Methyl Internal Rotation in Fruit Esters: Chain-Length Effect Observed in the Microwave Spectrum of Methyl Hexanoate

The gas-phase structures of the fruit ester methyl hexanoate, CH₃-O-(C=O)-C₅H₁₁, have been determined using a combination of molecular jet Fourier-transform microwave spectroscopy and quantum chemistry. The microwave spectrum was measured in the frequency range of 3 to 23 GHz. Two conformers were assigned, one with C_s symmetry and the other with C₁ symmetry where the γ-carbon atom of the hexyl chain is in a gauche orientation in relation to the carbonyl bond. Splittings of all rotational lines into doublets were observed due to internal rotation of the methoxy methyl group CH₃-O, from which torsional barriers of 417 cm⁻¹ and 415 cm⁻¹, respectively, could be deduced. Rotational constants obtained from geometry optimizations at various levels of theory were compared to the experimental values, confirming the soft degree of freedom of the (C=O)-C bond observed for the C₁ conformer of shorter methyl alkynoates like methyl butyrate and methyl valerate. Comparison of the barriers to methyl internal rotation of methyl hexanoate to those of other CH₃-O-(C=O)-R molecules leads to the conclusion that though the barrier height is relatively constant at about 420 cm⁻¹, it decreases in molecules with longer R.     

Internal Rotation of the Acetyl Methyl Group in Methyl Alkyl Ketones: The Microwave Spectrum of Octan-2-one

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 C_s 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 C_s conformer.     

Skeletal Torsion Tunneling and Methyl Internal Rotation: The Coupled Large Amplitude Motions in Phenyl Acetate

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 ΔE_A0/A1 and ΔE_E0/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.     

碘甲烷通过Ã态和C态的多光子电离

碘甲烷分子与激光相互作用,发生的光解、激发电离等化学或物理过程已由许多人进行过研究.本文报导用多光子电离飞行质谱和光电子能谱技术研究碘甲烷通过(?)态和(?)态的多光子过程的主要实验结果和结论.实验用Nd:YAG 激光器泵浦染料激光器,可调谐的染料(R590)激光经倍频或与YAG 基频1.06μm 混频后分别得到280nm 或366nm 附近的紫外光,能量为0.2—2mJ/脉冲,时间脉     

Sensing the Molecular Structures of Hexan-2-one by Internal Rotation and Microwave Spectroscopy

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.     

OCS的多光子电离高分辨光电子能谱

硫氧化碳OCS是线性三原子分子，这类小分子的激发态、离子态能级结构、能级之间的相互作用及电离过程，是研究中所关心的问题．Tanaka等[1]和Kopp[2]测量了OCS的VUV吸收光谱，Frey和Schlag等[3]以同步辐射光源，用光电离共振（PIR）谱方法、Kovac[4]和Wang，Shirley等[5]以Hel为电离光源，分别采用传统的光电子能谱和高分辨光电子能谱技术研究了CO2、CS2和OCS分子从电子振动基态吸收单个光子而电离的过程．Yang和Anderson等问为了作选态的离子一分子反应利用可调谐激光rt光子吸收将OCS选择激发到某一中间态，OCS再吸收光子后…     

Two Equivalent Internal Rotations in the Microwave Spectrum of 2,6-Dimethylfluorobenzene

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.     

Velocity Mapping Studies of Vibrational Energy Disposal Following Methyl Iodide Photodissociation

In this paper previous results are compared for two different types of velocity mapping studies which probe vibrational energy disposal following the A-band photodissociation of methyl iodide, CH₃I + hv → CH₃ (v) + 1(²P_3/2), 1*(²P_1/2). Full three-dimensional state-specific speed and angular distributions of the nascent fragments have been recorded for the photoelectrons, iodine atoms, and methyl radicals, using state- and mass-selective (2+1) resonance-enhanced multi-photon ionization (REMPI)/time-of-flight spectrometry. Two sources of information on the vibrational energy disposal are available from velocity mapping: (a) the photoelectron images, which give information on the initial stages of vibrational excitation in electronically excited CH₃I, and (b) methyl radical images, which indicate the final energy disposal channels. Even though the two signals are believed to probe very different time-scales of the dissociation process, good agreement between the two is found for the vibrational energy disposal trends. Several trends found in the data for methyl iodide photodissociation indicate that readjustment of the ab initio multi-dimensional potential energy surfaces calculated for this molecule appears to be needed.     

Microwave Spectra of o-Fluorotoluene and Its 13C Isotopic Species: Methyl Internal Rotation and Molecular Structure

The rotational spectra of o-fluorotoluene and its seven ¹³C isotopic species were recorded in the frequency range from 4 to 20 GHz with employment of pulsed molecular beam Fourier-transform microwave (MB-FTMW) spectrometers. The analysis of the spectra in the two lowest states of methyl internal rotation (torsional ground state, A and E species) was based on a asymmetric frame-rigid symmetric top Hamiltonian with inclusion of centrifugal distortion terms, yielding structural rotational constants, as well as the threefold barrier V ₃ to internal rotation and the angle(a,i) between the principal moment of inertia a axis and the internal rotor axis i. The rotational constants of all eight isotopomeres were used to derive the seven ¹³C r _s coordinates of the molecule.     

