37Cl16O218O

JOURNAL OF MOLECULAR SPECTROSCOPY ARTICLE NO.

175, 120–132 (1996)

0016

Microwave Fourier Transform Spectroscopy of Perchloryl Fluoride: 19 35/37 F Cl16O3 and 19F35/37Cl16O218O Holger S. P. Mu¨ller1 and Michael C. L. Gerry Department of Chemistry, The University of British Columbia, 2036 Main Mall, Vancouver, British Columbia, Canada V6T 1Z1 Received August 7, 1995; in revised form October 3, 1995 18 O-enriched, monolabeled perchloryl fluoride, 19F35/37Cl16O218O, has been synthesized. Its rotational spectrum has been studied in the 4–22 GHz region. In addition, the spectra of the symmetric isotopomers 19F35/37Cl16O3 have been reinvestigated. Rotational and centrifugal distortion constants, as well as chlorine nuclear quadrupole and chlorine and fluorine spin–rotation and spin–spin coupling constants, have been determined for the ground vibrational state. The ground state rotational data have been used to evaluate geometrical parameters. The harmonic force field has been refined. The results are compared with data of related molecules. q 1996 Academic Press, Inc.

I. INTRODUCTION

Perchloryl fluoride, FClO3, was first obtained by Bode and Klesper in 1951, but was believed to be chloryl hypofluorite, O2ClOF (1). In 1952, it was prepared and correctly identified by Engelbrecht and Atzwanger (2). Several further studies reported on the chemical and physical properties of FClO3 (3–5). Its C3£ symmetry was first established from its infrared spectrum (6), and was confirmed by electron diffraction (7) and microwave spectroscopic (8) measurements on the two symmetric isotopomers FClO3 and F37ClO3 (unlabeled atoms indicate 16O, 19F, and 35Cl). Rotational and centrifugal distortion constants, as well as chlorine nuclear quadrupole coupling constants, were determined (8). The molecule was also found to have a very small dipole moment of Ç0.025 D (9). This, combined with the small quadrupole coupling constants, suggests similar electronegativities for the F atom and the ClO3 group, and thus a rather covalent FCl bond. More recently, Christe et al. (10) obtained a reliable general valence force field (GVFF). For this purpose they used a combination of previously reported fundamentals for gaseous FClO3, 35/37Cl isotopic shifts from matrix IR spectra, Coriolis coupling constants, and an ab initio force field. In the past decade, the availability of Fourier transform techniques has led to several new studies at much higher resolution than was previously available. Heldmann and Dreizler (11) reinvestigated the rotational spectrum using a waveguide microwave Fourier transform (MWFT) spectrometer. They improved all previously reported spectroscopic constants except DJK (which was unavailable to them). In addition, they determined precisely the dipole moment 1

Present address: Jet Propulsion Laboratory 183-301, California Institute of Technology, Pasadena, CA 91109.

(0.02700(4)D) and evaluated some spin–rotation coupling constants. Burczyk et al. (12–14) reported extensive analyses of high-resolution IR and millimeter-wave spectra. In general, a sample monoisotopic in 35Cl was investigated, though in some instances a sample of natural isotopic composition was used. Precise spectroscopic constants were obtained for the ground and excited states of all fundamentals, as well as for some overtone, combination, and hot bands. An accurate A0 value for FClO3 was obtained by analysis of a DK Å {3 resonance in n5 (14). Fewer vibrational levels were studied for F37ClO3. Although r0 and re geometries were reported, their accuracies were difficult to assess; it was pointed out that further progress required spectral data of isotopomers containing 18O. Very recently, the C3£ species FCl18O3 and F37Cl18O3 were obtained as a by-product of an attempted synthesis of FCl18O2 (15). High-resolution IR spectra yielded molecular constants in the ground vibrational state and excited states of n1 and n2 (16). New r0 parameters were obtained which differed significantly from those obtained earlier (14). In this article, we report the first measurements of the pure rotational spectrum of the asymmetric isotopomers FClO218O and F37ClO218O. The work was prompted at this time by two main factors. (i) Even with the inclusion of B0 of FCl18O3 and F37Cl18O3, the geometrical parameters of perchloryl fluoride remain rather uncertain (13, 16). Further isotopic data should help to alleviate the situation. (ii) Because of the low dipole moment, the microwave transitions are rather weak. Recently, however, rotational spectra of several species with comparable or smaller dipole moments have been measured quite easily in this laboratory, using a pulsed jet cavity MWFT spectrometer (17). In the case of FClO3, the similarity of the masses of F and O causes a large rotation of the inertial axes (ú207) when an 16O atom is substituted by 18O.

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MICROWAVE FOURIER TRANSFORM SPECTROSCOPY OF FClO3

As a result, both a- and b-type transitions were expected to be observable. Accordingly, all three rotational constants should be accurately obtainable, thus significantly increasing the number of data available to determine the geometry of the molecule. In addition, we have remeasured the two lowest J transitions of the symmetric isotopomers FClO3 and F37ClO3. This was prompted because our available resolution is higher than that of the previous MWFT study (11). This has allowed us to improve the precision of previously obtained hyperfine constants, and to evaluate new ones for these isotopomers. These constants complement similar parameters obtained for the asymmetric species. II. EXPERIMENTAL DETAILS

FIG. 1. The FClO218O isotopomer in its principal axis system projected onto the ab-plane. The angle between the FCl bond and the a-axis (21.407) enabled b-type transitions to be observed.

