IONIZATION POTENTIAL AND HEATOF FORMATION OF THIOFORMALDEHYDE
4111
The Ionization Potential and Heat of Formation of Thioformaldehydel
by A. Jones2 and F. P. Lossing Division of Pure Chemistry, National Research Council of Canada, Ottawa, Ontario, Canada (Received July 3, 1967)
Monomeric thioformaldehyde has been prepared transiently from the pyrolysis of thiacyclobutane and subjected to electron impact in a mass spectrometer. The ionization potential is 9.44 f 0.05 v. From the appearance potential of H&S+ ion from thiacyclobutane, AHf(HzCS+)I 242 kcal/mole. Taken with IP(H2CS) = 9.44, this leads to AH~(H,CS)5 24 f 2.6 kcal/mole, much lower than previous estimates. Derived bond dissociation energies are D(H-H&S in CH3S.) = 45, D(CHrH2CS in CH&H2S.) = 32, and D(I&C=S) 124 kcal/mole.
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Introduction Thioformaldehyde, HzCS,is unstable as a monomeric species under normal conditionsa and all attempts to prepare it have given the cyclic polymer, trithioformaldehyde. At low pressures (ca. loV3torr), however, the monomer has an appreciable lifetime and we have observed it as a reaction product in the low-presphotosensitized decompositions of both sure Hg(3PP1) CH,SCH3 and CHaSSCH3.4 A thermochemical analysis of these reactions was not possible because of the lack of a reliable value for AHf(H&S(g)). It was the aim of this work to obtain a more accurate estimate of this quantity and to provide the first direct measurement of the ionization potential of HZCS. Two values for AHt(HzCS) have been employed in the literature. In an electron impact, study of sulfurcontaining compounds, Gallegos and Kisers used a value of 76 kcal/mole based on appearance potential measurements for which HzCS was one of the neutral fragments. This value was amended to 51 kcal/mole in later work.6 The method used here was based on appearance potential thresholds for the processes
CHzCHz
j
1
CH2S
+ e -+
H2CS+
+ C2H4+ 2e AH I AP(HzCS)+ (1)
H&S
+ e +H2CS+ + 2e
AH = IP(H&S+) (2)
The ionization potential measurement in (2) was carried out on HzCS generated at low pressures in a reaction stream. It follows that
AHr(HzCS) 5 AP(HzCS+) - IP(H2CS) AHr[(CHz)3Sl
+
- AHr(CzH4)
(3)
Experimental Section The monomeric H2CS was produced by pyrolysis a t 1OOO" of thiacyclobutane at low pressure in a fused-silica capillary furnace leading to the ionization chamber of a, mass spectrometer.' The other main product was ethylene. Small amounts of allyl radical and sllene were also detected. The appearance potential curves were compared to those of xenon by a curve-matching procedure' Results and Discussion With the furnace at room temperature, measurements of AP(HzCS+)from undissociated thiacyclobutane gave an average value of 10.40 0.1 v. With the furnace at 1OOO" thiacyclobutane was about 80% decomposed,
*
(1) Issued as National Research Council of Canada No. 9833. (2) National Research Council of Canada Postdoctorate Fellow, 1965-1967. (3) E. E. Reid, "Organic Chemistry of Bivalent Sulfur," Vol. 111, Chemical Publishing Co., Inc., New York, N. Y., 1960, Chapter 2. (4) A. Jones, S. Yamashita, and F. P. Lossing, submitted for publication. (5) E. J. Gallegos and R. W. Kiser, J. Phy8. Chem., 66, 136 (1962). (6) B. G. Hobrock and R. W. Kiser, ibid., 67, 1283 (1963). (7) R. Taubert and F. P. Lossing, J. A m . Chem. SOC.,84, 1523 (1962).
