Microwave Rotational Spectra Research Articles

The microwave spectrum, structure, and harmonic force field of the van der Waals trimer Ar2–OCS

The microwave rotational spectra of the van der Waals trimers Ar2–OCS, Ar2–OC34S, and Ar2–18OCS have been observed between 7.0 and 18.0 GHz using pulsed molecular beam microwave Fourier transform (MWFT) spectroscopy. Sixty-five transitions, both a and c type, have been assigned for Ar2–OCS, as well as 37 transitions and 19 transitions for Ar2–OC34S and Ar2–18OCS, respectively. The rotational constants and quartic centrifugal distortion constants have been determined. The trimer has been found to be a distorted tetrahedrally shaped highly asymmetric top (κ=∼0.377) with the OCS subunit lying in the Cs symmetry plane. An effective ground state structure has been determined. The Ar–Ar distance and the two ‘‘Ar–center of mass of OCS’’ distances are slightly longer than those in the corresponding dimers Ar2 and Ar–OCS, respectively. A harmonic force field analysis has been performed on the basis of the centrifugal distortion constants, to predict the van der Waals force constants and the corresponding vibrational frequencies. The Ar–Ar stretch force constant of the trimer is 0.008 42 mdyn Å−1, which compares favorably with 0.0078 mdyn Å−1 reported for the free dimer Ar2. A comparison of the geometry and spectroscopic properties of Ar2–OCS with those of Ar2 and of the Ar2–HX (X=F, Cl, CN) trimers is given.

Pulsed Molecular Beam Microwave Fourier Transform Spectroscopy of the van der Waals Trimer Ar2-CO2

The microwave rotational spectrum of the van der Waals trimer Ar2-CO2 has been observed between 7 and 18 GHz using a Balle-Flygare type pulsed molecular beam microwave Fourier transform (MWFT) spectrometer. Twenty-one transitions, all b-type, Ka + Kc = even, have been assigned. The rotational constants and centrifugal distortion constants have been determined. The trimer has been found to be a C2v distorted tetrahedral shaped highly asymmetric top (κ = ∼0.360). The effective Ar-Ar distance and the two Ar-C distances have been evaluated and have been found to be similar to the corresponding values reported for the Ar2 dimer and for Ar-CO2, indicating that pairwise additivity is the dominating influence in the trimer bondings. A harmonic force field analysis has been performed on the basis of the centrifugal distortion constants. The estimated Ar-Ar stretch frequency of Ar2-CO2 is 23.5 cm−1, which compares favorably with 25.7 cm−1 reported for the Ar2 dimer. A comparison of the structures and the van der Waals force constants of Ar2-CO2 with other reported Ar2-small molecule trimers is given.

The molecular structure of cyclopropylgermane from the rotational spectra of 41 isotopomers

The microwave rotational spectra of 41 isotopomers of cyclopropylgermane, C 3H 5GeH 3, many of them multiply substituted, have been measured, mainly by mw-mw double resonance. The ground state rotational constants were determined by least-squares fits to the spectra with fourth order centrifugal distortion included. A number of methods, some of them of recent origin, have been employed to determine the molecular structure of cyclopropylgermane from the large collection of substitution data. The results are summarized and compared: structures of the types r 0, r 1, ϵ = r Δ1, r B, α = r Δ B , r s, r s-fit, and r m ρ . A detailed structure, which was obtained with no assumptions other than molecular C s symmetry, is reported. Significant evidence for the CC bond length disparity, the germyl group tilt and deformation, and the methylene group deformation is given. The endo GeH bond and the two neighboring CH bonds are longer and more inclined towards each other than they would be without the departure from local bonding symmetry. This is presumably due to the interaction between the cyclopropyl ring and the substituent. The molecular electric dipole moment, essentially directed along the CGe bond, has been obtained from Stark effect measurements of the parent isotopomer, | μ a | = 0.684(7) D, μ b = 0, | μ c | = 0.261(8) D, |μ| = 0.732(9) D. From several doublet transitions of the first excited state of germyl torsion of the parent isotopomer, the internal rotation hindering potential has been calculated as V 3 = 5.58(20) kJ/mole.

