- Email: [email protected]
The unimolecular decay of difluoroethene cations. An experimental and quantum-chemical study
of Mass Spectrometly and Jon Processes, 61 {1984) 305-322 Elsevier Science Pubfshers B.V., Amsterdam - Printed in The Netherlands
~nternationaiJournal
THE UNIMOLECULAR AN EXPERIMENTAL
G. FRENKING, Institut
DECAY OF DIFLUOROETHENE CATIONS. AND QUANTUM-CHEMICAL STUDY
W. KOCH
ftir Organ&he
M. SCHAALE
305
Chemie der Technischen Universitiit Berlin, D - 1000 Berlin I2 (F.R. G.)
and H. BAUMGARTEL
*
Institut ftir Physikalische Chemie der Freien Universitiit Berlin, D - 1NW Berlin 33 (F. R.G.) (First received 13 February 1984; in final form S June 1984)
ABSTRACT The mechanism of the fragmentation reactions of l,l-, cis- and truns-difluoroethene cation in the threshold region are investigated by quantum mechanical calculations. The results elucidate the behaviour observed in appropriate experiments.
INTRODUCTION
The fragmentation of gaseous organic molecular ions has been studied by mass spectrometry for a long time. Usually, the decay mechanisms are discussed by analogy to the behaviour of solvated ions, for which extensive experimental material is available [1,2]. In physical chemistry, a statistical description of the fragmentation is favoured; it is based on the idea that the rate constant of unimolecular decay depends only on the internal energy of a molecular ion [3-51. Quite recently, the quantum-chemical interpretation of the ion fragmentation has been investigated carefully [6,23]. From this point of view, not only the amount of internal energy, but also its distribution over electronic, vibrational and rotational degrees of freedom is important. Consequently, a chemical reaction may be characterized by the quantum states of the educt and the products. Because of the high density of states that appear even in small molecules, it is not possible to observe a reaction from state to state except for diatom&. Therefore, the mechanism formulated for the unimolecular decay
* To whom correspondence should be addressed. 0168-1176/84,‘$03.00
6 1984 EIsevier Science Publishers B.V.
306
of a molecular ion represents either one representative decay channel or the average of several decay channels to identical products. Nevertheless, with the aid of different experimental techniques and quantum-chemical calculation, it is possible to evaluate for medium-sized molecules a very detailed picture of the reactions from which the fragmentation pattern results. This will be shown for the isomers of the difluoroethene cation. EXPERIMENTAL
RESULTS
Photoion spectra
Photoion spectra occupy an important position among the different experimental methods elucidating the fragmentation patterns of molecular ions. From the m/z ratio of the ionic products and the following energy balance, a statement concerning the structure of the products can be made. AHy8(educt)
+ AP = AHf9’(products)
+ ELcin(e-) + E *
where AHtga is the heat of formation of educt and products, AP the appearance potential of an ion, E,i,{e-) the kinetic energy of photoelectrons and E* the excess energy (internal energy and kinetic energy of products). Many times the kinetic energy of the photoelectrons and the excess energy vanishes at the appearance potential of a fragmentation.
TABLE
1
Compilation of experimental results from photoion and photoelectron spectra
A. Appearance potentials (AP) and maximum inner energy -(AP - f, ) of ions (in ev) l,l-C,H,F,
Reaction
AP
AP--I,
cb-C, H, F2
rrorz.~-C,H 2F2
AP
AP
AP-J,
AP-
C,H,F,
-+ C,H,Fg
+e-
10.29
C,H,F2
+ C,HF+
+HF+e-
14.18
3.89
13.75
3.55
13.70
3.55
C,H,F,
4 C,H,F’
+F+e-
14.37
4.08
14.00
3.80
13.97
3.82
C,H,F,
+ CH*F*
14.84
4.55
14.40
4.20
14.35
4.20
C,H,F,
---+CF+ + CH,F+
14.92
4.63
14.50
4.30
14.45
4.30
f CF+ ee-
10.20
10.15
B. Energy and assignment of photoelectron bands in the energy range 10-15.5 eV Compound
11
Type
12
Tse
l,l-C,H,F, c&C, H 2F2 truns-C, Hz F2
10.45 10.18 10.20
Wvc) b,(vc) a,(%z)
14.94 13.54 13.42
%(%-Iv n,)
b2(nCH2)
as(uCH,
nF)
13
Qw
15.16 15.10 15.38
al(Q) &(n,) &(n,)
f,
307
The photoion spectra of l,l-difluorethene [7] and f,2_difluoroethenes [S] have been reported. In Table lA, the appearance potentials of photoions and the results of thermochemical calculations are compiled. Photoelectron
spectra
The He(I)-photoelectron spectra of difluoroethenes have been measured by several groups 19,101. Threshold photoelectron spectra [S], where only photoelectrons without kinetic energy are recorded, are known. In Table lB, the ionization potentials, relevant for the following considerations, and their assignment are summarized. Fluorescence spectra The fluorescence of difluoroethene cations have been investigated by Maier et al. [ll]. Only the cis-difluoroethene cation shows a weak fluorescence from the first excited electronic state (A). Coincidence spectra
Both photoion-photoelectron coincidence spectra and photoion-fluorescence coincidence spectra have been measured [12,13]. Time-of-flight
mass spectra
TOF spectra reveal the kinetic energy of fragments. At the appearance potential of the HF abstraction, the kinetic energy of the fragments comes to about 1 eV. An identical amount of kinetic energy resuhs for the three isomers. From the shape of the metastable signal, a kinetic energy of 0.84 eV has been calculated [14,15]. Quantum-chemical
calculations
Precalculations of the difluoroethenes and additional isomer structures have been performed on a semiempirical level (MNDO) [16]. The resulting optimized geometries have been taken as the starting values for UHF/RHF ab initio calculations with the program GRADSCF 1171 (geometry optimization with a gradient procedure) using an STO-3G [42] basis set. Transition states were localized as saddle points on the energy hypersurface with one negative intrinsic value of the force constant matrix. With the ab initio optimized geometries, energy calculation was performed with the program Gaussian 76 [18] using a 6-31G * [43] basis set. The resulting energies
308 TABLE
2
Calculated total energies, E, and relative energies, Erel(mol- ‘) A. Educts and products No.
Compound
E
E rel (kcal mol-‘)
(a-u.) a 1 2 3 4 5 6 6s 6b 7 ?a 7b 8 sa 8b 9 9a 9b 10 11
l,l-C,H,F;truns-C2H 2Fc cis-C,H,F;’ H,FC-CF+’ HF,C-CH +. CH2F+ + CF’ CH,F+ CF’ * CH,F‘+CF* CH,F’ CF’. CH,CF++F’ CH,CF+ F’ HCCF++‘+HF HCCF+’ HF FC=CH-FH+’ FHC=C-FH+’
-
275.4075 275.3937 275.3924 275.3300 275.2924 275.2581 138.0877 137.1704 275.2396 138.3985 136.8411 275.2783 175.9133 - 99.3650 - 275.2776 - 175.2769 - 100.0007 - 275.2732 - 275.2615
a
0 9 9 49 72 94
106
81
82
84 91
a 1 a.u. = 27.21 eV; 1 eV = 23.06 kcal; 1 kcal = 4.18 kJ. B. Transition states Transition
Reaction
state TS 1 TS2 TS3 TS4 TS5 TS6 TS7 TSS TS9 TS 10
243 2+4 144 2,348 4+8 148 2,3410 2,34 11 4-9 l-+9
E
E rel
{a-u.) a
{kcal mol-
-
60 72 94 111 107 90 113 122 144 121
275.3117 275.2933 275.2575 275.2316 275.2371 275.2647 275.2281 275.2137 275.1783 275.2156
a 1 a.u. = 27.21 eV; 1 eV = 23.06 kcal; 1 kcal = 4.18
‘) a
kJ.
E(6-31G * /STO-3G) are listed in Table 2. The calculations were performed only for the electronic ground state (2) of the ions. The geometries of relevant ionic structures and transition states are given in Table 3.
309 TABLE
3
Calculated geometries of the most imp&cant ground states and transition states a
w 1
a
3
.
