Experimental Rotational Spectra Research Articles

Exploring the distinct conformational preferences of allyl ethyl ether and allyl ethyl sulfide using rotational spectroscopy and computational chemistry.

The conformational energy landscapes of allyl ethyl ether (AEE) and allyl ethyl sulfide (AES) were investigated using Fourier transform microwave spectroscopy in the frequency range of 5-23GHz aided by density functional theory B3LYP-D3(BJ)/aug-cc-pVTZ calculations. The latter predicted highly competitive equilibria for both species, including 14 unique conformers of AEE and 12 for the sulfur analog AES within 14 kJ mol-1. The experimental rotational spectrum of AEE was dominated by transitions arising from its three lowest energy conformers, which differ in the arrangement of the allyl side chain, while in AES, transitions due to the two most stable forms, distinct in the orientation of the ethyl group, were observed. Splitting patterns attributed to methyl internal rotation were analyzed for AEE conformers I and II, and the corresponding V3 barriers were determined to be 12.172(55) and 12.373(32) kJ mol-1, respectively. The experimental ground state geometries of both AEE and AES were derived using the observed rotational spectra of the 13C and 34S isotopic species and are highly dependent on the electronic properties of the linking chalcogen (oxygen vs sulfur). The observed structures are consistent with a decrease in hybridization in the bridging atom from oxygen to sulfur. The molecular-level phenomena that drive the conformational preferences are rationalized through natural bond orbital and non-covalent interaction analyses. These show that interactions involving the lone pairs on the chalcogen atom with the organic side chains favor distinct geometries and energy orderings for the conformers of AEE and AES.

A fitting program for structural determination of molecular clusters from rotational spectroscopy

The characterization of the structures of molecular clusters, which serve as building blocks for bulk substances, provides crucial insight into the interactions between constituent units. Chirped-pulse Fourier transform microwave (CP-FTMW) spectroscopy, combined with state-of-the-art quantum chemical calculations, is a powerful tool for characterizing the structures of molecular clusters, as the rotational spectra are directly related to the mass distribution of a molecule or cluster. However, determining the structures of large or complex clusters from experimental rotational spectra remains challenging due to their structural flexibility. Ab initio and density functional theory calculations for searching their stable structures could be significantly time-consuming and method-dependent. To address these challenges, we have developed an approach that relies on the experimental rotational constants to search for potential molecular structures without quantum chemical optimization. Our approach involves creating an initial set of conformers through either a semi-empirical sampling program or the quasi-Monte Carlo method. After-ward, the trust region reflective algorithm is utilized for structure fitting. This procedure enables us to quickly generate potential conformers and gain access to precise structural information. We apply our fitting program to water hexamer and benzaldehyde-water clusters, and the resulting topological structures align extremely well with the experimental results.

Accuracy of quantum chemistry structures of chiral tag complexes and the assignment of absolute configuration.

The absolute configuration of a molecule can be established by analysis of molecular rotational spectra of the analyte complexed with a small chiral molecule of known configuration. This approach of converting the analyte enantiomers, with identical rotational spectra, into diastereomers that can be distinguished spectroscopically is analogous to chiral derivatization in nuclear magnetic resonance (NMR) spectroscopy. For the rotational chiral tag method, the derivatization uses noncovalent interactions to install the new chiral center and avoids complications due to possible racemization of the analyte when covalent chemistry is used. The practical success of this method rests on the ability to attribute assigned rotational spectra to specific geometries of the diastereomeric homochiral and heterochiral tag complexes formed in the pulsed jet expansion that is used to introduce samples into the microwave spectrometer. The assignment of a molecular structure to an experimental rotational spectrum uses quantum chemistry equilibrium geometries to provide theoretical estimates of the spectrum parameters that characterize the rotational spectrum. This work reports the results of a high-sensitivity rotational spectroscopy study of the complexes formed between (3)-butyn-2-ol and verbenone. The rotational spectra of four homochiral and four heterochiral complexes are assigned. In addition, the 14 distinct, singly-substituted 13C isotopomer spectra of five of these species are assigned in natural abundance. Analysis of these spectra provides direct structural characterization of the complexes through determination of the carbon atom position coordinates. This data set is used to benchmark quantum chemistry calculations of candidate equilibrium geometries of the chiral tag complexes. The quantum chemistry calculations are limited to methods commonly used in the field of rotational spectroscopy. It is shown that the accuracy of the structures from quantum chemistry provides a high-confidence assignment of cluster geometries to the observed spectra. As a result, a high-confidence determination of the analyte (verbenone) absolute configuration is achieved.

