Torsional Potential Energy Function Research Articles

Morphing the Torsional Potential Energy Function from Local to Global Symmetry through a π Link: The Rotational Spectrum of α,α,α-Trifluoro-p-tolualdehyde

Morphing the Torsional Potential Energy Function from Local to Global Symmetry through a π Link: The Rotational Spectrum of α,α,α-Trifluoro-p-tolualdehyde

Torsion–rotation coupling and the determination of the torsional potential energy function of HSOH

Hindered torsional motion in a molecule such as HSOH leads to tunnelling splittings. Lying between the simpler limiting cases of strongly hindered torsion (the "we can simply neglect the tunnelling" limit) and that of free internal rotation (the "tunnelling pair of levels are just different vibrational states" limit) HSOH is particularly challenging due to strong and inherent torsion-rotation coupling. The low symmetry due to the different terminal moieties, SH and OH, further enriches the energy level pattern by allowing a systematic mixing within each quadruplet of torsion- and asymmetry-doubled levels. This mixing, which varies in a systematic manner with angular momentum quantum numbers, can reach 50% and is in addition to the mass-dependent and quasicyclic variation of the tunnelling splittings. We show that a carefully implemented reduced-dimension model, the Generalised SemiRigid Bender (GSRB), combined with a few modest ab initio calculations, is sufficient to account for the bulk of these physical effects. We also point out the reversal of energy pairing of torsional levels with increased energy. That is, at low torsional energy the lowest of the torsional level pair is the symmetric level while at high torsional energy it is the antisymmetric level which is the lowest. Finally, we present a purely empirical determination of the tunnelling splittings for K = 0, 1, 3, 4, and 5.

Collective and Noncollective Models of NMR Relaxation in Lipid Vesicles and Multilayers

NMR (13)C spin lattice relaxation (1/T(1)) rates of dipalmitoylphosphatidylcholine (DPPC) bilayers obtained from molecular dynamics simulations of 72 and 288 lipids are compared with each other, with experimental values from large liposomes obtained by magic angle spinning, and with previously published experimental data from small vesicles. The experimental results for multilayers and vesicles at the same frequencies differ only slightly. The simulation results indicate that T(1) relaxation in the 15.1 to 201.2 MHz carbon frequency range and up to 100 A length scale is dominated by fast isomerizations and slower lipid wobble (D perpendicular approximately 2.5 x 10(8) s(-1)). Rotational diffusion about the lipid long axis (described by D(parallel)) does not make a substantial contribution to the T(1). Modifications to the acyl chain torsional potential energy function used for the simulations substantially improve agreement with experiment.

An ab initio study of the structure, torsional potential energy function, and electric properties of disilane, ethane, and their deuterated isotopomers

Highly accurate ab initio computations of the molecular structure and properties, torsional potential energy function, and harmonic force field of disilane and ethane have been carried out. Equilibrium parameters as well as vibrational corrections have been evaluated. In addition, for these systems a vibrational averaging procedure has been employed for calculating the dipole moment of molecules which have no permanent dipole moment, i.e., SiH(3)SiD(3) and CH(3)CD(3). The molecular and spectroscopic properties calculated for ethane and its isotopomers provide a calibration against known experimental data, allowing us to estimate the reliability of our computed results for disilane for which there is much less experimental data. The goal of the present study is to predict the molecular parameters, with estimated uncertainties, that determine the microwave spectrum of SiH(3)SiD(3).

