3978
J. Phys. Chem. 1994,98, 3978-3984
Theoretical Study of the Structures and Stabilities of (SiC2H2)+ Species. The Reaction of Si+ with Acetylene Antonio Largo* and Carmen Barrientos Departamento de Quimica Fisica y Anafitica, Facultad de Quimica, Uniuersidad de Oviedo, 33006 Oviedo, Spain Received: July 16, 1993; In Final Form: January 27, 1994'
An a b initio theoretical study of the (SiC2H2)+ species has beeen carried out. Predictions have been made for some of the molecular properties (geometries, dipole moments, and harmonic vibrational frequencies) which could help in their possible experimental detection. A cyclic SiC2H2+(2B2) state has been shown to be the global minimum; however, a vinylidene-type structure, SiCCH2+(2B2),has been found to lie very close in energy (2.4 kcal/mol higher than SiC,H2+(,B2) a t the PMP4 level). Following in energy are a cyclic S ~ C Z H ~ + ( ~isomer A') and the insertion product HSiCCH+(2A'), which lie about 9 and 14.7 kcal/mol, respectively, above the cyclic 2B2 state. Another isomer, H2SiCC+, is shown to be quite unstable. The reaction of Si+ with acetylene to give SiC2+ H2 is found to be clearly endothermic, whereas the production of SiCCH+ seems to be only slightly exothermic. In addition, we have found a channel for the production of SiCCH+ which seems to proceed without activation energy. Therefore, according to our theoretical results, the reaction of Si+ with acetylene could take place under low-temperature conditions, such as in interstellar media.
+
Introduction Silicon interstellar chemistry has received much attention in recent years. It is certainly striking a t first sight that, although silicon has a relatively high cosmic abundance (twice that of sulfur, for instance), only a few molecules containing silicon have been detected in interstellar clouds. Of particular interest have been thedetections of silicon carbide molecules in the envelope of a carbon star. So far Sic,' SiCz,z and SiC43 have been identified, in the case of Sic2 through theoretical4 and experimental5 works which showed that this molecule has a cyclic CzUstructure. It is believed that ion-neutral reactions play an important role in the synthesis of molecules in space. Therefore, silicon carbide molecules could be produced via reaction of Si+ with hydrocarbons.6 In the case of SiC2, its production could be initiated by Si+
+ C,H2
-
(SiC,H)+
+H
(1)
followed by dissociative recombination of (SiC2H)+ (SiC2H)+
+e
-
Sic,
+H
(11)
Similar reaction schemes have been proposed as sources of SC, compound^.^ The reaction of Si+ with C2H2 has been experimentally studied by Creasy et a1.,8 using a Fourier transform mass spectrometer, and by Wlodeket a1.,9 using the selected-ion flow tube technique. In the second of these works,9 a rate constant of 3.5 X lQ-lo cm3 molecule-' s-1 was measured for the reaction of Si+with acetylene, and a branching ratio of 0.7 was observed for the production of SiCZH+ (the other product being the adduct, SiCzH2+, with a branching ratio of 0.3). The isomeric identity of the product, SiCzH+, was not determined in this study. However, a recent report on the generation and characterization of SiCzH and SiCzH+Io supports the theoretical prediction that the global minimum is linear SiCCH+.ll Given the conditions of low temperature and low density present in the interstellar medium, the reactions involved in interstellar chemical processes must be exothermic and must also have very low (or zero) activation energies. Therefore, it is important to e Abstract
published in Aduance ACS Abstracrs, March 1, 1994.
determine the enthalpy and possible activation barrier of reaction I to know if the production scheme initiated by the reaction of Si+with acetylenecould be a source of interstellar silicon dicarbide. This is the main purpose of the present paper. However, we will also provide a general study of SiCZHz+isomers in order to obtain theoretical information which could be useful in the gas-phase chemistry of silicon, since it should be recalled that SiCzH2+not only is of astrophysical interest but also is the focus of a number of experimental works. SiCzH2+ cations are observed after vaporization with a laser of silicon surfaces exposed to acetylene12 and generated in the reaction of acetylene with the Si+-benzene .rr complexl3 and, quite recently, in the electron impact ionization of C1Si(CH3)3.I4 We should mention that, although to the best of our knowledge there is no complete study of the SiC2Hz+species in the literature, a number of theoretical studies have been carried out on related systems. Among them of particular interest are those concerned with the neutral system and with the isoelectronic system AlC2H2.17-22 References will be made throughout the paper to some of their more important results.