5
’
Bond
c2,
kng~h (A>
c(2)-c(3) C(2)-F(l) c(2)-~(4)
2
Bond angks (degree) 1.46 1.30 1.09
123 119 122 114
C?V
lengths (A)
Bond
F(l)-c(2)-Y3) w4)-a3)--q2) )11(4)-CX3)-W6) F(l)-C(2)-F(S)
Bond angles {degree) 1.44 1.30 1.11
cc2)-cx3) C(2)-370) C(2)-H(5)
F(v-c(2)-C(3) H(S)-C(2)-C(3) F(l)-a2)-H(5)
121 121 119
3 =2h Fbnd lengths (A) 1.44 1.30 I.11
c(2)-C(3) C(2)-F(1) C(2)-W5) 1
F
Bond angles (degree) F(l)-c(2)-H(S) W5)-C-(2)-C(3) w)-c(2)-c(3)
119 122 119
4 3
4
3ond lengths (A) I=(2)-C(3) C(2)-F(l) C(3)-F(4) c(2)-W5) C(2)-H(6)
330nd angles (degree) 1.61 1.36 1.28 1.11 1.11
C(3)-CJ2)-W) t72)-C(3)-F(4) c(3)-~(2)-~(5) C(3)-C(2)-H(6)
F(IKW-HO)
F(l)-C(2)-H(6) H(5)-C(2)-H(6)
111 125 104 104 114 114 109
310 TABLE
3 (continued)
w 6
4
3
2
5
5
Bond lengths (A) C(21-C(31 C(2)-F(5) C(2)-F(6) C(2)-H(1) C(3)-H(4)
90
Bond angles (degree) 1.61 1.36 1.37 1.12 1.13
C(2)-C(3)-H(4) C(3)-C( 2)- H(1) C{3)-C(2)-F(5) C(3)-C(2)-F(6) F(5)-C(2)-F(6) H(l)-C(2)-F(5) H(l)-C(2)-F(6)
137 101 110 108 112 113 112
C-v
Bond lengths (A) 1.10 1.25 1.27
C-H c-c C-F
10
Bond lengths (;i> C(2)-C(5) C(2)-w4) C(5kW6) C(2)-Hfl) F(4)-H(3)
Bond angles (degree) 1.36 1.49 1.33 1.10 0.98
133 115 133 112 118
Bond angles (degree)
Bond lengths (A) C(2)-C(3) C(2)-W) CGbF(S) C(2)-H(4) F(5bH(4)
C(2)-C(S)-F(6) C(5)-C(2)-F(4) C(5)-C(2)-H(1) H(l)-C(2)-F(4) C(2)-F(4)-H(3)
1.38 1.35 1.46 1.10 0.97
F(l)-C(2)-C(3) C(2)-C(3)-F(5) C(3)-C(2)-H(6) H(6)-C(2)-F(1) C(3)-F(S)-H(4)
121 119 118 120 11s
311 TABLE
3 (continued)
@h-f 3
2
5
1
TS 1
Bond lengths (A) C(2)-C(3) C(2)-F(l) C(2)-H(6)
Bond angles (degree) 1.49 1.31 1.11
H(l)-C{2)-C(3) H(6)-C(2)-C(3) F(l)-C(2)-H(6)
120 122 118
TS2
Bond lengths (A) C(2)-C(3) C(3)- F(4) C(2)- F(6) C(2)- H(1) C(3)-H(1) C(2)- H(5)
Bond angles (degree) 1.43 1.31 1.33 1.40 1.37 1.11
121 129 119 108 113 59 58 62
Bond angles {degree ,I
Bond lengths (A) C(2)-C(3) C(3)-F(4) C(3)-F(l) C(2)-F(l) C(2)-W5) C(2)-H(6)
F(6)-C(2)-C(3) F(4)-C(3)-C(2) H(5)-C(2)-F(6) F(6)-C(2)-H(1) F(4)-C(3)-H( 1) C(2)-C(3)-H(1) C(3)-C(2)-H(1) C(2)-H(l)-C(3)
1.47 1.31 1.49 1.50 1.11 1.11
H(S)-C(2)-C(3) H(6)-C(2)-C(3) F(4)-C(3)-C(2) C(2)-C(3)-F(1) C(3)-C(2)-F(1) C(2)-F(l)-C(3) H(S)-C(2)-F(1) H(6)-C(2)-F(1) F(4)-C(3)-F(1) H(S)-C(2)-H(6)
120 120 175 61 60 59 111 111 124 117
TABLE
L
3 (continued)
3
4
TS5
Bond lengths (A) C(2)-C(3) C(2bWl) C(2)-F(5) CG-F(4)
Bond angles (degree) 1.42 1.10 2.05 1.30
1.38 2.04 1.26 1.11 I.10
Bond lengths (A) C(lbC(4) C(44)-F(5) W)-W2) C(4)- W3) CUbJX6) W2PW3)
C(2)-W)-W4) W5WW-WI
122 117 178 97
Bond angles (degree)
Bond lengths (&A) C(2wx3) CW-F(1) C(2)-F(6) C(3k-W4) C(3bW5)
H(l)-C(2)-C(3) H(6)-C(2)-C(3)
C(2)-C(3)-H(4) W+C(3)-W5) H(4)-C(3)-H(S) c/3)-Ct2)-m C(3)-C{2)-F(6) F(l)-C(2)-F(6)
124 110 121 114 146 97
Bond angles (degree) 1.51 1.28 1.42 1.53 1.10 1.09
C(l)-C(4)-H(3) C(4)-C(1)-F(2) C(l)-F(2)-H(3) C(P)-H(3)-F(2) C(l)-C(4)-F(5) C(4MvbW61 F(2)-C(l)-H(6) H(3)-C(4)-F(5)
72 92 90 106 122 142 116 165
313 TABLE
3 (continued)
T2-c 2
1
4
5
TSS
Bond angles (degree)
Bond lengths (A) 1.44 1.43 1.31 1.32 1.11
Cc2)-C(41 C{2)-~(3) C(4)-F(5) C(2)-H(1) C(4)-H(6)
H(lPX+IW C(2)-F(3)-H(1) F(3)-H(lbC(2) C(2)-C(4)-F(5) C(2)-C(4)-H(6) F(5)-C(4)-H(6) H(l)-~(2)-C(4) F(3)-c(2)-c(4)
2
49 60 71 122 119 118 164 115
3
A 1
4
6
TSIO
Bond angles (degree)
Bond lengths (A) C(lPCC4) C(l)-F(2) C(lPF(6) C(4)-H(3) C(4)-HC5) F(2ww31
1.43 1.46 1.30 1.30 1.10 1.11
F(2)-C(l)-C(4) F(2)-C(l)-F(6) C(l)-F(2)-H(3) C(l)-C(4)-H(3) C(l)-C(4)-H(5) F(2)-H(3)-C(4) H(3)-C(4)-H(5) F(6)-C(l)-C(4)
96 115 84 72 126 108 161 144
e For details of the structures and data of 6a, 4b, 7a, 7b, Sa and 9b, see ref. 44.
DISCUSSION
The combination of spectroscopic results and quantum-chemical model calculations gives evidence on the behaviour of the molecular ions at the appearance potential of fragmentations. According to the thermochemical energy balance from the appearance potentials and AH,?98 values from the literature (Table 4), the heats of formation of the fragments can be calculated. This gives a hint to the structure of the resulting fragment ions. The threshold photoelectron spectra confirm the formation of photoelectrons
314 TABLE
4
Heats of formations Compound
used for the thermochemical AH? (kcal mol-‘)
calculations
of C,H,F,+ Ref.