Torsion—rotation interactions in monodeuterated acetaldehyde

This paper presents an analysis of previously observed rotational transitions up to J = 17 of the sym and asym rotamers of monodeuterated acetaldehyde using a three-level Hamiltonian incorporating connections between rotational states of the vsym = 0, vasym = 0+ and vasym = 0 − torsional energy level manifolds. Rotational transitions observed to be significantly perturbed in earlier studies, where the spectrum was fitted using a single-level Hamiltonian for the vsym = 0 torsional state and a separate two-level Hamiltonian for the non-rigid asym states, are now fitted satisfactorily. It is confirmed that the perturbations detected in the earlier studies and tentatively attributed there to torsion-rotation interactions between the energetically distinct sym and asym manifolds do indeed arise as a consequence of such interactions. Rotational and quartic centrifugal distortion constants are given for each torsional state, together with torsion-rotation coupling constants primarily responsible for the perturbations noted in the earlier studies. Tunnelling between the energetically distinct sym and asym potential wells of the torsional potential energy function is also discussed.

Equilibrium Structure and Internal Rotation in B2F4 from Electron Diffraction and Spectroscopic Data and Quantum Chemical Calculations

Electron diffraction (ED) data for B2F4 recorded by Hedberg et al. over the temperature range −80 to +150°C have been used to obtain equilibrium geometry of this molecule in the framework of a large-amplitude motion model. The torsional coordinate has been adiabatically separated from the rest of vibrations. Two types of constraints applied to obtain ab initio torsional potential energy function (PEF) and the parameters of the geometry relaxation are discussed. The relations between anharmonic interaction force constants and the parameters of the geometry relaxation are briefly considered. Ab initio force constant matrices for rigid vibrational coordinates as well as large-amplitude torsional PEF have been scaled in the procedure of simultaneous fitting to the ED data and experimental vibrational frequencies. The resulting equilibrium geometry and potential function provided good fit to both ED and spectroscopic data. As expected, the results for the equilibrium geometry obtained from separate ED patterns recorded at different temperatures did not show noticeable temperature trend. The determined equilibrium structural parameters for B2F4 are: r e (B–B) = 1.719(4) A, r e (B–F) = 1.309(2) A, ∠BBF = 121.1(1)°. Uncertainties given in parentheses include three times standard deviation and a systematic error. The rotational barrier height was evaluated as 160(50) cm−1.

Laser-Induced Fluorescence Spectra and Torsional Potential Energy Functions of Jet-Cooled 4,4‘-Dimethyl-trans-Stilbene

The laser-induced fluorescence excitation spectra of jet-cooled 4,4‘-dimethyl-trans-stilbene have been recorded, and the phenyl and methyl internal rotation levels have been assigned for the S0 and the S1(π,π*) electronic states. Dispersed fluorescence spectra and vapor-phase Raman spectra were utilized to help assign the low-frequency vibrations in the electronic ground state. The methyl torsions have energy levels and potential functions very similar to those of m-xylene. The S0 state has V6 = 15 ± 15 cm-1 (negligible barrier), while S1 has V3 = 85 ± 10 cm-1 and V6 = −30 ± 20 cm-1. The barrier to simultaneous internal rotation of the phenyl groups in S0 was found to be 3100 cm-1. This is identical to that of trans-stilbene within experimental error. For the S1 state, the barrier is 3140 cm-1 as compared to 3000 cm-1 for trans-stilbene. The barrier to rotation about the CC bond in the S0 state was estimated to be 41 ± 5 kcal/mol based on one assigned frequency. In the S1 state the trans-twist barrier was...

Rotational isomeric state chains on a high coordination lattice: Dynamic Monte Carlo algorithm details

A high coordination lattice model for simulating coarse-grained rotational isomeric state (RIS) chains has been under development recently. Initially, the model was developed for chains with symmetric torsional potential energy functions, E(φ)=−E(φ). A single-bead move Monte Carlo algorithm was used and found to be effective in simulating polyethylene chains. A modification was subsequently developed to allow for the simulation of chains with an asymmetric torsional potential, E(φ)≠−E(φ). The single-bead move Monte Carlo (MC) algorithm employed previously was found to be ineffective following this modification. Similar kinetic effects have been seen previously with single-bead MC moves on the cubic lattice, which lead to the Hilhorst–Deutch modification (two-bead crankshaft MC move) of the Verdier–Stockmayer (single-bead move MC) algorithm. A reptation MC move applied to this model appears problematic. A multiple-bead MC move is developed using the pivot algorithm formalism in order to avoid the lattice model specific kinetic effect seen with only single-bead MC moves. This allows for the effective simulation of vinyl polymers with asymmetric torsional potentials such as polypropylene and polyvinyl chloride. Polypropylene (PP) and poly(vinyl chloride) (PVC) chains of varying stereochemical structure are simulated. The chains are found to relax with reasonable efficiency. Polypropylene and polyvinyl chloride chains are reverse mapped back to the fully atomistic description. The solubility parameters of the reverse-mapped atomistic structures are found to agree reasonably well with experimental values.

