Spin labeling methods in molecular biology : G.I. Likhtenshtein,John Wiley & Sons,New York,London, Sydney and Toronto, 1976, pp. xiii + 258, price £26.00

As a consequence of intramolecular vibrations distorted apparent structures may result from an electron diffraction analysis of molecules possessing symmetrical equilibrium configuration. The amount of torsional distortion gives information concerning the barrier height to internal rotation. An approach is suggested to estimate barrier heights on the basis of average torsional angles as determined from electron diffraction, and expressions of the rotation-dependent distances as obtained from a Taylor expansion by neglecting higher order terms.     

Zero point motion of the librating methyl group in p-hydroxyacetanilide

Single crystal pulsed neutron diffraction of p-hydroxyacetanilide (the analgesic paracetamol; C₈H₉NO₂) has been undertaken at ten temperatures between 20 K and 330 K. The librations of the methyl group have been analysed in terms of a torsional oscillator model assuming harmonic motion of the hydrogen atoms about the equilibrium configuration. Determination of the significant zero point motion by this method has allowed improved estimates of the force constant and barrier height for the methyl group oscillation to be obtained.     

Structural relaxations and energy effects of intramolecular motions inN,N-dimethylnitramine: A theoretical study

The equilibrium geometry of theN,N-dimethylnitramine molecule and changes in the energy and structural parameters due to the internal rotation of the nitro group and the inversion of the N atom in the amino fragment were calculated by the restricted Hartree-Fock (RHF) method and at the second-order Møller-Plesset (MP2) level of perturbation theory with inclusion of electron correlation using the 6–31 G^* and 6–31 G^** basis sets. The one-dimensional potential functions of these motions calculated at the RHF/6–31 G^* level were approximated by a truncated Fourier and power series, respectively. The frequencies of torsional and inversion transitions were determined by solving direct vibrational problems for a non-rigid model,i.e., taking into account the molecular geometry relaxation. The equilibrium conformation of the molecular skeleton ofN,N-dimethylnitramine is nonplanar. Transition states of the internal rotation of the nitro group and inversion of the amine N atom are characterized by pronounced concerted changes in its bond angles and the length of the N?N bond. In the MP2/6–31 G^* approximation, the height of the barrier to internal rotation calculated taking into account the difference in the zero-point vibrational energies is equal to 9.7 kcal mol^?1. Inversion in the amino fragment is accompanied by a relatively small energy change at the barrier height of ?1.0 kcal mol^?1 calculated in the same approximation.     

Torsional transitions and barrier to internal rotation of 1,2-butadiene

The far i.r. spectrum of 1,2-butadiene (methyl allene) has been recorded in the gas phase from 370 to 40 cm⁻¹ with a resolution of 0.1 cm⁻¹. The methyl torsional fundamental has been observed for the first time at 154.3 cm⁻¹, along with some accompanying torsional hot bands. From these data the barrier to internal rotation has been calculated to be 556 cm⁻¹ (1.59 kcal/mol). Detailed K-structure has also been observed for both A—A and E—E torsional transitions and considered in the analysis. SCF calculations have been made for the structure and energies of conformers, so that both kinetic and potential constants for internal rotation have been obtained. The a′ skeletal fundamental is observed at 201.8 cm⁻¹ as a much stronger band than the torsional mode, and the a″ skeletal fundamental gives rise to an even stronger band at 319.8 cm⁻¹.     

Equilibrium Structure and Internal Rotation in B2F4 from Electron Diffraction and Spectroscopic Data and Quantum Chemical Calculations

Electron diffraction (ED) data for B₂F₄ recorded by Hedberg et al. over the temperature range –80 to +150°C have been used to obtain equilibrium geometry of this molecule in the framework of a large-amplitude motion model. The torsional coordinate has been adiabatically separated from the rest of vibrations. Two types of constraints applied to obtain ab initio torsional potential energy function (PEF) and the parameters of the geometry relaxation are discussed. The relations between anharmonic interaction force constants and the parameters of the geometry relaxation are briefly considered. Ab initio force constant matrices for rigid vibrational coordinates as well as large-amplitude torsional PEF have been scaled in the procedure of simultaneous fitting to the ED data and experimental vibrational frequencies. The resulting equilibrium geometry and potential function provided good fit to both ED and spectroscopic data. As expected, the results for the equilibrium geometry obtained from separate ED patterns recorded at different temperatures did not show noticeable temperature trend. The determined equilibrium structural parameters for B₂F₄ are: r _e (B–B) = 1.719(4) Å, r _e (B–F) = 1.309(2) Å, BBF = 121.1(1)°. Uncertainties given in parentheses include three times standard deviation and a systematic error. The rotational barrier height was evaluated as 160(50) cm^–1.     

