J. Phys. Chem. 1995, 99, 10498-10505
10498
Infrared Matrix Isolation Studies of Methane-Nitric Acid System Z. Mielke," K. G. Tokhadze,+ and M. Hulkiewicz Institute of Chemistry, Wroctaw University, Joliot Curie 14, 50-383 Wroclaw, Poland
L. Schriver-Mazzuoli," A. Schriver, and F. Roux Luboratoire de Physique Moleculaire et Applications CNRS UPR 136, Universite Pierre et Marie Curie, Tour 13, Bte 76, 4 Place Jussieu, 75252 Paris Cedex 0.5, France Received: Februan 2, 1995; In Final Form: April 25, 1995@
The infrared spectra of CHI/HNO3/Ar matrices and their deuteurium analogs have been studied in the concentration range (l/l/lOOO)-( 16/2/1000). The spectra prove the formation of a well-determined 1:1 C& .HNO? complex and suggest an existence of (C&),HNO' aggregates of relatively well-defined geometry. The perturbed nitric acid fundamentals were identified for all 1: 1 complexes between CH4, CDJ and HNO3, DN03. The relatively strong perturbation of OH (OD) group vibrations indicates the complex geometry with the OH group directed towards a face of the methane tetrahedron and formation of the weak hydrogen bonding between nitric acid and methane molecules. The temperature reversible fine structure of the two absorptions due to the O H stretching and NOH in plane bending vibrations in the spectra of CD4**-HNOjcomplexes suggests that methane molecule undergoes hindered rotational motion in the complex. The formation of the CHI. mHNO3 complex does not affect, or affects insignificantly, the photolytical behavior of HNO! in solid argon.
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Introduction The study of van der Waals complexes and weak hydrogenbonded complexes continues to be an interesting area of physicochemical research leading to better understanding of the nature of intermolecular interaction. A new and interesting class of H-bonded systems are CHc**HX(X = C1, F, CN) complexes. The recent experimental and theoretical studies of the CHq .HX complexes'-* clearly indicate that the methane molecule acts as a proton acceptor in these systems which is a quite interesting result for understanding the nature of the hydrogen bonding. The existence and spectroscopic characteristics of the CH4. .HCl and CH1- * *HFcomplexes have been first reported by Barnes' and Andrews et a].: respectively, from their matrix isolation studies. Recently, the microwave spectra of the CHd**.HCN, CHI*.*HF, and CH4***HClcomplexes in the gas phase have been observed and analyzed by Legon et al.5.hand by Ohshima and Endo.' The obtained results allowed the conclusion that the CH4. * *HCNand CHc *HCI complexes have an effective Ci, geometry with the acidic proton pointed at the center of a face of the methane tetrahedron leading to single hydrogen bond. In the CH1.*.HCl complex the methane molecule has the capacity to undergo internal rotation;',' its mechanism has been recently studied by an ab initio quantum chemical method.x In the CH4-s .HF complex the interaction was found to be of the two-center type involving bonds CH. *F and C. -*HFwhich effectively quench the internal rotation of the CH5 subunit.h In this paper we report the results of matrix isolation studies of the CHJ-.*HNO~system. The nitric acid molecule is an interesting proton donor which contains both a strongly acidic proton and an electron-rich -NO? group. Investigation of the intermolecular interaction and structure of the CH4. sHNO3 complex can provide more information on the proton donor and
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Permanent address: Department of Molecular Spectroscopy. Institute of Physics. St. Petersburg University. Russia. Abstract published in Adi,utice ACS Ahsrracrs, June 1 . 1995.
'
proton acceptor properties of the CHd and HN03 molecules. The interaction between the two molecules should be well manifested in a perturbation of the multidimensional normal coordinate system of the HN03 molecule. Both nitric acid and methane are important atmospheric species. Nitric acid is known to act as a stratospheric reservoir for NO,. Recently, Barnes et aL9 have reported the results of infrared matrix isolation studies of nitric acid complexes with N?, CO, HzO, (CH3),0, and NH3; these complexes may play a role in atmospheric chemistry. Methane is the single naturally produced organic present in largest concentration in ambient air throughout the world. The major atmospheric sink for CHI is the reaction with OH: CH4 OH 4 CH3 f HrO. Nitric acid serves often as the photolytic source of OH radicals in the laboratory gas phase kinetic studies of the above reaction.'[' Recently, Cheng et al.' studied the photolytic behavior of nitric acid in low-temperature matrices and found that recombination of OH and NO? photofragments in the matrix cage led to formation of peroxynitrous acid HOONO. The aim of the present work was to investigate the interaction of nitric acid with methane: the photolytic behavior of the CH4*..HNO? system in an argon matrix was also studied.
+
Experimental Section Nitric acid was prepared by adding drop by drop concentrated (98%) sulfuric acid (or deuterated sulfuric acid) to solid potassium nitrate. In order to minimize the HNO3 decomposition the reaction vessel was kept at a temperature of 0 "C. The reaction product was vacuum distilled. The mixtures of methane (CH4 or CDd), nitric acid, and argon were prepared using standard manometric techniques. An overall CHdHNOdAr ratio was varied in the range (1/1/1000)-( 16/2/1000). 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). Infrared spectra were recorded with the matrix maintained at ca. 11 K. The spectra were registered
0022-3654/95/2099- 10498$09.00/0 0 1995 American Chemical Society
J. Phys. Chem., Vol. 99,No. 26, 1995 10499
IR Matrix Isolation Studies of Methane-Nitric Acid
0.6
1
A
0.5
E
5
0.4
W
V
z 0.3
s:m a
WAVENUMBER CM -1
0.2
Oal 0.0
Figure 2. The region of the NO1 asymmetric stretching vibration (A) and 0-NO2 stretching vibration (B) in the spectra of C H m N O d A r matrices of concentration 1/1/1000 (a), 2/1/1000 (b), and 8/1/400 (c).
t il, 3520
0.5
I
A
0.5
1
B
0.4
3500
3480
3460
0.3
WAVENUMBER CM -1
Figure 1. The region of the OH stretching vibration in the spectra of CH&INO3/Ar matrices of concentration 1/1/1ooO (a), 2/1/1000 (b), and 8/1/400 (c) and in the spectrum of CH&IN03 = 1/1000 matrix (d). The arrows indicate in all figures the methane-nitric acid product absorptions.
at 0.5 cm-' resolution in a reflection mode, with a Bruker 113v Fourier Transform infrared spectrometer. Two different light sources have been employed for photolysis: a medium pressure Hg lamp (200 W) and a Nd:YAG laser (266 nm, 20 Hz, Spectra Physics DCR-11).
