Khoa Minh
Tran
,
Andrew M.
McAnoy
and
John H.
Bowie
Department of Chemistry, The University of Adelaide, South Australia 5005
First published on 27th February 2004
Neutrals CCCO, CC13CO, CCCS and CC13CS have been prepared by one-electron vertical (Franck–Condon) oxidation of the precursor anion radicals (CCCO)−˙, (CC13CO)−˙, (CCCS)−˙ and (CC13CS)−˙ respectively in collision cells of a reverse sector mass spectrometer. Ionisation of the neutrals to decomposing cations shows the neutrals to be stable for the microsecond duration of the neutralisation–ionisation (−NR+) experiment. No rearrangement of the label in energised CC13CO or CC13CS occurs during these experiments. In contrast, minor rearrangement of (CC13CO)+˙ is observed [(CC13CO)+˙ → (OCC13C)+˙], while significant rearrangement occurs for (CC13CS)+˙ [(CC13CS)+˙ → (SCC13C)+˙]. Theoretical calculations at the CCSD(T)/aug-cc-pVDZ//B3LYP/6-31G(d) level of theory show that the cationic rearrangements occur by stepwise processes via key rhombic structures. Overall, the degenerate processes result in O and S migration from C-3 to C-1. The cations (CCCO)+˙ and (CCCS)+˙ require excess energies of ≥ 516 and ≥ 226 kJ mol−1 respectively to effect rearrangement.
Many cumulenes and heterocumulenes have been detected in interstellar dust clouds, and in circumstellar envelopes of carbon-rich suns.5–9 Two of the most abundant heterocumulenes to be reported are CCCO and CCCS. Sulfur compounds are particularly interesting since they are often more abundant than their oxygen analogues, even though the cosmic abundance of sulfur is less than that of oxygen.10–12
The aim of this study is to make CC13CO and CC13CS, and, using the neutralisation–reionisation (−NR+) technique as a probe, to determine whether any rearrangement occurs for energised neutrals. We are interested in comparing these two systems for two reasons: (i) because of the cumulene rearrangements described above, and (ii) because rearrangements occur for the related systems CCCHO and CCCHS. In the former case, CCCHO rearranges to HCCCO by H rearrangement,13 whereas CCCHS rearranges to HCCCS by competitive H and S rearrangement.14 Perhaps CCCS may undergo S rearrangement, but CCCO will be stable?
Neutral CCCO has been detected [together with the related propynal (HCC–CHO), from which CCCO may be formed] in the dark molecular cloud TMC-1.15 It has been made by a number of methods,16–19 including neutralisation of [CCCO]+˙17 and [CCCO]−˙.19 A number of theoretical studies have been devoted to cumulene oxides (including CCCO),20–25 and their rotational spectra have been measured.26,27 There are two stable forms of CCCO: the 1Σ singlet ground state is linear,27 while the 3A′ triplet state is bent and 271 kJ less stable than the ground state singlet (at the RCCSD(T)/aug-cc-pVDZ//B3LYP/6-31+G* level of theory).19
Sulfur carbon clusters occur in both dark clouds and circumstellar envelopes.28–30 Neutral CCCS has been detected in the circumstellar envelope of the star IRC+1021631,32 and has been synthesised in the laboratory.32,33 A number of theoretical studies on CCCS have been reported,28,29,32,34 and it has been proposed that CCCS may be formed from the interstellar radical CCCH, as shown in eqn. (1).34
CCCH˙ + S → CCCS + H˙ | (1) |
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Scheme 1 |
The syntheses commence with the TMS derivative of acetylene, which is allowed to react with dimethylformamide to yield the aldehyde. The aldehyde is converted to the acetal which on base treatment allows the addition of the second TMS group. Hydrolysis of the acetal furnishes the bis TMS ketone A, which is the precursor of (CCCO)−˙. The ketone A can be converted into the thioketone B by reaction with bis TMS disulfide. However the thioketone B is unstable, and is trapped in situ to form the Diels–Alder adduct C. This species is used (as shown below) to produce (CCCS)−˙. The same sequence was used for the formation of the two 13C labelled precursor molecules. The label is introduced in the first step of the synthesis, i.e. by the reaction between TMS acetylene and Me2N13CHO.
