3373
J . Phys. Chem. 1984, 88, 3373-3379
predicted effects are manifested by a variety of relatively inert s o l ~ t e s . ~ ~ ~ ~ ~ ~ ~ ~ There have been many discussions of the structure-making effects of certain solutes in aqueous solution that relates to the above ideas and extend them.7,’5,17,31,61”5,70,71 The emphasis here lies in showing that such effects are predictable from the properties of the random network, and thereby relating them to the anomalies of pure water. Several recent studies of the effects of hydrophobic solutes on the anomalies of water3s13-16 have been interpreted in terms of the cage model of E l e ~G, l~e ~~ , P~ al ~ l i n g , and ~ ~ ~Stillinger.’ *~ They offer persuasive evidence for the direct correspondence between the iceberg structures that form around inert solutes and the low-density species that give rise to the anomalies of pure water. 5. Summary The picture of the structure of cold water that follows from the postulates of section 2 is that the random network is differentiated into domains that are distinguishable on the basis of the 0-0-0-0 tetramer conformation (Figure 8). Those domains with a predominance of eclipsed tetramers contain pentagonal rings and cavities and they grow cooperatively as the water is cooled due to the self-replicating propensity of the pentagons. The expansion of water below 4 OC (1 atm) and the anomalies of supercooled water are attributed to the increasing proportion of the water in the low-density eclipsed domains. The conjectured “clathrate-like’’and the staggered domains “icelike”. The essential common element is the identification of species that have the (70) D. D. Eley, Trans. Faraday SOC.,35, 1281 (1939) (71) D. N. Glew, J . Phys. Chem., 66, 605 (1962).
stability limit at T, in supercooled water corresponds to the point at which a domain would grow to a macroscopic size if freezing could be avoided. The increasing range of the structural correlations due to domain growth implies a concomitant increase in structural relaxation times, which can account, for instance, for the apparently diverging viscosity’ of water as T T,. The pairing of cavities on either side of the plane of the pentagons, and the clustering of the pentagons, account for the phenomenon of hydrophobic association. The association between pentagons and cavities implies that hydrophobic solutes can induce the formation of pentagons in water and stabilize the eclipsed domains, which can account for “iceberg” formation and for the observation16 that clathrate-forming solutes can increase T,. The model has much in common with the clathrate-cage mode128,29363~64 as developed by Stillinger.7ss,61The correspondence could be emphasized by loosely calling the eclipsed domains qualities of self-replication and association with cavities. The advantages of the present model are that the presence of species with those qualities is deduced from simpler postulates and that the species (pentagons) are simple enough to be studied in computer simulation experiment^,^^,^^ thus allowing the model to be tested. Sections 3 and 4 show that it is not necessary to invoke the more complex polyhedral structures of the cage model to account for the anomalies of water and the simpler manifestation of the hydrophobic effect. “There is, after all, in flowers a flat pentagon, not a solid dodecahedron.”’ Johannes Kepler (16 1 1)
-
Registry No. Water, 7732-18-5.
Photodecomposition of Nitromethane Trapped in Solid Argon Marilyn E. Jacox Molecular Spectroscopy Division, National Bureau of Standards, Gaithersburg, Maryland 20899 (Received: December 6, 1983) When nitromethane isolated in solid argon at 14 K is exposed to the full light of a medium-pressure mercury arc, infrared absorptions of cis- and trans-CH30N0initially grow in intensity. On prolonged photolysis, these absorptions diminish in intensity, and there is continued growth in the infrared absorptions of the H2C0...HNO hydrogen-bonded complex and in those of the cis and trans rotamers of the recently discovered species nitrosomethanol. Detailed isotopic substitution studies are consistent with these identifications. In the later stages of the photolysis, absorptions of CO, S O , and HSCO and of the HzO.. .HSCO hydrogen-bonded complex become increasingly prominent. The mechanism by which these products are formed is discussed. When the enhanced role of cage recombination is taken into account, this mechanism is compatible with that determined from gas-phase studies of the photolysis of nitromethane.
Introduction The primary gas-phase photodecomposition products Of nitromethane, unlike those of most other molecules, show little apparent relationship to the products observed in solid argon. Both endproduct and time-resolved spectra observations indicate that CH3 and are the primary photodecomposition products of nitromethane in the gas phase. The first absorption of nitromethane has its onset near 330 nm, with a maximum near 278 nm, and there is very strong absorption beyond 240 nm.’ The absorption spectra Of both CH3 and were detected on a microsecond time scale in the flash photolysis study by Napier and N o r r i ~ hwhich ,~ marked the beginning of a new generation of time-resolved studies. Vibrationally excited NOz was also
-
(1) J. G. Calvert and J. N. Pitts, Jr., “Photochemistry”, Wiley, New York, 1966, pp 454, 479. (2) K. Honda, H. Mikuni, and M. Takahasi, Bull. Chem. SOC.Jpn., 45, 3534 (1972). (3) I. M. Napier and R. G. W. Norrish, Proc. R. SOC.London, Ser. A , 299, 317 (1967).
detected on a microsecond time scale in the laser-excited fluorescence studies of the 252-nm photolysis of nitromethane by Spears and Brugge.4 Although results of the crossed lasermolecular beam study at 266 nm by Lee and co-workers5 that photodissociation did not occur under collision-freeconditions, observed NOz on a picosecond time scale Schoen and co-workers6 in a study at 264 nm. Very recent molecular beam studies7,8 with 193-nm photolysis have demonstrated that N02 is formed both in the ground state and in an excited electronic state, In contrast, on mercury-arc photolysis of nitromethane isolated in solid argon, Brown and Pimentel9 detected infrared absorptions (4) K. G. Spears and S. P. Brugge, Chem. Phys. Lett., 54, 373 (1978). (5) H. S. Kwok, G. Z. He, R. K. Sparks, and Y. T. Lee, Int. J . Chem. Kinet., 13, 1125 (1981). (6) P. E. Schcen, M. J. Marrone, J. M. Schnur, and L. S. Goldberg, Chem. Phys. Lett., 90, 272 (1982). (7) L. J. Butler, D. Krajnovich, Y . T. Lee, G . Ondrey, and R. Bersohn, J . Chem. Phys., 79, 1708 (1983). (8) N. C. Blais, J . Chem. Phys., 79,1723 (1983). (9) H. W. Brown and G. C. Pimentel, J . Chem. Phys., 29,883 (1958).