Computational Methods
In order to allow direct comparison with previous work on related systems, in particular a theoretical study of SCzH2+isomers and the reaction of S+ with acetylene,23 we will employ similar theoretical methods as in that work. The geometries of the different species were obtained at the second-order Morller-Plesset (MP2) level with the split-valence plus polarization 6-3lG* basis ~et,2~-25 which includes d functions for heavy atoms. Electron correlation effects were included through complete fourth-order Morller-Plesset (MP4) perturbation theory,26J7 at the MP2/6-3 lG* geometries. For these correlated calculations we employed the 6-311G**Zs (which includes p functions for hydrogen) for first-row atoms and the McLean and Chandler (12,9)/[6,5] basis setz9for silicon (with triple-{character for the 3p orbitals) supplemented with d polarization functions. The basis set constructed in this way is usually denoted as MC-3 1lG**. It should be pointed out that inner-shell molecular orbitals were not taken into account in these correlated calculations ("frozencore" approximation).
0022-3654/94/2098-39~8~04.50/0 0 1994 American Chemical Society
The Journal of Physical Chemistry, Vola98, No. 15, 1994 3979
Reaction of Si+ with Acetylene Finally, harmonic vibrational frequencies were estimated at the MP2 level using the 6-31G* basis set, and these values were used to estimate zero-point vibrational energies. The calculations reported in this work were carried out with the GAUSSIAN 90 program package.30
/:\ /=Igg I \
3.554
\
Stable Species of the (SiCZHz)+ System Before discussing the reaction of Si+ with acetylene, we will present and discuss the results for the stable (SiC2H2)+ species. We have found stable structures for the following conformations: ring SiC2H2+ (Cb symmetry), SiCCH2+ (Cb symmetry, Sivinylidene type), and planar HSiCCH+ and H2SiCC+ (Cb symmetry). We have found four low-lying states for the SiC2H2+ ring conformation. Three of these correspond to the following electronic configurations, SiC2H,+(2B2): la,’ 2a12 lb; 3aI24 a I 2 lb,’ 2b: 3b:
si
si
H
H
158.3
-C-H
-&
1.220
1.069
zB*
si
SI
/x\
2.531
5a12
6a127a1’ 2b128a124b2’ (1)
175.4
lB,
... 9a1 ... 3b11
1)):
SiC2H;(’Al( SiC2H:(2B,):
1
(2)
Figure 1. MP2/6-31G*-optimized geometries for the cyclic Cb states of SiCzH2+. Distances are given in angstroms and angles in degrees.
(3)
Through a comparison of configurations 4-6 and 7, it is readily seen that this state does not correlate with Si+(2P) C2H2(’Zg+). The highest doubly-occupied al and b2 molecular orbitals of configuration 8 correspond to two formal Si-C bonds, whereas the unpaired electron is mainly located at silicon. Since, finally, the 2bl orbital corresponds to a *(C-C) bond, this state can be represented by the following valence structure
and therefore all of them correlate with Si+(zP) + C2H2(lZg+), bearing in mind that Si+(2P)splits, in Cb symmetry, into three spatially degenerate states with configurations Si+(’P,
’B,): la,’ 2aI2 lb: lb,’ 3aI24a122b2’ Si+(2P, 2Al):
... 5a, 1
Si+(2P, ’B,):
... 3b1’
(4)
‘A,): la,’ lb:
(6)
2a122b2’ 3a124a12 l b I 2 (7)
In fact these states can be better described as ion-molecule complexes resulting from the 7r interaction of Si+ with acetylene.
