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Electron spin resonance studies of the radiolysis of methyl isocyanate and methyl isothiocyanate
Radmt Phys Chem Vol 23, No 1-2, pp 127 135, 1984 Pnnted m Great Bntam
0146-5724/84 $ 3 0 0 + 00 Pergamon Press Ltd
ELECTRON SPIN RESONANCE STUDIES OF THE RADIOLYSIS OF METHYL ISOCYANATE A N D METHYL ISOTHIOCYANATEI" MARTYN C. R. SYMONS and PATRICK M. R. TROUSSON Department of Chemistry, The University, Leicester, LEI 7RH, England (Recewed 13 January 1983, accepted 7 March 1983)
Abstract--Exposure of ddute solutions of methyl lsocyanate m 2-methyl-tetrahydrofuran (MTHF) or methanol (CD3OD) to 6°Co v-rays at 77 K gave a triplet (A N ~ 7 G) assigned to the radical amon, (MeNt~O) -, together with weak satelhte features assigned to )3C (Ac ~ 110 G) These results suggest that the amon is bent at the cyanate carbon atom On photolysls the anions gave methyl radicals and cyanate ions, the former being directly detected m CD3OD, but gwmg solvent radicals in MTHF. When CD3OH or CD3OD/H20 mixtures were used an extra species exhibiting a total sphttmg of ca. 70 G was obtaaned. It ~s suggested that this specms ~s formed by protonatlon of the radical anion The me of protonation is discussed. Exposure of ddute solutions of methyl isocyanate in fluorotnchloromethane (freon) to 6°CO y-rays gave poorly defined features assigned to the radical cations, whose structure is discussed. These were not converted into H2(~NCO radicals by proton-loss However, when the pure compound was lrrad,ated, H2t~NCO radicals were a major product, being probably formed from the parent cauons The amons were trapped in the pure matenal, but methyl radicals were not detected on photolysls, presumably because they reacted to give H2t~NCO radmals. The latter were photolysed to give H2Clq + CO, the reaction being reversed on anneahng Weak outer features assigned to pair-trapped radicals were also detected m the lrradmted sohd In similar expenments vath methyl isothlocyanate (MeNCS), features for the parent amons (A s ~ 7 G, Ac ~ 100 G) were also obtained, and these also gave methyl radicals on photolysls with visible light. However, m this case, methyl radmals were present m low yield pnor to photolysls Ddute solutions m freon gave the radical cations, but concentrated solutions gave the dimer cations, (MeNCS)2 +, which are compared with the well known dimer anions (NCS)2- first discovered by Baxendale et al These were probably also present m the irradiated pure compound, together with H2~NCS radicals. These radmals were not photosensmve, m contrast with H2CNCO radicals.
INTRODUCTION ONE OF the many long-lasting discoveries of Baxendale et al (1) was that NCS. radicals readily add to thlocyanate anions to give the relatwely stable dimer, (NCS)2
(I)
Q NC--S
$--CN
(1)
NCS + N C S - --*(NCS)2-
These dimers are important examples of a class of radicals that we have labelled tr *°) since the semioccupied molecular orbital (SOMO) is p n m a n l y the sulphur-sulphur a * orbatal, as indicated m Insert I. Other examples of such radicals include the wellknown dihalogen amons ( V r centres), RS-" SR and R2S-" SR2 + Ions, (3-6) (Rhal-" halR)2 + cations, (7) and (R3P '--PR3)2 + cations. (s) We were later able to support Baxendale's structure by E S R spectroscopy on species clearly estabhshed to be (NCS)2 anions. (9) No such dimer anions were detected in the radiolysis of cyanates, in agreement with the general finding that oxygen-oxygen a* speoes are far less stable than their sulphur analogues. (~°'') tRadtatlon Mechamsms. Part 25
In the present study, whilst being generally concerned with the redox properties of M e N C O and MeNCS, and their general radiation chemistry, we were specifically interested in the possible formation of (MeNCS--" S C N M e ) ÷ radical cations, which m~ght be expected to be very similar to the ( N C S - ' S N C ) - anions. The effect of ionxzmg radiation on pure methyl lsocyanate was studied using E S R spectroscopy by Fujlwara et al., °2) who focused attention on the H2(~NCO radical and its photolysis. Trofimov and Chkheldze were primarily concerned with lowtemperature reactions of gas-phase hydrogen atoms with various orgamc ~socyanates and isothlocyanates. 03) Chung and Wilhams studied 127
M C R SYMONSand P M. R TROUSSON
128
frozen as small beads In hquld mtrogen after degassing They were irradiated at 77 K in a Vlckrad source with doses up to c a 1 Mrad ESR spectra were obtamed at 77 K with a Vanan El09 spectrometer cahbrated with a Hewlett-Packard 5246L frequency counter and a Bruker B-H 12E field probe, which were standardized with a sample of DPPH Generally, samples were annealed m the covered insert Dewar after decantmg the coolant, and were recooled to 77 K whenever slgmficant changes were observed m the ESR spectra When specific temperatures were required a Vanan variable temperature assessory was employed For resolution enhancement and other spectral manipulations, spectra were stored using a Hewlett-Packard HP9835 micro-computer. Resolution enhancement was achieved by two procedures, the results bemg ldenhcal In the first, a dlfferentmtlon routine usmg the third and fifth denvatwe was used with numerical differentmtlon so that there was no loss of mtens~ty The second procedure revolved modlfymg the Fourier transform of the spectrum
?-lrradtated methyl and ethyl lsocyanate in the expectation that their reactions mtght resemble those o f alkyl cyanides 115J6) which add electrons to gwe radical a m o n s or d~mer anions, these b e m g readily p h o t o lysed to gwe alkyl radicals and cyanide ions. S m c e , they failed to observe methyl or ethyl radicals, they concluded that these processes do not occur for the ~socyanates. Our results suggest that, m fact, there are strong parallels between the two reactions. C h u n g and Wilhams also stu&ed the formation o f t-butyl radxcals m irradiated t-butyl lsothxocyanate (17) EXPERIMENTAL Methyl lsocyanate and lsothlocyanate (Aldrich Chemical Co ) and all solvents were the best available grades, and their purity was verified using NMR spectroscopy Solutions were in the region of 0 1-4) 5% mole fracUon and were
a)
b) b)
lOG
gl 2OG I
MI(13C)J + ~ / 2
p
~H
1
x
FIG 1 First denvatwe X-band ESR spectra for methyhsothlocyanate after exposure to 6°Co ~/-rays at 77 K, (a) In MTHF showing features assigned to (MeNCS)- radical-amons after subtracting features for MTHF radicals and using resolution enhancement to narrow the components due to ~4N hyperfine coupling, (b) in CDtOD at high gain, showing features assigned to the radical-anions containing ~3C, and (c) In CD~OD/+ 5~o H20, showmg features assigned to the protonated radlcal-amons
Resonance studies of the radiolysls of methyl lsocyanate and methyl isothlocyanate RESULTS
superimposed on the normal broad multlplet from M T H F radicals. On careful annealing or by using computer subtraction, these features could be isolated. Both appeared to be amsotropic triplets, the tnplet splitting being clearly resolved using standard resolution enhancement techniques, as shown in Fig. 1. Simdar central hnes were obtained using CD3OD solvent but they were more effectively obscured by features from D2t~OD, (~D3 and e~- centres, though
In order to prepare the parent radical-anions, ddute solutions in M T H F and in CD3OD were studied. It is now well established that solutes do not give electron-loss centres m these media because the solvent cations are stabihzed. However, electrons are mobile and readily react with solutes having s]gmficant electron affinities. Both solutes in M T H F gave an intense central hne
32506
,,SG ,
"H
A
I •CH~ (- 3/z )
V
"CH3 (- 1/2)
I
I
CH3(÷1/l)
CH 3(÷3/z)
i|.
