COMMUNICATION TO THE EDITOR
2074
Vol. 66
COMMUNICATION TO THE EDITOR THE IOIC’IZATIOh’ POTENTIAL O F (1SO)THIOCYANIC ACID
Sir: In a mass spectrometric study of ethyl isothiocyanate; we have observed that the ion of m l e = 59 is 937, of the base peak (m/e = 87). The relative abundance of the isotopic mass peak at m/e = 61 indicates that the ion of m/e = 59 may be HNCS+ or HSCN+. The structure of t8he m/e = 59 ion is discussed further in the concluding paragraph of this communication. The ionization and dissociation of ethyl isothiocyanat’e leading to the m/e = 59 ion may be represented by CzH5KCS 4 HSCSf
+ C2H4
(1)
(the specific structure of the ion is not implied): if this is so, one would expect a fairly low appearance potential for the m/e = 59 ion. Equations similar to eq. 1 have been observed for a number of different systems, such as for 3,4-dithiahexaiie1 and triethylphosphite.2 Using a t’ime-of-flight mass ~pectrorneter,~ we have determined the appearance potential (by treating our experimental data in bhe manncr described by 1Yarren4) for the mle = 59 ioii to be 11.38 A 0.16 e.\’., t’hus indicating a reasonably simple process, such as t’hat given by eq. 1. The fieat, of‘ formation of C21&SCShas not, been reported in the literature, but AHf(CH,?I’CS(g)) = 27.1 kcal.,/mole.6 V7e therefore may estimak AH,(C2H,KCS(g)) = 22 kcal./mole using Franklin’s group calculations.6 Since AHf( C s H I ) = 12.406 k~al./mole,~ me calculate AH,+ (HKCS) = 272 kcal./mole, utilizing our measured appparance pot’cntial. The heat of formation of HXCS has not been reported, but we estimate it to be about 27 kcal./ mole by comparison of the heats of formation of a (1) R.
W.Ii-iser and B. C. Hobrock. J . PILUS. Chem., 6 6 , 1214 (1962).
(2) Y. Wada and R. W. Kiser, personal communioation.
(3) E. J. Gallegos a n d R. W. Kiser, J . A m . Chem. SOC.,83, 773 (1961). (4) J. W. Warren, Nature, 168, 811 (1960). ( 5 ) S. Sunner, Acta Chem. Scand., 9 , 837 (195.5). (6) 3 . L. Franklin, Ind. Eng. Chem., 41, 1070 (1949). (7) F. D. Rossini, D. D. Wngman, W. H. Evans, S. Levine, and I. Jaffe, “Selected Values of Chemical Thermodynamic Properties.” National Bureau of Standards Circular 500, U.S. Governnient Printing Office. Washington, D. C., 1952.
number of H X and CH,X molecules (X = OH, SH, CN, CHO). Therefore, I(HXCS) = AHf+(HKCS) - AHf(HNCS) = 272 - 27 = 245 kcal./mole, or 10.6 e.v. It is possible to calculate I(HSCS) based on Franklin’s group orbital method.* Taking I(CH&CS) = 9.25 e . ~ .I(C2H5XCS) ,~ = 9.10* 0.15 e.v. determined in the present study in our Laboratory (9.14 e.v. has been reported previouslyg) and the C-C interaction parameter = 1.55, me calculate I(HNCS) = 10.61 e.v., in better-than-expected agreement with the value we derived from the appearance potential of m/e = 59 from C2H5?;CS. As a by-product of this calculation, the C-XCS interaction parameter was determined to be 2.34. Extending these calculations, we find that I(CH,CH2CH2XCS) = 9.07 e.v., only slightly lower than I(C2H&CS). There is presently no experimental value with which to compare this calculated value. It is indeed tempting to assume that, in the dissociation leading to the elimination of CzH4 (eq. l ) , the structure of the ion formed is HSCS+. However, we presently have no experimental evidence that isomerization to HSC?;+ has not occurred. The referee of this communication has pointed out that a Franklin group orbital calculation of I(HSCN) from the ionization potentials for methyl and ethyl thiocyanateg 1c:tds t o a value of 10.9 e.\‘., a value not significantly diflerent from that calculated above for I(HNCS) or indirectly determined by us from the appearance potential of the m/e = 59 ion. Although we estimate that AHf(HNCS) and aHf(HSC?\‘) are very nearly the same, and that AHf+(I-ISCS) and AHf+(HSCN) are also very nearly the same, mf: are presently studying a series of alkyl thiocyanates in order to attempt to distinyuish between the structures of HSCN+ and HNCSf. The authors wish to thank the referee for his helpful comments. ( 8 ) J. L. Franklin, J. Chem. Phya., 22, 1304 (1954). (9) K. Watanabe, T. Kakayama, and J . Mottl, “Final Report on Ionization Potential of Molecules by a Photoionization Method,” December, 1959; Dept. Army SB99-01-004 ORD. TB-2-0001-00R1624, Contract No DA-04-200-ORD 480 and 737.
DEPARTMENT OF CHEMISTRY ROGERC. SHCXKEL BRICEG. HOBROCK KANSAS STATEUNIVERSITY MANHATTAN, KANSAS ROBERT W. KISBR RECEIVED AUGUST10, 1962
