J. Phys. Chem. 1983, 8 7 , 3126-3135
3128
Reaction of F Atoms with Nitromethane. Vibrational Spectra of the Addition Complex and of the Nitromethyl Free Radicalt Marllyn E. Jacox Molecular Spectroscopy Dlvision, Netionai Bureau of Standards, Washington, D.C. 20234 (Received: December 9, 1982)
When the products of the reaction between F atoms formed in a microwave discharge and CH3N02are frozen in a large excess of argon at 14 K, new infrared absorptions appear which, with the help of detailed isotopic substitution studies, have been assigned to the nitromethyl free radical. A large fraction of the nitromethyl is hydrogen bonded to HF trapped in an adjacent site. Analysis of the infrared spectrum for the HF complex indicates that the hydrogen bonding is relatively strong. The threshold for photodecomposition of nitromethyl lies between 300 and 280 nm. The resulting H2C0 and NO may be formed either directly or by the cage recombination of CH2and NOz. In the F + CH2DN02and CHD2N02reaction studies, H-atom abstraction occurs selectively,and in the F + CDBN02reaction study, prominent absorptions of an F-atom addition complex are present in the spectrum of the initial deposit. The threshold for the photodecomposition of this complex into CD2N02+ DF lies between 490 and 420 nm. These observations suggest that the initial attack of the F atom on nitromethane results in the formation of an addition complex which, when H atoms are present, eliminates HF-by a tunneling mechanism.
Introduction The predominance of CH3 and NO2 as the primary products of both the thermal decomp~sitionl-~ and the photodisso~iation~~~ of nitromethane has long been recognized. Cottrell and co-workers2 first suggested that the appearance of methane as a major product of the thermal decomposition of nitromethane could be explained by the occurrence of the reaction CH3 + CH3NO2 CHI + CH2N02 which also results in the formation of the nitromethyl free radical. The importance of this reaction, recognized in all of the subsequent work on the thermal decomposition of nitr~methane,~-~ implies that the subsequent reactions of nitromethyl play significant roles in the degradation of nitromethane in condensed-phase systems such as those of propellant ignition and explosive initiation. There have been several recent studies of the rates of H-atom abstraction from nitromethane by atoms and free radicals which may play significant roles not only in the chemistry of ignition and explosion but also in that of the upper troposphere. Ballod and co-worker~~.~ have determined the rate of the CH3 + CH3N02reaction over a wide range of temperatures. The rates of reaction of OH'O and O(3P)11J2with nitromethane have also been reported, and Salter and Thrush12 have proposed that nitromethyl is a primary reaction product in both systems. Since photochemically generated CH3may react with NO2in the upper atmosphere to form nitromethane and methyl nitrite,l3,l4 the OH and 0-atom reactions may also result in a sufficient concentration of nitromethyl for its participation in the chemistry of the upper troposphere. Very little is known about the properties of nitromethyl. Its ESR spectrum was first reported by Chachaty15J6in studies of the radiolysis of solid nitromethane. Although it was suggested that nitromethyl was nonplanar, a subsequent INDO calculation17was consistent with a planar
-
'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.
structure. No vibrational or electronic spectral data have been reported. The F + CH3N02reaction is particularly well suited to the preparation of nitromethyl under conditions favorable for direct spectroscopic observation. Setser and co-worke d 8 observed HF chemiluminescence in this reaction system, but found that the rate of HF formation was slow compared to the rate in the F + CH, reaction and that the peak in the vibrational energy distribution of the HF produced from CH3N02occurred at lower vibrational excitation than for HF produced from CHI. In the first of a series of F-atom reaction studies in this laboratory, in which the products of the reaction of F atoms produced in a discharge with a small molecule present in a large excess of argon carrier gas are rapidly frozen onto a cryogenic observation surface, the most prominent infrared (1)T.L. Cottrell and T. J. Reid, J . Chem. Phys., 18, 1306 (1950). (2)T.L. Cottrell, T. E. Graham, and T. J. Reid, Trans.Faraday Soc., 47, 584 (1951). (3)G. M. Nazin, G. B. Manelis, and F. I. Dubovitskii, Russ. Chem. Reu., 37,603 (1968). (4)C. G. Crawforth and D. J. Waddington, Trans. Faraday Soc., 65, 1334 (1969). (5)F. I. Dubovitakii and B. L. Korsunskii, Russ. Chem. Reu., 50,958 (1981). (6)1. M.Napier and R. G. W. Norrish, Proc. R. SOC.London, Ser. A , 299,317 (1967). (7)K. Honda, H.Mikuni, and M. Takahasi, Bull. Chem. SOC.Jpn., 45, 3534 (1972). (8) T. V. Fedorova, A. P. Ballod, and V. Ya, Shtern, Kinet. Katal., 11, 1052 (1970);Kinet. Catal. (Transl.), 11, 867 (1970). (9) A. P. Ballod,T. V. Fedorova, N. Chikvaidze, T. A. Titarchuk, and A. A. Kumova, Kinet. Katal., 21,1095(1980);Kinet. Catal. (Transl.),21, 781 (1980). (10)I. M. Campbell and K. Goodman, Chem. Phys. Lett., 36, 382 (1975). (11)I. M.Campbell and K. Goodman, Chem. Phys. Lett., 34, 105 (1975). (12)L. F. Salter and B. A. Thrush, J. Chem. Soc., Faraday Trans. I , 73,2025 (1977). (13)S. T.Levine and S. E. Schwartz, J. Photochem., 9,104 (1978). (14)W. D.Taylor, T. D. Allston, M. J. Moscato, G. B. Fazekas, R. Kozlowski, and G. A. Takacs, Int. J. Chem. Kinet., 12,231 (1980). (15)C. Chachaty, J . Chim. Phys., 62,728 (1965). (16)C. Chachaty and C. Rosilio, J. Chim. Phys., 64,777 (1967). (17)T.A. B. M. Bolsman. J. W. Verhoeven, and Th. J. de Boer, Tetrahedron, 31, 1015 (1975). (18)D. J. Smith, D. W. Setser, K. C. Kim, and D. J. Bogan, J. Phys. Chem., 81,898 (1977).
