Theoretical study of [Si,O,C,O] species: Prediction of new species on triplet potential energy surface

Theoretical Study of [Si,O,C,O] Species:Prediction of New Species on TripletPotential Energy Surface

BHASKAR MONDAL, DEEPANWITA GHOSH, ABHIJIT K. DAS

Department of Spectroscopy, Indian Association for the Cultivation of Science, Jadavpur,Kolkata 700032, India

Received 1 July 2009; accepted 13 July 2009Published online 9 December 2009 in Wiley Online Library (wileyonlinelibrary.com).DOI 10.1002/qua.22420

ABSTRACT: The intermediates [Si,O,C,O] of the Si þ CO2 reaction have beenstudied in detail using high level ab iniitio methods. Both singlet and triplet [Si,O,C,O]species are characterized structurally and energetically. On the singlet potential energysurface (PES), the vdw-OSi–CO isomer and in the triplet PES, the bent-SiOCO isomer isfound to be thermodynamically as well as kinetically most stable species. All possibleisomerization transition states (TS) are located on both singlet and triplet potentialsurfaces. On the triplet surface, the stability of the trans-OSiCO isomer is comparablewith that of the bent-SiOCO isomer. A non-planar cis-SiOCO isomer is located on thetriplet PES, which is predicted for the first time. Heats of formation at 0 K (DfH

�, 0 K)for all singlet and triplet species are computed using G3B3, G3MP2, and CBS-Qtheories. The discrepancy between G3B3 and the other two methods for the heat offormation value for triplet trans-OSiCO is discussed. The PESs for singlet as well astriplet species with their dissociation asymptotes are explored at the CCSD(T)/6-311G(d,p)//MP2/6-311G(d,p) level of theory. VC 2009 Wiley Periodicals, Inc. Int J QuantumChem 111: 606–615, 2011

Key words: [Si,O,C,O] system; potential energy surface; stability; thermochemistry

1. Introduction

C hemistry of silicon containing molecules inthe interstellar clouds has not yet clearly

understood. The presence of silicon containing

molecules in space with a cosmic abundance hasbeen confirmed so far with the detection of fewsilicon-species. In the circumstellar envelops ofcarbon-rich stars such as IRC þ 10�216, SiO andCO molecules were detected [1]. The dimers ofCO and SiO were studied extensively both theo-retically and experimentally [2–6]. But there arevery few studies on the mixed dimers [Si,O,C,O]of SiO and CO [7]. On the basis of these twoCorrespondence to: A. K. Das; e-mail: [email protected]

International Journal of Quantum Chemistry, Vol. 111, 606–615 (2011)VC 2009 Wiley Periodicals, Inc.

reported interstellar species SiO and CO,[Si,O,C,O] species may be a potential candidatefor astrochemical research. The [Si,O,C,O] speciesare also the intermediates of the reaction ofatomic silicon Si in its 3P ground state with CO2,which is of great importance for the atmosphericsilicon chemistry [8]. The reaction between Si andCO2 is also interesting because of its role in chem-ical vapor deposition (CVD) process for semicon-ductor industry as well as for producing new ce-ramic materials [9]. The reaction of Si-atom withCO2 at room temperature was first investigatedby Hussain and Norris [10] using atomic absorp-tion spectroscopy (AAS) and then the same reac-tion was studied at high temperature by Mickand Roth [9] using shock tube technique. TheSi þ CO2 reaction was explained by a non-adia-batic transition from the triplet reactant surface tothe singlet product surface [10]. The spin forbid-den bimolecular reaction can be represented asfollows:

