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101.
H. Tran B. Picquet-Varrault C. Viatte F. Hase 《Journal of Quantitative Spectroscopy & Radiative Transfer》2011,112(14):2287-2295
A theoretical approach is proposed to model line-mixing (LM) effects on absorption coefficients of O3 perturbed by N2 and air. It uses state-to-state rotational cross-sections calculated with a semi-classical approach and two empirical parameters, which enable switching from the state space to the line space. The first, associated with couplings within Q branches is deduced from a room temperature far-infrared spectrum. The second, governing line-couplings between R (or P) lines, is determined from a spectrum measured in the ν1+ν2+ν3 band. The model developed is then successfully compared with measurements performed at room temperature for a relatively large range of pressure (0.7-8 atm) and in four different bands (from 3 to 300 μm). Accurate predictions are, in particular, obtained in the 10 μm (ν1, ν3) region, which is widely used for remote sensing purposes. Consequences of LM effects on retrievals of ozone atmospheric volume mixing ratios are then studied using simulated atmospheric spectra. The results show that LM leads to systematic spectra fit residuals and errors on the retrieved ozone amounts, which are small but might be detectable in measured atmospheric spectra. 相似文献
102.
Siebert MR Zhang J Addepalli SV Tantillo DJ Hase WL 《Journal of the American Chemical Society》2011,133(21):8335-8343
Abietic acid, a constituent of pine resin, is naturally derived from abietadiene?--a process that requires four enzymes: one (abietadiene synthase) for conversion of the acyclic, achiral geranylgeranyl diphosphate to the polycyclic, chiral abietadiene (a complex process involving the copalyl diphosphate intermediate) and then three to oxidize a single methyl group of abietadiene to the corresponding carboxylic acid. In previous work (Nature Chem.2009, 1, 384), electronic structure calculations on carbocation rearrangements leading to abietadienyl cation revealed an interesting potential energy surface with a bifurcating reaction pathway (two transition-state structures connected directly with no intervening minimum), which links two products--one natural and one not yet isolated from Nature. Herein we describe direct dynamics simulations of the key step in the formation of abietadiene (in the gas phase and in the absence of the enzyme). The simulations reveal that abietadiene synthase must intervene in order to produce abietadiene selectively, in essence steering this reaction to avoid the generation of byproducts with different molecular architectures. 相似文献
103.
A model is proposed for performing microcanonical variational transition state theory calculations which incorporates ideas from vibrator and flexible variational transition state models. Vibrational frequencies, moments of inertia, and potential energy for the variational transition state are found by reaction path following as for the vibrator model. However, the transitional modes are treated as hindered rotors using an analytic potential and an analytic density of states, which are fit to barriers for hindered rotation determined from reaction path following. The model proposed here differs from the flexible transition state model in that the density of states for the transitional modes is analytic and transitional modes and external rotational angular momenta are uncoupled. For the H + CH3 ? CH4 system, rate constants calculated with this new model are only 6–23% smaller on average from those of the flexible transition state model for values of total angular momentum which correspond to average rotational temperatures of 0–2000 K. Harmonic frequencies calculated for the transitional modes from the hindered rotor Hamiltonian are in good agreement with the exact values found by a reaction path analysis. © 1994 John Wiley & Sons, Inc. 相似文献
104.
The method of conjugative interruption in conjunction with the CNDO/S and an ab initio method is applied to cyclopropene, cyclopropenone, cyclopropenthione, thiirene 1-oxide, and thiirene 1,1-dioxide. Thereby the orbital interactions between the π orbitals of the CC and M(MCH2, CO, CS, SO, and SO2) subunits a quantitatively analyzed as well as their consequences as regards the conjugative and inductive abilities of M, the aromaticities, and geometries of this chemically important series of compounds. A detailed comparison of the CNDO/S and ab initio results is made. Both sorts of calculation yield surprisingly well concurring results. Where the results of both methods basically differ (as to the sign of the aromaticities, or the splitting pattern of two interacting orbitals) it can be traced back to the zero differential overlap approximation made for the valence electron procedure. In addition, the paper presents the first ab initio data on cyclopropenethione, thiirene 1-oxide, and thiirene 1,1-dioxide. 相似文献
105.
Two new cembranoid diterpenes, cembrenene (1) and mayol (2), have been isolated from Sinularia mayi L?ttschw, along with the first reported cembrene (3) from marine organism. The structures were determined from spectral data and chemical transformations. 相似文献
106.
