Open Chain Trimer Research Articles

Synthesis, spectroscopic, and electronic properties of new tetrapyrazinoporphyrazines with eight peripheral 2,6-diisopropylphenol groups

Metal-free, magnesium, titanyl, and vanadyl tetrapyrazinoporphyrazines substituted with eight 2,6-diisopropylphenoxy groups at the peripheral positions were prepared and characterized by NMR, UV-Vis, magnetic circular dichroism (MCD), and mass spectrometry methods. In addition, the Pc(2,6iPrPhO)8VO complex was characterized by EPR spectroscopy and X-ray crystallography. Reaction between TiCl4 with 4,5-(2,6-diisopropylphenoxy)phthalonitrile in N,N-dimethylaminoethanol resulted in the formation of a red open-chain trimer, which was characterized by mass spectrometry and X-ray crystallography. Electronic structures of new compounds and their excited state properties were probed by Density Functional Theory (DFT) and Time-Dependent DFT (TDDFT) methods.

Ab initio molecular orbital calculations on hydrogen bonding in binary water-alcohol mixtures: (CH3OH)2(H2O), (CH3OH) (H2O)2, and (CF3CH2OH)(H2O)

Ab initio LCAO-SCF molecular orbital calculations have been performed on open-chain and cyclic trimers containing methanol and water and on dimers containing 2,2,2-trifluoroethanoI and water. These calculations were done in conjunction with experimental studies of molecular association in vapors of binary mixtures. Equilibrium structures and energies were determined using the minimal STO-3G basis set. The 2:1 and 1:2 methanol-water cyclic trimers have similar binding energies and are more stable than their respective open-chain trimers. Limited calculations were carried out to assess the importance of polarization functions and correlation energy in the methanol-water trimers. These calculations confirmed the STO-3G predictions. The most stable trifluoroethanol-water dimer has an O—H O and an H F hydrogen bond. It has a larger interaction energy than either a trifluoroethanol dimer or a water dimer.

The Probable Molecular Association in Liquid D-1-Propanol through Neutron Diffraction

The neutron scattering data of liquid D-1-propanol at room temperature has been carefully analyzed to extract the probable molecular association. The molecular conformation (quite elongated) being very different from its isomer 2-propanol (somewhat spherical) would suggest a different type of molecular association in 1-propanol. The probable molecular associations like open chain trimer (as in recent X-ray work) to pentamer (winding) as in earlier X-ray analysis in addition to hexameric ring clusters (as reported in 2-propanol and other alcohols) are compared. It is surprisingly seen that hexameric rings are most probable molecular association in 1-propanol too.

Stepwise, ring-closure synthesis and characterization of a homoleptic palladium(II)-pyrazolato cyclic trimer.

A triangular homoleptic complex, [Pd(mu-3-Ph-pz)2]3, is prepared by the stepwise formation of an open-chain trimer and subsequent ring-closure; D3 molecular symmetry results from the one-directional arrangement of the unsymmetrically substituted pyrazolato ligands; the palladium atoms are held in close proximity, with Pd-Pd distances of 2.997(1)-3.087(1) A.

Infrared laser induced isomerization of methanol polymers trapped in nitrogen matrix. I. Trimers

