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Gas-phase reactions of C+(2P) and Si+(2P) with oxygen bases. A G2 ab initio molecular orbital study
THEO CHEM ELSEVIER
Journal of MolecularStructure (Theochem) 310 (1994) 135-147
Gas-phase reactions ofC+ep) and Si+ep) with oxygen bases. A G2 ab initio molecular orbital study A. Luna, O. Mo, M. Yafiez" Departamento de Quimica, C-9. Universidad Autonoma de Madrid, Cantoblanco, 28049-Madrid. Spain
(Received 19 July 1993; accepted 13 August 1993)
Abstract
G2 ab initio molecular orbital theory has been used to study the relative stabilities of the different products of the reactions of C+ ep) and Si+ ep) with several oxygen bases in the gas phase. The reactions of sr' and C+ with formaldehyde and methanol as well as those ofC+ with silanone and silanol were considered. Significant differences in C+ and Si+ reactivity have been found in all cases, as a consequence of the greater electronegativity of carbon with respect to silicon and to the enhanced stability of the Si-O linkages, owing to their sizeable sr' -0- polarity. Consistent with this, methyl substitution, for instance, considerably stabilizes these oxygen-containing cations when the substituted center is the silicon rather than the oxygen atom. On the contrary, silyl substitution is more stabilizing when the substituted center is the oxygen atom.
1. Introduction Silicon is abundant in terrestrial [1, 2] and extraterrestrial [3, 4] environments. In the latter, silicon is mainly present as silicon monoxide, which is believed to be produced by neutralization of SiOH+. This particular ion is also abundant in our atmosphere, where sr' participates in many reactions. Silicon is introduced into the Earth's atmosphere by the ablation of meteors and is then readily ionized by charge transfer with 01" and NO+. The sr' ions formed in this way react with neutral components of the upper atmosphere, in particular with water, to yield the aforementioned SiOH+ cations. These factors, among others have motivated considerable research activity on the reactions of sr' with a large variety of "Corresponding author.
molecules in the gas phase [5-29], which has shown that most of the generated ions have unusual structural electronic and chemical properties compared to other molecular ions. In addition, the differences and similarities between silicon and carbon have been of interest to chemists for many years. In this respect, some studies comparing sr' and C+ reactions with small hydrocarbon molecules in the gas phase have been published recently [17] and have reported large differences between the reactivities of both monocations. For instance, while reactions of C+ with allene and propyne favor elimination of hydrogen and hydride transfer, reactions with sr' favor H atom and CH 3 elimination. Similarly, the reaction of diacetylene with C+ favors H atom elimination and charge transfer while with sr'. hydride transfer is the only channel. Nevertheless, to the best of our knowledge, no similar comparative studies regarding
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136
A. Luna et al.jl. Mol. Struct. (Theochem) 310 (1994) 135-147
gas-phase reactions of sr' and C+ with other neutrals, in particular with oxygen-containing molecules, have been reported in the literature so far. With the aim of providing some insight into this particular problem, we present in this paper highlevel ab initio molecular orbital calculations on the structures, relative stabilities and electronic charge distributions of complexes between C+ and sr' with oxygen bases, in particular with formaldehyde, silanone, methanol and silanol.
2. Computational details
Ab initio molecular orbital calculations at the G 1 [30] and G2 [31] levels of theory were carried out using the GAUSSIAN-90 program package [32]. G 1 theory [30] is a composite procedure based on Hartree-Fock (HF) and Meller-Plesset perturbation theory at second- and fourthorders (MP2 and MP4), and quadratic configuration interaction including single, double and triple excitations (QCISD(T» levels of theory. In the G 1 procedure the initial equilibrium structures are optimized at the UHF/6-31G* level in order to obtain the corresponding zero-point vibrational energies (ZPE) and to characterize the stationary points of the potential energy surface. These geometries are then refined at the MP2/6-3l G* level. Assuming additivity of different basis set enhancements at the MP4 level, and additivity of basis set and correlation effects between MP4 and QCISD(T), relative energies are obtained at effectively the QCISD(T) level with the 6-311 + G(2df,p) basis set. The final G 1 energy is obtained after adding a high-level correction which accounts for the isogyric effect and the ZPE value obtained in the SCF/6-31G* calculation scaled by the empirical factor 0.893. G2 theory [31] is a further refinement which corrects some shortcomings of the G 1 theory arising from the assumption of additivity of individual basis set improvements and from an overestimation of the isogyric effect. These corrections imply further calculations at the
MP2/6-311 + G(3df,2p) level with the result that the final energies are effectively of a QCISD(T)/6-3ll + G(3df,2p) quality. Although, in general, G2 ab initio theory [31] yields [31, 33] heats of formation, total atomization energies and ionization potentials which agree with the experimental values within ±O.l eV, it must be recalled that this theory, for open-shell systems, is based on the use of UMP2 optimized geometries and on several correlation contributions to the energy evaluated in the framework of the UMP theory, which has been shown [34-36] to converge slowly for molecules where the spin contamination from higher multiplicities is large. Hence, for some open-shell systems where spin contamination is significant, this theoretical scheme may lead to unreliable results. Therefore, in the present paper we have carried out additional calculations to avoid these possible shortcomings. In particular, for those species where a sizeable spin contamination has been detected, the geometry optimizations were performed at the QCISD(T)/ 6-3llG(d,p) level. The characteristics of the Si-X and C-X bonds and the nature of the interactions of the different neutrals with sr' and C+ were investigated by means of a topological analysis of the electronic charge density, p, and its Laplacian, V'2 p. As has been shown by Bader and co-workers [37-39], V'2p identifies regions of space where the electronic charge is locally depleted (V'2p > 0) or built up (''\12 P < 0). The former situation is typically associated with interactions between closed-shell systems (ionic bonds, hydrogen bonds and van der Waals molecules), while the latter characterizes covalent bonds, where the electronic charge concentrates between the nuclei. We have also located the relevant bond critical points (bcps), i.e. points where the electronic charge density is minimum along the bond and maximum in the other two directions, because the values of p and V'2 p at these points offer quantitative information on the strength and nature of the bonding. This topological analysis was carried out on the wavefunction correct to first-order, to take explicitly into account electronic correlation effects. For this purpose we have employed the AIMPAC series of programs [40].