Results The spectra obtained in the present work for nitric acid isolated in argon agree well with those previously rep~rted?~"-'~ C h , CD4 HNO3. The spectra of CH4/HNO3/Ar matrices deposited at 20 K showed a number of prominent new absorptions as compared to the spectra of parent molecules. Figure 1 presents the region of the OH stretching vibration in the spectra of CHdHN03/Ar matrices with increasing methane concentration. The most intense new band in the v(0H) region occurred at 3479.4 cm-' and was accompanied by a weak shoulder at 3478.0 cm-' on its low-frequency side and by a broad absorption at ca. 3486 cm-' on its high-frequency side. The intensity of the 3486 cm-' absorption relative to that of the 3479.4 cm-' band increased in the spectra of matrices with higher methane concentration. In the spectra of matrices with a high excess of CHq an additional broad shoulder also occurred at 3490 cm-', close to the vl(0H) frequency of nitric acid isolated in a methane matrix. For two-product absorptions, an intense band accompanied by a weaker broader one was also observed in the v2 NO2 asymmetric stretching region of the spectrum at 1698.3 and 1697.5 cm-I, in the v3 NOH in plane bending region at 1331.4 and 1329.0 cm-' (the latter absorption was observed as a shoulder), and in the v5 0-NO2 stretching region at 893.6 and 893.1 cm-' (see Figures 2 and 3a). The intensities of the 1697.5, 1329.0, and 893.1 cm-' bands increased relative to the 1698.3, 1331.4, and 893.6 cm-' ones in the spectra of matrices with higher methane concentration.
+
1320
1340
WAVENUMBER CM -1
Figure 3. The region of the NOH in plane bending vibration in the spectra of CHdHNOdAr matrices (A) of concentration 2/1/1000 (a) and 8/1/400 (b) and in the spectra of CD4/HNO3/Ar matrices (B) of concentration 2/1.5/1000 (a), 2/1/1000 (b), and 8/2/1000 (c).
One product absorption, apparently for the principal complex, was observed in the vg out of plane bending region at 764.8 cm-' and in the v9 OH torsion region of the spectrum at 495.0 cm-I. New absorptions were also observed in the vicinity of some overtones and combination transitions of nitric acid monomer, but their behavior was not always clear due to the small intensities of these bands. The weak product bands were observed at ca. 2988 cm-' and ca. 2586 cm-I, close to the 2990.1, v2 v4, and 2588.8, v2 v5 combination bands of HNO3 monomer in argon matrix. A new weak, broad band occurred at 2658.0 cm-I, close to the 2635.5 cm-' band assigned to the 2v3 overtone of HNO3 molecule in argon. Figure 4 presents the product bands appearing in the vicinity of the v2 v4 and 2v3 HNO3 transitions. All product absorptions observed in the spectra of HNO3/CWAr matrices are presented in Table 1. CD4 HNO3. Two experiments were performed with nitric acid and deuterated methane. In the spectrum of CD4/HN03/ Ar = 2/1/1000 matrix the CD4 HN03 product bands corresponding to all CHq HNO3 product absorptions were identified. The bands characteristic of the CDfiNO3 system were also observed in experiments with CD4 and nitric acid enriched in DNO3 (ca. 50%). In the spectra of CDJ(HNO3, DN03)/Ar matrices the same set of frequencies corresponding to the product absorptions of the CD4 HNO3 system was identified as in the spectra of the CD4/HNO3/Ar matrices. The bands observed at 3479.4, 1698.3, and 893.6 cm-' in the CHq HN03 experiment were shifted, respectively, to
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10500 J. Phys. Chem., Vol. 99, No. 26, 1995 I
Mielke et al. I
!
0.5
-
I'
I' J-
I
Id
r/i
c
0.4
5W 0.03 u z
2 0.02
s:m
r
2 0.3
4
3
w
0
10.0-1
2990
10.0- 1
2980
E e m 2660
2640
WAVENUMBER CM -1
Figure 4. The region of the vl + v4 (A) and 2 ~ 3(B) combination transitions of HNOi monomer i n the spectra of HNOdAr = 1/1000 matrix (a) and CHdHNOdAr = 2/1/1000 (b) and 8/1/400 (c) TABLE 1: Product Absorptions (cm-') of Methane-Nitric Acid Sample in Solid Argon before and after Photolysis with a Medium Pressure Hg Lamp new lines after photolysis product lines before photolysis cm-' assignment cm-I assignment 3490 3486 3479.4 2988 2658 2586 1698.3 1697.5 1331.4 1329.0 893.6 893.1 764.8 495.0
3562.7 3544.3 3538.7 3535.2 1876.4 1871.5 1842.0 1726.4 1708.3 1703.6 1600.4 1577.4 1562.8 1498.0 1390.0 1312.7 1364.4 1025.0 957.5 952.0 873.8 78 1.2 773.0
HOONO A HOONO B HOzNO?? HO?NO?? NO complexed NO ?
