Infra-red and Raman spectroscopy of free-base and zinc phthalocyanines isolated in matrices

Abstract

The infrared absorption spectra of matrix-isolated zinc phthalocyanine (ZnPc) and free-base phthalocyanine (H 2 Pc) have been recorded in the region from 400 to 4000 cm À1 in solid N 2 , Ar, Kr and Xe. Raman spectra have been recorded in doped KBr pellets. The isotopomers HDPc and D 2 Pc have been synthesised in an attempt to resolve the conflicting assignments that currently exist in the literature for the N-H bending modes in H 2 Pc spectra. A complete correlation between the vibrational modes of the three free-base isotopomers and ZnPc has been achieved. Comparison of the IR and Raman spectroscopic results, obtained with isotopic substitution and with predictions from large basis set ab initio calculations, allows identification of the in-plane (IP) and out-of-plane (OP) N-H bending modes. The largest IP isotope shift is observed in the IR at 1046 cm À1 and at 1026 cm À1 in Raman spectra while the largest effect in the OP bending modes is at 764 cm À1 . OP bending modes are too weak to be observed in the experimental Raman data. The antisymmetric N-H stretching mode is observed at B3310 cm À1 in low temperature solids slightly blue shifted from, but entirely consistent with the literature KBr data. With the exception of the N-H stretches, the recorded H/D isotope shifts in all the N-H vibrations are complex, with the IP bending modes exhibiting small n H /n D ratios (the largest value is 1.089) while one of the observed OP modes has a ratio o 1. DFT results reveal that the small ratios arise in particular from strong coupling of the N-H IP bending modes with IP stretching modes of C-N bonds. The unexpected finding of a n H /n D ratio smaller than one was analysed theoretically by examining the evolution of the frequencies of the free base by increasing the mass from H to D in a continuous manner. A consequence of this frequency increase in the heavier isotopomer is that the direction of the N-D OP bend is reversed from the N-H OP bend.

Solvent impact on the planarity and aromaticity of free and monohydrated zinc phthalocyanine: a theoretical study

“…It is worth mentioning that the recent X-ray studies [26] on three axial ZnPc•••n-alkylamines nicely agree with our data and prove the existence of similar nonplanar structures with d parameter of about 0.5 Å. In addition, the subsequent B3LYP/6-31+G(d) [70] b HF, B3LYP, and MP2 with 6-31G* result from [41] c UB3LYP/6-31G(d,p) data for ZnPc, from [71] d B3LYP/6-31G** data from [34] e B3LYP/6-31G* data for ZnPc, from [72] f X-ray data for ZnPc, from [20] g ED data from [21] h Reinvestigation of ED data from [22] i ED data for SnPc, from [22] j ED data for MgPc, from [22] k Laser-desorption supersonic jet spectroscopy data for H2Pc with D 2h symmetry; planar are also MgPc and ZnPc with D 4h symmetry, from [27] l ED data from [35] [75] e X-ray data for ZnPc•••n-hexylamine, from [24] calculations by these authors [26] closely reproduced the observed crystal structure of these solvates (d was about 0.43 Å …”

Free base tetraazaporphine isolated in inert gas hosts: Matrix influence on its spectroscopic and photochemical properties

“…This is currently the most effective theoretical method for obtaining ground state molecular vibrational frequencies. Moreover, as established in our previous study 27 of zinc and free base phthalocyanine, the Raman intensities are also reliable. A variety of basis sets were utilised during the course of this work -specifically the 6-31g(d), 6-311G++(2d,2p), and 6-311G++(3df,3pd) -but only the results of the largest one are utilised in this work.…”

A DFT study of reversed isotope shifts in H/D substitution of free-base porphyrin and related free-base tetrapyrroles

“…9 However, as will be shown later, the upward shifts in the heavier isotope were much larger than what we have seen with the force fields generated in the present quantum chemical method. The earlier work 8 only considered in-plane vibrations, i.e., the N-H stretches and in-plane bending modes, but as we have found experimentally 6 , the most easily identifiable instance of the reversed ISR effect in the phthalocyanines involved an out-of-plane N-H bending motion. A more recent DFT calculation of free-base phthalocyanine by Liu et al also found a reverse ISR.…”

“…Moreover, for the free-base tetrapyrroles, H/D substitution of the two inner hydrogens is particularly appealing as the D 2h molecular symmetry is conserved, allowing direct comparisons between the two isotopologues. However, in a recent combined IR/Raman and high-level density functional theory (DFT) study 6 of free-base phthalocyanine (H 2 Pc), we observed quite peculiar behaviour upon H/D substitution, whereby the frequency of one particular mode of the deuterium isotopologue was found to be higher than that of hydrogen. The mode in question is the IR-active out-of-plane bending vibration observed in Ar matrices at 722.7 cm -1 for H 2 Pc, but which shifts up to 729.9 cm -1 in D 2 Pc.…”

“…Moreover, as demonstrated in our previous study of both zinc and free-base phthalocyanine, the Raman intensities are also reliable. 6 Another advantage of this method is that it runs efficiently on multiprocessor computers. The present calculations were conducted with the Gaussian-03 suite of programmes 13 running, as described elsewhere 6 , on a Linux workstation with two quad-core processors.…”