The conformational behavior of N­ethylformamide as observed by rotational spectroscopy and quantum chemistry.

A peptide linkage (CO)NH containing molecule, N-ethylformamide, was investigated by rotational spectroscopy in order to determine the molecular constants of its highest-energy conformer, cis-ac. Its rotational spectrum was observed in two different frequency ranges, in the 4-26 GHz frequency region using a Fourier transform microwave spectrometer and at millimeter wave frequencies between 75 and 116 GHz, employing a broadband high-resolution rotational spectrometer. The measurements at low frequencies allowed to resolve the hyperfine structure components due to nitrogen nuclear quadrupole coupling while the data at higher frequencies provided spectroscopic information about high order centrifugal effects. From a merged fit using all the observational data we have determined a total of thirteen molecular constants that provide a more accurate spectral modelling of the cis-ac conformer and serves a basis for their astronomical search. We have also observed spectra of five singly substituted isotopologues for the cis-ac conformer, three 13C and one for each of 15N and the deuterated species on the N-D position, from which we derived a partial r0 structure, in fair agreement with an ab initio result. In addition, the rotational transitions of the deuterated species of the most stable trans-sc conformer were observed and assigned and three rotational, five centrifugal distortion constants and nuclear quadrupole coupling constants of the nitrogen and deuterium nuclei were determined.

Conformations and Low-Frequency Intramolecular Motions of 1-Butanol, 1-Butanethiol, Iso-butanol, and Iso-butanethiol Investigated by Fourier Transform Microwave Spectroscopy Combined with Quantum Chemical Calculations.

The rotational spectra of 1-butanol (1-BuOH), 1-butanethiol (1-BuSH), 2-methyl-1-propanol (iso-BuOH), and 2-methyl-1-propanethiol (iso-BuSH) were measured by Fourier transform microwave spectroscopy in the frequency region from 3.7 up to 25 GHz. The observed spectral lines were assigned by observation of the deuterium substitution effect and by ab initio or density functional theory calculations at the levels of MP2/6-311++G(d,p) or B3LYP and cam-B3LYP, respectively. For 1-BuOH and 1-BuSH, seven of the 14 conformations, anticipated to exist as stable, were detected, whereas four and three among the five possible conformations were identified for iso-BuOH and iso-BuSH, respectively. We further found that, of the seven conformers of 1-BuOH, five were trans and two gauche, with respect to the internal rotation axis: the C2-C3 bond, while three of iso-BuOH existed in gauche and one in trans. The most stable conformer of the two BuOH molecules was trans with respect to the C-O bond, while all the sulfur analogues were gauche to the C-S axis. The rare isotopomers examined included 13C and OD of 1-BuOH and OD of iso-BuOH, 34S, 13C, and SD of the two sulfur molecules, and the rotational constants obtained on these isotopomers were employed in the molecular structure derivation. The potential barrier to CH3 internal rotation and the deuterium quadrupole coupling constant, where available, were also derived from the spectral analysis, and the molecular parameters thus obtained were compared with those derived using quantum-chemical calculations; the values derived using cam-B3LYP/6-311++G(d,p) were in better agreement with the observed than those derived using MP2/6-311++G(d,p) and B3LYP/6-311++G(d,p). The TTg form of 1-BuOH and of 1-BuSH and the Tg form of iso-BuSH exhibited additional spectral splittings, which were interpreted as caused by the OH or SH group tunneling between the symmetric and antisymmetric states. Some of the J = 8 rotational levels of 1-BuSH happened to be near-degenerate with others, and the splittings in them caused by mutual repulsion could be precisely determined by the observation of the transitions involving those split levels. Such splittings were determined for 1-BuSH, 1-BuSD, and iso-BuSH to be 1694.1731 (22), 56.3174 (16), and 6.4678 (14) MHz, respectively. A natural bond orbital analysis was performed to show that the most stable conformation of the primary and secondary alcohols is Gt because of the charge transfer from the lone-pair electron of the oxygen atom to the antibonding orbital of the C-H bond in 1-BuOH, whereas in iso-BuOH, the charge transfer to the antibonding orbital of the C1-C2 bond.

