2267
PULSE RADIOLYSIS OF LIQUID AMIDES
Pulse Radiolysis of Liquid Amides by N. Hayashi,' E. Hayon,2*T. Ibata,l N. N. Lichtin,l and A. Matsumotol Chemistry Department, Boston University, Boston, Massachusetts, and Pioneering Research Laboratory, U . S . A r m y il'atick Laboratories, Xatick, Massachusetts 01760 (Received December 28, 1970) Publication costs assisted by the 77. S.A r m y Natick Laboratories
The pulse radiolysis of the following liquid amides has been investigated: formamide (F),N-methylformamide (NAIF), N,N-dimethylformamide (DATF), N,N-dimethylacetamide (DATA), and N-methylpropionamide (NlIP), Radiation-induced ionization and excitation processes have been observed on irradiation of solutions of anthracene (A) and naphthalene (X), as indicated by the characteristic spectra of A-, N-,AT, and NT. The yields of these species have been determined, based on known extinction coefficients. Transient absorptions with, , ,A -650 and 625 nm in DYF and DhIA, respectively, have been assigned to the absorption spectra of solvated electrons in these liquid amides. Absorption bands at, , ,A -350 nm due to the radicals HCOn"(CHP), HCON(CH,) (CH3), and CH3COS(CHz)(CH3) have been observed and characterized. The nature of some of the other transient absorptions produced is discussed. It is concluded that the absorption with, , ,A at 540 nm which is produced in F is, contrary to a previous assignment, not due to solvated electrons.
Liquid amides are of great interest t o the physical and physical organic chemist. They are polar liquids with high dielectric constants a t room temperature, e.g., formamide E = 109.5, N-methylformamide E = 182.4, dimethylformamide E = 36.7, dimethylacetamide E = 37.8, and good solvents for a number of organic and inorganic solutes. For the radiation chemist, these solvents are of particular concern with respect to solvation of the electron produced in radiation-induced ionization processes. Such a solvation is expected t o affect the escape probability of electrons and reduce the extent of geminate recombination of isolated ion pairs. I n addition, amides are potentially of interest to the understanding of the radiation chemistry of polypeptides, proteins, and related molecules. Relatively little attention has been given to the radiation or photochemistry of simple amides. Esr investigation of the radicals produced by electron irradiation of solid amides both in polycrystalline form and as single crystals has been carried Some of the products produced in the radiolysis of N,N-dimethylf~rmamide,~ and of acetamide in the solid and liquid s t a t e ~have , ~ been reported. Recently, a rather detailed study of the radiolysis of formamide has been done.B The results obtained from the pulse radiolysis of liquid formamide, N-methylformamide, and N,Ndimethylformamide have recently been reported briefly.' The sites of attack of OH radicals in aqueous solution on formamides, acetamides, propionamides, isobutyramides, and pivalamides have been identified, and the transient absorption spectra of the resulting intermediates have been determined.8 This paper reports a pulse radiolysis investigation of neat liquid formamide (F), N-methylformamide (NlIF), N,N-dimethylformamide (DhIF), N,N-dimethylacetamide (DMA), and N-methylpropionamide
(NRIP). Some of the intermediates produced have been identified, and the results are discussed.
