Dipole moment quantum-mechanical calculation

The IlyperChein log file includes calculated dipole moments of 111 oiccu les. To set th e am min t o f in form anon collected in th e log file, eh an gc the value of the Qu an turn Prin t Level set tin g in the eh em. in 1 File. Xote that the sign convention used in the quantum mechanical calculation of dipoles is opposite to that used in 111 oiccu lar mech an ics dipole calculation s this reflects th e differing sign conventions ofphysics and chemistry. 

For a quantum mechanical calculation, the single point calculation leads to a wave function for the molecular system and considerably more information than just the energy and gradient are available. In principle, any expectation value might be computed. You can get plots of the individual orbitals, the total (or spin) electron density and the electrostatic field around the molecule. You can see the orbital energies in the status line when you plot an orbital. Finally, the log file contains additional information including the dipole moment of the molecule. The level of detail may be controlled by the PrintLevel entry in the chem.ini file. 

Since the time of the quantum-mechanical calculations by Longuet-Higgins, many attempts have been made to calculate tt-electron densities, resonance energies, dipole moments, and optical transitions both by the LCAO-MO and the valence bond method.However, no agreement has been reached on the importance of pd-hybridization of the sulfur atom. This is considered by some workers an essential... 

On the basis of the dipole moment, Paik, values computed from the Helmholtz equation (2.21) and the alkali ion radius one can estimate the effective positive charge, q, on the alkali adatom, provided its coordination on the surface is known. Such calculations give q values between 0.4 and 0.9 e (e.g. 0.86e for K on Pt(lll) at low coverages) which indicate that even at very low coverages the alkali adatoms are not fully ionized.6 This is confirmed by rigorous quantum mechanical calculations.27,28... 

Values for the partial charges of atoms can be derived from quantum mechanical calculations, from the molecular dipole moments and from rotation-vibration spectra. However, often they are not well known. If the contribution of the Coulomb energy cannot be calculated precisely, no reliable lattice energy calculations are possible. 

The absolute determination of coverage is much more difficult. The formalism is well known, i.e., it is possible to go from the intensity of a spectral ion to the concentration material on the surface, but this can only be worked out if the transition dipole moment is known. A very rough order of magnitude version of this can, however, usually be obtained by analogy (rather than by quantum mechanical calculation) so that somewhat better than an order of magnitude calculation of the coverage of the surface radicals can be given. 

If one is interested in changes of the solute molecule, or if the structure of the surrounding solvent can be neglected, it may be sufficient to regard the solvent as a homogeneous dielectric medium, as was done in the older continuum theories, and to perform a quantum mechanical calculation on the molecule with a modified Hamiltonian which accounts for the influence of the solvent as has been done by Hylton et al. 18 5>. Similarly Yamabe et al. 186> substituted dipole-moment operators for the solvent in their perturbational treatment of solvent effects on the activation energy in the NH3 + HF reaction. 

Methods for quantum mechanical calculation of dipole moments and their IE s are well established. The dipole moment for any specified confirmation is defined by the nuclear coordinates of the various atoms n and their charges z , plus the electron density at each point in space expressed as a function of the vector re using the wave function... 

Transilion dipole moments are concerned with the dipole that exists in a molecular vibration when it adjusts itself to the frequency of the electromagnetic radiation with which it is in resonance (and about to undergo a transition). The values of such transition dipole moments can be calculated quantum mechanically (which is lengthy and approximate). If a bond is considered within a series of similar bonds for which reliably calculated transition dipole moments are available, an interpolation of the relevant value may be sufficient (Bockris and Carbajal, 1987)l... 

D higher than the dipole moment of uracil or thymine. All the quantum-mechanical calculations predicted a greater moment for cytosine (and guanine) than for uracil or thymine (and adenine). The agreement between the experimental dipole moment of cytosine and the values calculated by different methods is satisfactory (cf. Table XI)... 

We have performed a series of semiempirical quantum-mechanical calculations of the molecular hyperpolarzabilities using two different schemes the finite-field (FF), and the sum-over-state (SOS) methods. Under the FF method, the molecular ground state dipole moment fJ.g is calculated in the presence of a static electric field E. The tensor components of the molecular polarizability a and hyperpolarizability / are subsequently calculated by taking the appropriate first and second (finite-difference) derivatives of the ground state dipole moment with respect to the static field and using... 

Quantum mechanical calculations are also useful in predicting minimum energy conformations of chromophores and for predicting properties such as dipole moments. Calculated dipole moments are shown for several chromophores in Table 4. Such calculations are particularly useful in understanding how chromophore molecular conformation changes with addition of substituents. 

Tajiri and Winkler have conducted quantum mechanical calculations on some thiadiazolo-thiadiazoles (6)-(9) within the framework of the Parriser-Paw-Pople (PPP) method and have successfully employed their theoretical results to assist in the interpretation and prediction of the magnetic circular dichroism (MCD) spectra of these molecules <83ZN(A)1263>. Despite the fact that sulfur 3d orbital participation is more or less unimportant in the prediction of dipole moments, electronic absorption spectra, or photoelectron spectra, the authors suggest that its significance for the evaluation of magnetic moments of sulfur-containing heterocycles merits further consideration. 

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