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Isolated excited electronic states in the unimolecular gas-phase ion dissociation processes of the radical cations of isocyanogen and cyanogen
157
international Journal of Mass Spectrometry and Ion Processes, 103 (1991) 157-168 Elsevier Science Publishers B.V., Amsterdam
ISOLATED EXCITED ELECTRONIC STATES IN THE UNIMOLECULAR GAS-PHASE ION DISSOCIATION PROCESSES OF THE RADICAL CATIONS OF ISOCYANOGEN AND CYANOGEN
F. MATTHIAS BICKELHAUPT, NICO M.M. NIBBERING*
ROEL H. FOKKENS,
LEO J. DE KONING
and
Instituut voor Massaspectrometrie, Universiteit van Amsterdam, Nieuwe Achtergracht 129, 1018 WS Amsterdam (The Netherlands) EVERT JAN BAERENDS,
SIMON J. GOEDE and FRIEDRICH
BICKELHAUPT
Scheikundig Laboratorium der Vrije Universiteit, De Boelelaan 1083, 1081 HV Amsterdam (The Netherlands) (First received 29 March 1990; in final form 30 June 1990)
ABSTRACT The unimolecular gas-phase chemistry of CNCN+’ and NCCN+’ has been investigated by electron-impact mass spectrometry, mass-analysed ion kinetic energy mass spectrometry and a theoretical density-functional method. The CN+ formation appears to compete with the C:. formation in the microsecond time scale, in spite of an energy gap of 3.5 eV (CNCN) and 3.0eV (NCCN) between the two processes. This observation is explained by assuming that the fragmentation to CN+ proceeds via an isolated excited electronic state of the molecular ion. The C:. fragment ion must be formed from both isomeric Cz N:’ parent ions by a rearrangement reaction. While NCCN+’ is more stable than CNCN+’ by 0.6eV, the kinetic energies released in both rearrangements and theoretical calculations indicate that the corresponding intermediates are not only much closer in energy, but even reversed in energetic order with respect to their precursor ions. INTRODUCTION
There is considerable interest in the physics and chemistry of isocyanogen (CNCN) and its radical cation, because as one of the three linear CzN2 isomers (l-3) it was unknown until recently and because of its possible occurrence in interstellar space. Our major interest is the unimolecular gasphase chemistry of CNCN+’ . NEC-C=N 1
C=N-C-N
2
C=N-N=C
3
After Gay-Lussac [l] prepared cyanogen (NCCN; 1) for the first time, it took 173 years until, in 1988, Van der Does and Bickelhaupt [2] succeeded in 01681176/91/$03.50
0 1991 Elsevier Science Publishers B.V.
158
synthesizing the highly reactive isocyanogen (CNCN; 2) by flash vacuum thermolysis (FVT) of norbornadienonazine (4). I/ FVT 500 OC
~
/
C=N-C=N
+
2
(1) 0
/ 2 4
2
At first the structure of the FVT-product of reaction (1) was supposed to be that of diisocyanogen (3) [2,3]. However, since then high-resolution infrared and microwave (MW) studies by Stroh and co-workers [4,5], Yamada et al. [6] and Gerry et al. [7], electron diffraction (ED) experiments of Weiss et al. [8], high level theorectical calculations [9-121 and very recently X-ray diffraction measurements [13] have proved that the product is in fact isocyanogen. Thispaperpresentsthefirstmassspectrometric(MS)studyofisocyanogen(2), in which cyanogen (1) has been included. Moreover, for a better understanding, and to support the interpretation ofthe experimental results, theoreticalcalculations have been performed, usingadensity-functional MO-LCAOmethod commonly referred to as the Hartree-Fock-Slater (HFS) method [ 14-181. METHODS
Experimental The MS experiments were performed on a VG Analytical ZAB-2HF mass spectrometer [19] with reverse geometry, and the spectral data obtained were recordedon-line,storedandprocessedbytheVGl 1/250datasystem[19].CNCN [2]wasintroducedintothesourceviaaleakvalve, bygraduallyheatingthesample from - 196 “C to ca. - 115 “C using a pentane bath. NCCN (commercially available(Hoekloos);98.5%)wasintroducedintothesourceviaadirectgasinlet. Theion source was held at room temperatureup to a pressure ofca. 10v6 mbar, as indicated by the ion-gauge reading of the corresponding pump. Ionization was achieved by electron impact (EI). Most mass-analysed ion kinetic energy mass spectrometry (MIKES) experiments were performed without gas in thecollision cell, located in the region between the magnetic and electric sectors. Under a resolution of 10 000 at a 10 % valley definition, the signal at m/z 52 was shown to be a singlet and was confirmed to be from CzN,. The kinetic energy releases (KER) associated with the dissociation of the metastable ions, rn: + ml + m3, in the second field-free region (FFR) were measured by varying the electric sector voltage and using MIKE methodology [20]. The KER values, TO,s,were cal-
159
culated from the widths, d, of the peaks at half-height by the formula [21] T 0.5 =
(2)
where do and d are the widths of the main beam, m:, and the fragment ion peak, ml, respectively, in volts; UB and U, are the ion-accelerating voltage and the kinetic energy of the parent ions, m:, at the electric sector in volts, respectively. Appearance energies (AE) were obtained by the semi-log method of Morrison [22]. Consecutive two-step dissociations of parent ions located inside and after the collision cell in the second field-free region of the mass spectrometer were searched for by the application of the collision-cell potential dependence method as described by Maas and Nibbering [23]. The collision cell was filled with helium as target gas up to a pressure of 2 x 10e7mbar. Collisionally-induced dissociation (CID) spectra were acquired at collision-cell potentials in the range of approximately - 4000 to + 5OOOVwith intervals of % 1OOOV. Theoretical
The MOs were expanded in a large set of Slater-type orbitals (STOs). The basis is of double-l quality with two STOs per nl shell and a 3d polarization function added on each atom. Geometries were optimized with the simple Xa exchange-correlation potential [14] using gradient techniques [24]. For the systems 5,6 and 9 (see below) C,, point group symmetry was assumed. As the pure Xa-energies are too strongly bonding, the energy data reported have been obtained in the optimum geometry with more sophisticated densityfunctionals for exchange and correlation. Exchange is described with Slater’s p’j3 potential (Xa with CI= 2/3), with a non-local correction due to Becke [25-271. According to the suggestion by Stoll et al. [28], only correlation between electrons of different spin is introduced, for which electron-gas data (in the Vosko-Wilk-Nusair [29] parametrization) are used. Extensive studies [24, 30-321 show that with this approach interaction energies in systems involving main group elements and metals are described to an accuracy of a few tenths of an eV (of the order of 5 kcal mol-‘). Comparison of the results of this work with literature data indicates that the same order of accuracy holds for the present calculations. For example, our energy of isomerization of NCCN to CNCN (BE,,,) of 0.955 eV deviates by only - 0.126 eV from the CEPA-value of Botschwina et al. [12]. Furthermore, our permanent electric dipole moment of 0.670 D compares very well with the experimental value of 0.7074(52)D [7]. For comparison, Haese and Woods (CI-SD/DZP) calculated a dipole moment of 1.35 D [33] while the CEPA-value is 0.704D [12].
