Relationship between proton shielding constants and electric dipole moments in the hydrogen halides

The proton shielding constants of the hydrogen halides have been calculated employing a molecular wave function built from gauge-invariant atomic orbitals. Each of the molecular wave functions contains a single parameter which is determined from the observed dipole moment. Calculated and experimental values of the shielding constants for HF, HCl, HBr and HI agree to within approximately 10 per cent.     

Calculations of intermolecular interactions and their influences on structure and13C,1H magnetic shielding constants of solute molecules

On the basis of an improved method of molecular mechanics the spatial structures of molecular clusters modelling the solvate shell of methane in acetone and benzene, 2,9,10-trimethyl-1,3-dithia-5,6-benzocycloheptene in carbon disulfide have been calculated. The nuclear magnetic shielding constants for these structures are calculated by quantum chemical methods in the approach of the density functional theory of the B3LYP/6-31(d,p) level with the usage of gauge-invariant atomic orbitals. It is shown that the increase of the number of the solvent molecules results in better agreement of the calculated and experimental values of¹H and¹³C chemical shifts.     

Magneto-optical activity: Gauge-invariant calculations in the random-phase approximation

The use of London's gauge-invariant orbitals in the calculation of magnetic rotational strengths is investigated. Self-consistent molecular orbitals at non-zero magnetic field strengths have been determined in the Pariser-Parr-Pople model and these have been employed in the calculation of excitation energies and transition moments according to the mono-excited configuration interaction and the random phase approximations. Applications to the magneto-optical activity of aniline, phenol, and benzonitrile are reported. The results are origin-independent and compare well with experimental values.     

Orbital contributions to relativistic corrections of the NMR nuclear magnetic shielding tensor originated in scalar field-dependent operators

In this work relativistic corrections to the magnetic shielding constant (σM), which arise from scalar field-dependent operators (both linear and bilinear), are calculated and decomposed into contributions from molecular orbitals (MOs). Numerical results for the magnetic shielding constant of the heavy nucleus X in closed shell atomic ions X^? and HX compounds (X = F, Cl, Br, I) are presented. The relative importance of inner-shell and valence electrons in the definition of each one of these terms is thus assessed and its relation with their sensitivity to changes in a chemical environment is discussed.     

A new approach to bonding in transition metal clusters

The idea that the molecular orbitals of a cluster of atoms can be usefully classified according to their nodal structure, so that the energy increases with the number of nodes, is quantified and shown to apply in its usual form only to molecular orbitals constructed from atomic σ orbitals. The method is extended to deal with atomic π and δ orbitals by the use of vector and tensor surface harmonics respectively. In all cases the orbitals can be classified approximately in terms of angular momentum quantum numbers l and m, but in the π and δ cases there are two orbitals, with different parity, for each lm, and the energy is determined primarily by the parity. The method provides a classification scheme and an approximate energy ordering which does not depend on any point-group symmetry that the cluster may have, and therefore provides a useful framework for discussion of the bonding in cluster compounds such as the boron hydrides and the transition metal cluster carbonyls.     

Calculations of <Superscript>31</Superscript>P Magnetic Shielding Constants of Derivatives of Betaine and Phosphine Molecules Dissolved in Different Solvents by Using Supermolecular Model and Combined Methods of Quantum Chemistry and Molecular Mechanics

Spatial structures of molecular clusters modeling a solvate shell around phosphorus-containing methyl- and butyl-derivatives of phosphine and betaine molecules dissolved in different solvents (acetone, toluene, formamide) have been calculated by using different variants of density functional theory (unrestricted Becke three-parameter Lee–Yang–Parr [UB3LYP], Perdew–Burke–Ernzerhof [PBE], optimized exchange functional [OPTX] developed by Handy and Cohen in conjunction with Lee–Yang–Parr [LYP] correlational functional [OLYP]) with 6-31G(d,p) and 6-31G++(d,p) basis sets. The ³¹P magnetic shielding constants for the structures are calculated with the usage of gauge-including atomic orbitals in UB3LYP/6-31G(d,p) and 6-31G++(d,p) methods. The modeling of molecular clusters is done by using the supermolecular model, the molecular mechanics method and the combination of quantum chemistry and molecular mechanics methods (QM/MM). The own N-layered integrated molecular orbital method (ONIOM) has been applied for modeling and calculating of isotropic ³¹P nucleus magnetic shielding of clusters of trimethylphosphine and trimethylbetaine molecules dissolved in acetone using combinations of UB3LYP/6-31G(d,p) (higher level) and unrestricted Hartree–Fock (UHF)/6-31G(d,p) (lower level) methods. Applicability of the ONIOM approach and different ways of modeling to the calculation of ³¹P nucleus magnetic shielding constants is studied. A comparison of the results obtained by the density functional theory, ONIOM and MM methods is given.     

