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.     

Methyl isothiocyanate as a quasi-symmetric top. A reinterpretation of the microwave spectrum

The skeletal bending—internal rotation potential function for CH₃NCS has been determined from the microwave spectrum which gives the height of the barrier to linearity at 209 cm^?1 and the CNC equilibrium angle at 27.5°. An accidental near-resonance between the 2⁰ and 3³ skeletal bending states is found.     

Cooperative interactions of hydrogen bonds in proton-transfer processes involving water molecules. Simulation of biochemical systems

Quantum-chemical calculations of molecular complexes simulating the proton channel of influenza A virus and the proton-transfer system of the active site of carboanhydrase enzyme were performed. These complexes comprise a proton-donor and a proton-acceptor groups bridged by a chain of water molecules. Calculations of the methylimidazole (H⁺)-H₂O-CH₃COO^? complex as a model of influenza M₂ virus revealed free translation motion of the water molecule between the donor and acceptor, as well as concerted proton transfer in both H bonds. The barrier for proton transfer is independent of the position of the bridging water molecule and varies linearly with the difference in the electrostatic potentials between the donor and acceptor. With elongation of the H-bond bridge between the donor and acceptor groups, the H-bond lengths and proton shifts in the chain links vary periodically. This process can be defined as an H-bond deformation wave (proton wave). It was shown that motion of one proton along the H bond is associated with vibrational motion of protons in other links, which results in wave propagation along the chain. The calculation results allowed the rate of the proton wave and the time of proton transfer from the donor to acceptor to be estimated.     

Composite spectral density functions for the interpretation of the 13C NMR relaxation behavior of polymers containing hydrocarbon side chains. Free and restricted side chain motion

¹³C NMR NT₁ and NOE have been calculated by using composite spectral density functions describing polymer chain segmental motion and internal rotation of a hydrocarbon side chain attached to the polymer backbone. Numerical results at two magnetic fields are presented as a function of the various motional parameters characterizing the various models. NT₁ and NOE relaxation parameters are well behaved and appear to have practical value for describing the dynamics of these systems. The models have been applied to the relaxation data of poly(n-butyl methacrylate) and poly(n-hexyl methacrylate) in toluene solutions. The dynamics of the two polymers are characterized by a very localized backbone motion and restricted internal rotation about successive C? C bonds of the side chains. © 1995 John Wiley & Sons, Inc.     

NMR spectra of porphyrins. 26—conjugation versus steric repulsions in a planar chiral meso- Aminoporphyrin

The proton NMR spectra of the free base porphyrins γ-meso-dimethylaminopyrroetioporphyrin-XV (1) and the corresponding γ-diethylamino compound 2 show no temperature dependence, and are interpreted as being due to an essentially orthogonal conformation of the γ-NR₂ substituents. However, both of the corresponding dications 3 and 4, respectively, in CDCl₃–TFA solution, give temperature-dependent proton NMR spectra. Nuclear Overhauser enhancement difference spectra allow complete assignment of the spectrum of 3, and hence the interpretation of the temperature dependence. The spectra arise from a skew conformation of the ? NR₂ groups in which the barrier to rotation through the orthogonal conformation is ca 13.6 kcal mol (56.8 kJ mol^?1), and the barrier to rotation through the planar conformation is greater than this.     

Large Amplitude Torsions in Nitrotoluene Isomers Studied by Rotational Spectroscopy and Quantum Chemistry Calculations

Rotational spectra of ortho-nitrotoluene (2-NT) and para-nitrotoluene (4-NT) have been recorded at low and room temperatures using a supersonic jet Fourier Transform microwave (MW) spectrometer and a millimeter-wave frequency multiplier chain, respectively. Supported by quantum chemistry calculations, the spectral analysis of pure rotation lines in the vibrational ground state has allowed to characterise the rotational energy, the hyperfine structure due to the ¹⁴N nucleus and the internal rotation splittings arising from the methyl group. For 2-NT, an anisotropic internal rotation of coupled −CH₃ and −NO₂ torsional motions was identified by quantum chemistry calculations and discussed from the results of the MW analysis. The study of the internal rotation splittings in the spectra of three NT isomers allowed to characterise the internal rotation potentials of the methyl group and to compare them with other mono-substituted toluene derivatives in order to study the isomeric influence on the internal rotation barrier.     

Vibrational studies,barrier height and thermodynamic functions for biomolecules: 5-Trifluoromethyl uracil

Laser Raman (50–4000 cm⁻¹) and IR (200–4000 cm⁻¹) spectra of 5-trifluoromethyl uracil have been recorded and analysed. It has been possible to assign all the 39 (26a′+13a″) normal modes of vibration. Consistent assignments have been made for the internal modes of the CF₃ group, especially for the antisymmetric CF₃ stretching and bending modes. Using thus assigned vibrational frequencies and assumed structural parameters, thermodynamic functions, in the temperature range 100–1000 K, have been computed and the barrier to the internal rotation for the CF₃ top has been determined.     

