First High Resolution Spectroscopic Studies and Ab Initio Calculations

Feb 5, 2010 - The doublet structure observed in the rotational spectrum of synclinal conformer is attributed to tunneling motion of tellurol functiona...
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J. Phys. Chem. A 2010, 114, 2794–2798

First High Resolution Spectroscopic Studies and Ab Initio Calculations of Ethanetellurol Roman A. Motiyenko,*,† Laurent Margule`s,† Manuel Goubet,† Harald Møllendal,‡ Alexey Konovalov,‡ and Jean-Claude Guillemin§,| Laboratoire de Physique des Lasers, Atomes et Mole´cules, UMR CNRS 8523, UniVersite´ de Lille 1, F-59655 VilleneuVe d’Ascq, France, Centre for Theoretical and Computational Chemistry (CTCC), Department of Chemistry, UniVersity of Oslo, P.O. Box 1033 Blindern, NO-0315 Oslo, Norway, E´cole Nationale Supe´rieure de Chimie de Rennes, CNRS, UMR 6226, AVenue du Ge´ne´ral Leclerc, CS 50837, 35708 Rennes Cedex 7, France, and UniVersite´ europe´enne de Bretagne, Rennes, France ReceiVed: December 22, 2009; ReVised Manuscript ReceiVed: January 19, 2010

The millimeter-wave rotational spectrum of ethanetellurol has been recorded and assigned for the first time. The spectroscopic study has been complemented by high level ab initio calculations. Geometries, total electronic energies, and harmonic vibrational frequencies have been determined at the MP2 level. A small-core relativistic pseudopotential basis set (cc-pVTZ-PP) was employed to describe the tellurium atom. Two stable conformers, synclinal and antiperiplanar, have been identified. Both theory and experiment have shown the synclinal form to be more stable by 2 kJ/mol. The doublet structure observed in the rotational spectrum of synclinal conformer is attributed to tunneling motion of tellurol functional group. The energy difference between 0+ and 0- substates split by tunneling has been determined from the observed spectra. Introduction Few studies of the physical properties of tellurols have been reported in the past, presumably because conventional synthetic approaches were not always efficient.1 Their kinetic instability, toxicity, and repulsive smells are additional reasons for the sparse literature on these compounds. However, an effective synthetic procedure of tellurols, namely the reduction of ditellurides by tributyltin hydride (Bu3SnH), was reported recently.1 The use of diphenylditelluride as a catalyst in this reaction later proved to be beneficial.2,3 This new and effective synthesis of tellurols has made it possible to investigate the physical properties of this littleinvestigated class of compounds.2,3 The gas-phase acidities of several tellurols, including that of the title compound, ethanetellurol (CH3CH2TeH), were the themes of one of these investigations,2 which also included a quantum chemical study of such properties. In another recent study,3 photoelectron spectroscopy augmented by quantum chemical calculations were performed for several tellurols. In the present work, these investigations are extended to include the first study of the rotational spectrum of a tellurol, namely ethanetellurol, assisted by quantum chemical calculations. Ethanetellurol belongs to a class of compounds with the general formula CH3CH2XH, where X is a main-group 16 element (X ) O, S, Se, and Te). The rotational spectra of the other members of this group were reported as CH3CH2OH from the 1960s to very recently,4,5 CH3CH2SH6-11 in the 1960s and 1970s, and CH3CH2SeH the early 1980s.12,13 Two rotameric forms, where the C-C-X-H is antiperiplanar (ap; obsolete, trans), or synclinal (sc; obsolete, gauche), may exist for these compounds. This rotational isomerism is il* To whom correspondence should be addressed. Telephone: +33-320434943. Fax: +33-3-20337020. E-mail: [email protected]. † Universite´ de Lille 1. ‡ University of Oslo. § ENSCR. | Universite´ europe´enne de Bretagne.

