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Chemical Physics Letters 449 (2007) 44–52 www.elsevier.com/locate/cplett
A crossed beam investigation of the reactions of 1 þ 1 þ tricarbon molecules, C3ðX Rg Þ, with acetylene, C2H2ðX Rg Þ, 1 1 ethylene, C2H4ðX AgÞ, and benzene, C6H6ðX A1gÞ
Xibin Gu a, Ying Guo a, Alexander M. Mebel b, Ralf I. Kaiser a,*
a Department of Chemistry, University of Hawai’i at Manoa, Honolulu, HI 96822, United States b Department of Chemistry and Biochemistry, Florida International University, Miami, FL 33199, United States
Received 26 July 2007; in final form 10 October 2007 Available online 13 October 2007
Abstract
Crossed molecular beams experiments have been conducted to examine the chemical dynamics of the reactions of the tricarbon mole- 1 þ 1 þ 1 1 cule, C3ðX Rg Þ, with acetylene, C2H2ðX Rg Þ, ethylene, C2H4(X Ag), and benzene, C6H6(X A1g). All reactions proceeded via a tricarbon versus atomic hydrogen exchange pathway and are defined by characteristic threshold energies of 80–90 kJ mol 1 (acetylene), 40–50 kJ mol 1 (ethylene), and 90–110 kJ mol 1 (benzene) yielding the 2,4-pentadiynylidyne radical, HCCCCC(X2P) (acetylene), 2 2 1,2,3,4-pentatetraene-yl-1, HCCCCCH2(X B1) (ethylene), and phenyltricarbon, C6H5CCC(X A) (benzene) plus atomic hydrogen. Our findings suggest that tricarbon molecules can react in high temperature combustion flames with unsaturated hydrocarbon molecules to form hydrogen-deficient radicals. Published by Elsevier B.V.
1. Introduction key growth species in low power reactors [5], recent spectro- scopic investigations of CVD environments detected The energetics and dynamics of the reactions of small dicarbon molecules via laser induced fluorescence. More carbon molecules are of paramount importance in under- complex species such as diacetylene (C4H2), C4H3 isomers, standing combustion processes [1] and the chemical vapor and vinylacetylene (C4H4) of hitherto unknown origin were deposition of diamonds [2]. Particular attention has been further identified in diamond formation processes [6]. devoted to understand the reactions of small carbon mole- These processes are closely related to the growth of carbon cules and to incorporate these data into combustion and clusters in carbon-rich stars as well as to the synthesis of chemical vapor deposition models. Dicarbon and tricarbon diamonds in hydrogen-poor preplanetary nebulae. Due to are the simplest representatives of bare carbon molecules. the importance of dicarbon and tricarbon reactions, the Both species have been identified in high temperature com- kinetics of these species have been extensively investigated. bustion flames under fuel-rich conditions of incipient soot In these studies, the disappearance of, for instance, dicar- formation at concentrations near 1015 cm 3 [3]. Reactions bon in both the singlet ground state and the electronically of dicarbon and tricarbon are also important in chemical excited triplet state was followed. The reactions of 1 þ vapor deposition processes of nano diamonds [4]. C2ðX Rg Þ were found to be fast (of the gas kinetic order Although current models favor the methyl radical as the with rate constants of a few 10 10 cm3 s 1 when the molec- ular partner is an unsaturated hydrocarbon), whereas the 3 C2(a Pu) reactions were suggested to be systematically 1 * þ Corresponding author. Fax: +1 808 9565908. slower. Rate constants of C3ðX Rg Þ reactions with unsatu- E-mail address: [email protected] (X. Gu). rated hydrocarbons have been tackled, too. They were
