Experimentally Determined Structure of H2SiO by Rotational Spectroscopy and Isotopic Substitution

JOURNAL OF MOLECULAR SPECTROSCOPY ARTICLE NO.

175, 421–428 (1996)

0048

Experimentally Determined Structure of H2SiO by Rotational Spectroscopy and Isotopic Substitution Marcel Bogey,* Bruno Delcroix,* Adam Walters,* and Jean-Claude Guillemin† *Laboratoire de Spectroscopie Hertzienne, associe´ au CNRS, Universite´ des Sciences et Technologies de Lille, F-59655 Villeneuve d’Ascq Cedex, France, and †Laboratoire de Physico-chimie Structurale, associe´ au CNRS, Universite´ de Rennes 1, F-35042 Rennes Cedex, France Received: June 15, 1995; in revised form September 18, 1995

The rotational submillimeter-wave spectra of four isotopomers of silanone, H2 29Si16O, H2 30Si16O, H2 28Si18O, and D2 Si16O, have been measured for the first time. These reactive species were produced by a low-power ‘‘abnormal’’ glow discharge in a mixture of silane and oxygen. The observation of the deuterated species required the prior synthesis of SiD4. Anomalies in the spectrum of the parent species, measured previously, were shown to be due to a Stark splitting caused by the discharge. The fitted rotational constants were used to determine the rmr structure of silanone. The bond ˚ , Si{H 1.472 A ˚ , õHSiH 112.07. The precision of these lengths and angles have been determined as Si|O 1.515 A ˚ for the bond lengths and 0.27 for the bond angle. q 1996 Academic Press, Inc. results is expected to be better than 0.002 A

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1. INTRODUCTION

Interest has recently focused on small silicon-containing molecules following ab initio predictions that certain of them should exhibit geometries very different from analogous carbon-containing molecules (1, 2). These structural differences have been the subject of considerable theoretical speculation regarding the nature of p-type bonding in third-row elements. They have been attributed to the preference of silicon to have nonbonding electrons in atomic orbitals with a high percentage of s-character and to the possible involvement of d-orbitals in bonding (1). Despite recent experimental work showing that the most stable conformers of Si2H2 have structures very different from that of acetylene (3, 4), few experimentally determined structures of simple silicon-containing molecules are available for comparison with the theory. Silicon-containing molecules and ions of composition [Si, O, Hn] may play an important role in interstellar chemistry (5). The simplest closed shell molecules in this series are H2SiO (silanone), and HSiOH cis and trans. These molecules are highly interesting from a structural point of view since they contain, respectively, Si|O and Si{O bonds whose lengths, if measured, could be compared with the oxygen bond in unsaturated and saturated carbon-containing molecules. Despite the considerable experimental and theoretical attention which these molecules have attracted, due to their high reactivity, until recently they had only been observed solidified in argon matrices (6–8). Evidence for their existence in the gas phase was finally provided by chemiluminescent emission (9) and mass spectrometry (10). The study of H2SiO is also of technical interest since this molecule is predicted as a deposition precursor of silicon

dioxide films produced by the technique of remote plasmaactivated chemical-vapor deposition (11, 12). Ab initio calculations of the molecular structure of HSiOH and H2SiO have been carried out at various levels of theory: SCF with basis sets 3-21G and DZ / P (13), HF/3-21G* (1), HF/6-31G(d) (14), and MP2/6-31G* (15, 16). Recently Ma and Schaefer provided us, before publication, with structures calculated at a higher theory level [CCSD/TZ2P(f, d)] (17). Simultaneously Csa´sza´r calculated for us lower-level [SCF/DZP] ab initio values for the centrifugal distortion constants (18). These new calculations were sufficiently accurate to allow us to identify H2SiO created in a silane– oxygen plasma by electric discharge. We subsequently published the first high-resolution rotational spectrum of this molecule in the gas phase (19). Although a large number of lines in the experimental spectrum were not due to silanone, it has not yet been possible to identify the spectrum of HSiOH cis or trans. The analysis may be complicated by the presence of additional nonidentified reactive closed-shell molecules. The structure of a molecule can be determined using rotational spectroscopy and isotopic substitution, if it is assumed that the bond angles and lengths are unchanged on substitution. However, to determine a precise equilibrium structure it is necessary to take into account the effect of zero-point vibrations on the rotational constants. In principle this means that measurements must be taken in all excited vibrational states. This procedure is excessively time consuming and not suitable for reactive molecules whose spectra are already difficult to observe in the ground state. However, using certain approximations it is possible to estimate the structure of a molecule from the rotational constants of its isotopomers. This estimation is sufficiently accurate that the struc-

