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1.
The energies of the conformations of the serine dipeptide are computed by ab initio molecular orbital theory in order to demonstrate the existence of the hydrogen bonds, torsion barriers and non-bonded repulsions which govern the relative energies of these conformations. The Ramachandran map of the dipeptide is computed and found to resemble closely those of alanine and ethyl glycine. The side-chain maps are them computed and analysed to show the above physical effects in terms of the torsion barrier-free maps. There is generally good agreement between the experimental and theoretical results in the sense that all three side-chain conformations which are staggered about CCβ (1 = + 60, 180 and −60°) are computed to be close in energy and all three values of 1 occur with similar frequencies in the proteins examined. The fact that serine, unlike all other comparable residues except threonine, tends to favour a 1 value of + 60° in the bridge region is reproduced clearly by the computations and analysis of the physical factors involved shows that this conformation is stabilised by side chain to backbone hydrogen bonding. 相似文献
2.
Carnosine (β-alanyl-L-histidine) is a biologically active molecule involved in muscular metabolism. It crystallises in the C; space group with a = 24.725 Å b = 5,427 Å c = 8,004 Å β = 100,2° ( Z = 4) In the crystal, acid and basic groups are engaged in hydrogen bonds whose strength is evaluated through IR frequencies. Molecular conformation in the solid state is defined by τ1 = /t-177° τ2 = −38° φ = −96° ψ = +131° χ1 = 181° χ21 = 62° NMR study of carnosine in aqueous solution indicates that rotation about CH2-CH2 is free and that the other angles take the following values: Ø −150° or −90° and X1 = 165° or 315°. Infrared and Raman spectra suggest that τ2 undergoes small changes when going from crystal to solution while ψ is close to +150°. 相似文献
3.
Medium-resolution spectra of the N 2 b 1Π u-X 1Σ g+ band system were recorded by 1 + 1 multiphoton ionization. In the spectra we found different linewidths for transitions to different vibrational levels in the b 1Π u state: Δν 0 = 0.50 ± 0.05 cm −1, Δν 1 = 0.28 ± 0.02 cm −1, Δν 2 = 0.65 ± 0.06 cm −1, Δν 3 = 3.2 ± 0.5 cm −1, Δν 4 = 0.60 ± 0.07 cm −1, and Δν 5 = 0.28 ± 0.02 cm −1. From these linewidths, predissociation lifetimes τ ν were obtained: τ 0 = 16 ± 3 ps, τ 1 > 150 ps, τ 2 = 10 ± 2 ps, τ 3 = 1.6 ± 0.3 ps, τ 4 = 9 ± 2 ps, and τ 5 > 150 ps. Band origins and rotational constants for the b 1Π uν = 0 and 1 levels were determined for the 14N 2 and 14N 15N molecules. 相似文献
4.
A three-dimensional X-ray diffraction study has shown that the product obtained by bromination of
- or
-menthone is 2,4-dibromomenthone [2(a), 6(e)-dibromo-2(e)-isopropyl-5(e)methyl cyclohexanone]. The two Br atoms are trans, and the isopropyl and methyl groups are also trans. There are four molecules in the unit cell, which has symmetry P2 12 12 1 and dimensions a = 13·58, b = 13·81 and c = 6·25 Å. One intermolecular Br … Br contact of 3·52 ± 0·01 Å occurs, a distance which is about 0·4 Å shorter than the van der Waals distance. 相似文献
5.
The hydrothermal reactions of vanadium oxide starting materials with divalent transition metal cations in the presence of nitrogen donor chelating ligands yield the bimetallic cluster complexes with the formulae [{Cd(phen 2) 2V 4O 12]·5H 2O (1) and [Ni(phen) 3] 2[V 4O 12]·17.5H 2O (2). Crystal data: C 48H 52Cd 2N 8O 22V 4 (1), triclinic.
a=10.3366(10), b=11.320(3), c=13.268(3) Å, =103.888(17)°, β=92.256(15)°, γ=107.444(14)°, Z=1; C 72H 131N 12Ni 2O 29.5V 4 (2), triclinic.