0009-2614/$ - see front matter Published by Elsevier B.V. doi:10.1016/j.cplett.2007.10.041 X. Gu et al. / Chemical Physics Letters 449 (2007) 44–52 45 found to be steadily smaller then the related reactions the integrated signal intensity of an ion of distinct m/z ver- of dicarbon and seldom reached 10 12 cm3 s 1 at room sus the laboratory angle. Information on the chemical temperature [7]. This suggested the existence of characteris- dynamics was obtained by fitting these TOF spectra of tic threshold energies [8]. the reactively scattered products and the product angular However, information on the reaction products is still distribution in the laboratory frame (LAB) using a for- missing. How can these be identified? Crossed molecular ward-convolution routine [17]. This procedure initially beams experiments [9–11] have played an important role guesses an angular distribution T(h) and a translational in accessing the relevant regions of the potential energy sur- energy distribution P(ET) in the center-of-mass reference faces of important combustion intermediates and in eluci- frame (CM). TOF spectra and the laboratory angular dating possible reaction pathways how they might be distribution were then computed from the initial trial func- formed in combustion flames. The experimental dynamics tions, T(h) and P(ET). Best fits of the TOF and laboratory of the tricarbon molecule have only been exposed with angular distributions were achieved by refining the T(h) methylacetylene and allene as a co-reagent [12]. In the and P(ET) parameters. Since the reactions of tricarbon present Letter, we portray crossed molecular beams studies molecules with unsaturated hydrocarbons have characteris- 1 þ of the tricarbon molecule, C3ðX Rg Þ, with acetylene, tic threshold energies, E0 [18], we utilized an energy depen- 1 þ 1 C2H2ðX Rg Þ, ethylene, C2H4(X Ag), and benzene, dent cross section, r, via the line-of-center model through 1 C6H6(X A1g). Eq. (1) with the collision energy EC for EC P E0 in the fitting routine. 2. Experimental r ½1 E0=EC ð1Þ The experiments were carried out under single collision conditions in a crossed molecular beams machine at The 3. Results University of Hawaii [13,14]. Pulsed tricarbon beams were produced in the primary source by laser ablation of graph- 3.1. Laboratory data ite at 266 nm [15] (8–30 mJ per pulse; Table 1). Under these conditions, no carbon clusters higher than tricarbon are 3.1.1. C3/C2H2 system present in the primary beam. The ablated species were We detected reactive scattering signal at m/z =61 + þ seeded in neat carrier gas (helium, 99.9999%, Airgas) (C5H ) and 60 ðC5 Þ. However, at each laboratory angle, released by a Proch-Trickl pulsed valve. A four-slot chop- the TOF spectra at both mass-to-charge-ratios are – after per wheel mounted between the skimmer and the cold scaling – identical. This finding alone leads us to two con- shield selected a part out of the seeded tricarbon beam clusions. First, ions at m/z = 60 originate from dissociative [16]. This segment of the tricarbon beam crossed a pulsed ionization of the C5H neutral radical in the electron impact hydrocarbon beam of acetylene (99.9%; Airgas), ethylene ionizer. Secondly, only the tricarbon versus atomic hydro- (99.999%; Airgas), neon-seeded (about 5%) benzene gen reaction to form C5H radical(s) is open, but not the (+99%, Aldrich) released by a second pulsed valve in the molecular hydrogen elimination channel (Fig. 1a). How- interaction region of the scattering chamber. The reactively ever, for TOF spectra of ions recorded at lower mass-to- þ þ þ scattered species are monitored using a triply differentially charge ratios such as of 49 ðC4H Þ; 48 ðC4 Þ; 37 ðC3H Þ; þ pumped quadrupole mass spectrometric detector (QMS) in and 36 ðC3 Þ, the situation is more complex [19]. These the time-of-flight (TOF) mode after electron-impact ioniza- ions originated from dissociative ionization of the C5H tion of the neutral molecules. The detector can be rotated parent at m/z = 49, 48, 37, 36, from the ionized C4H parent within the plane defined by the primary and the secondary molecule formed in reaction of co-ablated dicarbon mole- reactant beams to allow taking angular resolved TOF spec- cules with acetylene (m/z = 49), from dissociative ioniza- tra. By integrating the TOF spectra at distinct laboratory tion of the C4H parent (m/z = 48, 37, 36), and from the