Local And Global Approaches To Treat The Torsional Barriers Of 4-methyl-acetophenone Using Microwave Spectroscopy

The Fourier transform microwave spectrum of 4-methylacetophenone recorded from 8 GHz to 18 GHz under jet-cooled conditions has revealed large tunneling splittings arising from a low barrier to internal rotation of the ring methyl group and small splittings from a high torsional barrier of the acetyl methyl group. The large splittings are especially challenging to model, while the small splittings are difficult to analyze due to the resolution limit of 120 kHz. The combination of two methyl groups undergoing internal rotations caused each rotational transition to split into five torsional species, which were resolved and fitted using a modified version of the \textit{XIAM} code and the newly developed \textit{ntop} code to a root-mean-square deviation close to measurement accuracy, providing an estimate of the \textit{V}$_{3}$ potential barriers of about 22 \wn ~and 584–588 \wn ~ for the ring and the acetyl methyl groups, respectively. The assignment was aided by separately fitting the five torsional species using odd-power order operators. Only one conformer in which all heavy atoms are located on a symmetry plane could be identified in the spectrum, in agreement with results from conformation analysis using quantum chemical calculations

Low barrier methyl rotation in 3-Pentyn-1-ol as observed by microwave spectroscopy

It is known that the barrier to internal rotation of the methyl groups in ethane (\textbf{1}) is about 1000 \wn.\footnote{R. M. Pitzer, \emph{Acc. Chem. Res.}, \textbf{1983}, \emph{16}, 207–210.} If a C-C-triple bond is inserted between the methyl groups as a spacer (\textbf{2}), the torsional barrier is assumed to be dramatically lower, which is a common feature of ethinyl groups in general. \newline \indent To study this effect of almost free internal rotation, we measured the rotational spectrum of 3-pentyn-1-ol (\textbf{3}) by pulsed jet Fourier transform microwave spectroscopy in the frequency range from 2 to 26.5 GHz. Quantum chemical calculations at the MP2/6-311++G(d,p) level of theory yielded five stable conformers on the potential energy surface. The most stable conformer, which possesses C$_{1}$ symmetry, was assigned and fitted using two theoretical approaches treating internal rotations, the rho axis method (\emph{BELGI-C$_{1}$}) and the combined axis method (\emph{XIAM}). The molecular parameters as well as the internal rotation parameters were determined. A very low barrier to internal rotation of the methyl group of only 9.4545(95) \wn \ was observed. \begin{wrapfigure}{r}{0pt} \includegraphics[scale=0.55]{3Py1.eps} \end{wrapfigure

14N quadrupole coupling in the microwave spectra of n-vinylformamide

The microwave spectra of two conformers, \emph{trans} and \emph{cis}, of the title compound were recorded using two molecular beam Fourier transform microwave spectrometers operating in the frequency range 2 GHz to 40 GHz, and aimed at analysis of their $^{14}$N quadrupole hyperfine structures. Rotational constants, centrifugal distortion constants, and nuclear quadrupole coupling constants (NQCCs) $\chi$$_{aa}$ and $\chi$$_{bb}$ - $\chi$$_{cc}$, were all determined with very high accuracy. Two fits including 176 and 117 hyperfine transitions were performed for the \emph{trans} and \emph{cis} conformers, respectively. Standard deviations of both fits are close to the measurement accuracy of 2 kHz. The NQCCs of the two conformers are almost exactly the same, and are compared with values found for other saturated and unsaturated formamides. \newline \indent Complementary quantum chemical calculations - MP2/6-311++G(d,p) rotational constants, MP2/cc-pVTZ centrifugal distortion constants, and B3PW91/6-311+G(d,p)//MP2/6-311++G(d,p) nuclear quadrupole coupling constants - give spectroscopic parameters in excellent agreement with the experimental parameters. B3PW91/6-311+G(d,p) calculated electric field gradients, in conjunction with eQ/h = 4.599(12) MHz/a.u., yields more reliable NQCCs for formamides possessing conjugated $\pi$-electron systems than does the B3PW91/6-311+G(df,pd) model recommended in Ref. \footnote{W. C. Bailey, \emph{Chem. Phys.}, \textbf{2000}, \emph{252}, 57.}, whereas this latter performs better for aliphatic formamides.\footnote{W. C. Bailey, Calculation of Nuclear Quadrupole Coupling Constants in Gaseous State Molecules, http://nqcc.wcbailey.net/index.html.} We conclude from this that f-polarization functions on heavy atoms hinder rather than help with modeling of conjugated $\pi$-electron systems