Fourier Transform Microwave Spectra of the Nitrogen Molecule-Ethylene Sulfide and Nitrogen Molecule-Dimethyl Sulfide Complexes.

We recorded the rotational spectra of N2-ethylene sulfide (ES) and N2-dimethyl sulfide (DMS) including the 15N2 and 15N14N isotopomers in the frequency range of 5-25 GHz by using a Fourier transform microwave spectrometer. The b-type transitions for the ortho and para states of 14N2-ES and 15N2-ES and c-type transitions of 14N2-DMS and 15N2-DMS were observed. The 15N14N-ES and 15N14N-DMS species were found to exist in two isomeric forms: inner (14N15N-ES and 14N15N-DMS) and outer (15N14N-ES and 15N14N-DMS). Neither the -ES nor -DMS complexes showed weak accompanying spectra, which had been observed for N2-ethylene oxide (EO). This is because the potential barriers to internal rotation of ES and DMS are higher than that of EO. The spectra were analyzed by an A-reduced asymmetric-top rotational program with less than 4 kHz standard deviation, except for the 15N14N-DMS and 14N15N-DMS complexes. Rotational, centrifugal distortion, and nuclear electric quadrupole coupling constants were determined by the spectral analysis. The V3 potential barrier to internal rotation of the two equivalent methyl groups of DMS in the ortho and para states of the 15N2-DMS complex was determined to be about 740 cm-1. We performed ab initio calculations in order to complement the information on the intracomplex motions obtained from the experimental spectra.

Molecular chirality: A new approach from a dynamical point of view.

The double minimum potential (DMP), which Hund assumed to explain the quantum-mechanical stability of enantiomers, was discussed, by citing three typical examples of DMP: inversion, internal rotation, and puckering. They expanded the classical scope of chirality, as defined by Kelvin, and indicated that a new bridge could be formed between the three low-frequency DMP modes and the asymmetric syntheses of chiral molecules.

Dimethyl Sulfide-Dimethyl Ether and Ethylene Oxide-Ethylene Sulfide Complexes Investigated by Fourier Transform Microwave Spectroscopy and Ab Initio Calculation.

The ground-state rotational spectra of the dimethyl sulfide-dimethyl ether (DMS-DME) and the ethylene oxide-ethylene sulfide (EO-ES) complexes were observed by Fourier transform microwave spectroscopy, and a-type and c-type transitions were assigned for the normal, (34)S, and three (13)C species of the DMS-DME and a-type and b-type transitions for the normal, (34)S, and two (13)C species of the EO-ES complexes. The transition frequencies measured for both the complexes were analyzed by using an S-reduced asymmetric-top rotational Hamiltonian. The rotational parameters thus derived for the DMS-DME were found to be consistent with a structure of Cs symmetry with the DMS bound to the DME by two C-H(DMS)···O and one S···H-C(DME) hydrogen bonds. Some high-Ka lines were found to be split, and we have interpreted these splittings in terms of internal rotations of the two methyl groups of the DMS and of the "free", i.e., outer group, of the DME. Some forbidden transitions were also observed in cases where Ka = 3 levels were involved, for the DMS-DME complex in the internal-rotation E state. The barrier height, V3, to internal rotation of the CH3 in the DME thus derived is smaller than that of the DME monomer, while the V3 of the CH3 groups in the DMS is nearly the same as that of the DMS monomer. For the EO-ES complex, the observed data were interpreted in terms of an antiparallel structure of Cs symmetry with the EO bound to the ES by two C-H(ES)···O and two S···H-C(EO) hydrogen bonds. An attempt was also made to observe a-type transitions of the DMS dimer without success. We have applied a natural bond orbital analysis to the DMS-DME and EO-ES to calculate the stabilization energy CT (= ΔEσσ*), which was correlated closely with the binding energy as found for other related complexes.

