首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献,搜索用时 31 毫秒
1.
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/∂τi)2+(∂P/∂τj)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°≤τ1≤180°; −180°≤τ1≤−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°≤τ1≤70°), there is a definite increase in the populations of τ2 and τ3 at −90 and −150°.  相似文献   

2.
3.
The syntheses and structural determination of NdIII and ErIII complexes with nitrilotriacetic acid (nta) were reported in this paper. Their crystal and molecular structures and compositions were determined by single-crystal X-ray structure analyses and elemental analyses, respectively. The crystal of K3[NdIII(nta)2(H2O)]·6H2O complex belongs to monoclinic crystal system and C2/c space group. The crystal data are as follows: a=1.5490(11) nm, b=1.3028(9) nm, c=2.6237(18) nm, β=96.803(10)°, V=5.257(6) nm3, Z=8, M=763.89, Dc=1.930 g cm−3, μ=2.535 mm−1 and F(000)=3048. The final R1 and wR1 are 0.0390 and 0.0703 for 4501 (I>2σ(I)) unique reflections, R2 and wR2 are 0.0758 and 0.0783 for all 10474 reflections, respectively. The NdIIIN2O7 part in the [NdIII(nta)2(H2O)]3− complex anion has a pseudo-monocapped square antiprismatic nine-coordinate structure in which the eight coordinate atoms (two N and six O) are from the two nta ligands and a water molecule coordinate to the central NdIII ion directly. The crystal of the K3[ErIII(nta)2(H2O)]·5H2O complex also belongs to monoclinic crystal system and C2/c space group. The crystal data are as follows: a=1.5343(5) nm, b=1.2880(4) nm, c=2.6154(8) nm, b=96.033(5)°, V=5.140(3) nm3, Z=8, M=768.89, Dc=1.987 g cm−3, μ=3.833 mm−1 and F(000)=3032. The final R1 and wR1 are 0.0321 and 0.0671 for 4445 (I>2σ(I)) unique reflections, R2 and wR2 are 0.0432 and 0.0699 for all 10207 reflections, respectively. The ErIIIN2O7 part in the [ErIII(nta)2(H2O)]3− complex anion has the same structure as NdIIIN2O7 part in which the eight coordinate atoms (two N and six O) are from the two nta ligands and a water molecule coordinate to the central NdIII ion directly.  相似文献   

4.
NMR field-cycling measurements of the Larmor frequency (v) and angular (Δ) dependences of the longitudinal proton spin relaxation time T1 for the nematic liquid crystals 5CB and 8CB allow a more detailed analysis of the underlying molecular motions than data available previously. All T1 (v, Δ) dispersion profiles essentially distinguish three frequency ranges where T1 is governed by either local field effects, collective motions (director order fluctuations), or rotational and translational diffusion of individual molecules or molecular groups, respectively. The angular dependence supports and extends previous conclusions about the significance of the order fluctuation term at low (kHz) and high (MHz) Larmor frequencies; in addition it is the basis for the disentanglement of local field effects, which involve Jeener's dipolar relaxation, and of the sophisticated rotational relaxation models suggested in the literature by Dong, Nordio and Vold. It is found that Vold's third rate concept gives the best explanation of the measurements. The results on the rotational diffusion processes essentially agree with deuteron studies from the literature, but also reveal clear distinctions with regard to the anisotropy parameter σ, essentially due to the improved separation from the order fluctuation contribution.  相似文献   

5.
The complexes formed by dimethylsulphide (DMS) and dimethyldisulphide (DMDS) with two isomers of nitrous acid have been observed, and characterised in argon and nitrogen matrices. The ν1 OH stretching vibration of the perturbed trans-HONO monomer is 425 and 294 cm−1 red shifted, respectively, for the DMS and DMDS complex in solid argon, and 441 and 301 cm−1 in solid nitrogen. A large blue shift is also observed for the ν3 NOH in-plane deformation mode: 101 and 80 cm−1 for DMS–HONO-trans in argon and nitrogen matrices and 46 cm−1 for DMDS–HONO-trans in nitrogen matrix. The results indicate formation of strong hydrogen bonds in the studied DMS–HONO and DMDS–HONO systems. The origin of the complicated shape of the ν1 OH absorption is discussed. Similarities and differences between argon and nitrogen matrices are considered.  相似文献   