a=12.305(3), b=13.172(6), c=15.133(4), =79.05(3)°, β=76.09(2)°, γ=74.66(3)°, Z=1. Data were collected on a Siemens P4 four-circle diffractometer at 293 K in the range 1.59° <θ<26.02° and 2.01°<θ<25.01° using the ω-scan technique, respectively. The structure of 1 consists of a [V 4O 12] 4− cluster covalently attached to two {Cd(phen) 2} 2+ fragments, in which the [V 4O 12] 4− cluster adopts a chair-like configuration. In the structure of 2, the [V 4O 12] 4− cluster is isolated. And the complex formed a layer structure via hydrogen bonds between the [V 4O 12] 4− unit and crystallization water molecules. 相似文献
6.
In connection with heliangine, extracted from leaves of Helianthus tuberosus L., the crystal structure of dihydroheliangine monochloroacetate, C 22H 29C 7Cl, has been determined by X-ray methods. The tetragonal unit cell with dimensions, A = 13·77, C = 11·95 Å, contains four molecules, the space group being P4 1 - C 42 or P4 3 - C 44. Using a three-dimensional Patterson function and minimum functions, a majority of atomic positions were determined. Further elucidation of the structure was continued by alternative application of successive least squares treatment and Fourier syntheses. The structure thus determined is fully consistent with the chemical results obtained by Morimoto et al. The final R-value is 0·134 (using 1680 data with sin 2 θ/λ 2 < 0·20) or 0·152 (using 2419 data with sin 2θ/λ 2 < 0·27). 相似文献
7.
Irradiation of the 30-electron Mo 2(η 5-C 5Me 5) 2(CO) 4 and Re 2(CO) 10 in toluene solution (containing H 2O) afforded (in 1–2% yields) a novel triangular metal cluster, (η 5-C 5Me 5) 3Mo 3(CO) 4(η 2-H)(η 3-O) (1), which was characterized by a single-crystal X-ray diffraction study. Compound 1, of pseudo Cs- m symmetry, has a triangulo-Mo 3(η 3-O) core with composite Mo---H---Mo and Mo---Mo electron-pair bonds along one unusually short edge (2.660(1) Å) and Mo--- electron-pair bonds along the other two edges (2.916(1) and 2.917(1) Å). The edge-bridged hydride ligand, which displays a characteristic high-field proton NMR resonance at δ −17.79 ppm, was not found from the crystallographic determination but was located via a quantitative potential-energy-minimization method. This procedure unambiguously established that the optimized hydrogen position, which corresponds to a distinct coordination site with identical Mo---H distances of 1.85 Å, is the only one that can be sterically occupied by a metal-bound hydride ligand. This 46-electron species is the first electron-deficient trimolybdenum cluster containing a monoprotonated Mo---Mo double bond; its existence is attributed to ligand overcrowding due to the bulky pentamethylcyclopentadienyl rings. Black (η 5- C 5Me 5) 3Mo 3(CO) 4(η 2-H)(η 3-O) · 1/2THF crystallizes with two formula species in a triclinic unit cell of P1 symmetry with a 8.603(4), b 11.115(4), c 19.412(11) Å, 80.69(4)°, β 101.10(4)°, and γ 98.88(3)° at −40° C. Least-squares refinement (RAELS with 221 variables) of one independent Mo 3 molecule and a centrosymmetrically-disordered THF molecule converged at R1( F) 5.62%, R2( F 6.88% for 8460 independent diffractometry data ( I0 ρ 3σ( I0 collected at −40° C with Mo- K radiation 相似文献
8.