*Si+ I I
I I
I I I
H-CeC-H For the 2A1and 2BI states this interaction becomes repulsive even for relatively long Si-C distances (especially for the former), whereas in the case of the 2B2 state the minimum is found at a much shorter S i 4 distance. The geometrical parameters are shown in Figure 1. Since long Si-C bond distances are found for the 2A1 and ZB1 states, the geometry of the HCCH group in both structures is very close to that of acetylene. In thecase of the 2B2group, the C-C distance is still not far from that of a formal triple bond and now the HCCH group is somewhat moredistorted, giving rise to a smaller HCC bond angle. We have also found another 2A1 state corresponding to the following electron configuration SiC2Hc(2A1(2)): l a I 2 2a12 lb: 3b:
3a124a12 lb122b:
ii+
(5)
and the electronic configuration for ground-state acetylene, in Cb symmetry, is the following: C2H2(’2;,
+
5a12
6ai2 7aI22b,’ 4b; 8a,’ (8)
H ’
‘H
This qualitative picture agrees roughly with the geometrical parameters shown in Figure 1. The C-C bond distance of 1.403 A, although greater than the value for ethylene (1.339 A), is still shorter than that of a single bond, whereas the Si-C distance of 1.764 A is considerably shorter than typical Si-C single bond distances (1.867 A in methylsilane31). Compared to the geometrical parameters of Sic2 at a similar level of theory (MP2/DZ+P: d(C-C) = 1.294 A, d(Si-C) = 1.835 A, LCSiC = 41.3°32),SiC2Hz+(2A1(2))shows a shorter Si-C bond and a longer C-C bond. In fact, the bond distances and angles of Sic2H~+(~A1(2)) are closer to those found by Schaefer et al.15 for the neutral system, silacyclopropenylidene. The major difference seems to be in the Si-C distance (1.806 A for SiCzH2). The Si-C bond distance in siC2H~+(~A1(2)) is certainly much closer to the suggested Si-C double bond distance (1.71 As3) than to typical values for Si-C single bonds. The harmonic vibrational frequencies (in cm-l) for the Cbring species, computed at the MP2/6-31G* level, are given in Table 1, along with the zero-point vibrational energies (in kcal/ mol). We also give estimates of the dipole moment (in Debyes) at the UHF/6-311G** level. It is worth noting that the values of the stretching normal modes 2al and 4al confirm the qualitative aspects of the bonding in the cyclic states. The C-C stretching frequency in 2B2 is much higher (more than 400 cm-I) than in the 2A1(2)state and is even higher (near 2000 cm-l) for 2Al(l) and 2B1. On the other hand, the S i 4 2 stretching frequency for 2A1(2) is more than 300 cm-1 higher than the value for the ZBz state, confirming a much stronger ring for the former than for the latter. For 2B1 and 2A1(1), very low Si-C2 stretching frequencies (especially for the latter) are found.
Largo and Barrientos
3980 The Journal of Physical Chemistry, Vol. 98, No. 15, 1994
TABLE 1: MP2/6-31G* Harmonic Vibrational Frequencies (in cm-l) and Zero-Point Vibrational Energies (in kcal/mol) for the Cyclic SiC2H2+ Species’ SiC2H2+ PB2)
SiC2H2+ (2Ai(l))
SiC2H2+
normal mode
SiC2H2+ (2Ai(2))
la1 C-H stretch 2a1 C-C stretch 3al C-H bend 4al Si-Cz stretch 1b2 C-H stretch 2b2 C-H bend 3b2 Si-C bend 1bl C-H wag la2 C-H asym wag
3336 1780 843 554 3268 976 578 725 695
3542 1987 784 98 3452 536 227 756 440
3473 1968 868 258 3378 618 140 751 468
3289 1330 953 861 3279 1308 815 737 991
ZPVE P
18.2 0.192
16.9 6.337
17.0 1.060
19.4 1.224
1.0931 1.778
1.189
si-c-c
/
lk\ 179.9
1.073
H-C-
‘A’
w
-si 1 7 5 2 1.206
179.7
119.6
H
a Dipole moments (D) have been computed at the UHF/6-311GS* level.