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Scheme 2 |
Before attempting the one-electron oxidations of (CCCO)−˙ and (CCCS)−˙ to the neutrals, it is necessary to be certain that the anions do not rearrange prior to or during the oxidation process. The collision induced mass spectra (MS/MS) of (CCCO)−˙ and (CCCS)−˙ show these radical anions to be very stable. The only fragment peaks in the spectra are of very small abundance. They correspond to loss of CO [from (CCCO)−˙] and the formation of (CS)−˙ [from (CCCS)−˙]. The two labelled anions fragment specifically as shown in eqns. (4) and (5). Thus there is no rearrangement of either system when collisionally activated.
(CC13CO)−˙ → (CC)−˙ + 13CO | (4) |
(CC13CS)−˙ → (13CS)−˙ + CC | (5) |
Although the above experimental results show that there is no rearrangement of either (CCCO)−˙ or (CCCS)−˙ during conditions equivalent to those used for the vertical one-electron oxidations of the anions to the neutrals, it is still of interest to examine theoretically how much excess energy is required to effect oxygen and sulfur rearrangement. These reactions have been studied at the CCSD(T)/aug-cc-pVDZ//B3LYP/6-31G(d) level of theory using Gaussian 98. The results are summarised in Figs. 1 and 2, with full details given in Tables 1 and 2. The lowest energy pathway for degenerate oxygen rearrangement is concerted with a barrier to the transition state of 356 kJ mol−1. This is far too high to allow rearrangement of (CCCO)−˙ in the first collision cell of the mass spectrometer. The S system is the more complex of the two (see Fig. 2). In this case there is no low-energy concerted reaction. The rearrangement process is a stepwise sequence proceeding through a “rhombic” transition state 219 kJ mol−1 above (CCCS)−˙. This is more energetically favourable than the oxygen rearrangement shown in Fig. 1. Even so, the process is not observed experimentally under conditions of collisional activation.
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Fig. 1 Degenerate rearrangement of (CCCO)−˙. Energies [kJ mol−1, relative to (CCCO)−˙, 0 kJ mol−1] at the CCSD(T)/aug-cc-pVDZ//B3LYP/6-31G(d) level of theory. For details of geometries and energies of minima and transition states see Table 1. |
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Fig. 2 Degenerate rearrangement of (CCCS)−˙. Energies [kJ mol−1, relative to (CCCS)−˙, 0 kJ mol−1] at the CCSD(T)/aug-cc-pVDZ//B3LYP/6-31G(d) level of theory. For details of geometries and energies of minima and transition states see Table 2. |
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a Energies are calculated at the CCSD(T)/aug-cc-pVDZ//B3LYP/6-31G(d) level of theory and are relative to CCCO−˙ (−188.9643951 Hartrees). b B3LYP/6-31G(d) geometries. | ||
State | 2A′ | 2A″ |
Relative energy (kJ mol−1)a | 0.0 | 356.1 |
Dipole moment (Debye)b | 3.45 | 2.96 |
C1C2 (Å) | 1.275 | 1.288 |
C2C3 | 1.363 | 1.673 |
C1C3 | 1.928 | |
C3O | 1.223 | 1.524 |
C1C2C3 (°) | 167.3 | 80.6 |
C2C3O | 142.8 | 49.3 |
C1C2C3O | 180.0 | 180.0 |
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a Energies are calculated at the CCSD(T)/aug-cc-pVDZ//B3LYP/6-31G(d) level of theory and are relative to CCCS−˙ (−511.5816670 Hartrees). b B3LYP/6-31G(d) geometries. | ||||
State | 2Π | — | 2B2 | 2A″ |
Relative energy (kJ mol−1)a | 0.0 | 118.3 | 30.3 | 218.7 |
Dipole moment (Debye)b | 4.17 | 2.36 | 1.59 | 2.74 |
C1C2 (Å) | 1.302 | 1.334 | 1.366 | 1.411 |
C2C3 | 1.296 | 1.363 | 1.427 | 1.642 |
C1C3 | 1.935 | 1.366 | 1.411 | |
C3S | 1.622 | 1.631 | 1.660 | 1.885 |
C1C2C3 (°) | 180.0 | 91.7 | 61.4 | 54.4 |
C2C3S | 180.0 | 167.0 | 151.4 | 64.2 |
C1C2C3S | 180.0 | −67.0 | 180.0 | 157.2 |
We conclude that the precursor radical anions (CCCO)−˙ and (CCCS)−˙ are suitable precursors to effect one-electron oxidation to CCCO and CCCS.
