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1.
Lynn Ferres Luca Evangelisti Assimo Maris Sonia Melandri Walther Caminati Wolfgang Stahl Ha Vinh Lam Nguyen 《Molecules (Basel, Switzerland)》2022,27(9)
The rotational spectrum of phenyl acetate, CH3COOC6H5, is measured using a free jet absorption millimeter-wave spectrometer in the range from 60 to 78 GHz and two pulsed jet Fourier transform microwave spectrometers covering a total frequency range from 2 to 26.5 GHz. The features of two large amplitude motions, the methyl group internal rotation and the skeletal torsion of the CH3COO group with respect to the phenyl ring C6H5 (tilted at about 70°), characterize the spectrum. The vibrational ground state is split into four widely spaced sublevels, labeled as A0, E0, A1, and E1, each of them with its set of rotational transitions and with additional interstate transitions. A global fit of the line frequencies of the four sublevels leads to the determination of 51 spectroscopic parameters, including the ΔEA0/A1 and ΔEE0/E1 vibrational splittings of ~36.4 and ~33.5 GHz, respectively. The V3 barrier to methyl internal rotation (~136 cm−1) and the skeletal torsion B2 barrier to the orthogonality of the two planes (~68 cm−1) are deduced. 相似文献
2.
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. 相似文献
3.
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. 相似文献
4.
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. 相似文献
5.
Maike Andresen Dr. Isabelle Kleiner Prof. Dr. Martin Schwell Prof. Dr. Wolfgang Stahl Dr. Ha Vinh Lam Nguyen 《Chemphyschem》2019,20(16):2063-2073
Using two molecular jet Fourier transform spectrometers, the microwave spectrum of hexan-2-one, also called methyl n-butyl ketone, was recorded in the frequency range from 2 to 40 GHz. Three conformers were assigned and fine splittings caused by the internal rotations of the two terminal methyl groups were analyzed. For the acetyl methyl group CH3 COC3H6CH3, the torsional barrier is 186.9198(50) cm−1, 233.5913(97) cm−1, and 182.2481(25) cm−1 for the three observed conformers, respectively. The value of this parameter could be linked to the structure of the individual conformer, which enabled us to create a rule for predicting the barrier height of the acetyl methyl torsion in ketones. The very small splittings arising from the internal rotation of the butyl methyl group CH3COC3H6 CH3 could be resolved as well, yielding the respective torsional barriers of 979.99(88) cm−1, 1016.30(77) cm−1, and 961.9(32) cm−1. 相似文献
6.
Alessio Melli Mattia Melosso Kevin G. Lengsfeld Luca Bizzocchi Víctor M. Rivilla Luca Dore Vincenzo Barone Jens-Uwe Grabow Cristina Puzzarini 《Molecules (Basel, Switzerland)》2022,27(10)
The processes and reactions that led to the formation of the first biomolecules on Earth play a key role in the highly debated theme of the origin of life. Whether the first chemical building blocks were generated on Earth (endogenous synthesis) or brought from space (exogenous delivery) is still unanswered. The detection of complex organic molecules in the interstellar medium provides valuable support to the latter hypothesis. To gather more insight, here we provide the astronomers with accurate rotational frequencies to guide the interstellar search of 3-aminoisoxazole, which has been recently envisaged as a key reactive species in the scenario of the so-called RNA-world hypothesis. Relying on an accurate computational characterization, we were able to register and analyze the rotational spectrum of 3-aminoisoxazole in the 6–24 GHz and 80–320 GHz frequency ranges for the first time, exploiting a Fourier-transform microwave spectrometer and a frequency-modulated millimeter/sub-millimeter spectrometer, respectively. Due to the inversion motion of the −NH2 group, two states arise, and both of them were characterized, with more than 1300 lines being assigned. Although the fit statistics were affected by an evident Coriolis interaction, we were able to produce accurate line catalogs for astronomical observations of 3-aminoisoxazole. 相似文献
7.