HO?NO?? HOONO A HOONO B 0 2
complexed
HNO complexed HNO
HO?? HOONO A HOONO B HOONO A HOONO B 9
HOONO A HOONO B
"m>n
+
3480.2, 1697.8, and 893.8 cm-I in the CD4 HNO3 experiment. They were accompanied by weaker and broader bands at ca. 3485, 1697, and 893 cm-I. On the low-frequency side of the 3480.2 cm-' vt(OH) band, an additional weak band was observed at 3478.5 cm-l. The 1331.4 cm-' band characteristic of the CH,c/HNO3/Ar matrices is shifted to 1332.6 cm-I in the spectra of the CD&INOJAr matrices. As can be seen in Figure 3b, the band is accompanied by two other absorptions at 1331.3 and 1329.5 cm-I in the spectrum of the matrix with small CD4 excess. In matrices with higher CD4 concentration an additional subband is observed at 1330.6 cm-I. The relative intensity of this component increases, with respect to the 1332.6 cm-I band, with an increase of CD4 concentration. The relative intensities of the other two satellite bands at 1331.3 and 1329.5 cm-' do not depend on CD4 excess in the matrix. The shift of the v4 methane absorption from ca. 1305 to ca. 990 cm-' after methane deuteration uncovered the v4 HNO3 band due to the 1:l CDJ
CI:
0 2 0.2
e
0.1 .
0.0
4 1330
1320
1310
WAVENUMBER C M - 1
Figure 5. Infrared spectra of the CDs/HN03/Ar = 2/1/1000 (a), CDJ (HNOj, DNO3)/Ar = 2/1.5/1000 (b), CHmN03IAr = 2/1/1000 (c). and CKJ(HNO3, DN03)/Ar = 2/1/1000 (d) matrices in the 1335-1305 cm-' region (HNO? 50% enriched in DNO?). HNO3 complex. The band was observed at 1306.5 cm-I in the spectra of CD&INO3/Ar matrices (see Figure 5 ) . The 764.8 cm-' product absorption in the region of the vg HN03 vibration was observed at the same frequency in both the CH4 HNO? and the CD4 HNO3 experiments. The weak product absorptions were also identified at 2989, 2660, and 2585 cm-I. The observed CD4 HNO3 product bands are compared with corresponding C h HN03 absorptions in Table 2. C h , CD4 DN03. Another series of experiments was performed with ca. 50% DNO3 enriched nitric acid. The strongest product absorptions for the CH/DNO3 system occurred at 2576.7, 1677.0, 1313.8, 1020.0, 893.8, a d 764.5 cm-l, and for the C D n N O 3 system at 2575.4, 1676.7, 1313.1, 1019.3, 893.7, and 764.3 cm-'. Figure 6 presents the vl(OH) and V I (OD) stretching regions in the spectrum of the CDJ(DNO3, HN03)/Ar = 1/4/500 matrix. One can note two shoulders, at 3485.6 and 3488.8 cm-I, on the high-frequency side of the 3480.2 V I(OH) stretching and the corresponding shoulders at 2577.4 and 2579.5 cm-' on the high-frequency side of the 2575.4 VI(OD)stretching vibration. These shoulders increase in intensity with CD4 concentration. On the low-frequency side of the vl(0D) band the additional weak absorptions are observed at 2563.7, 2566.1, and 2571.0 cm-I. Figure 5 presents the region of the v3 NOH (NOD) in-plane bending and the region of the v4 NO2 symmetric stretching vibrations in the spectra of the C W N O J A r and CD&N03/ Ar matrices; for comparison the spectra of the CH&-IN03/Ar and CD&NO3/Ar matrices are also presented. One can note the effect of replacement of CH4 by CD4 on the shape of the band occumng at ca. 1331 cm-' and assigned to NOH in-plane
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IR Matrix Isolation Studies of Methane-Nitric Acid
J. Phys. Chem., Vol. 99, No. 26, 1995 10501
TABLE 2: Absorptions (cm-l) Observed for the Perturbed HNO3 Modes in Methane-Nitric Acid 1:l Complex in Solid Argon HN03
cmo3
CD4HNO3
DNO3
3522.3 3519.0 2990.1 2635.5 2588.8 1699.4 1321.4 1304.4 896.9 763.6 450.3
3479.4
3480.2 3478.5 2989 2660 2585 1697.8 1332.6 1306.5 893.8 764.8
2601.5
2988 2658 2586 1698.3 1331.4 893.6 764.8 495.0
CbDN03
assignment
CD4DNO3
+ v4 2V3 v2 + vs VI, NO2 a-stretch v3, NOH i.p. bend v4, NO2 s-stretch VS. 0-NO2 stretch YE,ON02 0.p. bend vg, OH torsion v2
2567 1678.1 1013.5 1310.5 894.3 763.6
2564 1677.0 1020.0 1313.8 893.8 764.5
+
0.05 t
0.06
2564 1676.7 1019.3 1313.1 893.7 764.3
A
B 0.048. b
m0)
0.025
B2
0.036
P .-.
m
5
5 0.024 2
uz
0.012
t
I
w 0.00
0.0
+
I
3490
3480
I
I
1332.5
1327.5
WAVENUMBER CM -1
Figure 7. Temperature effects in the V I OH stretching region (A) and v3 NOH in plane bending region (B) (of complexed HNO3) in the spectra of matrix C D n N O J A r = 2/1/1000.