Microwave and submillimeter-wave rotational spectrum of methyl alcohol in the ground torsional state

The frequencies of 440 transitions of the spectrum of the ground torsional-vibrational state of methyl alcohol have been measured in the microwave and submillimeter regions, 130 of these belonging to the main isotopomer CH 3OH ( J ≤ 42, K ≤ 8), 190 to the isotopomer CD 3OH ( J ≤ 37, K ≤ 10), and 120 to CD 3OD ( J ≤ 27, K ≤ 5). For the first time, all the transitions were identified on the basis of directly calculated frequencies. These results were obtained through the use of a new effective torsional-rotational Hamiltonian. Its parameters were calculated based on the frequencies of all earlier known and new transitions of microwave and submillimeter-wave spectra of the ground torsional-vibrational state of methanol. The root-mean-square deviation is 710 kHz (800 lines) for the main isotopomer, 144 kHz (370 lines) for CD 3OH, and 316 kHz (150 lines) for CD 3OD.

J dependence of χa(14N) and χa(83Kr) for the Kr–HCN dimer

High-resolution microwave rotational spectra for 84Kr–HCN, 86Kr–HCN, and 83Kr–HC15N have been observed with the pulsed-nozzle, Fourier transform Balle/Flygare Mark II spectrometer. A new method of injecting the gas sample into the Fabry–Perot cavity along the axis of the microwave pulse was used for some transitions to narrow the linewidths. The present work extends that of the original study [J. Chem. Phys. 78, 3483 (1983)] over a wider frequency range (2–18 GHz) and with higher resolution. The 14N nuclear quadrupole coupling constant has been found to increase linearly with J(J+1) for 84Kr–HCN and 86Kr–HCN, with the slope Dχ one-third its value for the analogous Ar–HCN dimer. For 84Kr–HCN, the average HCN bending amplitude θ decreases from 26.85° for J=0 to 26.28° for J=7, while the average Kr to HCN center-of-mass (c.m.) distance R increases from 4.5202 to 4.5246 Å. Similar results are found for 86Kr–HCN. In addition, the 83Kr quadrupole coupling constant for 83Kr–HC15N is dependent on J, increasing from 7.5382 MHz for J=1 to 7.5713 MHz for J=4. This is interpreted with the long-range polarization model used previously to explain rare gas nuclear quadrupole coupling constants in Rg–HX dimers. In particular, the J dependence observed for χa(83Kr) is consistent with the J dependencies of θ and R inferred from χa(14N) in the 14N species. Radial and angular motions of HCN are strongly coupled.

The microwave spectrum of the dihydronitrosyl radical, H2NO (2B1)

The microwave rotational spectrum of the H2NO radical in the X̃ 2B1 ground vibronic state was observed by using a source-modulation spectrometer and a 1 m long free-space cell. The H2NO radical was generated in the cell by a reaction of hydroxylamine (NH2OH) with the products of 2450 MHz discharge in CF4. The spectral lines of H2NO were observed for the a-type R-branch transitions of N=1←0 to N=6←5, most of which showed resolved hyperfine structures due to the hydrogen and nitrogen nuclei. The 19 molecular constants including the hyperfine coupling constants of both the nuclei were precisely determined by the least-squares fit of 129 observed lines. The observed hyperfine structure shows that the H2NO radical in the ground electronic state is essentially planar with C2v symmetry. The nitrogen hyperfine coupling constants determined indicate that an unpaired electron occupies the pπ orbital perpendicular to the molecular plane, and the electronic wave function in the ground electronic state belongs to 2B1 symmetry. However, the small but negative inertial defect (Δ=−0.0051 uÅ2 ) suggests a possibility that H2NO may have a double-minimum potential function for the out-of-plane vibration.

Microwave spectrum of ortho-chlorotoluene. Rotational constants and nuclear quadrupole hyperfine structure

The microwave rotational spectra of orthochlorotoluene, C 6H 4CH 3Cl, have been measured in the frequency region 8–40 GHz. Spectra due to both isotopic species 35Cl and 37Cl have been observed. The hyperfine structure due to the chlorine nuclear quadrupole interaction in both isotopic species has been studied. Analysis of the spectra yields rotational and nuclear quadrupole coupling constants for both isotopic species. No splitting which could be attributed to the internal rotation of the methyl group was observed.

