 50-383 Wrocław, Poland
Emil Ratajczak Department of Physical Chemistry, Wrocław UniVersity of Medicine, Plac Nankiera 1, 50-140 Wrocław, Poland ReceiVed: July 28, 1995; In Final Form: September 28, 1995X
The complexes formed between cis- and trans-HONO isomers and ammonia have been observed and characterized in argon matrices. Five perturbed HONO vibrations and one perturbed NH3 deformation vibration were identified for the H3N‚‚‚HONO-trans complex, and one perturbed HONO vibration and perturbed NH3 deformation vibration were identified for the H3N‚‚‚HONO-cis complex. The OH stretching vibration in the H3N‚‚‚HONO-trans complex is ca. 800 cm-1 red-shifted and NOH bending vibration is ca. 190 cm-1 blueshifted with respect to the trans-HONO monomer, indicating formation of a very strong molecular hydrogen bond. Theoretical studies of the structure and spectral characteristics of the H3N‚‚‚HONO-trans and H3N‚‚‚ HONO-cis complexes were carried out on the electron correlation level and G-311+G(2df,2pd) basis set. The calculated binding energy at the MP2 level is -40.13 and -36.39 kJ mol-1 for the H3N‚‚‚HONO-trans and H3N‚‚‚HONO-cis complexes, respectively. The calculated spectra reproduce very well the frequencies and the intensities of the measured spectra.
Introduction Nitrous acid (HONO) plays an important role in atmospheric chemistry. It is one of the smallest molecules which exhibits a cis-trans conformational equilibrium; the cis-trans isomerization of nitrous acid was the first reported instance of an infrared photochemical reaction.1,2 For these reasons the HONO molecule has been of significant interest both experimentally1-20 and theoretically.21 The microwave spectra3-5 and the infrared spectra of the nitrous acid in the gas phase6-16 and in lowtemperature matrices1,2,17-20 have been extensively studied. Nitrous acid is unstable in the vapor phase and occurs in the complex equilibrium with the products of its decomposition. In spite of these difficulties the low-resolution infrared spectra6,7 and high-resolution FTIR12-16 and laser infrared spectra8-11 of cis- and trans-HONO have been extensively studied. McGraw et al.6,7 were the first who identified most of the fundamentals of the cis-HONO and trans-HONO molecules. The authors also estimated that the trans form of the molecule is more stable by about 1.6 kJ/mol than the cis form. This result was later confirmed by microwave studies5 which have shown that the barrier of interconversion between the two forms is approximately 40 kJ/mol. The performed ab initio calculations21 fairly well reproduced the experimental results. The matrix-isolated spectra of cis- and trans-HONO have been first reported by Baldeschwieler and Pimentel1 and by Hall and Pimentel.2 The authors produced nitrous acid in a nitrogen matrix by the photolysis of hydrazoic acid in the presence of O2. Their study was focused on the kinetics of the infrared photoisomerization reaction of the HONO molecule. The reaction was later reinvestigated by McDonald and Shirk.17,18 Guillory and Hunter19 obtained infrared spectra of nitrous acid molecules in solid argon by the reaction of H atoms with NO2. † On leave from Institute of Physics, St. Petersburg University, 198904 Russia. X Abstract published in AdVance ACS Abstracts, December 1, 1995.
0022-3654/96/20100-0539$12.00/0
Recently, Crowley and Sodeau20 isolated HONO in argon as the product of the reaction between the amidogen radical, NH2, and oxygen. In all above experiments HONO was stabilized in a nitrogen or argon matrix in the presence of large concentrations of other species. Although a lot of work has been done on the HONO molecule itself, very little work has been reported on complexes of nitrous acid. Pagsberg et al.22 studied recently the kinetics and thermochemistry of the reversible gas phase reaction between ammonia and HONO: HONO + NH3 S H3N‚‚‚HONO. The authors reported also the results of ab initio calculations on the thermochemistry of the H3N‚‚‚HONO complex, which was found to form a fairly strong hydrogen bond with the bond energy of D(H3N‚‚‚HONO)g ) 49.4 kJ mol-1. The H3N‚‚‚ HONO complex is of interest for several reasons. First, the reversible gas phase reaction between nitrous acid and ammonia is a possible pathway for the interconversion of nitrogen oxides to molecular nitrogen in the atmosphere. Second, in the H3N‚‚‚ HONO complex the HONO molecule acts as a proton donor, forming a N‚‚‚H-O type of hydrogen bond. A lot of infrared studies have been reported for strong hydrogen-bonded complexes in the gas phase and low-temperature matrices, but most of the work concerns B‚‚‚H-X complexes23 in which hydrogen halides play the role of proton donor. Comparison of the properties and spectral characteristics of bimolecular B‚‚‚H-X and B‚‚‚HO type of hydrogen-bonded complexes seems to be interesting. We report here the infrared study of the H3N‚‚‚HONO complex isolated in argon matrices. Crystalline ammonium nitrite was employed as a reliable source of gaseous nitrous acid.22 The experimental spectra are compared with the spectra predicted by ab initio molecular orbital theory. The paper demonstrates the capability of the theory in predicting the frequencies and infrared intensities of the H3N‚‚‚HONO complex. © 1996 American Chemical Society
540 J. Phys. Chem., Vol. 100, No. 2, 1996
Mielke et al.
