The intrinsic effect of the B-1u ruffling (ruf) distortion on the heme vibrational frequencies was evaluated using density functional theory (B3LYP/6-3 I G(d,p)). Ni(II) porphine (NiP) was constrained for a wide range ruffling angles (tau(ruf) = the cross macrocycle C-alpha-N---N-C-alpha 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 tau(ruf) = 22.8degrees 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 45degrees. In general, shifts to lower frequencies were seen for higher frequency modes and vice versa. The in-plane asymmetric C-alpha-C-m stretches nu(10)(B-1g), nu(19)(A(2g)), and nu(37)(E-u) showed the largest downshifts with ruffling, followed closely by their symmetric counterparts nu(3)(A(1g)), nu(28)(B-2g), and nu(39)(E-u) and the C-beta-C-beta stretches nu(2)(A(1g)), nu(11)(B-1g), and nu(38)(E-u). The Ni-N stretches nu(g) and nu(18) upshifted significantly, as did some of the in- and out-of-plane pyrrole motions, most notably nu(53)(E-u) and gamma(22)(E-g). The vibrational shifts were fit excellently by a cos(tau(ruf)) function, indicating that the major influence of OOP ruffling upon the vibrational spectrum, and probably porphyrin chemistry and photochemistry as well, is reduced pi-overtap.