Microwave spectrum, structure, and dipole moment of the trans-trans isomer of methylpropylether

Microwave spectra of the trans-trans (TT) isomer of methylpropylether and its 12 isotopically substituted species were measured. The r_s structure of this isomer was determined from the observed moments of inertia. Structural parameters of this isomer were roughly equal to those of the reported r_s structures of trans-ethylmethylether and propane. Dipole moments of the TT isomer for the normal and two deuterated species were determined by Stark-effect measurements. For the normal species, the dipole moment was μ_a = 0.082 ± 0.010, μ_b = 1.104 ± 0.013, and μ_total = 1.107 ± 0.013 D making angles of 4°17′ with the b-inertial axis, of 6°7′ with the bisector of the COC angle. The barrier to internal rotation of the CH₃C group was calculated to be 3300 ± 60 cal/mole from A-A splittings of the spectra in the CH₃C excited torsional state.     

Methyl barrier to internal rotation and evidence of torsion-torsion interaction in methyl thiolcyanoformate

The rotational spectrum of methyl thiolcyanoformate has been measured in the ground state and in four vibrationally excited states. The methyl group has the conformation syn with respect to the carbonyl group. The E component lines were assigned only in the ground state and the methyl group barrier to internal rotation has the value V₃ = 705 ± 20 cal/mole. The A species rotational transitions of the first excited state of the methyl and skeletal torsions do not follow a semirigid-rotor pattern probably because they strongly interact.     

Microwave spectrum,structure, dipole moment,and internal rotation of the GT isomer of fluoromethylethylether

Microwave spectra of fluoromethylethylether and its 13 isotopically substituted species have been measured. The r_s structure of the GT isomer of this molecule was determined from the observed moments of inertia. The structural parameters obtained are roughly close to those of fluoromethylmethylether and the GT isomer of chloromethylethylether. The dipole moments and their directions in the molecule were determined from the Stark effect measurements of several low-J transitions for the normal and two deuterated species. The dipole moment of the normal species was found to be 1.806 ± 0.012 D, making angles of 136°50′ and 107°40′ with the CF and FCH₂O bonds, respectively. From the A-E splittings of the spectra in the first excited methyl torsional state, the barrier to internal rotation of the methyl group was calculated to be 3150 ± 50 cal/mole in the one-top approximation.     

Barrier to internal rotation of the silyl group in cyclopropyl silane from the MW spectrum in the first excited torsional state

The MW spectrum of the first excited state of the internal rotation of the silyl group in cyclopropyl silane has been recorded and analyzed in the frequency region 6.9–38.0 GHz. Recordings were made with conventional Stark spectroscopy as well as with MW-MW double resonance. From the analysis of the torsional splittings the following parameters were derived: V₃=1917.6(44)cal/mole, I_α=5.888(14)u^*A², V_a=21.56(21)°     

Methyl and skeletal torsion interaction in methyl thiolfluoroformate

The microwave spectrum of methyl thiolfluoroformate (FCOSCH₃) is reported for the ground state and seven vibrational satellites. The methyl group is in the syn conformation to the carbonyl group. The dipole moment components are μ_a = 2.89(2) D, μ_b = 0.30(8) D, and μ_c = 0. Spacings of A and E levels due to methyl internal rotation are analyzed for the ground state, the first excited methyl torsional state, and the first excited skeletal torsional state. An anomalous sequence of A and E levels occurring in the latter satellite arises from torsional interaction, according to two-dimensional model calculations. Potential parameters consistent with the three observed level separations are V₃ = 304(5) cm⁻¹, V₆ = 23(1) cm⁻¹ for the methyl torsion and either k = 1.912 or k = 2.936 cm⁻¹ deg⁻² for the skeletal torsional force constant.     

The microwave spectrum, structure, chlorine nuclear quadrupole coupling constants, dipole moment, and centrifugal distortion constants of dichlorosilane

The microwave spectra of three isotopic species of dichlorosilane, SiH₂Cl₂, in its ground vibrational state, have been measured in the frequency region 8–40 GHz. The spectra have yielded values for the rotational constants, centrifugal distortion constants, and chlorine nuclear quadrupole coupling constants, as well as the molecular dipole moment, 1.13 ± 0.02 D. The molecule has C_2v symmetry, and the bond lengths and angles r(Si---Cl=2.033±Å, r(Si---H)=1.480±0.015Å, (Cl---Si---Cl)=109°43′±20±, (H---Si---H)=111°18′±40′ The centrifugal distortion constants have been compared with those calculated using a published force field.     

