How Does the Central Atom Substitution Impact the Properties of a

Article Cite This: J. Am. Chem. Soc. 2017, 139, 15446-15449

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How Does the Central Atom Substitution Impact the Properties of a Criegee Intermediate? Insights from Multireference Calculations Tarek Trabelsi, Manoj Kumar, and Joseph S. Francisco* Department of Chemistry, University of Nebraska-Lincoln, Lincoln, Nebraska 68588, United States S Supporting Information *

ABSTRACT: Complete active space self-consistent field (CASSCF) and multireference configuration interaction (MRCI)-based multireference calculations have been performed to better understand the ground state properties and the photodissociation mechanism of SiH2OO, a silicon analogue of the parent Criegee intermediate, CH2OO. The CASSCF/aug-cc-pV(T+d)Z results suggest that the ground state of SiH2OO is severely multireference in nature. This explains why SiH2OO could not be characterized in recently reported coupled cluster calculations. An important implication of this multireference character is the dramatically enhanced reactivity of SiH2OO, i.e., the calculated barrier for the cyclization of SiH2OO is only 4.4 kcal/mol, which is nearly 10 kcal/mol lower than that reported for the CH2OO case. The MRCI/aug-cc-pV(T+d)Z results on the evolution of the low-lying singlet electronic states along the OO bond suggest that SiH2OO absorbs strongly in the near UV−vis region. These results improve our fundamental understanding of the thermal and photobehavior of XH2OO (X = C, Si, Ge, and Sn) that serve as precursors for dioxiranes, an important class of oxidants for the synthesis of valueadded chemicals, and also find their applications in optodevices.

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INTRODUCTION Ever since the Criegee intermediate in the gas-phase has been directly detected,1 understanding Criegee chemistry has become a hot topic of research. Criegee intermediates are carbonyl oxides that are principally produced in the ozoneolefin cycloadditions.2 The Criegee reactions not only play a crucial role in the tropospheric budgets of hydroxyl radicals, organic acids, hydroperoxides, nitrates, sulfates, and secondary organic aerosols,3,4 but also provide a green synthetic route for pharmaceutical intermediates and other useful commodities.5−7 Because of its broad profile, factors influencing Criegee reactivity has been extensively investigated by means of experimental and theoretical means.3,4 However, only carbon-based Criegee chemistry has been deeply studied. Other members of group 14 may also form Criegee intermediates, but their reactivity profiles and photochemistry have rarely been examined. This is quite surprising because a large variety of olefin derivatives featuring a double bond between the identical heavier group IV(14)−V(15) elements have been synthesized8 following the discovery of the first stable disilene (>SiSiSiSi< = 2.14−2.29 Å), digermenes (>GeGe< = 2.21−2.51 Å) and distannenes (>SnSn< = 2.67−2.79 Å) could provide new synthetic routes for dioxiranes and XOn=0,1 materials. Dioxiranes find their synthetic utility as oxygen-atom transfer reagents in organic chemistry16 whereas XO n have wide ranging applications in optoelectronic devices.10,11,17,18 Moreover, it is well established that the nature and location of the substituents on the Criegee intermediate significantly tunes its reactivity,19 but it remains unclear how the nature of the central atom in the Criegee intermediate would influence its ground state geometry, electronic structure, chemical reactivity, and spectroscopic signatures. In this report, we have examined the electronic structure and rotational constants of the simplest non-carbon Criegee intermediate in its ground electronic state and the evolution of its low-lying electronic states along the OO reaction coordinate using multireference methods. Equilibrium geometries, rotational constants, and harmonic vibrational frequencies were evaluated using the complete active space self-consistent field (CASSCF)20 and two different basis sets, aug-cc-pV(T+d)Z and aug-cc-pV(Q+d)Z, whereas the evolution profiles of low-lying singlet excited states were studied using the multireference configuration interaction including the Davidson correction (MRCI+Q)21 method and aug-cc-pV(T +d)Z basis set. In the MRCI calculations, the CASSCF active space is constructed after considering the nine low-lying Received: August 14, 2017 Published: October 12, 2017 15446

