1,720,970 research outputs found
Ab initio molecular orbital calculations on NO+(H2O)(n) cluster ions. 2. Thermodynamic values for stepwise hydration and nitrous acid formation
No full textAb initio molecular orbital calculations have been used to compute thermodynamic constants (?H°, ?S°, ?G°) for the stepwise hydration reactions of NO+(H2O)(n), and for the competing rearrangement reaction which produces HONO and H+(H2O)(n) (for n less than or equal to 4). Geometry optimizations and harmonic frequency calculations were performed at the MP2/6-311++G(2d,p) level, and relative energies were computed at the MP2/augcc-pVTZ level with MP2/6-311++G(2d,p) optimized geometries. The geometry changes and energetics of these competing solvation and rearrangement reactions have been studied, and reasons are proposed to explain why NO+(H2O)(n+1) formation is the dominant process for n = 1 and n = 2 but HONO + H+(H2O)(n) formation contributes for n = 3 and becomes more important for n = 4
Anharmonic Franck-Condon factor analysis and simulation of the Cl20HeIphotoelectron spectrum
No full textAb initio study of low-lying electronic states of SnCl2+
No full textComplete active space self-consistent field (CASSCF), multireference configuration interaction (MRCI), and restricted-spin coupled-cluster singles-doubles with perturbative triples [RCCSD(T)] calculations have been carried out on low-lying doublet and quartet states of SnCl2+, employing basis sets of up to aug-cc-pV5Z quality. Effects of core correlation and off-diagonal spin-orbit interaction on computed vertical ionization energies were investigated. The best theoretical estimate of the adiabatic ionization energy (including zero-point vibrational energy correction) to the 2A1 state of SnCl2+ is 10.093 ± 0.010 eV. The first photoelectron band of SnCl2 has also been simulated by employing RCCSD(T)/ aug-cc-pV5Z potential energy functions and including Duschinsky rotation and anharmonicity
DFT and ab initio calculations on the reaction between fluorine atoms and the fire suppressant, 2-H heptafluoropropane
No full textBenchmark values for the reaction enthalpy (298 K) and the barrier height(0 K) of the reaction, CF3CHFCF3 + F -> CF3CFCF3 + HF, have been calculated at state-of-the-art ab initio level to be -34.7 +/- 1.0 and -0.9 +/- 0.9 kcal/mole, respectively. The B3LYP, BH&HLYP, BB1K, MPW1K, MPWB1K and TPSS1KCIS functionals and the model method, the integrated molecular orbital + molecular orbital (IMOMO) method, have also been used to study this reaction
The heat of formation of 2-H Heptafluoropropane by ab initio calculations
No full textThe heat of formation of the fire suppressant, 2-H Heptafluoropropane, which is a replacement for ozone depleting halons, AH(f.298) (K)(CF3CHFCF3) has been calculated at the B3LYP, MP2 and CCSD(T) levels with basis sets of up to cc-pVQZ quality, and with the model chemistry methods: G2(MP2), G2, G3 and CBS-Q, employing three reaction schemes. The CBS limit value of -370.6 +/- 2.3 kcal/mol obtained is believed to be the most reliable value currently available. This is recommended to be used in thermochemical/kinetic modelling of the inhibition chemistry of CF3CHFCF3 in flames
Determination of the photolysis rate coefficient of Monochlorodimethyl Sulfide (MClDMS) in the atmosphere and its implications for the enhancement of SO2 production from the DMS + Cl2 reaction
No full textIn this work, the photolysis rate coefficient of CH3SCH2Cl (MClDMS) in the lower atmosphere has been determined and has been used in a marine boundary layer (MBL) box model to determine the enhancement of SO2 production arising from the reaction DMS + Cl2. Absorption cross sections measured in the 28000-34000 cm(-1) region have been used to determine photolysis rate coefficients of MClDMS in the troposphere at 10 solar zenith angles (SZAs). These have been used to determine the lifetimes of MClDMS in the troposphere. At 0° SZA, a photolysis lifetime of 3-4 h has been obtained. The results show that the photolysis lifetime of MClDMS is significantly smaller than the lifetimes with respect to reaction with OH (?4.6 days) and with Cl atoms (?1.2 days). It has also been shown, using experimentally derived dissociation energies with supporting quantum-chemical calculations, that the dominant photodissocation route of MClDMS is dissociation of the C-S bond to give CH3S and CH2Cl. MBL box modeling calculations show that buildup of MClDMS at night from the Cl2 + DMS reaction leads to enhanced SO2 production during the day. The extra SO2 arises from photolysis of MClDMS to give CH3S and CH2Cl, followed by subsequent oxidation of CH3S
DFT and ab initio calculations on two reactions between hydrogen atoms and the fire suppressants 2-H heptafluoropropane and CF3Br
