J . Phys. Chem. 1989, 93, 558-564
558
Matrix Reactions of Oxygen Atoms with CH3CN. Infrared Spectra of HOCH,CN and CH3CNO Zofia Mielke? Michael Hawkins,$ and Lester Andrews* Chemistry Department, University of Virginia, Charlottesville, Virginia 22901 (Received: February 22, 1988)
Reactions of oxygen atoms and acetonitrile have been investigated in solid argon at 14-17 K. Primary photoproducts include hydroxyacetonitrile(HOCH2CN) and acetonitrile N-oxide (CH,CNO). Hydroxyacetonitrileforms hydrogen-bonded complexes with acetonitrile and acetonitrile N-oxide as the secondary products. Acetonitrile N-oxide is suggested to be formed by a simple bimolecular addition reaction of atomic oxygen with the nitrile nitrogen. The participation of O('D) atoms is considered to increase the yield of hydroxyacetonitrileas compared to acetonitrile N-oxide via H-atom abstraction or insertion reactions. The spectral characteristics of hydroxyacetonitrile, acetonitrile N-oxide, and hydrogen-bondedhydroxyacetonitrile-acetonitrile complex isolated in argon matrices are given.
Introduction There have been few gas-phase kinetic studies reported for reactions of oxygen atoms with nitriles.'-" In each of the O(,P) plus -C=N reactions, the major initial step was assumed to be interaction of the electrophilic O(,P) atom with the .rr bonds of the substrate molecules. In the O(,P) CH3CN reaction, displacement was suggested to be the major reaction channel, i.e., O(,P) + CH3CN CH3 + O C N (or CNO), which proceeded via addition followed by subsequent decomposition into free-radical fragment^.^ Direct molecular rearrangement and H-atom abstraction made little contribution to the overall O(,P) + CH3CN reaction. The minor contribution of the direct abstraction reaction O(3P) CH3CN O H + CH2CN contrasts with its exothermicity. The C-H bond in CH3CN is relatively weak (dissociation energy of 93 kcal mol-' 5 ) , and the abstraction reaction is exothermic4 by 9 kcal mol-'. The paper gives an account of a study of the acetonitrileoxygen atom reaction in solid argon at 15 K. The matrix isolation technique is well-suited for stabilizing transient species that react further in the gas phase. Matrix reactions of CH3CN and 0 atoms have been performed with the aim of getting more information about the mechanism of this reaction.
-
+
+
-
Experimental Section Apparatus. The cryogenic refrigeration system and vacuum vessel have been described previously.6 The spectra were recorded on Beckman IR-12 and Perkin-Elmer 983 spectrophotometers over the range 200-4000 cm-'. Regions of interest were examined with 0.3-cm-' resolution. The temperature of the CsI cold window was determined by a CTI Cryogenics temperature indicator/controller using an Au-Co vs Cu thermocouple. Samples were photolyzed for periods up to 160 min by using the 220-1000-nm output (full arc) of a BH-6 high-pressure mercury arc lamp (1000 W, Illumination Industries Inc.). A 10-cm water filter reduced the amount of infrared radiation incident upon the matrix during photolysis while replacement with aqueous NiS04/CoS04 solutions afforded selective transmission of near-UV light (240-340 nm). Chemicals. Ozone was generated by a static electric discharge (Tesla coil) of oxygen in a Pyrex tube and condensed with liquid N2.' Residual O2 was removed by pumping at 77 K. Normal isotopic O2 (Matheson) and O2enriched to 95% '*O(Yeda, Israel) were used directly. Acetonitrile (Aldrich), CD3CN (MSD Isotopes), and CH3CISN(Stohler Isotopic Chemicals) were condensed at 77 K and degassed. Procedure. Ozone and acetonitrile were diluted with argon and codeposited through two separate spray-on lines. The matrix ratios for acetonitrile and ozone were varied between 100:1 and 600: 1. On leave from Institute of Chemistry, Wroclaw University, Wroclaw, Poland. *Present address: Chemistry Department, University of Warwick, Coventry, United Kingdom. 0022-3654/89/2093-0558$0 1 .50/0
Both gases were deposited at average rates of 1.5 mmol h-' for 6-10 h.
Results CH,CN + 0,. Samples of acetonitrile (Ar/CH3CN = 200:l) and ozone ( A r / 0 3 = 1OO:l) were codeposited at 15 K for 6 h. The infrared spectrum recorded immediately after deposition is shown in Figure la. In addition to the precursor acetonitrile (A) and 03,small quantities of matrix-isolated water and carbon dioxide were detected. Irradiation of the matrix for a total of 90 min with the full output of the Hg arc (220-1000 nm) destroyed more than 60% of the O3 as determined from the change in intensity of the 704-cm-' absorption (Figure 1b). The photolysis of O3was accompanied by the appearance of a large number of bands throughout the entire 200-3700-cm-' spectral range. The positions of these absorptions are listed in Tables I, 11, and 111. The matrix annealing over 20 min at 35 K resulted in absorbance decrease for the bands denoted as X and listed in Table I at 3641, 2291,2262, 1274, 1209, 1061,971,901,888,355, and 256 cm-'. The bands denoted as Z and listed in Tables I1 and I11 at 3478, 3473, 1233, 1074,977, 878,553 (Table III), and 1311 cm-' (Table 11) simultaneously increased in absorbance. The set of bands denoted as Y and listed in Table I1 at 2309, 1381, 1332, 857, 780, and 579 cm-I showed less sensitivity to matrix annealing; their intensities remained approximately constant within experimental error. The concentration studies showed that the relative intensities of the X and Z bands depend also on the relative concentration of the reactants in the matrix. Figure IC presents the spectrum of a matrix obtained by codeposition of acetonitrile (Ar/CH3CN = 450:l) and ozone (Ar/03 = 1OO:l) samples and irradiated for a total of 90 min with the full arc. As can be seen in Figure IC, the relative intensities of the X bands increased in comparison with the Z bands when the CH3CN concentration in the matrix decreased. In several experiments the infrared spectrum of a matrix containing acetonitrile (200:l) and ozone (1OO:l) was recorded before and after 110-min irradiation with the NiS04/ C0S04 solution filter. This irradiation gave the same photoproduct yield as the full arc. I80and - 'SN-EnrichedReagents. Photolysis of argon matrices containing CH3CN + I8O3and CH3CI5N+ O3 resulted in the appearance of bands due to isotopically enriched photoproducts. Absorptions complementary to those labeled X, Y, and Z in Figure 1 were observed and are listed in the Tables. Of the bands denoted X, those at 3641 and 1061 cm-' showed 11- and 24-cm-' shifts, (1) Davies, P. B.; Thrush, B. A. Trans. Faraday SOC.1968, 64, 1836. (2) Bden, J. C.; Thrush, B. A. Proc. R. Sot. London, A 1968, 305, 107. (3) Arrington, Jr., C. A,; Cox, D. J. J . Phys. Chem. 1975, 79, 2584. (4) Bonanno, R. J.; Timmons, R. B.; Stief, L. J.; Klemm, R. B. J . Chem. Phys. 1977, 66, 92. ( 5 ) Handbook of Chemistry and Physics, 62nd ed.; Weast, R. C., Ed.; CRC Press: Boca Raton, FL, 1981-1982. (6) Andrews, L. J. Chem. Phys. 1971, 54, 4935. Kelsall, B. J.; Andrews, L. J . Phys. Chem. 1981,85, 1288. (7) Andrews, L.; Spiker, Jr., R. C. J . Phys. Chem. 1972, 76, 3208.