160 TABLE 1 EI and MIKES mass spectra, KER values (in meV) and AE values (in eV) for the CNCN and NCCN systems CNCN
NCCN
EI mass spectra C,N:’ C,N+ CN+ C,+. C+
100% 0 4 9 0
100 % 2 11 3 3
MIKES” C,‘N+ CN+ C,” C+
21% 31 100 10
54% 74 100 16
KER” CzN+ CN+ CC’
127meV 121 31
170meV 118 57
AE GN: (2:. C,N+ CN+
12.9eV 16.7 19.2b 20.T
13.4eV 18.2 20.2 21.2
“MIKES intensities and KER values are obtained from MIKES spectra which are averages over 100 scans. bCalculated [50]. There is no CzN+ signal detectable. “Calculated [50]. The AE of CNf from CNCN could not be determined because of a C2 H, impurity (ZE = 11.4 eV [36]) in the CNCN sample. This impurity, however, does not interfere with our determination of the AE of C:’ from CNCN, as the AE for C:’ from C,H, is considerably higher (reported values in ref. 37 are 18.2eV and 19SeV). RESULTS AND DISCUSSION
Table 1lists the mass spectrometric data obtained for CNCN and NCCN. The HFS-results are reported in Tables 2 (energies) and 3 (geometries). The main fragmentation processes according to the EI and MIKES mass spectra (Fig. 1) are the formation of C,+* (+ NJ and CNf (+ CN’). Other ions detected are C, N+ and C+ ’ but no signals from CN, +’ and N+ are observed in the EI mass spectrum of NCCN, in agreement with the results of Smith [34]. However, small peaks (less thanca. 5%) due to these ions and also NC ’ are detected in the MIKES spectrum. In accordance with EI literature data for NCCN [35], the peak due to
161 TABLE 2 HFS-energies (in eV) of Cz N2 systems CNCN
NCCN
Zsomerizution (relative energies)
A&o(M) bEis0 (M + ‘) AEisO(intermediateb)
0.955 0.615 -0.030
0 0 0
Rearrangement
M + + intermediateb
2.74
3.38
12.59 12.81 12.86
12.93 13.04 13.98
Ionization
ZE(adiabatic, n-‘) ZE(vertica1, K’) ZE(vertica1, a-‘) “NCCN+’
(n-l), CNCN+' (n-I). b5 in the case of NCCN, 6 in the case of CNCN.
C,+. (3%) is lessintense than that due to CN’ (1 1%), but for CNCN [2]the opposite is observed, i.e. the relative intensities of the peaks due to C:. and CN+ are 9% and 4%, respectively (see Table 1). This result is surprising, as the C, unit is already present in NCCN which is not the case for CNCN. TABLE 3 Bond lengths (in pm) in linear C,N, systems Method
CNCN
NCCN
C=N-
=N-_Cr
-C=N
NEC-
=c-_c-
119.0 119.1 118.1 117.5 118.1 119.5
129.4 131.8 132.2 131.4 131.2 130.0
117.2 117. 6 115.8 116.0 115.7 114.3
117.0 117.8 115.8 115.6’ -
135.7 138.1 139.5 138.8’ -
123.3
125.8
121.0
119.9
134.7
Neutrals
HFS MP2” CEPAb MW’ EDd X-ray’ Radical cation9
HFS
aNguyen (MP2/6-3 1lG*) [ 1I]. bBotschwina and Sebald [12]. “Stroh et al. [4]. dWeiss et al. [8]. eMorino et al. [38]. ‘Boese [ 131. pNCCN+’ (n-l), CNCN+’ (n-l).
162
CNCN+'
NCCN +*
Fig. 1. Metastable 70 eV EI-MIKES (CNCN+‘) and cyanogen (NCCN+‘).
mass spectra of the molecular ions of isocyanogen
C,+’ formation
In order to address the question of Cl* ion formation, the metastable fragmentations of the molecular ions CzN$’ (m/z 52) of both CNCN and NCCN in the second field-free region of the mass spectrometer have been studied. As shown in Table 1, fragmentation to C: ’ (+ NJ leads to the base peak in the MIKES spectra of both isomeric molecular ions. This process has to proceed via a rearrangement of the parent ions, as no consecutive two-step
163
NCCN +’ Fig. 2. Proposed qualitative features of the energetics of the rearrangement CNCN+’ and NCCN+’ leading to the formation of Ct. + N2 (see Eq. 3).
reaction
of
dissociations of both CNCN+’ and NCCN+’ leading to Cl * have been observed in the MIKES-CID spectra obtained by the collision-cell potential dependence method [23]. In Eq. 3 a mechanism for this rearrangement is presented which accounts for all experimental data summarized in Table 1.
+.
