High-resolution rotational spectroscopy of iminosilylene,HNSi

By means of Fourier transform microwave spectroscopy of a supersonic beam, the fundamental rotational transition of isotopic and vibrationally excited iminosilylene, HNSi, has been detected. In addition to seven isotopic species, vibrational satellite transitions from more than 30 vibrationally excited states, including the three fundamental modes, have been detected. Those from ν₂ are particularly intense, enabling detection of transitions from as high as (0,22⁰,0) (i.e. ～10,000 cm^?1 above ground). At high spectral resolution, well-resolved nitrogen quadrupole structure has been observed in nearly every transition. Excitation of ν₁ or ν₃ changes eQq(N) little, but eQq(N) systematically decreases with increasing excitation of the ν₂ bend, from a value of 0.376(5) MHz for (0,0⁰,0) to ?2.257(5) MHz for (0,20⁰,0). With the large amount of new data in hand, it has also been possible to determine the leading vibration–rotation constants (α_i and γ_i) for ν₂ or ν₃ to high precision, and derive a revised semi-empirical equilibrium structure for this fundamental triatomic molecule. Various electronic and molecular properties of iminosilylene have been calculated at the coupled cluster level of theory, and these generally agree well with experiment and previous calculations. An unsuccessful search for HSiN, a highly polar isomer calculated to lie nearly 3 eV above HNSi, is also reported.     

The rotational spectrum of methyl trifluoroacetate

ABSTRACT

The pulsed supersonic jet expansion microwave spectra of the parent and all three ¹³C mono-substituted isotopologues of methyl trifluoroacetate have been measured in the 6.5–18 GHz range. All observed transitions are split into two component lines, due to the internal rotation of the methyl group. The corresponding barrier has been determined to be V ₃ = 4.379(3) kJ/mol. Structural information has been obtained from the 12 available rotational constants.     

The Lamb-dip spectrum of methylcyanide: Precise rotational transition frequencies and improved ground-state rotational parameters

Methylcyanide, CH₃CN, is an important interstellar species, and therefore the accurate knowledge of precise rest frequencies for rotational transitions as well as ground-state rotational and hyperfine constants is needed. In this work the hyperfine structure of the millimeter- and submillimeter-wave spectra of CH₃CN has been further investigated. In addition, accurate THz measurements have been carried out for the first time. Consequently, the present investigation allowed us to provide the most accurate ground state rotational and hyperfine parameters known at the moment for CH₃C¹⁴N. To resolve the hyperfine structure of the rotational transitions observed, the Lamb-dip technique has been exploited. Both frequency-modulated and video-type detections have been employed.     

The millimeter-wave rotational spectrum of fluorobenzene

The room-temperature rotational spectrum of fluorobenzene was studied in the frequency region 167-318 GHz. Rotational transitions were assigned and measured in the ground vibrational state, and all six excited vibrational states with energies below 600 cm⁻¹, i.e., v₁₁ = 1, v₁₁ = 2, v_18b = 1, v_16a = 1, v_16b = 1, and v_6a = 1. Accurate quartic-level spectroscopic constants were determined for all states, allowing spectral predictions well into the submillimeter region. The states v_18b = 1 and v_16a = 1 were found to be connected by a strong Coriolis interaction, which allowed precise determination of their energy separation, ΔE = 7.455088(3) cm⁻¹. Unambiguous assignment of vibrational modes was made on the basis of the calculated inertial defect and nuclear spin statistical weights. Rotational constants for the ¹³C₄ isotopomer have also been redetermined and two new least-squares determinations of the geometry of fluorobenzene, r₀ and are reported.     

转动传能中的量子干涉--干涉角和转动量子数的关系

沙国河等人在CO(A1∏(v=0)~e3∑-(v=1))体系与He,Ne和Ar碰撞诱导转动传能中首次观测到了量子干涉效应,并测量了干涉度。从理论上进一步研究原子-双原子分子体系碰撞诱导转动传能中量子干涉效应与转动量子数以及能量间隔的关系是十分必要的。我们考虑长程相互作用势,应用一级玻恩近似和直线轨迹近似,分别计算了CO(A1∏(v=0)~e3∑-(v=1))体系和He,Ne,Ar碰撞诱导转动传能中不同转动量子数以及不同能量间隔下的干涉角,得到了干涉角随转动量子数和能量间隔的变化趋势。这些结果对设计、分析这种类型的实验有一定的指导意义。     

The magnetic hyperfine structure in the rotational spectrum of H2CNH

The hyperfine structure in the ground-state rotational spectrum of methanimine was studied in the frequency range of 64-172 GHz by means of the Lamb-dip technique. This allowed to resolve, in some hyperfine components due to the ¹⁴N nucleus, doublets separated by only some tenth of kHz. We explain the splittings as due to magnetic interactions of the three protons with their molecular environment. The analysis of the experimental spectrum has been guided by quantum-chemical calculations of the hyperfine parameters.     