137
A. Luna et al.jl. Mol. Struct. (Theochem) 310 (1994) 135-147 Table I G2 total energies (hartrees) of the minima of the [Hz, C, 0, Si]+ (a-e) and [Hz, C z, 0]+ (1-5) potential energy surfaces a
-403.07661 -403.06601 -403.03370 -403.02810 -403.02679 -152.01522 -151.94503 -151.93044 -151.91401 -151.91235
b
c d
e 1 2
3 4 5
3. Results and discussion
The G2 total energies of the different equilibrium conformations of [C, 0, H 2 , Si]+ and [C2 , 0, H 2 complexes are presented in Table 1. Their relative stabilities and structures are presented in Figs. 1 and 2, respectively. It must be mentioned that G I energies for several minima of the potential energy surface of [H 2 , C, 0, Si]+ have been reported before [23]. The present G2 values imply changes in their relative stabilities smaller than 1.6 kcal mor '. In
r
,+
40 5i
..,:.:..:...::..::...-.....5 i 1.524 0 113,
137.0 180.0
C
30
~+
HH
H 1.100
~5.4.., n..,. ..,_ I --------'= 30.4 1.471·~i~C 176.3
31.3
e
d
26.9
C
20
10
......-'-'-''"'-''.......c
1.~-t
175.9 6.6
b
o
0.0
a Fig. I. Relative stabilities (kcal mol ") and MP2j6-31G* optimized geometries of the minima of the [Hz, C, 0, surface. Values obtained at the G2 level of theory. Bond distances in A, bond angles in deg.
sW
potential energy
138
A. Luna et al./J. Mol. Struct. (Theochem) 310 (1994) 135-147
,+
75 H
H~~1095 1.1 70
1 080 C ' •
1,477
C
174,4
63.5
4
64.55
5
50 44.0
2
25
H
o
0.0
1 Fig. 2. Relative stabilities (kcalrnol") and MP2/6-31G* optimized geometries of the minima of the [Hz, C Z, OJ+ potential energy surface. Values obtained at the G2 level of theory. Bond distances in A, bond angles in deg.
this paper we shall designate by Latin letters the minima of the silicon-containing potential energy surfaces, namely [Hz, C, 0, Si}+ and [H4 , C, 0, Si]+, and by Arabic numbers those of [Hz, C z, 0]+ and [H4 , C z, OJ+. The different minima of the [Hz, C, 0, SW potential energy surface may be envisaged as the result of the gas-phase reactions between formaldehyde and Si+ep), silanone and C+ep), carbon monoxide and SiHtCZA\), and silicon monoxide
e
and CHt A z). As shown previously by various authors [21-23J, the most stable product of the first of these reactions is the sr' adduct (a), which is a molecular distonic ion, while the insertion of sr' into the C-O bond of formaldehyde (species e) is energetically much less favorable. On the contrary, the most stable product of the reaction between silanone and C+ corresponds to the insertion of this monocation into the Si-O bond (b), while the C+ adduct (c) lies much higher in energy.
A. Luna et al.jJ. Mol. Struct, (Theochem) 310 (1994) 135-147
H
\
r:
139
1 + 0
. ----51
H
P C-O O-Si
0.241 0.134
V 2p 0.470 1.345
Fig. 3. Contour maps of the Laplacian of the charge density of compound a of Fig. I. Positive values of '\72p are denoted by solid lines and negative values by dashed lines. Contour values (in atomic units) are ±O.05, ±O.25, ±O.50, ±O.75 and ±O.95. The numerical values (in atomic units) correspond to the charge density (p) and its Laplacian ('\72p) at the bcp of the indicated bonds.
Similar differences are found regarding the reactions of carbon monoxide and silicon monoxide with SiH! and CHi, respectively. In the first case the association of SiH! to the carbon atom of carbon monoxide is energetically preferred (yielding b), while CHi association to silicon monoxide leads to the global minimum a, i.e. to a molecular cation where the methylene cation is attached to the oxygen atom of silicon monoxide. These differences can be rationalized with the help of a topological analysis of their electronic
charge densities, p, and their Laplacians, 'V 2p. As shown in Fig. 3, the formaldehyde--Si" adduct a implies the appearance of a covalent O-Si bond. It can be seen that the Laplacian between both atomic centres is positive, showing that this bond is very polar, a fact which contributes to stabilizing the system. On the contrary, the attachment of C+ to silanone (see Fig. 4(a)) leads to a completely different situation, owing to the electronegativity differences between carbon and silicon. The formation of c implies a significant charge transfer from
A. Luna et al.L). Mol. Struct. ( Theochem ) 310 ( 1994) /3 5-/47
140
l 1-
HH
-:..~
!;i _
P Si-D D-C
O -
C
V2p
0.044 0. 162 00413 1.176
a
-,I +
H
H...~
...· \ i - C - O
Si-C C-D
P
V2p
0.D75
0.2 57
0.467
1.339
b
Fig. 4. Contour maps of the Laplacian of the charge densit y of: (a) compound c; (b) compound b of Fig . I . Same conventions as in Fig . 3.
