Structure électronique et origines des différences d'énergie entre isoméres de rotation: II. La formaldoxime

The semi-empirical CNDO and INDO methods are applied to the analysis of the electronic structure of various conformations of formaldoxime. In qualitative agreement with experimental data, the trans form of the CN-OH group is predicted to be more stable, by some 3 kcal mole^?1, than the cis form; from the stable form, the barrier to internal rotation is predicted to be equal to 4.6 kcal mole^?1. The computed dipole moment (0.36 ± 0.04 Debyes) is in good agreement with the value determined from Stark effect measurements (0.44 Debyes). Finally, bicentric partitioning of the total molecular energy reveals the principal factors that contribute to the shape of the potential curve.     

Vibrational dephasing of CH stretching vibrations in compounds containing the CH3 or the CCH group

Calculations are presented for the dephasing of the symmetric methyl stretching vibration taking account of the internal rotation of the methyl group in a rigid molecular environment of carbontetrachloride molecules, using two different liquid state models. The narrowing effect of the fast internal rotation on the bandshape is demonstrated. The Raman isotropic spectrum of the methyl stretching vibration in hexadiyne-2,4 is presented, and its second moment and bandwidth are shown to be represented fairly well by the calculations. The acetylenic CH stretch of 3,3-dimethylheptyne-1 has also been investigated. After correction for a hot band on the low frequency side its second moment and bandshape may be reasonably well well reproduced by calculations in the slow modulation limit. The implications for the modulation mechanism of repulsive and attractive forces in liquids are discussed.     

Molecular orbital studies of conformers and the barrier to internal rotation in 1,3-butadiene

The potential curve for rotation around the central bond in 1,3-butadiene has been estimated by ab initio calculations using Gaussian-type basis functions. The calculations, which also include limited geometry variation during rotation, suggest that in the SCF approximation the second stable form of the molecule is a gauche conformation rather than a cis. The predicted energy difference between the planar trans ground state and the stable gauche form is 2.7 kcal/mole and the barrier to internal rotation is found to be 6.0 kcal/mole using a (9,5) basis for carbon and 4s functions on hydrogen.     

NMR study of molecular motion in oriented long-chain alkanes. III. Oriented n-C32H66

The NMR second moment of a uniaxially oriented mat of single crystals of n-C₃₂H₆₆ (in the orthorhombic form) was measured at temperatures from ?170°C to 70°C and at various alignment angles γ between the orientation axis (preferential direction of the molecular chains) and the NMR magnetic field. Accurate expressions are given for the NMR second moment of an orthorhombic normal paraffin C_nH_2n+2 of arbitrary molecular chain length n for n ≥ 10, in the following states of molecular motion: no motion (a rigid lattice), rotation of CH₃ groups, and rotation of the chains around their axes with superimposed rotation of CH₃ groups. In addition to these well-known motions, n-C₃₂H₆₆ is found to exhibit an α process. The corresponding decrease of the NMR second moment shows the dependence on γ predicted for “flip-flop” motion, i.e., rotational jumps of the chain molecules around their axes through 180° and a simultaneous translation along these axes by one CH₂ group. The overall decrease in second moment occuring at the transition to the hexagonal rotator phase in n-C₃₂H₆₆ can be quantitatively accounted for. The dependence of this decrease on the alignment angle γ, however, is in disagreement with calculations based on a simple rotation of the chains around their axes. Considerable torsion of the chains superimposed on the rotation would improve agreement between theory and experiment.     

The illumination of molecular behavior by electron waves

Aspects of vapor-phase electron diffraction are discussed, singling out the seminal contributions of Yonezo Morino in collaboration with San-ichiro Mizushima and others. Recent developments leading to a precision approaching 0.0002Å are sketched, explaining why accuracy tends to fall short of this by one or two orders of magnitude, even if experimental intensities are error-free. The role of electron diffraction in studies of rotational isomerism is then outlined, with emphasis on hydrocarbon systems, to illustrate how experimental results have led to improved understanding, useful predictive procedures, and a new approach to probe the dynamics of internal rotation.     

