Internal Rotational Motion Research Articles

Pulsed field ionization zero kinetic energy photoelectron study on methylanisole molecules in a supersonic jet

Pulsed field ionization zero kinetic energy (PFI-ZEKE) photoelectron spectra were measured for jet-cooled o-, m-, and p-methylanisoles for the first time. The low-frequency bands observed at around the origin band in the spectra were assigned to the methyl internal rotational bands of the cations. The potential curves and the Franck–Condon factors for the methyl internal rotational motion were calculated by the one-dimensional free-rotor approximation. The potential barrier height was found to change drastically on ionization, suggesting that the electronic structure should mainly affect the potential barrier of the methyl internal rotation.

On the dissociation dynamics of the benzyl chloride radical anion. An ab initio dynamic reaction coordinate analysis study

The dynamics of the dissociation of the benzyl chloride radical anion to a benzyl radical and chloride anion was studied theoretically. To this end two different conformations of benzyl chloride were examined: one featuring a 90° angle between the Cl–C bond and the aromatic ring (conformation S), the other with the Cl–C bond and the aromatic moiety lying coplanar (conformation E). The potential-energy (PE) surface shows the existence of a conical intersection when studied as a function of the C–Cl bond length and of the rotation angle around the C(sp 2)–C(sp 3) bond. Information on the dynamics of this process was obtained using the dynamic reaction coordinate (DRC) algorithm for tracing reaction paths. The DRC method shows a qualitative difference in the dissociation mechanism of the two conformers. The lifetime of conformation E depends on the amount of kinetic energy assigned to the internal rotational mode motion, whereas that of conformation S does not.

A theoretical insight into the internal H-bond and related rotational motion and proton transfer processes of 1-hydroxy-2-acetonaphthone in the S 0 state

Ab initio calculations at the S 0 state of 1-hydroxy-2-acetonaphthone reveal the existence of several conformers that result from the hydroxyl and acetyl groups rotation and a coupled proton-transfer and twisting motion on the potential-energy surface. The most stable structure has an O–H...O=C internal H-bond. The bulk effect on the stability of the structures was studied through a continuum model. Specific interactions with two bridged molecules of water lead to the breaking and making of H-bonds and to the stabilization of rotamers.

Photoinduced Proton Transfer and Rotational Motion of 1-Hydroxy-2-acetonaphthone in the S1 State: A Theoretical Insight into Its Photophysics

The internal rotational motion, the intramolecular proton-transfer reaction, and the subsequent internal twisting process of 1-hydroxy-2-acetonaphthone (HAN) are studied in the first singlet excited electronic state (S1) using the ab initio electronic-structure method at the CIS/6-31G** level. The calculations show a multiple-well potential-energy surface (PES) with a low energy barrier (5.49 kcal/mol) between the excited OH···OC enol form (E*, the most stable structure at the ground state) and its keto tautomer (K*, result of intramolecular proton-transfer reaction). An internal rotation with activation energy of 7.53 kcal/mol in the produced K* may give a more stable twisted rotamer KR*. The energy of this structure is 6.23 kcal/mol lower than that of E*. The involvement of a twisting motion in the tautomer explains the reported structured fluorescence band, the low quantum yield, and short emission lifetime. The existence of energy barriers in the PES accords with the dependence of its emission spectroscopy and dynamics on the nature of the transferred isotope (H/D) and on the excess energy of excitation. Frequency modes analysis shows that in-plane and out-of-plane motions of the H-bonded chelate ring of E* will play a crucial role in the proton-transfer dynamics and spectroscopy. These theoretical results are in full agreement with previous experimental observations in the gas phase (Douhal, A.; et al. Chem. Phys. 1993, 178, 493. Lu, C.; et al. Chem. Phys. Lett. 1999, 310, 103) and in solution (Tobita, S.; et al. J. Phys. Chem. A. 1998, 102, 5206). However, they contrast with the conclusion of Catalán et al. stating the absence of proton transfer in the S1 state and the presence of a single minimum at the S0 state (Catalán, J.; et al. J. Am. Chem. Soc. 1993, 115, 4321; Chem. Phys. Lett. 1997, 269, 151). Moreover, the ER−OH and ER−CO isomers having rotated the OH and CO groups, are found at 13.95 and 17.36 kcal/mol above the E* structure, respectively. As a result of the pseudoaromaticity provided by the H-bonded chelate ring in the enol form, the electronic excitation induces in this one a stabilization of ∼2 kcal/mol with respect to ER−OH and ER−CO rotamers.