the oxygen atom of silanone to C+. Since the oxygen atom is highly electronegative it recovers part of this charge by depopulating the Si-O bond. In fact, the value of p at the Si-O bcp of silanone is O.165eu- 3 , while in c it is only O.044eu- 3 . The result is that this adduct closely resembles an ion-dipole complex between carbon monoxide
and SiHi. The aforementioned Si-O bond weakening is also mirrored in a dramatic red-shift of the Si-O harmonic stretching frequency. At the HF/631 G* level the Si-0 stretching displacement of silanone is predicted to appear (after scaling) at 1210 em - 1, while for c it appears at about 255cm- l . The same topological descriptions allow one to rationalize the relative stabilities of the reactions of carbon monoxide and silicon monoxide with methylene and silylene cations. As indicated before, Fig. 4(a) would also correspond to the SiHi association to the oxygen atom of carbon monoxide and clearly shows that this association is essentially electrostatic. On these grounds, it is reasonable to expect the association at the carbon atom to be more stable, owing to the favorable orientation of the carbon monoxide dipole. There are , however, other factors which enhance the stability of b. As shown in Fig . 4(b), SiH i attachment at the carbon atom also produces a sizeable polarization of the charge density around carbon, which is not present when association takes place at the oxygen atom (see Fig. 4(a» . Furthermore, in this latter case , again because of the high electronegativity of oxygen, the formation of complex c implies a certain activation of the C-O bond, which contributes to destabilizing this form. The strong sr' -0- polarity of silicon monoxide considerably favors the attachment of the methylene cation at the oxygen atom. In this case , because the oxygen atom already bears a significantly large negative charge in the neutral molecule, the charge transfer to the incoming CHi cation has quite a small activation effect on the Si-O bond, while a new and very stable C-O covalent bond is formed . In fact , the value of p at the Si-O bcp of a (see Fig . 3) is only O.02eu- 3 smaller than in the neutral silicon monoxide (p = O.154eu- 3 ) . Let us now consider the [C2 , 0, H 2] + potential energy surface, whose minima correspond to the reactions of formaldehyde and carbon monoxide with C+ and the methylene cation respectively, and which could be compared with the products of the reactions of the same neutrals with sr' and the silylene cation. As illustrated in Fig. 2, and in agreement with previous theoretical results of
A . Luna et al.jJ. Mol. S truct . t Theochem ) 310 ( 1994) 135-147 H
\
C- o - c
1
P CoO
o-c
0.165 0.39 4
\72p 0.349 1.384
a
~ /
1+ -c-o
P
c-c CoO
0.286 0.477
\72p -0.864 1.247
b Fig. 5. Contour maps of the Laplacian of the charge density of: (a) compound 4; (b) compound 1 of Fig. 2. Same convent ions as in Fig. 3.
Bouma et al. [41], the global minimum 1 corresponds to the ketene radical cation. Since this species presents a sizeable spin contamination at the HF/6-31G* and MP2/6-31G* levels (S2 = 0.80), its structure was refined at the QCISD(T)/6-311G(d.p) level. The geometrical parameters calculated do not differ significantly
141
from th ose obtained in the MP 2/6-3l G * optimization , but the spin contaminati on was considerably reduced . The great stability of 1 impl ies that insertion of the C+ cation into the C-O bond of formaldehyde is energetically much more favorable than oxygen associa tion to yield 4 . It sho uld be noted th at this co rrespo nds to ju st the opposite of what has been described above for the formaldehyde +Si + reaction s. The Laplacian of 4 (see Fig . 5(a)) offers a possible explanation. The attachment of C+ to the oxygen atom of formaldehyde leads to a significant activ ation of the C-O bond, due to a mechanism identical to that discussed above for the C+ attachment to silanone. Hence , with respect to neutral formaldehyde, the value of Pc a t the H 2C-O bcp decreases by as much as 0.23 e u- 3 . Accordingly, 4 looks like an ion dipole complex between carbon monoxide and the methylene ion (see Fig . 5(a)). Con siste nt with thi s, on going from the neutral fo rmald eh yde to 4, th e C=O stretching frequ ency und ergoes a redshift of 1367 cm - I . Ho wever , insertion of C + into the C=O bond of formaldehyde lead s to a C - C covalent linkage, as illustr ated in Fig. 5(b). Both forms ca n also be obtained by the gas- phase reaction between carbon monoxide and the methylene radical cation. Once more the att achment at the carbon atom is stro ngly favored with respect to attachment at oxygen. As mentioned a bove and shown in Figs. 5(a) and 5(b), in the first case a very stable covalent bond is formed between both reactants, while in the second case , the interaction is essentially electrostatic. It can also be observed that below 4, two other stable minima are found. 2 can be obtained from the global minimum by a T able 2 G2 tot al energies (hartrees) of the minima of the [C 2 , H 4 , potential energy surface 1 2
3 3' 4 5 5' 6 6' 7
- 153.21872 - 153.19803 - 153.19334 -153 .18697 - 153. 17864 -153.16441 - 153.15975 - 153.14202 - 153.14 196 - 153.08048
at
142
A. Luna et al.l J, Mol. Struct. (Theochem) 310 (1994) 135-147
1,3 H-shift, while 3 is the result of a 1,2 H-shift followed by a cyclization process.
The G2 total energies of the lowest minima of the [Cj, H 4 , 0]+ potential energy surface are given in Table 2. Those of the [C, H 4 , 0, sW potential energy surface were taken from Ref. 42. Their relative stabilities have been schematized in Figs. 6 and 7, respectively. Figure 8 presents the MP2/6-31G* optimized geometries of the most stable [H 4 , C z, 0]+ species and shows that in the reaction between methanol and sr' the most favorable process, from a thermodynamic point of view, is insertion into the C-O bond (a), while insertion into the 0-H bond yields a less stable species c. Once more the reactions between silanol and C+ show a different type of behaviour. Figure 6 indicates that, in this case, insertion into the O-H bond (h) is more favorable than insertion into the Si-O linkage (j). The great stability of a implies that methyl substitution has a considerably stronger stabilization effect in these kinds of cations when the substituted center is an Si atom as has been found in Ref. 42. On the contrary, the stability of h with respect to j indicates that silyl substitution is more stabilizing when the substituted center is the oxygen atom. To better quantify this effect one may consider the following isodesmic reactions: H-C-OH+eA') ---+
+ H 3SiOH
H 3Si-C-OH+eA')
D.H = -3.2kcalmol-
+ OH z
1
+ H 3SiOH ---+ H-C-OSiHteA') + OH z
H-C-OH+eA')
D.H = -20.7kcalmol- J It can be observed that although both processes are exothermic the stabilizing effect of silyl substitution is markedly stronger when it takes place at the oxygen atom, due to the considerable polarity of the Si-O linkage.
