THE JOURNAL OF
PHYSICAL CHEMISTRY Registered in U.8. Patent Ofice @ Copyright, 1971, by the American Chemical Society
VOLUME 75, NUMBER 13 JUNE 24, 1971
Loose and Tight Activated Complexes in the Ion Fragmentations of a Dimethylthiocarbamate and a Phosphorothioate by James C. Tou Chemical Physics Research Laboratory, The Dow Chemical Company, Midland, Michigan 48640 (Received December 5, 1970) Publication costs assisted by The Daw Chemical Company
The ion fragmentations involving rearrangement (r) and simple bond scission (s) of a dimethylthiocarbamate and a phosphorothioate are investigated. The energy dependence of the relative intensity of the rearrangement ion to the ion generated from a simple bond scission is interpreted based on the quasiequilibrium theory of mass spectra. The rate ratio, k,(E)/k,(E), can be expressed as W,*(E - eo,,)/We*(E - eo,,), and its value decreases monotonically from W ,*(eo,, - eo,,) and approaches ~ e r oasymptotically, as the ionizing energy increases.
Introduction The principles of the quasiequilibrium theory have been used in the interpretation of comparable ion fragmentation reactions comprising rearrangements and simple bond scissions.lb2 This reasoning has also been applied to the investigation of aryl participation in the expulsion of a bromine atom from the molecular ion of P-phenylethyl bromide3 which involves rearrangement mechanism. The approach aids in understanding the fundamental differences between the spectra of complex molecules a t 70 eV and at low voltages. I n this paper, the rearrangements occurring in the ion fragmentations of a dimethylthiocarbamate and a phosphorothioate are studied. The thermal isomerization of O-aryl dimethylthiocarbamates to X-aryl dimethylthiocarbamates has been reported, which provides a new efficient way of converting phenols into thiophenols. Similar rearrangement involving S=YOO=YS- isomerization, where Y = C or P, have also been observed in both thermal reaction (ArOC(=S)+
OAr -+ ArOC(=O)SAr, Schonberg rearrangement) and ion fragmentation (ArOC(=S. +)OR ArSC (=O. +)OR6and ROP(=S. +) = -+ RSP(=O+)='). -+
Results I n our recent mass spectrometric study of a series of dimethylthiocarbamatesls two intense peaks were observed a t m/e 88 and 72 with ion compositions C3HaNS (1) D.H. Williams and R. G. Cooks, Chem. Commun., 663 (1968). (2) (a) R. G.Cooks, I. Howe, and D. H. Williams, Org. Mass Spectrom., 2, 137 (1969); (b) A. N.H. Yeo and D. H. Williams, J . Amer. Chem. Soc., 92,3984 (1970). (3) R. H.Shapiro and T. F. Jenkins, Org. Mass Spectrom., 2, 771 (1969). (4) M. S. Newman and H. A. Karnes, J . Org. Chem., 31, 3980 (1966). (5) H. R. Al-Kaaimi, D. S. Tarhell, and D. Plant, J . Amer. Chem. Soc., 77, 2479 (1955). (6) J. B. Thomson, P. Brown, and C. Djerassi, ibi&., 88, 4049 (1966). (7) F. J. Biros and R. T. Ross, Paper G3, Eighteenth Annual Conference on Mass Spectrometry and Allied Topics, June 1970, San Francisco, Calif.
1903
JAMESC.Tou
1904
18
ao
f
248
70
r
124
I 40
60
I1
I
I
1111
1
I
II
80 100 120 140 160 180
I
I
200 220
II
283 xioo(hit)
L
240 260 280 300 320
- d e -> Figure 1. Mass spectrum of 0-2,3,6-trichlorophenyl dimethylthiocarbamate (150').