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ture can be compared with that of other molecules or with ab initio calculations (20). In this paper we present the first millimeter-wave spectra of H2 29Si16O, H2 30Si16O, H2 28Si18O, and D2 28Si16O. The observation of these isotopomers confirms the identification of the parent species. An analysis of their spectra has enabled us to estimate the equilibrium structure of the elusive silanone molecule. 2. EXPERIMENTAL PROCEDURE

The spectrometer used in this study has been described previously (21) as have several recent modifications and improvements (22, 23). Reactive molecules are produced by electric discharge in an all-glass 2.5-m-long, 5-cm-i.d. cell. Millimeter-wave radiation from a BWO enters and exits the cell through two Brewster-angle Teflon windows. A fraction of the incident power is extracted by means of a finely spaced wire grid and used to phase lock the source. Radiation leaving the cell is focused by a parabolic mirror onto a liquid-helium-cooled InSb bolometer. Source modulation at 40 kHz and lock-in detection at 80 kHz for improved sensitivity give a second-derivative lineshape. H2 29SiO and H2 30SiO were produced in natural abundance by an abnormal glow discharge (4 kV, 5 mA) in a mixture of unenriched oxygen, silane, and argon. The discharge cell was cooled to liquid nitrogen temperature, increasing line intensities by around 50-fold. In order to avoid condensation at the cell entrance, silane mixed with argon was introduced into the cell through an axial glass tube drilled all along its length. Oxygen was introduced separately to prevent blockage of the tube by SiO2 formation. Optimum conditions had already been determined by previous work (19) on the parent molecule H2 28Si16O: the partial pressures of silane and oxygen measured at room temperature were both 30 mTorr. Argon was added in order to obtain stable discharge conditions. The total pressure in the cooled cell never exceeded 70 mTorr. Transitions of D2SiO were too weak to be observed in natural abundance so that it was first necessary to synthesize SiD4. For the measurements of H2Si18O we used 18O2 in place of 16O2. 3. STARK EFFECT

In our previous paper (19), which presented the rotational spectrum of the parent molecule, H2 28Si16O, we noticed that certain close-lying Ka doublets were poorly fitted using a standard Watson’s A-reduced Hamiltonian (25). This effect was most evident when the two components were close in frequency but it was also observed for some well-separated lines, showing it could not be due simply to an instrumental convolution of the lineshapes; for instance two Ka Å 2 lines 94 MHz apart were each shifted by around 150 kHz. Some barely resolved doublets showed an unusual pattern, which

FIG. 1. 43 –33 Ka doublet of H2CO, with and without the electric field resulting from the discharge. In the former case several Stark components can be seen.

could not be reproduced by fitting to a convolution of two or even three typical lineshapes. After publication of that article we observed the same effect in the spectrum of H2CO produced by discharge in a mixture of methane and oxygen; the frequencies measured for the components of close-lying Ka doublets differed from the documented values. Formaldehyde being stable we were able to measure its spectrum in the absence of a discharge. In this case the doublets were not shifted and the frequency of each component was correctly determined. Figure 1 shows a typical doublet with and without the electric field resulting from the discharge. This example has been chosen since the Stark components are somewhat resolved. When these components are not resolved they blend together, causing an apparent shift of the line toward the center. Since the dipole moment of H2CO is known it should be possible to obtain a model of the electric field in the cell caused by the discharge. By observing a doublet of H2SiO under identical conditions it would then be possible to measure the dipole moment of this unstable species. 4. ROTATIONAL SPECTRA AND ANALYSIS

The rotational spectrum of the parent species has already been published (19). To find and identify the spectra of the four isotopomers it was necessary to have a prediction of their rotational constants. We first calculated the A, B, and C constants of the isotopomers using the ab initio structure of Schaefer and Ma (17). These constants were subsequently corrected by scaling them with the same factors as those obtained by comparing the ab initio constants of the parent molecule with the experimental values (26):

Aiscal Å Aiab in 1

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[1]

EXPERIMENTAL STRUCTURE OF H2SiO

TABLE 1 Observed Rotational Transitions (in MHz) of H2 29Si16O and H2 30Si16O in the Ground Vibrational State

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TABLE 2 Observed Rotational Transitions (in MHz) of H2Si18O in the Ground Vibrational State