Location of energy barriers. VII. Sudden and gradual late-energy-barriers

In the earlier papers of this series it was noted that barriers of “type II” (“Late barriers”, associated with substantially endothermic reaction) exhibited a large cross section if a given reagent energy was present predominantly as vibration in the bond under attack. It was found, however, that it was advantageous to retain in the reagents an amount of translation sufficient to surmount that part of the endothermic energy barrier which lay in the coordinate of approach; this could be identified with the repulsive part of the energy-release in the reverse, exothermic, reaction. In the present 3D classical trajectory study we compare S_r(V′)_T′+V′, for two endothermic surfaces with almost identical barrier heights and late barrier-crest locations, but differing fractions of the barrier in the coordinate of approach. For the surface IIS with a “sudden” rise to the barrier crest (implying substantial attractive energy-release in the reverse direction), S_r(V′_T′+V′ increased continually with V′, despite very low T′. By contrast surface IIG with “gradual” rise to the barrier crest (highly repulsive in the reverse direction), exhibited an S_r(V′_T′+V′ that passed through a maximum. Two further surfaces were investigated; surface IIHS resembling IIS but with a substantially higher barrier, and surface I, IIG with an intermediate barrier-location and hence a large fraction of the barrier along the coordinate of approach. The dynamics exhibited S-type (sudden) behaviour on IIHS, and G-type (gradual) behaviour on I, IIG. The presence of a significant fraction of the barrier on G-type surfaces along the endothermic coordinate of approach correlated with a more gradual curvature of the repulsive wall. This is likely to contribute to the greater availability of translation for barrier-crossing on G-type surfaces. Increased reagent rotation, R′, increased the reactive cross-section for endothermic reaction; the indications are that the mechanism by which rotation is effective involves vibration-rotation interaction.     

Molecular motions,the α relaxation,and chain transport in polyethylene crystals

Conformational energy calculations are used to investigate molecular motions in polyethylene crystals. From these a model is derived for the motion that accomplishes the net rotation translation that is believed to underlie the nuclear magnetic resonance (NMR) and dielectric α processes in polyethylenes and paraffins (and their dipole decorated derivatives). The resulting model is found to incorporate features of a number of previous models but differs significantly from all of them. The rotation is accomplished by means of a twisted (by 180°) region that propagates smoothly along the chain across the crystal. It differs from previous rotational models in that the twisted region is found to be rather localized (to ～12 CH₂ units). A dependence of activation energy on chain length (paraffins) or crystal thickness (polyethylenes), with the activation energy becoming independent of thickness in thick crystals, results not from the rotational lattice mismatch of the twisted region per se but from the translational lattice mismatch induced by the 180° rotation of one stem relative to the other. The twist differs from a stable point-defect twist previously proposed (Reneker defect) in that the chain torsion is relatively uniform through the twist and there is no shortening of the chain accompanying it. Further, the twist propagates smoothly without local barriers to its advance. Thus the propagating twist is to be through of as a transition state rather than a hopping defect. Detailed atomistic conformational energy calculations on C₂₂H₄₆ crystals were combined with a simplified elastic theory for translational stem mismatch in longer chains. From the combined calculations the activation parameters for twist propagation as a function of chain length or crystal thickness could be calculated. The results were compared with experiment for the dielectric α relaxation in paraffins containing dissolved ketones and polyethylenes containing carbonyl groups. The agreement is quite good, especially considering the paucity of adjustable parameters in the model. There is only some slight uncertainty in the calculated entropy of activation and a scattering correction was made a posteriori to account for a slow continued drop-off in relaxation rate in very thick crystals.     

An electron diffraction and CNDO/2 investigation of the molecular structure and internal rotation of hexafluorodisilane,Si2F6

The molecular structure and internal rotation of Si₂F₆ were investigated by electron diffraction of gases. The following r⁰_α -values for the geometric parameters were obtained: r(Si-Si) = 2.317 ± 0.006 Å, r(Si-F) = 1.564 ± 0.002 Å and ∠FSiF = 108.6° ± 0.3°. The barrier to internal rotation was found to be between 0.51 ± 0.10 and 0.73 ± 0.14 kcal mol^?1, depending on different assumptions of temperature drop due to gas expansion in the nozzle. Attempts were made to calculate the potential barriers for Si₂X₆ molecules with X as H, F and Cl, using the CNDO/2 approximation. When the 3d orbitais of silicon are taken into account, these results differ widely from the experimental values in the case of Si₂Fg₆ and Si₂Cl₆. Neglecting the 3d orbitais of silicon the theoretical and experi- mental potential barriers agree very well.     