Evaluation of machine learning methods for classification of rotational absorption spectra for gases in the 220–330 GHz range

Machine learning (ML) methods are implemented to classify rotational absorption spectra for gas-phase compounds in the THz region, specifically 220–330 GHz where experimental data is available. Eight ML methods were trained in both standard and one-versus-rest (OVR) implementations using simulated absorption spectra for 12 volatile organic compounds and halogenated hydrocarbons of interest in industrial and environmental gas sensing applications. The performance of the resulting ML classifiers was compared against simulated training spectra in both a 70–30 training–testing split and in tenfold cross-validation studies, with the classifiers exhibiting accuracies in the range of 88–99% for simulated spectra. The classifiers were then tested for their ability to classify noisy experimental rotational spectra for methanol, ethanol, formic acid, acetaldehyde, acetonitrile, and chloromethane. The OVR implementations of the support vector machine (SVM) classifier with both linear and radial basis function kernels and the multi-layer perceptron (MLP) classifier achieved average classification accuracies of 87–94% for the experimental dataset. The study shows that THz spectra in the present frequency region provide a sufficient spectral fingerprint for ML classifiers to learn and predict speciation, allowing automated gas sensing. The present methods can be extrapolated to different frequency ranges and compounds and conditions.

Rotational Spectrum of the β-Cyanovinyl Radical: A Possible Astrophysical N-Heterocycle Precursor.

A fundamental question in the field of astrochemistry is whether the molecules essential to life originated in the interstellar medium (ISM), and, if so, how they were formed. Nitrogen-containing heterocycles are of particular interest because of their role in biology; however, to date, no N-heterocycle has been detected in the ISM, and it is unclear how and where such species might form. Recently, the β-cyanovinyl radical (HCCHCN) was implicated in the low-temperature gas-phase formation of pyridine. While neutral vinyl cyanide (H2CCHCN) has been rotationally characterized and detected in the ISM, HCCHCN has not. Here, we present the first theoretical study of all three cyanovinyl isomers at the CCSD(T)/ANO1 level of theory and the experimental rotational spectra of cis- and trans-HCCHCN, as well as those of their 15N isotopologues, from 5 to 75 GHz. The observed spectra are in good agreement with calculations and provide a basis for further laboratory and astronomical investigations of these radicals.

Accuracy of Rotational Parameters Predicted by High-Level Quantum-Chemical Calculations: Case Study of Sulfur-Containing Molecules of Astrochemical Interest.

The accuracy of rotational parameters obtained from high-level quantum-chemical calculations is discussed for molecules containing second-row atoms. The main focus is on computed rotational constants for which two statistical analyses have been carried out. A first benchmark study concerns sulfur-bearing species and involves 15 molecules (for a total of 74 isotopologues). By comparing 15 different computational approaches, all of them based on the coupled-cluster singles and doubles approach (CCSD) augmented by a perturbative treatment of triple excitations, CCSD(T), we have analyzed the effects on computed rotational constants due to ( i) extrapolation to the complete basis-set limit, ( ii) correlation of core electrons, and ( iii) vibrational corrections to rotational constants. To extend the analysis to other molecules containing second-row elements, as well as to understand the effect of higher excitations, a second benchmark study involving 11 molecules (for a total of 54 isotopologues) has been performed. Finally, the rotational parameters of seven sulfur-containing molecules of astrochemical interest (CCS, C3S, C4S, C5S, HCCS+, HC4S+, and HOCS+/HSCO+) have been computed and compared to experimental data, when available, also addressing the direct comparison of simulated and experimental rotational spectra.

Spectral Analysis of the Amplified Spontaneous Emission in the Second Positive System of the N2 Molecule.