Sið3PÞ þ CO2ð1RÞ ! SiOð1RÞ þ COð1RÞ

On the basis of the above explanation, Hussainand Norris determined a rate coefficient k ¼ (1.1� 0.1) � 10�11 cm3/molecule/s, which is muchaway from the rate coefficient k � 1.2 � 10�14

cm3/molecule/s reported recently by Martin et al.[8] This discrepancy may be explained by adetailed theoretical study of the Si þ CO2 reactionmechanism. To understand the reaction mecha-nism, the isomerization processes and formationenthalpies of various reaction intermediates[Si,O,C,O] require detailed investigation. More-over, to correlate the theoretical results with theexperimental observations, a clear knowledgeabout their kinetic stability is also necessary. Thetheoretical work on [Si,O,C,O] intermediates wascarried out by Zhang and Qin [7], who studiedOSiCO and its isomers on the basis of the elec-tronic structures, vibrational frequencies, andthermodynamic stability of the species in theirsinglet and triplet states. Our main objective ofthis article is to investigate the isomerization proc-esses of the [Si,O,C,O] species, their kinetic stabil-ity in both singlet and triplet potential energysurfaces and formation enthalpies to get a pictureof the reaction mechanism for the spin forbiddenSi(3P) þ CO2 reaction. Our detailed theoreticalwork would initiate a further kinetic study thatcan help to find out the discrepancy between theexisting rate coefficient values.

2. Computational Details

All the equilibrium geometries and transitionstates are obtained using second order M

06oller-Plesset perturbation theory (MP2) [11] with a va-lence triple-n basis set 6-311G(d,p) [12, 13]. Thecharacter of the stationary points (either minimumwith energy points for which all vibrational fre-quencies are real or saddle points characterizedby one imaginary frequency) are confirmed byvibrational analysis done at the same MP2/6-311G(d,p) theoretical level. To locate the possibleinterconnecting transition states between thelocated minima at both singlet and triplet surfa-ces, we have used synchronous transit-guidedquasi-Newton (STQN) methods with QST2 andQST3 options [14]. The IRC algorithm has beenused to connect the located transition states tocorresponding minima [15].

The zero-point corrected MP2 energies are notgood enough to explain the thermodynamic sta-bility of the various [Si,O,C,O] species and theirtransition states. Therefore, to obtain accurateelectronic energies, single point calculations havebeen performed at the CCSD(T)/cc-pVTZ levelusing the MP2/6-311G(d,p) optimized geometries.The coupled cluster CCSD(T) method [16], whichincludes single and double excitations and an esti-mate of triple excitations by a perturbation treat-ment, in conjunction with the correlation-consist-ent polarized valence triple-n basis set cc-pVTZ[17, 18] is used for the calculation of single pointenergies. All the CCSD(T) energies are correctedfor zero point vibration energies (ZPVE) calcu-lated at the MP2/6-311G(d,p) level. The CCSD(T)predicted energy for the species are quite reliableas it is consistent with the previous study. Thesinglet and triplet potential energy surfaces are ex-plored at CCSD(T)/cc-pVTZ//MP2/6-311G(d,p)level of theory.

The unrestricted Hartree Fock (UHF) wave-functions are used in this study to describe thetriplet states. These wavefunctions are usuallyquite reliable and yield satisfactory optimizedgeometries. However, spin unrestricted wavefunctions are not the eigenfunctions of the S2 op-erator. This is normally not a significant problemunless spin contamination from higher spin statesis large which leads to distorted potential surfa-ces. For this reason, the hS2i value has been sys-tematically checked for each triplet optimizedstructure (minimum or transition state). The hS2i

THEORETICAL STUDY OF [Si,O,C,O] SPECIES

VOL. 111, NO. 3 DOI 10.1002/qua INTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY 607

value is found to be at most equal to 2.02. So,there was no significant spin contamination in thecorresponding wave function.

