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The microwave spectrum of 2,4-dimethylpyrrole was investigated using a Fourier-transform microwave spectrometer in a supersonic expansion. Torsional splittings arising from two inequivalent methyl internal rotors in combination with hyperfine splittings due to the nuclear quadrupole coupling of the 14N nucleus were observed. The experiments were accompanied by quantum chemical calculations. A total of 1561 rotational lines were assigned and fitted in global fits using the programs XIAM and BELGI-Cs-2Tops-hyperfine, both achieved the measurement accuracy of 4 kHz. Local separate fits were also performed to verify the correctness of the assignment. Accurate experimental molecular and internal rotation parameters could be deduced and compared to the calculated ones. The barrier to internal rotation of the 2-methyl rotor was determined to be 277.830(26) cm−1, essentially the same as the value of about 280 cm−1 found for 2-methylpyrrole but lower than the value of 317 cm−1 found for 2,5-dimethylpyrrole. The torsional barrier value of the 4-methyl rotor is 262.210(27) cm−1, slightly higher than the value of 246 cm−1 found for 3-methylpyrrole. Benchmarking the rotational constants for 2,4- and 2,5-dimethylpyrrole revealed that the MP2/6-31G(d,p) level could be helpful to guide the assignment of microwave spectra of pyrrole derivatives. [ABSTRACT FROM AUTHOR]

A number of internal rotation codes can deal with the combination of one or two internal rotors with one 14N quadrupole nucleus, but once it comes to two 14N nuclei, no such code is available even for the case of one internal rotor. We present here the extension of our internal rotor program called BELGI-2N using the rho-axis method global approach to deal with compounds containing one methyl rotor and two weakly coupling 14N nuclei. To test our new code, we applied it to the microwave data recorded for N-methylimidazole, 2-methylimidazole, 4-methylimidazole, and 5-methylimidazole using a chirped-pulse Fourier transform microwave spectrometer in the 7.0–18.5 GHz frequency range. Compared to the previously published study, BELGI-2N was able to (i) significantly increase the number of assigned and fitted lines, (ii) fit the complete datasets considering both the internal rotation and the 14N nuclear quadrupole coupling effects simultaneously, and (iii) achieve standard deviations within the measurement accuracy for all methylimidazole isomers. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The rotational spectra of coffee furanone (2-methyltetrahydrofuran-3-one) have been measured in the frequency range from 2.0 to 26.5 GHz using a molecular jet Fourier transform microwave spectrometer. Quantum chemical calculations used for the conformational analysis yielded two stable twist conformers, which were described using the Cremer-Pople notation for five-membered rings. The experimental spectrum of the more stable conformer with the methyl group in equatorial position was assigned and fitted using a rigid rotor model with centrifugal distortion corrections. The spectra of all five 13C-isotopologues could be observed in natural abundance. The carbon atom structure was experimentally deduced using Kraitchman's equations and compared with the structure calculated by quantum chemistry. Key words: rotational spectroscopy, conformational analysis, structure determination, five-membered ring. Resume : Nous mesurons les spectres de rotation du furanone de cafe (2- methyltetahydrofurane-3-un) dans le domaine 2.026.5 GHz, en utilisant un spectrometre microonde a transformee de Fourier avec jet moleculaire. Les calculs de chimie quantique utilises pour l'analyse de configuration donnent deux conformeres stables, qui sont decrits en utilisant la notation de CremerPople pour les anneaux a cinq membres. Le spectre experimental du conformere le plus stable avec le groupe methyl en position equatoriale est assigne et ajuste numeriquement au modele du rotateur rigide avec corrections de distorsions centrifuges. Les spectres des cinq isotopologues C sont observes selon les abondances isotopiques naturelles. La structure de l'atome de carbone est experimentalement deduite a l'aide de l'equation de Kraitchman et comparee avec la structure calculee en chimie quantique. [Traduit par la Redaction] Mots-cles : spectroscopie rotationnelle, analyse de configuration, determination de structure, anneau a cinq membres, 2-methyltetrahgydrofurane-3-un., 1. Introduction Coffee is a very popular drink in the daily life of millions of people in many countries as a first hot beverage in the morning. Several methods have [...]

