J. Phys. Chem. 1980, 84, 2503-2506
References and Notes (1) (a) P. L. Hanst, L. L. Spiller, D. M. Watts, J. W. Spence, and M. F. Miller, J. A t Polht. Confrol. Assoc., 25, 1220 (1975);(b) A. L. Torres, P. J. Maroulis, A. 8. Goldberg, and A. R. Bandy, Trans. Am. Geophys. Union, 58, 1082 (1978). (2)E. C. Y. Inn, J. F. Vsdder, B. J. Tyson, and D. Ohara, Geophys. Res. Lett., 6. 191 (1979). (3) P. J. Crutren, Geophys. Res. Left., 3, 73 (1976). (4) M. J. Kurylo, Chem. Phys. Lett., 58, 238 (1978). (5) R. Atkinson, R. A. Perry, and J. N. Pitts, Jr., Chem. Phys. Left., 54,
14 (1978). (6) R. D. Hudson and E. I. Reed, Eds., "The Stratosphere: Present and Future", NASA Reference Publlcation No. 1049,1979. (7) R. S. Iyer and F. S.Rowland, 14th Informal Conference on Photochemistry, Newport Beach, CA, April 1980,paper No. J4. (8) R. A. Cox and D. Sheppard, Nature (London), 284, 330 (1980). (9) H. W. Biermann, G. W. Harris, and J. N. Pitts, Jr., 14th Informal Conference on Photochemistty,Newport Beach, CA, April 1980,paper No. P2.
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(10) M. J. Kurylo and A. H. Laufer, J. Chem. Phys., 70, 2032 (1979). (11) F. J. Sandallsand S.A. Penkett, Afmos. Environ., 11, 197 (1977). (12) N. D. Sze and M. K. W. KO, Nature (London), 278, 731 (1979). (13) J. A. Logan, M. B. McElroy, S.C. Wofsy, and M. J. Prather, Nature (London), 281, 185 (1979). (14)R. P. Turco, R. C. Whitten, 0. B. Toon, E. C. Y. Inn, and P. Hamill,
J. Geophys. Res., submitted for publication. (15)P. H. Wine, N. M. Kreutter, and A. R. Ravishankara, J. Phys. Chem., 83, 3191 (1979),hnd references therein. (16) A. B. Callear, Proc. R . SOC. London, Ser. A , 276, 401 (1983). (17) S.W. Benson, "Themchemical Kinetics: Methods for the Estimation of Thermochemical Data and Rate Parameters", Wlley, New York,
1968. (18) R. F. Hampson, Jr., and D. Gamin, Eds., NBS Spec. Publ. ( U . S . ) , No. 513 (19771. (19)J. W. Rababls, i R. MCDOWI, v. Scheer, and s. P. McGlynn, ctwm. Rev., 71, 73 (1971). (20)J. 0.Calved and J. N. Pitts., Jr., "Photochemistry", Wlley, New York, 1966.
Acidity, Basicity, and Ion/Molecule Reactions of Isocyanic Acid in the Gas Phase by Ion Cyclotron Resonance Spectroscopy Charles A. Wight and J. L. Beauchamp' Arthur Amos Noyes Laboratory of Chemical Physics, California Institute of Technology, Pasadena, California 9 1125 (Received: April 7, 1980)
The acidity, basicity, and ion/molecule reactions of isocyanic acid have been examined in the gas phase by using the techniques of ion cyclotron resonance spectroscopy. Proton-transfer equilibria yield an acidity D(OCN--H+) = 344.7 f 2 kcal/mol for isocyanic acid, relative to D(F-H') = 371.3 kcal/mol. If one uses available thermodynamic data which give D(0CN-H) = 114.5 f 3 kcal/mol, the measured gas-phase acidity yields EA(0CN) = 3.6 f 0.2 eV for the electron affinity of cyanato radical. The base strength of isocyanic acid is comparable to formaldehyde with PA(HNC0) = 181.3 f 2 kcal/mol relative to PA(NH3) = 207.0 kcal/mol. Protonation on nitrogen is inferred from experiments designed to probe proton binding sites of HNCO. Several reactions which lead to formation of cyanate ion (OCN-) in gas mixtures were investigated.