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Fig. 3 Spectra of (CC13CO)−˙: A) −CR+, B) −NR+. VG ZAB 2HF mass spectrometer. For experimental conditions see Experimental section. |
(CCCO)−˙ | |
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−CR+ | 52(parent)100; 40(C)2.3; 36(O)2.9; 28(C2)1.2; 24(CO)2.0; 12(C2O)0.2 |
−NR+ | 52(parent)100; 40(C)2.5; 36(O)2.9; 28(C2)2.4; 24(CO)2.2; 12(C2O)0.2 |
(CCCS)−˙ | |
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−CR+ | 68(parent)100; 56(C)10; 44(C2)3.6; 36(S)0.6; 32(C3)1.6; 24(CS)0.2 |
−NR+ | 68(parent)100; 56(C)5.5; 44(C2)2.5; 36(S)1.5; 32(C3)1.7; 24(CS)0.2 |
The −NR+ spectrum (Fig. 3B) shows a pronounced parent peak (m/z 53) which indicates that a portion of the neutrals formed by one-electron oxidation are stable for the period of the NR experiment, i.e. ≥ 1 microsecond. The peak ratios of m/z 25 to 24 and m/z 28 to 29 are slightly less in the −NR+ spectrum (in comparison to those observed in the −CR+ spectrum). In addition, m/z 29 and 24 are of larger abundance in the −NR+ spectrum (than the corresponding peaks in the −CR+ spectrum). If neutral rearrangements occur, m/z 25 and 28 would be of greater abundance in the −NR+ than the −CR+ spectrum. Since this is not so, the rearrangement peaks observed in the −NR+ spectrum must originate from the cation, not from neutral CC13CO.
The above data show that CC13CO [formed by neutralisation of (CC13CO)−˙] is a stable species which does not undergo rearrangement during the microsecond duration of the NR experiment. Rearrangement of singlet CCCO has also been studied theoretically at the CCSD(T)/aug-cc-pVDZ//B3LYP/6-31G(d) level of theory. The reaction coordinate is shown in Fig. 4 with full details of all minima and transition states listed in Table 4. There is no low energy concerted process (cf.Fig. 1), instead, a high energy stepwise process proceeds through a symmetrical rhombic transition state, which lies 479 kJ mol−1 above singlet CCCO. Experiment and theory agree; there is no facile degenerate rearrangement of singlet CCCO.
