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not affect the ultimate fit of calculated and measured crystal

properties.

The same conclusion was obtained again when the consistent
We could then
conclude with confidence that, to a reasonable approximation, the

force field was extended to carboxylic acidsll.

hydrogen bond is a non—bonded interaction between two polar bonds,

where the attractive forces are mainly electrostatic.
Heals radius of the hydrogen atom in the OH or NH bond was found
to be so small as to be practically engulfed by the large Van der
waals radius of the electronegative atoms N or 0.‘-This allowed

the contact distance to become small enough, and the electrostatic
attraction large enough, to make this non—bonded interaction look

like a weak bond. As noted above, recent extended ab initio

The Van der

ca1Cu1ations support this new concept of the hydrogen bondls’lq.

The essence of the OFF method, may be illustrated by Table 1
from the "bench mark” paper23

Table l.

The table represents the root mean

Carboxylic Acids and Amides by Three Force Fields

Root Mean Square Deviations-of Properties Calculated for

 

 

 

No. of rms dev'
Property Units terms 12—6—1 9—6—1 MCMS

Acids
energy kcalémol 12 2.486 2.053 2.118
UCV length A 42 0.489 0.307 0.604
UCV angle dgg 17 3.456 2.856 4.465
volume g 14 15.911 16.772 18.876
d<4 % 14 0.247 0.190 0.322
H---O dist g 16 0.062 0.072 0.058
0---O dist A 16 0.047 0.071 0.041
C—O~‘-O angle deg 16 11.071 9.881 14.048
O-—-O=C angle deg 16 7.843 7.760 11.786
H...0=c angle deg 16 12.362 12.144 17.985
180°—0—H'--0 deg 16 8.491 7.732 11.710

Amides
energy kcalémol 6 1.574 1.930 8.446
UCV length A 36 0.208 0.235 0.261
UCV angle dgg 14 1.824 1.261 2.385
volume g 12 7.057 17.797 13.951
dd; .3. 12 0.145 0.145 0.164
H---0 dist g 30 0.049 0.059 0.056
N°"0 dist A 30 0.055 0.055 0.076
C—N---0 angle deg 22 3.337 3.575 4.071
N~-o0=C angle deg 22 5.931 5.502 9.257
H'--0=C angle deg 30 5.830 5.609 7.329
180°"NH--—0 deg 30 4.396 3.894 4.093

 

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