Introduction While spectroscopic evidence for the existence of cyanic acid (HOCN) has been observed in condensed media,l it is known that this compound exists entirely as isocyanic acid (HNCO) in the gas phase. Infrared2 and microwave3 spectra have shown the molecule to be a slightly asymmetric rotor having a nearly linear OCN group. The CNH bond angle of 134" is consistent with sp2hybridization of the nitrogen orbitals. The dipole moment4 is 1.592 f 0.01 D, with the negative charge concentrated at the oxygen end of the molecule. The photoelectron spectrum of "COS indicates that the parent ion has a structure very similar to that of the neutral since the lowest energy ionization is from the out-of-plane nonbonding T orbital a". In addition to the expected fragments NH+ and OCN', the mass spectrum exhibits a peak corresponding to HCO+.637 The latter species has been identified as the daughter ion in the metastable decomposition of HNCO+, leading some workers to postulate the rearrangement of the parent ion to a cyclic structure prior to decomposition. An alternative explanation involving spin-forbidden predissociation has been offered by otherselgto account for the presence of HCO+. It has been demonstrated that ion/molecule reactions play an important role in the synthesis of organic molecules in extraterrestrial environments.1° Although rotational 0022-3654/80/2084-2503$01 .OO/O
transitions attributed to HNCO have been observed in cometdl and interstellar clouds,12very little is known about the gas-phase ion chemistry of isocyanic acid. We wish to report here the ion/molecule reactions of isocyanic acid observed in an ion cyclotron resonance spectrometer. Study of its proton-transfer reactions has allowed the determination of the basicity of HNCO as well as the site of protonation. Similarly, the acidity of HNCO and the electron affinity of OCN radical have been determined by examination of proton-transfer reactions involving negative ions. Some other ion/molecule reactions leading to production of cyanate ion (OCN-) have also been investigated.
Experimental Section Ammonia and carbon dioxide (Baker Chemical) and formic acid (Allied Chemical) were used as supplied except for several freezepump-thaw cycles to degas the samples. Formaldehyde was prepared by heating paraformaldehyde (Matheson) under vacuum and allowing the monomer to distill into an evacuated sample bulb. Isocyanic acid was prepared by gently heating a small amount of cyanuric acid in a sealed Pyrex ampule under vacuum until the solid just disappeared. The ampule was broken under vacuum and distilled to remove carbon dioxide impurity. A mass spectrum indicated C 0 2 as the only detectable impurity, comprising less than 2% of the sample. Methyl nitrite was prepared from sodium nitrite and methanol13in acid so0 1980 American
Chemical Society
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The Journal of Physical Chemistry, Vol. 84, No. 20, 1980
Wight and Beauchamp
TABLE I: Major Ions Appearing in 70-eV Mass Spectra
1.0
of Isocvanic Acid
relative abundance
m/z
this work
44 43 42 29 28 15
2.6 100.0 15.5 17.7 16.1
0.5
ref 5 1.8
-w-
100.0 21.7 13.8 6.5 6.9
H
1
TABLE 11: Reaction Sequences Leading to Formation of Protonated Isocyanic Acid
-
no. reaction 1 CO' + HNCO HCO' + OCN 2 HCO' + HNCO H,NCO+ + CO 3 OCN+ + HNCO -+ HNCO+ t OCN 4 HNCO' + HNCO-. H,NCO+ + OCN 5 CD; + HNCO --t HDNCO' + CH, 6 C,D,' + HNCO -+ HDNCO' + C,D, 7 HDNCO+ + HNCO H,NCO' + DNCO --f
--f
a
Rate constants are in units of
0.2
\
U
0.05
ha 21.6 13.4 10.6 8.3 10.4 6.7 4.1
0.1
E
Y
?
4
f
2
-L
*2
0.020
~t 2
f
2
*2 *1
cm3 molecule-' s-',
400
200
Time (ms)
Figure 1. Temporal variation of ion abundance in HNCO at 9.8 X lo-' torr following ionization by a 70-eV, 10-ms electron beam pulse. I .o
I
lution. Most of the NO2 impurity was removed by distillation and did not interfere with the ion chemistry investigated here. ICR instrumentation and experimental techniques have been previously described in detail.14J5 Methods for obtaining equilibrium constants for proton-transfer reactions have also been discussed in detail elsewhere.16J7 All experiments were conducted at 298 K. Neutral pressures were determined by using a Schulz-Phelps ionization gauge calibrated at higher pressures against an MKS Instruments Model 90H1-E capacitance manometer for each reagent gas.