The molecular structures of n-hexane were determined by RHF/4-21G ab initio geometry optimization at 30° grid points in its three-dimensional τ1(C11–C8–C5–C1), τ2(C14–C11–C8–C5), τ3(C17–C14–C11–C8) conformational space. Of the resulting 12×12×12=1728 grid structures, 468 are symmetrically non-equivalent and were optimized constraining the torsions τ1, τ2, and τ3 to the respective grid points, while all other structural parameters were relaxed without any constraints. From the results, complete parameter surfaces were constructed using natural cubic spline functions, which make it possible to calculate parameter gradients, | P|=[(∂ P/∂τ i) 2+(∂ P/∂τ j) 2] 1/2, where P is a C–C bond length or C–C–C angle. The parameter gradients provide an effective measure of the torsional sensitivity of the system and indicate that dynamic activities in one part of the molecule can significantly affect the density of states, and thus the contributions to vibrational entropy, in another part. This opens the possibility of dynamic entropic conformational steering in complex molecules; i.e. the generation of free energy contributions from dynamic effects of one part of a molecule on another. When the conformational trends in the calculated C–C bond lengths and C–C–C angles are compared with average parameters taken from some 900 crystallographic structures containing n-hexyl fragments or longer C–C bond sequences, some correlation between calculated and experimental trends in angles is found, in contrast to the bond lengths for which the two sets of data are in complete disagreement. The results confirm experiences often made in crystallography. That is, effects of temperature, crystal structure and packing, and molecular volume effects are manifested more clearly in bond lengths than bond angles which depend mainly on intramolecular properties. Frequency analyses of the τ1, τ2 and τ3 torsional angles in the crystal structures show conformational steering in the sense that, if τ1 is trans peri-planar (170°≤τ 1≤180°; −180°≤τ 1≤−170°), the values of τ2 and τ3 are clustered closely around the ideal gauche (±60°) and trans (±180°) positions. In contrast, when τ1 is in the region (50°≤τ 1≤70°), there is a definite increase in the populations of τ2 and τ3 at −90 and −150°. 相似文献
9.
Variable temperature (−105 to −150 °C) studies of the infrared spectra (3500–400 cm −1) of 1,1-dimethylhydrazine, (CH 3) 2NNH 2, in liquid krypton have been carried out. No convincing spectral evidence could be found for the trans conformer which is expected to be at least 600 cm −1 less stable than the gauche form. The structural parameters, dipole moments, conformational stability, vibrational frequencies, and infrared and Raman intensities have been predicted from MP2/6-31G(d) ab initio calculations. The predicted infrared and Raman spectra are compared to the experimental ones. The adjusted r0 parameters from MP2/6-311+G(d,p) calculations are compared to those reported from an electron diffraction study. The energy differences between the gauche and trans conformers have been obtained from MP2 ab initio calculations as well as from density functional theory by the B3LYP method calculations from a variety of basis sets. All of these calculations indicate an energy difference of 650–900 cm −1 with the B3LYP calculations predicted the larger values. The potential function governing the conformational interchange has been predicting from both types of calculations and comparisons have been made. The barrier to internal rotation by the independent rotor model of the inner methyl group is predicted to have a value of 1812 cm −1 and that of the outer one of 1662 cm −1 from ab initio MP2/6-31G(d) calculations. These values agree well with the experimentally determined values of 1852±16 and 1558±12 cm −1, respectively, from a fit of the torsional transitions with the coupled rotor model. For the coupled rotor model the predicted V′ 33 (sin 3τ 0 sin 3τ 1 term) value which ranged from 190 to 232 cm −1 is in reasonable agreement with the experimental value of 268±3 cm −1 but the predicted V33 (cos 3τ 0 cos 3τ 1 term) value of −73 to −139 cm −1 is 25% smaller and of the opposite sign of the experimental value of 333±22 cm −1. These theoretical and spectroscopy results are compared to similar quantities of some corresponding molecules. 相似文献
10.
The optimized structures and proton transfer reactions of 3-methyl-5-hydroxyisoxazole and its water complexes (3-M-5-HIO · (H 2O) n · ( n = 0–3)) were computed at B3LYP and MP2 theoretical level. The results indicates that 3-M-5-HIO has four isomers (Ecis, Etrans, K1 and K2), and the keto tautomer, and K2 is the most stable isomer in the gas phase. Hydrogen bonding between 3-M-5-HIO and the water molecules can dramatically lower the barrier by the concerted transfer mechanism. Ecis · (H 2O) 3 → K1 · (H 2O) 3 and Ecis · (H 2O) 2 → K2 · (H 2O) 2 is found to be very efficient. Comparing with the proton transfer mechanism of 5-HIO shows that the methyl substitution prevents the intramolecular proton transfer. 相似文献
11.