This article not subject to U S . Copyright. Published 1984 by the American Chemical Society
3374 The Journal of Physical Chemistry, Vol. 88, No. 15, 1984
Jacox
TABLE I: Absorptions Which Appear on Mercury-Arc Photolysis of Nitromethane Isolated in Solid Argon cm-I peak optical density cm-I peak optical density I'C 163 '3C 163 normal I5N 35 rnin rnin 396 rnin assignment normal I5N 35 rnin min 396 min
0.000
500
0.005
0.014
1670
498 500 565
0.033
0.028
0.014
trans-CH3ONO
1733
0.019
0.015
0.007
cis-CH30N0
0.155
0.357
0.519
H,CO" HZCO
1704 1742 0.199
0.168
0.087
frans-CHBONO
1765
0.000
0.018
0.043
0.000
0.014
0.047
0.000
0.033
0.131
NO
0.009
0.084
0.244
co
0.009
0.030
0.045
HCNO ?
0.000
0.015
0.097
HNCO
0.000
0.010
0.086
HNCO
0.000
0.052
0.129
c02
0.000
0.029
0.070
0.000
0.014
0.044
HNOb
0.068
0.143
0.189
HICO, HNO'
1765 1770 1770 0.016
0.069
0.000
1875
0.000 0.022
858 985
0.000 0.014
0.016 0.007
1875 1842
cis-CH30N0
975 1029
0.000
0.015
2140 sh 2146 2098 2146
0.021
1029 -
2202
0.033
1041 br 1074
-
cis-CH30N0, 1
831 823
0.000
0.035 0.000
2178 2160
0.042 0.021
2230
1063
2202
-
-
0.000
1110
0.119
0.334
trans-CH2(NO)OH
2262
1093 1110
2234 2260 0.012
1128
0.053
0.133
cis-CH,(NO)OH
2345
1110 1127 1174
2278 0.017
0.041
0.055
H2CO"
2726
1162 1174 1180
2728 2722 0.017
0.025
2744
0.027
1172 -
2740 2738 0.000
1228
0.009
0.014
2802 2798 2802
1213 1226 0.000
1247
0.019
0.034
H2COa
2808 sh
-
0.000
1252
0.019
0.044
H2CO
2866
1242
0.000 0.000
0.000 0.014
0.014 0.040
1297 1306 0.000
1356
0.036
0.106
N2O CH4 cis-,fmns-CH2(NO)OH
h2c0a
-
2998 3290
0.153
0.215
H2CO
3322
0.016 0.000
0.027 0.024
0.048 0.130
H2CO HNCO
0.005
0.014
0.028
HCNO ?
0.000
0.000
0.022
0.000
0.014
0.041
cis-CH,(NO)OH
0.006
0.053
0.130
trans-CH2(NO)OH
0.000
0.039
0.115
H,O
3306 3320 0.023
0.048
0.074
HNO
3366 3368 3366
1506 1487 0.000
0.000
0.030
1595 1595 1613
h2c0
3289 3280 0.063
1596
0.121
2860 sh
1497 1497 1506
0.097
2878 sh
1350 1354 1498
0.047 2850 2866
-
1286 1306
h2c0"
2812 sh
1240 -
0.022
0.035
0.063
H2O
3474
NO, ?
3638
3476 3476
1613 1654
trans-CH30N0
1742 sh
803 794 815 sh 819 sh 839
0.039
1695 1733
624 596 807
0.072
1669 1638
560 561 625
assignment
0.089
3638 3638 0.028
0.034
0.030
3708 3710 3710
Hydrogen-bonded to HNO. bPerturbed by other molecules trapped in adjacent sites. 'Hydrogen-bonded to H,CO.
of trans-CH30N0, H2C0, HNO, CO, CO,,HNCO, H 2 0 , and N 2 0 , but not of NO2. Although the assignment of a prominent
absorption near 1570 cm-' to HNO has been confirmed, more recent studiesl02l1have demonstrated that prominent peaks at 1110
Photodecomposition of Nitromethane and 1125 cm-' and the moderately intense peak at 3300 cm-' must be contributed by other products. The matrix experiments differ from low-pressure gas-phase photolysis studies in having a high probability for the occurrence of cage recombination and of secondary photodecomposition. Because these characteristics should be common to nitromethane decomposition at higher pressures and in the condensed phase, the identification of the products and the understanding of the processes which occur in the matrix studies should be of more general importance. The following discussion reports the results of a detailed isotopic substitution study of the products of the mercury-arc photolysis of nitromethane isolated in solid argon, conducted in order to identify the unknown species and to clarify the photodecomposition mechanism in this system.