0 1989 American Chemical Society
Infrared Spectra of HOCHICN and CH3NO
The Journal of Physical Chemistry, Vol. 93, No. 2, 1989 559 1 6
i B E
B E
i
c
i
L
L
!-
I
4000
3000
2000
1600
1200
800
400
WAVENUMBER ( C P 4 - l )
Figure 1. Infrared spectra of an argon matrix containing acetonitrile and ozone: (a, b) A r / C H 3 C N = 400, Ar/03 = 200 recorded after (a) deposition and (b) a total of 90-min full Hg arc irradiation; (c) A r / C H 3 C N = 900, A r / 0 3 = 200 recorded after 90-min full Hg arc irradiation. X = hydroxyacetonitrile,Y = acetonitrile N-oxide or methyl isocyanate, and Z = complexes formed between hydroxyacetonitrileand acetonitrile or acetonitrile N-oxide. TABLE I: Observed Vibrational Frequencies (cm-I) and Assignments for Hydroxyacetonitrile Isolated in Solid Argon a t 15 K assienment isotopic frequencies protonated deuteriated H 1 4 1 6~0 ~ 1 4 ~ 1 8 0 ~ 1 5 ~ 1 6 0 ~ 1 4 ~ 1 6 0 ~ 1 4 ~ 1 8 0 molecule molecule
-
3641 2291 2262 1274
3630 2277 2243 1272
3634 2278 2249 1274
2688
1209 1061 97 1
1208 1061 97 1
90 1 888 355 256
1208 1037 969 909 90 1 '886 354 256
"v(C=N) and v(C-0)
+ S(C-OH).
dOH)
2677
dOD) . ,
Fermi doublet'
2248 1274 1134 897 980 724
1274 1128 897
4CD2) B(OD) v(C0) P(CD2)
B(OH)
960
+ w(CD2)
v(C0) P(CH2)
71 1
+ v(C0) + 4CC)
892 88 1 354 254
TABLE 11: Observed Vibrational Fundamentals (cm-') and Assimments for Acetonitrile N-Oxide Isolated in Solid Areon at 15 K O assianment 2309 1381 1332 (1311)b 780 851" 579"
2306 1374 1305 (1 285) 764 847 573
2298 1381 1323 (1 302) 778 853 577
2297
2282
1341 (1 322)
1314 (1296)
v((3N) B(CH3) u(N0)
MNO)COlnpld v(N0)
v(CC) 808 578
797 571
+ v(N0) + [6(CH3)] + v(CC)
+
v(NCO) + [6(cD3)1 B(NC0)
'The 857- and 579-cm-I bands and their isotopic counterparts, which have the same photochemical behavior, are assigned to the isomeric CH3NCO species as discussed in the text. *The 1311-cm-I band and its isotopic counterparts are identified as the "Z" bands due to the CHBCNO-HOCH2CN complex.
560
The Journal of Physical Chemistry, Vol. 93, No. 2, 1989
Mielke et al.
TABLE 111: Perturbed Vibrational Frequencies (cm-') of Hydroxyacetonitrile Molecule in the Complex with Acetonitrile Isolated in Argon Matrices at 15 K
frequency HI6NI60 3478 3473
Hl6NI80 3461 3462
1233 1074 977
1233 1049
H I5 ~ 1 3478 3413 1233 1074 977 917 898" 872
926 878
877
553
6 0
553
552
~ 1 4 ~ 1 6 0
assignment
~ 1 4 ~ 1 8 0
2572 1136 904 986 721 888" v(CC) 616' 412
614' 410
+ v(C0)
T(OH); r ( 0 D )
"The bands are due to the CH3CNO-HOCH2CN complex. bThe 616-cm-' band and its 614-cm-' counterparts are assigned to COD out-of-plane bending in the CH3CNO-DOCH2CN complex.
2600
2300
3000
3200
3400
3600
WAVENUMBER
Figure 3. Infrared spectra in the 2500-3800-~m-~region of an argon matrix containing acetonitrile (Ar/A = 400) and ozone (Ar/03 = 200) after irradiation with full output of the Hg arc: (a) CH3CN I6O3,hv, 90 min; (b) CH3CN + 180,, hu, 125 min; (c) CD,CN + I6O3,hu, 90 min.