The microwave spectrum of isopropyl fluoroformate is characterized by intense a-type R-branch transitions from one conformational species. The rotational constants of the ground state, A0 = 4967.0(8) MHz, B0 = 1704.69(2) MHz, C0 = 1468.86(1) MHz and κ = −0.8651(2) are consistent with a τ 1 (O=COC) = 0°, τ 2(COCH) ˜35° structure. This structure can be viewed as a combination of the two conformational species found in ethyl fluoroformate. Two vibrational satellites having rotational constants A0 = 4963(5) MHz, B0 = 1694.11(7) MHz. C0 = 1471.43(4) MHz and A0=4998(6) MHz, B0 = 1705.21(7) MHz, C0 = 1471.10(4) MHz have been assigned. 相似文献
12.
The vibrational spectra of σ-(C 3H 5)Mn(CO) 5 are reported. Assignment of bands is made and carbonyl force constants are calculated. The results indicate that the Mn(CO) 5 moiety has C4ν symmetry. The calculated angle between the axial and equatorial carbonyl groups is approximately 95°. The bonding in this compound is very similar to that in (CH 3)Mn(CO) 5. In the far-infrared region, seven bands are expected in C4ν symmetry (3A1 + 4E), and all are observed. 相似文献
13.
The reaction between RMgCl (two equivalents) and 1,2-W 2Cl 2(NMe 2) 4 in hydrocarbon solvents affords the compounds W 2R 2(NMe 2) 4, where R = allyl and 1− and 2-methyl-allyl. In the solid state the molecular structure of W 2(C 3H 5) 2(NMe 2) 4 has C2 symmetry with bridging allyl ligands and terminal W---NMe 2 ligands. The W---W distance 2.480(1) Å and the C---C distances, 1.47(1) Å, imply an extensive mixing of the allyl π-MOs with the WW π-MOs, and this is supported by an MO calculation on the molecule W 2(C 3H 5) 2(NH 2) 4 employing the method of Fenske and Hall. The most notable interaction is the ability of the (WW) 6+ centre to donate to the allyl π *-MO (π 3). This interaction is largely responsible for the long W---W distance, as well as the long C---C distances, in the allyl ligand. The structure of the 2-methyl-allyl derivative W 2(C 4H 7) 2(NMe 2) 4 in the solid state reveals a gauche-W 2C 2N 4 core with W---W = 2.286(1) Å and W---C = 2.18(1) Å, typical of WW and W---C triple and single bonds, respectively. In solution (toluene-d 8) 1H and 13C NMR spectra over a temperature range −80°C to +60°C indicate that both anti- and gauche- W 2C 2N 4 rotamers are present for the 2-methyl-allyl derivative. In addition, there is a facile fluxional process that equilibrates both ends of the 2-methyl-allyl ligand on the NMR time-scale. This process leads to a coalescence at 100°C and is believed to take place via an η 3-bound intermediate. The 1-methyl-allyl derivative also binds in an η 1 fashion in solution and temperature-dependent rotations about the W---N, W---C and C=C bonds are frozen out at low temperatures. The spectra of the allyl compound W 2(C 3H 5) 2(NMe 2) 4 revealed the presence of two isomers in solution—one of which can be readily reconciled with the presence of the bridging isomer found in the solid state while the other is proposed to be W 2(η 3-C 3H 5) 2(NMe 2) 4. The compound W 2R 2(NMe 2) 4 where R = 2,4-dimethyl- pentadiene was similarly prepared and displayed dynamic NMR behaviour explainable in terms of facile η 1 = η 3 interconversions. 相似文献
14.
