Exotic SiO2H2 Isomers: Theory and Experiment Working in Harmony

Letter pubs.acs.org/JPCL

Exotic SiO2H2 Isomers: Theory and Experiment Working in Harmony Michael C. McCarthy*,†,‡ and Jürgen Gauss§ †

Harvard−Smithsonian Center for Astrophysics, Cambridge, Massachusetts 02138, United States School of Engineering & Applied Sciences, Harvard University, Cambridge, Massachusetts 02138, United States § Institut für Physikalische Chemie, Universität Mainz, Mainz 55128, Germany ‡

S Supporting Information *

ABSTRACT: Replacing carbon with silicon can result in dramatic and unanticipated changes in isomeric stability, as the well-studied CO2H2 and the essentially unknown SiO2H2 systems illustrate. Guided by coupledcluster calculations, three SiO2H2 isomers have been detected and spectroscopically characterized in a molecular beam discharge source using rotational spectroscopy. The cis,trans conformer of dihydroxysilylene HOSiOH, the ground-state isomer, and the high-energy, metastable dioxasilirane c-H2SiO2 are abundantly produced in a dilute SiH4/O2 electrical discharge, enabling precise structural determinations of both by a combination of isotopic measurements and calculated vibrational corrections. The isotopic studies also provide insight into their formation route, suggesting that c-H2SiO2 is formed promptly in the expansion but that cis,trans-HOSiOH is likely formed by secondary reactions following formation of the most stable dissociation pair, SiO + H2O. Although less abundant, the rotational spectrum of trans-silanoic acid, the silicon analogue of formic acid, HSi(O)OH, has also been observed. by the selected-ion flow tube (SIFT) technique.17 In a related study, neutralization−reionization measurements of the Si(OH)+2 and HSi(O)OH+ ions18 concluded that the corresponding neutrals were stable in the gas phase. Replacing carbon with silicon results in a significant change in the stability of several isomers, with dihydroxysilylene HOSiOH rather than silanoic acid HSi(O)OH now predicted to be the global minimum on the potential energy surface. Additionally, singlet SiH2OO is calculated to be unstable or at best only marginally stable, likely undergoing facile ring closure9 to a more stable cyclic structure with C2v symmetry analogous to dioxirane (c-H2CO2). Figure 1 provides a comparison of the isomeric energy ordering between the two systems. Beyond comparisons to their carbon analogues, the SiO2H2 isomers are important in their own right; they are thought to be intermediates in silicon hydride oxidation processes, and dioxasiliranes (R1R2SiO2) have drawn attention as versatile oxo-transfer reagents20 for the subsequent oxygenation of organic substrates. Although not explicitly incorporated into chemical models of silicate formation, the SiO2H2 isomers may also be of astronomical interest because the reaction of SiO with water to yield HOSiOH is calculated to be exothermic and to possess only a small energy barrier. In this Letter, we report the gas-phase detection of three structural isomers of [Si,O2,H2] (Figure 2), cis,trans-HOSiOH,

A

lthough the [C,O2,H2] isomers, which include formic acid, dihydroxycarbene, and dioxirane, have been the subject of intense interest, driven in large part by the properties, photochemistry, and reactivity of the least stable singlet isomer, CH2OO, the simplest Criegee intermediate,1−3 relatively little theoretical and experimental effort has been devoted to the isovalent [Si,O2,H2] system. These studies are confined to (i) several direct rate measurements involving the reaction between silylene (SiH2) and oxygen;4−6 (ii) a modest number of quantum chemical calculations, which have reported the enthalpies of formation of possible intermediates, the potential energy surface, and optimized geometries of stable minima;7−14 and (iii) a spectroscopic investigation of silanoic acid,15,16 the silicon analogue of formic acid, HSi(O)OH. The SiH2 + O2 reaction surface is predicted6 to consist of a complicated set of pathways with as many as eight distinct stable minima and up to six pairs of dissociation fragments. Formation of the lowest-energy product pair (SiO + H2O) is thought to be the main product channel, but no product distributions have been measured so far, nor have any reactive intermediates been spectroscopically characterized in the gas phase. The absence of significant secondary reaction barriers is thought to account for the lack of a strong pressure dependence in the overall reaction rate. The only isomer spectroscopically identified to date is silanoic acid, which was observed at trace levels16 by matrix isolation infrared spectroscopy when SiH4 and O3 were codeposited on an Ar matrix at 17 K; subsequent quantum chemical calculations7 of its vibrational frequencies were found to be in good agreement with those measured. A neutral SiO2H2 isomer, possibly silanoic acid, was also detected © XXXX American Chemical Society

