Internal rotation in nitric acid

The torsional region of the gas phase infrared spectrum of nitric acid and nitric acid-d₁ has been reassigned, showing that besides a ground state torsional series also torsional series due to excited deformational states are present. Barriers to internal rotation have been calculated.     

Internal rotation and pyramidal inversion in triacylphosphines

MO calculations predict a P-C rotation barrier in triformylphosphine of less than 6 kcal/mol, and an inversion barrier of ca 14 kcal/mol.     

Muonium-substituted transient radicals observed by muon spin rotation

We report the first direct observation of radicals formed by formal addition of muonium to liquid organic compounds. They are characterized by muon precession frequencies in transverse magnetic fields of 0.3–5.0 kG. Comparison of the isotropic hyperfine coupling constants with those of hydrogen analogue reveals large isotope effects.     

Internal rotation in molecules dispersed in polymer matrices

Studies of certain polar or photochromic molecules dispersed in polymer matrices show that internal motion may be profoundly affected by the matrix. Thus, in polystyrene, dielectric studies of relatively small molecules show that the inversion motion of the central ring of xanthene becomes possible at temperatures below the recognized β and glass transition temperatures of the polymer. On the other hand, movement of the central oxazole ring in 2,5-diphenyl oxazole becomes marked at temperatures corresponding to the onset of polymer β motion, but dipole reorientation of diphenyl ether is not appreciable below the main glass transition.Considering a slightly larger molecule, the bleaching rate of the coloured isomer of 6′-nitro-1,3,3, trimethylindolinobenzopyrylospiran also depends on the nature of the polymer matrix. The observations are best explained in terms of a matrix-controlled internal rotation in which the photochromic molecules exist in a distribution of environments in the glassy matrix.     

ESR study of the rotation of peroxy radicals in polymers

The nature and the frequencies of rotation of peroxy radicals in polyethylene and polytetrafluorethylene are determined over a wide temperature range theoretically calculated ESR spectra.     

Internal rotation and framework relaxation in ethene thiol

New and existing data on the ground and excited CS torsional states of the stable rotamers of ethene thiol have been analysed to give a potential function for internal rotation around the CS bond. The barrier between the syn and anti rotamers is found to be 800 cm^?1 and that at the planar anti conformation itself to be 12 cm^?1. the syn conformation is 50 cm^?2 more stable than the ‘quasi-planar’ anti conformation Satisfactory reproduction of the observed trends in rotational constants with torsional excitations has been achieved by introducing extensive relaxation of the CCS angle and the CS bond length during internal rotation. The CCS angle reduces by up to 5° v'hilst the CS bond length increases by up to 0.02 A during rotation from the syn to anti conformation. The use of ab initio M.O. calculations to provide an indication of the most significant aspects of structure relaxation is described.     

Internal rotation in peroxynitrous acid (ONOOH)

Using higher levels of wave-function-based electronic structure theory than previously applied, as well as density functional theory (B-LYP and B3-LYP functionals), all theoretical models conclude that three ONOOH conformers are stationary point minima, in disagreement with some of the previous studies that we survey. In order of increasing energy, these are the cis-cis, cis-perp, and trans-perp conformers. Basis sets including diffuse functions seem to be needed to obtain a qualitatively correct representation of the internal rotation potential energy surface at higher levels of theory. Internal rotation about the peroxide bond involving the cis-cis, cis-gauche transition structure (TS), cis-perp, and cis-trans TS conformers is studied in detail. To help ascertain the relative stability of the cis-perp conformer, multireference configuration interaction energy calculations are carried out, and rule of thumb estimates of multireference character in the ground-state wave functions of the ONOOH conformers are considered. CCSD(T)/aug-cc-pVTZ physical properties (geometries, rotational constants, electric dipole moments, harmonic vibrational frequencies, and infrared intensities) are compared with the analogous experimental data wherever possible, and also with density functional theory. Where such experimental data are nonexistent, the CCSD(T) and B3-LYP results are useful representations. For example, the electric dipole moment |mu(e)| of the cis-cis conformer is predicted to be 0.97+/-0.03 D. CCSD(T) energies, extrapolated to the aug-cc-pVNZ limit, are employed in isodesmic reaction schemes to derive zero Kelvin heats of formation and bond dissociation energies of the ONOOH stationary point minima. In agreement with recent gas-phase experiments, the peroxide bond dissociation energies of the cis-cis and trans-perp conformers are calculated as 19.3+/-0.4 and 16.0+/-0.4 kcalmol, respectively. The lowest energy cis-cis conformer is less stable than nitric acid by 28.1+/-0.4 kcalmol at 0 K.     