However, the adducts ofSi+ and C+ with methanol and silanol (species d and m, respectively) lie quite high in energy with respect to the global minima since both correspond to electrostatic complexes between the monocation and the neutral. Another interesting feature is that b lies about 10 kcal mol- 1 above the global minima, while the equivalent structure e in the silanof-C" potential energy surface, lies about 23 kcal mol- J below structure j. The lower stability of b with respect to a seems to be related to the destabilizing effect of C=Si linkages. As shown by Hopkinson and Lien [43], the HzC-SiH+ cation is not stable and collapses to H 3C-Si+. We have found similarly that the HzC-SiHi cation is less stable than the H 3C-SiH+ system. This seems to imply that H-shifts from silicon to carbon are always favorable. This seems to happen because such an H-shift implies stabilization of the C-Si linkage due to an increase of the C - Si+ polarity. In all the species considered, and in particular, in j, e, b and a, the silicon atom always bears quite a high positive charge, which does not change significantly on going from j to e or from a to b. That is not the case with the net charge at the carbon atom. In j the net charge at the carbon atom is also quite positive due to the presence of an OH substituent. On going to species e this charge becomes slightly negative, since the carbon can partially depopulate the hydrogen now bonded to it. Consequently, the C-Si linkage becomes stronger and the system becomes more stable. On going from a to b, the situation is reversed. The net charge at the carbon atom is more negative in a than in band therefore, in the latter the C-Si linkage becomes weaker. Figure 6 also shows that CHi association with silanone or SiHi association with formaldehyde leads to three different minima (f, g and I). Quite surprisingly, the less stable conformation corresponds to the cyclic structure I. A topological analysis of its charge density indicates, however, that this compound is not a three-membered ring since no bcp between silicon and oxygen has been found. Hence the enhanced stabilities of f and g confirm the stabilizing role of the Si-O linkages in these kinds of systems.
A . Luna et al.jJ, Mol . Struct , ( Theochem ) 3/0 ( / 994) /35-/47
143
C++~S iOH
1
209. 1
H'o;l tI
H,.s~~
100
103·1
m
9 2. 8
H, 1
80
+
A~+ H
c
~
I
HrJi".,.H
HII'
HI'·Si H.... .....H
71.9
69.9
I
k
67. 0
j
60
~--lt
~H 4 9.6 h
h
H-...o.-s~+ HI'~~""" He::" H
4 3.4
e
40
36.7
d
~I+
20
~;'
I
H..... <-'H
10.1
b
o
H--'-t \ I.
,
Si,..o fYC",~
vH
0.0
a Fig. 6. Relative stabilities (kcal mol - I) of the minima of the [H4 , C, 0 , SW potential energy surface . Values obtained at the G2level of theory, taken from Ref. 42.
A . Luna et al.fJ . Mol. Stru ct . ( Theochem) 310 ( 1994) 135-147
144
100
1'1 ,
1
c. .....0"
+
I
HH~C"""H 86 .7
7
80
60 "Ill
lY"'c:..o 1'1
I
-~ 48.2
,
61
1'11+
40
( H""C
H? 'H 34.1
37.0
5'
5
20
J+
1'1,
c/
I
~I~C""""H 1 3.0
19.9
31
15.9
3
2
o
0.0
1
Fig. 7. Relat ive stab ilities (kcal mol" ) of the minima of the [H4 , theory.
c2 , at potential energ y surface. Values ob tained a t the G 2 level of
145
A. Luna et al.l.l. Mol. Struct. (Theochem} 310 (1994) 135-147
1
2
3
01 E-::11 5.1
4
6
5
cIt-
7
Fig. 8. MP2j6-310* optimized structures of the minima of the [H4 , C2 , 0)+ potential energy surface. Bond distances in in deg.
The reactivity between C+ and methanol seems to be more complex. As shown in fig. 7, the global minimum corresponds to the radical cation of vinyl alcohol, while the radical cation of acetaldehyde
A, bond angles
lies 13.0kcalmol- 1 higher. However, neither of these two minima can be produced in a direct reactive process between methanol and C+, and on the contrary, the methanol-C" adduct lies more than
146
A. Luna et at.ir. Mol. Struct. (Theochem) 310 (1994) 135-147
80 kcal mor ' above the global minimum. It seems then that the formation of 1 and 2 requires a previous step which corresponds to the insertion of C+ into the C-O bond of methanol to yield isomer 3. A 1,2 H-shift would then produce either 2 or 1. It must be observed that, again, the energy ordering of 1 and 3 is reversed with respect to their analogs a and b of the [C, H 4 , 0, Si]+ potential energy surface.
4. Conclusions
Si+ ep) and C+ ep) present very different behavior in their gas-phase reactions with oxygen bases. While the most stable product of the sr' + formaldehyde reaction corresponds to the formaldehyde.-Si" adduct, the C+ prefers to insert into the c=o bond yielding the ketene radical cation. A similar procedure is found for the reaction of C+ with silanone where the most stable product also corresponds to the C+ insertion into the Si=O bond. The low stability of the C+ adducts is due to the strong charge transfer which takes place from the neutral base to C+ in all cases and which results in a significant weakening of the X -0 linkages of the bases. This seems to be in agreement with the fact that in reactions between C+ and sr' with small hydrocarbon molecules, charge transfer was observed [17] with C+ but not with Si+. The most stable product of the sr' + methanol reaction corresponds to the insertion into the C-O bond, while in the C+ + silanol reaction, the insertion into the 0-H bond is preferred. This implies that the stabilization of [H 4 , C, 0, Si]+ cations by methyl substitution is greater when substitution takes place at the silicon atom. On the contrary, silyl substitution is more stabilizing when it takes place at the oxygen atom.
Acknowledgment
This work was partially supported by the DGICYT Project No. PB90-0228-C02-01.
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[27] R. Srinivas, J. Hrusak, D. Sulzle, D.K. Bohme and H. Schwarz, J. Am. Chern. Soc., 114 (1992) 2802. [28] R.S. Grev and H.F. Schaefer III, J. Chern. Phys., 97 (1992) 8389. [29] C.L. Darling and H.B. Schlegel, J. Phys. Chern., 97 (1993) 1368. [30] J.A. Pople, M. Head-Gordon, D.J. Fox, K. Raghavachari and L.A. Curtiss, J. Chern. Phys., 90 (1989) 5622. [31] L.A. Curtiss, K. Raghavachari, G.W. Trucks and J.A. Pople, J. Chern. Phys., 94 (1991) 7221. [32] M.J. Frisch, M. Head-Gordon, G.W. Trucks, J.B. Foresman, H.B. Schlegel, K. Raghavachari, M. Robb, J.S. Binkley, C. Gonzalez, D.J. DeFrees, D.J. Fox, R.A. Whiteside, R. Seeger, C.F. Melius, J. Baker, R.L. Martin, L.R. Kahn, J.J.P. Stewart, S. Topiol and J.A. Pople, GAUSSIAN 90, Revision I, Gaussian Inc., Pittsburgh, PA, 1990. [33] N.L. Ma, B.J. Smith and L. Radom, J. Phys. Chem., 96 (1992) 5804.