In previous research [J. Chem. Phys. 111, 3800 (1999)] a Hessian-based integration algorithm was derived for performing direct dynamics simulations. In the work presented here, improvements to this algorithm are described. The algorithm has a predictor step based on a local second-order Taylor expansion of the potential in Cartesian coordinates, within a trust radius, and a fifth-order correction to this predicted trajectory. The current algorithm determines the predicted trajectory in Cartesian coordinates, instead of the instantaneous normal mode coordinates used previously, to ensure angular momentum conservation. For the previous algorithm the corrected step was evaluated in rotated Cartesian coordinates. Since the local potential expanded in Cartesian coordinates is not invariant to rotation, the constants of motion are not necessarily conserved during the corrector step. An approximate correction to this shortcoming was made by projecting translation and rotation out of the rotated coordinates. For the current algorithm unrotated Cartesian coordinates are used for the corrected step to assure the constants of motion are conserved. An algorithm is proposed for updating the trust radius to enhance the accuracy and efficiency of the numerical integration. This modified Hessian-based integration algorithm, with its new components, has been implemented into the VENUS/NWChem software package and compared with the velocity-Verlet algorithm for the H(2)CO-->H(2)+CO, O(3)+C(3)H(6), and F(-)+CH(3)OOH chemical reactions. 相似文献
107.
Linear molecules with degenerate bending modes have states, which may be represented by the quantum numbers N and L. The former gives the total energy for these modes and the latter identifies their vibrational angular momentum jz. In this work, the classical mechanical analog of the N,L-quantum states is reviewed, and an algorithm is presented for selecting initial conditions for these states in quasiclassical trajectory chemical dynamics simulations. The algorithm is illustrated by choosing initial conditions for the N = 3 and L = 3 and 1 states of CO2. Applications of this algorithm are considered for initial conditions without and with zero-point energy (zpe) included in the vibrational angular momentum states and the C-O stretching modes. The O-atom motions in the x,y-plane are determined for these states from classical trajectories in Cartesian coordinates and are compared with the motion predicted by the normal-mode model. They are only in agreement for the N = L = 3 state without vibrational angular momentum zpe. For the remaining states, the Cartesian O-atom motions are considerably different from the elliptical motion predicted by the normal-mode model. This arises from bend-stretch coupling, including centrifugal distortion, in the Cartesian trajectories, which results in tubular instead of elliptical motion. Including zpe in the C-O stretch modes introduces considerable complexity into the O-atom motions for the vibrational angular momentum states. The short-time O-atom motions for these trajectories are highly irregular and do not appear to have any identifiable characteristics. However, the O-atom motions for trajectories integrated for substantially longer period of times acquire unique properties. With C-O stretch zpe included, the long-time O-atom motion becomes tubular for trajectories integrated to approximately 14 ps for the L = 3 states and to approximately 44 ps for the L = 1 states. 相似文献
108.
Recently found iron-pnictide superconductor (Ba,K)Fe2As2 and the heavy-fermion superconductor CeCu2Si2 both have the same crystal structure. In this paper we have calculated the electronic structure of LaFe2Si2 and LaFe2Ge2 from first-principles. These compounds also have the same crystal structure and closely related to both of (Ba,K)Fe2As2 and CeRu2Ge2. The obtained Fermi surfaces of LaFe2Si2 and LaFe2Ge2 resemble those of LaRu2Ge2, which are already found that they well explain the results of the dHvA experiments of CeRu2Ge2. Their density of states curves show the common feature with CaFe2As2. The density of states at the Fermi level strongly depends on the distortion of the FeX4 tetrahedra and/or the height of the X atom from the two-dimensional Fe plane, as also found in iron-pnictide system. The electronic specific heat coefficient is 11.8 mJ/mol K2 for LaFe2Si2 and 12.5 mJ/mol K2 for LaFe2Ge2, which is about 1/3 and 1/2 of experimental results, respectively. 相似文献
109.
M. Schneider F. Hase 《Journal of Quantitative Spectroscopy & Radiative Transfer》2009,110(17):1825-1839
Current atmospheric remote sensing experiments measuring high resolution spectra apply instruments of similar quality as the laboratory experiments that are especially equipped to derive spectroscopic parameters. We propose a method that uses high quality atmospheric spectra to improve the spectroscopic parameterization. It is an optimal estimation method that applies the uncertainty ranges given in current spectroscopic databases as a priori covariance and the residuals between the simulated and measured atmospheric spectra as new measurements. We test the method by updating the current HITRAN parameters of 49 mid-infrared water vapor lines (situated in 15 spectral windows between 795 and ) by the information present in high quality ground-based Fourier transform infra-red (FTIR) spectra. We show that the application of a speed dependent Voigt line shape model is important. The updated water vapor parameterization, compared to the original one, leads to lower residuals, larger measurement information content, and better agreement between remotely sensed and coincident in situ water vapor profiles. Using the new line parameterization, a state-of-the-art ground-based FTIR system is able to monitor upper tropospheric water vapor amounts and middle/upper tropospheric HDO/H2O ratios. 相似文献
110.
We investigate a superconducting Kosterlitz–Thouless transition in the two-dimensional (2D) Hubbard model using auxiliary quantum Monte Carlo method for the ground state. The pair susceptibility is computed for both the attractive and repulsive Hubbard model. The numerical results show that the s-wave pair susceptibility scales as χ L2 for the attractive case, in agreement with previous quantum Monte Carlo studies. The scaling χ L2 also holds for the d-wave pair susceptibility for the repulsive Hubbard model if we adjust the band parameter t′. 相似文献