Volume 71, Number 18 November 1067
4112
A. JONES AND F. P. LOSSING
and measurements on the HzCS produced gave IP(HzCS) = 9.44 f 0.05. The errors quoted represent standard deviations of several determinations rather than estimates of the absolute error. An attempt was made to measure AP(C&+) from (CHJ3S, and so obtain an independent value of AHf(HzCS) from the process (CH&S
+ e +CZH4+ + H2CS + 2e
(4)
The intensity of the m/e 28 peak from thiacyclobutane was less than 3% of the base peak, however, and excessive tailing of the CZH4+ curve precluded a reliable measurement of AP(CzH4+). As a check on our measurements, the ionization potential of thiacyclobutane was also determined. The observed value, 8.8 f 0.1 v, was in good agreement with two previous measurements of 8.9 0.155 (by electron impact) and 8.64 v8 (by photoionization). AHt(HzCS) and AHf(HzCS+). The appearance potential for HZCSf from (CHz)3Smeasured in this work, 10.40 f 0.1 v, is considerably lower than that found by Gallegos and K i ~ e r 11.8 , ~ f 0.2 v. The difference appears to be larger than can be accounted for by the difference in the method of comparing the ionization efficiency curves for the fragment ion with those for the standard gas. Values of AHf(H&S+) from the l i t e r a t ~ r e ,derived ~ ~ ~ ~ ~from electron-impact measurements on various sulfur compounds, show a spread from 215 to 252 kcal/niole, but in some cases the assumption has been made that the appearance potential includes the energy of dissociation of the neutral products as well. The present appearance potential and the assumption of the simplest dissociation process, reaction 1, lead to AHr(HzCS+) 5 242 kcal/mole, using AHt(C2H4) = 12.50 kcal/mole1° and AHf((CHZ),S) = 14.63 k c a l / n i ~ l e . ~ Substitution ~~'~ in eq 3 gives AHf(H2CS) 5 24 4 2.6 kcal/mole. Support for this lower value of AHf(H2CS) comes from a consideration of the observed temperature range for the thermal decomposition of thiacyclobutane in the reactor, as compared with the temperature range observed for other compounds for which activation energies of decomposition are known. On this basis, the activation energy at high pressure (assuming similar decreases in activation energy at low pressure) should not be greater than 30-55 kcal/mole for the dissociation
+
(CH2)SS +HzCS
+ CzH4
(5) From the relationship between the forward and reverse activation energies and the enthalpy for dissociation
E4 =
E5
- AH
and the experimental estimate of E5 The Journal of Physical (7hemistry
-
(6) 50 kcal/mole,
one can estimate E-5 for various values of AHf(H2CS). For example, a value of AHr(H2CS) = 51 kcal/mole would give AH = 49 kcal/mole and hence E-5 1 kcal/mole. Such a low value for E-s seems highly improbable. For analogous bimolecular association reactions, activation energies in the range 20-30 kcal/mole are generally found.I3 The value of AHt(H2CS) = 24 kcal/mole found in the present work leads to E-5 28 kcal/mole, which seems to be a reasonable value. It is of interest to calculate from the present data the heats of reaction for disproportionation and combination reactions of methylthio radicals
-
-
2CH3S.
4HzCS
+ CHSSH
(7)
2CHaS * +CH3SSCH3 (8) Using AHf(CH3S.) = 30.5 f 5,14 AHr(CH3SH) -5.46,lZ and AHf(CH8SzCH3) = -5.71 kcal/mole,12 AH7 beconies -42 f 8 kcal/mole and AH8 becomes -67 f 7 kcal/mole. For the isoelectronic CH30. radical the analogous reactions are 2CH30. +H2CO
+ CH30H
2CH30. +CHIOOCH,
(9)
(10)
Using AHf(CH30).= 2 f 2,15AHf(HzCO)= -27.7,16 and AHf(CH,OH) = -48.08lS kcal/mole, AHe is -78 f 2 kcal/mole and AHIOis -36.1 f 1k~al/mole.'~ It can be seen that for CH30 radicals the disproportionation is therniochemically favored, in contrast to CH3S for which the combination is favored over disproportionation, although by a smaller margin. It is interesting to note that the experimentally observed relative rates of these reactions are in qualitative agreement with the exothermicities: the combination of methoxyls is negligibly slow conipared to the disproportionation, whereas the combination of CH3S radicals is faster than the disproporti~nation.~!~~ (8) L. D. Isaacs, W. C. Price, and R. G. Ridley in "The Threshold of Space," Pergamon Press Ltd., London, 1957, p 143. (9) B.G.Hobrock and R. W. Kiser, J. Phys. Chsm., 66, 1648 (1962); 6 7 , 648 (1963). (10) F. D.Rossini, et al., "Selected Values of Physical and Tliermodynamic Properties of Hydrocarbons and Related Compounds," Carnegie Press, Pittsburgh, Pa., 1953. (11) W.N.Hubbard, C. Katz, and G. Waddington, J . Phys. Chem., 58, 142, 396 (1954). (12) H. Mackle and P. G. O'Hare, Tetrahedron, 19,961 (1963). (13) See, for example, Table XII-8 in S. W.Benson, "The Foundations of Chemical Kinetics," McGraw-Hill Book Co. Inc., New York, N. Y., 1960. (14) H. Mackle, Tetrahedron, 19, 1159 (1963). (15) J. A. Kerr, Chem. Rev., 66, 465 (1966). (16) "Selected Values of Thermodynamic Properties," National Bureau of Standards Circular 600, U. S. Government Printing Office, Washington, D. C., 1952.