In this formula A represents any rotational constant; the indices i and p refer to an isotopic substitution and to the parent molecule, respectively; scal, exp, and ab in denote the scaled, experimental, and ab inito values. Although this method has no real theoretical basis it has often provided the best estimate of rotational constants possible before their experimental determination, with the accuracy always being better for substitution of the heaviest atoms. The centrifugal distortion constants were taken as being the same as those determined experimentally for the parent species. All isotopic measurements were carried out between 335 and 475 GHz in the frequency range of our two BWOs where the spectrometer is most sensitive. Scans of less than 100 MHz were sufficient to identify the most intense transitions predicted using the previously estimated molecular constants. This good agreement confirms the previous assignment of the parent species. The frequencies of these transitions were then used in a preliminary least-squares analysis to refine the rotational constants and get better predictions. By using an iterative procedure it was then possible to measure the remaining transitions rapidly without further scanning. Silanone is a planar nearly symmetric prolate rotor. All the isotopomers considered possess C2£ symmetry with the symmetry axis along the SiO bond, and hence only a-type transitions can be observed. A permanent dipole moment of 03.82 D has been calculated by Schaefer and Ma for the parent species (17). The two equivalent hydrogen nuclei

induce a 3:1 intensity alternation corresponding, respectively, to transitions between oe and oo levels and those between eo and ee levels. For the dideuterated species the intensity alternation changes to 1:2 for transitions between oe and oo levels and those between eo and ee levels. The measured transitions of H2 29Si16O and H2 30Si16O are given in Table 1, those of H2 28Si18O in Table 2 and those of D2 28Si16O in Table 3. The data were fitted by using a standard Watson’s A-reduced Hamiltonian in the Ir representation (25). As observed for the parent species, the two components of certain Ka doublets were shifted due to interaction with the electric field caused by the discharge. The problem was hence to decide which frequencies to include in the fit. Nonaffected strong transitions were all measured within 30 kHz of the frequency calculated using the fitted constants. A symmetrical shift of both components of a doublet indicated a Stark interaction. When this shift exceeded 30 kHz we fitted the mean frequency of the two measured components to the central frequency of the doublet. These transitions are labeled with an asterisk in the column Kc. The difference between the measured and calculated frequencies is shown in each table. As can be seen the center frequency of each shifted doublet is well fitted. For these doublets the observed frequency of each component is compared with the frequency predicted using the fitted constants in order to show the size of the Stark shift. In this case the predicted frequency should be taken as better than the experimental frequency.

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EXPERIMENTAL STRUCTURE OF H2SiO

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TABLE 3 Observed Rotational Transitions (in MHz) of D2SiO in the Ground Vibrational State

The experimentally determined molecular constants of the four isotopomers are given in Table 4 and can easily be used to calculate the frequency of transitions either too weak to be measured or lying outside the range of our BWOs. In our first fits we allowed DK to vary. As expected the calculated value of DK was strongly correlated with that of A and badly determined for all isotopomers with the exception of the dideuterated species. In spite of the correlation these fits indicated for the monosubstituted species a value for DK close to that of the parent. For the dideuterated species, however, the calculated value of DK was significantly lower. We received the latest ab initio calculations of Schaefer for the isotopomers of H2SiO, which included predictions for the quartic centrifugal distortion constants (27), after the preliminary fits. His results independently verify our findings. As a further test of the validity of our calculations we carried out another fit including the ab initio value for DK (with an uncertainty of 10%) as an additional constraint. The A, B, and C constants thus obtained were consistent with those of the initial fits. Finally, for the monosubstituted spe-

cies we decided to constrain DK to the ab initio value, scaled using Eq. [1], in order to get a good determination of the other constants. The sextic and higher centrifugal constants could not be determined and we possess no ab initio values for the isotopomers. Hence, we constrained FJK to 4.76 Hz and FKJ to 047.7 Hz (the values determined for the parent species). Other higher order constants were constrained to zero. 5. MOLECULAR STRUCTURE

To eliminate the most important part of the residual centrifugal distortion effects in the rotational constants, they were first transformed to the so-called Watson’s determinable parameters A0, B0, and C0 (25, 28, 29) which are given in Table 5. The constants for the parent species are included and are based on the measured line frequencies published previously (19). The standard deviations for the rotational constants of this parent species have been reduced threefold by using the ab initio value of DK as an additional constraint.

