Conformational effect on the almost free internal rotation in 4-hexyn-3-ol studied by microwave spectroscopy and quantum chemistry

Low barriers to internal rotation in the microwave spectrum of 2,5-dimethylfluorobenzene

We investigated the rotational spectrum of 2,5-dimethylfluorobenzene containing coupled large amplitude motions of two methyl groups in the frequency range from 2 to 26.5 GHz using a pulsed molecular jet Fourier transform microwave spectrometer. The internal rotation of two inequivalent methyl groups with low torsional barriers (around 16 and 226 cm-1) causes splittings of all rotational transitions into quintets with separations of up to hundreds of MHz between the torsional components. Spectral analysis and modeling of the observed splittings were performed using the programs XIAM and BELGI-Cs-2Tops, whereby the latter achieved measurement accuracy. The methyl internal rotation can be used to examine the electronic and steric environments around the methyl group because they affect the methyl torsional barrier. Electronic properties play a particularly important role in aromatic molecules in the presence of a π-conjugated double bond system. The experimental results were compared with those of quantum chemistry. Benchmark calculations resulted in the conclusion that the B3LYP-D3BJ/6-311++G(d,p) level of theory can be recommended for predicting rotational constants to guide the microwave spectral assignment of dimethylfluorobenzenes in particular and toluene derivatives in general.

Two methyl internal rotations of 2-acetyl-4-methylthiophene explored by microwave spectroscopy and quantum chemistry

The microwave spectrum of 2-acetyl-4-methylthiophene (2A4MT) was recorded in the frequency range from 2 to 26.5 GHz using a molecular jet Fourier transform microwave spectrometer, revealing two conformers, syn and anti. Both methyl groups in the molecule, the acetyl methyl and the ring methyl groups, undergo internal rotation, causing resolvable splittings of all rotational transitions into quintets. The torsional barriers determined for the acetyl methyl and the ring methyl rotors are 324.919(94) cm^-1 and 210.7181(61) cm^-1 for the syn conformer; the respective values for anti-2A4MT are 281.201(17) cm^-1 and 212.9797(41) cm^-1. The experimentally deduced rotational constants and torsional barriers are compared to values obtained from quantum chemical calculations. The barriers to methyl internal rotation are also compared to those of related molecules in order to establish a "thiophene class" concerning the acetyl methyl torsion.

The LAM of the Rings: Large Amplitude Motions in Aromatic Molecules Studied by Microwave Spectroscopy

Large amplitude motions (LAMs) form a fundamental phenomenon that demands the development of specific theoretical and Hamiltonian models. In recent years, along with the strong progress in instrumental techniques on high-resolution microwave spectroscopy and computational capacity in quantum chemistry, studies on LAMs have become very diverse. Larger and more complex molecular systems have been taken under investigation, ranging from series of heteroaromatic molecules from five- and six-membered rings to polycyclic-aromatic-hydrocarbon derivatives. Such systems are ideally suited to create families of molecules in which the positions and the number of LAMs can be varied, while the heteroatoms often provide a sufficient dipole moment to the systems to warrant the observation of their rotational spectra. This review will summarize three types of LAMs: internal rotation, inversion tunneling, and ring puckering, which are frequently observed in aromatic five-membered rings such as furan, thiophene, pyrrole, thiazole, and oxazole derivatives, in aromatic six-membered rings such as benzene, pyridine, and pyrimidine derivatives, and larger combined rings such as naphthalene, indole, and indan derivatives. For each molecular class, we will present the representatives and summarize the recent insights on the molecular structure and internal dynamics and how they help to advance the field of quantum mechanics.

Internal rotation arena: Program performances on the low barrier problem of 4-methylacetophenone

In the rotational spectroscopy community, several popular codes are available to treat multiple internal rotors in a molecule. In terms of the pros and cons of each code, it is often a difficult task to decide which program to apply to a specific internal rotation problem. We faced this issue when dealing with the spectroscopic fingerprint of 4-methylacetophenone (4MAP), recently investigated in the microwave region, which we here extended into the millimeterwave region. The methyl group attached to the phenyl ring in 4MAP undergoes internal rotation with a very low barrier of only 22 cm^-1. The acetyl methyl group features a much higher barrier of about 580 cm^-1. The performances of a program using the so-called "local" approach in terms of Herschbach's perturbative treatment, SPFIT, as well as three programs XIAM, ERHAM, and ntop, representing "global" fits, were tested. The results aim at helping spectroscopists in the decision on how to tackle their own internal rotation problems.

Large Amplitude Motions in 2,3-Dimethylfluorobenzene: Steric Effects Failing to Interpret Hindered Methyl Torsion

The microwave spectrum of 2,3-dimethylfluorobezene, one of the six isomers of dimethylfluorobenzene, was recorded using a pulsed molecular jet Fourier transform microwave spectrometer operating in the frequency range from 2 to 26.5 GHz. The internal rotations of two inequivalent methyl groups, causing splittings of up to several hundred MHz of all rotational energy levels into quintets, were analyzed and modeled. The torsional barriers of the methyl groups at the ortho and the meta positions were determined to be 215.5740(56) cm^-1 and 488.53(11) cm^-1. A comparison with the barrier heights observed for the two isomers 2,6-dimethylfluorobenzene and 3,4-dimethylfluorobenzene has shown that the methyl group at the meta position seems to be invisible to its neighboring ortho-methyl group, while the meta-methyl group clearly senses the ortho one. Steric effects are not able to explain this observation, and electrostatic effects are most probably the reason. Highly accurate molecular parameters determined experimentally were compared with those obtained from quantum chemical calculations at different levels of theory.

