Internal Rotation of the Acetyl Methyl Group in Methyl Alkyl Ketones: The Microwave Spectrum of Octan-2-one

Methyl n-alkyl ketones form a class of molecules with interesting internal dynamics in the gas-phase. They contain two methyl groups undergoing internal rotations, the acetyl methyl group and the methyl group at the end of the alkyl chain. The torsional barrier of the acetyl methyl group is of special importance, since it allows for the discrimination of the conformational structures. As part of the series, the microwave spectrum of octan-2-one was recorded in the frequency range from 2 to 40 GHz, revealing two conformers, one with C₁ and one with C_s symmetry. The barriers to internal rotation of the acetyl methyl group were determined to be 233.340(28) cm⁻¹ and 185.3490(81) cm⁻¹, respectively, confirming the link between conformations and barrier heights already established for other methyl alkyl ketones. Extensive comparisons to molecules in the literature were carried out, and a small overview of general trends and rules concerning the acetyl methyl torsion is given. For the hexyl methyl group, the barrier height is 973.17(60) cm⁻¹ for the C₁ conformer and 979.62(69) cm⁻¹ for the C_s conformer.     

Does the HO2 radical react with ketones?

The possible reactions of HO₂ with five ketones were studied using a flow tube reactor equipped with a laser magnetic resonance detector. We did not observe reactive loss of HO₂ in any of the five reactions. We place upper limits of <8 × 10⁻¹⁶, <7 × 10⁻¹⁶, <5 × 10⁻¹⁶, <4 × 10⁻¹⁶, and <9 × 10⁻¹⁶ (in units of cm³; molecule⁻¹ S⁻¹) at 298 K for the reactions of HO₂ with CH₃COCH₃, CH₃COC₂H₅, CH₃COC₃H₇, C₂H₅COC₂H₅, and CH₃COC₄H₉, respectively, to give products other than an adduct. We conclude that their reactions with HO₂ are unlikely to be important loss processes for ketones in the atmosphere. © 2000 John Wiley & Sons, Inc. Int J Chem Kinet 32: 573–580, 2000     

Far-IR spectrum,conformation stability,barriers to internal rotation,vibrational assignment,and ab initio calculations of 3-chloro-2-methylpropene

The far-IR spectrum from 375 to 30 cm⁻¹ of gaseous 3-chloro-2-methylpropene, CH₂=C(CH₃)CH₂Cl, has been recorded at a resolution of 0.10 cm⁻¹. The fundamental asymmetric torsional mode for the gauche conformer is observed at 84.3 cm⁻¹ with three excited states falling to lower frequency. For the higher energy s-cis conformer, where the chlorine atom eclipses the double bond, the asymmetric torsion is observed at 81.3 cm⁻¹ with two excited states falling to lower frequency. Utilizing the s-cis and gauche torsional frequencies, the gauche dihedral angle and the enthalpy difference between conformers, the potential function governing the interconversion of the rotamers has been calculated. The determined potential function coefficients are (in reciprocal centimeters): V₁=189±12, V₂=−358±11, V₃=886±2 and V₄=−12±2 with an enthalpy difference between the more stable gauche and s-cis conformers of 150 ±25 cm⁻¹ (430 ± 71 cal mol⁻¹). This function gives values of 661 cm⁻¹ (1.89 kcal mol⁻¹), 1226 cm⁻¹ (3.51 kcal mol⁻¹) and 812 cm⁻¹ (2.32 kcal mol⁻¹), for the s-cis to gauche, gauche to gauche, and gauche to s-cis barriers, respectively. From the methyl torsional frequency of 170 cm⁻¹ for the gauche conformer, the threefold barrier of 678 cm⁻¹ (1.94 kcal mol⁻¹) has been calculated. The asymmetric potential function, conformational energy difference and optimized geometries of both conformers have also been obtained from ab initio calculations with both the 3–21G^* and 6–31G^* basis sets. A normal-coordinate analysis has also been performed with a force field determined from the 3–21G^* basis set. These data are compared with the corresponding data for some similar molecules.     

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).     

Polarized Raman spectra,thermodynamic functions and bidders to internal rotation of 2,3-dimethoxy toluene

Polarized Raman spectra of 2,3-dimethoxy toluene have been recorded in the region 50–4000 cm⁻¹ and IR spectra in the region 200–4000 cm⁻¹. All the 63 (40a′ + 23a″) normal modes of vibration have been assigned assuming a C_s point group. Consistent assignments for the internal modes of vibration of methyl (CH₃) and methoxy (OCH₃) groups have been proposed. In addition thermodynamic functions have been computed over the temperature range 100–1500 K on a MIGHTY II computer and barriers to internal rotations for the three methyl (CH₃) tops and the two methoxy (OCH₃) tops about their respective axes have been determined, using the assigned torsional frequencies and assumed structural parameter for the 2,3-dimethoxy toluene. The barrier heights have been found to be greater than 2.5 kcal mol⁻¹ for all five tops.     

