MNDO-UHF calculations

At the moment of their birth (i.e., when the excited singlet states of the aryl esters undergo lysis), the geminal radical pairs mustbe in positions that make their shape similar to that of their precursor ester the radical center of the acyl part is very near the oxygen atom of the aryloxy part, and all subsequent diffusion of the two species starts from this orientation. As a result, addition of an acyl radical to its aryloxy partner is favored spatially at the nearer adj acent ortho) position(s) than at the more distant meta and para positions. However, the ability of the acyl radical to add to each of the positions of an aryloxy radical is expected to depend on the energies of the adducts" ° (which are keto intermediates that enolize thermally with time to the eventual products Equation 13.5 shows an example of the keto intermediate for acyl addition to the ortho position of phenoxy). The spin densities at the 2-, 4-, and oxy-positions of 1-naphthoxy (and of phenoxy) from ESR measurements and HF/6-31G level calculations (in parentheses) are shown in Fig. 13.1. Those for the 2-naphthoxy radical by ESR measurements " and MNDO-UHF calculations are collected in Table 13.1. These considerations explain why the yields of products from addition at the meta positions are very low. 

TABLE 13.1 Spin Densities (Atomic Units) of Carbon Atoms and Oxygen of the 2-naphthoxy Radical from MNDO-UHF Calculations and ESR Data... 

IXDCf is faster than MINDO/3, MNDO, AMI, and PM3 and, unlike C XDO, can deal with spin effects. It is a particularly appealing choice for UHF calculations on open-shell molecules. It is also available for mixed mode calculations (see the previous section ). IXDO shares the speed and storage advantages of C XDO and is also more accurate. Although it is preferred for numerical results, it loses some of the simplicity and inierpretability of C XDO. 

The transition-state geometries for the cyclization of a range of electrophilic radicals have been obtained by MNDO semiempirical calculations.42 The regioselectivities observed were rationalized using a frontier orbital approach. Semiempirical calculations were not suited for describing the cyclization of the pent-4-en-l-oxyl radical.43 Instead, results indicated that ab initio (UHF/6-31G ) and (UBP/DZVP) were more reliable. 

A. T. Pudzianowski and G. H. Loew, /. Phys. Chem., 87, 1081 (1983). Hydrogen Abstractions from Methyl Groups by Atomic Oxygen. Kinetic Isotope Effects Calculated from MNDO/UHF Results and an Assessment of Their Applicability to Monooxygenase-Dependent Hydroxylations. 

Ab initio calculations. MNDO calculations. UHF calculations. ) MINDO/3 calculations. ... 

The constants ajso(N) and ajso(F) were obtained by a number of ab initio studies UHF ( Nesbet ) [14], UHF and UHFASA [15 to 19], comparison of the UHF, PUHF, PO, and SECI methods [20], RHF + CI [19], and some semiempirical calculations MNDO [21], MINDO within UHF and UHFASA formalisms [22], and INDO (s-electron spin densities aiso/a ef only given, see p. 233) [23]. 

MINDO/3, MNDO, MNDO/C, AM1, and PM3 are implemented in VAMP. Default calculations employ the RHF formalism (half-electron for open-shell species), but UHF and annihilated UHF (AUHF) are also available. 

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