(a) Chemicals

III. OBSERVED SPECTRA AND ANALYSIS

The preparation of F35/37ClO2 18O required as the first step the synthesis of monolabeled sodium perchlorate, NaClO318O. It was prepared following the method of Appelman (18). In a small Teflon-PFA container (Fluoroware) 533 mg NaClO3 (Ç5 mmol) were dissolved in ca. 0.6 ml H218O (98% 18O, MSD); 1 g of XeF2 (5.9 mmole, Aldrich) was added, and the mixture was allowed to react for 16 hr. At that point the water and HF remaining in the mixture were evaporated, and the product was dried for 4 hr at 1207C. FClO218O was prepared according to the method of Ref. (5). The dry product from the previous reaction was transferred to a 100-ml glass container, into which were then condensed ca. 2.5 ml HSO3F (Orange County Chemicals, technical grade, purified by single distillation at atmospheric pressure (19)), and ca. 1.5 ml SbF5 (Atochem, Ozark-Mahoning). The container was heated for 20 min to 1107C, and the products were separated by trap-to-trap condensation: SbF5 and HSO3F were trapped at 01107C; FClO218O and FClO3 (theoretical ratio ca. 3:1) were trapped at 01967C. FClO3 in natural isotopic composition was obtained by the same method from normal NaClO4. (b) Instrument The spectra have been measured in the frequency range 4–22 MHz using a Balle–Flygare type (20) cavity pulsed MWFT spectrometer, which is described in greater detail elsewhere (21). Samples were injected as pulsed jets of gas consisting of Ç0.5% FClO3 in neon at 1500–3000 mbar total pressure. Rotational temperatures achieved were (1 K. Linewidths of ca. 7 kHz were obtained, and lines 5 kHz apart could be resolved. The precision and accuracy of measurements for strong, well-resolved lines are about {0.5 and {1.0 kHz respectively. Precise transition frequencies, particularly for closely spaced lines, were determined from fits to the time-domain (‘‘decay’’) signals (22).

Rotational constants of FClO218O and F37ClO218O were initially predicted from the previously reported r0 structure (16). Both isotopomers are asymmetric, near spherical rotors (k Å 00.1027 and 00.1040, respectively) with Cs symmetry. Because of the similarities of the masses of O and F and of the ClO and ClF bond lengths, there is a large rotation of the axes from their directions in the symmetric species (Ç21.47; see Fig. 1). Consequently, both asymmetric isotopomers were expected to have significant, albeit small, dipole components along both the a- and b-axes (Ç0.025 and Ç0.010 D, respectively). 19F and 35/37Cl hyperfine constants were predicted from the results of Ref. (11); these included values for xab, which was expected to contribute up to Ç30 kHz to the transition frequencies, because of the presence of several near-degeneracies of the correct symmetry (see Fig. 2). Centrifugal distortion constants were derived from the force field in Ref. (10). The experimental conditions (e.g., flow rate of the sample, microwave pulse width etc.) were first optimized by recording a strong line of the FClO3 isotopomer. Several transitions of both asymmetric isotopomers were then located, including all hyperfine components of the 101 –000 and 111 – 000 transitions at ca. 10 GHz and some for the a-type R branch, J* –J 9 Å 2–1, near 20 GHz. The lines were found within Ç2 MHz of the predictions; these deviations were mainly due to differences between predicted and experimental A rotational constants, as indicated in Table 1. Because of the small dipole moment, the optimal excitation pulses were very long, about 10 and 24 msec for a-type and b-type lines, respectively. Thus only small sections of the spectrum could be recorded at a time. It was generally faster to record each Doppler component of a group of two or three lines, rather than to record the whole group using shorter pulses. Improved spectroscopic constants from these observed lines were used to find further hyperfine components of J* –J 9

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FIG. 2. Part of the rotational energy level diagram of FClO218O.

Å 2–1 transitions, including b-types, for both isotopomers. While some of the stronger lines were seen with a large signal-to-noise ratio (S/N) using only up to 100 averaging cycles, several weaker ones, notably those of the 220 –101 transition, required more than 1000 cycles. Part of the 202 – 101 transition of FClO218O is shown in Fig. 3. R-branch transitions with J § 3 are outside the frequency range of the spectrometer. Q-branch transitions within its range are rather high in J, and because of the low rotational temperatures were too weak to be observed. No evidence was found for the strongest hyperfine components of the 550 –533 and 642 –625 transitions, even after 2000 averaging cycles. The frequencies of the observed transitions of both asymmetric isotopomers are given in Table 2. The hyperfine splittings caused by the Cl nuclei were in general larger than those due to F. The assignments have thus been made using quantum numbers corresponding to the coupling scheme J / ICl Å F1, F1 / IF Å F. The investigations of the symmetric isotopomers were

TABLE 1 Comparison of Experimental Ground State Rotational Constants of F35/37ClO218O (MHz) with Those Calculated from r0 of Ref. (16)

FIG. 3. Detail of the power spectrum for the 202 –101 transition of FClO218O, showing splitting mainly due to 19F spin–rotation and 19F– 35Cl tensor spin– spin coupling; 2000 cycles were averaged. A stick diagram indicates the calculated relative intensities for an amplitude spectrum and the line positions from the decay fit. The lines are Doppler-split by Ç103 kHz. The quantum numbers F* 0 F9 are indicated; F=1 Å F01 Å 0.5.