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Fig. 4 Degenerate rearrangement of singlet CCCO. Energies [kJ mol−1, relative to CCCO, 0 kJ mol−1] at the CCSD(T)/aug-cc-pVDZ//B3LYP/6-31G(d) level of theory. For details of geometries and energies of minima and transition states see Table 4. |
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a Energies are calculated at the CCSD(T)/aug-cc-pVDZ//B3LYP/6-31G(d) level of theory and are relative to CCCO (−188.9262679 Hartrees). b B3LYP/6-31G(d) geometries. | ||||
State | 1Σ | 1A′ | 1A′ | 1A′ |
Relative energy (kJ mol−1)a | 0.0 | 385.6 | 376.4 | 478.6 |
Dipole moment (Debye)b | 1.95 | 2.66 | 1.88 | 0.55 |
C1C2 (Å) | 1.278 | 1.282 | 1.296 | 1.428 |
C2C3 | 1.299 | 1.483 | 1.478 | 1.428 |
C3O | 1.161 | 1.199 | 1.279 | 1.467 |
C2O | 1.876 | 1.483 | 1.731 | |
C1C2C3 (°) | 180.0 | 174.3 | 162.8 | 91.8 |
C2C3O | 180.0 | 88.1 | 64.6 | 73.4 |
C1C2C3O | 180.0 | 0.0 | 180.0 | 42.2 |
Why is m/z 29 more abundant in the −NR+ spectrum of (CC13CO)−˙ (see Figs. 3A and 3B)? The reason for this is apparent when the thermochemistries of the decompositions of CCCO and (CCCO)+˙ are considered. These are compared in Table 5. The losses of C and O noted in Fig. 3B are high energy decompositions of the cation. The only possible neutral decomposition is loss of CO from CCCO (+334 kJ mol−1). If some of the neutrals have energies ≥ 334 kJ mol−1 they may decompose to CC and CO. Thus the enhanced abundance of m/z 29 in the −NR+ spectrum of (CC13CO)−˙ is due to dissociation of some neutral CC13CO molecules prior to reionisation. This also explains the smaller abundance of m/z 28 in the −NR+ spectrum, since this is only produced following or accompanying rearrangement of the cation.
The −CR+ and −NR+ spectra of (CC13CO)−˙ show that the doublet cation radical (CC13CO)+˙ does undergo some rearrangement to (OCC13C)+˙. However this rearrangement is minor, and clearly unfavourable. The cation rearrangement has been investigated theoretically. The process is summarised in Fig. 5 with full details listed in Table 6. The cation rearrangement is quite similar to that already described for the neutral. Energetically, it is less favourable than the neutral rearrangement, requiring a minimum of 516 kJ mol−1 to effect the rearrangement. Thus it is not surprising that the cation rearrangement is only a minor process. The cations that rearrange are likely to obtain this excess energy (≥ 516 kJ mol−1) by keV collisions of parent cations occurring subsequent to the Franck–Condon vertical oxidation process.
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Fig. 5 Degenerate rearrangement of (CCCO)+˙. Energies [kJ mol−1, relative to (CCCO)+˙, 0 kJ mol−1] at the CCSD(T)/aug-cc-pVDZ//B3LYP/6-31G(d) level of theory. For details of geometries and energies of minima and transition states see Table 6. |
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a Energies are calculated at the CCSD(T)/aug-cc-pVDZ//B3LYP/6-31G(d) level of theory and are relative to CCCO+˙ (−188.5319672 Hartrees). b B3LYP/6-31G(d) geometries. | ||||
State | 2Σ | 2A′ | — | 2B2 |
Relative energy (kJ mol−1)a | 0.0 | 431.9 | 414.9 | 516.1 |
Dipole moment (Debye)b | 2.73 | 1.38 | 0.57 | 2.21 |
C1C2 (Å) | 1.221 | 1.239 | 1.347 | 1.389 |
C2C3 | 1.341 | 1.500 | 1.553 | 1.389 |
C3O | 1.134 | 1.217 | 1.290 | 1.851 |
C2O | 1.589 | 1.383 | 1.394 | |
C1C2C3 (°) | 180.0 | 175.6 | 151.3 | 166.8 |
C2C3O | 180.0 | 70.7 | 57.3 | 48.4 |
C1C2C3O | 180.0 | 180.0 | −139.2 | 0.0 |
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Fig. 6 Spectra of (CC13CS)−˙: A) −CR+, B) −NR+. VG ZAB 2HF mass spectrometer. For experimental conditions see Experimental section. |
The degenerate rearrangements of CCCS and (CCCS)+˙ have been investigated theoretically. There are two stable forms of CCCS; namely the linear 1Σ singlet and the bent triplet. It has been reported28 that these two structures are separated by 260 kJ mol−1, so we have not considered the triplet structure further. The degenerate stepwise rearrangement of singlet CCCS is shown in Fig. 7, with full details of minima and transition states listed in Table 7. There is no concerted rearrangement of lower energy. The S rearrangement is more favourable than the corresponding O rearrangement of CCCO: CCCS and CCCO require excess energies of ≥ 336 and 479 kJ mol−1 respectively to effect rearrangement. Even so, the S rearrangement is highly energetic in accord with the experimental observation that it does not occur under NR conditions. Interestingly, the reaction proceeds through a key rhombic C3S intermediate (216 kJ mol−1 above CCCS), while the corresponding rhombic species in the CCCO rearrangement is a transition state.