a
cis-C, H 2 F2
-71
trans-C, Hz F2
-70
8
1,1-C,
-80
38
HzFz
8
CH,F
-1
40
CH,F+
209
40
CF CF+ HF F
61
39
275
41
-65 19
fragmentation
39 39
a 1 kcal G 4.18 kJ
without kinetic energy at the appearance potential. Consequently, the corresponding molecular ions possess the internal energy AP-I, (1, is the first ionization potential). It should be mentioned, however, that in addition to zero kinetic energy electrons, photoelectrons with kinetic energy in the range zero to (AP - II) are emitted by irradiation with light of the energy hv = AP. Therefore, the totality of the primary originated C,H,F,+ distributes its internal energy on the electronic, vibrational and rotational states lying in the energy range zero to (AP - II). The consideration of specific vibrational and rotational states is not possible in the framework of our calculations and is, therefore, neglected in the following discussion. The comparison of appearance potentials and ionization potentials shows the formation of molecular ions in the electronic ground state (2) with high vibrational excitation in addition to those in the first excited electronic state (A) with low vibrational excitation. The analysis of the thermochemical energy balance concerning the structure of the fragment ions reveals the following facts. (1) The heat of formation of C,H,F+ produced by F abstraction from the isomers C,H 2FZ+ is 233 kcaI mol-I. The same value appears after H abstraction from C2H3Ff [7] and Cl abstraction from the three isomeric fluorochloroethene cations [19]. Quantum-chemical calculations [20] show a minimum on the hypersurface for the planar structures A and B [37]
where A is more stable than B. Therefore, the product of F abstraction in the threshold region is assigned to structure A. This result implies, at least for the 1,2_difluoroethene cations, an intramolecular rearrangement before the F abstraction is completed. (2) HF abstraction from C,H,Fc yields either cations of fluoroacetylene or fluorovinylidene. Thermochemical considerations reveal the formation of 1.1 eV (about 25 kcal mol-I) excess energy at the experimental threshold. The same value of excess energy is observed from l,l-, cis- and trans-C,H, F2f. The coincidence spectra and time of flight measurements confirm the kinetic energy of the fragments C,HF+ f HF to be about 1 eV. The difference of 0.1 eV excess energy is clearly less than the energetic difference between the cations of acetylene and vinylidene [21].From this result we conclude that, in addition to HF, the cation of fluoroacetylene is formed, probably in a vibrational excited state. (3) Either CH2F+ + CF or CF+ + CH,F results from the fragmentation of the C-C bond, indicating a complex mechanism for the reaction. The experimental appearance potentials agree with the calculated thermodynamic values based on the heat of formations of CH, F +, CH,F, CF+ , CF given in Table 4. The appearance potentials of the fragmentation reactions occur in the range of the second PE band, except for the HF + C,HF+ formation, which is definitely below the second ionization potential; no coincidence between appearance potentials and ionization potentials exists. From the investigation of the fluorescence of the cations 1111, it is known that only the cis isomer exhibits weak fluorescence from its first excited electronic state. From this result, it follows that the lifetime of the 2 states is limited by radiationless deactivation. As impact processes can be excluded on account of the experimental conditions, the mechanism of deactivation is a radiationless transition into the vibrationally excited ground state _%of the cation (internal conversion). These transitions are caused by vibronic coupling and have been the subject of detailed theoretical investigations [22,23] in the case of the ethene cation. It has been shown that especially the conical intersection of two electronic hypersurfaces, caused by the coupling through at least two vibrations, can frequently be expected in polyatomic molecules. The symmetry selection rules existing for this mechanism [23] are fulfilled by the difluoroethene cations. The experimental rate constants of the internal conversion 2-2 are ki, = 8 x 10” s-l for cis-C,H,F,+ and k, 3 10” s-i for tram-C, H 2F$ [12]. Therefore, the mechanism of the fragmentation in the threshold region is essentially determined by the hypersurface of the electronic ground state of the ions. In the following, we discuss the fragmentation processes of C,H,Fc after internal conversion into the ground state has been completed. The internal
316
energy of the molecular ions under discussion comes to 3.55-4.55 eV. In order to understand the strong uniform character of the fragmentation of the isomer% cations, we investigated the activation energy and the geometries of the transition states, which determine the cis-tram isomerization and the rearrangement of the l,l-difluoroethene cation into the l,%difluoroethene cations (see Fig. I). According to our calculations and in agreement with the experimental results, l,l-C,H,Fz+ is about 38 kJ mol-’ (9 kcal mol-‘) more stable than cd% and tram-C, H, Fz+; for all the isomer& cations, a planar structure results, in contrast to the ethene cation for which, from experimental and theoretical investigations, a C-C twisted geometry has been estabhshed [24-275 This difference is reasonable, as the distortion in the ethene cation is caused by a Jahn-Teller effect, which eliminates the energetic degeneracy by symmetry lowering. The bond lengths and bond angles in the difluoroethene cations (Table 3) hardly differ from the values reported for the ethene cation (C-C 1.44 A, C-H 1.07 A, HCH 118O) by Lorquet and co-workers [22J. The planar structure of the isomer+ difluoroethene cations I, 2 and 3 also results from MNDO calculations, which seems remarkable as MNDO proved to be a reliable method for this specific problem [283. The most favourable energetic reaction path for the cis-tram isomerization is the rotation of the C-C bond. The energy of the 90” twisted conformation (TS 1) is 214 kJ mol-’ (51 kcal mof-‘) compared with the planar ground states 2 and 3. The rearrangement of l,l-C,H,F,+ into the 1,2-isomers and vice versa is much more complex. The most favourable reaction path from the energetic point of view crosses a distinct minimum in the energy hypersurface. The minimum marks the intermediate 4 to which a fluoromethylcarbenium structure is assigned. The energy difference between 4 and 1 is 205 k3 mol’ r (49 kcal mol-‘) and 167 kJ mol-’ (40 kcal mol-r) between 2 and 3. Our results reveal similar energy differences as have been calculated on the 6-31G * level for the difference between the neutral fhroroethene [29j and the corresponding fhrorocarbene structure. The isomeric difhtoromethylcarbenium cation 5 is located about 96 kJ mol-I (23 kcal mol-‘) above 4. The transition state TS 2, which connects 4 and 2, 3 is located 96 kJ mol-’ (23 kcal mol-I) above 4. TS 3, connecting 4 and 1 is located 147 M mol-’ (35 kcal mol-‘) above 4_ The energy profile of the isomerization of 1, 2 and 3 is shown in Fig. 1. The transition state TS 3 coincides energetically with the appearance potential of the HF abstraction, which needs the smallest amount of internal energy. The geometries of the transition states TS 2 and TS 3 are similar. Lorquet proposed in the mechanism of H, abstraction from CZHf( 2) a transition state, which leads to the formation of the methylcarbenium intermediate and which has an analogous geometry to TS 2. The energy profile (Fig. 1) shows that the
317
internal energy of the ground state difluoroethene cations at the threshold of fragmentations is sufficient to cross the energy barriers of isomerization. The X,2-difluoroethenes need an excitation energy of 13.7 eV, l,l-difluoroethene an excitation energy of 14.1 eV, to isomerize. As the appearance potentials of the fragmentation reactions exceed these values, it is reasonable that no structure-specific differences in the fragmentation of difluoroethene cation is observed.
The abstraction of fluorine occurs without formation of excess energy, so that the energetic asymptote calculated according to thermochemistry is observed experimentily. For the mechanistic interpretation, one has to determine whether this is possible from the vibrationally excited ground states of 1,2, 3 and 4 or not. The quantum-chemical calcuIations exhibit two transition states (TS 3 and TS 6) with different geometries, which are favourable from the energetic point of view. TS 6 can be reached from 1; at the threshold of G,H, F+, TS 3 is hardly a barrier for the cations 2 and 3 to reach TS 6 via 1, because in our calculations the energy of TS 3 is about 17 kJ molWf (4 kcal mol- ‘) higher than that of TS 6, but it is known that the energy of transition states,
I 140 130 120 -
-;^
z
90
E
80
;
70
-z
60
w’
50
Fig. 1. Caiculated
energy profile for F loss and isomerization
of 1, 2 and 3.
318
especially of those with a cyclic structure, decreases relative to the ground states if the basis sets are improved and configuration interaction is included in the calculation [30]. From our calculations, a small reverse activation energy is expected; this discrepancy with experiment is negligible considering the level of the model calculations. Nevertheless, it cannot be decided which of the transition states TS 3 of TS 6 is rate determining for the F abstraction from cis- and tram-C, I-I, F2‘+, but there is good evidence that the isomerization is a necessary step on the fragmentation pathway of these two ions at the appearance potential_ If the internal energy of the molecular ions is increased slightly (about IO-15 kcal mol-i), two additional transition states, TS 4 and TS 5, have to be considered_ TS 4 is the top of direct F abstraction from 2 and 3, TS 5 is the transiton state which controls F abstraction from the intermediate 4. In contrast to the situation at the appearance potential of the F abstraction, where only one reaction channel has to be discussed, at slightly higher energies, the reaction mechanism principally consists of three reaction channels and at present there is no evidence on the contribution of each channel to the yield of C2H2F+. It should be stressed that the previously mentioned considerations are valid for ground state ions; the structures of transition states of ions in excited electronic states will differ from those given in Table 3. The F abstraction from 1,2_difluoroethene cations in their c and 8 states according to coincidence measurements [12] occurs directly without internal conversion. HF abstraction (Fig. 2)
The transition states controlling the HF abstraction are located above the thermodynamically calculated appearance potential because a considerable amount of excess energy is observed_ This is confirmed by the energies of the transition states TS 7, TS 8 and TS 10 resulting from ;he model calculations. The 1,2 elimination of HF is completed via weak minima [lO,ll] in the energy hypersurface; compounds of this type have been identified experimentally and theoretically [32-351. In addition to the 1,2 elimination, the 1,l elimination via TS 10 may be discussed from energetic considerations, but this reaction channel first yields the fluorovinylidene cation, which should isomerize to the fluoroacetylene cation by analogy with the neutral fluorovinylidene practically without an activation barrier [31,36]. As can be seen from the energy scheme (Fig. 2), even at the appearance potential, three different reaction channels (TS 7, TS 8 and TS 10) are energetically possible for HF abstraction; it is not possible to distinguish between 1,2- and l,l-elimination of HF. The uniform behaviour of the three isomers is caused
319
by the relative low energy barriers for the isomerization and not by fragmentation of the intermediate 4. TS 9 is clearly too high in energy to be significant for HF abstraction at threshold. The lack of F2 formation and the very low yield of C,F,-C resulting from Hi, abstraction is remarkable, especially as the cation of vinyl fluoride produces high amounts of C,HF+ by H, abstraction. The H, abstraction from the ethene cation starts by a Jahn-Teller distortion of the intermediate methylcarbonium ion (C,,,). The corresponding fluorinated intermediate in the C,H,F,+ system is 4, which has C, symmetry, and therefore the mechanism operating in the 1,l elimination of H, from the ethene cation is not possible. In the framework of our calculations, no transition states for H 2 or F, abstration have been recognized. CF + and CH,F
+ formation
(Fig. 3)
After fragmentation of the C-C double bond, one expects fragments like CF,+ , CFH+, etc. We observed CH,F+ and CF+. The formation of these ions requires a rearrangement of C, H,F2+ before fragmentation. The model calculations give clear evidence that the intermediate 4 is the precursor ion of C-C fragmentation. In agreement with the experimental results, the calculations reveal the lower appearance potential for CH,F+ compared with CF+. The result that no reverse activation energy is involved confirms the lack of excess energy.
