5656
J . Phys. Chem. 1988, 92, 5656-5666
Molecular Orbital Studies of Methylation Effects in Aromatic Hydrocarbons. 1. Ab Initio Calculations of the Structure, Electronic Properties, and Energy of Toluene, 1-Methylnaphthalene, and 2-Methyinaphthalene Philip George: Charles W. Bock,* John J. Stezowski,s Thomas Hildenbrand,s and Jenny P. Glusker* Department of Biology, Leidy Laboratories, University of Pennsylvania, Philadelphia, Pensyluania 19104-6018, Philadelphia College of Textiles and Science, Henry Avenue and School House Lane, Philadelphia, Pennsylvania 19144, Institut f u r Organische Chemie, Biochemie und Isotopenforschung, Universitat Stuttgart, Pfafenwaldring 55, 0-7000 Stuttgart 80, Federal Republic of Germany, and The Institute for Cancer Research, The Fox Chase Cancer Center, 7701 Burholme Avenue, Philadelphia, Pennsylvania 191 1 1 (Received: December 15, 1987)
Ab initio geometry, energy calculations, and Mulliken population analyses have been carried out for toluene, naphthalene, and the two monomethylnaphthalenes to obtain information on the effect of methyl groups in aromatic systems. The staggered-eclipsed (s-e) and orthogonal rotamers of toluene were calculated by using the 6-31G and 6-31G* (SD) basis sets; the staggered-eclipsed, orthogonal, and eclipsedstaggered (e-s) rotamers of 1-methylnaphthalene (1-MN) and 2methylnaphthalene (2-MN) were calculated by using the 6-31G basis set. When the methyl C atoms and the ring system were kept coplanar, the most stable rotamers were found to be s-e toluene, s-e 1-MN, and e-s 2-MN, with energy barriers for rotation of the methyl groups of 0.05, 10.2, and 3.1 kJ mol-', respectively. Additional calculations for orthogonal toluene, with relaxation of the planarity constraint, gave a nonplanar structure that is more stable by 0.03 kJ mol-l, so that the barrier to rotation is reduced to 0.02 kJ mol-'. Methylation has a marked effect on the geometry of the aromatic rings, and significant changes accompany the interconversion of the various rotamers-not only in the methylated rings in 1-MN and 2-MN but in the other rings as well. Upon rotation, the length of a given methyl H-C bond and the charge on this H atom pass through a maximum when the bond is perpendicular to the ring plane, whereas the overlap population passes through a minimum. In the most stable rotamers of 1-MN and 2-MN a methyl H atom eclipses the adjacent carbon-carbon bond in the ring that has the greater r-bond order, and there is a positive (i.e., bonding) overlap population between this H atom and the nearby ring H atom. This structural feature, which also characterizes the most stable rotamer of many aliphatic molecules of the type CHpCH=X, has been invoked to predict the most stable rotamers of methyl derivativesof larger polycyclic aromatic hydrocarbon ring systems and appears to agree with some preliminary solid-state results by X-ray crystallographictechniques.
Introduction It has long been known that methylation can exert a profound influence on the carcinogenic potency of polycyclic aromatic hydrocarbons (PAHs)-inducing it, enhancing it, reducing it compared to the parent molecule, or abolishing it altogether.' More recent research has shown that oxidative metabolites of PAHs, that is, epoxides, diols, and diol epoxides, are more directly responsible for the carcinogenic activity.* The changes in the aromatic ring systems caused by methylation nevertheless provide us with a basis of understanding in more detail those particular structural and electronic properties of PAHs in which the carcinogenic activity originates. The very wide range of values for the methyl group rotation barrier and the lack of any straightforward correlation with steric effects show the interaction between the methyl group and the ring to be quite specific and complicated in nature. In toluene (I, Chart I) the energy barrier determined from microwave measurements3 is extremely small, only 0.059 kJ mol-', which would seem to indicate almost free rotation. Electron diffraction: microwave s t u d i e ~and , ~ several molecular orbital (MO) calculations using a variety of basis setsb* demonstrate that the bond lengths and angles in the ring of toluene, especially near the methyl group, are significantly different from those for benzene. In o-xylene (11), where two methyl groups are in close proximity, the barrier is much larger, ranging from 6.2 to 9.2 kJ mol-' in seven different experimental investigation^.^ On the other hand, in 1-methylnaphthalene (111) and 1,4-dimethylnaphthalene (IV) the barriers derived from nuclear spin relaxation time measureUniversity of Pennsylvania. *Philadelphia College of Textiles and Science. Universitat Stuttgart. *Towhom correspondenceshould be addressed at The Institute for Cancer Research.
CHART I
ie II
I
Me
111, 1-MN
Me
IV
VI
VII. 2-MN
ments lie in the same range, 8.8, 9.0, and 9.6 kJ mol-','*'* and 9.2 kJ rnol-',lo respectively, even though there might appear to (1) (a) Schoental, R. In Polycyclic Hydrocarbons;Clar, E., Ed.; Academic: London, 1964; Chapter 18, pp 133-160. (b) Badger, G. M. Er. J . Cancer 1948.2, 309-350. (c) Dipple, A. In Chemical Carcinogens;Searle, C. E., Ed.; ACS Mongraph 173; American Chemical Society: Washington, DC, 1976; Chapter 5, p 245-314. (d) Hecht, S. S.; Amin, S.; Melikian, A. A.; LaVoie, E. J.; Hoffman, D. In Polycyclic Hydrocarbons and Carcinogenesis;Harvey, R. G., Ed.; ACS Symposium Series 283; American Chemical Society: Washington, DC, 1985; Chapter 5 , pp 85-105.
0 1988 American Chemical Society
Methylation Effects in Aromatic Hydrocarbons
The Journal of Physical Chemistry, Vol. 92, No. 20, 1988 5657
i s-e toluene
Figure 2. Staggered methyl groups in o-xylene.
S-8
1-MN
Pa I
I
H5
H4
Figure 1. Conformations of methyl groups and definition of terms.
be far less steric strain in the region of the single methyl group. Furthermore in 1,8-dimethylnaphthalene (V), where steric hindrance between the two methyl groups in the peri position might seem to be considerably more unfavorable to rotation than that in o-xylene, the barrier is not all that much larger, namely, 11.7 kJ mol-'.'O In 7,12-dimethylbenz[a]anthracene(VI) there is the expected contrast. The barrier for the methyl group in position 7 is much less than that for the group in position 12 (the bay region), the former being
