Testing a new analytical approach for determination of vibrational transition moment directions in low symmetry planar molecules: 1-D- and 2-D-naphthalene

A new analytical approach for improving the precision in determination of vibrational transition moment directions of low symmetry molecules (lacking orthogonal axes) is discussed in this paper. The target molecules are partially uniaxially oriented in nematic liquid crystalline solvent and are studied by IR absorption spectroscopy using polarized light. The fundamental problem addressed is that IR linear dichroism measurements of low symmetry molecules alone cannot provide sufficient information on molecular orientation and transition moment directions. It is shown that computational prediction of these quantities can supply relevant complementary data, helping to reveal the hidden information content and achieve a more meaningful and more precise interpretation of the measured dichroic ratios.The combined experimental and theoretical/computational method proposed by us recently for determination of the average orientation of molecules with Cs symmetry has now been replaced by a more precise analytical approach. The new method introduced and discussed in full detail here uses a mathematically evaluated angle between two vibrational transition moment vectors as a reference. The discussion also deals with error analysis and estimation of uncertainties of the orientational parameters. The proposed procedure has been tested in an analysis of the infrared linear dichroism (IR–LD) spectra of 1-D- and 2-D-naphthalene complemented with DFT calculations using the scaled quantum mechanical force field (SQM FF) method.

Testing a new analytical approach for determination of vibrational transition moment directions in low symmetry planar molecules: 1-D- and 2-D-naphthalene, partially oriented in nematic liquid crystal.Figure optionsDownload as PowerPoint slideHighlights
► Target molecules are partially uniaxially oriented in nematic liquid crystalline solvent.
► Analytical approach gives an exact expression for calculation of orientational parameters.
► Average orientation of 1-D- and 2-D-naphthalene is slightly different from that of naphthalene.
► Transition moment directions of weak C–D bands were predicted with good accuracy.
► Mathematical method for evaluation angle between two vibrational transition moment vectors.