Methyl group internal rotation A–E line splittings in several torsionally excited states of methyl glycolate and 2-methoxyethanol

The rotational spectra of several torsional satellites of methyl glycolate and 2-methoxyethanol have been investigated.The methyl barrier to internal rotation in methyl glycolate increases with the torsional quantum number of the C-C skeletal torsion, for which A–E line splittings have been measured up to v_SK=3. The V₃ value determined from the A–E line splittings in the first excited state of the methyl internal rotation is nearly the same of that previously determined in the ground state.For 2-methoxyethanol the V₃ barrier as determined from the A–E line splittings in the ground state is about 20% lower than the value previously obtained from the splittings observed in the first excited state of the methyl internal rotation. The sequence of the A–E splittings in the first excited state of the O-C skeletal torsion (v_SK=1) is probably reversed with respect to the ground state, while in the v_SK=2 state the sequence is like that in the ground state.     

The Structure and Torsional Dynamics of Two Methyl Groups in 2‐Acetyl‐5‐methylfuran as Observed by Microwave Spectroscopy

The molecular‐beam Fourier transform microwave spectrum of 2‐acetyl‐5‐methylfuran is recorded in the frequency range 2–26.5 GHz. Quantum chemical calculations calculate two conformers with trans or cis configuration of the acetyl group, both of which are assigned in the experimental spectrum. All rotational transitions split into quintets due to the internal rotations of two nonequivalent methyl groups. By using the program XIAM, the experimental spectra can be simulated with standard deviations within the measurement accuracy, and yield well‐determined rotational and internal rotation parameters, inter alia the V₃ potentials. Whereas the V₃ barrier height of the ring‐methyl rotor does not change for the two conformers, that of the acetyl‐methyl rotor differs by about 100 cm^?1. The predicted values from quantum chemistry are only on the correct order of magnitude.     

Matrix and gas infrared spectra of nitroethane isotopic species

IR and FIR gas phase and Ar matrix isolation spectra of the isotopic species of nitroethane, CH₃CH₂NO₂, CH₃CD₂NO₂ and CD₃CD₂NO₂ are reported. An assignment will be proposed, based mainly on matrix spectra and normal coordinate analysis. An approximate 25 parameter valence force field is given. The analysis of the gas phase spectra is complicated by the fact that only very few band contours comparable to quasirigid model envelopes occur. The phenomenon and unusual band widths and splittings in the matrix spectra are related to the nearly free internal rotation of the nitro group. The Q branch sequence associated with the methyl torsion shows a remarkable splitting into doublets probably also related to the NO₂ rotation. A value of 1080 ± 20 cm^?1 for the methyl torsional barrier is derived.     

Probing the Methyl Torsional Barriers of the E and Z Isomers of Butadienyl Acetate by Microwave Spectroscopy

The Fourier transform microwave spectra of the E and Z isomers of butadienyl acetate were measured in the frequency range from 2 to 26.5 GHz under molecular‐jet conditions. The most stable conformer of each isomer, in which all heavy atoms are located in a symmetry plane, was identified after analyzing the spectrum by comparison with the results from quantum‐chemical calculations. The barriers to internal rotation of the acetyl methyl group were found to be 149.1822(20) and 150.2128(48) cm^?1 for the E and Z isomers, respectively, which are similar to that of vinyl acetate. A comparison between two theoretical approaches treating internal rotation, the rho axis method and combined axis method, was also performed. The influence of the alkyl R chain on the methyl torsional barriers in CH₃ ‐COOR acetates was explored.     

Microwave Spectrum of Two-Top Molecule: 2-Acetyl-3-Methylthiophene

The microwave spectrum of 2-acetyl-3-methylthiophene (2A3MT) was recorded in the frequency range from 2 to 26.5 GHz using a molecular jet Fourier transform microwave spectrometer and could be fully assigned to the anti-conformer of the molecule, while the syn-conformer was not observable. Torsional splittings of all rotational transitions in quintets due to internal rotations of the acetyl methyl and the ring methyl groups were resolved and analyzed, yielding barriers to internal rotation of 306.184(46) cm⁻¹ and 321.813(64) cm⁻¹, respectively. The rotational and centrifugal distortion constants were determined with high accuracy, and the experimental values are compared to those derived from quantum chemical calculations. The experimentally determined inertial defect supports the conclusion that anti-2A3MT is planar, even though a number of MP2 calculations predicted the contrary.     

