Rotational Constants Research Articles

A rotational/roto-translational constraint method for condensed matter.

Molecular rotations influence numerous condensed matter phenomena but are often difficult to isolate in molecular dynamics (MD) simulations. This work presents a rotational/roto-translational constraint algorithm designed for condensed matter simulations. The method is based on the velocity Verlet scheme, ensuring a direct constraint on velocity and simplifying implementation within material simulation software packages. We implemented the algorithm in a customized version of a CP2K package and validated its effectiveness through MD simulations of molecule and crystal. The results demonstrate successful selective constraint of rotational and roto-translational motions, enabling stable long-term simulations. This capability opens avenues for studying rotation-related phenomena (e.g., paddle-wheel mechanism in solid-state electrolytes) and constrained sampling.

Space and laboratory discovery of iminopentadienylidene, HNC5

We report the discovery of HNC$_5$ in TMC-1. Six lines have been found in harmonic relation, with quantum numbers $J$=12-11 up to $J$=17-16. The lines can be reproduced with the standard frequency relation for linear molecules with $B$=1361.75034pm 0.00033 MHz and $D$=32.2pm 0.7 Hz. The assignment of the carrier to iminopentadienylidene was achieved through examining the possible candidates at a high level of theoretical ab initio calculations. Motivated by the good agreement between the observed $B$ and the calculated value for HNC$_5$, we searched for it in the laboratory and observed the transitions $J$=5-4 to 7-6. The derived rotational and distortion constants are 1361.74998pm 0.00040 MHz and 26.5pm 5.5 Hz, respectively. Hence, we solidly conclude that the carrier of the lines found in TMC-1 is HNC$_5$. The calculated dipole moment for this species is 7.7\,D and the derived column density is (1.3pm 0.2)times 1010 $. We used the new QUIJOTE data to improve previous observations of HC$_4$NC and found that the abundance ratio HC$_4$NC/HNC$_5$ is 10pm 2. The abundance ratio of HC$_5$N and its two isomers HC$_4$NC and HNC$_5$ is 500pm 80 and 5100pm 800, respectively. These abundance ratios are higher by a factor of sim 10 than those of the equivalent isomers of HC$_3$N. Chemical models reproduce the observed abundances reasonably well when a chemistry similar to that of the smaller species C$_3$HN isomers is adopted. The formation of HNC$_5$ and HC$_4$NC arises from the dissociative recombination with electrons of the cations HC$_5$NH$^+$ and HC$_4$NCH$^+$.

Collective rovibronic dynamics of a diatomic gas coupled by cavity

We consider an ensemble of homonuclear diatomic molecules coupled to the two polarization directions of a Fabry-Pérot cavity via fully quantum simulations. Accompanied by analytical results, we identify a coupling mechanism mediated simultaneously by the two perpendicular polarizations, and inducing polaritonic relaxation towards molecular rotations. This mechanism is related to the concept of light-induced conical intersections (LICIs). However, unlike LICIs, these nonadiabatic pathways are of a collective nature, since they depend on the intermolecular orientation of all electronic transition dipoles in the polarization plane. Notably, this rotational mechanism directly couples the bright upper and lower polaritonic states, and it stays in direct competition with the collective relaxation towards dark states. Published by the American Physical Society 2024

Controlled interconversion of macrocyclic atropisomers via defined intermediates

Macrocyclic conformations play a crucial role in regulating their properties. Our understanding of the determinants to control macrocyclic conformation interconversion is still in its infancy. Here we present a macrocycle, octamethyl cyclo[4](1,3-(4,6)-dimethylbenzene)[4]((4,6-benzene)(1,3-dicarboxylate) (OC-4), that can exist at 298 K as two stable atropisomers with C2v and C4v symmetry denoted as C2v-OC-4 and C4v-OC-4, respectively. Heating induces the efficient stepwise conversion of C2v- to C4v-OC-4 via a Cs-symmetric intermediate (Cs-OC-4). It differs from the typical transition state-mediated processes of simple C–C single bond rotations. Hydrolysis and further esterification with a countercation dependence promote the generation of C2v- and Cs-OC-4 from C4v-OC-4. In contrast to C2v-OC-4, C4v-OC-4 can bind linear guests to form pseudo-rotaxans, or bind C60 or C70 efficiently. The present study highlights the differences in recognition behavior that can result from conformational interconversion, as well as providing insights into the basic parameters that govern coupled molecular rotations.