CDzOD (a) 3230 (3 t5G
I I CH3( - 3/2)
i
I
'CH3(+1/21
CH3 (-1/21 ~,
129
ii.
CH3(÷3/z)
/
CO20D (b) FIG, 2. First denvatwe X-band ESR spectra for so|ut]ons m CD3OD after exposure to 6°Co 7-rays and subsequently to visible hght at 77 K(a) for CH3NCS showing the appearance of signals due to CH 3 radicals which were absent prior to photolys,s, and (b) for CH3NCO, showing the enhanced signals due to 'CH3 radscals as a result of photolysls.
130
M C R SYMONSand P M R TROUSSON
the last of these was greatly suppressed, and could be mlnlm~sed by using relatively high power levels In all cases, weak satellite features in the 100-100 G sphttlng region were detected which we assign to radical anions containing ~C (Fig lb) These were shown to decay concomitantly with the decay of the central triplets, but they were too poorly resolved to reveal the ~N splitting which would have made our identification completely unambiguous For solutions in CD~OD containing CD3OH or H20, extra satellite lines separated by ca 70 G were obtained (Fig lc) which were absent m pure CD~OD, and m the M T H F systems Solutions of M e N C S in CD~OD gave clear features for methyl radicals (1 3 3 1, A n = 2 2 5 G ) which were greatly enhanced on exposure to visible light (Fig 2a) These features were absent for M e N C O solutions, but intense methyl radical features grew in on photolysls (Fig 2b) The central triplet features and the 100--IlOG satelhte lines were lost during these photolyses Exposure of pure methyl lsocyanate to ),-rays gave a central line superimposed on a triplet of triplets clearly due to H2(~NCO radicals These became better resolved on annealing or photobleachlng to remove the central components (Fig 3a) A range of weak outer features were detected at high gain (Fig. 3b) including the 100 l l 0 G doublets assigned to the parent anions containing t3C and several lines separated by Ca 2 0 0 G On photolysls, clear features assigned to H2CIq radicals ~Tv~ grew in (Fig 3c) After loss of the central hnes, there was a concomitant loss of the H~(~NCO triplet features, and on anneahng to c a 150 K, the H2CI~ features were lost, with return of the H~CNCO lines The original features observed in the vicinity of the H~CN lines were rapidly lost on anneahng these cannot be due to H~CIq radicals They are labelled species c~ alpha In the following d~scusslon Irradiated methyl lsOthlocyanate gave similar lealures for H~CNCS radicals (F~g. 4) but, in th~s case, methyl radical features were also clearly present These were strongly enhanced on exposure to v~s~ble light, with concomitant loss of a central component. Satellite lines assigned to m~C were also lost. In contrast with H~(~NCO radicals, the H2(~NCS radicals were stable to visible hght, and we were unable to generate H~C]~ radicals therefrom. The centre regions of the spectra were never well defined and there was evidence for a broad, underlying asymmetric feature This had parameters within the range expected for the dimer cations, whose presence was also suggested by the intense red colourat~on which developed on ~rradlation To check this hypothesis we studied solutions in freon (CFCI0. Just as solutions in M T H F c>r C I ) . O D ~re exnec'tecl
32506
lOG
0
MI (I~'N)
P
.-H
I
• L • +I
0
-I
~L
0
l *1
MI(IH) L~ ]
1# + .L)O
-I
~ (#),4
(#) -I
_i
li!T2S°G2SG ....+ ~H
:l If
~+ H~CN(+11
,, + H2CN(-1 )
I1
32&OG 2S6 ~
~H
-~,.2 r
/
£
1
4"
MI(~N)
[
MI(~H)
m
ii
l
• I
+I
-I
FI~ 3 First derivative X-band ESR spectra for pure MeNCO after exposure to ~°Co 7-rays at 77 K, (a) after photolysJs and annealing to ~a 150 K, showing features assigned to H2CNCO radicals, (b) at high gain showing extra wlng lines possibly due to radical-pairs (c() and (c) after photolysls, showing feature assigned to H2CN radicals The Mt(IH) = 0 component also contains weak hnes due to
131
Resonance stu&es of the radiolysls of methyl ]socyanate and methyl lsothlocyanate
c-O
3230G
CHs
,1Or; ,
"H
(III)
(II)
to give electron adducts, so dilute solutions m freons are expected to give excluswely the electron-loss centres. It has recently been estabhshed that a wzde range of compounds with ionization potentials less than c a . 11.8 eV will selectively lose electrons in such solvents. (2°-25) This arises because these solvents are very efficzent at trapping electrons, but the freon caUons are mobile wa electron transfer, so that mzgration of charge occurs until substrate molecules react. It is our view that for very dilute solutions the only products that solutes will form are the parent radical cahons or ummolecular breakdown products thereof. We therefore conclude that poorly defined spectra obtained from M e N C O and M e N C S are due to the catzons ( M e N C O ) ÷ and (MeNCS) ÷ The most noticeable feature of these broad ill-defined spectra is
I
HI (1H,_CH~II
I
-3/2
(b)
1
3230G
t lOG I ., H
3230G
I ,lOG
,
~H
I
I_ I I I I 1 I 1_ 1 rl I IE r I •1
0
-1.1
(-11
0
(o)
-I *1
0
-1
(.11
? MI(1H' CH3)
Hi(1H HzCNCS)
I
)/2
+1
Fig 4(a)
0
(c) FIG 4 F~rst derivative X-band ESR spectra for pure CH3NCS after exposure to 6°Co 7-rays at 77 K, (a) showing features assigned to CH 3 (_3, _½, +½, +I) and H2(2NCS ( - 1 , 0, + 1) ra&cals [features fl are assigned to ra&calamons and the &mer cations, (CH3NCS)2 + ] and (b) after photolys,s, showing the growth m CH 3 ra&cals and loss of central components (the gain m (a) is c a 4 × that m (b)) and (c) after anneahng to c a 150 K, showing features assigned to H2~NCS ra&cals, wgh loss of those due to CH~ radicals
M C R SYMONSand P M R TROUSSON
132
3200G
i 256 I
I
~H
Jl
/
/
/
/
Z