Molecular mechanics assessment of the configurational statistics of polyoxyethylene

Previous investigators have shown that statistical mechanical averages for configuration-dependent physical properties of long unperturbed polyoxyethylene chains are sensitive to the gauche–trans energy difference for rotation about C? C bonds. Agreement between theory and experiment could be obtained only by significant adjustment of this energy away from values predicted by semiempirical conformational energy computations. The present work examines the success of MM 2 in evaluating conformational properties of long unperturbed polyoxyethylene chains. Calculations are performed which identify the rotational isomers, and their energies, for the indicated bonds in CH₃OCH₂CH₂O? CH₂? CH₂? OCH₂CH₂OCH₃. These energies are used to assign statistical weights utilized in the configuration partition function for a rotational isomeric state chain with symmetric threefold interdependent rotations. The customary generator matrix scheme is employed to evaluate the mean-square unperturbed end-to-end distance, mean-square unperturbed dipole moment, and their temperature coefficients. Contrary to computational schemes employed previously, MM 2 is found to provide an estimate of the gauche–trans energy difference for rotation about C? C which is in harmony with the known dimensions and dipole moments of the unperturbed polymer. MM 2 also provides good estimates for most of the other parameters required in the rotational isomeric state treatment. A notable exception is provided by the gauche–trans energy difference for rotation about the C? O bond. This energy difference is overestimated by MM 2.     

Stereomutations of atropisomers of sterically hindered salophen ligands

[structure: see text] The stereomutations in nonsymmetrical salophen ligands 1-4 were studied by means of dynamic NMR and HPLC methods. DNMR experiments showed that in DMSO-d(6) hindered ligands 2-4 exist in two chiral conformations, depending on whether the imine carbon atoms are in a cis or trans disposition with respect to the plane of the central o-phenylenediamine ring, the latter being more stable by 1.0 kcal mol(-1). Owing to its higher dipole moment, in the apolar solvent C(6)D(6) the cis conformer is destabilized with respect to the trans one, in agreement with the results of ab initio calculations. In DMSO-d(6) solution the two conformers are in equilibrium through the less hindered rotation about the C6-N7 bond aligned to the a(6,7) axis, and the interconversion barriers range from 18.4 to 19.3 kcal mol(-1). The enantiomerization process is a two step-process that implies sequential rotations around the C6-N7 and the C1-N8 bonds, so that the rate determining step is the slower rotation around the more hindered C1-N8 bond aligned to the a(1,8) axis, and the energy barriers range from 21.4 to 21.9 kcal mol(-1). These values compare well with those determined by chromatography on an enantioselective HPLC column at low temperature, thus confirming that DNMR and DHPLC can be conveniently employed as complementary techniques.     

Molecular orbital calculations of poly(vinylidene fluoride) and its model compounds

Calculations of dipole moments of poly(vinylidene fluoride)(PVDF) and its model compounds were performed by the MNDO method. 2,2-difluoropropane as a model compound was found to have a dipole moment of 8.97 × 10^?30 C m (2.69 D). It was in satisfactory agreement with a previously obtained experimental value, 8.01 × 10^?30 C m. Dipole moments of two other model compounds, tetrafluoropentane and octafluorononane, were calculated to be 1.70 and 3.24 × 10^?29 C m, respectively. Ratios of repeat unit moments of the second and third compounds to the first compound moment were equal to 0.95 and 0.90, respectively. These were nearly identical with a theoretical ratio, 0.96, derived from the free rotation model of a polymer chain. The calculated dipole moments were considered to reflect the molecular structures in which free rotations of nearly tetrahedral bond angles might be allowed around C—C links. Dipole moments of each monomer unit for three polymorphs of PVDF, Form I, II, and III were calculated to be 7.64, 5.40, and 5.07 × 10^?30 C m, respectively. Ratios of the three moments to the first model compound moment were found to be 0.85, 0.60, and 0.57. The decreasing order of the three factors suggests that orientations of monomer unit dipoles are more and more interdependent, and free internal rotations around skeletal bonds are more and more hindered, when the conformation varies from Form I to III. Also it was confirmed that the atomic charge distributions of the three polymorphs were very similar, and that the difference in dipole moments were primarily caused by conformation changes of the polymer chain.     