Microwave spectrum,dipole moment and AB initio structure of iminopyruvonitrile (CH3C(CN)NH)

The microwave spectrum of iminopyruvonitrile has been investigated from 19 to 51 GHz. Rotational transitions have been assigned for the vibrational ground state and the rotational and centrifugal distortion constants have been adjusted. The electric dipole components μ_a = 1.806(6), μ_b = 0.759(21) and μ_total = 1.958(10) D have been deduced from Stark splittings. Some rotational transitions showed splittings arising from the internal rotation of the methyl group, the barrier to which has been determined to be V₃ = 593.5(89) cm⁻¹. Different conformations have been predicted via ab initio calculations and are compared to the microwave results.     

Sensing the Molecular Structures of Hexan-2-one by Internal Rotation and Microwave Spectroscopy

Using two molecular jet Fourier transform spectrometers, the microwave spectrum of hexan-2-one, also called methyl n-butyl ketone, was recorded in the frequency range from 2 to 40 GHz. Three conformers were assigned and fine splittings caused by the internal rotations of the two terminal methyl groups were analyzed. For the acetyl methyl group CH₃ COC₃H₆CH₃, the torsional barrier is 186.9198(50) cm⁻¹, 233.5913(97) cm⁻¹, and 182.2481(25) cm⁻¹ for the three observed conformers, respectively. The value of this parameter could be linked to the structure of the individual conformer, which enabled us to create a rule for predicting the barrier height of the acetyl methyl torsion in ketones. The very small splittings arising from the internal rotation of the butyl methyl group CH₃COC₃H₆ CH₃ could be resolved as well, yielding the respective torsional barriers of 979.99(88) cm⁻¹, 1016.30(77) cm⁻¹, and 961.9(32) cm⁻¹.     

Acetyl Methyl Torsion in N‐Ethylacetamide: A Challenge for Microwave Spectroscopy and Quantum Chemistry

The gas‐phase structures and parameters describing acetyl methyl torsion of N‐ethylacetamide are determined with high accuracy, using a combination of molecular beam Fourier‐transform microwave spectroscopy and quantum chemical calculations. Conformational studies at the MP2 level of theory yield four minima on the energy surface. The most energetically favorable conformer, which possesses C₁ symmetry, is assigned. Due to the torsional barrier of 73.4782(1) cm^?1 of the acetyl methyl group, fine splitting up to 4.9 GHz is found in the spectrum. The conformational structure is not only confirmed by the rotational constants, but also by the orientation of the internal rotor. The ¹⁴N quadrupole hyperfine splittings are analyzed and the deduced coupling constants are compared with the calculated values.     

Conformational Equilibria and Large‐Amplitude Motions in Dimers of Carboxylic Acids: Rotational Spectrum of Acetic Acid–Difluoroacetic Acid

We report the rotational spectra of two conformers of the acetic acid–difluoroacetic acid adduct (CH₃COOH–CHF₂COOH) and supply information on its internal dynamics. The two conformers differ from each other, depending on the trans or gauche orientation of the terminal ?CHF₂ group. Both conformers display splittings of the rotational transitions, due to the internal rotation of the methyl group of acetic acid. The corresponding barriers are determined to be V₃(trans)=99.8(3) and V₃(gauche)=90.5(9) cm^?1 (where V₃ is the methyl rotation barrier height). The gauche form displays a further doubling of the rotational transitions, due to the tunneling motion of the ?CHF₂ group between its two equivalent conformations. The corresponding B₂ barrier is estimated to be 108(2) cm^?1. The increase in the distance between the two monomers upon OH→OD deuteration (the Ubbelohde effect) is determined.     