the large shift for one c o m p o n e n t of" the g - t e n s o r This is expected for the d~storted (bent) cation structure for field along the long axis o f the molecule In fact, the parent molecules are only "quas~" hnear, t~6~ and this bending will be greatly magnified on electron-loss because o f the resulting splitting o f the ~z, and 7r~ orbltals. The expected distortion, shown in Inserts II and III, generates a " and a ' orb~tals, the S O M O (semi-occupied molecular orbital) being a " Because o f the p o o r defimtlon m these spectra, we prefer to state only that the expected large gl,-shlft is certainly observed We are currentl) trying to ~mprove the resolution in these spectra However, when m o r e concentrated solutions o f M e N C S were irradiated and annealed shghtly above 77 K a better defined spectrum was obtained (Fig 5) together with a clear red colouratlon This we interpret in terms o f the d]mer cation, ( M e N C S ) ~ , whlch Is probably also formed m the pure c o m p o u n d The derived parameters are collected in Table l, together with those o f others DISCUSSION
ili x
FIt. 5 First derivative X-band ESR spectrum for MeNCS (1°o) in CFCI~ showing extra features, absent for dilute solutions, assigned to the dlmer cations, (MeNCS)2 + The low-field absorption (y) is thought to be due to (MeNCS) cations
TABLF
]
ESR
Stru~ ture o/radical anions We think that the radicals responsible for the narrow, almost lsotrop~c central triplets obtained m M T H F and CD~OD are the parent radical anions This ~s based on expectation for these solvents, and the fact that small fatrly lsotrop~c hyperfine c o u p h n g to ~4N is expected for radicals bent at carbon The
PARAMETERS FOR V A R I O U S R A D I ( ALS FORMED IN |RRAD1AFEI) METHYL ISO('YANATI:, METHXtl ISOTHIO( YANATt ~ AND RELATED C O M P O U N D S
Source
Solvent
Radical
MeNCO
MTHF
(MelqCO)
MeNCO
CD~OD
(MeN(~'O)
MeNCS
MTHF
(MeN~?S)
Nucleus
14N I~C 14N J~( ~N
Hyperi]ne coupl]ng,,G+
6+ I 70 90 6+ I
J~("
MeN('S
CD~OD
(MeN~'S)
MeNCO
MeNCO
H:(~NCO
MeNCS
MeNCS
H2(~NCS
MeNCS (NCS)
CFCI~ KNCS{
(MeNCS) 2 ~ (NCS)~
"~G = i0 4T, {Not resolved § _+0 ++Ref 9
001
t4N ~C tH 14N ~H J4N ~4N 14N s~S
70,90 16 5 47 16 5
6 _+ 1 < 120 + 70-90 6_+ I < 120 { 76 90 16 5 47 16 5
6
6
~ 3 0+ 3 80
~ 3 0 _+ 3 80
~,,-values~
6± I
6± I
2 002
2 002
2 002
114 6_+ I
85 98 6~ I
2002
2002
2005
110 26 47 24
83-97 19 5 47 19
2 002
2 002
2 002
2 002
2 002
2 005
2 021 2 021
2 012 2 019
2 002 1 998
7
~3 32 56 5
6 3
~3 ~3 24 2
133
Resonance studies of the radlolysls of methyl lsocyanate and methyl isothlocyanate
the anisotroplc components (Table 1). However, Chung and Wilhams estimated that A,~o('H) was ca. 26 G. This IS equal to our parallel value, and suggests that they may possibly have had some preferential alignment. This value was recognised by these workers as being anomalous. "4) We were unable to detect any g-value variation, which must be less than ca. +_ 0 002 However, for the sulphur analogue, there IS clear g-asymmetry, which largely explains the curious form of this spectrum (Fig. 4c). We have interpreted the spectrum in terms of axial symmetry, but expect that a small degree of x-y-splitting would give an improved fit. The axial symmetry observed for the proton coupling in the - C H 2 unit is characteristic of such groups when they rotate freely or exhibit rapid proton interchange. In both cases, there is ca. 80~ spindensity on the H2(2 unit as estimated from the ~H hyperfine coupling, so that delocahsation onto the - N C O or - N C S unit cannot be more than ca. 20~ The gz-Shlft for the latter is ln&catlve of spin on sulphur. The photosensitivity of H2(2NCO to give H2Clq + CO is remarkable, as is the facile thermal return, which suggests that the fragments never separate. The detailed pathway for this photolysls probably involves a nx-,ny optical transition because, in the conversion, the unpaired electron moves from the nx into a 2py orbital on nitrogen as indicated in (1), where the unpaired electron is shown localised on carbon and nitrogen purely for ease of representatlon This will clearly facilitate loss of CO because no further electron shift from n ~ n ~ is required The fact that H2~NCS does not break down on photolysis probably reflects the greater stability of CO than CS molecules
weak satellite features with the 110 G splitting are of about the correct intensity for '3C satellites, and the large isotroplc sphtting is in accord with the bent structure. This can be converted in the usual way (27) into an approximate orbital population of ca. 9% 2s on carbon. The anlsotropic coupling is, unfortunately, poorly defined, but approximate values m the region of 43-50~ results from our most reasonable interpretation for both (MeNI20)- and (MeN(2S)-. Central features for these anions were very poorly defined m the pure materials, but the 100 G satelhte lines were still detected. It, therefore, seems unlikely that anion dimers of the type detected m irradiated cyanomethane "5'~6) are formed m either of these compounds Structure o f radical cations
Although we are satisfied that the parent radical cations are formed in dilute freon solutions, the ESR spectra are rather uninformatwe. In the limiting linear structures we should observe a very large positive gll-Value, whereas the experimental values are quite close to that of the free-spin (2.0023). This shows that there must be major distortions from hnearity for both ions, as expected. The marked change in spectrum for concentrated solutions of MeNCS strongly supports the postulate that dimer cations are indeed favoured. The g-values are similar to those that we obtained for Baxendale's (NCS)2 anions, whach shows that the bonding is similar for both ions. Also the red colouration indicatlve of a band at ca. 550nm, is in accord with expectation since (NCS)~- anions have their maxlmum at 475 nmJ ~) Our more detailed results for (NCS)~-, which Included the observatmn of 33S hyperfine coupling (Table 1) can best be interpreted m terms of structure I, the antlbonding a* orbital (SOMO) being essentially 3p on sulphur For (NCS)2- anions, the g-tensor was nearly axial, suggesting that 0 ~ 90 ° (the "open-book" structure) However, for (MeNCS)2+ the g-tensor is far from axial, the results being more in accord with expectation for the planar structure (0 = 180°).
