Internal Rotation In Molecules Research Articles

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.

Internal Rotation of Molecules with Two Inequivalent Methyl Groups: The Microwave Spectrum of N-Methylethylidenimine

The microwave spectra of two isotopic species of trans-N-methylethylidenimine (CH3CH=NCH3) were recorded between 8 and 40 GHz. The rotational transitions clearly demonstrate the coupling with the internal motions of the two methyl groups, which are inequivalent in this molecule. The theory for the over-all and internal rotation was extended to molecules with two inequivalent symmetric internal rotors. The internal axis method (IAM) was used to calculate the internal rotors' splittings of the rotational transitions. They were split into five components according to the different irreducible representations of the symmetry group. In addition to this, the spectra were further complicated by the nuclear quadrupole multiplets of the nitrogen. A least squares analysis of the measured transition frequencies yielded the following rotational and quadrupole coupling constants for CH3CH=NCH3: A=38258.627± 0.048 MHz, B=4078.1354± 0.0058 MHz, C=3862.7391± 0.0072 MHz, χaa=1.578± 0.039 MHz, χbb=−4.601± 0.037 MHz, χcc=3.023± 0.041 MHz; and for CH3CD=NCH3: A=32982.409± 0.050 MHz, B=4076.6142± 0.0066 MHz, C=3799.5528± 0.0079 MHz, χaa=1.551± 0.038 MHz, χbb=−4.595± 0.023 MHz, χcc=3.044± 0.036 MHz. The mean values for the barriers of the two methyl groups were found to be 1642± 19 cal/mole for the C–CH3 top and 2109± 84 cal/mole for the N–CH3 top. A dipole moment of 1.499± 0.007 D was determined from the measured Stark splittings.

Role of nonbonded intramolecular forces in barriers to internal rotation in molecules with two equivalent methyl groups

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTRole of nonbonded intramolecular forces in barriers to internal rotation in molecules with two equivalent methyl groupsKenneth C. InghamCite this: J. Phys. Chem. 1972, 76, 4, 551–553Publication Date (Print):February 1, 1972Publication History Published online1 May 2002Published inissue 1 February 1972https://doi.org/10.1021/j100648a017RIGHTS & PERMISSIONSArticle Views19Altmetric-Citations6LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (378 KB) Get e-Alerts

Hindered Internal Rotation and Dielectric Relaxation

A model calculation of the dielectric behavior of a liquid whose molecules have conformation-dependent dipole moment components is presented. The molecules contain two groups of equal size which interact according to parabolic and cosine potentials of varying symmetry. The calculated dielectric behavior is discussed in terms of the model parameters. Measurements of dielectric permittivities and losses of 4-bromobiphenyl in dilute solution with carbon tetrachloride at 20, 30, and 40°C are interpreted in light of the model calculation. Measurements of dielectric permittivities and losses of 2,2′-bipyridine and 2,2′-dibromobiphenyl in dilute solution with carbon tetrachloride at 10–40°C are interpreted in light of a model calculation involving hindered internal rotation reported previously. The advantages and limitations of the dielectric method for characterizing hindered internal rotation in molecules in the liquid state are discussed.

The study of internal rotation in molecules by microwave spectroscopy

This article outlines some of the ways in which microwave spectroscopy can yield information pertaining to internal molecular movements. The main experimental features of the technique are summarized and the effect of hindered internal rotation of a methyl group on the overall rotational spectrum is briefly discussed. Illustrative examples from current research work include the determination of methyl barriers in substituted methyl formates, tunnelling of hydrogen in propargyl alcohol, cis and trans isomerism in acrylic acid and internal rotation of two methyl groups in isopropyl fluoride.

On Polytypism and Internal Rotation

The ordered stacking of eclipsed (E) and staggered (S) layers in SiC or ZnS crystals leads to formation of polytype lattices. The short-range forces acting between layers are considered to be analogous to those determining the barriers to internal rotation in molecules. This is applied to an understanding of eclipsing in covalent lattices (SiC, diamond) and in the more ionic lattices such as ZnS. The Madelung energies are calculated for the prevalent polytype structures, and there is a small effect favoring a cooperative formation of the completely eclipsed lattice, supporting Schneer's proposal that the zincblende–wurtzite transition is second-order. Configurational and vibrational entropy effects are not found to contribute to the stabilization of polytype lattices. The progressive distortions in these lattices with increasing numbers of E layers are attributed to “internal-rotation” forces and may be the explanation of polytype formation. A simple model, successfully applied to ethane, is used to calculate possible distortions in the recently prepared hexagonal diamond lattice. An undistorted lattice, not significantly more stable than the cubic structure, was obtained. Electron–nuclear interaction terms dominate over nuclear–nuclear terms, just the opposite of ethane.

