DENISJ. BOGANAND CLIFFORD W. HAND
1532 has obtained breakdown data which do not agree with those of vonKoch. Brehm’s data can only be interpreted as implicating CH4+ ions with energy greatly in excess of that required for reaction I and yet not fragmenting. Such a result would require “isolated st,ates” and hence is incompatible with the fundamental premise of the quasiequilibrium model. Again this would warrant the most careful scrutiny. Experiments such as those of Brehm and of Ottinger are taking much of the guess work out of unimolecular fragment,at,ion st,udies. The formulation of quasiequilibrium theory that has been presented here is hoped to have a similar result by obviating any need
to estimate transition-state parameters. Our illustration of its use has, by contrast, involved only the most transparent of approximations. And, of course, there is the not inconsiderable advantage that this formalism satisfies microscopic reversibility. Accordingly, while discrepancies between theory and experiment persist, one can be reasonably certain that these are not artifacts of the theory but rather worthy of a more measured study.
Acknowledgment. Discussions with Professors Ch. Ottinger and J. H. Futrell and with Dr. H. C. Schweinler are gratefully acknowledged.
Mass Spectrum of Isocyanic Acid1
by Denis J. Bogan Carnegie-itfellon University and, the University of Alabama, University, Alabama
36486
and Clifford W . Hand* Department of Chemistry, University of Alabama, University, Alabama
36486
(Received January 4, 1971)
Publication costs assisted by The University of Alabama
Energies of several possible structures for the HNCO+ ion have been calculated by INDO methods. Compared with the linear HNCO+, a cyclic structure was found to be more stable by 80 kcal. A fragmentation pattern and appearance potentials for the major fragment ions are reported, and a mechanism involving the cyclic structure is presented. The mechanism explains the observed features, including a metastable transition, of the mass spectrum of HNCO. The successful use of INDO calculations in this problem suggests that semiempirical methods may be generally useful in the interpretation of mass spectra.
Introduction The major product of the thermal decomposition of cyanuric acid has the empirical formula CHON, corresponding to the two possible structures HOCK’ (cyanic acid) and HNCO (isocyanic acid). Compelling evidence of the true structure comes from an infrared study2in which all the spectral features were accounted for by the structure HNCO, with a linear KCO group and an HNC angle of ca. 130”. The structural constants were later very accurately determined by millimeter wave t e c h n i c ~ e s . ~Thus, although some chemiCa1 evidence4 suggests the existence, in solution, of a tautomeric with 1301, H°CN present’ and photolytically generated HOCN has been observed in low temperature matriceSJ6the conclusion is inescapable that gaseous samples at room temperature consist almost exclusively of HNCO. The mass spectrum Of HNC06’7shows a large peak at m/e 29, corresponding to an ion of formula HCO+;
-
The Journal of Physical Chemistry, Vol. 76, No. 10, 1&71
this species is the second most abundant fragment ion in the spectrum. This is a curious feature inasmuch as the HCO+ fragment cannot be formed without extensive rearrangement of the parent structure. Mass peaks corresponding to CH+, OH+, and NO+ also appear, and on this basis Smith and Jonassen’ suggested that HNCO+ might have a “triangular structure.” (1) Presented in part at the Southeast-Southwest Regional Meeting of the American Chemical Society, New Orleans, La., Dec 2, 1970. (2) G.Heriberg and C. Reid, Discuss. Faraday Soc., 9, 92 (1950); C.Reid, J . Chem. Phys., 18, 1544 (1950). (3) R. Kewley, K. L. V. N. Bastry, and M. Winnewisser, J. Mol. Spectrosc., io, 418 (1963). 78, 6234 (1956); (b) N. (4) (a) A. Amell,, J . Amer. Chem. SOC., Groving and A. Holm, Acta Chem. Scand., 19, 1768 (1965). (5) M. E.Jacox ahd D. E. Milligan, J . Chem. Phys., 40,2467 (1964). (6) J. M. Ruth and R . J. Philippe, Anal. Chem., 38, 720 (1966). (7) 8. R. Smith and H.B. Jonassen, J. Inorg. Nucl. Chem., 29, 860 (1967).
