3246 (h) J. 8. Hendrickson, J. Am. Chem. SOC.,89, 7036 (1967). (2) H. van Bekkum, M. A. Hoefnagel. L. de Lavieter, A. van Veen, P. E. Verkade, A. Wemmers. B. M. Wepster, J. H. Palm, L. Schfer. H. Dekker, C. Mosselman, and G. Somsen, R e d Trav. Chim. Pays-Bas, 86, 1363 (1967). (3) E. L. Eliel, "Stereochemistry of Carbon Compounds", McGraw-Hill. New York. N.Y.. 1962. D 207. (4) The value given is based on A@ = 5.7 kcal/mol and ASo = 4 eu.lb (5) F. A. L. Anet and M. Squillacote, preceding paper. (6) R . J. Abremski, C. W. Brown, and E. R. Lippincott, J. Chem. Phys., 49, 185 (1 968). (7) The band system near 1350 cm-' is moderately strong in the crystal6 but is virtually absent in the matrix. (8)H. E. Hallam in "Vibrational Spectroscopy of Trapped Species", H. E. Hallam, Ed., Wiley, New York, N.Y., 1973, p 67. (9) For assignments of the vibrational spectra of cyclohexane (and CeDI2). see ref 6 and references therein, and K. B. Wiberg and A. Shrake, Spectrochim. Acta, Part A, 27, 1139 (1971); H. M. Pickett and H. L. Strauss, J. Chem. Phys., 53, 376 (1970); M. A. Moskaleva and G. N. Zhizhin. Opt. Spectrosc. (USSR).36, 535 (1974). (10) Nonequilibrium rotamer populations have been reported in films deposited from vapors at room temperature onto cold surfaces, but quantitative aspects of this phenomenon do not appear to have been investigated: R . G. Snyder and J. H. Schachtschneider, J. Mol. Spectrosc., 30, 290 (1969); G. A. Crowder and H. K. Mao. J. Mol. Struct., 18, 33 (1973); G. A. Crowder and P. Pruettiangkura, ibid., 18, 177 (1973); A. J. Barnes and H. E. Hailam, Trans. Faraday SOC.,86, 1932 (1970); see also ref 8, pp 115 and 116. (11) On further warming to 170'K and cooling back to 1O0K, a very sharp infrared spectrum identical with that described6 for (annealed) crystalline phase li cyclohexane is obtained showing that no chemical decomposition of the cyclohexane took place during the brief heating at 800'. (12) S. Benson, "The Foundations of Chemical Kinetics", McGraw-Hill. New York, N.Y., 1960, p 250. (13) Interconversions between rotamers have been observed in an inert gas matrix at 35'K, 0. L. Chapman, C. L. McIntosh, and J. Pacansky, J. Am. Chem. SOC., 95, 244 (1973); photochemical interconversions have been reported at 20°K. A. Krantz, T. D. Goldfarb. and C. Y . Lin, ibid., 94, 4022 (1972). (14) F. A. L. Anet and A. J. R. Bourn. J. Am. Chem. SOC.,89, 760 (1967). (15) National Science Foundation Predoctorai Fellow.
M. Squillacote, R. S. SheridanI5 0.L. Chapman,* F. A. L. Anet* Contribution No. 3443, Department of Chemistry University of :California Los Angeles, California 90024 Received January 20, 1975
I
400
350
A.nm
'
660
Figure 1. Quenching of 9CNP-p-BA exciplex by fumaronitrile: kqr,ralr = 90 IU-1,
Table I. Rate Constants for Excidex Fluorescence OuenchinP ~~~~
~~~~
Quenchers Ph,N
9CNP-trans 9CNPAnethole p-BA Anth-DEA' 11.3 (8.0)b 39 (16)b 120 (22)b 1.9 x l o 8 1.5 x lo8 1.5 x i o 8 2.0 x io8 1.4 x 107 3u(F,), 20 < 52.9O); Mn(N4C44H28).2C,Hg, triclinic; a = 11.320 (6), b = 11.465 (6), c = 10.487 (6) A; a = 110.63 (2), p = 103.34 (3), and y , 107.80 (3)'; pcaicd = 1.258 g / cm3; Z = 1; space group Pi; R= 0.092 and R2 = 0.068; 5659 unique data (F, > 2u(F,), 20 < 63.7O). T h e structure of Mn(TPP) provides one answer to an interesting question of porphyrin stereochemistry, namely, whether independent of coordination number, a large metal ion takes an out-of-plane position from the porphinato nitrogen atoms. With one molecule per the point group symmetry (real or statistically effective) required of the Mn(TPP) molecule is Ci. Refinement with the Mn(I1) ion positioned a t the center of symmetry (center of the porph rin molecule) leads to a Mn-N bond distance of 2.082 , possibly consistent with a high-spin ion.9 However, the anisotropic thermal parameter of the Mn( 11) ion, perpendicular to the mean porphinato plane, is unrealistically large (root mean square displacement 0.351 A). An alternate refinement in which the Mn(I1) ion was allowed to take an out-of-plane position, while retaining a statistical center of symmetry, leads to a slightly less unrealistic thermal parameter (root mean square 0.28, Figure l ) despite a n identical R factor. Either alternative suggests that the M n atom is out-of-plane in accord with theoretical calculations'0
-
8:
Communications to the Editor