Experimental Section Infrared Matrix Isolation Studies. Gas mixtures composed of HONO, NH3, and Ar were prepared by adding argon into a 1 L bulb 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. A series of experiments were carried out by varying the initial pressures of added argon in the range 12-32 mbar, so the initial concentration of the gaseous mixtures HONO/ NH3/Ar was between 1/1/300 and 1/1/800. The prepared mixture was deposited onto a cold mirror until the total pressure decreased about ca. 4 mbar; then argon gas was added to the residual mixture to recover the initial pressure, and the deposition was continued. The deposition was monitored by the infrared spectrum of the matrix. The approximate ratio of the most concentrated mixture varied during deposition in the range 1/1/300-1/1/200 and the most diluted one in the range 1/1/ 800-1/1/700; the concentration of the other deposited mixtures varied in a similar way. Three experiments were performed with the matrices containing an excess of NH3. The matrices were prepared by simultaneous deposition of HONO/NH3/Ar ) 1/1/400 mixture (the initial concentration is given) and NH3/ Ar ) 1/400, 1/200, or 1/100 mixture. 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 mixtures were deposited onto a cold mirror by means of a ca. 50 cm long, 5 mm o.d. stainless steel tubes. In two experiments the stainless steel tubes were treated with concentrated HF/Ar mixture before the HONO/ NH3/Ar mixtures of initial concentrations 1/1/300 and 1/1/800 were deposited. In a set of experiments the deposited matrices were irradiated through KBr window with the output of 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. NH4NO2 was prepared and purified according to ref 24. Computational Details. Ab initio calculations were carried out using the GAUSSIAN-9225 package of computer codes. The structures of the H3N‚‚‚HONO complex and the isolated monomers HONO and NH3 were fully optimized by using the second-order Moller-Plesset perturbation theory (MP2) with the 6-31+G(d,p) basis set,26,27 which is a split-valence plus polarization basis augmented with diffuse functions on nonhydrogen atoms. Vibrational frequencies and intensities were computed both for the two monomers and for the complex. Single-point calculations in which the valence electrons were correlated at the MP2 level were carried out with the 6-311G+ (2d,2p)27,28 basis set for all species to evaluate the binding energy of the complex. Interaction energies were corrected by the Boys-Bernardi full counterpoise correction29 at both the SCF and MP2 levels. The optimized geometries of the monomers and of the H3N‚‚‚HONO complexes are presented in Table 1. Results and Discussion All bands observed in the spectra of matrices obtained by deposition of the equilibrium vapor pressure above solid NH4NO2 diluted with argon are shown in Figures 1-7 and listed in Table 2. The observed bands reveal several species. The bands due to cis- and trans-HONO molecules and to NH3 are identified on the basis of comparison with earlier gaseous and matrix spectra.2,6-19,30,31 Matrix-isolated water, nitrogen oxide, and nitrogen dioxide were also detected (see Figures 1B and 4). The H2O, NO, and NO2 molecules are possibly formed in the
Figure 1. ν(OH) stretching region in the spectrum of the matrix obtained by deposition of the NH3/HONO/Ar mixture of the ratio varying in the range 1/1/800-1/1/700 (B); the ν1(OH) absorptions of trans-HONO and cis-HONO monomers isolated in argon (A).
TABLE 1: Calculated Geometry of the H3N···HONO-trans and H3N···HONO-cis Complexes and Their Monomers trans-HONO
cis-HONO
H3N···HONO HONO/NH3 H3N···HONO HONO/NH3 r(NdO) r(N-O) r(O-H) ∆r(O-H) R(O···N) r(N-H1) r(N-H2) θ(ONO) θ(NOH) θ(NON) θ(ONH1) θ(ONH2) θ(H1NH2) θ(H2NH3) θ(H1NON)
1.209 1.381 1.003 0.029 2.766 1.014 1.013 111.8 102.5 100.6 106.5 114.0 107.3 107.3 0.0
1.194 1.430 0.974 1.011 1.011 110.6 102.1
108.1 108.1
1.220 1.363 1.014 0.031 2.777 1.014 1.013 113.9 108.9 111.0 104.9 114.6 107.6 107.3 0.0
1.207 1.392 0.983 1.011 1.011 113.1 105.4
108.1 108.1
surface-catalyzed HONO decomposition reaction 2HONO S NO + NO2 + H2O. The surface of the narrow stainless steel tubes by means of which the mixture was deposited onto a cold mirror probably catalyzed the HONO decomposition reaction. In addition to the isolated monomeric species, the H3N‚‚‚HONO adduct and a small quantity of the H3N‚‚‚H2O complex and possibly the H2O‚‚‚HONO complex were also detected. When the gaseous mixtures were deposited by means of the tubes treated with HF, the bands due to NH3 species and the bands assigned to H3N‚‚‚H2O and H3N‚‚‚HONO complexes were not observed anymore in the recorded spectra. (The frequencies of these bands are given in parantheses in Table 2.) Ammonia was removed from the mixtures by reacting with HF adsorbed on the tube walls. The absorptions due to the H3N‚‚‚HONO adduct were identified on the basis of the following data. In the spectra of all matrices obtained by deposition of NH4NO2 equilibrium vapor pressure diluted with argon, a set of bands was observed at 2765.2, 2737.5, 1455.0, 1434.0, 1086.7, 1076.8, 949.0, 947.5, 883.5, and 702.2 cm-1. These bands gained in intensity relative to HONO absorptions in the spectra of matrices with an excess of NH3. When the NH4NO2 equilibrium vapor pressure was passed by tubes treated with HF and ammonia was removed
Complexes between Nitrous Acid and Ammonia
J. Phys. Chem., Vol. 100, No. 2, 1996 541
Figure 5. Region of the NH3 deformation vibration in the spectra of matrices HONO/Ar ) 1/800 (a) and NH3/HONO/Ar ) 1/1/800 (b); NH3/HONO/Ar ) 1/1/300 (c). The approximate initial ratios of the gaseous mixtures are given. The bands due to the H3N‚‚‚HONO complexes are indicated by arrows.