0.00 3520
3500
3480
WAVENUMBER CM -1 Figure 6. The region of the v(0H) (A) and v(0D) (B) stretching vibrations in the spectrum of CDJ(HNO3, DN03)/fb = 4/1/500 matrix (HN03 50% enriched in DNO3).
bending vibration (as will be discussed later). The second band in this region is due to v4 NO2 symmetric stretching vibration. It is observed at 1306.5, 1313.8, and 1313.1 cm-' for CDd HNO3, CH4/DNO3, and CD4/DN03 systems, respectively. The 1313.1 cm-' band characteristic of the CD4/DN03 system also shows some structure with subpeaks at 1312.1 and 1311.2, which is in contrast to the structureless 1313.8 cm-' band of the CH4/DNO3 system. Temperature Studies. In order to find the origin of the fine structure of the 1332.5 cm-' band in the spectra of the CDd HN03 system, the effect of temperature increase from 11 K up to 32 K on the spectra of the CD4HNO3/Ar matrices has been studied. Most sensitive to temperature increase were the bands due to OH (OD) group vibrations in the spectra of the CD4/ (€€NO3, DN03)/Ar matrices. As mentioned earlier, the band occurring at 1332.6 cm-I in the spectra of CDdHNOs/Ar matrices has a characteristic structure with two subpeaks at 1331.0 and 1329.5 cm-I. The corresponding band at 1331.4 cm-I in the spectra of CI-I&INOd Ar matrices is relatively broad but smooth (see Figure 3). An increase of the temperature of the CDdHNO3/Ar matrix up to 32 K is accompanied by the pronounced change of the 1332.6
cm-' band shape as illustrated in Figure 7B. The principal peak shifts from 1332.6 cm-' at 11 K to 1331.4 cm-' at 32 K; the other two subpeaks broaden and diffuse. The observed evolution of the band shape is completely reversible; when the matrix is cooled down to 11 K, the original band is reproduced again. Similar, although less pronounced, temperature dependence is observed for the band due to the OH stretching vibration in the spectra of the CD4/HNO3/Ar matrices. The main absorption peak shifts from 3480.2 cm-' at 11 K to 3482.1 cm-' at 32 K; both the main peak and the subpeak at 3478.4 cm-l broaden considerably with temperature increase, and the subpeak is not observed anymore at 32 K (see Figure 7A). Photolysis of c-03 System in Argon. Photolysis of the CH4/HNO3/Ar matrices with a medium pressure mercury arc led to the same products as photolysis with a 266 nm laser line. Irradiation of the matrices was accompanied by a strong decrease of the absorptions due to methane-nitric acid complexes and simultaneous but weaker increase of HN03 absorptions. Figure 8 illustrates the effect of photolysis with a medium pressure Hg lamp on the YI(OH)region of the CH&NO3/Ar = 8/1/400 matrix. Irradiation of this matrix for a total 7 h with the full output of the Hg arc (220-1000 nm) destroyed more than 80% of the CH4. * + I N 0 3complex, as determined from the change in intensity of the YI(OH)absorption at 3479.4 cm-I, and augmented the concentration of HNO3 monomer in the matrix by ca. 10%. Simultaneously, the irradiation gave rise to a set of new weak absorptions. The positions of these absorptions are listed in Table 1. Most of the new absorptions are due to HNO3 photodecomposition products. The two sets of bands appearing at 3562.7, 1708.3, 1372.7,957.5, and 781.2 cm-' and at 3544.3, 1703.6, 1364.4, 952.0, and 773.0 cm-I
Mielke et al.
10502 J. Phys. Chem., Vol. 99, No. 26, 1995 0.4
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acid molecules. The absorption at 3479.4 cm-' is due to the vl(0H) stretching vibration of HN03 interacting with C h . This vibration is down 42.9 cm- from the corresponding vibration of free HNO3 in solid argon. The comparison of the relative AVIYHA values (where Av = YHA - Y H A . .B) for C& +fNO3 and CH4. **HFcomplexes shows that HNO3 acid is a very strong proton donor, much stronger than HF.4 The formation of the CH4. *m03complex leads to perturbation of all HNO3 modes. The complex absorption at 1698.3 cm-' is 1.1 cm-l red shifted from the v;?NO2 asymmetric stretching mode of HNO, monomer. The band at 1331.4 cm-' is attributed to the perturbed v3 mode of complexed HN03. The corresponding mode occurs at 1321.4 cm-' in the spectra of free HN03 in argon" and corresponds to the mixed NOH in plane bending and NO;?symmetric stretching coordinates.'" The main contribution to the 1331.4 cm-' mode in the CH". * *HNO3 complex is probably the NOH in-plane bending coordinate. The 893.6 and 764.8 cm-I product bands correspond, respectively, to the v5 O-NO2 stretching and vg ON02 out of plane bending vibrations of HN03 in the complex. These modes occur, respectively, at 896.9 and 763.6 cm-' in the spectra of the acid monomer. The 495.0 cm-' product band is due to the v g OH torsion vibration of the complexed HN03. This band is 44.7 cm-' blue shifted from the corresponding band of free HNOi. The band observed at 2586 cm-' is assigned to the v? v5 combination transition in the bonded HNO3 molecule. The 2.8 cm-' red shift of this band from the 2588.8 cm-' absorption of free HN03 is in agreement with lower frequency values of both v 2 and v5 fundamentals in the complex as compared to those of HNO3 monomer. The 2988 cm-' product band is tentatively assigned to the v2 v4 transition in the bonded acid molecule. The band is 2.1 cm-' red shifted from the corresponding absorption of free HNO3. The 2658.0 cm-' product band is 22.5 cm-I blue shifted from the 2635.5 cm-' HN03 monomer absorption assigned to the 2v3 overtone. The blue shift of the 2v3 overtone in the bonded HNO3 molecule is in agreement with the 10.0 cm-' blue shift of the v3 fundamental in the complex as compared to that of HN03 monomer. The observed perturbation of the modes of HNO3 molecule in the CH"**.HNO3 complex indicates that HN03 acts as a proton donor forming a weak hydrogen bond with methane molecule. This is confirmed by relatively strong perturbation of the OH group vibrations. As was discussed above, the OH stretch is 42.9 cm-' shifted toward lower frequencies compared to that of HN03 monomer whereas the NOH in plane bending and OH torsion are shifted 10.0 and 44.7 cm-I, respectively, toward higher frequencies. The 0-NO;? group vibrations are only weakly perturbed by complex formation, and there is no evidence that the -NO2 group interacts directly with the methane molecule. The perturbed v5 O-NO2 stretch vibration is 3.3 cm-' shifted toward lower frequencies compared to that of the HNO3 monomer. In spite of the careful examination of the obtained spectra, we did not identify, unfortunately, any of the perturbed C& modes. The v3 F2 C h asymmetric stretching and v4 F2 CH4 deformation modes are expected to be only weakly perturbed by the complex formation. The absorptions corresponding to these modes are probably covered by the intense absorptions of nonbonded CH" molecules. Davis and Andrews4 observed in the spectra of CH4. *HFcomplexes the v1 A1 CHq stretch activated by the complex formation. Despite careful analysis of the studied spectra, we did not identify the corresponding mode for the CH4. **HNO3complex. Perhaps the absorption characteristic for the perturbed V I CH4 vibration in the CH4. aHNO3 complex is weaker than the corresponding absorption for the CHg *HF complex, and the concentration
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3560
3540
3520
3500
3480
W A V E W E R CM -1
Figure 8. The V I OH stretching region in the infrared spectra of the CHdHNOl/Ar = 8/1/400 matrix after sample deposition (a) and after 3 h (b) and 7 h (c) irradiation with full output of mercury arc. The 3560-3520 cm-' region of spectrum c scaled by a factor of 10 is presented as c'.