straightforward as a consequence of earlier measurements (11). The new measurements for both isotopomers are given in Table 3 along with their assignments. The spectroscopic constants of each isotopomer were obtained from global least-squares fits using Pickett’s program SPFIT (23). The line frequencies were weighted as the inverse squares of their uncertainties, which were generally 0.5 kHz (1.0 kHz for weak lines near much stronger ones). Blended lines were given uncertainties derived from the calculated splittings for the individual components, which were weighted according to the squares of the intensities. For the asymmetric isotopomers it was not possible to obtain all the quartic centrifugal distortion constants. For FClO218O at least one constant had to be fixed to its value from the force field (vide infra). The best result (relatively small error bars and correlations) was obtained when d2 was held fixed. However, the uncertainty in DJK was larger than its value, even though this uncertainty was comparable to those of the remaining constants. Consequently the value of DJK was also fixed at its value from the force field; the result is listed under Fit I in Table 4. For comparison, two further fits were carried out, in which DJK was released, but d2 was assigned uncertainties of 3 and 10 times its value. The results of the latter are given in Table 4 under Fit II; the uncertainties, though larger than those of Fit I, are still comparable, suggesting that the frequencies are relatively insensitive to d2. For F37ClO218O, three distortion constants were fixed to their values from the force field calculation. The distortion constants which could be determined experimentally agreed in general within their error limits with those calculated from the force field. The derived constants for the symmetric isotopomers are

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MICROWAVE FOURIER TRANSFORM SPECTROSCOPY OF FClO3

TABLE 2 Observed Frequencies (MHz) of Rotational Transitions of F35/37ClO218O and Residuals (o 0 c)a (kHz)

also in Table 4. In these cases the only determinable rotational and distortion constants were B0 and DJ. A combined fit of the rotational data from the present and earlier work is given in the Appendix. To obtain satisfactory standard deviations of the fits, it

TABLE 2— Continued

was necessary to take Cl–F spin–spin interaction into account. In some instances, the contributions were as large as Ç4 kHz. For the asymmetric isotopomers, the scalar, electron-coupled spin–spin coupling constants JClF were barely determined and were omitted from the final fits. The acomponents, Saa, of the tensor part of the spin–spin coupling were moderately well determined, whereas the differences of the b- and c-components, S0, were essentially indetermi-

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TABLE 3 Observed Frequencies (MHz) of Rotational Transitions of F35/37ClO3 and Residuals (o 0 c)a (kHz)

nate. The standard deviations of the fits of (0.3 kHz are within the measurement uncertainties, and imply that all relevant effects have been accounted for. Although the moderate number of lines might suggest these standard deviations are artificially small, a similar value (Ç0.39 kHz) was obtained for 32S16O35Cl2, where 262 lines were fit to 24 parameters (24). IV. DISCUSSION

(a) Rotational Constants and Molecular Geometry This work has produced the first experimental values for the rotational constants of the two asymmetric isotopomers FClO218O and F37ClO218O. Precise values have been obtained for all three constants for both species. For the symmetric isotopomers FClO3 and F37ClO3, our new values of B0, as well as those from the combined fits (see the Appendix), agree generally very well with earlier ones, in particular

those obtained from rotational data alone (11, 13); the precision of the present values is greater. The new results represent a significant increase in the data available to determine the molecular geometry. They were used for this purpose along with earlier rotational constants of other isotopomers, including the symmetric species FCl18O3 and F37Cl18O3, whose B0 values are well known, though to a somewhat lower precision (ca. one order of magnitude) (16). This lower precision should have a negligible effect in an r0 fit, but might be more important with more sophisticated structure models. The value of A0 for FClO3 was also included; since it has been determined through a DK Å {3 resonance in n5 (13), its precision is ˚ 2 for the lower (0.21 MHz, corresponding to 0.0034 amu A moment of inertia), which may be significant even for an r0 structure determination. In order to employ the fitting program as described below, values for A0 were also estimated for the remaining symmetric isotopomers in the rigid rotor approximation: Cl substitution would produce no change in the A0 value, whereas substitution of three O atoms would produce a change inversely proportional to the atomic masses. Sets of r0 and rDP geometrical parameters were calculated with Rudolph’s least-squares fitting program RU111J (25, 26). The two bond lengths and the FClO bond angle were fit to the principal planar moments, or their differences, respectively, for the two models. Following Rudolph’s recommendations (27), PB and PC were omitted from the fit for C3£ species substituted at Cl; similarly, PC was omitted for the asymmetric isotopomers with Cs symmetry. The initial r0 fit was carried out with all moments weighted ˚ 2, equally. The standard deviation obtained, ca. 0.01 amu A 2 ˚ was considerably larger than ca. 0.003 amu A expected for a heavy-atom molecule with normal vibrational contributions to precisely determined ground state rotational constants (26). The differences between experimental and calculated constants indicate that this somewhat unsatisfactory situation arises primarily through the rather uncertain A0 constants. In order to account for the effects of the weights on the geometries, two further fits were carried out. In one, the uncertainties were taken to be proportional to the rotational constants themselves. In the other, they were increased by factors of 1.25 for B0 of F35/37Cl18O3, of 5 for A0 of FClO3, and of 25 for A0 of the remaining C3£ isotopomers. In both fits, the weights were inversely proportional to the squares of the uncertainties. The resulting structural parameters are in Table 5, columns e and f; they agree within their uncertainties, and are also close to those in Ref. (16) (column d). They differ more greatly from the parameters in Ref. (13) (column c), which was obtained using only three rotational constants of F35/37ClO3. The results of two rDP fits, using the same sets of weights as above, are also in Table 5. These fits have reduced the