+
2250
I
I
1
1
2270
2290
2710
WAVtNUMBER ( C M -
1
Figure 2. Infrared spectra in the 2240-2320-cm-I region of an argon matrix containing acetonitrile (Ar/A = 400) and ozone (Ar/03 = 200) after 90-min irradiation with full output of the Hg arc: (a) CH3CN + I6O3before photolysis; (b) CH$N 1603, hu; (c) CH3CN + 1803, hu; (d) CH3CI5N I6O3,hv; (e) CD3CN 1603, hu.
+
+
+
respectively, with 1 8 0 3 . The X bands at 971 and 888 cm-' exhibited 2-cm-] l80 shifts. Most sensitive to the ISN enrichment are the two X bands at 2291 and 2262 cm-I, which showed a 13-cm-I shift in the spectrum of lsN-enriched sample as shown in Figure 2. The two bands at 901 and 888 cm-' are shifted 9 and 7 cm-I, respectively, on photolysis of the CH3ClSN/O3sample. Five of the six bands denoted Y undergo shifts in frequency on both 180and 3 15N enrichment. Those at 1332 and 2309 cm-' showed 27- and 3-cm-' shifts with I8O3and 9- and 11-cm-l shifts with lSN,respectively. The bands located at 857, 780, and 579 cm-' are shifted 10, 16, and 6 cm-' on CH3CN/1803photolysis and 4, 2, and 2 cm-' on CH3ClSN/03photolysis, respectively. Of the bands denoted Z (see Table 111) those at 3478, 3473 cm-' (doublet), and 1074 cm-l show 1 1-, 11-, and 25-cm-' shifts,
respectively, with I8O3. The 878-cm-I Z band is sensitive to 15N enrichment and is shifted to 872 cm-' on CH,C'SN/03 photolysis. The 1311-cm-I Z band (see Table 11) is located at 1285 cm-' in CH3CN/'803spectra and at 1302 cm-' in CH3C'SN/03spectra. It can be noticed in Tables I and 111 that there are X, Z band pairs which have relatively close frequencies and show the same behavior on isotopic substitution. Photolysis of a CH3CN sample with scrambled isotopic ozone gave a composite spectrum in the CEN region of Figure 2, an isotopic doublet at 1332 and 1305 cm-', and single l 6 0 bands at 1074 and 1061 cm-I without evidence for mixed isotopic comIt . thus appears ponents (the I8O region was covered by 1603) that the product species contain single 0 atoms. CD,CN + O3and 1803. In Figures 3 and 4 the infrared spectra of (Ar/CH3CN = 200 + A r / 0 3 = 100) and (Ar/CD3CN = 200 + A r / 0 3 = 100) matrices after full Hg arc irradiation are compared. As can be seen, many photoproduct bands suffer large shifts in frequency after deuterium enrichment. The matrix annealing and concentration studies allow identification of absorptions complementary to those labeled X, Y, and Z . The 2688-cm-I band disappeared, and the 2572-cm-' band increased in intensity when the sample was annealed for 20 min at 35 K. The 2688and 2572-cm-' bands are the deuterium counterparts of the 3641-cm-' X band and the 3478-3473-cm-' Z pair with the isotopic shift ratios 1.35 and 1.35. In the 180-enrichedsample the bands are located at 2677 and 2555 cm-', respectively. Only one X band at 2248 cm-' was observed in the -C=N stretching region. The 980-cm-' (X) and 986-cm-' ( Z ) bands correspond to the 1061-cm-' (X) and 1074-cm-' ( Z ) pair in the spectra of the CH3CN/0, matrices (see Figure 4), as concluded from their high
The Journal of Physical Chemistry, Vol. 93, No. 2, 1989 561
Infrared Spectra of HOCH2CN and C H 3 N 0
'
300
'
I
500
'
1
7 -
'
'
-rL
'
,ic
bvaveiunker
,300-' I
~cn-
Figure 4. Infrared spectra in the 200-1500-c1n-~ region of an argon matrix containing acetonitrile (Ar/A = 400) and ozone (Ar/03 = 200) after 90-min irradiation with full output of the Hg arc: (a) CH$N + I6O3;(b) CDJCN + I6O3.
relative intensity and the large I8Oisotopic shift. The bands are located at 960 and 967 cm-I on CD3CN/1803photolysis, suffering 20- and 19-cm-I I8O shifts. The X, Z pair at 897, 904 cm-l shows little or no I8O shift in the spectra of I80-enriched samples, similarly as the 1209, 1233 Cm-' X, Z pair in the protonated samples. The isotopic shift ratios between the 897- and 904-cm-I bands and their 1209- and 1233-cm-I counterparts are 1.35 and 1.36, respectively. The medium-intensity X band at 772 cm-I and its medium-intensity counterpart at 888 cm-I in protonated matrices both show 2-cm-I I8O isotopic shifts. The only reasonable counterparts of the 724,721-cm-I X, Z pair and 412-cm-I Z band are the 971, 977 cm-I X, Z pair and the 553-cm-I Z band, respectively, in the spectra of protonated samples; the corresponding isotopic shift ratios are 1.34 (971/724), 1.35 (977/721), and 1.34 ( 5 53/4 12). A few additional X and Z bands were observed in the spectra of deuteriated samples with no corresponding bands in the spectra of protonated matrices. Those are the intense X, Z pair at 1134, 1136 cm-l and the Z bands at 888 and 616 cm-'. The 1134- and 616-cm-' bands show 6- and 2-cm-I I8O shifts; the other bands were not identified on CD3CN/1803photolysis. The bands denoted Y in the spectra of protonated matrices suffer less shift on deuterium substitution. The 2297-, 1341-, 808-, and 578-cm-I Y bands in D spectra have their counterparts at 2309, 1332, 857, and 579 cm-I in H spectra. The corresponding bands are located at 2282, 1314, 797, and 571 cm-I on CD3CN/I8O3photolysis. Photolysis of CD3CN with an isotopic ozone mixture gave 2297-2282- and 1341-1 3 14-cm-l doublets without intermediate components and otherwise the same spectrum as pure isotopic samples.