147
[34] N.C. Handy, P.J. Knowles and K. Somasundram, Theor. Chim. Acta, 68 (1985) 87. [35] R.H. Nobes, J.A. PopIe, L. Radom, N.C. Handy and PJ. Knowles, Chern. Phys. Lett., 138 (1987) 481. [36] C.W. Murray and N.C. Handy, J. Chern. Phys., 97 (1992) 6509. [37] R.W.F. Bader and H. Essen, J. Chern. Phys., 80 (1984) 1943. [38] R.W.F. Bader, P.J. MacDougall and C.D.H. Lau, J. Am. Chern. Soc., 106 (1984) 1594. [39] R.W.F. Bader, Atoms in Molecules. A Quantum Theory, Oxford University Press, New York, 1990. [40] The AIMPAC programs package has been provided by J. Cheeseman and R.F.W. Bader. [41] W.J. Bouma, P.M.W. Gill and L. Radom, Org. Mass Spectrom., 19 (1984) 610 [42] A. Luna and M. Yanez, J. Phys. Chem., 97 (1993) 10659. [43] A.C. Hopkinson and M.H. Lien, J. Chern. Soc .. Chern. Commun., (1980) 107.
Journal of MolecularStructure (Theochem) 310 (1994) 135-147
Gas-phase reactions ofC+ep) and Si+ep) with oxygen bases. A G2 ab initio molecular orbital study A. Luna, O. Mo, M. Yafiez" Departamento de Quimica, C-9. Universidad Autonoma de Madrid, Cantoblanco, 28049-Madrid. Spain
(Received 19 July 1993; accepted 13 August 1993)
Abstract
G2 ab initio molecular orbital theory has been used to study the relative stabilities of the different products of the reactions of C+ ep) and Si+ ep) with several oxygen bases in the gas phase. The reactions of sr' and C+ with formaldehyde and methanol as well as those ofC+ with silanone and silanol were considered. Significant differences in C+ and Si+ reactivity have been found in all cases, as a consequence of the greater electronegativity of carbon with respect to silicon and to the enhanced stability of the Si-O linkages, owing to their sizeable sr' -0- polarity. Consistent with this, methyl substitution, for instance, considerably stabilizes these oxygen-containing cations when the substituted center is the silicon rather than the oxygen atom. On the contrary, silyl substitution is more stabilizing when the substituted center is the oxygen atom.
1. Introduction Silicon is abundant in terrestrial [1, 2] and extraterrestrial [3, 4] environments. In the latter, silicon is mainly present as silicon monoxide, which is believed to be produced by neutralization of SiOH+. This particular ion is also abundant in our atmosphere, where sr' participates in many reactions. Silicon is introduced into the Earth's atmosphere by the ablation of meteors and is then readily ionized by charge transfer with 01" and NO+. The sr' ions formed in this way react with neutral components of the upper atmosphere, in particular with water, to yield the aforementioned SiOH+ cations. These factors, among others have motivated considerable research activity on the reactions of sr' with a large variety of "Corresponding author.
molecules in the gas phase [5-29], which has shown that most of the generated ions have unusual structural electronic and chemical properties compared to other molecular ions. In addition, the differences and similarities between silicon and carbon have been of interest to chemists for many years. In this respect, some studies comparing sr' and C+ reactions with small hydrocarbon molecules in the gas phase have been published recently [17] and have reported large differences between the reactivities of both monocations. For instance, while reactions of C+ with allene and propyne favor elimination of hydrogen and hydride transfer, reactions with sr' favor H atom and CH 3 elimination. Similarly, the reaction of diacetylene with C+ favors H atom elimination and charge transfer while with sr'. hydride transfer is the only channel. Nevertheless, to the best of our knowledge, no similar comparative studies regarding
0166-1280/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved SSDI 0166-1280(93)03503-Y
136
A. Luna et al.jl. Mol. Struct. (Theochem) 310 (1994) 135-147
gas-phase reactions of sr' and C+ with other neutrals, in particular with oxygen-containing molecules, have been reported in the literature so far. With the aim of providing some insight into this particular problem, we present in this paper highlevel ab initio molecular orbital calculations on the structures, relative stabilities and electronic charge distributions of complexes between C+ and sr' with oxygen bases, in particular with formaldehyde, silanone, methanol and silanol.
2. Computational details
Ab initio molecular orbital calculations at the G 1 [30] and G2 [31] levels of theory were carried out using the GAUSSIAN-90 program package [32]. G 1 theory [30] is a composite procedure based on Hartree-Fock (HF) and Meller-Plesset perturbation theory at second- and fourthorders (MP2 and MP4), and quadratic configuration interaction including single, double and triple excitations (QCISD(T» levels of theory. In the G 1 procedure the initial equilibrium structures are optimized at the UHF/6-31G* level in order to obtain the corresponding zero-point vibrational energies (ZPE) and to characterize the stationary points of the potential energy surface. These geometries are then refined at the MP2/6-3l G* level. Assuming additivity of different basis set enhancements at the MP4 level, and additivity of basis set and correlation effects between MP4 and QCISD(T), relative energies are obtained at effectively the QCISD(T) level with the 6-311 + G(2df,p) basis set. The final G 1 energy is obtained after adding a high-level correction which accounts for the isogyric effect and the ZPE value obtained in the SCF/6-31G* calculation scaled by the empirical factor 0.893. G2 theory [31] is a further refinement which corrects some shortcomings of the G 1 theory arising from the assumption of additivity of individual basis set improvements and from an overestimation of the isogyric effect. These corrections imply further calculations at the
MP2/6-311 + G(3df,2p) level with the result that the final energies are effectively of a QCISD(T)/6-3ll + G(3df,2p) quality. Although, in general, G2 ab initio theory [31] yields [31, 33] heats of formation, total atomization energies and ionization potentials which agree with the experimental values within ±O.l eV, it must be recalled that this theory, for open-shell systems, is based on the use of UMP2 optimized geometries and on several correlation contributions to the energy evaluated in the framework of the UMP theory, which has been shown [34-36] to converge slowly for molecules where the spin contamination from higher multiplicities is large. Hence, for some open-shell systems where spin contamination is significant, this theoretical scheme may lead to unreliable results. Therefore, in the present paper we have carried out additional calculations to avoid these possible shortcomings. In particular, for those species where a sizeable spin contamination has been detected, the geometry optimizations were performed at the QCISD(T)/ 6-3llG(d,p) level. The characteristics of the Si-X and C-X bonds and the nature of the interactions of the different neutrals with sr' and C+ were investigated by means of a topological analysis of the electronic charge density, p, and its Laplacian, V'2 p. As has been shown by Bader and co-workers [37-39], V'2p identifies regions of space where the electronic charge is locally depleted (V'2p > 0) or built up (''\12 P < 0). The former situation is typically associated with interactions between closed-shell systems (ionic bonds, hydrogen bonds and van der Waals molecules), while the latter characterizes covalent bonds, where the electronic charge concentrates between the nuclei. We have also located the relevant bond critical points (bcps), i.e. points where the electronic charge density is minimum along the bond and maximum in the other two directions, because the values of p and V'2 p at these points offer quantitative information on the strength and nature of the bonding. This topological analysis was carried out on the wavefunction correct to first-order, to take explicitly into account electronic correlation effects. For this purpose we have employed the AIMPAC series of programs [40].