飞秒时间分辨的光电子影像技术研究2-氯吡啶超快内转换动力学

利用时间分辨的飞秒光电子影像技术结合时间分辨的质谱技术, 研究了2-氯吡啶分子激发态的超快过程. 实时观察到了2-氯吡啶分子第二激发态(S₂)向第一激发态(S₁)高振动能级的的超快内转换过程,该内转换的时间常数为(162±5)fs. 实验结果表明, 通过S₂/S₀的锥形交叉衰减到基态的衰减通道也是退布居的重要通道, 其时间尺度为(5.5±0.3) ps.     

飞秒时间分辨的光电子影像研究氯化苄分子内转换动力学(英文)

结合时间分辨的飞秒光电子影像(TRPEI)技术和时间分辨的质谱技术,研究了氯化苄(BzCl)分子内转换动力学过程.从光电子影像中获得了光电子动能分布和角度分布.氯化苄分子吸收两个400nm的光子后从基态跃迁到S4态和S2态.获得的母体离子随泵浦-探测时间延迟变化的曲线可以用两个指数函数进行拟合,包括一个时间常数为50fs的快速组分和一个时间常数为910fs的慢速组分.通过分析光电子动能分布随延迟时间的变化,我们认为分子被激发到S4态后在很短的时间内与S2态发生耦合迅速弛豫到S2态,然后再经内转换(IC)弛豫到S1态.最初布居的激发态分子经过内转换弛豫到S1态的时间尺度为50fs.910fs的慢速时间组分反映了分子弛豫到S1态后,经内转换向基态S0的弛豫.光电子角度分布的各向异性参数从零时刻的0.87增加到25fs时的0.94,然后逐渐减小到190fs时刻的0.59的现象,也反映了氯化苄分子从S4态耦合到S2态,然后内转换到S1态的动力学过程.     

The Impact of Leaving Group Anomericity on the Structure of Glycosyl Cations of Protected Galactosides

It has been reported that fragments produced by glycosidic bond breakage in mass spectrometry-based experiments can retain a memory of their anomeric configuration, which has major implications for glycan sequencing. Herein, we use cryogenic vibrational spectroscopy and ion mobility-mass spectrometry to study the structure of B-type fragments of protected galactosides. Cationic fragments were generated from glycosyl donors carrying trichloroacetimidate or thioethyl leaving groups of different anomeric configuration. The obtained infrared signatures indicate that the investigated fragments exhibit an identical structure, which suggests that there is no anomeric memory in B-type ions of fully protected monosaccharides.     

用13C的NMR T1研究苯环上甲基的内旋转

提出了通过对样品进行邻位取代和间位取代以从实验上测定自旋-内旋转弛豫速率的处理方法.在28-90℃的温度范围内测量了邻二甲苯和间二甲苯的甲基碳的T₁和NOE因子.分离得间二甲苯上甲基碳的自旋-内旋转弛豫速率和自旋-整体旋转弛豫速率,并计算得甲基的内旋转速率.     

溴甲烷在强激光场中的电离和解离

利用自制的反射式飞行时间质谱仪(RTOF-MS)研究了多原子分子CH₃Br在强激光场中的电离解离. 得到了溴甲烷在强激光场中电离解离的飞行时间质谱, 基于RTOF-MS的高分辨率(M/ΔM>2000), 测量了分子库仑爆炸产生的系列碎片离子的动能释放(KER), 用多光子解离和库仑爆炸解释了实验结果. 与碘甲烷在强场中的实验结果对比发现: (1) 在相同的激光场强下, 碘甲烷电离解离的最高价碎片离子为I⁶⁺而溴甲烷为Br³⁺; (2) 溴甲烷质谱中存在母体离子的脱氢产物CH_mBr⁺ 和CH_mBr²⁺, 而对于碘甲烷, 没有检测到这些通道, C-I键首先断开; (3) 质谱中存在H⁷⁹Br⁺和H⁸¹Br⁺, 而碘甲烷的电离解离中不存在HI产物; (4) 溴甲烷库仑两体爆炸的有效电荷间距随着两碎片电荷乘积的增大而增大, 而对于碘甲烷此间距几乎不随电荷乘积变化; (5) CH_m⁺(m=0, 1, 2)的主要生成通道可能与碘甲烷不同, 不是来自CH₃⁺的顺序脱氢, 而是来自脱氢母体离子的直接解离.     