Other centres
The features which fall close to those for H2Clq molecules pnor to photolysls (Fig. 3b) were originally thought to be '3C features for the radical-anions, but this cannot be correct since no such features were obtained for solutions in M T H F or CD3OD. We are inclined to the view that they are due to small concentrations of radical-pairs, but we are not clear which radicals are involved. We looked carefully for half-field transitions, but were unsuccessful
HzI~NCO and H212NCS radicals Our results for H2(2NCO radicals agree with those for Fujlwara et al. °2) except that we have estimated
x_
y
T (2)
~ ---~--s~,~,
N-c-o
..
c
c-o
,
c=
+
CO
M C R SYMONS and P M. R TROUSSON
134
The features separated by ca 70 G which develop only in the m e t h a n o h c systems c o n t a i n i n g O J H groups a n d are absent in pure C D 3 O D or C D 3 O D + D 2 0 systems are t h o u g h t to be due to p r o t o n a t e d a n i o n s (Fig lc). Even if the p r o t o n s (~H) are entirely responsible for the splitting, the features for the c o r r e s p o n d i n g (2H) derivatives, which should be separated by ca. 22 G would be lost b e n e a t h the intense signals due to C D 2 O D a n d CD3 radicals, so we are unable to verify this postulate. N o r is it clear which t a u t o m e r will be f a v o u r e d o n p r o t o n a t i o n . If, as seems p r o b a b l e , the p r o t o n coupling Is in the region o f 70 G, then the m o s t hkely structure is (IV).
H ~ / C =0 (IV)
(V)
H3C/C
This resembles the radical cation o f acetaldehyde (V) which we showed recently has a p r o t o n coupling o f ca 1 3 7 G t281 The large hyperfine coupling for ( C H a C H O ) + is expected in terms of the positive charge effect, ~2~ but IS nevertheless surprisingly large relative to t h a t for IV, if this is the correct structure The alternative site for p r o t o n a t i o n is nitrogen, to give either (VI) or (VII) T h e radical H 2 N C O shows
~N C/0
H/
- .~
H~ N C~_/0
(VI) ~ /
- ~_~ (VII)
strong c o u p h n g (ca 3 0 G ) to one p r o t o n a n d very weak coupling (ca 1 G ) to the other, the J4N coupling being in the 2 0 G region a n d fairly lsotroplc. °°'3° Structure (VII) which is expected to have the large p r o t o n coupling is therefore possible, since the total splitting is ca 7 0 G U n f o r t u n a t e l y , the central features were always so intense t h a t we were only able to resolve the o u t e r m o s t lines for this radical (Fig lc) a n d hence we are at present u n a b l e to distinguish between these alternatives Finally, we m e n t i o n that, in all cases, the E S R p a r a m e t e r s for methyl radicals were normal. It is often f o u n d that for alkyl radicals formed f r o m R - X by dissociative electron capture, there is a slight residual interaction with X which results in reduced ESR p a r a m e t e r s for R (32]3) This is n o t the case for
NCO or N C S , but it is n o t e w o r t h y t h a t some modification was observed for Me3C radicals in irradiated t-butyl lsothlocyanate. "7) Acknowledgement -We thank the Commlssmn of the European Communities for a grant to PMRT
REFERENCES 1 J H BAXENDALE, P L T BEVAN and D A SCOTT. Trans Faraday Soc 1968, 64, 2389 2 M C R SYMONS, Pure Appl Chem 1981, 53, 223 3 R L PETERSEN, D J NELSON and M C R SYMONS,J Chem. Soc Perkm H, 1977, 2005, 1978, 225 4 B C GILBERT, D K C HODGEMAN and R O C NORMAN, J Chem Soc, Perkm II, 1973, 1748 5 J H HADLEY and W GORDY, Proe Nat Acad Stl U S A 1977, 74, 216 6 K AKASAKA, S ! OHNISHI, T SUITA and J N1TTA, J Chem Phys 1964, 40, 3110 7 S P MISHRA and M C R SYMONS, J Chem Sot, Perkm If, 1975, 1492 8 A R LYONS and M C R SYMONS, J Chem Soc, Faraday Trans II 1972, 68, 1589 9 I S GINNSand M C R SYMONS,J Chem Soe. Dalton Tran~ 1973, 3 10. M C R SYMONSand S P MISHRA, J Chem Res (S), 1981, 214 l l M C R SYMONS and B W WREN, J Chem Sot. Chem Commun 1982, 817 12 M FUIIWARA, N TAMURA and H HIRAI, Bull Chem Soc Japan, 1973, 46, 701 13 V I TROFIMOVand I I CHKHEIDZE, Khtm~ya Vysoklkh Energn 1971, 5, 404 14 Y J CHUNG and F WILLIAMS,J Phys Chem 1971,75. 1893 15 R EGLAND and M C R SYMONS, J Chem Soc (.4) 970, 1326 16 K TAKEDAand F. WILLIAMS,J Phys Chem, 1970, 74. 4007, E D SPRAGUE, K TAKEDA and F WILLIAMS, Chem Phys Lett 1971, 10, 299 17 Y J CHUNG and F WILLIAMS,J Phy~ Chem 1972, 76, 1792 18 F S ADRIAN, E L COCHRAN and V A BOWERS. J ('hem Phys 1962, 36, 1938 19 J A BRIVATI,K D J ROOT, M C R SYMONsandD J A TINLING, J Chem Soc (A), 1969, 1942 20 T SHIDA and T KATO, J Am Chem Sot' 1979, 101, 6869 21 M. C R SYMONsand I G SMITH, J Chem Res (S), 1979, 382 22 K TORIYAMA, K NUNOME and M IWASAKI, J Phys Chem 1981, 85, 2149 23 J T WANG and F WILLIAMS,J Phys Chem 1980, 84, 3156 24 M C R SYMONS,A HASEGAWAand P MAJ, Chem Phys Lett 1982, 89, 254 25 T SHIDA, Y EGAWAand H KUBODERA,J Phy~ Chem. 1980, 73, 5963 26 B M GIMARCand D A WOOOCOCK,J Mol Structure 1981, 85, 37 27 M C R SYMONS,Chemmal and Btochemtcal Aspects o! Electron Spin Resoname Spettrowopy Van Nostrand Reinhold, Woklngham, 1978 28 M C R SYMONSand P J BOON. Chem Phys Lett 1982, 89. 516
Resonance studies of the radlolysls of methyl lsocyanate and methyl lsothlocyanate 29 M C. R SYMONSand L. HARRIS, J Chem Res. (S), 1982, 268, (M) 1982, 2746. 30 H. BOWER, J. McRAE and M. C. R. SYMONS, J. Chem. Soc (A), 1971, 2400 31 R LIVINGSTONand H ZELDES,J Chem. Phys. 1967, 47, 4173
135
32. C. M. L KERR and F WILLIAMS,J Amer Chem Soc 1971, 93, 2805. 33 S P. MtSHRA and M. C. R SYMONS, J Chem. Soc., Perkm Trans. II 1973, 391; M C. R. SYMONSand I. G. SMITH, J. Chem Soc, Perkm Trans II 1981, ll80
0146-5724/84 $ 3 0 0 + 00 Pergamon Press Ltd
ELECTRON SPIN RESONANCE STUDIES OF THE RADIOLYSIS OF METHYL ISOCYANATE A N D METHYL ISOTHIOCYANATEI" MARTYN C. R. SYMONS and PATRICK M. R. TROUSSON Department of Chemistry, The University, Leicester, LEI 7RH, England (Recewed 13 January 1983, accepted 7 March 1983)
