High-resolution Rotational Spectra Research Articles

Microwave spectroscopy and large amplitude motion of chlorosulfonic acid (ClSO2OH)

The high-resolution rotational spectrum of chlorosulfonic acid (ClSO2OH) has been studied using both broadband and cavity-based Fourier transform microwave spectrometers over the frequency range of 5–18 GHz. a-, b-, and c-type transitions have been recorded for both the 35Cl and 37Cl isotopologues. The observation of c-type lines establishes that the molecule lacks a plane of symmetry and suggests that the OH group can undergo large amplitude motion between equivalent structures. Interconversion between these structures can be achieved via internal rotation through two inequivalent barriers occurring at Cl-S-O-H torsional angles of 0 or 180degrees. As in previous work on triflic and methanesulfonic acids, two states are observed and are treated as tunneling states which are presumed to arise primarily due to motion through the lower of the two barriers. The a- and c-type transitions occur within each of these states while the b-type transitions cross between them. Rotational, centrifugal distortion, and chlorine nuclear quadrupole coupling constants, as well as the energy difference between the two tunneling states and associated coupling constants, have been determined. The experimental tunneling energies, ΔE, for the 35Cl and 37Cl isotopologues are 52.6926(16) MHz and 52.6397(46) MHz, respectively. Quantum chemical calculations were carried out using MP2 and B3LYP density functional theory (DFT) methods with an aug-cc-pVTZ basis set. The rotational constants from the optimized structures were in good agreement with the experimental values. The lowest energy barrier for OH motion was calculated to be 2.63 kcal/mol at the B3LYP/aug-cc-pVTZ level. The effects of the large amplitude motion are similar to those recently reported for triflic acid (CF3SO2OH) and methanesulfonic acid (CH3SO2OH). However, while the tunneling splittings in chlorosulfonic and triflic acids are virtually identical, they differ significantly from that of methanesulfonic acid.

Accurate Geometries of Large Molecules by Integration of the Pisa Composite Scheme and the Templating Synthon Approach.

An effective yet reliable computational workflow is proposed, which permits the computation of accurate geometrical structures for large flexible molecules at an affordable cost thanks to the integration of machine learning tools and DFT models together with reduced scaling computations of vibrational averaging effects. After validation of the different components of the overall strategy, a panel of molecules of biological interest have been analyzed. The results confirm that very accurate geometrical parameters can be obtained at reasonable cost for molecules including up to about 50 atoms, which are the largest ones for which comparison with high-resolution rotational spectra is possible. Since the whole computational workflow can be followed employing standard electronic structure codes, accurate results for large-sized molecules can be obtained at DFT cost also by nonspecialists.

Laboratory Measurements and Astronomical Search for Methoxyacetone and Methyl Methoxyacetate.

High-resolution pure rotational spectra of methoxyacetone and methyl methoxyacetate have been recorded and analyzed by using pulsed jet-expansion Fourier transform microwave spectroscopy with the aid of quantum calculations. The global minima for both target molecules have been detected in pulsed jet, whose spectra are featured with the splittings raised from the methyl internal rotations. On the basis of the spectroscopic results, a radio-astronomical search of methoxyacetone and methyl methoxyacetate was carried out toward the high-mass star-forming region Sgr B2(N) using the Shanghai Tianma 65 m radio telescope. No lines belonging to either of the target molecules were detected, and the upper limits to the column density were derived.

Structure of benzene from mass-correlated rotational Raman spectroscopy.

We present high resolution rotational Raman spectra and derived geometry parameters for benzene. Rotational Raman spectra with sub-5 MHz resolution were obtained via high-resolution mass-correlated rotational alignment spectroscopy. Isotopologue spectra for C6H6, 13C–C5H6, C6D6, and 13C–C5D6 were distinguished through their correlated mass information. Spectra for 13C6H6 were obtained with lower resolution. Equilibrium and effective bond lengths were estimated from measured inertial moments, based on explicit assumptions and approximations. We discuss the origin of significant bias in previously published geometry parameters and the possibility to derive H,D isotope-specific bond lengths from purely experimental data.