Received: March 20, 2016 Accepted: May 3, 2016

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DOI: 10.1021/acs.jpclett.6b00632 J. Phys. Chem. Lett. 2016, 7, 1895−1900

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Figure 1. Comparison of the relative energy ordering of the [C,O2,H2] and [Si,O2,H2] singlet isomers. Energies for the carbon-containing isomers are from ref 19, calculated using the HEAT-345(Q) scheme, while those of their silicon analogues are from this work, calculated using a similarly accurate composite scheme (see text) including zero-point vibrational corrections (see Table S1). Numbers in parentheses indicate the number of possible conformers with that atom connectivity.

the putative ground state, trans-silanoic acid HSi(O)OH, and dioxasilirane c-H2SiO2 by means of Fourier-transform (FT) microwave spectroscopy of a molecular beam, guided by highlevel quantum chemical calculations of their molecular structures. Two of these isomers, cis,trans-HOSiOH and cH2SiO2, are abundantly produced, allowing precise semiexperimental (remp e ) geometries to be derived from isotopic measurements and zero-point vibrational corrections calculated using second-order vibrational perturbation theory. The deuterium and 18O isotopic investigations also provide strong evidence as to their formation pathways; dioxasilirane is formed by a prompt, that is direct, mechanism, very likely from SiH2 + O2, while HOSiOH appears to be produced via a secondary set of reactions, likely proceeding through SiO + H2O. Searches for the rotational spectra of the various SiO2H2 isomers were based on structures computed at the coupledcluster (CC) level.21 Initial calculations were performed at the CC singles and doubles (CCSD) level augmented by a perturbative treatment of triple excitations (CCSD(T))22 together with a very large basis set (i.e., cc-pCVQZ23,24), while the final calculations were carried out at a composite level,25 in which the frozen-core (fc) CCSD(T) energy was extrapolated to the basis set limit and augmented by corrections for core correlation (computed at the CCSD(T)/cc-pCV5Z level23,24), full triples contributions (computed at the cc-pVTZ level26), and quadruples contributions (computed at the ccpVDZ level26). For a full description of this scheme, which is denoted in the following as fc-CCSD(T)/cc-pV∞Z+Δcore/ccpCV5Z+ΔT/cc-pVTZ+ΔQ/cc-pVDZ, the reader is referred to the Supporting Information. The equilibrium rotational constants obtained in this way were in a further step corrected for zero-point vibrational contributions27 obtained at the fcCCSD(T)/cc-pVTZ level using second-order vibrational perturbation theory (VPT2).28 From past experience, the

Figure 2. Three SiO2H2 isomers detected in this work. Structural parameters (remp e ) were determined for cis,trans-HOSiOH (a) and dioxasilirane (c), c-H2SiO2, from a least-squares fit (see Tables 1 and 2) using a combination of experimental rotational constants and calculated vibrational corrections (see Tables S5, S6, S13, and S14); estimated uncertainties are also provided in Tables 1 and 2 (see the text).

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Figure 3. Lower rotational levels of c-H2SiO2, showing the a-type lines measured in the range of the FT microwave spectrometer (↔) or by double resonance (←----→ ). A typical line (the transition highlighted in red) is shown for the normal and 29Si isotopic species. The double-peaked line profile is instrumental in origin, owing to Doppler splitting, which arises when the supersonic molecule beam and the two traveling waves that compose the mode of the confocal Fabry−Perot microwave cavity are coaxially aligned. Partially resolved structure in the c-H229SiO2 line (of order 10 kHz) arises from the interaction of the I = 1/2 spin of the 29Si nucleus with the rotationally induced magnetic field.

spectrometer operation and experimental conditions are provided in the Supporting Information. On the basis of predicted rotational spectra, searches for the fundamental a-type rotational transition of a number of SiO2H2 isomers were undertaken. Because cis,cis- and trans,transHOSiOH only possess b-type spectra, however, these conformers were not initially targeted for detection. Strong lines were detected within 0.2% of those predicted for both cis,transHOSiOH and c-H2SiO2, and subsequent chemical assays established that the carriers of both lines were dischargedependent and required the presence of SiH4 in addition to O2. For these reasons, searches for higher-J, a-type lines and for cis,trans HOSiOH, the lowest b-type transition, were then performed. As new lines were found, double-resonance experiments were routinely performed to establish if these lines were simply interlopers from unrelated species or instead if they arose from the same carrier, in which the two transitions shared either a common upper or lower state, as required from the predicted pattern of rotational lines. By this iterative procedure, approximately 10 lines up to J = 4 and originating from both the Ka = 0 and 1 ladders of both isomers were detected (e.g., see Figure 3 and Tables S2 and S3). Following more careful optimization of the source chemistry, a renewed search for trans-HSi(O)OH was performed, and ultimately seven a- and b-type lines were found (Table S4). Searches were also undertaken for cis,cis- and trans,trans-HOSiOH, but owing