NCCN+‘1+* 5
\ C2+’ + N,
(3)
CNCN+’ 2 +* 6
After a 1,2-shift of a terminal nitrogen in l+’ and 2+‘, respectively, the intermediates 5 and 6 may decompose to C,+’ + N, by coupling of the two nitrogen or carbon atoms, respectively. As appears from Table 1, Cc’ formation with respect to all other fragmentations is considerably higher for CNCN+’ compared with NCCN+‘. Furthermore, the metastable formation of Cc* from NCCN+’ is accompanied by a kinetic energy release (KER) which is nearly twice as large as the KER accompanying the metastable CT ’ formation from CNCN+‘. In general, the KER value, T, has two main sources: the non-fixed energy (E’) leading to Tt and the reverse activation energy (Er) leading to T”‘. The total KER is given by T = Tz + Fe’ [20]. For metastable ion decompositions, including rearrangements, Et and, therefore
164
p, is usually negligible. Furthermore, it is well known that rearrangement reactions typically have reverse activation energy barriers, so that TS is small compared to T”“. As a consequence, T x T”’ holds and the higher KER for Cl ’ formation from NCCN+’ may be related to a higher reverse activation energy (Ey(Cl’ + N2 + 5) > Er CC,‘*+ N, + 6)) [20]. If it is assumed that both rearrangement processes furnish the products C,+* + N, in the same electronic (ground) state, this means that the transition state (TS) for the dissociation of 6 is (slightly) lower in energy than the TS for dissociation of 5. Furthermore, the “CNCN+’ intermediate” 6 is calculated to be 0.03 eV more stable than the “NCCN+’ intermediate” 5, as can be seen in Table 2. Of course, 0.03 eV is a very small energy difference. Nevertheless, the qualitative picture tits well with the Hammond postulate [39] which for mechanistically similar reactions predicts a higher energy TS when the energy of the reactants (intermediates 5 or 6) rises (see Fig. 2). The different KER values for the C:’ formation from CNCN+’ and NCCN+’ also exclude the occurrence of a common intermediate in the microsecond time scale, such as 5 which, in the case CNCN+‘, could be formed by a 1,2-shift of the terminal carbon atom, i.e. 5 and 6 are supposed not to interconvert before fragmentation. Further support for the mechanism is provided by the geometries of the intermediates 5 and 6, as inferred from our HFS-calculations, which fit in well with dissociation to the products C,‘* + N, as the subsequent and final step. The CN-distance which gives the separation between the two C2- and N,units, is already rather large, while the distance between the two atoms that are to be coupled is quite short (5; CC = 136.7 pm, CN = 138.4 pm, NN = 130.9pm, 6; NN = 114.5pm, CN = 152.1 pm, CC = 127.5pm). Moreover, the major part of the positive charge is concentrated on the C, fragment in both cases (Q,,, in5) = + 0.91 electron; Q(c-, 6) = + 0.72 electron). For the sake of completeness, another conceivable rearrangement mechan-
c C2+’ + N2
NCCN+'1+-
C-N CNCN+‘2+’
+
InI
N-C
+* 1
NrN7C i TS 11
(4)
165
ism leading to C2+. formation and presented in Eq. 4 should be discussed. After a 1,4 ring closure, the cyclic structures 9 or 10 could decompose to the products Cl. + N,. In this mechanism, C,’ ’ formation from 2+’ would proceed via the tetrahedral, and probably highly energetic, transition state (TS) 11,making this reaction path very unattractive relative to that proposed in Eq. 3. Therefore, and for economic reasons, no calculations on 10 and 11 have been performed. However, 9 might be a suitable structure for a low energy reaction path leading to the products Cc ’ + N, by simple bond-breaking between the C2 and N2 fragments. However, calculations on 9 show that the energy of this structure is about 1.5 eV higher than that of the NCCN intermediate 5. We therefore conclude that the most likely mechanism for Cl. formation from CNCN+’ and NCCN+’ from those discussed is that shown in Eq. 3. CN’ formation In contrast to Cl’ formation, there is no need for rearrangement when CNCN+’ and NCCN+’ decompose to CN+ + CN’ . In fact, the corresponding direct cleavage reaction for CN+ formation is expected to be the dominating process at high energy, because it is well known that the specific frequency factor in the expression for the unimolecular rate constant, k (E), is higher for direct cleavage processes than for reactions which prevail at low energy [20, 40-421. As shown in Table 1, the appearance energy (AE) measurements for CN+ formation from CNCN and NCCN indicate that the corresponding critical energies are indeed 3.5 and 3.0 eV, respectively, higher than those for C,” formation, Most surprisingly, however, is that, notwithstanding this considerable energy gap, the two reaction pathways are found to compete with each other in the metastable time window (10-6-10-5 s), as can be seen from the MIKES data in Table 1. This delay of the high energy fragmentation can be understood by assuming that the two processes of CN+ and C,+’ formation take place from molecular ions that are generated in the ion source in different electronic states. As a consequence, one must assume that the two reaction channels can be in competition with each other because of the occurrence of molecular ions in isolated excited electronic states! Alternatively, this competition could be explained in terms of crossing k(E) vs. E curves for the two dissociation processes. However, crossing k(E) vs. E curves for this system imply kinetic shifts for the Ct ’ formation of 3.5-3.0 eV. To account for such large values, frequency factors of about 10’s_’ are needed, two orders of magnitude below the lowest value for rearrangement processes proposed by Levsen [40]. These considerations make the hypothesis of crossing k(E) vs. E curves very unlikely. Moreover, this alternative mechanism can not explain why Cl ’ and CN+ have comparable intensities in the
166
metastable MIKES spectra (Fig. 1). A slowly increasing C:’ curve would have an intersection with the metastable window which is much larger than that for the steep CN+ curve. As a consequence, the C,+* forming process would contribute to the metastable dissociation of the molecular ion over a much larger interval on the internal energy scale, which in its turn, would result in a strongly dominating C: ’ formation. Electronically-excited (NCCN+‘)* is known not to decay radiatively [43], so that non-radiative transitions are the only means of relaxation. Because of the close similarity, the same may be true for (CNCN+‘)*. From photoelectron-photoion (PEPICO) and photoion-fluorescence-photon (PIFCO) coincidence experiments, it is known that excited electronic states or rovibronic levels of the homologous series of XCN+’ open-shell cations (X = Cl, Br, I) have lifetimes in the microsecond range [44-47]. From emission spectral data of XCN+’ systems excited in the gas phase by electron impact, Allan and Maier [48] have determined lifetimes of 4.4 ps and 0.205 ps for the A 2Z+ state and the B211 state of ClCN+‘, respectively. These values have been confirmed by more recent PEPICO and PIFCO studies [45-461. In spite of the obvious structural and spectroscopic differences, CNCN+’ and NCCN+’ may also be considered as members of the XCN+’ series (X = the pseudo-halogens CN and NC, respectively). Consequently, lifetimes of the order of microseconds for the A and B excited states of CNCN+’ and NCCN+’ could be expected and could explain why the high energy fragmentation to CN+ + CN’ competes with C: ’ + N2 production in the metastable time window. We therefore propose that CN+ formation proceeds via such an isolated excited electronic state of the molecular ion ((M+‘)*). Possible candidates for such a state are found in the photoelectron spectra (PES) of CNCN (IEs: 12.873 (a-‘), 12.911 (7~~I), 14.390 (6-l) and 16.460eV (rr-‘) [3]) and NCCN (IEs: 13.36 (rrg’), 14.49 (a;‘), 14.86 (a;‘) and 15.47eV (7~;‘) [49]). These PES spectra prove the existence of three bonding excited electronic states of the molecular ion. Fragmentation of (M+‘)* to CNf + CN’ would then take place by predissociation to the vibrational continuum of the ground state or by predissociation to a yet unknown repulsive electronic state. Both possibilities can account for the relatively high KER values of 121 and 118 meV for CN+ formation from CNCN+ ’ and NCCN+ ‘, respectively (see Table 1). Reaction scheme 5 summarizes this proposed mechanism. For comparison, C,+* formation is included in this scheme.