Abstract--Exposure of ddute solutions of methyl lsocyanate m 2-methyl-tetrahydrofuran (MTHF) or methanol (CD3OD) to 6°Co v-rays at 77 K gave a triplet (A N ~ 7 G) assigned to the radical amon, (MeNt~O) -, together with weak satelhte features assigned to )3C (Ac ~ 110 G) These results suggest that the amon is bent at the cyanate carbon atom On photolysls the anions gave methyl radicals and cyanate ions, the former being directly detected m CD3OD, but gwmg solvent radicals in MTHF. When CD3OH or CD3OD/H20 mixtures were used an extra species exhibiting a total sphttmg of ca. 70 G was obtaaned. It ~s suggested that this specms ~s formed by protonatlon of the radical anion The me of protonation is discussed. Exposure of ddute solutions of methyl isocyanate in fluorotnchloromethane (freon) to 6°CO y-rays gave poorly defined features assigned to the radical cations, whose structure is discussed. These were not converted into H2(~NCO radicals by proton-loss However, when the pure compound was lrrad,ated, H2t~NCO radicals were a major product, being probably formed from the parent cauons The amons were trapped in the pure matenal, but methyl radicals were not detected on photolysls, presumably because they reacted to give H2t~NCO radmals. The latter were photolysed to give H2Clq + CO, the reaction being reversed on anneahng Weak outer features assigned to pair-trapped radicals were also detected m the lrradmted sohd In similar expenments vath methyl isothlocyanate (MeNCS), features for the parent amons (A s ~ 7 G, Ac ~ 100 G) were also obtained, and these also gave methyl radicals on photolysls with visible light. However, m this case, methyl radmals were present m low yield pnor to photolysls Ddute solutions m freon gave the radical cations, but concentrated solutions gave the dimer cations, (MeNCS)2 +, which are compared with the well known dimer anions (NCS)2- first discovered by Baxendale et al These were probably also present m the irradiated pure compound, together with H2~NCS radicals. These radmals were not photosensmve, m contrast with H2CNCO radicals.
INTRODUCTION ONE OF the many long-lasting discoveries of Baxendale et al (1) was that NCS. radicals readily add to thlocyanate anions to give the relatwely stable dimer, (NCS)2
(I)
Q NC--S
$--CN
(1)
NCS + N C S - --*(NCS)2-
These dimers are important examples of a class of radicals that we have labelled tr *°) since the semioccupied molecular orbital (SOMO) is p n m a n l y the sulphur-sulphur a * orbatal, as indicated m Insert I. Other examples of such radicals include the wellknown dihalogen amons ( V r centres), RS-" SR and R2S-" SR2 + Ions, (3-6) (Rhal-" halR)2 + cations, (7) and (R3P '--PR3)2 + cations. (s) We were later able to support Baxendale's structure by E S R spectroscopy on species clearly estabhshed to be (NCS)2 anions. (9) No such dimer anions were detected in the radiolysis of cyanates, in agreement with the general finding that oxygen-oxygen a* speoes are far less stable than their sulphur analogues. (~°'') tRadtatlon Mechamsms. Part 25
In the present study, whilst being generally concerned with the redox properties of M e N C O and MeNCS, and their general radiation chemistry, we were specifically interested in the possible formation of (MeNCS--" S C N M e ) ÷ radical cations, which m~ght be expected to be very similar to the ( N C S - ' S N C ) - anions. The effect of ionxzmg radiation on pure methyl lsocyanate was studied using E S R spectroscopy by Fujlwara et al., °2) who focused attention on the H2(~NCO radical and its photolysis. Trofimov and Chkheldze were primarily concerned with lowtemperature reactions of gas-phase hydrogen atoms with various orgamc ~socyanates and isothlocyanates. 03) Chung and Wilhams studied 127
M C R SYMONSand P M. R TROUSSON
128
frozen as small beads In hquld mtrogen after degassing They were irradiated at 77 K in a Vlckrad source with doses up to c a 1 Mrad ESR spectra were obtamed at 77 K with a Vanan El09 spectrometer cahbrated with a Hewlett-Packard 5246L frequency counter and a Bruker B-H 12E field probe, which were standardized with a sample of DPPH Generally, samples were annealed m the covered insert Dewar after decantmg the coolant, and were recooled to 77 K whenever slgmficant changes were observed m the ESR spectra When specific temperatures were required a Vanan variable temperature assessory was employed For resolution enhancement and other spectral manipulations, spectra were stored using a Hewlett-Packard HP9835 micro-computer. Resolution enhancement was achieved by two procedures, the results bemg ldenhcal In the first, a dlfferentmtlon routine usmg the third and fifth denvatwe was used with numerical differentmtlon so that there was no loss of mtens~ty The second procedure revolved modlfymg the Fourier transform of the spectrum
?-lrradtated methyl and ethyl lsocyanate in the expectation that their reactions mtght resemble those o f alkyl cyanides 115J6) which add electrons to gwe radical a m o n s or d~mer anions, these b e m g readily p h o t o lysed to gwe alkyl radicals and cyanide ions. S m c e , they failed to observe methyl or ethyl radicals, they concluded that these processes do not occur for the ~socyanates. Our results suggest that, m fact, there are strong parallels between the two reactions. C h u n g and Wilhams also stu&ed the formation o f t-butyl radxcals m irradiated t-butyl lsothxocyanate (17) EXPERIMENTAL Methyl lsocyanate and lsothlocyanate (Aldrich Chemical Co ) and all solvents were the best available grades, and their purity was verified using NMR spectroscopy Solutions were in the region of 0 1-4) 5% mole fracUon and were
a)
b) b)
lOG
gl 2OG I
MI(13C)J + ~ / 2
p
~H
1
x
FIG 1 First denvatwe X-band ESR spectra for methyhsothlocyanate after exposure to 6°Co ~/-rays at 77 K, (a) In MTHF showing features assigned to (MeNCS)- radical-amons after subtracting features for MTHF radicals and using resolution enhancement to narrow the components due to ~4N hyperfine coupling, (b) in CDtOD at high gain, showing features assigned to the radical-anions containing ~3C, and (c) In CD~OD/+ 5~o H20, showmg features assigned to the protonated radlcal-amons
Resonance studies of the radiolysls of methyl lsocyanate and methyl isothlocyanate RESULTS
superimposed on the normal broad multlplet from M T H F radicals. On careful annealing or by using computer subtraction, these features could be isolated. Both appeared to be amsotropic triplets, the tnplet splitting being clearly resolved using standard resolution enhancement techniques, as shown in Fig. 1. Simdar central hnes were obtained using CD3OD solvent but they were more effectively obscured by features from D2t~OD, (~D3 and e~- centres, though
In order to prepare the parent radical-anions, ddute solutions in M T H F and in CD3OD were studied. It is now well established that solutes do not give electron-loss centres m these media because the solvent cations are stabihzed. However, electrons are mobile and readily react with solutes having s]gmficant electron affinities. Both solutes in M T H F gave an intense central hne
32506
,,SG ,
"H
A
I •CH~ (- 3/z )
V
"CH3 (- 1/2)
I
I
CH3(÷1/l)
CH 3(÷3/z)
i|.
