2528 BENTZ B. HOWARD by Ballinger. 6 Using these values to calculate the dipoledipole energy, we get, e.g., for w= ()O, Edipole(Mulliken) = -0.01476 a.u. and Edipole (Ballinger, Lowdin) = -0.00610 a.u. On comparing these numbers with results in Tables I and II it can be seen that dipole-dipole contribution must be overwhelmingly the largest term in the multipole energy, and that the remaining part of the total energy is not very different in the two approximations. The experimental dipole moment is 0.720 a.u.,I6 so that the true value of Eel can be guessed as being about 0.002 a.u. lower than the results in Table II for small angles. The penetration terms are rather sensitive to the assumed falloff of the HB electron density with distance, however, so that this estimate 16 G. A. Kuipers, quoted in Ref. 6, p. 147. is very rough. For example, the Hartree-Fock orbital for the hydrogen atom is, of course, the hydrogenic Is, the use of which could be expected to add a larger contribution from the penetration terms.Finally, although this calculation does not really help to explain the observed 17 equilibrium value for w of about 35°-40°, it does not make it unreasonable, either. For the Lowdin approximation Eel is constant within 0.1 kcaljmole from 0° to about 45°; in fact, the actual (very shallow) minimum occurs at about 30° rather than 0°, as might have been expected intuitively from the recognition that the dipole-dipole contribution is the largest single term. 17 Since the isolated HF dimer has not been observed, we use the value for the linear chain polymer.Monomolecular films of chlorophyll a and pheophytin a on aqueous subphases have been prepared and studied. The preparation and preservation of these pigments in a state of high purity has required procedural refinements which are described. Chemical degradation in solution and in the monolayer can be prevented by appropriate precautions. At surface pressures below 22 dyn/cm for chlorophyll and 10 dyn/cm for pheophytin, the films on pH 8.0 buffer are mechanically stable in the dark at 20°C. At these pressures, there is little difference in the force-area curves of chlorophyll and pheophytin, although the surface potentials and surface dipole moments of the two pigments are markedly different. The visible absorption spectrum of the pigment monolayers on the aqueous subphase is rapidly altered by illumination, although under optimum conditions it shows little change in the dark over periods of several hours. These experiments were performed with a monolayer spectrometer, which permits measurements of the absorption spectra of monolayers in situ. This instrument is briefly described. The unified set of quantitative measurements of this study, together with previous observations in the literature, provide evidence for a monolayer structure in which the pigment molecules, with a folded configuration (both porphyrin plane and phytol sidechain rising above the water surface), are closely packed but randomly oriented in the two-dimensional array. There is no e...