E1
M+-
rearrangement
*
Cz+’ + N2
*
CN+
l-
predissociation
+
CN'
CN+ + CN’
(5)
167 ACKNOWLEDGEMENT
We thank Dr. J.G. Snijders for helpful discussions and Mr. P. Vernooijs for help with the calculations. REFERENCES 1 L.J. Gay-Lussac, Ann. Chim. (Paris), 95 (1815) 175. 2 T. van der Does and F. Bickelhaupt, Angew. Chem., 100 (1988) 998. 3 0. Grabandt, C.A. de Lange, R. Mooyman, T. van der Does and F. Bickelhaupt, Chem. Phys. Lett., 155 (1989) 221. 4 F. Stroh and M. Winnewisser, Chem. Phys. Lett., 155 (1989) 21. 5 F. Stroh, B.P. Winnewisser, M. Winnewisser, H.P. Reisenauer, G. Maier, S.J. Goede and F. Bickelhaupt, Chem. Phys. Lett., 160 (1989) 105. 6 K.M.T. Yamada, M.W. Markus, G. Winnewisser, W. Joentgen, R. Kock, E. Vogel and H.-J. Altenbach, Chem. Phys. Lett., 160 (1989) 113. 7 M.C.L. Gerry, F. Stroh and M. Winnewisser, J. Mol. Spectrosc., 140 (1990) 147. 8 I. Weiss, H. Oberhammer, S.J. Goede, P.J.K.M. Eeken and F. Bickelhaupt, personal communication (1989). 9 L.S. Cederbaum, F. Tarantelli, H.-G. Weikert, M. Scheller and H. Kiippel, Angew. Chem., 101 (1989) 770. 10 M.K. Scheller, H.G. Weikert, L.S. Cederbaum and F. Tarantelli, J. Electron Spectrosc., 51 (1990) 75. 11 M.T. Nguyen, Chem. Phys. Lett., 157 (1989) 430. 12 P. Botschwina and P. Sebald, Chem. Phys., 141 (1990) 311. 13 R. Boese, personal communication (1990). 14 J.C. Slater, Quantum Theory of Molecules and Solids, Vol. 4, McGraw-Hill, New York, 1974. 15 E.J. Baerends, D.E. Ellis and P. Ros, Chem. Phys., 2 (1973) 41. 16 E.J. Baerends and P. Ros, Chem. Phys., 2 (1973) 52. 17 E.J. Baerends and P. Ros, Chem. Phys., 8 (1975) 412. 18 W. Heijser, A. Th. van Kessel and E.J. Baerends, Chem. Phys., 16 (1976) 371. 19 VG Analytical Ltd., Wythenshawe, Manchester M23 9LE, U.K. 20 R.G. Cooks, J.H. Beynon, R.M. Caprioli and G.R. Lester, Metastable Ions, Elsevier, Amsterdam, 1973. 21 A.-M. Dommrijse and H.-Fr. Grtitzmacher, Int. J. Mass Spectrom. Ion Processes, 76 (1987) 95. 22 J.D. Morrison, J. Chem. Phys., 19 (1951) 1305. 23 W.P.M. Maas and N.M.M. Nibbering, Int. J. Mass Spectrom. Ion Processes, 95 (1989) 171. 24 L. Versluis and T. Ziegler, J. Chem. Phys., 88 (1988) 322. 25 A.D. Becke, Int. J. Quantum Chem., 23 (1983) 1915. 26 A.D. Becke, J. Chem Phys., 85 (1986) 7184. 27 T. Ziegler, V. Tschinke and A. Becke, Polyhedron, 6 (1987) 685. 28 H. Stoll, E. Golka and H. Preus, Theor. Chim. Acta, 55 (1980) 29. 29 S.H. Vosko, L. Wilk and M. Nusair, Can. J. Phys., 58 (1980) 1200. 30 T. Ziegler, V. Tschinke and C. Ursenbach, J. Am. Chem. Sot., 109 (1987) 4825. 31 T. Ziegler, V. Tschinke, L. Versluis and E.J. Baerends, Polyhedron, 7 (1988) 1625. 32 L. Fan and T. Ziegler. J. Chem. Phys., 92 (1990) 3645.
168 33 N.N. Haese and R.C. Woods, J. Chem. Phys., 73 (1980) 4521. 34 0.1. Smith, Int. J. Mass Spectrom. Ion Processes, 54 (1983) 55. 35 NBS Library Compilation, disk library number 55, Finnigan Corporation, San Jose, CA, 1984. 36 S.G. Lias, J.E. Bartmess, J.F. Liebman, J.L. Holmes, R.D. Levin and W.G. Mallard, J. Phys. Chem. Ref. Data, 17 (suppl. 1) (1988). 37 H.H. Rosenstock, K. Draxl, B.W. Steiner and J.T. Herron, J. Phys. Chem. Ref. Data, 6 (suppl. 1) (1977). 38 Y. Morino, K. Kuchitsu, Y. Hori and M. Tanimoto, Bull. Chem. Sot. Jpn. 41 (1968) 2349. 39 G.S. Hammond, J. Am. Chem. Sot., 77 (1955) 334. 40 K. Levsen, Fundamental Aspects of Organic Mass Spectrometry, Verlag Chemie, Weinheim, 1978. 41 W. Forst, Theory of Unimolecular Reactions, Academic, New York, 1973, Chapter 2. 42 B.J. McClelland, Statistical Thermodynamics, Chapman and Hall, London, 1973, Chapter 12. 43 J. Falura, S. Leutwyler, J.P. Maier and U. Spittel, J. Phys. Chem., 89 (1985) 3190. 44 J.P. Maier and F. Thommen, in M.T. Bowers (Ed.), Gas Phase Ion Chemistry, Academic, Orlando, 1984, Chap. 25. 45 J.P. Maier, M. Ochsner and F. Thommen, Faraday Discuss. Chem. Sot., 75 (1983) 77. 46 0. Braitbart, E. Castellucci, G. Dujardin and S. Leach, J. Phys. Chem., 89 (1985) 3252. 47 S. Leutwyler, J.P. Maier and U. Spittel, J. Chem. Phys., 83 (1985) 506. 48 M. Allan and J.P. Maier, Chem. Phys. Lett., 41 (1976) 231. 49 J.M. Hollas and T.A. Sutherley, Mol. Phys., 24 (1972) 1123. 50 The AE values for C: and CN+ formation from CNCN are calculated by subtracting our HFS energy difference of 0.955 eV between NCCN and CNCN (Table 2) from the corresponding AE values for NCCN. This procedure leads to a value of 17.2 eV for the AE of C:’ from CNCN. The deviation of + 0.5 eV from the measured value is within the experimental error obtained for our AE measurements.