Figure 1. The calculated structure of the sc (a) and ap (b) conformers of ethanetellurol.

lustrated in Figure 1 in the case of the title compound, where the two forms are abbreviated ap and sc, respectively. Precise experimental values for the energy difference between the antiperiplanar and synclinal forms are now known for

10.1021/jp912082b  2010 American Chemical Society Published on Web 02/05/2010

Rotational Spectrum of Ethanetellurol CH3CH2OH, where the antiperiplanar form is 0.5 kJ/mol more stable than the synclinal form,5 whereas the opposite conformational preference is found for the sulfur and selenium analogues. The synclinal rotamers are more stable than the antiperiplanar conformers in the cases of CH3CH2SH11 and CH3CH2SeH13 by 1.70(18) kJ/mol and 0.79 kJ/mol, respectively. A successful investigation of a delicate conformational equilibrium such as the one presented by ethanetellurol requires experimental methods possessing high resolution. Rotational spectroscopy meets this requirement because of its superior accuracy and resolution, making this method especially well suited for conformational studies of gaseous species. The spectroscopic work has been augmented by high-level quantum chemical calculations, which were conducted with the purpose of obtaining information for use in assigning the rotational spectrum and investigating properties of the potential energy hypersurface. To our knowledge, there is no published investigation of the potential energy surface of ethanetellurol. Its gas phase acidity has been previously studied, together with other sulfur, selenium, and tellurium containing compounds, using a G2(ECP) approach.2 Although the accuracy of this level of theory is certainly good enough to account for the thermochemical properties, calculations have been here performed at a higher level of theory, using larger basis sets and a smallcore relativistic pseudopotential. Indeed, more accurate values of the spectroscopic parameters are needed to help in interpreting the rotational spectra. Experimental Methods Caution. Ethanetellurol is malodorous and potentially toxic. All reactions and handling should be carried out in a wellventilated hood. Synthesis. The synthesis of this compound, which has already been reported,1 is repeated in the Supporting Information for the convenience of the reader. Stark Spectroscopy Experiment. The spectrum of ethanetellurol was studied in the 21.8-79 GHz frequency interval by Stark-modulation spectroscopy using the microwave spectrometer of the University of Oslo. Details of the construction and operation of this device have been given elsewhere.14,15 The spectrometer has a resolution of about 0.5 MHz and measures the frequency of isolated transitions with an estimated accuracy of about 0.1 MHz. The experiments were performed at a pressure of roughly 10 Pa with the cell cooled to about -30 °C by dry ice. The spectrum observed under these conditions was of intermediate to low intensity and quite dense. It is assumed that the spectral weakness was caused at least in part by the presence of considerable amounts of impurities. The high Ka aR-type transitions of this prolate rotor (κ ∼ -0.99) were first recorded and seen to be the strongest absorptions of the spectrum. The intensities of these peaks were checked from time to time and were found to diminish slowly with time, presumably because of reactions taking place in the cell. The cell therefore had to be filled with fresh sample every two hours. Conventional Absorption Spectroscopy Experiment. Initial experiments with the microwave spectrum of ethanetellurol have been carried out using the spectrometer in Lille. The rotational spectrum has been recorded in the frequency range 75-300 GHz. The accuracy of the frequency measurement for an isolated line is estimated to be better than 0.03 MHz. The spectrometer is built according to typical scheme, [source of radiation]-[absorption cell]-[detector]. As a source of radiation, we use two-step frequency multiplication system, based on Agilent E8752D