This article not subject to U.S.Copyright. Published 1983 by the American Chemical Society
Reaction of F Atoms wlth Nitromethane
The Journal of Physical Chemistry, Vol. 87, No. 16, 1983 3127
TABLE I: Peak Optical Densities of Product Absorptions in Ar:CH,NO, = 200 t Discharged Ar:NF, = 200 Experiment Before and After Exposure of Sample t o Mercury-Arc Radiation of Wavelength Greater Than 280 nm cm-'
initial
h
> 280 nm assignment'
434 0.034 0.034 ( HF )n 476 0.028 0.028 N N 534 0.009 0.009 1500 1100 1300 1200 1100 100 600 557 0.012 D 57 8 0.288 0.105 B 606b 0.356 0.128 B 639 0.497 0.296 B 652b 0.176 D 693 0.151 0.102 B 719 0.357 0.133 B 939 0.025 0.025 N 0.010 B 986 0.030 1060 0.021 0.011 B 1090 0.034 0.016 B 1095 0.359 0.179 B I , , 1 1139 0.028 0.034 N 4000 3800 3600 1900 ldS0 1150 1100 cm-1 1179 0.053 H,COd Figure 1. (-) 4.07 mmol Ar:CH,NO, = 200 codeposited over period 1209 0.053 0.017 B of 201 min with 3.00 mmol discharged Ar:NF, = 200. (----) 95-mln 1253 0.011 0.049 H,COd subsequent mercury-arc photolysis, X > 280 nm. 1297 0.284 0.083 B 1307 0.053 0.030 B absorption of the products of the F CHI reaction was A 1361 0.025 141gb 0.055 0.015 B that contributed by CH3.19 Subsequently, vibrational 1461 0.694 0.244 B spectra of several other free radicals have been observed 1484 0.656 0.224 B as a result of H-atom abstraction by F atoms.2@22In the 1500 0.156 H,COd following discussion, results of an analogous infrared A 1557' spectroscopic study of the primary products of the F 1594 0.017 H,O? CH3N02reaction are presented. 1605 0.017 1613 0.085 0.098 NO 2 Experimental Details 1617 0.085 0.098 NO, 1692 0.017 A Samples of nitromethane (Eastman Organic Chemicals 1731 0.392 H,COd Spectro Grade), of CHJ5N02 (Prochem), of nitromethane 0.017 H,COe 1742 enriched to 90% in carbon-13, and of completely and 0.017 D 1781 partially deuterium-substituted nitromethane (Merck 1883 0.117 NOf Sharp and Dohme of Canada, Ltd.) were freed of traces 2738 0.046 0.072 D of relatively volatile impurities by pumping on the solid 2836 0.130 D deposit at 77 K. Isotopically pure samples of the partially 2900 0.052 D 3002 0.023 D deuterated species were not available; the mass analysis 3055 0.036 0.012 B of one sample indicated that it contained 24.7% CH3N02 3200 0.060 0.023 B and 75.3% CH2DN02and of the other that it contained 3520 0.845 0.645 B 4.9% CH3N02,24.6% CH2DN02,and 70.5% CHD2N02. 3625 0.170 0.049 B Ar:nitromethane samples of mole ratio 200 were prepared 3740 0.043 0.012 B by standard manometric procedures. Ar:NF3 samples of D 3760 0.024 3780 0.012 D mole ratio ranging from 200 to 800 were used as F-atom 3886 0.018 0.018 HF sources. Details of the preparation of the k N F 3 samples 3920 0.031 0.038 H F and of the discharge configuration used to produce F atoms 3966 0.052 0.053 HF have previously been des~ribed,'~ as have been the cryogenic equipment and sample observation c~nfiguration.~~ a A, diminishes on 300-nm photolysis; B, diminishes on 280-nm photolysis; N, little change on 280-nm photolysis; All observations were conducted at 14 K. After the inD, appears on 280-nm photolysis. Overlapped by CH,frared spectrum of the initial deposit had been recorded, NO, absorption. Broad, weak shoulder on very strong the sample was subjected to the full or filtered radiation CH,NO, absorption. Perturbed by H F and NO trapped of a medium-pressure mercury arc in order to determine Perturbed by HF and in adjacent sites. e Isolated. H,CO trapped in adjacent sites. the threshold and products of secondary photodecompol
1
,
1
8
1
1
+
+
sition. Although a variety of glass filtem was used, Corning filters of glass type 0160 and 7740, with short wavelength cutoffs of 300 and 280 nm, respectively, were most frequently employed. 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-' (19)M.E.Jacox, Chem. Phys., 42, 133 (1979). (20)M.E. Jacox, Chem. Phys., 59, 199 (1981). (21)M.E.Jacox, Chem. Phys., 59, 213 (1981). (22)M.E. Jacox, Chem. Phys., 69, 407 (1982). (23)M.E.Jacox, Chem. Phys., 7, 424 (1975).
between 400 and 2000 cm-' and 2 cm-' between 2000 and 4000 cm-'. Observations
When an Ar:CH3N02 = 200 sample was codeposited with a discharged Ar:NF3 = 200 sample, a rich product spectrum resulted. Spectral regions of especial interest are shown for a typical experiment in the solid trace of Figure 1,and the positions and peak optical densities of the product absorptions characteristic of this reaction system are summarized in the first two columns of Table I. In addition to the absorptions given in Table I, peaks
3128
Jacox
The Journal of phvsical Chemistry, Vol. 87, No. 16, 1983
were contributed by the unreacted species, by NF2and NF produced in the discharge, and by FNO and F02, which result from the reaction of F atoms with trace atmospheric impurities in the dis~harge.'~ The presence of absorptions at 3886,3920, and 3966 cm-', characteristic of HF isolated in solid argon,%indicates that the abstraction of an H atom from CH3N02is represented among the F-atom reactions which occur in this system. It is often useful to classify products of potentially complicated reaction systems according to their photodecomposition thresholds and products. In such studies on this system, very few changes in the infrared spectrum resulted on exposure of the sample to radiation of wavelength longer than 300 nm; only a weak to moderately intense peak a t 1361 cm-', a broad absorption near 1557 cm-', on the low-frequency shoulder of the very intense CH3N02absorption, and a weak peak at 1692 cm-' disappeared under these conditions, and no new peaks appeared. In the subsequent discussion, peaks having this photolysis behavior will be designated as type A. When mercury-arc photolysis was resumed with a 280-nm cutoff filter, marked changes occurred. The resulting spectrum is shown in part in the broken-line trace of Figure 1, and the peak optical densities of the product absorptions in a typical experiment are summarized in the third column of Table I. Product absorptions which did not exhibit type A photolytic behavior could be classified as type B, N, or D, depending on whether they diminished in intensity, remained approximately constant, or grew in intensity on exposure of the sample to 280-nm cutoff radiation. Supplementary observations on an Ar:CH3N02 = 200 deposit in the absence of an F-atom source demonstrated that the infrared spectrum is unchanged on prolonged exposure of the deposit to mercury-arc radiation with a 280-nm cutoff. The most prominent product absorptions exhibited type B photolysis behavior. In the experiment here tabulated, these peaks decreased on photolysis to approximately 40% of their intensity in the initial sample deposit. A few of them, including peaks a t 639, 693, 1307, and 3520 cm-', decreased somewhat more slowly. Such behavior could be explained either by the assignment of these peaks to another product of type B photolysis behavior or by contributions from an overlapping peak of type N or D photolysis behavior. The latter explanation is strongly favored for the 3520-cm-' absorption, which is shifted upward by a few cm-' as a result of photolysis. Isotopic Substitution experiments, considered in the following discussion, have provided further clarification of the type B absorptions. Relatively little can be said about the type N peaks. The three HF peaks between 3880 and 4000 cm-' fall into this group. In the F + CHI reaction study,lg an absorption at 434 cm-' was attributed to (HF),. A similar assignment is possible for the 435-cm-' peak of the present series of experiments. The most prominent of the remaining type N peaks are of only weak to moderate intensity. Assignments are proposed in Table I and Figure 1 for several of the more prominent type D peaks, which grew on 280-nm photolysis. Arguments supporting these identifications will be presented in the following discussion. The positions of the type B absorptions in experiments in which nitromethane enriched in either carbon-13 or nitrogen-15 was used are summarized in Table 11. The peaks at 578, 639, 3625, and 3740 cm-' and, within the experimental error, that at 3520 cm-' were unshifted on isotopic substitution, a necessary but not a sufficient condition for their assignment to HF vibrations in a hy(24) M.G. Mason, W. G. Von Holle, and D.W. Robinson, J. Chem. Phys., 54, 3491 (1971).