On the OH torsional motion in solid pentachlorophenol

After theoretical and experimental studies, an assignment is made at 409 cm^?1 for the OH torsional vibration in solid pentachlorophenol. An optimization method is applied to calculate the V₂ torsional barrier, which provides larger values than those obtained from the conventional methods used by other authors.     

Ring inversion of dihydronaphthalene by laser-induced fluorescence in a supersonic jet

The laser-induced fluorescence excitation (FE) and the single vibronic level fluorescence (SVL) emission spectra of 1,2-dihydronaphthalene have been studied in a pulsed supersonic jet. The 128 cm⁻¹ progression in FE and the 140 cm⁻¹ progression in SVL have been assigned to the low-frequency large-amplitude torsional motion between the two symmetry-related non-planar conformations of the excited and the ground states, respectively. Comparison of frequencies and intensifies with corresponding values calculated from a model double-well potential leads to a barrier height of 640 cm⁻¹ for the excited state and greater than 1000 cm⁻¹ for the ground state.     

Sensing the Molecular Structures of Hexan-2-one by Internal Rotation and Microwave Spectroscopy

Using two molecular jet Fourier transform spectrometers, the microwave spectrum of hexan-2-one, also called methyl n-butyl ketone, was recorded in the frequency range from 2 to 40 GHz. Three conformers were assigned and fine splittings caused by the internal rotations of the two terminal methyl groups were analyzed. For the acetyl methyl group CH₃ COC₃H₆CH₃, the torsional barrier is 186.9198(50) cm⁻¹, 233.5913(97) cm⁻¹, and 182.2481(25) cm⁻¹ for the three observed conformers, respectively. The value of this parameter could be linked to the structure of the individual conformer, which enabled us to create a rule for predicting the barrier height of the acetyl methyl torsion in ketones. The very small splittings arising from the internal rotation of the butyl methyl group CH₃COC₃H₆ CH₃ could be resolved as well, yielding the respective torsional barriers of 979.99(88) cm⁻¹, 1016.30(77) cm⁻¹, and 961.9(32) cm⁻¹.     

Computational Insights of Selective Intramolecular O-atom Transfer Mediated by Bioinspired Copper Complexes

The stereoselective copper-mediated hydroxylation of intramolecular C−H bonds from tridentate ligands is reinvestigated using DFT calculations. The computational study aims at deciphering the mechanism of C−H hydroxylation obtained after reaction of Cu(I) precursors with dioxygen, using ligands bearing either activated ( L¹ ) or non-activated ( L² ) C−H bonds. Configurational analysis allows rationalization of the experimentally observed regio- and stereoselectivity. The computed mechanism involves the formation of a side-on peroxide species ( P ) in equilibrium with the key intermediate bis-(μ-oxo) isomer ( O ) responsible for the C−H activation step. The P/O equilibrium yields the same activation barrier for the two complexes. However, the main difference between the two model complexes is observed during the C−H activation step, where the complex bearing the non-activated C−H bonds yields a higher energy barrier, accounting for the experimental lack of reactivity of this complex under those conditions.     

Two Equivalent Internal Rotations in the Microwave Spectrum of 2,6-Dimethylfluorobenzene

Large amplitude motion of methyl groups in isolated molecules is a fundamental phenomenon in molecular physics. The methyl torsional barrier is sensitive to the steric and electronic environment in the surrounding of the methyl group, making the methyl group a detector of the molecular structure. To probe this effect, the microwave spectrum of 2,6-dimethylfluorobenzene, one of the six isomers of dimethylfluorobenzene, was measured using two pulsed molecular jet Fourier transform microwave spectrometers operating in the frequency range from 2 to 40 GHz. Due to internal rotations of two equivalent methyl groups with relatively low torsional barriers, all rotational transitions split into quartets with separations of up to several hundreds of MHz. The splittings were analyzed and modeled to deduce a torsional barrier of 236.7922 (21) cm⁻¹. The results are compared to those obtained from quantum chemical calculations and with other fluorine substituted toluene derivatives of the current literature where the methyl group is adjacent to a fluorine atom.     

