4378
NOTES
not resolved completely. The appearance of the ion a t 2.2 eV may be due to the reaction PH3
+ e+PH-+
Hz
+ e -+
PH-
+ 2H
+ e +PH- + H + + H + e
(8)
leading to EA(PH) 2 0.5 eV. P - ions. Three resonance maxima are observed. For the lowest peak appearing a t 5.8 eV, the reaction PH3
+ e+P-
+ Hz + H
(10)
requires AP(P-) 2 4.8 eV. The second peak, starting at about 8.4 eV, cannot be due to the process
PI%
+ e +P- + 3H
(11)
which would acquire AP(P-) 1 9.3 eV. The intensity of this peak is linearly related to the gas pressure in the ion source. Thus it is not a secondary ion-molecule reaction product. Possibly this P- peak is due to process 10, but involving kinetic and internal excitation of the fragments. The reaction proposed for the ion pair formation is PH3
+ e -+ P- + H+ + 2H + e
(12)
Thus, AP(P-) 2 22.9 eV, in good agreement with the observed value, 22.6 eV. The steep increase in ion currents in the ion-pair region for PHZ-, PH- and P- is preceeded by a 3-4 eV wide section of shallow shoulders and broad maxima. We propose that the processes in this region may be due to a series of successive excitations of the newly formed hydrogen atom to different atomic levels, converging in the limit to ionization to H + (reactions 5, 8, and 12.) These processes are therefore analogous to the Lyman series in the atomic spectrum of hydrogen. In the atomic spectrum the first line (Lyman-a) appears a t 10.15 eV above the ground level, and the ionization limit occurs at 13.53 eV. Thus we expect that the onset for the process forming the excited H atoms should be in each case a t 3.4 eV below the onset of the ion-pair processes. As seen in the figure, the onset of such shoulders can be discerned at about 3 eV below the steep onset for ion-pair production. The Journal of Physical Chemistry
by S. H. Daniel and Yi-Noo Tang Department of Chemistry, Texas A & M Uniuersity, College Station, Texas 77843 (Received January 13,1969)
(7)
which results in EA (PH) 2 0.4 eV. The intense peak of PH- at 8.2 eV is more difficult to explain. Possibly it is due to formation of an excited, long-lived sec) PH- ion. I n the ion pair formation region, the reaction proposed is
PH3
with Methysilanes and Alkanes
(6)
Hence, EA (PH) 2 0.1 eV. The stronger peak at 6.3 eV fits the reaction PH3
Competitive Reactions of Recoil Tritium Atoms
Although the reactions of recoil tritium atoms with carbon-skeletal molecules have been extensively studied, the corresponding work for the reactions with siliconskeletal molecules is barely initiated.1-3 I n 1967, Cetini and coworkers studied the recoil tritium reactions with monosilane and concluded that (a) monosilane is more reactive than methane probably because of the weaker Si-H bonds, and (b) kinetic theory for hot atom reactions as proposed by Wofgang, et al., can be extended to silane ~ y s t e m s . ~ Earlier studies on methylsilanes which were either published or submitted for publication during the process of this research included the results of Witkin and Wolfgang's work on tetramethyl-, trimethyl-, and monomethylsilanes, and the work on the first two molecules by Tominaga, Hosaka, and R ~ w l a n d . ~I n, ~ the present work, we would like to report recoil tritium reactions with tetra-, tri-, and the previously unstudied dimethylsilanes in one-component systems, as well as in two-component mixtures with their corresponding member of the alkane series. With these competitive studies, we can directly and quantitatively evaluate the relative reactivities of recoil tritium reactions with the silicon-skeletal and carbon-skeletal compounds.