Figure 2. ν(OH) absorption of H3N‚‚‚HONO-trans complex in the spectra of matrices obtained by deposition of HONO/Ar mixture (a) and NH3/HONO/Ar mixtures of the initial concentrations 1/1/800 (b), 4/1/800 (c), and 1/1/300 (d). Spectrum e shows the effect of 30 min irradiation on the matrix presented in (d).
Figure 6. The 980-775 cm-1 region in the spectra of the same matrices as presented in Figure 5.
Figure 3. Absorptions due to the NOH in-plane bending vibration in trans-HONO monomer (A) and in H3N‚‚‚HONO-trans complex (B) in the spectra of matrices obtained by deposition of the HONO/Ar mixtures of initial concentration 1/800 (a) and NH3/HONO/Ar mixtures of initial concentration 1/1/800 (b) and 1/1/300 (c).
Figure 7. The 710-510 cm-1 region in the spectra of the same matrices as presented in Figure 5.
Figure 4. Region of the ν2(NdO) stretching vibration in the spectrum of the matrix obtained by deposition of the NH3/HONO/Ar mixture of initial ratio 1/1/800.
from the matrix mixtures, these bands were not observed anymore in the spectra. The above experimental facts prove that the bands are due to H3N‚‚‚HONO complexes. The 3702.5 and 3435.6 cm-1 bands observed in the spectra
of matrices obtained by deposition of argon diluted equilibrium vapor pressure agree well with frequencies reported earlier by Nelander32 for the argon-isolated H3N‚‚‚H2O complex. The 3322.9 cm-1 band is tentatively assigned to the H2O‚‚‚HONO complex. The band is observed in both the spectra of matrices containing NH3 and those free of NH3; its intensity depends on water concentration in the matrix. HONO Monomers. When NH3 was removed from the argon-diluted NH4NO2 equilibrium vapor pressure, the spectra of obtained matrices showed only the bands due to cis- and trans-HONO monomers and to HONO decomposition products: H2O, NO, and NO2. Figures 1A, 3a, 4, 6a, and 7a present the spectra of cis- and trans-HONO monomers in the regions
542 J. Phys. Chem., Vol. 100, No. 2, 1996
Mielke et al.
TABLE 2: Frequencies (cm-1) and Absorbances of the Absorptions Appearing after Deposition of NH4NO2 Equilibrium Vapor Pressure Diluted with Ara freq
A
3776.8 3756.4 3711.5 (3702.5) (3573.5 sh) 3572.4 3568.4 (3435.6) 3412.4 sh 3410.7 3322.9 (3242.3) (2765.2) (2737.5) 1871.8 1689.1 1688.0 1634.0 1632.8 1624.0 1611.8 1610.8 1593.2 1589.3 (1455.0) (1434.0)
0.06 0.15 0.06 0.02 0.13 0.12
assignt H2O H2O H2O NH3H2O NH3 HONO HONO NH3H2O
0.03 0.02
HONO HONOH2O
0.01 0.01 0.02 0.22 0.62 0.21 0.64 0.14 0.18 0.47 0.06 0.06 0.01 0.001
NH3HONO NH3HONO NO HONO HONO HONO HONO H2O NO2 NO2 H2O H2O NH3HONO NH3HONO
freq 1319.2 1265.8 1263.9 1259 sh (1086.7) (1076.8) (1035.2) (1013.9) (1000.2) (974.6) (961.6) (949.0) (947.5) 913.1 (883.5) 853.1 850.2 800.4 796.6 (702.2) 639.3 638.4 618.8 608.7 549.4 548.2
A
TABLE 3: Comparison of Calculated and Experimental Frequencies and Intensities for trans-HONO and NH3 Monomers and H3N‚‚‚HONO-trans Complex
assignt
0.31 0.30
HONO HONO
0.03 0.015 0.06 0.02 0.02 0.2 0.03 0.015 0.015 0.02 0.03 0.05 0.25 0.06 0.11
NH3HONO NH3HONO NH3 NH3 NH3 NH3 NH3 NH3HONO NH3HONO ? NH3HONO HONO HONO HONO HONO NH3HONO
0.12 0.06 0.17
HONO ? HONO
0.15
HONO
a
The frequencies in parantheses indicate bands which disappear when NH3 is removed from equilibrium vapor above NH4NO2.
