Theoretical Analysis of the Conformation of the Skeletal Base of Epoxysaccharide Molecules by the Vibrational Spectra

To explain the influence of the conformation of the skeletal base of the epoxysaccharide molecule on the frequencies and forms of normal vibrations, we have made a comparative theoretical analysis of the frequencies, forms, and potential energy distributions of normal vibrations in the 4001500 cm^–1 spectral range of five conformers of the epoxysaccharide methyl3,4anhydroD,Lallopyranoside molecule that approximate conformations of the hexapyranose rings close to the halfchair and boat forms. It is shown that a change in the structure of the molecule leads to frequency shifts and distortions of the forms of normal vibrations practically throughout the spectral range under consideration. The character of these changes is specific for different spectral ranges. The most sensitive to conformational changes in the pyranose ring are normal vibrations localized mainly within this ring, and the least sensitive ones are those localized on the bonds lying on the molecule's periphery. The frequency shift of normal vibrations comparable in form in the lowfrequency region in the series of conformers can reach 100 cm^–1. It is shown that the 650800 cm^–1 spectral range is the most convenient for identifying the form of the epoxypyranose ring. The changes in the force field associated with the conformation transitions of the pyranose ring are analyzed.     

Electron diffraction and quantum-chemical studies of the molecular structure of 2-methylbenzenesulfochloride

A combined electron diffraction and quantum-chemical (MP2/6-31G**) study of the molecular structure of 2-methylbenzenesulfochloride at 336(5) K was carried out. It was found that the gas phase contained only one conformer, C ₁. The following structural parameters were obtained: r _h1(C-H)_av = 1.095(8) Å, r _h1(C-C)_Ph = 1.402(4) Å, r _h1(C_Ph-C_meth) = 1.507(13) Å, r _h1(C_Ph-S) = 1.763(6) Å, r _h1(S=O) = 1.418(4) Å, r _h1(S-Cl) = 2.048(5) Å, ∠(H-C-H)_meth/av = 107.3(96)°, ∠(Cl-S-O)_av = 106.4(3)°, ∠C_Ph-S-Cl = 100.8(9), ∠O=S=O = 120.8(10)°. The C_C-C_S-S-Cl torsion angle that defines the position of the S-Cl bond relative to the plane of the benzene ring is 75.6(20)°. The B3LYP/6-311+G** calculated barriers of internal rotation of the methyl and sulfochloride groups are 1.2 kcal/mol and V ₀₁ = 10.2 (V ₀₂ = 4.1) kcal/mol, respectively.     

Joint X-ray spectroscopic and quantum-chemical study of the electronic structure of pentafluorophenylalkyl ethers

The high resolution X-ray emission O-Kα spectra of pentafluorophenylalkyl ethers C₆F₅OR (R=Et, Prⁱ, and Bu^t) exhibit differences related to a change in the electronic structure of the compounds as R is varied. The search for stable conformers was performed by the semiempirical PM3 method. The most probable structures of C₆F₅OR were determined by the comparison of the experimental and theoretical X-ray spectra plotted for each conformer usingab initio calculations in the 6–31 G basis set. Substituent R in pentafluorophenylalkyl ethers is situated outside of the ring plane. The fluorination of the benzene ring changes the energy level of the lone electron pair of oxygen relative to the levels of orbitals of the ring and substituent R and leads to an increase in the efficiency of interactions in the σ-system. Deceased. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 12, pp. 2443–2450, December, 1998.     

IR spectra temperature changes and conformational structure of 4-butyl-4′-cyanobiphenyl

IR spectra of 4-butyl-4′-cyanobiphenyl in the liquid and solid-crystal states were measured in the 400–4000 cm^–1 region at 28–70°C. Changes of the spectra as a function of temperature are due to the conformational fluxionality of the molecules according to calculated IR spectra of the possible 4-butyl-4′-cyanobiphenyl conformers. Calculations were carried out using the method of fragments and the LEV-100 program package. Bands sensitive to the conformational changes in the molecular structure were found. IR spectra have been interpreted. Conclusions have been made about the conformations of the sample in the solid-crystal and liquid phases. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 76, No. 3, pp. 325–333, May–June, 2009.     

Ranking the effect of [1A(ax), 1B(eq)] versus [1A(eq), 1B(ax)] cyclohexane ring substitution on the H chemical shifts of γ-methylene cyclohexane ring protons using 2,2-disubstituted adamantanes as models

When two different substituents are placed in the nonbridgehead position of adamantane, the two [1A(ax), 1B(eq)] and [1A(eq), 1B(ax)] cyclohexane chair conformers are modeled and features of their NMR spectra can be studied from a single spectrum at 298 K. The effect of [1A(ax), 1B(eq)] and [1A(eq), 1B(ax)] cyclohexane ring substitution on the ¹H resonance separation within the γ-CH₂s of cyclohexane ring is compared for various substituent pairs; this aim is approached by measuring the ¹H chemical shift separation within the 4′,9′-H and 8′,10′-H methylenes from the ¹H NMR spectrum of the model 2A,2B-disubstituted adamantane at 298 K.     