TABLE 11: Heavy-Atom Isotopic Shifts (cm-') in Type B Absorptionsa CH,NO, '3CH,N0, CH,'5N0, assignment 578 s 606 s 639 s 693 m 719 s 986 w-m 1060 w, br 1090 w, sh 1095 s 1209 w-m 1297 s 1307 sh 1419 w-m 1461 S-vs 1484 s-vs 3055 w 3200 w-m 3520 vs 3625m-s 3740 w-m
578 s 606 s 639 S-vs 679 w-m 685 w-m 714 s 974w-m
578m-s 600 m-s 639 s
HFdef H F def
690 w-m 709 m-s 982w
1088 m-s 1207 w-m 1292 sh 1297vs
1094 m-s
1413 w-m 1451 s 1460 m 1478 vs 1484 m 3050 w 3185 w-m 3518 vs 3625m 3740 w-m
1413 w-m 1435 s 1439 m-s 1471 w-m 1478 w-m
2( 606) 1272s
3200 w 3518 vs 3625m
NO, sym stretch NO, asym stretch
CH, sym stretch CH, asym stretch H F stretch
a vw, very weak; w, weak; m, medium; s, strong; vs, very strong; sh, shoulder; br, broad.
TABLE 111: Heavy-Atom Isotopic Shifts (cm-') in Type D Absorptionsa CH3N0, 13CH,N0, CH315N0, assignment 557 w 652 s 1179 w-m 1253 w-m 1500 s 1594 w 1605 w 1731 vs 1742 w 1781 w 1883 m 2738 w-m 2836 m-s 2900 w-m 3002 w-m 3760 w-m 3780 w
652 m 1168 w-m 1244 w-m 1500 m
652 m 1179 w 1253 w-m 1500 w-m
1605 vw 1693 s 1705 w
1731 s
1883 m 2832 m 2880 w 2972 w-m
H,COb H,COb H,COb HlO H,COb H,COb NOd
2836 w-m 2900 w-m
a Intensity estimates for photolyzed samples. w, weak; Perturbed by H F m, medium; s, strong; vs, very strong. and NO trapped in adjacent sites. Isolated. Perturbed by HF and H,CO trapped in adjacent sites.
drogen-bonded complex of HF with another reaction product. The behavior of the type D absorptions on heavy-atom isotopic substitution in the nitromethane is summarized in Table 111. Of this group of absorptions, only that at 652 cm-' could be assigned to an HF vibration in a hydrogen-bonded complex. Spectral regions containing prominent product absorptions are shown in the solid trace of Figure 2 for an Ar: CDBNOz= 200 sample codeposited with a discharged Ar:NF3 = 200 sample. The positions of the product absorptions characteristic of this system are summarized in the first column of Table IV, and their peak optical densities are given in the fifth column. The presence of a small concentration of CH3N02in this particular experiment led to the appearance of weak to moderately intense peaks corresponding to its product absorptions. These have been omitted from Table IV. New product absorptions at 1358 and 1530 cm-' were extremely prominent. Noteworthy was
The Journal of Physlcal Chemistty, Vol. 87, No. 16, 1983 3129
Reaction of F Atoms with Nitromethane
TABLE IV: Dependence of Peak Optical Densities of Product Absorptions in Ar:CD,NO, t Discharged Ar:NF, Experiments on F-Atom Concentration and on Short Wavelength Cutoff (nm ) of Photolysis Radiation Ar:NF, = 200 Ar:NF, = 800 cminitial h > 420 A > 280 initial h > 300 h > 280 assignment
429 432 436 481 488 532 590 632 662 694 821 841 880 905 993 1038 1073 1296 1307 1321 1358 1365 1460 1511 1517 1530 1540 1684 1883 2100 2596 2662
0.014 0.087 0.101 0.128 0.206
0.014
0.019 0.028 0.098
0.061 0.014
0.065 0.018 0.014
0.005
0.053 0.039
sh 0.040 0.076 0.009 0.010 0.612 0.057 0.189 0.064 0.192 1.016 0.081
0.147 0.021
A
0.034 sh sh 0.009
0.274 0.066 0.183 0.036 0.651 0.010 0.038 0.266 0.045
0.223 0.061
0.482 0.005 0.170 0.051 0.083 0.597 0.066
0.136 0.131 0.122 0.228
0.073 0.071 0.122 0.162
0.028
0.028
C C D C A N
A
0.009 0.191 0.005 0.075 0.039 0.090 0.010 0.010 0.622 0.054 0.239 0.074 0.190 0.934 0.091
0.015 0.092 0.020 0.019 0.115 0.077 0.010 0.045
0.048
C
0.113 0.031 0.042 0.033
A, CD,NO,
0.336 0.084
0.138 0.044
0.010
0.010
A A
0.709
0.316
C
0.021 0.055 0.011 0.029 0.532 0.103
0.119 0.020
C D,COb A, CD,NO, A C C A
A A A A
0.021 0.215 0.074 0.099 0.459 0.046
D,COb NOC D C C
a A, diminishes on 420-or 300-nm photolysis; B, unchanged on 420-or 300-nm photolysis, diminishes on 280-nm photolysis, C, grows on 420-or 300-nm photolysis, diminishes on 280-nm photolysis; N, little change on photolysis; D, appears Perturbed by DF and D,CO Perturbed by DF and NO trapped in adjacent sites. on 280-nmphotolysis. sh, shoulder. trapped in adjacent sites.