J. Phys. Chem. A, Vol. 114, No. 8, 2010 2795 synthesizer (12.5-17 GHz) whose frequency is multiplied by 6 and amplified in the frequency range 75-110 GHz (Spacek Laboratories Inc. active sextupler) at the first stage and additionally multiplied by 2, 3, or 5 (Virginia Diodes Inc. multipliers) at the second. The emitted microwave signal passes through the absorption cell and is detected by an InSb liquid He-cooled bolometer. A 1.2 m Pyrex glass tube with Teflon windows is used as an absorption cell. During the experiments the cell was kept at room temperature. Therefore, to minimize the observation of spectra of decomposition products, the sample of ethanetellurol was continuously injected via a side opening at one end of the cell and pumped out via another side opening at the other end. The sample was evaporated at -70 to -80 °C. This temperature was found to be optimal to provide enough vapor pressure in the cell (about 1 Pa) and at the same time to avoid fast decomposition of the sample. The spectra recorded under these conditions seemed to contain no significant impurities, since all the strongest features observed were assigned to ethanetellurol. Computational Methods All the calculations of this study were performed using the Gaussian03 software package.16 The geometries were fully optimized and the frequencies were calculated at the MP2 level. All electrons were included in the correlation calculation (MP2(full)). In the case of heavy atoms like tellurium, inner core electrons are very close to the nucleus, so that their velocity is close to the speed of light. Therefore, relativistic effects are taken into account. For this reason, the small-core relativistic pseudopotential basis set (cc-pVTZ-PP) was employed to describe the tellurium atom.17 Consequently, the cc-pCVTZ basis set with extra core/valence functions was used for the carbon atoms.18,19 The standard correlation-consistent polarized triple valence basis set (cc-pVTZ) of Dunning and co-workers18,19 was employed to describe the electrons of the hydrogen atoms. Basis sets were obtained from the EMSL basis set library.20,21 The combination of MP2 level of theory with correlation-consistent polarized triple valence basis sets type was chosen because it is well-known to offer the best compromise between calculation time and accuracy for analogous molecules.22,23 Finally, the transition states existing between the stable conformations were characterized using the QST2 procedure, as implemented in the Gaussian03 software package. Geometry optimizations led to two stable conformations denoted hereafter ap (a planar skeleton with a C-C-Te-H dihedral angle of 180°) and sc (with a C-C-Te-H dihedral angle of 63° from syn). The stable conformers are displayed in figure 1 and geometrical parameters are given in Table S1 in the Supporting Information. Calculations with other starting point geometries have been performed, for example, for the syn geometry. In each case, geometry optimizations converged to one of the two aforementioned conformations, showing that these are the only stable forms of ethanetellurol. The transition states along the tellurol internal rotation coordinate (C-Te bond torsion) have also been characterized. Two saddle points, exhibiting only one imaginary frequency, have been found, one with a syn conformation (C-C-Te-H dihedral angle of 0°, denoted hereafter syn-TS) and another one with an anticlinal conformation (C-C-Te-H dihedral angle of 128°, denoted hereafter ac-TS). It is worth noting that the sc and ac-TS forms have two equivalent conformations with opposite sign of the C-C-Te-H dihedral angle. In addition, the barrier to internal rotation of the methyl group (C-C bond torsion) have been examined for the sc conformer (denoted hereafter rot-TS). The

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TABLE 1: Relative Energies with Respect to the sc Conformer ∆Esca

a

conformation

∆Esc/kJ.mol-1

sc ap syn-TS ac-TS rot-TS

0.00 2.01 (2.18) 4.49 (5.23) 4.46 (4.70) 14.98 (15.43)

The ZPE corrected energies are reported in parentheses.