Internal rotation and phase transitions in lanthanum tetrahydroborates

Conclusion The investigation carried out shows that in the low-symmetry compouds La(BH₄)₃ and La(BH₄)₃·3THF, as in the high-symmetry MBH₄, phase transitions occur. An unexpected result is the fact that the symmetry of the environment of the boron ions for these phase transitions practically does not change and remains close to cubic. The activation energy for reorientation of the BH _– ⁴ ion (1.2 kcal/mole) is somewhat overstimated, if we start from the difference in the electronegativity of the boron and hydrogen ions (2.0 and 2.15). This may mean that the nature of the barrier to reorientation of the BH _– ⁴ anion in lanthanum tetrahydroborates is connected with the van der Waals interaction of the tetrahedral BH _– ⁴ groups with the environment. From the¹³⁹La NMR data it follows that the coordination sphere of the triply-charged lanthanum is characterized by low-symmetry of the arrangement of the BH_4– and THF (OC₄H₈) ligands, and the La³⁺-(BH₄)^– bond has predominantly ionic character. Consequently, the potential energy of the BH _– ⁴ anions in the lattice is the sum of the strong isotropic ionic component and the substantially weaker anisotropic van der Waals component. In the low-temperature phases, for both compounds we observe structurally nonequivalent positions for the BH _– ⁴ anions, retained in the high-temperature phase. The phase transitions in the studied compounds are connected with a change in the nature of the hydrogen atom distribution. Due to the low height of the barriers, the reorientational mobility sets in even at low temperatures, and in the phase transition region the value of _c reaches 10¹⁰–10¹¹ Hz. For such a high reorientational frequency, there may be substantial effects from correlation rotation of the anions, leading to a change in the hydrogen atom distribution in the structure and an order-disorder type phase transition.Institute of Inorganic Chemistry, Siberian Branch, Academy of Sciences of the USSR. Translated from Zhurnal Strukturnoi Khimii, Vol. 29, No. 1, pp. 69–72, January–February, 1988.     

Internal rotation and rotational isomerism in substituted ethanes

The types of potential energy curves of internal rotation in substituted ethanes were studied. Formulas for the calculation of the energy differences in rotational isomers and of the potential barriers to internal rotation in these derivatives were obtained by Bernstein's method. As a rule, the calculated results are in agreement with experiment.     

Internal rotation spectra of HONO and HSSH

Two numerical methods are used to solve the time independent Schrödinger equation for the torsional potential functions for HONO and HSSH. High accuracy energy levels are obtained with a relatively low computational cost. Theoretical results are compared with the experimental data and different aspects of the dynamic of internal rotation are discussed.     

Internal rotation in conjugated ethylenes: Dynamic nmr study

The kinetic and activation parameters for the rotation about the CC double bond and the C-N single bond in the compounds listed in Table 1 were determined by variable temperature NMR spectroscopy. Complete line shape analyses, using a computer REEP program, were carried out on several systems. The main objective of the present study is to determine the effect of the ring size in II on the two types of kinetic processes.     

Internal rotation study of some sixfold barrier molecules

Molecular orbital calculations of sixfold barriers in nitromethane, methyl boron difluoride, and trifluoronitromethane were performed by various Hartree-Fock and electron correlation methods. In those calculations, staggered and eclipsed conformations are of primary concern. These results indicated that for CH₃NO₂ and CH₃BF₂ the staggered conformations are more stable, while CF₃NO₂ has a more stable conformation in an eclipsed form. Both conformations do not differ significantly, which may account for the low internal rotational barrier of each molecule. Values of the barrier calculated by the Møller-Plesset perturbation and the quadratic configuration interaction methods did not match the experimental results. However, better internal rotational barrier values of each molecule were observed when the improved better basis sets and the Hartree-Fock method were selected. © 1997 John Wiley & Sons, Inc.