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MICROWAVE FOURIER TRANSFORM SPECTROSCOPY OF FClO3

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TABLE 4 Spectroscopic Constantsa of Perchloryl Fluoride, FClO3, Obtained in This Study

effects of vibrations on the ground state rotational constants, resulting in smaller standard deviations and hopefully more reliable geometries (25, 26). For FClO3, the standard deviations from the r0 fits are reduced by a factor of Ç3 in the corresponding rDP fits. This change is rather small when compared with fits for the similar molecule ClSO2F, where the standard deviation was reduced by a factor of Ç30 (28). In addition, the two sets of rDP parameters show differences in the FCl bond lengths. In the fit where the uncertain-

ties of all rotational constants were proportional to their magnitudes, this bond length is essentially the same as in the corresponding r0 fit. This seems rather unreasonable, since the F atom is unsubstituted, and the rather large residuals and high correlations strongly affect structural parameters involving the F atom (26). In the second rDP fit, this bond length is 0.78 pm shorter than that from the respective r0 fit. Although this value is only about twice the combined error bars, it may be noteworthy that it corresponds to

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TABLE 5 Structural Parametersa (pm, degree) of Perchloryl Fluoride

roughly the estimate of rz 0 re of ca. 0.90 pm: rDP is intended to produce a better estimate of re than r0 (vide supra). In a third approach, a ground state average structure (rz) was calculated for the main isotopomer, FClO3. The harmonic contributions to the a-constants were obtained from the force field (vide infra) and subtracted off the measured effective rotational constants Bg0. The geometrical parameters were fit to the resulting Bgz using Typke’s program MWSTR. Isotopic variations in the bond lengths were accounted for using the equations (29) rz Å re / 3/2 a »u2… 0 K 2

[1]

drz Å dre / 3/2 a d»u … 0 dK.

[2]

Here »u2… and K are respectively the zero-point mean square amplitude of a given bond and its perpendicular amplitude correction for the respective isotopomer, both obtained from the force field. The constants ‘‘a’’ are Morse anharmonicity parameters, which were approximated by values from the corresponding diatomics (30). Values of 1.933 and 2.021 ˚ 01, derived from Refs. (31) and (32) were used for the ClF A and ClO bonds, respectively. Variations in the bond angles with isotopomer are neglected in this model (29). The resulting parameters, using the same sets of weights as above, are also in Table 5. The rz structure allows a direct comparison with the results from electron diffraction. Although the equivalent parameters, r0a, have not been reported, they are close to the rg(1) values, whose values are known (7). The corrections producing r0a values from the rg(1) parameters can be calculated from the force field (33). The resulting differences (rg(1) 0 r0a) are 0.01 and 0.05 pm for the ClF and ClO bond length respectively, well below the quoted uncertainties; changes in nonbonding distances are neglected.

Table 5 also allows a comparison of both sets of rz values with the rg(1) values. There is agreement between the two sets of rz parameters within their error limits. They agree within or close to the error limits of the corresponding rg(1) parameters. The equilibrium bond lengths, re, have been estimated using Eq. [1]. About 0.90 and 0.47 pm were obtained for rz 0 re of the ClF and ClO bond respectively. The angles are assumed to be those of the rz structure (29) under heading (f). The resulting estimate of the equilibrium structure is also in Table 5, in comparison with that of Ref. (13). Both estimated re geometries required assumptions and must be viewed cautiously. The most severe one in the present case is the bond angles: in a test calculation for OClO, \e and \z differed by Ç0.10; this caused an error of Ç0.2 pm in re. For the earlier geometry (13), an estimated aeB was required for F37ClO3: a change of 1% in aeB caused a change of 1.7 pm in re(ClF). The source of the problem is that the Cl atom is close to the center of mass of the molecule. Ultimately, a larger number of properly determined equilibrium rotational constants will be needed if a reliable re structure is to be obtained. In all fits the best determined parameter is r(ClO); its values show much smaller variations than do those of r(FCl). Although the values of the bond angles in Table 5 show some variations, those of the present study and of Ref. (16) are close to 102.07 for \(FClO) and 115.87 for \(OClO). Selected properties of FClO3 are compared with those of related compounds in Table 6. In this comparison, the geometries used for FClO3 were the rDP values with reduced weights of the A0 constants (Table 5, column rDP, f), and the rg(1) values. It was pointed out earlier (42) that, where there is no significant vibrational mixing, the ClO bond lengths decrease strongly with increasing average ClO stretching fre-

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MICROWAVE FOURIER TRANSFORM SPECTROSCOPY OF FClO3

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TABLE 6 Comparison of Selected Properties (pm, 100 N m01, cm01, degree) of FClO3 with Those of Related Compounds

quencies and force constants. These relations were used to derive ClO bond lengths for ClClO2 (15) and ClO3 (43). They also hold for the compounds listed in Table 6, if one compares related structural parameters. Thus it is not surprising that the ClO bond in FClO3 is not only quite short among ClO compounds in general, but also among those where chlorine has the formal oxidation number VII. The ClO bonds in the ions ClOF2/ and ClO2F2/ can be expected to be even shorter still, since for them nav(ClO) Å 1333 and 1360 cm01 respectively (5). As was outlined earlier (42) these short ClO bond lengths indicate considerable double bond character, whereas the lengths of the bridging ClO bonds in Cl2O7 (see Table 6) are fairly typical for ClO single bonds. An even larger value was obtained for ClOO, with r(ClO) É 214 pm (44). The FCl bond length and force constant in FClO3 are fairly typical of a rather covalent FCl bond (see Table 6 and Refs. (5, 37)). Greater bond lengths and smaller force constants have been interpreted in terms of highly polar (p– p*) s-bonds (for FClO and FClO2) or mainly semi-ionic three center-four electron bonds (5, 37). However, in general, the relations between FCl bond lengths and force constants are not as simple as those outlined for ClO bonds. Indeed, opposite relationships (increasing bond lengths and increasing force constants) have been observed for related compounds involving bonds between F and another electronegative element (45). Among related compounds with similar coordination sphere the angles \(FClO) and \(OClO) show only small variation. (b) Harmonic Force Field The harmonic force field used in the present work, particularly in estimating centrifugal distortion constants of the