Experimental Section Most of the work was carried out using the Febetron 705 (Field Emission Corp.) pulsed radiation source. This machine produces single pulses of electrons of 2.3RIeV energy and -3O-nsec duration. The experimental conditions have been described el~ewhere.~ Initial experiments'O were performed using a pulsed Van de Graaf accelerator a t Brookhaven National Laboratory. A number of preliminary experiments, as well as the results presented in Figure 5 , were obtained using the Satick linear accelerator. Electrons of 7-9MeV energy were used and single pulses of 1.2-1.6 psec employed. Details are given elsewhere.l 1 (1) Department of Chemistry, Boston University, Boston, Mass. (2) U. S. Army Katick Laboratories, Natick, Mass. (3) E. J. Burrell, Jr., J . A m e r . Chem. SOC., 83, 571 (1961); M. T. Rogers, S. Bolte, and P. S. Rao, ibid., 87, 1875 (1965), and references cited therein: P. J. Hamrick, Jr., H . W. Shields, and S. H. Parkey, ibid., 90, 5371 (1968). (4) N. Colebourne, E. Collinson, and F. S. Dainton, Trans. Faraday Soc., 59, 886 (1963). (5) K. N. Rao and A. 0. Allen, J . P h y s . Chem., 72, 2181 (1968). (6) A. Matsumoto, N. Hayashi, and N. N. Lichtin, Radiat. Res., 41, 299 (1970). (7) N. S. Fel', P. I. Dolin, and V. I. Zolotarevskii, Khim. V y s . Energ., 1, 154 (1967); K. S. Fel', P. I. D o h , and V. A. Sharpatyi, ibid., 2 , 189 (1968). (8) E. Hayon, T. Ibata, N. N. Lichtin, and M. Simic, J . A m e r . Chem. Soc., 9 2 , 3898 (1970); E. Hayon, T. Ibata, N. N. Lichtin, and AM.Simic, {bid., in press. (9) M. Simic, P . Neta, and E. Hayon, J . P h y s . Chem., 73, 3794 (1969); E. Hayon, J . Chem. Phys., 51, 4881 (1969). (10) A. hIatsumoto and N. N. Lichtin, Advan. Chem. Ser., 82, 547 (1968). (11) R. AI, Danziger, E. Hayon, and M . E. Langmuir, J . P h y s . Chem., 72, 3842 (1968); E. D. Black and E. Hayon, $bid., 74, 3199 (1970). The Journal of Physical Chemistry, Vol. 76, No. 16. 1.972
N. HAYASHI, E. HAYON,T. IBATA, N. IY.LICHTIN,AND A. MATSUMOTO
2268
purity. In early experiments, an absorption band Tyith -410 nm was observed (in addition to the 540-nm band) but could not be reproduced in later experiments, The spectra shown in Figure 1 (top) are the results presently considered to be more reliable. In the region below 420 nm, the overall absorption is made up of the overlap of two (or more) transients. One of the tran-320 nm could be due t o the CONH, sients with, , ,A radical, its formation has been suggesteds in the pulse radiolysis of aqueous solutions of formamide by the reaction A,,
0.4
0.3
0.2
0.1
d
OH
o
0: 0.4
0.3
0.2
0. I
0 A, nm.
Figure 1. Transient absorption spectra produced in the pulse radiolysis (dose ~ 4 krads/pulse) 1 of formamide: top, in presence of argon (1 atm), OD read a t 0.2 (O),1.3 (a), 2.3 (A), and 28.0 (8)psec after the 30-nsec electron pulse; bottom, in presence of O2 (1 atm), (a), and 0.1 M "821 (O), OD read a t 0.2 psec after the pulse.
Dosimetry was done using KCKS in aqueous solution as mentioned p r e v i o ~ s l y . ~The quartz optical cell used had an optical path of 2 cm, and fresh solutions were used for each electron pulse. Formamide and N-methylformamide were purified as described in ref 6. Spectrograde D M F and DMA gave similar results compared to the liquids used after multiple distillations at low pressure. XIIP was obtained from Eastman, distilled, bp 66" at 0.2 Torr. The extinction coefficient of anthracene radical anion of Gill, et aZ.,12 e720 = 0.99 X lo4 M-' cm-l, was used. The value for triplet excited naphthalene was that given by Land,la €413 = 2.26 X lo4M-l cm-l.