TS 9 I..
TS 10
_f
90
Z E
80
$
70
5
60
ldJ
50
Fig. 2. Calculated
energy profile for HF elimination
from 1, 2 and 3.
320
1Lil 130 120 110
APcl=+) =lU2 eV
CF= CH,F I
l
loo
AP(CH,F+) =I&.64 ev
CyFtCF
40 30 20 10
Fig. 3. Calculated
energy profile for C-C
cleavage_
CONCLUSIONS
The results of different experimental methods and quantum-chemical model calculations give evidence that the fragmentation of dif’luoroethene cations in the threshold region of the most intense fragmentation processes occurs in the electronic ground state of the molecular ion. It is possible to explain the mechanism of the different decay reactions with the aid of quantum-chemical calculations. They reveal two main reasons for the uniform behaviour of the different isomers. (a) The activation barriers for the isomerization are lower than for the fragmentation processes and (b) fluoromethylfluorocarbenium, the precursor ion of C-C fragmentation, can be easily accessed by l,l- and 1,2-difluoroethene cations. The reaction mechanism, even in the case of simple C-F fragmentation, is a complex succession of elementary steps and can be described by superpositioning reaction channels. ACKNOWLEDGEMENTS
Financial support from the Fonds der Chemischen Industrie is gratefully acknowledged, especially for a Liebig grant (G-F.) and a Ph.D. grant (MS.).
321
Additional financial and technical support has been given by Gesellschaft der Freunde der Technischen Universitat and Zentrales Rechenzentrum der Technischen Universittit. REFERENCES 1 F.W. McLafferty, Interpretation of Mass Spectra, University Science Books, Mill Valley, CA, 1980. R.D. Bowen, D.H. Williams and H. Schwarz, Angew. Chem., 91 (1979) 484. W. Forst, Theory of Unimolecular Reactions, Academic Press, New York, 1973. R.A. Marcus, J. Chem. Phys., 20 (1952) 359. ELM. Rosenstock, M-3. Waflenstein, A.L. Wahrhaftig and I-Z. Eyring, Proc. Natl. Acad. Sci. U.S.A., 38 (1952) 667. 6 J.C. Lorquet, Org. Mass Spectrom., 16 (1981) 469. 7 D. Reinke, R. Kr&ssig and H. Baumg--1, Z. Naturforsch. Teil A, 28 (1973) J.021. 8 H.W. Jochims, W. Lohr and H. Baumg&rtel, Nouv, J. Chim., 3 41979) 109. 9 C.R. Brundle, M.B. Robin, N.A. Kuebler and H. Basch, J. Am. Chem. Sot., 94 (1972) 1451. 10 J.A. Sell and A. Kuppermann, J. Chem. Phys., 71 (1979) 4703. 11 J.P. Maier, 0. Marthaler and G. Bieri, Chem. Phys., 44 (1979) 131. 12 J.P. Stadelmann and 3. Vogt, Int. J. Mass Spectrom. Ion Phys., 35 (1980) 83. 13 R. Frey, Dissertation, Universittit F&burg, 1974. 14 R.G. Cooks, J.H. Beynon, R.M. Gaprioli and G.R. Lester, Metastable Ions, Elsevier, Amsterdam, 1973. 15 T.H. Heir& and J. Vogt, unpublished results. 16 M,J.S. Dewar and W. Thiel, J. Am. Chem. Sot., 99 (1977) 4889, 4907. 17 A. Komornicki, available from Quantum Chemical Program Exchange. 18 J.S. Binldey, RA. Whit&de, P.C. Haviharan, R. Seeger, J.A. Pople, W.J. Hehre and M.D. Newton, Quantum Chemical Program Exchange No. 368. 19 G. Tornow and H. BaumgUtel, unpubiished results. 20 P. Kolhnan, 5. Nelson and S. Rothenberg, J. Phys. Chem., 82 (1978) 1403. 21 P. Rusmus, P. Botschw&x and J.P. Maier, Chem. Phys. Lett,, 84 (1981) 71. 22 C. Sannen, G. Raseev, C. Gallay, G. Fauville and J.C. Lorquet, J. Chem. Phys., 74 (1981) 2402. 23 H. K&ppel, L.S. C=ederbaum, W. Domcke and S.S. Shaik, Angew. Chem., 95 (1983) 221 and literature cited therein. 24 R.S. Mull&en and C.C.J. Roothan, J. Chem. Phys., 41 (1947) 219. 25 A.J. Merer and L-J. Schonveld, J. Chem. Fhus., 48 (1968) 522. 26 H. IMppel, W. Domcke, L-S, Cederbaum and W. vcm Niessen, J. Chem. Phys., 69 (1978) 4252. 27 R.J. Buenker, SD. Peyetioff and H.L. Hsu, Chem. Phys. L&t., 11 (1971) 65, 28 D-J. Bellville and N.L. Bauld, J. Am. Chem. Sot., 104 (1982) 294. 29 S. Kato and H. Morokuma, J_ Chem. Phys., 74 (1981) 6285. 30 K. Raghavachari, M.J. Frisch, J.A. Pople and P.v.R. Schleyer, Chem. Phys, Lett., 85 (1982) 145. 31 M.J. Frisch, R. Krishnan, J.A. Pople and P.Y.R. Schleyer, Chem. Phys. Lett., 81 (1981) 421. 32 N-J. 3ouma, J.K_ Mcceod and L. Radom, J. Am. Chem. Sot., 104 (1982) 2930.
322 33 J.L. Holmes, F.P. Lossing, J.K. Terlouw and P.C. Bingers, J. Am. Chem. Sot., 104 (1982) 2931. 34 H. Halim, B. Ciommer and H. Schwarz, Angew. Chem., 94 (1982) 547. 35 Y. Apeloig, B. Ciommer, G. Frenking, M. Kami, A. Mandelbaum, H. Schwarz and A. Weisz, J. Am. Chem. Sot., 105 (1983) 2186. 36 G. Frenking, Chem. Phys. Lett., 100 (1983) 484. 37 E. Keller, Chiuz, 2 (1980) 56. 38 J.H. Simmie and E. Tschuikow-Roux, J. Phys. Chem.., 74 (1970) 4075. 39 D.R. Stull and H. Prophet, JANAF Thermochemical Tables, Natl. Bur. Stand., NSRDSNBS, 37 (1971). 40 J.C. Franklin, J.G. Dillard, H.M. Rosenstock, J.T. Herron, K. Draxl and F.H. Field, Natl. Bur. Stand., NSRDS-NBS, 26 (1969). 41 J. Phys. Chem. Ref. Data, 3 (2) (1974). 42 W. J. Hehre, R.F. Stewart and J.A. Pople, J, Chem. Phys., 51 (1969) :627. 43 J.S. Binkley and J.A. Pople, J. Chem. Phys., 66 (1979) 879. 44 R.A_ Whiteside, M.J. Frisch, J.S. Binkley, D.J. DeFrees, H.P. Schlegel, K. Raghavachavi and J.A. Pople, Carnegie-Mellon Quantum Chemistry Archive, 2nd edn., 1981, CamegieMellon University, Pittsburgh, PA.