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.     

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.     

Far-infrared spectra and barriers to internal rotation of ethyl nitrate

The far-infrared spectra of gaseous and solid ethyl nitrate, CH₃CH₂ONO₂, have been recorded from 500 to 50 cm⁻¹. The fundamental asymmetric torsion of the trans conformer which has a heavy atom plane has been observed at 112.50 cm⁻¹ with two excited states failing to lower frequencies, and the corresponding fundamental torsion of the gauche conformer was observed at 109.62 cm⁻¹ with two excited states also falling to lower frequencies. The results of a variable temperature Raman study indicate that the trans conformer is more stable than the gauche conformer by 328 ± 96 cm⁻¹ (938 ± 275 cal mol⁻¹). An asymmetric potential function governing the internal rotation about the CH₂O bond is reported which gives a trans to gauche barrier of 894 ± 15 cm⁻¹ (2.56 ± 0.04 kcal mol⁻¹) and a gauche to gauche barrier of 3063 ± 68 cm⁻¹ (8.76 ± 0.20 kcal mol⁻¹) with the trans conformer more stable by 220 ± 148 cm⁻¹ (0.63 ± 0.42 kcal mol⁻¹). Transitions arising from the symmetric CH₃ and NO₂ torsions are observed for both conformers, from which the threefold and twofold periodic barriers to internal rotation have been calculated. For the trans conformer the values are 1002 cm⁻¹ (2.87 kcal mol⁻¹) and 2355 ± 145 cm⁻¹ (6.73 ± 0.42 kcal mol⁻¹) and for the gauche conformer they are 981 cm⁻¹ (2.81 kcal mol⁻¹) and 2736 ± 632 cm⁻¹ (7.82 ± 1.81 kcal mol⁻¹) for the CH₃ and NO₂ rotors, respectively. These results are compared to the corresponding quantities for some similar molecules.     

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.     

The methyl and methoxyl torsional modes and the lattice vibration in the low-frequency Raman spectrum of 1,4-dimethoxybenzene

The low-frequency (10–450 cm^?1) Raman spectra of solid (at 300 K and 130 K) and liquid (at 335 K) 1,4-dimethoxybenzene-d₀ and 1,4-dimethoxybenzene-d₅ have been measured. The methyl nad methoxyl torsional transitions have been identified and the corresponding torsional barriers calculated. Upon deuleration the methyl torsional barrier is reduced by 450 cm^?1, implying a coupling between the methyl torsion and a low-frequency ring mode. As far as the torsions are considered, the internal dynamic situation in 1,4-dimethoxybezene resembles that in amisole. A tentative assignment of the observed lattice bands in given. Certain changes in the spectrum when going from the solid to the melt are attributed to the coexistence of both cis and trans conformers in the liquid 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.     

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.     

Kinetics and products of propargyl (C3H3) radical self‐reactions and propargyl‐methyl cross‐combination reactions

Propargyl (HCC CH₂) and methyl radicals were produced through the 193‐nm excimer laser photolysis of mixtures of C₃H₃Cl/He and CH₃N₂CH₃/He, respectively. Gas chromatographic and mass spectrometric (GC/MS) product analyses were employed to characterize and quantify the major reaction products. The rate constants for propargyl radical self‐reactions and propargyl‐methyl cross‐combination reactions were determined through kinetic modeling and comparative rate determination methods. The major products of the propargyl radical combination reaction, at room temperature and total pressure of about 6.7 kPa (50 Torr) consisted of three C₆H₆ isomers with 1,5‐hexadiyne(CHC CH₂ CH₂ CCH, about 60%); 1,2‐hexadiene‐5yne (CH₂CC CH₂ CCH, about 25%); and a third isomer of C₆H₆ (∼15%), which has not yet been, with certainty, identified as being the major products. The rate constant determination in the propargyl‐methyl mixed radical system yielded a value of (4.0 ± 0.4) × 10⁻¹¹ cm³ molecule⁻¹ s⁻¹ for propargyl radical combination reactions and a rate constant of (1.5 ± 0.3) × 10⁻¹⁰ cm³ molecule⁻¹ s⁻¹ for propargyl‐methyl cross‐combination reactions. The products of the methyl‐propargyl cross‐combination reactions were two isomers of C₄H₆, 1‐butyne (about 60%) and 1,2‐butadiene (about 40%). © 2000 John Wiley & Sons, Inc. Int J Chem Kinet 32: 118–124, 2000     

Large Amplitude Torsions in Nitrotoluene Isomers Studied by Rotational Spectroscopy and Quantum Chemistry Calculations

Rotational spectra of ortho-nitrotoluene (2-NT) and para-nitrotoluene (4-NT) have been recorded at low and room temperatures using a supersonic jet Fourier Transform microwave (MW) spectrometer and a millimeter-wave frequency multiplier chain, respectively. Supported by quantum chemistry calculations, the spectral analysis of pure rotation lines in the vibrational ground state has allowed to characterise the rotational energy, the hyperfine structure due to the ¹⁴N nucleus and the internal rotation splittings arising from the methyl group. For 2-NT, an anisotropic internal rotation of coupled −CH₃ and −NO₂ torsional motions was identified by quantum chemistry calculations and discussed from the results of the MW analysis. The study of the internal rotation splittings in the spectra of three NT isomers allowed to characterise the internal rotation potentials of the methyl group and to compare them with other mono-substituted toluene derivatives in order to study the isomeric influence on the internal rotation barrier.     