relative energies of each calculated conformation with respect to the sc conformer are listed in Table 1. For all forms, the total electronic energy has been ZPE corrected with subtraction of the frequency, in the stable conformers, corresponding to the imaginary one in the TS (i.e., the C-Te torsional mode). Calculated harmonic frequencies for the sc and ap conformers are listed in Table S2 in the Supporting Information. These results warrant comments. First, two stable conformers, ap and sc were expected because the torsion angles associated with the C-Te bond put the groups attached to these two adjacent atoms in a staggered conformation. Moreover, it agrees very well with analogous molecules containing atoms of the sixteenth column (ethanol, ethanethiol, and ethaneselenol). Second, in the present case, the sc conformer is calculated to be more stable than the ap by about 2 kJ/mol. This is also similar to the results for the analogous thiol and selenol. For ethanol, the ap and sc conformers have about the same energy, with a slight preference for ap (about 0.5 kJ/mol).5 For ethanethiol, sc is unambiguously the most stable rotamer from considerations of relaxation processes in a supersonic expansion.24 A previous study yielded an energy of sc of approximately 1.7 kJ/mol below the energy of ap.11 For ethaneselenol, sc is estimated to be more stable than ap by about 1 kJ/mol from semiempirical calculations.13 Finally, the exploration of the potential energy surface agrees with the results from the analogous molecules as well. For ethanol, ac-TS and syn-TS were found to lie about 4.7 and 5 kJ/mol, respectively, above the stable conformations.5 For ethanethiol, ac-TS and syn-TS were found to lie about 6 and 6.5 kJ/mol, respectively, above the sc conformer.11 For ethaneselenol, ac-TS and syn-TS were found to lie about 5.2 and 5.8 kJ/mol, respectively, above the energy of sc.13 In each case, the syn-TS is at a higher energy than the energy of ac-TS, so that the most favorable way of interconversion from sc to ap is to pass by the ac-TS. One should note that all the energy values for ethanol, ethanethiol, and ethaneselenol were obtained semiempirically, while the results on ethanetellurol obtained in this study come only from ab initio calculations. Therefore, the comparative analysis presented above should be considered with caution. Assignment and Analysis of the Spectra The results of ab initio calculations facilitated significantly the assignment of the rotational spectrum of ethanetellurol. On the basis of the calculated dipole moment components (µa ) 1.48 D, µb) 0.3 D for sc and µa ) 1.43 D, µb ) 0.6 D for ap conformation) one could expect to observe a strong µa spectrum. Indeed, the first spectra recorded using the Lille spectrometer in the frequency range 150-170 GHz revealed the presence of a series of strong aR0,1 bands. For this type of bands, the separation between their origins is approximately equal to the sum of the rotational constants, ∆ν ) B + C.25 The experimental value of ∆ν was found to be ∼5.7 GHz and close to theoretical values of 5.69 GHz for sc- and 5.85 GHz for ap-ethanetellurol.

Motiyenko et al. We have also found that each band consisted of at least three subbands. The position of subbands and the distribution of their relative intensities corresponded well to isotopic shifts (lighter species spectrum is shifted to higher frequencies) and isotopic abundances of the tellurium atom (130Te, 34%; 128Te, 30%; and 126 Te, 18%). Finally, the high resolution analysis has shown that each line in the band has a doublet structure, indicating the presence of internal rotation in the molecule. The separation between doublet components varies from 0.7 to 1.5 MHz depending on frequency range. We have analyzed two possible origins of internal rotation. First CH3 functional group torsion has been excluded from consideration since the calculated value of the torsional barrier (15 kJ/mol) is too high to provide large splittings. For example, for a similar molecule such as ethyl cyanide, the barrier height is estimated to be 12.9 kJ/mol and no torsional splittings have been observed in millimeter-wave spectra.26,27 Second, we have found that the separation between the components of a doublet depends on the J quantum number, and does not depend on the position within a band, that is, the Ka quantum number. According to the theoretical estimations, such splittings can be caused by Te-H functional group tunneling motion. Therefore, the observed rotational transitions having doublet structure were assigned to synclinal conformation of ethanetellurol which has double minima potential and a barrier to Te-H tunneling of medium height (about 5 kJ/mol). This value is comparable with corresponding values of barrier height to S-H (6 kJ/mol) and Se-H (5 kJ/mol) tunneling of ethanethiol and ethaneselenol. For both of these molecules, the tunneling splittings have been observed in the spectra of sc conformation. The initial assignment of the antiperiplanar conformation on the basis of millimeter-wave measurements in Lille was quite problematic due to the high density of the spectra and presumably low intensity of rotational lines. Therefore, we turned to less crowded lower frequency spectra recorded using the spectrometer in Oslo. The differences of the rotational constants between ap and sc obtained in the quantum chemical calculations were added to the experimental rotational constants of sc and used to predict the aR-spectrum of ap. The spectrum of the most abundant isotopologue (130Te) of ap was found in this manner close to its prediction. The 128Te and 126Te isotopologue spectra were then readily assigned. The pseudoinertial defect Ic - Ia Ib ) -6.47 × 10-20 u m2 is taken as an additional evidence that ap had indeed been assigned, because this value is typical for a compound having a symmetry plane and two pairs of sp3hybridized out-of-plane hydrogen atoms. The corresponding values for the antiperiplanar forms of the other CH3CH2XH compounds are -6.405(2) × 10-20 u m2 for CH3CH3OH,28 -6.389(2) × 10-20 u m2 for CH3CH332SH,9 and -6.424(3) × 10-20 u m2 for CH3CH380SeH.11 The following analysis has also lead to the assignment of ap conformation of ethanetellurol in the spectra recorded in Lille. Analysis of sc Conformation. Because of the effect of tunneling through the barrier to tellurol internal rotation, the ground state of sc-ethanetellurol is split into symmetric and antisymmetric 0+ and 0- substates. In the internal axis system the ab plane is formed by three heavy atoms C-C-Te, and for sc conformation the C-C-Te-H dihedral angle is about 63° from syn. Taking this configuration into account one can see that the tunneling motion does not change the sign of µa and µb dipole moment components, or equivalently, that µa and µb are even functions of TeH internal rotation angle. In order that the transition dipole moment matrix element 〈ψi|µx|ψj〉(x ) a,b) to be nonvanishing, the wave functions ψi and ψj should be of the same symmetry. Thus, for sc-ethanetellurol a- and