~nternationaiJournal
THE UNIMOLECULAR AN EXPERIMENTAL
G. FRENKING, Institut
DECAY OF DIFLUOROETHENE CATIONS. AND QUANTUM-CHEMICAL STUDY
W. KOCH
ftir Organ&he
M. SCHAALE
305
Chemie der Technischen Universitiit Berlin, D - 1000 Berlin I2 (F.R. G.)
and H. BAUMGARTEL
*
Institut ftir Physikalische Chemie der Freien Universitiit Berlin, D - 1NW Berlin 33 (F. R.G.) (First received 13 February 1984; in final form S June 1984)
ABSTRACT The mechanism of the fragmentation reactions of l,l-, cis- and truns-difluoroethene cation in the threshold region are investigated by quantum mechanical calculations. The results elucidate the behaviour observed in appropriate experiments.
INTRODUCTION
The fragmentation of gaseous organic molecular ions has been studied by mass spectrometry for a long time. Usually, the decay mechanisms are discussed by analogy to the behaviour of solvated ions, for which extensive experimental material is available [1,2]. In physical chemistry, a statistical description of the fragmentation is favoured; it is based on the idea that the rate constant of unimolecular decay depends only on the internal energy of a molecular ion [3-51. Quite recently, the quantum-chemical interpretation of the ion fragmentation has been investigated carefully [6,23]. From this point of view, not only the amount of internal energy, but also its distribution over electronic, vibrational and rotational degrees of freedom is important. Consequently, a chemical reaction may be characterized by the quantum states of the educt and the products. Because of the high density of states that appear even in small molecules, it is not possible to observe a reaction from state to state except for diatom&. Therefore, the mechanism formulated for the unimolecular decay
* To whom correspondence should be addressed. 0168-1176/84,‘$03.00
6 1984 EIsevier Science Publishers B.V.
306
of a molecular ion represents either one representative decay channel or the average of several decay channels to identical products. Nevertheless, with the aid of different experimental techniques and quantum-chemical calculation, it is possible to evaluate for medium-sized molecules a very detailed picture of the reactions from which the fragmentation pattern results. This will be shown for the isomers of the difluoroethene cation. EXPERIMENTAL
RESULTS
Photoion spectra
Photoion spectra occupy an important position among the different experimental methods elucidating the fragmentation patterns of molecular ions. From the m/z ratio of the ionic products and the following energy balance, a statement concerning the structure of the products can be made. AHy8(educt)
+ AP = AHf9’(products)
+ ELcin(e-) + E *
where AHtga is the heat of formation of educt and products, AP the appearance potential of an ion, E,i,{e-) the kinetic energy of photoelectrons and E* the excess energy (internal energy and kinetic energy of products). Many times the kinetic energy of the photoelectrons and the excess energy vanishes at the appearance potential of a fragmentation.
TABLE
1
Compilation of experimental results from photoion and photoelectron spectra
A. Appearance potentials (AP) and maximum inner energy -(AP - f, ) of ions (in ev) l,l-C,H,F,
Reaction
AP
AP--I,
cb-C, H, F2
rrorz.~-C,H 2F2
AP
AP
AP-J,
AP-
C,H,F,
-+ C,H,Fg
+e-
10.29
C,H,F2
+ C,HF+
+HF+e-
14.18
3.89
13.75
3.55
13.70
3.55
C,H,F,
4 C,H,F’
+F+e-
14.37
4.08
14.00
3.80
13.97
3.82
C,H,F,
+ CH*F*
14.84
4.55
14.40
4.20
14.35
4.20
C,H,F,
---+CF+ + CH,F+
14.92
4.63
14.50
4.30
14.45
4.30
f CF+ ee-
10.20
10.15
B. Energy and assignment of photoelectron bands in the energy range 10-15.5 eV Compound
11
Type
12
Tse
l,l-C,H,F, c&C, H 2F2 truns-C, Hz F2
10.45 10.18 10.20
Wvc) b,(vc) a,(%z)
14.94 13.54 13.42
%(%-Iv n,)
b2(nCH2)
as(uCH,
nF)
13
Qw
15.16 15.10 15.38
al(Q) &(n,) &(n,)
f,
307
The photoion spectra of l,l-difluorethene [7] and f,2_difluoroethenes [S] have been reported. In Table lA, the appearance potentials of photoions and the results of thermochemical calculations are compiled. Photoelectron
spectra
The He(I)-photoelectron spectra of difluoroethenes have been measured by several groups 19,101. Threshold photoelectron spectra [S], where only photoelectrons without kinetic energy are recorded, are known. In Table lB, the ionization potentials, relevant for the following considerations, and their assignment are summarized. Fluorescence spectra The fluorescence of difluoroethene cations have been investigated by Maier et al. [ll]. Only the cis-difluoroethene cation shows a weak fluorescence from the first excited electronic state (A). Coincidence spectra
Both photoion-photoelectron coincidence spectra and photoion-fluorescence coincidence spectra have been measured [12,13]. Time-of-flight
mass spectra
TOF spectra reveal the kinetic energy of fragments. At the appearance potential of the HF abstraction, the kinetic energy of the fragments comes to about 1 eV. An identical amount of kinetic energy resuhs for the three isomers. From the shape of the metastable signal, a kinetic energy of 0.84 eV has been calculated [14,15]. Quantum-chemical
calculations
Precalculations of the difluoroethenes and additional isomer structures have been performed on a semiempirical level (MNDO) [16]. The resulting optimized geometries have been taken as the starting values for UHF/RHF ab initio calculations with the program GRADSCF 1171 (geometry optimization with a gradient procedure) using an STO-3G [42] basis set. Transition states were localized as saddle points on the energy hypersurface with one negative intrinsic value of the force constant matrix. With the ab initio optimized geometries, energy calculation was performed with the program Gaussian 76 [18] using a 6-31G * [43] basis set. The resulting energies
308 TABLE
2
Calculated total energies, E, and relative energies, Erel(mol- ‘) A. Educts and products No.
Compound
E
E rel (kcal mol-‘)
(a-u.) a 1 2 3 4 5 6 6s 6b 7 ?a 7b 8 sa 8b 9 9a 9b 10 11
l,l-C,H,F;truns-C2H 2Fc cis-C,H,F;’ H,FC-CF+’ HF,C-CH +. CH2F+ + CF’ CH,F+ CF’ * CH,F‘+CF* CH,F’ CF’. CH,CF++F’ CH,CF+ F’ HCCF++‘+HF HCCF+’ HF FC=CH-FH+’ FHC=C-FH+’
-
275.4075 275.3937 275.3924 275.3300 275.2924 275.2581 138.0877 137.1704 275.2396 138.3985 136.8411 275.2783 175.9133 - 99.3650 - 275.2776 - 175.2769 - 100.0007 - 275.2732 - 275.2615
a
0 9 9 49 72 94
106
81
82
84 91
a 1 a.u. = 27.21 eV; 1 eV = 23.06 kcal; 1 kcal = 4.18 kJ. B. Transition states Transition
Reaction
state TS 1 TS2 TS3 TS4 TS5 TS6 TS7 TSS TS9 TS 10
243 2+4 144 2,348 4+8 148 2,3410 2,34 11 4-9 l-+9
E
E rel
{a-u.) a
{kcal mol-
-
60 72 94 111 107 90 113 122 144 121
275.3117 275.2933 275.2575 275.2316 275.2371 275.2647 275.2281 275.2137 275.1783 275.2156
a 1 a.u. = 27.21 eV; 1 eV = 23.06 kcal; 1 kcal = 4.18
‘) a
kJ.
E(6-31G * /STO-3G) are listed in Table 2. The calculations were performed only for the electronic ground state (2) of the ions. The geometries of relevant ionic structures and transition states are given in Table 3.
309 TABLE
3
Calculated geometries of the most imp&cant ground states and transition states a
w 1
a
3
.