the appearance of the 1530-cm-' peak almost 50 cm-' above the highest frequency product absorption in this spectral region for the undeuterated molecule. Other product absorptions were relatively weak, and no absorptions could be assigned to isolated DF.24 In order to test for the occurrence of secondary F-atom reactions, an experiment was also conducted substituting a discharged h N F 3 = 800 sample and depositing the same amounts of both the Ar:CD3N02and the k N F 3 samples as in the more concentrated F-atom experiment. The peak optical densities of the product absorptions in this experiment are summarized in the second column of Table IV. The product absorptions appeared with similar intensities in both experiments. The peaks at 880 and 1038 cm-' are superposed on CD3N02absorptions, making it difficult to estimate their peak optical densities. The very strong product absorptions at 1358 and 1530 cm-' diminished significantly in intensity when a sample was exposed to the Nernst glower infrared background for 90 min, and the high-frequency satellite absorptions at 1365 and 1540 cm-' almost completely disappeared, prompting a detailed filtered photolysis study of the 1250-1550-~m-~ spectral region for the CD3N02system. The threshold for photodecomposition of the carrier of the 1358- and 1530-cm-' absorptions was found to lie between 490 and 420 nm. A summary of the changes in the absorptions in this spectral region after exposure of the dilute F-atom sample to mercury-arc radiation of wavelength longer than 420 nm for 32 min is given in the third column of Table IV. While the 1358- and 1530-cm-I peaks were reduced to about 30% of their original intensity, absorp-
1000
90nn 'Y""
900
Icnn '"VU
700
i i n n I C I ifinn I,"" ,".I" I"""
600
500
400
iI Ti n" "n
i8""" m
CIII-I
"111
Figure 2. (-) 3.85 mmol Ar:CD3N02= 200 codeposited over period of 173 min with 2.03 mmol discharged Ar:NF, = 200. (----) 32-min subsequent mercury-arc photolysis, A > 300 nm. 0 decreases, grows on subsequent mercury-arc photolysis, A > 280 nm.
+
tions at 1296 and 1460 cm-I more than tripled in intensity. These peaks, in turn, diminished in intensity when a
3130
The Journal of Physical Chemistty, Vol. 87, No. 16, 1983
Jacox
TABLE V: Isotopic Shifts (cm-I) in Type A Absorptionsa o n Deuterium Substitution assign-
CH,NO,
CH,DNO,
CHD,NO,
CD,NO,
ment
429 w-m 532 w-m 632 m 880 m 1038 m 1073 w-m 1321 w 1361 w-m 1358 w-m 1359 m 1358 vs NO, sym 1369 w-m 1365 m stretch 1511 m 1517 s 1557 w, br 1530 w-m 1530 m 1530 vs NO, asym 1692 w
stretch
a w, weak; m, medium; s, strong; vs, very strong; br, broad.
1500
1400
1300 CM-l
Figure 3. (a) (-) 4.07 mmol Ar:CH,NO, = 200 codeposited over period of 201 mln with 3.00 mmol discharged Ar:NF, = 200. (b) (-) 3.85 mmol Ar:CH.J)NO, = 200 codeposited over period of 194 min with 2.57 mmol discharged Ar:NF, = 200. (----) 29-min subsequent mercury-arc photolysis, A > 300 nm. (c) (-) 4.39 mmol Ar:CHD$JO, = 200 codeposited over period of 202 min with 3.53 mmol discharged Ar:NF, = 200. (- -) 52-mln subsequent mercury-arc photolysis, A > 300 nm. (d) (-) 3.85 mmol Ar:CD,NO, = 200 codeposited over period of 173 min with 2.03 mmol discharged Ar:NF, = 200. (----) 32 min subsequent mercuygrc phototysis, X > 300 nm. 0 decreases on subsequent mercury-arc photolysis, A > 280 nm.
--
280-nm cutoff filter was substituted, and new absorptions, comparable to the type D absorptions of the studies on undeuterated samples, appeared. In the more concentrated F-atom experiment of Figure 2, exposure of the sample to 300-nm cutoff radiation led to the same spectral changes between 1250 and 1550 cm-' as had been observed with 420-nm cutoff radiation. As is shown in the broken-line trace of Figure 2, several very prominent new absorptions appeared at this stage of the experiment. The peak optical densities of all of the product absorptions after this 300-nm cutoff photolysis are summarized in the sixth column of Table IV. In the subsequent discussion, peaks which grew in intensity on 300-nm cutoff irradiation of the sample are referred to as having type C photolysis behavior. Substitution of a 280-nm cutoff filter for a second period of photolysis led to further dramatic changes. The peaks which are marked by filled circles in Figure 2 diminished in intensity. In general, type C peaks showed this type of behavior, as type B peaks had in the studies on the undeuterated samples. Prominent type D absorptions appeared at this stage of the experiment. The peak optical densities of the product absorptions after 280-nm cutoff photolysis are given in the seventh column of Table IV, and the photolysis behavior of each of the absorptions is summarized in the final column. Spectra observed between 1275 and 1550 cm-' for unphotolyzed Ar:CH3N02-d, samples codeposited with dis-
charged k N F 3 are shown comparatively in the solid traces of Figure 3. Product absorptions which diminish on exposure of the sample to 280-nm cutoff radiation are marked by fiied circles, in order to distinguish them from prominent CH3N02-d, absorptions in this spectral region. Except for weak absorptions at 1461 and 1484 cm-', attributable to products of the F-atom reaction with CH3N02 in the sample, the spectrum shown in trace b for the CH2DN02sample is distinct from that shown in trace a for CH3N02. There is a small splitting in the two highest frequency peaks in trace b. The spectrum of trace c, for an Ar:CHD,NO, sample, is also unique, with small contributions which correspond to the F-atom reaction product for the residual CH2DN02present in the sample. The two most prominent product absorptions of trace c are also present in trace d, for CD3N02,but these are considerably more prominent in the solid trace c than in the solid trace d. The broken-line traces of Figure 3 show spectral regions in which changes occurred on 300-nm cutoff photolysis. Since the only change in this spectral region for CH3N02 samples was the disappearance of the weak to moderately intense 1361-cm-l peak, the broken-line trace a was omitted. As the extent of deuterium substitution in the nitromethane was increased, the intensities of the type A peaks, which disappeared on 3Wnm cutoff irradiation, also increased, and, as already noted, for CD3N02they become very prominent. The positions and approximate relative intensities of all of the type A absorptions in the deuterium-substitution experiments are summarized in Table V. Changes in the intensities of other absorptions as the type A absorptions were destroyed by photolysis were small for all of the systems in which the nitromethane possesses one or more H atoms. However, in the CD3N02 experiments photolysis of the carrier of the type A absorptions resulted in a marked growth in the intensities of the product absorptions which are common t o the CHD2N02experiments. Spectra observed between 3450 and 4000 cm-' and between 2550 and 2950 cm-' for unphotolyzed deposits of Ar:CH3N02-d, samples codeposited with discharged Ar: NF3 are compared in Figure 4. The three absorptions of isolated HF persist with similar intensity even in the experiment using a CHD2N02sample. Furthermore, the 3520-cm-' peak remains extremely strong as long as the nitromethane molecule retains one or more H atoms. A weak to moderately intense 3520-cm-' peak in experiments on Ar:CD3N02 samples, not shown in Figure 4, can be attributed to CH3N02impurity in the sample. Absorptions of isolated DF24were not detected in the spectra of any
Reactlon of F Atoms wkh Nitromethane
The Journal of Physical Chemlstty, Vol. 87, No. 10, 1983 3131
TABLE VI: Isotopic Shifts (cm-l) in Type B and C Absorptionsa on Deuterium Substitution ~
~~
CH,NO,
CH,DNO,
CHD,NO,
CD,NO,
C 432w-m
C 432m C 436m C 488s
assignment
D F def
C 518w-m C 539w-m HF def HF def
B 578s B 606s B 639s
B 586vs
C 585vs
B 639vs
B 638vs B 669m
HF def
B 679m-s B 693m
C 694m
C 694m
B 906m
C 905m
B 1297 vs
C 1296 vs
C 1309 w-m B 1314 w-m, sh
C 1307 m
B 1460 vs B 1472 m B 1477 m
C 1460 vs
NO, asym stretch
C 2598 w-m
C 2596 vs C 2662 m
DF stretch
B 703m-s B 719s B 879w-m B 986w-m B 998m B 1005m B B B B
1060 w, br 1090 w,sh 1095s 1209 w-m B 1263 m B 1283 s
B 1297 s
NO, sym stretch
B 1302 s B 1307 sh B 1313 s B 1419 w-m B 1 4 6 1 S-vs B 1484 8-vs
B 1452 s B 1455 s C 1460 m B 1472 vs B 1476 vs C 1483 w
B 3055 w B 3200 w-m B 3516 vs B 3518 vs HF stretch B 3520 vs B 3624 m B 3625 m B 3625 m-s B 3738 w B 3740 w B 3740 w-m a vw, very weak; w, weak; m, medium; s, strong; vs, very strong; sh, shoulder; br, broad. Intensities estimated for type C absorptions when fully developed by photolysis with 300-nm cutoff.
