The Ground and First Torsional States of CD(3)CHO

Strong Torsion-Vibration Interaction in N-Methylpyrrole Observed by Far-Infrared Spectroscopy

An interaction between methyl torsion and the low-lying out-of-plane methyl wag vibration has been observed in toluene, p-fluorotoluene, and m-fluorotoluene, contravening the traditional assumption used when analyzing spectra that methyl torsion can be treated independently of the small-amplitude vibrations. When a methyl group is attached to a planar frame, out-of-plane methyl wag vibrations always occur, and hence this type of interaction between methyl torsion and vibration is potentially extensive. To probe whether this coupling occurs beyond toluene and its derivatives, we have studied the far-infrared absorption band for the out-of-plane methyl wagging mode in N-methylpyrrole. The torsional sequence structure reveals a particularly strong torsion-vibration interaction. Spectral simulations yield a torsion-vibration coupling matrix element of 34.0 cm^-1, over twice the value for toluene. The large torsion-vibration coupling constant implies that there is a significant tilting of the methyl group out of plane. Quantum chemistry calculations reveal a much larger out-of-plane methyl tilt angle in N-methylpyrrole compared to toluene, qualitatively consistent with this expectation. This is the first nontoluene derivative for which this type of torsion-vibration interaction has been reported and shows that the effect extends beyond toluenes. When present, this interaction links small-amplitude vibrations to the methyl torsion, providing a mechanism to bring the increased density of states into play and accelerate the rate of intramolecular vibrational energy redistribution.

Substitution Reactions in the Pyrolysis of Acetone Revealed through a Modeling, Experiment, Theory Paradigm

The development of high-fidelity mechanisms for chemically reactive systems is a challenging process that requires the compilation of rate descriptions for a large and somewhat ill-defined set of reactions. The present unified combination of modeling, experiment, and theory provides a paradigm for improving such mechanism development efforts. Here we combine broadband rotational spectroscopy with detailed chemical modeling based on rate constants obtained from automated ab initio transition state theory-based master equation calculations and high-level thermochemical parametrizations. Broadband rotational spectroscopy offers quantitative and isomer-specific detection by which branching ratios of polar reaction products may be obtained. Using this technique, we observe and characterize products arising from H atom substitution reactions in the flash pyrolysis of acetone (CH₃C(O)CH₃) at a nominal temperature of 1800 K. The major product observed is ketene (CH₂CO). Minor products identified include acetaldehyde (CH₃CHO), propyne (CH₃CCH), propene (CH₂CHCH₃), and water (HDO). Literature mechanisms for the pyrolysis of acetone do not adequately describe the minor products. The inclusion of a variety of substitution reactions, with rate constants and thermochemistry obtained from automated ab initio kinetics predictions and Active Thermochemical Tables analyses, demonstrates an important role for such processes. The pathway to acetaldehyde is shown to be a direct result of substitution of acetone's methyl group by a free H atom, while propene formation arises from OH substitution in the enol form of acetone by a free H atom.

The Case for Methyl Group Precession Accompanying Torsional Motion

For molecules containing a methyl group, high precision fits of rotational line data (microwave spectra) that encompass several torsional states require considerably more constants than are required in comparable rigid molecules. Many of these additional terms are ‘torsion-rotation interaction’ terms, but their precise physical meaning is unclear. In this paper, we explore the physical origins of many of these additional terms in the case where the methyl group is attached to a planar frame. We show that torsion-vibration coupling, which has been observed in toluene and several substituted toluenes, provides the dominant contribution to a number of the torsion-rotation constants in toluene. It is further demonstrated that this coupling is intimately related to precession of the methyl group. A number of the constants required in the high resolution fits of rotational line data are shown to arise as a natural consequence of methyl precession. By considering several molecules whose rotational line spectra have been fit to high precision, we demonstrate that the experimental evidence is consistent with the occurrence of methyl group precession. Quantum chemistry calculations of the optimised molecular structures at key torsional angles provide further evidence that methyl precession occurs. There is both a torsional angle dependent tilt of the Cmethyl-frame bond and of the methyl group principal rotation axis relative to the Cmethyl-frame bond.

The Ground and First Excited Torsional States of Acetic Acid

A global fit of microwave and millimeter-wave rotational transitions in the ground and first excited torsional states (v(t) = 0 and 1) of acetic acid (CH(3)COOH) is reported, which combines older measurements from the literature with new measurements from Kharkov, Lille, and NIST. The fit uses a model developed initially for acetaldehyde and methanol-type internal rotor molecules. It requires 34 parameters to achieve a unitless weighted standard deviation of 0.84 for a total of 2518 data and includes A- and E-species transitions with J </= 30 and K(a) </= 15. While these results represent a significant improvement over past fitting attempts, it should be cautioned that the present data set is dominated by v(t) = 0 transitions, and no direct infrared measure of the v(t) = 1 <-- 0 torsional interval is available. Copyright 2001 Academic Press.