Force fields for planar and non-planar modes of fluorobenzene revisited

The force field calculations for planar and non-planar modes of fluorobenze ne have been revised using more accurately observed values of fundamentals [J Mol Spectrosc, 73 (1978) 290] of the parent molecule and its two deutera ted analogues namely, fluorobenzene -p-d and -d(5). As many as 38 planar an d 17 non- planar parameter force fields have been determined leading to alt eration of the earlier proposed vibrational assignments for some of the nor mal modes. In addition, two types of ring torsional coordinates, namely, I: (Delta phi(cccc) + Delta phi(xccx)) and II : (Delta phi(xccc) + Delta phi( cccc) + Delta phi(xccx) + Delta phi(xccx)) were used separately to study th e effect of the definition on the force constants. It is found that the pri ncipal ring torsional force constant in the set (I) are nearly four times o f those in the set (II). As expected non-diagonal force constants are also seen to be affected, however, to small degrees. In the present case 3 princ ipal force constants corresponding to the 3 pairs of the C-C bond stretchin g motions have been taken. Similarly, corresponding to 3 types of the C-H s tretching, 3 types of the C-H bending and 4 types of the C-C-C angle bendin g motions, the number of principal force constants are 3, 3 and 4 respectiv ely. The variation of the 3 C-C stretching principal force constants from t heir average value is within 2.3%. Similarly, the C-H stretching, the C-H p lanar bending and the C-C-C angle bending principal force constants show va riations within 0.9%, 7.6% and 29.4% respectively from their average values . Variations of the non-planar principal force constants involving the C-H non-planar bendings and the C-C torsions ale within 14.6% and 1.3% for the set I and within 10.5% and 23.4% for the set II.