INFRARED SPECTRUM OF 15NF3 AND FORCE FIELD

15NF3 was prepared and its infrared spectrum studied. The band due to ν1 was resolved and analyzed, yielding the parameters ν1 = 1008.93, B″ = 0.3541, and α1B = -0.00128 cm-1. The band due to ν3 was analyzed by computer simulation and the values ν3 = 886.3, α3B = +0.0027, α3C = +0.0007 cm-1, ζ3 = +0.77 obtained. 14N-15N isotope shifts of 2.32 and 0.60 cm-1, respectively on ν2 and ν4 were obtained by comparison of the Q branch contours. The isotope shift Δν4 together with the ζ3 and ζ4 values for 14NF3 and 15NF3 define identically two sets of force constants in the E species, neither of which can be disregarded on the basis of existing experimental evidence. These two sets predict precisely equal frequencies, all possible ζ constants, and contributions to the centrifugal distortion constants. Of the two sets of force constants defined by Δν2 in the A1 species, only one yields contributions to the centrifugal distortion constants which give DJ and DJK values in good agreement with observed ones. The importance of examining all possible stes of force constants fitting experimental data is stressed. © 1969.