Bond Length Alternation and Internal Dynamics in Model Aromatic Substituents of Lignin

Broadband microwave spectra were recorded over the 2-18 GHz frequency range for a series of four model aromatic components of lignin; namely, guaiacol (ortho-methoxy phenol, G ), syringol (2,6-dimethoxy phenol, S ), 4-methyl guaiacol ( MG ), and 4-vinyl guaiacol ( VG ), under jet-cooled conditions in the gas phase. Using a combination of ¹³C isotopic data and electronic structure calculations, distortions of the phenyl ring by the substituents on the ring are identified. In all four molecules, the r_C(1)-C(6) bond between the two substituted C-atoms lengthens, leading to clear bond alternation that reflects an increase in the phenyl ring resonance structure with double bonds at r_C(1)-C(2), r_C(3)-C(4) and r_C(5)-C(6). Syringol, with its symmetric methoxy substituents, possesses a microwave spectrum with tunneling doublets in the a-type transitions associated with H-atom tunneling. These splittings were fit to determine a barrier to hindered rotation of the OH group of 1975 cm⁻¹, a value nearly 50 % greater than that in phenol, due to the presence of the intramolecular OH⋅⋅⋅OCH₃ H-bonds at the two equivalent planar geometries. In 4-methyl guaiacol, methyl rotor splittings are observed and used to confirm and refine an earlier measurement of the three-fold barrier V₃=67 cm⁻¹. Finally, 4-vinyl guaiacol shows transitions due to two conformers differing in the relative orientations of the vinyl and OH groups.     

用乙酰氯淬灭法测定Ziegler-Natta催化剂的活性中心数

The reactions of Solvay TiCl₃-AlR₃ (R=C₂H₅,i-C₄H₉) catalysts with acetyl chloride were studied. The main reaction, which is much faster than the reactions between A1R₃ and CH₃COC₁, is as follows:It is found that when CH₃COC₁ was introduced into 1-octene polymerization systems catalyzed by TiCl₃-AlR₃ for 5-15min, it reacted selectively and completely with the active centers to produce acetyl-ended polymer chains.With CH₃COC₁ as a quenching agent, the number of active centers in .these polymerization systems were determined by measuring the content of carbonyl in the polymer,and the results consist with those obtained by kinetics-molecular-weight method.     

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.     

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.     

Spectres de vibration et conformations du propionate et de l'isobutyrate de méthyle

Infrared and Raman spectra (3600–3620cm^?1) of methyl propionate CH₃CH₂-COOCH₃, CH₃CH₂COOCD₃ and methyl isobutyrate (CH₃)₂CHCOOCH₃, (CH₃)₂CHCOOCD₃, in liquid and crystalline states, have been recorded. Rotational isomerism, by rotation around the C-C bond α to the carbonyl group, is detected and the energy difference between the conformers is 1.1 ±0.3 kcal mol^?1 for methyl propionate and 0.5 ±0.1 kcal mol^?1 for methyl isobutyrate. Vibrational assignments in terms of group frequencies are proposed for each conformer, only the more stable being present in the crystal.     

Negative ion chemical ionization mass spectra of (η6-arene)Cr(CO)3

The negative ion chemical ionization mass spectra, with ammonia and methane as reagent gases, of the (η⁶-arene)Cr(CO)₃ complexes, where the arene is C₆H₅COCH₃, C₆H₅COC₂H₅, C₆H₅COC₃H₇, C₆H₅COC(CH₃)₃, 2-CH₃C₆H₄COC₃H₇, C₆H₅COOCH₃, C₆H₅CH₃, 1,3,5-(CH₃)₃C₆H₃ and C₆H₅CH₂COC₂H₅, are reported. Similar behaviour is observed with the two reagent gases, but ammonia shows a much higher abundance for the ions produced by reactions of [NH₂]^? with sample molecules. The compounds containing the C₆H₅CO group display an abundant [M]^{? ˙}, whereas the other compounds exhibit [M? H]^? as base peak, produced by ion/molecule reactions. A comparison of the negative ion chemical ionization mass spectra of the (η⁶-arene)Cr(CO)₃ complexes with those of the corresponding ligands shows the strong electron withdrawing power of the Cr(CO)₃ group in the gas phase.