The calculation of standard heats of formationat 0 K (DfH

�, 0 K) is performed by means of atom-ization scheme [19] using the energies obtainedfrom G3B3, G3MP2, and CBS-Q methods. TheG3B3, G3MP2, and CBS-Q are composite methodsthat predict the energy accurately using the opti-mized geometry from a computationally cheaperlevel and the steps involved can be found else-

where [20–22]. There is a deviation of G3B3 geom-etry from MP2/6-311G(d,p) geometry for triplettrans-OSiCO species and this deviation is reflectedin the heats of formation value. Therefore, the useof other two composite methods G3MP2 andCBS-Q gives consistent results. The frozencore (FC) approximation is employed for elec-tronic structure and single point energy calcula-tions. All calculations have been performed usingthe Gaussian 03 suite of quantum chemical pro-gram [23].

FIGURE 1. Optimized geometries of the singlet species with geometrical parameters at MP2/6-311G(d,p) level oftheory.

MONDAL, GHOSH, AND DAS

608 INTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY DOI 10.1002/qua VOL. 111, NO. 3

3. Results and Discussions

3.1. THE [Si,O,C,O] SPECIES ON THELOWEST SINGLET POTENTIALENERGY SURFACE

On the singlet PES of [Si,O,C,O] species wehave located six isomers and four transition statesstructures at MP2/6-311G(d,p) level of theorywithin the energy range of 95 kcal/mol. Figure 1represents the fully optimized geometries withthe geometrical parameters of the singlet[Si,O,C,O] and related species. The weaklybonded van der Waals complex vdw-OSiCO is sta-ble over its dissociation asymptotes SiO(1R) þCO(1R) and Si(3P) þ CO2 (1R) by 0.81 kcal/moland 79.80 kcal/mol, respectively, and this is thethermodynamically most stable isomer among allsinglet isomers. The vdw-OSiCO lies 0.80 kcal/mol below the dissociation fragments SiO(1R) þCO(1R), suggesting that the former is very weaklybound. The structure of the vdw-OSiCO is quiteinteresting. The OSi and CO units are situatedalmost parallel maintaining a SiAO distance3.794 A, and the angles ffOSiC and ffOCSi are59.7� and 119.2�, respectively, calculated at MP2/6-311G(d,p) level. The use of correlation consist-ent basis set with MP2 method deforms the struc-

ture by increasing the SiAO distance and decreas-ing the ffOSiC angle by about 0.6 A and 29.0�,respectively. Same deformation is also observedat DFT/B3LYP level of theory. This suggests thathigh flexibility of this isomer depends largely onthe level of theory and the basis set used. The rel-ative thermodynamic stability for rest of the spe-cies is as follows:

SiðOCÞO > cyc-OSiCO > butterfly-OSiCO

> bent-SiOCO > lin-OSiCO

The relative energies for all the singlet[Si,O,C,O] species in Table I are calculated withrespect to the most stable singlet vdw-OSiCO spe-cies. Total energies with ZPE correction for MP2as well as CCSD(T) level and relative energies atCCSD(T) level for all singlet species are shown inTable I. Among the six isomeric [Si,O,C,O] spe-cies, only the butterfly-OSiCO is non-planar with adihedral angle ffOSiCO ¼ 163.61�. The non-planarbutterfly-OSiCO can be derived from the planarcyclic isomer cyc-OSiCO by changing the dihedrald-OSiCO, which corresponds to the out-of-planebending mode of the cyc-OSiCO isomer when Siand C atoms moves in the opposite direction ofthe two O atoms. Except the linear isomer lin-OSiCO, the other [Si,O,C,O] species lies below

TABLE ITotal energies (a.u.) and relative energies (kcal/mol) of the singlet [Si,O,C,O] and related species.

