Microwave spectrum,conformation, quadrupole coupling constants,and barrier to internal rotation of methoxyamine

The microwave spectra of three isotopic species of methoxyamine (CH₃ONH₂) have been studied. For the normal species the ground-state rotational constants are A = 42488 ± 150 MHz, B = 10049.59 ± 0.03 MHz, and C = 8962.85 ± 0.03 MHz. From these data and those from the -NHD and -ND₂ species, the amino protons have been shown to occupy a symmetrical trans position relative to the methyl group. The barrier to internal rotation of the methyl group has been found to be 873 ± 15 cm^?1 by analysis of ground-state splittings. Analysis of hyperfine splittings has yielded the ¹⁴N quadrupole coupling constants, which have the following values for the normal isotopic species: χ_aa = 3.63 ± 0.03 MHz, χ_bb = ?3.69 ± 0.07 MHz, and χ_cc = 0.06 ± 0.07 MHz.     

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.     

Methyl group tilt and structure from rotational spectra of the CH2DNHCl monodeuterated species

The structure and tilt of the methyl group of CH₃NHCl have been determined by analysis of the three different ground state rotational spectra of the methyl monodeuterated species. The tilt was found to be ?3.5° which is of the same order of magnitude and towards the unshared pair of electrons at the nitrogen atom as in methylamine and its derivatives.The barrier to internal rotation was determined form the A-E splittings of the previously measured transitions and found to be 3781 ± 14 cal/mole for CH₃NHCl and 3784 ± 15 cal/mole for CH₃NDCl, while the values obtained under the assumptions of no tilt and of asymmetric methyl group as in methylamine were 3707 ± 30 cal/mole for CH₃NHCl and 3726 ± 25 cal/mole for CH₃NDCl. Some information has also been deduced on the orientation of the z axis of the principal quadrupole coupling tensor.     

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.     

Microwave spectrum,structure, dipole moment,and internal rotation of the trans isomer of ethyl methyl sulfide

The microwave spectra of the trans isomer of ethyl methyl sulfide and its 10 isotopic species were measured. The r_s structure of this isomer was determined from the observed moments of inertia. The dipole moment and its direction in the molecule were determined by Stark effect measurements of low J transitions for the normal and CH₃CH₂SCD₃ species. The barrier to internal rotation of the SCH₃ group was calculated from the observed A-E splittings of the transitions. The present results were compared with those for the analogous molecules.     

Spectra and structure of organogermanes: Microwave and Raman spectra of methyltrichlorogermane

The microwave spectrum of methyltrichlorogermane has been investigated in the region 26.5 to 40.0 GHz. The ground state rotational constants, B, were found to be 1602.19, 1601.42, 1601.10, 1600.71, 1600.02, 1537.84, 1537.10, and 1536.36 MHz for the symmetric top molecules CH₃⁷⁰Ge³⁵Cl₃, CH₃⁷²Ge³⁵Cl₃, CH₃⁷³Ge³⁵Cl₃, CH₃⁷⁴Ge³⁵Cl₃, CH₃⁷⁶Ge³⁵Cl₃, CH₃⁷⁰Ge³⁷Cl₃, CH₃⁷²Ge³⁷Cl₃, and CH₃⁷⁴Ge³⁷Cl₃, respectively. For the asymmetric top molecules CH₃⁷²Ge³⁵Cl₂³⁷Cl and CH₃⁷⁴Ge³⁵Cl₂³⁷Cl the ground state rotational constants A, B, and C were found to be 1597.96, 1559.31, 1203 and 1597.17, 1558.59, 1207 MHz, respectively. From the rotational constants the r_s values for the GeCl bond distance of 2.135 ± 0.006 Å and the CGeCl bond angle of 106.0 ± 0.7° were obtained. The centrifugal distortion constant for the CH₃Ge³⁵Cl₃ species was calculated to be 0.35 ± 0.08 kHz. The Raman spectra of methyltrichlorogermane has been recorded in the gas phase and the methyl torsional overtone (Δν = 2) was observed. From the observed frequency shift the barrier to internal rotation has been calculated to be 1.45 kcal/mole.     