It is worth pointing out that the 3b2 $$e, associated with the rotation movement of Si around the center of the C-C bond, has a relatively high value (81 5 cm-1) for the 2A1(2) state, indicating (together with the high Si-C2 stretching frequency) that this structure is a relatively deep minimum on the potential energy surface. Cgnversely, small values are found for the other states. In fact, an imaginary frequency (1 58i cm-l) is found for 2B2 at the UHF/6-31G* level. Therefore ’B2 is not actually a true minimum on the UHF surface, and distortion of the Cbsymmetry should lead to a more stable structure. Optimization in C, symmetry leads to an incorrect asymetrical minimum, corresponding to a (cyclic) cis-SiCHCH+ structure, and accordingly the entire potential surface is mapped at the MP2 level, since this gives a Cb cyclic minimum. It is interesting to point out that in the case of the isoelectronic AlC2H2 system a similar behavior was observed.19 The ’A1 state was shown to be a real minimum, but an imaginary frequency was also found in the case of the 2B2 state at the SCF level with a DZ+P basis set. However, at correlated levels the ’B2 state was shown to be a real minimum, and the cis-AlCHCH structure collapsed to the C, structure. We werealsoable tolocateat theUHFlevela trans-SiCHCH+ structure (2A’ electronic state). Its electronic configuration is SiCHCH+(’A’): la” 2a” 3a” 4a” 5a” la”’ 6a” 7a” 8a” 9a” loa” 1la” 2a”’ 12a” (9) and the unpaired electron resides at a carbon atom. The C - C bond distance corresponds to a double bond, and therefore, this state can be depicted as
\
Figure 2. MP2/6-31G*-optimized geometries for the SiCCH2+(2B2) and H2SiCC+(2A~)isomers, CISD/6-3 1G* for HSiCCH+ (2A’). Distances are given in angstroms and angles in degrees.
TABLE 2 MP2/6-31C* Harmonic Vibrational Frequencies (in cm-’) and Zero-Point Vibrational Energies (in kcal/mol) for the SiCCH2+ (Si-Vinylidene) Species’ normal mode la1 C-H stretch 2al C-C stretch 3al C-H scissor 4al Si-C stretch 1b2 C-H stretch 2b2 C-HI rock 3b2 SiCC bend 1bl CH2 wag 1a2 SiCC bend
3156 1917 1405 697 3245 892 278 1168 26 1
ZPVE
18.6 0.162
fi
The dipole moment (D) has been computed at the UHF/6-31 lG** level.
The lowest-lying state for the SiCCH2+(Si-vinylidene type) confdrmation is ’B2, corresponding to the following electronic configuration SiCCH;(’B,):
:Si+
\c
119.3
la,’ 2a,’ 3alZ4a,’ lb,’ 5a,’ lb; 6a1’ 7a,’ 8a122b; 2b,’ galZ3b,’ (10)
/
c.
/
H
It is clear that the interaction between the unpaired electron on the carbon atom and the empty silicon 3p orbital is precluded in the trans conformation. On the other hand, this interaction is enhanced in the cis isomer, giving rise to a structure with a SiCC bond angle much smaller, which is closer to the symmetric C, ZBi state than t o the trans isomer. In fact a significant delocalization of the unpaired electron between silicon and the CZcarbon atom is noticed for the cis isomer. However, optimization of the trans isomer at the MP2 level also leads, as in the case of the cis isomer, to the symmetric Cb structure (2B2 state). It is clear then that electron correlation decisively favors delocalization of the unpaired electron also in an unfavorable anti arrangement.
Its MP2/6-31GS-optimized geometry is shown in Figure 2 and the corresponding vibrational frequencies in Table 2. The unpaired electron is mainly located at the carbon atom bonded to silicon (the 3b2 orbital is essentially a 2p(C1) orbital), whereas the C-C bond distance corresponds to a double bond. This is not surprising, since SiCCH2+(2B*)correlates with Si+(2P,2Al) and triplet vinylidene (3B2) CCH2(’BZ): la,’ 2a,’ 3alZ4a,’ lb:
lb,’ 2b2’5a,’
(11)
On the other hand, the S i 4 bond distance is shorter than that of a typical single bond but still longer than the standard value for a double bond, 1.71 A.32 Therefore, this state can be represented by the following valence bond picture
-+
:Si-C=C
/H
‘ti
Reaction of Si+ with Acetylene
The Journal of Physical Chemistry, Vol. 98, No. 15, 1994 3981
TABLE 3 CISD/6-31C* Harmonic Vibrational Frequencies (in cm-1) and Zero-Point Vibrational Energies (in kcal/mol) for the HSiCCH+ Species’ HSiCCH+ normal mode (2A‘) la’ C-H stretch 3583 2a‘ Si-H stretch 2739 2305 3a’ C-C stretch 4a’ CCH bend 847 5a‘Si-C stretch 778 6a’ HSiC bend 722 7a’ HSiCCH bend 205 887 la” HSiCCH wag 2a” HSiCCH torsion 195 ZPVE 17.5 c1 1.561 a The dipole moment (D) has been computed at the UHF/6-311GS* level.