angles, we obtain the laboratory angular distribution, i.e., ionized C3H parent molecule formed in reaction of co- ablated carbon atoms with acetylene [20]. Signal at þ m=z ¼ 36 ðC3 Þ has contributions from (i) dissociative ioni- zation of the C5H, C4H, and C3H parents, (ii) reactive scat- Table 1 tering signal of the carbon atom versus molecular hydrogen Peak velocities (v ), speed ratios (S) and the center-of-mass angles (H ), p CM exchange in the carbon–acetylene reaction [19], and – for together with the nominal collision energies (EC) of the tricarbon– acetylene, ethylene, and benzene systems those TOFs taken close to the primary beam – from inelas- 1 1 tically scattered tricarbon molecules. The corresponding Beam vp (ms ) SEC (kJ mol ) HCM 1 þ LAB distribution of the ions at m/z = 61 is very narrow C2H2ðX Rg Þ 902 ± 2 16.0 ± 1.0 – – 1 þ and extends only by about 15° in the scattering plane C3ðX Rg Þ 4018 ± 122 1.5 ± 0.1 128 ± 8 9.2 ± 0.3 1 C2H4(X Ag) 893 ± 3 15.7 ± 1.0 – – defined by both beams (Fig. 1b); TOF could not be 1 þ C3ðX Rg Þ 2899 ± 113 2.7 ± 0.3 73 ± 5 13.4 ± 0.5 recorded at angles of less than 4° with respect to the pri- 1 C6H6(X A1g) 762 ± 3 21.6 ± 1.0 – – mary source beam due to the enhanced helium gas load C X1Rþ 3680 ± 153 1.8 ± 0.2 174 ± 14 24.2 ± 1.0 3ð g Þ at angles in close proximity to the beam. 46 X. Gu et al. / Chemical Physics Letters 449 (2007) 44–52
þ þ þ Fig. 1a. Selected time-of-flight data for m=z ¼ 61 ðC5H Þ; m=z ¼ 63 ðC5H3 Þ and m=z ¼ 113 ðC9H5 Þ recorded for the reactions of tricarbon molecules with acetylene (left), ethylene (center) and benzene (right) and at various laboratory angles. The circles indicate the experimental data, the solid lines the calculated fit.
Fig. 1b. Laboratory angular distribution of the C5H, C5H3,C9H5 radical products recorded at m/z = 61, m/z = 63, and m/z = 113 for the reactions of tricarbon with acetylene (left), ethylene (center) and benzene (right). Circles and error bars indicate experimental data, the solid line the calculated distribution with the best-fit center-of-mass functions.
þ 3.1.2. C3/C2H4 system and 60 ðC5 Þ. Ion counts at lower mass to charge ratios In case of the tricarbon–ethylene system, signal was originate in dissociative ionization of the parent molecule þ þ þ detected at m=z ¼ 63 ðC5H3 Þ; 62 ðC5H2 Þ; 61 ðC5H Þ; –hereC5H3 – in the electron impact ionizer of the detector. X. Gu et al. / Chemical Physics Letters 449 (2007) 44–52 47
Therefore, the tricarbon versus atomic hydrogen pathway is open (Fig. 1). We also monitored ion counts at þ þ lower mass-to-charge ratios of 51 ðC4H3 Þ; 50 ðC4H2 Þ; þ þ þ þ þ 49 ðC4H Þ; 48 ðC4 Þ; 39 ðC3H3 Þ; 38 ðC3H2 Þ; 37 ðC3H Þ; þ and 36 ðC3 Þ [19]. They result from dissociative ionization of the C5H3 parent molecules, from ionization of the i- C4H3 (m/z = 51) radical formed in the reaction of dicarbon with ethylene [21], and from ionization of the propargyl 2 radical, C3H3(X B1), synthesized in the carbon atom–ethyl- ene reaction (m/z = 39). Within the experimental signal-to- noise ratio, no other channels were observed. These parent species also fragment in the electron impact ionizer to give signal at m/z = 50–48 and 38–36. Finally, we also have to account for an elastic scattering of the tricarbon molecule with ethylene at angles close to the primary beam for m/z = 36. Similarly to the acetylene reactant, the labora- tory angular distribution of ions at m/z = 63 is narrow and spread over only 25° in the scattering plane (Fig. 1b).
3.1.3. C3/C6H6 system Considering the tricarbon–benzene reaction, we moni- þ tored ion counts from m=z ¼ 113 ðC9H5 Þ down to þ m=z ¼ 108 ðC9 Þ. The TOFs at lower mass to charge ratios overlap with the patterns of the TOF recorded at m/z = 113. Therefore, signal in the range of m/z = 112– 108 originates in dissociative ionization of the parent mol- ecule in the electron impact ionizer of the detector. There- fore, the tricarbon versus atomic hydrogen pathway is open (Fig. 1).