Microwave spectra and barriers to internal rotation of z- and e-1-propenyl isocyanide

A synthetic procedure yielding a mixture of Z- and E-1-propenyl isocyanide (CH(3)CH═CHNC) is described. The microwave spectrum of this mixture has been recorded in the 12-100 GHz spectral range, and the spectra of the Z and E isomers have been assigned for the first time. Most transitions of the Z form were split into two components of equal intensity due to tunneling of the methyl group, which allowed the barrier to internal rotation of this group to be determined as 4.0124(12) kJ/mol by fitting 568 transitions with a maximum value of J = 46 using the computer program Xiam. This fit had a root-mean-square deviation as large as 4.325. The same transitions were therefore fitted anew using the more sophisticated program Erham. This fit has a rms deviation marginally better (4.136) than the Xiam fit. No split MW lines were found for E-1-propenyl isocyanide. The absence of splittings is ascribed to a barrier to internal rotation of the methyl group that is significantly higher than the barrier of the Z isomer. It is concluded that the barrier must be larger than 6 kJ/mol for the E form. The experimental work was augmented by quantum chemical calculations at CCSD/cc-pVTZ, B3LYP/cc-pVTZ, and MP2/cc-pVTZ levels of theory. The CCSD method predicts rotational constants of the Z and E forms well. The B3LYP barriers to internal rotation of a series of substituted propenes were calculated and found to be in good agreement with experiments. Calculations of the quartic centrifugal distortion constants of the two 1-propenyl isocyanides by the B3LYP and MP2 methods were less successful.     

A New Insight into the Stereoelectronic Control of the Pd0-Catalyzed Allylic Substitution: Application for the Synthesis of Multisubstituted Pyran-2-ones via an Unusual 1,3-Transposition

Pyran-2-ones 3 undergo a novel Pd⁰-catalyzed 1,3-rearrangement to afford isomers 6 . The reaction proceeds via an η²-Pd complex, the pyramidalization of which (confirmed by quantum chemistry calculations) offers a favorable antiperiplanar alignment of the Pd−C and allylic C−O bonds ( C ), thus allowing the formation of an η³-Pd intermediate. Subsequent rotation and rate-limiting recombination with the carboxylate arm then gives isomeric pyran-2-ones 6 . The calculated free energies reproduce the observed kinetics semi-quantitatively.     

Wetting vs Droplet Aggregation: A Broadband Rotational Spectroscopic Study of 3-Methylcatechol⋅⋅⋅Water Clusters

Two competing solvation pathways of 3-methylcatechol (MC), an atmospherically relevant aromatic molecule, with up to five water molecules were explored in detail by using a combination of broadband rotational spectroscopy and computational chemistry. Theoretically, two different pathways of solvation emerge: the commonly observed droplet pathway which involves preferential binding among the water molecules while the solute serves as an anchor point for the formation of a water cluster, and an unexpected wetting pathway which involves interactions between the water molecules and the aromatic face of MC, i.e., a wetting of the π-surface. Conclusive identification of the MC hydrate structures, and therefore the wetting pathway, was facilitated by rotational spectra of the parent MC hydrates and several H₂¹⁸O and ¹³C isotopologues which exhibit splittings associated with methyl internal rotation and/or water tunneling motions. Theoretical modelling and analyses offer insights into the tunneling and conversion barriers associated with the observed hydrate conformers and the nature of the non-covalent interactions involved in choosing the unusual wetting pathway.     

Microwave spectrum,structure, dipole moment and internal rotation of ethyl fluorosilane

Microwave spectra of the trans and gauche isomers of ethyl fluorosilane and their eleven isotopically substituted species have been measured. The r_s structures of the two isomers were determined from the observed moments of inertia. The molecular structures found for the two isomers in the present study are compared with those of analogous molecules. Dipole moments of the two isomers were determined by Stark-effect measurements and are also compared with those of analogous molecules. The energy difference between the trans and gauche isomers was obtained from the relative intensity measurements of the spectra and the barrier to internal rotation of the methyl group for the gauche isomer was obtained from the A—E splittings of the spectra in the first excited methyl torsional state. The V₃ value was 2775 ± 25 cal mol^?1.     