Rotational spectra and structures of small clusters containing the HCN dimer: (HCN)2–Y with Y=HF, HCl, HCF3, and CO2

Microwave rotational spectra have been observed for a number of isotopic species of the (HCN)2–HF, –HCl, –HCF3, and –CO2 trimers. The observations were made with the pulsed nozzle, Fourier transform, Flygare/Balle Mark II spectrometer. The trimers have structures which are composites of the linear (HCN)2 dimer and the HCN–Y dimers, the latter linear for Y=HF and HCl, a symmetric top for Y=HCF3, and T-shaped with C2v symmetry for Y=CO2. The rotational constants for the most abundant species of each trimer are as follows: For Y=HF and HCl, B0 is 699.204 and 467.408 MHz, respectively, and DJ is 162 and 87 Hz; for Y=HCF3, B0 is 305.742 MHz and DJ and DJK are 51 and 471 Hz; for Y=CO2, treated as a symmetric top, (B0+C0)/2 is 452.426 MHz and DJ is 1.057 kHz. Hyperfine interaction constants were determined for several species. The B0’s for each trimer were analyzed by a combination of isotopic substitution and fitting procedures to determine the distances r1 and r2 between the centers of mass (c.m.) of adjacent monomers. The B0’s are relatively insensitive to the position of the central HCN but give r1+r2 accurately. With this limitation, r1 and r2 in the trimers are compared with the corresponding distances in the dimers, which are longer. For Y=HF, HCl, HCF3, and CO2, respectively, the shrinkages found in r1 are 0.069, 0.054, 0.030, and 0.004 Å and in r2, 0.043, 0.062, 0.042, and 0.052 Å. The shrinkage in r1 and several other properties of the trimers exhibit some correlation with the pseudodiatomic stretching force constant in the HCN–Y dimer.

Rotational spectrum and structure of the linear CO2–HCN dimer: Dependence of isomer formation on carrier gas

A linear hydrogen-bonded dimer, OCO–HCN, has been identified and characterized via its microwave rotational spectrum. The study was made using the pulsed nozzle Fourier transform method with the Flygare/Balle Mark II spectrometer. A T-shaped HCN–CO2 dimer was reported earlier by the Klemperer group. Rotational constants have been determined for all seven monoisotopically substituted species of the linear form. B0 , DJ , and χaa (14 N) for the normal isotopic dimer are 1057.9397(2) MHz, 1.372(8) kHz, and −4.2466(5) MHz, respectively. The average torsional displacements of the OCO and HCN monomers about their center of mass (c.m.) are found to be 7.66° and 12.40°, based on the substitution O–C and C–N bond distances for the dimer. With these values for α and γ, the B0 for the normal isotopic dimer corresponds to a c.m. to c.m. distance R=5.035 Å. Bending and stretching force constants and the well depth (ε∼590 cm−1 ) are estimated from the centrifugal distortion. The relative concentrations of the linear and T-shaped isomers are unusually sensitive to the carrier gas used in the supersonic jet expansion. The linear form could not be detected at all with argon as the carrier gas but gave a strong signal in neon first run (70% Ne, 30% He). In contrast, the T form gave strong signals in both carrier gases. However, a carrier-gas effect was not found for the N2 O/HF dimer pair, which has a high barrier between the bent NNO–HF and linear FH–NNO isomers. Similar results were obtained for chlorocyclohexane (CCH) and ethyl formate (EF), which have two conformational isomers. In CCH which has a high barrier to a↔e interconversion, the two conformers gave strong signals in both Ar and He. In EF, with a low barrier, the gauche conformer could not be detected in Ar but gave a strong signal in He, while the trans form gave strong signals in both carrier gases.

Microwave rotational spectrum and properties of a hydrogen-bonded dimer formed by trimethylamine and acetylene

The rotational spectra of the four isotopic species (CH 3) 3 14N⋯HCCH, (CH 3) 3 14N⋯DCCH, (CH 3) 3 14N⋯HCCD and (CH 3) 3 14N⋯DCCD of a hydrogen-bonded dimer formed between trimethylamine and acetylene have been observed by pulsed-nozzle, Fourier-transform microwave spectroscopy. The ground-state spectroscopic constants determined for these symmetric-top molecules are: Isotopic species B 0 (MHz) D J (kHz) D JK (kHz) H JK (Hz) H KJ (Hz) χ( 14N) (MHz) (CH 3) 3 14N⋯HCCH 1293.2561(3) 1.425(5) 964.6(1) 156(2) −347(18) −4.918(3) (CH 3) 3 14N⋯DCCH 1269.5182(2) 1.401(4) 958.4(2) 157(2) −257(31) −4.896(14) (CH 3) 3 14N⋯HCCD 1228.0614(4) 1.292(6) 933.1(3) 153(3) −184(53) −4.856(23) (CH 3) 3 14N⋯DCCD 1225.1186(4) 1.264(7) 929.9(3) 157(3) −379(55) −4.923(24) For (CH 3) 14N⋯HCCH, it has been established that the 14N-nuclear quadrupole coupling constant varies linearly with K 2 according to χ( K)=χ( K=0)+(6.6×10 −3) K 2 MHz. The following molecular properties have been determined for the dimer from the spectroscopic constants: r(N⋯C)=3.254(4) Å, θ av=15.43(9)° and k σ=5.82(2) N m −1.

