Concerted hydrogen atom exchange between three HF molecules

We have investigated the termolecular reaction involving concerted hydrogen exchange between three HF molecules, with particular emphasis on the effects of correlation at the various stationary points along the reaction. Using an extended basis, we have located the geometries of the stable hydrogen-bonded trimer, which is of C(sub 3h) symmetry, and the transition state for hydrogen exchange, which is of D(sub 3h) symmetry. The energies of the exchange reation were then evaluated at the correlated level, using a large atomic natural orbital basis and correlating all valence electrons. Several correlation treatments were used, namely, configration interaction with single and double excitations, coupled-pair functional, and coupled-cluster methods. We are thus able to measure the effect of accounting for size-extensivity. Zero-point corrections to the correlated level energetics were determined using analytic second derivative techniques at the SCF level. Our best calculations, which include the effects of connected triple excitations in the coupled-cluster procedure, indicate that the trimer is bound by 9 +/- 1 kcal/mol relative to three separate monomers, in excellent agreement with previous estimates. The barrier to concerted hydrogen exchange is 15 kcal/mol above the trimer, or only 4.7 kcal/mol above three separated monomers. Thus the barrier to hydrogen exchange between HF molecules via this termolecular process is very low

Density-functional-based predictions of Raman and IR spectra for small Si clusters

We have used a density-functional-based approach to study the response of silicon clusters to applied electric fields. For the dynamical response, we have calculated the Raman activities and infrared (IR) intensities for all of the vibrational modes of several clusters (SiN with N=3-8, 10, 13, 20, and 21) using the local density approximation (LDA). For the smaller clusters (N=3-8) our results are in good agreement with previous quantum-chemical calculations and experimental measurements, establishing that LDA-based IR and Raman data can be used in conjunction with measured spectra to determine the structure of clusters observed in experiment. To illustrate the potential of the method for larger clusters, we present calculated IR and Raman data for two low-energy isomers of Si10 and for the lowest-energy structure of Si13 found to date. For the static response, we compare our calculated polarizabilities for N=10, 13, 20, and 21 to recent experimental measurements. The calculated results are in rough agreement with experiment, but show less variation with cluster size than the measurements. Taken together, our results show that LDA calculations can offer a powerful means for establishing the structures of experimentally fabricated clusters and nanoscale systems

Ab Initio Calculation of Harmonic Force Fields and Vibrational Spectra of the Fluoroarsines AsH_nF_3–n (n = 0–3) and the Fluoroarsoranes AsH_nF_5–n (n = 0–5)

Ab initio self-consistent-field calculations using effective care potentials and polarized double-ζ basis sets are reported for the title compounds and the analogous phosphorus compounds. The most stable trigonal-bipyramidal isomers of the fluoroarsoranes are those with the maximum number of axial fluorine atoms, as in the case of the fluorophosphoranes. For the known molecules AsH3, AsF3, and AsF5, the calculated geometries and vibrational frequencies agree well with the experimental data, and improvements to the current empirical force field of AsF5, are suggested. Predictions are made for the vibrational spectra of the other molecules, which are unknown, and plots of the calculated gas-phase infrared spectra at 300 K are presented for ASH5, AsH4F, and AsH3F2, as an aid to their spectroscopic identification. Trends in the calculated properties of the fluoroarsoranes are discussed and found to be very similar to those for the fluorophosphoranes

Analytical Second Derivatives for Effective Core Potentials

Analytical first and second derivatives for effective core potentials are reported. The computational implementation of the derivative formulas makes use of new integral routines which take advantage of the shell concept. Test calculations for H3SnBr and F3AsS demonstrate the efficiency of the analytical determination of harmonic force fields using effective core potentials. The spectroscopic constants of the unknown molecule F3AsS are predicted

Ab Initio Calculation of Harmonic Force Fields and Vibrational Spectra for the Fluorophosphoranes PH_nF_5−n (n = 0−5)