137
A. Luna et al.jl. Mol. Struct. (Theochem) 310 (1994) 135-147 Table I G2 total energies (hartrees) of the minima of the [Hz, C, 0, Si]+ (a-e) and [Hz, C z, 0]+ (1-5) potential energy surfaces a
-403.07661 -403.06601 -403.03370 -403.02810 -403.02679 -152.01522 -151.94503 -151.93044 -151.91401 -151.91235
b
c d
e 1 2
3 4 5
3. Results and discussion
The G2 total energies of the different equilibrium conformations of [C, 0, H 2 , Si]+ and [C2 , 0, H 2 complexes are presented in Table 1. Their relative stabilities and structures are presented in Figs. 1 and 2, respectively. It must be mentioned that G I energies for several minima of the potential energy surface of [H 2 , C, 0, Si]+ have been reported before [23]. The present G2 values imply changes in their relative stabilities smaller than 1.6 kcal mor '. In
r
,+
40 5i
..,:.:..:...::..::...-.....5 i 1.524 0 113,
137.0 180.0
C
30
~+
HH
H 1.100
~5.4.., n..,. ..,_ I --------'= 30.4 1.471·~i~C 176.3
31.3
e
d
26.9
C
20
10
......-'-'-''"'-''.......c
1.~-t
175.9 6.6
b
o
0.0
a Fig. I. Relative stabilities (kcal mol ") and MP2j6-31G* optimized geometries of the minima of the [Hz, C, 0, surface. Values obtained at the G2 level of theory. Bond distances in A, bond angles in deg.
sW
potential energy
138
A. Luna et al./J. Mol. Struct. (Theochem) 310 (1994) 135-147
,+
75 H
H~~1095 1.1 70
1 080 C ' •
1,477
C
174,4
63.5
4
64.55
5
50 44.0
2
25
H
o
0.0
1 Fig. 2. Relative stabilities (kcalrnol") and MP2/6-31G* optimized geometries of the minima of the [Hz, C Z, OJ+ potential energy surface. Values obtained at the G2 level of theory. Bond distances in A, bond angles in deg.
this paper we shall designate by Latin letters the minima of the silicon-containing potential energy surfaces, namely [Hz, C, 0, Si}+ and [H4 , C, 0, Si]+, and by Arabic numbers those of [Hz, C z, 0]+ and [H4 , C z, OJ+. The different minima of the [Hz, C, 0, SW potential energy surface may be envisaged as the result of the gas-phase reactions between formaldehyde and Si+ep), silanone and C+ep), carbon monoxide and SiHtCZA\), and silicon monoxide
e
and CHt A z). As shown previously by various authors [21-23J, the most stable product of the first of these reactions is the sr' adduct (a), which is a molecular distonic ion, while the insertion of sr' into the C-O bond of formaldehyde (species e) is energetically much less favorable. On the contrary, the most stable product of the reaction between silanone and C+ corresponds to the insertion of this monocation into the Si-O bond (b), while the C+ adduct (c) lies much higher in energy.
A. Luna et al.jJ. Mol. Struct, (Theochem) 310 (1994) 135-147
H
\
r:
139
1 + 0
. ----51
H
P C-O O-Si
0.241 0.134
V 2p 0.470 1.345
Fig. 3. Contour maps of the Laplacian of the charge density of compound a of Fig. I. Positive values of '\72p are denoted by solid lines and negative values by dashed lines. Contour values (in atomic units) are ±O.05, ±O.25, ±O.50, ±O.75 and ±O.95. The numerical values (in atomic units) correspond to the charge density (p) and its Laplacian ('\72p) at the bcp of the indicated bonds.
Similar differences are found regarding the reactions of carbon monoxide and silicon monoxide with SiH! and CHi, respectively. In the first case the association of SiH! to the carbon atom of carbon monoxide is energetically preferred (yielding b), while CHi association to silicon monoxide leads to the global minimum a, i.e. to a molecular cation where the methylene cation is attached to the oxygen atom of silicon monoxide. These differences can be rationalized with the help of a topological analysis of their electronic
charge densities, p, and their Laplacians, 'V 2p. As shown in Fig. 3, the formaldehyde--Si" adduct a implies the appearance of a covalent O-Si bond. It can be seen that the Laplacian between both atomic centres is positive, showing that this bond is very polar, a fact which contributes to stabilizing the system. On the contrary, the attachment of C+ to silanone (see Fig. 4(a)) leads to a completely different situation, owing to the electronegativity differences between carbon and silicon. The formation of c implies a significant charge transfer from
A. Luna et al.L). Mol. Struct. ( Theochem ) 310 ( 1994) /3 5-/47
140
l 1-
HH
-:..~
!;i _
P Si-D D-C
O -
C
V2p
0.044 0. 162 00413 1.176
a
-,I +
H
H...~
...· \ i - C - O
Si-C C-D
P
V2p
0.D75
0.2 57
0.467
1.339
b
Fig. 4. Contour maps of the Laplacian of the charge densit y of: (a) compound c; (b) compound b of Fig . I . Same conventions as in Fig . 3.