6.
This study examines a linear variation of the specific heat CP with the frequency shifts 1/ν(∂ν/∂T) for the Brillouin frequencies of the L-mode [010], [001] and [100] in the ferroelectric phase of NaNO2 according to our spectroscopically modified Pippard relation. We obtain this linear relationship for those modes studied and calculate dTC/dP in the ferroelectric phase of NaNO2. Our calculated values of dTC/dP for the [001] and [100] modes are in good agreement with the values given in the literature.  相似文献   

7.
Powder X-ray diffraction, 119Sn NMR spectra, and 1H NMR spin–lattice relaxation times, T1, were measured for (CH3)nNH4−nSnCl3 (n=1–4). From the Rietveld analysis, it is shown that all four compounds crystallize into deformed perovskite-type structures at room temperature. The temperature dependence of 1H T1 was analyzed in terms of the CH3 reorientation and other motions of the whole cation. Except for the phase transition in CH3NH3SnCl3, which is from monoclinic to rhombohedral at 331 K, 1H T1 was continuously changed at other phase transitions in this compound as well as in the n=2–4 compounds, suggesting that the transitions are not caused by the change of the motional state of the cation but by an instability of the [SnCl3]nn perovskite lattice.  相似文献   

8.
The infrared spectra of phosphinic acid R2POOH dimers (R=CH3, CH2Cl, C6H5) have been studied in CCl4 and CH2Cl2 solutions (T=300 K). The infrared spectra of deuterated R2POOD dimers (R=CH3, CH2Cl) were also studied in the gas phase (T=400–550 K) and solid state (T=100–300 K). They are compared with previously studied spectra of the light (non-deuterated) dimers in the gas phase, in the solid state and in low-temperature argon matrices (T=12–30 K) in the 4000–400 cm−1 spectral region. It is found that the strong and broad ν(OH) dimer bands have similar shapes, nearly equal values of bandwidth and low-frequency shift, and possess the Hadzi ABC structure irrespective of the type of acid, significant differences of dimerization enthalpies, influence of solvent, the type of H-bonded complexes (cyclic dimers in the gas phase, in solutions, and in inert matrices, and infinite chains in the solid state), and temperature in the range 12–600 K. Isotopic ratio of the first moments of light and deuterated acid bands has been measured. Analysis of the ν(OH/OD) band of hydrogen bonded dimers of phosphinic acids shows that the interaction between the two intermolecular bonds O–HOP in a cyclic complex plays virtually no role in the mechanism of the ν(OH/OD) band formation; the shape of ν(OH/OD) band is controlled mainly by the POOH(D)O fragment; and the band shape of strong hydrogen bonded complexes is formed by a number of vibrational transitions from the ground state to different combination levels in the region 3500–1500 cm−1.  相似文献   

9.
A method for predicting an analytical equation of state for polymer mixtures and blends from surface tension and liquid state density at normal (ordinary) temperature (γn, ρn), as scaling constants, is presented. B2(T) follows a promising corresponding-states principle. Calculation of (T) and b(T), the two other temperature-dependent constants of the equation of state, are made possible by scaling. As a result, γn and ρn are sufficient for determination of thermophysical properties of polymer mixtures and blends.

We applied the procedure to predict liquid density of poly(ethylene glycol) (PEG-200) + 1-octanol solutions and poly(propylene glycol) (PPG) + poly(ethylene glycol) (PEG-200) blends at compressed state with temperature range from 298.15 to 338.15 K and pressures up to 40 MPa. In this work, the ISM EoS is extended to polymer mixtures and blends as well as pure case without proposing any mixing rule.  相似文献   


