Reaction of perfluoro-1-methylindan with SiO2-SbF5

The reaction of perfluoro-1-methylindan with SiO₂-SbF₅ depending on the amount of SiO₂ led to the formation after hydrolysis of the reaction mixture of perfluoro-3-methylindan-1-one, perfluoro-4-methylisochromen-1-one, 6-(1-carboxy-2,2,2-trifluoro-ethyl)-2,3,4,5-tetrafluoro-benzoic and 6-(carboxymethyl)-2,3,4,5-tetrafluorobenzoic acids. Heating in the SbF₅ medium perfluoro-1-methylindan in a glass ampoule at 130°C, or perfluoro-3-methylindan-1-one at 70°C provided a solution of a perfluoro-4-methylisochromenium salt that on treating with anhydrous HF was converted into perfluoro-4-methyl-1H-isochromen, and on hydrolysis, into perfluoro-4-methylisochromen-1-one.     

Molecular conformations from distance matrices

Two algorithms are introduced that show exceptional promise in finding molecular conformations using distance geometry on nuclear magnetic resonance data. The first algorithm is a gradient version of the majorization algorithm from multidimensional scaling. The main contribution is a large decrease in CPU time. The second algorithm is an iterative algorithm between possible conformations obtained from the first algorithm and permissible data points near the configuration. These ideas are similar to alternating least squares or alternating projections on convex sets. The iterations significantly improve the conformation from the first algorithm when applied to the small peptide E. coli STh enterotoxin. © 1993 John Wiley & Sons, Inc.     

Spectroscopic observation of jet-cooled 2-fluoro-m-xylyl radical

The jet-cooled 2-fluoro-m-xylyl radical was generated and vibronically excited in a corona excited supersonic expansion from precursor 2-fluoro-m-xylene seeded in a large amount of carrier gas helium. The well-resolved visible vibronic emission spectrum of the jet-cooled 2-fluoro-m-xylyl radical was recorded using a long-path monochromator. From the analysis of the spectrum, we determine an accurate electronic energy of the D(1) → D(0) transition and the frequencies of vibrational modes in the ground electronic state by comparison with those of ab initio calculations and the known spectroscopic data of 2-fluoro-m-xylene for the first time.     

Molecular structures and conformations: Experiment and theory

Theoretical calculations in combination with experimental gas phase structure research can be performed in two ways. The first is to support and improve experimental analyses by including additional data from theoretical calculations. This is to the advantage of the experiment. The second way is a comparison of geometric structures and conformational properties obtained with different theoretical methods with the experimental result. This comparison indicates which theoretical method or methods are suitable for a specific compound. This approach is to the advantage of the theory. © 1998 John Wiley & Sons, Inc. J Comput Chem 19: 123–128, 1998     

Molecular conformations of aminophenylimidazoles exhibiting antiulcer activities

To experimentally clarify a possible stereostructure-activity relationship proposed for H2-receptor antagonists, three 5-aminophenylimidazoles (1, 2 and 3), in which respective amino groups are located on the ortho, meta and para positions of the benzene ring, were synthesized and examined for their conformational characteristics using X-ray diffraction and proton nuclear magnetic resonance (1H-NMR) methods, and for antiulcer activities on rats and H2-receptor antagonist activities in guinea pig. The ortho isomer 1, which preferentially formed an intramolecular N-H (amino)...N (imidazole) hydrogen bond, showed the highest antiulcer activity with half the efficacy of cimetidine. On the other hand, none of 1, 2 and 3 showed significant H2-receptor antagonist activity. Based on these results, the conformational characteristic for the exhibition of antiulcer activity has been discussed.     

Molecular conformations,hydrodynamics and optics of ladder polymers

Hydrodynamic (diffusion, sedimentation, viscosity), dynamo-optical and electro-optical properties of several ladder polysiloxanes with different substituents in the side groups have been investigated.In accordance with the theories of hydrodynamic properties of worm-like chains, the equilibrium rigidity of the main chain of these macromolecules was evaluated quantitatively. High equilibrium rigidity of ladder polysiloxanes is a direct consequence of the double-chain structure of their molecular chains.Values of reduced birefringence in electrical and mechanical fields in solutions of ladder polysiloxanes increase with molecular weight but tend to a limit, which is characteristic for semirigid macromolecules.Birefringence in an electrical field for solutions of all ladder polymers investigated is by two orders higher than the value of electro-optical effect in polymers with flexible chains and its sign (negative) coincides with that of flow birefringence. In a variable (sinusoidal) field in the region of high frequencies, strong frequency dependence of birefringence is characteristic for all samples. Unique electrooptical properties in constant and variable fields prove that, in contrast to polymers with flexible chains, highly organized orientational long-range order exists in ladder macromolecules, moreover, it is not only an axial order but also a polar one. The occurrence of long-range correlation in the orientation of polar groups and bonds of the molecular chain gives rise to high total moment of the macromolecule, which is responsible for its rotation in an electrical field. The direction of this dipole moment coincides with the long axis of the molecule since the Kerr effect is of the same sign as flow birefringence.     