High-resolution laser spectroscopy and magnetic effect of the B̃(2)E(')←X̃(2)A2(') transition of the (15)N substituted nitrate radical.

Rotationally resolved high-resolution fluorescence excitation spectra of the 0-0 band of the B̃(2)E(')←X̃(2)A2(') transition of the (15)N substituted nitrate radical were observed for the first time, by crossing a jet-cooled molecular beam and a single-mode dye laser beam at right angles. Several thousand rotational lines were detected in the 15 080-15 103 cm(-1) region. We observed the Zeeman splitting of intense lines up to 360 G in order to obtain secure rotational assignment. Two, nine, and seven rotational line pairs with 0.0248 cm(-1) spacing were assigned to the transitions from the X̃(2)A2(') (υ″ = 0, k″ = 0, N″ = 1, J″ = 0.5 and 1.5) to the (2)E3/2(') (J' = 1.5), (2)E1/2(') (J' = 0.5), and (2)E1/2(') (J' = 1.5) levels, respectively, based on the ground state combination differences and the Zeeman splitting patterns. The observed spectrum was complicated due to the vibronic coupling between the bright B̃(2)E(') (υ = 0) state and surrounding dark vibronic states. Some series of rotational lines other than those from the X̃(2)A2(') (J = 0.5 and 1.5) levels were also assigned by the ground state combination differences and the observed Zeeman splitting. The rotational branch structures were identified, and the molecular constants of the B̃(2)E1/2(') (υ = 0) state were estimated by a deperturbed analysis to be T0 = 15 098.20(4) cm(-1), B = 0.4282(7) cm(-1), and DJ = 4 × 10(-4) cm(-1). In the observed region, both the (2)E1/2(') and (2)E3/2(') spin-orbit components were identified, and the spin-orbit interaction constant of the B̃(2)E(') (υ = 0) state was estimated to be -12 cm(-1) as the lower limit.

Intermolecular interaction in the formaldehyde-dimethyl ether and formaldehyde-dimethyl sulfide complexes investigated by Fourier transform microwave spectroscopy and ab initio calculations.

The ground-state rotational spectra of the formaldehyde-dimethyl ether (H2CO-DME) and formaldehyde-dimethyl sulfide (H2CO-DMS) complexes have been studied by Fourier transform microwave spectroscopy. The a-type and c-type rotational transitions have been assigned for the normal and deutrated formaldehyde-containing species of both complexes. In the case of H2CO-DME, doublets were observed with the splitting 10-300 kHz, whereas no such splittings were observed for H2CO-DMS, D2CO-DME, and D2CO-DMS. The observed rotational spectra were found consistent with a structure of Cs symmetry with DME or DMS bound to H2CO by two types of hydrogen bonds: C-H(DME/DMS)---O(H2CO) and O(DME)/S(DMS)---H-C(H2CO). The R(cm) distances between the centers of mass of the component molecules in the H2CO-DME and H2CO-DMS complexes were determined to be 3.102 and 3.200 Å, respectively, which are shorter than those in most related complexes. The spectral and NBO analyses showed that H2CO-DMS has a stronger charge transfer interaction than H2CO-DME does and that the binding energy of H2CO-DMS is larger than that of H2CO-DME.

High-resolution laser spectroscopy and magnetic effect of the B̃2E' ← X̃2A2' transition of 14NO3 radical.

