Presence Of Internal Rotation Research Articles

Determination of rotational constants in the presence of methyl internal rotation: 2-acetylfuran and 2-acetylthiophene as examples

A simple method to determine unperturbed rotational constants in the presence of internal rotation is proposed and applied on the microwave data sets of 2-acetylfuran and 2-acetylthiophene, featuring intermediate barriers to methyl internal rotation of about 300 cm−1. Two general approaches, global fits of all torsional species simultaneously and local fits of each torsional species separately, were compared. Based on the results, we conclude that the rotational constants derived from both approaches can be trusted for structure determination purposes, as long as the high barrier approximation is valid.

Theoretical study of internal rotational barriers of electrons donating and electrons withdrawing groups in aromatic compounds

The presence of internal rotation in sigma bonds is essential for conformational analysis of organic molecules and its understanding is of great relevance in chemistry, as well as in several other areas. However, for aromatic compounds that have substituent groups, withdrawers or donors of electron, there are no data in the literature to explain their rotational barriers. In this context, the work studied the internal rotational barriers of electron donating and withdrawing groups in aromatic compounds using the MP3, MP4, and CCSD(T) methods and the influence of substituents' nature on barrier heights was investigated through calculations based on the theory of Natural Bond Orbitals (NBO) and Quantum Theory of Atoms in Molecules (QTAIM). The results obtained showed that the CCSD(T) method is the one that best describes the internal rotational barriers, followed by MP4 and MP3 and the electron donating groups decrease the barrier, whereas electron withdrawing groups increase. Through the NBO analysis it was possible to observe that for withdrawing groups the interaction of the molecular orbitals is more accentuated promoting the increase of the rotational barrier of these compounds. Through the QTAIM analysis it was possible to show that, for electron donating groups, the internal rotation is influenced by the loss of electronic density when the substituents is perpendicular to the ring plane, however, for withdrawing groups the density is little influenced, regardless of the two conformations (minimum and maximum energy). Two molecules showed free rotation, trichloromethylbenzene and methylbenzene, and the theoretical calculations NBO and QTAIM showed that for these species there is no difference in the properties studied when there is rotation of the dihedral angle.

Ordered rate constitutive theories for non-classical thermoviscoelastic solids with memory incorporating internal and Cosserat rotations

This paper considers conservation and balance laws for non-classical solid continua in the presence of internal rotations ( $${}_i \pmb {\varvec{\varTheta }}$$ ) due to the Jacobian of deformation and Cosserat rotations ( $${}_e \pmb {\varvec{\varTheta }}$$ ) at each material point. In these balance laws, internal rotations are completely defined as functions of the displacement gradient tensor, but Cosserat rotations are additional three degrees of freedom at each material point. When these rotations are resisted by the deforming matter, conjugate moments are created. For thermoviscoelastic solids with memory, these result in additional energy storage, dissipation mechanism, and rheology. This paper presents a thermodynamically consistent derivation of constitutive theories for such solids based on the entropy inequality in conjunction with representation theorem. Material coefficients are derived and discussed. The constitutive theories are presented in the absence as well as in the presence of the balance of moment of moments as additional balance law for non-classical continuum theories, and the resulting theories are compared with classical continuum theories in which this balance law is not needed. Retardation moduli corresponding to the Cauchy stress tensor as well as the Cauchy moment tensor are derived. In this paper we only consider small strain, small deformation physics.

MILLIMETER/SUBMILLIMETER SPECTROSCOPY OF PH2CN (${\tilde{X}}$1A′) AND CH3PH2(${\tilde{X}}$1A′): PROBING THE COMPLEXITY OF INTERSTELLAR PHOSPHORUS CHEMISTRY

Millimeter/submillimeter spectra of PH{sub 2}CN ( X-tilde {sup 1}A') and CH{sub 3}PH{sub 2} ( X-tilde {sup 1}A') have been recorded for the first time using direct absorption techniques. This work extends previous measurements of both molecules beyond the 10-50 GHz range. Both species were created in the presence of an AC discharge by the reaction of phosphorus vapor and either cyanogen and hydrogen (PH{sub 2}CN) or methane (CH{sub 3}PH{sub 2}). Twelve rotational transitions of PH{sub 2}CN were recorded over the region 305-422 GHz for asymmetry components K{sub a} = 0 through 8. For CH{sub 3}PH{sub 2}, eight rotational transitions were measured from 210-470 GHz with K{sub a} = 0 through 16; these spectra exhibited greater complexity due to the presence of internal rotation, which splits the K{sub a} = 1, 2, and 3 asymmetry components into A and E states. Combined analyses of the millimeter/submillimeter and previous microwave data were performed for both molecules. For PH{sub 2}CN, the spectra were fit with a Watson S-reduced asymmetric top Hamiltonian, resulting in more accurate rotational and centrifugal distortion constants. In the case of CH{sub 3}PH{sub 2}, an asymmetric top internal-rotation Hamiltonian was employed in the analysis, significantly improving the rotational and torsionalmore » parameters over previous microwave estimates. Searches for both molecules were subsequently conducted toward Sgr B2(N), using the 12 m telescope of the Arizona Radio Observatory (ARO). Neither species was identified, with abundance upper limits, relative to H{sub 2}, of f (PH{sub 2}CN/H{sub 2}) 2 and >200, respectively.« less