Experimental Section Recoil tritium atoms were produced by employing the nuclear transmutation He3(n,p)H3 for the gas phase reactions and Li6(n,a)H3for the liquid phase studies. Gas samples containing He3, methylsilane, and other additives were prepared with a high vacuum system. For scavenged samples, 5-10 cm of oxygen was added. For liquid samples, the components were condensed into capillary tubes containing a small amount of LiF. Iz or DPPH were used as liquid phase scavengers. These samples mere irradiated at the Texas A & h!I University Nuclear Science Center Research Reactor with a neutron flux of 1 X 10l2 neutrons/(cm2 sec) for 30 min. The analysis of tritium-labeled products after irra(1) R. Wolfgang, Progr. Reaction Kinetics, 3, 97 (1965). (2) R. Wolfgang, Ann. Rev. Phys. Chem., 16, 15 (1965). (3) F. Schmidt-Bleek and F. 5. Rowland, Angew. Chem., Intern. Ed., 3,769 (1964). (4) G. Getini, 0 . Gambino, M. Castiglioni, and P. Volpe, J . Chsm. Phys., 46, 89 (1967). (5) J. Witkirl and R. Wolfgang, S.Phys. Chem., 72, 2631 (1968). (6) T . Tominaga, A. Hosaka, and F. S. Rowland, ibid., 73, 465 (1969).
NOTES
4379
Table I : Relative Yields of Products from Recoil Tritium Reactions with Methylsilanes" Reacting Compound
%
HT
CHsT
T-for-H substitution
T-for-CHa substitution
Reference
(CH3)zSiHe (CH3)aSiH
10 10 1
(CH3)rSi
10 5
168 rt 3 157 f 2 227 97 -Ir 2 99 167
13 f 1 13 -f 1 18 15 =t1 14 20
100 100 100 100 100 100
9.4 f 0.3 8.8 rt 0 . 4 7.7 6.7 f 0 . 3 5 6.4
This work This work Ref 5 This work Ref 6 Ref 5
021
mole
1 a
Relative to T-for-H substitution reaction as 100.
Table I1 : Calculation of Specific Activity Ratio per Bond for Methylsilanes and Alkanes from Recoil Tritium Competitive Reactions Obsd activity(first aliquot) Methylsilanet Alkanet
7 -
Methylsilanes, om
Alkanes, cm
(CHW 37.7 34.6 (CH8)aSiH 36.0 37.0 (CH8)zSiHz 34.8 35.2
(CHa)aC 36.9 36.7 (CHahCH 39.3 43.6 (CHs)zCHz 35.4 35.1
alkane
Sp act ratio (whole sample)
278 ,000 183,800
192,800 166,700
1.17 1.17
1 . 1 7 i 0.02 1.17 f 0 . 0 2
33,750 89,100
26,100 88,350
1.29 1.29
1.30 =k 0.02 1.22 f 0.04
159,550 148,200
104,600 96,800
1.55 1.53
1.55 Z!C 0.01 1.55 f 0.02
diation was performed by radio-gas chromatography.' The majority of the separations was performed with a 50-ft dimethyl sulfolane (DMS) column at 25" which separates well all the methylsilanes. A 50-ft safrole column was also used. However, for the competition experiments, an additional 30-ft DMS column was connected in series with the 50-ft one in order to get a clean separation between the silanes and the hydrocarbons. A 50-ft propylene carbonate column (coated on alumina) a t 0" was used to separate dimethylsilane from propane. It also separates H T and CH3T. The methylsilanes were obtained from Peninsular Chemical Co. .while the hydrocarbons were obtained from the Matheson Co. with an impurity level less than 0.5%. He3 with a tritium content of less than 2 X l0-l1% was obtained from n'lonsanto Research Corp.