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TABLE 4 Rotational and Centrifugal Distortion Constants for the Ground Vibrational State of the Four Measured Isotopomers of Silanone Using Watson’s A-Reduced Hamiltonian

TABLE 5 Watson’s Determinable Constants for All Available Isotopomers of Silanone

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EXPERIMENTAL STRUCTURE OF H2SiO

TABLE 6 Structure of Silanone: Comparison between ab Initio and Experimental Bond Lengths and Angles

It should be noted that the value of DK obtained is still a true fitted value. The standard deviation for the constants of H2 29Si16O, H2 30Si16O and H2 28Si18O appears smaller since DK was not fitted for these isotopomers. For D2 28Si16O correlation between DK and A is much smaller and the constants are well determined in spite of the fact that DK has also been fitted. It was then necessary to consider the effect of zero-point vibrations on these constants. The moment of inertia of a molecule I0 in the ground vibrational state differs from the equilibrium value Ie and we can write (29) I0 Å Ie / e,

rÅ

[2]

where e is the rotation–vibration parameter. An experimentally determined structure close to that in equilibrium can only be obtained from measurements in the vibrational ground state if certain approximations are made. The bond lengths and angles determined experimentally using several different approximations are compared in Table 6 with the ab initio calculations of Schaefer et al. (17). The simplest but least accurate assumption that can be made is that e is small, random, and normally distributed around zero. Using a least-squares method an effective structure (r0) can then be calculated using the rotational constants measured for several isotopomers. A much more reliable assumption is that e remains constant when an isotopic substitution is made (30). Using Kraitchman’s equations (see Refs. 29, 31), it is then possible to calculate the Cartesian coordinates of the substituted atom. The substitution structure (rs) of the molecule is determined by substituting each atom in turn. This method gives the best results if DI @ De. When H atoms are replaced by D atoms (as is the case here) the change of mass and hence De are large so that the approximation is less good. Watson has defined a ‘‘mass-dependence’’ (rm) method which is derived from a first-order treatment of isotope effects (32). This method is based on the relation between the moments of inertia Im Å 2Is 0 I0,

where Im is the mass-dependence estimate of the equilibrium value Ie, with Is and I0 being, respectively, the substitution and zero-point values. The determination of the rm structure is not only unreliable (20) but also requires several different estimates of the rs structure to be calculated. Each atom of the molecule must then be substituted by several isotopes. However, Harmony and co-workers (33–36) have improved this method and made it easier to use. They assume that the quotient

[3]

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remains constant. They have checked this assumption for many molecules. Once r is known, it becomes easy to calculate Im for different isotopic species using the determined values of I0: Imr Å (2r 0 1)I0.

[5]

A least-squares fir of Imr then gives the rmr structure. This is the method which we believe gives the best structural approximation for H2SiO from our measurements. The good agreement between the different structures and the ab initio values confirms the validity of our calculations. A statistically significant standard deviation cannot be associated with the structural parameters calculated since the fit is based on only four isotopomers and the parent species. However, this method, even when based on a relatively small number of isotopic species, usually gives results with a precision of ˚ for bond lengths and 0.27 for angles. better than 0.002 A 6. DISCUSSION

˚ experimentally deterThe Si|O bond length of 1.515 A ˚ determined mined for H2SiO is very close to that of 1.510 A ˚ for SiO (37). Ma and Schaefer calculate 1.521 A for H2SiO at the CCSD level (17), significantly closer to the experimental ˚ , calculated in previous ab initio value than that of 1.545 A studies (15, 16) at the MP2 level. The same CCSD calcula-

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˚, tions predict an Si{O bond length of 1.658 and 1.662 A respectively, in HSiOH cis and trans. For silanol an Si{O ˚ has been calculated at the MP2 level bond length of 1.670 A (16). These results can be compared with experimentally determined carbon oxygen bond lengths: a C|O bond ˚ in carbon monoxide (38) and of 1.203 A ˚ length of 1.128 A in formaldehyde (36), and a C{O bond length of 1.4214 ˚ in methanol (39). The double silicon oxygen bond would A appear to undergo much less variation than the double carbon oxygen bond. Using the theoretical results for Si{O, the relative shortening of the silicon oxygen double bond with respect to the single bond is less than that undergone by carbon. To our knowledge there has been no precise experimental determination of an Si{O bond length, which adds extra interest to attempts to identify the spectra of HSiOH cis and trans. We would also like to study whether silanol is formed in the discharge, since many unidentified lines remain in our spectra. 7. CONCLUSION

An analysis of the submillimeter-wave spectra of H2 Si16O, H2 30Si16O, H2 28Si18O, and D2 28Si16O has led to the determination of accurate molecular parameters for these isotopomers of silanone. These parameters, together with those measured previously for the parent species, have been used to obtain a good approximation of the equilibrium structure of this reactive molecule. The experimental structure confirms the validity of recent ab initio calculations by Schaefer and Ma (17). 29

ACKNOWLEDGMENTS The present study was jointly supported by the CNRS (GDR ‘‘PhysicoChimie des Mole´cules et des Grains Interstellaires’’), the French government and the region Nord-Pas de Calais. The authors thank H. F. Schaefer for providing further ab initio calculations for the isotopomers of H2SiO. They are very grateful to J. Demaison for his advice on structural determination.

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