Rotational Spectrum of Ethanetellurol

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TABLE 2: Spectroscopic Constants for Various Isotopomers of sc Conformation of Ethanetellurol 130

A (MHz) B (MHz) C (MHz) DJ (kHz) DJK (kHz) DK (kHz) d1 (kHz) d2 (kHz) HJ (Hz) HJK (Hz) HKJ (Hz) E*J (kHz) E*2 (kHz) E*JJ (Hz) E*JK (Hz) ∆E (MHz) Fbc (MHz) N σ (MHz) σwb a

Te

25605.359(73) 2865.67988(30) 2721.05960(30) 1.108067(58) -9.02495(45) [175.1]a -0.109246(18) -0.003192(13) 0.000406(10) -0.007534(77) -0.81180(95) -6.736(38) -5.259(24) 0.1615(93) 2.85(10) 656.59(14) 6.3757(27) 734 0.067 0.83

130

128

Te theory

25948.14 2891.240 2746.340 1.070 -10.31 175.1 -0.112 -0.00630

Te

25607.048(89) 2873.11816(35) 2727.77227(35) 1.113647(57) -9.04527(46) [175.1]a -0.110129(21) -0.003212(14) 0.000403(10) -0.007499(82) -0.8216(10) -6.798(38) -5.289(27) 0.1663(91) 3.00(11) 657.34(14) 6.4058(28) 752 0.057 0.86

126

Te

25608.577(71) 2880.78479(35) 2734.69051(35) 1.119542(55) -9.06333(46) [175.1]a -0.111040(18) -0.003271(13) 0.000423(10) -0.007484(86) -0.8223(10) -6.867(35) -5.339(24) 0.1621(88) 2.83(12) 656.90(21) 6.4408(35) 674 0.056 0.74

125

Te

25609.261(93) 2884.702134(42) 2738.223547(42) 1.122542(77) -9.06981(78) [175.1]a -0.111512(21) -0.003319(16) 0.000421(14) -0.00664(19) -0.8264(29) -6.974(49) -5.342(28) 0. 193(12) 3.00(23) 657.25(31) 6.4496(41) 433 0.026 0.83

124

Te

25610.353(83) 2888.693977(52) 2741.820195(52) 1.125723(82) -9.0783(11) [175.1]a -0.111946(25) -0.003270(16) 0.000438(14) -0.00618(21) -0.8266(41) -6.799(58) -5.402(30) 0.138(13) 2.47(29) 658.98(20) 6.4419(52) 375 0.029 0.73