are readily assigned to the two conformers of the HOONO molecule.''~12The bands observed at 1871.5 and 1562.8 cm-' are due to NO and HNO species." The other new bands were observed at 3538.7, 3535.2, 1726.4, and 1390.0 cm-' (in the vicinity of the HOONO absorptions), at 1876.4 and 1577.4 cm-I (in the vicinity of the NO and HNO bands), and at 1842.0, 1600.4, and 873.8 cm-I.
Discussion Identification. The product absorptions occurring after deposition of nitric acidmethanelargon matrices and not observed in the spectra of parent molecules are assigned to the complexes formed between nitric acid and methane. The set of absorptions observed in the spectra of diluted CH4/HN03/ Ar matrices at 3479.4, 1698.3, 1331.4, 893.6, 764.8, and 495.0 cm-I, close to the fundamental HN03 absorptions, and at ca. 2988, 2658, and 2586 cm-I, close to the HN03 overtone or combination bands, are assigned to the perturbed HNO3 vibrations in the 1:l CH4.**HNO3complex. The corresponding bands were identified in the spectra of all studied isotopomers, their frequencies are collected in Table 2. When the methane concentration in the matrix increased additional absorptions occurred in the vicinity of the 1:l complex bands. In the CH4/ HNO3/Ar = 8/1/400 matrix they were observed at ca. 3486 and at 1697.5, 1329.0, and 893.1 cm-'. These bands are much broader than the bands due to the 1:1 complex and their relative intensities increase with respect to the bands assigned to the 1 :1 complex when the methane concentration increases. The bands are assigned to higher order (CH&HNO3 aggregates. Spectral Characteristics and Geometry of the C&.+INO3 Complex. The obtained results show that a weak complex of well-defined structure is formed between methane and nitric
+
J. Phys. Chem., Vol. 99, No. 26, 1995 10503
IR Matrix Isolation Studies of Methane-Nitric Acid of the complex in the matrix was too low for the V I AI mode to be observed. Effect of Isotopic Substitution. The bands corresponding to perturbed nitric acid fundamentals were also identified for the 1 :1 CD4* oHNO3, CH4. *DNO3, and CD4* oDNO3 complexes; the frequencies of these bands are collected in Table 2. The 3479.4cm-l OH stretch in the CH4. *m03complex is shifted to 2576.7cm-I in the C& oDNO3 complex which gives an isotopic shift ratio of 3479.4/2576.7= 1.35. The 1331.4 cm-I vibration characteristic of the CH4. complex is shifted to 1020.0 cm-l in the complex with deuterated acid giving a 1331.4/1020.0= 1.305 cm-l ratio. The relatively low isotopic shift ratio for this mode, as compared to the 1.36 value expected for the harmonic NOH in-plane bending vibration, is due to contribution of the NO2 symmetric stretch to the 1331.4 cm-l mode. The frequencies of the perturbed nitric acid vibrations in methane-nitric acid 1 :1 complexes are also slightly affected by the replacement of methane by deuterated methane molecule. The OH stretching vibration in the methane-HNO3 complex is shifted toward higher frequencies, from 3479.4 to 3480.2 cm-l, when CHq is replaced by CD4. On the other hand, the OD stretching vibration in the methane-DN03 complex is shifted toward lower frequencies, from 2576.7to 2575.4cm-l, on CD4 substitution. In the spectra of the C&**HNO3 complexes the most sensitive to methane deuteration is the 1331.4 cm-l absorption which is shifted to 1332.6cm-I in the spectra of the CD4 *HNO3 complex. Methane deuteration affects both the position and the shape of this absorption. The corresponding absorptions in the spectra of the CH4.**DNO3 and CD4***DNO3complexes were observed at 1020.0 and 1019.3 cm-'. CD4 substitution affects also strongly the frequency and the shape of the 1313.8 cm-' band of the C&**DNO3 complex. The band is due to the perturbed v4 DNO3 mode to which the main contribution gives the NO2 symmetric stretching coordinate. The absorption shifts from 1313.8 to 1313.1 cm-I on CD4 substitution, and additional subbands occur at 1312.1 and 131 1.2cm-l. The corresponding absorption is observed as a single band at 1306.5 cm-l in the spectra of CD4* eHNO3 complexes and is covered by an intense v2 F2 CHq band in the spectra of the CH4.**HNO3complexes. The frequencies of 0-NO2 group are also weakly perturbed by CD4 substitution as can be seen in Table 2. Internal Rotation of Methane-HNO3 Complex? The temperature reversible fine structure of the bands due to the and v3 modes in the spectra of the CDJ-JNOdAr matrices deserves notice. The most distinct structure at 1 1 K shows the absorption due to the v3 mode (see Figure 7). As was discussed earlier, the three components of this absorption can be assigned with confidence to the 1:l CD4***HNO3complex. When the temperature increases to ca. 28 K, the fine structure collapses and a broad band remains. This effect is completely reversible; lowering the temperature to 1 1 K leads to reproduction of the original band structure. From the possible matrix effects which could be responsible for the observed spectral features aggregation should be excluded as it should give irreversible spectral changes. Site effects may be reversible, but it seems unlikely that they affect only the two modes of the CD4***HNO3 complex. The explanation that seems most likely is the assignment of the fine structure to the more or less hindered rotation of methane in the complex. The recent studies of the rotational spectra of the CH4. *HCl and C& *HCN complexes provide evidence that the methane molecule undergoes internal rotation motion in these comp l e x e ~ . ~As - ~ was already mentioned, the two complexes have
1
d
k.. c
b4
c d
Figure 9. The proposed structures for the C&**HNO3 complex.