Selective vibrational excitations of OH stretching modes of methanol aggregates trapped in solid nitrogen were carried out in the range 3500–3370 cm−1. This proved an efficient means of interconverting trimers and possibly branched polymers absorbing in this domain. Irradiation in the absorption band of the cyclic trimer (CT) around 3450 cm−1 leads to a cycle opening and to the appearance of several groups of bands assigned to different forms of open chain trimer (OTi, i=1–3). Irradiations at the absorption frequencies of OT1 and OT2 interconverted these open species and partially regenerate CT. Isotopic dilution experiments enable to improve vibrational analysis of the open trimers, previously advanced from data obtained with irradiations performed with polychromatic light [Chem. Phys. Lett. 217, 123 (1994)]. Two other bands at 3424 and 3370 cm−1, growing upon annealing as the high polymer bands around 3250 cm−1, also turn out to be sensitive to selective irradiation. OH/OD and 16O/18O isotopic substitutions lead to the conclusion that the species (X) responsible for these absorptions is a branched trimer generated by fixation of one extra molecule on a cyclic trimer. Among the various products of photolysis one unstable species, with a lifetime of 8 min at 7 K, has been identified as a branched open chain trimer with one methanol molecule acting as proton biacceptor. Isomerization kinetic studies were followed in the ν CO region. According to the nature of the precursor, more or less complicated conversion schemes were identified. A quantitative analysis was carried out in only two cases, namely the photolysis of CT and OT1 upon irradiation at 3450 and 3389 cm−1, respectively. In both cases the kinetics were interpreted in term of first order processes, leading to quantum yield values between 10−2 and 10−3.

Infrared photoisomerization of the methanol cyclic trimer trapped in a nitrogen matrix

Condensation of nitrogen/methanol mixtures at 15 K revealed broad 3450 and 3284 cm −1 absorptions assignable to (CH 3OH) n aggregates, n⩾3. The 3450 cm −1 band is infrared photosensitive and progressively disappears upon irradiation in the beam of the spectrometer, with appearance of three main v OH bands at 3661.9, 3433.3 and 3389.2 cm −1 growing at the same rate. Mixed CH 3OH/CH 3OD samples revealed several mixed isotopic satellites of these three bands in both v OH and v OD regions, which allows us to conclude that the stable cyclic trimer responsible for the 3450 cm −1 absorption is converted upon OH (OD) vibrational excitation into an open chain trimer consisting of three non-equivalent subunits. This conclusion agrees with complementary observations in the v CO region.

Substituent modification in tri-o-thymotide and its effects on host geometry and guest enclathration .1. synthesis

A new synthesis is described for the preparation of tri-o-thymotide ( 1 , TOT) and some TOT analogues. The methodology is based on the sequential coupling of appropriately substituted and protected salicylic acid monomers followed by cyclization of the deprotected open-chain trimers. A variety of protecting methodologies and coupling sequences are discussed. The procedure seems generally applicable for the preparation of salicylides and has been used to prepare TOT in 25% overall yield (for the coupling-deprotection-coupling-deprotection-cyclization sequence). In addition, two new modified TOT-analogues 9 (25%, and 10 (14% were prepared in which the isopropyl group(s) ortho to the phenolic units is TOT is replaced by one ( 9 ) or two ( 10 ) ethyl groups. A third analogue 66 . where the methyl group ortho to the carboxyl group is removed in one of the salicylic acid monomer units of TOT, requires only the last cyclization step for completion of its synthesis. This methodology represents an important breakthrough for the controlled preparation of selected thymotide (salicylide) trimers and allows easy access to a variety of modified thymotides (salicylides) for structural, conformational and host-guest studies.

Topological studies of the molecular species that characterize lower alkanol + methylene bromide mixtures: molar excess volumes and molar excess enthalpies

Molar excess volumes, V E, and molar excess enthalpies, H E, have been determined at 298.15 and 308.15 K for the binary mixtures of methanol (A) or ethanol (A) with methylene bromide (B). The V E data for these mixtures have been analysed in terms of an approach that employs the graph theoretical connectivity parameters of the third degrees of the A and the B molecular entities to show, for the first time, that (i) in the pure states while methanol exists mainly as cyclic dimers (with perhaps open chain dimers and trimers), ethanol exists mainly as open chain trimers, and that (ii) in the (A+B) mixtures A (= a n ( n = 1 and/or 2))B molecular species characterize these mixtures. The H E data of these (A+B) mixtures have been utilized to understand the energetics of the various interactions that characterize these binary mixtures. These conclusions have further been substantiated with an analysis of the V E- and the H E data of ROH (A) (R = -CH 3 or -C 2H 5)+CHX 3 (B) (X = Cl or Br) mixtures.