the oxygen atom of silanone to C+. Since the oxygen atom is highly electronegative it recovers part of this charge by depopulating the Si-O bond. In fact, the value of p at the Si-O bcp of silanone is O.165eu- 3 , while in c it is only O.044eu- 3 . The result is that this adduct closely resembles an ion-dipole complex between carbon monoxide
and SiHi. The aforementioned Si-O bond weakening is also mirrored in a dramatic red-shift of the Si-O harmonic stretching frequency. At the HF/631 G* level the Si-0 stretching displacement of silanone is predicted to appear (after scaling) at 1210 em - 1, while for c it appears at about 255cm- l . The same topological descriptions allow one to rationalize the relative stabilities of the reactions of carbon monoxide and silicon monoxide with methylene and silylene cations. As indicated before, Fig. 4(a) would also correspond to the SiHi association to the oxygen atom of carbon monoxide and clearly shows that this association is essentially electrostatic. On these grounds, it is reasonable to expect the association at the carbon atom to be more stable, owing to the favorable orientation of the carbon monoxide dipole. There are , however, other factors which enhance the stability of b. As shown in Fig . 4(b), SiH i attachment at the carbon atom also produces a sizeable polarization of the charge density around carbon, which is not present when association takes place at the oxygen atom (see Fig. 4(a» . Furthermore, in this latter case , again because of the high electronegativity of oxygen, the formation of complex c implies a certain activation of the C-O bond, which contributes to destabilizing this form. The strong sr' -0- polarity of silicon monoxide considerably favors the attachment of the methylene cation at the oxygen atom. In this case , because the oxygen atom already bears a significantly large negative charge in the neutral molecule, the charge transfer to the incoming CHi cation has quite a small activation effect on the Si-O bond, while a new and very stable C-O covalent bond is formed . In fact , the value of p at the Si-O bcp of a (see Fig . 3) is only O.02eu- 3 smaller than in the neutral silicon monoxide (p = O.154eu- 3 ) . Let us now consider the [C2 , 0, H 2] + potential energy surface, whose minima correspond to the reactions of formaldehyde and carbon monoxide with C+ and the methylene cation respectively, and which could be compared with the products of the reactions of the same neutrals with sr' and the silylene cation. As illustrated in Fig. 2, and in agreement with previous theoretical results of
A . Luna et al.jJ. Mol. S truct . t Theochem ) 310 ( 1994) 135-147 H
\
C- o - c
1
P CoO
o-c
0.165 0.39 4
\72p 0.349 1.384
a
~ /
1+ -c-o
P
c-c CoO
0.286 0.477
\72p -0.864 1.247
b Fig. 5. Contour maps of the Laplacian of the charge density of: (a) compound 4; (b) compound 1 of Fig. 2. Same convent ions as in Fig. 3.
Bouma et al. [41], the global minimum 1 corresponds to the ketene radical cation. Since this species presents a sizeable spin contamination at the HF/6-31G* and MP2/6-31G* levels (S2 = 0.80), its structure was refined at the QCISD(T)/6-311G(d.p) level. The geometrical parameters calculated do not differ significantly
141
from th ose obtained in the MP 2/6-3l G * optimization , but the spin contaminati on was considerably reduced . The great stability of 1 impl ies that insertion of the C+ cation into the C-O bond of formaldehyde is energetically much more favorable than oxygen associa tion to yield 4 . It sho uld be noted th at this co rrespo nds to ju st the opposite of what has been described above for the formaldehyde +Si + reaction s. The Laplacian of 4 (see Fig . 5(a)) offers a possible explanation. The attachment of C+ to the oxygen atom of formaldehyde leads to a significant activ ation of the C-O bond, due to a mechanism identical to that discussed above for the C+ attachment to silanone. Hence , with respect to neutral formaldehyde, the value of Pc a t the H 2C-O bcp decreases by as much as 0.23 e u- 3 . Accordingly, 4 looks like an ion dipole complex between carbon monoxide and the methylene ion (see Fig . 5(a)). Con siste nt with thi s, on going from the neutral fo rmald eh yde to 4, th e C=O stretching frequ ency und ergoes a redshift of 1367 cm - I . Ho wever , insertion of C + into the C=O bond of formaldehyde lead s to a C - C covalent linkage, as illustr ated in Fig. 5(b). Both forms ca n also be obtained by the gas- phase reaction between carbon monoxide and the methylene radical cation. Once more the att achment at the carbon atom is stro ngly favored with respect to attachment at oxygen. As mentioned a bove and shown in Figs. 5(a) and 5(b), in the first case a very stable covalent bond is formed between both reactants, while in the second case , the interaction is essentially electrostatic. It can also be observed that below 4, two other stable minima are found. 2 can be obtained from the global minimum by a T able 2 G2 tot al energies (hartrees) of the minima of the [C 2 , H 4 , potential energy surface 1 2
3 3' 4 5 5' 6 6' 7
- 153.21872 - 153.19803 - 153.19334 -153 .18697 - 153. 17864 -153.16441 - 153.15975 - 153.14202 - 153.14 196 - 153.08048
at
142
A. Luna et al.l J, Mol. Struct. (Theochem) 310 (1994) 135-147
1,3 H-shift, while 3 is the result of a 1,2 H-shift followed by a cyclization process.
The G2 total energies of the lowest minima of the [Cj, H 4 , 0]+ potential energy surface are given in Table 2. Those of the [C, H 4 , 0, sW potential energy surface were taken from Ref. 42. Their relative stabilities have been schematized in Figs. 6 and 7, respectively. Figure 8 presents the MP2/6-31G* optimized geometries of the most stable [H 4 , C z, 0]+ species and shows that in the reaction between methanol and sr' the most favorable process, from a thermodynamic point of view, is insertion into the C-O bond (a), while insertion into the 0-H bond yields a less stable species c. Once more the reactions between silanol and C+ show a different type of behaviour. Figure 6 indicates that, in this case, insertion into the O-H bond (h) is more favorable than insertion into the Si-O linkage (j). The great stability of a implies that methyl substitution has a considerably stronger stabilization effect in these kinds of cations when the substituted center is an Si atom as has been found in Ref. 42. On the contrary, the stability of h with respect to j indicates that silyl substitution is more stabilizing when the substituted center is the oxygen atom. To better quantify this effect one may consider the following isodesmic reactions: H-C-OH+eA') ---+
+ H 3SiOH
H 3Si-C-OH+eA')
D.H = -3.2kcalmol-
+ OH z
1
+ H 3SiOH ---+ H-C-OSiHteA') + OH z
H-C-OH+eA')
D.H = -20.7kcalmol- J It can be observed that although both processes are exothermic the stabilizing effect of silyl substitution is markedly stronger when it takes place at the oxygen atom, due to the considerable polarity of the Si-O linkage.