Reducing aerodynamic vibration of rigid rotors with retreating side active control avoidance

This paper proposes a new approach to eliminate aerodynamic lift oscillation, called the Dominant Sector Individual Blade Control (DS-IBC) method for rigid rotor helicopters. An Advancing Blade Concept (ABC) rotor model for aerodynamic analysis based on the free-wake method is applied. DS-IBC avoids applying active control on the rotor’s retreating side by employing and restricting active control inputs to a sector area of the rotor disc. Outside this sector, only primary collective and cyclic pitch control are used. Each blade takes turns entering the sector, creating a “relay” active control form to ensure continuous control inputs. The method also includes outer-trim and inner-trim iteration modules. Results show that DS-IBC can eliminate aerodynamic lift oscillation using much smaller control inputs than the sine-trim method. By focusing active control on the rotor’s advancing side, DS-IBC improves the effective lift-to-drag ratio and reduces the implementation difficulty of active rotor control for aerodynamic oscillation elimination, especially at a large lift-offset.

End jitter suppression using FONFTSMC for rigid-flexible coupling systems of PMSpM based on NDO

A permanent magnet spherical motor (PMSpM) with three degrees of freedom rotation characteristics in the rigid rotor has broad application prospects. But the adding flexible material will inevitably produce end jitter issue in the process of the rigid-flexible coupling system (RFCs) movement. To solve the above problem, a fractional order non-singular fast terminal sliding mode control method with a disturbance observer was proposed for the rigid-flexible coupling system of a permanent magnet spherical motor (RFCs-PMSpM). Firstly, a dynamic model of RFCs-PMSpM was established by using the Hamilton principle and Euler–Bernoulli beam theory. Secondly, the influences on the end jitter were discussed from the perspectives of load mass, material parameters, and driving torque of the flexible shaft. The sensitivity analysis is carried out, and the key factors affecting the jitter are obtained. Then, a fractional order non-singular fast terminal sliding mode controller based on a nonlinear disturbance observer (NDO-FONFTSMC) is proposed. The stability of the closed-loop control system was proved by the Lyapunov method. Finally, the effectiveness of the proposed jitter suppression strategy was validated and compared with existing approaches, which also provides an important reference for the future application of RFCs-PMSpM in high-precision industry.

Twins in rotational spectroscopy: Does a rotational spectrum uniquely identify a molecule?

Rotational spectroscopy is the most accurate method for determining structures of molecules in the gas phase. It is often assumed that a rotational spectrum is a unique "fingerprint" of a molecule. The availability of large molecular databases and the development of artificial intelligence methods for spectroscopy make the testing of this assumption timely. In this paper, we pose the determination of molecular structures from rotational spectra as an inverse problem. Within this framework, we adopt a funnel-based approach to search for molecular twins, which are two or more molecules, which have similar rotational spectra but distinctly different molecular structures. We demonstrate that there are twins within standard levels of computational accuracy by generating rotational constants for many molecules from several large molecular databases, indicating that the inverse problem is ill-posed. However, some twins can be distinguished by increasing the accuracy of the theoretical methods or by performing additional experiments.

Theoretical investigation of the electronic structure of the Rhodium Halides molecules RhF and RhCl with dipole moment calculation

The current study involves an ab initio exploration of the ground and low-lying excited electronic states of the rhodium halide molecules RhF and RhCl using the complete active space self-consistent field (CASSCF) with multireference configuration interaction (MRCI+Q) method including single and double excitations and with Davidson corrections. We investigated the potential energy curves, the transition and permanent electric dipole moments, the electronic energy relative to the ground state Te, the harmonic frequency ωe, the internuclear distance Re, and the rotational constant Be corresponding to each of the bounded states. Our findings demonstrate good agreement with the available experimental data. Notably, this work represents the inaugural theoretical investigation of the excited states of RhF and RhCl molecules, identifying the ground state of both to be X3Π, as observed in the sole two experimental investigations.