Microwave spectrum of propionyl chloride

The microwave spectrum of propionyl chloride has been investigated in the region 18.0–40.0 GHz, and transitions due to a cis conformer have been assigned. This form has a heavy atom planar configuration and the methyl group and the carbonyl oxygen atom are cis to each other. Using the substitution structures of propionic acid and acetyl chloride as molecular models for the propionyl chloride molecule, good agreement is found between observed and calculateò effective rotational constants. For the ³⁵Cl species satellite spectra assigned to the first four excited states of the C-C torsional mode have been observed together with the first excited state of the methyl torsional mode. The ground state spectrum has also been assigned for the ³⁷Cl species. Relative intensity measurements yielded the lowest C-C torsional vibration frequency of 86 ± 10 cm^?1. The CH₃ internal rotation frequency was found to be 197 cm^?1. Nuclear quadrupole coupling constants were determined for the ground state of the ³⁵Cl and ³⁷Cl species. From observed A-E splittings of ^bQ-branch transitions of the first excited state of the methyl torsional mode a barrier to internal rotation was estimated to be V₃ = 2480 ± 40 cal mol^?1 (867 ± 14 cm^?1).     

Methyl Internal Rotation in Fruit Esters: Chain-Length Effect Observed in the Microwave Spectrum of Methyl Hexanoate

The gas-phase structures of the fruit ester methyl hexanoate, CH₃-O-(C=O)-C₅H₁₁, have been determined using a combination of molecular jet Fourier-transform microwave spectroscopy and quantum chemistry. The microwave spectrum was measured in the frequency range of 3 to 23 GHz. Two conformers were assigned, one with C_s symmetry and the other with C₁ symmetry where the γ-carbon atom of the hexyl chain is in a gauche orientation in relation to the carbonyl bond. Splittings of all rotational lines into doublets were observed due to internal rotation of the methoxy methyl group CH₃-O, from which torsional barriers of 417 cm⁻¹ and 415 cm⁻¹, respectively, could be deduced. Rotational constants obtained from geometry optimizations at various levels of theory were compared to the experimental values, confirming the soft degree of freedom of the (C=O)-C bond observed for the C₁ conformer of shorter methyl alkynoates like methyl butyrate and methyl valerate. Comparison of the barriers to methyl internal rotation of methyl hexanoate to those of other CH₃-O-(C=O)-R molecules leads to the conclusion that though the barrier height is relatively constant at about 420 cm⁻¹, it decreases in molecules with longer R.     

Chloromethane–Water Adduct: Rotational Spectrum,Weak Hydrogen Bonds,and Internal Dynamics

The rotational spectra of four isotopologues of the 1:1 complex between chloromethane and water revealed the presence of only one rotamer in a pulsed jet expansion. The two subunits are linked through two weak hydrogen bonds, O? H???Cl (R_H???Cl=2.638(2) Å) and C? H???O (R_H???O=2.501(2) Å), forming a five‐membered ring. All transitions display the hyperfine structure due to the ³⁵Cl (or ³⁷Cl) nuclear quadrupole effects. Dynamical features in the spectrum are caused by two large‐amplitude motions. Each component line appears as an asymmetric doublet with a relative intensity ratio of 1:3. The splittings led to the determination of barrier to internal rotation of water around its symmetry axis, V₂=320(10) cm^?1. Finally, an unexpected small value of the inertial defect (?0.96 uŲ rather than ?3.22 uŲ) allowed the estimation of the barrier to the internal rotation of the CH₃ group, V₃≈8 cm^?1.     