Singlet species MP2 (FC)a ZPVE Corrected CCSD(T) (FC)b Erel (kcal/mol)

vdw-OSiCO (1A0) �477.178662 0.008069 �477.170593 �477.344234 0.00SiO (1R) þ CO (1R) �477.176503 0.007590 �477.168913 �477.342944 0.81Si(OC)O (1A0) �477.124092 0.010948 �477.113144 �477.295308 30.70cyc-OSiCO (1A0) �477.095368 0.009573 �477.085795 �477.283913 37.85butterfly-OSiCO (1A) �477.095635 0.010050 �477.085585 �477.283151 38.33bent-SiOCO (1A0) �477.043931 0.011901 �477.032030 �477.217438 79.56Si (1D) þ CO2 (1R) �477.039316 0.010148 �477.029168 �477.217052 79.80lin-OSiCO �477.018738 0.010597 �477.008141 �477.193262 94.731TS1 �477.021275 0.008942 �477.012333 �477.199151 91.041TS2 �477.044292 0.011347 �477.032945 �477.217095 79.781TS3 �477.086762 0.008777 �477.077985 �477.272497 45.011TS4 �477.123011 0.009841 �477.113170 �477.294562 31.17CO2 (1R) �188.199130 0.010148 �188.188982 �188.317040SiO (1R) �364.101747 0.002724 �364.099023 �364.192251CO (1R) �113.074755 0.004866 �113.069889 �113.150693Si (1D) �288.840186 0.000000 �288.840186 �288.900011

aMP2(FC) energies are with 6-311G(d,p) basis set.bCCSD(T) (FC) energies are corrected with ZPE taken from MP2 (FC)/6-311G(d,p) level.



their dissociation asymptotes Si(3P) þ CO2(1R).

The structures of all the isomers are found con-sistent with the previous study of Zhang and Qin[7]. A normal mode vibrational analysis at theMP2/6-311G(d,p) level shows that the cyc-OSiCOand lin-OSiCO are appeared to be a first ordersaddle point having one imaginary frequency foreach, and hence, these two isomers are omittedfrom the singlet PES. The imaginary vibrationalmode of cyc-OSiCO corresponds to the out-of-plane bending mode in which the Si and C atomsmove opposite to the O atoms. A more detailanalysis of the vibrational frequencies of these iso-mers can be found in the work of Zhang and Qin[7]. In the planar bent-SiOCO isomer, the Si and Oatoms are datively bonded maintaining a SiAObond distance 2.731 A, which is relatively largerthan the SiAO distances in other isomers. TheSi(OC)O isomer has a side-on type structurewhere one O atom is side-on bonded through adistance of 1.190 A. The singlet PES with fourminima and four isomerization transition states isexplored at CCSD(T)/cc-pVTZ level in Figure 2.The singlet transition states corresponding to theconversion of bent-SiOCO to vdw-OSiCO andSi(OC)O are labeled as 1TS1 and 1TS2, respec-tively. Between these two isomerization processes,the second one is easier with a very low activa-tion barrier 0.22 kcal/mol. The transition statescorresponding to the conversion of most stableisomer vdw-OSiCO to butterfly-OSiCO andSi(OC)O are labeled as 1TS3 and 1TS4, respec-tively. These two isomerization processes haverelatively high activation barrier 45.01 kcal/mol

and 31.17 kcal/mol, respectively. All the transi-tion states are planar except 1TS3, which connectsvdw-OSiCO and butterfly-OSiCO and this observa-tion is corroborated with the equilibrium geome-tries of the isomers. It is now important to lookinto the kinetic stabilities of the isomers. From thesinglet PES it can be found that the vdw-OSiCOisomer is kinetically the most stable singlet spe-cies with the lowest isomerisation barrier of 31.17kcal/mol [vdw-OSiCO ! Si(OC)O]. Such isomer-isation barrier is enough to ensure that isomervdw-OSiCO can exist in the low temperature con-ditions of the laboratory and interstellar space.The lowest isomerisation barriers of the remainingspecies bent-SiOCO, butterfly-OSiCO, and Si(OC)Oare 0.22 (bent-SiOCO ! Si(OC)O), 6.68 (butterfly-OSiCO ! vdw-OSiCO), and 0.47 (Si(OC)O ! vdw-OSiCO) kcal/mol, respectively. For these remain-ing isomers, the least isomerisation barriers aresmaller than 10.0 kcal/mol, and therefore, theseisomers are kinetically less important. In this con-text it is important to note that the ZPE correctedMP2/6-311G(d,p) energies for 1TS2 and 1TS4 liebelow their connecting parent bent-SiOCO andSi(OC)O isomers. This observation implies that toget accurate energies, an additional CCSD(T) sin-gle point energy calculation is necessary for allthe species involved in the PES.