The millimeter wave rotational spectrum of lactic acid

The results of a comprehensive investigation of the rotational spectrum of lactic acid over the frequency region 171-318 GHz are reported. Some supersonic expansion measurements at 8-16 GHz have also been made. A complete set of octic level constants in the asymmetric rotor Hamiltonian has been determined for the ground vibrational state from a fit to over 1000 measured transition frequencies. Spectroscopic constants have also been determined for the first five excited states of the low frequency, 60 cm⁻¹, torsional vibrational mode, and for four other vibrationally excited states. Vibrational states become rather crowded above 200 cm⁻¹, with seven different states only in the next 100 cm⁻¹, and almost all of the measured states in this energy region show evidence of perturbations. The analysis was carried out with the newly developed AABS software package for Assignment and Analysis of Broadband Spectra.     

Millimeter-wave spectroscopy of deuterated hydrogen sulfide, SH2D

The monodeuterated isotopologue of the sulfonium ion, the asymmetric species SH₂D⁺, was produced in a magnetically confined negative glow discharge and detected for the first time by means of its rotational spectrum in the region 324-637 GHz. The determined rotational constants, along with those for the symmetric isotopologues and for obtained in previous measurements, enabled to compute a semi-experimental equilibrium structure, where the contribution of the zero-point vibrations is assumed from available ab initio calculations. In addition, thanks to the large body of quartic centrifugal distortion constants now available, the harmonic force field of the sulfonium ion has been refined by a least-squares procedure.     

The pure rotational spectrum of ZnCl (XΣ): Variations in zinc halide bonding

The radical ZnCl (X²Σ⁺) has been studied using millimeter-wave direct-absorption techniques. Pure rotational spectra of ⁶⁷Zn³⁵Cl, ⁶⁶Zn³⁷Cl, ⁶⁸Zn³⁵Cl, ⁶⁴Zn³⁵Cl, ⁶⁴Zn³⁷Cl, and ⁶⁶Zn³⁵Cl were measured in the vibrational ground state and data were also recorded for the latter three in the v = 1 and v = 2 states. Every rotational transition was found to be split into a doublet due to spin-rotation interactions. For ⁶⁷Zn³⁵Cl, each doublet exhibited additional splittings arising from hyperfine coupling of the ⁶⁷Zn (I = 5/2) nucleus. Rotational, fine structure, and hyperfine constants have been determined from these data, and equilibrium parameters calculated. The equilibrium bond length of ⁶⁴Zn³⁵Cl is found to be 2.13003305(24) Å, in good agreement with recent theoretical predictions. Interpretation of hyperfine constants indicates that the 12σ orbital is ∼70% Zn(4s) in character, suggesting that the zinc chloride bond is relatively ionic.     

Millimeter wave measurement and assignment of the rotational spectrum of aniline

Previous work involving the rotational spectrum of aniline was limited to the lower frequencies of 8-40 GHz with very few lines being assigned. This work extends the earlier studies. Here we present a much more extensive measurement and assignment of the rotational spectrum of aniline in the frequency range of 75-110 GHz. The observed frequencies have been assigned to the ground (0⁺), first (0⁻), and second excited (1⁺) states in the inversion vibration. With the newly assigned lines, significantly improved rotational constants and all five centrifugal distortion constants have been obtained.     

一种获取双原子分子的转动常数的方法

本文给出了对于任何双原子分子或双原子自由基分子,由其振动常数结合RKR方程、结合从头计算,获得势能曲线及其转动常数的方法.并对CN的基态和激发态进行了计算,结果与实验值符合得非常一致.A     

The rotational spectrum of silacyclohexane

The rotational spectra of the normal and Si-d₂ isotopomers of the chair form of silacyclohexane have been measured by microwave absorption spectroscopy. A partial r₀ structure has been obtained. The rotational spectra of some, the most intense, vibrational satellites have also been measured. They belong to the ring-puckering motions. Their vibrational energies and their shifts of planar moments of inertia with respect to the ground state indicate that the amplitude of these vibrations is larger than in cyclohexane. The dipole moment has also been determined: μ_a = 0.75(2), μ_c = 0.280(2), and μ_tot = 0.80(2) D.     

The rotational spectrum of epifluorohydrin measured by chirped-pulse Fourier transform microwave spectroscopy

The rotational spectrum of epifluorohydrin measured by chirped-pulse Fourier transform microwave (CP-FTMW) spectroscopy is presented. A new CP-FTMW spectrometer capable of measuring the entire 7.5-18.5 GHz spectrum with a single polarizing pulse is described briefly. The CP-FTMW spectrometer takes advantage of recent advances in digital electronics by utilizing a 4.2 GS/s arbitrary waveform generator as a frequency source and a 12 GHz digital oscilloscope to digitize the down converted molecular free induction decay (FID). Signal averaging in the time domain is used to increase the signal-to-noise ratio. The rotational constants of three unique conformers of epifluorohydrin were measured, as well as the rotational constants of the three unique ¹³C isotopomers and the ¹⁸O isotopomer (in natural abundance) of the most stable conformer. The rotational constants of the two less stable conformers differ significantly from those previously reported [F.G. Fujiwara, J.L. Painter, H. Kim, J. Mol. Struct. 41 (1977) 169-175]. Ab initio calculations were performed for all three conformations and are compared to experimental values.     

