PAM analysis of the microwave spectra of thioacetic acid,CH3COSH and CH3COSD

The microwave spectra of two isotopic species of thioacetic acid, CH₃COSH and CH₃COSD, have been studied. Using the principal axis method (PAM), including terms through n = 6 in the perturbation series and the denominator correction, the spectra were analyzed and 45 lines for CH₃COSH and 40 lines for CH₃COSD were assigned. The parameters obtained by the least-squares analysis are A = 9913.29 ± 0.56 MHz, B = 4923.11 ± 0.23 MHz, C = 3354.60 ± 0.24 MHz, θ = 57.080 ± 0.030°, s = 6.2980 ± 0.0012, and I_α = 3.198 ± 0.020 amu$\text{A}\text{?}{}^2$ for CH₃COSH, and A = 9662.80 ± 0.78 MHz, B = 4810.74 ± 0.26 MHz, C = 3273.92 ± 0.18 MHz, θ = 55.097 ± 0.024°, s = 5.9742 ± 0.0016, and I_α = 3.171 ± 0.020 amu$\text{A}\text{?}{}^2$ for CH₃COSD. The barrier to internal rotation of the methyl group is V₃ = 222.6 ± 1.4 cal/mole for CH₃COSH and V₃ = 212.9 ± 1.4 cal/mole for CH₃COSD. The Stark effect measurements of A species transitions for CH₃COSH led to the dipole moment μ = 1.821 ± 0.013 D with the components μ_a = 0.191 ± 0.010 D and μ_b = 1.811 ± 0.013 D.     

The microwave spectrum of a second N-gauche rotamer of allylamine

The microwave spectra of the ground and five excited states of a second gauche rotamer of allylamine have been measured and assigned. Three of the excited states belong to the same mode, most probably the CC torsion, the second and third vibrational states present a symmetrical splitting due to tunneling effect. The spectrum was conclusively identified as due to the N-gauche, lone-electron-pair trans form by means of the N-quadrupole coupling constants and dipole moment components. The variation observed for the quadrupole coupling constants in the different vibrationally excited states was explained by a suitable model. The ground state constants are (in MHz) A₀ = 23 957.05 ± 0.048, B₀ = 4 229.96 ± 0.025, C₀ = 4 154.91 ± 0.025, χ_aa = ? 1.48 ± 0.04, χ_bb - χ_cc = ? 1.42 ± 0.04, and (in D) ∥μ_a∥ = 0.766 ± 0.010, ∥μ_b∥ = 0.700 ± 0.005, ∥μ_c∥ = 0.290 ± 0.020.The excited states of the N-cis, lone-electron-pair trans form were also measured and assigned; two of these states appear to belong to the CC torsion as indicated by their intertial defects. The potential hindering the internal CC rotation was calculated using the relative intensity data of the N-cis and N-gauche forms as well as the tunneling splittings. A three-term cosine potential was fitted to the data yielding (in cm^?1) V₁ = ? 77 ± 85, V₂ = 170 ± 126, V₃ = 663 ± 95. The Dennison-Uhlenbeck potential was used for an approximate calculation of the N-trans barrier separating the two identical N-gauche forms. The barrier obtained was 1.9 ± 0.3 Kcal/mole.     

Infrared,Raman, and microwave spectra of ethaneselenol and the determination of the barriers to internal rotation

The infrared, Raman, and microwave spectra of gaseous ethaneselenol have been investigated. The rotational constants for both the more stable gauche and for the trans conformers are reported for the Et⁷⁸SeH, Et⁷⁸SeD, Et⁸⁰SeH, and Et⁸⁰SeD isotopic species. A proposed structure has been derived from a least-squares analysis of the moments of inertia. Dipole moment components have been obtained from each conformer using second-order Stark effects. For the gauche conformer, they are μ_a = 1.42 ± 0.01, μ_c = 0.37 ± 0.03, and μ_total = 1.47 ± 0.01 D. For the trans isomer they are μ_a = 1.217 ± 0.002, μ_b = 0.850 ± 0.001, and μ_total = 1.485 ± 0.002 D. The methyl barrier to internal rotation was calculated using observed frequencies obtained from the infrared and Raman spectra; a value of 3.59 ± 0.01 kcal/mole was obtained. Asymmetric potential functions have been calculated for both the EtSeH and EtSeD isotopic species. For the light species the potential constants for internal rotation around the CSe bond are V₂ = ?96.4 ± 1, V₃ = 432 ± 4, and V₆ = ?20 ± 2 cm^?1. The difference between ground-state energy levels of the two conformers was found to be 66 cm^?1. A vibrational assignment based on infrared and Raman spectra of the gaseous phase is presented.     