3.2. Center-of-mass functions
We would like to direct the discussion now to the derived center-of-mass translational energy distributions, P(ET)(Fig. 2). First, we can obtain information on the maximum translation energy of the reaction products in the center-of-mass frame. For the acetylene, ethylene, and benzene, best fits of the TOF and LAB data were achieved with a single channel fit and translational energy distributions extending up to 170 kJ mol 1 (acetylene), 120 kJ mol 1 (ethylene), and 75 kJ mol 1 (benzene). Secondly, all P(ET)s peak well away from zero transla- tional energy at about 80 kJ mol 1, 40 kJ mol 1, and 25 kJ mol 1; the distributions depict a Gaussian-like shape. A comparison of these patterns with related reactions of carbon atoms [11] and dicarbon molecules with unsatu- rated hydrocarbon molecules suggests that a significant fraction of energy is being released into the translational Fig. 2. Center-of-mass translational energy flux distributions for reaction of tricarbon with acetylene (top), ethylene (middle) and benzene (bottom) degrees of freedom of the reaction products; most likely, to form C5H, C5H3 and C9H5 radical(s) and atomic hydrogen; the hatched the reaction also goes through short-lived reaction interme- areas comprise fits within the experimental error limits. In case of the diates on the singlet C5H2,C5H4,andC9H6 and potential acetylene and benzene systems, the distribution of collision energies is energy surfaces. It should be stressed that in order to get overlaid in gray. an acceptable fit of the data, it was important to include an energy-dependence of the reactive cross section via and 90 and 110 kJ mol 1 for the acetylene, ethylene, ben- Eq. (1). Here, we varied the threshold energies between zene reactions, respectively. 10 and 150 kJ mol 1. Best fits were derived for threshold It should be emphasized that the very narrow range of energies between 80 and 90 kJ mol 1, 40 and 50 kJ mol 1, the reactive scattering signal as evident from the LAB dis- 48 X. Gu et al. / Chemical Physics Letters 449 (2007) 44–52 tributions combined with the low speed ratio of the tricar- discrepancy be accounted for? In Fig. 2, the P(ET) bon beams make it problematical to derive quantitative extracted from the tricarbon–acetylene reaction is overlaid information from the center-of-mass angular distributions, by the distribution of collision energies. The latter has been T(h). Recall that to generate fast tricarbon beams, we have computed from the peak velocities (vp) and speed ratios (S) to utilize laser ablation of graphite and must select the early of the reactant beams. The effect of the limited speed ratio part of the ablation beam in which the tricarbon molecules of the tricarbon beam and of the mass combinations of 61 are poorly helium-seeded [28]. This resulted in only limited amu (C5H) plus 1 amu (H) is dramatic: we can see that speed ratios (Table 1). Unfortunately, there is currently no energies up to a few 100 kJ mol 1 present a significant frac- alternative to produce high velocity tricarbon beams tion of this distribution. Therefore, we acknowledge that in (2900 ms 1 and faster) except pulsed laser ablation. Based case of the tricarbon–acetylene reaction, it is not feasible to on the limited speed ratio, the fluctuations of the tricarbon provide reliable data on the reaction energies. As a matter beam velocity, and the narrow range of the reactive scatter- of fact, the only firm information from the experimental ing signal, we cannot make a definite conclusions if the data is the existence of a tricarbon versus atomic hydrogen T(h)s are forward or backward scattered with respect to channel and the presence of a characteristic threshold the tricarbon beam. However, it is necessary to fit the data energy to reaction between 80 and 90 kJ mol 1. Further, with intensity over the complete angular range from 0° to we can predict that the molecular hydrogen elimination 180°; this suggests that the reactions are indirect and pathway is closed. Also, since the collision energy in this 1 involve C9H6 intermediate(s). However, we can change system is extremely high (128 kJ mol ), we would antici- the intensity ratios at the poles, I(0°)/I(180°), from 4.0 to pate a short life time of any intermediate involved. This 0.2 in each system without having a significant effect on is expected to prevent any hydrogen atom migrations [11] the fit. such as the isomerization from i3 to i5 and from i1 to i4 (Fig. 3). Note that the latter intermediate would be 