TABLE 4 MP2/6-31G* Harmonic Vibrational Frequencies (in cm-’) and Zero-Point Vibrational Energies (in kcal/mol) for the H@iCC+ Species’ HZSiCC+ normal mode (‘AI) la, Si-H stretch 2428 2al C-C stretch 2406 979 3al C-H scissor 4al S i 4 stretch 767 lb2 Si-H stretch 248 1 2b2 SiH2 rock 695 3bz CCSi bend 258 1bl SiH2 wag 753 2bl SiCC bend 24 1 ZPVE 15.7 c1 2.406 a The dipole moment (D) has been computed at the UHF/6-31G** level.
It is interesting to note the near-zero dipole moment of SiCCH2+(2Bz),as shown in Table 2. We should mention that other possible low-lying states were studied. Particularly, we considered the possible structure
where the 9al orbital, occupied by the unpaired electron, is mainly localized at the terminal carbon. The following valence bond picture may be proposed for a qualitative description of this state
~~~~
+
.Si=
H
C=C it‘
corresponding to a ’AI state, as a likely candidate for the ground state. However, this structure with two formal double bonds (and bond distances in agreement with that qualitative picture, d(Si-C) = 1.656 A and d(C-C) = 1.323 A at the UHF/6-31G** level) was found to lie much higher in energy, about 50 kcal/mol above ’Bz a t the PMP4/MC-311G** level. In the case of the HSiCCH+ conformation, the lowest-lying state (’A’) corresponds to the following electronic configuration la” 2ar2 3a” 4ar2 5ar2la’” 6a” 7ar2 82” 9a” loar21 la” 2a”‘ 12a” (12)
HSiCCH+(’A’):
where the 12a’ orbital is essentially a 3p(Si) orbital and can be described by the valence bond structure H
which agrees quite well with the geometrical parameters shown in Figure 2. The C - C stretching vibrational mode (Table 4) also agrees with a triple C-C bond, with a value which is in fact the highest of all (SiCzH2)+ isomers. It should be mentioned that this state exhibits the most severely spin-contaminated U H F wave function, with (S2) = 1.184 (when the 6-311G** basis set is used). The main contribution to spin-contamination comes from the rupture of a r(C-C) bond, resulting in the quartet state H \+
ti
\
.
Si
/
-C = C *
We also searched for other structures with a Si-H bond, namely the syn and anti HSiCHC+ conformers. In both cases the lowestlying state, ’A’, corresponds to the electronic configuration
Si+-CeC-H
Although this structure is easily obtained at the U H F level, we could not obtain a minimum at the MP2 level, even with different optimization algorithms. This problem seems to be related to spin contamination, which, although not especially severe((S2) = 0.849at theUHF/6-31G*level),isquiteagreeable with the geometry. Therefore, unprojected MP2 theory is not able to give a proper minimum for this state, so we decided to employ configuration interaction with single and double excitations (CISD) to obtain this state. The CISD/6-31G* geometrical parameters are shown in Figure 2, and the corresponding vibrational frequencies in Table 3.It can be seen in Figure 2 that this state has a quasilinear SiCCH backbone, since there is a formal C-C triple bond (d(CC) = 1.206 A). This strong C-C bond is also reflected in the C-C stretching frequency (3a’vibrational mode in Table 3), which is very high. Finally, the lowest-lying state for the H2SiCC+ conformation is ’AI, with the following electronic configuration
where again the unpaired electron occupies an almost pure silicon orbital. Bond distances and angles are very close for both conformers. Since the C-C bond distance suggests a double bond and there is a lone pair at the terminal carbon, we may propose the following valence bond structure for these species H
:c=c
\H
H2SiCC+(’AI): la,’ 2a123a124a12 lb,’ 5a12 lb? 6a1’
7a,2 8a,2 2b;
2b12
3b;
(13)
However, we were not able to locate these structures at correlated levels, since all optimizations we tried evolved toward
...
