Showing papers on "Intramolecular force published in 1993"

Crystallographic studies of inter- and intramolecular interactions reflected in aromatic character of .pi.-electron systems

Abstract: An old quantitative measureof aromaticcharacter named HOMA (harmonic oscillator measureof aromaticity) is reformulated and related to the Jug and Koester concept of resonance coordinate. In its new version, the HOMA index can be used to estimate an aromatic character of r-electron systems (molecules, ions, or their fragments) built up of the following bonds: CC, CN, CO, CP, CS, NN, and NO. It is shown that even if aromatic stabilization is due to a-electron structure inclined to maintain equal bond lengths, the aromatic character is mainly a result of r-electron properties of the system. Application of the HOMA index to benzene rings in various molecular and ionic systems showed that the aromatic character depends rather weakly on deformation from planarity whereas it may be strongly dependent on the intermolecular H-bonding net as well as on cooperative substituent interactions of push-pull type and on the topology of a mutual link between benzene rings in condensed benzenoid hydrocarbons,

Excited-state proton transfer reactions II. Intramolecular reactions

Abstract: Excited-state intramolecular proton transfer reactions are reviewed. Special emphasis is given to the intrinsic processes and to the mechanisms of proton transfers in relation to the nature of the intramolecular hydrogen bond ring.

Fluorescent Sensors of Molecular Recognition. Modified Cyclodextrins Capable of Exhibiting Guest-Responsive Twisted Intramolecular Charge Transfer Fluorescence

Abstract: α-, β-, and γ-cyclodextrin derivatives bearing a p-(dimethylamino)benzoyl (DMAB) moiety (DMAB-αCyD, DMAB-βCyD, and DMAB-γCyD, respectively) have been synthesized as fluorescent sensors of molecular recognition. These compounds show dual fluorescence emission arising from normal planar (NP) and twisted intramolecular charge transfer (TICT) exited states, and among them strong TICT emission was observed for DMAB-βCyD. The induced circular dichroism spectra of the derivatives suggest that only DMAB-βCyD among other derivatives binds the DMAB moiety into its own cavity, forming an intramolecular inclusion complex. This conformation was confirmed by the analysis of its 1 H-NMR data and was related to its strong TICT emission

Long-lived, highly luminescent rhenium(I) complexes as molecular probes: intra- and intermolecular excited-state interactions

Abstract: A series of isonitrile complexes of the form fac-LRe(CO) 3 CNR + , where L is an α-diimine and R=t-Bu or n-alkyl, have been synthesized and characterized. These complexes can have extraordinarily high quantum yields (>0.7) and long excited-state lifetimes (>100 μs) in fluid solutions at room temperature. The lowest excited state (charge transfer or ligand localized) can be controlled by varying either L or the temperature. Intramolecular foldback can affect the luminescence properties. These complexes are bihydriphobic because they have two spacially separated hydrophobic binding sites (i.e. L and R). By suitably engineering the complex, one can control which group will bind to hydrophobic targets such as cyclodextrins

Fragmentation of protonated peptides: surface-induced dissociation in conjunction with a quantum mechanical approach.

Abstract: This paper describes the results of a systematic investigation designed to assess the utility of surface-induced dissociation in the structural analysis of small peptides (500-1800u). A number of different peptides, ranging in mass and amino acid sequence, are fragmented by collision with a surface in a tandem mass spectrometer and the spectra are compared with data obtained by gas-phase collisional activation. The surface-induced dissociation spectra provide ample sequence information for the peptides. Side-chain cleavage ions of type w, which are generally detected upon kiloelectronvolt collisions with gaseous targets but not upon electronvolt collisions with gaseous targets, are detected in the ion-surface collision experiments. A theoretical approach based on MNDO bond order calculations is suggested for the description of peptide fragmentation. This model, supplemented by ab initio calculations, serves as a complement to the experimental work described in the paper and explains (i) the easy cleavage of the amide bond, (ii) charge-remote backbone and side-chain cleavages, and (iii) the influence of intramolecular H-bonding.