Results and Discussion The primary reactions of recoil tritium atoms with methylsilanes are the abstraction of and the substitution for hydrogens, and the reaction a t the Si-C bond to give either CHIT or a formation of the corresponding Si-T bond. These reactions are illustrated below with dimethylsilane as an example. (a) H-abstraction HT CH3SiHZCH2. (la) (CH3)zSiHz4- T* + HT (CH3)2SiH. (lb)
+ +
Sp act ratio (first aliquot) '; methylsilane/
Av sp act ratio per bond
(CH,)aSi/ (CHB)IC 1.17 i 0.02 (CHshSi I/ (CH3)3CH 1.26 i 0.06 (CHahSiH2 / (CHs)zCHn 1.55 f 0.02
(b) T-for-H substitution (CH&SiH2
+ T*+
CHsTSiH2CH3 (CH3)zSiHT
+H
+H
(e) reactions at Si-C bond
+ T* CH3T + CH3SiH2. (CH&SiH2 + T* -+ CH3SiH2T + CH,. (CH3)zSiHz
-3
From the one-component systems of the methylsilanes, (CH3).SiH4-,, every expected primary product has been experimentally detected as an appreciable yield. These include HT, CHaT, (CH,)..-BiHc,T and (CH3).SiH4-,-t as illustrated above in reactions 1 to 4 in the case of (CH3)2SiH2. The experimental results are as shown in Table I, together with the previously published data from ref 5 and 6 for comparison. For the tetramethylsilane system, the agreement between our data and those from ref 6 is excellent. When allowance is made for different oxygen concentrations, there is basically no disagreement between cur data on trimethyl- and tetramethylsilane with those of ref 5. However, data in Table I are relative to the Tfor -H substitution reaction in methylsilanes as an (7) J. K. Lee, E. K. C. Lee, B. Musgrave, Y.-N. Tang, J. W. Root, and F. S . Rowland, Anal. Chem., 34, 741 (1962).
Volume 75, Number 1% December 1969
NOTES
4380 ~~~~~~~
~
Table 111: Comparison of Normalized Product Yields of hIethylsilanes and Corresponding Alkanes from Recoil Tritium Reactions"
Molecule
HT
T-for-H substitution
(CH3)zCHz (CH3)ZSiHz
185 260
100 155
10.9 20.2
4.5 14.6
300 450
(CH3)sCH (CH3)&H
192 198
100 126
14.1 16.4
3.0 11.1
309 352
(CH3)rC (CHa)dSi
139 114
100 117
21.0 17.5
2.5 7.8
263 256
' Values for alkanes come from ref
Total
CHIT
T-for-CHs eubstitution
yield
1.
arbitrary standard. Their values cannot be directly compared with those of the hydrocarbon systems unless the two systems can be normalized. The normalization factor for each methylsilane-alkane pair can be obtained by carrying out a two-component competition reaction between the pair and calculate their specific reactivity ratio. For such competiton runs, 1: 1 mixtures of the pairs (CH3)2SiH2-C3Hs,(CH3)3SiH-i-C4H10,and (CH3)&i(CH3)4C were studied. Table I1 shows the experimental data of these competitions together with the calculation which gives the specific activity ratio for T-for-H substitution in silane over that in the corresponding hydrocarbon for the above three pairs. Their values are, respectively, 1.55 h 0.02, 1.26 f 0.06, and 1.17 f 0.02 with the tetramethyl compounds showing the least difference in T-for-H substitution reactivity. A liquid competition of the (CH3)4Si-(CH3)4Csystem shows a specific activity ratio of 1.21 i 0.07 which is similar to the 1.17 gas phase value. This fact coupled with the low yield of CHzTI detected in the 12-scavenged systems indicates that either the decomposition of the primary products is not serious or these two compounds decompose t o a similar degree, or b0th.~,9 Therefore the gas phase relative yields can be used to represent the ratio of the primary hot yields, at least as a first approximation. Consequently, by assuming that the moderating abilities of the molecules in each pair are similar, the observed data for the individual methylsilanes were normalized to those of alkanes by multiplying them by tjhe above specific activity ratios. The results of each pair, as shown in Table 111,can then be directly compared. The major conclusion to these competition experiments is the quantitative comparison of the reactivities of alkylsilanes and their corresponding hydrocarbons. The values in Table I11 clearly show that the total reactivities of (CH,),Si and (CH3)qC are approximately the same, quantitatively confirming the estimation of Tominaga, et al. However, for the other two pairs, The Journal of Physical Chemistry