Results The purification of formamide presented some difficulties, and results varied from one purified batch to another. I n all cases, the transient optical absorption -540 nm, was observed on pulse radiband, with A,, olysis of air-free formamide, see Figure 1. However, the transient absorption below -430 nm was found not to be reproducible. Taking the conductivity of formamide as an indication of the purity of the solvent does not appear to be the only, or main, criterion of The Journal of Physical Chemistry, Vol. 76, N o . 16,2071
+ HC09Hz
--f
CONHz
+ HzO
The transient absorption with A,, 4 4 0 nm decays relatively quickly with r = -6 psec. Decay in this and other regions of the spectrum displayed in Figure 1 (top) follows neither first- nor second-order kinetics, Neither the initial transient spectrum produced by pulse radiolysis of neat formamide nor its rate of decay is altered by saturating with 1 atm of N 2 0 (-5 X 10-2 AP114). As shown in Figure 1 (bottom) however, pulsing of the sohtions formed by saturation with 1 atm of O2 or dissolution of 0.1 M NHdC1 gives a transient spectrum in which the 540-nm band is not detectable and in which other qualitative changes are also apparent. Results identical with those obtained with 0.1 M NH&l are obtained with 0.1 M (NH&S04. Varying the concentration of (NH&S04 over the range down to ! f showed that the effect of this reagent is to 5 X accelerate the decay of the 540-nm peak. The initial OD of this peak is not affected. The pulse radiolysis of air-free N-methylformamide gives rise t o two absorption bands, with maxima a t -360 and -570 nm, Figure 2 . I n the presence of 1 atm of X20, a slight overall decrease in transient absorption is observed, Figure 2, which could be due to trace amounts of O2 in the K20 gas used. The 570-nm band was found to decay by a first-order process, Table I, with = 2.5 psec. The transient with A,, -360 nm follows a second-order decay, with 2k/e E 1.3 X lo6. This band and its decay are similar to those of a transient absorption produced by attack of OH radicals in the pulse radiolysis of aqueous solutions of NAIF and assigned*to the HCOKH(CH2) radical. Radiolysis of N,N-dimethylformamide produces transients with maxima a t -365 and 650 nm, Figure 3. I n the presence of K20 (1 atm) or 1.4 X M SHdC1, the 650-nm band disappears, while the rest of the spectrum remains unchanged, Figure 3. The 650-nm band decays by a first-order process with r = 3.3 psec. The transient with an absorption maximum a t -366 nm and with increasing absorption below 300 nm is closely similar to the spectrum assigned to the HCOK(CH2)(12) D. Gill, J. Jagur-Grodzinski, and M . Szwaro, Trans. Faraday Soc., 6 0 , 1424 (1964). (13) E. J. Land, Proc. R o y . SOC.,Ser. A , 305, 467 (1968).
(14) D. A . Head and D. C. Walker, Can. J . Chem., 48, 1657 (1970).
2269
PULSE RADIOLYSIS OF LIQUIDAMIDES Table I : Decay Kinetics of Transient Species Produced in the Pulse Radiolysis of Liquid Amides Amide
N-Methylformamide
HCONH(CH~)
...
360 570 570 C 365 €ICON (CH2)(CH3) 365 €ICON(CHs)(CH3) 650 esOlvCHICON(CHZ)(CH~) 350 625 enolve esolv-
4 . 1 x 105 5 . 2 X lofid
C
N , A'-Dimethylformamide N,A'-Dimethylacetamide iV-Methylpropionamide
... ... 3
x
1.3
x
109
... ...
...
x
,..
2 . 5 X lo6 ( 1 . 5 X lo6) 2 . 8 x 106
1.6
...
...
1 . 3 X lo6 ( 0 . 8 X lo6)
2 . 0 X lo3
106
...
1 . 0 x 103
1 . 3 X l W ( 1 . 4 X lo6)
9 . 3 x 105 1 . 3 X lo6
...
103
...
4.0
x 109 ... ...
2 . 6 X log
...
I . .
...
a Values given in parentheses were those obtained8 for the same radical in the pulse radiolysis of the corresponding amides in aqueous I n the presence of 1 atm of NzO solution. * Extinction coefficients used are those derived in aqueous solutions, ref 8. c See text. (-5 x 10-2 M ) . e Due to very broad absorptions in the 450-700-nm region, the absorption maximum cannot be given.
