Isolated impact of ruffling on the vibrational spectrum of Ni porphyrins. Diagnosing out-of-plane distortions

Hui Hsu Gavin Tsai, M. Cather Simpson

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22 Scopus citations

Abstract

The intrinsic effect of the B1u ruffling (ruf) distortion on the heme vibrational frequencies was evaluated using density functional theory (B3LYP/6-31 G(d,p)). Ni(II) porphine (NiP) was constrained for a wide range ruffling angles (τruf = the cross macrocycle Cα-N---N-Cα angle), but otherwise freely geometry optimized, and vibrational frequencies were calculated at each point. This approach allowed the impact of the out-of-plane distortion to be isolated from complicating factors arising from peripheral substituents, the environment, and the metal-macrocycle interaction. The potential energy surface revealed a minimum energy structure at τruf = 22.8° and a small barrier at the planar conformation of 0.162 kcal mol-1. A clear pattern of vibrational shifts driven by the ruffling distortion was observed. Seven degenerate and 17 nondegenerate modes shift by > 10 cm-1 upon ruffling by 45°. In general, shifts to lower frequencies were seen for higher frequency modes and vice versa. The in-plane asymmetric Cα-Cm stretches v10(B1g), v19(A2g), and v37(Eu) showed the largest downshifts with ruffling, followed closely by their symmetric counterparts v3(A1g), v28(B2g), and v39(Eu) and the Cβ-Cβ stretches v2(A1g), v11(B1g), and v38(Eu). The Ni-N stretches v8 and ν18 upshifted significantly, as did some of the in- and out-of-plane pyrrole motions, most notably v53(Eu) and γ22(Eg). The vibrational shifts were fit excellently by a cos(τruf) function, indicating that the major influence of OOP ruffling upon the vibrational spectrum, and probably porphyrin chemistry and photochemistry as well, is reduced π-overlap.

Original languageEnglish
Pages (from-to)1224-1232
Number of pages9
JournalJournal of Physical Chemistry A
Volume108
Issue number7
DOIs
StatePublished - 19 Feb 2004

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