DOI: 10.1021/jacs.7b08412 J. Am. Chem. Soc. 2017, 139, 15446−15449

Article

Journal of the American Chemical Society

[0.94(10a211a212a2) + 0.20(10a211a213a2)]. The key orbitals along with their natural occupation numbers are shown in Figure 1b. See Table 1 for more details. The 16a orbital in

molecular orbitals as doubly occupied (frozen) ones and the remaining valence orbitals as active ones. All configurations in the CI expansion of the CASSCF wave functions having a weight larger than 0.005 were considered. This lead to >4.8 × 107 (2.1 × 109) contracted (uncontracted). These gas-phase calculations were performed in the C1 symmetry group and using MOLPRO201522 software. For additional method details, see the Supporting Information.

Table 1. CASSCF/aug-cc-pV(T+d)Z Calculated Dominant Electron Configurations, Transition Dipole Momentum (μ, in Debye), and the Oscillator Strength Calculated at the Equilibrium Geometry of SiH2OOa

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RESULTS AND DISCUSSION Ground State Properties of SiH2OO. Considering that CH2OO has noticeable diradical character,19 that prompted us to explore the ground state properties of SiH2OO at the CASSCF level. The CASSCF optimized equilibrium geometry of SiH2OO is shown in Figure 1. The comparative analysis

state

dominant electron configuration

μ

f

Te

X1A

(0.83)(Core)(14a)2(15a)2(16a)2 (0.49)(Core)(14a)2(15a)2(17a)2 (0.75)(Core)(14a)2(15a)1(16a)2(17a)1 (0.55)(Core)(14a)2(15a)2(16a)1(17a)1 (0.94)(Core)(14a)2(15a)1(16a)2(19a)1

0.15

0.0

0.68

2.48

0.1

3.94

11A 21A a

The weights of the configuration in the corresponding wavefunctions are also given in parentheses. The vertical excitation energies (Te, in eV) calculated at the MRCI+Q level of theory are also given.

SiH2OO is quite similar to the 12a orbital in CH2OO except for the fact that it is more localized on the Si atom. This localization of the 16a orbital in SiH2OO accounts for the relatively longer Si−O bond (1.70 Å) in SiH2OO than the analogous C−O bond (1.29 Å) in CH2OO. Moreover, the extent of multireference or diradical character in SiH2OO is appreciably higher than in CH2OO, as is evidenced from the natural occupation numbers of molecular frontier orbitals. In SiH2OO, the natural occupation number of 17a orbital is 0.53, which is nearly 4 times than that of the analogous 13a orbital (0.13) in CH2OO. It is interesting to note that though the unimolecular chemistry of SiH2OO has been previously studied using theoretical means,28,29 the ground state wave function of SiH2OO was never explored. We next calculated the energetics for the cyclization of SiH2OO at the MRCI/aug-cc-pV(T+d)Z//CASSCF/aug-ccpV(Q+d)Z level to see if the nature of central atom or the extent of biradical character influences the thermal properties of a Criegee intermediate. The results suggest that the SiH2OO cyclization barrier is just 4.4 kcal/mol. This barrier is significantly lower compared to that for the cyclization of a carbon-based Criegee intermediate, CH2OO (∼20−25 kcal/ mol).26−29 Our calculated barrier is slightly higher than the previously CASSCF/6-31G(d)//GVB/6-31G(d) calculated value of 2.4 kcal/mol24 but is in good agreement with the MRMP2/cc-pVTZ//MCSCF/6-31G(d) calculated value of 4.5 kcal/mol.25Moreover, the SiH2OO cyclization has a reaction energy of −68.1 kcal/mol, which is nearly 28−31 kcal/mol more favorable than that for the CH2OO reaction,26−29 see Table S2 for the structural details of all three stationary points characterized on the cyclization potential energy surface. We also calculated the key natural orbitals of TScyc and cyc-SiH2OO at the CASSCF/aug-cc-pV(T+d)Z level of theory (Figure 2). The 17a orbital in TScyc has an appreciable occupation number of 0.64, which is slightly larger than that of the analogous orbital in SiH2OO. This suggests that the TScyc has significant multireference character. On the other hand, the occupation number of 17a orbital in cyc-SiH2OO is only 0.11, indicating that cyc-SiH2OO is a monoreference system. The multireference nature of TScyc and monoreference nature of cycSiH2OO were also verified via the T1 diagnotic30 analysis (see the Supporting Information for details). These results suggest that as we move down group IV(14), the reactivity of Criegee-type intermediate increases dramati-

