12 The Difluoramino Radical FREDERIC A. JOHNSON
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Redstone Arsenal Research Division, Rohm&Haas Co., Huntsville, Ala.
Results
of studies of the tetrafluorohydrazine
difluoramino radical equilibrium
(N F 2
2
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NF ) 2
are reviewed; good agreement of the four diverse methods indicates ΔH = 20.0 ± 1.0 kcal. A detailed analysis of the NF infrared spectrum has established the F-N-F angle as about 104°. Various other properties of the difluoramino radi cal which have been studied include the mass spectrum and ionization potential, the broad un resolved EPR spectrum, and the ultraviolet spec trum. Reactions leading to the inorganic prod ucts HNF , CINF , and ONNF are summarized, with such discussion of the equilibria with C l and NO as the limited data permit. 298
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During the past few years the dissociation of tetrafluorohydrazine (N F ) into difluoramino free radicals (NF ) has been studied intensively. Reactions of N F have also been reported. A review of the published material on these subjects would appear to be in order. The dissociation of N F into N F has been the subject of a number of recent papers (5, 9,11, 19). Four independent methods of investigation were used; the summarized results are shown in Table I. 2
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Table I. Method
Dissociation of N F 2
AH, Kcal.
4
AS, E.U.
K
P)
25°C.
Ultraviolet spectra (77)
21.8 ± 2
45 ± 5
8.8 Χ 10"
Q^*/(77)
19.9
=b 0.5
40 ±
1.2 X 10"* (extrap.)
E P R (70)
19.3
± 1
Mass spectra (0)
21.9 + 1.5 20.0
2
7
—
—
—
—
zb 1
The agreement of the enthalpy changes found by such diverse methods is gratifying and surely indicates that the true value is within the indicated range. The K values are also in good agreement, considering the extremes of pressure and temperature under which they were determined. p
123 COLBURN; FREE RADICALS in Inorganic Chemistry Advances in Chemistry; American Chemical Society: Washington, DC, 1962.
124
ADVANCES IN CHEMISTRY SERIES
Thefiguresgiven indicate that the radical is present at room temperature and 1-atm. pressure to the extent of only 0.05%. The radical concentration reaches 90% only at 300° C. and 1 atm., at 150° C. and 1 mm., or at 25° C. and 10~ atm. It is not practical to obtain a large enough concentration of N F for the usual methods of characterization. Modern methods of spectral investigation, however, are applicable and fruitful here. Mass spectra of N F (5, 9) show clearly that the main species present under ionizing chamber conditions—i.e., 1 0 mm. and 175° C — is the radical (NF ). The radical molecular weight of 52 is, of course, obtained directly in mass spectral work; the parent ion of N F is observed only in molecular beams or with an unheated ionization section. The existence of this equih'brium ( N F +± 2NF ) caused some initial misinterpretation of electron impact data, but good agree ment now exists among the studies of Loughran and Mader (15), Herron and Dibeler (9), and Kennedy, Colburn, and Johnson (5, 11, 13). An ionization po tential of 11.8 e.v. is found for N F (11, 15). The average bond strengths of the two NF bonds in N F is 70 kcal. from the heat of formation (I) of N F and the Ν—Ν bond dissociation energy (11). In lieu of further experimental results, D ( F - N F ) and D ( N - F ) are now assumed (13) to be equal to the 70 kcal. average. The electron paramagnetic resonance (EPR) spectra (19) indicate a single unpaired electron, as expected, in NF . The absorption band is broad and feature less, although fine structure might be expected from interaction with the N nuclei, as well as with molecular rotational levels. The cause of the lack of struc ture is found in the relatively high pressure and temperature of this study (40 mm. and above 80° C ) . There would appear to be little hope of more detailed EPR data on this molecule except possibly in matrix studies, since good concentration necessitates either high pressure or high temperature, both of which are incom patible with maximum resolution. The fact that no signal is observed under con ditions where sufficient numbers of radicals are present for studies of other species is due to the extreme broadness of the line (104 gauss), whereas many radicals have line widths of the order of 10 gauss. For N H the coupling of 24 gauss to H is about twice that to N (3). In other systemsfluorinecouplings are gen erally larger than those to hydrogen. The 104-gauss line width is reasonable, therefore, as is the large g value of 2.010 arising from the same strong interaction. Investigation of the infrared spectra of N F by Harmony, Meyers, Schoen, Lide, and Mann (8) and Johnson and Colburn (10) gives some details of the structure of the radical. Fundamental bands are found at 930 and 1074 c m . ; these are two of the three infrared active bands expected for the molecule. One of the reviewers kindly furnished the information that R. T. Myers has observed the third fundamental band recently at 575 c m . - From the 4-cm. spacing of rotational levels in the 1074-cm. band (symmetric stretch) Harmony et al. (8) derive a bond angle of 104.2° using an assumed bond length of 1.37 A. (as found in N F ) . By analogy with OF , which has been extensively analyzed by Bernstein and Powling (2), a third fundamental at 510 c m . was estimated. The ultraviolet spectrum (11) of N F consists of a band at 260 m/x with a width of 20 τημ at half height. The poorly resolved vibrational spacing is approxi mately 370 c m . , probably corresponding to the "scissors" frequency of the excited vibrational state. The lack of resolution is due in part to significant ab sorption by the first excited vibrational state of the ground electronic state—this participation of excited vibrational states is due to the low scissors frequency of NF , which requires some 9% of the molecules to be in vibrationally excited states 10
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COLBURN; FREE RADICALS in Inorganic Chemistry Advances in Chemistry; American Chemical Society: Washington, DC, 1962.