of the ν1 OH stretch, ν3 NOH bend, ν2 NdO stretch, ν4 N-O stretch and ν5 ONO bend, and ν6 torsion fundamentals, respectively. In Figure 1B the ν(OH) bands of nitrous acid and water contaminant are presented, and in Figure 4 the ν(NdO) bands of nitrous acid and nitrogen dioxide can be compared. As can be seen (see also Table 2), the concentration of HONO decomposition products is relatively low, which makes possible the studies of HONO complexes. All six fundamentals of the trans-HONO monomer and four fundamentals of the cis-HONO monomer were identified in the spectra. The frequencies of observed fundamentals of cis- and trans-HONO molecules are collected in Tables 3 and 4. The irradiation of matrices with the output of the Xe lamp diminished the bands due to the trans isomer and increased the bands due to the cis form. Hall and Pimentel2 showed that trans to cis photolysis of nitrous acid occurs during both ultraviolet and infrared radiation, so the observed effect is probably due in part to ultraviolet radiation from the Xe lamp and in part to infrared radiation. Our assignment of the observed bands to trans and cis isomers is in accord with the reported spectral characteristics of the two isomers in nitrogen matrices.1,2 However, there is some disagreement between our analysis and recently reported spectral analysis of nitrous acid in argon.19,20 We assigned the 1265.8, 1263.9, and 608.7 cm-1 bands to the trans form and not to the cis form as recently reported for nitrous acid in argon. First, the two bands decrease after irradiation similarly as all the other bands due to trans-HONO. Second, in the gas phase spectra of cis-HONO isomer the band corresponding to ONO bending vibration is very weak, and the absorption due to the NOH bending vibration has not been identified14 whereas the two vibrations are characterized by intense bands in the spectra of trans-HONO isomer. These experimental data are confirmed by calculations which predict that the bands due to the NOH and ONO bending vibrations of the trans isomer are ca. 30 and ca. 5 times, respectively, more intense than the corresponding bands for cis isomer.
trans-HONO freq (cm-1) exp calcd exp calcd exp calcd exp calcd exp calcd exp calcd
H3N‚‚‚HONO-trans
I (km I/ I (km freq mol-1) INb (cm-1) mol-1)
3572.6 3568.5 3783.7 1689.1 sa 1688.0 1655.4 1265.8 1263.9 1289.0 800.4 s 796.6 814.4 608.7 583.8 549.4 s 548.2 608.2
1.1 92
0.5 1.5
103
0.6 1.2
189
1.0 1.0
186 123 167
2765.2 2737.5 3170.2
assignt ν1 OH stretch
3.9 1688
4.8 ν2 NdO stretch
1650.5 1455.0 1434.0 1491.9 883.5
27
351
0.9
122
1097.7
0.07 1.0 ν3 NOH bend
270
1.0 954.0 0.8 702.2 0.7 717.2 0.7 (1230?)
NH3 calcd calcd calcd calcd calcd exp calcd
I/ INb
0.8 1.0
ν4 N-O stretch
1.0 0.1 0.1
30
ν5 ONO bend ν6 OH torsion
0.3
H3N···HONO-trans
3738.2
7
3567.8 1707.9
1 29
974.6 1047.2
247
3708.8 3704.5 3550.1 1702.9 1698.3 1086.7 1186.5
22 22 0 24 27
NH stretch NH stretch NH stretch NHN bend NHN bend NHN bend
0.6 0.6
202
a s ) less intense of the two component bands corresponding to each fundamental. b IN ) Iν(N-O).
TABLE 4: Comparison of Calculated and Experimental Frequencies and Intensities for cis-HONO and NH3 Monomers and H3N···HONO-cis Complex cis-HONO
H3N···HONO-cis
I (km freq I (km freq (cm-1) mol-1) I/INb (cm-1) mol-1) I/INb exp calcd exp calcd calcd exp calcd calcd exp calcd
3412.4 3410.7 3623.2 1634.0 s 1632.8 1610.0 1324.0 853.1 850.2 902.1 635.5 638.4 695.4
38
0.1 3003.4 0.7
1355
147 6
0.4 1599.9 0 1465.9 1.0 947.5
198 27
0.5 0.07 ν3 NOH bend 1.0 ν4 N-O stretch
392 27
1.0 1035.9 0.1 710.9 0.2 0.3 1129.5
396 11
1.0 0.1
128
0.3
123
NH3
calcd calcd calcd calcd calcd exp calcd
3.4 ν2 NdO stretch
ν5 ONO bend ν6 OH torsion
H3N···HONO-cis
freq (cm-1)
I (km mol-1)
freq (cm-1)
I (km mol-1)
3738.2
7
3567.8 1707.9
1 29
3709.1 3705.1 3547.6 1700.0 1699.4 1076.8 1177.8
21 22 0 17 27
974.6 1047.2
assignment ν1 OH stretch
0.1
204
I/INb
assignment
0.05 0.05 0 0.0 0.1 0.8 0.5
NH stretch NH stretch NH stretch NHN bend NHN bend NHN bend
s ) less intense of the two component bands corresponding to fundamental. b IN ) Iν(N-O). a
As can be seen in Figures 1A, 3a, 4, 6a, and 7a and in Tables 3 and 4, the argon matrices displayed two bands for each observed fundamental of cis- and trans-HONO (except the ν6 ONO bending mode for which only one band was observed). This could be due either to matrix site effects or to aggregation.