Structures of guaiacol and the guaiacol-argon van der waals complex from rotational spectroscopy of guaiacol isotopologues

High-resolution rotational spectra were recorded for guaiacol and eight isotopologues as well as the guaiacol-argon van der Waals complex using a resonant-cavity Fourier transform microwave spectrometer. The monomer spectra were assigned to an anti-syn conformation with an internal hydrogen bond from the hydroxyl group to the methoxy oxygen. Atomic coordinates of the carbon and hydroxyl hydrogen atoms were determined from the experimental moments of inertia using the Kraitchman method, and these coordinates are in excellent agreement with the corresponding coordinates in the MP2/6-311++G(d,p) model structure. The recorded rotational transitions showed no evidence for methyl internal rotation tunneling, and a potential energy scan (ωB97XD/6-311++G(d,p)) identified a 1006.0 cm−1 barrier to methyl internal rotation. The rotational spectrum of guaiacol-argon was used determine the location of the argon with respect to the anti-syn monomer conformation: RCM = 3.533 Å, θ = 105.4 °, and χ = -100.0 °. The argon is positioned towards the hydroxyl and methoxy groups because of relatively strong van der Waals interactions between argon and the oxygen atoms.

Microwave measurements of rotational transitions and nitrogen quadrupole coupling for 2-aminopyridine

The high-resolution microwave rotational spectrum was measured for 2-aminopyridine in the 4.5–13.2 GHz range using a pulsed beam Fourier transform microwave spectrometer. The measured transitions for the parent isotopologue in the 0+ vibrational state of the amino group inversion vibration were used to determine accurate rotational constants and quadrupole coupling constants for the two 14N atoms in the molecule. The molecular parameters determined for the 2-aminopyridine in 0+ vibrational state have the following values: A = 5780.374(1) MHz, B = 2733.5017(3) MHz, C = 1857.6768(3) MHz, 1.5χaa (14N1) = 3.5789(45) MHz, 0.25(χbb – χcc) (14N1) = 1.5033(24)MHz, 1.5χaa (14N2) = -0.0958(70) MHz, and 0.25(χbb – χcc) (14N2) = −1.1615(21) MHz. The measured quadrupole coupling constants are in excellent agreement with those calculated using different computational methods and basis sets. It is noted that the calculation using the MP2 functional with cc-pvDz basis set closely reproduces the experimental values for quadrupole coupling constants for the two 14N atoms.

Microwave spectrum and structure of 2-(trifluoromethyl)pyridine

The high resolution rotational spectrum of 2-(trifluoromethyl)pyridine in 2 20 GHz was recorded and analyzed. Spectroscopic parameters including rotational constants, nuclear quadrupole coupling constants of 14N as well as the centrifugal distortion constants were determined. The rotational spectra of five mono-substituted 13C and one 15N isotopologues were also measured and assigned in natural abundance. Experimental results complemented by ab initio calculations lead to an accurate determination of the skeleton structure. The values of the planar moment inertia Pcc were determined to be 44.46 uÅ2 for all the measured isotopologues, indicating a Cs symmetry of this molecule. The molecular electrostatic surface potential was calculated to illustrate the trifluoromethyl substitution effects on the electron distribution.

The rotational spectrum of 1,2,3,4-tetrahydroquinoline: The extended study

In the present study, the millimeter and submillimeter high-resolution rotational spectra of the most stable conformer of 1,2,3,4-tetrahydroquinoline were recorded up to 500 GHz. Almost 3800 new ground state a-type R-branch, b-type R-branch and c-type R-branch rotational transitions were assigned and analyzed in detail. Based on the analysis of the transition frequencies, the rotational and quartic centrifugal distortion constants were obtained with significantly higher accuracy and the sextic centrifugal distortion constants were determined for the first time.

Automated assignment of rotational spectra using artificial neural networks.

A typical broadband rotational spectrum may contain several thousand observable transitions, spanning many species. While these spectra often encode troves of chemical information, identifying and assigning the individual spectra can be challenging. Traditional approaches typically involve visually identifying a pattern. A more modern approach is to apply an automated fitting routine. In this approach, combinations of 3 transitions are searched by trial and error, to fit the A, B, and C rotational constants in a Watson-type Hamiltonian. In this work, we develop an alternative approach-to utilize machine learning to train a computer to recognize the patterns inherent in rotational spectra. Broadband high-resolution rotational spectra are perhaps uniquely suited for pattern recognition, assignment, and species identification using machine learning. Repeating patterns of transition frequencies and intensities are now routinely recorded in broadband chirped-pulse Fourier transform microwave experiments in which both the number of resolution elements and the dynamic range surpass 104. At the same time, these high-resolution spectra are extremely sensitive to molecular geometry with each polar species having a unique rotational spectrum. Here we train the feed forward neural network on thousands of rotational spectra that we calculate, using the rules of quantum mechanics, from randomly generated sets of rotational constants and other Hamiltonian parameters. Reasonable physical constraints are applied to these parameter sets, yet they need not belong to existing species. A trained neural network presented with a spectrum identifies its type (e.g., linear molecule, symmetric top, or asymmetric top) and infers the corresponding Hamiltonian parameters (rotational constants, distortion, and hyperfine constants). The classification and prediction times, about 160 µs and 50 µs, respectively, seem independent of the spectral complexity or the number of molecular parameters. We describe how the network works, provide benchmarking results, and discuss future directions.