Internal hydrogen bond,torsional motion,and molecular properties of 2-methoxyethylamine by microwave spectroscopy: Methyl barrier to internal rotation for 2-methoxyethanol

The microwave spectra of four substituted isotopic species of 2-methoxyethylamine (NH₂, NHD, NDH, ND₂) have been assigned. The molecule is found to exist in a gauche form with an intramolecular hydrogen bond of the NH?O type. The four possible sets of the amino hydrogen r_s corrdinates give different H?H distances, probably because the -NH₂ group is involved in large amplitude vibrations and because of changes in the heavy atom positions arising from the deuteration of the hydrogen bond. For the most abundant species many vibrational states have been analyzed and assigned to the two possible CO torsions in the molecule. A value V₃ = 3150 ± 50 cal/mol was found for the methyl torsional barrier and V₁ = 9 ± 3 kcal/mol for the other CO torsional barrier. A third set of observed vibrational satellites is probably assignable to the CC torsion. The determination of the dipole moment and of the quadrupole coupling constants gave values which were not in good agreement with those predicted from nonhydrogen bonded molecules. In addition a value V₃ = 3100 ± 100 cal/mol was calculated for the CH₃ torsional barrier in the related 2-methoxyethanol, using previous experimental data (Canad. J. Chem.50, 1149–1156 (1972)).     

predictions of PMNS and CKM angles

Generalizing a previous model to accommodate the third quark family and CP violation, we present a T^′ model which predicts tribimaximal neutrino (PMNS) mixings while the central predictions for quark mixings are |V_td/V_ts|=0.245 and |V_ub/V_cb|=0.237 with a predicted CP violating KM phase δ_KM=65.8°. All these are acceptably close to experiment, including the KM phase for which the allowed values are 63°<δ_KM<72°, and depend only on use of symmetry T^′×Z₂ to define the model and no additional parameters.     

Internal rotation in acetaldehyde

The microwave spectrum of acetaldehyde has been investigated in the frequency range from 7 to 40 GHz. A rather complete assignment of rotational transitions in the ground and torsionally excited states has been found with the help of microwave-microwave double resonance techniques. The spectral data have been analyzed using three different models for the overall and internal rotation problem including a nonrigid model. The threefold component of the internal rotation barrier was determined to be V₃ = 400 ± 2 cm⁻¹. The sixfold contribution V₆ = −10.9 ± 0.3 cm⁻¹ could only be adjusted reliably from data for both ground and torsionally excited states using the nonrigid model. The methods of barrier determinations have been critically reviewed. In an appendix, the Hamiltonian for a nonrigid model is derived based on structure relaxation of the methyl top during internal rotation.     

Infrared and microwave spectra, molecular conformation and electric dipole moment of methyl thiolformate

The microwave spectrum (41-10 GHz) and the infrared spectrum (4000-50 cm⁻¹) of methyl thiolformate have been obtained and analyzed. The spectra are consistent with a single molecular conformation having a planar array of heavy atoms and with the alkyl group cis to the carbonyl group. The measured rotational constants are: A, 11042.22 MHz; B, 5118.27 MHz; C, 3562.03 MHz (κ = −0.5839). No internal rotation doublets were observed in the microwave spectrum for the ground vibrational state, which implies that the barrier hindering internal rotation of the methyl group is either much larger or much smaller than the corresponding value for methyl formate. If the former is true then a lower limit of 10.5 kJ mol⁻¹ may be placed on the barrier height.The dipole moment of methyl thiolformate was measured using the Stark effect to be 1.58 ± 0.05 Debyes (μ_A = 1.52 D; μ_B = 0.43 D) for the vapor, and for dilute solutions in benzene at 295 K the value of 1.6 ± 0.1 D was found from capacitance measurements.SCF computations using minimal basis sets of STO/3G atomic orbitals and extended basis sets of STO/4.31G atomic orbitals have been carried out for methyl thiolformate and methyl formate. Energy differences between rotational isomers and estimates of barrier heights are given together with the calculated dipole moments.     

Rotational spectrum of the Ar–dimethyl sulfide complex

The rotational spectra of three isotopomers of the Ar–dimethyl sulfide (DMS) complex – normal, ³⁴S, and ¹³C species – were measured in the frequency region from 3.7 up to 24.1 GHz by Fourier transform microwave spectroscopy. The normal species yielded 43 a-type and 79 c-type transitions. No Ar tunneling splitting was observed, while many transitions were split by the internal rotation of the two methyl tops of the DMS unit. In cases where the K-type splitting was close to that due to methyl internal-rotation, several forbidden transitions were observed that followed b-type selection rules. All of the observed transition frequencies were analyzed simultaneously using a phenomenological Hamiltonian also used in previously published work describing the Ar–dimethyl ether (DME) and Ne–DME complexes. The rotational and centrifugal distortion constants and the potential barrier height to methyl-top internal rotation, V₃, were determined. The rotational constants were consistent with an Ar–DMS center of mass (cm) distance of 3.796 (3) Å and a S–cm–Ar angle of 104.8 (2)°. The V₃ potential barrier obtained, 736.17 (32) cm⁻¹, was 97.8% of the DMS monomer barrier. By assuming a Lennard–Jones-type potential, the dissociation energy was estimated to be 2.4 kJ mol⁻¹, which was close to the value for Ar–DME, 2.5 kJ mol⁻¹.     