Microwave spectrum of gauche-isopropylphosphine

The microwave spectra of isopropylphosphine has been recorded in the region 12.4–40.0 GHz. Both a- and b-type transitions were observed and assigned. The rigid rotor rotational constants were determined to be A = 7633.34 ± 0.09, B = 4243.36 ± 0.02, and C = 3045.84 ± 0.02 MHz for (CH₃)₂CHPH₂ and A = 7226.47 ± 0.05, B = 4041.06 ± 0.02, and C = 2946.85 ± 0.02 MHz for (CH₃)₂CHPD₂. Dipole moment components of |μ_a| = 1.15 ± 0.01, |μ_b| = 0.43 ± 0.01, |μ_c| = 0.03 ± 0.02 and |μ_t| = 1.23 ± 0.01 were determined from the Stark effect. From the microwave spectra, the Stark effect and the experimental rotational constants, the assigned spectrum has been identified to result from the gauche form and this conformer is believed to be more stable than the other form which is present at room temperature.     

Microwave spectrum,barrier to internal methyl rotation,dipole moment,and partial structure of dimethylketene

From the microwave spectrum of dimethylketene which has been recorded from 8 to 37 GHz, the following rotational constants were derived: A = 8 267.832 ± 0.8, B = 3 884.101 ± 0.03, C = 2 728.826 + 0.03 MHz. The dipole moment is μ_a = 1.94 ± 0.01 D. Substitution coordinates for all methyl group atoms have been obtained by investigating the spectra of six isotopic species of the molecule. The potential barrier V₃ hindering internal rotation of the methyl tops has been fitted to the multiplet width of a number of high-J ground state ^aQ-transitions which were observed as triplets. V₃ is 2065 cal/mole, keeping fixed I_α = 3.132 amu Ų and angle (methyl-top to a-axis) = 58.94° as obtained from the partial substitution studies.     

Analysis of torsional splittings in the microwave spectra of dimethylether,CH3OCH3 and its isotopes,CD3OCH3 and CD3OCD3

The multiplet splitting patterns of microwave transitions in the ground state and the first two torsional excited states of CH₃OCH₃, CD₃OCD₃, and CD₃OCH₃ were analyzed in terms of the semirigid rotor models C_2vF-C_3vT-C_3vT and $\text{C}{}_3\text{F-}\text{C}{}_{\mathrm{3v}}\text{T-}\text{C}{}_{\mathrm{3v}}\text{T}\text{?}$. The following nonzero potential coefficients were obtained for CH₃OCH₃: V₃₀ = V₀₃ = 909.05 ± 0.49 cm^?1, V′₃₃ = 5.06 ± 1.60 cm^?1; for CD₃OCH₃: V₃₀(CD₃) = 897.18 ± 2.41 cm^?1, V₀₃(CH₃) = 910.45 ± 0.33 cm^?1; for CD₃OCD₃: V₃₀ = V₀₃ = 897.00 cm^?1. These results are compared to earlier microwave studies of these molecules.     

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.     

Microwave spectrum of tertiary-butylphosphine

The microwave spectra of tertiarybutylphosphine (CH₃)₃CPH₂, (CH₃)₃CPHD, and (CH₃)₃CPD₂ have been recorded in the region 26.5–40.3 GHz. Both a- and c-type transitions were observed and assigned for the “light” and “heavy” molecules and a-type transitions were observed and assigned for the d₁ species. The rigid rotor rotational constants were determined to be A = 4397.63 ± 0.04, B = 2878.88 ± 0.02, and C = 2870.86 ± 0.02 MHz for (CH₃)₃CPH₂ and A= 4261.98 ± 0.04, B = 2769.82 ± 0.02, and C = 2752.71 ± 0.02 MHz for (CH₃)₃CPD₂ and A = 4330 ± 2, B = 2831.45 ± 0.02, and C = 2801.50 ± 0.02 MHz for (CH₃)₃CPHD. Dipole moment components of |μ_a| = 1.06 ± 0.02, |μ_c| = 0.49 ± 0.02 and |μ_t| = 1.17 ± 0.02D were determined from the Stark effect. By assuming reasonable structural parameters for the tertiarybutyl and phosphine groups, a least-squares fit of the rotational constants gave $\text{λ}{}_{\mathrm{<mtext>P-C</mtext>}}\text{= 1.896}\text{A}\text{?}$ and $\text{?CPH = 95.7°}$. No splitting was observed of the first excited state of the phosphine torsional mode.     