The microwave rotational spectrum of X 2Σ LaO

The microwave absorption spectrum of LaO has been observed in the frequency range between 84 and 275 GHz. A precise set of Dunham coefficients for the X 2Σ + ground state has been determined. Fine and hyperfine constants were found to be in agreement with previous data. An upper limit for the electric dipole moment of μ < 2 D can be estimated from the saturation of the transitions.

Rotational spectrum and structure of the linear HCN trimer

Microwave rotational spectra have been observed for 22 isotopic species of an HCN, hydrogen-bonded trimer with the pulsed nozzle, Fourier transform method using the Flygare/Balle Mark II spectrometer. The 14N nuclear quadrupole hyperfine structure was analyzed and the interaction constants and line centers determined. The line centers were fitted to obtain ground vibronic state rotational constants. For the normal isotopic species of (HCN)3, B0 was found to be 469.3073(1) MHz and DJ, 82.6(1) Hz. The quadrupole coupling constants χ(n) are −4.049(2), −4.251(2), and −4.375(1)MHz for n=1, 2, and 3, respectively, in HCN(1)HCN(2)HCN(3). The trimer has a linear or very near linear equilibrium structure. The B0’s are insensitive to the position and torsional oscillations of the central HCN but they determine the outer HCNs quite accurately. An isotopic substitution method gives R, the c.m. distance between the outer HCN’s, to be 8.790 Å in the 14–14–14 species. A slightly smaller value 8.788 Å is obtained from a fit of the B0’s which includes the effects of isotopic substitution on the H/D–C and C–N bond lengths in the monomer. The distance between c.m.(1) and c.m.(2) may be somewhat shorter (∼0.01 Å) than that between c.m.(2) and c.m.(3). The average c.m. separation in the trimer (4.395 Å) is halfway between that of the dimer (4.447 Å) which is known to be linear and the infinite linear chains in the solid (4.34 Å). It is shown that the smaller quadrupole coupling constants in the trimer compared to the monomer are caused by a combination of torsional and charge redistribution effects. The torsional vibrations have average angular displacements of 12.60° and 8.55° for HCN(1) and HCN(3). The charge redistribution effect upon χ in the trimer (6.5%) is about 1.8 times that in the HCN dimer. The dipole moment of the trimer was found to be 10.6 D, a value enhanced by 1.8 D over the vector sum of the three vibrating monomers (8.83 D), an enhancement about 1.7 times that of the dimer.

Rotational spectra and structures of small clusters: The Ar4–H/DF pentamers

The 40Ar4–HF and –DF clusters have been identified and characterized by their microwave rotational spectra using a Flygare–Balle FT spectrometer with a pulsed supersonic nozzle to generate and detect the clusters. The observed J=2→3 to 6→7 transitions have fine structure limited to K=0, ±3 and ±6, indicative of symmetric tops with a threefold axis of symmetry produced by I=0 nuclei. For Ar4–HF the rotational constants B0, DJ, and DJK are 623.4539(1) MHz, 1.062(1) kHz, and 0.611(31) kHz, respectively; and for Ar4–DF, 618.5846(1) MHz, 1.030(1) kHz, and 0.665(4) kHz. The results show that the clusters consist of the trigonal Ar3–H/DF tetramer reported earlier, with a fourth argon on the back side of the Ar3 group to form a tetrahedral or near tetrahedral Ar4. In both clusters the H/DF lies along the threefold axis of the Ar3 group, with the H/D end pointed at its face. If one assumes an Ar–Ar distance of 3.85 Å for both clusters, the Ar3 plane to HF c.m. distance for Ar4–HF with a tetrahedral Ar4 is 2.713 Å compared to 2.735 Å for Ar3–HF. The nuclear hyperfine structure of the rotational transitions shows that the H/DF experiences large amplitude bending vibrations with respect to the C3 symmetry axis. The average amplitude for HF (40.4°) is comparable with that reported for the smaller Arm–HF complexes (m=1, 2, 3). However, for DF the apparent 25.1 (1.6)° amplitude found is 10° less than that in the smaller complexes. The difference is attributed to an increased electric field gradient at the D in the pentamer. A possibly related feature is an Ar4 c.m. to F distance which is 0.013 Å longer for Ar4–DF than for Ar4–HF.

