Ab initio MO study of dipole—dipole interactions in acetonitrile dimers

MO STO-3G ab initio calculations have been carried out for the antiparallel dipole and the head-to-tail dipole model of acetonitrile dimers. The optimized interaction enthalpy is about half of the lowest experimental estimate. The calculated interaction distance for the antiparallel dipole model is very close to the sum of intermolecular radii of N and C; the distance for the head-to-tail model is about 20% higher than the sum of N and H intermolecular radii. The discussion of the interaction in terms of the supermolecule MO's suggests for both models a bonding of mainly electrostatic character. The shortcomings of the STO-3G basis set in dealing with this problem are compared with those reported in the literature. The influence of the basis set on the calculated electron distribution in acetonitrile monomer was examined as a preliminary part of the present study, and is also reported in the paper.     

Ab initio calculation of the dipole polarizabilities of unsaturated hydrocarbons

Finite-field SCF MO calculations of polarizability are reported for ethene, benzene and naphthalene using three good gaussian basis sets Double-zeta wavefunctions yield values of α_LL and α_MM in agreement with experiment but the calculated α_NN is half the experimental value. Polarization functions when present preferentially improve α_NN. (L,M) and N refer to the molecular long, medium and normal axes, respectively.) STO-4G results are given for benzene, naphthalene, azulene, anthracene and phenanthrene, and the agreement with the experiment is reasonable after scaling the calculated α_LL and α_MM by a constant factor. The calculated α_MM are an order of magnitude too low.     

Ab initio SCF computation of force constants for CO2

Force constants for CO₂ have been evaluated using SCF wave functions. The effect ofd basis functions and geometry are investigated. Comparison with experimental values shows that a large error, due to neglect of electron correlation, occurs for theK ₁₂ interaction stretch force constant.     

Ab initio calculation of the dipole moment function of hydrogen iodide

SCEP/CEPA and MC SCF potential energy and dipole moment functions for hydrogen iodide have been calculated. Spectroscopic constants and vibrational dipole matrix elements obtained from the CEPA functions are in good agreement with experimental data. In contrast to previous results for hydrogen fluoride, the MC SCF dipole moment function is less accurate than the CEPA function.     

Geometrical consequences of resonance-assisted intramolecular hydrogen-bond formation from Ab initio MO calculations on 2-nitroresorcinol

Ab initio orbital calculations on phenol, nitrobenzene, and 2-nitroresorcinol have been performed with the GAUSSIAN 92 series of programs. Initial RHF/6-31G^* and RHF/6-31G^** optimizations were followed by second-order MØller-Plesset MP2(FC)/6-31G^* optimizations. The general geometrical features of these molecules, and, in particular, the characteristic changes as going from phenol to 2-nitroresorcinol and from nitrobenzene to 2-nitroresorcinol are in good agreement with recent gas-phase electron diffraction studies and with the notion of resonance-assisted intramolecular hydrogen-bond formation in 2-nitroresorcinol.     

Ab initio study of force constants and intensities of 1,3,5-trioxane

The structure, vibrational force constants and infrared and Raman intensities have been calculated for 1,3,5-trioxane using a 3-21G basis set. These results have been used to identify some possible inaccuracies in experimental diffraction based structures and in vibrational assignments. It is demonstrated that there is a marked contrast between the trends in the vibrational force constants and in the King's effective atomic charges of the axial and equatorial CH bonds in the series cyclohexane, 1,4-dioxane and 1,3,5-trioxane. The axial CH stretching force constant decreases by 0.04 mdyne Å⁻¹ for each adjacent oxygen atom, whereas that of the equatorial CH bond increases by 0.15 mdyne Å⁻¹ per oxygen. In trioxane the effective atomic charge of the axial hydrogen is twice that of the equatorial. Atomic polar tensors are calculated in a bond oriented frame, and the effect of the oxygens on CH stretching and bending mode intensities discussed. Some properties are also calculated using the 4-31G basis.     

Ab initio computation of force constants: Part VI. Applications of the force relaxation method for geometry optimization

Techniques to improve the computational efficiency of the force relaxation method are discussed. Force constants for fragments in previously computed smaller molecules can be transferred to construct a guess force constant matrix. Additional force constants that may be needed can be computed by a procedure which uses only one additional force calculation per diagonal force constant required. A scaling technique to improve convergence on the optimized geometry is discussed.     

Calculations of force constants and dipole moment derivatives in silane

Force constants and dipole moment derivatives were calculated for the symmetric stretching and asymmetric stretching and bending vibrational modes for the silane molecule. The orbital exponents of small basis sets of Slater orbitals were optimized for silane in several geometrical configurations. It was determined that the addition of d orbitals to the silicon basis set has negligible effect on the calculated properties.     