Experimental Details1* The sources and isotopic compositions of the nitromethane samples used for these experiments have previously been described.13 Ar:nitromethane samples of mole ratio 200 (for most of the experiments), 400, and 1600 were prepared by using standard manometric procedures and were deposited on the cryogenic observation surface, maintained at 14 K, without photolysis. The cryogenic equipment and sample observation configuration have also previously been described.14 After the infrared spectrum of the initial deposit had been recorded, the sample was subjected to the full or, in a few experiments, the filtered radiation of a medium-pressure mercury arc. In the filtered photolysis studies, Corning filters of glass type 7740 and 7058, with short-wavelength cutoffs of 280 and 260 nm, respectively, were used. Infrared spectra were recorded with a Beckman IR-9 spectrophotometer. Under the scanning conditions typical of these experiments, the resolution and the relative and absolute frequency accuracies are estimated to be 1 cm-' between 400 and 2000 cm-' and 2 cm-' between 2000 and 4000 cm-'. Observations An initial study of the photolysis of an Ar:CH3N02 = 200 sample by mercury-arc radiation of wavelength longer than 280 nm for 100 min showed no evidence for product formation. A small amount of photodecomposition was observed when a sample was exposed to radiation of wavelength longer than 260 nm for approximately 30 min. Subsequent experiments were, therefore, conducted using unfiltered mercury-arc radiation. The positions of the absorptions which appeared on mercury-arc photolysis of an Ar:CH,N02 = 200 sample are summarized in the first column of Table I, and their peak optical densities on increasing duration of photolysis are summarized in the third, fourth, and fifth columns of Table I. Spectral regions of particular interest in this experiment are shown in Figure 1, in which the solid trace designates the spectrum of the unphotolyzed deposit and the broken-line trace the spectrum of the sample after 396 min of photolysis. As has previously been reported,1s*16 after the first, 35-min period of photolysis, the most prominent absorptions of both cis- and trans-CH30N0 were present, but on extended photolysis first the absorptions of the cis rotamer and later those of the trans diminished in intensity. Photolysis of the initially formed C H 3 0 N 0 was accompanied by a growth in the absorptions of H 2 C 0 and HNO, with perturbations in their spectrum because of the hydrogen-bonding interaction between The (10) M. E. Jacox and D. E. Milligan, J. Mol. Spectrosc., 48,536 (1973). ( 1 1 ) P. N. Clough, B. A. Thrush, D. A. Ramsay, and J. G. Stamper, Chem. Phys. Lett., 23, 155 (1973). (12) Certain commercial instruments and materials are identified in this
paper in order to specify adequately the experimental procedure. In no case does such identification imply recommendation or endorsement by the National Bureau of Standards, nor does it imply that the instruments or materials identified are necessarily the best available for the purpose. (1 3) M. E. Jacox, J . Phys. Chem., 87, 3 126 (1 983). (14) M. E. Jacox, Chem. Phys., 7, 424 (1975). (15) F. L. Rook and M. E. Jacox, J . Mol. Spectrosc., 93, 101 (1982). (16) M. E. Jacox and F. L. Rook, J . Phys. Chem., 86, 2899 (1982). (17) R. P. Muller, P. Russegger, and J. R. Huber, Chem. Phys., 70, 281 (1982).
The Journal of Physical Chemistry, Vol. 88, No. 15, 1984 3375
-
I
" I '
' I '
"
1
/
1550 1500 1200 1100 850 800
w ,
3600
600
500
0 A
I
,
!
3400 2250 2150 1750 1700 cm - 1
Figure 1. Ar:CH3N02 = 200, 14 K: (-) 24.1 pmol of CH3NO2 deposited over period of 224 min, without photolysis; (---) 396-min subsequent unfiltered mercury-arc photolysis.
1560-1570-cm-' spectral region, in which the most prominent H N O absorption lies, was obscured by the extremely strong absorption of unphotolyzed CH3N02. Very recently, Muller and Huber'* have assigned absorptions at 3638, 1555, and 1106.5 cm-' to the previously unreported species trans-nitrosomethanol and absorptions at 3477.5, 1559, and 1130.5 cm-' to cis-nitrosomethanol. In the present series of experiments, peaks at 11 10 and 3638 cm-' grew at an approximately linear rate throughout the course of the photolysis and were prominent in the broken-line trace of Figure 1. Less prominent peaks with similar growth behavior on prolonged photolysis appeared at 1128 and 3474 cm-l. The very strong absorption of C H 3 N 0 2usually obscured the region near 1560 cm-' even after prolonged photolysis. However, in one experiment approximately 90% of the CH3N02was destroyed, and a rather broad shoulder at 1561 cm-' could be distinguished on the residual C H 3 N 0 2absorption. The 1356-cm-' absorption, which was quite prominent at the later stages of photolysis, also lies close to the moderately intense peaks at 1352 and 1355 cm-' which have been assigned to trans- and cis-CH,(NO)OH, respectively, in a more detailed studylg of the infrared spectra of these species. N o evidence was obtained for 0-atom detachment. Infrared absorptions of nitrosomethane,20its tautomer formaldoxime,2' and methyl nitrate,22a possible product of 0-atom addition, did not appear at any stage in the matrix experiments. On prolonged photolysis, the absorptions of N O and CO became prominent, and small concentrations of N20, CO,, H20, and CHI appeared. A rather broad absorption at 1613 cm-' probably was contributed by NO2. A pair of moderately intense, broad bands at 2230 and 2262 cm-' corresponded closely with the most prominent absorptions of H N C O isolated in solid argon.23 All of the remaining unassigned peaks except those at 3290 and 3708 cm-' were of weak to moderate intensity. The absorptions attributed to CH2(NO)OH persisted in an experiment on an Ar:CH3N02 = 400 sample, and in the Ar: CH3N02= 1600 study the 1110-cm-' peak was moderately intense (18) R. P Muller and J. R. Huber, J . Phys. Chem., 87, 2460 (1983). (19) R. P. Muller, J. R. Huber, and H. Hollenstein, J . Mol. Spectrosc., 104, 209 (1984). (20) A. J. Barnes, H. E. Hallam, S. Waring, and J. R. Armstrong, J . Chem. Soc., Faraday Trans. 2, 72, 1 (1976). (21) R. L. Arnett and B. L. Crawford, Jr., J . Chem. Phys., 18, 118 (1950). (22) J. C. D. Brand and T. M. Cawthon, J . Am. Chem. SOC.,77, 319 (1955). (23) M. E. Jacox and D. E. Milligan, J . Chem. Phys., 40,2457 (1964).