0.6
-
-
02
0.D. 0,4
-
-
-
0
OD
0.2
"
1
-
300
400
500
600
700
X, n m Figure 2. Transient absorption spectra produced in the pulse radiolysis (dose -19 krads/pulse) of N-methylformamide: in presence of argon (1 atm), OD read a t 0.2 (0)and 8.0 ( 0 ) psec after the 30-nsec pulse; and in presence of K 2 0 (1 atm), OD read a t 0.2 psec after the pulse ( 0 ) .
(CH,) radical in the pulse radiolysis of aqueous solution, by reaction with OH radicals (see Table I). The decay of the 365-nm absorption is second order with 2 k / e equal to 2.5 X lo6, similar to that observed in aqueous solution. The pulse radiolysis of N,N-dimethylacetamide gives rise to a transient absorption, Figure 4, somewhat similar to that observed for DNF. The presence of NzO (1 atm) eliminates the formation of the band with Amax -625 nm, while it has little effect on the rest of the spectrum. The transient at 625 nm follows a first-order decay with T = 10.7 Msec. The shoulder at X -350 nm decays by a second-order process, with a value of 2 k / e similar to that obtained8 for the transient produced by attack of OH in the pulse radiolysis of DRIF in aqueous solution and assigned to the radical CH3CON(CH3) (CH,) (see Table I). Pulse radiolysis of N-methylpropionamide produces a
1, n m
Figure 3. Transient absorption spectra produced in the pulse radiolysis (dose -34 krads/pulse) of dimethylformamide: top, in presence of Ar (1 atm) OD read a t 0.2 (0)and 3.3 ( 0 )psec after the 30-nsec pulse; bottom, in presence of XZO(1 atm) ( 0 ) and in presence of 1.4 X 10+ M NH&l (a), OD read at 0.2 psec after pulse.
rT------'
ao
02 04'1_1.,
0 100
I
1
I
I
T-l 1
bo I.dppzkI 400
500
600
700
h, nm
Figure 4. Transient absorption spectra produced in the pulse radioIysis (dose -34 krads/pulse) of dimethylacetamide in presence of 1 atm of Ar ( 0 ) and N20 (O), OD read a t 0.2 rsec after the pulse. Insert: transient absorption below -300 nm, in Ar. Pulse width -30 nsec. The Journal of Physical Chemistry, Vol. 75, N o . 15, 1971
N. HAYASHI, E. HAYON, T. IBATA, N. N. LICHTIN,AND A. MATSUMOTO
2270
Table I1 : Yield and Decay Rate of A- and NT Produced in the Pulse Radiolysis of Anthracene (A) and Kaphthalene (Ii)in Liquid Amides [AI, M
Amide
Formamide N-Methylformamide A7,N-Dimethylformamide N,N-Dimethylacetamide N-Methylpropionamide a
I .5 a 2.0 1.7 2.6
Decay A - , seo-1
1x 1x 1x 1x
... ... 6 . 5 X lo4 2 . 6 X 105 2 . 2 x 104
.
10-2 10-2 10-2 10-2 I
.
0.19 0.14 0.43 0.32
0.08 0.10 0.24 0.11
. .
...
Low GA- values probably due to limited solubility of anthracene in these amides and/or presence of impurities,
transient spectrum with A,, -340 nm and a weaker very broad absorption over the wavelength region 400700 nm. Addition of S z O (1 atm) or 0.1 Ill KH,Cl completely suppresses the absorption in the visible region of the spectrum and causes only a small decrease in the 340-nm region. Pulse radiolysis of KRIP in aqueous solutions produces a transient absorption by attack of OH radicals, which has maxima a t 242 and 350 nm and which has been assigned to the CH,CH2CONHCH, radical. Based on the extinction coefficients derived8 in aqueous solutions, the yields of the following radicals produced in the radiolysis of neat KMF, DNF, and DMA can be estimated: G[HCOKH(CH,)] 2.0, G[HCOK(eHJ(CH,)] 1.6, and G[CH,COS(eH,)(CH,)] 5 2.0. N
N
Discussion Absorption of high-energy radiation is known to produce ionization and excitation processes in liquids, e.g.