Microwave spectrum of chloromethyl methyl ether

The microwave spectrum of chloromethyl methyl ether has been studied in the region 12.4–40 GHz. For ³⁵Cl species, a- and c-type transitions have been assigned for the ground state, the first excited state of the chloromethyl torsional mode, and the first excited state of the methyl torsional mode. Assignments were also made for the ground state of ³⁷Cl species. The assigned transitions are due to the gauche conformer. The nuclear quadrupole coupling constants were determined for the ground state of ³⁵Cl and ³⁷Cl species. The observed A-E splittings of the rotational transitions arising from the three vibrational states indicate a strong coupling between the two torsional vibrations. A model calculation based on the Hamiltonian previously used by Butcher and Wilson (J. Chem. Phys.40, 1671 (1964)), was carried out to account for the splittings and the vibrational frequencies of the two torsional modes. The barrier to internal rotation of the methyl group is estimated to be V₃ = 647 ± 17 cm^?1 (1.84 ± 0.05 kcal/mole).     

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 the heterodimers: CH3OH-CO2 and CH3OH-H2CO

Molecular complexes, dimers and heterodimers often show interesting structures, large amplitude internal motions and orientations for reaction coordinates. These properties were the motivations for the current study of the rotational spectra of the heterodimers, CH₃OH-CO₂ and CH₃OH-H₂CO, in a pulsed nozzle Fourier-transform microwave (FTMW) spectrometer. In addition to studying the normal isotopic forms, several isotopologues containing ¹³C or deuterium substituted atoms of each heterodimer were analyzed in order to obtain structural data of the complexes. All species showed splittings from internal rotation of the methyl group and splittings on the b-type transitions of the CH₃OH-H₂CO species suggesting rotation of the H₂CO group between equivalent structural forms. Stark effect measurements on each of the parent species provided dipole moment components. Theoretical ab initio results are compared to the experimentally determined molecular parameters.     

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.     

Variation of Br quadrupole coupling with isotopic substitution in methyl bromide

The J = 0 ← 1 transitions in CH₃⁷⁹Br (I), CH₃⁸¹Br (II), CD₃⁷⁹Br (III), and CD₃⁸¹Br (IV) were measured using a Stark-cell spectrometer constructed from C-band waveguide. High-resolution spectra yielded precise values for the bromine quadrupole coupling strength. Values obtained were eqQ(I) = ?577.08 ± 0.15 MHz, eqQ(II) = ?482.18 ± 0.15 MHz, eqQ(III) = ?575.66 ± 0.15 MHz, and eqQ(IV) = ?480.89 ± 0.15 MHz. The observed center frequencies for the J = 0 ← 1 transitions are ν₀(I) = 19136.35 ± 0.03 MHz, ν₀(II) = 19063.62 ± 0.03 MHz, ν₀(III) = 15429.23 ± 0.03 MHz, and ν₀(IV) = 15362.41 ± 0.03 MHz. A 0.26 ± 0.02% decrease in bromine quadrupole coupling is observed when the methyl group is fully deuterated. This is in agreement with, and supports interpretations given for, previous results on methyl chloride.     

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 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 spectrum and barrier to internal rotation of 1-chloro-2-butyne

Rotational transitions of the μ_a and μ_b type have been identified with microwave-microwave double resonance measurements for 1-chloro-2-butyne in the ground vibrational state. In the first excited state of the methyl torsion only μ_a-type transitions have been identified. The A-type transitions of the ground vibrational state can be described perfectly by the rigid rotor approximation with centrifugal corrections. Using the internal axis method the barrier to internal rotation was determined from the A,E splittings: V₃ = 10.05 ± 0.09 cm⁻¹. A model which allowed for geometry relaxation upon internal rotation was used to fit one set of parameters to the transition frequencies of both ground state and first excited torsional state. The sixfold contribution to the barrier was found to be negligible: V₆ = −0.4 ± 0.3 cm⁻¹.     