PHOTOREACTION OF THE ACIDIFIED FORM OF BACTERIORHODOPSIN AND ITS 9-CIS DERIVATIVE IN PURPLE MEMBRANE AT LOW TEMPERATURES

Abstract— The photoreaction of the acidified form of bacteriorhodopsin and its 9-cis derivative was studied by low temperature spectroscopy.
A short exposure of the acidified form of bacteriorhodopsin, which was prepared by adding 2 m M HC1 to purple membrane suspension in 67% glycerol at 0°C, to red light at – 72°C resulted in the blue-shift of the spectrum. The feature of the shift was very similar to that accompanied by the formation of stable 9- cis acidified form of bacteriorhodopsin at 0°C, but only 13- cis - and all- trans -retinals were found in the extract from this product. No blue-shifted product was found on irradiation at – 190°C.
Irradiation of the 9- cis form of acidified bacteriorhodopsin at -72°C with blue light caused the isomerization of its 9- cis -retinylidene chromophore to 13- cis and all- trans forms without a significant spectral change. It became greater only after the sample was warmed above – 24°C. These results indicate the presence of the light-induced product which has trans configuration on the 9-10 double bond and exhibits the 9- cis type spectrum.     

Quantum-chemical calculation of the hydroxylamine molecule and its complex with the proton

The cis and trans conformations of the hydroxylamine molecule were calculated by the SCF LCAO-MO method in the CNDO/2 approximation with optimization of the geometry. It was shown that the cis conformer is more stable in the free state; the barrier to internal rotation of the OH group about the N-O axis was determined. Various schemes for the protonation of hydroxylamine were investigated. The proton affinity of the molecule was calculated (for the N- and O-complexes). It was shown that protonation of NH₂OH at the nitrogen is more favorable, and the trans form of hydroxylamine has the strongest proton affinity.Translated from Teoreticheskaya i éksperimental'naya Khimiya, Vol. 24, No. 1, pp. 122–125, January–February, 1988.     

NMR observations of drawn polymers. VII. Nylon 66 fibers

Wide-line NMR spectra have been obtained on an oriented sample of drawn nylon 66 fibers at temperatures between ?196°C and 200°C and at alignment angles between the fiber axis and the magnetic field of 0°, 45°, and 90°. At ?196°C, 20°C, and 180°C, the complete angle dependence of the NMR spectrum has been measured. The second moments of these spectra have been compared to theoretical second moments calculated for various models of chain segmental motion in an attempt to elucidate the mechanisms involved in the low-temperature segmental motion (γ process) and the high-temperature segmental motion (α_c process). In agreement with earlier suggestions, the present results indicate that the γ process consists of segmental motion in noncrystalline regions. The overall decrease in second moment caused by the γ process is consistent with a model in which all noncrystalline segments rotate around axes nearly fixed in space. Furthermore, this decrease shows a pronounced dependence on the alignment angle. It is believed that this is due to tie molecules which become highly oriented along the fiber axis during drawing; their axes of rotation will therefore be nearly parallel to the fiber axis. The segments in noncrystalline entities such as chain folds and chain ends are less well oriented along the fiber axis and make an essentially isotropic contribution to the second moment decrease. The second moment at 180°C indicates the presence of considerable motion in the crystalline regions, and this motion is denoted the α_c process. The second moment S_c of the crystalline regions is strongly dependent on the alignment angle, the predominant feature being a relatively high value of the second moment when the fiber axis is directed parallel to the magnetic field. This is in qualitative, but not quantitative, agreement with the motional model recently advanced by McMahon, which assumes full rotation of the chains around their axes. Excellent quantitative agreement with experiment has been obtained by superimposition of rotational oscillation around the chain axis of amplitude roughtly 50°, and torsion of the chains with neighboring CH₂ groups oscillating around the C? C bond with a relative amplitude of about 40°. A model in which the chains perform rotational jumps of 60° between two equilibrium sites has also been considered (60° flip-flop motion). A distinction between this model and rotational oscillation has not been possible.     

Calculation of internal rotation spectra of three molecules with increasing asymmetry factors

Internal rotation energy levels for phenol, fluoracetaldehyde, and difluoracetaldehyde are estimated with two methods: first, by means of the reduced inertia moment formula introduced by Pitzer; second, with use of an internal axis system and by taking into account the influence of the overall rotation through the inertia factor. Two methods have been used to estimate potential barriers (CNDO /2 and PCILO ). Results are related to the asymmetry of the tops. Theoretical far IR spectra for the three molecules are calculated. In the case of phenol, good agreement is found with experimental data.     

