摘要 :
The existence of C-H···F-C hydrogen bonds in the complexes of trifluoromethane and cyclic molecule (oxirane, cyclobutanone, dioxane, and pyridine) has been experimentally proven by Caminati and co-workers. This study presents a...
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The existence of C-H···F-C hydrogen bonds in the complexes of trifluoromethane and cyclic molecule (oxirane, cyclobutanone, dioxane, and pyridine) has been experimentally proven by Caminati and co-workers. This study presents a theoretical investigation on these C-H···F-C hydrogen bonds at B97D/6-311++G?? and MP2/6-311++G?? levels, in terms of C-H vibrational frequency shifts, atoms in molecules characteristics, and the bonding feature of C-H···F-C hydrogen bonds. It is found that in three important aspects, there are significant differences in properties between C-H···F-C and conventional hydrogen bonds. The C-H···F-C hydrogen bonds show a blueshift in the C-H vibrational frequencies, instead of the X-H normal redshift in X-H···Y conventional hydrogen bonds. The natural bond orbital (NBO) analyses show that σ and p types of lone pair orbitals of the F atom to an antibonding σ?H-C orbital form a dual C-H···F-C hydrogen bond. Such a dual hydrogen bonding leads to the proton acceptor directionality of the C-H···F-C hydrogen bond softer. Our studies also show that the Laplacian of the electron density (▽～2_(ρBCP)) is not always a good criterion for hydrogen bonds. Therefore, we should not recommend the use of the Laplacian of the electron density as a criterion for C-H···F-C hydrogen bonds.
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The kinetic of hydrogen bonding of tetramethyl-1,4-benzoquinone (duroquinone, DQ) anion radical and dianion with methanol and ethanol was investigated in DMSO. The hydrogen bonding equilibrium constant and the (hydrogen bonding) r...
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The kinetic of hydrogen bonding of tetramethyl-1,4-benzoquinone (duroquinone, DQ) anion radical and dianion with methanol and ethanol was investigated in DMSO. The hydrogen bonding equilibrium constant and the (hydrogen bonding) rate constants were evaluated through the use of linear scan-and cyclic-voltammetry. It was found that the monoanion radical hardly ' undergoes any hydrogen bonding with the above mentioned hydrogen bonding agents. The hydrogen bonding equilibrium constant, K_(cq), for the process DQ~(-·) + e ← DQ~(2-) , DQ~(2-) + m [HB] ←DQ~(2-)(HB)_m, (where HB is a hydrogen bonding agent and [ ] means large excess concentration) was found to be in the range of 2.3 × 10~3 M~(-m) to 2.2 × 10~6 M~(-m), whereas the rate constants k_m were found to be [O] 10~6 M~(-m) s~(-1) for ethanol and [O] 10~7 M~(-m) s~(-1) for methanol. The hydrogen bonding number m was found to be ~3 for both alcohols.
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摘要 :
The kinetic of hydrogen bonding of tetramethyl-1,4-benzoquinone (duroquinone, DQ) anion radical and dianion with methanol and ethanol was investigated in DMSO. The hydrogen bonding equilibrium constant and the (hydrogen bonding) r...
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The kinetic of hydrogen bonding of tetramethyl-1,4-benzoquinone (duroquinone, DQ) anion radical and dianion with methanol and ethanol was investigated in DMSO. The hydrogen bonding equilibrium constant and the (hydrogen bonding) rate constants were evaluated through the use of linear scan-and cyclic-voltammetry. It was found that the monoanion radical hardly ' undergoes any hydrogen bonding with the above mentioned hydrogen bonding agents. The hydrogen bonding equilibrium constant, K_(cq), for the process DQ~(-·) + e ← DQ~(2-) , DQ~(2-) + m [HB] ←DQ~(2-)(HB)_m, (where HB is a hydrogen bonding agent and [ ] means large excess concentration) was found to be in the range of 2.3 × 10~3 M~(-m) to 2.2 × 10~6 M~(-m), whereas the rate constants k_m were found to be [O] 10~6 M~(-m) s~(-1) for ethanol and [O] 10~7 M~(-m) s~(-1) for methanol. The hydrogen bonding number m was found to be ~3 for both alcohols.
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A structural feature of hydrogen bonding chains found in the crystals of trifluoromethylated amino alcohols is reported.Hydrogen bondings of 3-(N,N-dialkylamino)-l,l,l-trifluoro-2-propanols construct chiral spiral hydrogen bonding...