3982 The Journal of Physical Chemistry, Vol. 98, No. 15, 1994
TABLE 5 Relative Ener ‘es (in kcal/mql) of the Different Species of the (SiCfi2)+ &stem with Respect to Si+(2P) C2H2(1Z +), Calculated at Different Levels of Theory with the MC-311&** Basis Set at the MP2/631G* Geometries’ species UHF MP4 PMP4 (S2) SiC2H2+(2B2) -35.6 -45.9 -46.0 0.761 -11.2 -11.2 0.759 SiC2H2+(2AI(1)) -10.9 -19.3 -21.7 -21.9 SiC2H2+(2BI) 0.763 -36.5 -37.0 0.772 SiC2H2+(2Al(2)) -28.1 cis-SiCHCH+(2A’)b -36.4 -43.2 -44.0 0.779 rruns-SiCHCH+(ZA’)b -30.1 -25.1 -30.3 1.014 SiCCH2+(2Bz) -45.6 -40.9 -43.6 0.877 HSiCCH+(ZA’) -29.5 -30.2 -31.3 0.810 16.4 1.184 H2SiCC+(2AIjzA,)b 4.3 25.3 syn-HSiCHC ( 6.8 18.1 14.8 0.994 unri-HSiCHC+(2A’)b 8.2 20.1 17.5 0.955 * MP2/6-31G* ZPVEcorrections have been used (CISD/6-31G* for HSiCCH+). Structures only obtained at the UHF level.
+
.
Largo ana aarrientos global minimum is the sulfur-vinylidene structure SCCH2+(2B~), followed by cyclic SC2H2+(2BI)and HSCCH+(2A”), lying about 19 and 29 kcal/mol, respectively, higher in energy.23 In the case of the isoelectronic (AlC2H2)s y ~ t e m the , ~ cyclic ~ , ~ ~A I C Z H ~ ( ~ A , ) state lies below the corresponding *-bonded state, AICZHZ(~B~), although by a small quantity (-3 kcal/mol) at correlated levels of theory. In addition, both the vinylidene-type structure, AlCCH2(2B2), and the C-H insertion product, HAlCCH(2A’), lie lower in energy than cyclic A1CzH2(2Al) by small amounts.
Reaction of Si+ with Acetylene In this section we consider the reaction of Si+ with acetylene as a possible initial step for the production of interstellar silicon carbide. Charge transfer, leading to Si + C ~ H Z +should , not be a competing process, since the ionization potentials of silicon and acetylene are respectively 8.2 and 11.41 eV. Therefore, the reaction may proceed in principle toward two different products: (SiC,H)+ and (SiC2)+.
geometries close to the HSiCCH+ isomer, through migration of silicon from one carbon to the other. The relative energies of the different (SiC2H2)+ species with respect to Si+(2P) C2H2(lXB+),obtained a t different levels of theory with the MC-311G** basis set (at the MP2/6-31G* S?(2P) + C2H2(‘&*) geometries, CISD/6-31G* for HSiCCH+), are given in Table 5 . (Si,)’ + H2 In this table weonly providetheunrestricted Hartree-Fock (UHF) and the highest-order correlated (complete fourth-order MdlerPrevious theoreticalll and experimentallo works have shown Plesset, MP4) results. However, since there is a certain degree that the ground state of SiC2H+ is a linear isomer SiCCH+ (lying of spin contamination in some of the U H F wave functions, we more than 70 kcal/mol below the next lowest-lying isomer at have also included the projected34 fourth-order M P values correlated levels of theory in our calculations.l1 (PMP4). The Sz expectation values of the respective UHF/ In the case of SiC2+ we found in previous that there MC-311G** wave functions are also shown in Table 5 . Even are two isomers, a cyclic C, structure and a linear SiCC+ species, though both cis- and rrans-SiCHCH+(2A’) structures, as well as which lie very close in energy. MP4 calculations show the cyclic the syn- and anti-HSiCHC+ isomers, were found to collapse to isomer (*Al state) to be more stable than the linear one (lXg other structures when geometry optimizations at the MP2/6state) by 7.9 kcal/mol (a difference which is reduced to just 1 3 l G * level are carried out, we have included the MP4 and PMP4 kcal/mol if projected MP4 energies are considered). On the results forr these species for comparison. other hand, CI calculations place the linear isomer below the C, It is readily seen in Table 5 that the lowest-lying isomer a t the structure by only 1.6 kcal/mol. U H F level is the vinylidene-type structure SiCCH2+(2B2). At Since higher-order correlated calculations seem to favor the that level of theory, cyclic S ~ C Z H ~ + (is~not B ~a )true minimum cyclic isomer, we suggested that quite