of the deuterium-containing samples, and the 2596-cm-' peak, while invariant in position for all of these samples, attained moderate intensity only for the fully deuteriumsubstituted sample. Although DF absorptions are considerably weaker than their H F counterparts, these observations indicate that there is a marked isotopic selectivity for H-atom abstraction. The positions and approximate relative intensities of all of the type B and C absorptions observed in experiments on CH3N02-d, samples are summarized in Table VI. The 639-cm-' peak remained invariant in position as long as one or more H atoms was present in the parent molecule, consistent with its assignment to a complex with HF. With increasing deuterium substitution, there is a small drift to lower frequencies for the 3520-cm-' peak. However, the intensity and breadth of this absorption are so great that this frequency shift is within the range of experimental error. Several of the peaks in the CH2DNOzand CHD2NO2 experiments showed type C photolysis behavior, but in general the increase in their intensities was rather small. The principal differences between the type B and C absorption pattern in the CHD2N02experiments and that in the CD3N02experiments is the disappearance of the peaks attributed to the HF complex, the appearance of prominent new absorptions at 488 and 2662 cm-', and the intensification of the absorptions at 432 and 2596 cm-' for
fully deuterated samples. The presence of a type B or C absorption at 669 cm-I in the CD3N02 experiments is uncertain, since there was a weak to moderately intense background absorption at 669 cm-' in this system. The isotopic shifts in type D absorptions on deuterium substitution are summarized in Table VII. A broad, moderately intense peak appears at 653 f 2 em-' as long as H atoms are present in the system and is replaced by a peak at 481 cm-l in CD3N02experiments. Again, most of the other peaks are common to the experiments on CHD2NOzand CD3N02samples.
Discussion Type D Absorptions. All of the major type D absorptions in the 700-1800-cm-' spectral region may be assigned to H2CO-d,. For the normal and fuUy deuterated systems, these peaks lie close to, and have similar relative intensities to,major absorptions in the infrared spectra of H2C0 and D2C0 isolated in an argon matrix.25 The positions of absorptions attributed to HDCO in the summary of Table VI1 also lie near the band centers of absorptions reported for HDCO in the gas phase,2e and all of the carbon- and (25)H.Khoshkhoo and E. R. Nixon, Spectrochim. Acta, Part A , 29, 603 (1973). (26)D. W.Davidson, B. P. Stoicheff, and H. J. Bernstein, J . C h e n . Phys., 22,289 (1954).
3132
The Journal of Physical Chemistry, Vol. 87, No. 16, 1983
Jacox
of formaldehyde are sensitive to hydrogen-bonding interactions, and many satellite peaks have been assigned to dimers and higher aggregates. When methyl nitrite isolated in solid argon is photolyzed, HzCO is selectively produced in sites adjacent to HN0,27928resulting in hydrogen-bonding perturbations in the spectra of both molecules. In the present experiments, a weak absorption at 1742 cm-' can be assigned to isolated H2C0and the very strong peak at 1731 cm-' to H2C0 hydrogen-bonded to another photolysis product. The relative intensities of these two peaks suggest that only a small fraction of the HzCO precursor is formed before the initial F-atom reaction products are trapped in the solid deposit. Type D absorptions between 2700 and 3050 cm-' may also be attributed to hydrogen-bonded H2C0,but absorptions in this spectral region have previously been found to be relatively strongly perturbed by hydrogen bonding, and a more detailed assignment will not be attempted. The identification of HzCO among the photodecomposition fragments of the CH3N02 + F reaction product IlO% suggests that the other fragments, trapped in adjacent I sites, may be NO and HF. The 1883-cm-' peak lies only 8 cm-' above the absorption of NO isolated in solid argon. 1000 3800 3600 3100 2800 2600 CN-l This peak is invariant on carbon-13 and on deuterium Flgure 4. (a) 4.07 mmol Ar:CH,NO, = 200 codeposited over period substitution and is shifted on nitrogen-15 substitution. of 201 min with 3.00 mmol discharged Ar:NF, = 200. (b) 3.85 mmol The absorption of 15N0 isolated in solid argon has been Ar:CH,DN02 = 200 codeposited over period of 194 min with 2.57 observed in a supplementary experiment at 1840 cm-', mmol discharged Ar:NF, = 200. (c) 4.39 mmol Ar:CHD,NO, = 200 suggesting that the absorption of perturbed 15N0should codeposited over period of 202 min with 3.53 mmol discharged Ar:NF, = 200. (d) 3.85 mmol Ar:CD,NO, = 200 codeposited over period of lie at 1848 cm-'. This absorption would be obscured by 173 min with 2.03 mmol discharged Ar:NF, = 200. the prominent 1852-cm-' absorption of FNO, present in all of the NF, discharge experiments because of trace atTABLE VII: Isotopic Shifts (cm-I) in Type D mospheric impurities in the NF, sample. Absorptions' on Deuterium Substitution The rather broad type D peak at 653 cm-' lies in the assignspectral region characteristic of HF deformation vibrations CH,NO, CH,DNO, CHD,NO, CD,NO, ment in hydrogen-bonded complexes. The frequency ratio of 481s DFdef the strong 481-cm-' peak, unique to the experiments on 557 w fully deuterated nitromethane, to the 653-cm-' peak is 0.74, 653 m 655 m HF de! 652 s consistent with the assignment of the 481-cm-' peak to the 993 w 993 w-m D,CO DF deformation. In the more concentrated F-atom study HDCOb 1032 w-m summarized in Table IV, the 481-cm-' peak appeared to H,COb 1179 w-m remain constant in intensity on 280-nm cutoff photolysis, H,COb 1253 w-m H D C ~ ~ while the other type D absorptions continued to grow. 1398 w-m H,CO 1500 s However, because this peak was only partially resolved 1594 w H2O from the intense 488-cm-' type C absorption, estimates of 1605 w its peak optical density are subject to an atypically large 1619 w-m uncertainty. D,COb 1684 s 1684 s There is no high-frequency absorption which can be HDCO~ 1707 vs 1706 w-m H,COb 1731vs 1731 w definitively assigned to a type D HF-stretching absorption. H,COC 1742 w However, there is an upward shift in the very prominent 1781 w type B absorption, which decays more slowly than most NOd 1883m 1883m 1883 w-m 1 8 8 3 m of the other type B peaks as 280-nm photolysis proceeds. 