REPRESENTATION OF NONPLANAR STRUCTURES OF NICKEL(II) 5,15-DISUBSTITUTED PORPHYRINS IN TERMS OF DISPLACEMENTS ALONG THE LOWEST-FREQUENCY NORMAL COORDINATES OF THE MACROCYCLE
Xz. Song et al., REPRESENTATION OF NONPLANAR STRUCTURES OF NICKEL(II) 5,15-DISUBSTITUTED PORPHYRINS IN TERMS OF DISPLACEMENTS ALONG THE LOWEST-FREQUENCY NORMAL COORDINATES OF THE MACROCYCLE, Journal of the American Chemical Society, 118(51), 1996, pp. 12975-12988
The influence of substituents with increasing steric demands on the st
ructure of nickel(II) 5,15-disubstituted porphyrins has been investiga
ted with X-ray crystallography, UV-visible absorption spectroscopy, re
sonance Raman spectroscopy, molecular energy optimization calculations
, and INDO/s molecular orbital calculations. Nickel 5,15-diphenylporph
yrin is predicted by molecular mechanics calculations to be a mixture
of planar and nonplanar conformers. All of the nickel dialkylporphyrin
s (where the alkyl group is propyl, isopropyl, and tert-butyl) are cal
culated to be in a predominantly gabled (gab) conformation resulting f
rom an alpha alpha orientation of the substituents with respect to the
macrocycle. This nonplanar gab distortion is made up of a linear comb
ination of distortions along the lowest-frequency out-of-plane macrocy
cle normal coordinates of A(2u) (doming) and B-1u (ruffling) symmetry
types. A higher energy stable ap conformer is also predicted for dialk
ylporphyrins, and its nonplanar structure can be represented as an equ
al combination of distortions along the x- and y-components of the low
est-frequency E(g) (waving) normal coordinate. The nonplanar structure
s calculated by using molecular mechanics have been structurally decom
posed into the displacements alone the lowest-frequency normal coordin
ate of each symmetry type, and the contributions of each type to the t
otal distortion in the calculated structures agree well with contribut
ions obtained from structural decompositions of the available X-ray cr
ystal structures. The predicted gab distortion is confirmed most convi
ncingly by the X-ray crystal structure of [5,15-di-tert-butylporphinat
o]nickel(II) which is found to be in a gab alpha alpha conformation. F
inally, INDO/s calculations show that the red shifts in the absorption
spectra of the nickel disubstituted porphyrins are caused by the incr
easing nonplanarity resulting from increasing steric crowding within t
he series.