Rotational spectra and structures of small clusters containing the HCN dimer: (HCN)2–Ar, a T-shaped trimer

Microwave rotational spectra have been observed for the four 14N/15N isotopic species of an (HCN)2–Ar trimer with the pulsed nozzle Fourier transform method using the Flygare Mark II spectrometer. Thirteen J→J′ asymmetric top transitions were measured in the 2.5 to 10 GHz region for the parent 14N,14N trimer and nine for each of the other three species. The 14N nuclear quadrupole hyperfine structure was analyzed for the transitions and the interaction constants and line centers determined. The line centers were fitted to obtain ground vibronic state rotational and quartic centrifugal distortion constants. For the 14N/14N trimer these are (in MHz) for A″, B″, and C″: 2013.5993(10), 1759.2756(3), 932.3709(2); and for τ1, τ2, τaaaa, τbbbb, and τcccc: −0.3017(1), −0.0660(1), −0.1700(9), −0.013 71(1), and −0.0088(1). The inertial defect is 3.7881 amu Å2. The zero-point vibrationally averaged geometry is planar and T shaped with the structure of the linear HCN dimer remarkably unperturbed by presence of the Ar atom. The substitution N⋅⋅⋅N distance in the cluster is 4.453 Å compared with 4.461 Å in the free dimer. The argon is attracted by the π electrons of the two –C≡N groups, being equidistant from them. The Ar⋅⋅⋅N(1) and Ar⋅⋅⋅N(2) distances are 3.671 and 4.316 Å, respectively.

Rotational spectrum and structure of the Ar3–HCl symmetric top

The Ar3–HCl tetramer has been identified via the observation of its microwave rotational spectrum using the pulsed nozzle Fourier transform technique. The rotational spectroscopic constants of the ground vibrational state have been obtained for the 35Cl/37Cl isotopes from measurements of the J=1→2 to J=4→5 transitions and their hyperfine structure. For Ar3–H35Cl, the values found for B0, DJ, DJK, and χaa are 843.8974(1) MHz, 2.333(2) and 1.818(6) kHz, and 31.006(5) MHz, respectively, and for Ar3–H37Cl, 825.9811(2) MHz, 2.211(5) and 1.940(12) kHz, and 24.481(8) MHz. The cluster is a symmetric top with the HCl along the C3 axis, its H end directed towards the Ar3 trimer. In its torsional oscillations, the HCl makes an average angle of 36.91° with the symmetry axis. The chlorine isotopic substitution leads to values of 3.8510 and 4.0024 Å for the Ar–Ar and Ar–Cl distances. The results of the present study are compared with those reported earlier for the Arm–HF (m=1,2,3) and Arm–HCl (m=1,2) clusters.

Spectroscopic ramifications due to cancellation effects in special degenerate electronic states

Various cancellation effects occurring in special kinds of spatially degenerate electronic states are discussed. The ramifications of the cancellation effects on the IR and Raman vibrational spectra, microwave rotational spectra, and electronic spectra are elucidated.