Microwave spectra of 2-phenylethyl methyl ether and 2-phenylethyl methyl ether-argon: Conformation-dependent tunneling and complexation

High-resolution rotational spectra were recorded for 2-phenylethyl methyl ether and the 2-phenylethyl methyl ether – argon complex using a cavity-based Fourier transform microwave spectrometer. The spectra were assigned to the aao and g−ao monomer conformations and the g−ao – argon complex. Small tunneling splittings arising from more than one internal motion were resolved for eight transitions of the aao conformer; no tunneling splittings were observed in the spectra of the g−ao conformer or the argon complex. The calculated barriers (ωB97XD/6-311++G(d,p)) for methyl and phenyl internal rotations were found to be very similar for both conformers. The observation of tunneling splittings is limited to the aao conformer by relatively large barriers for the internal motions and sensitivity to the rotor axis angles within the two conformers.

Ab initio conformational analysis of 1,2,3,4-tetrahydroquinoline and the high-resolution rotational spectrum of its lowest energy conformer.

The saturated part of the 1,2,3,4-tetrahydroquinoline (THQ) molecule allows for the possibility of multiple conformers' existence. High-resolution microwave spectroscopy, supported by high-level quantum chemistry calculations, was used to determine the precise molecular structures of the conformers of THQ. Via the MP2 calculations, we were able to discriminate four stable conformations, i.e. two pairs of energetically equivalent enantiomorphic conformers. The results of the calculations also indicate that energetically non-equivalent conformers are separated by a low energy barrier (104 cm-1) that allows for conformational cooling to occur. The high resolution rotational spectrum with resolved hyperfine structure in the frequency range of 7-20 GHz was obtained using both the In-phase/quadrature-phase-Modulation Passage-Acquired-Coherence Technique (IMPACT) and the coaxially oriented beam resonator arrangement (COBRA) to perform Fourier transform microwave (FTMW) spectroscopy. The precise values of the rotational constants, 14N nuclear hyperfine coupling parameters and centrifugal distortion parameters were determined from the measured transition frequencies. Based on our experimental results, only the most stable enantiomeric pair of THQ contributes to the rotational spectrum under the conditions of our experiment as the less stable conformers seem to efficiently relax to the lower energy conformers. Thus the experimentally evaluated molecular constants unambiguously define the lowest energy conformer of 1,2,3,4-tetrahydroquinoline.

High-Resolution Rotational Spectrum, Dunham Coefficients, and Potential Energy Function of NaCl.

We report laboratory spectroscopy for the first time of the J = 1-0 and J = 2-1 lines of Na35Cl and Na37Cl in several vibrational states. The hyperfine structure has been resolved in both transitions for all vibrational levels, which permit us to predict with high accuracy the hyperfine splitting of the rotational transitions of the two isotopologues at higher frequencies. The new data have been merged with all previous works at microwave, millimeter, and infrared wavelengths and fitted to a series of mass-independent Dunham parameters and to a potential energy function. The obtained parameters have been used to compute a new dipole moment function, from which the dipole moment for infrared transitions up to Δv = 8 has been derived. Frequency and intensity predictions are provided for all rovibrational transitions up to J = 150 and v = 8, from which the ALMA data of evolved stars can be modeled and interpreted.

Probing the Electronic Environment of Methylindoles using Internal Rotation and (14)N Nuclear Quadrupole Coupling.

High-resolution rotational spectra were recorded in the 10.5-21.0 GHz frequency range for seven singly methylated indoles. (14)N nuclear quadrupole hyperfine structure and spectral splittings arising from tunneling along the internal rotation of the methyl group were resolved for all indole species. The nuclear quadrupole coupling constants were used to characterize the electronic environment of the nitrogen atom, and the program XIAM was used to fit the barrier to internal rotation to the measured transition frequencies. The best fit barriers were found to be 277.1(2), 374.32(4), 414.(5), 331.6(2), 126.8675(15), 121.413(4), and 426(3) cm(-1) for 1-methylindole through 7-methylindole, respectively. The fitted barriers were found to be in good agreement with barriers calculated at the ωB97XD/6-311++G(d,p) level. The complete set of experimental barriers is compared to theoretical investigations of the origins of methyl torsional barriers and confirms that the magnitude of these barriers is an overall effect of individual hyperconjugative and structural interactions of many bonding/antibonding orbitals.