an effective C3,, geometry with the acidic proton pointed at the center of a face of the methane tetrahedron. Ohshima and Endo7 postulated the two-dimensional internal rotation of methane in the CH4.**HClcomplex whereas Craw et a1.,* on the basis of their quantum-chemical studies, concluded that methane undergoes 3-fold rotation about 1 of 3 CH bonds. It is well known that methane molecules trapped in lowtemperature matrices undergo rotational m o t i ~ n . l ~Evidence -~~ for hindered rotation of CHq and CD4 when trapped in argon, krypton, and xenon was first presented by Cabana et al.I5 More recently, Nelander18 reported that methane rotates also in solid nitrogen. Jones et al.I9 recorded lately high-resolution infrared spectra of CHq trapped in rare gas solids. They observed two dominant sites for the CHq molecule trapped in argon, krypton, and xenon. The fine structure of one site was that predicted for a tetrahedral molecule rotating in an octahedral field; the barrier for this rotation was not clearly defined. The second site had probably C3,, symmetry. But the spectra did not allow for a conclusion of whether the hindered rotation for this site took place about only the C3 axis or about axes perpendicular to C3 as well. As discussed above, the 1 :1 complex between methane and nitric acid in solid argon has geometry of the type C&**HONO2 in which the OH group interacts with a methane molecule forming a single hydrogen bond. There is no evidence of direct interaction between the -NO;! group and methane. Let us assume that the orientation of the OH group toward CHq is similar to the orientation of HCl toward CHq in the CH4. *HCl c0mplex.6.~ So, we assume that the OH group is directed toward the center of a face of the methane tetrahedron and collinear with CHa bond (see Figure 9). The complex has an effective C, symmetry if the angle 8 formed between the plane of the HNO3 molecule and H T H b plane assumes values 8 = n60" where n is an integral number. The effective symmetry of the complex is lowered to Cl for all other 8 angles. The equilibrium geometry is different for 8 with even (8 = 0, 120,240")and odd (8 = 60,180,300") n values, and consequently, the potential energy should be slightly different for these two structures. However, if there is no direct interaction between the NO2 group and methane molecule, one may expect close energy values for the two structures. This indicates a relatively low 3-fold potential barrier for the internal rotation of methane about CHa in the complex. So, methane may undergo hindered rotational motion in the complex. The above conclusion concerning the possibility of rotation of methane in the CH4.**HNO3complex is supported by the rotations of ammonia and ammonia complexes in low-temperature matrices. The rotation of ammonia in noble gas
Mielke et al.
10504 J. Phys. Chem., Vol. 99, No. 26, 1995
matrices20-22and nitrogen mat rice^?^-^' has been the subject of several studies. Ammonia has been shown to rotate around its C3 axis both as a monomer and bound to C1224or to C02.25 The evidence for internal rotation of methane in the C m eHN03 complex would be the temperature reversible fine structure of the v3 and v4 absorptions of bonded methane molecule. Unfortunately, we do not observe any absorption due to the perturbed methane. Instead, we do observe temperature reversible fine structure of the absorptions due to the V I and v3 modes of bonded HN03 in the CD4-.-HN03 complex. One may presume that the angular momentum associated with the internal rotation of methane in the complex may be coupled with vibrational momenta associated with other normal modes of the complex. Ohshima and Endo' attributed the inconsistency between the estimated anisotropic parameters for the CH4. *HC1 and CDc..DCl complexes to the higher order effects of the coupling between the internal rotation and the other vibrational modes of the complexes. The direct interaction between the OH group and methane might be responsible for the coupling between the OH group modes and internal rotation of the methane molecule. The vibrational momentum associated with the NOH in plane bending of bonded HN03 should have a measurable component along the angular momentum associated with methane rotation which would explain the interaction between these modes. So, we tentatively assign the temperature reversible fine structure of the V I and v3 modes of the perturbed HNO3 in the CD4. +IN03 complex to the vibrational-rotational coupling between the OH group modes and internal rotation of methane. The question arises as to why the fine structure occurs for the CD4*.