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A structural feature of hydrogen bonding chains found in the crystals of trifluoromethylated amino alcohols is reported.Hydrogen bondings of 3-(N,N-dialkylamino)-l,l,l-trifluoro-2-propanols construct chiral spiral hydrogen bonding chains.Lone pairs on the nitrogen atoms of the amino alcohols participate in two hydrogen bondings.Detailed structural analysis of the hydrogen bonds of the 3-(N,N-dimethylamino)-l,l,l-trifluoro-2-propanol suggested that the chain built up with alternating intermolecular-medium and intramolecular-weak hydrogen bonds.The medium intermolecular hydrogen bond,which transfers a proton from the hydroxy group to the amino nitrogen,would make a tentative zwitterionic form of the molecule.Then,electrostatic attraction between the charges in the zwitterion centers induced a weak intramolecular hydrogen bond.
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The initial folding of the backbone of peptides and the subsequent arrangement of the molecule into a supramolecular assembly is primarily associated with NH...O=C hydrogen bonds where the N...O distance is 2.8-3.0 A and to a lass...
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The initial folding of the backbone of peptides and the subsequent arrangement of the molecule into a supramolecular assembly is primarily associated with NH...O=C hydrogen bonds where the N...O distance is 2.8-3.0 A and to a lasser extent with water molecules that act as donors and acceptors for hydrogen bonds, both within a peptide molecule and between peptide molecules. In helical peptides, the transition between 3_(10)-helices (with 10 atoms in a hydrogen-bond ring) and #alpha#-helices (with 13 atoms in a hydrogen bond ring) merely requires a shift from one C=O to an adjacent C=O and is often quite facile. Among examples to be presented are two crystal structures of a heptapeptide wherein the helices are the 3_(10)-type when the amino end group is Boc and the #alpha#-type when the amino end is Ac. A new type of regular helix was synthesized and characterized by x-ray diffraction analysis using #beta#-amino acid residues, rather than #alpha#-amino acid residues. The new helix was named #beta#-foldamer and it is characterized by 14-atom hydrogen-bond rings rather than the 13-atom rings present in #alpha#-helices. An NH group will not always form a hydrogen bond with any C=O (or H_2O) that is present in a crystal. Examples are shown in which the N...O distance is > 3.30 A and also examples of structures in which there is no O acceptor or any other kind of acceptor within possible reach of an available NH. Assemblies with 'free' NH groups are presented in which these in no possibility for a 'cooperative hydrogen bond' between NH and the center of a phenyl group, although other structures with a close NH...phenyl approach will be discussed.
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Background: Compared with the wild type of lipase (WTL), mutant lipase 6B has twelve mutations (A15S, F17S, A20E, N89Y, G111D, L114P, A132D, M134E, M137P, I157M, S163P, N166Y). The melting temperature of 6B (78.2 degrees C) is muc...
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Background: Compared with the wild type of lipase (WTL), mutant lipase 6B has twelve mutations (A15S, F17S, A20E, N89Y, G111D, L114P, A132D, M134E, M137P, I157M, S163P, N166Y). The melting temperature of 6B (78.2 degrees C) is much higher than that of WTL (56 degrees C). Hydrogen bond (HB) play an important role in stabilizing the protein. It is important to analyze how mutations affect hydrogen bond and hydrogen bond network and explain how hydrogen bond and hydrogen bond network affect lipase thermostability by the change of the intensity of HB and HB networks with temperature changing.
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In this work, the excited-state hydrogen bonding dynamics of photoexcited coumarin 102 in aqueous solvent is reconsidered. The electronically excited states of the hydrogen bonded complexes formed by coumarin 102 (C102) chromophor...
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In this work, the excited-state hydrogen bonding dynamics of photoexcited coumarin 102 in aqueous solvent is reconsidered. The electronically excited states of the hydrogen bonded complexes formed by coumarin 102 (C102) chromophore and the hydrogen donating water solvent have been investigated using the time-dependent density functional theory method. Two intermolecular hydrogen bonds between C102 and water molecules are considered. The previous works (Wells et al., J Phys Chem A 2008, 112, 2511) have proposed that one intermolecular hydrogen bond would be strengthened and the other one would be cleaved upon photoexcitation to the electronically excited states. However, our theoretical calculations have demonstrated that both the two intermolecular hydrogen bonds between C102 solute and H_2O solvent molecules are significantly strengthened in electronically excited states by comparison with those in ground state. Hence, we have confirmed again that intermolecular hydrogen bonds between C102 chromophore and aqueous solvents are strengthened not cleaved upon electronic excitation, which is in accordance with Zhao's works.