3376 The Journal of Physical Chemistry, Vol. 88, No. 15, 1984 after an 80-min photolysis period. The positions of the product absorptions in experiments in which the nitromethane sample was highly enriched in carbon-13 or nitrogen-15 are summarized in the second column of Table I. Because the product yield in the nitrogen-15 study was somewhat smaller than in many of the other experiments, a few relatively weak, broad peaks, generally typical of the later stages of the photolysis, were not identified. The isotopic shifts in the absorptions of the stable products were consistent with the assignments given in Table I. The 1487-cm-' peak of the nitrogen-15 substitution study agreed closely with the 1487.5-cm-I absorption attributed by Muller and co-workersi7to HISNO hydrogen-bonded to H2C0. These workers also attributed an absorption at 2802 cm-I to the NH-stretching vibration of H N O in the hydrogenbonded complex. In the present experiments, the absorption at 2802 cm-' shows a moderate carbon-13 shift but no nitrogen-15 shift, suggesting that it is contributed predominantly by isolated H2C0. The weak to moderately intense peaks at 2726 and 2744 cm-I may be contributed by H N O trapped in less highly perturbed sites. However, such an assignment is relatively uncertain for the 2744-cm-I peak, which may have had a small carbon-13 shift. This absorption was weak in the nitrogen-15 study. Within the 2-cm-' experimental error, the 3474- and 3638-cm-I peaks were unshifted, behavior appropriate for their assignment to the OH-stretching fundamentals of cis- and trans-CH2(NO)OH, respectively. The isotopic shifts of the other fundamentals of these two species agree very well with those observed by Muller and c o - w o r k e r ~ , ~ who ~J~ found carbon-13 and nitrogen-15 shifts of 16.5 and 1.0 cm-', respectively, for the 1110-cm-I absorption of the trans rotamer and of 16.5 and 0.5 cm-I for the 1128-cm-' absorption of the cis rotamer. In the present experiments, the corresponding pairs of isotopic shifts were 17, 0 and 18, 1 cm-l. The carbon-13 and nitrogen-15 shifts observed by Muller et al. for the fundamentals near 1356 cm-' were 5.5 and 0.5 cm-1 for the trans species and 6.5 and 1.0 cm-l for the cis species, compared to 6 and 2 cm-I in the present study. The broad 2230- and 2262-cm-' absorptions, attributed to HNCO, were each shifted by 28 cm-I on carbon-13 substitution, but there was a shift of only 2 cm-l in the 2262-cm-l peak of the nitrogen-15 study, in which the counterpart of the 2230-cm-I peak was not observed. Although infrared spectra of HNI3COand HISNCOhave not been reported, the isotopic shifts for the antisymmetric stretching fundamental of the N C O free radical24are 53 and 7 cm-I, suggesting that the nitrogen-15 shift for the corresponding fundamental of HNCO may be small. The unassigned absorption at 3290 cm-I, which corresponds well with the prominent 3300-cm-' peak of the earlier NaCl prism study,9 showed a substantial nitrogen-15 isotopic shift but little or no carbon-13 shift, dictating its assignment to an NH-stretching vibration. Product absorptions at various stages in the photolysis of an Ar:CD3N0, = 200 sample are summarized in Table 11, and regions of especial interest in the spectra of this sample before photolysis (solid trace) and after 424 min of unfiltered mercury-arc photolysis (broken-line trace) are shown in Figure 2. The assignments given in Table I1 for cis- and trans-CD30N0 and for the D2C0. .DNO complex are consistent with the earlier observations of these specie^.'^^'^ Muller and HuberlShave assigned the OD-stretching fundamentals of cis- and trans-CD2(NO)OD at 2571 and 2686.5 cm-', respectively, in excellent agreement with the values of 2572 and 2688 cm-' given for these two absorptions in Table 11. In order of decreasing absorption intensity, Muller and c o - w ~ r k e r shave ' ~ assigned other vibrational fundamentals of trans-CD2(NO)OD at 1172.5, 1555, 983.5, 692, 816.5, and 1028 cm-I and of cis-CD,(NO)OD at 1191, 1558.5, and 929 cm-I. The agreement with the assignments given in Table I1 is good, except that absorptions in the 1550-1565-cm-' spectral region were obscured by the extremely strong CD3N02absorption. The assignment of the absorptions which grew relatively rapidly in intensity in the later stages of photolysis is also consistent with (24) D. E. Milligan and M. E. Jacox, J . Chem. Phys., 47, 5157 (1967).