4. Discussion required if a molecular hydrogen elimination pathway was observed experimentally. These considerations corre- 4.1. Energetics late nicely with a recent theoretical study of this system [23]. Here, the authors predicted a dominating hydrogen To extract the underlying reaction dynamics, we would elimination pathway to form the linear HCCCCC isomer; like to comment first on the energetics of the title reactions. branching ratios for the molecular hydrogen channel plus Recall that the maximum translational energy of the center- pentacarbon were computed to be less than 0.7%. Due to of-mass translational energy distribution portrays the sum the limitations on the experimental information on the tri- of the absolute of the reaction energy plus the collision carbon–acetylene system, the following paragraphs only energy. Therefore, by subtracting the latter, we find that discuss the possible dynamics of the tricarbon–ethylene in case of the ethylene and benzene systems, the reaction and tricarbon–benzene reactions in depth. to form the C5H3 isomer(s) plus atomic hydrogen (ethylene reaction) and C9H5 isomer(s) plus atomic hydrogen (ben- 4.1.1. Tricarbon–ethylene zene reaction) is exoergic by about 47 ± 10 kJ mol 1 (ethyl- We make an attempt now to resolve the underlying reac- ene) and endoergic by about 100 ± 20 kJ mol 1 (benzene). tion dynamics leading to the synthesis of the 1,2,3,4-penta- We can compare now the experimentally derived energetics tetraene-yl-1 radical, HCCCCCH2, by comparing our with those obtained from calculations (Fig. 3) [22,23]. Here, experimental findings with the computed potential energy the experimental data agree very well with the formation of surface (Fig. 3) [22]. We first correlate the structure of 2 the 1,2,3,4-pentatetraene-yl-1 radical, HCCCCCH2(X B1) the tricarbon and ethylene reactants with the reaction 2 (ethylene) and of phenyltricarbon, C6H5CCC(X A) – a product and suggest then feasible reaction intermediates species related to the recently identified 1,3-diphenylpropy- on the singlet surface. Recall that although we could not nylidene (C6H5CCCC6H5) [24] – in both cases, atomic extract quantitative information from the center-of-mass hydrogen represents the co-product. Based on this informa- angular distribution, the experimental data indicate that tion, we can also calculate the fraction of energy channeling the reactions must involve C5H4 reaction intermediates. on average into the translational modes of the reaction Here, the carbon backbone in the 1,2,3,4-pentatetraene- products to be about 50 ± 5% for both systems. This large yl-1 radical, HCCCCCH2, is expanded by three carbon fraction suggests that the reaction happens on a very short atoms compared to the ethylene reactant (H2CCH2). To time scale [11]. connect the HCCCCCH2 structure to the H2CCH2 reac- The center-of-mass translational energy distribution of tant via C5H4 reaction intermediate(s), it is very likely that the reaction of tricarbon with acetylene to form C5H+H the reverse reaction, i.e. a hydrogen atom addition to the presents a tricky problem. Based on the P(ET), we would radical center at the CH group of the HCCCCCH2 product 1 derive an exoergicity of about 20 kJ mol . However, a forms a pentatetraene intermediate (H2CCCCCH2). To close look at the computed C5H2 potential energy surface formally connect the H2 CCCCCH2 intermediates with tri- (Fig. 3) indicates that the reaction to yield C5H+H is carbon plus ethylene, it is necessary to expand the carbon– endoergic by at least 100 kJ mol 1. How can this obvious carbon backbone by three carbon atoms. In a similar way X. Gu et al. / Chemical Physics Letters 449 (2007) 44–52 49
a
+ H
110.5 117.1
85.8 102.1 73.2 57.7 57.3 l-C5H + H -1 46.9 41.4 31.8 25.5 15.9 0.0 -3.8 -7.5 l-C5 + H2 i0 C3 + C2H2
-72.0 -67.8 Relative energy/kJ mol i6 -95.8 i7
-228.0 -237.2-236.4 i1 -245.6 -252.3 i2 i3' -244.3 i4 i3 -308.4
i5
b
48.1 42.3 26.4 0.0 12.6 4.2 C +C H 3 2 4 p1 -60.7 c
-1 -55.6 -116.3 104.3 -1
i3 + H -185.4 43.3
Relative energy/kJ mol i2 0.0 C3 + C6H6
Relative energy/kJ mol -59.9 i1
-407.1
i4