Internal methyl group rotation in o-cresol studied by pulsed field ionization-ZEKE photoelectron spectroscopy

Pulsed field ionization-ZEKE photoelectron spectroscopy and (1+1) R2PI spectroscopy have been applied to cis- and trans-o-cresol. The internal rotational structure in S1 has been re-assigned for the cis-isomer, and the potential curve for the internal rotation has been determined. In the PFI-ZEKE spectra recorded via different internal rotational levels in the S1 state, well-resolved low-frequency bands have been observed. The low-frequency bands are assigned to the internal rotational motion of the methyl group in the cation. Level energies and relative transition intensities are reproduced well by a one-dimensional rotor model with a three-fold axis potential. Potential curves for the internal rotation have been determined for both cis- and trans-o-cresol cations. The barrier height for the internal rotation is different for the two isomers in the cation, while it becomes similar in S1. Contributions of steric and electronic factors to the rotational barrier are discussed.

Isokinetic performance and shoulder mobility in elite volleyball athletes from the United Kingdom

Objectives—To evaluate the differences in strength and mobility of shoulder rotator muscles in the dominant and non-dominant shoulders of elite volleyball players. Methods—Isokinetic muscle strength tests were performed at speeds...

Torsional Vibrations of Ester Groups as a Mechanism of Dielectric Relaxation of Poly(p-hydroxybenzoic Acid)

Dielectric relaxation of poly(p-hydroxybenzoic acid) (PHBA) due to torsional vibrations of the dipoles of ester groups in the main chain is considered. A dynamic model of elastically coupled rotators interacting with the crystal lattice is applied. The model is characterized by two sets of force constants which determine intrachain correlations between rotators along a given chain and interchain interactions in the crystal lattice. Correlation functions for the cosines of the rotation angles of different rotators in a chain are calculated. Fluctuations of the components of dipole moments normal to the chain axis (transverse components) and fluctuations of the components along the chain axis (longitudinal components) are considered. The transverse component is connected with the previously considered crank-shaft type of internal rotational motions of ester groups. The origin of the longitudinal component is ascribed to a complicated superposition of internal rotations and bending vibrations. The fluctuatio...

Sub-Millimeter-Wave Spectroscopy of the Ar·H+3and Ar·D+3Ionic Complexes

An investigation of the sub-millimeter-wave spectra of the ionic complexes Ar·H+3and Ar·D+3is presented. These complexes were produced in a negative glow electric discharge, in mixtures of argon with either H2or D2. About 80 new transitions were assigned in the 485–680 GHz frequency range using a sub-millimeter-wave spectrometer built with Russian made backward wave oscillators (BWO) sources. These measurements enabled us to observe the firstKa= 2 transitions for Ar·H+3and the firstKa= 3 transitions for Ar·D+3. Analyses of the line frequencies were carried out using an IAM-like approach, which accounts for the large amplitude internal rotation motion displayed by both species. Insights into the geometry of the intermediate configuration for this large amplitude motion were gained.

Dynamically Modified Torsional Eigenstates for Single-Top Molecules

We present a semiclassical treatment for the inclusion of kinetic energy-dependent variations in the moment of inertia for certain molecules exhibiting internal rotational motion. The model is applied to trifluoroacetyl bromide and the resulting dynamically modified transition energies yield an improved spectral deviation in contrast to the rigid model.

