PCILO calculations

Compound Energy (koal/mole), with respect to the individual global minimum, from PCILO calculation. Angle of rotation, (C4-N8), in degrees Barrier heights6 to rotation (kcal/mole) - from CNDO/2 calcu-lation (Rao189,197)... 

Fig. 31. Conformation of the disaccharides forming repeating unit of the backbone of mureine from PCILO calculations...

The foregoing relationship was utilized in conformational analysis of methyl 4-thio-a-maltoside in solution.153 The VC H values of 2.95 and 5.15 Hz measured151 for H-4 —C-4 —S—C-l—H-l differ from those predicted from Eq. 45 37c 4. h-i = 3.1 Hz and 37C-i,h-4 = 3.8 Hz for the conformation in the crystal154 as defined by H = 25.6° and PH = 3.6°. The observed VC H values were not reproduced by the calculated values for any of the stable conformations of methyl 4-thio-a-maltoside, but are in good agreement with the ensemble average (37c.h) values 2.67 and 5.24 Hz calculated for the compound in water solution, using PCILO-calculated potential surfaces. 

TABLE 12 PCILO CALCULATED ENERGIES AND EXTENT OF CHARGE TRANSFER IN H-BONDED COMPLEXES OF OPIATE... 

The map for Sympath-1 (Figure 3g) appears to locate the minima properly, however, the shape is somewhat different from that calculated by PCILO. We show a larger repulsion about the center of the map. This could be a function of the resolution used in the PCILO calculations, but more likely involves basic differences between the two methods. For our purposes this map is sufficiently accurate. 

Conformational results are often sensitive to the input geometry used. For instance, Pullman at at. (68) carried out two PCILO calculations for mescaline, using the x-ray crystal geometries obtained for the hydrobromide and hydrochloride salts these salts exist in gauche and extended conformations, respectively. 

The most extensive studies of phenylethylamines have been done by Pullman and coworkers (59,65,67/68). Their PCILO calculations indicated that gauche and trans conformers have nearly identical energies in the majority of phenylethylamines and phenylethanolamines studied. AI studied were carried out using three different basis sets (67) these all indicated a slight but definite preference for gauche conformers. These authors noted the preponderance of extended conformers in crystal structures and in solution they attributed this to environmental forces. To test this hypothesis, computations were repeated with the inclusion of water molecules, and a tendency toward the extended conformation was indeed observed. 

Fig. 15. Conformation map of the glycyl residue (PCILO calculations). Isoenergy curves in kcal/mole with respect to the global minimum +. + Local minima...

It is seen that in both cases the quantum-mechanical calculations impose fewer restrictions on the allowed or preferred conformational space than do the empirical ones. Thus, while for GLY this space is made up in the empirical computations by four separate zones A, B, C and D, these zones communicate between themselves in the PCILO calculations. In the case of ALA, the PCILO calculations substantially increase the limits of this space, with re-... 

Limits of PCILO calculations within 6 kcal/mole above above the global minimum... 

PCILO calculations 69Head to the conformational energy curve of Fig. 29, in complete agreement with experiment. 

Similar PCILO calculations on methyl a- and )9-D-glucopyranoside established that the a anomer is the more stable, and the calculated energy difference is 4.2 kj.mol". A preference for the sc orientation about the C-1-0-1 bond was suggested by calculations for both anomers. The ap arrangements about the anomeric C-1 -0-1 bond in the a and)9anomers are... 

PCILO Calculated Bond Lengths (pm) and Bond Angles (degrees) for 2-Substituted Oxane Derivatives, Compared with... 

PCILO Calculated Net Atomic Charges" and Dipole Moments of the Most Stable Conformers of 2-Methoxyoxane ... 

Remko, M., and J. Polcin. 1980. MO investigations on lignin model compounds. XII. PCILO calculations of conformational structure and hydrogen bonds in 2-2 -dihydroxybihenyl, 2-2 -dimethoxybiphenyl and 2-2 dihydroxy-3 -methoxybiphenyl. Z Phys. Chem. (Wiesbaden), 120(1) 1-8. 

NMR studies and PCILO calculations of the tautomerism of triazinethiones 243 (R = H, Me) favor the imino-thione form 243a both in the gas phase and in solution (79RTC503). 

In the isolated molecule of barbituric acid and its 5-mono-, 5,5-di-, and 1,3,5,5-tetrasubstituted derivatives, all with the same 1,3-substituents, a flat or almost flat pyrimidine ring is symmetric about the C2 line (C-5 C-2)149,150 (Fig. 4). CNDO/2 and PCILO calculations show that for the isolated molecule of 5,5-diethylbarbituric acid a distribution of atomic charges is symmetric about that C2 line.60,151... 

Yadav JS, Barnickel G, Bradaczek H. et al. Quantum chemical studies on the conformational structure of bacterial peptidoglycan. II. PCILO calculations on the monosaccharides. J Theor Biol 1982 95 151-166. 

Table III. PCILO calculated electron densities for selected carbon and oxygen atoms as a function of hydroxymethyl groups orientations of methyl 3-D-cellobioside in crystal conformation. 

QCFF/PI and PCILO calculations , as well as a combined MM3, NMR X-ray crystallography and structure comparison study , were used to identify biologically active... 

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