likely that would be the and lies slightly higher in energy than cis-SiCHCH+(2A’). global minimum of Sic2+. In any case, both isomers will lie very However, correlation effects clearly favor the cyclic 2B2 state, close in energy. which lies lowest in energy at both MP4 and PMP4 levels. At The relative enthalpies (in kcal/mol) for the possible products the PMP4 level of theory, the energy difference between SiC2H2+of the reaction of Si+with acetylene and of the different (SiC2H2)+ (2B2) and S ~ C C H Z + ( ~isBjust ~ ) 2.4 kcal/mol. This small value isomers, with respect to the energies of the reactants, are given prevents us from drawing a definitive conclusion about the global in Figure 3. Only stable structures a t correlated level are included minimum of the (SiC2H2)+ system. However, it is expected in in this figure. We provide the MP4 values and the PMP4 ones view of our results that higher-order correlated calculations will (in parentheses) obtained with the MC-311G** basisset including show the cyclic structure as the ground state. It is confirmed that correlation effects favor S ~ C Z H ~ + ( ~ B Z )ZPVE differences obtained a t the MP2/6-31GS level (CISD/ 6-31G* for the HSiCCH+ isomer). over cis and trans isomers. Among the other cyclic states studied in this work, SiC~H2+(2A1(2))is the most stable, lying above It is readily seen in Figure 3 that the production of SiC2+ is SiCzH2+(2B2) by -9 kcal/mol a t all levels of theory. The 2B1 clearly endothermic by more than 30 kcal/mol. Although inclusion of correlation effects lowers the endothermicity (the and zAl(1) cyclic states still lie well below Si+ CzH2. corresponding value a t the UHF/MC-3 1 1G** level is 45.2 kcal/ It is also worth noting that HSiCCH+(ZA’), whichcan beviewed mol), thisvalue is too high to leave any doubt about the energetics as the product of insertion of Si+ into a C-H bond of acetylene, is quite stable, lying only 15 kcal/mol above S ~ C Z H Z + ( and ~ B ~ ) of this process. We have employed the results for the cyclic SiC2+ isomer, but very small differences (always increasing the about 3 1 kcal/mol below Si+ acetylene. Undoubtedly, the fact endothermicity) are obtained with the energy values for the linear that in this structure the C-C triple bond of acetylene is preserved SiCC+ isomer. contributes to its relatively high stability. The other structures with Si-H bonds syn- and anri-HSiCHC+(2A’) and H2SiCC+On the other hand, production of SiCCH+is slightly exothermic (zAl), are much less stable. All of them lie above Si+ + C2Hzby just 1.1 kcal/mol. The corresponding reaction enthalpy at (IXg+)by amounts which are increased when correlation effects the U H F level is +4.6 kcal/mol, indicating that electron are taken into account. correlation tends to favor the exothermicity of this process. On It is interesting to compare our results for the relative stability the other hand, extension of the basis set seems to have a quite of the (SiC2H2)+ isomers with those found in related systems. In different effect, since we found a value of -4.6 kcal/mol at the the case of the neutral system, (SiC2H2), a similar result is MP4/6-31G** level (+0.7 at the UHF/6-31G** level). Therefore we have two opposite effects which make it difficult to obtained. Frenking et a1.l5 found that the global minimum is cyclic SiC2H2, with SiCCH2 and HSiCCH lying 17 and 22 kcal/ establish beyond any doubt whether this process is exo- or mol, respectively, higher in energy a t the CI level. On the other endothermic. In any case this reaction will be very close to hand, in the case of the sulfur analogue system, (SC2H2)+, the thermoneutrality.
+
+
+
The Journal of Physical Chemistry, Vol. 98, No. 15, 1994 3983
Reaction of Si+ with Acetylene
SICP++ HP CCSiH1+
E
(32.31
36.8
25.3 (16.4)
(kcal/mol)
TS3 YP4IYC.IllO"
TS2
-
14.0 (9.3)
Si++ C2H2
+H
SiC2Hi
0.0 (0.0)
TS4
SiCPHP+(2Al(i)) -11.2 (-11.2)
TS1
-1.l
-5.6 (-8.9)
-11.1 (-11.4)
SiC2H~+(2BP) -21.7 (-21.9)
-36.5 (37.0)
SiC2H