分子中基团的运动方式、机制对分子体系性质、分子功能的表达等具有重要作用。微波波谱法在研究分子系统内部动力学、分子结构、构象变化、弱相互作用、基团大幅度运动以及探索量子溶剂等方面具有独特的能力,特别适合研究分子的精细结构、分子系统基团的内部转动运动,具有高灵敏度、高分辨率的特点。本文讨论了微波波谱法在研究分子系统基团大幅度运动动力学方面的应用,包括分子系统中甲基基团的内部转动、OH基团的运动、氨和氨基化合物的反演以及环状有机分子环运动等的动力学,同时结合作者使用微波波谱法研究的部分体系进行了分析。 相似文献
8.
Maike Andresen Damian Schöngen Dr. Isabelle Kleiner Prof. Dr. Martin Schwell Prof. Dr. Wolfgang Stahl Dr. Ha Vinh Lam Nguyen 《Chemphyschem》2020,21(19):2206-2216
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 Cs 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 Cs conformer. 相似文献
9.
Javix Thomas Dr. Oleksandr Sukhorukov Prof. Dr. Wolfgang Jäger Prof. Dr. Yunjie Xu 《Angewandte Chemie (International ed. in English)》2014,53(4):1156-1159
Hydration of chiral molecules is a subject of significant current interest in light of recent experimental observations of chirality transfer from chiral solutes to water in solution and the important roles which water plays in biological events. Using a broadband chirped pulse and a cavity based microwave spectrometer, we detected spectroscopic signatures of the mono‐ and dihydrates of methyl lactate, a chiral hydroxy ester. Surprisingly, these small hydration clusters show highly specific binding preferences. Not only do they strongly prefer the insertion H‐bonding topology, but they also favor specific pointing direction(s) for their non‐H‐bonded hydroxy group(s). We observed that the particular dihydrate conformer identified is not the most stable one predicted. This work highlights the superior capability of high‐resolution spectroscopy to identify specific water binding topologies, and provides quantitative data to test state‐of‐the‐art theory. 相似文献
10.
Vadim V. Ilyushin Dr. Laura B. Favero Dr. Walther Caminati Prof. Dr. Jens‐Uwe Grabow Dr. 《Chemphyschem》2010,11(12):2589-2593
The microwave spectroscopic signatures of multiple torsional states of the CF3 internal rotation in benzotrifluoride (α,α,α‐trifluorotoluene) are reported. Individual rotational transitions are observed in a total of eight different torsional states, a quite challenging task for heavy tops even with Fourier transform microwave techniques. Accidental mixings of m=0 and m=3 torsional states as well as m=1 and m=2 torsional states, which can complicate the assignment of the spectra severely, are observed. These accidental mixings are probably systematic for molecules with heavy tops exhibiting an almost free internal rotation, and give an opportunity to determine the sign in the (1/2) V6 (1±cos6τ) potential function hindering internal rotation and in consequence the orientation of the CF3 top versus C6H5 frame. A recently developed torsion–rotation program reproduces all line positions within an experimental accuracy of about 2.0 kHz. The V6 barrier is determined to be 3.229949(32) cm?1. The corresponding torsional spacings are determined with the seven‐digit accuracy underlying the supersonic‐jet Fourier transform microwave technique. 相似文献
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Probing the Methyl Torsional Barriers of the E and Z Isomers of Butadienyl Acetate by Microwave Spectroscopy 
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下载免费PDF全文 Atef Jabri Vinh Van Dr. Ha Vinh Lam Nguyen Prof.Dr. Wolfgang Stahl Dr. Isabelle Kleiner 《Chemphyschem》2016,17(17):2660-2665
The Fourier transform microwave spectra of the E and Z isomers of butadienyl acetate were measured in the frequency range from 2 to 26.5 GHz under molecular‐jet 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 the results from quantum‐chemical calculations. The barriers to internal rotation of the acetyl methyl group were found to be 149.1822(20) and 150.2128(48) cm?1 for the E and Z isomers, respectively, which are similar to that of vinyl acetate. A comparison between two theoretical approaches treating internal rotation, the rho axis method and combined axis method, was also performed. The influence of the alkyl R chain on the methyl torsional barriers in CH3 ‐COOR acetates was explored. 相似文献
14.