Fixed to ab initio value. b Unitless rms deviation of the fit.

b-type type transitions can occur only within each substate (selection rules: 0+ r 0+ and 0- r 0-). Because the µc dipole moment component changes sign with tunneling, c-type transitions connect two substates (selection rules: 0+ r 0- and 0r 0+). The Hamiltonian used for treating the tunneling substates was set up by a combination of pure rotational and Coriolis coupling terms: H ) HR + HC. The rotational term is defined in such a way that it allows one to fit the rotational constants averaged for 0+ and 0- substates.29 It is written as HR ) HS + H∆ where HS represents the standard Watson’s S-reduction Hamiltonian30 in Ir representation and H∆ is a Hamiltonian of centrifugal distortion corrections. In terms of rotational angular momentum J and its components, Jz and J( ) Jx ( iJy, the term H∆ is expressed as 2 2 2 2 4 H∆ ) E* + E*J + E*J J K z + E*(J 2 + + J-) + E* JJJ +

E*JKJ2Jz2 + ... (1) where E* is a half energy difference between 0+ and 0substates, - 2E* ) ∆E ) E(0+) - E(0-). Compared to a typical approach when each substate is described by a separate rotational Hamiltonian, the model chosen in the present study is proved to provide fewer correlations between rotational constants, which can be very important especially at the initial stage of an assignment. In addition, a unique set of rotational constants allows more straightforward comparison with corresponding ab initio values. The Coriolis coupling term describing the interaction between tunneling substates is defined as

HC ) Fbc(JxJy + JyJx)

(2)

It should be noted that in eqs 1 and 2 the Ir representation (x ) b, y ) c, z ) a) is used. The predictions and fits for the sc-ethanetellurol spectrum have been undertaken using Pickett’s SPCAT/SPFIT programs.31 The rotational parameters obtained as the results of the fit for all isotopologues studied are listed in the Table 2. The frequencies of assigned rotational transitions are listed in Tables

S3-S7 of Supporting Information. Besides the most abundant isotopic species of tellurium 130Te, 128Te, and 126Te, we have observed the spectra of ethanetellurol containing less abundant species like 125Te (7%) and 124Te (4.7%). In Table 2, we have also listed the theoretical values of rotational and centrifugal distortion constants calculated for CH3CH2130TeH species. In comparison with experimental values one can see that the rotational constants have been calculated with nearly 1% error which is a good result for ab initio calculations. For the centrifugal distortion constants, the agreement is less good, but at least the order of magnitudes and signs agree for all of them. Because of the lack of b- and c-type transitions, the DK parameter could not be determined and it has been fixed in the fit to its ab initio value. The value of the energy difference between 0+ and 0- substates ∆E found as a global solution of the tunneling problem is about 660 MHz (0.022 cm-1) and seems to vary slightly with isotopic species. Analysis of ap Conformation. The initial difficulties in the assignment of the spectrum of the antiperiplanar conformation of ethanetellurol were caused by relatively low intensities of its lines. It was found that typically its lines are 2-2.5 times less intense than those of sc. This is in agreement with theoretical results predicting the sc conformation to be 2 kJ/ mol (160 cm-1) more stable than ap. As an illustration of the observed relative intensities we provide a portion of spectra (Figure 2) of unresolved a-type doublet 347-337 for both conformations. The observed microwave transitions of ap ethanetellurol were fitted to the usual Watson S-reduction Hamiltonian in Ir representation. However, we have found that this model has a limitation in the case of ap conformation; it allows one to fit within experimental accuracy only low Ka transitions up to Ka ) 4. In principle we can also fit higher Ka transitions by including centrifugal distortion correction of higher orders, but it leads then to severe convergence problems and unrealistic values of the corresponding centrifugal distortion parameters. The possible explanation of the observed perturbations comes from consideration of the potential function which can no longer be expanded in a fast convergent Taylor series. Theoretical estimations provide the value of the barrier to Te-H internal rotation for the ap conformation to be only 2.5 kJ/mol (200

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Motiyenko et al. quantum chemical calculations, is now being applied to another molecule containing a tellurium atom, vinyltellurol (H2Cd CHTeH). Results of this study will be published shortly. Acknowledgment. A.K. thanks The Research Council of Norway for financial assistance through Contract 177540/V30. J.-C.G. thanks the Centre National d’Etudes Spatiales (CNES) for financial support. Jean Demaison is gratefully acknowledged for helpful discussions on basis set choice.

