A general procedure to calculate non-orthogonal, strictly local molecular orbitals (NOLMOs) expanded using only a subset of the total basis set is presented. The energy of a single determinant wave function is minimised using a Newton-Raphson approach. Total energies and barriers to internal rotation for CH4, NH3, H2O, CH3CH3, CH3NH2, CH3OH, NH2NH2, NH2OH and HOOH, and certain properties of the NOLMOs present in these molecules, are investigated using the 4-31G basis set. 相似文献
The FOGO method is used to calculate absolute proton affinities of the molecules H2, HF, NH3, H2O, CH3OH, C2H5OH, H2O2, CH2O, CO, and CH2CO. Comparison with experimental values demonstrates that the geometrical and energetical data resulting from this type of ab initio calculation are of chemical accuracy. Predictive data for higher energy isomers, such as hydroxymethylene and ethynol are given as possible aid for the identification of these species. 相似文献
A general method of generating radicals in cold supersonic expansions in the gas phase is presented. The method relies on excimer laser photolysis of suitable precursor molecules in a thin quartz capillary mounted at the orifice of a pulsed gas nozzle and can easily be combined with vacuum‐UV photoionization mass spectrometry and high‐resolution photoelectron spectroscopy to study the reactivity and the rovibronic energy level structure of neutral radicals and their ions, as well as to determine highly accurate adiabatic ionization energies. The characteristics of the radical source are described in detail, and its performance is illustrated by mass spectrometric and high‐resolution photoelectron spectroscopic investigations of NH2, CH2, CH3, C2H, C2H3, and C2H5. The radical source is not only suitable to produce cold samples (rotational temperature of ca. 30 K) of radicals of moderate reactivity, such as NH2, CH3, or C2H5, but it is also useful to prepare highly reactive radicals (e.g., C2H) for spectroscopic investigations. 相似文献
New estimates of Hartree–Fock limit energies (ERHF) for selected AH and AHn hydrides, diatomic and linear polyatomic molecules have been made utilizing ESCF values recently reported in the literature for HF, N2, CO, NH3, and CH4 which are very close to the respective limits. These new values have been used to investigate the applicability of Ermler and Kern's procedure for estimating ERHF: i.e., a factor f is first evaluated from data for reference molecules, where f = ERHF/ESCF, which is then used with ESCF values for other molecules to obtain their ERHF values. f has been evaluated for three groups of reference molecules? HF, H2O, NH3, CH4, N2, and CO; CH4, C2H2, C2H4, and C2H6; and C2H2, HCN, and N2? utilizing ESCF data in the literature for many Gaussian-type orbital (GTO) basis sets together with some new values calculated at the (9,5,1) to (13,8,2) levels. Trends in the variation of f within each group of reference molecules from one basis set to another, and the trends in f from one group of reference molecules to another, are discussed in detail. To minimize the influence of these effects in an ERHF estimate it is recommended that the f value should be derived from reference molecules which possess a similar combination of structural features, i.e., bonded hydrogen, single, double, or triple bonds, and the number of lone-pair electrons. Further calculations show that an f value based on data for closed-shell molecules is not applicable to open-shell species. 相似文献
The total Mulliken charges on the C and N atoms, populations of the S-trans-(N1) conformers, and rotation barriers in the molecules of 2-vinyl-5-R-tetrazoles (R = H, CH3, CH = CH2, C6H5, CH2Cl, CF3) were calculated ab initio (HF/6-31G**, MP2/6-31G**). The results were compared with the 1H and 13C NMR data for these compounds. 相似文献
We explored the interactions of gas molecules such as H2, CH4, C2H4, C2H6, CO2, and CS2 sandwiched by two pyrazine (Pz) molecules, which were employed as a model of organic linker in the Hofmann-type metal?Corganic framework (MOF). The MP2.5/aug-cc-pVTZ method was employed here, because this method presents almost the same binding energy as that calculated by the CCSD(T)/aug-cc-pVDZ with MP2.5-evaluated basis set extension effects to aug-cc-pVTZ basis set. The binding energy of the gas molecule increases in the order H2?<?CH4?<?CO2?<?C2H4????C2H6?<?CS2. The energy decomposition analysis of the interaction energy indicates that the electrostatic term presents the largest contribution to the interaction energy at the Hartree?CFock level. However, the dispersion interaction provides dominant contribution to the total binding energy at correlated level. We newly found a linear correlation between the z-component of polarizability of gas molecules and dispersion energy, where the z-axis was taken to be perpendicular to two Pz rings. These results are useful for understanding and predicting the binding energy of the gas molecule with the organic linkers of MOF. 相似文献