asymmetric isotopomers, is an improved version of that in Ref. (10), which dates from 1981. More recent data were used in the evaluation. The calculation was carried out with the program ASYM20 (46). The geometrical parameters were the rDP parameters under heading (f) in Table 5. The symmetry coordinates are the same as those in Ref. (10) except for S3, which was adjusted to account for slightly different bond angles, following the method of Aldous and Mills (47). It is given as footnote to Table 7. The data used came entirely from the symmetric isotopomers. (The only available data for the asymmetric species, the centrifugal distortion constants, were less well determined than those of the symmetric isotopomers. Their inclusion did not affect the final results.) Experimental wavenumbers were taken from Refs. (10, 12–14, 16). Harmonic wavenumbers of the most abundant isotopomer, FClO3, were evaluated using anharmonic constants xij from Refs. (12–14), and estimating the remainder from related vibrations of FClO3. A value of 08 cm01 was assumed for x14; it was estimated from a comparison with the corresponding xij of FClO2 (48). The ratios vi /ni obtained were 1.0187, 1.0173, 1.0145, 1.0118, 1.0113, 1.0064 for i from 1 to 6, respectively. Harmonic wavenumbers of the other isotopomers were estimated as described in (44). Centrifugal distortion constants were taken from Ref. (16) and from this work (see Table 10); equilibrium distortion constants could not be derived because even for the major isotopomer four of twelve vibration rotation interaction corrections could not be determined. The Coriolis coupling constants z4, z5, and z6 for the major isotopomer were taken from Refs. (13, 14). Their sum, which should equal B/2A, was actually smaller. In the initial fits, the Coriolis constants were scaled to fulfil this relation. At

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TABLE 7 Harmonic Force Constantsa (100 N m01) and Potential Energy Distributionb (PED) of FClO3

a later stage we had available preliminary values for n3, n5, n6, z5, and z6 of FCl18O3 and F37Cl18O3 (49). These values indicated that the sum rule was breaking down in FClO3 because z4 alone may be too small. Because n4 is in strong Fermi resonance with n2 / n5, this suggestion does not seem unreasonable. Consequently, z4 was omitted from the final fit, and z5 and z6 were used unscaled. The relative weights attributed to the experimental data were inversely proportional to the squares of their assigned uncertainties, designed to give particular emphasis to the harmonic wavenumbers and to reproduce the remaining constants satisfactorily. For the wavenumbers of the fundamentals, these were 0.001 cm01, except for n4 and n5 of F37ClO3, where they were larger by a factor of 10 and 3, respectively because of significantly lower precision of their isotopic shifts. The experimental uncertainties were attributed to the distortion constants DJ and DJK; the uncertainties of z5 and z6 were 10 times their experimental values. The force constants of Ref. (10) were used as starting values. They are presented in Table 7 together with the final force field and the resulting potential energy distribution (PED). Different weighting of the input data or inclusion of the preliminary data of FCl18O3 and F37Cl18O3 (49) had only very small effects on the diagonal force constants and F23, whereas in some instances significant changes were obtained for the off-diagonal force constants in the symmetry class

E, implying that additional input data in this symmetry class may further improve the force field. A comparison between experimental constants and those calculated from the force field is given in Table 8. The anharmonic wavenumbers were generally reproduced very well. In part, this is a result of using harmonized values in the fit (44). The distortion constants were also reproduced very well, except for DK of FClO3, which was not obtained directly from a spectroscopic fit, and which was not used in the force field calculation. The higher weight of z6 contributes to its better reproduction in comparison to z5. The present force field agrees reasonably well with that of Ref. (10). Some of the changes are due to the harmonic corrections applied to the vibrational wavenumbers in the present fit, whereas some are certainly caused by the larger set of input data, especially for the force constants in the symmetry class A1. The PED of the present force field is also quite similar to the earlier one (10); as was outlined there n1, n4, n5, and n6 are typical rather unmixed vibrations, whereas n2 and n3 are strongly mixed. Inspection of the Cartesian displacements of the normal coordinates suggests these two modes might be better described as asymmetric and symmetric combination, respectively, of n(FCl) and ds(ClO3). This can be symbolized by n2 Å [n(FCl) / ds(ClO3)]as and n3 Å [ds(ClO3) / n(FCl)]s; the first contribution is the more dominant.

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TABLE 8 Comparison of Experimental Vibrational Wavenumbers, Coriolis Constants (cm01), and Centrifugal Distortion Constants (kHz) with Those Calculated from the Force Field

(c) Hyperfine Constants Ground state nuclear quadrupole and spin–rotation coupling constants of the symmetric (C3£) isotopomers have been determined with greater precision than in the previous studies (11). The respective constants of the asymmetric (CS) isotopomers are of comparable quality. Values for the spin– spin coupling constants have been obtained for the first time. The principal values of the Cl quadrupole tensor of the asymmetric isotopomers are in Table 4. They agree well with those of the respective symmetric isotopomers. As expected, the angles uza agree quite well with the angles between the FCl bond and the a-axis: ua,FCl Å 21.407 and 21.077 for FClO218O and F37ClO218O, respectively. The small differences between uza and ua,FCl may be due to uncertainties in the structure and to vibrational contributions to the ground state hyperfine constants. As was pointed out in the introduction the small value of xzz indicates a rather symmetric charge distribution around the Cl atom and thus a rather covalent FCl bond. The situation is quite similar to that of HOClO3 where xaa, which is expected to be close to xzz, is 014.26(15) MHz (50). For the symmetric isotopomers, since no transitions with Ka ú 0 were observable, C\ Å Caa could not be determined directly. However, it could be estimated by diagonalizing the spin–rotation tensors of the asymmetric isotopomers. These values are given in Table 4. The values for C⊥ (ÅCbb Å Ccc), obtained by the same procedure, are 3.306(154) and 22.682(310) kHz for F and Cl in FClO3, and 2.57(56) and 22.45(81) kHz for F and Cl in F37ClO3, respectively. They agree well with those obtained directly (see Table 4).