2100m 2100m A band near 3570 cm-l has previously been assigned to this 2125 w-m absorption of the H,CO-HF complex.21 Thus, the HF 2166 w-m stretching absorption of the [H2C0+ NO HF] complex 2738 w-m 2836 m-s may well lie near 3530 cm-'. 2896 w-m Peaks assigned to the HF-stretching vibration of the 2900 w-m CO-.HF complex appeared at 3776 and 3792 cm-' in the 3002 w-m study of the reaction of F atoms with CH30H.21 When 3760 w-m 3760 w-m CH3 was trapped in an adjacent site, this absorption of 3780 w CO-HF appeared at 3770 cm-l. Thus, the rather weak ' Intensity estimates for photolyzed samples. vw, very 3760- and 3780-cm-' absorptions may be contributed by weak; w, weak; m, medium; s, strong; vs, very strong. a small concentration of CO-HF perturbed by NO trapped Perturbed by NO and HF or DF trapped in adjacent in an adjacent site. sites. Isolated. Perturbed by H,CO-d, and H F or DF trapped in adjacent sites. Type B and C Absorptions. Under the sampling conditions used for these experiments, it has been estimatedm deuterium-isotopic shifts between 700 and 1800 cm-' correspond to those observed in other studies in which (27) M.E. Jacox and F. L. Rook, J. Phys. Chem., 86, 2899 (1982). formaldehyde is formed.21 There were no spectral shifts (28) R. P.Muller, P. Rwegger, and J. R. Huber, Chem. Phys., 70,281 on nitrogen-15 substitution. The vibrational absorptions (1982).
I
+
Reaction of F Atoms wkh Nitromethane
that F atoms and the reactant molecule may interact in the gas phase over a period of approximately 10” s before the mixture is quenched on the cryogenic observation surface. F atoms can continue to diffuse through solid argon at 14 K, leading to the preferential stabilization of radicals formed by H-atom abstraction in sites adjacent to HF. As in earlier experiments using this sampling technique,lS2’ infrared absorptions of both the isolated radical and the radical complexed to HF would be expected to appear. Spectral perturbations of radicals complexed to H F have typically amounted to a few cm-’ but occasionally have been as large as 50 or 60 cm-l. Heavy-atom isotopic substitution should lead to similar shifts in a given vibration of both the isolated and the HF-complexed radical. Despite the great intensity of several of the absorptions, such pairs are not evident in the summary of Table 11. In the earlier experiments using the discharge configuration for F-atom production and similar reactant concentrations,’S22 there was little evidence for secondary F-atom reactions. The F-atom concentration study of Table IV is consistent with a predominant role of the primary reaction in the F CH3N02studies, as well. Given the great intensity of the 3520-cm-’ absorption and its failure to be significantly shifted on carbon-13 substitution, it is most likely to be contributed by HF hydrogen bonded to the free radical product, CH2N02. The observed products of the photolysis of the carrier of the type B and C absorptions, H2C0and NO perturbed by HF trapped in an adjacent site, are consistent with this identification. The conclusion that the CH2N02is almost completely stabilized as the HF complex is consistent with the appearance of only a weak absorption of isolated H2C0 at 1742 cm-’ after 280-nm cutoff photolysis. The weak to moderate intensity of the absorptions of isolated HF also implies that a relatively small concentration of isolated CH2N02is produced in these experiments. The deuterium substitution experiments are also consistent with the assignment of the type B and C absorptions to CH2N02hydrogen bonded to HF. These two types of absorptions have the same photodecomposition threshold and could be contributed by the same species if there is an intermediate in its formation and if the stability of this intermediate is enhanced by deuterium substitution. The type A absorptions, which are weak for the CH3N02and CH2DN02reaction systems, somewhat more prominent for CHD2N02,and very strong for CD,NO2, provide evidence for such a species, and the growth in the type C absorptions can be correlated with the destruction of the type A absorptians. Theselectivity of H-atom abstraction to form nitromethyl also accounts for the relatively small overlap between the product peaks for CH2DN02and CHD2N02and for the similarity of the product absorptions of CHD2N02and CD3N02(excluding, of course, peaks contributed by HF or DF vibrations in the hydrogen-bonded complex). The structure of CH2N02is not known with certainty. INDO calculations” have predicted that it is planar. Ab initio calculations for the indicate that it is also planar. The semiempirical arguments of W a l ~ for h ~H2CX ~ molecules suggest that both the uncharged species and the
+
(29) J. N. Murrell, B. Vidal, and M. F. Guest, J.Chem. SOC.,Faraday Trans. 2, 71, 1577 (1975). (30)P. G. Mezey, A. J. Kresge, and I. G. Csizmadia, Can. J. Chem., 54, 2526 (1976). (31) I. G. Csizmadia, G. Theodorakopoulos, H. B. Schlegel, M.-H. Whangbo. and S. Wolfe. Can. J. Chem., 55.986 (1977). (32r A.’D. Walsh, J. Chem. SOC.,2296 (1953).