Microwave spectrum of the gauche and trans isomers of methylsilanethiol: Determination of molecular structure,dipole moment,and internal rotation potential functions of methyl and mercapto groups

Microwave spectra of methylsilanethiol and three of its deuterated species were measured and assigned for the gauche and trans isomers. The double minimum splitting due to internal rotation of the mercapto group in the gauche isomer was directly observed in c-type transitions for all the species measured. Rotational constants and the pure torsional energy difference, Δν, between the (+) and (−) states in the gauche isomer of the parent species were determined to be A(+) = 15 567.654 ± 0.040, B(+) = 3663.038 ± 0.004, C(+) = 3179.727 ± 0.005, ΔA = −4.328 ± 0.021, ΔB = −0.220 ± 0.012, ΔC = −0.008 ± 0.011, and Δν = 2826.371 ± 0.045 MHz, where A(+) represents the A rotational constant of the (+) state and ΔA = A(−) – A(+) and so on. For the trans isomer of the parent species, the following rotational constants were determined: A = 14 745.953 ± 0.051, B = 3841.291 ± 0.010, C = 3220.350 ± 0.010. Additional splittings due to internal rotation of the methyl group were also observed for both of the isomers. Analysis of these splittings derived barrier heights of the methyl internal rotations to be 1581 ± 26 and 1729 ± 23 cal/mol for the trans and gauche isomers of the parent species, respectively. Dipole moments were obtained from Stark effect measurement to be 1.056 ± 0.006 and 1.604 ± 0.006 D for the trans and gauche isomers of the parent species, respectively. Potential function of the mercapto internal rotation and plausible structures for both the isomers were discussed.     

The microwave spectrum,structure, and dipole moment of the unstable molecule phosphaethene,CH2PH

A detailed rotational analysis of the microwave spectrum between 26.5 and 40 GHz of phosphaethene, CH₂PH, has been carried out. This molecule is the simplest member of a new class of unstable molecules—the phosphaalkenes. The species can be produced by pyrolysis of (CH₃)₂PH, CH₃PH₂ and also somewhat more efficiently from Si(CH₃)₃CH₂PH₂. Full first-order centrifugal distortion analyses have been carried out for both ¹²CH₂³¹PH and ¹²CH₂³¹PD yielding: A₀ = 138 503.20(21), B₀ = 16 418.105(26), and C₀ = 14 649.084(28) MHz for ¹²CH₂³¹PH. The 1₀₁-0₀₀μ_A lines have also been detected for ¹³CH₂PH, cis-CDHPH and trans-CHDPH. These data have enabled an accurate structure determination to be carried out which indicates: $\text{r(}\text{H}{}_{\mathrm{c}}\text{C}\text{) = 1.09 ± 0.015}\text{A}\text{?}\text{, ∠(}\text{H}{}_{\mathrm{c}}\text{CP}\text{) = 124.4 ± 0.8°; r(}\text{H}{}_{\mathrm{t}}\text{C}\text{) = 1.09 ± 0.015}\text{A}\text{?}\text{, ∠(}\text{H}{}_{\mathrm{t}}\text{CP}\text{) = 118.4 ± 1.2°; r(}\text{CP}\text{) = 1.673 ± 0.002}\text{A}\text{?}\text{, ∠(}\text{HCH}\text{) = 117.2 ± 1.2°; r(}\text{PH}\text{) = 1.420 ± 0.006}\text{A}\text{?}\text{, ∠(}\text{CPH}\text{) = 97.4 ± 0.4°}$. The dipole moment components have been determined as μ_A = 0.731 (2), μ_B = 0.470 (3), μ = 0.869 (3) D for CH₂PH; μ_A = 0.710 (2), μ_B = 0.509 (10), μ = 0.874 (7) D for CH₂PD.     

Internal rotation in methyl silane by avoided-crossing molecular-beam spectroscopy

