Isotope effect in the infrared spectra of free-base phthalocyanine and its N,N-dideuterio-derivative: density functional calculations

Abstract

Infrared frequencies and intensities for the free-base phthalocyanine H 2 Pc and its N , N -dideuterio-derivative D 2 Pc have been calculated at density functional B3LYP, RBLYP and SVWN levels using the 6-31G ∗ basis set to investigate the isotope effect in the IR spectra. The calculated vibrational frequencies were scaled by the factors 0.9613, 0.9940 and 0.9833 for 6-31G ∗ basis set at B3LYP, RBLYP and SVWN levels, respectively. Detailed assignments of N–H related vibrational bands in the IR spectra have been made on the basis of comparison between the calculated data of H 2 Pc and D 2 Pc as well as the available experimental results of corresponding β form compounds. The normal mode descriptions were obtained with assistance of animated pictures produced on the basis of the normal coordinates. In the IR spectrum of H 2 Pc, the typical N–H stretching, in-plane bending (IPB) and out-of-plane bending (OPB) modes are assigned to the bands observed at 3273, 1250 and 752 cm −1 , respectively. The bands observed at 1539, 1441, 1043 and 735 cm −1 that show isotope effects are also interpreted. The characteristic band of H 2 Pc at 1005 cm −1 is re-interpreted as a C–N (pyrrole) in-plane bending mode instead of the N–H in-plane or out-of-plane bending mode assigned by traditional empirical method. Calculations at B3LYP/6-31G ∗ and RBLYP/6-31G ∗ levels are found more suitable for further vibrational spectroscopic investigation of phthalocyanine and its derivatives. Animated pictures of the vibrational modes are useful for exact assignment of the correspondence between the vibrational modes of H 2 Pc and D 2 Pc.