Figure 1. (a) CASSCF/aug-cc-pV(T+d)Z calculated equilibrium geometry of SiH2OO along with key structural parameters. The bond distances are given in Å units whereas the bond angle and the dihedral angle are given in degree units. For the comparison sake, the previously reported MCSCF/aug-cc-pVTZ values of structural parameters for CH2OO are shown in parentheses. (b) Relevant natural orbitals for SiH2OO along with their occupation numbers according to the CASSCF wave function. The lower panel in Figure 1b shows the natural orbitals for CH2OO.

shows that the Si−O bond in SiH2OO (1.70 Å) is relatively longer than the C−O bond in CH2OO (1.27 Å). Moreover, SiH2OO (φ(HSiOO) = 24.9°) is relatively less planar than CH2OO (φ(HCOO) = 0.0°). The MCSCF/cc-pVTZ optimized structural parameters of CH2OO are taken from a recent study.23 See Table S1 for additional structural details. The calculations suggest that SiH2OO is a multireference system. Interestingly, the extent of multireference character in SiH2OO is dramatically higher than in CH2OO. From an electronic point of view, the ground state wave function of SiH2OO is mainly dominated by two electronic configurations [0.83(....15a2 16a2) + 0.49(....15a2 17a2)], whereas that of CH2OO is dominated by two electronic configurations 15447

DOI: 10.1021/jacs.7b08412 J. Am. Chem. Soc. 2017, 139, 15446−15449

Article

Journal of the American Chemical Society

Figure 3. The MRCI+Q/aug-cc-pV(T+d)Z calculated evolution of the singlet electronic states of SiH2OO along the OO bond. The remaining coordinates were kept fixed at their equilibrium values.

Figure 2. MRCI/aug-cc-pV(T+d)Z//CASSCF/aug-cc-pV(Q+d)Z calculated reaction profile for the cyclization of SiH2OO. The zeropoint uncorrected electronic energies (298 K, 1 atm) are given in kcal/ mol units. The key natural orbitals involved in the cyclization of SiH2OO along with their occupation numbers according to the CASSCF wave function are also given. Note that the CASSCF orbitals of SiH2OO, TScyc and cyc-SiH2OO were calculated using the aug-ccpV(T+d)Z basis set.

below the lowest dissociation asymptote H2SiO + O and has a deep potential well. As it can be seen from Table 1, SiH2OO absorbs strongly in the UV−vis region. The transition dipole momentum |Re| for a transition between the ground state X 1A and the 21A electronic state in SiH2OO is calculated to be 2.48 D at the CASSCF/aug-cc-pV(T+d)Z level of theory. In this region, the 21A state can be populated after UV−vis absorption 3.94 eV (314 nm) from the ground state. Excitation 21A ← X1A at 3.94 eV leads the wavepacket to 21A electronic state, which has a flat potential along the OO bond. The wavepacket will subsequently explore the flat potential, resulting in the large amplitude motion of the molecule along the OO bond. The flat potential energy surface will lead the wavepacket directly to H2SiO(1A1) + O(1D) and, if enough energy is provided, the small barrier at 2.2 Å will be overcome. It is interesting to compare the UV−vis spectrum of SiH2OO with that of CH2OO. Both these systems absorb strongly in the UV−vis region around 4 eV. The calculated transition dipole momentum for both systems is nearly similar, i.e., the 21A′ ← X1A′ transition momentum at the equilibrium geometry of the CH2OO ground state is computed to be 2.59 D, which is within 0.1 D of that of SiH2OO. Finally, the photodissociation mechanism of SiH2OO is similar to that of CH2OO,33 which is a direct photodissociation through the 21A′ electronic state.

