Abstract
The ab initio variational method for handling the Renner–Teller effect in tetraatomic molecules, developed in a previous work, is extended to enable the calculation of the vibronically averaged values for the hyperfine coupling constants. The approach is applied to compute the vibronic mean values for the isotropic hyperfine coupling constants (hfcc's) of hydrogen and boron centers in theX2Πuelectronic state of[formula]. The present study clearly shows that for the isotropic hfcc of the boron center the incorporation of the vibronic effects is essential already for the lowest vibronic state. The isotropic hfcc's calculated for the lowest vibronic state (Aiso(11B) = 12.6 MHz,Aiso(H) = 47.2 MHz) differ considerably from the isotropic hfcc's calculated at the equilibrium geometry (Aiso(11B) = 1.3 MHz,Aiso(H) = 47.9 MHz). The strong influence of the vibronic effects can also be seen inAiso(11B) calculated for[formula]which should be identical to that of[formula]within the Born–Oppenheimer approximation. Including vibronic effects, the values differ by about 20%.
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