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Fig. 7 Degenerate rearrangement of singlet CCCO. Energies [kJ mol−1, relative to CCCS, 0 kJ mol−1] at the CCSD(T)/aug-cc-pVDZ//B3LYP/6-31G(d) level of theory. For details of geometries and energies of minima and transition states see Table 7. |
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a Energies are calculated at the CCSD(T)/aug-cc-pVDZ//B3LYP/6-31G(d) level of theory and are relative to CCCS (−511.5287282 Hartrees). b B3LYP/6-31G(d) geometries. | |||||
State | 1Σ | 1A′ | 1A′ | 1A′ | 1A1 |
Relative energy (kJ mol−1)a | 0.0 | 336.1 | 266.2 | 278.6 | 216.3 |
Dipole moment (Debye)b | 3.04 | 4.11 | 2.17 | 2.52 | 2.86 |
C1C2 (Å) | 1.286 | 1.281 | 1.309 | 1.303 | 1.420 |
C2C3 | 1.294 | 1.420 | 1.431 | 1.469 | 1.473 |
C3S | 1.551 | 1.609 | 1.741 | 1.788 | 1.762 |
C2S | 2.292 | 1.818 | 1.740 | 1.420 | |
C1C2C3 (°) | 180.0 | 177.8 | 159.4 | 103.9 | 58.7 |
C2C3S | 180.0 | 98.2 | 69.1 | 63.7 | 65.3 |
C1C2C3S | 180.0 | 0.0 | 180.0 | 180.0 | 180.0 |
The −CR+ and −NR+ spectra of (CC13CS)−˙ show that significant rearrangement occurs within the parent radical cation. The results of molecular modelling the reaction coordinate of the degenerate rearrangement of (CCCS)+˙ are shown in Fig. 8, with full details of minima and transition states listed in Table 8. The rearrangement is a stepwise process. No lower energy concerted process was found. This stepwise rearrangement is more energetically feasible than any other rearrangements considered in this paper. It is very similar in mechanism to the neutral CCCS rearrangement considered above. The cation requires an excess energy of ≥ 226 kJ mol−1 in order to effect the degenerate rearrangement which proceeds through a symmetrical ‘rhombic’ C3S cation lying 189 kJ mol−1 above CCCS. The neutral and cationic CCCS have very similar geometries (see Tables 6 and 7), thus the excess energy of the cation as a function of the Franck–Condon vertical process will be small. The Franck–Condon excess energy can be calculated since it is the difference in energy between doublet (CCCS)+˙ and of the cation with neutral geometry on the cationic surface. This energy is only 17 kJ mol−1 at the level of theory used in this investigation, hence rearranging (CCCS)+˙ cations must obtain the additional energy by keV collisions with the collision gas in the collision cell.