Two equivalent methyl internal rotations in 2,5-dimethylthiophene investigated by microwave spectroscopy

The microwave spectrum of 2,5-dimethylthiophene, a sulfur-containing five-membered heterocyclic molecule with two conjugated double bonds, was recorded using two molecular beam Fourier transform microwave spectrometers operating in the frequency range from 2 to 40 GHz. Highly accurate molecular parameters were determined. The rotational constants obtained by geometry optimizations at different levels of theory are in good agreement with the experimental values. A C$_{2v}$ equilibrium structure was calculated, where one hydrogen atom of each methyl group is antiperiplanar to the sulfur atom, and the two methyl groups are thus equivalent. \newline \indent Transition states were optimized at different levels of theory using the Berny algorithm to calculate the barrier height of the two equivalent methyl rotors. The fitted experimental torsional barrier of 247.95594(30) \wn \ is in reasonable agreement with the calculated barriers. Similar barriers to internal rotation were found for the monomethyl derivatives 2-methylthiophene (194.1 \wn) and 3-methylthiophene (258.8 \wn). A labeling scheme for the group G$_{36}$ written as the semi-direct product (C$_{3}$$^{I}$ x C$_{3}$$^{I}$) (x C$_{2v}$ was introduced

Probing the methyl torsional barriers of the E and Z isomers of butadienyl acetate by microwave spectroscopy

The Fourier transform microwave spectra of the \textit{E} and the \textit{Z} isomer of butadienyl acetate have been measured in the frequency range from 2 to 26.5 GHz under molecular beam 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 results from quantum chemical calculations. The barrier to internal rotation of the acetyl methyl group was found to be 149.1822(20) cm$^{-1}$ and 150.2128(48) cm$^{-1}$ for the \textit{E} and the \textit{Z} isomer, respectively, which are similar to that of vinyl acetate \footnote{B. Velino, A. Maris, S. Melandri, W. Caminati, J. Mol. Spectrosc. 2009, 256, 228}$^{,}$\footnote{H. V. L. Nguyen, A. Jabri, V. Van, and W. Stahl, J. Phys. Chem. A, 2014, 118, 12130}. A comparison between two theoretical approaches treating internal rotations, the rho axis method (using the program \textit{BELGI-C$_{s}$}) and combined axis method (using the program \textit{XIAM}), is also performed. Since several years we study the barriers to internal rotation of the acetyl methyl group in acetates, CH$_{3}-$COOR. Currently, we assume that all acetates can be divided into three classes. Class I contains $\alpha$,$\beta$ saturated acetates, where the torsional barrier is always close to 100 cm$^{-1}$. Examples are a series of alkyl acetates such as methyl acetate and ethyl acetate. Class II contains $\alpha$,$\beta$-unsaturated acetates where the C$=$C double bond is located in the COO plane. This is the case of vinyl acetate and butadienyl acetate. Finally, in class III with isopropenyl acetate and phenyl acetate as two representatives, $\alpha$,$\beta$-unsaturated acetates, in which the double bond is not located in the COO plane, are collected. There, we observed a barrier height around 135 cm$^{-1}$. This observation will be discussed in details

The molecular structure of phenetole studied by microwave spectroscopy and quantum chemical calculations