Electrostatic forces that determine O,O-trans versus O,S-cis conformers in the aminated porphyrin complexes of Cd(N-NHCO-2-C4H3O-tpp)(OAc) and Cd(N-NHCO-2-C4H3S-tpp)(OAc)

Two new diamagnetic, mononuclear and aminated porphyrin complexes of O,O-trans-Cd (3-trans) and O,S-cis-Cd (4-cis) have been synthesized and characterized by ¹H, ¹³C NMR spectroscopy. The crystal structures of (acetato)(N-2-furancarboxamido-meso-tetraphenylporphyrinato)cadmium(II) [Cd(N-NHCO-2-C₄H₃O-tpp)(OAc); 3-trans] and (acetato)(N-2-thiophenecarboxamido-meso-tetraphenylporphyrinato)cadmium(II) [Cd(N-NHCO-2-C₄H₃S-tpp)(OAc); 4-cis] were determined. The coordination sphere around Cd²⁺ is a distorted square-based pyramid in which the apical site is occupied by a bidentate chelating OAc⁻ group for 3-trans and 4-cis. The plane of three pyrrole nitrogen atoms [i.e., N(1), N(2), N(4) for 3-trans and N(1), N(2), N(3) for 4-cis] strongly bonded to Cd²⁺ is adopted as a reference plane 3N. The N(3) and N(4) pyrrole rings bearing the 2-furancarboxamido (Fr) and 2-thiophenecarboxamido groups in 3-trans and 4-cis, respectively, deviate mostly from the 3N plane, thus orienting separately with a dihedral angle of 33.4° and of 31.0°. In 3-trans, Cd²⁺ and N(5) are located on different sides at 1.06 and −1.49 Å from its 3N plane, while in 4-cis, Cd²⁺ and N(5) are also located on different sides at 1.04 and −1.53 Å from its 3N plane. An attractive electrostatic interaction between the Cd²⁺ and O(4)⁻ atoms in furan stabilizes the O,O-trans conformer of 3. A repulsive electrostatic interaction between Cd²⁺ and S(1)⁺ destabilizes the O,S-trans conformer of 4. Both of these repulsive and the mutually attractive interactions between S(1)⁺ and O(3)⁻ atoms favor the O,S-cis rotamer of 4 both in the vapor phase and in low polarity solvents. NOE difference spectroscopy, HMQC and HMBC were employed for the unambiguous assignment of the ¹H and ¹³C NMR resonances of 3-trans and 4-cis in CDCl₃ at 20 and −50 °C.     

The millimeter- and submillimeter-wave spectrum of the trans-gauche conformer of diethyl ether

The results of a first investigation of the rotational spectrum of the trans-gauche conformer of diethyl ether are reported. Two spectrometers have been used to measure the spectrum in the millimeter-wave and submillimeter-wave regions and a total of 1090 absorption line frequencies in the range 108-366 GHz were obtained and analyzed. Of these lines, 902 were measured with a spectrometer employing the fast scan submillimeter spectroscopic technique (FASSST) at Ohio State and the remaining 188 were measured with the phase-lock two-loop system (PLL) in Warsaw. The spectrum was fit to within experimental accuracy with the use of the A-reduced Watson Hamiltonian. Based on relative intensity measurements, the percentage of diethyl ether at room temperature in the trans-gauche conformer was found to be 30.5(13)%, in good agreement with prior spectroscopic values and an ab initio determination based on an energy difference of 5.40 kJ mol⁻¹ (452 cm⁻¹) between the excited trans-gauche and ground trans-trans conformers. This work also stimulated a critical evaluation of the data acquisition and calibration procedure of the FAAAST spectrometer, the results of which will be discussed.     

Peculiarities of the structure and intermolecular interactons in 4-n-ALKYL-4′-cyanobiphenyls studied by IR spectroscopy

Experimental and theoretical methods of IR spectroscopy have been used to study the peculiarities of the structure and of intermolecular interactions in the homologous series of 4-n-alkyl-4′-cyanobiphenyls (abbreviated to nCBs, where n is the number of carbon atoms in the alkyl radical). IR absorption spectra of nCBs (n = 4, 8, 9) were measured in the frequency range from 400 cm^?1 to 4000 cm^?1 and in the temperature range from 26° to 100°C, which corresponded to the crystalline, liquid crystalline and liquid states of the samples. The analysis of the measured spectra was performed with regard to available X-ray data on the structures of crystalline phases and to the theoretical modeling of IR spectra; a comparison with previously studied spectra of nCBs with n = 2 and 5 was made. Characteristic differences between the spectra of higher and lower homologues of nCBs were found. Changes in the conformation of nCB molecules with the change of temperature and the length of the alkyl radical are discussed.     

Corannulene Molecular Rotor with Flexible Perfluorobenzyl Blades: Synthesis,Structure and Properties

Two members of a new class of organic‐acceptor perfluorobenzyl corannulenes were prepared by gas‐phase and highly‐selective solution‐phase reactions at elevated temperatures. The peculiar single‐crystal X‐ray structure of C₅‐C₂₀H₅(CF₂C₆F₅)₅ revealed two high‐energy conformers with drastically different bowl depths and orientations of perfluorobenzyl blades; the conformers are alternating in columnar packing arrangements and every pair is sandwiched by toluene molecules.     

Relative stabilities and the spectral signatures of stacked and hydrogen-bonded dimers of serotonin

The O–H???N hydrogen-bonded dimer of serotonin is shown to be more stable than the stacked dimer in its ground electronic state, by using the Møller–Plesset second-order perturbation theory (MP2) and the 6–31g^** basis set. The vertical excitation energy for the lowest π?→?π* transition for the monomer as well as the dimer is predicted by time-dependent density functional theory. The experimentally observed red shift of excitation wavelength on oligomerisation is explained in terms of the change in the HOMO–LUMO energy gap due to complex formation. The impact of dimer formation on the proton magnetic resonance spectrum of serotonin monomer is also examined.