The microwave spectrum of 2,4-dimethylfluorobenzene was recorded using a molecular jet Fourier transform microwave spectrometer in the frequency range from 2.0 to 26.5 GHz. The spectral assignment and modeling were challenging due to the large tunnelling splittings resulting from the very low barrier to internal rotation of the p-methyl group that approaches the free rotor limit. Internal rotation splittings arising from two inequivalent o- and p-methyl groups were observed, analysed and modelled using the modified version of the XIAM code and the BELGI-Cs-2Tops code, giving a root-mean-square deviation of 549.1 kHz and 4.5 kHz, respectively, for a data set of 885 rotational lines. The torsional barriers of the o- and p-methyl groups were determined to be 227.039(51) cm−1 and 3.23(40) cm−1, respectively. The V3 barrier observed for the p-methyl group is lower than in any other para-methyl substituted toluene derivatives with coupled internal rotations, becoming the lowest value ever observed to date. The barrier to internal rotation of the o-methyl group next to a fluorine atom is consistently around 220 cm−1, as confirmed by comparing it to barriers observed in other toluene derivatives. The experimental rotational constants were compared to those obtained by quantum chemical calculations. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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-C2v-2Tops-hyperfine code, a modified version of the BELGI-C2v-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. [ABSTRACT FROM AUTHOR]

The spectra of N-ethyl methyl amine, CH3(NH)CH2CH3, were measured using a molecular jet Fourier transform microwave spectrometer in the frequency range of 2 GHz–26.5 GHz. Splittings due to proton inversion tunneling, Coriolis coupling, 14N quadrupole coupling, and methyl internal rotation were fully resolved. The experimentally deduced rotational constants are A = 25 934.717(21) MHz, B = 3919.8212(23) MHz, and C = 3669.530(21) MHz. The proton tunneling causes (+) ↔ (−) splittings of about 1980.9 MHz for all c-type transitions between the lowest symmetric and the higher anti-symmetric energy levels. The splittings of the (+) ← (+) and (−) ← (−) levels, mainly influenced by Coriolis coupling, were also observed and assigned for b-type transitions, yielding the coupling constants Fbc = 0.3409(71) MHz and Fac = 163.9(14) MHz. The 14N quadrupole coupling constants were determined to be χaa = 2.788 65(55) MHz and χbb − χcc = 4.630 45(91) MHz. Fine splittings arising from two inequivalent methyl rotors are in the order of 150 kHz, and the torsional barriers are determined to be 1084.62(41) cm−1 for the CH3NH methyl group and 1163.43(80) cm−1 for the CH2CH3 methyl group. The experimental results are in good agreement with those of quantum chemical calculations. [ABSTRACT FROM AUTHOR]

The rotational spectrum of 2-methylthiazole was recorded using two pulsed molecular jet Fourier transform microwave spectrometers operating in the frequency range of 2–40 GHz. Due to the internal rotation of the methyl group, all rotational transitions were split into A and E symmetry species lines, which were analyzed using the programs XIAM and BELGI-Cs-hyperfine, yielding a methyl torsional barrier of 34.796 75(18) cm−1. This value was compared with that found in other monomethyl substituted aromatic five-membered rings. The 14N quadrupole coupling constants were accurately determined to be χaa = 0.5166(20) MHz, χbb − χcc = −5.2968(50) MHz, and χab = −2.297(10) MHz by fitting 531 hyperfine components. The experimental results were supplemented by quantum chemical calculations. [ABSTRACT FROM AUTHOR]

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 XIAM code and the newly developed ntop code to a root-mean-square deviation close to measurement accuracy, providing an estimate of the V3 potential barriers of about 22 cm−1 and 584–588 cm−1 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. [ABSTRACT FROM AUTHOR]