our data show that (CH3)3SiH and (CIQ2SiH2 are, respectively, about 20 and 50% more reactive than their hydrocarbon counterparts, qualitatively in agreement with what Witkin and Wolfgang suggested. But, yields from compounds containing Si-H bonds may remain slightly sensitive to the scavenger concentration even a t the level of 10% 02.6 As for the comparison of the individual yields, the specific reactivity ratios show that the average T-for-H substitution in methylsilanes is about 20-50% more reactive than the corresponding reaction in hydrocarbons. The H T yields indicate that hydrogen abstraction a t the C-H bonds in tetramethylsilane is only 0.82 times as likely as those in neopentane. The close relationship between H T yields from recoil tritiurn reactions and bond dissociation energies has been well established by a series of studies in CzD4moderating systerns.'On1' The value 0.82 indicates that the C-H bond dissociation energy in tetramethylsilane is larger than 99 kcal/mol and is probably around 101 kcal/ mol.1° For the other two pairs, the data indicate that H abstraction is about the same for both isobutane and trimethylsilane, but, on the average, it is easier to abstract from dimethylsilane than from propane. Data in Table I11 reveal that the total reactivities at the Si-C bonds are always higher than those at the corresponding C-C bonds. Moreover, if we use R-CH3 to represent both the methylsilanes and the alkanes, the ratios of CH3T/RT from methylsilanes are always much smaller than those from their corresponding hydrocarbons although the R's from the silanes always have the larger mass. For example, the CH3T/RT ratio from tetramethylsilane is 2.2 while that from neopentane is 8.4. This observation might be attributed to the incoming T being closer to the normal bond angle of the Si-T than it is to the C-T bond, and to the slower bending vibration of the Si-T bond as proposed by Witkin and Wolfgang.5 However, other effects might also be operating such as the following: (a) the larger size of Si in comparison with that of C and therefore a higher chance t o collide with the incoming T ; (b) the availability and direct use of d orbitals of Si; and (c) the electropositive nature of Si. But, it is not possible from the present data to determine what the correct explanations are. Acknowledgments. The authors wish to thank Dr. F. S. Rowland of the University of California, Irvine, E. K. C. Lee and F. 9. Rowland, J . Amer. Chem. SOC.,8 5 , 897 (1963). (9) Y.-N. Tang, E. K. C. Lee, and F. S. Rowland, ibid., 86, 1280 (1964). (10) J. W. Root, W. Breckenridge, and F. 9. Rowiand, J . Chem. Phys., 43, 3694 (1965). (11) E. Tachikawa and F. S.Rowland, J . Amer. Chem. SOC.,9 0 , 4767 (1968). (8)
4381
NOTES and Dr. D. H. O'Brien of Texas A & M University for their helpful discussions. This research was SUPported by the Research Council and the Chemistry University. I It is also Depart,ment of Texas A & & partially supported by Research Corporation. ACknowledgment is also made to the donors of the Petroleum Fund (PRF-1447-G), administered by the American Chemical Society, for partial support of this research.
Photoconductivity of Electron Acceptors.
11. 2,7-Dibromofluoren-Aga-malononitrile by Tapan K. Ahkherjee Energetics Branch, Air Force Cambridge Research Laboratories, Bedford, Massachu.retts 01730 (Received February 20, 1969)
I n our previous work' on the photoconductivity in nitro derivatives of fluoren-Aga-malononitrile it was noted that, in the case of the 2,7-dinitro derivative (DDF; X = NOz), the maximum photocurrent was obtained by excitation with energy less than that required to raise the photoconductor to the first singlet
state. The exact reason for this difference was not clearly understood. Replacement of the NOz-group by a heavy atom would enhance the probability of direct singlet-triplet taansition. If the S -+ T transition is involved in any way in the generation of photocarriers, then one would expect the photocurrent maximum to be shifted to a wavelength considerably longer than the first singlet-singlet absorption band. To test this possibility, 2,7-dibromo-A9"-malononitrile (DBF, X = Br) was synthesized, and its light absorption and photoconductivity characteristics were studied.z DBF, synthesized from 2,7-dibromofl~orenone~ by the previously described procedure, was purified by repeated crystallization from dimethylformamide, ethyl acetate, and chloroform4 (red crystals, mp 346-348'). Spectroscopic and photoconductivity measurements were performed as before.'