5
’
Bond
c2,
kng~h (A>
c(2)-c(3) C(2)-F(l) c(2)-~(4)
2
Bond angks (degree) 1.46 1.30 1.09
123 119 122 114
C?V
lengths (A)
Bond
F(l)-c(2)-Y3) w4)-a3)--q2) )11(4)-CX3)-W6) F(l)-C(2)-F(S)
Bond angles {degree) 1.44 1.30 1.11
cc2)-cx3) C(2)-370) C(2)-H(5)
F(v-c(2)-C(3) H(S)-C(2)-C(3) F(l)-a2)-H(5)
121 121 119
3 =2h Fbnd lengths (A) 1.44 1.30 I.11
c(2)-C(3) C(2)-F(1) C(2)-W5) 1
F
Bond angles (degree) F(l)-c(2)-H(S) W5)-C-(2)-C(3) w)-c(2)-c(3)
119 122 119
4 3
4
3ond lengths (A) I=(2)-C(3) C(2)-F(l) C(3)-F(4) c(2)-W5) C(2)-H(6)
330nd angles (degree) 1.61 1.36 1.28 1.11 1.11
C(3)-CJ2)-W) t72)-C(3)-F(4) c(3)-~(2)-~(5) C(3)-C(2)-H(6)
F(IKW-HO)
F(l)-C(2)-H(6) H(5)-C(2)-H(6)
111 125 104 104 114 114 109
310 TABLE
3 (continued)
w 6
4
3
2
5
5
Bond lengths (A) C(21-C(31 C(2)-F(5) C(2)-F(6) C(2)-H(1) C(3)-H(4)
90
Bond angles (degree) 1.61 1.36 1.37 1.12 1.13
C(2)-C(3)-H(4) C(3)-C( 2)- H(1) C{3)-C(2)-F(5) C(3)-C(2)-F(6) F(5)-C(2)-F(6) H(l)-C(2)-F(5) H(l)-C(2)-F(6)
137 101 110 108 112 113 112
C-v
Bond lengths (A) 1.10 1.25 1.27
C-H c-c C-F
10
Bond lengths (;i> C(2)-C(5) C(2)-w4) C(5kW6) C(2)-Hfl) F(4)-H(3)
Bond angles (degree) 1.36 1.49 1.33 1.10 0.98
133 115 133 112 118
Bond angles (degree)
Bond lengths (A) C(2)-C(3) C(2)-W) CGbF(S) C(2)-H(4) F(5bH(4)
C(2)-C(S)-F(6) C(5)-C(2)-F(4) C(5)-C(2)-H(1) H(l)-C(2)-F(4) C(2)-F(4)-H(3)
1.38 1.35 1.46 1.10 0.97
F(l)-C(2)-C(3) C(2)-C(3)-F(5) C(3)-C(2)-H(6) H(6)-C(2)-F(1) C(3)-F(S)-H(4)
121 119 118 120 11s
311 TABLE
3 (continued)
@h-f 3
2
5
1
TS 1
Bond lengths (A) C(2)-C(3) C(2)-F(l) C(2)-H(6)
Bond angles (degree) 1.49 1.31 1.11
H(l)-C{2)-C(3) H(6)-C(2)-C(3) F(l)-C(2)-H(6)
120 122 118
TS2
Bond lengths (A) C(2)-C(3) C(3)- F(4) C(2)- F(6) C(2)- H(1) C(3)-H(1) C(2)- H(5)
Bond angles (degree) 1.43 1.31 1.33 1.40 1.37 1.11
121 129 119 108 113 59 58 62
Bond angles {degree ,I
Bond lengths (A) C(2)-C(3) C(3)-F(4) C(3)-F(l) C(2)-F(l) C(2)-W5) C(2)-H(6)
F(6)-C(2)-C(3) F(4)-C(3)-C(2) H(5)-C(2)-F(6) F(6)-C(2)-H(1) F(4)-C(3)-H( 1) C(2)-C(3)-H(1) C(3)-C(2)-H(1) C(2)-H(l)-C(3)
1.47 1.31 1.49 1.50 1.11 1.11
H(S)-C(2)-C(3) H(6)-C(2)-C(3) F(4)-C(3)-C(2) C(2)-C(3)-F(1) C(3)-C(2)-F(1) C(2)-F(l)-C(3) H(S)-C(2)-F(1) H(6)-C(2)-F(1) F(4)-C(3)-F(1) H(S)-C(2)-H(6)
120 120 175 61 60 59 111 111 124 117
TABLE
L
3 (continued)
3
4
TS5
Bond lengths (A) C(2)-C(3) C(2bWl) C(2)-F(5) CG-F(4)
Bond angles (degree) 1.42 1.10 2.05 1.30
1.38 2.04 1.26 1.11 I.10
Bond lengths (A) C(lbC(4) C(44)-F(5) W)-W2) C(4)- W3) CUbJX6) W2PW3)
C(2)-W)-W4) W5WW-WI
122 117 178 97
Bond angles (degree)
Bond lengths (&A) C(2wx3) CW-F(1) C(2)-F(6) C(3k-W4) C(3bW5)
H(l)-C(2)-C(3) H(6)-C(2)-C(3)
C(2)-C(3)-H(4) W+C(3)-W5) H(4)-C(3)-H(S) c/3)-Ct2)-m C(3)-C{2)-F(6) F(l)-C(2)-F(6)
124 110 121 114 146 97
Bond angles (degree) 1.51 1.28 1.42 1.53 1.10 1.09
C(l)-C(4)-H(3) C(4)-C(1)-F(2) C(l)-F(2)-H(3) C(P)-H(3)-F(2) C(l)-C(4)-F(5) C(4MvbW61 F(2)-C(l)-H(6) H(3)-C(4)-F(5)
72 92 90 106 122 142 116 165
313 TABLE
3 (continued)
T2-c 2
1
4
5
TSS
Bond angles (degree)
Bond lengths (A) 1.44 1.43 1.31 1.32 1.11
Cc2)-C(41 C{2)-~(3) C(4)-F(5) C(2)-H(1) C(4)-H(6)
H(lPX+IW C(2)-F(3)-H(1) F(3)-H(lbC(2) C(2)-C(4)-F(5) C(2)-C(4)-H(6) F(5)-C(4)-H(6) H(l)-~(2)-C(4) F(3)-c(2)-c(4)
2
49 60 71 122 119 118 164 115
3
A 1
4
6
TSIO
Bond angles (degree)
Bond lengths (A) C(lPCC4) C(l)-F(2) C(lPF(6) C(4)-H(3) C(4)-HC5) F(2ww31
1.43 1.46 1.30 1.30 1.10 1.11
F(2)-C(l)-C(4) F(2)-C(l)-F(6) C(l)-F(2)-H(3) C(l)-C(4)-H(3) C(l)-C(4)-H(5) F(2)-H(3)-C(4) H(3)-C(4)-H(5) F(6)-C(l)-C(4)
96 115 84 72 126 108 161 144
e For details of the structures and data of 6a, 4b, 7a, 7b, Sa and 9b, see ref. 44.
DISCUSSION
The combination of spectroscopic results and quantum-chemical model calculations gives evidence on the behaviour of the molecular ions at the appearance potential of fragmentations. According to the thermochemical energy balance from the appearance potentials and AH,?98 values from the literature (Table 4), the heats of formation of the fragments can be calculated. This gives a hint to the structure of the resulting fragment ions. The threshold photoelectron spectra confirm the formation of photoelectrons
314 TABLE
4
Heats of formations Compound
used for the thermochemical AH? (kcal mol-‘)
calculations
of C,H,F,+ Ref.