Collisional alignment and molecular rotation control the chemi-ionization of individual conformers of hydroquinone with metastable neon.

The relationship between the shape of a molecule and its chemical reactivity is a central tenet in chemistry. However, the influence of the molecular geometry on reactivity can be subtle and result from several opposing effects. Here, using a crossed-molecular-beam experiment in which individual rotational quantum states of specific conformers of a molecule are separated, we study the chemi-ionization reaction of hydroquinone with metastable neon atoms. We show that collision-induced alignment of the reaction partners caused by geometry-dependent long-range forces influences reaction pathways, which is, however, countered by molecular rotation. The present work provides insights into the conformation-specific stereodynamics of complex polyatomic systems and illustrates the capability of advanced molecule-control techniques to unravel these effects.

A Tale of Two Tails: Rotational Spectroscopy of N-Ethyl Maleimide and N-Ethyl Succinimide.

Broadband microwave spectra of N-ethyl maleimide (NEM) and N-ethyl succinimide (NES) have been recorded using chirped pulse Fourier transform microwave spectroscopy in the Ka-band (26.5-40 GHz). The spectra for both molecules were fit to a Watson A-reduced Hamiltonian in the Ir representation to obtain best fit experimental rotational constants (NEM: A0 = 2143.1988(29), B0 = 1868.7333(22), C0 = 1082.98458(36); NES: A0 = 2061.47756(14), B0 = 1791.73517(12), C0 = 1050.31263(11)), centrifugal distortion constants, and nuclear quadrupole coupling constants. While the heavy atoms of the five-membered ring of both molecules are planar, the ethyl chain has its terminal methyl group perpendicular to the ring. Along the relaxed potential energy curve for the ethyl dihedral angle (θ1 = C(6)-C(5)-N-C(4)), the ethyl group experiences significant steric strain when it is in the plane of the ring, associated with the interaction of the ethyl group with the two carbonyl oxygens. This leads to calculated barriers of θ1 = 1469 cm-1 and θ1 = 1680 cm-1 in N-ethyl maleimide and N-ethyl succinimide, respectively.

Evidence for NMR Relaxation Enhancement in a Protic Ionic Liquid by the Movement of Protons Independent of the Translational Diffusion of Cations.

The molecular dynamics, thermal stability, and ionic conductivity were studied in the protic ionic liquid 1-methylimidazolium bis(trifluoromethylsulfonyl)imide ([MIm][TFSI]). The relaxation of the 1H spin-lattice of cations in the measured frequency range (10 kHz to 20 MHz) and temperature (298 to 343 K) is sensitive mainly to slow processes occurring in the molecular dynamics of protic ionic liquid and dominated by the contribution of intermolecular translational diffusion. Molecular rotations give only a constant contribution and become significant in the higher frequency range. An interesting feature is the observed enhancement of the 1H spin-lattice relaxation below 0.03 MHz attributed to the exchange of protons (order of 10-5 s) between imidazolium cations. The measurements of the self-diffusion coefficient of hydrogen atoms of cation from 298 to 343 K additionally confirm the observed phenomenon. The coefficient for exchangeable protons -NH is higher than for the cation. The nuclear magnetic resonance (NMR) experiments provide unambiguous evidence for proton transport decoupled from molecular diffusion of ions and support the conclusion that the charge transport mechanism in the studied PIL includes contributions from both the vehicular and Grotthus mechanisms. The protic ionic liquid is thermally stable to about 573 K as shown by thermogravimetric analysis and its electrical conductivity is 5 × 10-2 S/cm at 423 K.