COMPREHENSIVE ANALYSIS OF PREBIOTIC PROPENAL UP TO 660 GHz

Since interstellar detection of propenal is only based on two rotational transitions in the centimeter wave region, its high resolution rotational spectrum has been measured up to 660 GHz and fully characterized by assignment of more than 12,000 transitions to provide direct laboratory data to the astronomical community. Spectral assignments and analysis include transitions from the ground state of the trans and cis isomers, three trans-13C isotopologues, and ten excited vibrational states of the trans form. Combining new millimeter and submillimeter data with those from the far-infrared region has yielded the most precise set of spectroscopic constants of trans-propenal obtained to date. Newly determined rotational constants, centrifugal distortion constants, vibrational energies, and Coriolis and Fermi interaction constants are given with high accuracy and were used to predict transition frequencies and intensities over a wide frequency range. Results of this work should facilitate astronomers further observation of propenal in the interstellar medium.

HIGH-RESOLUTION FOURIER-TRANSFORM MICROWAVE SPECTROSCOPY OF METHYL- AND DIMETHYLNAPTHALENES

High-resolution pure rotational spectra of four alkylnaphthalenes were measured in the range of 6–15 GHz using a molecular-beam Fourier-transform microwave spectrometer. Both a- and b-type transitions were observed for 1-methylnaphthalene (1-MN), 1,2-dimethylnaphthalene (1,2-DMN), and 1,3-dimethylnaphthalene (1,3-DMN); only a-type transitions were observed for 2-methylnaphthalene (2-MN). Geometry optimization and vibrational analysis calculations at the B3LYP/6-311++G(d,p) level of theory aided in the assignments of the spectra and the characterization of the structures. Differences between the experimental and predicted rotational constants are small, and they can be attributed in part to low-lying out-of-plane vibrations, which distort the alkylnaphthalenes out of their equilibrium geometries. Splittings of rotational lines due to methyl internal rotation were observed in the spectra of 2-MN, 1,2-DMN, and 1,3-DMN, and allowed for the determination of the barriers to methyl internal rotation, which are compared to values from density functional theory calculations. All four species are moderately polar, so they are candidate species for detection by radio astronomy, by targeting the transition frequencies reported here.

Correlation Between Hydroxide ion Dynamics and Transport Properties of Hydroxide/Chloride Melts According to Raman Spectroscopy Data

UDC 541.454;543.42;535.375.5 Using data obtained in analysis of the band contour for the totally symmetric stretching vibration of the OH - anion in Raman spectra of hydroxide/chloride ionic melts, a correlation has been established between the orientational relaxation time and effective moment of inertia of the hydroxide ion and the transport properties of the electrolyte. Study of the structure of ionic compounds and its controlling infl uence on physical and chemical properties is one of the fundamental scientifi c problems in physical chemistry. Today the appearance of new and improvement of conventional experimental methods for studying the structure of matter and also creation of high-tech device components has permitted considerable expansion of the range of systems that can be studied, including high-temperature ionic melts. Development of methodological approaches has led to a signifi cant increase in the amount of information that can be extracted and an increase in the reliability of this information. Raman spectroscopy is a physical method for investigation of matter which, based on recorded vibrational spectra of the analyte system, allows us to carry out structural analysis, to determine the vibrational frequencies, and to calculate force constants and the moment of inertia of a molecule and the rates of relaxation processes. In contrast to x-ray diffraction and neutron diffraction methods, determining the structure of materials (the symmetry type, the interatomic distances, the coordination numbers), the Raman spectroscopy method allows us to obtain a much greater amount of information: the recorded vibrational frequencies correspond to the position of the vibrational levels in the molecule, the width of the vibrational bands contain information about its rotational motion, and the intensity of the Raman spectra directly determines the bond ionicity or covalency (1, 2). Today a methodological approach has been developed that allows us, from analysis of the contour of the vibrational bands and the width of their isotropic and anisotropic components, to obtain important information about the dynamics of the particles in the picosecond time range (1-4). It has been shown (4) that information about relaxation processes can be obtained by calculating the temporal correlation functions, the vibrational and orientational relaxation times for the structural units. Unique information on estimation of the moments of inertia of particles, which is practically inaccessible by other methods for studying the condensed state of matter and which can be obtained only using high-resolution rotational spectra of gas molecules, can be extracted by analyzing the shape of the contour of vibrational bands using the spectral moments method (4). According to this method, the moment of inertia can be calculated from the equation (3): I = 6kT/4π 2 c 2 Mor(2),