cally. This is because when we go from C → Si, the multireference character of a Criegee intermediate is significantly enhanced, as a result of which, the Si−O bond acquires a dominant single bond character and the bending of SiOO angle becomes relatively easier. This may help in better understanding the results of a recent experimental study31 on the SiH4 + O2 reaction. In that experimental study, which was guided by the coupled cluster calculations, three SiO2H2 isomers, namely, cyc-SiH2OO and cis, trans-HOSiOH were detected and spectroscopically characterized. Although less abundant, the rotational spectrum of trans-silanoic acid, the silicon analogue of formic acid, HSi(O)OH, was also observed. Interestingly, they did not observe SiH2OO in their experiments or the coupled cluster calculations. There are two main reasons for that (i) SiH2OO is a severe multireference system, which cannot be described using the monoreference methods such as the coupled-cluster methods, and (ii) even if SiH2OO is formed in their experiments, it decomposes immediately into the cyc-SiH2OO as it is very reactive and cyclizes into cycSiH2OO with a very moderate barrier of 4.4 kcal/mol. Excited State Properties of SiH2OO. The photochemistry of CH2OO is previously studied using first-principles and multireference methods.32,33 However, the photobehavior of noncarbon Criegee intermediates is yet to be explored. Herein, we have studied the evolution of the low-lying singlet electronic states of SiH2OO along the OO bond using the MRCI+Q/augcc-pV(T+d)Z method. The remaining coordinates of SiH2OO were kept fixed at their equilibrium values. The inclusion of tight d functions ensures a better description of core polarization effects and valence orbital correlation effects in second row atoms. For studying the lowest singlet electronic states of SiH2OO at the MRCI/aug-cc-pV(T+d)Z level, we started our calculations from the CASSCF/aug-cc-pV(Q+d)Z optimized equilibrium geometry of SiH2OO. The evolution of these states is shown in Figure 3. From this figure, the 11A electronic state is found to be stable along the OO stretch since it is located

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CONCLUSIONS

In summary, our multireference calculations show that as we move down the group IV(14), the multireference character of a Criegee intermediate increases dramatically. An important implication of this multireference character is the dramatically enhanced reactivity of SiH2OO due to reduction in the double bond character of the Si−O bond. The MRCI/aug-cc-pV(T +d)Z results on the evolution of the low-lying singlet electronic states along the OO bond suggest that SiH2OO absorbs strongly in the near UV−vis region. The electronic structure of the ground state and the thermal reactivity of noncarbon Criegee intermediates is significantly different from that of the carbon-based Criegee intermediate. 15448

DOI: 10.1021/jacs.7b08412 J. Am. Chem. Soc. 2017, 139, 15446−15449

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Journal of the American Chemical Society

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.7b08412. Tables containing the key structural parameters of SiH2OO, cyc-SiH2OO, and TScyc; harmonic vibrational frequencies and rotational constants of SiH2OO and cycSiH2OO; and their Cartesian coordinates (PDF)

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AUTHOR INFORMATION

Corresponding Author

*[email protected] ORCID

Joseph S. Francisco: 0000-0002-5461-1486 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the Holland Computing Center of the University of Nebraska-Lincoln. REFERENCES

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DOI: 10.1021/jacs.7b08412 J. Am. Chem. Soc. 2017, 139, 15446−15449