CDzOD (a) 3230 (3 t5G
I I CH3( - 3/2)
i
I
'CH3(+1/21
CH3 (-1/21 ~,
129
ii.
CH3(÷3/z)
/
CO20D (b) FIG, 2. First denvatwe X-band ESR spectra for so|ut]ons m CD3OD after exposure to 6°Co 7-rays and subsequently to visible hght at 77 K(a) for CH3NCS showing the appearance of signals due to CH 3 radicals which were absent prior to photolys,s, and (b) for CH3NCO, showing the enhanced signals due to 'CH3 radscals as a result of photolysls.
130
M C R SYMONSand P M R TROUSSON
the last of these was greatly suppressed, and could be mlnlm~sed by using relatively high power levels In all cases, weak satellite features in the 100-100 G sphttlng region were detected which we assign to radical anions containing ~C (Fig lb) These were shown to decay concomitantly with the decay of the central triplets, but they were too poorly resolved to reveal the ~N splitting which would have made our identification completely unambiguous For solutions in CD~OD containing CD3OH or H20, extra satellite lines separated by ca 70 G were obtained (Fig lc) which were absent m pure CD~OD, and m the M T H F systems Solutions of M e N C S in CD~OD gave clear features for methyl radicals (1 3 3 1, A n = 2 2 5 G ) which were greatly enhanced on exposure to visible light (Fig 2a) These features were absent for M e N C O solutions, but intense methyl radical features grew in on photolysls (Fig 2b) The central triplet features and the 100--IlOG satelhte lines were lost during these photolyses Exposure of pure methyl lsocyanate to ),-rays gave a central line superimposed on a triplet of triplets clearly due to H2(~NCO radicals These became better resolved on annealing or photobleachlng to remove the central components (Fig 3a) A range of weak outer features were detected at high gain (Fig. 3b) including the 100 l l 0 G doublets assigned to the parent anions containing t3C and several lines separated by Ca 2 0 0 G On photolysls, clear features assigned to H2CIq radicals ~Tv~ grew in (Fig 3c) After loss of the central hnes, there was a concomitant loss of the H~(~NCO triplet features, and on anneahng to c a 150 K, the H2CI~ features were lost, with return of the H~CNCO lines The original features observed in the vicinity of the H~CN lines were rapidly lost on anneahng these cannot be due to H~CIq radicals They are labelled species c~ alpha In the following d~scusslon Irradiated methyl lsOthlocyanate gave similar lealures for H~CNCS radicals (F~g. 4) but, in th~s case, methyl radical features were also clearly present These were strongly enhanced on exposure to v~s~ble light, with concomitant loss of a central component. Satellite lines assigned to m~C were also lost. In contrast with H~(~NCO radicals, the H2(~NCS radicals were stable to visible hght, and we were unable to generate H~C]~ radicals therefrom. The centre regions of the spectra were never well defined and there was evidence for a broad, underlying asymmetric feature This had parameters within the range expected for the dimer cations, whose presence was also suggested by the intense red colourat~on which developed on ~rradlation To check this hypothesis we studied solutions in freon (CFCI0. Just as solutions in M T H F c>r C I ) . O D ~re exnec'tecl
32506
lOG
0
MI (I~'N)
P
.-H
I
• L • +I
0
-I
~L
0
l *1
MI(IH) L~ ]
1# + .L)O
-I
~ (#),4
(#) -I
_i
li!T2S°G2SG ....+ ~H
:l If
~+ H~CN(+11
,, + H2CN(-1 )
I1
32&OG 2S6 ~
~H
-~,.2 r
/
£
1
4"
MI(~N)
[
MI(~H)
m
ii
l
• I
+I
-I
FI~ 3 First derivative X-band ESR spectra for pure MeNCO after exposure to ~°Co 7-rays at 77 K, (a) after photolysJs and annealing to ~a 150 K, showing features assigned to H2CNCO radicals, (b) at high gain showing extra wlng lines possibly due to radical-pairs (c() and (c) after photolysls, showing feature assigned to H2CN radicals The Mt(IH) = 0 component also contains weak hnes due to
131
Resonance stu&es of the radiolysls of methyl ]socyanate and methyl lsothlocyanate
c-O
3230G
CHs
,1Or; ,
"H
(III)
(II)
to give electron adducts, so dilute solutions m freons are expected to give excluswely the electron-loss centres. It has recently been estabhshed that a wzde range of compounds with ionization potentials less than c a . 11.8 eV will selectively lose electrons in such solvents. (2°-25) This arises because these solvents are very efficzent at trapping electrons, but the freon caUons are mobile wa electron transfer, so that mzgration of charge occurs until substrate molecules react. It is our view that for very dilute solutions the only products that solutes will form are the parent radical cahons or ummolecular breakdown products thereof. We therefore conclude that poorly defined spectra obtained from M e N C O and M e N C S are due to the catzons ( M e N C O ) ÷ and (MeNCS) ÷ The most noticeable feature of these broad ill-defined spectra is
I
HI (1H,_CH~II
I
-3/2
(b)
1
3230G
t lOG I ., H
3230G
I ,lOG
,
~H
I
I_ I I I I 1 I 1_ 1 rl I IE r I •1
0
-1.1
(-11
0
(o)
-I *1
0
-1
(.11
? MI(1H' CH3)
Hi(1H HzCNCS)
I
)/2
+1
Fig 4(a)
0
(c) FIG 4 F~rst derivative X-band ESR spectra for pure CH3NCS after exposure to 6°Co 7-rays at 77 K, (a) showing features assigned to CH 3 (_3, _½, +½, +I) and H2(2NCS ( - 1 , 0, + 1) ra&cals [features fl are assigned to ra&calamons and the &mer cations, (CH3NCS)2 + ] and (b) after photolys,s, showing the growth m CH 3 ra&cals and loss of central components (the gain m (a) is c a 4 × that m (b)) and (c) after anneahng to c a 150 K, showing features assigned to H2~NCS ra&cals, wgh loss of those due to CH~ radicals
M C R SYMONSand P M R TROUSSON
132
3200G
i 256 I
I
~H
Jl
/
/
/
/
Z