Figure 2. An example of relative intensities distribution for 347-337 transition of ap and sc conformers of ethanetellurol.

TABLE 3: Spectroscopic Constants for Various Isotopomers of ap Conformation of Ethanetellurol 130

A (MHz) B (MHz) C (MHz) DJ (kHz) DJK (kHz) DK (kHz) d1 (kHz) d2 (kHz) HJ (Hz) N σ (MHz) σwb a

Te

25051.85(18) 2973.3668(12) 2751.64087(82) 1.38393(16) -11.73(12) [169.4]a -0.16489(24) -0.00524(13) 38.6(57) 133 0.099 1.05

130

Te theory

25292.86 3011.15 2783.97 1.339 -14.40 169.4 -0.156 -0.00366

128

Te

25057.61(43) 2980.9628(17) 2758.18551(85) 1.39207(71) -11.05(15) [169.4]a -0.16819(63) -0.00550(42) 75.5(88) 85 0.091 0.85

126

Te

25060.98(24) 2988.7754(18) 2764.93365(90) 1.39749(27) -11.36(20) [169.4]a -0.16886(36) -0.00628(20) 68.(10) 103 0.084 0.98

Fixed to ab initio value. b Unitless rms deviation of the fit.

cm-1). A rigorous treatment of the rotational spectra of the ap conformation requires the use of a more appropriate model which takes into account the shape of potential function and is beyond the scope of the present study. Therefore we present here only the results obtained using the S-reduction. The rotational parameters of ap-ethanetellurol obtained as the result of least-squares fit are listed in the Table 3. The frequencies of assigned rotational transitions are listed in Tables S8-S10 of Supporting Information. Conclusions This paper presents the results of the studies of the ethanetellurol millimeter-wave rotational spectrum, which has been recorded and assigned for the first time. The assignment was supported by high level ab initio calculations taking into account relativistic effects of the tellurium atom. In the present study, a good agreement between the results of quantum chemical calculations and experimental observations has been achieved. As expected, two stable conformations (ap and sc) have been identified in the spectra and the sc conformer is found to be the most stable by 2 kJ/mol from theoretical consideration as well as from experimental observations. Calculated and experimental values of the rotational constants of sc and ap conformations agree within 1% accuracy. For both rotamers, the rotational spectra of the three most abundant isotopic species of the tellurium atom have been studied. In addition, the more intense spectrum of sc-ethanetellurol has allowed us to perform an analysis of its two less abundant species, namely 125Te and 124Te. The approach used in the present study, which is a combination of high resolution millimeter-wave spectroscopy and high level