3.2. THE [Si,O,C,O] SPECIES ON THELOWEST TRIPLET POTENTIALENERGY SURFACE

On the triplet potential energy surface of[Si,O,C,O] species, we have located five isomericspecies and five isomerization transition states atthe MP2/6-311G(d,p) level of theory. Among thelocated equilibrium structures of the isomers, oneis non-planar, and the rest are planar. In the pre-vious work of Zhang and Qin [7], only four pla-nar isomers were reported at the MP2 and B3LYPlevels with 6-311G(d) basis set. The structures ofall the triplet species with their optimized geo-metrical parameters are shown in Figure 3. Nor-mal mode vibrational analysis at the MP2/6-311G(d,p) level of theory confirms that all the tri-plet isomers have global minima. A non-planartriplet cis-SiOCO isomer having C1 symmetry isnewly predicted with a dihedral d-SiOCO ¼30.5�. The calculated molecular parameters alongwith vibrational frequencies for cis-SiOCO atMP2/6-311G(d,p) level are presented in Table IIand compared them with the existing B3LYP val-ues [8]. This isomer is stable over its dissociation

FIGURE 2. Singlet PES of [Si,O,C,O] species atCCSD(T)/cc-pVTZ//MP2/6-311G(d,p) level.



asymptotes SiO (3P) þ CO (1R) and SiO (1R) þCO (3P) by 34.9 kcal/mol and 74.27 kcal/mol,respectively, at CCSD(T)/cc-pVTZ level of theory.Among the isomers, the planar bent form bent-SiOCO is thermodynamically most stable, and itlies very close in energy (1.23 kcal/mol) to its dis-sociation asymptotes Si (3P) þ CO2 (1R). In thebent-SiOCO isomer, the Si atom is dativelybonded to the OACAO unit by a relatively large

SiAO distance 2.773 A. Therefore, the bent-SiOCOisomer can be treated as a weakly bound Si–OCOcomplex and its binding energy is 1.23 kcal/mol.The two trans isomers and one cyclic isomer,namely trans-OSiCO, trans-SiOCO, and cyc-OSiCOhave comparable thermodynamic stability andtheir relative energies are 4.53 kcal/mol, 9.10kcal/mol, and 7.83 kcal/mol, respectively, atCCSD(T) level. The relative energies for all the

FIGURE 3. Optimized geometries of the triplet species with geometrical parameters at MP2/6-311G(d,p) level oftheory (CBS-Q and G3B3 values are shown in bold and italics faces, respectively, for trans-OSiCO isomer).



triplet [Si,O,C,O] species are calculated withrespect to the most stable triplet bent-SiOCO spe-cies. In the previous study [7], the bent-SiOCOand trans-OSiCO isomers were predicted to benearly degenerate with a negligible separation ofenergy 0.26 kcal/mol. Our calculated separationof energy between the isomers is 4.53 kcal/mol atCCSD(T) level. The same energy gap calculated atG3B3, G3MP2, and CBS-Q levels are �2.70 kcal/mol, 7.67 kcal/mol, and 5.62 kcal/mol, respec-tively. So, it is very much clear that the thermody-namic stability of bent-SiOCO and trans-OSiCO issignificantly dependent on the level of theory. Allthe isomers are quite stable over the spin-alloweddissociation limit SiO (3P) þ CO (1R) and highly

unstable over the spin-forbidden dissociation limitSiO(1R) þ CO(1R). The spin-forbidden dissociationasymptotes SiO(1R) þ CO(1R) lie 56.98 kcal/molbelow the bent-SiOCO isomer. It should be notedhere that this spin-forbidden dissociation is pre-dicted by the experimental analysts [8]. All rela-tive energies are calculated with respect to themost stable triplet intermediate bent-SiOCO. Totalenergies with ZPE correction for MP2 as well asCCSD(T) level and relative energies at CCSD(T)level for all triplet species are shown in Table III.The triplet PES containing five global minima,five isomerization transition states and dissocia-tion asymptotes is explored at CCSD(T)/cc-pVTZ//MP2/6-311G(d,p) level in Figure 4. The