The electronic spectra of jet-cooled 5-phenyltropolone: Phenyl torsion and its effect on proton tunneling

The S1–S0 electronic spectra of jet-cooled 5-phenyltropolone were measured to elucidate the features of phenyl torsion and its effect on intramolecular proton transfer. Several bands in the fluorescence excitation spectra were assigned to phenyl torsion and other low-frequency modes with the aid of the ab initio calculation. The torsional potential energy function was investigated with the one-dimensional torsional potential function analysis for the S0 state. Presence of significant potential barriers due to the large H-atom steric repulsion between the phenyl ring and the tropolone ring were anticipated at the torsional angle φ=0° in both the S0 and S1 states. It was suggested that the large steric repulsion significantly influenced both phenyl torsion and proton transfer.

Potential Energy Surface and Vibrational−Rotational Energy Levels of Hydrogen Peroxide

The six-dimensional potential energy surface of hydrogen peroxide, H2O2, has been determined from large-scale ab initio calculations using the coupled-cluster method, CCSD(T), with the basis set of quadruple-ζ quality, cc-pVQZ. The effects of core-electron correlation on the calculated structural parameters and the torsional potential energy function have been investigated. The anharmonic quartic force field has been determined. The vibrational−rotational energy levels of the molecule have then been calculated using the variational method. The calculated molecular properties are found to be in good agreement with experimental data.

Mapping of rotational isomeric state chains with asymmetric torsional potential energy functions on a high coordination lattice: Application to polypropylene

A high coordination lattice model was recently introduced for simulating coarse-grained rotational isomeric state (RIS) chains in which the bonds have symmetric torsional potential energy functions, E(φ)=E(−φ). This symmetry was exploited in the coarse-graining and mapping onto the high coordination lattice, thereby making the procedure unsuitable (without modification) for application to chains where one or more bonds has an asymmetric torsion potential energy function, E(φ)≠E(−φ). The necessary modification is described here, and then documented by mapping previously described RIS models for isotactic and syndiotactic polypropylene onto the high coordination lattice. Each bead on the high coordination lattice represents a monomer unit, C2H4, of polypropylene. The conditional probabilities derived from the RIS model form the basis for the acceptance of the single bead moves used in the Monte Carlo simulations on the 2nnd lattice. The simulated chains have reasonable mean-square end-to-end distances and mean-square radii of gyration. The relaxation of the end-to-end vector follows the stretched exponential behavior, exp[−(t/τ)β], where β=0.5 and τ is the correlation time. The elaboration retains the ability to correctly treat chains in which the bonds have symmetric torsional potential energy functions, as shown by application to polyethylene, where each bead on the high coordination lattice represents C2H4.