Results Mass Spectrum and Ion/Molecule Reactions of Positive Ions from HNCO. When isocyanic acid undergoes 70-eV electron impact ionization, several fragment ions are observed in addition to HNCO'. Table I compares the relative ion abundances observed in this study with those reported using conventional mass spectrometers. The results are comparable except for a somewhat larger peak at m/z 28 in the ICR spectrum and the absence of the peak at m1.z 15. With the ICR spectrometer set up for trapped-ion studies, reactions 1-4 were observed leading to the proCO+ + HNCO HCO+ + OCN (1) HCO' + HNCO H2NCO+ + CO (2) OCN+ + HNCO HNCO+ + OCN (3) (4) HNCO' + HNCO H2NCO+ + OCN duction of H2NCO+as the only ion present at long trapping times. Figure 1 shows a semilogarithmic plot of normalized ion abundance vs. trapping time for the ions derived from isocyanic acid. All of the reaction pathways are confirmed by double resonance ejection of the reactant ions, and the rate constants are listed in Table 11. To determine the proton binding site of HNCO, a chemical ionization study using a large excess of CD4with HNCO was performed. Deuteration of HNCO by CD5+ and C2D5+in reactions 5 and 6 is followed by proton transfer to HNCO in reaction 7, indicating that deuteration CD5+ + HNCO HDNCO' + CD4 (5) C2D5+ + HNCO HDNCO' + C2D4 (6) (7) HDNCO' + HNCO H2NCO++ DNCO
---
--
-
Time (s)
Figure 2. Temporal variation of ion abundance in a 10: 1 mixture of CD4 and HNCO at a total pressure of 1.8 X 10" torr following a 7 0 4 , 10-ms electron beam pulse.
causes the proton to become labile in subsequent encounters with HNCO. Figure 2 illustrates the temporal variation of ion abundances for the most abundant species in the reaction system. The reaction rates are given in Table 11. Basicity of Isocyanic Acid. When a mixture of isocyanic acid and formaldehyde was ionized by 70-eV electrons, the two ions in reaction 8 were observed in equilibrium at H2COH++ HNCO
H2NCO++ H2C0
(8)
trapping times longer than 1 s. The forward and reverse reaction rate constants were determined by double resoand (3.8 f 0.8) nance techniques to be (2.6 f 0.5) X X cm3 molecule-l s-l, respectively. The equilibrium constantla K , = k f / k , = 6.8 f 2 yields a Gibbs free energy change for reaction 8 of AGO = -RT In K,, = -1.1 f 0.2 kcal/mol. Since most proton-transfer reactions do not involve major changes in molecular structure, ASo values are usually estimated by accounting for rotational sym-
The Journal of Physical Chemistry, Vol. 84, No. 20, 1980 2505
Isocyanic Acid in the Gas Phase
metry numbers only.17 If one uses eq 9 with B1 = HzCO
and B2 = HNCO and assumes that protonated isocyanic acid has a structure with C2,-type symmetry, the standard enthalpy change for reaction 8 is AHo = -2.0 f 0.5 kcal/mol. TJsing the known proton affinity of formaldehyde,17one calculates a value of PA(HNCO)= 181.3 f 2 kcal/mol relative to PA(",) = 207.0 f 2 kcal/mol.lg I t should be noted that proton affinity is defined here by using AHf(H+) = 365.7 kcal/mol." Acidity of Isocyanic Acid. Methyl-d, nitride is known to undergo dissociative electron attachment with thermal electrons to form CD30- and DNO-. Both of these ions react with HNCO in the ICR spectrometer to form OCN(eq 10 and 11) as the only persistent ion in the system at CD30- + HNCO .-* OCN- + CD3OH DNO- + HNCO
-
OCN- + HD
+ NO
TABLE 111: Reactions Leading to Formation of Cvanate Ion no. reaction
ha
1 0 CD,O- t HNCO-. OCN- t CD,OH 11 DNO- t HNCO-. OCN- t HD t NO 1 4 NH,- t CO, -+ OCN- t H,O 1 5 CN- t C,H40b .+ OCN- t C,H4 16 CN- t N,O+ OCN- t N,
0.9 * 0.2 1.1f 0.2 8.6 1.0