The Bonding Nature of the Canonical Molecular Orbitals in Simple Diatomic Molecules

The bonding nature of the canonical molecular orbitals 2σg, 2σu and 3σg in the molecules N₂,O₂, F₂ and the related analogous molecular orbitals in the molecules P₂ and CO, is analysed using Weinhold's natural bond orbital set. When the canonical molecular orbitals can be well localized into natural bond orbitals, the covalent bond can be completely attributed to the bonding type natural bond orbitals. The decomposition of canonical molecular orbitals into the natural bond orbital basis then gives the weighted bond order as the component of the bonding portion in the canonical molecular orbital. The weighted bond order results match the photoelectron spectroscopy assignment quite satisfactorily.     

脱氢松香酸甲酯类衍生物的合成

以脱氢松香酸甲酯为原料, 经溴代、硝化、还原、重氮化等步骤合成了6种12位取代的脱氢松香酸甲酯衍生物, 研究了12位上不同取代基对它们紫外、荧光性质的影响; 然后再分别以得到的12-氯脱氢松香酸甲酯和12-溴脱氢松香酸甲酯为起始物, 经双硝化、氧化、脱异丙基硝化等步骤合成了一系列重要的脱氢松香酸甲酯卤代硝化衍生物, 并对12-氯脱氢松香酸甲酯、12-溴-13,14-二硝基脱氢松香酸甲酯的单晶进行了X射线衍射表征.     

Millimeter-Wave Spectroscopy of Methylfuran Isomers: Local vs. Global Treatments of the Internal Rotation

Methylfurans are methylated aromatic heterocyclic volatile organic compounds and primary or secondary pollutants in the atmosphere due to their capability to form secondary organic aerosols in presence of atmospheric oxidants. There is therefore a significant interest to monitor these molecules in the gas phase. High resolution spectroscopic studies of methylated furan compounds are generally limited to pure rotational spectroscopy in the vibrational ground state. This lack of results might be explained by the difficulties arisen from the internal rotation of the methyl group inducing non-trivial patterns in the rotational spectra. In this study, we discuss the benefits to assign the mm-wave rotational-torsional spectra of methylfuran with the global approach of the BELGI-Cs code compared to local approaches such as XIAM and ERHAM. The global approach reproduces the observed rotational lines of 2-methylfuran and 3-methylfuran in the mm-wave region at the experimental accuracy for the ground vt=0 and the first torsional vt=1 states with a unique set of molecular parameters. In addition, the V3 and V6 parameters describing the internal rotation potential barrier may be determined with a high degree of accuracy with the global approach. Finally, a discussion with other heterocyclic compounds enables the study of the influence of the electronic environment on the hindered rotation of the methyl group.     

Jahn–Teller Effects in Molecular Cations Studied by Photoelectron Spectroscopy and Group Theory

The traditional “ball‐and‐stick” concept of molecular structure fails when the motion of the electrons is coupled to that of the nuclei. Such a situation arises in the Jahn–Teller (JT) effect which is very common in open‐shell molecular systems, such as radicals or ions. The JT effect is well known to chemists as a mechanism that causes the distortion of an otherwise symmetric system. Its implications on the dynamics of molecules still represent unsolved problems in many cases. Herein we review recent progress in understanding the dynamic structure of molecular cations that have a high permutational symmetry by using rotationally resolved photoelectron spectroscopy and group theory. Specifically, we show how the pseudo‐Jahn–Teller effect in the cyclopentadienyl cation causes electronic localization and nuclear delocalization. The fundamental physical mechanisms underlying the vaguely defined concept of “antiaromaticity” are thereby elucidated. Our investigation of the methane cation represents the first experimental characterization of the JT effect in a threefold degenerate electronic state. A special kind of isomerism resulting from the JT effect has been discovered and is predicted to exist in all JT systems in which the minima on the potential‐energy surface are separated by substantial barriers.     

Large Amplitude Torsions in Nitrotoluene Isomers Studied by Rotational Spectroscopy and Quantum Chemistry Calculations

Rotational spectra of ortho-nitrotoluene (2-NT) and para-nitrotoluene (4-NT) have been recorded at low and room temperatures using a supersonic jet Fourier Transform microwave (MW) spectrometer and a millimeter-wave frequency multiplier chain, respectively. Supported by quantum chemistry calculations, the spectral analysis of pure rotation lines in the vibrational ground state has allowed to characterise the rotational energy, the hyperfine structure due to the ¹⁴N nucleus and the internal rotation splittings arising from the methyl group. For 2-NT, an anisotropic internal rotation of coupled −CH₃ and −NO₂ torsional motions was identified by quantum chemistry calculations and discussed from the results of the MW analysis. The study of the internal rotation splittings in the spectra of three NT isomers allowed to characterise the internal rotation potentials of the methyl group and to compare them with other mono-substituted toluene derivatives in order to study the isomeric influence on the internal rotation barrier.     

Microwave Spectrum of Two-Top Molecule: 2-Acetyl-3-Methylthiophene

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.