The role of hydrogen bonding in carbohydrates: molecular dynamics simulations of maltose in aqueous solution

Abstract: Molecular dynamics simulations of maltose in two different conformations in vacuum and aqueous (TIP3P) solution have been used to examine the types of hydrogen bonds made by this carbohydrate molecule. Maltose was found to be extensively hydrogen bonded to solvent molecules in aqueous solution, and these hydrogen bonds were found to have potential conformational consequences. The exchange of an intramolecular O2-O3' hydrogen bond found in the crystal structure for hydrogen bonds to solvent was observed to produce significant changes in the solvation of the sugar molecule

Synthesis of a rigid "ball-and-chain" donor-acceptor system through Diels-Alder functionalization of buckminsterfullerene (C60)

Abstract: Encouraged by our recent results in the functionalization of C[sub 60] through Diels-Alder reactions, we sought to develop a novel type of derivative of C[sub 60] by creating ball-and-chain systems, in which the C[sub 60] unit and another functional group, G, are attached to a rigid polycyclic chain or bridge. These systems should provide unprecedented insight into a variety of long-range intramolecular processes between C[sub 60] and G, including energy and electron transfer, and would complement the corresponding intermolecular processes studied so far. We are pleased to report the synthesis of our first ball-and-chain molecule in which the chain comprises a rigid polynorbornyl-bicyclo[2.2.0]hexyl bridge which has been used extensively by the UNSW group in the construction of several donor-bridge-acceptor systems. The key feature of the present work is the Diels-Alder reaction between C[sub 60] and a novel bridge diene affording the ball-and-chain adduct. 18 refs., 1 fig.

Intramolecular charge transfer in amino- benzonitriles: Requirements for dual fluorescence

Abstract: Dual fluorescence and intramolecular charge transfer (ICT) are observed with aminobenzonitriles when two excited state levels (Sl and S2(CT) in DMABN) have an energy gap sufficiently small for vibronic coupling: a solvent-induced pseudo-Jan-Teller effect. It is argued that the N-inversion of the amino group acts as a promoting mode. These conclusions are based on a comparison of absorption spectra and photostationary and time-resolved fluorescence data. Dual fluorescence is also observed with MMD, in which the dimethylamino group is twisted towards a perpendicular configuration with respect to the phenyl ring.

Theoretical study of intramolecular vibrational relaxation of acetylenic CH vibration for v=1 and 2 in large polyatomic molecules (CX3)3YCCH, where X=H or D and Y=C or Si

Abstract: Quantum calculations are reported for the intramolecular vibrational energy redistribution and absorption spectra of the first two excited states of the acetylenic CH stretch vibration in the polyatomic molecules (CX3)3YCCH, where X=H or D and Y=C or Si. Using approximate potential energy surfaces, comparison is made with the corresponding recent experimental spectra. It is found that a model of intramolecular vibrational relaxation based on the assumption of sequential off-resonance transitions via third and fourth order vibrational couplings (as opposed to direct high order couplings) is in agreement with experimental results on spectral linewidths. In a semiclassical limit this type of relaxation corresponds to a dynamic tunneling in phase space. It is shown that the local density of resonances of third and fourth order, rather than the total density of states, plays a central role for the relaxation. It is found that in the Si molecule an accidental absence of appropriate resonances results in a bottleneck in the initial stages of relaxation. As a result, an almost complete localization of the initially prepared excitation occurs. It is shown that an increase of the mass alone of the central atom from C to Si cannot explain the observed difference in the C and Si molecules. The spectral linewidths were calculated with the Golden Rule formula after prediagonalization of the relevant vibrational states which are coupled in the molecule to the CH vibration, directly or indirectly. For the spectral calculations, in addition to the direct diagonalization, a modified recursive residue generation method was used, allowing one to avoid diagonalization of the transformed Lanczos Hamiltonian. With this method up to 30 000 coupled states could be analyzed on a computer with relatively small memory. The efficiency of C programming language for the problem is discussed.

On the stability of nucleic acid structures in solution: enthalpy - entropy compensations, internal rotations and reversibility

Abstract: The thermodynamics of self-association (stacking) of free bases and nucleotides, intramolecular stacking in dinucleotides, nearest-neighbour base pair stacking interactions in duplex DNA and RNA, and the formation of hairpin loops illustrate enthalpy/entropy compensations. Large stacking exothermicities are associated with large negative entropy changes that ensure that delta G is small, permitting readily reversible associations in solution. We rationalise enthalpy/entropy compensations with reference to residual motions and torsional vibrations which make a larger entropic contribution to binding when - delta H approximately kT (thermal energy at room temperature), than when - delta H >> kT. We present a factorisation of experimental free energies for helix formation in terms of approximate contributions from the restriction of rotations, hydrophobic interactions, electrostatic interactions due to base stacking, and contributions from hydrogen bonding, and estimate the adverse free energy cost per rotor (mainly entropy) of ordering the phosphate backbone as between 1.9 and 5.4 kJ mol-1 [averaged over 12 rotors per base pair for A-U on A-U stacking (lower limit), and G-C on C-G stacking (upper limit)]. The largest cost is associated with the most exothermic stacking interactions, while the range of values is consistent with earlier conclusions from data on the fusion of hydrocarbon chains (lower value), and with entropy changes in covalent isomerisations of small molecules involving severe restrictions (upper value).

