Molecular-orbital calculations dimerization

Ab initio molecular orbital calculations for the model systems RCN3S2 (R = H, NH2) show that these dithiatriazines are predicted to be ground state singlets with low-lying triplet excited states (Section 4.4). The singlet state is stabilized by a Jahn-Teller distortion from C2v to Cj symmetry. In this context the observed dimerization of these antiaromatic (eight r-electron) systems is readily understood. 

Molecular orbital calculations using PM3 were performed on the ehalcogenides, their radical cations, and the dimer dications. The radical cations were calculated to be 77 radicals, with the unpaired electron in a p orbital perpendicular to the plane containing the two carbons and the chalcogen atom (Fig. 40), as observed experimentally for the S and Se cases. Similar modeling of the corresponding... 

In bridged metal-metal bonded dimeric complexes, the relative importance of metal-metal and bridging ligand effects are more difficult to unravel. Dahl and his co-workers have elegantly exploited systematic crystallographic analyses to detail the stereochemical consequences of valence-electron addition or removal in dimeric metal complexes (46, 47, 65, 230) and clusters (66, 88, 204, 205, 213, 216, 222). Their experimental work has been neatly underpinned by nonparameterized approximate Hartree-Fock molecular orbital calculations (217) on the phosphido-bridged dimers [Cr2(CO)80ti-PR2)2]n"2 and [Mn2(CO)g(/i.-PR2)2]n (rt = 0, + 1, or +2) ... 

The characteristic features of hydroboration of alkenes—namely, regioselec-tivity, stereoselectivity, syn addition, and lack of rearrangement—led to the postulation of a concerted [2 + 2] cycloaddition of borane353,354 via four-center transition state 37. Kinetic studies, solvent effects, and molecular-orbital calculations are consistent with this model. As four-center transition states are unfavorable, however, the initial interaction of borane [or mentioned monobridged dimer, Eq. (6.56)] with the alkene probably involves an initial two-electron, three-center interaction355,356(38, 39). 

Investigations of the electronic structure of quadruply bound dimers have relied heavily on electronic spectra to reveal the nature of the excited-state configurations. The availability of quantitative molecular orbital calculations coupled with single-crystal polarized electronic absorption spectral studies of quadruply bound dimers at low temperature has firmly established certain features of the excited electronic states of these compounds. A discussion of electronic spectra follows and a tabulation of energies associated with 6 - 6 transitions in quadruply bound dimers is given in Table 111. 

The experimental observations on the mechanism(s) of electroreduction of 2-thio-pyrimidines have been interpreted on the basis of their electronic structures as calculated with the aid of the CNDO/2 and Huckel procedures l42). The energies of LUMO (lowest unoccupied molecular orbitals), calculated for pyrimidine and its 2-oxo-and 2-thioderivatives, were compared with the reduction half-wave potentials (Table V). These show that the presence of a carbonyl or thione substituent at C2 enhances the electron acceptor properties of the molecule, which are correlated with formation of a dimer susceptible to photooxidation. 

An ab initio molecular-orbital calculation for the NH3-HC1 dimer was performed and the existence in it of a hydrogen bond was predicted [98-700]. This result was confirmed experimentally when it was found that three N-H bonds are shorter than the fourth one [707], NH4C1 crystallizes in the CsCl-type structure, where NH4 has tetrahedral symmetry, because of the stabilization effect of the Madelung energy... 

Although the geometries of the corannulene moieties in these dimers could not be obtained from NMR experiments, semiempirical MNDO molecular orbital calculations clearly favor a stacked-bowl geometry (convex face to concave face). The equivalence of the four external lithium cations is explained in terms of rapid intermolecular exchange. Moreover, the equivalence of the two corannulene units is accounted for by a rapid bowl-to-bowl inversion of both corannulene decks. The validity of this proposal was supported both theoretically71 and experimentally73. 

The hydrogen-bonded dimers with biologically interesting functional groups that have been studied by these methods are the OwH Ow bond in the water dimer and the OH -0=C bond in the formic and acetic acids dimers. They are important because they refer more directly to the intrinsic properties of a particular bond and provide experimental data for comparison with the ab-initio molecular orbital calculations on simple systems described in Chapter 4. 

Table 4.3. Ab-initio molecular orbital calculations on the hydrogen-bonded water dimer. Variation of geometry with level of approximation [290]...

Concerted two-electron transfer and reversible metal-metal bond cleavage in phosphine-bridged dimers have been investigated, and extended Hiickel molecular orbital calculations have shown that the redox-active orbital is a metal-metal antibonding orbital. A Ru-Ru-bonded dimeric cation [Ru(Cp)2]2++ has been prepared and characterized electrochemically. The electrochemistry of these dimers may give insight into more complex clusters and polymeric metals. 

The formation of a H-bond causes certain changes in the internal geometries of each monomer. For example, the bridging hydrogen moves a little farther away from the donating atom. In addition to the deformations of the nuclear positions, the H-bond formation is accompanied by real redistributions in the electronic structure of each subunit. These polarizations are readily apparent from experimental spectra, as well as from molecular orbital calculations. Taking into account the above perturbations of the two molecules involved in a H-bonded dimer, it is reasonable to presume that the ability of either of these two molecules to form another H-bond is altered by their participation in the first bond . 

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