Ab initio self-consistent field calculations at the 6-31G** level are reported for the title compounds. The theoretical force fields are scaled with the use of scale factors derived previously for the fluorophosphines. Improvements to the current empirical force field for PF5 are suggested. The original assignments of the observed vibrational spectra are revised or extended for PHF4, PH2F3, and PH3F2. Predictions are made for the vibrational spectra of the unknown molecules PH4F and PH5, and the accuracy of these predictions is estimated. Plots of the calculated gas-phase infrared spectra at 300 K with rotational fine structure are presented for the unknown molecules, as an aid to their spectroscopic identification. Trends in the calculated properties of the fluorophosphoranes are discussed

Ab Initio Calculation of Harmonic Force Fields and Vibrational Spectra for the Phosphine Oxides and Sulfides R₃PY (R = H, F, CH₃; Y = O, S)

Ab initio self-consistent field calculations at the 6-31G* level are reported for the title compounds R3PY and the corresponding phosphines R3P. The calculated geometries, rotational constants, vibrational frequencies, centrifugal distortion constants, Coriolis coupling constants, and infrared band intensities are compared with the available experimental data. Detailed assignments are suggested for the vibrational spectra of the trimethyl compounds. The scaled theoretical interaction force constants for F3P0 and F3PS are believed to be more reliable than the current empirical ones. Predictions for H3P0 are compared with recent results from a matrix-isolation study. Plots of the calculated infrared spectra at 300 K with rotational structure are presented for H3P0 and H3PS, as an aid to their spectroscopic identification in the gas phase. Trends in the calculated force constants and dissociation energies for PO and PS bonds are discussed

CALCULATION OF PHOTOIONIZATION CROSS SECTIONS USING AB-INITIO WAVEFUNCTIONS AND THE PLANE WAVE APPROXIMATION

A method is outlined for the calculation of photoionization cross sections representing the initial state by a minimal basis set STO NG wavefunction and the final state by the plane wave approximation. Numerical results for several small molecules compare well with the band intensities in published ESCA spectra, and are superior to ESCA band intensities obtained from valence electron wavefunctions. For the UV region, ab-initio and valence electron calculations lead to comparable results

Methodological assumptions of modelling of passenger vehicle traffic on national and voivodeship roads crossing the borders of poviats in Poland (project KoMaR )

Celem artykułu jest prezentacja założeń metodycznych oraz wstępnych rezultatów modelowania ruchu pojazdów osobowych na odcinkach dróg krajowych i wojewódzkich przekraczających granice powiatów w Polsce w ramach projektu: „Kompleksowe modelowanie osobowego ruchu drogowego w Polsce wraz z identyfikacją jego lokalnych uwarunkowań społeczno-ekonomicznych (KoMaR)”. Zaprezentowano podobieństwa i różnice między Krajowym Modelem Ruchu [3], a założeniami modelowymi w projekcie KoMaR. Założenia w projekcie bazują przede wszystkim na przeglądzie kompleksowych badań ruchu przeprowadzonych w ostatnich latach w Polsce na poziomie regionalnym oraz aglomeracyjnym. Zakłada się uwzględnienie również innych baz danych, w tym macierzy międzygminnych dojazdów do pracy oraz migracji na poziomie krajowym. W artykule przedstawiono wstępne wyniki rozkładu ruchu dla podróży krótkich oraz długich, z uwzględnieniem podróży zewnętrznych, po porównaniu wyników modelowych z Generalnym Pomiarem Ruchu z 2010 r. Głównym rezultatem projektu jest wydzielenie obszarów kraju na poziomie powiatowym, dla których lokalne uwarunkowania społeczno-ekonomiczne decydują o odmiennym od typowego dla Polski rozkładu ruchu.The purpose of the article is to present the methodological assumptions and the preliminary results of the modelling of the traffic of passenger vehicles on sections of national and voivodeship roads crossing the borders of poviats in Poland within the framework of the project “Complex modelling of road traffic in Poland with the identification of local socio-economic determinants”. The similarities and differences between the project and the National Traffic Model [3] are presented. The comprehensive travel studies carried out at the agglomeration and regional level and databases (commuting and migration matrices) at the national level are going to be taken into account. The preliminary results of the traffic distribution for both short and long trips are compared with the results of General Traffic Measurement in 2010