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Local stretching modes for 69 different DH single bonds and 58 H···A H-bonds are calculated at the ?B97X-D/aug-cc- pVTZ level of theory to describe the changes in donor D and acceptor A upon forming the hydrogen-bonded complex....
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Local stretching modes for 69 different DH single bonds and 58 H···A H-bonds are calculated at the ?B97X-D/aug-cc- pVTZ level of theory to describe the changes in donor D and acceptor A upon forming the hydrogen-bonded complex. The intrinsic strength of the DH and AH interactions is determined utilizing the properties of a well-defined set of local, uncoupled vibrational modes. The local mode stretching force constant k_a(HA) provides a unique measure of bond strength for both covalently and electrostatically bonded complexes. Generally applicable bond orders are derived, which can be related to the binding energies of the hydrogen bonded complexes. Although the red shifts in the DH stretching frequencies can be used to detect hydrogen bonding, they are not sufficient to assess the strength of hydrogen bonding. It is demonstrated that the calculated BSSE-corrected binding energies of hydrogen bonded complexes are related to the sum of bond order changes caused by hydrogen bonding. The covalent character of charge assisted hydrogen bonds is explained. Because local mode frequencies can also be derived from experimental normal mode frequencies, a new dimension in the study of hydrogen bonding is gained.
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The synthesis, ~1H and ~13C NMR spectra, and X-ray structures are described for three dialkoxy ethynylnitrobenzenes that differ only in the length of the alkoxy chain, namely 1-ethynyl-2-nitro-4,5-dipropoxybenzene, C_14H_17NO_4, 1...
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The synthesis, ~1H and ~13C NMR spectra, and X-ray structures are described for three dialkoxy ethynylnitrobenzenes that differ only in the length of the alkoxy chain, namely 1-ethynyl-2-nitro-4,5-dipropoxybenzene, C_14H_17NO_4, 1,2-dibut-oxy-4-ethynyl-5-nitrobenzene, C_16H_21NO_4, and 1-ethynyl-2-nitro-4,5-dipentoxy-benzene, C_18H_25NO_4. Despite the subtle changes in molecular structure, the crystal structures of the three compounds display great diversity. Thus, 1-ethynyl-2-nitro-4,5-dipropoxybenzene crystallizes in the trigonal crystal system in the space group R3, with Z = 18, 1,2-dibutoxy-4-ethynyl-5-nitrobenzene crystallizes in the monoclinic crystal system in the space group P21/c, with Z = 4, and 1-ethynyl-2-nitro-4,5-dipentoxybenzene crystallizes in the triclinic crystal system in the space group P1, with Z = 2. The crystal structure of 1-ethynyl-2-nitro-4,5-dipropoxybenzene is dominated by planar hexamers formed by a bifurcated alkoxy sp-C-H???O,O' interaction, while the structure of the dibutoxy analogue is dominated by planar ribbons of molecules linked by a similar bifurcated alkoxy sp-C-H???O,O' interaction. In contrast, the dipentoxy analogue forms ribbons of molecules alternately connected by a self-complementary sp-C-H???O_2N interaction and a self-complementary sp~2-C-H???O_2N interaction. Disordered solvent was included in the crystals of 1-ethynyl-2-nitro-4,5-dipropoxybenzene and its contribution was removed during refinement.
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The hydrogen bonded polymer complex bulk and thin film was prepared by solution mixing and layer-by-layer assembly, respectively. Poly(vinylpyrrolidone) (PVPON) and poly(ethylene oxide) (PEO) were hydrogen bonding acceptor polymer...
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The hydrogen bonded polymer complex bulk and thin film was prepared by solution mixing and layer-by-layer assembly, respectively. Poly(vinylpyrrolidone) (PVPON) and poly(ethylene oxide) (PEO) were hydrogen bonding acceptor polymers while poly(acrylic acid) (PAA) and poly(methacrylic acid) (PMAA) were hydrogen bonding donor polymers. The detachment of hydrogen bond between the chains in polymer complexes was investigated during the dissolution in alkaline solution, ionic liquid and tertiary amine N-oxide. We compared the dissolution process of the polymer complex bulk with the polymer complex thin film, and discussed the polymer chain length, chain entanglement degree and temperature effect on hydrogen bond detachment and dissolution of polymer complexes.
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