Rotationally resolved high-resolution fluorescence excitation spectra of (14)NO3 radical have been observed for the 662 nm band, which is assigned as the 0-0 band of the B̃(2)E' ←X̃(2)A2' transition, by crossing a single-mode laser beam perpendicularly to a collimated molecular beam. More than 3000 rotational lines were detected in 15,070-15,145 cm(-1) region, but it is difficult to find the rotational line series. Remarkable rotational line pairs, whose interval is about 0.0246 cm(-1), were found in the observed spectrum. This interval is the same amount with the spin-rotation splitting of the X̃(2)A2' (υ = 0, k = 0, N = 1) level. From this interval and the observed Zeeman splitting up to 360 G, seven line pairs were assigned as the transitions to the (2)E'(3/2) (J' = 1.5) levels and 15 line pairs were assigned as the transitions to the (2)E'(1/2) (J' = 0.5) levels. From the rotational analysis, we recognized that the (2)E' state splits into (2)E'(3/2) and (2)E'(1/2) by the spin-orbit interaction and the effective spin-orbit interaction constant was roughly estimated as -21 cm(-1). From the number of the rotational line pairs, we concluded that the complicated rotational structure of this 662 nm band of (14)NO3 mainly owes to the vibronic interaction between the B̃(2)E' state and the dark Ã(2)E″ state through the a2″ symmetry vibrational mode.

Fourier transform microwave spectrum of the nitrogen molecule-ethylene oxide complex: intracomplex motions.

The rotational spectra of the N2-ethylene oxide (EO) complex were measured in the frequency region from 4 to 27 GHz by Fourier transform microwave spectroscopy, paying particular attention to intracomplex motions. The isotopologues with enriched (15)N2 or (15)NN as a moiety were also investigated. We have observed spectra of a strong/weak pair for each of the ortho and para states of the (14)N2-EO and (15)N2-EO species, which indicated that the complex existed in four distinct states. We interpreted, on the basis of the observed relative intensities, that these states were generated primarily by the exchange of the nitrogen atoms of the N2 moiety, followed by that of the two CH2 groups in the EO molecule. The (15)NN-EO species was found to consist of two isomers, one with the (15)N in the inner expressed as N(15)N-EO and the other in the outer position designated as (15)NN-EO, and the spectra of both isomers were accompanied by one weak set of satellites. The observed spectra were rotationally assigned by using sum rules and were analyzed by the asymmetric-rotor program of S-reduction, with the standard deviation of less than 10 kHz. We have found some of the molecular parameters like A, D(JK), and D(K) to be correlated between the two pairs of the spectra, and also, to much less extent, between the strong and weak members. The differences in these molecular parameters between the four sets were explained by the first-order Coriolis interaction between the "ground" and "excited" states generated by a combination of the two internal motions corresponding to the exchanges of the equivalent atoms and/or groups in the N2 and EO constituents of the complex. These internal motions were simulated by the 2-fold internal rotations of the two moieties. We have carried out ab initio molecular orbital calculations at the level of MP2 with basis sets 6-311++G(d,p), aug-cc-pVDZ, and aug-cc-pVTZ, to complement the information on the intracomplex motions obtained from the observed rotational spectra.

Microwave studies on 1,4-pentadiene: CH2═CH-CH2-CH═CH2; transformations among the three rotational isomers.

In order to examine significant roles of conformations played in various research fields, a molecule with two internal-rotation axes of high symmetry, 1,4-pentadiene, was studied in detail through the observation of its rotational spectra by using various types of microwave spectroscopy, Stark modulation and Fourier transform in the centimeter-wave region, direct absorption in the millimeter-wave region, and centimeter-/millimeter-wave combinations for double resonance, along with ab initio molecular orbital calculations. The molecule was confirmed to exist in three rotameric forms: skew-skew, cis-skew, and skew-skew'. For the cis-skew form, rotational spectra not only in the ground vibrational state, but also in three excited C-C torsional states were detected. Rotational and centrifugal distortion constants were precisely determined by the analysis of all the observed spectra, in addition to the relative energies of the three isomers and the torsional frequencies for the cis-skew form, as estimated from the observed spectral line intensities. The skew-skew form was found to be the most stable among the three isomers, the cis-skew form higher in energy than the skew-skew by 172 ± 66 cm(-1), and the skew-skew' form higher in energy than the cis-skew by 44 ± 26 cm(-1). These experimental results were compared with those derived from a two-dimensional potential energy surface calculated by ab initio molecular orbital methods, in order to obtain a global view of molecular dynamics taking place on the surface, while paying attention to unique features of internal rotation characteristic of two dimensions.