*HNO3 complexes and is not observed for the CH4.. .HNO3 ones. We find two possible explanations of this fact. Bratos and Martin28and E ~ i n showed g ~ ~ that perturbation of the molecule in a van der Waals complex depends on the height of the potential barrier and on the rotational spacing: AEJ-J+~= (4J 6)B, where B is rotational constant. The B(CH4) value is approximately twice the B(CD4) value, and ~ spacing for C h is twice consequently, the A E J - J Arotational the spacing between the corresponding levels in CD4. The difference in rotational spacing of these two molecules may lead to different perturbation of the internal rotation of CHq and CD4. Thus, the CH4 and CD4 rotational motion may be manifested in a different way in the low-temperature spectra of their van der Waals complexes with HNO3. The other possible explanation for the observed difference between CHJ. * eHNO3 and CD4. oHNO3 complexes might be the different vibrational relaxation times from the levels v3 = 1 (where v3 is the perturbed HN03 mode) in these complexes. A much faster relaxation from the v3 = 1 level in the CH4. * complex than in CD4-*.HNO3 may increase the widths of the fine structure components of the v3 absorption of CH4**.HN03 complex leading to their coalescence. Jones et a1.I9 noted, for methane in solid argon, that the width of the fine structure components of the v4 vibrational-rotational band increases with concentration. They attributed this property to the resonance energy exchange or inhomogenous broadening for the v4 = 1 methane level. Nelander'* explained the large difference in the width of the rotational components of different vibrational bands in the spectra of methane isotopomers in solid nitrogen by different population relaxation for the corresponding levels. In the CH4. eHNO3 complex the 1331.4 cm-' perturbed v3 HN03 mode has an energy value very close to that of the v4 C& mode (ca. 1305 cm-I). There might be very fast energy transfer from the v3 = 1 level of HNO3 to the v4 = 1 level of CH4 which may affect the width of the fine structure components of the v3
-
-
+
HN03 absorption. In the CD4***HNO3complex the v4 CD4 vibration is shifted to ca. 995 cm-' which is too low to affect the relaxation time from the v3 = 1 HNO3 level. (CH&HN03 Aggregates. The absorptions assigned to the perturbed HNO3 modes in the 1:1 complex are accompanied by weaker and broader bands. The relative intensities of these bands with respect to the corresponding 1:l complex bands increase with CH4 concentration. This fact seems to suggest that (CH&HN03 ( n > 1) aggregates of relatively well-defined structure exist in the studied matrices. The 3486 cm-I band assigned to the (CH4),HNO3 aggregate is 6.6 cm-l blue shifted from the 3479.4 cm-I band due to the v,(OH) vibration in the 1:l CH?..*HNO3 complex. The band is 3.5 cm-l red shifted from the 3489.5 cm-' absorption corresponding to HN03 monomer in a methane matrix. The 1329.0 cm-I aggregate band is observed as a shoulder on the 1331.4 cm-I absorption due to the v3 NOH in plane bending vibration in the 1:l complex. The bands due to the v? NO? asymmetric stretch and vg O-NO2 stretch are shifted from 1698.3 and 893.6 cm-' for the CH4**.HN03complex to the 1697.5 and 893.1 cm-I, respectively, for the (CHd),,HN03 aggregate. The blue shift of the vl(0H) vibration and small red shift of the v3 NOH in-plane bending vibration in the (CHd),,HN03 aggregate compared to those of the CHc * .HN03 complex suggest that the hydrogen bonding is slightly weakened when the second and farther CHq molecules are attached to the 1: 1 C&*.HNO3 complex. The second and farther CH4 molecules have to interact with the -0-NO2 group which is demonstrated by stronger perturbation of the O-NO2 group vibrations in the aggregate compared to those in the 1:1 complex. The obtained data do not allow us to draw a conclusion about the structure of the (Cb),HN03 aggregates. Photolysis. The photolysis of the CH&INO3/Ar matrices with the 266 nm YAG laser line or with a medium pressure Hg arc output markedly diminished the bands due to the CHJ.*.HNO3 complexes and increased the absorptions due to HNO3 monomer (see Figure 8). However, the photolysis did not affect the overall intensity of the CH4 bands. Simultaneously, the new bands that appeared in the spectra are due to HNO3 photolysis products as discussed below. The frequencies of the bands appearing after matrix photolysis are collected in Table 1. The main bands occurring after photolysis are due to the HN03 photodissociation products: HOONO, HNO, and NO. The frequencies of these bands agree very well with results reported recently by Cheng et a1.l1 from their studies of photolytic behavior of HN03 in solid argon. Some additional very weak bands not reported by Cheng were also observed in the spectra. The weak bands observed at 1876.4 and 1577.4 cm-', close to the 1871.5 cm-I NO and 1562.8 cm-' HNO bands, are assigned to the perturbed vibrations of the NO and HNO species possibly interacting with CH4 or 0 2 molecules. Another sharp weak band was observed at 1600.4 cm-'. and a set of very weak absorptions occurred at 3538.7, 3535.2, 1726.4, 1498, and 1390 cm-I. The intensities of the latter bands increased with respect to HOONO absorptions with prolonged photolysis time which suggests that they are due to secondary photolysis products. The 1600.4 cm-l band is tentatively assigned to the activated 0 2 stretching vibration. There are two possible sources of O(3P) atoms in the studied matrices: the direct photofragmentation of HONO? (along the path HONO2 0 HONO) and secondary photolysis of NOz.I2 The oxygen atoms can migrate through the argon lattice even at 8 K and recombine forming 0 2 . There is also a possibility that 0 2 can be directly formed =)
+