Jacox TABLE II: Peak Optical Densities of Absorptions Which Appear on Mercury-Arc Photolysis of Nitromethane-d, Isolated in Solid Argon
peak optical density cm-' 550 597 691 741 775 800 819 910 920 931 947 981 sh 986 1027 1064 1082 1098 1128 1133 sh 1158 1165 sh 1169 sh 1174 1184 sh 1189 1287 1613 1654 1668 1681 sh 1688 1698 sh 1754 1768 1875 2052 2075 2082 sh 2148 2186 2195 sh 2228 2260 2345 2472 2572 2620 2635 2654 2688 2742 br
30 min 0.037 0.014 0.000 0.000 0.147 0.016 0.000 0.017 0.000 0.000 0.080
184
min 0.027 0.014 0.035 0.015 0.137 0.013 0.033 0.017 0.000 0.028 0.069
424 min 0.022 0.004 0.060 0.015 0.058 0.005 0.079 0.000 0.022 0.048 0.033
assignment trans-CD,ONO cis-CD30N0 trans-CD2(NO)OD trans-CD,ONO cis-CD,ONO trans-CD,(NO)OD cis-CD30NO cis-CD2(NO)OD trans-CDSONO trans-CDz(NO)OD, DzCO trans-CD2(NO)OD
0.021 0.037 0.015
0.162 0.025 0.021 0.028 0.054 0.020
0.000
0.015
0.037
0.024
0.264
0.564
DNO" DNO" DNO" trans-CD,(NO)OD
0.000 0.000 0.047 0.011 0.086
0.043 0.000 0.061 0.044 0.062
0.067 0.021 0.054 0.037 0.027
cis-CD2(NO)OD
0.071
0.366
0.477
D2COb D2CO
0.000 0.010 0.000 0.000 0.069
0.010 0.034 0.077 0.031 0.286
0.016 0.062 0.185 0.033 0.296
0.000 0.049
0.076 0.155
0.200 0.151
0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000
0.068 0.012 0.049 0.038 0.03 1 0.026 0.010 0.015 0.088 0.016
0.314 0.032 0.141 0.183 0.079 0.035 0.019 0.029 0.174 0.071
0.019 0.000 0.000 0.000 0.000 0.000
0.116 0.020
0.000
DZCO
N20 NO2,cis-CD30N0
trans-CD,ONO
NO
DNO' D2COb DXOb CO" D2COb D,COb DNCO, N2O HNCO
co2
DNCO" cis-CD2(NO)OD DNCO trans-CD,(NO)OD DzO'
a Perturbed by other molecules trapped in adjacent sites. Hydrogen-bonded to DNO.
that proposed in the Ar:CH3N02photolysis study. The identification of CO, C 0 2 , NO, and N 2 0 is straightforward. The persistence of a moderately intense, broad absorption at 1613 cm-' after most of the cis-CD30N0 has been photolyzed supports the assignment of this absorption to NO,. The prominent absorption at 2228 cm-I and the weak to moderately intense peak at 2620 cm-I correspond well with the positions of the NCO antisymmetric stretching and N D stretching modes of DNCO isolated in solid argon.23 A persistent weak to moderately intense absorption at 2260 cm-l may have been contributed by a small concentration of H N C O resulting from incomplete deuterium of the sample. All of the remaining unassigned absorptions were relatively weak except that at 1768 cm-', within the experimental error unshifted on deuterium substitution, and the peaks at 2472 and 2742 cm-'. The ratios of the frequencies of these two peaks to those at 3290 and 3708 cm-I in the Ar:CH3NO2photolysis experiments are 0.75 and 0.74, respectively, supporting their assignment to the deu-
The Journal of Physical Chemistry, Vol. 88, No. 15, 1984 3311
Photodecomposition of Nitromethane
TABLE 111: Peak Optical Densities of Absorptions Which Appear on Mercurv-Arc Photolvsis of Nitromethane-d Isolated in Solid Argon
1
1200
1100
1
4
-
1000
I
900
I
Y
800
700
/
71
H ,
n
2800
2600
0
z
2400 1900 1850
1700
1600 cm-'
Figure 2. Ar:CD3NO2= 200, 14 K: (-) 24.1 pmol of CD3NO2deposited over period of 238 min, without photolysis; (---) 424-min subsequent unfiltered mercury-arc photolysis.
terium-stretching counterparts of the corresponding absorptions in the undeuterated system. The product absorptions which resulted in a prolonged photolysis study of an Ar:CH2DN02= 200 sample are summarized in Table 111. The assignments proposed for cis- and trans-CH,DONO are simply related to those for the undeuterated species and are consistent with the behavior of the corresponding absorptions on prolonged photolysis. Although the unphotolyzed sample deposit was completely absorbing between 1560 and 1572 cm-', in this particular experiment most of the CH2DN02was destroyed during the photolysis, permitting identification of product absorptions in this region. The assignment of absorptions to hydrogen-bonded H2C0. .HNO pairs having partial deuterium substitution follows from the earlier studie~,'~*'' together with the vibrational assignment for HDC0.25 Four singly deuterium-substituted nitrosomethanol species are possible. In order of decreasing relative intensity, Muller and co-w~rkers'~ have assigned peaks at 1128.5, 1555.5, 2686, and 1247 cm-' to trans-CH,(NO)OD, at 1551, 1162, and 2569 cm-' to cis-CH,(NO)OD, at 1264, 1557,3638, 1100, and 919 cm-' to trans-CHD(NO)OH, and at 1147 and 3477.5 cm-I to cis-CHD(N0)OH. These peaks are in good agreement with the peaks reported in Table 111. Until the late stages of photolysis, the 1264-cm-I peak of trans-CHD(N0)OH was obscured by an absorption of CH2DN02. In the final trace, an absorption of peak optical density 0.058 persisted at 1268 cm-I. The relatively great intensity of the peak at 3638 cm-' and the presence of the 1110-cm-' peak resulted from the photolysis of the 24.7% undeuterated nitromethane present in the sample. As in the studies on the undeuterated and fully deuterated systems, the absorptions of the cis products were relatively weak the OHand OD-stretching vibrations of these two products were not detected. The assignments for the remaining peaks which persisted or grew on prolonged photolysis are supported by the observations on the photolysis of CH2DN02. The broad peak at 161 1 cm-I was again contributed by NO2, with an overlapping cisCH2DON0 absorption in the earlier stages of the photolysis. No new peaks appeared between 2228 and 2268 cm-', consistent with the hypothesis that HNCO and DNCO contributed to these two absorptions. Weak to moderately intense peaks at 3510 and 2628 ( 2 5 ) D. W. Davidson, B. P. Stoicheff,and H. J. Bernstein, J . Chem. Phys., 22, 289 (1954).