TABLE IIITotal energies (a.u.) and relative energies (kcal/mol) of the triplet [Si,O,C,O] and related species.

Species MP2 (FC)a ZPVE Corrected CCSD(T) (FC)b Erel (kcal/mol)

SiO (1R) þ CO(1R) �477.176503 0.007590 �477.168913 �477.342944 �56.98bent-SiOCO (3A00) �477.095695 0.011920 �477.083775 �477.252131 0.00Si (3P) þ CO2 (1R) �477.091325 0.010148 �477.081177 �477.250163 1.23trans-OSiCO (3A00) �477.072712 0.011184 �477.061528 �477.244904 4.53cyc-OSiCO (3A00) �477.062692 0.011288 �477.051404 �477.239649 7.83trans-SiOCO (3A) �477.064171 0.010679 �477.053492 �477.237626 9.10cis-SiOCO (3A) �477.061084 0.010673 �477.050411 �477.235028 10.73SiO (3P) þ CO (1R) �477.008633 0.011859 �476.996774 �477.179417 45.63SiO (1R) þ CO (3P) �476.939227 0.008464 �476.930763 �477.116676 85.003TS1 �477.034184 0.008869 �477.025315 �477.217040 22.023TS2 �477.020926 0.009474 �477.011452 �477.216350 22.453TS3 �477.060258 0.010335 �477.049923 �477.235000 10.753TS4 �477.008041 0.008530 �476.999511 �477.202868 30.913TS5 �477.056108 0.010451 �477.045657 �477.231384 13.02Si (3P) �288.892195 0.000000 �288.892195 �288.933122CO (3P) �112.837479 0.005742 �112.831737 �112.924426SiO (3P) �363.933877 0.006992 �363.926885 �364.028724

aMP2(FC) energies are with 6-311G(d,p) basis set.bCCSD(T) (FC) energies are corrected with ZPE taken from MP2 (FC)/6-311G(d,p) level.

TABLE IICalculated molecular parameters for non-planar triplet cis-SiOCO at MP2/6-311G(d,p) Level.

Species l (Debye) Rotational constants (GHz) Vibrational frequencies (cm�1)

cis-SiOCO (3A) 2.2733 (2.3104)a 26.078 (46.878)a 111.6 (21.9)a

C1 3.953 (3.265) 228.7 (173.5)3.449 (3.110) 661.1 (539.0)

711.0 (740.5)1094.3 (1045.9)1817.3 (1855.2)

a Respective literature values [8] are shown in the parentheses.



isomerization of bent-SiOCO to trans-SiOCO andtrans-OSiCO is characterized by triplet transitionstates 3TS1 and 3TS2, respectively, and these twoisomerizations require almost equal amount ofactivation energy 22.02 kcal/mol and 22.45 kcal/mol. The isomerization of trans-SiOCO and trans-OSiCO to cis-SiOCO occurs via 3TS3 and 3TS4,respectively. Conversion of trans-OSiCO to cis-SiOCO requires a very small activation barrier1.65 kcal/mol. The activation barrier for the con-version of cis-SiOCO to cyc-OSiCO via 3TS5 is alsosmall, 2.28 kcal/mol. The two low-lying isomersbent-SiOCO and trans-OSiCO are kinetically im-portant due to their relatively higher conversionbarriers 22.02 kcal/mol for bent-SiOCO ! trans-

SiOCO and 17.92 kcal/mol for trans-OSiCO !bent-SiOCO, respectively. Thus these two tripletisomers are kinetically stable and therefore detect-able at low temperature under appropriate experi-mental condition. The remaining triplet isomersare kinetically less important having the least con-version barriers 1.65 kcal/mol for trans-SiOCO !bent-SiOCO, 2.28 kcal/mol for cis-SiOCO ! cyc-OSiCO and 5.18 kcal/mol for cyc-OSiCO ! cis-SiOCO, respectively.