NMR and rotational angles in solution conformation of polypeptides

Professor San-Ichiro Mizushima and Professor Yonezo Morino's classical contributions provided unique means and firm basis for understanding of conformational states and internal rotation in polypeptide molecules. Now the NMR spectroscopy is the best choice to study molecular conformation, mechanism of action and structure-functional relationships of peptide and proteins in solution under conditions approaching those of their physiological environments. Crucial details of spatial structure and interactions of these molecules in solution are revealed by using proton-proton and carbon-proton vicinal coupling constants, proton nuclear Overhauser effect and spectral perturbation techniques. The results of NMR conformational analysis are presented for valinomycin “bracelet”, gramicidin A double helices, honey-bee neurotoxin apamin, scorpion insectotoxins and snake neurotoxins of long and short types.     

Microwave Spectra of o-Fluorotoluene and Its 13C Isotopic Species: Methyl Internal Rotation and Molecular Structure

The rotational spectra of o-fluorotoluene and its seven ¹³C isotopic species were recorded in the frequency range from 4 to 20 GHz with employment of pulsed molecular beam Fourier-transform microwave (MB-FTMW) spectrometers. The analysis of the spectra in the two lowest states of methyl internal rotation (torsional ground state, A and E species) was based on a asymmetric frame-rigid symmetric top Hamiltonian with inclusion of centrifugal distortion terms, yielding structural rotational constants, as well as the threefold barrier V ₃ to internal rotation and the angle(a,i) between the principal moment of inertia a axis and the internal rotor axis i. The rotational constants of all eight isotopomeres were used to derive the seven ¹³C r _s coordinates of the molecule.     

Anisotropy of dielectric relaxation in crystal form II of poly(vinylidene fluoride)

The anisotropy of the crystalline relaxation (α relaxation) in oriented poly(vinylidene fluoride) in crystal form II has been studied. The dielectric increment Δε is analyzed on the basis of the two-site model. A linear relation between Δε/χξ and cos²θ is obtained, where χ is the degree of crystallinity, ξ is the ratio of the internal field to the applied field, and θ is the angle between the applied electric field and the molecular axis. The dipole moment changes direction only along the molecular axis in the relaxation in crystal form II; the molecular motion cannot be explained by chain rotation around the molecular axis. Possible models for the α relaxation are proposed: change in conformation with internal rotation can occur in the crystalline chains, and defects in the crystalline regions play an important role in the α relaxation.     

An intrinsic reaction coordinate calculation of the torsion-internal rotation potential of hydrogen peroxide and its isotopomers

Intrinsic reaction coordinate (IRC) calculations of the internal rotation (torsional) potentials for H(2)O(2) and its isotopomers HDO(2) and D(2)O(2) were carried out at the CCSD(T)/CBS//aug-cc-pVDZ level. Two extrapolation methods were used to obtain energies in the complete basis set (CBS) limit. The full IRC potential was constructed from scans from the C(2v) (cis) and C(2h) (trans) transition states to the equilibrium C(2) (gauche) structure. The IRC potential for H(2)O(2) was fit to a five-term Fourier function; coefficients were compared with values obtained from spectroscopic data. The twofold IRC torsional potentials were used to obtain torsional eigenvalues, which yielded values of the transitions between various ntau states. These results compare favorably with Raman and near-infrared data. Our calculations provide values of the cis and trans barriers of 2495 and 364 cm(-1), respectively, which are in good agreement with both previously calculated and experimentally derived values. It appears that coupling between torsional motion and other degrees of freedom is not significant in these molecules.     