The Journal of Physical Chemistty, Vol. 87, No. 16, 1983 3133
anion should be pyramidal but that the radical, with one less valence electron, would more closely approach planarity than would the anion. The planar molecule, with C2”symmetry, should have 1 2 vibrational fundamentals, 11 of which are infrared active. The approximate descriptions of these are as follows: a,
a,
CH, sym stretch b, CH, “scissors” NO, sym stretch b, CN stretch NO, “scissors” torsion (infrared inactive)
H,CN out-of-plane CNO, out-of-plane CH, asym stretch NO, asym stretch CH, in-plane rock NO, in-plane rock
The assignment of several of the type B(C) absorptions to planar CH2N02would be straightforward. The 3200and 3055-cm-’ peaks lie at suitable positions for their assignment to the CH2antisymmetric and symmetric stretch, respectively. The large shift on nitrogen-15 substitution and the small shift on deuteration support the assignment of the very prominent 1461- and 1297-cm-’ absorptions to the two NO2stretching vibrations, which typically are very strongly absorbing. By analogy with nitromethane, the higher frequency peak is assigned to the antisymmetric stretching vibration, of bz symmetry. In addition to the NO2 antisymmetric stretching fundamental, the CH2 “scissors” vibration, of al symmetry, and the CH2 in-plane rocking vibration, of b2 symmetry, may lie between 1400 and 1500 cm-’. Furthermore, the mean of the vibration frequencies typical of C-N and C=N bonds lies near 1450 cm-’, suggesting that, if the unpaired electron is localized in a CN-bonding orbital, the totally symmetric CN-stretching vibration may also lie in this spectral region. The two CH2-deformation fundamentals would be greatly shifted on deuterium substitution in the molecule, whereas the CN-stretching and NO2 antisymmetric stretching absorptions should shift only slightly. Two prominent, structured absorptions appear between 1400 and 1500 cm-l in the spectrum of CHDN02, but only one for CD2N02. While the two stretching absorptions of CD2N02may be superposed, since they are of different symmetry, several alternate explanations may account for the appearance of two prominent absorptions for CHDN02. These include Fermi resonance interaction and the occurrence of two different stereoisomeric configurations for the HF complex, possible only for this isotopomer. The CHD “scissors”vibration may appear in the 1300-cm-’ spectral region, where it would interact strongly with the N O , symmetric stretching mode. The spectrum of CHDN02includes strong absorptions at 1283, 1302, and 1313cm-’, consistent with the occurrence of such an interaction. The CD2“scissors”mode of CD2N02may have been obscured by the 1115-cm-’ absorption of NF. The out-of-plane vibration of CH, has been a s ~ i g n e d ~ ~ , ~ at 606 cm-’. While substitution of the heavy NO2 group would tend to lower the frequency of the H2CN out-ofplane mode, any partial double bond character in the CN bond would tend to raise it. Both the H2CN out-of-plane deformation and the NO2 “scissors” vibration probably lie between 500 and 750 cm-’. The hydrogen-bonding of HF to CH2N02would add six more vibrational fundamental absorptions to the infrared spectrum. The stretching of the HF unit typically occurs above about 3000 cm-’. The 3520-cm-’ peak, which shifts to 2596 cm-’ when DF is substituted in the complex, has been assigned to this vibration. The shift of this absorption from that of isolated HF provides a measure of the (33) D. E. Milligan and M. E. Jacox, J. Chem. Phys., 47,5146 (1967). (34) C. Yamada, E. Hirota, and K. Kawaguchi, J. Chen. Phys., 75, 5256 (1981).
3134
The Journal of phvsical Chemistty, Vol. 87, No. IS, 1983
strength of the complex. Gas-phase studies of H20.-HF by Thomas35 resulted in the assignment of the HFstretching vibration at 3608 cm-' and of two H-deformation fundamentals near 700 cm-'. The corresponding studies of the HF complexes with HCN and with CH3CN36gave HF-stretching fundamental frequencies of 3710 and 3627 cm-', respectively. Argon-matrix studies in this laboratory have yielded values of 3764,3854,3600, and 3380 cm-' for the HF-stretching frequencies in its complexes with CH3,I9 H2CF,20HzCOH,21and H2CH=0,22 respectively. The relatively large shift in the HF-stretching absorption for CH2N02-HF from that of isolated HF (3966 cm-') suggests that the complex is rather tightly bound. The F-H-X hydrogen deformation vibration, which is degenerate for a sufficiently symmetric species X, frequently appears in the 500-700-cm-' spectral region, and a comparatively high frequency can be correlated with a relatively strong hydrogen bond. For CH20H, one such peak lies near 570 cm-1,21and for the strongly bound HF complex with CH2CH=0 such a vibration was assigned22 at 740 cm-'. The symmetry of the CH2N02-HF complex should be sufficiently low for removal of the degeneracy of this H-deformation vibration. The assignment of the 639- and 57&cm-' peaks to the two H-deformation modes of the complex would be reasonable and would be consistent with the formation of a relatively strong complex. D-deformation vibrations should lie at frequencies about 0.75 of the values for the HF complex. The frequency ratio of the 488-cm-' peak of the CD3N02experiments to the 639-cm-' peak of the CH3N02experiments is 0.76 and of the 432- to the 578-cm-' peak is 0.75, in good agreement with this assignment. However, the 578-cm-' peak shifts to 586 cm-I for the HF complexes with CHDN02 and CD2N02. Such a shift could be explained by a weak coupling of the H-deformation vibration with a CHz vibration of similar frequency in the CH2N02complex. The expected appearance of the CH2N out-of-plane vibration in this spectral region has already been noted. The large frequency shift in this out-of-plane deformation mode on deuterium substitution would remove such a coupling. As noted by Thomas,35the remaining three HF vibrations should lie in the far-infrared. These vibrations may participate in combination bands in the observed spectra. The high-frequency satellites to the 3520- and 2596-cm-' HF- and DF-stretching absorptions may exemplify such combination bands. The infrared spectra give relatively little information regarding the structure of the HF complex, which may involve the unpaired electron of the free radical, the 7relectrons in the N=Q bonds, or the lone-pair electrons on the 0 atoms. There are several pairs of absorptions of similar contour and intensity, including those at 9981005, 1302-1313,1452-1455, and 1472-1476 cm-l, in the spectrum of the CHDN02 complex. These pairs could be explained if the HF were complexed to one of the N=O bonds or 0 atoms and if there were a slight barrier to rotation about the CN bond, permitting stabilization of two rotamers:
F-ti
F-
h
While the observed products of the photolysis of CHzNOz, H2C0and NO, are consistent with this identification, (35)R. K. Thomas, Proc. R. SOC.London, Ser. A, 344, 579 (1975). (36)R. K.Thomas, R o c . R. SOC.London, Ser. A, 325, 133 (1971).
Jacox
TABLE VIII: Comparison of Type A Absorptionsa (cm-') in C D , N O , t F Reaction Study with "hose of CD,NO, Isolated in Solid Argon
~-
Type A 429 w-m 532 w-m 632 m 880 m
CD,NO, 542 m 626 s 880 s 892 m-s 1042 m, br
1038 m 1073 w-m 1321 w 1385 vs 1365 m 1511 m 1517 s 1561 ws 1530 vs a w, weak; m, medium; s, strong; vs, very strong; ws, extremely strong; br, broad.