Ab initio MO calculation of hydration energies of ammonium ions and their solvation effect

The hydration energies for the NH⁺₄ and CH₃NH⁺₃ ions were calculated by an ab initio MO method. The aqueous solvation energy difference between these two ions was found to be accounted for by the interactions of the ions with a few solvent molecules.     

Ab initio rotational constants of interstellar species: Cyanoacetylene hydrogenated derivatives

By means of ab initio calculations, the rotational constants and dipole moments of H_nC₃N (n = 1, 3, 5, 7, and 9) species have been calculated at the HF / 6-31G * level of theory. Selected cases have been also calculated at the MP 2/6-31G * level and the influence of calculation level on rotational constant values is briefly discussed. Some of these species were discovered in the interstellar medium, while others have still not been detected there, although their existence is very probable. The results given here could help in their detection. © 1993 John Wiley & Sons, Inc.     

An ab initio MO study on the thymine dimer and its radical cation

Ab initio SCF calculations with the 6-31G basis set for the thymine dimer (cys-syn form) and the thymine dimer radical cation are reported. The fusion of the thymine bases at the C5 and C6 positions involves the formation of a cyclobutane ring with puckering. The puckering causes a notable difference in the electronic structures of the two bases of the thymine dimer. The density of the HOMO orbital of the thymine dimer is localized on the O2, N1, and C6 atoms of both thymine rings, with the higher density on one of the rings. The HOMO orbital has a bonding character on the C6(SINGLEBOND)C6 bond. In the thymine dimer radical cation, the unpaired electron is localized mainly on the lengthened C6(SINGLEBOND)C6 bond with the higher density on one of the C6 atoms and to a lesser extent on the N1 atoms of both rings. © 1996 John Wiley & Sons, Inc.     

Ab initio investigation of the simultaneous motion of protons in the hydrogen bonds in the cyclic dimer of formamide

The simultaneous motion of protons in interacting H-bonds plays an important role in biochemistry. By means of the simple example of the cyclic dimer of formamide some significant properties of the simultaneous motion of protons are discussed.Starting from the calculations of Dreyfus, Maigret, and A. Pullman [9] an energy surface as well as a surface of dipole moment for the linear motion of the two protons is calculated point by point using a selfwritten program of the SCF-MO-LC (LCGO)-method and using a Gaussian basis set of intermediate size.The resulting surface of total energy is further used to determine the states of vibration of the simultaneous motion of the protons. Whereas the surface of dipole moment is used to compute the relative transition intensities. These spectroscopic properties are finally used to find a criterion for estimating the kind of motion - mainly correlated or mainly independent.Held at the 6^th Jerusalem Symposium, April 9–13, 1973.     

Ab initio calculation of magnetic susceptibilities and screening constants using gauge invariant Gaussian orbitals

The magnetic susceptibility and screening constant tensors are calculated using an ab initio finite perturbation SCF method, with gauge invariant Gaussian orbitals. The isoelectronic BH BeH^?, and CH⁺ molecules have been studied. The calculated values for BH are relatively different from those obtained by other methods. The CH⁺ molecule seems to exhibit a stronger temperature independent paramagnetism than BH, while the BeH^? molecule would be diamagnetic. The screening constants of the heavy atom of these molecules present a similar variation.     

Ab initio study of spectroscopic constants and anharmonic force field of 74GeCl2

The equilibrium structure, spectroscopy constants, and anharmonic force field of germanium dichloride have been calculated at MP2, B3LYP, and CCSD(T) levels of theory employing two basis sets, cc-pVDZ and cc-pVTZ, respectively. The computed geometries, rotational constants, and vibration-rotation interaction constants, and quartic centrifugal distortion constants are compared with the available experimental data. The harmonic frequencies, anharmonic constants, and cubic and quartic force constants are predicted. The calculated results show that the MP2 results are in excellent agreement with experiment and represent a substantial improvement over the results obtained from B3LYP. The CCSD(T) method is also an advisable choice to study anharmonic force field of molecules.     

Ab initio studies of the ground-state potential energy surface of formamide

Ab initio calculations employing a standard double-zeta basis set augmented with various polarization functions have been used to investigate the lowest energy region of the ground-state potential energy surface of the formamide molecule. Hartree-Fock calculations with only d polarization functions on the nonhydrogen atoms located two stable minima, that with geometry distorted from planarity having slightly lower energy; only one stable minimum with planar structure is found when p polarization functions on the hydrogens are included. In contrast optimizations, which account approximately for the correlation energy using second-order Møller-Plesset perturbation theory consistently favor a single slightly nonplanar minimum energy geometry.     

2-苯基苯并咪唑衍生物非线性光学性质的从头算研究

采用HF／6-31G^*方法优化分子构型，在此基础上用CPHF方法系统地研究了多种基团取代的2-苯基苯并咪唑衍生物的二阶非线性光学系数βvec，并对βvdc的影响因素进行了探讨，为进一步设计综合性能优良的有机非线性光学材料提供理论指导．