The Centrifugally Induced Pure Rotational Spectrum and the Structure of Sulfur Trioxide. A Microwave Fourier Transform Study of a Nonpolar Molecule

Abstract

The pure rotational spectrum of sulfur trioxide has been observed for the first time. A total of 25 high-J transitions could be assigned. The rotational constants, two quartic centrifugal distortion constants, and three sextic centrifugal distortion constants were determined as: B= 10 449.0667(23) MHz, C = 5216.0330(12) MHz, DJ = 9.2651 (18) kHz, DJK = -16.3922(18) kHz, HJ, = -8.8(34) • 10-3 Hz, HJK= -15.8(73) • 10-3 Hz, and HKJ = 34.2(73) • 10-3 Hz. An r0- and an re -structure are presented: r0= 1.4198(7) Å (calculated from B), r0 = 1.4210(7) Å (calculated from C), and re = 1.4175 Å

The ν3 and 2ν3 bands of 32S16O3, 32S18O3, 34S16O3, and 34S18O3

“…We note that the C-rotational constant for the ground state has been experimentally determined only for 32 S 16 O 3 [10], while those for the other isotopomers are based on the use of the planarity condition, I C ¼ 2I B minus the inertial defect observed for 32 S 16 O 3 and scaled by the magnitude of B. The experimental moments of inertia of 32 S 16 O 3 in the ground state are I C ¼ 1:608 888 26 Â 10 À45 kg m 2 and I B ¼ 0:803 135 32 Â 10 À45 kg m 2 so that the inertial defect D ¼ I C À 2I B ¼ 0:002 617 62 Â 10 À45 kg m 2 ¼ 0.157 636 u A A 2 , a value which is in excellent agreement with the value D 0 ¼ 0.1581 u A A 2 calculated by Watson [37] from an empirical harmonic potential [38] with Coriolis contribution.…”

“…DD JK  10 10 (9) 6.44 (11) 5.45 (5) 5.06 (8) g K  10 8 )1.87 (6) )2.02 (13) )1.04 (9) )1.27 (12) g K  10 8 )1.62 (9) )1.93 (12) )1.10 )1.11 (10) q  10 4 )1.361 (2) )1.417 (3) )1.046 (3) )1.094(1) Q  10 5 5.35 (2) 5.53 (2) 3.95 (2) 4.04 (1) q J  10 9 )2.59 (5) )2.32 (17) )2.07 (14) )1.90 ( (32) )1.242 (5) )0.985 (10) )1.006 (16) q J  10 9 The H constants of the ground state were assumed. c From [27], but modified to be consistent with the analysis of the data for the other isotopomers.…”

“…The resulting constants, however, represent averages over the ground and low lying, thermally populated vibrational states. Even though the absence of a permanent electric dipole moment precludes the appearance of a microwave pure rotation spectrum, the rotation of a molecule with D 3h symmetry nevertheless generates a centrifugal distortion-induced dipole moment in the moleculeÕs plane and a pure rotational microwave spectrum due to such a moment was indeed observed for 32 S 16 O 3 [10]. Such a spectrum allows not only the usual determination of the rotation constant B 0 , but also, due to a different selection rule (DK ¼ AE3), a direct determination of the rotation constant with respect to the threefold symmetry axis, C 0 .…”

The HITRAN 2004 molecular spectroscopic database

“…This entry was based on the experimental data of Maki and co-workers [499][500][501][502] with relative transition intensities scaled by the absolute values computed ab initio by Underwood et al [503] . Also included were 25 pure-rotation transitions observed by Meyer et al [504] for which the ab initio intensities of Underwood et al [503] were used. SO 3 is a planar, non-polar molecule but with sufficient rotational excitation that the molecule can distort and undergo pure-rotational transitions.…”

On the determination of C0 (or A0), , and some dark states for symmetric-top molecules from infrared spectra without the need for localized perturbations

“…All three molecules are ideal choices since, although all are nonpolar, the centrifugally-distorted molecules have a small dipole and sensitive microwave measurements have been made of weak DK = ±3 transitions for each. [12][13][14][15][16] These microwave measurements can be analyzed to yield accurate values of C 0 so that a comparison can be made with the results obtained here.…”