the large shift for one c o m p o n e n t of" the g - t e n s o r This is expected for the d~storted (bent) cation structure for field along the long axis o f the molecule In fact, the parent molecules are only "quas~" hnear, t~6~ and this bending will be greatly magnified on electron-loss because o f the resulting splitting o f the ~z, and 7r~ orbltals. The expected distortion, shown in Inserts II and III, generates a " and a ' orb~tals, the S O M O (semi-occupied molecular orbital) being a " Because o f the p o o r defimtlon m these spectra, we prefer to state only that the expected large gl,-shlft is certainly observed We are currentl) trying to ~mprove the resolution in these spectra However, when m o r e concentrated solutions o f M e N C S were irradiated and annealed shghtly above 77 K a better defined spectrum was obtained (Fig 5) together with a clear red colouratlon This we interpret in terms o f the d]mer cation, ( M e N C S ) ~ , whlch Is probably also formed m the pure c o m p o u n d The derived parameters are collected in Table l, together with those o f others DISCUSSION
ili x
FIt. 5 First derivative X-band ESR spectrum for MeNCS (1°o) in CFCI~ showing extra features, absent for dilute solutions, assigned to the dlmer cations, (MeNCS)2 + The low-field absorption (y) is thought to be due to (MeNCS) cations
TABLF
]
ESR
Stru~ ture o/radical anions We think that the radicals responsible for the narrow, almost lsotrop~c central triplets obtained m M T H F and CD~OD are the parent radical anions This ~s based on expectation for these solvents, and the fact that small fatrly lsotrop~c hyperfine c o u p h n g to ~4N is expected for radicals bent at carbon The
PARAMETERS FOR V A R I O U S R A D I ( ALS FORMED IN |RRAD1AFEI) METHYL ISO('YANATI:, METHXtl ISOTHIO( YANATt ~ AND RELATED C O M P O U N D S
Source
Solvent
Radical
MeNCO
MTHF
(MelqCO)
MeNCO
CD~OD
(MeN(~'O)
MeNCS
MTHF
(MeN~?S)
Nucleus
14N I~C 14N J~( ~N
Hyperi]ne coupl]ng,,G+
6+ I 70 90 6+ I
J~("
MeN('S
CD~OD
(MeN~'S)
MeNCO
MeNCO
H:(~NCO
MeNCS
MeNCS
H2(~NCS
MeNCS (NCS)
CFCI~ KNCS{
(MeNCS) 2 ~ (NCS)~
"~G = i0 4T, {Not resolved § _+0 ++Ref 9
001
t4N ~C tH 14N ~H J4N ~4N 14N s~S
70,90 16 5 47 16 5
6 _+ 1 < 120 + 70-90 6_+ I < 120 { 76 90 16 5 47 16 5
6
6
~ 3 0+ 3 80
~ 3 0 _+ 3 80
~,,-values~
6± I
6± I
2 002
2 002
2 002
114 6_+ I
85 98 6~ I
2002
2002
2005
110 26 47 24
83-97 19 5 47 19
2 002
2 002
2 002
2 002
2 002
2 005
2 021 2 021
2 012 2 019
2 002 1 998
7
~3 32 56 5
6 3
~3 ~3 24 2
133
Resonance studies of the radlolysls of methyl lsocyanate and methyl isothlocyanate
the anisotroplc components (Table 1). However, Chung and Wilhams estimated that A,~o('H) was ca. 26 G. This IS equal to our parallel value, and suggests that they may possibly have had some preferential alignment. This value was recognised by these workers as being anomalous. "4) We were unable to detect any g-value variation, which must be less than ca. +_ 0 002 However, for the sulphur analogue, there IS clear g-asymmetry, which largely explains the curious form of this spectrum (Fig. 4c). We have interpreted the spectrum in terms of axial symmetry, but expect that a small degree of x-y-splitting would give an improved fit. The axial symmetry observed for the proton coupling in the - C H 2 unit is characteristic of such groups when they rotate freely or exhibit rapid proton interchange. In both cases, there is ca. 80~ spindensity on the H2(2 unit as estimated from the ~H hyperfine coupling, so that delocahsation onto the - N C O or - N C S unit cannot be more than ca. 20~ The gz-Shlft for the latter is ln&catlve of spin on sulphur. The photosensitivity of H2(2NCO to give H2Clq + CO is remarkable, as is the facile thermal return, which suggests that the fragments never separate. The detailed pathway for this photolysls probably involves a nx-,ny optical transition because, in the conversion, the unpaired electron moves from the nx into a 2py orbital on nitrogen as indicated in (1), where the unpaired electron is shown localised on carbon and nitrogen purely for ease of representatlon This will clearly facilitate loss of CO because no further electron shift from n ~ n ~ is required The fact that H2~NCS does not break down on photolysis probably reflects the greater stability of CO than CS molecules
weak satellite features with the 110 G splitting are of about the correct intensity for '3C satellites, and the large isotroplc sphtting is in accord with the bent structure. This can be converted in the usual way (27) into an approximate orbital population of ca. 9% 2s on carbon. The anlsotropic coupling is, unfortunately, poorly defined, but approximate values m the region of 43-50~ results from our most reasonable interpretation for both (MeNI20)- and (MeN(2S)-. Central features for these anions were very poorly defined m the pure materials, but the 100 G satelhte lines were still detected. It, therefore, seems unlikely that anion dimers of the type detected m irradiated cyanomethane "5'~6) are formed m either of these compounds Structure o f radical cations
Although we are satisfied that the parent radical cations are formed in dilute freon solutions, the ESR spectra are rather uninformatwe. In the limiting linear structures we should observe a very large positive gll-Value, whereas the experimental values are quite close to that of the free-spin (2.0023). This shows that there must be major distortions from hnearity for both ions, as expected. The marked change in spectrum for concentrated solutions of MeNCS strongly supports the postulate that dimer cations are indeed favoured. The g-values are similar to those that we obtained for Baxendale's (NCS)2 anions, whach shows that the bonding is similar for both ions. Also the red colouration indicatlve of a band at ca. 550nm, is in accord with expectation since (NCS)~- anions have their maxlmum at 475 nmJ ~) Our more detailed results for (NCS)~-, which Included the observatmn of 33S hyperfine coupling (Table 1) can best be interpreted m terms of structure I, the antlbonding a* orbital (SOMO) being essentially 3p on sulphur For (NCS)2- anions, the g-tensor was nearly axial, suggesting that 0 ~ 90 ° (the "open-book" structure) However, for (MeNCS)2+ the g-tensor is far from axial, the results being more in accord with expectation for the planar structure (0 = 180°).