Triple resonance for a three-level system of a chiral molecule.

A new spectroscopic method of triple resonance is proposed for studying chirality of a molecule of C1 symmetry. Each enantiomer of such a molecule is of mixed parity and thus exhibits all three a-, b-, and c-types of rotational spectra. The present study concludes, by using time-dependent perturbation theory, that the transition probability between two of the three rotational levels under triple resonance differs for different enantiomer. This result can thus be of some significance for enantiomer differentiation.

Intermolecular interaction between CO or CO2 and ethylene oxide or ethylene sulfide in a complex, investigated by Fourier transform microwave spectroscopy and ab initio calculations.

The rotational spectra of the CO-ethylene oxide (EO), CO-ethylene sulfide (ES), CO(2)-EO, and CO(2)-ES complexes were measured by Fourier transform microwave spectroscopy in the frequency region from 4 up to 31 GHz. The isotopologues with a single (13)C atom in the EO or ES, (18)O in the EO, (34)S in the ES, and (13)C in the CO(2) moiety, respectively, were observed in natural abundance, and enriched samples, (13)CO or C(18)O in the CO-EO or CO-ES complex and C(18)OO and C(18)O(2) in the CO(2)-EO or CO(2)-ES complex, were employed to record respective rotational transitions. The rotational spectra observed for the CO-EO, CO-ES, CO(2)-EO, and CO(2)-ES complexes were analyzed by using an asymmetric-rotor S-reduced Hamiltonian to determine rotational and centrifugal distortion constants. The r(s) coordinates of the atoms in the four complexes, which were calculated from the observed rotational constants, led to a structure in which the CO or CO(2) moiety is located in a plane perpendicular to the EO or ES skeletal plane and bisecting the COC or CSC angle. We have also carried out ab initio molecular orbital calculations at the level of MP2 with basis sets 6-311++G(d,p) and aug-cc-pVDZ using the Gaussian 09 package. The MP2/6-311++G(d,p) calculations yield rotational constants in better agreement with the experimental values than with the other basis set; in other words, the molecular structures calculated using this basis set are close to those experimentally found for the ground state. The estimated dissociation energies of the complexes, including the zero-point vibrational energy corrections ΔZPV and the basis set superposition errors (BSSE) calculated with the counterpoise correction (CP), are in good agreement with the experimentally obtained binding energies E(B). We have applied an NBO analysis to the complexes to calculate the stabilization energy CT (=ΔE(σσ*)), which we found are closely correlated with the binding energies E(B). We have thus achieved a consistent overview on the intermolecular interaction in the complexes under consideration. It is to be noted that the spectral intensities of the inner OC(18)O-EO and OC(18)O-ES complexes were larger by a factor of 2 than those of the outer (18)OCO-EO/ES complexes. This observation was explained by the zero-point energy of the inner conformer being a little smaller than that of the outer one.

Chirality of and gear motion in isopropyl methyl sulfide: A Fourier transform microwave study.

Isopropyl methyl sulfide (CH(3))(2)CHSCH(3) was investigated by Fourier transform microwave spectroscopy. Two rotational isomers gauche and trans were detected. The rotational spectra of gauche were found fit to an asymmetric rotor pattern, except for being split by the internal rotation of CH(3) attached to S with the potential barrier V(3) of 601.642 (65) cm(-1) and for exhibiting the effect of tunneling between the two equivalent gauche forms in a few high-K transitions. The tunneling was discussed from a viewpoint of chirality. The trans spectra appeared generally similar to those of gauche, with V(3) to the S-CH(3) internal rotation of 559.00 (11) cm(-1), but satellite lines accompanied the ground torsional state lines in some high-K transitions. These satellites were ascribed to the excited state of the C(isop)-S torsion. In fact, the potential function for this torsion was shown by an ab initio calculation to be flat or even of double minima around the trans position, which was presumably caused by a gear coupling between the two methyl groups of the isopropyl group and the one in the S-CH(3).