Discussion The photoproducts characteristic of CH3CN + O3samples are identified, and their vibrational assignments are given. Attention is paid to the mechanism of the reaction between 0 atoms and acetonitrile in an argon matrix. Hydroxyacetonitrile, HOCH2CN. The X species is identified as HOCH2CN, hydroxyacetonitrile, on the basis of the following data. The X product absorption a t 3641 cm-I is characteristic of the OH group, a conclusion supported by the 11-cm-I I8O isotopic shift. The absolute frequency of the intense band at 1061 cm-I together with its 24-cm-l I8O isotopic shift indicates the presence of a C - O bond in species X. The medium-intensity band at 1209 cm-I and intense band at 256 cm-I are characteristic of the two COH bending modes in a - C H 2 0 H group; their negligible oxygen and nitrogen shifts and large deuterium shifts support this assignment. The fact that all four COH group vibrations correspond to single-component bands with mixed isotopic ozone further suggests that only one COH group is present in the X
species. The presence of the CH2 group in X gains support from a medium-intensity band at 971 cm-I. This band shows a 2-cm-I I8O isotopic shift and a large deuterium shift and is assigned to CH2 deformation. The bands observed at 2291 and 2262 cm-' with 13-cm-I I5N isotopic shifts demonstrated the presence of a -C=N group in the X species. The identification of vibrational bands in the infrared spectrum of X characteristic of C H 2 0 H and -C=N modes together with the stoichiometry of the reaction between an 0 atom and acetonitrile argues strongly that species X is hydroxyacetonitrile, HOCH2CN. The association behavior of species X, as discussed in the following section, confirms its identification. The observed vibrational fundamentals and assignments for five isotopic hydroxyacetonitrile molecules are given in Table I. Assignment of the 0 - H stretching mode is straightforward; however, the C=N stretching region is more complicated. In particular, the observation of two bands at 2291 and 2262 cm-' is reminiscent of CH3CN itself, which exhibits Fermi resonance between v2 (the C=N mode) and v3 v4 (the symmetric %H3 deformation and C-C stretch).8 The presence of these two strong bands for CH3CN unfortunately complicates the product spectrum, but product bands are observed superimposed on each CH3CN band. In the I 8 0 case, the bands are completely resolved at 2277 and 2243 cm-', and one is resolved at 2249 cm-I for I5N with another observed as a shoulder at 2278 cm-I on the side of CH3CISNat 2283 cm-l as illustrated in Figure 2. The strongest X fundamentals that might give rise to a combination band are the C-O stretch at 1061 cm-I and the C-O-H bend at 1209 cm-', and their sum is 2270 cm-', in very good agreement with the 2262-cm-l shoulder which is in Fermi resonance with the 2291cm-I absorption on the side of the acetonitrile band at 2293 cm-I. In the I8O case, the fundamentals shift to 1208 and 1037 cm-I and their sum is 2245 cm-I, low enough to reduce Fermi resonance and allow the upper band to relax to 2277 cm-I; the combination component was observed at 2243 cm-I. In the I5N case, the fundamentals were observed at 1208 and 1061 cm-I and the Fermi doublet components are displaced 13 cm-I from the I4N values primarily due to the I5N shift expected for the C=N stretch (35 cm-I total calculated for harmonic diatomic). The I5N shift is distributed between both components of the Fermi doublet due to the extensive mixing and interaction even though the fundamentals that make up the combination band had little ISN shift. In the CD3CN case, only one X band was observed at 2248 cm-I since the fundamentals (897 and 980 cm-I) come too low to cause Fermi resonance. The C-O stretching vibration is readily identified at 1061 cm-' from its intensity and absolute frequency value, which is very close to the C-0 fundamental in other alcohols.9-" The mode is expected to be mixed with C-C stretching and CH2 deformation modes as is the case in ethanol: chloromethanol,I0 and allyl alcohol." The 24-cm-I '*O isotopic shift exhibited by the 1061-cm-I band indicates that the contribution of other internal modes to the CO stretch is small in the protonated molecule. On deuteriation the band shifts to 980 cm-' as a result of mixing with CD2 deformation modes. The 20-cm-I I8O isotopic shift exhibited by this band is less than expected for a pure C-0 stretch, which confirms mixing of the C-0 vibration with other internal modes. There are two CD2 deformation modes that appear in the vicinity of the C-0 stretch. One mode is observed at 1134 cm-I in the spectra of CD3CN I6O3samples and is shifted to 1128 cm-I in the spectra of CD3CN I8O3samples, indicating mixing with the CO stretch. Unfortunately, the counterpart in CH3CN samples was not observed; it is probably obscured by the intense CH3 deformation bands in acetonitrile. The 1134-cm-I band is probably due to the CD2 wagging mode. A normal-coordinate analysis for allyl alcohol" shows that the CD2 wagging mode is
+
+
+
(8) Johnson, G. L.; Andrews, L. J . Phys. Chem. 1983, 87, 1852. (9) Barnes, A. J.; Hallam, H. E. Trans. Faraday SOC.1970, 66, 1910, 1932. (10) Perttila, M.; Murto, J.; Halonen, L. Spectrochim. Acta, Part A 1972, 28A, 1375. (1 1) Silvi, B.; Perchard, J. P. Spectrochim. Acta, Parr A 1978, 34A, 469.