Switching of Circularly Polarized Luminescence via Dynamic Axial Chirality Control of Chiral Bis(Boron Difluoride) Complexes with Salen Ligands

AbstractThe intensity and handedness of circularly polarized luminescence (CPL) were successfully controlled by dynamic molecular motion in solutions. Bis(boron difluoride) complexes with chiral salen ligands were synthesized and their photophysical properties were investigated. Although these complexes showed rapid molecular rotation about the C−N bond axis in solution at room temperature, two conformers assigned as atropisomers were observed in the NMR spectra at low temperature. Furthermore, the equilibrium of these atropisomers was found to change depending on the external environment, such as the solvent and temperature, allowing precise control of the intensity and handedness of CPL without luminescence color shifts. Theoretical calculations based on density functional theory (DFT) revealed that intramolecular chiral exciton coupling is the key to changes in CPL properties.

Accurate structures and rotational constants of bicyclic monoterpenes at DFT cost by means of the bond-corrected Pisa composite scheme (BPCS).

The structural, conformational, and spectroscopic properties in the gas phase of 20 bicyclic monoterpenes and monoterpenoids have been analyzed by a new accurate, reduced-cost computational strategy. In detail, the revDSD-PBEP86 double-hybrid functional in conjunction with the D3BJ empirical dispersion corrections and a suitable triple-zeta basis set provides accurate geometrical parameters, whence equilibrium rotational constants, which are further improved by proper account of core-valence correlation. Average deviations within 0.1% between computed and experimental rotational constants are reached when taking into account the vibrational corrections obtained by the B3LYP functional in conjunction with a double-zeta basis set in the framework of second-order vibrational perturbation theory. In addition to their intrinsic interest, the studied terpenes further extend the panel of systems for which the proposed strategy has provided accurate results at density functional theory cost. Therefore, a very accurate yet robust and user-friendly tool is now available for systematic investigations of the role of stereo-electronic effects on the properties of large systems of current technological and/or biological interest by experimentally oriented researchers.

High resolution far-infrared synchrotron spectroscopy of 2-furfural conformers: Fundamental and hot bands.

In the continuity of a previous jet-cooled rovibrational study of trans and cis conformers of 2-furfural in the mid-infrared region (700-1750cm-1) [Chawananon et al., Molecules 28 (10), 4165 (2023)], the present work investigates the far-infrared spectroscopy of 2-furfural using a long path absorption cell coupled to a high-resolution Fourier transform spectrometer and synchrotron radiation at the AILES beamline of the SOLEIL synchrotron. Guided by anharmonic calculations, vibrational energy levels and excited-state rotational constants are sufficiently predictive for a complete assignment of all fundamental and combination bands up to 700cm-1, as well as the rovibrational analysis of 4 (1) low-frequency modes of trans-(cis-)2-furfural. A global rovibrational simulation, including far-infrared rovibrational lines and microwave and millimeter-wave rotational lines assigned in a previous study [Motiyenko et al., J. Mol. Spectrosc., 244, 9 (2007)] provides a reliable set of ground- and excited-state rotational parameters involving ring torsion, bending, and ring puckering modes of 2-furfural. In a second step, a rovibrational analysis of several hot band sequences, mainly involving the lowest frequency ring CHO torsion mode, is carried out. Reliable values of some anharmonic coefficients are obtained experimentally and could serve as a benchmark for validating advanced anharmonic calculations related to these large amplitude motions of flexible molecules.

Millimeter to THz Spectroscopy of HC18O+ and HC17O+: Accurate Rest Frequencies for Astrophysical Studies

Heavy oxygen isotopic species of HCO+ are important optically thin astrophysical tracers. The ground-state rotational spectrum of HC18O+, DC18O+, HC17O+, and DC17O+ has been recorded in the laboratory in the frequency range from 85 GHz to 1.3 THz. The ions have been produced in the negative column of a glow-discharge plasma, and their spectrum has been recorded in absorption using a frequency-modulation submillimeter-wave spectrometer. Various sources of systematic error have been carefully accounted for in order to obtain highly accurate line-position measurements. Theoretical estimates of the molecular parameters and of the collision effects on the line shape have been obtained by high-level ab initio calculations. The analysis yielded much improved rotational and centrifugal distortion constants, thus bringing the spectroscopic characterization of these rare isotopic variants to the same level of the parent species. Also, the first experimental rotational data for DC17O+ have been provided. These results allow for the calculation of an updated set of rest frequencies to support current and future astrophysical studies. The derived data set for the widely used HC18O+ tracer reaches an accuracy of a few parts in 109 up to 1.5 THz. Such accuracy is important for the analysis of astrophysical objects targeted by Atacama Large Millimeter/submillimeter Array observations at the submillimeter regime.