a
cis-C, H 2 F2
-71
trans-C, Hz F2
-70
8
1,1-C,
-80
38
HzFz
8
CH,F
-1
40
CH,F+
209
40
CF CF+ HF F
61
39
275
41
-65 19
fragmentation
39 39
a 1 kcal G 4.18 kJ
without kinetic energy at the appearance potential. Consequently, the corresponding molecular ions possess the internal energy AP-I, (1, is the first ionization potential). It should be mentioned, however, that in addition to zero kinetic energy electrons, photoelectrons with kinetic energy in the range zero to (AP - II) are emitted by irradiation with light of the energy hv = AP. Therefore, the totality of the primary originated C,H,F,+ distributes its internal energy on the electronic, vibrational and rotational states lying in the energy range zero to (AP - II). The consideration of specific vibrational and rotational states is not possible in the framework of our calculations and is, therefore, neglected in the following discussion. The comparison of appearance potentials and ionization potentials shows the formation of molecular ions in the electronic ground state (2) with high vibrational excitation in addition to those in the first excited electronic state (A) with low vibrational excitation. The analysis of the thermochemical energy balance concerning the structure of the fragment ions reveals the following facts. (1) The heat of formation of C,H,F+ produced by F abstraction from the isomers C,H 2FZ+ is 233 kcaI mol-I. The same value appears after H abstraction from C2H3Ff [7] and Cl abstraction from the three isomeric fluorochloroethene cations [19]. Quantum-chemical calculations [20] show a minimum on the hypersurface for the planar structures A and B [37]
where A is more stable than B. Therefore, the product of F abstraction in the threshold region is assigned to structure A. This result implies, at least for the 1,2_difluoroethene cations, an intramolecular rearrangement before the F abstraction is completed. (2) HF abstraction from C,H,Fc yields either cations of fluoroacetylene or fluorovinylidene. Thermochemical considerations reveal the formation of 1.1 eV (about 25 kcal mol-I) excess energy at the experimental threshold. The same value of excess energy is observed from l,l-, cis- and trans-C,H, F2f. The coincidence spectra and time of flight measurements confirm the kinetic energy of the fragments C,HF+ f HF to be about 1 eV. The difference of 0.1 eV excess energy is clearly less than the energetic difference between the cations of acetylene and vinylidene [21].From this result we conclude that, in addition to HF, the cation of fluoroacetylene is formed, probably in a vibrational excited state. (3) Either CH2F+ + CF or CF+ + CH,F results from the fragmentation of the C-C bond, indicating a complex mechanism for the reaction. The experimental appearance potentials agree with the calculated thermodynamic values based on the heat of formations of CH, F +, CH,F, CF+ , CF given in Table 4. The appearance potentials of the fragmentation reactions occur in the range of the second PE band, except for the HF + C,HF+ formation, which is definitely below the second ionization potential; no coincidence between appearance potentials and ionization potentials exists. From the investigation of the fluorescence of the cations 1111, it is known that only the cis isomer exhibits weak fluorescence from its first excited electronic state. From this result, it follows that the lifetime of the 2 states is limited by radiationless deactivation. As impact processes can be excluded on account of the experimental conditions, the mechanism of deactivation is a radiationless transition into the vibrationally excited ground state _%of the cation (internal conversion). These transitions are caused by vibronic coupling and have been the subject of detailed theoretical investigations [22,23] in the case of the ethene cation. It has been shown that especially the conical intersection of two electronic hypersurfaces, caused by the coupling through at least two vibrations, can frequently be expected in polyatomic molecules. The symmetry selection rules existing for this mechanism [23] are fulfilled by the difluoroethene cations. The experimental rate constants of the internal conversion 2-2 are ki, = 8 x 10” s-l for cis-C,H,F,+ and k, 3 10” s-i for tram-C, H 2F$ [12]. Therefore, the mechanism of the fragmentation in the threshold region is essentially determined by the hypersurface of the electronic ground state of the ions. In the following, we discuss the fragmentation processes of C,H,Fc after internal conversion into the ground state has been completed. The internal
316
energy of the molecular ions under discussion comes to 3.55-4.55 eV. In order to understand the strong uniform character of the fragmentation of the isomer% cations, we investigated the activation energy and the geometries of the transition states, which determine the cis-tram isomerization and the rearrangement of the l,l-difluoroethene cation into the l,%difluoroethene cations (see Fig. I). According to our calculations and in agreement with the experimental results, l,l-C,H,Fz+ is about 38 kJ mol-’ (9 kcal mol-‘) more stable than cd% and tram-C, H, Fz+; for all the isomer& cations, a planar structure results, in contrast to the ethene cation for which, from experimental and theoretical investigations, a C-C twisted geometry has been estabhshed [24-275 This difference is reasonable, as the distortion in the ethene cation is caused by a Jahn-Teller effect, which eliminates the energetic degeneracy by symmetry lowering. The bond lengths and bond angles in the difluoroethene cations (Table 3) hardly differ from the values reported for the ethene cation (C-C 1.44 A, C-H 1.07 A, HCH 118O) by Lorquet and co-workers [22J. The planar structure of the isomer+ difluoroethene cations I, 2 and 3 also results from MNDO calculations, which seems remarkable as MNDO proved to be a reliable method for this specific problem [283. The most favourable energetic reaction path for the cis-tram isomerization is the rotation of the C-C bond. The energy of the 90” twisted conformation (TS 1) is 214 kJ mol-’ (51 kcal mof-‘) compared with the planar ground states 2 and 3. The rearrangement of l,l-C,H,F,+ into the 1,2-isomers and vice versa is much more complex. The most favourable reaction path from the energetic point of view crosses a distinct minimum in the energy hypersurface. The minimum marks the intermediate 4 to which a fluoromethylcarbenium structure is assigned. The energy difference between 4 and 1 is 205 k3 mol’ r (49 kcal mol-‘) and 167 kJ mol-’ (40 kcal mol-r) between 2 and 3. Our results reveal similar energy differences as have been calculated on the 6-31G * level for the difference between the neutral fhroroethene [29j and the corresponding fhrorocarbene structure. The isomeric difhtoromethylcarbenium cation 5 is located about 96 kJ mol-I (23 kcal mol-‘) above 4. The transition state TS 2, which connects 4 and 2, 3 is located 96 kJ mol-’ (23 kcal mol-I) above 4. TS 3, connecting 4 and 1 is located 147 M mol-’ (35 kcal mol-‘) above 4_ The energy profile of the isomerization of 1, 2 and 3 is shown in Fig. 1. The transition state TS 3 coincides energetically with the appearance potential of the HF abstraction, which needs the smallest amount of internal energy. The geometries of the transition states TS 2 and TS 3 are similar. Lorquet proposed in the mechanism of H, abstraction from CZHf( 2) a transition state, which leads to the formation of the methylcarbenium intermediate and which has an analogous geometry to TS 2. The energy profile (Fig. 1) shows that the
317
internal energy of the ground state difluoroethene cations at the threshold of fragmentations is sufficient to cross the energy barriers of isomerization. The X,2-difluoroethenes need an excitation energy of 13.7 eV, l,l-difluoroethene an excitation energy of 14.1 eV, to isomerize. As the appearance potentials of the fragmentation reactions exceed these values, it is reasonable that no structure-specific differences in the fragmentation of difluoroethene cation is observed.
The abstraction of fluorine occurs without formation of excess energy, so that the energetic asymptote calculated according to thermochemistry is observed experimentily. For the mechanistic interpretation, one has to determine whether this is possible from the vibrationally excited ground states of 1,2, 3 and 4 or not. The quantum-chemical calcuIations exhibit two transition states (TS 3 and TS 6) with different geometries, which are favourable from the energetic point of view. TS 6 can be reached from 1; at the threshold of G,H, F+, TS 3 is hardly a barrier for the cations 2 and 3 to reach TS 6 via 1, because in our calculations the energy of TS 3 is about 17 kJ molWf (4 kcal mol- ‘) higher than that of TS 6, but it is known that the energy of transition states,
I 140 130 120 -
-;^
z
90
E
80
;
70
-z
60
w’
50
Fig. 1. Caiculated
energy profile for F loss and isomerization
of 1, 2 and 3.
318
especially of those with a cyclic structure, decreases relative to the ground states if the basis sets are improved and configuration interaction is included in the calculation [30]. From our calculations, a small reverse activation energy is expected; this discrepancy with experiment is negligible considering the level of the model calculations. Nevertheless, it cannot be decided which of the transition states TS 3 of TS 6 is rate determining for the F abstraction from cis- and tram-C, I-I, F2‘+, but there is good evidence that the isomerization is a necessary step on the fragmentation pathway of these two ions at the appearance potential_ If the internal energy of the molecular ions is increased slightly (about IO-15 kcal mol-i), two additional transition states, TS 4 and TS 5, have to be considered_ TS 4 is the top of direct F abstraction from 2 and 3, TS 5 is the transiton state which controls F abstraction from the intermediate 4. In contrast to the situation at the appearance potential of the F abstraction, where only one reaction channel has to be discussed, at slightly higher energies, the reaction mechanism principally consists of three reaction channels and at present there is no evidence on the contribution of each channel to the yield of C2H2F+. It should be stressed that the previously mentioned considerations are valid for ground state ions; the structures of transition states of ions in excited electronic states will differ from those given in Table 3. The F abstraction from 1,2_difluoroethene cations in their c and 8 states according to coincidence measurements [12] occurs directly without internal conversion. HF abstraction (Fig. 2)
The transition states controlling the HF abstraction are located above the thermodynamically calculated appearance potential because a considerable amount of excess energy is observed_ This is confirmed by the energies of the transition states TS 7, TS 8 and TS 10 resulting from ;he model calculations. The 1,2 elimination of HF is completed via weak minima [lO,ll] in the energy hypersurface; compounds of this type have been identified experimentally and theoretically [32-351. In addition to the 1,2 elimination, the 1,l elimination via TS 10 may be discussed from energetic considerations, but this reaction channel first yields the fluorovinylidene cation, which should isomerize to the fluoroacetylene cation by analogy with the neutral fluorovinylidene practically without an activation barrier [31,36]. As can be seen from the energy scheme (Fig. 2), even at the appearance potential, three different reaction channels (TS 7, TS 8 and TS 10) are energetically possible for HF abstraction; it is not possible to distinguish between 1,2- and l,l-elimination of HF. The uniform behaviour of the three isomers is caused
319
by the relative low energy barriers for the isomerization and not by fragmentation of the intermediate 4. TS 9 is clearly too high in energy to be significant for HF abstraction at threshold. The lack of F2 formation and the very low yield of C,F,-C resulting from Hi, abstraction is remarkable, especially as the cation of vinyl fluoride produces high amounts of C,HF+ by H, abstraction. The H, abstraction from the ethene cation starts by a Jahn-Teller distortion of the intermediate methylcarbonium ion (C,,,). The corresponding fluorinated intermediate in the C,H,F,+ system is 4, which has C, symmetry, and therefore the mechanism operating in the 1,l elimination of H, from the ethene cation is not possible. In the framework of our calculations, no transition states for H 2 or F, abstration have been recognized. CF + and CH,F
+ formation
(Fig. 3)
After fragmentation of the C-C double bond, one expects fragments like CF,+ , CFH+, etc. We observed CH,F+ and CF+. The formation of these ions requires a rearrangement of C, H,F2+ before fragmentation. The model calculations give clear evidence that the intermediate 4 is the precursor ion of C-C fragmentation. In agreement with the experimental results, the calculations reveal the lower appearance potential for CH,F+ compared with CF+. The result that no reverse activation energy is involved confirms the lack of excess energy.