Internal Rotation in Completely Asymmetric Molecules. I. A General Theory and Analysis of the Microwave Rotational Spectrum of CH2DCOH, CD2HCOH, and CHOOCH2D

A theory for the internal rotation in molecules where the internal rotor is not a symmetric top has been formulated. The principal feature of the theory is the expansion of the rotational constants for the molecule in a converging series of sinnα and cosnα. This allows for the straightforward evaluation of certain integrals appearing in the development of the theory. In addition, the theory is applied specifically to CH2DCOH, CD2HCOH, and CHOOCH2D in the analysis of the microwave rotational spectra for these molecules. For CH2DCOH the height of the potential barrier hindering the internal motion is found to be V=410 cm—1, which is consistent with the barrier height for other species of acetaldehyde. To fit the spectra of CD2HCOH and CHOOCH2D a slight rotation of the effective principal axes must be made to obtain a consistent potential barrier. The results of the calculation indicate that (1) the theory is readily applicable in the determination of the barrier heights of the more general molecules, and (2) investigation of the deuterated-methyl internal rotors could provide information on the structure of the rotor.

Internal Rotation in Molecules with Two Internal Rotors: Microwave Spectrum of Acetone

The theory of internal rotation in molecules is extended to the case of two internal symmetric rotors attached to a molecule with C2v symmetry. The calculation of the internal rotational energy levels and of the interaction with the over-all rotational energy levels is discussed both for the high barrier and the low barrier approximation. The symmetry of the Hamiltonian for a molecule with the acetone structure is considered and the selection rules and relative intensities of the various fine structure components are derived. The microwave spectra of acetone and of completely deuterated acetone have been measured and assigned. From the rotational constants, the molecular structure has been partially determined. The most probable values of the structural parameters are r0(C=O) = 1.215 A (assumed), r0(C–C) = 1.515±0.005 A, r0(C–H) = 1.086±0.010 A, ∠CCC = 116°14′±1°, and ∠HCC = 110°16′±1°. The angle between the axes of the methyl groups is approximately 3° larger than the ∠CCC. The dipole moment has been determined to be 2.90 D by a measurement of the Stark effect on the microwave transitions. Each of the rotational transitions is split into a number of components by the internal rotation. From an analysis of these patterns the barrier height to internal rotation is found to be 274 cm—1 for (CH3)2CO and 258 cm—1 for (CD3)2CO. The difference in the two calculated barrier heights and the errors in the method are discussed.

Valence Bond Calculation of the Barrier to Internal Rotation in Molecules

An approximate calculation of the barrier to internal rotation in ethane was made using a set of covalent valence bond structures involving only s and p orbitals. The calculated result is sufficiently great that it does not appear that other effects are needed to explain the barrier height.

Internal Rotation in Double Internal Rotor Molecules: The Microwave Spectrum of Dimethyl Silane

Internal Rotation in Double Internal Rotor Molecules: The Microwave Spectrum of Dimethyl Silane

Polytypism and the Origin of the Potential Barrier Hindering Internal Rotation in Molecules

Polytypism and the Origin of the Potential Barrier Hindering Internal Rotation in Molecules

Internal rotation of methyl groups in cadmium dimethyl.

In view of current interest in the problem of free or restricted internal rotation in molecules, we may mention briefly some results which indicate free rotation of methyl groups about the Cd—C bond in cadmium dimethyl. We have already described1 measurements on the rotational fine structure of some vibrational bands of zinc dimethyl and mercury dimethyl, which provided strong evidence for such free internal rotation in these molecules. The infrared absorption spectrum of cadmium dimethyl has now been measured with greater resolving power than that previously used by Gutowsky2, particularly in the region of 3 µ.

Internal Rotation in Molecules with Two or More Methyl Groups

Internal Rotation in Molecules with Two or More Methyl Groups