Theoretical rate constants have been calculated for O(3P) with five saturated hydrocarbons, CH4, C2H6, C3H8, iso-C4H10, and neo-C5H12. The method of choice is bond energy–bond order (BEBO) with activated complex theory (ACT). Because the BEBO method is empirical, O(3P) + CH4 is evaluated first, and the theoretical results are compared to more rigorous calculations and to the empirical transition state method. Comparisons are also made between predictions and experimental results. All of these comparisons show that the BEBO-ACT method gives results which are consistent with experiment and other theory. Because the method is successful, the other four cases are then considered. Ambiguity arises for the higher hydrocarbons from the problem of internal rotations in the activated complexes, and three cases are evaluated. Best agreement with experiment is obtained if the primary rotor(s) in the complexes are considered to be free. Predictions of rate constants are made from 500 to 2500 K. Throughout the discussion issues of theory which are common to any ACT calculation from any method of potential energy evaluation (LEP, LEPS, or ab initio quantum mechanics) are featured. 相似文献
The relative rate constants for the hydrogen atom abstraction by CCl3CH?CH· radical from CH2Cl2, CHCl3, CH3COCH3, CH3CN, C6H5CH3, C6H5OCH3, CH3CHO, and CH3OH in the liquid phase at 20°C have been measured. It was shown that these reaction rate constants are correlated by the two-parameter Taft equation with ρ* = 0.726 ± 0.096, r* = 1.22 ± 0.16. A relationship between r* and bond dissociation energy D(R? H) has been found for the abstraction reactions of different free radicals. 相似文献
Total cross sections for electron scattering on “quasi spherical” (CH4, SiH4, GeH4) molecules have been analyzed phenomenologically over a wide energy range. Regions, at low and high energies can be usefully represented by simple analytical formulae. Regularities associated with characteristic points such as the Ramsauer minima have been exposed. Comparison with other simple hydrides (NH3, H2O, H2S) allows the demonstration of a possible correlation between the maximum value of the total cross section and the bond length. Some points of contact with first-principles theory are noted and in particular the energy at which the maximum cross section occurs, is related to the occurrence of a partial wave resonance. In the absence of complete data for GeH4, prediction of characteristic points in the low energy cross section proves possible via the phenomenological analysis. Similarly, in the high energy regime, predictions of the cross section for SnH4 is made from data on the lighter molecules of the series, using non-relativistic Thomas-Fermi self consistent field scaling. 相似文献
The harmonic force constants, vibrational frequencies and integrated intensity ratios of CH2, H2O, CH2O, C2H2, CO2, HCN, CH3, CH4, and C2H4 have been calculated using the MINDO—FORCES program and the Pulay method for the calculation of the molecular force constants. The results obtained are in general quite satisfactory when compared with available literature values. The results are, however, not as satisfactory in case of molecules containing heteroatoms, due to the neglect of some dipolar repulsion integrals for the heteroatoms by the MINDO/3 method. Calculated integrated intensities for CH3 and C2H4 agree well with experimental results. The calculated integrated intensities for other molecules are obtained for the first time and no comparison with published data is therefore possible.Part of the M.Sc. Thesis of K. H. A. 1978. 相似文献