theoretical predictions of the rotational constants obtained in this way are typically accurate to a few tenths of one percent.29 In addition, the quantum chemical calculations provided information about the energetic ordering of the various isomers and values for the components of the dipole moment along the principal axes as well as for the quartic centrifugal distortion constants. The highly sensitive technique of FT microwave spectroscopy was used to detect rotational lines of the new isomers of SiO2H2, in which a low repetition rate (6 Hz) pinhole nozzle source was used to both produce and rotationally cool to within a few degrees of absolute zero the discharge products of a very dilute gas mixture (1%) of silane and oxygen in Ne. This gas mixture is known to produce copious amounts of SiO30,31 and also the isomeric radical pair HSiO and SiOH32,33 in good yield. Although an electrical discharge source is generally a nonspecific molecule source, potentially producing many different species simultaneously, owing to the very narrow line widths of individual rotational lines (5 kHz FWHM) in this type of spectroscopy, the spectral resolution is quite high (of order 0.1 ppm). Rotational spectra of different species are thus rarely overlapped, and spectral congestion or confusion from multiple species is infrequent. For the isotopic measurements, SiD4 and a statistical mixture of 50% 18O/16O−O2 were used in place of SiH4 and 16O2, respectively. Additional details of the 1897

DOI: 10.1021/acs.jpclett.6b00632 J. Phys. Chem. Lett. 2016, 7, 1895−1900

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four for c-H2SiO2 owing to its C2v symmetry. As a check on the validity of the perturbative approach used and on the adequacy of the vibrational corrections, we note that the inertial defects of normal and isotopic cis,trans-HOSiOH are uniformly reduced by an order of magnitude, from Δ0 ≈ 0.165 amu Å2 to no more than 0.018 amu Å2 for all seven species; no such comparison is possible for c-H2SiO2 owing to its nonplanar geometry. The best-fit and equilibrium structural parameters are summarized in Table 1 for cis,trans-HOSiOH and in Table 2

to their C2v symmetry and relatively high frequency b-type lines, there are very few transitions in the range of our spectrometer, making definitive detections difficult. A standard asymmetrictop Hamiltonian including centrifugal distortion was leastsquares fitted to the rotational spectra of each of the three new SiO2H2 isomers; best-fit constants are compared to those predicted in Tables S5−S7. By comparing line intensities with those of a stable gas at known concentration (i.e., 0.1% OCS in Ne), taking into account differences in the rotational partition functions, computed dipole moment values (see Table S8), and the instrument response function with frequency, we estimate that on the order of ∼5 × 1011 cis,trans-HOSiOH molecules and ∼2.5 × 1011 c-H2SiO2 molecules are produced per gas pulse. Lines of trans-HSi(O)OH are considerably weaker (by more than an order magnitude) under the same experimental conditions, implying an abundance of roughly 1010 molecules per pulse for this isomer. Owing to the very high signal-to-noise ratio (SNR) of the cis,trans-HOSiOH and c-H2SiO2 lines, extensive isotopic spectroscopy was feasible, as evidenced from detection of the 29 Si and 30Si isotopic species of each (see the inset in Figure 3), despite the low fractional abundance of these isotopes (4.7 and 3.1%, respectively). Searches for these rare species, as well as those with both singly and doubly substituted 18O or D, were based on scaled rotational constants, in which the quantum chemically determined rotational constants were scaled to those measured for the normal species. Such scaling is generally accurate to ±1% of the predicted frequency shifts for all but the lightest atoms (i.e., when D replaces H) and is only somewhat worse (∼2%) for these. Using these predictions, all five singly substituted isotopic species were detected for cis,trans-HOSiOH and all but c-HDSiO2 for dioxasilirane. In the course of these investigations, a strong set of rotational lines of c-H2SiO2 in a low-lying vibrational state was detected (Table S9); on the basis of the close agreement of the derived rotation−vibration coupling constants (Table S10), these almost certainly arise from the ν7 mode, which is calculated (at the fc-CCSD(T)/ccpVTZ level) to lie ∼480 cm−1 above ground. No isotopic spectroscopy was attempted for trans-silanoic acid owing to the faintness of its lines. Rotational spectra were analyzed in the same manner as the normal species, but because most D species possess well-resolved quadrupole hyperfine structure owing to the I = 1 spin of this nucleus, several χ(D) terms could be determined as well; the diagonal elements of the 29Si nuclear spin−rotation tensor have also been determined for cis,transHO29SiOH based on the hyperfine structure that arises from the interaction of this I = 1/2 spin with molecular rotation. The isotopic measurements are summarized in Table S2, S3, S11, and S12, while the best-fit constants are given in Tables S5 and S6. Highly accurate, semi-experimental (remp e ) structures have been derived for cis,trans-HOSiOH and c-H2SiO2, using a combination of isotopic measurements and vibrational corrections determined at the fc-CCSD(T)/cc-pVTZ level (Tables S13 and S14). Using a now standard procedure,34 the vibrationally averaged, experimental rotational constants (B0) have been corrected for the effects of zero-point vibrational motion using second-order vibrational perturbation theory. The resulting constants (Bemp e ; Tables S13 and S14) are then used in a least-squares structural optimization to determine the best-fit bond lengths and angles. For cis,trans-HOSiOH, there are seven structural parameters (assuming planarity), and there are only