RCONR2
-w+
RCOKR2*
+ (RCONR2)+ + e-
(1)
The isolated ion pairs produced in reaction 1 can undergo geminate combination and give rise t o a further yield of excited molecules (RCOKR2)+
+ e-
4RCONRz*
(21
The extent of this combination is dependent on a number of factors, including the degree of solvation afforded by the solvent to the electron, the occurrence of proton transfer, ion-molecule reactions, the temperature, and the presence of solutes capable of reacting with either or both of the primary ions produced in the radiolysis. Recently, a marked dependence on the static dielectric constant of the liquid has been found15 for the yield of electrons which escape primary combination. The free radicals produced on radiolysis (the precursors of many of the observed products) are presumed to be formed from the dissociation of the excited solvent molecules. They could also, however, be produced from the dissociation of the parent ions and/or in secondary reactions with free radicals. The formation of electrons and excited state molecules in the radiolysis of the liquid amides studied can The Journal of Physical Chemistry, Vol. 7 6 , No. 16, 2971
be shown by irradiating amides containing anthracene (A), Figure 5. Here the characteristic transient spec-720 nm (with tra due to A- radical anions, with, , ,A shoulder and peak a t -G5O and 570 nm) were observed and correspond to the reported1, spectrum of A-. From the known12 extinction coefficient of A-, and assuming it to be independent of the solvent, the yield of A-, and therefore of e-, produced was derived (see Table 11) e-+A+A-
(3)
The values of G,- observed in formamide and N methylformamide are considered to be low due to the limited solubility of A in these solvents, the dependence of G,- on [A] and/or the possible presence of electronscavenging impurities ( e . g . , the corresponding acid, HCOOH, has a reactivity” with eaq- of -lo8 M-’ see-’, and the presence of 0.1-1.0% of this impurity would compete effectively with A). The yield of Gein D l I F and DRIA of -2.0 and -1.7, respectively, is considered to represent the yield of free ions produced in these liquids. These yields are considerably lower than the limiting yield, Ge- -4.0, expected for the total ionization yield in liquids. I n the presence of high concentrations of electron scavengers, this optimum yield can be obtained. The limiting yield of K2 produced from the radiolysis of formamide in the presence of up to -8 X 10-2 174 K20is reported14to be 3.3 f: 0.3 (see also ref G ) , possibly corresponding to the reaction
e-
+ K 2 0+N2 + 0-
(4)
It is suggested that the transient absorptions with maxima a t -650 and 4 3 2 5 nm obtained in the pulse radiolysis of neat DRIF and DlIil, respectively, Figures 3 and 4, are due t o the solvated electron in these liquids: on addition of NzO, 02,or NH4+ ions these transient absorptions cannot be observed. These species also decay by first-order processes (see Table I). These results are usually characteristic for the behavior (15) E. Hayon, J . Chew. Phys., 53, 2353 (1970). (16) P. Balk, G. J. Hoijtink, and J. W. H. Schreurs, R e d . Trav. Chim. P a y s - B a s , 76, 813 (1957); E. deBoer and S. I. Weissman, ibid., 76, 824 (1957). (17) M. Anbar and P. Neta, Int. J. A p p l . Radiat. Isotopes, 18, 493 (1967).