Hydrogen-bonding effect on the photophysical properties of 7-aminocoumarin derivatives

Abstract: The photophysical properties of 7-amimocoumarin derivatives with different alkylation degrees are studied in water and in different monoalcohols. Specific hydrogen bonds between the solute and protic solvents are considered to explain spectral shifts and internal conversion process. The mechanism for the internal conversion process via a twisted intramolecular charge transfer state and the umbrella-like motion model is discussed

The Role of Aromatic Rings as Hydrogen-Bond Acceptors in Molecular Recognition

Abstract: Observations of phenol-benzene and ammonia—benzene complexes in the gas phase show that hydrogen bonds link their proton donors to the π electrons of the benzene with a bond energy of between 2 and 4 kcal mol -1 , large enough to be biologically significant. Intramolecular hydrogen bonds between OH and NH donors and aromatic acceptors have also been found in crystal structures of organic compounds. NH-aromatic interactions stabilize x-helices if donors and acceptors occur at successive turns of the helix. These interactions also contribute to the stability of several proteins and play an important part in cellular and synaptic signal transmission.

Structure-photophysics correlations in a series of 4-(dialkylamino)stilbenes: intramolecular charge transfer in the excited state as related to the twist around the single bonds

Abstract: 4-(Dimethylamino)stilbene and eight regioselectively bridged 4-(dialkylamino)stilbene derivatives have been synthesized and their solvatochromism, fluorescence quantum yields, and lifetimes measured versus the solvent polarity and the temperature. When the single bond connecting the dialkylanilino group to the ethylene group (bond 2) is bridged, strong fluorescence quenching is observed; when this bond is flexible, the fluorescence quenching is strongly reduced, and lifetime maxima at intermediate temperature indicate the involvement of a further TICT emitting state. Fluorescence quenching is reduced in polar solvents and suppressed when the double bond is included in a five-membered carbocyclic ring

Effect of end substitution on electrochemical and optical properties of oligothiophenes

Abstract: A series of oligothiophenes (from bithiophene to sexithiophene) substituted at the ends of the chain by various electron donor of acceptor groups or atoms (methoxy, nitro, or bromo) have been prepared. Their electrochemical oxidations have been studied using microelectrodes in order to determine accurately the formal redox potential of the radical-cation generation. The stability of the latter can be deduced from the required potential scan rate. Electron donor substituents (methoxy and bromo) stabilize the radical-cation forms, and the dications can also be obtained from these substituted terthiophenes. The optical properties of the compounds have also been investigated. Only the nitro-substituted group induces a significant increase of the fluorescence quantum yield and of the lifetime of the excited state. For solution study, this positive effect is only limited to the short oligomeric chains for which an intramolecular charge transfer is evidenced by solvatochromic effects. 21 refs., 5 figs., 3 tabs.

Synthetic applications of imidotitanium-alkyne [2+2] cycloadditions. A concise, stereocontrolled total synthesis of the antifungal agent (+)-preussin

Abstract: An efficient stereoselective total synthesis of (+)-preussin, (25,3S,SR)-1-methyl-5-nonyl-2-(phenylmethyl)-3-pyrrolidinol (1), was achieved by way of a convergent, intramolecular imidotitanium-alkyne [2+2] cycloaddition-acyl cyanide condensation sequence

Catalytic asymmetric synthesis of either enantiomer of physostigmine. Formation of quaternary carbon centers with high enantioselection by intramolecular Heck reactions of (Z)-2-butenanilides

Abstract: A practical method for preparing either enantiomer of physostigmine and congeners is reported. The Z stereochemistry of the butenanilide cyclization substrate is required to obtain high enantioselection (95% ee) in the key asymmetric Heck insertion