3.3. THERMODYNAMICS

Heats of formation at 0 K (DfH�,0 K) for all sin-

glet and triplet isomeric species are computedusing atomization scheme at three different levelsof theory namely, G3B3, G3MP2, and CBS-QB3and are listed with total energies in Table IV. Theheats of formation of all molecular species are cal-culated using the heats of formation at 0 K of theatomic species Si(3P), 446 � 8 kJ/mol; O(3P),246.98 � 0.08 kJ/mol; C(3P), 711.7 � 0.4 kJ/mol[24]. The heats of formation for most of the singletspecies are negative, and that for all triplet spe-cies are positive. The G3B3 DfH

� value for themost stable singlet vdw-OSiCO isomer is �53.57kcal/mol. There is lack of literature values forheats of formation of [Si,O,C,O] species for com-parison with our calculated values. Recently, Mar-tin et al. [8] have computed the heats of formationof two triplet [Si,O,C,O] isomers using CBS-Q incontext of the reaction Si þ CO2. They designatedthe isomers as non-planar SiOCO (3A) and planarOSiCO (3A00), and the corresponding DfH

� valueswere 15.13 kcal/mol and 13.10 kcal/mol. Our

TABLE IVTotal energies (a.u.) and heats of formation at 0 K DfH8 (0 K) of various [Si,O,C,O] species.

Species G3B3 G3MP2 CBS-Q

DfH� (0 K)

G3B3 G3B3 CBS-Q

vdw-OSi–CO(1A0) �477.830181 �477.424199 �477.406893 �53.57 �54.67 �55.05Si(OC)O(1A0) �477.783439 �477.375099 �477.359755 �24.24 �23.85 �25.47cyc-OSiCO(1A0) �477.770239 �477.362007 �477.347227 �15.96 �15.64 �17.61butterfly-OSiCO(1A) �477.769281 �477.361995 �477.347065 �15.35 �15.63 �17.51bent-SiOCO(1A0) �477.702108 �477.292447 �477.274096 26.80 28.00 28.27lin-OSiCO �477.684265 �477.278129 �477.261687 38.00 37.00 36.06bent-SiOCO(3A00) �477.734874 �477.326366 �477.307136 6.23 6.72 7.54trans-OSiCO(3A00) �477.739184 �477.314136 �477.298177 3.53 14.40 13.16cyc-OSiCO(3A00) �477.722151 �477.314067 �477.299216 14.21 14.44 12.51trans-SiOCO(3A) �477.721798 �477.313425 �477.298277 14.44 14.84 13.10cis-SiOCO(3A) �477.719289 �477.310710 �477.294933 16.01 16.54 15.20

FIGURE 4. Triplet PES of [Si,O,C,O] species atCCSD(T)/cc-pVTZ//MP2/6-311G(d,p) level.



calculated CBS-Q values for the cis-SiOCO (3A)and trans-OSiCO (3A00) isomers are 15.20 kcal/moland 13.16 kcal/mol, respectively, that are consist-ent with the CBS-Q values of Martin et al. Fromour calculated electronic structures and computedDfH

� values, we can conclude that the isomersreported by Martin et al. are nothing but the cis-SiOCO and trans-OSiCO (3A00). So, the existence ofthe non-planar cis-SiOCO isomer is completelyverified by means of the thermochemical calcula-tions. Note that the G3B3 DfH

� value 3.53 kcal/mol is much away from the G3MP2 and CBS-Qvalues for trans-OSiCO (3A00) isomer. This is dueto the fact that the G3B3 geometry [B3LYP/6-31G(d) geometry] is different from G3MP2[MP2(Full)/6-31G(d) geometry] and CBS-Q[MP2/6-31G(d0) geometry] geometries for trans-OSiCO (3A00) (refer to Fig. 3), but the G3B3 geome-try is close to the MP2/6-311G(d,p) and the litera-ture geometry. So, from the calculation it isobserved that the theoretical heat of formationvalue for trans-OSiCO is highly method depend-ent. The heats of formation values for other sin-glet and triplet species are consistent at differentlevels of theory.