Complexes between Nitrous Acid and Ammonia The first of these two explanations is more likely on the basis of the obtained data. First, the relative intensity of the twocomponent bands of each fundamental was constant within experimental error in the studied concentration range. Second, the relative intensity of the two components was independent of the concentration of HONO decomposition products (H2O, NO, NO2) in the matrix which excluded their assignment to HONO‚‚‚NO, HONO‚‚‚NO2, HONO‚‚‚H2O aggregates. In addition, the frequency difference between the two components of ν(OH), δ(NOH), and τ(OH) fundamentals is too small to assign one of the two components to HONO‚‚‚H2O or the (HONO)2 aggregate and the second one to the HONO monomer. One expects stronger perturbation of the corresponding HONO fundamentals in these aggregates. In the spectra of nitrous acid isolated in nitrogen only one band was observed for each fundamental.33 All the above facts indicate that the two observed components of HONO fundamentals are due to the matrix site effect. In Tables 3 and 4 the observed frequencies and intensities of both HONO isomers are compared with their infrared spectra computed in double harmonic approximation. The calculated MP2 frequencies well reproduce the observed frequencies of the two isomers with the exception of the observed OH stretching frequencies, which are somewhat lower than the predicted ones. The observed relative intensities of the two isomers were measured with respect to their ν(N-O) bands. These bands are calculated to be the most intense ones in the spectra of monomers. In the intensity measurement of each fundamental were included the two component bands corresponding to different sites. One can note a discrepancy between calculated and observed intensities of the ν(OH) and ν(NdO) bands of the trans form. The discrepancy between the calculated and observed intensities can be in part due to the interaction between the HONO molecule and matrix cage. The effect of strong enhancement of the ν(OH) band intensity in hydrogenbonded complexes is a well-known phenomenon. The relative intensities of the absorptions due to the two forms were approximately constant in the spectra measured after matrix deposition. Baldschwieler and Pimentel showed1 that the absorption coefficient of the ν(N-O) band of cis-HONO is about double the absorption coefficient of the corresponding band of trans-HONO. This result matches very well with calculated intensities of these bands (see Tables 3 and 4). It was interesting to estimate the relative concentration of the trans to cis isomer in the deposited matrices. The measured relative intensity of the ν(N-O) band of the trans form with respect to the cis form indicates that similar concentrations of trans-HONO and cis-HONO isomers were present in the studied matrices. H3H‚‚‚HONO Complexes. Optimized Geometry and Calculated Spectra. Searches for the potential energy surface (PES) were limited to those regions where strong hydrogen bonds are likely to be formed. Because the nitrous acid molecule has better acidic than basic properties, the structures with nitrous acid acting as a proton donor were taken into consideration. The structures corresponding to those optimized at the MP2 level with the 6-31+G(d,p) basis set stationary points on the PES are shown in Figure 8. Two cis and two trans forms were localized which differ in relative orientation of the NH3 subunit with respect to cis- or trans-HONO molecule. The vibrational spectra of all four possible structures of the H3N‚‚‚HONO complex were analyzed within the framework of the 6-31+G(d,p) basis set at the MP2 level. The frequencies and intensities were computed by the double-harmonic approximation. The structures labeled in Figure 8 as trans II and cis II have an imaginary frequency corresponding to the rotation
J. Phys. Chem., Vol. 100, No. 2, 1996 543
Figure 8. Possible structures of the NH3‚‚‚HONO complexes.
of NH3 molecule around the pseudo C3 axis. This means that the two structures correspond to the saddle point (or transition state) on the PES. The true minima correlate with structures labeled as trans I and cis I. The fully optimized geometrical parameters of the trans I and cis I complexes as well as the geometrical parameters for isolated subunits are presented in Table 1. The optimized geometry of the trans I complex has been published previously22 but is presented in Table 1 for comparison with the cis I complex. The energetics of binding of the two H3N‚‚‚HONO complexes were also calculated. Values of interaction energy (∆E) were corrected for the basis set superposition error using the Boys-Bernarde counterpoise procedure. In order to obtain a better determination of the binding energy, additional singlepoint calculations were performed with a larger 6-311+G(2d, 2p) basis set applying the MP2 level and the previously optimized geometry. The best estimate of binding energy at the MP2 level (∆EMP2) with the 6-311+G(2d,2p) basis set is -40.13 and -36.39 kJ mol-1 for the trans-HONO and cisHONO complexes, respectively. The structure of the complex with trans-HONO form was found to be more stable than the structure with cis-HONO form by about 3.7 kJ mol-1 independently of the used basis set. The calculated frequencies and intensities of the intramolecular