STUDY OF THE MOLECULAR MOTION IN POLYEPICHLOROHYDRIN BY HIGH RESOLUTION NMR

The molecular motion in polyepichlorohydrin (PEPCH), in solution and bulk, was studied by high resolution NMR by means of line width, spin-lattice relaxation time T_1 and nuclear Overhauser effect NOE. The results show that the VJGM model can describe the main chain motion of PEPCH in solution perfectly. In bulk state, the relationship between the line width and the temperature is consistent with WLF equation, but that between the high frequency molecular motion correlation time (in T_1 scale ) and temperature is consistent with Arrhenius equation. The motion parameters of PEPCH in both states were calculated. The internal rotation motion of side—CH_2Cl group was analyzed by using equal three-site jump and diffusion internal rotation model in both states.     

Conformational Equilibria and Large‐Amplitude Motions in Dimers of Carboxylic Acids: Rotational Spectrum of Acetic Acid–Difluoroacetic Acid

We report the rotational spectra of two conformers of the acetic acid–difluoroacetic acid adduct (CH₃COOH–CHF₂COOH) and supply information on its internal dynamics. The two conformers differ from each other, depending on the trans or gauche orientation of the terminal ?CHF₂ group. Both conformers display splittings of the rotational transitions, due to the internal rotation of the methyl group of acetic acid. The corresponding barriers are determined to be V₃(trans)=99.8(3) and V₃(gauche)=90.5(9) cm^?1 (where V₃ is the methyl rotation barrier height). The gauche form displays a further doubling of the rotational transitions, due to the tunneling motion of the ?CHF₂ group between its two equivalent conformations. The corresponding B₂ barrier is estimated to be 108(2) cm^?1. The increase in the distance between the two monomers upon OH→OD deuteration (the Ubbelohde effect) is determined.     

Origin of threefold methyl torsional potential in methylindoles

In this article, we present a systematic study on mono-methylindoles to investigate the electronic origin of the threefold symmetric component (V ₃) of the methyl torsional potential barrier in the ground electronic state (S ₀). The structures and the torsional potential parameters of these molecules were evaluated from ab initio calculation using Hartree-Fock (HF), second order Mollar Plesset perturbation (MP2) and B3LYP density functional level of theories and Gaussian type basis set 6-31G(d, p). Natural bond orbital (NBO) analysis of these molecules were carried out using B3LYP/6-31G(d, p) level of calculation to understand the formation of the threefold V ₃ term arising from the changes of various non-covalent interactions during methyl rotation. Our analysis reveals that the contributions from π orbitals play a dominant role in the barrier height determination in this class of molecules. The threefold term in the barrier arises purely from the interactions non-local to the methyl group in case when the methyl group has two single bonds vicinal to it. On the other hand, it is the local interaction that determines the potential energy barrier when the methyl group has one single bond and one double bond vicinal to it. However, in all these cases, the magnitude of the energy barrier depends on the resonance structure formation in the benzene ring frame upon rotation of the methyl group and, therefore, the energetics of the barrier cannot be understood without considering the molecular flexing during methyl rotation.     

Dynamic model and tunneling splittings in LMH4 non-rigid hydrides

A non-rigid model for the molecules LiBH₄. NaBH₄⁺. LiCH₄⁺ is suggested on the basis of ab initio data. The model is used in numerical calculations of the levels of large-amplitude deformation-migration motion of the cation in these molecules. It is shown that the lowest states can be regarded as highly anharmonic bending vibrations, while levels above the barrier should be considered as hindered rotation of the MH₄ group. T_d symmetry is used to classify the states. Tunneling splittings of the vibrational states are 0.01–1 cm^?1.     

Molecular structure of octachlorotrisilane Si<Subscript>3</Subscript>Cl<Subscript>8</Subscript>

The parameters of the geometrical configuration of octachlorotrisilane Si₃Cl₈ were determined by quantum-chemical and gas-phase electron diffraction methods at 303±2 K. The calculated barrier to the internal rotation of SiCl₃ groups relative to the Si-Si bond was calculated.     

Internal motion in organosilicon polymers. I. Linear dimethylpolysilazane

A linear dimethylpolysilazane polymer has been prepared and its internal motion studied by the techniques of broadline nuclear magnetic resonance. Second moments and line widths are reported as a function of temperature from ?196 to 30°C. The results are compared to those for linear dimethylpolysiloxane, as well as to those for the crosslinked siloxane and silazane. The experimental low-temperature second moment of 7.25 ± 0.5 gauss² corresponds to a C₃ reorientation of methyl groups about the silicon–carbon bond. The motions occurring in the silazane are found to be the same as those in the corresponding siloxane. The transition to low second moments occurs approximately 30°C. higher in the linear silazane than in the linear siloxane. This is attributed to a somewhat greater resistance to motion in the silazane.     