TS 9 I..
TS 10
_f
90
Z E
80
$
70
5
60
ldJ
50
Fig. 2. Calculated
energy profile for HF elimination
from 1, 2 and 3.
320
1Lil 130 120 110
APcl=+) =lU2 eV
CF= CH,F I
l
loo
AP(CH,F+) =I&.64 ev
CyFtCF
40 30 20 10
Fig. 3. Calculated
energy profile for C-C
cleavage_
CONCLUSIONS
The results of different experimental methods and quantum-chemical model calculations give evidence that the fragmentation of dif’luoroethene cations in the threshold region of the most intense fragmentation processes occurs in the electronic ground state of the molecular ion. It is possible to explain the mechanism of the different decay reactions with the aid of quantum-chemical calculations. They reveal two main reasons for the uniform behaviour of the different isomers. (a) The activation barriers for the isomerization are lower than for the fragmentation processes and (b) fluoromethylfluorocarbenium, the precursor ion of C-C fragmentation, can be easily accessed by l,l- and 1,2-difluoroethene cations. The reaction mechanism, even in the case of simple C-F fragmentation, is a complex succession of elementary steps and can be described by superpositioning reaction channels. ACKNOWLEDGEMENTS
Financial support from the Fonds der Chemischen Industrie is gratefully acknowledged, especially for a Liebig grant (G-F.) and a Ph.D. grant (MS.).
321
Additional financial and technical support has been given by Gesellschaft der Freunde der Technischen Universitat and Zentrales Rechenzentrum der Technischen Universittit. REFERENCES 1 F.W. McLafferty, Interpretation of Mass Spectra, University Science Books, Mill Valley, CA, 1980. R.D. Bowen, D.H. Williams and H. Schwarz, Angew. Chem., 91 (1979) 484. W. Forst, Theory of Unimolecular Reactions, Academic Press, New York, 1973. R.A. Marcus, J. Chem. Phys., 20 (1952) 359. ELM. Rosenstock, M-3. Waflenstein, A.L. Wahrhaftig and I-Z. Eyring, Proc. Natl. Acad. Sci. U.S.A., 38 (1952) 667. 6 J.C. Lorquet, Org. Mass Spectrom., 16 (1981) 469. 7 D. Reinke, R. Kr&ssig and H. Baumg--1, Z. Naturforsch. Teil A, 28 (1973) J.021. 8 H.W. Jochims, W. Lohr and H. Baumg&rtel, Nouv, J. Chim., 3 41979) 109. 9 C.R. Brundle, M.B. Robin, N.A. Kuebler and H. Basch, J. Am. Chem. Sot., 94 (1972) 1451. 10 J.A. Sell and A. Kuppermann, J. Chem. Phys., 71 (1979) 4703. 11 J.P. Maier, 0. Marthaler and G. Bieri, Chem. Phys., 44 (1979) 131. 12 J.P. Stadelmann and 3. Vogt, Int. J. Mass Spectrom. Ion Phys., 35 (1980) 83. 13 R. Frey, Dissertation, Universittit F&burg, 1974. 14 R.G. Cooks, J.H. Beynon, R.M. Gaprioli and G.R. Lester, Metastable Ions, Elsevier, Amsterdam, 1973. 15 T.H. Heir& and J. Vogt, unpublished results. 16 M,J.S. Dewar and W. Thiel, J. Am. Chem. Sot., 99 (1977) 4889, 4907. 17 A. Komornicki, available from Quantum Chemical Program Exchange. 18 J.S. Binldey, RA. Whit&de, P.C. Haviharan, R. Seeger, J.A. Pople, W.J. Hehre and M.D. Newton, Quantum Chemical Program Exchange No. 368. 19 G. Tornow and H. BaumgUtel, unpubiished results. 20 P. Kolhnan, 5. Nelson and S. Rothenberg, J. Phys. Chem., 82 (1978) 1403. 21 P. Rusmus, P. Botschw&x and J.P. Maier, Chem. Phys. Lett,, 84 (1981) 71. 22 C. Sannen, G. Raseev, C. Gallay, G. Fauville and J.C. Lorquet, J. Chem. Phys., 74 (1981) 2402. 23 H. K&ppel, L.S. C=ederbaum, W. Domcke and S.S. Shaik, Angew. Chem., 95 (1983) 221 and literature cited therein. 24 R.S. Mull&en and C.C.J. Roothan, J. Chem. Phys., 41 (1947) 219. 25 A.J. Merer and L-J. Schonveld, J. Chem. Fhus., 48 (1968) 522. 26 H. IMppel, W. Domcke, L-S, Cederbaum and W. vcm Niessen, J. Chem. Phys., 69 (1978) 4252. 27 R.J. Buenker, SD. Peyetioff and H.L. Hsu, Chem. Phys. L&t., 11 (1971) 65, 28 D-J. Bellville and N.L. Bauld, J. Am. Chem. Sot., 104 (1982) 294. 29 S. Kato and H. Morokuma, J_ Chem. Phys., 74 (1981) 6285. 30 K. Raghavachari, M.J. Frisch, J.A. Pople and P.v.R. Schleyer, Chem. Phys, Lett., 85 (1982) 145. 31 M.J. Frisch, R. Krishnan, J.A. Pople and P.Y.R. Schleyer, Chem. Phys. Lett., 81 (1981) 421. 32 N-J. 3ouma, J.K_ Mcceod and L. Radom, J. Am. Chem. Sot., 104 (1982) 2930.
322 33 J.L. Holmes, F.P. Lossing, J.K. Terlouw and P.C. Bingers, J. Am. Chem. Sot., 104 (1982) 2931. 34 H. Halim, B. Ciommer and H. Schwarz, Angew. Chem., 94 (1982) 547. 35 Y. Apeloig, B. Ciommer, G. Frenking, M. Kami, A. Mandelbaum, H. Schwarz and A. Weisz, J. Am. Chem. Sot., 105 (1983) 2186. 36 G. Frenking, Chem. Phys. Lett., 100 (1983) 484. 37 E. Keller, Chiuz, 2 (1980) 56. 38 J.H. Simmie and E. Tschuikow-Roux, J. Phys. Chem.., 74 (1970) 4075. 39 D.R. Stull and H. Prophet, JANAF Thermochemical Tables, Natl. Bur. Stand., NSRDSNBS, 37 (1971). 40 J.C. Franklin, J.G. Dillard, H.M. Rosenstock, J.T. Herron, K. Draxl and F.H. Field, Natl. Bur. Stand., NSRDS-NBS, 26 (1969). 41 J. Phys. Chem. Ref. Data, 3 (2) (1974). 42 W. J. Hehre, R.F. Stewart and J.A. Pople, J, Chem. Phys., 51 (1969) :627. 43 J.S. Binkley and J.A. Pople, J. Chem. Phys., 66 (1979) 879. 44 R.A_ Whiteside, M.J. Frisch, J.S. Binkley, D.J. DeFrees, H.P. Schlegel, K. Raghavachavi and J.A. Pople, Carnegie-Mellon Quantum Chemistry Archive, 2nd edn., 1981, CamegieMellon University, Pittsburgh, PA.