The spin–rotation constants of a given nucleus can be described using two terms (51). The first of these, the nuclear term C n, depends on the positions of the nuclei; the second, electronic term C e, is a second-order term dependent on the electronic structure. The derived values are in Table 9. C e, in turn, is directly proportional to the paramagnetic part sp of the magnetic shielding. The total shielding sav is itself the sum of sp and sd, the diamagnetic part. There is an approximate method for calculating sd from C n (52, 53). A pronounced shielding anisotropy of the 19F nucleus was derived earlier from NMR measurements on FClO3 both in the liquid phase (54) and enchlathrated in D2O (55). In both studies, the results were used to predict the 19F spin–rotation constants. Maryott et al. (54) gave two choices: C\ Å 01.2 and C⊥ Å 27 kHz; and C\ Å 36.4 and C⊥ Å 8.2 kHz. Garg et al. (55) gave C\ Å 6.38 and C⊥ Å 22.64 kHz. The latter

TABLE 9 Nuclear, C n, and Electronic, C e, Contributions to the Experimental Spin–Rotation Constants C (kHz) of FClO3

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TABLE 10 Spectroscopic Constants of FClO3 and F37ClO3 from a Combined Fit of Rotational Data in Comparison with Previous Results

work used a more sophisticated method to estimate sd (53), and predicted values, particularly C⊥, in excellent agreement with the experimental results. To permit comparison with related molecules, the 19F spin–rotation constants have been used to calculate the paramagnetic shielding sp (51). The resulting values are sp\ Å 0192 ppm and sp⊥ Å 01014 ppm; the latter agrees with the value in Ref. (55), but there is a small discrepancy in sp\ , paralleling the discrepancy in C\ mentioned above. The average paramagnetic shielding s(F) p is 0740 ppm, very close to the experimental value in F2: 0750 ppm (56). In Ref. (5), the recommended value for the fluorine chemical shift of FClO3 is 0241.5 ppm relative to CCl3F, taken from Ref. (57). An earlier value of 0287 ppm from a solution in CCl3F (58) was rejected. It should be pointed out that the latter value is in agreement with the isotropic value obtained by Garg et al., whereas the former is not. With the absolute shielding of 188.7 ppm for CCl3F (59), values of s(F) av Å 053 or 099 ppm, respectively, are obtained for the shielding. (F) (F) The diamagnetic shielding is thus s(F) d Å sav 0 sp Å 687 or 641 ppm. Both values are slightly different from the value estimated earlier, 666 ppm (55). However, both are comparable to other FCl bonds (60). These include ClF, for which (F) s(F) d Å 575 ppm; the anomalous value of sav for ClF arises (F) through sp (61). From the 35Cl spin–rotation constants of the main isotopomer, FClO3, the paramagnetic shieldings of 35Cl are found

to be sp\ Å 01546 ppm, sp⊥ Å 01237 ppm, and spav Å 01340 ppm. Although the spin–spin coupling constants are not especially well determined, there are some conclusions to be drawn from the present results. The elements of the spin– spin coupling tensor are the sums of contributions from two sources, namely, a direct dipole–dipole coupling between the nuclei, whose constants, designated by d, can be calculated from the structure, and an indirect, electron-coupled term (62). The sign of S is chosen in a way that d is negative for homodiatomics. For FClO3 and F37ClO3, values of daa Å d\ Å 02.64 and 02.20 kHz, respectively, were estimated from the structure. Thus, considering the uncertainties of the experimental values, the electron-coupled contribution is not significant. The same holds for the asymmetric isotopomers, for which the estimated daa and d0 are 02.12 and 00.18 kHz, for FClO218O, and 01.76, and 00.15 kHz for F37ClO218O, respectively. This is in accordance with more general empirical findings that the direct term is the dominant one for bonds between rather light elements (62). The scalar, electron-coupled spin–spin constants are barely determined, but in all instances they are negative, even for the asymmetric isotopomers when they are included in the fit. The magnitude of J is frequently available with high precision from NMR measurements, but usually not the sign. From simulations of the experimental NMR spectrum of liquid FClO3 at room temperature a value ÉJÉ Å

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MICROWAVE FOURIER TRANSFORM SPECTROSCOPY OF FClO3

0.278(5) kHz was determined for the main isotopomer, FClO3 (57, 63). The present results are consistent with this value. APPENDIX

Since our new measurements are more precise than those available previously for the corresponding transitions, we have carried out a combined fit of all measured rotational transitions, in an attempt to improve the rotational and centrifugal distortion constants. The data used were the present results for the symmetric isotopomers (uncertainties 0.5 kHz), along with the measurements of J* –J9 Å 3–2 lines from Ref. (11) (uncertainty 10 kHz), and those of Ref. (8), including Cl hyperfine splittings and their quoted uncertainties. The millimeter-wave data of Ref. (13) were used as reported (no hyperfine splittings, 100 kHz uncertainty). The results are in Table 10. The reduced standard deviations are 0.728 and 0.558 for FClO3 and F37ClO3, respectively. The precision of the rotational constants, and of all other constants except DJ and DJK, is higher than that of previous studies. The uncertainties of the spectroscopic constants are somewhat sensitive to the weighting scheme employed, which may explain larger uncertainties for DJ and DJK in this study compared to Ref. (13). The residuals (nobs 0 ncalc) are in general much smaller than the quoted uncertainties, particularly for F37ClO3. ACKNOWLEDGMENTS We thank Professor F. Aubke for providing the samples of HSO3F and SbF5, Professor G. Graner for making available some preliminary data on F35/37Cl18O3, and Professor E. E. Burnell for helpful discussion. Funding by the Natural Sciences and Engineering Research Council of Canada (NSERC) is gratefully acknowledged.