modes of trans I and cis I complexes are displayed in Tables 3 and 4, respectively, and compared with experimental data. The calculated frequencies of the intermolecular modes are not included in the tables as they are computed with relatively large errors due to strong anharmonicity of these modes. The intermolecular modes were not observed in the performed experiments. ObserVed Spectra. As already discussed, the set of bands at 2765.2, 2737.5, 1455.0, 1434.0, 1086.7, 1076.8, 949.0 sh, 947.5, 883.5, and 702.2 cm-1 can be assigned with confidence to the H3N‚‚‚HONO complexes. Irradiation of the matrix with Xe lamp considerably decreased the 2765.2, 2737.5, 1455.0, 1434.0, 1086.7, 883.5, and 702.2 cm-1 bands and increased the 1076.8 and 947.5 cm-1 bands. This fact allowed us to assign with confidence the bands decreasing after matrix photolysis to the H3N‚‚‚HONO-trans complex and the two bands increasing after matrix photolysis to the H3N‚‚‚HONO-cis complex. Isomerization of the trans-HONO monomer to cis-HONO form during matrix irradiation is expected to result in a decrease of the number of H3N‚‚‚HONO-trans complexes. Simultaneously, the number of H3N‚‚‚HONO-cis complexes in the matrix may increase if the produced cis-HONO molecules will interact with the NH3 molecules occupying the same cage. The observed frequencies and intensities of the H3N‚‚‚HONOtrans and H3N‚‚‚HONO-cis complexes are presented in Tables 3 and 4. Similarly as in the case of the HONO monomers, the relative band intensities were measured with respect to the bands
544 J. Phys. Chem., Vol. 100, No. 2, 1996 assigned to the ν(N-O) vibrations in the H3N‚‚‚HONO-trans and H3N‚‚‚HONO-cis complexes, respectively. These are the best defined bands in the spectra of the two complexes. Five perturbed trans-HONO vibrations and one perturbed NH3 vibration were identified for the H3N‚‚‚HONO-trans complex. Figure 2 shows the region of the OH stretching vibration in the spectra of the H3N‚‚‚HONO-trans complex. The doublet at 2765.2 and 2737.5 cm-1 is assigned to the OH stretching fundamental in the 1:1 complex. As can be seen in Figure 2, the relative intensity of the two components is independent of the matrix concentration, and the matrix irradiation effects the two components in the same way. So, the possible explanation of the observed doublet could be matrix site effect. The two components observed for the NOH inplane bending vibration (see Figure 3B) seem to support such an explanation. However, the characteristic shape of the ν(OH) stretching absorption suggests also another possibility. The ν(OH) stretching band in the spectrum of the complex is ca. 800 cm-1 red-shifted from the corresponding absorption of the trans-HONO monomer. It is a well-known fact that such a strong shift of the OH stretching band in the spectra of hydrogen-bonded complexes is accompanied by a strong increase of band intensity and by band broadening. Indeed, we do observe the strong increase of the intensity of the ν(OH) stretching band in the complex with respect to the trans-HONO monomer; this effect is also predicted by ab initio calculations (see Table 3). The observed shape of the ν(OH) absorption seems to suggest that in fact the two peaks at 2765.2 and 2737.5 cm-1 are the components of the same broad absorption which has a characteristic, Fermi resonance window34 with minimum at 2750 cm-1. The 2δ(NOH) overtone would be a good candidate for Fermi resonance interaction with ν(OH) fundamental transition. The δ(NOH) fundamental transition is observed at the 1455.0, 1434 cm-1 (site). Due to expected, relatively strong anharmonicity of this mode, its overtone may fall in the region of the broad ν(OH) absorption. The shape of the observed ν(OH) band is very similar to the shape of the band observed for hydrogen stretching vibration in dimethylacetamide-HCl strongly hydrogen-bonded complex.35 The two bands at 1455.0 and 1434.0 cm-1 presented in Figure 3B are assigned to the NOH in-plane bending vibration of the 1:1 complex. The bands are ca. 190 ( 10 cm-1 shifted toward higher frequencies from the corresponding bands of transHONO monomer; such a large blue shift is expected for strongly hydrogen-bonded complex. The frequency of the NOH in-plane bending mode in trans-HONO monomer and its perturbation by complex formation are well predicted by ab initio calculations (see Table 3). The appearance of the two bands is probably due to matrix site effect. The aggregation is rather excluded as the 1455.0 and 1434.0 cm-1 bands display the same relative intensity independent of the matrix concentration (see Figure 3B). Matrix irradiation also affects the two bands in the same way; they diminish after irradiation, but their relative intensity remains constant within experimental error. The fact that the two site components are observed only for the NOH in-plane bending vibration can be reasonably justified. The OH group vibrations are particularly sensitive to the interaction with the environment. The broadness of the ν(OH) absorption may hide the component bands corresponding to different sites whereas they may become observable for the NOH in-plane vibrations. A weak and diffuse absorption at ca. 1230 cm-1 is tentatively assigned to the OH torsion vibration in the complex. The band has higher frequency than predicted by ab initio calculations (1097.7 cm-1). The N-O stretching vibration of the H3N‚‚‚HONO-trans