We present quantum mechanical partition functions, free energies, entropies, and heat capacities of 1,3-butadiene derived from ab initio calculations. Our technique makes use of a reaction path-like Hamiltonian to couple all 23 vibrational modes to the large-amplitude torsion, which involves heavy asymmetric functional groups. Ab initio calculations were performed at the B3LYP, MP2, and CCSD(T) levels of theory and compared with experimental values as a reference case. By using the ab initio potentials and projected frequencies, simple perturbative expressions are presented for computing the couplings of all the vibrational modes to the large-amplitude torsion. The expressions are particularly suited for programming in the new STAR-P software platform which automatically parallelizes our codes with distributed memory via a familiar MATLAB interface. Using the efficient parallelization scheme of STAR-P, we obtain thermodynamic properties of 1,3-butadiene for temperatures ranging from 50 to 500 K. The free energies, entropies, and heat capacities obtained from our perturbative formulas are compared with conventional approximations and experimental values found in thermodynamic tables. 相似文献
15.
Acetyl Methyl Torsion in N‐Ethylacetamide: A Challenge for Microwave Spectroscopy and Quantum Chemistry 下载免费PDF全文
下载免费PDF全文 Raphaela Kannengießer Marcel J. Lach Prof. Dr. Wolfgang Stahl Dr. Ha Vinh Lam Nguyen 《Chemphyschem》2015,16(9):1906-1911
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. 相似文献
16.
We present a partially automated method for the thermodynamic treatment of large-amplitude motions. Starting from the molecular geometry and the Hessian matrix, we evaluate anharmonic partition functions for selected vibrational degrees of freedom. Supported anharmonic vibration types are internal rotation and inversion (oscillation in a double-well potential). By heuristic algorithms, we identify internal rotations in most cases automatically from the Hessian eigenvectors, and we also estimate the parameters of anharmonic partition functions (e.g., potential barrier, periodicity, and symmetry number) with thermodynamically sufficient precision. We demonstrate the validity of our schemes by comparison to pointwise calculated ab initio potential curves. 相似文献
17.
The microwave spectra of the natural substance coumarin, a planar aromatic molecule with the specific scent of maibowle, a popular fruit punch served in spring and early summer, were recorded using a molecular jet Fourier transform microwave spectrometer working in the frequency range from 4.0 to 26.5 GHz. The rotational constants and centrifugal distortion constants were determined with high precision, reproducing the spectra to experimental accuracy. The spectra of all singly-substituted 13C and 18O isotopologues were observed in their natural abundances to determine the experimental heavy atom substitution rs and semi-experimental equilibrium reSE structures. The experimental bond lengths and bond angles were compared to those obtained from quantum chemical calculations and those of related molecules reported in the literature with benzene as the prototype. The alternation of the C−C bond lengths to the value of 1.39 Å found for benzene reflects the localization of π electrons in coumarin, where the benzene ring and the lactone-like chain −CH=CH−(C=O)−O− are fused. The large, negative inertial defect of coumarin is consistent with out-of-plane vibrations of the fused rings. 相似文献
18.
Rihab Hakiri Dr. Najoua Derbel Prof. Dr. Wolfgang Stahl Dr. Halima Mouhib 《Chemphyschem》2020,21(1):20-25
To accurately characterize the large amplitude motions and soft degrees of freedom of isolated molecules, sampling their conformational landscape by molecular mechanics and quantum chemical calculations may provide a valuable insight into the structure and dynamics. However, the resulting models need to be validated by a reliable experimental counterpart. For ethyl pentanoates, which belong to the family of fruit esters, benchmark calculations at different levels of theory showed that the C−C bond in proximity to the ester carbonyl group exhibits a large amplitude motion that is extremely sensitive to the choice of quantum chemical method and basis set. In such cases, insights from high-resolution molecular jet techniques are ideal to accurately identify and characterize soft degrees of freedom. Here, we report on the most abundant conformer of ethyl 2-ethyl butyrate using Fourier-transform microwave spectroscopy. We show that – unlike other structurally related pentanoates for which gas-phase and crystallographic data is available – ethyl 2-ethyl butyrate possesses a Cs symmetry plane under molecular jet conditions. 相似文献
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