CONCENTRIC AND ISOMETRIC ACTIONS IN PROPRIOCEPTIVE NEUROMUSCULAR FACILITATION STRETCHING TECHNIQUES

931 The purpose was to compare proprioceptive neuromuscular facilitation (PNF) stretching techniques to improve range of motion (ROM) in hip flexion and shoulder internal rotation. PNF techniques included an isometric action of the target muscle group (hip extensors or shoulder external rotators) using 0%, 50%, or 100% of maximal voluntary isometric force (MVIF) measured using a Biodex® dynamometer, and either did or did not include a concentric action of the muscles opposing the target muscles. Twelve students of mean (± SD) age 21 ± 3 yr volunteered. ROM was the largest joint angle attained with assistance from the investigator (i.e., “passive” ROM). Stretching was performed on five separate days. The mean of the five baseline ROM measures served as the value for “no concentric, 0% MVIF”. Mean values (in degrees): TableTableResults of two-way repeated measures ANOVAs revealed significantly greater ROM (p < 0.001) at both the hip and shoulder after PNF techniques that included both a concentric action and a high percentage of MVIF. The two components of PNF techniques appear to contribute independently to the improved ROM, and the effect of the isometric action is dose-dependent.

REPRODUCIBILITY AND RELIABILITY OF MEASURING GLENOHUMERAL VERSUS SCAPULOTHORACIC MOTION UPON PHYSICAL EXAMINATION.

438 PURPOSE: In a previous study in cadavers we have established that glenohumeral motion (GH) in internal (IR) and external rotation (ER) can be distinguished from scapulothoracic motion (ST) upon physical examination. The goal of this study was to evaluate the reliability and reproducibility of this new concept of shoulder motion. MATERIALS AND METHODS: Both shoulders of twenty volunteers were examined by a fellowship trained shoulder surgeon (SG) and an experienced musculoskeletal physical therapist(PT). All shoulders were re-examined four weeks later by both examiners who were blinded to each other and to the previous measures. Statistics included student t-test, two way ANOVA interclass coefficient correlations. RESULTS: With the subject standing and the elbow at the side, the glenohumeral external rotation was 42 +/- 13 for SG and 66 +/- 17 for PT (p<0.05). In this position the combined motion was 82 +/- 14 for SG and 85 +/- 14 for PT (p>0.05). With the arm abducted ninety degrees, GH-ER was 81 +/- 16 for SG and 90 +/- 11 for PT (p>0.05), while combined motion was 109 +/- 16 for SG and 109 +/- 13 for PT (p>0.05). With the arm abducted 90 degrees GH-IR was 20 +/- 15 for SG and 33 +/- 13 for PT (p<0.05) while combined motion was 88 +/- 20 and 80 +/- 14 respectively(p>0.05). Intraclass correlation coefficients showed intrarater reliability was good (>0.80) for all measures except for GH-IR abducted 90 degrees. CONCLUSIONS: This study validates that GH motion can be reproducibly distinguished from ST motion in IR and ER upon physical examination. This study reveals the importance of careful documentation of the exact conditions of measurement of shoulder motion.

The involvement of rotational processes in the intramolecular proton‐transfer cycle

AbstractEncapsulation processes and picosecond time‐resolved transient absorption spectra of 2‐(2î‐hydroxyphenyl) imidazo[1,2‐a]pyridine have been used to elucidate the fluorescence band shift and the transient absorption dynamics (in the picosecond time scale) of the proton transfer phototautomer in cyclodextrins and in cyclohexane, respectively. Restriction of the internal motion of the phototautomer by cyclodextrins leads to a 70 nm blue shift of the proton transfer fluorescence band. A shaping time (14 ps) of the transient absorption spectrum has been observed and assigned to a combination of this internal rotational motion and a vibrational‐cooling process of the phototautomers in the excited state. Both studies suggest the involvement of rotational processes in the intramolecular proton‐transfer cycle.