Mielke et al. complex is identified at 883.5 cm-1 (see Figure 6b,c). The band is ca. 87 cm-1 shifted toward higher frequencies from the corresponding absorption of the trans-HONO monomer. The blue shift of the ν(N-O) frequency in the complex agrees with calculated shortening of the N-O bond, but the calculated ν(N-O) frequency (954.0 cm-1) is overestimated. The ONO bending vibration is observed as a weak band at 702.2 cm-1 (see Figure 7b,c); its position is well predicted by calculations. Only one band is observed for perturbed NH3 vibrations. The band occurs at 1086.7 cm-1 and is due to NH3 symmetric deformation vibration observed at 974.6 cm-1 for NH3 monomer (see Figure 5). The calculations well predicted the frequency change of δ(NH3) mode in the complex with respect to HONO monomer (∆νobs ) 112.1 cm-1 and ∆νcalc ) 139.3 cm-1) although they overestimate the absolute value of δ(NH3) frequency for both the complex and NH3 monomer. As discussed above, the frequencies predicted by ab initio calculations for the H3N‚‚‚HONO-trans complex agree reasonably well with the observed frequencies. One may also note (see Tables 3 and 4) that the predicted relative intensities match remarkably well the measured intensities. The six observed fundamentals of H3N‚‚‚HONO-trans complex are predicted to be the most intense bands of the complex; there is almost perfect agreement between the predicted and measured relative intensities for these bands. The calculations also prove why the attempts to locate ν(NdO) mode and other perturbed NH3 modes (in addition to the identified mode) were unsuccessful as these modes are predicted to have very low intensities. The existence of the H3N‚‚‚HONO-cis complex in the studied matrices is well proved by the measured spectra. But in contrast with the H3N‚‚‚HONO-trans complex, only two bands were identified for the cis complex. The 947.5 cm-1 band (see Figure 6b,c) is assigned to ν(N-O) vibration and the 1076.8 cm-1 (see Figure 5) band to perturbed NH3 deformation vibration. Attempts to locate other fundamentals proved fruitless. The two observed bands are predicted to be the most intense ones for the H3N‚‚‚HONO-cis complex, neglecting the ν(OH) absorption. As can be seen in Figures 5 and 6, the observed bands are weak, and the identification of absorptions still weaker may present difficulties. The concentration of the H3N‚‚‚HONO-cis complexes in the studied matrices is probably too low for the weak absorptions to be detected. As concerns the ν(OH) absorption, the band is probably broad and diffuse which makes its detection difficult. We estimated from the obtained spectra the ratio of the H3N‚‚‚HONO-trans to H3N‚‚‚HONO-cis complexes in the deposited matrices. As discussed earlier, the spectra indicate similar concentration of cis- and trans-HONO monomers in the deposited matrices. The ratio of the H3N‚‚‚HONO-trans to H3N‚‚‚HONO-cis complexes can be estimated from the relative intensities of the bands due to NH3 deformation vibrations in the two complexes. It is expected and is predicted by calculations that the absorption coefficients of these bands have similar values for the two complexes (see Tables 3 and 4), so the relative intensities of the δ(NH3) bands corresponding to the two complexes will give directly the relative concentration of the T / two species in the studied matrices. The estimated Iδ(NH 3) C Iδ(NH3) ratio is 2.5, which means that the number of trans complexes in the deposited matrices is more than double the number of the cis complexes. In spite of similar concentrations of cis- and trans-HONO monomers in the deposited gaseous mixtures, there is much less H3N‚‚‚HONO-cis complexes formed than H3N‚‚‚HONO-trans complexes, which is most probably due to lower stability of the H3N‚‚‚HONO-cis complexes.
Complexes between Nitrous Acid and Ammonia
J. Phys. Chem., Vol. 100, No. 2, 1996 545
TABLE 5: HX Stretching Frequencies, Relative Frequency Shifts, and NH3 Deformation Frequencies of Ammonia Complexes with Nitric Acid, Nitrous Acid, and Hydrogen Halides acid
νmon
νcom
∆ν/ν
δcom(NH3)
HONO HONO2 HFc HCld HBre
3572.6a 3522a 3919 2871 2559
2750b 1870 3041 1371 729
0.23 0.46 0.22 0.52 0.71
1086.7 1117 1093 1072 1112
a The frequency corresponding to the more intense band of the doublet. b The frequency corresponds to Fermi resonance window on the ν(OH) absorption. c Johnson, G. L.; Andrews, L. J. Am. Chem. Soc. 1982, 104, 3043. d Barnes, A. J.; Beech, T. R.; Mielke, Z. J. Chem. Soc., Faraday Trans. 2 1984, 80, 455. e Barnes, A. J.; Wright, M. P. J. Chem. Soc., Faraday Trans. 2 1986, 82, 153.