international Journal of Mass Spectrometry and Ion Processes, 103 (1991) 157-168 Elsevier Science Publishers B.V., Amsterdam
ISOLATED EXCITED ELECTRONIC STATES IN THE UNIMOLECULAR GAS-PHASE ION DISSOCIATION PROCESSES OF THE RADICAL CATIONS OF ISOCYANOGEN AND CYANOGEN
F. MATTHIAS BICKELHAUPT, NICO M.M. NIBBERING*
ROEL H. FOKKENS,
LEO J. DE KONING
and
Instituut voor Massaspectrometrie, Universiteit van Amsterdam, Nieuwe Achtergracht 129, 1018 WS Amsterdam (The Netherlands) EVERT JAN BAERENDS,
SIMON J. GOEDE and FRIEDRICH
BICKELHAUPT
Scheikundig Laboratorium der Vrije Universiteit, De Boelelaan 1083, 1081 HV Amsterdam (The Netherlands) (First received 29 March 1990; in final form 30 June 1990)
ABSTRACT The unimolecular gas-phase chemistry of CNCN+’ and NCCN+’ has been investigated by electron-impact mass spectrometry, mass-analysed ion kinetic energy mass spectrometry and a theoretical density-functional method. The CN+ formation appears to compete with the C:. formation in the microsecond time scale, in spite of an energy gap of 3.5 eV (CNCN) and 3.0eV (NCCN) between the two processes. This observation is explained by assuming that the fragmentation to CN+ proceeds via an isolated excited electronic state of the molecular ion. The C:. fragment ion must be formed from both isomeric Cz N:’ parent ions by a rearrangement reaction. While NCCN+’ is more stable than CNCN+’ by 0.6eV, the kinetic energies released in both rearrangements and theoretical calculations indicate that the corresponding intermediates are not only much closer in energy, but even reversed in energetic order with respect to their precursor ions. INTRODUCTION
There is considerable interest in the physics and chemistry of isocyanogen (CNCN) and its radical cation, because as one of the three linear CzN2 isomers (l-3) it was unknown until recently and because of its possible occurrence in interstellar space. Our major interest is the unimolecular gasphase chemistry of CNCN+’ . NEC-C=N 1
C=N-C-N
2
C=N-N=C
3
After Gay-Lussac [l] prepared cyanogen (NCCN; 1) for the first time, it took 173 years until, in 1988, Van der Does and Bickelhaupt [2] succeeded in 01681176/91/$03.50
0 1991 Elsevier Science Publishers B.V.
158
synthesizing the highly reactive isocyanogen (CNCN; 2) by flash vacuum thermolysis (FVT) of norbornadienonazine (4). I/ FVT 500 OC
~
/
C=N-C=N
+
2
(1) 0
/ 2 4
2
At first the structure of the FVT-product of reaction (1) was supposed to be that of diisocyanogen (3) [2,3]. However, since then high-resolution infrared and microwave (MW) studies by Stroh and co-workers [4,5], Yamada et al. [6] and Gerry et al. [7], electron diffraction (ED) experiments of Weiss et al. [8], high level theorectical calculations [9-121 and very recently X-ray diffraction measurements [13] have proved that the product is in fact isocyanogen. Thispaperpresentsthefirstmassspectrometric(MS)studyofisocyanogen(2), in which cyanogen (1) has been included. Moreover, for a better understanding, and to support the interpretation ofthe experimental results, theoreticalcalculations have been performed, usingadensity-functional MO-LCAOmethod commonly referred to as the Hartree-Fock-Slater (HFS) method [ 14-181. METHODS
Experimental The MS experiments were performed on a VG Analytical ZAB-2HF mass spectrometer [19] with reverse geometry, and the spectral data obtained were recordedon-line,storedandprocessedbytheVGl 1/250datasystem[19].CNCN [2]wasintroducedintothesourceviaaleakvalve, bygraduallyheatingthesample from - 196 “C to ca. - 115 “C using a pentane bath. NCCN (commercially available(Hoekloos);98.5%)wasintroducedintothesourceviaadirectgasinlet. Theion source was held at room temperatureup to a pressure ofca. 10v6 mbar, as indicated by the ion-gauge reading of the corresponding pump. Ionization was achieved by electron impact (EI). Most mass-analysed ion kinetic energy mass spectrometry (MIKES) experiments were performed without gas in thecollision cell, located in the region between the magnetic and electric sectors. Under a resolution of 10 000 at a 10 % valley definition, the signal at m/z 52 was shown to be a singlet and was confirmed to be from CzN,. The kinetic energy releases (KER) associated with the dissociation of the metastable ions, rn: + ml + m3, in the second field-free region (FFR) were measured by varying the electric sector voltage and using MIKE methodology [20]. The KER values, TO,s,were cal-
159
culated from the widths, d, of the peaks at half-height by the formula [21] T 0.5 =
(2)
where do and d are the widths of the main beam, m:, and the fragment ion peak, ml, respectively, in volts; UB and U, are the ion-accelerating voltage and the kinetic energy of the parent ions, m:, at the electric sector in volts, respectively. Appearance energies (AE) were obtained by the semi-log method of Morrison [22]. Consecutive two-step dissociations of parent ions located inside and after the collision cell in the second field-free region of the mass spectrometer were searched for by the application of the collision-cell potential dependence method as described by Maas and Nibbering [23]. The collision cell was filled with helium as target gas up to a pressure of 2 x 10e7mbar. Collisionally-induced dissociation (CID) spectra were acquired at collision-cell potentials in the range of approximately - 4000 to + 5OOOVwith intervals of % 1OOOV. Theoretical
The MOs were expanded in a large set of Slater-type orbitals (STOs). The basis is of double-l quality with two STOs per nl shell and a 3d polarization function added on each atom. Geometries were optimized with the simple Xa exchange-correlation potential [14] using gradient techniques [24]. For the systems 5,6 and 9 (see below) C,, point group symmetry was assumed. As the pure Xa-energies are too strongly bonding, the energy data reported have been obtained in the optimum geometry with more sophisticated densityfunctionals for exchange and correlation. Exchange is described with Slater’s p’j3 potential (Xa with CI= 2/3), with a non-local correction due to Becke [25-271. According to the suggestion by Stoll et al. [28], only correlation between electrons of different spin is introduced, for which electron-gas data (in the Vosko-Wilk-Nusair [29] parametrization) are used. Extensive studies [24, 30-321 show that with this approach interaction energies in systems involving main group elements and metals are described to an accuracy of a few tenths of an eV (of the order of 5 kcal mol-‘). Comparison of the results of this work with literature data indicates that the same order of accuracy holds for the present calculations. For example, our energy of isomerization of NCCN to CNCN (BE,,,) of 0.955 eV deviates by only - 0.126 eV from the CEPA-value of Botschwina et al. [12]. Furthermore, our permanent electric dipole moment of 0.670 D compares very well with the experimental value of 0.7074(52)D [7]. For comparison, Haese and Woods (CI-SD/DZP) calculated a dipole moment of 1.35 D [33] while the CEPA-value is 0.704D [12].