Change in enthalpies at 298 K (DrH�298 K) for

the spin forbidden as well as spin allowed proc-esses are calculated at three different levels oftheory and are shown in Table V. The calcu-lated DrH

�298 K values are found to be consistent

for G3B3, G3MP2, and CBS-Q theoretical levels.It should be noted here that only the spin for-bidden process Si (3P) þ CO2 (1R) ! SiO (1R) þCO (1R) is highly exothermic, and the G3B3value for exothermicity is �63.07 kcal/mol,which is in very good agreement with previ-ously reported exothermicity value �64.30 kcal/mol calculated at B3LYP/6-311þG(2d,p) level[8]. The rest of the two spin allowed processesare highly endothermic in nature, and the G3B3endothemicities are 30.74 kcal/mol and 74.34kcal/mol.

4. Conclusions

Using MP2 and CCSD(T) methods, we haveexplored the isomerization pathways of several[Si,O,C,O] species on its singlet as well as tripletPES. Three accurate energy prediction methods,namely G3B3, G3MP2, and CBS-Q are also usedto compute the heats of formation values for all[Si,O,C,O] isomers and compared them with theexisting literature values. The vdw-OSi–CO com-plex is the thermodynamically most stable spe-cies on the singlet PES and also kinetically moststable one with the least conversion barrier 31.17kcal/mol. On the triplet PES, the bent-Si–OCOcomplex is thermodynamically most stable. Thetwo isomers on the triplet PES, bent-SiOCO andtrans-OSiCO have comparable kinetic stabilitywith the least conversion barriers of 22.02 and17.92 kcal/mol, respectively. Thus, the singletvdw-OSi–CO, triplet bent-Si–OCO, and trans-OSiCO species may be produced or detected inthe laboratory and interstellar space. The exis-tence of a new non-planar complex cis-SiOCO onthe triplet PES is confirmed by high level ther-mochemical calculations followed by a compari-son with the literature. Our calculated heat offormation 13.16 kcal/mol for the non-planar tri-plet cis-SiOCO isomer agrees very well with13.10 kcal/mol reported recently for the non-pla-nar SiOCO. Low activation barrier for someisomerization channels on both singlet and tri-plet PESs indicates the high possibility of inter-conversion of the [Si,O,C,O] isomers. This theo-retical characterization of [Si,O,C,O] species willhelp to understand the underlying mechanismfor the spin forbidden process Si (3P) þ CO2 (1R)! SiO (1R) þ CO (1R).

ACKNOWLEDGMENT

B. M. is grateful to the Council of Scientific andIndustrial Research (CSIR), Government of India,

TABLE VReaction enthalpies (DrH8298 K) (kcal/mol) of the Si 1 CO2 processes.

Reaction

DrH�298 K

G3B3 G3MP2 CBS-Q

Si (3P) þ CO2 (1R) ! SiO (1R) þ CO (1R) �63.07 �63.54 �62.68Si (3P) þ CO2 (1R) ! SiO (3P) þ CO (1R) 30.47 32.88 35.50Si (3P) þ CO2 (1R) ! SiO (1R) þ CO (3P) 74.34 74.61 76.95



for providing him a junior research fellowship.Thanks are due to the reviewer for his construc-tive comments to improve the manuscript.

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Theoretical study of [Si,O,C,O] species: Prediction of new species on triplet potential energy surface