the present experiments do not definitively establish the photodecomposition mechanism. An alternate mechanism to the direct photolysis would be CH2NOZ
+ hv
+
CH2
+ NO2
followed by CH2 + NO2 -* HzCO
+ NO
The typically great reactivity of CHz may favor the occurrence of the CH, + NOz recombination reaction in the condensed phase. Type A Absorptions. The spectral data do not suffice for a detailed determination of the nature of the species which contributes the type A absorptions. This species possesses strong absorptions shifted to lower frequencies from the two prominent NO2 stretching fundamentals of CH3N02and of its deuterated counterparts by approximately 25 cm-', suggesting that it may be contributed by a complex in which the F atom is weakly bound to CH,NO2. This hypothesis is supported by the comparison given in Table VI11 between the absorptions observed below 2000 cm-I for CDBN02isolated in solid argon and the type A absorptions observed in the F + CD3N02experiments. Five of these absorptions correspond quite closely. Most of the remaining type A absorptions are weak, except for the two near 1500 cm-', which may be attributed to site effects in the argon lattice. Studies of F-atom addition reactions in this laboratory have shown that the F atom attacks the multiple bond of unsaturated hydrocarbon^,^'-^^ resulting in stabilization of a free radical in which the F atom is bonded to one of the carbon atoms. The relatively large shift in the NO2 stretching vibrations, compared to that of the lower frequency vibrations, suggests that this complex is more likely to involve interaction of the F atom with an N=O bond or with one of the 0 atoms than interaction at an H atom to provide a metastable F-H-CH2N02 species. The formation of the type A intermediate is consistent with the observation by Setser and co-workersls of a relatively small rate constant for the formation of HF and of partial relaxation of the HF vibrational excitation in the gas-phase studies of the F + CHsN02 reaction. The low yield of isolated HF and CHzN02 in the matrix experiments suggests that the type A complex persists for a time of the order of magnitude of that of the presence of the reactants in the gas phase, s. (37)M.E. Jacox, Chem. Phys., 53, 307 (1980). (38)M.E.Jacox, Chem. Phys., 58, 289 (1981). (39)M.E.Jacox, J.Phys. Chem., 86, 670 (1982).
J. Phys. Chem. 1983, 87, 3135-3138
The high isotopic selectivity for HF formation in these experiments can be explained by the occurrence of tunneling. Other examples of extreme isotopic selectivity attributed to tunneling phenomena in low-temperature hydrogen-abstraction reactions have recently been reviewed by Le Roy, Murai, and Williams.4o One possible intermediate in the formation of HF in the F + CH3NOz reaction might involve a five-center transition state having a conformation of the type A complex in which the F atom forms a “bridge” between an H atom on the methyl group and the 0 atom to which the F atom is loosely bound. The H-atom “jump” from the CH bond to the stronger HF bond might then occur by a tunneling mechanism.
Conclusions The initial reaction of F atoms with nitromethane involves complexing of the F atom either to the lone-pair electrons on one of the oxygen atoms or to ?r-electronsin an N=O bond. While F-atom addition to CH3NOZ-d,is followed by decomposition of the complex to form HF and (40) R. J. Le Roy, H. Murai, and F. Williams, J.Am. Chem. SOC.,102, 2325 (1980).
3135
the corresponding nitromethyl radical when n I2, a number of prominent type A absorptions can be attributed to the addition complex with CD3N02. The high degree of isotopic selectivity for the decomposition of the complex to form HF and CHZNO2-d,can be explained by a tunneling mechanism. The threshold for the photodecomposition of this complex into nitromethyl radical and HF or DF lies between 490 and 420 nm. Prominent type B and C absorptions can be assigned to the nitromethyl free radical, which in these experiments is predominantly formed in the solid and, therefore, is hydrogen bonded to HF. The infrared spectrum suggests that this hydrogen bond is relatively strong. The threshold for the photodecomposition of nitromethyl to produce HzCO and NO either directly or by the cage recombination of CH2 and NOz lies between 300 and 280 nm. Acknowledgment. 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,NR 659-804. Registry No. CH3N02,75-52-5; CHZNOz,16787-85-2;HF, 766439-3;Hz, 1333-740;F, 14762-948;NF3,7783-542;CHZDNOB, 23171-70-2; CHD2NOz,86013-71-0;CD3N02, 13031-32-8.
One-Electron Transfer Reactions of the Couple NAD/NADH Jan Grodkowskl,’ P. Neta,*2 Brlan W. C a r l s ~ n and , ~ Larry Millers Radiation Laboratory‘ and hpattment of Chemistry, University of Notre Dame, Notre Dame, Indiana 46556; National Bureau of Standards, Washington, D.C. 20234; and Depattment of Chemistry, UniverstYy of Minnesota, Minneapolis, Minnesota 55455 (Received: January 3, 1983)
One-electrontransfer reactions involving the couple NAD./NADH were studied by pulse radiolysis in aqueous solutions. One-electron oxidation of NADH by various phenoxyl radicals and phenothiazine cation radicals was found to take place with rate constants in the range of 105-108 M-’ s-l, depending on the redox potential of the oxidizing species. In all cases, NAD. is formed quantitatively with no indication for the existence of the protonated form (NADH+.). The spectrum of NAD., as well as the rates of oxidation of NADH by phenoxyl and by (chlorpromazine)’. were independent of pH between pH 4.5 and 13.5. Reaction of deuterated NADH indicated only a small kinetic isotope effect. All these findings point to an electron transfer mechanism. On the other hand, attempts to observe the reverse electron transfer, Le., one-electron reduction of NAD. to NADH by radicals such as semiquinones, showed that k was less than 104-106 M-’ s-*, so that it was unobservable. Consequently, it was not possible to achieve equilibrium conditions which would have permitted the direct measurement of the redox potential for NAD./NADH. One-electronreduction of NAD. appears to be an unlikely process.
Introduction The biological importance of the NAD+/NADH couple has stimulated numerous chemical studies on the reversible reaction NAD+ + H+ + 2e F! NADH (1) in the absence of enzymes. The possibility of observation of each of the two one-electron transfer steps independently has attracted special attention.&” NAD+ + e F! NAD. (2) NAD. + e + H+ e NADH (3) (1) University of Notre Dame. On leave of absence from the Institute of Nuclear Research, Warsaw, Poland. (2) University of Notre Dame and National Bureau of Standards. (3) University of Minnesota. (4) The Radiation Laboratory is supported by the Office of Basic This is Document No. Energy Sciences of the Department of Energy. -. NDRL-2411. (5) Land, E. J.; Swallow, A. J. Biochim.Biophys. Acta 1968,162,327. 0022-3654/83/2087-3 135W 1.50/0
k
R
NAD‘
A
NAD.
NADH
R = adenosine diphosphoribosyl
The energetics of the first one-electron reduction step has been recently studied by pulse radiolysi~.~J~ The redox (6) Willson. R. L. Chem. Commun. 1970. 1005. (7) Land, E. J.; Swallow, A. J. Biochim.hiophys. Acta 1971,234,34. (8) Neta, P. Radiat. Res. 1972, 52, 471. (9) Farrington, J. A.; Land, E. J.; Swallow, A. J. Biochim.Biophys. Acta 1980,590, 273. (IO) Anderson, R. F. Biochim. Biophys. Acta 1980,590, 277. (11) Bresnahan, W. T.; Elving, P. J. J.Am. Chem. SOC.1981,103,2379,
and references therein.
0 1983 American Chemical Society