cm-' 446 475 49 1 558 62 1 78 1 803 813 sh 920 986 1027 1103 1110 1128 1144 1147 1158 1228 1404 1497 1506 1550 1596 1611 1669 1705 sh 1710 1721 1732 1744 1754 1788 1875 2050 2106 2140 sh 2148 2202 2228 2268 br 2282 2345 2472 2492 2628 2655 2686 2726 2802 2854 sh 2860 sh 2866 3288 3510 3638
peak optical density 33 151 365 min min min 0.000 0.000 0.014 0.000 0.000 0.018 0.000 0.000 0.014 0.023 0.016 0.000 0.014 0.009 0.007 0.054 0.036 0.000 0.117 0.066 0.026
assignment
Zrans-CHzDONO cis-CH,DONO trans-CHzDONO cis-CHzDONO trans-CHD(NO)OH cis-CHzDONO
0.000 0.040 0.038 0.019 0.000 0.029 0.000 0.000 0.000 0.005 0.000 0.032 0.037 0.000
0.017 0.023 0.086 0.092 0.010 0.113 0.000 0.015 0.019 0.024 0.000 0.067 0.078 0.068
0.021 0.000 0.059 0.098 0.024 0.138 0.015 0.015 0.045 0.010 0.029 0.052 0.084 0.152
0.000 0.044 0.075
0.000 0.062 0.034
0.045 0.050 0.000
0.128 0.000 0.069 0.000 0.010 0.000 0.000 0.000 0.000
0.269 0.056 0.134 0.016 0.01 1 0.005 0.071 0.000 0.019
0.185 0.153 0.105 0.034 0.01 1 0.016 0.163 0.032 0.019
0.040 0.013 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.024
0.209 0.027 0.043 0.018 0.000 0.000 0.020 0.000 0.01 1 0.000 0.016 0.023 0.043
0.385 0.028 0.187 0.055 0.045 0.158 0.099 0.021 0.022 0.01 1 0.028 0.054 0.030
CO" HCNO ? HNCO, DNCO HNCO
0.045 0.000 0.000 0.016
0.088 0.061 0.017 0.065
0.066 0.272 0.023 0.087
HzCO HNCOb HNCO trans-CHD(NO)OH, trans-CHz(NO)OH
trans-CHD(N0)OH tr~ns-CHz(N0)OH trans-CHz(NO)OD cis-CHD(N0)OH DNO" HDCO HZCO HNO DNO, trans-CHD(NO)OH, trans-CHZ(NO)OD HZ0 cis-CH,DONO, NOz trans-CH,DONO HDCO~
HDCO HzCOb HZCO NO DNO'
co
co2
DNCO' DNCO trans-CHZ(NO)OD HzCO
'Perturbed by other molecules trapped in adjacent Sites. Hydrogen-bonded to HNO (DNO). cm-I may also be assigned to isolated HNCO and DNCOSz3The 3288- and 2472-cm-' absorptions grew greatly during the final period of photolysis. The interpretation of the data for the photolysis of Ar: C H D 2 N 0 2samples, summarized in Table IV, parallels that for the other systems. Although spectral data for cis- and transC H D 2 0 N 0 have not been reported, assignments of the most prominent absorptions of these expected products are straightforward. Because significant absorption at 798 cm-' persisted after the final period of photolysis, it is suggested that absorption of another product underlies that of cis-CHD20NQ. Muller and
Jacox
3378 The Journal of Physical Chemistry, Vol. 88, No. 15, 1984 TABLE I V Peak Optical Densities of Absorptions Which Appear on Mercury-Arc Photolvsis of Nitromethane-dz Isolated in Solid Argon peak optical density 152 300 min min min 0.025 0.009 0.000 0.024 0.009 0.000 0.000 0.019 0.025 0.054 0.031 0.128 0.065 0.049 0.027 0.028 0.011 0.000 0.014 0.000 0.014 0.024 0.000 0.024 0.010 0.000 0.025 0.005 0.000 0.015 0.020 0.020 0.000 0.072 0.072 0.037 0.037 0.084 0.078 0.011 0.120 0.141 0.000 0.022 0.022 0.000 0.056 0.069 0.068 0.270 0.312 0.000 0.022 0.033 0.000 0.023 0.009 0.000 0.018 0.018 0.000 0.019 0.024 0.025 0.046 0.030
HNO
0.059 0.113
0.059 0.051
0.046 0.031
c~s-CHD~ONO, NO2 trans-CHD20N0
0.070 0.006
0.142 0.042
0.125 0.051
D2COb D2CO
32
cm-' 555 599 710 777 798 812 867 875 91 1 963 972 986 1026 1134 1150 1158 1167 1188 1202 1232 br 1318 1506 1573 sh 1609 1668 1682 sh 1687 1698 1705 sh 1710 1720 sh 1733 1754 1769 br 1875 2038 2050 2062 2074 2080 sh 2140 sh 2146 2185 2226 2264 br 2344 2472 2488 2568 2622 2684 2862 br 3288 3476 3638 3700 br
-
assimment
trans-CHD20N0 c~s-CHDIONO trans-CHD20N0 c~s-CHD~ONO, ?