The spectrum of the 0-0 band of the [Formula: see text] electronic transition of the N2 molecule presents a considerable difference in its distribution of intensities, as a function of the wave number, when the emission spectrum by glow discharge is compared to an amplified spontaneous emission (ASE) regime spectrum, commonly known as the N2 "laser". In the present paper, this particularity, due to gain of the transition, is analyzed from an experimental and theoretical point of view, and for the first time has its experimental and theoretical intensities fully compared. An experimental rotational spectrum is obtained for this transition and a model for the ASE intensities has been carried out in order to retrieve the experimental conditions. The theoretical calculations of the gain have been carried out through a model proposed by other authors, as explained in the article. For the comparison among the ASE experimental and theoretical intensities, the fast and slow relaxation approximations proposed have been used, being the first one that best reproduces the experimental spectrum. For the first time, the experimental and theoretical spectra are compared directly, allowing the precise determination of the vibrational coefficient of inversion and temperature, showing the possible problems arising from the approximation. A good agreement between experimental and theoretical results is observed showing the reasonable validity of the model for the gain.

Systematic study of rotational energy formulae for superdeformed bands in La and Ce isotopes

The experimental rotational spectra of superdeformed (SD) bands of 130La, 131Ce(1,2), 132Ce(1,2,3) and 133Ce(1,2,3) in the A ∼ 130 mass region are systematically analyzed with the four parameter formula, power index formula, nuclear softness formula, and VMI model. It is observed that out of all the formulae, the four parameter formula suits best for the study of the 130La, 131Ce(1,2), 132Ce(2,3) and 133Ce(1,2,3) SD bands. The four parameter formula works efficiently in determining the band head spin of the 130La, 131Ce(1,2) 132Ce(2,3) and 133Ce(1,2,3) SD bands. Good agreement is seen between the calculated and observed transition energies whenever the accurate spin is assigned. In 132Ce(1), the power index formula is found to work better than the other three formulae. The dynamic moment of inertia is also calculated for all the formulae and its variation with the rotational frequency is investigated.

The benzoic acid–water complex: a potential atmospheric nucleation precursor studied using microwave spectroscopy and ab initio calculations

The pure rotational, high-resolution spectrum of the benzoic acid-water complex was measured in the range of 4-14 GHz, using a cavity-based molecular beam Fourier-transform microwave spectrometer. In all, 40 a-type transitions and 2 b-type transitions were measured for benzoic acid-water, and 12 a-type transitions were measured for benzoic acid-D2O. The equilibrium geometry of benzoic acid-water was determined with ab initio calculations, at the B3LYP, M06-2X, and MP2 levels of theory, with the 6-311++G(2df,2pd) basis set. The experimental rotational spectrum is most consistent with the B3LYP-predicted geometry. Narrow splittings were observed in the b-type transitions, and possible tunnelling motions were investigated using the B3LYP/6-311++G(d,p) level of theory. Rotation of the water moiety about the lone electron pair hydrogen-bonded to benzoic acid, across a barrier of 7.0 kJ mol(-1), is the most likely cause for the splitting. Wagging of the unbound hydrogen atom of water is barrier-less, and this large amplitude motion results in the absence of c-type transitions. The interaction and spectroscopic dissociation energies calculated using B3LYP and MP2 are in good agreement, but those calculated using M06-2X indicate excess stabilization, possibly due to dispersive interactions being over-estimated. The equilibrium constant of hydration was calculated by statistical thermodynamics, using ab initio results and the experimental rotational constants. This allowed us to estimate the changes in percentage of hydrated benzoic acid with variations in the altitude, region, and season. Using monitoring data from Calgary, Alberta, and the MP2-predicted dissociation energy, a yearly average of 1% of benzoic acid is expected to be present in the form of benzoic acid-water. However, this percentage depends sensitively on the dissociation energy. For example, when using the M06-2X-predicted dissociation energy, we find it increases to 18%.

Calculated and experimental rotational spectra of 3,3,3-trifluoro- and 2,2,3,3-tetrafluoropropionyl chloride

Quantum chemical calculations have been performed on the potential energy surfaces of the two title molecules. The calculations have located the two lowest energy conformations for each molecule. For all four structures, centrifugal distortion constants and chlorine nuclear quadrupole coupling tensor components have been calculated. Analyses of the experimental rotational spectra have yielded the identification of only one conformer for each molecule. These conformers have spectroscopic constants closely matching those of the calculated lowest energy conformers. The chlorine nuclear quadrupole coupling tensor components of the title molecules have been compared with those for propionyl chloride and perfluoropropionyl chloride.