Vibrational Dynamics of H5+and Its Deuterated Isotopomers

The vibrational dynamics of the H5+complex and its deuterated isotopomers H4D+, H3D2+, H2D3+, HD4+, and D5+are investigated using a model Hamiltonian which is based on the assumption that on the potential energy hypersurface the barriers for the internal rotation motions are infinitely high except for the practically free propeller-like motion. According to our previous studies on H5+, the propeller-like rotation essentially does not interact with the remaining vibrations and is therefore neglected in the present calculations. Within the framework of the adiabatic approximation the resulting eight-dimensional vibrational problem is separated into two smaller subproblems which are solved numerically applying the same scheme as previously. The calculations are performed using a new extended potential energy function which also provides a reliable description of the interactions between the degenerate stretching motions and the remaining vibrations. As in our previous calculations, the high-frequency fundamentals obtained for H5+are in good agreement with their experimental counterparts, whereas the reliability of the present results for the low-frequency motions is considerably improved as a result of the appropriate description of the relevant interactions. Predictions of the vibrational energies of the other deuterated isotopomers are made on the same accuracy level. The zero-point energies derived from the present calculations are believed to be accurate enough for a quantitative determination of the binding energies of the different isotopomers.

Evidence for heavy atom large amplitude motions in RG-cyclopropane van der Waals complexes (RG=Ne, Ar, Kr) from rotation-tunneling spectroscopy

Rotation-tunneling spectra of the van der Waals complex Ne-cyclopropane, and pure rotational spectra of Ar-cyclopropane and Kr-cyclopropane, were measured with a Fourier transform microwave spectrometer in the frequency range from 4–19 GHz. The observed transitions are all of a-type and are in accord with those of prolate symmetric top molecules where the rare gas atom is positioned on the C3 axis of the cyclopropane subunit above its symmetry plane. Altogether transitions of five isotopomers of the Ne complex with J values ranging from 0 to 4 and K values from 0 to 1 were measured. Three isotopomers of the Ar complex (J from 1 to 5 and K from 0 to 2) and nine isotopomers of the Kr complex (J from 1 to 8 and K from 0 to 2) were observed. Effective separations between the center-of-mass of cyclopropane and the rare gas atoms were obtained from the ground state rotational constants and were found to be 3.673, 3.802, and 3.906 Å for the Ne-, Ar-, and Kr-containing complexes, respectively. The derived centrifugal distortion constants indicate that the complexes are comparatively rigid in the radial coordinate. Transitions of two isomers were observed for complexes that contain monodeuterated cyclopropane. The intensities of transitions of the D-bonded species were considerably higher compared to those of the H-bonded isomers indicating that the angular motion of the rare gas atom is much less restricted. In the case of the parent Ne-cyclopropane complex, and its Ne22- and mono C13-isotopomers, all transitions appeared as doublets. This is attributed to a tunneling internal rotation motion of the cyclopropane unit within the complex.

Coordinates, Hamiltonian, and Symmetry Operations for the Small-Amplitude Vibrational Problem in Methyl-Top Internal-Rotor Molecules like CH3CHO

Motivated by attempts to understand the mechanism(s) by which internal rotation of methyl tops strongly enhances intramolecular vibrational energy redistribution, a vibration–torsion–rotation Hamiltonian has been constructed for molecules containing oneC3vrotor. Internal rotation is taken to be slow compared to the other vibrations, so that the small-amplitude vibrational problem is solved for each value of the torsional angle, and the angular momentum operator associated with internal rotation motion is grouped with the three components of the total angular momentum operator. Even though the molecular symmetry group for two common basis-function sets is isomorphic withC3v, small-amplitude vibrational eigenvectors resulting from diagonalization of the FG matrix may change sign when the methyl top is rotated by 2π, so that a double group ofC3vmust be used to classify vibrational eigenvectors. This sign change seems to be related to the fact that the CH3CHO molecule passes through two chemically differentCsconfigurations during the internal rotation motion. Some changes in theGmatrix are required because the torsional constraint is not orthogonal to traditional bond stretching and angle bending vibrations. Several remaining questions concerning application of the present formalism are pointed out. These will probably be answered only when the formalism is applied to vibrational data (particularly torsionally mediated perturbations) of example molecules like acetaldehyde and methanol.