The stabilities of the hydrated uranyl phosphates (UO 2) 3(PO 4) 2 · 4 H 2O, UO 2HPO 4 · 4 H 2O, and UO 2(H 2PO 4) · 3 H 2O have been reinvestigated. The compounds identified by thermal analysis have been prepared isothermally and characterized by their strongest X-ray reflections. During dehydration, oxygen was not evolved and the crystalline compounds (UO 2) 3(PO 4) 2, (UO 2) 2P 2O 7, UO 2(PO 3) 2, and probably (UO 2) 3P 4O) 13 were found. At still higher temperatures, the uranyl phosphates are reduced. The decomposition products lose phosphorus oxide above 1300–1400°C. The present results are summarized in a tentative pseudo-binary phase diagram UOx(x = 3 to 2)—UO2(PO3)2. 相似文献
15.
The compound [RU 3(μ 3,η 2- -ampy)(μ 3η 1:η 2-PhC=CHPh)(CO) 6(PPh 3) 2] (1) (ampy = 2-amino-6-methylpyridinate) has been prepared by reaction of [RU 3(η-H)(μ 3,η 2- ampy) (μ,η 1:η 2-PhC=CHPh)(CO) 7(PPh 3)] with triphenylphosphine at room temperature. However, the reaction of [RU 3(μ-H)(μ 3, η 2 -ampy)(CO) 7(PPh 3) 2] with diphenylacetylene requires a higher temperature (110°C) and does not give complex 1 but the phenyl derivative [RU 3(μ 3,η 2-ampy)(μ,η 1:η 2 -PhC=CHPh)(μ,-PPh 2)(Ph)(CO) 5(PPh 3)] (2). The thermolysis of complex 1 (110°C) also gives complex 2 quantitatively. Both 1 and 2 have been characterized by0 X-ray diffraction methods. Complex 1 is a catalyst precursor for the homogeneous hydrogenation of diphenylacetylene to a mixture of cis- and trans -stilbene under mild conditions (80°C, 1 atm. of H 2), although progressive deactivation of the catalytic species is observed. The dihydride [RU 3(μ-H) 2(μ 3,η 2-ampy)(μ,η 1:η 2- PhC=CHPh)(CO) 5(PPh 3) 2] (3), which has been characterized spectroscopically, is an intermediate in the catalytic hydrogenation reaction. 相似文献
16.
The reaction between Ru 3(CO) 12 and a cyclic olefin ( cis-cyclooctene or trans-cyclododecene) at 100 °C for several hours gives the title compounds (μ-H) 2RU 3(CO) 9(μ 3-η 2-C 8H 12) (1), and (μ-H)RU 3(CO) 9(μ 3-η 3-C 12H 19) (2), both of which have been characterized by X-ray diffraction studies, IR and NMR spectral measurements and elemental analysis. The prolonged reaction between Ru 3(CO) 12 and cis-cyclooctene gives compound HRu 3(CO) 9(C 8H 11) (3). Compound 3 has been characterized with IR and NMR spectral analyses. In 1 the cyclooctene ring is linked via a μ 3-η 2-alkyne type of bonding to the face of the Ru 3 cluster. It is formally σ-bonded to two of the three Ru atoms and π-bonded to the third Ru. The two hydrides in 1 are bridging Ru---Ru bonds. In 2 the cyclododecene ring is bonded to the Ru 3 face via the μ 3-η 3-CCHC linkage. There are two formal σ-bonds from the allyl part to the hydrido-bridged Ru atoms and the η 3-allyl linkage to the third Ru atom. 相似文献
17.