Occupancy of rotational population in molecular spectra based on nonextensive statistics

The procedure to obtain gas temperature in plasmas is to fit the experimental rotational spectrum to a theoretical one based on the Boltzmann distribution. For many systems a single distribution fails to account for the occupation of the levels. Researchers have improved the fitting by coupling two distributions and obtaining two distinct temperatures. They assigned the lowest temperature to the gas. Here, we show that these systems should be described by Tsallis nonextensive statistics and its unique associated temperature. Experimental and simulated spectra are tested and excellent agreement is obtained.

The rotational spectrum of Roesky’s ketone

The experimental rotational spectrum of 5-oxo-1,3,2,4-dithiadiazole (Roesky’s ketone) has been recorded and the experimental rotational constants have been determined. The latter have been used to evaluate the performance of a large number of quantum chemical methods combined with different basis sets, by comparing the calculated with the experimental values. The results of this comparison indicate that, in general, the wave-function-based methods perform better than those from Density Functional Theory. Four of the 42 investigated method/basis set combinations prove to be the most valuable, i.e., MP4(SDQ)/(aug-)cc-pVTZ, B3PW91/cc-pV(T+d)Z and MPW1PW91/aug-cc-pVTZ, as they produce rotational constants with a root-mean-square deviation from the experimental values of only about 5 MHz.

Comparison between formulas of rotational band for axially symmetric deformed nuclei**Supported by National Natural Science Foundation of China (10575004, 10675007)

The experimental rotational spectra of the deformed nuclei available in even-even and odd-A nuclei in the rare-earth and actinide regions are systematically analyzed with several rotational spectra formulas, including Bohr-Mottelson's I(I+1)-expansion, Harris' ω2-expansion, ab and abc formulas. It is shown that the simple 2-parameter ab formula is much better than the widely used 2-parameter Bohr-Mottelson's AB formula and Harris' αβ formula. The available data of the rotational spectra of both ground-state band in even-even nuclei and one-quasiparticle band in odd-A nuclei can be conveniently and rather accurately reproduced by ab formula and abc formula. The moment of inertia and the variation with rotational frequency of angular momentum can be satisfactorily reproduced by ab and abc formulas.

Morphing the He–OCS intermolecular potential

A potential energy surface for He–OCS that agrees with experimental rotational spectra to within 1 MHz is presented. The potential was first calculated at a grid defined in prolate spheroidal coordinates, which give stabler interpolations than Jacobi coordinates. Coupled cluster calculations at the CCSD(T) level were carried out with an aug-cc-pVTZ basis set. The potential was then morphed, a procedure that scales the energy and the intermolecular distance in a coordinate-dependent way. The parameters of the function used for morphing were determined by a least-squares fit to the experimental data. The global minimum of the recommended potential, at −50.2 cm−1, is 4.8 cm−1 deeper than the unscaled potential of Higgins and Klemperer [J. Chem. Phys. 110, 1383 (1999)]. The morphing procedure increases the well depth by more at the sulfur end than at the oxygen end.

Measurement of Rotational Temperature by Simulated Molecular Spectra

Abstract The measurement of temperature of heavy particles in a thermal plasma is based on the comparison of experimental rotational spectrum obtained by optical emission Spectroscopy and synthetic spectra calculated for different temperatures. The calculation principle of the synthetic spectra is detailed as well as the temperature accuracy obtained. This method is used to measure the temperature of an Ar-CO2 mixture plasma produced in a wall stabilized arc with molecular spectra of C2 (Swan system) and CN (violet system).

Electrorotation of erythrocytes treated with dipicrylamine: mobile charges within the membrane show their "signature" in rotational spectra.

In this study, electrorotation spectra of individual cells (that is, frequency dependence of cell rotation speed) have been proved to yield information not only about the passive electric properties of cell constituents, but also about the presence of mobile charges within the plasma membrane being part of ion carrier transport systems. Experiments on human erythrocytes pretreated with the lipophilic anion dipicrylamine (DPA) gave convincing evidence that these artificial mobile charges adsorbed to the plasma membrane contributed significantly to the rotational spectrum at relatively low conductivity of the external medium (2-5 mS m-1). Theoretical integration of the mobile charge concept into the single-shell model (viewing the cell as a homogenous sphere surrounded by a membrane) led to a set of equations which predicted electrorotational behavior of DPA-treated cells in dependence on medium conductivity. The quantitative data on the partition and the transmembrane translocation rate of the DPA anion extracted from the experimental rotational spectra agreed well with the corresponding literature values.