10.
The vibrational characteristics of deuterated acetonitrile dissolved in isopropanol, dimethyl formamide (DMF), and dimethyl sulfoxide (DMSO) have been studied. Observed vibrational bands show substantial frequency shifts, the amounts of which vary almost linearly with concentration. The absorption feature in the region of 2220–2280 cm−1 was deconvoluted to the consisting absorption bands. The band at 2258 cm−1 of pure CD3CN, which is on the low frequency side of the monomer CN stretch (ν2), is attributed to the CN stretch of the dimer (ν′2). The shoulder found on the further low frequency side of the ν2 band, particularly in dilute solution, is believed to be due to ν5, and its frequency and intensity vary largely as a function of concentration along with those of other vibrational bands involved with the CD3 group. The ν5 band of pure CD3CN is believed to be active and located at about 2251 cm−1. Ab initio calculations have also been performed for the solute–solvent complexes, CD3CN–DMF and CD3CN–DMSO, at the MP2/6-31+G(2d,p) level assuming anti-parallel configurations. The calculated results show a good agreement with the observed results.  相似文献   

11.
A theory is developed to describe the profiles of hydrogen stretching infrared bands of hydrogen-bonded solids taking into account the anharmonic coupling between the high-frequency stretching vibration, ν(XH), and low-frequency lattice phonons, Ωi, as well as multi-Fermi resonances between states involving the ν(XH) stretching and overtones or combinations of some internal modes. The theory has been constructed in the framework of the extended molecular exciton Davydov approach. Model calculations show that the strong couplings between the high frequency ν(XH) and low-lattice vibration frequencies, Ωi, generate the broadness of the ν(XH) band but the multi-Fermi resonances between the ν(XH) state and overtones or combinations of internal modes generate the complicated substructure band which is observed experimentally.  相似文献   

12.
Tert-butyl chloride has been broadly studied experimentally through various techniques such as X-ray crystallography, DTA, and NMR. It was also studied experimentally through nuclear quadrupole resonance (NQR), but this study was limited and incomplete. In this paper, we present a more detailed study of TBC through the NQR of 35Cl. Our results show that near 120 K, the onset of the CH3 groups semirotations around symmetry axis C3 takes place with an activation energy U=16.1 kJ mol−1. This intramolecular movement produces a T1 minimum near 148 K and is the dominant mechanism of the nuclear spin-lattice relaxation in phase III of this compound. In phase II of TBC, we show that there are not only methyl groups semirotations, but also semirotations of the whole molecule around a different axis from the symmetry axis C3 (C–Cl bond) with an activation energy of E=10.4 kJ mol−1.  相似文献   

13.
A T1ρ filter for the NMR imaging of solid polymers is presented. T1ρ is the relaxation time in the rotating frame, which is sensitive to the molecular motion and whose spectral density falls in the range of slow dynamics, namely motion characterized by correlation times ranging between 10−2 and 10−5 s. The method allows one to obtain a good spatial resolution without the need of strong field gradients and without the effects related to the spatial encoding manipulation of the spin system.  相似文献   

14.
Carbon oxides of the form COn (n = 2–8) have long been known as important molecules in atmospheric and solid state chemical reactions. Here, we report on the first infrared spectroscopic detection of the cyclic (Cs) isomer of carbon hexaoxide (12C16O6) via its ν1 vibrational mode centered around 1876 cm−1 under matrix isolation conditions; the identification of the 12C18O6, 13C16O6, and 13C18O6, isotopologues supported by ab initio calculations confirm the assignments. We also discuss possible formation routes of this molecule.  相似文献   

15.
Calculations with Hartree—Fock electron densities for the rare gas atoms He through Xe show that the gradient expansion for the kinetic energy functional, T[] = T0[] + T2[] + T4[] + … = ∫t() dτ, approximates the kinetic energy by averaging over the shell structure present in the true local kinetic energy density, t(), and that the accuracy of the gradient expansion improves with increasing atomic number. Components of t(), t0(), t2() and t4(), are exhibited and discussed. The defined function t() is everywhere positive.  相似文献   