Kinetics and mechanism of a cobalt(III) complex catalyzed hydration of nitriles

Abstract: [Co(cyclen)(OH 2 ) 2 ] 3+ (1) efficiently catalyzes hydration of acetonitrile to acetamide in a three-step cycle at pH 7, 40 o C (cyclen=1,4,7,11-tetraazacyclododecane). The mechanism involves (a) equilibrium coordination of acetonitrile to 1 (K 1 =2.5 M -1 ) followed by (b) intramolecular metal hydroxide attack on the coordinated nitrile resulting in formation of the chelated acetamide (k 2 =4.7×10 -3 s -1 ) and (c) dissociation of the chelated amide (k 3 =3.3×10 -4 s -1 )

Intramolecular Coordination in Organotin Chemistry

Abstract: Publisher Summary This chapter overviews organotin compounds containing C,Y-chelating ligands particularly, (1) structures in the solid state, (2) fluxional behavior in solution, and (3) stereochemical aspects. Attention is also given to the enhanced reactivity of tin–carbon bonds in tetraorganotin compounds as a result of intramolecular coordination. The first example of an organotin compound having C,Y-chelating ligands, i.e., bis [1, 2-bis(ethoxycarbonyl)ethyl]tin dibromide, for which the structure in the solid state was unambiguously established by an X-ray crystal structure determination. By using the steric and electronic properties of C,Y-chelating ligands, organometallic compounds that have interesting properties and special reactivities can be isolated. Special features of some of these compounds are (1) stabilization of organometallic compounds in which the metal has an unusual oxidation number, e.g., Fe(II1), Co(I1), and Ni(II1); (2) the trapping of organometallic species that are supposed to be intermediates in reactions, e.g., organoplatinum compounds in which a di-iodine molecule is coordinated end-on to platinum as a first step in oxidative-addition reactions; (3) the use of a novel organonickel compound as a catalyst in the selective Karasch addition of polyhaloalkanes to olefines; and (4) the unexpected reactivity of organocopper compounds toward acetylenes.

Intramolecular kinetic isotope effects in alkane hydroxylations catalyzed by manganese and iron porphyrin complexes

Abstract: Intramolecular kinetic isotope effects (KIEs) in alkane hydroxylations catalyzed by manganese and iron porphyrins have been studied with 1,3-dideuterioadamantane as substrate. This substrate is highly suitable for determination of primary isotope effects in metalloporphyrin-catalyzed hydroxylations: the high chemical reactivity of tertiary C-H (D) bonds avoids side reactions which complicate product analyses, and tertiary C-H and C-D bonds have the same steric and stereochemical environment. Kinetic isotope effects have been determined with adamantane-1,3-d 2 (98 atom % D) and iron and manganese complexes of meso-tetrakis(2,6-dichlorophenyl)porphyrin and meso-tetramesitylporphyrin [M III (TDCPP)Cl and M III (TMP)Cl,M III =Fe or Mn] as catalysts

Cation complexation by chemically modified calixarenes. 5. Protonation constants for calixarene carboxylates and stability constants of their alkali and alkaline-earth complexes

Abstract: The complexing properties of some calix[4]arenes in which carboxylic acid ligating groups have been progressively attached to the lower rim through the phenolic hydrogen atoms have been measured with alkali and alkaline-earth metal cations in methano by means of potentiometric titrations. The acid-base characteristics of these compounds were first determined. the weak acidities generally observed may be explained by the formation of intramolecular hydrogen bonding.

Combined neutron diffraction and computer simulation study of liquid dimethyl sulphoxide

Abstract: The structure of liquid dimethylsulphoxide (DMSO) at 25 °C is explored using a combination of neutron diffraction with isotope substitution and computer simulation techniques. The potentials used in the computer simulation consist of Coulomb and 6‐12 Lennard‐Jones interactions for each of the carbon, oxygen, and sulfur sites on the molecule. To interpret the neutron diffraction data most effectively, it is necessary to refine both the molecular internal structure and the intermolecular contributions to the measured structure factors at the same time in order to separate the intermolecular terms correctly, because there is a large degree of overlap between intramolecular and intermolecular distances. This renders the data far more sensitive to the intermolecular forces than if this analysis were not performed. Direct comparison of neutron diffraction data and computer simulation results indicates that existing models of the molecular force field give a sensible description of the liquid structure, although there are some discrepancies which are not fully understood at this time. The question of whether this material can be regarded as an associated liquid, as it is frequently referred to, is discussed. All tests for association that have so far been applied to both the diffraction data and the computer simulation results do not indicate a highly ordered molecular association in the liquid.