Microwave spectrum of 3-methoxypropionitrile

The microwave spectrum of 3-methoxypropionitrile, CH₃OCH₂CH₂CN, has been investigated in the region 16 to 40 GHz. The spectrum reveals the presence of only one rotational isomer, the fully-trans form (with C₈ symmetry). For the ground vibrational state the rotational constants are A = 17 821 ± 14, B = 1425.526 ± 0.005, and C = 1353.909 ± 0.005MHz and the centrifugal distortion constants are D_J = 0.162 ± 0.010 and D_JK = ?10.28 ± 0.03kHz. Several series of vibrational satellite lines have been assigned to the torsional motions about the CH₂CH₂ and CH₂O bonds and to the CC≡N bending motion.     

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.     

Infrared spectroscopic studies of partially deuterated ethanes and the r0, rz, and re structures

Samples of CH₃CH₂D, CH₃CHD₂, CD₃CH₂D, and CD₃CHD₂ have been prepared, and their infrared spectra recorded. Analysis of type B or type C “perpendicular” bands has enabled the rotational parameter ($\text{A}{}_0\text{-}\text{B}{}_0$) to be determined for all four species. These have been combined with existing infrared, Raman, and microwave data for CH₃CH₃, CD₃CD₃, and CH₃CD₃ species, to determine the ground state (r₀) and ground state average (r_z) structures within narrow limits. Zero point energy effects on the average structure are determined to be a CH bond shortening of 0.0015(3) Å and an HCC angle opening of 0.010(5)° on deuteration. These effects enable the equilibrium structure of ethane to be estimated. The r_z(CC) bond length is determined to be 1.5351(2) Å, which is significantly longer than previous estimates involving electron diffraction data.     

Microwave spectrum,dipole moment,and conformation of 3,6-dioxabicyclo[3.1.0.]hexane

The microwave spectrum of 3,6-dioxabicyclo[3.1.0.]hexane has been obtained. The rotational lines of one ring conformation only have been observed and assigned. Ground state rotational constants are A₀ = 6287.302 ± 0.011 MHa, B₀ = 4683.546 ± 0.008 MHz, and C₀ = 3358.517 ± 0.089 MHz. The diploe moment components obtained from Stark effect measurements are μ_a = 0.276 ± 0.010 D and μ_c = 2.47 ± 0.04 giving μ = 2.485 ±0.040 for the dipole moment of the molecule. The rotational constants and dipole moment components obtained experimentally can be satisfactorily explained only if the boat form is the most stable ring conformation.     

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°.     

Microwave spectrum and rotational isomerism of 3,3-difluoropropene CHF2CHCH2

The analysis of the microwave spectrum of 3,3-difluoropropene has confirmed the existence of two rotational isomers, cis and gauche. The rotational constants in the ground vibrational state are A = 9126.08 MHz, B = 3722.120 MHz, and C = 2946.598 MHz for the cis form and A = 8901.64 MHz, B = 4192.759 MHz, and C = 3107.718 MHz for the gauche form. The dipole moment and its components along the principal axes of intertia are μ_a = 2.369 ± 0.015 D, μ_c = 0.70 ± 0.03 D, and μ_t = 2.47 ± 0.03 D for the cis form and μ_a = 1.535 ± 0.015 D, μ_b = 0.53 ± 0.04 D, μ_c = 1.36 ± 0.03 D, and μ_t = 2.12 ± 0.05 D for the gauche form. The relative intensity measurement indicates that the cis form is more stable than the gauche form by 260 ± 80 cm^?1. The energy of the first excited state with respect to the ground state was found to be 63 ± 8 cm^?1 for the cis form and 85 ± 10 cm^?1 for the gauche form. Two Fourier coefficients of the potential function restricting the torsion around the CC bond were determined to be V₁ = 266 ± 40 cm^?1 and V₃ = 508 ± 200 cm^?1, using the available data. The potential function thus obtained is compared to a prediction model which is derived assuming additivity of the potential as a function of substitution.     

