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J . Phys. Chem. 1985,89, 5274-5278
k8 to k , , in the simulation did not change the value of kz by more than 15%. Another cause of error is the initial concentration of H atom which depends on the ratio (gH 4- gOH)/g, -. The temperature coefficients of gH, gOH, and g%- in alkaline sohion are not known. But in acid solution between 2 and 65 “cgeg-+ gH and goH increase respectively by 0.10 and 0.18% deg.IP It is reasonable to suppose that the ratio (gH gOH)/g,, - would not change by more than 0.2%/deg giving a variation with H atom concentration of 9% over the temperature range 15-60 OC. The total error on the determination of k, is probably less than f20%. Reaction between Hand OH-. The activation enthalpy is given by the equation AH* = E - R T where E is the experimental activation energy. From the relation AG* = AH* - TAS* the activation entropy of reaction 2 is AS2* = -6 f 2 cal/deg. Comparison with AS1* = -32 cal/deg for the reverse reaction 1 indicates that the transition state must be closer to OH- than to eaq-. For a bimolecular reaction with a normal frequency factor AS* is around -12 cal/deg.zl This means that the charge is probably more delocalized in the transition state than on the hydroxyde anion. In the same way the large entropy decrease from e a i to the transition state can be accounted for by localization of the charge of eaq- on a single water molecule. Thermodynamic Properties of eaq-. The absolute enthalpy of formation AHof and the standard entropy So of the hydrated electron are calculated in Table I1 by taking for Has+ AGOf = 103.8 kcal/mol, AHof = 98.8 kcal/mol, and So = -1.17 cal/(mol deg)3,2z*23 and for Has AGOf = 53.03 kcal/mol, AHOf = 51.09 kcal/mol, and So = 9.08 cal/(mol deg).23 Generally, the values of the thermodynamic properties for the individual ions in aqueous solution are based on the usual convention that the values of Wf, AGOf, and So for Has+are zero in the standard state.24 With this convention AGOf = 66.3 kcal/mol, AHof = 66.2 kcal/mol,
+
Gottschall, W. C.; Hart, E. J. J. Phys. Chem. 1967, 71, 2102. Laider, K. J. “Chemical Kinetics”; McGraw-Hill: London, 1965. Noyes, R. M. J. Am. Chem. SOC.1964,86, 971. (23) Noyes, R. M. “Radiation Chemistry I”; American Chemical Society: Washington, DC, 1968; Adv. Chem. Ser., No. 81, p 65. (24) Nut. Bur. Stand. Circ. 1952, No. 500.
and So = 15.6 cal/(mol deg) for eaq-. Enthalpy and Entropy of Hydration. The enthalpy and standard entropy of the electron in the gas phase at 298 K are respectively 1.48 kcal/mol and 4.998 cal/(mol deg).25 With the same convention as in ref 3 (see Table 11) the hydration enthalpy and entropy of e, - are AHs = -34 f 1.6 kcal/mol and ils, = 1 1.7 5.4 cal/(mol Jeg). A previous estimation of hydration entropy was done by extrapolation of the corresponding thermodynamic properties of halides ions.z2 A value of -1.9 cal/(mol deg) was obtained for a radius of 2.98 A.3 The difference between the two values indicates that ea; is more efficient at breaking the water structure than a hypothetical halide anion of same radius. Comparison between Water and Ammonia In liquid ammonia at 25 OC the solvation entropy of the electron is 11.4 cal/(deg mol) and its standard entropy So 16.4 cal/(deg In water the values are respectively 11.7 and 16.7 cal/(deg mol) (Table 11). For spherical ions with no specific interactions with the solvent, the solvation entropy is more negative in ammonia than in waterz7 because water is more structured than ammonia. If the solvated electron behaves like other spherical anions one would expect a lower solvation entropy in liquid ammonia than in water. The exceptional value of So and ASo for the ammoniated electron28 is partly due to the formation of a large cavity in the solvent. In water the cavity size is much smaller than in ammoniaz6but this is compensated for by the stronger structure of the solvent. This is probably the reason the solvation entropy ASs and the standard entropy So are about the same in water and in liquid ammonia at 25 OC.