The microwave spectra of ortho-fluorotoluene with the asymmetric internal rotors CH2D and CD2H

The rotational spectra of αd₁- and αd₂-ortho-fluorotoluene in the ground state of the methyl group torsion have been measured. The evaluation of the spectra has been based on the theory for the internal rotation of an asymmetric internal top formulated earlier by several authors. The barrier potential being threefold symmetric (V₃), each torsional level consists of three nondegenerate substates, designated as sy and ±asy. The sy-state is assigned to the conformation with the unique methyl hydrogen isotope within the molecular heavy-atom plane (sy-rotamer), while the ±asy-states belong to the respective out-of-plane conformation (asy-rotamer). In the torsional ground state the level spacing between the ±asy substates is very small and numerous accidental close degeneracies are present between the rotational level systems based on these torsional substates. The rotational levels involved are strongly perturbed by the coupling between molecular overall rotation and internal rotation. Large deviations from a rigid rotor spectrum and (+) ? (?) intersystem (“tunneling”) transitions are observed. The spectrum of the asy-rotamer can be well reproduced by a “two-dimensional” Hamiltonian containing 11 “rotational constants,” 9 of which are determined by a fit to the spectrum. Several are sufficiently barrier-dependent to derive V₃. We obtain (in cal/mole) 567 ± 48 for αd₁-ortho-fluorotoluene, 711 ± 40 for the αd₂-isotope. The deviations from 649 cal/mole for the normal isotope are appreciable, probably indicating shortcomings of the semirigid model. The sy-rotamer presents a rigid rotor spectrum.     

Internal rotation and inversion doubling in the microwave spectrum of CH3NHCl

The microwave “a” and “c” type spectra of four isotopic species of CH₃NHCl in the ground state and of CH₃NHCl³⁵ and CH₃NDCl³⁵ in the first excited torsional state have been analyzed. From the A-E torsional splittings of the excited state the torsional barrier height has been determined to be V₃ = 3710 ± 46 cal/mole. The “c” type transitions show an inversion doubling of 4.60 ± 0.10 MHz in the ground state and of 5.25 ± 0.10 MHz in the first excited torsional state. Such doublings are independent on the rotational quantum numbers within the experimental errors. The height of the inversion barrier has been roughly evaluated by using the Dennison-Uhlenbeck potential.     

The microwave spectrum,substitution structure,internal rotation barrier,and dipole moment of thioacetaldehyde,CH3CHS

The microwave rotational spectrum of the unstable species thioacetaldehyde, CH₃CHS, has been studied in a flow pyrolysis system. Eight isotopic variants have been studied allowing an accurate substitution structure to be derived. Most of the spectral lines show splittings due to internal rotation, analysis of which has allowed a barrier study to be made. For the torsional ground state of the most abundant species, V₃ = 1572 ± 30 cal/mole or 375.7 ± 7 J/mole. The dipole moment is μ = 2.33 ± 0.02 D with components μ_A = 2.26 ± 0.02 and μ_B = 0.56 ± 0.01 D.     

The microwave spectrum, structure, and barrier to internal rotation of selenoacetaldehyde, CH3CHSe

The rotational spectrum of the unstable molecule selenoacetaldehyde, CH₃CHSe, has been studied by microwave spectroscopy between 26.5 and 40 GHz. Transitions have been measured for five abundant selenium isotopic variants. These measurements have, together with structural information from the related molecules CH₃CHS and CH₃CHO, allowed reliable data on the C=Se bond length (1.758 ± 0.01 Å) and the e angle (125.7 ± 0.3°) to be derived. The spectral lines show splittings due to hindered internal rotation and using these together with the derived structure, barrier heights of 1602 cal mole⁻¹ (6703 J mole⁻¹) and 1648 cal mole⁻¹ (6859 J mole⁻¹) have been determined for the ground and first torsionally excited states, respectively.     