16.
A new technique, called interpolation method, with general application in the kinetic analysis of processes studied by thermogravimetry (TG) under linear temperature programming is developed. It is based on the linear relationship, with slope 1, between log g() and log I(γ, θ) for the appropriate kinetic function, where I(γ, θ) is the normalized temperature integral, θ the normalized temperature (θ=T/T0) and γ a dimensionless activation energy (γ=E/RT0). Values of log I(γ, θ) are calculated by linear interpolations in a pre-built table. This method can easily be programmed and implemented in a personal computer, where the results (kinetic parameters and quality of regressions for the kinetic functions considered) are typically obtained in a very short time. The method is validated by analyzing different simulated thermogravimetric curves and comparing the results with those determined with some classic methods taken from the literature. In addition, the results are compared with the values obtained by a similar method, also developed and explained in this paper, which involves the evaluation of all the values of the temperature integral by numerical integration, therefore, demanding a much larger calculation time. The interpolation method is found to be more accurate than other published methods, particularly in the case of thermogravimetric curves corresponding to processes with low activation energies. The results obtained are always similar to those determined by the integration method, which is taken as reference. Application of the technique to experimental data for various types of reactions shows that the results are in agreement with the published parameters and kinetic laws.  相似文献   

17.
Polarised IR and Raman spectra for KH2PO3 single crystal samples were measured at room temperature. Additionally, the IR spectra for the Xb(Z) sample were also measured at low temperatures (300–14 K). The spectra are discussed on the basis of oriented gas model and group theory. The stretching νOH vibrations of the hydrogen bonds with the OO distances of 2.547 and 2.529 Å give characteristic broad ABC-type bands in the IR (polarised parallel to the X and to the b(Z) directions) and Raman (xx, xz and yx) spectra. The Davydov-type (correlation field or factor group) splitting is not observed for the νOH modes. The presence of two independent hydrogen bonds in the crystal is manifested by splitting of the C band into two (C′, C″) components and by the different frequencies of the out-of-plane bending γOH vibrations. The in-plane bending modes δOH are strongly mixed/coupled with the stretching vibrations of the PO3 groups.

The C bands (C′ and C″) change into quite sharp bands on lowering of the temperature. Various simplified models for internal vibrations of the phosphite anions are applied for finding a correlation between the crystal structure and polarised vibrational spectra. The stretching vibrations of the νPH groups manifest their unequivalence in two symmetry-independent hydrogenphosphite anions.  相似文献   


18.
The rate constants, k1 and k2 for the reactions of C2F5OC(O)H and n-C3F7OC(O)H with OH radicals were measured using an FT-IR technique at 253–328 K. k1 and k2 were determined as (9.24 ± 1.33) × 10−13 exp[−(1230 ± 40)/T] and (1.41 ± 0.26) × 10−12 exp[−(1260 ± 50)/T] cm3 molecule−1 s−1. The random errors reported are ±2 σ, and potential systematic errors of 10% could add to the k1 and k2. The atmospheric lifetimes of C2F5OC(O)H and n-C3F7OC(O)H with respect to reaction with OH radicals were estimated at 3.6 and 2.6 years, respectively.  相似文献   

19.
The crystalline Br, I and CH3 derivatives of 9-hydroxyphenalenone (5X–9HPO) and their deuteroxy analogues (5X–9DPO) are treated by application of the Ising model approaches. The molecular field parameter (J0) as well as the tunneling parameter (Ω) are evaluated for each material with the help of different quantum chemistry procedures. As our evaluations show both relations Ω(D)/J0 and Ω(H)/J0 are less than unity in the case of 5CH3–9(H/D)PO derivatives that leads to an appearance of the low-temperature ordered (anti-ferroelectric) phase. A relatively small Ω(D)/J0 (0.2–0.4) values derived in the case of 5Br– and 5I–9DPO imply the tendency to transition into the similar phase in these species. At the same time a rather large values of Ω(H)/J0 (0.9–1.7) derived in the case of their hydroxy analogues speak in favor of their quantum paraelectric behavior. The obtained theoretical estimations and conclusions are compared with the available experimental data.  相似文献   

20.
Pulsed-EPR spectroscopy was used to study a modified C60 molecule (1) in its photoexcited triplet state. The analysis of the triplet EPR lineshape shows a break of the high symmetry of the lowest populated triplet state of C60 due to the modification of the C60 unity. The reported temperature dependence of the spectra is influenced by relaxation effects. Lineshape simulations based on a triplet Hamiltonian including anisotropic T1/T2 relaxation were performed. The results are discussed in relation to the behaviour of pure 3C60.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号