Microwave spectra of CH314NC and CH315NC in the 4ν8 state

The J → J + 1 transitions (J = 4, 5, 6, 7, 8) in the microwave spectra of methyl isocyanide and its ¹⁵N derivative have been obtained and analyzed in the 4ν₈ degenerate vibrational state. Theoretical analytical expressions are given for the rotational frequencies in a 4ν_E state, separately for the l = 0, ±2, ±4 values. These formulas could only be used as a starting point for the assignment and analysis, because of the complexity of the spectrum and the number of accidental resonances appearing in many l = 0 and ±2 lines for low K values. A detailed analysis was obtained through a diagonalization of the energy matrix. Many types of A₁A₂ doublings could be localized; in particular for CH₃¹⁵NC the K, l = ±1, ±4 doubling allowed the calculation of the g₆ coefficient of the 〈K, l|H|, l ± 6〉 term. As in the 3ν₈ state, some lines seem to undergo the effects of a vibrational resonance. A set of constants is given for both species, and a comparison is made with the other states.     

Microwave spectra of deuterated ethylenes: Dipole moment and rz structure

The pure rotational spectra of three deuterated ethylenes, CH₂CD₂, CH₂CHD, and cis-CHDCHD, were observed by microwave spectroscopy, and the rotational and centrifugal distortion constants were determined precisely. The dipole moment of CH₂CD₂ was calculated from the Stark effects to be 0.0091 ± 0.0004 D. From the observed rotational constants the average structure was calculated to be $\text{r}{}_{\mathrm{z}}\text{(CC)}\text{= 1.3391 ± 0.0013}\text{A}\text{?}$, $\text{r}{}_{\mathrm{z}}\text{(CH)}\text{= 1.0869 ± 0.0013}\text{A}\text{?}$, θ_z(CCH) = 121.28 ± 0.10°, and $\text{r}{}_{\mathrm{z}}\text{(CH)}\text{- r}{}_{\mathrm{z}}\text{(CD)}\text{= 0.00137 ± 0.00037}\text{A}\text{?}$, where the errors include one standard deviation in the fitting and errors due to an uncertainty (±0.03°) in θ_z(CCH) - θ_z(CCD).     

Microwave spectrum and rotational isomerism in isopropyl mercaptan

A microwave investigation of isopropyl mercaptan has established the existence of both trans and gauche conformers, the trans being more stable by 57 cal mole^?1. Stark effect measurements give the dipole moments as 1.61 ± 0.2 D for the trans and 1.53 ± 0.2 D for the gauche species. The spectra of the isotopic species (CH₃)₂CH³²SD, (CH₃)₂CH³⁴SH, and (CH₃)₂CH³⁴SD of the trans form have also been analyzed, providing a limited amount of structural data.The rotational spectrum of the gauche isomer is noticeably influenced by inversion. Interactions between energy levels in the two lowest inversion states have been satisfactorily accounted for in terms of rotational constants, coupling parameters (G_a and G_c), and ΔE₀, the inversion level splitting. ΔE₀ is found to be 562.4 MHz for the ground state of (CH₃)₂CHSH and 10.0 MHz for (CH₃)₂CHSD. A value of 1.98 kcal mole^?1 has been calculated for the barrier to internal rotation of the -SH group in terms of a V₃ potential.     

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)).     

The microwave spectrum of trans-ethylamine

The microwave spectrum of normal trans-ethylamine CH₃CH₂NH₂ and that of the -NHD and -ND₂ species were measured and assigned. The obtained rotational constants for the ground state of the normal species are (in MHz): A = 31 758.33 ± 0.08, B = 8749.157 ± 0.025, and C = 7798.905 ± 0.025. The fitted dipole moment components are (in Debye): |μ|_a = 1.057 ± 0.006, |μ_b| = 0.764 ± 0.009, and |μ_t| = 1.304 ± 0.011. The quadrupole coupling constants were fitted as (in MHz): χ⁺ = 1.62 ± 0.035 and χ^? = ?1.89 ± 0.08. Analysis of the HFS of the deuterated species -ND₂ allowed the experimental determination of the principal quadrupole tensor values (in MHz): χ_zz = ?4.68 ± 0.20, χ_yy = 1.75 ± 0.06, and χ_xx = 2.93 ± 0.20. The angle between the CN bond and the direction of the χ_zz quadrupole tensor component was fitted as 108.9° ± 0.6° and agreed with the expected general direction of the lone electron pair.     

Microwave spectrum of CH3OD

The microwave spectrum of CH₃OD has been observed in the frequency region between 14 and 92 GHz. All the ground-state transitions with J ≤ 8 and J = 2 ← 1, a-type transitions in the excited torsional states (v = 1 and v = 2) have been observed. The spectrum has been analyzed and rotational constants, torsional constants, torsion-vibration-rotation interaction constants, and centrifugal distortion constants have been evaluated. The Stark effect measurements have been made and the dipole moment components have been determined as μ_a = 0.833 ± 0.008 D and μ_b = 1.488 ± 0.015 D.