弯曲振动引致的过渡态的混沌

Molecular vibration is correlated to the motion of a pendulum and the lowly excited states are corresponding to that of the pendulum around its stable fixed point while the highly excited states are to the unstable fixed point. Specifically,the transitional state due to internal rotation is also corresponding to the unstable fixed point of the pendulum. As for a perturbed pendulum,chaos occurs first around its unstable fixed point,which is a reasonable consequence that the highly excited state and the transitional state are full with intrinsic chaotic motion. With this conjecture,HCN,its isomer HNC and the excited delocalized transitional state due to the internal rotation of H-C around the skeleton of C-N are interpreted with Morse oscillators in resonance. It is stressed that the delocalized transitional state is in multiple resonances between the H - C stretch and the bending due to that the classical bending frequency is lowered as the transitional state is approached. Multiple resonances,or the overlapping of resonances,lead to chaos as noted by Chirikov. Hence,the delocalzed transitional state can be in a chaotic state. Besides,the internal rotational state of HCP due to H atom is analyzed by this physical picture. For this purpose,an algebraic Hamiltonian for HCN,its isomer HNC and the delocalized transitional state is proposed with its coefficients elucidated by fitting with the quantal levels adopted from the literature by the quantum mechanical algorithm. The result shows that both the transitional state due to the internal rotation of H atom and the highly excited states of HCN and HNC are full of multiple resonances. Therefore,chaos is expected for these systems. Finally,all these ideas are compounded by a proposed model for unfolding the characteristics of chaos in the molecular system.     

Spectra and structure of small ring compounds. LXII. Conformational stability,structural parameters,ab initio calculations and vibrational assignment of cyclopropylmethyl ketone

The conformational stability, barriers to internal rotation, and fundamental vibrational frequencies of cyclopropylmethyl ketone, c-C₃H₅C(CH₃)O, have been obtained from Hartree—Fock ab initio calculations with the RHF/3-21G and RHF/6-31G* basis sets, as well as the 6-31G* basis set with electron correlation at the MP2 level, and the results are compared to those obtained from experiment. The data are consistent with the predominant rotamer having the cis conformation (carbonyl bond cis to the ring). A second form, having a “near” trans structure, is calculated to have a larger total dipole moment than the cis form, which accounts for its increased abundance in the liquid compared to that in the gas. A complete vibrational assignment is proposed based on experimental data and normal coordinate results from the ab initio calculations. The asymmetric torsional barrier has been calculated to be approximately 2000 cm⁻¹ and this result along with others is compared to the corresponding data obtained from both experiment and theory for the cyclopropylcarbonyl halides.     

Gas-phase Raman spectra and the potential energy function for the internal rotation of 1,3-butadiene and its isotopologues

The gas-phase Raman spectra of 1,3-butadiene and its 2,3-d(2), 1,1,4,4-d(4), and -d(6) isotopologues have been recorded with high sensitivity in the region below 350 cm(-1) in order to investigate the internal rotation (torsional) vibration. Based on more accurate structural information, the internal rotor constants F(n) were calculated as a function of rotation angle (?). The data for all the isotopologues were then fit using a one-dimensional potential energy function of the form V = (1)/(2)∑V(n)(1 - cos ?). Initial V(n) values were based on those generated from theoretical calculations. The agreement between observed and calculated frequencies is very good, although bands not taken into account were present in the spectra. The energy difference between the trans and gauche forms was determined to be about 1030 cm(-1) (2.94 kcal/mol), and the barrier between the two equivalent gauche forms was determined to be about 180 cm(-1) (0.51 kcal/mol), which agrees well with high-level ab initio calculations. An alternative set of assignments also fits the data quite well for all of the isotopologues. For this model, the energy difference between the trans and gauche forms is about 1080 cm(-1) (3.09 kcal/mol), and the barrier between gauche forms is about 405 cm(-1) (1.16 kcal/mol).     

Barrier to internal rotation in 2,3- and 3,3-difluoropropenal and their methyl derivatives

The conformational stability and structure of 2,3-dimethylpropenal, 2,3-difluoropropenal and their 3,3-dimethyl and 3,3-difluoro derivatives were investigated utilizing ab initio calculations with 3-21G and 6-31G basis sets. For 2,3-dimethylpropenal and 3,3-difluoropropenal the s-trans was predicted to be the low-energy form. In the case of 3,3-dimethylpropenal and 2,3-difluoropropenal the s-cis was predicted by both levels of calculation to be the more stable conformer. Full optimization was performed at the transition states and the barriers to internal rotation were calculated. Methyl and fluorine substitution were found to significantly increase the barrier to interconversion in propenal. The relative change in the barrier depends on the position and the type of the substituent. The trans to cis barrier in 2,3-dimethylpropenal was calculated to be about 3 kcal mol⁻¹ greater than that in 3,3-dimethylpropenal, while the cis to trans barrier in 2,3-difluoropropenal was predicted to be about 7 kcal mol⁻¹ higher than the corresponding one in 3-3-difluoropropenal.