Fluorescence Spectra and Torsional Potential Energy Functions for 4-Methoxy-trans-stilbene in its S0 and S1(.pi.,.pi.) Electronic States

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTFluorescence Spectra and Torsional Potential Energy Functions for 4-Methoxy-trans-stilbene in its S0 and S1(.pi.,.pi.) Electronic StatesWhe-Yi Chiang and Jaan LaaneCite this: J. Phys. Chem. 1995, 99, 31, 11823–11829Publication Date (Print):August 1, 1995Publication History Published online1 May 2002Published inissue 1 August 1995https://doi.org/10.1021/j100031a006RIGHTS & PERMISSIONSArticle Views114Altmetric-Citations15LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (670 KB) Get e-Alerts

An ab initio study on the equilibrium structure and torsional potential energy function of hydrogen peroxide

The molecular parameters of hydrogen peroxide, H 2O 2, have been determined in large-scale ab initio calculations using the coupled cluster method, CCSD(T), and basis sets of double- to quadruple-zeta quality. With the largest basis set employed, cc-pVQZ, the equilibrium parameeters are determined to be: r( OO) = 1.4525 A ̊ , r( OH) = 0.9627 A ̊ , < ( OOH) = 99.91°, <( HOOH) = 122.46°, V trans = 377 cm −1 and V cis = 2545 cm −1 .

The torsional potential energy function of N2O4

The infrared spectrum of dinitrogen tetroxide, N2O4, has been measured in the region 200650 cm−1 at different resolutions and temperatures. Analysis of the sequence of weak bands near 540 cm−1 involving a combination of the v6 (NO2 rocking) and v4 (torsion) modes has been performed. As a result, the shape of the potential energy function governing internal rotation about the NN bond has been determined. In the ground vibrational state, the height of the barrier at the staggered conformation is determined to be 1900 ± 200 cm−1.

Effective internal rotation potential energy function of acryloyl fluoride, CH2 CH  CF O

The geometry of acryloyl fluoride was optimized completely at the RHF/6-31G* level of theory at 10 points on the theoretical potential energy curve for internal rotation. The energies obtained were used to determine a six term cosine expansion of the torsional potential energy function. This function was then refined using the experimental torsional transition frequencies in the s-trans and s-cis wells in conjunction with the geometrical parameters optimized at the RHF/6-31G* level. The effective potential function obtained is compared with previous results. The necessity of accounting for relaxation of the geometry upon internal rotation is stressed. © 1992 by John Wiley & Sons, Inc.

Allowed conformations for protonated and unprotonated lidocaine: three-dimensional isopotential energy surfaces

The conformational energies of unprotonated and protonated lidocaine were calculated using nonbonded, electrostatic, and torsional potential energy functions. Calculations were performed varying three central torsion angles in the molecules from 0 to 360° at 10° intervals. The results of the energy calculations were represented as three-dimensional isopotential energy surfaces from which the regions of low-energy conformations allowed for each molecule could be selected. Each of these calculations was repeated as a fourth dihedral angle was varied in increments of 20° between 0 and 360°. In this manner, a global minimum was found for unprotonated and for protonated lidocaine. A comparison of the three-dimensional isopotential energy surfaces for unprotonated lidocaine with that of protonated lidocaine shows a striking difference for the global minimum for the N–CO–CH2–N dihedral angle. The angle is near 70° for unprotonated lidocaine and 220° for protonated lidocaine, resulting in a more extended structure for the protonated molecule. The three-dimensional isopotential surfaces also reveal that the conformational freedom allowed for lidocaine is greatly reduced when it is protonated. The difference in the electrostatic interaction between the two forms of lidocaine is responsible for the large differences in conformational preference. The threedimensional isopotential energy surfaces suggest minimum-energy pathways by which the molecules may interconvert from one to another low-energy conformational region.

Ab initio calculations of phenylene ring motions in polyphenylene oxide

AbstractAn important class of polymers is comprised, in part, of 1,4‐disubstituted phenylene groups. It is widely believed that large amplitude phenylene torsional motions play a critical role in determining such physical properties as the toughness and degree of crystallinity of these polymers. We have studied what is perhaps the simplest polymer in this class, polyphenylene oxide (PPO), using ab initio quantum chemistry methods to determine the conformational properties, torsional potential energy functions, and vibrational frequencies. From our calculations on dimer (diphenyl ether) and trimer (para‐diphenoxy benzene) fragments emerges a qualitative description of the mechanism of phenylene rotation in the polymeric material.