Laser spectroscopy and dynamics of the jet-cooled AsH2 free radical

The A (2)A(1)-X (2)B(1) electronic transition of the jet-cooled AsH(2) free radical has been studied by laser-induced fluorescence (LIF), wavelength-resolved emission, and fluorescence lifetime measurements. The radical was produced by a pulsed electric discharge through a mixture of arsine (AsH(3)) and high pressure argon at the exit of a pulsed valve. Nine vibronic bands were identified by LIF spectroscopy in the 505-400 nm region, including a long progression in the bending mode and two bands (1(0) (1) and 1(0) (1)2(0) (1)) involving the excited state As-H symmetric stretch. Single vibronic level emission spectra showed similar activity in the bending and symmetric stretching frequencies of the ground state. High-resolution spectra of the 0(0) (0) band exhibited large spin splittings and small, resolved arsenic hyperfine splittings, due to a substantial Fermi contact interaction in the excited state. The rotational constants obtained in the analysis gave effective molecular structures of r"(0)=1.5183(1) A, theta"(0)=90.75(1) degrees and r'(0)=1.4830(1) A, theta'(0)=123.10(2) degrees . The excited state fluorescence lifetimes vary dramatically with rovibronic state, from a single value of 1.4 micros to many with lifetimes less than 10 ns, behavior which the authors interpret as signaling the onset of a predissociative process near the zero-point level of the ground state.     

Molecular structure of nitrobenzene in the planar and orthogonal conformations

The molecular structure and ring distortions of nitrobenzene have been determined by gas-phase electron diffraction and ab initio molecular orbital (MO) calculations as well as from the structures of six derivatives studied by X-ray crystallography. The experimental value of the ring angle at the ipso position is = 123.4 ± 0.3° in the free molecule; this is about 1.5° less than the hitherto reported values. Regression analysis of the ring angles in the six derivatives studied by X-ray crystallography yields = 122.7(1)° for nitrobenzene in a crystalline environment. The small difference in the two values of a is interpreted as an effect of intermolecular interactions in the crystal. The value produced by the MO calculations, = 122.3° at the 6–31G^* (5D) level, is smaller than either of the experimental results. As regards the ring angles at the meta and para positions, the three techniques of structure determination consistently indicate that these are larger than 120° by a few tenths of a degree. Other important geometrical parameters from the electron diffraction study are r _g (C-C) = 1.399 ± 0.003 Å,r _g (C-N) = 1.486 ± 0.004 Å,r _g (N-O) = 1.223 ± 0.003 Å, and A sO-N-O = 125.3 ± 0.2°. X-ray diffraction experiments on 3,5-dimethyl-4-nitrobenzoic acid and 3,5-dimethylbenzoic acid and ab initio MO calculations provide solid evidence that the geometry of nitrobenzene is little affected when the nitrogroup is twisted by 90° out of the planar equilibrium conformation. This indicates that the extent of -electron transfer from the benzene ring to the nitro group is small. The barrier to rotation is estimated to be 17 ± 4 kJ mol^–1 from the electron diffraction data.     

Molecular conformations at the cellulose–water interface

¹³C-NMR chemical shifts were measured for C-4 and C-6 in a collection of eight crystalline glucoses and glucosides. The influence of the hydroxymethyl conformation was greater at C-4 than at C-6, with mean chemical shifts for gauche–trans molecules displaced 3.1 ppm (C-4) and 2.5 ppm (C-6) relative to gauche–gauche molecules. This information was used to interpret ¹³C-NMR spectra of crystalline celluloses. Chemical shifts for C-4 in the crystallite cores of celluloses I and II differed by just 0.2 ppm, but the corresponding chemical shifts for well-ordered crystallite surfaces differed by 3.0 ppm. The separation between crystallite-surface signals was attributed to different hydroxymethyl conformations at the cellulose–water interface, i.e., gauche–gauche and gauche–trans on crystallites of cellulose I and cellulose II, respectively. A broad C-4 signal in the spectrum of cellulose II indicated gauche–gauche conformations in disordered cellulose. Chemical shifts for C-6 were consistent with these conformations.     

Molecular arrangements and conformations of liquid unbranched alkanes in narrow slits

Realistic, atomistic models of liquid tridecane in broad slits (>3 nm) and in narrow slits of thickness 1,2 nm and 1,0 nm have been obtained using the Monte Carlo technique. The setup of the models is such that the molecules in the slits are in equilibrium with the bulk liquid. The surfaces of the plates are modelled as two-dimensional arrays of hexagonally packed units having the same size and interaction parameters of a methylene group. The regions adjacent to the plates in slits with thickness > 3 nm are characterized by a well defined tendency to form partially ordered layer structures, while molecules at a distance from the plates larger than 1,5 nm are unperturbed. The simultaneous presence of two plates increases the tendency to form layer structures when their distance is 1,2 nm, while this tendency is almost totally destroyed when the slit is squeezed down to a thickness of 1,0 nm. This is also associated with a 10% decrease of the density in the latter slit.     