Local vs global approaches to treat two equivalent methyl internal rotations and 14N nuclear quadrupole coupling of 2,5-dimethylpyrrole

The microwave spectrum of 2,5-dimethylpyrrole was recorded using a molecular jet Fourier transform microwave spectrometer operating in the frequency range from 2 to 26.5 GHz. Only one stable conformer was observed as expected and confirmed by quantum chemical calculations carried out to complement the experimental analysis. The two equivalent methyl groups cause each rotational transition to split into four torsional species, which is combined with the quadrupole hyperfine splittings in the same order of magnitude arising from the ¹⁴N nucleus. This results in a complicated spectrum feature. The spectral assignment was done separately for each torsional species. Two global fits were carried out using the XIAM code and the BELGI-C_2v-2Tops-hyperfine code, a modified version of the BELGI-C_2v-2Tops code, giving satisfactory root-mean-square deviations. The potential barriers to internal rotation of the two methyl groups were determined to be V₃ = 317.208(16) cm^-1. The molecular parameters were obtained with high accuracy, providing all necessary ground state information for further investigations in higher frequency ranges and on excited torsional-vibrational states.

Steric effects on two inequivalent methyl internal rotations of 3,4-dimethylfluorobenzene

The microwave spectrum of 3,4-dimethylfluorobenzene was measured using a pulsed molecular jet Fourier transform microwave spectrometer operating in the frequency range from 2.0 to 26.5 GHz with the goal of quantifying steric effects on barriers to internal rotation of the two inequivalent methyl groups. Due to these torsional motions, splittings of all rotational transitions into quintets were observed and fitted with residuals close to measurement accuracy. The experimental work was supported by quantum chemical calculations, and the B3LYP-D3BJ/6-311++G(d,p) level of theory yielded accurate optimized geometry parameters to guide the assignment. The three-fold potential values of 456.20(13) cm^-1 and 489.78(15) cm^-1 for the methyl groups at the meta and para position, respectively, deduced from the experiments are compared with the predicted values and those of other toluene derivatives.

Understanding (coupled) large amplitude motions: the interplay of microwave spectroscopy, spectral modeling, and quantum chemistry

A large variety of molecules contain large amplitude motions (LAMs), inter alia internal rotation and inversion tunneling, resulting in tunneling splittings in their rotational spectrum. We will present the modern strategy to study LAMs using a combination of molecular jet Fourier transform microwave spectroscopy, spectral modeling, and quantum chemical calculations to characterize such systems by the analysis of their rotational spectra. This interplay is particularly successful in decoding complex spectra revealing LAMs and providing reference data for fundamental physics, astrochemistry, atmospheric/environmental chemistry and analytics, or fundamental researches in physical chemistry. Addressing experimental key aspects, a brief presentation on the two most popular types of state-of-the-art Fourier transform microwave spectrometer technology, i.e., pulsed supersonic jet expansion–based spectrometers employing narrow-band pulse or broad-band chirp excitation, will be given first. Secondly, the use of quantum chemistry as a supporting tool for rotational spectroscopy will be discussed with emphasis on conformational analysis. Several computer codes for fitting rotational spectra exhibiting fine structure arising from LAMs are discussed with their advantages and drawbacks. Furthermore, a number of examples will provide an overview on the wealth of information that can be drawn from the rotational spectra, leading to new insights into the molecular structure and dynamics. The focus will be on the interpretation of potential barriers and how LAMs can act as sensors within molecules to help us understand the molecular behavior in the laboratory and nature.

Internal Motions and Sulfur Hydrogen Bonding in Methyl 3-Mercaptopropionate

The effect of sulfur hydrogen bonding on the conformational equilibrium of methyl 3-mercaptopropionate was investigated using microwave spectroscopy in a supersonic jet expansion. The two most stable conformers (I and II) were assigned in the rotational spectra, and complex splitting patterns owing to the methyl internal rotation and SH tunneling motion were resolved and analyzed in detail. For both conformers, the experimental torsional barriers for the methyl top are similar and about 5.1 kJ mol^-1, revealing that their geometrical differences do not affect the methyl internal rotation. The experimentally derived rotational and centrifugal distortion constants, along with the methyl internal rotation barriers, are discussed and compared with results from density functional theory and ab initio calculations. Quantum theory of atoms in molecules, noncovalent interactions, and natural bond orbital analyses show that the global minimum geometry (I), which has the thiol hydrogen oriented toward the carbonyl of the ester, is stabilized by an SH···O=C hydrogen bond. The presence of a hydrogen bond is confirmed by the derivation of an accurate experimental geometry that reveals a hydrogen bond distance and S-H-O angle of 2.515(4) Å and 117.4(1)°, respectively. These results are key benchmarks to expand the current knowledge of sulfur hydrogen bonds and the relationship between internal motions and conformational preferences in esters.

A strong dependence of the CH3 internal rotation barrier on conformation in thioacetic acid: Microwave measurements and an energy decomposition analysis

Rotational spectra of thioacetic acid (CH₃COSH) have been observed by pulsed-nozzle Fourier transform microwave spectroscopy. Spectroscopic constants are reported for both the syn and anti conformers of the parent species, as well as the ³⁴S and ¹³C carbonyl isotopologues. Transitions arising from the lowest A and E internal rotor states of the methyl group have been observed and analyzed. Experimental values of the three-fold internal rotation barrier, V₃, for the syn and anti conformers of the parent isotopologue are 76.300(12) and 358.056(51) cm^-1, respectively, indicating a large effect of the S-H orientation on the CH₃ internal rotation potential. M06-2X/6-311+G(d,p) calculations are in good agreement with these results. The block localized energy decomposition method has been applied to understand the origins of this strong dependence of V₃ on conformation. The results indicate that π conjugation from the SH to the carbonyl group and steric repulsion between the SH and the methyl group in the anti form are main contributors to the difference.