Equilibrium structure in the presence of internal rotation: A case study of cis-methyl formate

The Born–Oppenheimer (BO) equilibrium molecular structure (r BO e )o fcis-methyl formate has been determined at the CCSD(T) level of electronic structure theory using Gaussian basis sets of at least quadruple-f quality and a core correlation correction. The quadratic, cubic and semi-diagonal quartic force field in normal coordinates has also been computed at the MP2 level employing a basis set of triple-f quality. A semi-experimental equilibrium structure (r SE e ) has been derived from experimental ground-state rotational constants and the lowest-order rovibrational interaction parameters calculated from the ab initio cubic force field. To determine r SE e structures, it is important to start from accurate ground-state rotational constants. Different spectroscopic methods, applicable in the presence of internal rotation and used in the literature to obtain ‘‘unperturbed” rotational constants from the analysis and fitting of the spectrum, are reviewed and compared. They are shown to be compatible though their precision may be different. The r BO e and r SE e structures are in good agreement showing that, in the particular case of cis-methyl formate, the methyl torsion can still be treated as a small-amplitude vibration. The best equilibrium structure obtained for cis-methyl formate is: r(Cm–O) = 1.434 A, r(O–Cc) = 1.335 A, r(Cm–Hs) = 1.083 A, r(Cm– Ha) = 1.087 A, r(Cc–H) = 1.093 A, r(C@O) = 1.201 A, \(COC) = 114.4, \(CCHs) = 105.6, \(CCHa) = 110.2, \(OCH) = 109.6, \(OCO) = 125.5, and s(HaCOC) = 60.3. The accuracy is believed to be about 0.001 A for the bond lengths and 0.1 for the angles.

Rotational modes of Ca2+-liganded calmodulin, as determined by time-domain fluorescence.

The time decay of fluorescence anisotropy was monitored as a function of pH and temperature for complexes of 2,6-toluidinylnaphthalenesulfonate with calmodulin, with its proteolytic fragments, and with the 1:1 complex of calmodulin and melittin. For all the conditions examined the anisotropy decay of native calmodulin involved at least two rotational modes. These corresponded to a short correlation time of 2-3 ns, reflecting a localized motion in the vicinity of the binding site and a longer correlation time which arises from the rotation of a major portion of the molecule. The relative amplitudes of the two rotational modes were dependent upon temperature in the range 11-40 degrees C, the contribution of the more rapid mode increasing with temperature. The maximum immobilization of the probe occurred at pH 5.0 and 12 degrees C. While these results indicate the presence of internal rotations in Ca2+-liganded calmodulin, the magnitude of the longer correlation time is consistent with the crystallographic structure.

Spin—rotational relaxation study of molecular reorientation in the presence of internal extended rotational diffusion

Molecular reorientation in the presence of internal rotation is investigated and an analytical expression for the spin—rotational rate of a nucleus attached to the internal rotor is obtained in terms of the internal angular-momentum correlation time. A model of a symmetric-top molecule undergoing anisotropic rotational diffusion is extended to include a modified extended diffusion of internal rotation. The result is applied to liquid toluene and the internal angular-momentum correlation time is evaluated from the 13C nuclear spin—rotational relaxation rate of the methyl carbon. A comparison with the previous result on the dipole—dipole relaxation data is made and the consistency of the present theory is discussed.

Properties of the complexes formed by 1-anilinonaphthalene-8-sulfonate with phosphorylase kinase and calmodulin.

AbstractPhosphorylase kinase contains four approximately equivalent binding sites for 1‐anilinonaphthalene‐8‐sulfonate (1,8‐ANS). Measurements of the time decay of fluorescence anisotropy have failed to give any indication of internal degrees of rotational freedom involving a significant portion of the tertiary structure. In the presence of 1 mM Ca2+, calmodulin binds one molecule of 1,8‐ANS. No binding occurs in the absence of Ca2+. The binding is strongly temperature‐dependent, a decrease in binding occurring with increasing temperature. Determinations of the time decay of fluorescence anisotropy indicate the presence of internal rotations, which become more important with increasing temperature. Complex formation between phosphorylase kinase and calmodulin reduces the binding of 1,8‐ANS.