Supporting Information Available: Synthesis of ethanetellurol, calculated molecular structure and harmonic vibrational frequencies, rotational line assignments, measured frequencies, experimental uncertainties and deviations from the final fits for studied isotopologues of sc and ap conformations of ethanetellurol. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Guillemin, J.-C.; Bouayad, A.; Vijaykumar, D. Chem. Commun. 2000, 1163. (2) Guillemin, J.-C.; Riague El, H.; Gal, J.-F.; Maria, P.-C.; Mo´, O.; Ya´nez, M. Chem.sEur. J. 2005, 11, 2145. (3) Khater, B.; Guillemin, J.-C.; Bajor, G.; Veszpre´mi, T. Inorg. Chem. 2008, 47, 1502. (4) Michielsen-Effinger, J. Bull. Acad. Roy. Belg. 1962, 48, 438. (5) Pearson, J. C.; Brauer, C. S.; Drouin, B. J. J. Mol. Spectrosc. 2008, 251, 394. (6) Kadzhar, C. O.; Abbasov, A. A.; Imanov, L. M. Opt. Spektrosk. 1968, 24, 629. (7) Kadzhar, C. O.; Abbasov, A. A.; Imanov, L. M. IzV. Akad. Nauk Azerb. SSR, Ser. Fiz.-Tekh. Mat. Nauk 1968, 71. (8) Hayashi, M.; Imaishi, H.; Ohno, K.; Murata, H. Bull. Chem. Soc. Jpn. 1971, 44, 872. (9) Hayashi, M.; Imaishi, H.; Kuwada, K. Bull. Chem. Soc. Jpn. 1974, 47, 2382. (10) Schmidt, R. E.; Quade, C. R. J. Chem. Phys. 1975, 62, 3864. (11) Nakagawa, J.; Kuwada, K.; Hayashi, M. Bull. Chem. Soc. Jpn. 1976, 49, 3420. (12) Durig, J. R.; Bucy, W. E. J. Mol. Spectrosc. 1977, 64, 474. (13) Nakagawa, J.; Okutani, H.; Hayashi, M. J. Mol. Spectrosc. 1982, 94, 410. (14) Møllendal, H.; Leonov, A.; de Meijere, A. J. Phys. Chem. A 2005, 109, 6344. (15) Møllendal, H.; Cole, G. C.; Guillemin, J.-C. J. Phys. Chem. A 2006, 110, 921. (16) Gaussian03, A quantum chemistry program exchange, 2004; http:// www.gaussian.com. (17) Peterson, K. A.; Figgen, D.; Goll, E.; Stoll, H.; Dolg, M. J. Chem. Phys. 2003, 119, 11113. (18) Dunning, T. H., Jr. J. Chem. Phys. 1989, 90, 1007. (19) Woon, D. E.; Dunning, T. H., Jr. J. Chem. Phys. 1995, 103, 4572. (20) Feller, D. J. Comp. Chem. 1996, 17, 1571. (21) Schuchardt, K. L.; Didier, B. T.; Elsethagen, T.; Sun, L.; Gurumoorthi, V.; Chase, J.; Li, J.; Windus, T. L. J. Chem. Inf. Model. 2007, 47, 1045. (22) Petitprez, D.; Demaison, J.; Wlodarczak, G.; Riague, El, H.; Guillemin, J.-C. J. Phys. Chem. A 2004, 108, 47. (23) Senent, M. L.; Smeyers, Y. G.; Dominguez-Gomez, R.; Villa, M. J. Chem. Phys. 2000, 112, 5809. (24) Choi, S.; Kang, T. Y.; Choi, K.-W.; Han, S.; Ahn, D.-S.; Baek, S. J.; Kim, S. K. J. Phys. Chem. A 2008, 112, 7191. (25) Gordy, W.; Cook, R. L. MicrowaVe molecular spectra, 3rd edition; John Wiley & Sons, Inc.: New-York, 1984. (26) Demaison, J.; Margule`s, L.; Ma¨der, H.; Sheng, M.; Rudolph, H. D. J. Mol. Spectrosc. 2008, 252, 169. (27) Brauer, C. S.; Pearson, J. C.; Drouin, B. J.; Yu, S. Astrophys. J., Suppl. Ser. 2009, 184, 133. (28) Takano, M.; Sasada, Y.; Satoh, T. J. Mol. Spectrosc. 1968, 26, 157. (29) Mu¨ller, H. S. P.; Christen, D. J. Mol. Spectrosc. 2004, 228, 298. (30) Watson, J. K. G. Vibrational Spectra and Structure; Elsevier: Amsterdam, 1977; Vol. 6, p 1. (31) Pickett, H. M. J. Mol. Spectrosc. 1991, 148, 371; see also the web page at http://spec.jpl.nasa.gov.

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