A model potential for the internal rotation of neighbouring rings of bithiophene and bipyrrole

Based upon ab initio Hartree-Fock calculations we propose a model potential for simplifying the study of the internal rotation of neighbouring rings. The quality of the proposed analytic potential was numerically verified in the case of bithiophene and bipyrrole by determining the deviations of the predicted energy values along the torsional angle with respect to 19 independent reference values. It is found that our model potential describes correctly the torsional motion the average deviations of the predicted energies along the torsional angle with respect to the ab initio reference energies are 0.20 and 0.22 kJ mol⁻¹ for bithiophene and bipyrrole, respectively. In both molecules, when going from a coplanar to a perpendicular conformation, two stable isomers have been detected. These isomers are separated by a barrier height of about 4 kJ mol⁻¹. Full optimization of the structural parameters along the torsional angle shows that they remain almost constant along the isomerization process.     

Microwave spectrum and the structure of benzoyl chloride

The microwave spectrum of benzoyl chloride was observed in the frequency range 12–18.6 GHz. Rotational constants have been obtained for the ground vibrational state, the first three excited torsional states of the COCl group, and one of the out-of-plane bending states. The residual inertial defect obtained from the ground and the torsional excited states indicates that the equilibrium conformation is planar. Ab initio MO calculations (STO-3G) showed the potential energy curve as a function of the COCl torsional angle to be rather flat around zero degrees.     

Internal axis molecular parameters, torsional energies and matrix elements of methyl mercaptan-D2

In this paper an internal axis method Hamiltonian model has been applied to evaluate the torsional rotational molecular parameters of asymmetrically substituted methyl mercaptan (CHD2SH) using previously observed microwave transitions. The torsional potential barrier function V2 has been obtained. The pure torsional energies and matrix elements between various torsional sub-levels up to the fourth excited torsional state in the ground vibrational state have been determined. The matrix elements and the torsional energies will be of great value to researchers seeking the spectrum of this molecule.     

One-photon mass-analyzed threshold ionization spectroscopy of 2-chloropropene (2-C3H5Cl) and its vibrational assignment based on the density-functional theory calculations

A high-quality mass-analyzed threshold ionization (MATI) spectrum of 2-chloropropene, 2-C3H5Cl, is reported. Its ionization energy determined for the first time from the 0-0 band position was 9.5395+/-0.0006 eV. Almost all the peaks in the MATI spectrum could be vibrationally assigned utilizing the frequencies calculated at the B3LYP6-311++G(3df,3pd) level and the Franck-Condon factors calculated with the molecular parameters obtained at the same level. In particular, the observed methyl torsional progression could be reproduced very well through quantum-mechanical calculations using the molecular parameters obtained at this level. Dramatic lowering of the torsional barrier inferred from the experimental data was entirely compatible with the B3LYP6-311++G(3df,3pd) results. The torsional barrier and the internal rotational constant determined by fits to six torsional peaks were 53.6 and 5.20 cm(-1), respectively. A brief discussion at the level of molecular orbital is presented to account for the dramatic lowering of the torsional barrier upon ionization.     

The experimental determination of the torsional barrier and shape for disilane

The torsional spectrum of disilane was recorded for the first time under high-pressure-pathlength conditions and at a spectral resolution of 0.007 cm(-1) using a Bruker IFS-120 HR Fourier transform spectrometer. The spectrum shows six distinct Q branches. The most prominent Q branch is near 130 cm(-1) which is a blend of four components of the torsional fundamental. Of the remaining five, four were assigned to the first torsional hot band (v(4)=2<--1) and one to the second torsional hot band (v(4)=3<--2). Over 350 transitions were identified. An analysis of the torsional fundamental, the first torsional hot band, and the lower state combination differences from frequencies of the vibrational bands nu(9) and nu(9)+nu(4)-nu(4) was made to characterize the torsion-rotation Hamiltonian in the ground vibrational state. The barrier height, barrier shape, and the rotational constant about the Si-Si bond were determined to be 404.344(83) cm(-1), 2.255(65) cm(-1), and 43208(28) MHz, respectively. Comparison of simulated and the experimental spectra yielded (mu||-mu(perpendicular))/mu(perpendicular)= -4(1) for the torsional dipole moments. This ratio compares well with -3.39(6) for ethane. A comparison of molecular parameters obtained here is made with those for methyl silane and ethane.     