Table 1. Experimental and Calculated Structures of cis,transHOSiOH parametera

r0

remp e

r(HcisO) θ(HcisOSi) r(OcisSi) θ(OSiO) r(SiOtrans) θ(SiOHtrans) r(HtransO)

0.973(26) 117(2) 1.639(9) 100.2(2) 1.657(10) 120(10) 0.920(62)

0.960(3) 117.7(1) 1.635(1) 99.59(5) 1.654(1) 117(1) 0.946(8)

b rtheor e

0.9615 117.65 1.6352 99.47 1.6553 116.24 0.9584

a

Bond lengths are in Å, and bond angles are in degrees (°). Estimated uncertainties (in parentheses) are 1σ in units of the last significant digit. All three rotational constants were included in the least-squares optimization. bCalculated at the fc-CCSD(T)/cc-pV∞Z+Δcore/ccpCV5Z+ΔT/cc-pVTZ+ΔQ/cc-pVDZ level; see the text.

Table 2. Experimental and Calculated Structures of c-H2SiO2 parametera

r0

remp e

r(SiO) θ(OSiO) r(SiH) θ(HSiH)

1.642(1) 59.1(3) 1.462(1) 113.0(7)

1.6373(5) 58.94(10) 1.4641(5) 113.24(14)

b rtheor e

1.6378 58.74 1.4648 113.31

a

Bond lengths are in Å, and bond angles are in degrees (°). Estimated uncertainties (in parentheses) are 1σ in units of the last significant digit. All three rotational constants were included in the least-squares optimization. bCalculated at the fc-CCSD(T)/cc-pV∞Z+Δcore/ccpCV5Z+ΔT/cc-pVTZ+ΔQ/cc-pVDZ level; see the text.

for c-H2SiO2. For both molecules, the main difference between the r0 and remp structures is the much higher precision of the e latter, with most parameters improved by a factor of 3−10. This improvement clearly is due to the fact that the definition of the re structure is more rigorous and provides a consistent way of analyzing rotational constants. Furthermore, the remp structures e agree very closely, that is, within 2σ, with the re structures determined at the highest level of theory. The isotopic studies have also enabled us to infer with a high degree of confidence how the two isomers are formed in our gas-discharge expansion. As summarized in Table 3, by comparing the intensity of different isotopic species when starting with isotopically labeled precursors, for example, SiD4 or 16O/18O−O2, we find that c-H2SiO2 is made by a prompt gas-phase reaction, that is, very likely by the reaction SiH2 + O2, followed by collisional stabilization in the resulting expansion. Indeed, a sizable barrier (up to ∼26 kcal mol−1) is calculated6 for subsequent rearrangement or dissociation of c-H2SiO2. In contrast, the presence of isotopic contamination in the isotopic measurements of cis,trans-HOSiOH strongly suggests that this isomer is formed by secondary reactions, in which the reaction (SiH2 + O2) proceeds over this barrier to form the most stable product pair (SiO + H2O) owing to the very high exothermicity 1898

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Table 3. Relative Abundances of Isotopic c-H2SiO2 and cis,trans-HOSiOH with Either SiD4 or 16O/18O−O2 isotopic speciesa

SiD4 + O2

SiH4 + 16O/18O−O2b

c-H2SiO2 c-D2SiO2 c-HDSiO2 c-H2Si18OO c-H2Si18O2 HOSiOH DOSiOH HOSiOD DOSiOD H18OSiOH HOSi18OH H18OSi18OH