CHD2ONO CHDZONO D,CO HDCO trans-CHD(N0)OD ? DNO" DNO" D2O"
0.125
0.243
0.222
HDCO~
0.011 0.011 0.000 0.017 0.014 0.000 0.000 0.057
0.017 0.022 0.023 0.105 0.034 0.024 0.025 0.111
0.023 0.017 0.023 0.134 0.034 0.020 0.031 0.111
H2COb NO DNO" DCNO ? D2COb D2COb
co 0.052 0.032 0.000 0.000 0.000 0.000 0.000 0.000 0.018 0.012 0.044 0.000 0.000 0.000
0.239 0.062 0.136 0.011 0.105 0.060 0.000 0.030 0.043 0.083 0.088 0.056 0.012 0.085
0.291 0.053 0.199 0.028 0.169 0.104 0.015 0.032 0.038 0.094 0.082 0.071 0.019 0.100
CO"
HNCO, DNCO HNCO
co2
DNCO" cis-CHD(NO)OD DNCO trans-CHD(N0)OD H2CO HNCO" cis-CD2(NO)OH trans-CD,(NO)OH H20," HDO"
"Perturbed by other molecules ti.apped in adjacent sites. bHydrogen-bonded to HNO (DNO).
co-w~rkers'~ did not study the spectra of nitrosomethanol-dz. The OH-stretching modes of cis- and trans-CD2(NO)OH and the OD-stretching modes of cis- and trans-CHD(N0)OD should be unshifted from their counterparts in the other isotopic studies, in agreement with the observations. The failure to detect absorptions at either 1110 or 819 cm-' is consistent with the presence of three hydrogen (deuterium) atoms in the carriers of these absorptions. The 1555-1 570-cm-' spectral region, in which prominent NO-stretching absorptions should appear, was obscured by the absorption of unphotolyzed CHD2NOZ.The very strong 1134-crn-' peak was probably contributed by tram-CHD(NO)OD, since the trans species is generally more abundant than the cis under the photolysis conditions of these experiments and since
on the basis of stoichiometry the probability of migration of a D atom to the oxygen should be twice that for the migration of an H atom. Once again, there was no shift in the peaks attributed to the antisymmetric NCO-stretching vibrations of HNCO and DNCO. The peak at 2622 cm-' may have been contributed in part by isolated DNCO, although some absorption appeared at this frequency at an early stage in the photolysis. Prominent peaks again appeared at 2472 and 3288 cm-'.
Discussion
-
The primary photodissociation reaction CH3N02 hv CH3 NO2
+
+
(1)
is known to occur in the gas phase over a wide range of wavelengths. As will be shown, the argon-matrix observations are also consistent with this primary process. On the basis of thermochemical arguments, Grayz6first suggested that the most probable reaction of the free radicals produced in reaction 1 should be CH3 NO2 C H 3 0 N O (2)
-
+
+
The gas-phase kinetic study by Gutman and c o - ~ o r k e r of s ~the ~ reaction between CH3 and NO2 indicated that reaction 2 predominated; nitromethane was not detected. In solid argon, the cage recombination
+ -
CH30 + NO
cis-, trans-CH30N0
(3)
would be expected to follow reaction 2. However, the reaction CH30
NO
H2C0 + H N O
(4)
occurs readily at room t e m p e r a t ~ r eand ~ ~ predominates ,~~ in studies of the photolysis of C H 3 0 N 0 in an argon m a t r i ~ . ' ~ .In' ~the solid, the products of reaction 4 form the hydrogen-bonded HzCO-.. H N O complex. The mechanism by which nitrosomethanol is formed in the present experiments is somewhat less certain. Honda2 has discussed the possible occurrence of a second, less important photodissociation channel CH3N02 + hu CH3NO + 0 (5) -+
which would account for the observation of C H 3 N 0 among the photolysis products. Although 0 atoms may undergo at least limited diffusion through solid argon at 14 K, they may also insert into a C-H bond of nitrosomethane at the site of their photoproduction: CH3NO
+0
CHz(N0)OH
-
(6)
Muller and Huber18 observed the reaction
-
H2C0. .HNO
+ hu
trans-CHz(NO)OH
(7)
on prolonged exposure of the sample to narrow-band radiation at 345 nm. The earlier studies in this laboratory16of the photolysis of methyl nitrite by mercury-arc radiation with a short-wavelength cutoff of 345 nm gave very high yields of H,CO..-HNO, but even after several hours of irradiation almost no nitrosomethanol was present. On the other hand, when Ar:CH30N0 or Ar:CD30N0 samples were irradiated for approximately 1 h by the full light of a medium-pressure mercury arc, prominent absorptions not only of formaldehyde-nitroxyl hydrogen-bonded pairs but also of trans-nitrosomethanol were present. Muller and Huber18 have suggested that reaction 7 involves the initial photoexcitation of formaldehyde. Since absorption associated with the A 'A"-X 'A, transition of formaldehyde extends from 353 to 230 nm,30 the higher yield of trans-nitrosomethanol when methyl nitrite is ex(26) P.Gray, Trans. Faraday Soc., 51, 1367 (1955). (27) F. Yamada, I. R. Slagle, and D. Gutman, Chem. Phys. Lett., 83,409 (1981). (28) L. Batt, R.T. Milne, and R. D. McCulloch, Int. J . Chern. K i n d . , 9, 567 (1977). (29) N. Sanders, J. E. Butler, L. R. Pasternack, and J. R. McDonald, Chern. Phys., 48, 203 (1980).