1533
MASSSPECTRUM OF ISOCYANIC ACID
The present work was undertaken in the hope of establishing the structure of the HNCO+ ion. We report the results of INDO calculations and of redetermination of the molecular ion and principal fragment ion appearance potentials in the mass spectrum of isocyanic acid. A mechanism which satisfactorily explains all of the experimental data is proposed.
Mixtures of ca. 2 :3 Ar :HNCO were admitted to the TOF from a 2.3-1. reservoir via a gold leak. Reservoir pressure was measured by a capacitance manometer (MKS Baratron Type 144). The effusion rate through the pinhole was measured for an air sample, and the pressure drop was found to be about 15%/hr. Therefore, txe pressure drop was taken to be linear with time over the shorter duration of appearance potential and fragmentation pattern measurements, and the data were corrected accordingly. Since Smith and Jonassen7 found that isocyanic acid trimerizes to cyanuric acid by a first-order mechanism lvith a half-life of 18 hr at 30°, the pressure drop due to polymerization is also linear over a time span of 1hr or less. lo Background pressure in the ion source of the TOF was less than lo-’ Torr, and a pressure of 15 Torr in the reservoir resulted in a source pressure of 3 X lop6 Torr. The background mass spectrum was sufficiently low that the fragmentation pattern required correction only at m/e 17. INDO calculations were performed using the pro~ from the Quantum Chemgram C N I N D O ~obtained istry Program Exchange, Indiana University, Bloomington, Ind., and the Univac 1108 time sharing system of the University of Alabama Research Institute at Huntsville, Ala.
Experimental Section
Results
Isocyanic acid was prepared by the thermal decomposition of cyanuric acid under v a c ~ u m . The ~ mass spectrum of the crude product showed l0-20% carbon dioxide a n d a lesser amount of water. Although hydrogen cyanide has been reported to be a product of this r e a ~ t i o n the , ~ mass spectrum showed that it could not have been present in more than trace quantities. Water was removed by passing the product gas through phosphorus pentoxide four times, followed by passage through a -20” trap. Carbon dioxide was removed by successive freeze, pump, thaw cycles at - 84”. The process was repeated, usually 8-12 cycles, until the mass spectra showed no further change in the ratio m/e 44:43. The final value of this ratio was 0.018, in good agreement with the ratio 0.015 expected from known isotopic abundances. The infrared spectrum of the purified product was identical with that reported by Herzberg and Reid.2 Argon was Matheson ultrahigh purity and was passed through a -196” trap before introduction into the vacuum system. Most of the experimental work was done with a Bendix Model 12-107 time-of-flight mass spectrometer (TOF) equipped with a 14-107 ion source, in which the pressure was monitored with a cold cathode gauge. For comparison of results, two measurements of the appearance potential of the molecular ion were made with a magnetic deflection mass spectrometer, the CEC 21-104.
The fragmentation pattern and appearance potentials of the major fragments from HKCO are shown in Tables I and 11, together with previously reported values. The appearance potential for the parent ion is the result of five determinations, all of which agreed to within 0.05 eV, made with both TOF and magnetic instruments. Appearance potentials were measured relative to that of argon, used as an internal standard, and the data were treated by the extrapolated potential difference method.12 This method is discussed by Field and Franklin13 and is considered to be the best method of obtaining appearance potentials from electron impact data, exclusive of those methods utilizing monoenergetic electron beams. Ionization efficiency curves for HNCOf and Ar+ were of the same shape at low ion currents, and therefore the simpler linear ex-
Appearance potentials for HNCO+, NCO+, HCO+, and NH+ from HNCO have been reported by Rowland, et ~ 1 . ~who 8 also observed a metastable peak a t m/e 19.6, corresponding to the process HNCO+ --f HCO+
+N
(1)
The metastable peak has a flat-top width corresponding to a kinetic energy release of 0.53 eV. The existence of a metastable peak for such a small molecule was explaineds in terms of a slow spin-forbidden predissociation to a repulsive surface, with consequent kinetic energy release in the fragmentation processes. That is, the reaction path (a), (2a) was considered more likely than the simpler reaction 3.