Fig. 3. Simplified potential energy surfaces involved in the reactions of tricarbon with acetylene (a), ethylene (b), and benzene (c). to the reaction of singlet dicarbon with ethylene [14,21],we carbon–carbon double bond of the ethylene reactant to propose that the tricarbon reactant eventually ‘inserts’ into form initially cyclic C5H4 collision complex(es) on the sin- the carbon–carbon double bond of the ethylene molecule. glet surface. Based on our experiment, this process was Since it is not feasible for the tricarbon reactant to ‘insert’ found to have a characteristic threshold energy of 40– in a single step, we propose that tricarbon adds to the 50 kJ mol 1. The initial cyclic reaction intermediate(s) 50 X. Gu et al. / Chemical Physics Letters 449 (2007) 44–52 can isomerize to form eventually the pentatetraene interme- accurate to ±10 kJ mol 1. Having identified phenyltricar- diate (H2CCCCCH2). bon, C6H5CCC, as the reaction product, we now connect The computations confirm this postulated reaction the molecular structures of the tricarbon and benzene mechanism (Fig. 3) [22]. Here, tricarbon can add in one reactants with the reaction product. Formally, a carbon step either side on or end-on to the carbon–carbon double atom of the benzene molecule is replaced by the tricarbon bond forming the intermediates i3 and i1, respectively. molecular unit. This exchange is similar to the reactions Both addition processes have characteristic entrance barri- of, for instance, the reactions of singlet dicarbon benzene 1 ers of about 26 and 48 kJ mol . Intermediate i1 can rear- leading to the phenylethynyl radical (C6H5CC). Here, range stepwise via i2 to i3. Ultimately, i3 ring opens to we propose that tricarbon attacks the aromatic ring lead- form the postulated pentatetraene intermediate i4 ing to a C6H6CCC intermediate; the latter may lose a (H2CCCCCH2) on the singlet surface. However, the com- hydrogen atom to form the phenyltricarbon radical puted surface suggests that the pentatetraene molecule (C6H5CCC). loses a hydrogen atom via a loose exit transition state, The computations of the relevant parts of the C9H6 i.e. a simple bond rupture process. This should be reflected potential energy surface support the proposed reaction in a center-of-mass translational energy distribution peak- mechanism (Fig. 3) [22]. Here, tricarbon can add to the ing at or close to zero translational energy. This is clearly aromatic ring via an entrance barrier to yield a weakly not observed (Fig. 2). How can this be explained? The large bound bicyclic intermediate stabilized by about fraction of the available energy channeling into the transla- 60 kJ mol 1 [33]. Vala and coworkers calculated this inter- tional degrees of freedom of about 50% suggest that the mediate to lie 52 and 46 kJ mol 1 below the reactants at the decomposing intermediate is very short-lived – possible at less accurate density functional B3LYP level with the 6- the borderline between direct and indirect dynamics. Con- 31G** and 6-311+G** basis sets, respectively. They also sidering the large collision energy, this finding is not sur- found weakly bound p-complexes of tricarbon with ben- prising. The very short life time can result in the zene stabilized by 8 kJ mol 1 between the reactants and experimentally observed off-zero peaking of the center-of- the bicyclic intermediate. The transition state leading to mass translational energy distributions. Previous crossed this intermediate from tricarbon plus benzene (via the p- beam experiments, of for instance electronically excited complexes) is located 43 kJ mol 1 above the reactants at carbon atoms with acetylene [25], ethylene, and propylene our G3(MP2,CCSD) level, somewhat lower than [26] at high collision energies of at least 45 kJ mol 1showed 54 kJ mol 1 obtained in B3LYP calculations [33]. Vala explicitly that – although the decomposing intermediates and coworkers also considered various isomerization path- formally reside in deep potential energy wells – the transla- ways of the bicyclic C9H6 intermediate, which are appar- tional energy distributions peak well away from zero trans- ently not relevant to the hydrogen atom elimination lational energy. Therefore, our data of the ethylene– reaction leading to the C6H5CCC product identified in tricarbon experiment indicate the presence of a short-lived our experiment. We have found here that, accompanied H2 CCCCCH2 intermediate decomposing rather ‘directly’ by a three-member ring opening, the bicyclic intermediate than via a long-lived complex behavior. emits a hydrogen atom to synthesize the phenyltricar- bon radical. The overall reaction to form phenyltricarbon 4.1.2. Tricarbon–benzene plus atomic hydrogen was found to be endoergic by To propose the underlying reaction dynamics, we follow 104 kJ mol 1. This value correlates nicely with the experi- a similar approach as utilized in the tricarbon–ethylene mentally derived energetics and also with the