227 1
PULSE RADIOLYSIS OF LIQUIDAMIDES I
I
I
I
I
0.2
0.I
C
0.6
0,4
ci
d 2
0.2
0,
.e a c
C
0.2
C
I
I
I
I
I
400
500
600
700
800
h,
nm
Figure 5 . Transient absorption spectra produced in the pulse radiolysis of anthracene (A) in liquid amides: (a) 2 X 10-8 M A in formamide, OD read a t 4.0 ( 0 ) and 8.0 ( 0 )psec after a 1.5-psec pulse; (b) 1.8 X M A in dimethylformamide, OD read a t 3 psec after a 1.5 psec pulse; (c) 1 X loF2M A in dimethylacetamide, OD read a t 3 psec after a 1.5-psec pulse.
of the solvated electron in polar liquids. Based on this interpretation, and the yields of G,- given in Table 11, one can determine the extinction coefficient of the solvated electron in DLIF and DMA. Values of €650 -1000 J4-l cm-1 for D M F and €025 -1500 M-' cm-I for DMA were derived. I t is t o be noted that these values are considerably lower than those for the solvated electron in water or aliphatic alcohols, where extinction coefficients of -1-2 X lo4 M - I cm-l were obtained.18 The absorptions with A,, -540 and -570 nm in F and KMF, Figures 1 and 2, do not appear to be those of the solvated electron in these liquids, contrary to the suggestion made by Fel', et al.,' for formamide. This conclusion is based primarily on (1) the presence of these bands in solutions containing 5 X M NzO, snd ( 2 ) the almost quantitative formation of these bands in the presence of anthracene. The observation14 that typical electron scavengers, e.g., acids and silver ions, sharply reduce the yields of Nzproduced by irradiation of their cosolutions with S Z O in formamide,
supports the view that NzO either scavenges solvated electrons, eq 4, or some other highly reactive donor. Clearly, the transient absorption with A, at -540 nm cannot be such a donor. The nature of these transients formed in F and SAIF is discussed further below. The yield of free ions produced in the radiolysis of organic liquids appears to depend (among other factors) on the static dielectric c a s t a n t of the liquid. The observed G,- values for the liquid amides studied (Table 11) are in accord with this approximate correlation. These results are further discussed elsewhere.I5 The formation of excited solute molecules was observed in the pulse radiolysis of solutions of anthracene (A) or naphthalene in F, N-ZIF, DMF, and DMA. The characteristic transient absorption with A,, -410-420 nm for AT was found (see Figure 5). The yields of excited states were determined using IT and are given in Table 11. I t is interesting t o note that G N is~ significantly lower in the presence of the electron scavenger NzO, indicating a decrease of reaction 2. For further discussion on these yields see ref 15. I n the pulse radiolysis of DAIF and DAIA the transient species absorbing a t -365 and -350 nm, respectively, have spectra and decay rates which correspond closely to the radicals HCOK(CHz)(CH3) and CH,CON(CH,) (CH3), see Table I, observed recently.8 Similarly, the transient absorption with, , ,A -340 nm produced by pulse radiolysis of NlIP resembles that produced by attack of hydroxyl radical an aqueous NMP and ascribed8 to CH3CH2CONHCHz. I n the radiolysis of DMF, the following products were measured4: CO (G = 2.6), (CH&?rTH (G = 2.6 0.5), Hz (G = 0.14), and CH, (G = 0.93). Of these products, the (CH3)2x radical (the probable precursor of dimethylamine) due to the high electronegativity of the nitrogen atom should be electrophilic and, therefore, effectively abstract an H atom from DMF. N o information is available on the spectrum of CON(CHJ2, but the RkCOCH, radical (R = H or CH3) is expected8t o absorb below -280 nm. Intermediates produced in the pulse radiolysis of F and NMF, Figures 1 and 2, are more difficult to characterize. If the 540- and 570-nm bands are not due t o the solvated electron, then other possibilities such as one or more of the following species must be considered: HCO, .CONH2, .COSHCH3, HCONH, kH,, SHCH3, anions and cations of F and NAIF. I n aqueous solutions, the sCONH2 and H C O k H radicals have been shown* to absorb with, , ,A -320 nm and A,,