Organolanthanide chloride complexes [(CH 3OCH 2CH 2C 5H 4) 2Ln(μ-Cl)] 2 (Ln = La, Pr, Ho and Y) react with excess NaH in THF at 45°C to give the dimeric hydride complexes [(CH 3OCH 2CH 2C 5H 4) 2Ln(μ-H)] 2, which have been characterized by IR, 1H NMR, MS and XPS spectroscopy, elemental analyses and X-ray crystallography. [(CH 3OCH 2CH 2C 5H 4) 2Y(μ-H)] 2 crystallizes from THF/n-hexane at −30°C, in the triclinic space group P1 with a = 8.795(2) Å, b = 11.040(1) Å, c = 16.602(2) Å, = 93.73(1)°, β = 91.82(1)°, γ = 94.21(1)°, Dc = 1.393 gcm −3 for Z = 2 dimers. However, crystals of [(CH 3OCH 2CH 2C 5H 4) 2Ho(μ-OH)] 2 were obtained by recrystallization of holmium hydride in THF/n-hexane at −30°C, in the orthorhombic space group Pbca with a = 11.217(2) Å, b = 15.865(7) Å, c = 17.608(4) Å, Dc = 1.816 gcm −3 for Z = 4 dimers. In the complexes of yttrium and holmium, each Ln atom of the dimers is coordinated by two substituted cyclopentadienyl ligands, one oxygen atom and two hydrogen atoms (for the Y atom) or two hydroxyl groups (for the Ho atom) to form a distorted trigonal bipyramid if the C(η 5)-bonded cyclopentadienyl is regarded as occupying a single polyhedral vertex. 相似文献
18.
A high-resolution emission spectrum of a low-pressure Ar-diluted CO + N 2O → CO 2 + N 2 flame catalyzed by Na metal vapor has been obtained and examined for vibrational disequilibrium. Emission in the 1900-2400 cm −1 spectral region, which includes the fundamental and “hot” bands of CO, CO 2(ν 3), and N 2O(ν 3), was recorded with high resolution and the CO emission was analyzed in detail to determine vibrational and rotational temperatures which were found to be unequal, Tv = 2050°K and TR = 1100°K. An examination of vib-vib and vib-trans energy transfer mechanisms results in the conclusion that an excess of 14% of the chemical energy is preferentially deposited in the resonantly-coupled N 2, CO, CO 2 (ν 3), and N 2O(ν 3) vibrational modes. It is further observed that CO vibrational levels for ν > 4 are excessively populated, presumably due to quenching of Na*(3p) by CO; the flame is accompanied by intense Na D-line chemiluminescence. 相似文献
19.
The interaction between Mo 2(O 2CCH 3) 4, Me 3SiI and I 2 in THF resulted in oxygen abstraction from the solvent and formation of [Mo 2(μ-O)(μ-I)(μ-O 2CCH 3) I 2(THF) 4] +[MoOI 4(THF)] − and I---(CH 2) 4---I. The molybdenum complex has been characterized by X-ray diffractometry. Crystal data: triclinic, space group P
, a = 13.827(3) Å; b = 15.803(7) Å; c = 9.950(3) Å; = 93.34(4)°; β = 102.40(2)°; γ = 90.09(2)°; V = 2120(2) Å 3; Z = 2; dcalc = 2.559 g cm −3; R = 0.0476 ( Rw = 0.0613) for 370 parameters and 3938 data with F02> 3σ( F02). The metal-metal distance in the cation is 2.527(2) Å and indicates a strong interaction. The magnetic behavior is consistent with the assignment of one unpaired electron to the Mo 27+ core of the cation and one to the d1 Mo(V) center of the anion. The interaction between Mo(CO) 6 and I 2 in THF also results in the formation of 1,4-diiodobutane. 相似文献
20.
Reaction of [Ru 3(CO) 12 with (CF 3) 2P---P(CF 3) 2 in p-xylene at 140°C yielded the compounds [Ru 4(CO) 13{μ-P(CF 3) 2} 2] (1), [Ru 4(CO) 14{μ-P(CF 3) 2} 2] (2) and [Ru 4(CO) 11{μ-P(CF 3) 2} 4] (3). Reaction with [(μ-H) 4Ru 4(CO) 12] under similar conditions yielded [(μ-H) 3Ru 4(CO) 12{μ-P(CF 3) 2}] (4). All four compounds have been characterised by X-ray crystallography. The fluxional behaviour of the hydrides in 4 has also been studied by variable-temperature NMR spectroscopy. Compounds 1, 2 and 4 were also obtained from the reactions of Ru 3(CO) 12 with (CF 3) 2PH in dichloromethane at 80°C. 相似文献
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