562
The Journal of Physical Chemistry, Vol. 93, No. 2, 1989
strongly mixed with the C-0 stretch; on this basis the 1134-cm-I band is assigned to the CD, wagging mode. The medium-intensity band at 971 cm-I is assigned to a CH2 rocking mode, which is in good agreement with frequency values observed for the corresponding mode in chloromethanol10and allyl alcohol." The p(CH2)/p(CD2) = 1.34 isotopic shift ratio observed for the 971-cm-I band supports its assignment to CH2 rocking. The 2-cm-l I8O isotopic shift exhibited by this band indicates some mixing with the C-0 stretch. The C-C stretch is identified as 888 cm-' with a frequency identical with the 888-cm-I C-C stretching mode in ethan01.~On deuteriation it shifts to 772 cm-' as the result of mixing with the CD2 rocking mode. In the acetonitrile molecule a similar shift of the C-C stretch on deuteriation is observed as a result of the coupling of the C-C stretch with the CD, deformation mode.'* The C-C stretch exhibits a 2-cm-I I 8 0 isotopic shift in both the protonated and deuteriated molecules, which indicates small contribution of the C - O stretch to the C-C stretch in both molecules. The 1209-cm-' band is readily assigned as the C O H in-plane bending mode. It appears at a very similar frequency as the corresponding mode in ethanol (1241 ~ m - ' ) chloroethanol ,~ (1201 cm-'),'O and allyl alcohol (1226 and 1326 cm-').ll The negligible I 8 0 and I5N isotopic shifts are expected, and the large deuterium shift (isotopic ratio of 1.35) is characteristic of this mode. Its association behavior, appearance of the 1233-cm-I statellite band due to a perturbed C O H in-plane vibration in the X-B (where B = CH3CN or another X) molecular complex, gives credence to this assignment. The out-of-plane C O H deformation or O H torsion is identified as an intense band a t 256 cm-I. The corresponding band appears in the 200-300-~m-~frequency range in the spectra of saturated alcohols (271 cm-I for MeOH,I3 21 1 cm-I for EtOH).9 The 256-cm-I band shows a large deuterium shift in agreement with its assignment, but its deuterium counterpart was not observed as it shifts beyond the studied frequency range. Its association behavior is characteristic of a torsional mode. Acetonitrile N-Oxide, C H , G N - O . Identification of the Y product is less straightforward. The reaction of O(,P) atoms with CH3CN in the gas phase involves electrophilic oxygen attack on the -C=N group and formation of the CH3 and N C O or C N O free radicals as a first step." Recombination of these two radicals in the matrix cage can lead to formation of acetonitrile N-oxide. Acetonitrile N-oxide is unstable at room temperature, undergoing dimerization to form 3,4-dimethylf~roxan.~~The monomer spectrum is known from spectra recorded in CCl,, CS,, and cyclohexane solutions, since bands due to 3,4-dimethylfuroxan can be s ~ b t r a c t e d . ' ~ Four of the six observed Y bands fit well with the reported C H 3 C N 0 spectrum. These are the 1332-, 2309-, 1381-, and 780-cm-' matrix bands corresponding to the 1319-cm-' N - 0 stretch, the 2315-cm-' C=N stretch, the 1381-cm-I CH, deformation, and 778-cm-' C-C stretching modes. The 1332-cm-' band shows 27-cm-I '*O and 9-cm-' I5N isotopic shifts in agreement with assignment td a symmetrical skeletal stretching mode with a significant N - 0 stretching component; the 9-cm-l blue deuterium shift implies mixing of normal vibrations in the C H 3 C N 0 molecule. The 1311-cm-' satellite band of the 1332cm-I absorption appearing at approximately the same frequency shift, 20 f 2 cm-l, in spectra of all isotopic mixtures is assigned to a perturbed N - O stretch in a CH3CNO-HOCH2CN complex. The presence of the satellite band supports assignment of the 1332-cm-l band to a N - 0 stretch as oxygen is a good site for intermolecular interaction. The 16-cm-' '*O and 2-cm-l I5N isotopic shifts of the 780-cm-I band imply strong mixing of the skeletal -C-=N-O stretching modes in good agreement with the observed D isotopic shift of the 1332-cm-l mode. The two weaker bands at 857 and 579 cm-' were not reported in the C H 3 C N 0 spectrum. Recombination of CH, and N C O (1 2) Freedman, T. B.; Nixon, E. R. Spectrochim. Acta Part A 1972,28A, 1375. Ducan, J. L.Spectrochim. Acta 1964, 20, 1197. (1 3) Serrallach, A.; Meyer, R.; Gunthard, H. Hs. J. Mol. Spectrosc. 1974, 52, 94. (14) Isner, W. G.; Humphrey, G. L. J . Am. Chem. SOC.1967.89, 6442.
Mielke et al. radical could also lead to formation of methyl isocyanate, CH3NCO, the most stable of the C H 3 C N 0 isomers,I5and the presence of this species in the photolyzed samples must also be considered. Infrared spectra of the vapor and liquid states of C H 3 N C 0 have been reported.I6 None of the major photoproducts X, Y, or Z can be identified as CH,NCO, but it cannot be excluded that more than three different products contribute to the X, Y, or Z band pattern. The three most intense and characteristic bands of the C H 3 N C 0 molecule appear at 2288 cm-I, v,,(NCO), 852 cm-I, v(C-N), and 596 cm-I, 6(NCO), in the spectrum of liquid C H 3 N C 0 . The C=N stretching region is complicated in the matrix spectra; bands due to parent CH3CN molecule and the HOCH2CN and C H 3 C N 0 photoproducts appear in this region, and any absorption due to an additional photoproduct could be overlapped by these bands. The two weak Y bands at 857 and 579 cm-I have similar frequencies as the v(C-N) and 6(NCO) bands in the spectrum of C H 3 N C 0 . This fact suggest that C H 3 N C 0 also contributes to the photolysis product spectrum. Bending fundamentals of NCO groups have been observed at 573 cm-I for H N C O in solid argon" and a t 596 cm-' for liquid CH3NCOi6whereas C N O bending modes occur at 539 cm-I for HCNO in solid neon" and at 478 cm-' for CH,CNO in s~lution.'