Analyses of the infrared absorption bands of (15)NO(3) in the 1850-3150 cm(-1) region.

We have observed the infrared spectrum of (15)NO(3) by a high resolution Fourier transform infrared (FT-IR) spectrometer using the reaction of F atoms with H(15)NO(3). Five (2)E'-(2)A(2)' bands are identified in the 1850-3150 cm(-1) region. The rotational analyses indicate that these bands have the lower state in common, which coincides with the ground state of planar D(3h) symmetry. The upper (2)E' states more or less suffer from perturbations by close-lying dark states. Among them, those of the 2004, 2128, and 2492 cm(-1) bands are analyzed to determine molecular parameters in these states by fixing the ground-state constants to those derived by a combination difference method. The spin-orbit and Coriolis coupling constants in the (2)E' states are substantially different for different vibronic states. The vibrational assignments of NO(3) in the ground electronic state are discussed using experimental data heretofore available, supplemented by those obtained by the present study.

The spectroscopic characterization of the methoxy radical. II. Rotationally resolved A (2)A(1)-X (2)E electronic and X (2)E microwave spectra of the perdeuteromethoxy radical CD(3)O.

Rotationally resolved laser-induced fluorescence (LIF) and stimulated emission pumping (SEP) A (2)A(1)-X (2)E spectra of the perdeuteromethoxy radical (CD(3)O) have been observed. These data directly connect the two spin-orbit components (E(1/2) and E(3/2)) of the ground electronic state with high precision. Molecular constants for both electronic states are determined in a global fitting that involves LIF, SEP, and pure rotational spectra in the microwave region. For the microwave transitions, the resolved hyperfine structure is analyzed providing molecular parameters characterizing it and hyperfine-free transitions for the global fitting. A complete "experimental" geometry for the methoxy radical at the C(3v) conical intersection is determined from the rotational constants of its isotopologs. The experimental isotopic dependence of other parameters in the effective Hamiltonians is compared to the theoretically expected variation. These comparisons allow considerable insight into the physical significance of a number of parameters in the effective Hamiltonian. In particular, experimental evidence is found for a previously predicted vibrational correction to the A rotational constant of a Jahn-Teller active molecule.

Fourier transform microwave spectrum of CO-dimethyl ether.

Two sets of 32 rotational transitions were observed for the carbon monoxide-dimethyl ether (CO-DME) complex and two sets of 30 transitions for both (13)CO-DME and C(18)O-DME, in the frequency region from 3.5 to 25.2 GHz, with J ranging from 1<--0 up to 7<--6, by using a Fourier transform microwave spectrometer. The splittings between the two sets of the same transition varied from 2 to 15 MHz, and the two components were assigned to the two lowest states of the internal rotation of CO with respect to DME governed by a twofold potential. A preliminary analysis carried out separately for the two sets of the observed transition frequencies by using an ordinary asymmetric-rotor Hamiltonian indicated that the heavy-atom skeleton of the complex was essentially planar, as evidenced by the "pseudoinertial defects," i.e., the inertial defects, which involve the contributions of the out-of-plane hydrogens of the two methyl groups, I(cc)-I(aa)-I(bb) of -5.764(23) and -5.753(16) uA(2) for the symmetric and antisymmetric states, respectively. All of the observed transition frequencies were subsequently analyzed simultaneously, by using a phenomenological Hamiltonian which was described in a previous paper on Ar-DME and Ne-DME [Morita et al., J. Chem. Phys. 124, 094301 (2006)]. The rotational constants thus derived were analyzed to give the distance between the centers of gravity of the two component molecules, DME and CO, to be 3.682 A and the angle between the CO and the a-inertial axes to be 75.7 degrees ; the C end of the CO being closer to the DME. Most a-type transitions were observed as closely spaced triplets, which were ascribed to the internal rotation of the two methyl tops of DME. The V(3) potential barrier was obtained to be 772(2) cm(-1) from the first-order Coriolis coupling term between the internal rotation and overall rotation, which is about 82% of V(3) for the DME monomer, whereas the second-order contribution of the coupling to the B rotational constant led to V(3) of 705(3) cm(-1). By assuming a Lennard-Jones-type potential, the dissociation energy was estimated to be E(B)=1.6 kJ mol(-1), to be compared with 1.0 and 2.5 kJ mol(-1) for Ne-DME and Ar-DME, respectively.