Other centres
The features which fall close to those for H2Clq molecules pnor to photolysls (Fig. 3b) were originally thought to be '3C features for the radical-anions, but this cannot be correct since no such features were obtained for solutions in M T H F or CD3OD. We are inclined to the view that they are due to small concentrations of radical-pairs, but we are not clear which radicals are involved. We looked carefully for half-field transitions, but were unsuccessful
HzI~NCO and H212NCS radicals Our results for H2(2NCO radicals agree with those for Fujlwara et al. °2) except that we have estimated
x_
y
T (2)
~ ---~--s~,~,
N-c-o
..
c
c-o
,
c=
+
CO
M C R SYMONS and P M. R TROUSSON
134
The features separated by ca 70 G which develop only in the m e t h a n o h c systems c o n t a i n i n g O J H groups a n d are absent in pure C D 3 O D or C D 3 O D + D 2 0 systems are t h o u g h t to be due to p r o t o n a t e d a n i o n s (Fig lc). Even if the p r o t o n s (~H) are entirely responsible for the splitting, the features for the c o r r e s p o n d i n g (2H) derivatives, which should be separated by ca. 22 G would be lost b e n e a t h the intense signals due to C D 2 O D a n d CD3 radicals, so we are unable to verify this postulate. N o r is it clear which t a u t o m e r will be f a v o u r e d o n p r o t o n a t i o n . If, as seems p r o b a b l e , the p r o t o n coupling Is in the region o f 70 G, then the m o s t hkely structure is (IV).
H ~ / C =0 (IV)
(V)
H3C/C
This resembles the radical cation o f acetaldehyde (V) which we showed recently has a p r o t o n coupling o f ca 1 3 7 G t281 The large hyperfine coupling for ( C H a C H O ) + is expected in terms of the positive charge effect, ~2~ but IS nevertheless surprisingly large relative to t h a t for IV, if this is the correct structure The alternative site for p r o t o n a t i o n is nitrogen, to give either (VI) or (VII) T h e radical H 2 N C O shows
~N C/0
H/
- .~
H~ N C~_/0
(VI) ~ /
- ~_~ (VII)
strong c o u p h n g (ca 3 0 G ) to one p r o t o n a n d very weak coupling (ca 1 G ) to the other, the J4N coupling being in the 2 0 G region a n d fairly lsotroplc. °°'3° Structure (VII) which is expected to have the large p r o t o n coupling is therefore possible, since the total splitting is ca 7 0 G U n f o r t u n a t e l y , the central features were always so intense t h a t we were only able to resolve the o u t e r m o s t lines for this radical (Fig lc) a n d hence we are at present u n a b l e to distinguish between these alternatives Finally, we m e n t i o n that, in all cases, the E S R p a r a m e t e r s for methyl radicals were normal. It is often f o u n d that for alkyl radicals formed f r o m R - X by dissociative electron capture, there is a slight residual interaction with X which results in reduced ESR p a r a m e t e r s for R (32]3) This is n o t the case for
NCO or N C S , but it is n o t e w o r t h y t h a t some modification was observed for Me3C radicals in irradiated t-butyl lsothlocyanate. "7) Acknowledgement -We thank the Commlssmn of the European Communities for a grant to PMRT
REFERENCES 1 J H BAXENDALE, P L T BEVAN and D A SCOTT. Trans Faraday Soc 1968, 64, 2389 2 M C R SYMONS, Pure Appl Chem 1981, 53, 223 3 R L PETERSEN, D J NELSON and M C R SYMONS,J Chem. Soc Perkm H, 1977, 2005, 1978, 225 4 B C GILBERT, D K C HODGEMAN and R O C NORMAN, J Chem Soc, Perkm II, 1973, 1748 5 J H HADLEY and W GORDY, Proe Nat Acad Stl U S A 1977, 74, 216 6 K AKASAKA, S ! OHNISHI, T SUITA and J N1TTA, J Chem Phys 1964, 40, 3110 7 S P MISHRA and M C R SYMONS, J Chem Sot, Perkm If, 1975, 1492 8 A R LYONS and M C R SYMONS, J Chem Soc, Faraday Trans II 1972, 68, 1589 9 I S GINNSand M C R SYMONS,J Chem Soe. Dalton Tran~ 1973, 3 10. M C R SYMONSand S P MISHRA, J Chem Res (S), 1981, 214 l l M C R SYMONS and B W WREN, J Chem Sot. Chem Commun 1982, 817 12 M FUIIWARA, N TAMURA and H HIRAI, Bull Chem Soc Japan, 1973, 46, 701 13 V I TROFIMOVand I I CHKHEIDZE, Khtm~ya Vysoklkh Energn 1971, 5, 404 14 Y J CHUNG and F WILLIAMS,J Phys Chem 1971,75. 1893 15 R EGLAND and M C R SYMONS, J Chem Soc (.4) 970, 1326 16 K TAKEDAand F. WILLIAMS,J Phys Chem, 1970, 74. 4007, E D SPRAGUE, K TAKEDA and F WILLIAMS, Chem Phys Lett 1971, 10, 299 17 Y J CHUNG and F WILLIAMS,J Phy~ Chem 1972, 76, 1792 18 F S ADRIAN, E L COCHRAN and V A BOWERS. J ('hem Phys 1962, 36, 1938 19 J A BRIVATI,K D J ROOT, M C R SYMONsandD J A TINLING, J Chem Soc (A), 1969, 1942 20 T SHIDA and T KATO, J Am Chem Sot' 1979, 101, 6869 21 M. C R SYMONsand I G SMITH, J Chem Res (S), 1979, 382 22 K TORIYAMA, K NUNOME and M IWASAKI, J Phys Chem 1981, 85, 2149 23 J T WANG and F WILLIAMS,J Phys Chem 1980, 84, 3156 24 M C R SYMONS,A HASEGAWAand P MAJ, Chem Phys Lett 1982, 89, 254 25 T SHIDA, Y EGAWAand H KUBODERA,J Phy~ Chem. 1980, 73, 5963 26 B M GIMARCand D A WOOOCOCK,J Mol Structure 1981, 85, 37 27 M C R SYMONS,Chemmal and Btochemtcal Aspects o! Electron Spin Resoname Spettrowopy Van Nostrand Reinhold, Woklngham, 1978 28 M C R SYMONSand P J BOON. Chem Phys Lett 1982, 89. 516
Resonance studies of the radlolysls of methyl lsocyanate and methyl lsothlocyanate 29 M C. R SYMONSand L. HARRIS, J Chem Res. (S), 1982, 268, (M) 1982, 2746. 30 H. BOWER, J. McRAE and M. C. R. SYMONS, J. Chem. Soc (A), 1971, 2400 31 R LIVINGSTONand H ZELDES,J Chem. Phys. 1967, 47, 4173
135
32. C. M. L KERR and F WILLIAMS,J Amer Chem Soc 1971, 93, 2805. 33 S P. MtSHRA and M. C. R SYMONS, J Chem. Soc., Perkm Trans. II 1973, 391; M C. R. SYMONSand I. G. SMITH, J. Chem Soc, Perkm Trans II 1981, ll80