Conformational properties of methyl vinyl sulfone: Ab initio geometry optimization and vibrational analysis

An ab initio conformational analysis of methyl vinyl sulfone (CH₂CHSO₂CH₃) has been carried out. Molecular geometry optimizations have been performed at the HF and MP2 levels of the theory. Relative energies of the stationary points have been determined by using different approaches, including electron correlation corrections up to the third order. The IR and Raman spectra of the liquid have been measured and a vibrational assignment is proposed. The height of the barrier to the methyl group internal rotation has been estimated. Theoretical calculations and vibrational spectra have shown that the predominant conformation of methyl vinyl sulfone has the C=C bond eclipsed with one of the S=O bonds. Similar eclipsed forms have been found in vinyl fluoro sulfone, vinyl chloro sulfone and divinyl sulfone by ab initio HF calculations.     

Two Equivalent Internal Rotations in the Microwave Spectrum of 2,6-Dimethylfluorobenzene

Large amplitude motion of methyl groups in isolated molecules is a fundamental phenomenon in molecular physics. The methyl torsional barrier is sensitive to the steric and electronic environment in the surrounding of the methyl group, making the methyl group a detector of the molecular structure. To probe this effect, the microwave spectrum of 2,6-dimethylfluorobenzene, one of the six isomers of dimethylfluorobenzene, was measured using two pulsed molecular jet Fourier transform microwave spectrometers operating in the frequency range from 2 to 40 GHz. Due to internal rotations of two equivalent methyl groups with relatively low torsional barriers, all rotational transitions split into quartets with separations of up to several hundreds of MHz. The splittings were analyzed and modeled to deduce a torsional barrier of 236.7922 (21) cm⁻¹. The results are compared to those obtained from quantum chemical calculations and with other fluorine substituted toluene derivatives of the current literature where the methyl group is adjacent to a fluorine atom.     

Analysis of torsional spectra of molecules with two internal C3ν, rotors—XXVI. A far infrared study of 1,1-dimethylhydrazine

The infrared spectra of 1,1-dimethylhydrazine, (CH₃)₂NNH₂, and two isotopomers, (CD₃)₂NNH₂ and (CH₃)₂NND₂, have been recorded in the region between 600 and 100 cm⁻¹. Very rich and complex spectra were obtained and analysis of the data has been carried out. The interpretation of the spectra arising from the two methyl torsional modes of the −d₀ compound was carried out using a semi-rigid model, and the resulting potential function obtained is V₃₀ = 1685 ± 12 cm⁻¹ (4.82 ± 0.04 kcal mol⁻¹); V₀₃ = 1827 ± 16 cm⁻¹ (5.22 ± 0.05 kcal mol⁻¹); V₆₀ = −92±5cm⁻¹ (−0.26 ± 0.02 kcal mol⁻¹); V₀₆ = −41 ± 6cm⁻¹ (−0.12 ± 0.02 kcal mol⁻¹) and V^′₃₃ = −51 ± 5 cm⁻¹ (−0.15 ± 0.01 kcal mol⁻¹). Ab initio gradient calculations were carried out employing the 3–21G and 6–31G* basis sets, as well as the 6–31G* basis set with electron correlation at the MP2 level. The structural parameters, conformational stability, and three-fold barriers to internal rotation have been determined and the gauche conformer is calculated to be more stable than the trans form by 783 cm⁻¹ (2.24 kcal mol⁻¹) with the MP2/6–31G* basis set. These calculations were also used to re-evaluate the previously reported assignment of the fundamental modes, and to obtain a potential function for the asymmetric torsion. All of these results are discussed and compared with corresponding quantities for some similar compounds.     

Tunneling dynamics of internal rotation in the nitric acid molecule

The Hamiltonian of internal rotation about theC ₂ axis in the HNO₃ molecule and its H/D-, O¹⁸/O¹⁶-, and N¹⁵/N¹⁴-isotopomers was reconstructed using the results of quantumchemical calculations. The Fermi resonance between the torsional (2v₉) and ONO bending (v₅) vibrations is a characteristic feature of the molecule. Tunneling splittings in the ground and excited states were calculated using the perturbative instanton approach. Abnormally large changes in the splittings upon isotope substitution of heavy atoms are predicted. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 11, pp. 2052–2060, November, 1999.