160 TABLE 1 EI and MIKES mass spectra, KER values (in meV) and AE values (in eV) for the CNCN and NCCN systems CNCN
NCCN
EI mass spectra C,N:’ C,N+ CN+ C,+. C+
100% 0 4 9 0
100 % 2 11 3 3
MIKES” C,‘N+ CN+ C,” C+
21% 31 100 10
54% 74 100 16
KER” CzN+ CN+ CC’
127meV 121 31
170meV 118 57
AE GN: (2:. C,N+ CN+
12.9eV 16.7 19.2b 20.T
13.4eV 18.2 20.2 21.2
“MIKES intensities and KER values are obtained from MIKES spectra which are averages over 100 scans. bCalculated [50]. There is no CzN+ signal detectable. “Calculated [50]. The AE of CNf from CNCN could not be determined because of a C2 H, impurity (ZE = 11.4 eV [36]) in the CNCN sample. This impurity, however, does not interfere with our determination of the AE of C:’ from CNCN, as the AE for C:’ from C,H, is considerably higher (reported values in ref. 37 are 18.2eV and 19SeV). RESULTS AND DISCUSSION
Table 1lists the mass spectrometric data obtained for CNCN and NCCN. The HFS-results are reported in Tables 2 (energies) and 3 (geometries). The main fragmentation processes according to the EI and MIKES mass spectra (Fig. 1) are the formation of C,+* (+ NJ and CNf (+ CN’). Other ions detected are C, N+ and C+ ’ but no signals from CN, +’ and N+ are observed in the EI mass spectrum of NCCN, in agreement with the results of Smith [34]. However, small peaks (less thanca. 5%) due to these ions and also NC ’ are detected in the MIKES spectrum. In accordance with EI literature data for NCCN [35], the peak due to
161 TABLE 2 HFS-energies (in eV) of Cz N2 systems CNCN
NCCN
Zsomerizution (relative energies)
A&o(M) bEis0 (M + ‘) AEisO(intermediateb)
0.955 0.615 -0.030
0 0 0
Rearrangement
M + + intermediateb
2.74
3.38
12.59 12.81 12.86
12.93 13.04 13.98
Ionization
ZE(adiabatic, n-‘) ZE(vertica1, K’) ZE(vertica1, a-‘) “NCCN+’
(n-l), CNCN+' (n-I). b5 in the case of NCCN, 6 in the case of CNCN.
C,+. (3%) is lessintense than that due to CN’ (1 1%), but for CNCN [2]the opposite is observed, i.e. the relative intensities of the peaks due to C:. and CN+ are 9% and 4%, respectively (see Table 1). This result is surprising, as the C, unit is already present in NCCN which is not the case for CNCN. TABLE 3 Bond lengths (in pm) in linear C,N, systems Method
CNCN
NCCN
C=N-
=N-_Cr
-C=N
NEC-
=c-_c-
119.0 119.1 118.1 117.5 118.1 119.5
129.4 131.8 132.2 131.4 131.2 130.0
117.2 117. 6 115.8 116.0 115.7 114.3
117.0 117.8 115.8 115.6’ -
135.7 138.1 139.5 138.8’ -
123.3
125.8
121.0
119.9
134.7
Neutrals
HFS MP2” CEPAb MW’ EDd X-ray’ Radical cation9
HFS
aNguyen (MP2/6-3 1lG*) [ 1I]. bBotschwina and Sebald [12]. “Stroh et al. [4]. dWeiss et al. [8]. eMorino et al. [38]. ‘Boese [ 131. pNCCN+’ (n-l), CNCN+’ (n-l).
162
CNCN+'
NCCN +*
Fig. 1. Metastable 70 eV EI-MIKES (CNCN+‘) and cyanogen (NCCN+‘).
mass spectra of the molecular ions of isocyanogen
C,+’ formation
In order to address the question of Cl* ion formation, the metastable fragmentations of the molecular ions CzN$’ (m/z 52) of both CNCN and NCCN in the second field-free region of the mass spectrometer have been studied. As shown in Table 1, fragmentation to C: ’ (+ NJ leads to the base peak in the MIKES spectra of both isomeric molecular ions. This process has to proceed via a rearrangement of the parent ions, as no consecutive two-step
163
NCCN +’ Fig. 2. Proposed qualitative features of the energetics of the rearrangement CNCN+’ and NCCN+’ leading to the formation of Ct. + N2 (see Eq. 3).
reaction
of
dissociations of both CNCN+’ and NCCN+’ leading to Cl * have been observed in the MIKES-CID spectra obtained by the collision-cell potential dependence method [23]. In Eq. 3 a mechanism for this rearrangement is presented which accounts for all experimental data summarized in Table 1.
+.