Analyticity of density of states in a gauge-invariant model for disordered electronic systems

Then-orbital gauge-invariant model of disordered electronic systems proposed by Wegner is studied in the regime of dominant diagonal disorder. Analyticity of the density of states is established in two cases: (a) when the number of orbitals is small, (b) when the number of orbitals is large and the energy is in the expected extended states region.     

中高能电子和N_2（~1∑_g~＋）、CO（~1∑~＋）弹性散射全截面的计算

此文采用由原子轨道线性组合构成的自洽场分子轨道（ＳＣＦ－ＬＣＡＯ－ＭＯ），用一级玻恩近似（ＦＢＡ）计算了能量为１００－５０００ｅＶ的电子与弹性散射的微分截面和全截面。由于采用了Ｇａｕｓｓ函数的线性组合拟合Ｓｌａｔｅｒ函数的方法（ＳＴＯ－ＫＧ），得到了弹性散射微分截面的解析表达式，使计算大大简化。计算得到的电子和Ｎ２，ＣＯ散射的总截面和实验结果进行了比较，当入射电子的能量大于１ｋｅＶ时，理论值和实验值之间符合得很好。     

The electronic structure of hydrogen fluoride

Self-consistent field (SCF) calculations have been carried out for hydrogen fluoride at the experimental internuclear distance using a single determinantal wave function composed of molecular orbitals (MO's) expressed as linear combinations of atomic orbitals (LCAO's). The AO's used are of Slater-type in which the parameters have been varied within the framework of the variational method. The improvement in the energy and other physical quantities brought about by the variation of the Slater parameters has been discussed in relation to the results of other methods of obtaining improved wave functions for this molecule.     

Valency electron molecular orbital calculations

The self-consistent perturbation method used previously with the CNDO/2 approximations is generalized to obtain diamagnetic susceptibilities and nuclear shielding constants, as well as polarizabilities, for N₂, BF, CO and F₂ both with a minimal basis set and with a slightly extended set containing an extra 2p orbital. The importance of origin-dependent terms is pointed out. For quantities that depend on the first-order perturbed molecular orbitals, inclusion of the extra 2p orbitals produces results in better agreement with experiment.     

Sternheimer shielding in molecules

The effect of perturbing electric fields and electric-field gradients on the nuclear quadrupole coupling in molecules is investigated. The theory for the first-order effects is developed and it is shown that not only a perturbing field gradient but also a perturbing field, and higher terms, induce a finite field gradient at a nucleus in a molecule. The so-called Sternheimer (anti)shielding is thus considerably more complex in molecular than in atomic systems. The different shielding components have been calculated for HCl, HCN and NH₄ ⁺ using a finite perturbation ab initio molecular orbital method. The calculated values are then used to rationalize experimentally determined ¹⁴N quadrupole coupling constants in condensed media for HCN and NH₄ ⁺.     

The hydrogen molecule in an arbitrarily oriented magnetic field

Summary The modifications induced by a magnetic field of arbitrary direction and intermediate strength (i.e not larger than 2.35·10⁵ tesla, the ?atomic tesla?) on the lowest singlet and triplet energy states of the hydrogen molecule are studied. Using a linear combination of products of field-modified atomic orbitals, it is found that increasing the field strength the depth of the singlet energy well increases and the equilibrium internuclear distance decreases, yielding more rigid and localized nuclear vibrations. For sufficiently strong fields perpendicular to the internuclear axis, the triplet state exhibits a bonding behaviour. An explanation of the above results is given in terms of the field-modified electronic-charge distributions in the internuclear region. Based on the thesis submitted by S. Basile to the University of Palermo for graduation in Physics.     

Electrostatic binding in the first-row AH and A2 diatomic molecules

The charge migration that converts two overlapping spherical atoms into a bound molecule produces attractive Hellmann-Feynman fields at the two nuclear positions, offsetting the repulsive field that each nucleus encounters on penetrating inside the charge cloud of its neighbour. The magnitudes of these fields correlate positively with the molecular dissociation energies. The contribution of the σ valence orbitals to the field at a first-row nucleus varies regularly from binding in lithium to anti-binding in fluorine. The electrostatic effect of these orbitals is weakened by reversed polarization inside the 2s nodal sphere and is further opposed by a tiny but effective exchange polarization of the core. The major binding role in the first row thus falls to the π orbitals, which, like the σ orbital in hydrogen, have no radial node to inhibit a uniformly binding forward polarization.