IONIZATION POTENTIAL AND HEATOF FORMATION OF THIOFORMALDEHYDE
Calculations of the endothermicities of the radical dissociation processes
+ HzCS
RCHzS. + R *
(11)
which are analogous to the well-known alkoxy radical dissociations RCHZO. +R .
+ HzCO
(12)
have recently been performed by Friswell and Gowenlockla using AHf(HzCS) = 51 kcal/mole. Values recalculated using AHf(H2CS) = 24 kcal/mole are given below, together with (in brackets) the corresponding values for reaction 12: R = H, 45 (22); R = CHS, 32 (13); R = C2H5,30 (9); R = n-C3H,, 30 (9). The heats of formation of alkyl and alkoxy radicals16 and the alkylthio radicals14 are taken from the literature. Although the thiyl radicals are on this basis less stable than the earlier estimates1* indicated, they are still considerably more stable than the corresponding alkoxy radicals. The Bond Dissociation Energy D(HzC=S). From the equation HZCS +CHz
+S
(13)
4113
(HzCO) = -27.7 kcal/mole.16 Although these dissociation energies are subject to large errors, the difference D(H2C-0) - D(H&=S) is more precise since AHf(CH2) cancels. From the present data, this difference is 45.6 kcal/mole. IP(HzCS). The measured value of 9.44 i 0.05 v found in this work is much higher than the value of 6.7 v derived by Gallegos and Kiser, which was based on their initial value of AHr(H2CS) = 76 k~al/mole.~It might be noted that IP(HzCO) = IP(CHIOH), 10.87 and 10.85 v, respectively.21 Correspondingly, the present value for IP(H2CS) is in good agreement with that of CH3SH,9.44 Peak Ratios in Mass Spectrum of HZCS. The ratio of peak intensities for the parent, P-1 and P-2 peaks of HzCS were measured by stripping contributions from undecomposed thiacyclobutane from the spectrum a t 1000”. The ratios were m/e 46 (80%), m/e 45 (loo%), and m/e 44 (19%). Peaks at lower mass numbers were subject to possible interference by other reaction products.22 Acknowledgment. The authors wish to thank Professor B. G. Gowenlock for correspondence concerning ref 18 in advance of publication.
it can be seen that D(HzC=S) = AHf(CH2)
+
(17) R. P. Steer, B. L. Kalra, and A. R. Knight, J . Phys. Chem.,
AHr(S) - AHf(H2CS) (14)
71, 783 (1967); P.M.Rao, J. A. Copeck, and A. R. Knight, Can. J . Chem., 45, 1369 (1967). (18) N. J. Friswell and B . G. Gowenlock in “Advances in Free
The heat of formation of CHz is not known precisely. AHr(CHz) used here is 86 f 6 kcal/mole from data given by Bell and Kistiako~sky.’~The heat of formation of atomic sulfur, AHf(S(g)), has been the subject of some uncertainty, but a value of 65.9 kcal/mole appears to be now established.lZ Substituting these quantities in reaction 14 gives D(HzC=S) = 124 kcal/mole with an uncertainty of about 6-8 kcal/mole. The analogous dissociation energy D(HzC=O) is 173 rt 6 kcal/mole, using AHt(0) = 59.520and AHf-
Radical Chemistry,” Vol. 2, G. H . Williams, Ed., Logos Press, London, 1967, p 27. (19) J. A. Bell and G. B . Kistiakowsky, J. Am. Chem. SOC.,84,3417 (1962). (20) T. L. Cottrell, “The Strengths of Chemical Bonds,” 2nd ed, Butterworth and Co. Ltd., London, 1958. (21) R. W. Kiaer, “Introduction to Mass Spectrometry, and its Applications,” Prentice Hall, Inc., Englewood Cliffs, N. J., 1965. (22) NOTEADDEDIN PROOF.Reaction 11, with R = H, has been observed at 925’ in this reactor [T.F. Palmer and F . P. Lossing, J. A m . Chem. Soc., 84,4661 (1962) 1. Decomposition of CHsS at this temperature is consistent with D(H-HzCS) = 45 kcal/mole as given, but is incompatible with the 73 kcal/mole required for this reaction by the higher value for AHr(HzCS).
Volume 71, Number 12 November 1967