Syntheses of Polycyclic Natural Products Employing the Intramolecular Double Michael Reaction

Abstract: The one-pot intramolecular double Michael reaction of compounds having two different α, β-unsaturated carbonyl groups to form polycyclic compounds can be carried out by three different methods: in the first the substrate is treated with lithium hexamethyldisilazide, in the second with chlorotrimethylsilane, triethylamine, and zinc chloride at an elevated temperature, and in the third with tert-butyldimethylsilyl trifluoromethanesulfonate and triethylamine. The reaction proceeds with complete regioselectivity and high stereoselectivity following a stepwise mechanism. Spiro-fused bicyclo[2.2.2]octane derivatives can be constructed with high stereoselectivity by the intromolecular double Michael reaction by using the first method. Enantiomerically pure atisine and the enantiomer of atisirene were synthesized stereoselectively by application of this methodology. The syntheses of steroids and angular triquinane-type sesquiterpenoids were possible with the second method. Heterocyclic compounds with a bridgehead nitrogen atom were obtained by the reaction of α, β-unsaturated amides following the second and third methods. The asymmetric synthesis of tylophorine with diastereofacial control was achieved by the intramolecular reaction according to the third method. The sulfur-mediated intramolecular double Michael reaction utilizing the third method produced trans-hydroindane derivatives. Furthermore, the intramolecular Michael-aldol reaction can be employed in synthesizing polycyclic systems with cyclobutane units by treatment with tert-butyldimethylsilyl trifluoromethanesulfonate in the presence of triethylamine. The intermolecular double Michael reaction and related reactions will also be described.

Stereoselective intramolecular bis-silylation of alkenes promoted by a palladium-isocyanide catalyst leading to polyol synthesis

Abstract: Stereoselective Intramolecular Bis-Silylation of Alkenes Promoted by a Palladium-Isocyanide Catalyst Leading to Polyol Synthesis

Intramolecular electronic energy transfer between rigidly linked naphthalene and anthracene chromophores

Abstract: The rate of intramolecular singlet-singlet electronic energy transfer (EET) between naphthalene and anthracene chromophores that are held in a well-defined orientation and separation by a rigid bis(norbornyl)bicyclo-[2.2.0]hexane bridge, six σ-bonds in length, is measured to be 2.9 × 1010 s-1 (n-hexane). EET between the lowest-energy naphthalene 1Lb and anthracene 1La states (via a direct Coulombic mechanism) is forbidden in this molecule because of the orthogonality of donor and acceptor transition dipoles. EET involving a Coulombic interaction between the naphthalene 1Lb and the nonfluorescent anthracene 1Lb electronic states is also considered. Although Coulombic EET is vibronically allowed, to account for the high measured rate, it is proposed that relayed mechanisms, involving the bridge, play a dominant role. The significance of these results to EET in aromatic polymers and other multichromophoric systems is discussed. © 1993 American Chemical Society.

Palladium-catalyzed cyclizations of polyenynes. A palladium zipper

Abstract: The cycloisomerization of polyenynes with a catalyst derived from Pd(0), acetic acid, and a ligand to polycycles depends upon the juxtaposition of unsaturation. The process involves the stages of initiation (by addition of Pd-H to an acetylene), propagation (by intramolecular carbopalladation), and termination (by β-hydrogen elimination). With 2-homopropargylated 1,5- and 1,6-dienes, monocyclizations via 5-exo and 6-endo modes dominatewith the ratio dependent upon ligand. The 5-exo mode is favored by tri-o-tolylphosphine, whereas triphenylstibine favors the 6-endo mode. On the other hand, a 3-homoallyl-3-homopropargylcyclopentene undergoes smooth bicyclization even when triphenylphosphine is used as ligand

Toward the photostability mechanism of intramolecular hydrogen bond systems. The photophysics of 1'-hydroxy-2'-acetonaphthone

Abstract: The photophysical behavior of 1'-hydroxy-2'-acetonaphthone in dry cyclohexane, methanol, and dimethyl sulfoxide was studied. The compound was found to exhibit a single fluorescence band (both at room temperature and at 77 K) that was scarcely dependent on the solvent used as well as an anomalously small Stokes shift (ca. 6000 cm -1 ) for an excited-state intramolecular proton-transfer mechanism and a fluorescence spectrum similar to the mirror image of its absorption spectrum. The compound was also found to exhibit a single fluorescence band both in gas phase at 368 K and in crystal phase at 298 K