Structural and vibrational study of isochroman

The fully optimized equilibrium structure of isochroman has been obtained from DFT calculations by using the hybrid B3LYP and B3PW91 functionals in combination with different basis sets (6-31G∗, 6-311G∗∗ and 6-311++G∗∗). Due to the absence of any experimental geometry data for isochroman, the optimized molecular structure has been compared to those of other compounds like naphthalene, benzene, cyclohexane, tetralin, 2H-pyran and chroman. The vapor, liquid, and solution (CCl4 and CS2) infrared (IR) and liquid Raman spectra have been recorded. The harmonic force field generated with B3LYP/6-31G∗, given in terms of the natural internal coordinates, has been scaled by the recommended scale factors by Rauhut and Pulay, using their multiple scaling (SQM) force field procedure. In order to get the best possible fitting between the calculated and experimental frequencies, the scale factors were refined by a linear least squares procedure obtaining a final rms of 7.5 cm−1. Furthermore, supposing a Lorentzian band contour, the infrared spectrum of isochroman has been simulated using the theoretical and scaled force fields. On the basis of the frequencies and infrared intensities calculated from the refined SQM force field, the first vibrational assignment of the normal modes of the individual isochroman molecule is presented, having resulted the infrared intensity calculation decisive in this case for assigning the fundamentals in the low energy region where the most of the uncertainties appeared. In addition, a new method has been developed to characterize the similarity between the experimental and calculated spectra: the inspection of their scalar product, yielding the best result for isochroman in the case of scaled force fields.