Rotational spectra and structures of the Ar2–H35Cl/37Cl trimers

Microwave rotational spectra have been observed for both Cl isotopes of the Ar2–HCl trimer with the pulsed nozzle Fourier transform method using the Flygare Mark II spectrometer. The Cl nuclear quadrupole hyperfine structure was analyzed for each of the transitions and the coupling constants and line centers determined. Sixteen transitions were observed in the 2 to 15 GHz region for the 35Cl species and 11 for 37Cl. The line centers were fitted to obtain ground state rotational and quartic centrifugal distortion constants A″, B″, C″, τ1, τ2, τaaaa, τbbbb, and τcccc. For Ar2–H35Cl, the values are 1733.857, 1667.932, 844.491, −0.1170, −0.0292, −0.1199, −0.0802, and −0.0079 MHz, respectively, and for Ar2–H37Cl: 1733.824, 1606.877, 828.497, −0.1121, −0.0279, −0.1205, −0.0737, and −0.0075 MHz. The equilibrium geometry is determined to be T shaped with C2v symmetry and the H end of the HCl closest to the Ar2. Large amplitude slightly anisotropic torsional motion of the HCl is evident from the hyperfine constants. The chlorine isotopic substitution enables the torsional displacement of the Ar2 dimer to be estimated. It is found to be an average of 7.3° with respect to the b axis. With allowance for the torsional effects, a value of 3.861 Å is obtained for the Ar–Ar distance and 4.005 Å for the Ar to Cl distance. An approximate, harmonic force field analysis is based on the centrifugal distortion constants. A comparison is given of the force constants and Ar–Ar distances for the Ar2–HCl and Ar2–H/DF trimers.

ChemInform Abstract: The Rotational Spectrum of Acetone: Deuterated Species, Internal Rotation, and Structure

Abstract The microwave and millimeter-wave rotational spectra of CH 3 COCH 2 D, CD 3 COCD 2 H, and CD 3 COCD 3 have been measured. For these three isotopic species the rotational constants, the centrifugal distortion constants and the three internal rotation parameters (barrier, methyl top moment of inertia, and angle between the internal rotation axis and the a inertial axis) have been accurately determined using two computer programs based on the Internal Axis Method. The decrease of the potential barrier upon deuteration is evident. The substitution coordinates of the hydrogen atoms could also be determined. It is confirmed that the in-plane methyl hydrogen has a position cis with respect to the oxygen atom.

Rotational spectra and structures of the Ar2–H/DF trimers

The feasibility of studying weakly bound, neutral van der Waals clusters through their microwave rotational spectra has been shown by identifying and characterizing Ar2–H/DF. Detection of the trimeric complexes was accomplished with the Flygare Mark II Fourier transform spectrometer, modified to better utilize its inherent sensitivity. Twenty-six b-dipole transitions (all ee↔oo) were observed in the 3–18 GHz region for Ar2–HF, and 19 for Ar2–DF, and their hyperfine structure analyzed. Fitting of the line centers with Kirchhoff’s NBS program gave values (all in MHz) for Watson’s determinable parameters of 3576.508(1), 1739.139(1), and 1161.054(1) for the rotational constants A″, B″, and C″ in Ar2–HF, and of 3506.791(1), 1744.056(1), and 1155.636(1) in Ar2–DF. The centrifugal distortion constants τ1, τ2, τaaaa, τbbbb, and τcccc were found to be −0.2039(3), −0.0522(1), −0.5353(5), −0.1159(1), and −0.02021(1) in Ar2–HF, and −0.2116(7), −0.0528(2), −0.4941(7), −0.1123(1), and −0.0191(1) in Ar2–DF. Three small sixth order terms were necessary to fit the data within experimental error. The trimer is found to be a planar T-shaped, highly asymmetric top (κ=−0.521) with the equilibrium position of the H/DF on the C2v figure axis (the b axis), the H/D end pointing toward the two argon atoms. The structure can be viewed as a composite of three dimers, the Ar2 dimer and two Ar–H/DF’s. In the trimer the Ar–Ar and the two Ar–H/DF distances are a bit longer (0.04 and 0.01 Å) than in the dimers, suggesting the presence of second-order effects. This picture is supported by an approximate, pseudotriatomic analysis of the force field, based on the four independent planar centrifugal distortion constants. The cluster is quite ‘‘floppy.’’ The hyperfine structure shows that torsional oscillations of the H/DF are comparable with those in the Ar–H/DF dimers, but anisotropic, being 3° larger in-plane than out-of-plane. The in-plane torsional oscillation of the Ar2 is considered.

The rotational spectrum of acetone: Deuterated species, internal rotation, and structure

The microwave and millimeter-wave rotational spectra of CH 3COCH 2D, CD 3COCD 2H, and CD 3COCD 3 have been measured. For these three isotopic species the rotational constants, the centrifugal distortion constants and the three internal rotation parameters (barrier, methyl top moment of inertia, and angle between the internal rotation axis and the a inertial axis) have been accurately determined using two computer programs based on the Internal Axis Method. The decrease of the potential barrier upon deuteration is evident. The substitution coordinates of the hydrogen atoms could also be determined. It is confirmed that the in-plane methyl hydrogen has a position cis with respect to the oxygen atom.