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Fig. 8 Degenerate rearrangement of (CCCS)+˙. Energies [kJ mol−1, relative to (CCCS)+˙, 0 kJ mol−1] at the CCSD(T)/aug-cc-pVDZ//B3LYP/6-31G(d) level of theory. For details of geometries and energies of minima and transition states see Table 8. |
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a Energies are calculated at the CCSD(T)/aug-cc-pVDZ//B3LYP/6-31G(d) level of theory and are relative to CCCS+˙ (−511.1557742 Hartrees). b B3LYP/6-31G(d) geometries. | |||||
State | 2Σ | 2A′ | 2A1 | 2A″ | 2A1 |
Relative energy (kJ mol−1)a | 0.0 | 125.8 | 108.6 | 225.8 | 188.5 |
Dipole moment (Debye)b | 1.35 | 1.05 | 1.08 | 2.51 | 0.19 |
C1C2 (Å) | 1.227 | 1.300 | 1.304 | 1.521 | 1.345 |
C2C3 | 1.336 | 1.453 | 1.511 | 1.554 | 1.509 |
C1C3 | 1.632 | 1.304 | 1.382 | 1.345 | |
C3S | 1.510 | 1.533 | 1.542 | 1.646 | 1.757 |
C2S | 1.914 | 1.757 | |||
C1C2C3 (°) | 180.0 | 72.5 | 64.5 | 53.4 | 55.9 |
C2C3S | 180.0 | 161.2 | 154.5 | 73.4 | 64.6 |
C1C2C3S | 180.0 | 180.0 | 180.0 | 180.0 | 180.0 |
The −NR+ spectrum of (CC13CS)−˙ is weak in comparison to the −CR+ spectrum, but it shows an area ratio of close to 3 : 1 for the losses of C and 13C, and 1 : 1 for losses of CC and C13C. In the case of (CC13CS)+˙, the key intermediate is the ‘rhombic’ structure shown in Scheme 3. This intermediate can ring open to give the two three membered ring systems shown in Scheme 3. Equilibration of this system accompanied by ring opening will give (CC13CS)+˙, (C13CCS)+˙ and (13CCCS)+˙ in the ratio 1 : 1 : 1. This will lead to the decomposition ratios (i) C to 13C loss (2 : 1), and (ii) CC to C13C loss (1 : 2), assuming there is no 13C kinetic isotope effect operating. The experimentally observed ratios indicate that incomplete carbon scrambling accompanies the fragmentations outlined above. The losses of C and CC from (CCCS)+˙ are both high energy processes [+784 to +581 kJ mol−1 respectively (at the CCSD(T)/aug-cc-pVDZ//B3LYP/6-31G(d) level of theory)].
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Scheme 3 |
2 The majority of neutrals CCCO and CCCS formed by this Franck–Condon process are stable for at least a microsecond, but a small proportion may decompose to form CC and CO, and S and CCC respectively. Neither oxygen (CCCO to OCCC) nor sulfur rearrangement (CCCS to SCCC) occurs during the NR timeframe.
3 The structures of the neutrals CCCO and CCCS were probed by a consideration of the decompositions of the corresponding cations (produced by ionisation of the neutrals). A minority of the (CCCO)+˙ cations undergo the degenerate rearrangement to (OCCC)+˙.
In contrast, the analogous (CCCS)+˙ to (SCCC)+˙ rearrangement is significant under NR conditions.
4 The reason why CCCS does not undergo S rearrangement while CCCHS does,14 is due to the lower energy requirement for rearrangement of CCCHS [CCCHS (+100 kJ mol−1);14 CCCS (+ 393 kJ mol−1)]. This difference is primarily a consequence of the different geometries of the two systems: for CCCHS and CCCS, the CCS angles are 114.4°14 and 180° respectively.
Neutralisation–reionisation37–39 (−NR+) experiments were performed for mass-selected anions utilising the dual collision cells located between the magnetic and electric sectors. Neutralisation of anions was effected by collisional electron detachment using O2 at 80% transmittance (of the main beam) as the collision gas in the first collision cell, while reionisation to cations was achieved by collision of the neutrals with O2 (80% transmittance) in the second collision cell. In order to detect a reionisation signal due to the parent neutral, the neutral species must be stable for the one microsecond timeframe of this experiment. Charge reversal (−CR+) spectra40,41 were recorded using single collision conditions in collision cell 1 (O2, 80% transmission of main beam).
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