Acknowledgment. The authors thank H. Corfitzen, T. Johansen, and P. Genske for technical assistance during the work. Registry No. HzO,7732-18-5; H, 12385-13-6; OH-, 14280-30-9. (25) “JANAF Thermochemical Tables”, Nut. Stand. Re$ Dura Ser., Nut. Bur. Stand. 1971, No. 37. (26) Lepoutre, G.; Jortner, J. J. Phys. Chem. 1972, 76, 683. (27) Schindewolf, U. Ber. Bunsenges. Phys. Chem. 1982, 86, 887. (28) Lepoutre, G.; Demortier, A. Ber. Bunsenges. Phys. Chem. 1971, 75, 647.
Proton Affinity and Gas-Phase Ion Chemistry of Methyl Isocyanate, Methyl Isothiocyanate, and Methyl Thiocyanate Z. Karpas,*+ W. J. Stevens, T. J. Buckley, and R. Metz* Center f o r Chemical Physics, National Bureau of Standards, Gaithersburg, Maryland 20899 (Received: May I O , 1985) The gas-phase ion chemistry of CH3NC0, CH3NCS, and CH3SCN was investigated by pulsed ICR techniques and their proton affinities were determined as being 184.5 0.5, 193.0 f 0.4, and 192.6 f 0.5 kcal/mol, respectively. The main reaction of the molecular ion in the three compounds is production of the protonated molecule. The CH2Xt ions, where X = NCO, NCS, or SCN, are unreactive toward the parent molecule. The fragment ions CH,Y+, where n = 0-3 and Y = 0 or S, react by charge transfer or proton transfer. The protonated molecules react very slowly with their parent compounds. Although protonated dimers were observed their production is inefficient. Ab initio calculations at the SCF level were used to determine the structures of the neutral and protonated molecules. The calculated proton affinities, 188.5, 188.6, and 193.7 kcal/mol for CH3NC0, CH,SCN, and CH3NCS, respectively, are in good agreement with the experimental values. The favored site of protonation was found to be the nitrogen atom in the former two, while in CH3NCS the sulfur atom is the preferred site of protonation. Introduction As a result of the recent accident in Bhopal, India, in which heavy casualties resulted from the release of a large quantity of methyl isocyanate, C H 3 N C 0 , considerable attention has been
focused on its chemistry. Surprisingly, no studies of its gas-phase ion chemistry have been published and even its mass spectrum has only been reported recently.’ Methyl isothiocyanate, CH3NCS, and methyl thiocyanate, C H W N , which are closely
‘Permanent address: Nuclear Research Center, Negev, P. 0. Box 9001, Beer-Sheva, Israel. ‘Summer student.
(1) Heller, E. R.; Milne, G. W. A,; Gevantman, L. H.“EPA/NIH Mass Spectral Data Base“; Natl. Stand. Ref: Data Ser. (V.S.,Natl. Bur. Stand.) 1983, NSRDS-NBS 63, SUPPI. 2, 5271.
This article not subject to U.S. Copyright. Published 1985 by the American Chemical Society
The Journal of Physical Chemistry, Vol. 89,No. 24, 1985 5275
Ion Chemistry of C H 3 N C 0 , CH,NCS, and CH3SCN
TABLE I: Summary of the Ion-Molecule Reactions in CH3X (X = NCO, NCS, and SCN)
LI (kAnd," IO-'' cm'/s
---- -
reaction
CH3X' + CH3X CH3XH' + CH2X CH2X+ + CH3X CO+ + CH3NC0 CH3NCO++ CO CHO+ + CH3NC0 CH,NCOH+ + CO CH3S' + CH3X CH3XH' + HZCS CHZS' + CH3X CH3X" + HzCS CH,XH+ + HCS CHS' + CH3X CH3XH++ CS CS+ + CH,X CH3X++ CS CH3XH+ + CH3X products -+
X = NCO
X = NCS
11.6 f 0.8 (20.3)