NCCN+‘1+* 5
\ C2+’ + N,
(3)
CNCN+’ 2 +* 6
After a 1,2-shift of a terminal nitrogen in l+’ and 2+‘, respectively, the intermediates 5 and 6 may decompose to C,+’ + N, by coupling of the two nitrogen or carbon atoms, respectively. As appears from Table 1, Cc’ formation with respect to all other fragmentations is considerably higher for CNCN+’ compared with NCCN+‘. Furthermore, the metastable formation of Cc* from NCCN+’ is accompanied by a kinetic energy release (KER) which is nearly twice as large as the KER accompanying the metastable CT ’ formation from CNCN+‘. In general, the KER value, T, has two main sources: the non-fixed energy (E’) leading to Tt and the reverse activation energy (Er) leading to T”‘. The total KER is given by T = Tz + Fe’ [20]. For metastable ion decompositions, including rearrangements, Et and, therefore
164
p, is usually negligible. Furthermore, it is well known that rearrangement reactions typically have reverse activation energy barriers, so that TS is small compared to T”“. As a consequence, T x T”’ holds and the higher KER for Cl ’ formation from NCCN+’ may be related to a higher reverse activation energy (Ey(Cl’ + N2 + 5) > Er CC,‘*+ N, + 6)) [20]. If it is assumed that both rearrangement processes furnish the products C,+* + N, in the same electronic (ground) state, this means that the transition state (TS) for the dissociation of 6 is (slightly) lower in energy than the TS for dissociation of 5. Furthermore, the “CNCN+’ intermediate” 6 is calculated to be 0.03 eV more stable than the “NCCN+’ intermediate” 5, as can be seen in Table 2. Of course, 0.03 eV is a very small energy difference. Nevertheless, the qualitative picture tits well with the Hammond postulate [39] which for mechanistically similar reactions predicts a higher energy TS when the energy of the reactants (intermediates 5 or 6) rises (see Fig. 2). The different KER values for the C:’ formation from CNCN+’ and NCCN+’ also exclude the occurrence of a common intermediate in the microsecond time scale, such as 5 which, in the case CNCN+‘, could be formed by a 1,2-shift of the terminal carbon atom, i.e. 5 and 6 are supposed not to interconvert before fragmentation. Further support for the mechanism is provided by the geometries of the intermediates 5 and 6, as inferred from our HFS-calculations, which fit in well with dissociation to the products C,‘* + N, as the subsequent and final step. The CN-distance which gives the separation between the two C2- and N,units, is already rather large, while the distance between the two atoms that are to be coupled is quite short (5; CC = 136.7 pm, CN = 138.4 pm, NN = 130.9pm, 6; NN = 114.5pm, CN = 152.1 pm, CC = 127.5pm). Moreover, the major part of the positive charge is concentrated on the C, fragment in both cases (Q,,, in5) = + 0.91 electron; Q(c-, 6) = + 0.72 electron). For the sake of completeness, another conceivable rearrangement mechan-
c C2+’ + N2
NCCN+'1+-
C-N CNCN+‘2+’
+
InI
N-C
+* 1
NrN7C i TS 11
(4)
165
ism leading to C2+. formation and presented in Eq. 4 should be discussed. After a 1,4 ring closure, the cyclic structures 9 or 10 could decompose to the products Cl. + N,. In this mechanism, C,’ ’ formation from 2+’ would proceed via the tetrahedral, and probably highly energetic, transition state (TS) 11,making this reaction path very unattractive relative to that proposed in Eq. 3. Therefore, and for economic reasons, no calculations on 10 and 11 have been performed. However, 9 might be a suitable structure for a low energy reaction path leading to the products Cc ’ + N, by simple bond-breaking between the C2 and N2 fragments. However, calculations on 9 show that the energy of this structure is about 1.5 eV higher than that of the NCCN intermediate 5. We therefore conclude that the most likely mechanism for Cl. formation from CNCN+’ and NCCN+’ from those discussed is that shown in Eq. 3. CN’ formation In contrast to Cl’ formation, there is no need for rearrangement when CNCN+’ and NCCN+’ decompose to CN+ + CN’ . In fact, the corresponding direct cleavage reaction for CN+ formation is expected to be the dominating process at high energy, because it is well known that the specific frequency factor in the expression for the unimolecular rate constant, k (E), is higher for direct cleavage processes than for reactions which prevail at low energy [20, 40-421. As shown in Table 1, the appearance energy (AE) measurements for CN+ formation from CNCN and NCCN indicate that the corresponding critical energies are indeed 3.5 and 3.0 eV, respectively, higher than those for C,” formation, Most surprisingly, however, is that, notwithstanding this considerable energy gap, the two reaction pathways are found to compete with each other in the metastable time window (10-6-10-5 s), as can be seen from the MIKES data in Table 1. This delay of the high energy fragmentation can be understood by assuming that the two processes of CN+ and C,+’ formation take place from molecular ions that are generated in the ion source in different electronic states. As a consequence, one must assume that the two reaction channels can be in competition with each other because of the occurrence of molecular ions in isolated excited electronic states! Alternatively, this competition could be explained in terms of crossing k(E) vs. E curves for the two dissociation processes. However, crossing k(E) vs. E curves for this system imply kinetic shifts for the Ct ’ formation of 3.5-3.0 eV. To account for such large values, frequency factors of about 10’s_’ are needed, two orders of magnitude below the lowest value for rearrangement processes proposed by Levsen [40]. These considerations make the hypothesis of crossing k(E) vs. E curves very unlikely. Moreover, this alternative mechanism can not explain why Cl ’ and CN+ have comparable intensities in the
166
metastable MIKES spectra (Fig. 1). A slowly increasing C:’ curve would have an intersection with the metastable window which is much larger than that for the steep CN+ curve. As a consequence, the C,+* forming process would contribute to the metastable dissociation of the molecular ion over a much larger interval on the internal energy scale, which in its turn, would result in a strongly dominating C: ’ formation. Electronically-excited (NCCN+‘)* is known not to decay radiatively [43], so that non-radiative transitions are the only means of relaxation. Because of the close similarity, the same may be true for (CNCN+‘)*. From photoelectron-photoion (PEPICO) and photoion-fluorescence-photon (PIFCO) coincidence experiments, it is known that excited electronic states or rovibronic levels of the homologous series of XCN+’ open-shell cations (X = Cl, Br, I) have lifetimes in the microsecond range [44-47]. From emission spectral data of XCN+’ systems excited in the gas phase by electron impact, Allan and Maier [48] have determined lifetimes of 4.4 ps and 0.205 ps for the A 2Z+ state and the B211 state of ClCN+‘, respectively. These values have been confirmed by more recent PEPICO and PIFCO studies [45-461. In spite of the obvious structural and spectroscopic differences, CNCN+’ and NCCN+’ may also be considered as members of the XCN+’ series (X = the pseudo-halogens CN and NC, respectively). Consequently, lifetimes of the order of microseconds for the A and B excited states of CNCN+’ and NCCN+’ could be expected and could explain why the high energy fragmentation to CN+ + CN’ competes with C: ’ + N2 production in the metastable time window. We therefore propose that CN+ formation proceeds via such an isolated excited electronic state of the molecular ion ((M+‘)*). Possible candidates for such a state are found in the photoelectron spectra (PES) of CNCN (IEs: 12.873 (a-‘), 12.911 (7~~I), 14.390 (6-l) and 16.460eV (rr-‘) [3]) and NCCN (IEs: 13.36 (rrg’), 14.49 (a;‘), 14.86 (a;‘) and 15.47eV (7~;‘) [49]). These PES spectra prove the existence of three bonding excited electronic states of the molecular ion. Fragmentation of (M+‘)* to CNf + CN’ would then take place by predissociation to the vibrational continuum of the ground state or by predissociation to a yet unknown repulsive electronic state. Both possibilities can account for the relatively high KER values of 121 and 118 meV for CN+ formation from CNCN+ ’ and NCCN+ ‘, respectively (see Table 1). Reaction scheme 5 summarizes this proposed mechanism. For comparison, C,+* formation is included in this scheme.
