The equilibrium geometries of the methyl halides

Values of A e and B e for the H 3 and D 3 methyl chlorides, bromides, and iodides are calculated from the best present estimates of values of A 0 , the microwave values of B 0 , and the α r a and α r b data. The theoretical A e (CH 3 X/ A e (CD 3 X) ratio is well obeyed in methyl iodide and bromide, but a discrepancy of 1 % is observed in methyl chloride. It is concluded that A e (CD 3 Cl) is slightly high, possibly as a result of the lack of precise data available from the strongly overlapped and Coriolis interacting fundamentals v 2 and v 5 . The equilibrium rotational constants predict the molecular parameters: methyl chloride r e (CH) = 1.086 ± 0.004 A, R e (CCl) = 1.778 ± 0.002 A, α e (HCH) = 110° 40'± 40'; methyl bromide r e (CH) = 1.086 ± 0.003A, R e (CBr) = 1.933 ± 0.002 A, α e (HCH) = 111° 10'± 25'; methyl iodide r e (CH) = 1.085 ± 0.003 A, R e (Cl) = 2.113 ± 0.002 A, α e (HCH) = 111° 17'± 25'; For methyl fluoride, for which the α r a and α r b data are not so complete, the A 0 and b 0 values are corrected to A e and B e by considering the trends in ( A e – A 0 ) and ( B e – B 0 ) in the other halides. Almost precise agreement with the theoretical ratio is obtained with the A e values. The equilibrium rotational constants predict the molecular parameters: methyl fluoride r e (CH) = 1.095 A, R e (CF) = 1.382 A, α e (HCH) = 110° 27'. No estimates of uncertainty are made, as the results depend entirely on the validity of the extrapolation process used to obtain A e and B e values. The considerably larger CH bond length in comparison with the other halides is supported by the observations that the unperturbed CH 3 and CD 3 stretching vibration frequencies are much lower, and that the CH stretch force constant is significantly smaller, than in the other halides.