REFERENCES 1. H. Bode and E. Klesper, Z. Anorg. Allg. Chem. 266, 275–280 (1951). 2. A. Engelbrecht and H. Atzwanger, Monatsh. Chem. 83, 1087–1089 (1952). 3. M. Schmeisser and K. Bra¨ndle, Adv. Inorg. Chem. Radiochem. 5, 41– 89 (1963). 4. M. Becke-Go¨hring (Ed.), ‘‘Gmelins Handbuch der Anorganischen Chemie. Chlor,’’ 8th ed., Syst. No. 6, Part B, No. 2, Verlag-Chemie, Weinheim, 1969. 5. K. O. Christe and C. J. Schack, Adv. Inorg. Chem. Radiochem. 18, 319–398 (1976). 6. (a) D. R. Lide and D. E. Mann, J. Phys. Chem. 25, 1128–1131 (1956); (b) R. P. Madden and W. S. Benedict, J. Phys. Chem. 25, 594–595 (1956). 7. A. H. Clark, B. Beagley, D. W. J. Cruickshank, and T. G. Hewitt, J. Chem. Soc. (A), 872–875 (1970). 8. D. R. Lide, J. Phys. Chem. 43, 3767–3768 (1965). 9. (a) A. A. Maryott and S. J. Kryder, J. Phys. Chem. 27, 1221–1222 (1957); (b) L. Frenkel, W. Smith, and J. J. Gallagher, J. Phys. Chem. 45, 2251–2253 (1966); (c) R. L. DeLeon and J. S. Muenter, J. Mol. Spectrosc. 83, 469–470 (1980).

10. K. O. Christe, E. C. Curtis, W. Sawodny, H. Ha¨rtner, and G. Fogarasi, Spectrochim. Acta Part A 37, 549–556 (1981). 11. C. Heldmann and H. Dreizler, Z. Naturforsch. A 45, 811–813 (1990). 12. K. Burczyk and H. Bu¨rger, J. Mol. Spectrosc. 118, 288–297 (1986). 13. K. Burczyk, H. Bu¨rger, M. LeGuennec, G. Wlodarczak, and J. Demaison, J. Mol. Spectrosc. 148, 65–79 (1991). 14. K. Burczyk, H. Bu¨rger, and L. Halonen, J. Mol. Spectrosc. 159, 265– 273 (1993). 15. H. S. P. Mu¨ller and H. Willner, Inorg. Chem. 31, 2527–2534 (1992). 16. F. Meguellati, G. Graner, H. Bu¨rger, K. Burczyk, H. S. P. Mu¨ller, and H. Willner, J. Mol. Spectrosc. 164, 368–378 (1994). 17. See, for example, (a) Y. Xu, M. C. L. Gerry, D. L. Joo, and D. J. Clouthier, J. Chem. Phys. 97, 3931–3939 (1992); (b) Y. Xu, W. Ja¨ger, I. Ozier, and M. C. L. Gerry, J. Chem. Phys. 98, 3726–3731 (1993); (c) Y. Xu, W. Ja¨ger, and M. C. L. Gerry, J. Mol. Spectrosc. 157, 132– 140 (1993); (d) W. Ja¨ger, Y. Xu, and M. C. L. Gerry, J. Chem. Phys. 99, 919–927 (1993); (e) W. Ja¨ger, Y. Xu, N. Heineking, and M. C. L. Gerry, J. Chem. Phys. 99, 7510–7520 (1993); (f) Y. Xu, W. Ja¨ger, and M. C. L. Gerry, J. Chem. Phys. 100, 4171–4180 (1994); (g) W. Ja¨ger, M. C. L. Gerry, C. Bissonnette, and F. R. W. McCourt, J. Chem. Soc. Faraday Discuss. 97, 105–118 (1994). 18. E. H. Appelman, Inorg. Chem. 10, 1881–1888 (1971). 19. J. Barr, R. J. Gillespie, R. C. Thompson, Inorg. Chem. 3, 1149– (1964). 20. T. J. Balle and W. H. Flygare, Rev. Sci. Instrum. 52, 33–45 (1981). 21. (a) Y. Xu, W. Ja¨ger, and M. C. L. Gerry, J. Mol. Spectrosc. 151, 206– 216 (1992); (b) T. Ogata, W. Ja¨ger, I. Ozier, and M. C. L. Gerry, J. Chem. Phys. 98, 9399–9404 (1993). 22. J. Haekel and H. Ma¨der, Z. Naturforsch. A 43, 203–206 (1988). 23. H. M. Pickett, J. Mol. Spectrosc. 148, 371–377 (1991). 24. H. S. P. Mu¨ller and M. C. L. Gerry, J. Chem. Soc. Faraday Trans. 90, 3473–3481 (1994). 25. K. Epple and H. D. Rudolph, J. Mol. Spectrosc. 152, 355–376 (1992). 26. H. D. Rudolph, in ‘‘Advances in Molecular Structure Research’’ (I. Hargittai and M. Hargittai, Eds.), Vol. 1, JAI Press, Greenwich, CT, in press. 27. H. D. Rudolph, J. Mol. Spectrosc. 89, 460–464 (1981). 28. H. S. P. Mu¨ller and M. C. L. Gerry, J. Chem. Soc. Faraday Trans. 90, 2601–2610 (1994). 29. (a) K. Kuchitsu, J. Chem. Phys. 49, 4456–4462 (1968); K. Kuchitsu, T. Fukuyama, and Y. Morino, J. Mol. Struct. (b) 1, 463–479 (1968); (c) 4, 41–50 (1969). 30. K. Kuchitsu and Y. Morino, Bull. Chem. Soc. Jpn. 38, 805–813 (1965). 31. H. Bu¨rger, P. Schulz, E. Jacob, and M. Fa¨hnle, Z. Naturforsch. A 41, 1015–1020 (1986), and references therein. 32. J. B. Burkholder, P. D. Hammer, C. J. Howard, A. G. Maki, G. Thompson, and C. Chackerian, Jr., J. Mol. Spectrosc. 124, 139–161 (1987). 33. A. G. Robiette, in ‘‘Molecular Structure by Diffraction Methods,’’ Chemical Society Specialist Peridical Reports, Vol. I, The Chemical Society, London, 1973. 34. R. E. Willis, Jr., and W. W. Clark III, J. Chem. Phys. 72, 4946–4950 (1980). 35. L. Andrews, F. K. Chi, and A. Arkell, J. Am. Chem. Soc. 96, 1997– 2000 (1974). 36. A. G. Robiette, C. R. Parent, and M. C. L. Gerry, J. Mol. Spectrosc. 86, 455–464 (1981). 37. K. O. Christe and H. Oberhammer, Inorg. Chem. (a) 20, 297–299 (1981); (b) 21, 273–275 (1982), and references therein. 38. J. D. Witt and R. M. Hammaker, J. Chem. Phys. 58, 303–313 (1973), and references therein. 39. B. Casper, H.-G. Mack, H. S. P. Mu¨ller, H. Willner, and H. Oberhammer, J. Phys. Chem. 98, 8339–8342 (1994). 40. J. Ortigoso, R. Escribano, J. B. Burkholder, and W. J. Lafferty, J. Mol. Spectrosc. 155, 25–43 (1992), and references therein.