The net electrostatic interaction of two spherical atoms, derived with neglect of antisymmetry, is always binding. Especially in the heavier first-row A₂ molecules, this classical Coulomb attraction provides more of the binding energy than either charge accrual on the bond axis, which is opposed by the exclusion principle, or contraction towards the nuclei, which is characteristic of hydrogen but hardly occurs in first-row atoms.     

开孔矩形腔体的近场电磁屏蔽效能研究

基于扩展的等效电路方法, 建立了电偶极子和磁偶极子天线近场照射下开孔矩形腔体电磁屏蔽效能计算的近似解析模型, 计算分析了场源–腔体距离对电场和磁场屏蔽效能的影响规律. 结果表明在近场区, 屏蔽效能随场源–腔体距离的减小而明显减小, 近场屏蔽效能小于远场屏蔽效能. 基于Bethe小孔耦合理论, 得出了描述近场和远场屏蔽效能关系的解析公式, 并用该公式检验了等效电路方法计算结果的可信性. 关键词： 电磁屏蔽 矩形腔体 近场 Bethe 理论     

Spectroscopic Parametrization of the Indo Method Including Vacant Atomic Orbitals in an Extended Basis

An extended basis for a semi-empirical INDO method including wave functions of valent and vacant atomic orbitals is proposed for calculating molecules whose spectra have Rydberg transitions. The parameters for vacant atomic orbitals of oxygen and hydrogen are calibrated against electronic spectra of water clusters. Electronic spectra of water clusters and hydrogen peroxide are examined by the INDO method using the parameters chosen. The results obtained agree reasonably well with the available theoretical and experimental data.     

Analytical energy gradients for local second-order Møller-Plesset perturbation theory using intrinsic bond orbitals

ABSTRACT

We present the theory and an implementation for PAO-LMP2 analytical energy gradients (local second-order Møller-Plesset perturbation theory with domains of projected atomic orbitals), using intrinsic bond orbitals (IBOs). The accuracy of optimised LMP2 geometry parameters relative to canonical MP2 and CCSD(T) ones is benchmarked for more than 100 small- to medium-sized molecules. Using augmented triple-ζ basis sets, the deviations of computed LMP2 bond lengths and bond angles from the corresponding MP2 ones are small and quickly decrease with increasing domain sizes. For small domains, a systematic compensation between the intrinsic error of MP2 and the errors due to the local approximations is found, so that the LMP2 results deviate less from CCSD(T) than the MP2 ones. On the other hand, with extended domains, the differences between LMP2 and MP2 geometry parameters become negligible.     

Infrared and microwave spectra, molecular conformation and electric dipole moment of methyl thiolformate

The microwave spectrum (41-10 GHz) and the infrared spectrum (4000-50 cm⁻¹) of methyl thiolformate have been obtained and analyzed. The spectra are consistent with a single molecular conformation having a planar array of heavy atoms and with the alkyl group cis to the carbonyl group. The measured rotational constants are: A, 11042.22 MHz; B, 5118.27 MHz; C, 3562.03 MHz (κ = −0.5839). No internal rotation doublets were observed in the microwave spectrum for the ground vibrational state, which implies that the barrier hindering internal rotation of the methyl group is either much larger or much smaller than the corresponding value for methyl formate. If the former is true then a lower limit of 10.5 kJ mol⁻¹ may be placed on the barrier height.The dipole moment of methyl thiolformate was measured using the Stark effect to be 1.58 ± 0.05 Debyes (μ_A = 1.52 D; μ_B = 0.43 D) for the vapor, and for dilute solutions in benzene at 295 K the value of 1.6 ± 0.1 D was found from capacitance measurements.SCF computations using minimal basis sets of STO/3G atomic orbitals and extended basis sets of STO/4.31G atomic orbitals have been carried out for methyl thiolformate and methyl formate. Energy differences between rotational isomers and estimates of barrier heights are given together with the calculated dipole moments.     

Mag netic properties of $$\hbox{Ni}_{13- n}\hbox{Al}_ n$$ clusters with n = 0–13

We report the magnetic properties of small Ni_13-nAl_n\hbox{Ni}_{13-n}\hbox{Al}_n clusters with n = 0–13 calculated in the framework of density functional theory. The cluster magnetic moment decreases with the sequential substitution of Ni by Al atoms, which can be attributed to a greater degree of hybridization that forces the pairing of the electrons in the molecular orbitals of Ni and Al. For Ni₇Al₆, the complete quenching of the cluster magnetic moment appears to be due to the antiferromagnetic alignment of atomic spins as revealed by the spin density plots.