The avoided-crossing molecular-beam electric-resonance technique was applied to methyl silane in the ground torsional state. A new type of anticrossing is introduced which breaks the torsional symmetry and obeys the selection rules ΔJ = 0, K = +1 /a3 ?1. For these “barrier” anticrossings, the values of the crossing fields E_c yield directly the internal rotation splittings; the E_c are independent of the difference (A-B) in the rotational constants. Such anticrossings were observed for J from 1 to 6. Studies were also conducted of several “rotational” anticrossings (J, K) = (1, ±1) /a3 (2, 0) for which E_c does depend on (A-B). The normal rotational transition (J, K) = (1, 0) ← (0, 0) was observed in the ground torsional state using the molecular beam spectrometer. The present data on CH₃²⁸SiH₃ were combined with Hirota's microwave spectra and analyzed with the torsion-rotation Hamiltonian including all quartic centrifugal distortion terms. In addition to evaluating B and several distortion constants, determinations were made of the moment of inertia of the methyl top I_α = 3.165(5) amu-Ų, the effective rotational constant A^eff = 56 189.449(32) MHz, and the effective height of the threefold barrier to internal rotation V₃^eff = 592.3359(73) cm^?1. The correlations leading to these two effective constants are discussed and the true values of A and V₃ are determined within certain approximations. For the isotopic species CH₃³⁰SiH₃, barrier and rotational anticrossings were observed. The isotopic changes in A and V₃ were determined, as well as an upper limit to the corresponding change in I_α.     

Dynamical structure of peptide molecules: Fourier transform microwave spectroscopy of N-methylpropionamide

In order to clarify the dynamical aspects of the peptide structure, N-methylpropionamide (NMPA) was investigated as an example of peptide molecules: XCONHY (X=CH₃CH₂ and Y=CH₃ for NMPA), paying special attention to the internal rotation of the two methyl groups. NMPA was found to have an almost planar skeleton with an extended syn/trans conformation, as indicated by the observed value of I_aa+I_bb−I_cc, and its rotational spectra were interpreted in terms of group G₁₈ consisting of six symmetry species: A₁, A₂, E₁, E₂, E₃, and E₄. The A₁ and E₂ spectra were observed split in most of b-type transitions, yielding the internal-rotation potential barrier V₃ of 796 (21) cm⁻¹ for CH₃ in the ethyl group referred to as C-CH₃. The spectra of the three E species: E₁, E₃, and E₄ appeared several tens to thousands MHz apart from the corresponding A₁ spectra, suggesting the internal-rotation potential barrier of CH₃ bonded to the nitrogen, called N-CH₃, to be quite low. In sharp contrast with the A₁ spectra, which were well fitted to the ordinary asymmetric-rotor spectral pattern, a few higher-order terms were required to reproduce the E₁ spectra, presumably because of the low N-CH₃ barrier. The spectral analysis thus performed, in fact, led to the V₃ of 80.06487 (14) cm⁻¹, an order of magnitude lower than that of C-CH₃. The E₃ and E₄ spectra were found to form triplets with the corresponding E₁ lines at the center, and the E₃-E₁ and E₄-E₁ splittings were explained essentially by the contributions of the C-CH₃ internal rotation combined with the kinetic-energy coupling between the two methyl groups. The torsion around the C-C bond between the ethyl and carbonyl groups was suggested by an ab initio calculation to be of double minimum nature, but the observed A₁ spectra did not show any indication of such a double-minimum potential for the C-C torsion, although the possibility of a small hump being present at a planar conformation could not be entirely eliminated. The present results on NMPA along with those obtained on other peptide molecules will be of some significance in clarifying important problems of structural biology such as protein folding and signal transfer through biological systems.     

Microwave and Raman spectra,dipole moment,barrier to internal rotation,and vibrational assignment of silylphosphine-d2

The microwave spectra of SiH₃PD₂ have been recorded in the range 26.5–40.0 GHz. Both a- and c-type transitions were observed and assigned. The rigid rotor rotational constants were determined to be A = 37589.06 ± 0.11, B = 5315.70 ± 0.02, and C = 5258.70 ± 0.02 MHz. The barrier to internal rotation has been calculated from the A-E splittings to be 1512 ± 26 cal/mole. The dipole moment components of |μ_a| = 0.22 ± 0.01, |μ_c| = 0.56 ± 0.01, and |μ_t| = 0.60 ± 0.01 D were determined from the Stark effect. By using previously determined microwave data for SiH₃PH₂, several structural parameters have been calculated and their values are compared to similar ones in other compounds. The Raman (0–2500 cm^?1) spectra of gaseous, liquid, and solid SiH₃PH₂ and gaseous SiH₃PD₂ have been recorded and interpreted in detail on the basis of C_s molecular symmetry.