Ammonia Complexes with Nitric, Nitrous Acids, and Hydrogen Halides. In Table 5 the HX stretching frequencies and relative frequency shifts are compared for ammonia complexes with nitric, nitrous acids, and hydrogen halides. The frequencies of the perturbed NH3 deformation modes in the complexes are also presented. The series of ammonia complexes with HF, HCl, HBr, and HI in argon matrices provides examples of the three types of hydrogen bonds with molecular asymmetric (H3N‚‚‚H-F, H3N‚‚‚H-Cl), molecular symmetric (H3N‚‚H‚‚ Br), and ion-pair type (H3N-H+‚‚‚I-) of hydrogen bond. As can be seen in Table 5, the relative shift of the ν(OH) stretching frequency in the H3N‚‚‚HONO complex is comparable with the relative shift of the ν(HF) frequency in the H3N‚‚‚ HF complex, whereas the relative shift in the H3N‚‚‚HONO2 complex has a larger value. This indicates that ammonia forms a weaker hydrogen bond with nitrous than with nitric acid and that the H3N‚‚‚HF and H3N‚‚‚HONO complexes exhibit the same type of strong, molecular hydrogen bond. The performed calculations predict small elongation of the OH bond (0.03 Å) of the HONO molecule on complex formation, which clearly indicates the molecular type of hydrogen bond. The relative shift of the ν(HX) frequency serves as a good indicator of hydrogen bond strength not only for the same type of hydrogen bond (as within the B‚‚‚HX series) but also for different types of hydrogen bonds with HX and OH groups as proton donors. Conclusions Infrared matrix isolation studies and ab initio calculations indicate that the trans- and cis-HONO isomers form with ammonia strong, molecular hydrogen-bonded complexes with OH group directed toward the lone pair of NH3. The binding energy at the MP2 level with the G-311+G(2d,2p) basis set is -40.13 and -36.39 kJ mol-1 for H3N-HONO-trans and H3N‚‚‚ HONO-cis complexes, respectively. Five perturbed HONO vibrations and one perturbed NH3 deformation vibration were identified for the H3N‚‚‚HONOtrans complex isolated in an argon matrix. Only one perturbed HONO vibration and one perturbed NH3 deformation vibration were identified for the H3N‚‚‚HONO-cis complex due to much smaller concentration of the cis complex with respect to the trans complex in the studied matrices. The perturbed OH stretching vibration in the H3N‚‚‚HONO-trans complex is ca. 800 cm-1 red-shifted and the NOH in-plane bending vibration
is ca. 190 cm-1 blue-shifted from the corresponding absorptions of trans-HONO monomer. Strong perturbations of the N-O stretch and ONO bending vibrations also occur; the observed ca. 80 cm-1 blue shift of the N-O stretching vibration agrees well with predicted shortening of the N-O bond after complex formation. The calculated spectra reproduce very well the frequencies and the intensities of the measured spectra of the two complexes. Acknowledgment. Z.M. gratefully acknowledges financial support from the Polish State Committee for Scientific Research (Grant KBN No 2 0848 91 01). References and Notes (1) Baldeschwieler, J. D.; Pimentel, G. C. J. Chem. Phys. 1960, 33, 1008. (2) Hall, R. T.; Pimentel, G. C. J. Chem. Phys. 1963, 38, 1889. (3) Cox, P. A.; Brittain, A. H.; Finnigan, D. J. Trans. Faraday Soc. 1971, 67, 2179. (4) Finnigan, D. J.; Cox, A. P.; Brittain, A. H. J. Chem. Soc., Faraday Trans. 2 1972, 68, 548. (5) Varma, R.; Curl, R. F. J. Phys. Chem. 1976, 80, 402. (6) McGraw, G. E.; Bernitt, D. L.; Hisatsune, I. C. J. Chem. Phys. 1966, 45, 1932. (7) McGraw, G. E.; Bernitt, D. L.; Hisatsune, I. C. J. Am. Chem. Soc. 1970, 92, 775. (8) Allegrini, M.; Johns, J. W. C.; McKellar, A. R. W.; Pinson, P. J. J. Mol. Spectrosc. 1980, 79, 446. (9) Maki, A. G.; Wells, J. S. J. Mol. Spectrosc. 1980, 82, 427. (10) Maki, A. G.; Sams, R. L. J. Mol. Struct. 1983, 100, 215. (11) Maki, A. G. J. Mol. Spectrosc. 1988, 127, 104. (12) Deeley, C. M.; Mills, I. M. J. Mol. Struct. 1983, 100, 199. (13) Deeley, C. M.; Mills, I. M. Mol. Phys. 1985, 45, 23. (14) Holland, S. M.; Stickland, R. J.; Ashfold, M. N.; Newnham, D. A.; Mills, I. M. J. Chem. Soc., Faraday Trans. 1991, 87, 3461. (15) Guilmot, J. M.; Godefroid, M.; Herman, M. J. Mol. Spectrosc. 1993, 160, 387. (16) Guilmot, J. M.; Me´len, F.; Herman, M. J. Mol. Spectrosc. 1993, 160, 401. (17) McDonald, P. A.; Shirk, J. S. J. Chem. Phys. 1982, 77, 2355. (18) Shirk, A. E.; Shirk, J. S. Chem. Phys. Lett. 1983, 97, 549. (19) Guillory, W. A.; Hunter, C. E. J. Chem. Phys. 1971, 54, 598. (20) Crowley, J. N.; Sodeau, J. R. J. Phys. Chem. 1989, 93, 4785. (21) Coffin, J. M.; Pulay, P. J. Phys. Chem. 1991, 95, 118 and references therein. (22) Pagsberg, P.; Ratajczak, E.; Sillesen, A.; Latajka, Z. Chem. Phys. Lett. 1994, 227, 6. (23) Barnes, A. J. J. Mol. Struct. 1983, 100, 259. (24) Gmelin Handbuch der anorganischen chemie; 1955; Vol. 23, p 85. (25) Frisch, M. J.; Tricks, G. W.; Head-Gordon, M.; Gill, P. M. W.; Wong, M. W.; Foresman, J. B.; Johnson, B. G.; Schlegel, H. B.; Robb, M. A.; Replogle, E. S.; Gomperts, R.; Andres, J. L.; Raghavachari, K.; Binkley, J. S.; Gonzalez, C.; Martin, R. L.; Fox, D. J.; Defress, D. J.; Baker, J.; Stewart, J. J. P.; Pople, J. A. GAUSSIAN-92; Gaussian, Inc.: Pittsburgh, PA, 1992. (26) Hehre, W. J.; Ditchfield, R.; Pople, J. A. J. Chem. Phys. 1972, 56, 2257. Hariharan, P. C.; Pople, J. A. Theor. Chim. Acta 1973, 28, 213. (27) Frisch, M. J.; Pople, J. A.; Binkley, J. S. J. Chem. Phys. 1984, 80, 3265. (28) Krishnan, R.; Binkley, J. S.; Seeger, R.; Pople, J. A. J. Chem. Phys. 1980, 72, 650. (29) Boys, S. F.; Bernardi, F. Mol. Phys. 1970, 19, 553. (30) Abouaf-Marguin, L.; Jacox, M. E.; Milligan, D. E. J. Mol. Spectrosc. 1977, 67, 34. (31) Suzer, S.; Andrews, L. J. Chem. Phys. 1987, 87, 5131. (32) Nelander, B.; Nord, L. J. Phys. Chem. 1982, 86, 4375. (33) Mielke, Z.; Tokhadze, K.; Kolodziej, J.; Latajka, Z. To be published. (34) Evans, J. C. Spectrochim. Acta 1960, 16, 994. (35) Mielke, Z.; Barnes, A. J. J. Chem. Soc., Faraday Trans. 2 1986, 82, 437.
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