Correlation-consistent basis sets are developed for the Ti atom. The polarization functions are optimized for the average
of the 3F and 5F states. One series of correlation-consistent basis sets is for 3d and 4s correlation, while the second series includes 3s and 3p correlation as well as 3d and 4s correlation. These basis sets are tested using the Ti 3F–5F separation and the dissociation energies of TiCl X4Φ, TiH X4Φ, and TiH+ X3Φ. The CCSD(T) complete basis set limit values are determined by extrapolation. The Douglas–Kroll approach is used to compute
the scalar relativistic effect. Spin-orbit effects are taken from experiment and/or are computed at the CASSCF level. The
Ti 3F–5F separation is in excellent agreement with experiment, while the TiCl, TiH, and TiH+ bond energies are in good agreement with experiment. Extrapolation with the valence basis set is consistent with other atoms,
while including 3s and 3p correlation appears to make extrapolation more difficult.
Received: 20 January 1999 / Accepted: 26 February 1999 / Published online: 7 June 1999 相似文献
Optical time stretch imaging (OTSI), providing the capability of capturing the dynamics of fast single-shot or random events, overcomes the fundamental trade-off between imaging speed and sensitivity in ultrafast imaging regions. Lying at the heart of the OTSI is dispersive Fourier transformation, being capable of using large chromatic dispersion to map the spectrum of a broadband ultrashort optical pulse into a stretched time-domain waveform. Dispersive grating pair (DGP) is a unique solution to generate large chromatic dispersion for dispersive Fourier transformation at the wavebands, in which dispersion compensation fibers commonly suffer from high dispersion-to-loss ratio. Here we characterize the performances of DGP-based OTSI modality and analyze the crucial parameters that strongly impact on the temporal as well as spatial resolutions, and further discuss its merits and challenges. Our results demonstrate DGP-based OTSI, allowing creation of high resolution images, is an effective modality compared to fiber-based OTSI. 相似文献
The relative stabilities of the geometrical and rotational isomers of 2-alkylideneoxacycloalkanes (-oxiranes, -oxetanes, -tetrahydrofurans, and -tetrahydropyrans; alkyl = Et, Pr, i-Bu, 2,2-dimethylpropyl) have been studied by DFT calculations. Independent of the size of the alkyl group, the E and Z isomers of alkylideneoxiranes have almost comparable stabilities (the Z form, however, being slightly favored), but, with increasing size of the heterocyclic ring, the relative stability of the E isomer decreases. This is particularly prominent for the tetrahydropyran derivatives with alkyl = 2,2-dimethylpropyl, in which marked repulsive interactions between the t-Bu group and the 3-CH2 group of the tetrahydropyran ring make the E form, ca. 13 kJ mol–1 less stable than the Z isomer. On the other hand, for alkyl = Et, Pr, and i-Bu, the relative stabilities of the geometrical isomers are almost independent of the alkyl group. Besides the relative stabilities of the geometrical isomers, energetics of the rotational isomerism of the alkyl group about the C(sp3)—C(sp2) bond is also surveyed. 相似文献
It is argued that the preservation of algebraic equivalence between the Allen and Laidler bond-energy schemes for nonconjugated
alkenes logically determines that the Allen scheme should apply to a classical structure of a conjugated hydrocarbon exactly
as it stands, i.e. no additional parameters are needed. Extending the requirement of equivalence to conjugated alkenes implies
that, in the Laidler scheme, the bond energy of the pure single CC bond in a conjugated system is a combination of the bond-energies
of the semiconjugated and normal CC single bonds: E(Cd—Cd)=2E(Cd—C)−E(C—C). This result is a deduction and is not an independent hypothesis. The equivalence of the two schemes for conjugated
hydrocarbons is demonstrated numerically, by calculating the resonance energies of some selected molecules by both methods.
Received: 5 December 1999 / Accepted: 5 March 2000 / Published online: 5 June 2000 相似文献
Ab initio and some density functional theory calculations of bond lengths in fluoro- and chloro-ethanes and disilanes are reported with a precision of ±0.0001 Å under strictly comparable conditions. The resulting changes in MH and MX (M=C, Si; X=F, Cl) bond length are analysed for the effects of halogens substituted in geminal (), or vicinal (gauche or trans) positions. The shortening effect of halogen on an MH bond is markedly reduced or even reversed by the introduction of electron correlation at the MP2 or B3LYP level. MX bonds are little affected. gauche halogen consistently shortens both MH and MX bonds, while trans halogen has no effect on an MH bond but a small and variable effect on the MX bond.
The reality of these calculated changes in bond length is tested in two ways. MH bond lengths are plotted against experimental values of the isolated stretching frequencies νisMH, which themselves correlate well with experimental r0 bond lengths. Agreement on the resulting substituent effects is generally good for the gauche and trans effects of halogen but variable for effects. Unobserved νisMH values are predicted from computed bond lengths in fluoroethanes, chloroethanes and chlorodisilanes.
Calculated MX and MM bond lengths are compared with experimental values, notably those from electron diffraction studies amongst the ethanes. Most calculations underestimate the changes found experimentally in CF and CCl bond lengths. CC bond length changes are underestimated in fluoroethanes and overestimated in the chloro-compounds.
The ‘offset’ value (re(calc)−re(true)) for a CH or SiH bond calculated with a given basis set and level of theory in most cases varies markedly throughout the series of compounds. The same is true for CF, CCl, CC and SiSi bonds if the corresponding offset values for the ra lengths are constant.
The need is stressed for extended experimental work on many of the compounds, especially the disilanes. It is recommended that structures should be refined with ab initio derived constraints on the bond lengths involved and differences between spectroscopic and diffraction-based geometries reconciled through the calculation of rz structures. 相似文献
Following widespread damage to bridge joints in the San Francisco region from the 1989 Loma Prieta earthquake, the necessity for establishing an alternative method for seismic design of bridge joints was identified. Recognizing that conventional joint design practice based directly on shear forces results in congested reinforcement details, which are difficult to implement in practice, a rational design procedure was sought through large-scale testing of bridge joint systems and subsequent finite element and strut-and-tie analyses. The finite element part of the study is presented in this paper, which focuses on (a) identification of compression force flow and thus the load path across the joint, (b) examination of an efficient joint force transfer model, and (c) influence of cap beam prestressing. Combining the experimental and analytical results, a joint design method has been established in which reduction of joint reinforcement was achieved by treating joint shear as part of the complete force transfer across the joint, rather than as an independent action. The proposed design approach has been validated in a laboratory test on a full-scale multiple-column bridge bent. 相似文献