Molecular structure and conformations of 2,2-di-tert-butyl-1,3-diaza-2-silacyclopentane: gas electron diffraction and quantum chemical calculations

The geometric structure and conformational properties of the saturated five-membered-ring compound 2,2-di-tert-butyl-1,3-diaza-2-silacyclopentane, (t-Bu)(2)Si(NH)(2)(CH(2))(2), was investigated by gas electron diffraction and quantum chemical methods (B3LYP and MP2 with 6-31G basis sets). The compound exists as a mixture of two conformers, both possessing a twist conformation and C(2) symmetry. In the prevailing form (76(6) % at 305 K) the N-H bonds stagger the adjacent CH(2) groups, and in the minor form the N-H bonds eclipse the CH(2) groups. This conformational mixture corresponds to a free energy difference of DeltaG degrees = 0.69(19) kcal/mol. The B3LYP method predicts a preference for the eclipsed conformer. The largest torsion occurs around the C-C bond with tau(NCCN) = 29.2(24) degrees. The degree of puckering in the title compound is considerably smaller than that in silacyclopentane with tau(CCCC) = 49.7(14) degrees. This has been rationalized by larger angle strain in the title compound.     

Conformational study of jet-cooled L-phenylglycine

We investigated the conformational structures of L-phenylglycine in the gas phase by photoionization and double resonance spectroscopy techniques as well as high-level ab initio calculations. The UV-UV and IR-UV double resonance spectroscopy suggested that there exists only one conformer that has a free OH band for the carboxyl group. Rotational contour analysis combined with ab initio calculation indicated that the conformer we detected by resonant two-photon ionization was not one of those found by Sanz et al. in their microwave spectroscopic study [Chem. Eur. J. 12, 2564 (2006)]. Different methods of vaporization along with different expansion and cooling conditions and different detection methods are believed to be the culprit for such intriguing discrepancy. The identical hydrogen bonding structure of our phenylglycine conformer with the most abundant conformer of glycine found in helium droplets and their nearly identical OH frequencies suggest that the skeletal structure of glycine is not significantly altered by phenyl substitution.     

Molecular orbital treatment of the conformations of composite molecules: Phenylthiophenes

A correlative study of the theoretical determination of the conformation of composite molecules is given. Results of EH, M-I-M and $\text{CNDO}\text{2}$ treatments are compared for the model composite systems; 2- and 3-phenylthiophenes. The systematic failure of the EH method reveals its inadequacy for conformational analysis of composite molecules. The transition energies, oscillator strengths and dipole moments of the two isomers, as calculated by the CNDO method, agree satisfactorily with experiment and confirm a planar equilibrium conformation for the studied composite molecules. The total charge distribution calculated by the CNDO method shows an appreciable σ-charge contribution and a comparatively small charge-transfer contribution. The formal σ-charges on the atoms of the two isomers are shown to be polarized in a direction opposed to that of the π-polarization. The molecular orbitals of the two isomers are found to possess similar nodal properties which indicates that the extent of cross conjugation in the 3-phenyl isomer is very limited.     

Rotationally resolved spectroscopy of jet-cooled NbMo

Rotationally resolved resonant two-photon ionization spectra of jet-cooled NbMo are reported for the first time. A vibronic spectrum of NbMo was recorded in the 17 300-22 300 cm(-1) spectral region. Although the observed bands could not be grouped into electronic band systems, four excited vibronic levels with Omega=2.5 and two excited levels with Omega=3.5 were identified. The ground state of NbMo has been assigned as (2)Delta(52), deriving from a 1sigma(2)1pi(4)1delta(3)2sigma(2) configuration of the valence electrons. Rotational analysis of six bands provides a ground state rotational constant of B(0) (")=0.087 697(26) cm(-1), corresponding to a bond length of r(0) (")=2.008 09(30) A for (93)Nb(98)Mo. Correction for the effects of the spin-uncoupling operator changes the estimated bond length only slightly to r(0) (")=2.008 02(30) A. The experimentally determined value of r(0) (") is compared to that predicted using previously determined multiple bonding radii of Nb and Mo. A comparison to the known diatomic molecules composed of group V and VI metal atoms is also made.     

Electronic spectroscopy of jet-cooled YbNH3

We report the first spectroscopic study of a complex consisting of a rare earth atom in combination with ammonia. Using two-color resonance-enhanced multiphoton ionization (REMPI) spectroscopy, the lowest energy electronic transition of YbNH(3) has been found in the near-infrared. The spectrum arises from a spin-forbidden transition between the (1)A(1) ground electronic state and the lowest (3)E excited electronic state. The transition is metal centered and approximately correlates with the Yb 6s6p (3)P ← 6s(2) (1)S transition. The observation of clear spin-orbit structure in the spectrum confirms the C(3v) symmetry of YbNH(3). Vibrational structure is also observed in the REMPI spectrum, which is dominated by excitation of the Yb-N stretching vibration.