Bulk viscosity of the rigid rotor one-component plasma.

Bulk viscosity of a plasma consisting of strongly coupled diatomic ions is computed using molecular dynamics simulations. The simulations are based on the rigid rotor one-component plasma, which is introduced as a model system that adds two degrees of molecular rotation to the traditional one-component plasma. It is characterized by two parameters: the Coulomb coupling parameter, Γ, and the bond length parameter, Ω. Results show that the long-range nature of the Coulomb potential can lead to long rotational relaxation times, which in turn yield large values for bulk viscosity. The bulk-to-shear viscosity ratio is found to span from small to large values depending on the values of Γ and Ω. Although bulk viscosity is often neglected in plasma modeling, these results motivate that it can be large in molecular plasmas with rotational degrees of freedom.

Anharmonic Vibrational Spectroscopy of Germanium-Containing Clusters, GexC4-x and GexSi4-x (x = 0-4), for Interstellar Detection.

An extensive, high-level theoretical study on tetra-atomic germanium carbide/silicide clusters is presented. Accurate harmonic and anharmonic vibrational frequencies and rotational constants are calculated at the CCSD(T)-F12a(b)/cc-pVT(Q)Z-F12 levels of theory. With growing capabilities to discern more of the chemical composition of the interstellar medium (ISM), an accurate database of reference material is required. The presence of carbon is ubiquitous in the ISM, and silicon is known to be present in interstellar dust grains; however, germanium-containing molecules remain elusive. To begin understanding the presence and role of germanium in the ISM, we present this study of the vibrational and rotational spectroscopic properties of various germanium-containing molecules to aid in their potential identification in the ISM with modern observational tools such as the James Webb Space Telescope. Structures studied herein include rhomboidal (r-), diamond (d-), and trapezoidal (t-) tetra-atomic molecules of the form GexC4-x and GexSi4-x, where x = 0-4. The most promising structure for detection is r-Ge2C2 via the ν4 mode with a frequency of 802.7 cm-1 (12.5 μm) and an intensity of 307.2 km mol-1. Other molecules that are potentially detectable, i.e., through vibrational modes or rotational transitions, include r-Ge3C, r-GeSi3, d-GeC3, r-GeC3, and t-Ge2C2.

Equilibrium Values for the Si-H Bond Length and Equilibrium Structures of Silyl Iodide and Halosilylenes.

The equilibrium structures of silyl iodide, SiH3I, and silylene halides, SiHX (X = F, Cl, Br, I), were determined by using the mixed regression method, where approximate values of the rotational constants are supplemented by the structural parameters of a different origin. For this goal, it is shown that the r(Si-H) bond length can be determined by using the isolated SiH stretching frequency and that an accurate estimation of the bond angles is obtained by an MP2 calculation with a basis set of triple zeta quality. To check the accuracy of the experimental structures, they were also optimized by means of all electron CCSD(T) calculations using basis sets of quadruple zeta quality.

The influence of organic molecular rotation on carrier Dynamics: A case of MAPbI3

At room temperature, organic molecules undergo continuous rotation, driven by rotational barriers lower than kBT. However, consensus regarding the impact of organic molecule rotation on carrier dynamics remains elusive, as conventional research methods struggle to distinguish between organic molecule rotation and inorganic frame vibrations. This study successfully achieved the goal of modulating the rotation rate of organic molecules by adjusting the relative atomic mass of nitrogen atoms. In the investigation of carrier dynamics, the rotational rate of organic molecules served as the sole independent variable. The findings indicate that a slower rotation rate correlates with an extended carrier lifetime. This correlation is attributed to the heightened charge separation and reduced motion of nuclei. This research provides valuable insights for prolonging the carrier lifetime of organic–inorganic hybrid perovskites.

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.