Aryl Ring Rotation in Porphyrins. A Carbon-13 NMR Spin−Lattice Relaxation Time Study

Overall tumbling and internal rotational motions in porphyrins bearing meso aryl substituents and, in some cases, flanking alkyl groups at the β-pyrrolic positions have been determined using 13C spin−lattice relaxation time measurements. In deuteriochloroform solution at 303 K, the overall reorientation of all three porphyrins investigated occurs with diffusion coefficients of ∼1 × 109 s-1. In porphyrins with only hydrogen at the β-pyrrolic positions, the meso phenyl rings undergo rotations about their single bonds to the porphyrin with diffusion coefficients of ∼4 × 109 s-1. Introduction of methyl substituents at the β-pyrrolic positions adjacent to the phenyl rings reduces these motions, but only to ∼1 × 109 s-1. Thus, significant internal motions are present in both types of molecules. These motions occur on the time scale of many photoinduced electron and energy transfer processes in porphyrins covalently linked to electron or energy donors or acceptors through meso aryl groups. Thus, the internal lib...

The vibrational dependence of the hydrogen and oxygen nuclear magnetic shielding constants in OH− and OH− · H2O

Abstract The effect of hydrogen binding and vibrational motions on the oxygen and hydrogen nuclear magnetic shielding constants in the OH − and mono-hydrated OH − and mono-hydrated OH − ion (H 3 O 2 − ) is investigated by ab initio calculations. A large down-field shift in 17 O shieldings and a small down-field shift in the 1 H shieldings is found for H 3 O 2 − relative to OH − . The dependence of the nuclear magnetic shielding constants in H 3 O 2 on the strongly anharmonic symmetric and antisymmetric O···H···O stretching motions and on the internal rotation motion of the outer hydrogens is studied with the non-rigid bender model Hamiltonian [V. Spirko, W. P. Kraemer and A. Cejchan, J. Mol. Spectrosc. 136 (1989) 340] at the level of the random phase approximation (RPA). The dependence of the shielding constants in OH − on the bond length is investigated at the level of the RPA and the second order polarization propagator approximation (SOPPA). Pertinent (analytic) nuclear magnetic shielding functions are obtained by fitting to the ab initio shielding points and these functions are used to calculate the vibrational averages using the corresponding vibrational eigenfunctions. The predicted effective shielding constants of H 3 O 2 − exhibit a sizable and non-monotonic dependence on the stretching vibrational quantum numbers, whereas the dependence on the internal rotation is practically negligible. The effective shielding constants of OH − show an even larger dependence on the vibrational quantum number. The effect of the end-over-end rotational motion, however, is small.

Internal rotation of methyl group in o- and m-toluidine cations as studied by pulsed field ionization–zero kinetic energy spectroscopy

Internal rotational levels of the methyl group in o- and m-toluidine cations have been observed by pulsed field ionization–zero kinetic energy photoelectron spectroscopy. Level energies and transition intensities were reproduced by a one-dimensional rotor model with a free-rotor basis set, and the potential curves of the internal rotation in the cations have been determined. Analysis for m-toluidine shows a drastic increase of the barrier height for internal rotational motion from the neutral to the corresponding cation. On the other hand, the barrier in o-toluidine slightly decreases by ionization. The mechanism of the change in barrier height will be discussed in terms of geometrical and electrical change by ionization. It is suggested that the drastic change of internal rotational motion is mainly determined by the electronic structure.