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132

¨ LLER AND GERRY MU

41. K. Miyazaki, M. Tanoura, K. Tanaka, and T. Tanaka, J. Mol. Spectrosc. 116, 435–449 (1986). 42. (a) E. A. Robinson, Can. J. Chem. 41, 3021–3033 (1963); (b) A. J. Downs and C. J. Adams, ‘‘The Chemistry of Chlorine, Bromine, and Astatine,’’ Pergamon, Oxford, 1975. 43. H. Grothe and H. Willner, Angew. Chem. 106, 1581–1583 (1994); Angew. Chem. Int. Ed. Engl. 33, 1482–1484 (1994). 44. H. S. P. Mu¨ller and H. Willner, J. Phys. Chem. 97, 10589–10598 (1993). 45. (a) H. G. Mack, D. Christen, and H. Oberhammer, J. Mol. Struct. 190, 215–226 (1988); (b) D. Christen, O. D. Gupta, J. Kadel, R. L. Kirchmeier, H. G. Mack, H. Oberhammer, and J. M. Shreeve, J. Am. Chem. Soc. 113, 9131–9135 (1991); (c) P. Politzer and D. Habibollahzadeh, J. Chem. Phys. 98, 7659–7660 (1993). 46. L. Hedberg and I. M. Mills, J. Mol. Spectrosc. 160, 117–142 (1993). 47. J. Aldous and I. M. Mills, Spectrochim. Acta 18, 1073–1091 (1962). 48. H. S. P. Mu¨ller, Ph.D. Thesis, University of Hanover, Germany, 1992. 49. F. Meguellati, G. Graner, G. Pawelke, H. Bu¨rger, and P. Pracna, in preparation. 50. J. J. Oh, M. Birk, R. R. Friedl, and E. A. Cohen, in preparation. 51. W. H. Flygare, J. Chem. Phys. 41, 793–800 (1964). 52. W. H. Flygare and J. Goodisman, J. Chem. Phys. 49, 3122–3125 (1972).

53. T. D. Gierke and W. H. Flygare, J. Am. Chem. Soc. 94, 7277–7283 (1972). 54. A. A. Maryott, T. C. Farrar, and M. S. Malmberg, J. Chem. Phys. 54, 64–71 (1971). 55. S. K. Garg, J. A. Ripmeester, and D. W. Davidson, J. Chem. Phys. 72, 567–572 (1980). 56. (a) I. Ozier, L. M. Crapo, J. W. Cederberg, and N. F. Ramsey, Phys. Rev. Lett. 13, 482–484 (1964); (b) I. Ozier, Ph.D. Thesis, Harvard University, Cambridge, MA, 1965. 57. S. Brownstein, Can. J. Chem. 38, 1597–1599 (1960). 58. H. Agahigian, A. P. Gray, and G. D. Vickers, Can. J. Chem. 40, 157– 158 (1962). 59. D. K. Hindermann and C. D. Cornwell, J. Chem. Phys. 48, 4148–4154 (1968). 60. J. Mason, J. Chem. Soc. Dalton Trans., 1426–1431 (1975), and references therein. 61. C. D. Cornwell, J. Chem. Phys. 44, 874–880 (1966). 62. E.g., J. S. Muenter, in ‘‘Molecular Structure and Properties’’ (A. D. Buckingham, Ed.), MTP International Review of Science, Physical Chemistry, Series II, Vol. 2, Butterworths, London, 1975. 63. J. Bacon, R. J. Gillespie, and J. W. Quail, Can. J. Chem. 41, 3063– 3069 (1963).

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