Dipole moments, semiempirical molecular orbital

Quantum mechanical calculations of molcular orbitals have been performed on five examples (8-azapurine, -hypoxanthine, -guanine, -adenine, and -xanthine) by two methods (a) a semiempirical approximation, which included contributions from the a electrons of the skeleton, and (b) the CNDO approximation, which included contributions from all the valence electrons of the molecule. The results were tabulated in parallel for each of the three possible positions of the triazole proton. In all 15 entries, the highest occupied and the lowest unoccupied molecular orbitals were calculated and also the dipole moment, the molecular energy, and the UV absorption maxima (the last-named showed only a modest agreement with experimental results). It was concluded that both types of calculation indicated that relative stabilities for the three tautomers (in each of the five sets) should decrease in the order HN-9, HN-7, and HN-8, and that the HN-8 tautomers should be 85 to 125 kJ (20-30 kcal) per mol less stable than the other two. However, it had to be admitted that, in all sets of three isomers examined experimentally, the HN-8 member has never been found inferior in stability. ... 

Murdachaew et used the SCP-NDDO semiempirical molecular orbital model to calculate an increase in the dipole moment from the equilibrium gas-phase value to the liquid-phase value from 2.16 D to 2.8 D, an increase of 30%, whereas with the older PM3 and PM6 NDDO-t3q)e method, which significantly underestimate the polarizability of water, they found that the increase was only 9% and 11%, respectively. 

The type of conjugation is also reflected in the frontier orbital profile, the charge distribution, and the permanent dipole moments. The results of semiempirical calculations on l-methylpyridinium-3-olate (16), Malloapeltine (17), Trigollenine (18), and Homarine (19) are presented in Scheme 7. Characteristically for the class of conjugated mesomeric betaines, the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) are distributed over the entire molecule as examplifled for l-methylpyridinium-3-olate. It was shown that 90% of the... 

Molecular orbital calculations, whether by ab initio or semiempirical methods, can be used to obtain structures (bond distances and angles), energies (such as heats of formation), dipole moments, ionization energies, and other properties of molecules, ions, and radicals—not only of stable ones, but also of those so unstable that these properties cannot be obtained from experimental measurements." Many of these calculations have been performed on transition states (p. 279) this is the only way to get this information, since transition states are not, in general, directly observable. Of course, it is not possible to check data obtained for unstable molecules and transition states against any experimental values, so that the reliability of the various MO methods for these cases is always a question. However, our confidence in them does increase when (1) different MO methods give similar results, and (2) a particular MO method works well for cases that can be checked against experimental methods. ... 

For this work, the molecular structures derived using an AMI semiempirical method (20) served as input for an INDO SCF procedure (27), which generated the unoccupied molecular orbitals. (Note The summation in equation 2 over the vibrational subspace of each electronic state is approximated as unity and is valid for all off-resonance processes.) The dipole moment matrix and transition energies corresponding to these Cl states were calculated and inserted directly into equation 2. 

Molecular orbital computational analysis by PM3 CI UHF semiempirical methods have been used to support the contention that preferable HSOMO-LUMO interactions produce a favored biradical and explain the site selectivity in the sensitized photochemical [2 + 2] cycloadduct formation of 2-pyrones with electron-deficient ethylenes <92BCJ354>. The lowest ionization energies, dipole moment, and dominant electronic configurations of a 5-methylidenated version of 7-nitroso-oxazolo[4,5- ]cyclopenta[e]pyrimidine of unknown origin were calculated by the ADC(3) ab initio method <92CPHii>. An extensive semiempirical and ab initio investigation into the mechanism of oxidation of methanol by PQQ is cited in Section 7.22.12.4. 

Among the variety of compounds emerging from prototype structures 10-28 mentioned in the Introduction (Table I), X-ray structural determinations have been performed on six representative examples the mesomeric betaines 55, 116, 117, and 120 the higher homologue 119 and the novel aza analogue of sesquifulvalene with a betaine character 118 (Scheme 9 and Table VIII). As mentioned earlier, the experimental dipole moments for molecules 55,116-119 were found to be in the range 9 to 13 D (III,B, Scheme 8 and Table Vll). Comparison of the experimental molecular geometries and dipole moment values with those obtained from semiempir-ical molecular orbital calculations is discussed below (III,D). 

Molecular-orbital calculations, whether by ab initio or semiempirical methods, can be used to obtain stmctures (bond distances and angles), energies (e.g., heats of formation), dipole moments, ionization energies, and other properties of molecules,... 

Semiempirical method of molecular orbitals with PM3 parametrization of Hamiltonian [12] has been used to calculate electric dipole moments of functionalized (5,5) nanotube. Two cases were considered adsorption of hydrogen and fluorine atoms at the opposite open edges of the nanotube segments (case A, see Fig. 1 A) and adsorption of hydrogen atoms at one open edge of the nanotube (case B, see Fig. IB). The calculated electric dipole moments are d = 4.536-10 29 and d = 7.397-KT29 C m for cases A and B, respectively. 

By method is meant the set of approximations used. These approximations must satisfy several criteria. Some criteria are theoretic if any of these are violated, then the method is not a valid one. For example, the results must be rotationally invariant This means that the results of a calculation should not depend on the orientation of the system in Cartesian space. Here results refers to any scalar ob.servable, such as the heat of formation, dipole moment, or interatomic distance or angle. (Some results are not observables, an example of which would be the molecular orbitals or eigenvectors, which are composed of a linear combination of atomic orbitals. Since the atomic orbitals are defined in terms of the Cartesian coordinate system, the coefficients of those atomic orbitals, which have angular dependence, will change as the system is rotated.) This is a rigorous and essential criterion, and all semiempirical methods in current use pass this test. Another theoretical requirement is that the results of a calculation on two well-separated (i.e., noninteracting) systems should be the sum of the results of calculations of the two isolated systems. 

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