However, the adducts ofSi+ and C+ with methanol and silanol (species d and m, respectively) lie quite high in energy with respect to the global minima since both correspond to electrostatic complexes between the monocation and the neutral. Another interesting feature is that b lies about 10 kcal mol- 1 above the global minima, while the equivalent structure e in the silanof-C" potential energy surface, lies about 23 kcal mol- J below structure j. The lower stability of b with respect to a seems to be related to the destabilizing effect of C=Si linkages. As shown by Hopkinson and Lien [43], the HzC-SiH+ cation is not stable and collapses to H 3C-Si+. We have found similarly that the HzC-SiHi cation is less stable than the H 3C-SiH+ system. This seems to imply that H-shifts from silicon to carbon are always favorable. This seems to happen because such an H-shift implies stabilization of the C-Si linkage due to an increase of the C - Si+ polarity. In all the species considered, and in particular, in j, e, b and a, the silicon atom always bears quite a high positive charge, which does not change significantly on going from j to e or from a to b. That is not the case with the net charge at the carbon atom. In j the net charge at the carbon atom is also quite positive due to the presence of an OH substituent. On going to species e this charge becomes slightly negative, since the carbon can partially depopulate the hydrogen now bonded to it. Consequently, the C-Si linkage becomes stronger and the system becomes more stable. On going from a to b, the situation is reversed. The net charge at the carbon atom is more negative in a than in band therefore, in the latter the C-Si linkage becomes weaker. Figure 6 also shows that CHi association with silanone or SiHi association with formaldehyde leads to three different minima (f, g and I). Quite surprisingly, the less stable conformation corresponds to the cyclic structure I. A topological analysis of its charge density indicates, however, that this compound is not a three-membered ring since no bcp between silicon and oxygen has been found. Hence the enhanced stabilities of f and g confirm the stabilizing role of the Si-O linkages in these kinds of systems.
A . Luna et al.jJ, Mol . Struct , ( Theochem ) 3/0 ( / 994) /35-/47
143
C++~S iOH
1
209. 1
H'o;l tI
H,.s~~
100
103·1
m
9 2. 8
H, 1
80
+
A~+ H
c
~
I
HrJi".,.H
HII'
HI'·Si H.... .....H
71.9
69.9
I
k
67. 0
j
60
~--lt
~H 4 9.6 h
h
H-...o.-s~+ HI'~~""" He::" H
4 3.4
e
40
36.7
d
~I+
20
~;'
I
H..... <-'H
10.1
b
o
H--'-t \ I.
,
Si,..o fYC",~
vH
0.0
a Fig. 6. Relative stabilities (kcal mol - I) of the minima of the [H4 , C, 0 , SW potential energy surface . Values obtained at the G2level of theory, taken from Ref. 42.
A . Luna et al.fJ . Mol. Stru ct . ( Theochem) 310 ( 1994) 135-147
144
100
1'1 ,
1
c. .....0"
+
I
HH~C"""H 86 .7
7
80
60 "Ill
lY"'c:..o 1'1
I
-~ 48.2
,
61
1'11+
40
( H""C
H? 'H 34.1
37.0
5'
5
20
J+
1'1,
c/
I
~I~C""""H 1 3.0
19.9
31
15.9
3
2
o
0.0
1
Fig. 7. Relat ive stab ilities (kcal mol" ) of the minima of the [H4 , theory.
c2 , at potential energ y surface. Values ob tained a t the G 2 level of
145
A. Luna et al.l.l. Mol. Struct. (Theochem} 310 (1994) 135-147
1
2
3
01 E-::11 5.1
4
6
5
cIt-
7
Fig. 8. MP2j6-310* optimized structures of the minima of the [H4 , C2 , 0)+ potential energy surface. Bond distances in in deg.
The reactivity between C+ and methanol seems to be more complex. As shown in fig. 7, the global minimum corresponds to the radical cation of vinyl alcohol, while the radical cation of acetaldehyde
A, bond angles
lies 13.0kcalmol- 1 higher. However, neither of these two minima can be produced in a direct reactive process between methanol and C+, and on the contrary, the methanol-C" adduct lies more than
146
A. Luna et at.ir. Mol. Struct. (Theochem) 310 (1994) 135-147
80 kcal mor ' above the global minimum. It seems then that the formation of 1 and 2 requires a previous step which corresponds to the insertion of C+ into the C-O bond of methanol to yield isomer 3. A 1,2 H-shift would then produce either 2 or 1. It must be observed that, again, the energy ordering of 1 and 3 is reversed with respect to their analogs a and b of the [C, H 4 , 0, Si]+ potential energy surface.
4. Conclusions
Si+ ep) and C+ ep) present very different behavior in their gas-phase reactions with oxygen bases. While the most stable product of the sr' + formaldehyde reaction corresponds to the formaldehyde.-Si" adduct, the C+ prefers to insert into the c=o bond yielding the ketene radical cation. A similar procedure is found for the reaction of C+ with silanone where the most stable product also corresponds to the C+ insertion into the Si=O bond. The low stability of the C+ adducts is due to the strong charge transfer which takes place from the neutral base to C+ in all cases and which results in a significant weakening of the X -0 linkages of the bases. This seems to be in agreement with the fact that in reactions between C+ and sr' with small hydrocarbon molecules, charge transfer was observed [17] with C+ but not with Si+. The most stable product of the sr' + methanol reaction corresponds to the insertion into the C-O bond, while in the C+ + silanol reaction, the insertion into the 0-H bond is preferred. This implies that the stabilization of [H 4 , C, 0, Si]+ cations by methyl substitution is greater when substitution takes place at the silicon atom. On the contrary, silyl substitution is more stabilizing when it takes place at the oxygen atom.
Acknowledgment
This work was partially supported by the DGICYT Project No. PB90-0228-C02-01.
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