Microwave spectrum of alkali metal tetrahydroborate. III. Rotational transitions of LiBD4, NaBD4, and KBH4 in the ground vibrational states and molecular structures of MBH4 (M=Li, Na, and K)

In order to determine the molecular structure of alkali metal tetrahydroborates in a systematic way, we have extended the measurements of rotational transitions to LiBD4, NaBD4, and KBH4 in the ground vibrational states. The observed spectra, which all conformed well to the pattern expected for a symmetric top molecule, yielded rotational and centrifugal distortion constants for 7Li11BD4, 7Li10BD4, 6Li11BD4, 6Li10BD4, Na11BD4, Na10BD4, 39K11BH4, 39K10BH4, and 41K11BH4. The four observed rotational constants of LiBD4 gave rs(Li–B) to be 1.931 09(14) Å, which, when combined with an assumption that θ(Db–B–Dt)=113.0°, led to r(B–Db) and r(B–Dt) to be 1.250±0.025 Å and 1.212∓0.032 Å, respectively, where the uncertainties of the B–D distances are primarily due to that of θ(Db–B–Dt) estimated to be ∓1.0°. [The suffixes attached to the deuterium atom, b and t, denote bridge and terminal, respectively.] The rs(Li–B) distance in LiBD4 is significantly shorter than that in LiBH4, 1.939 38(10) Å. This large secondary isotope effect is ascribed partly to the large amplitude rocking (or internal rotation) motion of the BH4 group. By fixing the Li–B distance to the respective rs values, all of the eight rotational constants of LiBH4 and LiBD4 were simultaneously analyzed to determine the structure of the BH4 group, where the isotope effect was taken into account only for the B–H distances in a form δ=r(B–H)–r(B–D). Again θ(Hb–B–Ht)=θ(Db–B–Dt) had to be fixed to 113.0∓1.0°. The isotope shift δ was found to be not very dependent on the value of θ and was determined to be 0.006 26(6) Å. The two B–H distances were obtained to be r(B–Hb)=1.257±0.025 Å and r(B–Ht)=1.218∓0.032 Å. The data on NaBD4 were analyzed by assuming θ(Db–B–Dt)=111.0∓1.0° and r(B–Db)−r(B–Dt)=0.04(I) or 0.03(II) Å, yielding r(Na–B)=2.2978(I) or 2.2987(II)±0.0055 Å and r(B–Db)=1.269(I) or 1.258(II)∓0.040 Å. An alternative way of the analysis is to combine the data on the H and D isotopic species; the assumptions on θ(Hb–B–Ht)=θ(Db–B–Dt)=111.0∓1.0° and on the difference r(B–Hb or Db)–r(B–Ht or Dt)=0.04(I) or 0.03(II) Å lead to the following results: r(Na–B)=2.3075(I) or 2.3080(II)±0.0028 Å, r(Na–B) (in NaBH4)–r(Na–B) (in NaBD4)=0.0097(I) or 0.0092(II)∓0.0028 Å, r(B–Hb)=1.278(I) or 1.267(II)∓0.040 Å, and δ=0.0086∓0.0001 Å for both I and II. The rs(K–B) distance was obtained from the three observed rotational constants to be 2.656 41(20) Å, which, combined with the assumption of θ(Hb–B–Ht)=110.8∓1.0°, led to r(B–Hb)=1.272±0.030 Å and r(B–Ht)=1.233∓0.030 Å.

Internal Rotation of the Methyl Group in Cations of Toluene Derivatives

AbstractThe potential of the internal rotation of the methyl group was determined for o‐, m‐, and p‐fluorotoluene cations by pulsed field ionization spectroscopy. The potential of the internal rotational motion was also surveyed for other toluene derivative cations. It was found that the barrier height generally increases by ionization. The increase in the barrier height has been discussed in connection with the reduction of the internal rotational constant B by ionization. The geometrical distortion of the methyl group during the internal rotation has been suggested.