No full textReaction enthalpies and barrier heights of the reactions CF3Br + H CF3 + HBr {reaction (1)} and CF3CHFCF3 + H CF3CFCF3 + H2 {reaction (2)} have been calculated at the near state-of-the-art ab initio level, and also by employing the B3LYP, BH&HLYP, BB1K, MPW1K, MPWB1K and TPSS1KCIS functionals. In addition, the integrated molecular orbital + molecular orbital (IMOMO) method has been used to study reaction (2). The ab initio benchmark values of the reaction enthalpy (298 K) and barrier height (0 K) of reaction (2) are reported for the first time {-(0.7 ± 0.7) and 13.3 ± 0.5 kcal/mole respectively}. When density functional theory (DFT) results are compared with ab initio benchmarks for both reactions (1) and (2), the MPWB1K functional is found to have the best performance of the six functionals used. The IMOMO method with the RCCSD/aug-cc-pVTZ and/or RCCSD(T)/aug-cc-pVTZ levels, as the high levels of calculation on the model system, gives reaction enthalpies and barrier heights of reaction (2), which agree with ab initio benchmark values to within 1 kcal/mole. Computed key geometrical parameters and imaginary vibrational frequencies of the transition state structures of reactions (1) and (2) obtained at different levels of calculation are compared. The magnitudes of the computed imaginary vibrational frequencies of the transition states of both reactions considered are found to be very sensitive to the levels of calculation used to obtain them. The heat of formation (298 K) of CF3CFCF3 calculated at the near state-of-the-art level has a value of -(318 ± 3) kcal/mole
Accurate potential energy curves for Zn+-Rg (Rg = He-Rn): Spectroscopy and transport coefficients
No full textHigh-quality ab initio potential energy curves are presented for the Zn+–Rg series (Rg = He–Rn). Calculations are performed at the RCCSD(T) level of theory, employing aug-cc-pV5Z quality basis sets, with ‘small core’ relativistic effective core potentials being used for Kr–Rn. We present spectroscopic information for the titular species, derived from our potential energy curves, and compare to previous results. We also present calculated ion transport data and show that mobility minima occur for a number of these systems, albeit at low gas temperatures for some of them
Can cyclen bind alkali metal azides? a DFT study as a precursor to synthesis
No full textCan cyclen (1,4,7,10-tetraazacyclododecane) bind alkali metal azides? This question is addressed by studying the geometric and electronic structures of the alkali metal azide-cyclen [M(cyclen)N3] complexes using density functional theory (DFT). The effects of adding a second cyclen ring to form the sandwich alkali metal azide-cyclen [M(cyclen)2N3] complexes are also investigated. N3? is found to bind to a M+(cyclen) template to give both end-on and side-on structures. In the end-on structures, the terminal nitrogen atom of the azide group (N1) bonds to the metal as well as to a hydrogen atom of the cyclen ring through a hydrogen bond in an end-on configuration to the cyclen ring. In the side-on structures, the N3 unit is bonded (in a side-on configuration to the cyclen ring) to the metal through the terminal nitrogen atom of the azide group (N1), and through the other terminal nitrogen atom (N3) of the azide group by a hydrogen bond to a hydrogen atom of the cyclen ring. For all the alkali metals, the N3-side-on structure is lowest in energy. Addition of a second cyclen unit to [M(cyclen)N3] to form the sandwich compounds [M(cyclen)2N3] causes the bond strength between the metal and the N3 unit to decrease. It is hoped that this computational study will be a precursor to the synthesis and experimental study of these new macrocyclic compounds; structural parameters and infrared spectra were computed, which will assist future experimental work
Study of pentafluoroethane and its thermal decomposition using UV photoelectron spectroscopy and ab initio molecular orbital calculations
No full textThe UV photoelectron spectrum of CF3CHF2 has been recorded and assigned using EOM-CCSD calculations. For the first band, the adiabatic ionization energy (AIE) and vertical ionization energy (VIE) have been measured as (12.71 ± 0.05) and (13.76 ± 0.02) eV, respectively. The measured AIE is higher than the recommended value from state-of-the-art ab initio calculations of (12.26 ± 0.02) eV because of a large geometry change on ionization, mainly arising from a significant increase in the C-C bond length, which results in poor Franck-Condon factors in the adiabatic region. The experimental VIE also shows poor agreement with the computed value of 14.05 ± 0.06 eV because, in the higher energy region of the first photoelectron band, dissociation of CF3CHF2+ to CHF 2- + CF3 occurs. This has a calculated thermodynamic onset of (12.89 ± 0.20) eV. Recommendations are made for the heats of formation, ΔHØf,298, of CF 3CHF2 and CF3CHF2+, based on the results of the ab initio calculations. Pyrolysis of flowing CF 3CHF2 diluted in argon shows evidence of production of C2F4 and HF at lower temperatures and CF2 and CF3H at higher temperatures. The relative temperature dependence of the observed photoelectron bands associated with these molecules is interpreted in terms of two decomposition reactions of CF3CHF2 as well as the reaction C2F4 →2CF2.Department of Building Services Engineerin
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