and GHeNO, respectively. The mass spectrum of 0-2,3,6-trichlorophenyl dimethylthiocarbamate at 150' is shown in Figure 1. The spectrum is simple. The molecular ion was detected with extremely weak intensity and shows a loss of C1. in the generation of a fragment ion at m/e 248. No significant metastable ion transitions to the ions a t m/e 88 and 72 were observed in the beam defocusing experiment. It would not be illogical, however, to assume the following reaction scheme Scheme I
----+
+S=CN(CHJ, a, mle 88
where (M. +)* represents the excited molecular ions, (MI. +)I* and (MI. +)2* the activated complexes for the reactions of the simple bond scission and the rearrangement leading to the formation of ions at m/e 88 and 72 respectively, and z = 2 and (or) 3. In order to determine the extent of thermal rearrangement of -OC(=S)to -SC(=O)-4 prior to ionization, the temperature effect on the ion fragmentation was investigated. The intensity ratio of the peaks a t m/e 72 and 88, 172/188, were recorded a t the sample temperatures 150, 200, and 250" as a function of residence time in the inlet system. The results are shown in Figure 2. It is noted that a rapid isomerization, -OC(=S)to S C ( = O ) - , took place a t 250" and reached equilibrium in about 25 min. However, the ratio shows only a very slight increase with residence time a t 150". Therefore, the ion at m/e 72 shown in Figure 1 (150") is purely generated from the molecular ion, (MI. +)*, upon electron impact through rearrangement reaction 2. This presents another example of the similarity between two energetic processes, thermolysis and ion fragmentati~n.~ Under the condition where the thermal effect upon ion fragmentation can be eliminated, i.e., at 150 and loo", the ratio, Iy2/I@ was studied as a function of ionizing energy. The results are shown in Figure 3. A drastic increase of the ratio near threshold was observed. This indicates that reaction 2 involving rearrangement over(8) J. C. Tou and R. M. Rodis, unpublished work. (9) A. Maccoll in "Modern Aspects of Mass Spectrometry," R. I. Reed, Ed., Plenum Press, New York, N. Y . , 1968, pp 143-168, and the references therein.
Th.e Journal of Phyakal ChembtTy, Vol. 76, No. IS,1071
1905
LOOSEAND TIGHT ACTIVATED COMPLEXEB 4 99
300 200
F
250°C
i-z I' :::I 100 90 80 70 60
," 30
.=
rl
m
20
ri
Y
10 9 m 8 m 7 Y 0
:
CI
5
4 1
3
2
1 0.9 0.8
0.7 0.6 0.5 0.4
0.1
I
1
I
10
Figure 2. Temperature dependence ofI,p/Iag spectrum of the phosphorothioate,
I
I
I
I
I
I
I
40 50 r e s i d e n c e time (min,) i h t h e i n l e t .
20
30
I
I 60
I
I 70
( 0 )in the mass spectrum of the dimethylthiocarbamate and I I , , / I w (0) ~ in the mass
whelmingly overcomes reaction 1 involving simple bond scission in low-energy electron impact. I n contrast to the above case, two comparable rearrangement processes occurring in the ion fragmentation of O-(3~6-dichloro-2-pyridyl)-O~O-diethyl phosphorothioate were investigated. The mass spectrum is shown in Figure 4. The molecular ion shows stepwise losses of one chlorine atom and two ethylene molecules in the generation of fragment ions at m/e 280, 252, and 224 involving one hydrogen rearrangement in each of the last two steps. This is similar to the case of 0,O-diethyl phtha1imidophosphorothioates)lO but different from that of triethyl phosphate where a double-hydrogen rearrangement" is involved. The ion at m/e 97 is
reasonably assumed to be formed from a simple 0-P bond scission of the ion a t m/e 224, and has a plausible structure +S=P /OH.
The mechanism of the genera-
\OH tion of the ion at m/e 174 is unknown. The empirical formula of this ion was determined, however, to be (10) (a) G. J. Kallos and L. A. Shadoff, "Mass Spectral Studies of Organophosphorus Compounds," The Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, 1967; (b) G. J. Kallos, unpublished data. (11) H. Budzikiewicz, C. Djerassi, and D. H. Williams, "Mass Spectrometry of Organic Compounds," Holden-Day, San Francisco, Calif., 1967, p 651, and the references therein.
The Journal of Physical Chemistry, Vol. 76,No. 18,1971
JAMESC.Tou
1906
0 10
12
-
10
-
8-
n
z (II
r.
d
Y
$4
6-
0 m m
Y
\ N
r
n
4-
I 2-
x10 I
I
Figure 3. Energy dependence of Ir2/Iss (0, 150'; the mass spectrum of the phosphorothioate.