Calculations and measurements of the deuterium tunneling frequency in the propiolic acid-formic acid dimer and description of a newly constructed Fourier transform microwave spectrometer

The concerted proton tunneling frequency for the propiolic acid-formic acid dimer was calculated using a relaxed ab initio double-well potential in the imaginary-frequency mode of the saddle point, and new measurements were made for the deuterated propiolic acid-formic acid (ProOD-FAOD) isotopologue. It is important to have consistent calculated tunneling frequency values between normal and deuterated isotopologues since parameters can be readily adjusted to get good agreement with one isotopologue. High-resolution rotational spectra of deuterated (ProOD-FAOD) dimer were measured using a newly constructed Fourier Transform microwave spectrometer. The new spectrometer has mirror size: 30 cm in diameter with a radius of curvature of 59 cm and is equipped with multiple-FID data collection (5-10 FID's for each gas pulse). For the deuterated (ProOD-FAOD) isotopologue, 45 rotational lines (a type: 34; b type: 11) were measured in the lowest tunneling states range between 6.5 GHz and 15.5 GHz. With the new high-resolution measurements of the tunneling doublets (b-dipole transitions), the double potential well responsible for the deuterium tunneling was depicted much more precisely. The two tunneling states are separated by 3.48 MHz. The rotational constants obtained in this work are quite helpful for further structure analysis as well.

On the Trimerization of Cyanoacetylene: Mechanism of Formation of Tricyanobenzene Isomers and Laboratory Detection of Their Radio Spectra

In support of a deeper understanding of the chemistry of cyanoacetylene--a known constituent of planetary atmospheres and interstellar space--theoretical and experimental studies address the chemical mechanism of dimerization and trimerization, and provide high-resolution rotational spectra of two of the trimeric products, 1,2,3- and 1,2,4-tricyanobenzene. Analysis of the rotational spectra is particularly challenging because of quadrupolar coupling from three (14)N nuclei. The laboratory rotational spectra provide the basis for future searches for these polar aromatic compounds in interstellar space by radio astronomy.

ChemInform Abstract: Internal Rotation and Structure of Isobutylene.

Abstract The high-resolution rotational spectrum of isobutylene was measured for transitions with J ≤ 7, using a modified Balle-Flygare Fourier transform microwave spectrometer with a pulsed supersonic nozzle as the sample source. Previously improved by the addition of a computer-based IBM PC-AT controller and averager [C. Chuang et al., Rev. Sci. Instrum. 61, 1629–1635 (1990)], the spectrometer has been fitted with software which provides Autosearch operation. Analysis of the fine structure of the observed transitions, caused by internal rotation of the two methyl groups, gives the V3 barrier to be 2176(3) cal/mole and the angle θ between the methyl top and b-axes to be 58.5(3)°, with the latter perhaps decreasing slightly at higher J. Rotational constants reported previously for six isotopic species of isobutylene and those now obtained for (CH3)2 13CCH2 at natural abundance were fitted to obtain an effective (r0) structure. The results indicate that the methyl groups are distorted slightly from C3ν symmetry. This tilts the CH3 axis 0.8(2)° away from coaxiality with the CC bond axis toward the double bond.

The rotational spectra of the 6 1, 7 1, 8 1, 9 1 and 5 1/9 2 vibrational states of H 15NO 3

The high-resolution rotational spectrum of H 15NO 3 has been recorded in the range between 74 and 850 GHz and used to complete an extensive analysis of the six vibrational states below 1000 cm −1 that include the isolated 8 1 and 9 1 states along with the weakly interacting 6 1 and 7 1 states and strongly interacting 5 1 and 9 2 dyad. The 6 1 and 7 1 states couple via a weak Coriolis interaction while the 5 1 and 9 1 states couple through strong Fermi and weaker Coriolis interactions. The Hamiltonian models account for the observed torsional splitting in the 9 1 and 9 2 states of 2.4 and 70 MHz, respectively, and the induced torsional spitting of 15 MHz in the 5 1 state due to the strong Fermi mixing with the 9 2 state. The transitions from each state are fit to within the experimental accuracy and the resulting spectroscopic constants agree well with the main isotopologue.