observed reaction and compare our experimental data with the com- reaction threshold of 90–110 kJ mol 1. In a similar manner puted potential energy surface (Fig. 2). In our calculations, to the tricarbon–ethylene system, the surface indicates that the reactants, products, intermediates, and transition states the intermediate should emit the hydrogen atom via a lose on the relevant part of the C9H6 potential energy surface exit transition state. This is expected to result in a center- 1 þ for the C3ðX Rg Þ plus benzene reactions were optimized of-mass translational energy distribution peaking at or using the hybrid density functional B3LYP method with close to zero translational energy. Again, this is not the 6-311G(d,p) basis set [27,28]. Vibrational frequencies observed (Fig. 2). Here, the significant fraction of the avail- were computed at the same level of theory for the charac- able energy channeling into the translational degrees of terization of stationary points and to obtain zero-point freedom of about 50% indicates that the fragmenting inter- energy (ZPE) corrections. More accurate relative energies mediate is short-lived. Also, considering the large collision were recalculated using the G3(MP2,CCSD)//B3LYP energy, this finding is not surprising, and dynamical effects method [29], which approximates the coupled cluster [30] likely result in the observed off-zero peaking of the center- CCSD(T)/G3MP2large energy. All calculations were car- of-mass translational energy distributions. ried out using the GAUSSIAN 98 [31] and MOLPRO 2000 [32] programs. Because the average absolute deviation of 5. Summary the G3(MP2,CCSD)//B3LYP method for enthalpies of formation in the G3 molecular test set is 5 kJ mol 1,we In our crossed beams experiments, we investigated the 1 þ expect that our calculations of relative energies are reaction of tricarbon molecules, C3ðX Rg Þ, with acetylene X. Gu et al. / Chemical Physics Letters 449 (2007) 44–52 51
(C2H2), ethylene (C2H4), and benzene (C6H6). The experi- Acknowledgements ments suggested that all reactions proceeded via a tricar- bon versus atomic hydrogen exchange pathway. Further, The work was supported by the US Department of the reactions were characterized by threshold energies, Energy-Basic Energy Sciences (DE-FG02-03ER15411 1 1 E0, of 80–90 kJ mol , 40–50 kJ mol , and 90–110 (RIK) and DE-FG02-04ER15570 (AMM)). kJ mol 1. In case of the acetylene reactant, it was not fea- sible to derive detailed dynamical information on the tri- carbon–acetylene reaction on the experiments alone. References However, the tricarbon–ethylene and tricarbon–benzene [1] Faraday Discussion 119, Combustion Chemistry: Elementary Reac- reactions were found to proceed via initial addition pro- tions to Macroscopic Processes, 2001. cesses of the tricarbon molecule to the unsaturated hydro- [2] P. John, J.R. Rabeau, J.I.B. Wilson, Diam. Relat. Mater. 11 (2002) carbon on the singlet surfaces. In case of ethylene, the 608. cyclic intermediates isomerized to yield eventually the acy- [3] A.G. Gordon, The Spectroscopy of Flames, Chapman and Hall Ltd., clic pentatetraene intermediate (H CCCCCH ). The actual Wiley, New York, 1974. 2 2 [4] T. Shiomi, H. Nagai, K. Kato, M. Hiramatsu, M. Nawata, Diam. lifetime of this intermediate is proposed to be rather short; Relat. Mater. 10 (2001) 388. it was found to decompose via atomic hydrogen elimina- [5] S.A. Redman, C. Chung, K.N. Rosser, M.N.R. Ashfold, Phys. Chem. tion to form the 1,2,3,4-pentatetraene-yl-1 radical, Chem. Phys. 1 (1999) 1415; 2 J.S. Foord, K.P. Loh, N.K. Singh, R.Z.B. Jackman, G.J. Davies, J. HCCCCCH2(X B1). Note that the experimentally derived threshold energy of the tricarbon–ethylene reaction is sim- Cryst. Growth 164 (1996) 208. [6] A.G. Loewe, A.T. Hartlieb, J. Brand, B. Atakan, K. Kohse- ilar as found for the reaction of tricarbon with allene and Hoeinghaus, Combust. Flame 118 (1999) 37. 1 methylacetylene, i.e. 40–50 kJ mol . Together with the [7] H.H. Nelson, L. Pasternack, J.R. Eyler, J.R. McDonald, Chem. Phys. endoergic reaction of tricarbon with benzene and acetylene, 60 (1981) 231; these data can rationalize now the low rate constants of less M.L. Lesiecki, K.W. Hicks, A. Orenstein, W.A. Guillory, Chem. than 10 12 cm3 s 1 derived previously in kinetics studies of Phys. Lett. 71 (1980) 72. [8] H.H. Nelson, H. Helvajian, L. Pasternack, J.R. McDonald, Chem. these reactions [7]. This is in strong contrast to the reac- Phys. 73 (1982) 431.
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