~ The 579-cm-I matrix band is more appropriate for a NCO bending mode based on these data. The small 4-cm-I I5N isotopic shift observed for the 857-cm-I band rules out a pure C-N stretching mode, but strong mixing of stretching vibrations within the H,C-N=C=O skeleton is expected. This is supported by the large 49-cm-' CD, shift for the 857-cm-I band, indicating contribution of the symmetric CD, deformation to the 857-cm-' mode. Hence, band position and isotopic shift favor assignment of the weak 857- and 579-cm-' matrix bands to the C H 3 N C 0 isomer. The HOCH2CN Complexes with CH,CN, CH3CN0, and (HOCH,CN), Dimers. The examination of the X and Z pattern spectra shows that many X bands have counterparts in the Z bands. There are the 3641-cm-' X band and the 3478- and 3473-cm-I Z doublet in the 0-H stretch region. The 1209- and 1061-cm-' X bands have counterparts in the 1233- and 1074-cm-' Z bands, respectively. The two other X and Z doublets are the 888- and 878-cm-l bands and the 1134- and 1136-cm-I bands (observed in the spectrum of CD3CN + I6O3only). Similar isotopic shifts observed for the X bands and their Z counterparts give evidence for this assignment. The Z counterparts of the X bands have to be due to perturbed vibrations of X molecules that are characteristic of 2 species. This will be the case when a Z molecule forms an intra- or intermolecular hydrogen-bonded complex with an X molecule as a proton donor or/and proton acceptor. The concentration studies and annealing experiments strongly support this suggestion; the intensity ratio of the X and Z bands, I(X)/Z(Z), decreases in spectra of the concentrated matrices and annealed samples. Three different hydrogen-bonded complexes that may be formed by the HOCH2CN molecule in the matrices studied are presented: N
i
11 I
C
C H$\O
Hf\dH
H lb
10
(HOCHzCN),
CHjCNO- HOCHzCN
CH3CN-HOCHzCN
2
3
4
Although the HOCH2CN molecule may possibly form two rotational isomers, the O H is sufficiently far from the C%N group (15) Poppinger, D.; Radom, L. J. Am. Chem. SOC.1978, 100, 3674. (16) Hirschmann, R. P.; Kniseley, R. N.; Fassel, V. A. Spectrochim. Acta 1965, 21, 2125.
(17) Bondybey, V. E.; English, J. H.; Mathews, C. W.; Contolini, R. J. J. Mol. Spectrosc. 1982, 92, 431. (18) Mielke, Z.; Andrews, L., to be published.
Infrared Spectra of HOCHzCN and C H 3 N 0 that any intramolecular hydrogen bonding is expected to be very weak. Relatively strong perturbation of the COH group vibrations in Z species and strong dependence of the X/Z yields ratio on matrix concentration exclude this isomer as the Y species. In fact, the HOCH2CN monomer can have the l b structure. Species 2 and 3 are the 1:l complexes formed between photoproducts; 2 is an alcohol dimer, and 3 is a heterodimer between an alcohol and acetonitrile N-oxide molecules. Complex 4 is a heterodimer between an alcohol and acetonitrile molecules. All three complexes may be formed as the secondary step products once the primary photoproduct molecules are present in the matrix sample. In all complex possibilities, the alcohol molecule acts as a proton donor and the proton acceptors are the alcohol, acetonitrile N-oxide, and acetonitrile molecules, respectively. It is expected that the proton affinity (PA) of the acetonitrile N-oxide is higher than that of the acetonitrile molecule due to the high electron density on the oxygen atom. For comparison, the PA values for trimethylamine and its N-oxide are equal to 972 and 983 kJ mol-', respecti~e1y.I~ Acetonitrile has a PA value equal to 787 kJ mol-I, which is very In hydroxyclose to that of ethanol equal to 788 kJ acetonitrile there are two relatively strong base centers: nitrogen and oxygen atoms. The presence of the strongly electronegative -C=N group probably slightly increases the proton donor properties of the HOCH2CN as compared to ethanol, weakening simultaneously the proton acceptor properties of the oxygen base center. One may expect that complex 3 contains the strongest hydrogen bond while complexes 2 (which can have different structures) and 4 contain weaker hydrogen bonds. In the high-frequency region there is only one Z band that has a characteristic structure and shows two subpeaks at 3478 and 3473 cm-I; its assignment to the perturbed 0-H stretching vibration is self-evident. This frequency is close to the corresponding mode in the (CH30H), dimer and CH30H-NCCD3 complex isolated in argon where the O H stretches are observed at 3505 and 3560 cm-I, respectively.20 Consequently, the band is assigned to complex 2 or 4. A lower frequency for this band as compared to the corresponding bands in methanol complexes is due to the higher gas-phase acidity of the HOCH2CN molecule as compared to methanol. The absolute frequency of the 3478/3473-cm-' band is consistent with either complex 2 or 4; however, the composition of the matrices studied suggests that the band is due to complex 4 formed between HOCHzCN and CH3CN molecules. The photoproduct yield under the conditions studied was lower than 30% as estimated from the intensity decrease of the C-C stretch of the acetonitrile molecule. In both HOCH2CN and C H 3 C N 0 the C-C stretch is shifted toward lower frequencies as compared to the CH3CN molecule, so the decrease in absorbance of the C-C stretch band in acetonitrile molecule gives a rough estimation of the photoproduct yield. Consequently, the CH3CN/HOCHzCN ratio on photolysis was higher than 2:1, which favored the formation of the heterodimer complex 4 rather than 2 or 3. Hence, the 3478- and 3473-cm-' bands are tentatively assigned to the perturbed O H stretch in the CH3CN-HOCH2CN complex, and it is assumed that the yield of this complex in the studied samples is higher than the yield of the (HOCHZCN), dimers. The most perturbed vibrations of the HOCHICN molecule upon complex formation are the -COH group vibrations. The C - 0 stretch shifts from 1061 cm-' in HOCHzCN monomer to 1074 cm-' in the complex. The COH in-plane bending (S(0H)) shifts from 1209 cm-' in monomer to 1233 cm-' in the complex; a relatively small shift of this mode toward higher frequencies is expected for the medium-strength hydrogen bond. The COH out-of-plane mode ( r ( 0 H ) ) is very sensitive to complex formation and shifts from 256 cm-' in HOCH2CN monomer to 553 cm-' in acetonitrile complex; its frequency is close to the corresponding modes in alcohol dirnem2' The S(OH)/6(OD) = 1233 cm-'/904 cm-' = 1.36 and s(OH)/7(0D) = 553 crn-'/412 cm-' = 1.34 (19) Lias, S.G.; Liebman, J. F.; Levin, R. D. J . Phys. Chem. Re$ Data 1984, 13, 695.