Results and Diricussion I n methylene dichloride, DBF shows a low intensity broad absorption between 550-425 mp (maximum a t 480 mp; log E , 2.49). Due to the limited solubility of DBF, the solvent dependence of the absorption spectrum could not be studied. Hence, the assignment of the lorn energy itransition (480 mp) as a -A--A* state was done by indirect means. A comparison of the absorption spectra of DBF and
Table I : Absorption Spectra in Methylene Dichloride m p (log E )
Compound
380 (2.42), 327 310 (3.28) 420 (2.72), 312 310 (3.79) 435 (2.51), 365 350 (3.53),295 480 (2.49), 360 348 (4.29), 317
Fluorenone 2,7-Dibromofluorenone
Fluoren-Aga-malononitri~e (X = H) DBF (X = Br)
(3 .OO) (3.68) (3.45) (3.35) (4.25) (4.22)
fluoren-Aga-malononitrile(X = H) with those of 2,7-dibromofluorenone and fluorenone (Table I) shows that the long wavelength peaks are of similar intensities. From theoretical molecular orbital calculations5 and other experimental evidence,6 the weak absorption near 380 mp in fluorenone has been assigned to a -A--A* state, rather than to a n-a* state as previously thought. By analogy, the band a t 420 mp in 2,7-dibromofluorenone is assigned the -A--A* level. It is also known that the well characterized n--A* bands in aldehydes and ketones disappear when carbonyl oxygen (=O) atom is replaced
( )(:.
by dicyanomethylene group' =C
The positions
and the intensities of the first low-energy bands that appear in the products have the properties of -A--A* transitions.8 Table I also shows that in DBF, the low energy band is red-shifted compared to the unsubstituted fluoren-Aga-malononitrile. Similar red shift was observed in fluorenone with the electron donating substituent in the "two" position, and was considered a good test of -A--T* t r a n ~ i t i o n . ~ The absorption spectrum of the vacuum evaporated film (Figure 1) of DBF shows some vibrational structure in the long wavelength region. All peaks are somewhat red-shifted compared to those of the solution spectrum. Since the excited singlet levels in the crystals are often found to be lower than the singlet levels in solution by a few tenths of an electron volt,lo the difference between the solid and solution spectrum is not unexpected, (1) T. K. Mukherjee, J . Phys. Chem., 70,3848 (1966). (2) The monobromo derivative, as well as the 2,7-diamino and 2,7-dihydroxy derivatives were found to be nonphotoconductive. 2,7-Dibromofluorenone did not yield any photocurrent. (3) Ch. Courtot, Ann. Chim., 14,99 (1930). (4) Anal. Calcd. for ClsH~BrzNz:C, 49.77; H, 1.56; Br, 41.40; N, 7.25. Found: C, 49.70; H, 1.80; Br, 41.15; N, 7.28. The upper limit of purity was ascertained from a single spot on TLC and maximum molecular extinction coefficient. Repeated melting of a sample showed some decomposition, hence zone refining was not attempted. (5) H. Kuroda and T. L. Kunii, Theor. Chim. Acta, 9 , 51 (1967). (6) K. Yoshihara and D. R. Kearns, J . Chem. Phys., 45, 1991 (1966). (7) E. Campaigne, R. Subramaya, and D. R. Maulding, J . Org. Chem., 28, 623 (1963). There are a large number of examples scattered in the literature. (8) This test of n-a* transition may be used as a supplement to the oximation test described by Yasihara and Kearns in ref 6. (9) A. Kuboyama, Bull. Chem. SOC.Jap. 37, 1540 (1964). (10) F. Gutman and L. E. Lyons, "Organic Semiconductors," John Wiley and Sons, Inc., New York, N. Y., 1967, Table I, p 653.
Volume 78, Number 12 December 1969