We have measured the synchrotron‐induced photofragmentation of isolated 2‐deoxy‐D ‐ribose molecules (C5H10O4) at four photon energies, namely, 23.0, 15.7, 14.6, and 13.8 eV. At all photon energies above the molecule′s ionization threshold we observe the formation of a large variety of molecular cation fragments, including CH3+, OH+, H3O+, C2H3+, C2H4+, CHxO+ (x=1,2,3), C2HxO+ (x=1–5), C3HxO+ (x=3–5), C2H4O2+, C3HxO2+ (x=1,2,4–6), C4H5O2+, C4HxO3+ (x=6,7), C5H7O3+, and C5H8O3+. The formation of these fragments shows a strong propensity of the DNA sugar to dissociate upon absorption of vacuum ultraviolet photons. The yields of particular fragments at various excitation photon energies in the range between 10 and 28 eV are also measured and their appearance thresholds determined. At all photon energies, the most intense relative yield is recorded for the m/q=57 fragment (C3H5O+), whereas a general intensity decrease is observed for all other fragments— relative to the m/q=57 fragment—with decreasing excitation energy. Thus, bond cleavage depends on the photon energy deposited in the molecule. All fragments up to m/q=75 are observed at all photon energies above their respective threshold values. Most notably, several fragmentation products, for example, CH3+, H3O+, C2H4+, CH3O+, and C2H5O+, involve significant bond rearrangements and nuclear motion during the dissociation time. Multibond fragmentation of the sugar moiety in the sugar–phosphate backbone of DNA results in complex strand lesions and, most likely, in subsequent reactions of the neutral or charged fragments with the surrounding DNA molecules. 相似文献
A method is described for extrapolating existing experimental data on the reactions of OH radicals with alkanes to higher temperatures using conventional transition-state theory. Expressions are developed for the estimation of the structural properties of the activated complex necessary for calculating ΔS± and ΔH±. The vibrational frequencies and internal rotations of the activated complex are given by those of the reacting alkane or the analogous alcohol and a set of additional internal modes that is the same for all OH + alkane reactions considered. Differences between primary, secondary, and tertiary hydrogen attack are discussed, and the validity of representing the activated complexes of all OH + alkane reactions by a fixed set of vibrational frequencies and other internal modes is evaluated. Calculations are presented for the reaction of OH with CH4, C2H6, C3H8, n-C4H10, i-C4H10, c-C4H8, c-C5H10, c-C6H12, (CH3)2CHCH(CH3)2, (CH3)3CCH(CH3)2, (CH3)4C, and (CH3)3CC(CH3)3, and the results are compared with experiments. 相似文献
The proton NMR spectra at 220 MHz of two series of substituted cyanopropionates, have been investigated. In addition the 13C spectra at 15 MHz of the series I were also studied. In I, where the R groups are diastereotopic, differences are observed in the proton chemical shifts of the CH3 groups in R for R = CH3, C2H5, n-C3H7, i-C3H7, n-C4H9, n-C5H11 and n-C6H13. In II [R′ = n-C3H7, CH(CH3)2, CH2CH(CH3)2 and C(CH3)3] diastereoisomers are found with substantial differences in chemical shifts between corresponding protons. Coupling constants are interpreted in terms of conformational preferences for certain molecules in both series. 相似文献
An analysis of the transformation of localized orbitals into restricted alternant orbitals is proposed. This approach has the advantage of expressing the wave-function in an orbital product while some electron correlation is introduced permitting the study of dissociation reactions. All applications of the orbital technique may be made as easily as with RHF, but with the additional possibility of studying chemical radicals. Some illustrations of this fact are shown for the molecules HF, H2O, NH3, CH4, C2H6 and for the dissociation reactions of CH4 and C2H6 generating CH3 radicals. 相似文献
Synthetic routes for the preparation of 3-alkyl-6-phenyl-4(3H)-pteridinones 6 and their corresponding 8-oxides 5 (R = CH3, C2H5, (CH2)2CH3, (CH2)3CH3, CH(CH3)C2H5, CH(CH3)2 and CH(C2H5)CH2OCH(OC2H5)2 are described and their reactivities towards xanthine oxidase from Arthrobacter M-4 are determined. Only the 3-methyl derivative of 6-phenyl-4(3H)-pteridinone and its 8-oxide i. e. 6a and 5a are found to be substrates although their reactivities are still very low. Oxidation takes place at C-2 of the pteridinone nucleus. All the 3-alkyl derivatives are less tightly bound to the enzyme than 6-phenyl-4(3H)-pteridinone. Introduction of the N-oxide at N-8 considerably lowers the binding of the substrates. Inhibition studies have revealed that 3-methyl-6-phenyl-4(3H)-pteridinone ( 6a ) is a non-competitive inhibitor with a Ki-value of 47 μM and the 3-ethyl derivative ( 6b ) an uncompetitive one with a Ki-value of 19.6 μM. 相似文献