(20) Burneau, A,; Loutellier, A.; Schriver, L. J . Mol. Struct. 1980,61, 397. (21) Falk, M.; Whallcy, E. J . Chem. Phys. 1961, 34, 1554. Malarski, Z.; Szostak, R.; Sorriso, S. Lett. Nuouo Cimento 1984, 40, 261.
The Journal of Physical Chemistry, Vol. 93, No. 2, 1989 563 isotopic shift ratios confirm assignment of the 1233- and 553-cm-l bands to the OH in-plane and out-of-plane modes. The I8O, IsN, and D isotopic shifts of the perturbed -COH group modes follow the isotopic shift pattern of their counterparts of the -COH group in the HOCH2CN monomer. The 878-cm-I Z band is tentatively assigned to the perturbed C-C stretch in the CH3CN-HOCH2CN complex; it is 10 cm-' red-shifted from the C-C stretch in the alcohol itself. The CHI rocking mode shifts from 971 to 977 cm-' in the complex. The CDz wagging (observed only in deuteriated samples) shows a small blue shift upon complex formation; it appears at 1134 cm-' in DOCDzCN and is shifted to 1136 cm-' upon complex formation. The appearance of the perturbed C H 3 C N 0 modes suggests that complex 3 is also present in the samples studied. The mode expected to be most strongly perturbed upon complex formation is the N - 0 stretch. The mode is identified at 1332 cm-' in C H 3 C N 0 with its Z counterpart a t 131 1 cm-I. In the spectra of all isotopic species the frequency difference between the N - 0 stretch and its Z counterpart is approximately equal (20 f 2 cm-I), which strongly supports the assignment of the Z counterpart to the perturbed N - 0 stretch in a C H 3 C N 0 complex. The 20 f 2 cm-' frequency difference indicates that it is a hydrogen-bonded complex as the perturbation of the N - 0 stretch should be much less in the CH3CNO-CH3CN0 and CH3CNO-CH3CN pairs. Due to the stronger proton acceptor properties of the C H 3 C N 0 as compared to CH3CN, the three -OH group vibrations of the HOCHzCN molecule are expected to be more strongly perturbed in the C H 3 C N 0 complex than they are in the CH3CN complex. The 616-cm-l band in the spectra of CD3CN photoproducts is tentatively assigned to the O D out-of-plane mode in the C H 3 C N 0 complex. The O H stretch and O H in-plane bending modes and their deuterium counterparts were not identified. The other HOCH2CN modes in complex 3 are probably similarly perturbed as in complex 4. N o additional bands were observed that could be assigned to (HOCH,CN), dimers. It is reasonable to assume that (HOCH2CN)2 dimers are also present at low concentrations in the studied samples. In fact, many of the HOCHzCN modes in the dimer can be similarly perturbed as the corresponding modes in the CH3CN-HOCH2CN complex. Mechanism. The failure to observe splittings in precursor bands in the initial sample deposit and the lack of long-wavelength photolysis in these experiments both show that a precursor complex does not contribute to the photochemical process. Any interaction between CH3CN and O3 does not alter the photochemistry of ozone itself. Similar results for CH4, C2H4, and SiH4 ozone s t ~ d i e s are ~ ~ contrasted -~~ by the definite role of H3P-03 and P4-03 complexes in the red photochemistry of the latter complexes.25 Formation of the hydroxyacetonitrile which occurs under full arc (220 < X < 1000 nm) irradiation involves insertion of oxygen atoms into C-H bonds. The insertion of 0 atoms into C-H and Si-H bonds has been observed in similar studies with CH,,22 C Z H ~ ,SiH4,24 '~ SiHC13,z6and methyl-substituted silanes.27 The alcohol yield in the CH3CN + 0 reaction is higher than that in similar CH4 0 and CzH4 0 matrix reactions. The higher alcohol yield in the present studies is consistent with the lower dissociation energy of the C H bond in the CH3CN molecule; the C-H dissociation energy in acetonitrile is about 93 kcal mol-' as compared to 104 kcal mol-' in methane or 108 kcal mol-' in ethene.5 In fact, the reaction may proceed by H abstraction to give OH radical which combines with CHzCN radical in the matrix cage.
+
+
CH3CN
+0
-+
HOCHZCN
(22) Withnall, R.; Andrews, L., unpublished results, 1986. (23) Hawkins, M.; Andrews, L. J . Am. Chem. SOC.1983, 105, 2523. (24) Withnall, R.; Andrews, L. J . Phys. Chem. 1985,89, 3261. (25) Withnall, R.; Andrews, L. J . Phys. Chem. 1987, 91, 784; J . Am. Chem. SOC.1988, 110, 5606. (26) Shirk, A. E.; Shirk, J. S. J . Mol. Spectrosc. 1982, 92, 218. (27) Withnall, R.; Andrews, L. J . Phys. Chem. 1988, 92, 4610.