A pulsed molecular beam Fourier transform microwave spectrometer operating in the frequency range 2 - 26.5 GHz was used to measure the spectrum of phenetole (ethyl phenyl ether or ethoxybenzene, C$_{6}$H$_{5}$OC$_{2}$H$_{5}$). The conformational landscape is completely determined by the orientations of the phenyl ring and the ethyl group. A two-dimensional potential energy surface was calculated at the MP2/6-311++G(d,p) level of theory. Two conformers were found: The trans conformer has a C$_{s}$ symmetry, and the gauche conformer has the ethyl group tilted out of the phenyl plane by about $70^{\circ}$. \newline \indent Totally 186 rotational transitions were assigned to the more stable planar trans conformer, and fitted using a semi-rigid rotor model to measurement accuracy of 2 kHz. Highly accurate rotational and centrifugal distortion constants were determined. Several method and basis set combinations were applied to check for convergence and to compare with the experimentally deduced molecular parameters. The inertial defect of the observed conformer $\Delta$$_{c}$ = (\emph{I$_{c}$} - \emph{I$_{a}$} - \emph{I$_{b}$}) = -6.718 u\AA$^{2}$ confirms that the heavy atom skeleton is planar with two pairs of hydrogen atoms out of plane. All lines in the spectrum could be assigned to the trans conformer, which confirms that the gauche conformer cannot be observed under our measurement conditions. In agreement with the rather high torsional barrier of the methyl group (\emph{V}$_{3}$ = 1168 \wn) calculated by quantum chemical methods, all assigned lines appeared sharp and no signs of splittings were observed for the methyl internal rotation

LOWERING THE TORSIONAL BARRIERS BY STERICAL HINDRANCE: MICROWAVE SPECTRUM OF THE THREE-TOP MOLECULE 2,6-DIMETHYLANISOLE

The title molecule 2,6-dimethylanisole (26DMA) is one of the six isomers of dimethylanisole systematically studied by microwave spectroscopy. The spectrum of 26DMA was recorded using a pulsed molecular jet Fourier transform spectrometer. The experimental part was supported by quantum chemical calculations carried out at the B3LYP/6-311++G(d,p) level of theory. As calculated and experimentally proven for the three mono-methylanisoles (\textit{o}-,\footnote{L. Ferres, H. Mouhib, W. Stahl, and H. V. L. Nguyen, \textit{ChemPhysChem} \textbf{18}, 1855-1859, (2017).} \textit{m}-,\footnote{L. Ferres, W. Stahl, H. V. L. Nguyen, \textit{J. Chem. Phys.} \textbf{148}, 124304, (2018).} and \textit{p}-methylanisole\footnote{L. Ferres, W. Stahl, I. Kleiner, and H. V. L. Nguyen, \textit{J. Mol. Spectrosc.} \textbf{343}, 44-49, (2018).}) and three dimethylanisoles (2,3-DMA,\footnote{L. Ferres, K-N. Truong, W. Stahl, H. V. L. Nguyen, \textit{ChemPhysChem} \textbf{19}, 1781-1788, (2018).} 3,4-DMA,\footnote{L. Ferres, J. Cheung, W. Stahl, H. V. L. Nguyen, \textit{J. Phys. Chem. A} \textbf{123}, 3497-3503, (2019).} and 2,4-DMA\footnote{L. Ferres, W. Stahl, H. V. L. Nguyen, \textit{J. Chem. Phys.} \textbf{151}, 104310, (2019).}), the barrier to internal rotation of the methoxy methyl rotor surpasses 1000 cm$^{-1}$, causing unresolvable torsional splittings in the microwave spectrum. With both ortho positions substituted by a methyl group in 26DMA, the methoxy part is highly sterically hindered. It is thus forced to tilt out of the plane spanned by the heavy atoms of the phenyl ring by an angle of 90$^{\circ}$. Many experimental studies have shown that sterical hindrance often increases the barrier to internal rotation. Surprisingly, in the case of 26DMA, the torsional barrier decreases dramatically to about 460 cm$^{-1}$, leading to observable fine splittings in the microwave spectrum. Thus, 26DMA represents a three-top molecule, featuring two equivalent aryl methyl rotors and one methoxy methyl rotor