Dynamical structure of peptide molecules.

In view of the importance of the peptide linkage in structural biology, we have carried out intensive investigations on peptide molecules consisting of a peptide linkage with one or two substituents in the gas phase by Fourier transform microwave spectroscopy, paying special attention to the internal rotation of the substituents relative to the central linkage framework. We have found that, in sharp contrast with the stiff structure around the central C-N bond of the linkage, the internal rotations of the substituents are of low frequency and thus of large amplitude and are extremely susceptible to their local environment such as the presence of other substituents.

Internal motions in a complex consisting of a rare gas atom and a C2v molecule: theoretical formulations and their applications to Fourier transform microwave spectra of Ne-dimethyl ether and Ar-dimethyl ether.

The internal motion of the rare gas atom, i.e., the relative motion of the two constituents, in a complex shown in the title was discussed by paying special attention to its effect on the rotational motion of the complex in order to extract as much precise information on this motion as possible from the observed rotational spectra. We have set up two theoretical formulations. One is based on a coordinate axis system attached to the C2v molecule, but its origin is floating with the motion of the rare gas atom, while keeping the orientation parallel to the original C2v molecule-fixed coordinate system. The second approach starts with counting the number of equivalent potential minima, which are well separated from the others by high potential barriers, and then collects all permutation-inversion operations, which transform the system from one minimum to another, to set up a group appropriate for the complex. By using the symmetry properties thus derived, a phenomenological Hamiltonian is set up to fit the observed spectra. The two formulations result in alike rotational energy matrices, and we have applied them to analyze the internal motions in the two complexes of present concern: neon-dimethyl ether (Ne-DME) and argon-dimethyl ether (Ar-DME). Some of the transitions observed by the present study exhibited additional splittings, which were interpreted as due to an internal rotation of the methyl groups in DME and were analyzed by the second formulation. For Ar-DME the splittings appeared only in high-K transitions, yielding the V3 potential barrier to be 778(1) cm(-1), whereas those observed for Ne-DME were ascribed to the effects of the CH3 internal rotation on the inversion splitting.

Internal Rotation of the Hydroxyl Group in Isopropanol and the Chirality of the Gauche Form: Fourier Transform Microwave Spectroscopy of (CH(3))(2)CHOD.

The rotational spectrum of the deuterated isopropanol (CH(3))(2)CHOD has been observed by Fourier transform microwave spectroscopy and analyzed to yield tunneling splitting of 4431.4613 (17) MHz, between the antisymmetric and the symmetric gauche forms, which is much larger than the 2400 MHz estimated from the internal-rotation potential function reported in the literature. The potential function for the OH internal rotation has been examined in view of the discrepancy between the observed and estimated tunneling splitting, and it was accounted for by taking into account isotope effects on the potential constants. The deuterium quadrupole coupling effect has been included together with the Coriolis terms in the off-diagonal block of the Hamiltonian matrix for the gauche form. The deuterium quadrupole coupling constants obtained for the trans form were employed to calculate the components of the coupling constants as functions of the internal-rotation angle, and the components at around 120 degrees were compared with the values observed for the gauche form, thereby leading to unambiguous determination of the signs of the constants in the off-diagonal block; the signs are not obtainable from an ordinary analysis of the rotational spectra. The chirality of the gauche form was discussed by placing special emphasis on the effect of intermolecular interactions between two chiral molecules. Copyright 2001 Academic Press.