0, 100') in the mass spectrum of the dimethylthiocarbamate and I1,0/I16a(A)in
100
f
163
90 80
70 60
280
179 174
40
60
80
100
120
140
160 -de
Figure 4.
180
200
220
->
Mass spectrum of 0-(3,6-dichloro-2-pyridyl)-0,O-diethylphosphorothoate.
The Journal of Physical Chemistry, Vol. 76, No. 13, 19'71
240
260
280
300
320
LOOSEAND TIGHT ACTIVATED COMPLEXE~
1907
CyHeNCI%,which indicates the occurrence of R. possible skeletal rearrangement involving one of the alkyl groups. The ions of most interest are the ions a t m/e 163 and 179 with empirical formulas C6HsONCl2and CsHaNC12S,respectively, determined by high-resolution mass spectrometry. No significant metastable ion transition was detected for the generation of these two ions in the beam defocusing experiment. Two possible schemes are assumed. Scheme IIa
The notations are similar to the ones defined previously. These two comparable rearrangement reactions were studied in detail. The monoisotopic intensity ratio of these two ions, 11,~/1183, were investigated as a function of residence time at 250 and 200" as well as of ionizing energy at 200". The results were plotted in Figures 2 and 3, respectively. The constancy of the ratios at 200" in Figure 2 demonstrates the negligible thermal effects upon the ratio-energy curve in Figure 3. The results show a slight increase of the curve as the ionizing energy decreases in contrast to the 1 7 2 / 1 8 8 curve in the case of the dimethylthiocarbamate.
Discussion The data given above clearly show that the rearrangement reaction is increasingly important relative to the simple bond scission reaction as the ionizing energy decreases. However, the comparison of two rearrangement reactions indicates that one is not preferentially favored over another. According to the quasiequilibrium theory of mass spectra, the rate of dissociation of an isolated excited ion as with internal energy, E, can be expressed12*1s
d,mle 179
c, mle 163
t
t
where h is Planck's constant, W , the number of states of the ion with internal energy less than and equal to E, W * that of the activated complex leading to the reaction with activation energy €0. The rates for the and the simple rearrangement reaction [k,(E), bond scission reaction [k,(E), Q,,] can then be written
1' asc1 1' aoC1
c1
c1
H
H
Scheme IIb
f, mle 259
The experimental data frequently show e o , r < EO,^ for two comparable reactions. The activated complex, like (M1.+)2*, (Mz.+)l*, or (Mz.+)z*, etc., of a rearrangement reaction involves new bond formation and hence certain vibrational frequencies will increase and internal rotation will be stopped. Therefore, the activated complex of this type is called a tight complex. of a simple bond The activated complex, like (M1. scission reaction only involves stretching of a bond along the reaction coordinate. The electronic interactions between the atoms of two recoiling groups will be reduced and hence certain vibrational frequencies will decrease and certain torsional and skeletal vibrations in the ground state of the molecular ion might change to allow free internal rotation in the activated state. This
1.5
(12) H. M. Rosenstock, M. B. Wallenstein, A. L. Wahrhaftig, and H. Eyring, Proc. Nat. Acad. Sci. U . S., 38, 667 (1952). (13) J. C. Tou, L. P. Hills, and A. L. Wahrhaftig, J. Chem. Phys.,
(Mz.+)'
c1
0-P=O
I
c1
II
0
OC2HS e, m/e 287
+d, m/e 179
(3)
c, m/e 163
45, 2129 (1966).
The Journal of Physical Chemistry, Vol. Y6,No. IS, l Q Y l
JAMESC,Tou
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-Internal
Energy, E -
Figure 5. Rate curves for reactions involving rearrangement and simple bond scission.
type of activated complex is called a loose complex. Therefore, W,*increases much faster than Wr* as a function of active internal energy, E By definition, the rate constants are then being affected.'j The effect is shown in the quantitative calculations of the transitions 86. + -t 71 + CH8 (simple bond scission) and 86. ++ 70. + CHs (rearrangement reaction) in the case of 2-rnethylpentane.'* It is clear that the rates involving simple bond scission increase faster than those involving rearrangement even though the former process has a higher activation energy than the latter. These typical curves are plotted in Figure 5. The ratio of the two rate constants can be expressed as a function of internal energy.
+
+
kr(E) - Wr*(E' kdE) %*(E For E E