Nuclear quadrupole coupling interactions in the rotational spectrum of tryptamine

Four conformers of tryptamine have been detected in a supersonic expansion and characterized by laser ablation molecular beam Fourier transform microwave spectroscopy LA-MB-FTMW in the 5–10 GHz frequency range. The quadrupole hyperfine structure originated by two ¹⁴N nuclei has been completely resolved for all conformers and used for their unambiguous identification. Nuclear quadrupole coupling constants of the nitrogen atom of the side chain have been used to determine the orientation of the amino group involved in N–H?π interactions: to the π electronic system of the pyrrole unit in the Gauche-Pyrrole conformers (GPy) or to the phenyl unit in the Gauche-Phenyl ones.     

Millimeter-wave investigation, simplified interpretation of the fourfold rotational spectrum, and dynamics of the internal motions of acetaldehyde-argon

The rotational spectra of the acetaldehyde-argon van der Waals complex have been measured by free jet absorption millimeter-wave spectroscopy in the frequency range 60-78 GHz. Each rotational transition is split into four hyperfine component lines, which is evidence of the occurring of two different internal motions. The splittings have been interpreted in terms of a coupled Hamiltonian that precisely determines the separation of energy levels due to the tunneling of the rare gas atom between two equivalent minima, while the information on the barrier to internal rotation of the methyl group is obtained from the pattern of the component A-E lines due to this motion. The interaction of the rare gas atom with the acetaldehyde moiety is reflected in a reduction of the V₃ barrier to internal rotation in going from the molecule to the weakly bound complex of about 20%. The barrier to the Ar tunnelling has been estimated to be 26 cm⁻¹.     

Millimeter wave measurement and assignment of the rotational spectrum of 2-aminopyridine

Previous work involving the rotational spectrum of 2-aminopyridine was limited to the lower frequencies of 4-40 GHz with very few lines being assigned. This work extends this earlier study. Here we present a much more extensive measurement and assignment of the rotational spectrum of 2-aminopyridine in the frequency range of 75-110 GHz. The observed frequencies have been assigned to the ground (0⁺ state) and the first excited state in the inversion vibration (0⁻ state). Measurements of these two states have been extended up to J=46. With the newly assigned lines, significantly improved rotational constants and all five centrifugal distortion constants have been obtained.     

Millimeter and submillimeter wave rotational spectrum of pyridine in the ground and excited vibrational states

The pure rotational spectra of the ground and five excited vibrational states of pyridine were measured, assigned and fit in the 75-110 and 260-370 GHz frequency bands. An improved set of spectroscopic constants was obtained for the ground state, and all the rotational and quartic centrifugal distortion constants were obtained for the excited vibrational states.     

Accurate rotational spectroscopy of sulfur dioxide, SO2, in its ground vibrational and first excited bending states, v2 = 0, 1, up to 2 THz

Approximately 150 pure rotational transitions each have been recorded for SO₂, v₂ = 0 and 1, in selected frequency regions up to 2 THz. The J and K_a quantum numbers reach very high values: 92 and 23, respectively, for the ground vibrational state and 81 and 21, respectively, for the first excited bending state. The highest levels accessed are almost 3000 cm⁻¹ above ground. The relative experimental uncertainties Δν/ν are about 10⁻⁸ for several medium to strong, isolated lines, and generally better than 2.5 × 10⁻⁷. Improved spectroscopic parameters have been obtained for both states, particularly for the excited bending state. In fact, the accuracies with which the energy levels of the v₂ = 1 state are known depend essentially only on the accuracy with which the vibrational spacing is known from infrared spectroscopy.     

Acrylic acid (CH2CHCOOH): the rotational spectrum in the millimetre range up to 397 GHz

In order to facilitate its detection in astronomical observations, the measurement of the rotational spectrum of acrylic acid has been extended to 397 GHz using a free space cell at room temperature. 295 new lines were assigned to the s-cis conformer and 286 new lines to the s-trans conformer of acrylic acid. Using the determined experimental parameters, the predictions of the rotational transition frequencies up to 900 GHz and their intensities were obtained at temperatures of 100 and 200 K and at room temperature. Based on these predictions, a search of the most intense transitions of acrylic acid in star-forming regions was performed using published data from the HERSCHEL Science Archive. No lines were found but the possibility of observing rotational transitions of acrylic acid in astronomical objects is discussed.