The microwave spectrum and skeletal torsion of 2,6,7-trioxa-1-phosphabicyclo[2.2.2]octane

The microwave spectrum of 2,6,7-trioxa-1-phosphabicyclo[2.2.2]octane was measured in the region from 12.4 to 26.5 GHz. Vibrational satellites for eleven states of the skeletal torsion were observed and measured. Vibrational transition frequencies were obtained from the solid-phase laser Raman spectrum and also by microwave intensity measurements. From these data the quantitative form of the potential function for the skeletal vibration was determined to be V(X) = 7.47(−0.702X² + X⁴) cm⁻¹. The barrier height is calculated to be 0.920 ± 0.20 cm⁻¹. As this is well below the energy of the v = 0 state, the molecule exists in the untwisted (C_3v) configuration.     

Raman and infrared spectra,conformational stability,vibrational assignment,barriers to internal rotation and ab initio calculations of but-2-enoyl chloride

The Raman (3500–10 cm⁻¹) and infrared (3200–50 cm⁻¹) spectra were recorded for the fluid and solid phases of but-2-enoyl chloride (crotonyl chloride), trans-CH₃CHCHCClO, where the methyl group is trans to the CClO group, and a complete vibrational assignment is proposed. These data were interpreted on the basis that the s-trans (anti) form (two double bonds oriented trans to one another) is the most stable form in the fluid phases and the only conformer remaining in the solid state. The asymmetric torsional fundamental of the more stable s-trans and the higher energy s-cis (syn) form were observed at 97.5 and 86.9 cm⁻¹, respectively. From these data the asymmetric potential function governing the internal rotation about the C C bond was determined. The potential coefficients are V₁ = −111 ± 2, V₂ = 1860 ± 48, V₃ = 6 ± 2, V₄, = −43 ± 24 and V₆ = −22 ± 6. The s-trans to s-cis and s-cis to s-trans barriers were determined to be 1890 and 1785 cm⁻¹, respectively, with an enthalpy difference between the conformers of 105 ± 52 cm⁻¹ [300 ± 149 cal mol⁻¹ (1 cal = 4.184 J)]. Similarly, the barrier governing internal rotation of the CH₃ group for the s-trans conformer was also determined to be 912 ± 30 (2.61 ± 0.09 kcal mol⁻¹) from the torsional fundamental observed in the far-infared spectrum of the gas. All these data were compared with the corresponding quantities obtained from ab initio Hartree–Fock gradient calculations employing the RHF/3–21G*, RHF/6–31G* and/or MP2/6–31G* basis sets. These results were compared with the corresponding quantities for some similar molecules.     

Influence of the spectral power distribution of a LED on the illuminance responsivity of a photometer

The measurement accuracy in the photometric quantities measured through photometer head is determined by the value of the spectral mismatch correction factor (c(S_t,S_s)), which is defined as a function of spectral power distribution of light sources, besides illuminance responsivity of the photometer head used. This factor is more important when photometric quantities of the light-emitting diode (LED) style optical sources, which radiate within relatively narrow spectral bands as compared with that of other optical sources, are being measured. Variations of the illuminance responsivities of various V(λ)-adopted photometer heads are discussed. High-power-colored LEDs, manufactured by Lumileds Lighting Co., were used as light sources and their relative spectral power distributions (RSPDs) were measured using a spectrometer-based optical setup. Dependences of the c(S_t,S_s) factors of three types of photometer heads (f₁′=1.4%, f₁′=0.8% and f₁′=0.5%) with wavelength and influences of the factors on the illuminance responsivities of photometer heads are presented.     

Microwave spectra of partially deuterated acetic acid: Internal rotation,potential energy function,and methyl group geometry

The microwave spectra of CH₂DCOOH and CHD₂COOH have been studied by means of microwave-microwave double resonance. For the asy rotamers torsional splittings (5898 and 530 MHz, respectively) and effective rotational constants were determined in the ground state. Effective barrier parameters were provisionally estimated and used to predict excited-state spectra. Here significant interaction between sy and asy rotamers occurred, and a Hamiltonian based on an extension of the IAM method to the case of an asymmetric internal rotor was used to account for the spectra. A few direct sy-asy transitions were observed as well as spectra originating from the second excited torsional state. Effective potential energy coefficients, V₁ through V₆, were determined accurately; apart from V₃ and V₆, which are comparable to values in CH₃COOH and CD₃COOH, large V₂ terms occur (28.5 cm^?1 in CH₂DCOOH and ?25.4 cm^?1 in CHD₂COOH). These terms provide localization in the ground state wave functions, and can be rationalized as arising from the zero-point energies of the other normal vibrations. Also determined were Fourier components of the rotational constants, which were in fair agreement with results from model calculations when geometry relaxation was included. After correction of the ground state inertial moments for effects of the torsion a consistent set of inertial moments was obtained for the various isotopic species, and a complete substitution structure could be determined. The HCH angles in the methyl group were found to differ by 2.7°.