J . Phys. Chem. 1984, 88, 3379-3382
posed to the full light of the medium-pressure mercury arc may be a consequence of the much greater radiation intensity in the 250-260-nm region. Mechanisms for subsequent photodecomposition reactions which lead to acceleration in the production of such molecules as CO and N O in the later stages of the experiment are also of some interest. The formation of these two molecules is readily explained by the photodecomposition of HzCO and HNO, respectively. As in the C H 3 0 N 0 photolysis study,16 the appearance of the absorption maximum for CO at 2146 cm-' rather than at 2138 cm-', the value appropriate for CO isolated in solid argon, is consistent with the trapping of C O in a site adjacent to N O or to HNO. The stabilization of H N C O would require more extensive rearrangement. Photoelimination of HzO from either nitrosomethanol or the well-known aci-nitromethane, which may possibly be formed by migration of a hydrogen atom to one of the oxygen atoms of the nitro group and which may undergo further photodecomposition, would produce H C N O CHz(N0)OH
+ hv -,H C N O + H20
or CH3N02
+ hv
followed by H,C-N(O)-OH
-
HCNO
+ hv
-
3379 HNCO
(11)
The occurrence of this rearrangment would explain the formation of a substantial concentration of HNCO on prolonged mercury-arc photolysis of matrix-isolated nitromethane. The N-H bond can participate in relatively strong hydrogenbonding interactions. Since these interactions generally shift hydrogen-stretching vibrations to longer wavelength, the assignment of the 3290-cm-' absorption to the N-H-stretching fundamental of H N C O hydrogen-bonded to H 2 0 is suggested. The broad 3708-cm-' absorption, which also grew at an accelerated rate in the later stages of the photolysis, may be contributed by v3 of the H 2 0moiety in the complex. The peaks at 2472 and 2742 cm-' in the studies of the photolysis of CD3N02could be attributed to hydrogen-bonded DNCO and DzO, respectively. The appearance of the 2472- and 3290-cm-' peaks, but not of the 2742and 3708-cm-I peaks, in the partially deuterated nitromethane studies, in which H D O would be an important product, is consistent with such an assignment.
(8) Conclusions
H,C-N(O)-OH
+ hv
+
HCNO
+ HZO
(9) (10)
Weak absorptions at 2202 and 3322 cm-' may have been contributed by HCNO, the two highest frequency fundamentals of which have been observed in an argon matrix3' at 2193 and 3318 cm-I. Trapping of the H C N O in a site adjacent to HzO may lead to small spectral perturbations from the frequencies reported for the isolated molecule. Prominent absorptions of DzCO and DNO would obscure the 2063-cm-I absorption of DCNO in solid argon.31 However, the 2062-cm-' peak in the CHDzN02photolysis study may have been contributed by DCNO. Bondybey and co-workers3' have reported the photoinduced rearrangement of H C N O to H N C O in a neon matrix: (30) G. Herzberg, -.Molecular Spectra and Molecular Structure. 111. Electronic Spectra and Electronic Structure of Polyatomic Molecules", Van Sostrand, Princeton, NJ, 1966, pp 518-522, 612. (31) V. E. Bondybey. J. H. English, C. W. Mathews, and R. J. Contolini, J. Mol. Spectrosc., 92, 431 (1982).
The recent infrared spectroscopic identifications of cis- and tramnitrosomethanol have been confirmed, and these species have been demonstrated to be important products of the mercury-arc photolysis of nitromethane isolated in solid argon. In both the gas phase and an argon matrix, the primary photodecomposition of nitromethane leads to the formation of CH, and NO,. However, cage recombination occurs in the matrix, resulting in the formation of cis and trans-CH30N0 and of the hydrogen-bonded HzCO-..HNO complex. Nitrosomethanol may result either from the photoexcitation of this complex or from the cage recombination of 0 atoms with CH,NO, products of a second, less important reaction channel for the photodissociation of nitromethane. Prolonged irradiation of the sample leads to the stabilization of HNCO and of H 2 0 , which may also form hydrogen-bonded pairs. Acknowledgment. I thank Dr. R. P. Muller and Prof. J. R. Huber for helpful conversations and for sharing with me in advance of publication the results of their studies of the infrared spectra of isotopically substituted nitrosomethanol. This work was supported in part by the U S . Army Research Office under Research Proposal 17710-C and by the Office of Naval Research under Contract No. N00014-83-F-0038, N R 659-804.
Electrostatic and Magnetic Field Effects on the Behavior of Radical Pairs Derived from Ionic Benzophenones' J. C. Scaiano* and D. J. Lougnot** Division of Chemistry, National Research Council of Canada, Ottawa, Ontario, Canada K I A OR6 (Received: October 4, 1983)
The photochemistry of cationic, anionic, and surfactant benzophenones has been examined in sodium dodecyl sulfate and dodecyltrimethylammonium chloride micelles. The triplet behavior is not very dependent upon electrostatic interactions, although lifetimes are shorter when the benzophenoneand the surfactant have opposite charges. The efficiency of radical-pair separation, following hydrogen abstraction, is markedly dependent upon electrostatic effects and can be enhanced by application of an external magnetic field. The percentage of radical pairs that undergo geminate recombination can vary from 16% to 98% depending on the nature of the hydrogen donor and the role of electrostatic and magnetic field effects.
The behavior of radical pairs (RP) in organized systems, in particular micelles, is a subject of current It has now
been established that the predominant modes of decay of triplet RPs in compartamentalized systems involve a competition between
(1) Issued as NRCC 22931. (2) NRCC visiting scientist, on leave from the Ecole Nationale SuNrieure de Chimie, Laboratoire de Photochimie Generale, ERA no. 386-Mulhouse, France.
(3) Turro, N. J.; Kraeutler, B. Acc. Chem. Res. 1980, 23, 369-377 and references therein. (4) Turro, N. J. Proc. Natl. Acad. Sci. U.S.A. 1983, 80, 609-621. (5) Scaiano, J. C.; Abuin, E. B. Chem. Phys. Lett. 1981, 81, 209-213.
0022-3654/84/2088-3379$01.50/0
Published 1984 American Chemical Society