Methyl rotation in polydimethylsiloxane studied by neutron transmission

Neutron transmission of polydimethylsiloxane has been measured as a function of neutron wavelength in the range 4–10 Å, at room temperature. Scattering cross sections per hydrogen atom have been obtained and the slope (12.2 ± 0.2) barns/Å has been derived. Comparison with calibration curves relating the slope to the barrier hindering internal rotation as well as comparison with calculated neutron cross sections using the Krieger–Nelkin formalism for different dynamical situations indicates practically free rotation of CH₃ groups about their C₃ symmetry axes.     

Molecular structure of C(SiCl<Subscript>3</Subscript>)<Subscript>4</Subscript> tetrakis-(trichlorosilyl)methane

Methods of quantum chemistry and gas phase electron diffraction (at a temperature of 303±5 K) are applied to determine parameters of the geometric configuration of C(SiCl₃)₄ tetrakis(trichlorosilyl)methane. Frequencies of the vibrational spectrum are calculated. The value of the internal rotation barrier of SiCl₃ groups relative to the Si-C bond is 148.7 kJ/mol.     

Photophysics and photochemical dynamics of methylanisole molecules in a supersonic jet

The fluorescence excitation, dispersed fluorescence and hole burning spectra, and fluorescence lifetimes of jet-cooled o-, m-, and p-methylanisoles (MA) were measured. The low-frequency ring methyl internal rotational bands observed for their S₀ and S₁ states were assigned. In the case of m-MA, the rotational isomers of cis and trans conformers, which arise from the orientation of the OCH₃ group with respect to the CH₃ group, were assigned by hole-burning spectroscopy. The observed level energies and relative intensities of the methyl internal rotation were reproduced by a calculation using a free rotor basis set. Furthermore, their potentials in the S₀ and the S₁ states were determined. The potential barrier heights for the S₀ states of m- and p-MA were quite low, suggesting that the methyl groups are freely rotating, while changing from S₀ to S₁ states, the potential barrier height increases. The potential barrier heights of o-MA drastically decreased in going from S₀ to S₁ states. The decrease would be due to the hydrogen bonding between O atom and one H atom of the methyl group. The torsional bands of the methoxy group (–OCH₃) were also observed for p- and o-MA. The –OCH₃ modes are found to couple with the level of the e species for the methyl internal rotation.Fluorescence lifetimes (τ_f) of the methyl internal rotational bands in the S₁ states of o-, m-, and p-MA were measured in order to investigate the photochemical dynamics. The values of the nonradiative rate constant (k_nr) were estimated from the τ_f values and Franck–Condon factors. The k_nr values drastically increased with the excitation of methyl internal rotation. Accordingly, the methyl internal rotation should enhance the nonradiative process, presumably intersystem crossing (ISC). The enhancement should be caused by the increase of the state density (ρ) effectively coupled with triplet manifolds. The drastic increase in the ρ value should be caused by level mixing. In addition, the methyl internal rotational motion may enhance the increase of the coupling matrix elements through the vibronic coupling between the excited singlet states. The remarkable rotational quantum species dependence on the ISC rate constant (k_ISC) value clearly appeared in m-MA. The dependence should result from the difference of the ρ value between a₁ and e species, since the e species are doubly degenerate. The species dependence was apparently related to the potential barrier height, suggesting that the large barrier height should have an influence on the ρ value of the triplet states.     

Partitioning of methyl internal rotational barrier energy of thioacetaldehyde

The nature of methyl internal rotational barrier in thioacetaldehyde has been investigated by relaxation effect, natural bond orbital (NBO) analysis and Pauling exchange interactions. The true experimental barrier can be obtained by considering fully relaxed rotation. Nuclear-electron attraction term is a barrier forming term in the fully relaxed rotation, but it appears as an antibarrier for rigid rotation. It is seen that during methyl rotation, the torsional mode is coupled with the aldehydic hydrogen out-of-plane wagging motion. Natural bond orbital analysis shows that the principal barrier forming term originates from the C-C bond. The lengthening of the C-C bond is explained by considering charge transfer interaction between several bonding and antibonding orbitals in the C-C bond region, which leads to higher bonding overlap for the eclipsed conformer compared to the staggered conformer. S-C(σ)/C^me-Hp and C-H_ald/C_me-H_op interactions appear to be the main barrier-forming Pauling exchange terms but have less contribution to make to the barrier compared to the C-C bond interaction.