IR Matrix Isolation Studies of Methane-Nitric Acid in the photofragmentation reaction of HOONO (HOONO 4 0 2 HNO). The relatively strong interaction between the HNO radical and 0 2 molecule trapped in the same cage might activate the 0 2 vibration. The products of the photodissociation of the peroxynitrous acid are not yet established. The 3538.7,3535.2, and 1726.4 cm-' bands have frequencies close to the two strongest absorptions of peroxynitric acid observed at 3540.1, 3536.0, and 1728.3 cm-' in the gas phase ~ p e c t r u m . ~The ~ , ~peroxynitric ' acid is known to be formed in the reaction between NO2 and HO2. Such a reaction might occur in the studied matrices. If the migrating 0 atoms react with OH radicals to form HO2, then, in turn, H02 may react with NO2 trapped in the same cage to produce HO2N02. The 1390 cm-' band can be tentatively assigned to H02. We cannot exclude the possibility that HO2NO2 is formed in an addition reaction of 0 atom to HOONO molecule. The additional absorptions occumng in our spectra and not reported by Cheng et al." are very weak, and their assignment is only tentative. The obtained results show that formation of the CHg**HNO3complex does not affect (or affects insignificantly) the photolysis of HNO3 in solid argon. The OH radicals obtained in HONO2 photofragmentation do not react with C& molecules but recombine with NO2 to give HOONO or HON02. This is due to the fact that OH radicals obtained in HON02 photofragmentation are not energized enough to overcome the activation energy barrier for the C& OH reaction. The strong decrease of the bands due to CHg**HNO3 complexes and simultaneous increase of the HNO3 absorptions during photolysis confirm that OH radicals do not react with CHq in the studied matrices. In similar experiments performed with C2H4 HNOdAr matrices the photolysis destroyed both the C2&**HN03 complex and HN03.30
+
+
Conclusions The infrared spectra of matrices obtained by deposition of mixtures and their deuterated isotopomers with excess of argon gave evidence for a well-defined 1:l methanenitric acid complex. The perturbed nitric acid vibrations were observed for the 1:l CHg..HNO3, CDc**HNO3,CHg**HNO3, and CD4***DNO3complexes. The spectra suggest also the formation of the (CH&HN03 aggregates of relatively welldefined geometry. The relatively strong perturbation of the OH group vibrations in the 1:l complex suggests that the complex geometry is similar to the geometry of the CH4. qHC1 and CHg *HCNcomplexes with the OH group directed toward the center of a face of the methane tetrahedron. The temperature reversible fine structure of two absorptions due to the OH stretching and NOH in-plane bending vibrations in the spectra of CDc complexes is explained in terms of hindered rotation of the methane molecule in the complex. c 0 3
J. Phys. Chem., Vol. 99, No. 26, 1995 10505 Photolysis of CHJ-iNO3/Ar matrices with the full output of a mercury arc leads esseptially to the same main products as the photolysis of HNO3 in solid argon. The formation of the CHg N " 3 complex does not affect, or affects insignificantly, the photolytical behavior of HNO3 in an argon matrix.
Acknowledgment. Z.M. gratefully acknowledges financial support from the Polish State Committee for Scientific Research (Grant KBN No. 2 0848 91 01) and the receipt of an MRT (France) fellowship. References and Notes (1) Legon, A. C.; Millen, D. J. Chem. Rev. 1986,86,635. (2) Buckingham, A. D.; Fowler, P. W.; Huston, J. Ch. Chem. Rev. 1988, 88, 963. (3) Barnes, A. J. J . Mol. Struct. 1983,100,259. (4) Davis, S. R.; Andrews, L. J . Chem. Phys. 1987,86,3765. (5) Legon, A. C.; Wallwork, A. L. J . Chem. SOC.,Chem. Commun. 1989,588. (6) Legon, A. C.; Roberts, B. P.; Wallwork, A. L. Chem. Phys. Lett. 1990,173,107. (7) Ohshima, Y.; Endo, Y. J . Chem. Phys. 1990,93,6256. (8) Craw, J.; Bone, R. G. A,; Bacskay, G. B. J . Chem. SOC.,Faraday Trans. 1993,89,2363. (9) Barnes, A. J.; Lasson, E.; Nielsen, C. J. J . Mol. Struct. 1994,322, 165. (10) Finlayson-Pitts, B. J.; Pitts, J. N., Jr. Atmospheric Chemistry: Fundamentals and Experimental Techniques; Wiley: New York, 1986. (11) Cheng, B. M.; Lee, J. W.; Lee, Y. P. J . Phys. Chem. 1991,95, 28 14. (12) Cheng, W. J.; Lo, W. J.; Cheng, B. M.; Lee, Y. P. J . Chem. Phys. 1992,97,7167. (13) Guillory, W. A.; Bemstein, M. L. J . Chem. Phys. 1975,62,1058. (14) Mc Graw, G. E.; Bemitt, D. L.; Hisatsune, F. C. J. Chem. Phys. 1965,42,237. (15) Cabana, A.; Savitsky, G. B.; Homig, D. F. J . Chem. Phys. 1963, 39,2942. (16) Frayer, F. H.; Ewing, G. E. J . Chem. Phys. 1968,48, 781. (17) Chamberland, A.; Belzile, R.; Cabana, A. Can. J . Chem. 1970,48, 1128. (18) Nelander, B. J . Chem. Phys. 1985,82,5340. (19) Jones, L. H.; Ekberg, S. A.; Swanson, B. I. J . Chem. Phys. 1986, 85,3203. (20) Hopkins, H. P.; Curl, R. F.; Pitzer, K. S. J . Chem. Phys. 1968,48, 2959. (21) Cugley, J. A.; Pullin, A. D. E. Chem. Phys. Lett. 1972,17,406; Spectrochim. Acta 1973,29A, 1665. (22) Abouaf-Marguin, L.; Jacox, Ch. E.; Milligan, D. E. J . Mol. Spectros. 1977,67,34. (23) Ribbegard, G.Chem. Phys. 1975,8, 185. (24) Fredin, L.; Nelander, B.; Ribbegard, G. Chem. Phys. 1976,12,153. (25) Fredin, L.; Nelander, B. Chem. Phys. 1976,15,473. (26) Fredin, L.; Nelander, B. Chem. Phys. 1981,60, 181. (27) Nelander, B.; Nord, L. J . Phys. Chem. 1982,86,4375. (28) Niki, H.; Maker, P. D.; Savage, C. M.; Breitenbach, L. P. Chem. Phys. Lett. 1977,45,564. (29) Graham, R. A.; Winer, A. M.; Pitts, J. N., Jr. Chem. Phys. Lett. 1977,51,215. (30) Schriver, L.; Schriver, A.; Mielke, Z. To be published. JP950335G
