Abstract
The influence of vibration on rotatory strength in the adiabatic approximation is considered and vibronic band rotatory strengths are developed. At a level of approximation which gives ``forbidden'' character and mixed polarization in ordinary absorption, circular dichroism with mixed sign can be expected. At this level a nonvanishing contribution to integrated rotatory strength obtains from vibrational perturbation, even in the low-temperature limit. Under such circumstances the sign of rotatory strength is not simply related to molecular conformation but depends: (1) on symmetry of the perturbing vibration(s), (2) the sign of ``borrowed'' rotatory strength, and (3) the degree of electronic—vibrational coupling. An upper limit to coupled rotatory strength is estimated as 10−2 Debye—Bohr magneton units, corresponding to molar ellipticity values for reasonable uv bands of 103 deg·cm2 dmole−1. The classic case of β-octyl nitrite as well as the many recent cases of organic carbonyl compound dichroisms of mixed sign are considered as examples of the phenomena.
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