E1
M+-
rearrangement
*
Cz+’ + N2
*
CN+
l-
predissociation
+
CN'
CN+ + CN’
(5)
167 ACKNOWLEDGEMENT
We thank Dr. J.G. Snijders for helpful discussions and Mr. P. Vernooijs for help with the calculations. REFERENCES 1 L.J. Gay-Lussac, Ann. Chim. (Paris), 95 (1815) 175. 2 T. van der Does and F. Bickelhaupt, Angew. Chem., 100 (1988) 998. 3 0. Grabandt, C.A. de Lange, R. Mooyman, T. van der Does and F. Bickelhaupt, Chem. Phys. Lett., 155 (1989) 221. 4 F. Stroh and M. Winnewisser, Chem. Phys. Lett., 155 (1989) 21. 5 F. Stroh, B.P. Winnewisser, M. Winnewisser, H.P. Reisenauer, G. Maier, S.J. Goede and F. Bickelhaupt, Chem. Phys. Lett., 160 (1989) 105. 6 K.M.T. Yamada, M.W. Markus, G. Winnewisser, W. Joentgen, R. Kock, E. Vogel and H.-J. Altenbach, Chem. Phys. Lett., 160 (1989) 113. 7 M.C.L. Gerry, F. Stroh and M. Winnewisser, J. Mol. Spectrosc., 140 (1990) 147. 8 I. Weiss, H. Oberhammer, S.J. Goede, P.J.K.M. Eeken and F. Bickelhaupt, personal communication (1989). 9 L.S. Cederbaum, F. Tarantelli, H.-G. Weikert, M. Scheller and H. Kiippel, Angew. Chem., 101 (1989) 770. 10 M.K. Scheller, H.G. Weikert, L.S. Cederbaum and F. Tarantelli, J. Electron Spectrosc., 51 (1990) 75. 11 M.T. Nguyen, Chem. Phys. Lett., 157 (1989) 430. 12 P. Botschwina and P. Sebald, Chem. Phys., 141 (1990) 311. 13 R. Boese, personal communication (1990). 14 J.C. Slater, Quantum Theory of Molecules and Solids, Vol. 4, McGraw-Hill, New York, 1974. 15 E.J. Baerends, D.E. Ellis and P. Ros, Chem. Phys., 2 (1973) 41. 16 E.J. Baerends and P. Ros, Chem. Phys., 2 (1973) 52. 17 E.J. Baerends and P. Ros, Chem. Phys., 8 (1975) 412. 18 W. Heijser, A. Th. van Kessel and E.J. Baerends, Chem. Phys., 16 (1976) 371. 19 VG Analytical Ltd., Wythenshawe, Manchester M23 9LE, U.K. 20 R.G. Cooks, J.H. Beynon, R.M. Caprioli and G.R. Lester, Metastable Ions, Elsevier, Amsterdam, 1973. 21 A.-M. Dommrijse and H.-Fr. Grtitzmacher, Int. J. Mass Spectrom. Ion Processes, 76 (1987) 95. 22 J.D. Morrison, J. Chem. Phys., 19 (1951) 1305. 23 W.P.M. Maas and N.M.M. Nibbering, Int. J. Mass Spectrom. Ion Processes, 95 (1989) 171. 24 L. Versluis and T. Ziegler, J. Chem. Phys., 88 (1988) 322. 25 A.D. Becke, Int. J. Quantum Chem., 23 (1983) 1915. 26 A.D. Becke, J. Chem Phys., 85 (1986) 7184. 27 T. Ziegler, V. Tschinke and A. Becke, Polyhedron, 6 (1987) 685. 28 H. Stoll, E. Golka and H. Preus, Theor. Chim. Acta, 55 (1980) 29. 29 S.H. Vosko, L. Wilk and M. Nusair, Can. J. Phys., 58 (1980) 1200. 30 T. Ziegler, V. Tschinke and C. Ursenbach, J. Am. Chem. Sot., 109 (1987) 4825. 31 T. Ziegler, V. Tschinke, L. Versluis and E.J. Baerends, Polyhedron, 7 (1988) 1625. 32 L. Fan and T. Ziegler. J. Chem. Phys., 92 (1990) 3645.
168 33 N.N. Haese and R.C. Woods, J. Chem. Phys., 73 (1980) 4521. 34 0.1. Smith, Int. J. Mass Spectrom. Ion Processes, 54 (1983) 55. 35 NBS Library Compilation, disk library number 55, Finnigan Corporation, San Jose, CA, 1984. 36 S.G. Lias, J.E. Bartmess, J.F. Liebman, J.L. Holmes, R.D. Levin and W.G. Mallard, J. Phys. Chem. Ref. Data, 17 (suppl. 1) (1988). 37 H.H. Rosenstock, K. Draxl, B.W. Steiner and J.T. Herron, J. Phys. Chem. Ref. Data, 6 (suppl. 1) (1977). 38 Y. Morino, K. Kuchitsu, Y. Hori and M. Tanimoto, Bull. Chem. Sot. Jpn. 41 (1968) 2349. 39 G.S. Hammond, J. Am. Chem. Sot., 77 (1955) 334. 40 K. Levsen, Fundamental Aspects of Organic Mass Spectrometry, Verlag Chemie, Weinheim, 1978. 41 W. Forst, Theory of Unimolecular Reactions, Academic, New York, 1973, Chapter 2. 42 B.J. McClelland, Statistical Thermodynamics, Chapman and Hall, London, 1973, Chapter 12. 43 J. Falura, S. Leutwyler, J.P. Maier and U. Spittel, J. Phys. Chem., 89 (1985) 3190. 44 J.P. Maier and F. Thommen, in M.T. Bowers (Ed.), Gas Phase Ion Chemistry, Academic, Orlando, 1984, Chap. 25. 45 J.P. Maier, M. Ochsner and F. Thommen, Faraday Discuss. Chem. Sot., 75 (1983) 77. 46 0. Braitbart, E. Castellucci, G. Dujardin and S. Leach, J. Phys. Chem., 89 (1985) 3252. 47 S. Leutwyler, J.P. Maier and U. Spittel, J. Chem. Phys., 83 (1985) 506. 48 M. Allan and J.P. Maier, Chem. Phys. Lett., 41 (1976) 231. 49 J.M. Hollas and T.A. Sutherley, Mol. Phys., 24 (1972) 1123. 50 The AE values for C: and CN+ formation from CNCN are calculated by subtracting our HFS energy difference of 0.955 eV between NCCN and CNCN (Table 2) from the corresponding AE values for NCCN. This procedure leads to a value of 17.2 eV for the AE of C:’ from CNCN. The deviation of + 0.5 eV from the measured value is within the experimental error obtained for our AE measurements.