Impact of molecular conformation on barriers to internal methyl rotation: the rotational spectrum of m-methylbenzaldehyde

The ground state spectrum of m-methylbenzaldehyde (m-MBA) was measured with a chirped-pulse Fourier transform microwave (CP-FTMW) spectrometer. The methyl rotor on m-MBA introduces an internal rotation barrier, which leads to splitting of the torsional energy level degeneracy into A and E states. Ab initio calculations predict a low torsional barrier for both the O-cis and O-trans conformers, resulting in a large doublet splitting up to several gigahertz in the frequency spectrum. The rotational constants, distortion terms, and V(3) values for both species have been determined from the ground state rotational spectrum using the BELGI-C(s) fitting program. There are significant differences in the torsional potential for the O-cis and O-trans m-MBA conformers. Molecular orbitals and resonance structures for each conformer are analyzed to understand the difference in torsional barrier height as well as the irregular shape of the O-trans torsional potential.     

A CNDO/2 study of the OH torsion in phenol and its hydrogen bonded complex with pyridine

The CNDO/2 molecular orbital method has been applied to the study of the OH torsion, in phenol and phenol—pyridine hydrogen bonded complex. The calculated torsional barrier (13.58 kJ mol^?1) and force constant (5.4 × 10^?20 J rad^?2) of phenol agree well with the experimental quantities. The calculated force constant of the corresponding vibration in phenol—pyridine is increased sixfold, reproducing closely the rise in the torsional frequency observed when phenol is complexed to strong acceptors. It is shown that according to CNDO theory, most of the increase can be attributed to the influence of the intermolecular force field and not to a major change in the torsional force constant.     

Analytical investigation of energy spectrums of beta rays emitted from <Superscript>90</Superscript>Sr and <Superscript>204</Superscript>Tl radioisotopes

The energy spectra of beta rays emitted from ⁹⁰Sr and ²⁰⁴Tl radioisotopes were obtained by using a silicon surface barrier detector with a 1000 μm depleted layer and 50 mm² effective area. The detector response function is interpreted by making use of range distributions of mono-energetic electrons in matter and by assuming a linear energy loss along the range in the depleted layer of the detector. An analytical expression is given for pulse height distribution obtained in the surface barrier detector. A good agreement is observed between the experimental results and theoretical interpretation.     

A conformational analysis of the six isomers of bipyridinium dication

The conformational analysis of the bipyridinium dications have been investigated using ab initio STO-3G calculations in which the inter-ring bond length was optimized at each torsional angle. The variation of the inter-ring separation has been rationalized using a heuristic valence bond model (which is electrostatic in nature) and which was suggested by the molecular orbital calculations. Furthermore, all six isomers show a double potential minima at the optimium torsional angles of ～ 50° and ～ 130° respectively. Moreover, all barrier heights are calculated to be less than 8 kJ mol^?1, thereby suggesting that the barrier heights are not large enough to prevent rapid interconversion between different rotamers.     

The methyl and methoxyl torsional modes and the lattice vibration in the low-frequency Raman spectrum of 1,4-dimethoxybenzene

The low-frequency (10–450 cm^?1) Raman spectra of solid (at 300 K and 130 K) and liquid (at 335 K) 1,4-dimethoxybenzene-d₀ and 1,4-dimethoxybenzene-d₅ have been measured. The methyl nad methoxyl torsional transitions have been identified and the corresponding torsional barriers calculated. Upon deuleration the methyl torsional barrier is reduced by 450 cm^?1, implying a coupling between the methyl torsion and a low-frequency ring mode. As far as the torsions are considered, the internal dynamic situation in 1,4-dimethoxybezene resembles that in amisole. A tentative assignment of the observed lattice bands in given. Certain changes in the spectrum when going from the solid to the melt are attributed to the coexistence of both cis and trans conformers in the liquid state.