JOHNSON
125
The Difluoramino Radical
at 25° C. From the band area an oscillator strength of 0.003 is calculated (16), indicating a forbidden transition. The generally symmetric shape of the band— i.e., with no band origin—is also indicative of a forbidden transition. The band is due without doubt to the nitrogen nonbonding pair or to the odd electron, rather than the N—F bonding electrons orfluorinenonbonding electrons. The band may be due to a η#-σ* transition such as appears from 200 to 250 m/A in amines, but the presence of the unpaired electron probably makes this assignment an over simplification. The reported reactions of N F which yield inorganic products are few—three in fact. Of these, two are known to be equilibria: the reactions with NO and Cl . The third reaction to give H N F will be discussed last. 2
Downloaded by UNIV OF CALIFORNIA SAN DIEGO on December 21, 2016 | http://pubs.acs.org Publication Date: January 1, 1962 | doi: 10.1021/ba-1962-0036.ch012
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Nitrosodifluoramine Nitric oxide and tetrafluorohydrazine react to give nitrosodifluoramine (4). This blue-violet compound is responsible for the color of condensed N F as handled by the usual vacuum techniques in glass systems. Its formation in the condensed state is favored by a high concentration of N F radicals in the gas phase prior to condensation. If N F is condensed from low pressures and tem peratures near 25° C , as little as 0.1% of NO gives a deep color to the solid N F . However, if the same sample is chilled prior to condensation by passage through a cold coil or by the addition of helium which prevents rapid condensation, the N F radicals recombine and only N F and NO are deposited. Nitrosodifluoramine is stable indefinitely at —160° C , but appears to decom pose above —140° C. The mode of reaction appears to involve vaporization, then decomposition in the gas phase with subsequent recondensation of N F . Relatively pure samples can be prepared by condensing at —196° C. a 10 to 1 mixture of NO and N F which has been passed through a hot capillary. The combination of expansion and residual cold NO gas ensures that the gas mixture cools so rapidly that the weak NO—NF bond persists when formed. The excess NO can then be pumped off at —183° C , leaving a nearly black solid. On warming, the N F NO decomposes to a 2 to 1 mixture of NO and N F . The F NMR spectrum of the black liquid at —140° C. shows two absorp tion peaks. The peak at —5736 cycles downfield from TFA is due to NF NO; the second peak is due to N F (—5485 cycles from TFA). The initially greater NF NO peak decreases with time at —80° C , while the N F peak increases. The visible absorption spectrum of NF NO has been determined in liquid N F and liquid N F at - 1 6 0 ° and - 1 9 6 ° C , respectively. The absorbance at the 570-πΐμ maximum was found to be a linear function of nitric oxide concentra tion when the condensation conditions were such that the N F concentration was many times the NO concentration. An absorptivity of NF NO of 110 liters per mole-cm. at 570 πΐμ was found. An oscillator strength of 0.001 is calculated for this band. The position of the absorption and the oscillator strength are typical of η -π* nitroso group transitions, as shown by the following comparison. 2
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Ν
Solvent Ether Vapor Ethyl alcohol N F , - 1 6 0 ° G.
tert-BuNO GF —NO (GH ) N—NO F N—NO 3
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A. 6650 6925 3610 5700
e
20 24 125 (110)
The equilibrium 2 NO + N F
-> 2 H N F + 2