HNCO+ (2A”) +HNCO+ (4A”)
(2)
+ N (4S) (2a) HCO+ (lZ+) + N (2D) (3)
HNCO+ (4A”) +HCO+ (‘2+)
HNCO+ (2A”) ---t
(8) C. G. Rowland, J. H. D. Eland, and C. J. Danby, Chem. Commun., 1535 (1968). (9) M.Linhard, Z.Anorg. AlZg. Chem., 236, 200 (1938). (10) I n the present work, a sample of isocyanic acid kept in the reservoir for 3 days showed no mass spectral evidence of trimer formation. (11).(a) J. A. Pople and D. L. Beveridge, “Approximate Molecular Orbital Theory,” McGraw-Hill, New York, N. Y.,1970, Appendix A ; (b) J. A. Pople and G. A. Segal, J. Chem. Phys., 44, 3289 (1966); (0) J. A. Pople, D. L. Beveridge, and P. A. Dobosh, {bid., 47, 2026 (1967). (12) (a) J. W. Warren, Nature, 165, 810 (1950); (b) J. W. Warren and C. A. McDowell, Discuss. Faraday Soc., 10, 63 (1951). (13) F. H.Field and J. L. Franklin, “Electron Impact Phenomena and the Properties of Gaseous Ions,” Pure and Applied Physics Series, Vol. 1, Academic Press, New York, N. Y . , 1957, Section II-D.
The Journal of Physical Chemistry, VoZ. ‘76,No. 10, 1971
DENISJ. BOGANAND CLIFFORD W. HAND
1534 trapolation method yields the same value for V(HNCO +) as the extrapolated potential difference method. Four measurements were taken using the linear extrapolation method in order to check on the reproducibility of results. Table I : Fragmentation Pattern of HNCO at 70 eV Ionizing Energy -Intensity
bindi nq
structure charge
C.
same as above
multiplicity energy (a.u.)
+I
a
work
45 44 43 42 30 29 28 27 26 22 21.5 21 17 16 15 14 13.5 13 12
0.1 1.8 100.0 21.7 1.5 13.8 6.5 2.0 2.4 0.0
See ref 6.
Ref
Ref b
a
0.5 10 100.0 26 3 27 12 5 7
0.10 1.56 100.0 22.77 2.04 17.77 4.08 3.53 3.51 0.02 1.83 0.21 0.09 0.94 13.49 3.35 0.03 0.35 4.39
1
1.0
0.06 0.08 0.4 6.9 1.2 0.0 0.08 1.2
1 2 14 3
0.4 5
HNCO NCO NO HCO NH a
See ref 8.
2
-1.094
4
-0.837
a,
same a s above
C-H 0.
+I
7
2
-1.21
4
-1.014
2
-1.105
1.33 0
~z~l.”o
$1
2 -0.956
1.330
Figure 1. Structures and energies calculated by INDO. All structures are planar, and all bond lengths are in bngstroms. Structures a, b, and d are the result of an optimization procedure to find the geometry of minimum binding energy. Energies of structures c, e, f, and g are the result of a single calculation of the designated geometry.
See ref 7. Ionizing energy not stated.
Table I1 : Mass Spectrometric Appearance Potentials of Ions Arising from HNCO Ion
-1.504
relative to m/e 43 aooording to-
This d e
I
Appearanoe potentials in eV according to This work Ref a
12.15 16.66 15.76 15.76 17.26
0.05
11.60b