LOCAL AND GLOBAL APPROACHES TO TREAT THE TORSIONAL BARRIERS OF 4-METHYL-ACETOPHENONE USING MICROWAVE SPECTROSCOPY

The Fourier transform microwave spectrum of 4-methylacetophenone recorded from 8 GHz to 18 GHz under jet-cooled conditions has revealed large tunneling splittings arising from a low barrier to internal rotation of the ring methyl group and small splittings from a high torsional barrier of the acetyl methyl group. The large splittings are especially challenging to model, while the small splittings are difficult to analyze due to the resolution limit of 120 kHz. The combination of two methyl groups undergoing internal rotations caused each rotational transition to split into five torsional species, which were resolved and fitted using a modified version of the \textit{XIAM} code and the newly developed \textit{ntop} code to a root-mean-square deviation close to measurement accuracy, providing an estimate of the \textit{V}$_{3}$ potential barriers of about 22 \wn and 584–588 \wn for the ring and the acetyl methyl groups, respectively. The assignment was aided by separately fitting the five torsional species using odd-power order operators. Only one conformer in which all heavy atoms are located on a symmetry plane could be identified in the spectrum, in agreement with results from conformation analysis using quantum chemical calculations

THE ROTATIONAL SPECTRUM AND CONFORMATIONAL STRUCTURES OF METHYL VALERATE

Methyl valerate, C$_{4}$H$_{9}$COOCH$_{3}$, belongs to the class of fruit esters, which play an important role in nature as odorants of different fruits, flowers, and wines. A sufficient explanation for the structure�odor relation of is not available. It is known that predicting the odor of a substance is not possible by knowing only its chemical formula. A typical example is the blueberry- or pine apple-like odor of ethyl isovalerate while its isomers ethyl valerate and isoamyl acetate smell like green apple and banana, respectively. Obviously, not only the composition but also the molecular structures are not negligible by determining the odor of a substance. Gas phase structures of fruit esters are thus important for a first step towards the determination of structure�odor relation since the sense of smell starts from gas phase molecules.\ \For this purpose, a combination of microwave spectroscopy and quantum chemical calculations (QCCs) is an excellent tool. Small esters often have sufficient vapor pressure to be transferred easily in the gas phase for a rotational study but already contain a large number of atoms which makes them too big for classical structure determination by isotopic substitution and requires nowadays a comparison with the structures optimized by QCCs. On the other hand, the results from QCCs have to be validated by the experimental values.\ \About the internal dynamics, the methoxy methyl group -COOCH$_{3}$ of methyl acetate shows internal rotation with a barrier of 424.581(56) wn. A similar barrier height of 429.324(23) wn was found in methyl propionate, where the acetyl group is extended to the propionyl group. The investigation on methyl valerate fits well in this series of methyl alkynoates. In this talk, the structure of the most energetic favorable conformer as well as the internal rotation shown by the methoxy methyl group will be reported. It could be confirmed that the internal rotation barrier of the methoxy methyl group remains by longer alkyl chain

TWO EQUIVALENT INTERNAL ROTATIONS OBSERVED IN THE MICROWAVE SPECTRUM OF 2,6-DIMETHYLFLUOROBENZENE

The microwave spectrum of 2,6-dimethylfluorobenzene, one of the six isomers in the dimethylfluorobenzene family, was measured using a pulsed molecular jet Fourier transform microwave spectrometer in the frequency range from 2 to 40 GHz. Quantum chemical calculations were performed at the B3LYP/6-311++G(d,p) and MP2/6-31G(d,p) levels of theory to obtain optimized molecular geometries. The latter level yielded values of the rotational constants which were in almost exact agreement with the experimental values, and had eased tremendously the spectral assignment. Due to the internal rotation of the two equivalent methyl groups, all rotational transitions split into quartets with separations of up to several hundreds of MHz. The splittings were analyzed and modeled using the \textit{XIAM}\footnote{H. Hartwig, H. Dreizler, \textit{Z. Naturforsch.} \textbf{51a}, 923, (1996).} and the \textit{ntop} code\footnote{L. Ferres, W. Stahl, H.V.L. Nguyen, \textit{J. Chem. Phys.} \textbf{151}, 104310, (2019).} to measurement accuracy. The deduced \textit{V}$_{3}$ potential value of 206.4 cm$^{-1}$ is in reasonable agreement with the values predicted by quantum chemistry