Ab initio studies on clusters of polar molecules. stability of cyclic versus open-chain trimers of hydrogen fluoride

Large-scale ab initio calculations have been performed on cyclic and open-chain trimers of hydrogen fluoride including partial geometry optimization. The cyclic trimer turned out to be more stable by ≈ 2 kcal/mole. Equilibrium geometries and hydrogen bond energies are compared with corresponding quantities of (HF) 2 and of the infinite chain (HF)∞.

A b i n i t i o calculations as a source of intermolecular potential functions. Ethanol–water with a minimal basis set

The potential energy surface of the (trans)ethanol–water dimer has been explored with ab initio quantum mechanical calculations employing a minimal basis set to obtain a potential function suitable to computer simulations of the relevant aqueous solution. In all, 410 interaction energy values have been calculated. A part of the complex geometries has been produced approaching the water molecule to the ethanol along 17 different axes, and a part with an iterative random procedure. On this data base a fit with the experimental geometry of the water molecule and one with the ST2 model have been performed, with standard deviations of 0.69 and 0.73 kcal/mole, respectively. A limited test concerning the role of the three body term in open chain trimers has also been carried out. Finally, these SCF calculations have been used to check the possibility of the electrostatic approximation and of the pair potential library given by Clementi, to provide reliable intermolecular potential energy values in these kinds of systems.

Electrochemical oxidation of barbituric acids in aqueous solution in the absence of chloride ion

Barbituric acid, 1-methylbarbituric acid, 1,3-dimethylbarbituric acid and 1,3-diethylbarbituric acid are electrochemically oxidized at the pyrolytic graphite electrode by way of two principal voltammetric oxidation processes. The reaction schemes and products formed in these processes were studied in most detail for 1,3-dimethylbarbituric acid. The first voltammetric peak I a involves theoretically 1.33 e per molecule of 1,3-dimethylbarbituric acid oxidized. The reaction scheme involves an initial 1 e oxidation of 1,3-dimethylbarbituric acid to give a neutral radical that dimerizes at the 5-position to give hydurilic acid. This is then immediately further electrooxidized in a 2 e/1H + process to give a carbonium ion that reacts further with barbituric acid to give 5,5-di[1′,3′-dimethyl-2′,4′,6′-trioxopyrimid(5)yl]-1,3-dimethyl-2,4,6-trioxopyrimidine, the so-called open chain trimer of 1,3-dimethylbarbituric acid. The second voltammetric oxidation peak II a is rather more complex. Electrochemical oxidation of the open chain trimer of 1,3-dimethylbarbituric acid at peak II a potentials involves a 2 e/2H + reaction to give 5,6-dihydro-1,3-dimethyl-5,6-di[1′,3′-dimethyl-2′,4′,6′-trioxopyrimid-(5′,5′)yl]furo[2,3-d]uracil, the so-called cyclic trimer of 1,3-dimethylbarbituric acid. Direct electrooxidation of 1,3-dimethylbarbituric acid at peak II a potentials however, involves two reaction routes. The first via an initial 1 e process to a radical that dimerizes and ultimately forms open chain trimer as in the peak I a process, however the open chain trimer is then further electrooxidized to the cyclic trimer. The second route appears to involve a further 1 e oxidation of the initially formed barbituric acid radical to give a carbonium ion. This is rapidly attacked by water to give dialuric acid which in turn is immediately further oxidized to 1,3-dimethylalloxan. Barbituric acid and other N-substituted barbituric acids appear to be electrochemically oxidized by similar mechanisms.

Theoretical Study of Open Chain Dimers and Trimers Containing CH3OH and H2O

Ab initio minimal basis LCAOSCF molecular orbital calculations have been performed on open chain dimers and trimers containing methanol and water. The equilibrium structures and energies of the dimers are determined and compared. The rigidity of the hydrogen bond in each dimer is discussed in terms of calculated intermolecular force constants. The energies of seven open chain trimers are presented and analyzed. It is found that the energies of all trimers deviate from additivity, indicating the existence of a cooperative effect in hydrogen bonding in these systems.