564
J. Phys. Chem. 1989, 93, 564-570
The high yield of an alcohol product in the present reaction as contrasted with small contribution of H abstraction reaction in the gas-phase reaction CH3CN + O(3P) is possibly due to the participation of excited O('D) atoms, in addition to O(3P) atoms in an overall CH3CN 0 matrix reaction. In the reaction of ethene with atomic oxygen in solid argon, the relative yield of vinyl alcohol was enhanced via H-atom abstraction or insertion of O(lD) atoms.23 The first step in the major reaction channel of the gas-phase CH3CN + O(3P)reaction4 was reported to involve displacement of C H 3 by O(3P) and formation C H 3 and OCN (or CNO) radicals:
+
CH3CN + O(3P)
-
CH3 + O C N (or CNO)
-
The formation of C H 3 C N 0 via CH3 and C N O radicals might C N O in the matrix cage require the rearrangement O C N followed by cage recombination of CH3 and CNO. As recently reported,28 the C N O radical is easily converted photolytically to the OCN radical, but the reverse rearrangement does not occur. This leads to the conclusion that C H 3 C N 0 is formed in simple bimolecular addition reaction of 0 atom to the nitrile nitrogen, the excess energy being quenched by the matrix cage, which contrasts the gas-phase reaction m e ~ h a n i s m . ~ It is of interest to conjecture on the branching ratio between HOCH2CN and C H 3 C N 0 in the matrix photolysis experiments. It is suggested that photolysis of ozone in close proximity to CH3CN where the excited O('D) photoproduct can react before relaxation gives HOCHzCN preferentially, but longer diffusion of 0 atoms before reaction provides an opportunity for relaxation to O('P) where the C H 3 C N 0product is favored. This is supported by annealing experiments. When matrices were annealed, the C H 3 C N 0 monomer concentration was relatively constant even though the amount of CH3CNO-HOCH2CN complexes increased. (The Y bands showed little sensitivity to matrix annealing, but the 1311-cm-' Z band due to the complex increased in absorbance.) During matrix annealing the quenched O(3P) (28) Bondybey, V. E.; English, J. H.; Mathews, C. W.; Contolini, R. J . Chem. Phys. Lert. 1981, 82, 208.
atoms diffuse through the matrix and react with CH3CN forming C H 3 C N 0 and keeping the C H 3 C N 0 monomer concentration approximately constant. The argon matrix is a convenient vehicle for studying different 0-atom addition product in ozone photochemistry.
Conclusions Full arc photolysis of O3 in argon matrices containing acetonitrile leads to the formation of hydroxyacetonitrile and acetonitrile N-oxide as primary products, and there is also evidence for methyl isocyanate as a minor product. Hydroxyacetonitrile forms hydrogen-bonded complexes with acetonitrile and with acetonitrile N-oxide molecules as secondary products. The alcohol yield in the CH3CN + 0 reaction is higher than in similar CH4 + 0 and C2H4+ 0 matrix reactions, in accord with the lower dissociation energy of the C H bond in the CH3CN molecule as compared to methane or ethene. Hydroxyacetonitrile is formed by insertion of oxygen atom into a C-H bond or H-atom abstraction by oxygen atom followed by recombination of OH and CH2CN radicals. Acetonitrile N-oxide is believed to be formed by a simple bimolecular addition reaction of oxygen atom to the nitrile nitrogen. It is suggested that participation of excited O('D) atoms, in addition to O(3P),increases the relative yield of hydroxyacetonitrile as compared to acetonitrile N-oxide. The vibrational spectra of different C H 3 C N 0 isotopomers evidence strong mixing of internal modes within the C H 3 C N 0 molecule. There is an extensive interaction between the N - 0 stretch, C=N stretch, C-C stretch, and CH3 deformation modes. The hydroxyacetonitrile molecule exhibits Fermi resonance between the C=N stretch mode and combination between C-0 stretch and C-OH in-plane bending modes. The perturbation of the hydroxyacetonitrile C O H group vibrations in the hydroxyacetonitrile-acetonitrile complex evidences formation of a medium-strength hydrogen-bonded complex. Acknowledgment. We gratefully acknowledge support from N S F Grant CHE85-16611 and helpful discussions with R. Withnall. Registry No. 0, 17778-80-2; CH,CN, 75-05-8; HOCH,CN, 16-4; CH,CNO, 7063-95-8; Ar, 7440-37-1.
107-
Ultraviolet/Vkible Spectra of Halogen Molecule/Arene and Halogen Atom/Arene ?r-Molecular Complexes' K. D. Raner,* J. Lusztyk,* and K. U. Ingold* Division of Chemistry, National Research Council of Canada, Ottawa, Ontario, Canada, K1 A OR6 (Received: April 20, 1988)
Ultraviolet/visible spectra have been recorded for chlorine, bromine, and iodine atom *-molecular complexes with a number of aromatic compounds. The visible spectra of the I'/arene complexes show charge-transfer (CT) bands whose transition energy correlates linearly with the vertical ionization potential of the corresponding arene. Comparison of the spectra of these complexes with the spectra of the corresponding Clz/arene, BrJarene, and 12/arene complexes leads to the conclusion that the I'/arene complexes are classical hexahapto *-molecular complexes. The CT transition energies of the Cl'/arene and the Br'/arene complexes correlate well with one another, yet the CT transition energies of these complexes are seemingly independent of the vertical ionization potential of the arene.
Thirty years ago, R u s ~ e l Ireported ~ * ~ that the photochlorination of 2,3-dimethylbutane gave a greater ratio of tertiary to primary monochloride products in the presence of benzene and certain other arenes. This enhanced selectivity was attributed to the formation (1) (2) (3) (4)
Issued as NRCC No. 29568. NRCC Research Associate 1986-1988. Russell, G. A. J. Am. Chem. SOC.1957, 79, 2977-2978. Russell, G. A. J . Am. Chem. SOC.1958, 80, 4987-4996.
0022-3654/89/2093-0S64.$01.50/0
of a chlorine atom/arene *-complex which acted as a much more selective hydrogen atom abstracting agent than the free chlorine a t ~ m . Ten ~ , ~years later, Biihler5" discovered that transient spectra of chlorine atom/arene complexes were produced during the pulse radiolysis of carbon tetrachloride/arene mixtures. These spectra were a ~ s i g n e dto ~ , charge-transfer ~ (CT) chlorine atom/arene ( 5 ) Biihler, R. E.; Ebert, M. Nature (London) 1967, 224, 1220-1221. (6) Biihler, R. E. Helu. Chim. Acto 1968, 52, 1558-1571.
Published 1989 by the American Chemical Society
